Q.137 A diesel engine crankcase ventilation system is provided to Ans. remove the combustible gases. Inside the crankcase, the oil is getting spread all around due to the moving parts of the engine. At the same time, this oil is coming in contact with the hot surfaces thus causing evaporation of oil. If this evaporated oil is allowed to condense, it forms oil mist. This oil mist is a potential hazard for the crankcase explosion. The only thing that lacks is a spark. If there is any spark present due to the blowpast or running of any hot part, crankcase explosion will take place. In order to avoid such occurrence, crankcase is fitted with ventilation so as to remove the combustible gases which are formed. Q.138 Most fuel injector nozzles are opened by Ans. (a) fuel oil pressure. Most of the fuel injector nozzles presently utilized on the internal combustion engines are hydraulically operated. This hydraulic pressure is produced by the fuel itself which is going to burn in the cylinder. Hence when the fuel pump plunger is moving upwards, oil pressure is developed which acts on the underside annular space of the fuel valve needle. This results in the lifting up of the needle and the oil, under high pressure, finds its way through the already unseated fuel injector needle and is forced through the fuel injector holes as a fine stream. As soon as the pressure drops, the spring pressure will be more than the hydraulic pressure and the valve snap closes on the seat. Q.139 The bearing used to support the crankshaft are generally called Ans. (c) main bearings. Main bearings are the bearings which support the crankshaft. The full load or the full power developed by the engine is transmitted to the crankshaft through various moving parts such as pistons, connecting bearing, crosshead bearing, bottom end bearing and ultimately to the crankshaft. From crankshaft, it is transmitted to the propeller shaft. At the same time, the downward forces acting on the crankshaft are taken care of by the bearings which supports the crankshaft. Hence they are called the main bearings as they are the most important bearing of the system. If they are not of proper strength or are worn out, the crankshaft may get deflected and there is a possibility that the crankshaft may crack. Also, the complete alignment of the engine is fully dependent on these bearings; hence they are called main bearings. Q.140 Injection pressure in a common rail fuel system is controlled Ans. (d) by a bypass valve. In common rail system of fuel injection, common piping supplies fuel to the individual injectors by a branch pipe which is the feature of the common rail system. This pipe and the full system is under high pressure i.e. the injection pressure. To control this, a bypass valve is utilized which is fitted at the end of the common pipe. By closing or opening this valve, more or less, the fuel is bypassed from the system back to the suction of the pump, thus reducing the pressure. Q.141 In internal combustion engines, the crankcase vent outlets must be equipped with Ans. (b) corrosion resistant flame screens. Corrosion resistant flame screens are required on the vent outlets of the crankcase, in internal combustion engines, in order to prevent spark from outside to enter the crankcase. As the crankcase atmosphere is full of oil mist, it is only waiting for the spark to come in contact to have an explosion. Under any circumstances, the spark must be prevented from entering the crankcase. For this purpose, it is necessary to have the flame arrester fitted on the crankcase breather. At the same time, this flame arrester should be maintained in clean condition so that it should have better heat transfer property so that it can extinguish the flame before entering the crankcase. It is therefore necessary that there should be not any rust formation on the screen because of which the heat transfering capacity and the efficiency of the arrester would be reduced. Taking into consideration the above conditions, it is necessary to provide a rustproof flame arrester screen on the crankcase vent outlet. Q.142 What is to be installed on an internal combustion engine if a cylinder bore exceeds 200mm? Ans. (d) Explosion relief valves. The engines having cylinder bore that exceeds 200mm are generally highly rated engines with high peak pressures. At the same time, most of these engines are compressed air started engines. Taking into consideration these conditions, it is necessary to prevent the excessive build-up of pressure inside the cylinder as this excessive pressure may put the parts of the engine under heavy stresses, which may cause mechanical failure of these parts. Hence, in order to avoid this, it is necessary to provide a cylinder relief valve on these engines. Q.143 Cylinder lubricating oil for a low-speed main propulsion diesel engine is admitted to each cylinder during Ans. (b) the compression stroke. Cylinder lubricating oil, for lubricating the piston rings and the liner, has to be admitted when the piston, piston rings and the liner are in cool condition and the piston is moving upward so that oil can be retained on the piston rings and sprayed by the piston rings on the liner walls. This is only possible during the compression stroke. Otherwise, the piston is hot and if the lubricating oil is sprayed on it, it will evaporate very fast and will not carry out any work of lubrication. At the same time, if lubricating oil is injected during the expansion stroke, i.e. when the piston is moving downwards, it will have a scrapping effect rather than lubrication. Q.144 What is the purpose of the window installed in the housing of an individual jerk-type fuel pump? Ans. (a) To allow the pump to be timed with the engine. Through the window of the fuel pump housing, it is possible to see the movement of the sleeve up and down. The sleeve has one marking only. At the same time, the sides of the window also have one reference mark. When the reference mark on the sleeve starts moving upward, it indicates that the plunger is moving upward. When the mark on the upward moving sleeve coincides with the mark on the pump body, it indicates that the upward moving plunger has closed the suction port of the fuel pump. This is the beginning of the injection. This
can be checked on the flywheel and, if required, can be adjusted. The end of the injection will be dependent on the amount of fuel to be injected. Q.145 Which of the following listed construction details of the internal combustion engines is required? Ans. (a) A warning notice of caution against the opening of a hot crankcase for a specified period of time after shutdown. On most of the modern engines, the crankcase doors has been put up with a warning or notice as not to open before particular time has lapsed. The crankcase is full of oil spray. This oil spray will be spread over the hot surfaces of the crankcase and will get evaporated into oil mist. This oil mist, when comes in contact with the hotspots, causes the explosion. Most of the times these crankcase explosions do not occur as the atmosphere in there is too rich to cause an explosion. If the crankcase is hot due to some reason and oil mist has been already formed, under such circumstances, if the crankcase door is opened before allowing sufficient amount of cooling, there will be ingress of fresh air. This will try to dilute the crankcase atmosphere and bring it into explosive range and an explosion might occur. Thus it is necessary to wait for specified amount of time before opening a crankcase door. Q.146 Which statement regarding the arrangement and location of explosion relief valve used on internal combustion engines is true? Ans. (b) Minimising the danger from emission of flames is a key consideration. When a crankcase explosion takes place, the crankcase relief door opens and allows the hot gases to get relieved through the relief door. Along with this, there is a possibility of a flame emerging from the crankcase relief door. An arrangement should be made on the crankcase relief door so that this flame and hot gases will not emerge directly and create danger for the personnel working in the engine room. Hence it has to be deflected to a safer place. This is carried out by a deflector plate on the outside of the crankcase relief doors. At the same time, inside of the crankcase relief door is provided with a flame arrester valve which also cools the flame before it emerges in the engine room atmosphere. Thus it prevents the spreading of fire in the engine room. Q.147 The term “proper metering” as applied to a diesel engine injection system can be best defined as Ans. (a) delivering the same quantity of fuel to each cylinder for each power stroke depending upon the engine load. It is necessary to inject particular quantity of fuel oil, for a specified load, in order to have proper combustion of the fuel and the best thermal efficiency of the engine. Also it is necessary that this fuel oil should be injected at the right time. In addition, it is necessary that all the units get equal quantity of fuel so that the engine will be mechanically and thermally balanced. For this purpose metering is done. The process of injecting the right quantity of oil at the right time and to distribute equally for all the cylinders so as to have the full power balancing of the engine is called metering. Actually, metering is the word used for measuring the fuel equally for all units. Q.148 One function of the fuel pump delivery chest valve is to Ans. (c) help the injector needle reset without dribbling at the injection nozzle holes. The fuel pump delivery valve is designed in such a manner that it has got one portion of the valve in the form of a piston. This piston form of the valve will help in preventing the smart closing of the valve. In other words, it will delay the closing of the valve because of the drag effect. This delayed closing of the valve will help in relieving the pressure wave caused due to sudden closing of the fuel valve needle on the seat. As this pressure wave is reduced, it will not have any effect on the fuel valve needle. If this pressure wave is not reduced, then as the delivery valve of the fuel pump is closed, it will cause the fuel injector to open for the second time thus causing secondary injection. This is not desirable as it would increase the exhaust temperature and reduce the power generated and increase the specific fuel oil consumption. Hence the fuel pump delivery valve does this function quite efficiently. Q.149 The rate of fuel injection in a diesel engine cylinder depends primarily on Ans. (a) the size of holes in the fuel nozzle. Taking into consideration the size of the holes, the volume of the fuel injected through each hole will depend upon the diameter of the hole because the length of the hole is constant for a particular fuel injector nozzle. Also, as the hole size of the injector increases, the droplet size increases. As the droplet size increases, it has more penetration into the cylinder volume. Hence it can overcome the compression pressure of the cylinder. Taking into consideration both these aspects, the diameter of the hole of nozzle will be the main governing factor for the rate of fuel injection. Q.150 The fuel injection system meters, atomize, and Ans. (c) inject the fuel at the right time. Fuel injection system has many functions to perform. One of the functions is to inject the proper amount of the fuel which is determined by load on the engine and through the governor. Secondly, it has to atomize the fuel by passing the high-pressure fuel through the fuel injector nozzle having tiny holes so that it would be spread in the form of oil mist. One more important function of the system is to inject the fuel at the proper time. Right time fuel injection is the most important aspect of the efficient burning of the fuel. This is carried out by the helix cut on the plunger or with the help of the * spring valve. Hence, in most of the engines, the proper timing is the beginning of injection time because the end of the injection timing is governed by the quantity of the fuel injected. This is achieved by the time when the upward moving plunger closes the inlet port of the fuel pump or when the fuel pump plunger moves upward and the suction valve closes. Q.151 The valve cam slope angle determines the Ans. (b) acceleration rate of valve closing and opening. As steeper the slope angle of the valve, more abrupt is the opening and closing of the valve. At the same time, this gives lot of stresses on the operating mechanism of the cams, the rollers and the pushrods. The best possible efficiency can be obtained from the cam when the flank of the cam is kept tangential to the base surface which gives snap opening and snap closing, without undue stresses on the operating mechanism of the cams. Especially in the case of the inlet and exhaust valves, the closing and opening should be fast enough to
avoid the throttling effect which may be encountered during the opening and closing. Steeper the angle, the valve will open and close faster. Q.152 The engine displacement is equal to the cylinder Ans. (b) area x stroke x the number of the cylinder. The displacement of the engine is the displacement volume caused by the engine or the swept volume of the engine piston, together for all units. Hence the displacement is calculated by multiplying the cross-sectional area of the cylinder and the stroke of the piston which gives the swept volume for one cylinder. When this is multiplied by the number of cylinders, it gives the total swept volume for the engine. Q.153 Cylinder diameters greater than 230mm require additional safety devices when the scavenging spaces are openly connected to the cylinders. Which of the following devices will be used to protect such spaces? Ans. (b) Explosion relief valves. When the under piston scavenge spaces are directly connected to the cylinder, in such conditions if there is blowpast due to the any reasons (such as broken piston rings, excessive liner wear, seized piston rings in the groove), it will cause scavenge fire. When scavenge fire takes place, there is a possibility of explosion occurring inside the scavenge space because of the availability of combustible material, hot gases or sparks blowing past and plenty air available in the scavenge space. In order to avoid such explosion, it is necessary to relieve the pressure if it gets generated. This is carried out by the explosion relief valves. Q.154 A loop or cross scavenge engines utilizes the motion of its piston and turbocharger to provide scavenge air. Which of the listed mechanical design prevents the air under the piston from being pumped back through the scavenge ports during the piston travel? Ans. (b) length of the piston skirt.
Q.159 The duration of fuel injection developed by the individual port and helix fuel injection pump is determined by Ans. (b) effective pump stroke. Effective pump stroke is the stroke when fuel is delivered by the pump. It will not take into consideration the idle stroke before the upmoving fuel pump plunger covers the suction port or the movement of the fuel pump plunger after the completion of the delivery stroke. Hence the position the fuel pump will be decided by the time through which the fuel is actually delivered. When the fuel pump plunger is moving upwards, as soon as it covers the ports, fuel injection is started. This is the beginning of the effective delivery stroke. As the plunger moves upwards, depending upon the load on the engine, the rack position of fuel pump will rotate the fuel pump plunger. Thus, for every load, there will be a definite disruptive position between the helix cut on the fuel pump plunger surface and the suction port. When the plunger is moving upwards, a point will come when the top of the plunger, i.e. the high pressure side, will get connected to the suction side thus relieving the high pressure and causing the end of injection. This is the end of effective delivery stroke. The quantity of the fuel injected will vary as per the effective delivery stroke which is controlled by the governor through fuel pump rack. Q.160 The plunger in a jerk pump is rotated until the release port is uncovered. If the port remains uncovered all the time which of the listed operation will occur? Ans. (a) no fuel. When the plunger is moving upwards, the fuel pump as soon as the top of plunger covers the suction port, the oil on top of the plunger will get compressed and pressure will be developed and injection will commence. As soon as the top of the plunger will get connected through the helix to the suction port, the suction and the delivery side will get short-circuited and hence no pressure will be developed on top of the plunger which will cause the end of the delivery stroke. If the rack position is nearly zero, even though the plunger is moving upwards and is covering the suction port, the helix position is such that it will always connect the discharge side of the pump with the suction side of the pump thus causing the short-circuiting and no pressure will be developed throughout the stroke of the plunger. If no pressure is developed throughout the stroke of the plunger in that case there will be no delivery of the fuel throughout the stroke of the plunger. This is called as a zero delivery stroke. Q.161 When the lower edge of the spiral begins to uncover the release port, in a jerk pump, Ans. (b) effective pumping stroke of the plunger ends. As the plunger is moving upwards and the fuel is getting compressed, the pressure is developed and it will cause the fuel injector nozzle to get lifted and the fuel is injected. Depending on the position of the rack which governs the position of the helix with respect to the suction port, a point will come when the helix bottom portion will come in contact with the suction port. This will cause the short-circuiting of the fuel oil between the high pressure and the low-pressure side i.e. between the top of the plunger and the suction side. This will cause a sudden drop in the delivery pressure due to which the fuel valve will close under the spring pressure thus causing the end of fuel injection and hence the end of effective delivery stroke. Q.162 The amount of fuel delivered by a helical plunger fuel injection pump is controlled by Ans. (c) the rotation of the pump plunger. The surface of the pump plunger has a helix cut on it. These helical grooves on the surface will be responsible for changing the effective delivery stroke of the plunger. The moment at which the helical groove connects the delivery side of the plunger with the suction side will be the end of injection. The position of the helical groove can be changed with respect to the fuel pump barrel suction hole by means of rotating the fuel pump plunger while it is moving up and down in vertical position. This movement of the plunger with respect to the pump barrel is due the fuel pump rack which is connected to the governor. Hence depending upon the fuel load, the governor will change the position of the rack. Due to this, the fuel pump plunger will rotate and change the position of the helix with respect to the suction holes. Hence the quantity of fuel delivered will vary as this causes change in the end of injection timing.
Q.163 Which of the following will occur when lower edge of the spiral on the plunger of a jerk pump uncovers the spill ports? Ans. The pressure drops and the fuel delivery stops. When the lower spiral edge cut of the helix cut on the fuel pump plunger surface uncovers the suction port or the spill port, it will have a short-circuiting effect between the top of the plunger and the suction side. As the top of the plunger is high-pressure side, when it is connected to suction low-pressure side the pressure will drop suddenly on the delivery side. As a result, the fuel pump will have the end of effective delivery stroke and the fuel valve will close smartly. Q.164 In a diesel engine, the pistons are attached to the crankshaft by Ans. (c) connecting rods. The transfer of the power from a piston to the crankshaft is achieved by the use of connecting rod and gudgeon pin assembly or connecting rod and crosshead assembly. This connecting rod also helps in converting the reciprocating motion of the piston into rotary movements of the crankshaft. This part of engine connects the piston to the crankshaft hence it is called the connecting rod. Q.165 Which of the component listed is not found in a single acting engine? Ans. (b) Double crankshaft. In single acting engine, the action of the fuel burning and load transmission is carried out only on the top of the piston and during the downward movement of the piston Hence it requires only one crankshaft to transfer the power. Q.166 In diesel engines designed with a crosshead the motion of the engine can be described as Ans. (c) straight reciprocating. In an engine with crosshead, the piston has a piston rod which is rigidly connected to the bottom of the piston and the other end is connected to the crosshead. Hence when the piston moves up and down, the piston rod also moves in a vertical direction thus causing the reciprocating movement which follows the movement of the piston. At the same time it is passes through a stuffing box which separates the scavenge space from the crankcase space. This vertical movement of the piston rod is converted into the rotary movement by the use of crosshead and connecting rod combination. Q.167 Cast iron piston used in large propulsion diesel engines are constructed with Ans. (d) the crown being tapered and smaller than the skirt. The crown of a large engine is always subjected to more heat rather than the skirt and hence the expansion of the crown is higher than the skirt. Clearance is provided for the expansion. Otherwise, the crown will expand to such an extent that it may start to touch the liner and it may cause a damage. In order to avoid such things, clearance is kept at higher valve at the top of the piston. This is achieved by using a tapered crown so that the clearance is maximized at the top of the crown where the temperature is maximum and reduced at the bottom where the temperature is minimum. Q.168 Which of the manufacturing method is recommended diesel engine fuel injection line piping? Ans. (c) Seamless drawn. The diesel engine fuel injection piping is subjected to very high pressure, in the range of about 1200bar. Also, there is sudden increase and decrease of pressure which causes the fatigue in the material. Taking into consideration these two things it is necessary that the material and the manufacturing process of the piping should be such that it should be able to withstand the high pressure and fatigue. It is also of prime importance, from the point of view of safety of the ship and fire hazard, that the fuel pipe should not get cracked under these high pressures. In order to achieve these requirements, it is necessary to use fuel pump pipes made up of seamless, solid drawn tubes. Q.169 The surface of combustion in the cylinder of a diesel engine depends on (i) fine atomizing. (ii) high temperature. (iii) good mixing of fuel and air. (iv) All of the above. Ans. (iv) All of the above. The efficient combustion of fuel will depend on the proper mixing of the fuel and air in the combustion space and evaporation of the fuel as fast as possible so as to form a combustible mixture. This is achieved by breaking the fuel into fine particles and to spread the fuel in the entire combustion space available. This is carried out by the fuel injector which will atomize the fuel into fine spray due to which the fuel droplets will be broken down into small particles and cause the evaporation of the fuel. Higher the temperature of the fuel or the compressed air, the time required for the fuel to evaporate and form a combustible mixture reduces and hence greater combustion effect is achieved. As good mixing of fuel and air takes place, entire volume of air is available for combustion. Hence full quantity of fuel can be burnt effectively as there are no pocket formations which will cause local combustion. Q.170 Which of the combustion parameters listed is use in diesel engine but not related to the injection system? Ans. (c) Effective stroke. Effective stroke is not directly related with the injection system. It will control only the quantity of fuel which is injected. It will not be related to any of the parameters required for efficient combustion such as atomization, metering or penetration. Q.171 Fuel oil penetration in the cylinder of a diesel engine is Ans. (b) reduced by the finer atomization. Atomization is carried out by injecting the fuel with high pressure through the fuel injector nozzle holes and it breaks the fuel into very minute particles. If the atomization is too much, the particle size will be very small which will have less weight and hence it will not have sufficient amount of kinetic energy to overcome the pressure of compression which is existing inside the cylinder at the time of injection.
Hence it will not be possible for the fuel to penetrate deep into the combustion space, which is filled with high-pressure compressed air, at the end of compression stroke. This may cause the improper burning of the fuel as pockets of high concentration of fuel will be created and complete amount of air available will not be use for combustion. Q.172 Proper atomization of fuel in the diesel engine combustion chamber will Ans. (b) improve the combustion. Proper atomization means breaking the fuel oil droplets into fine particles or to inject the fuel under the influence of high pressure which will convert the fuel oil into fine spray. This will cause more surface area of the fuel available for the contact with air. As the surface area increases, the fuel and air mixing will be properly done. At the same time, because of availability of more area, the fuel will evaporate fast and will form the combustible mixture which is the main important point from the point of view of greater combustion of the fuel and increasing the thermal efficiency of the engine. Q.173 What is required for crosshead type engines that have a scavenging space in open connection to the cylinder? Ans. (c) The scavenge space to be permanently connected through an approved fire extinguishing system, entirely separated from the extinguishing system of the engine room. When the scavenge space is connected to the under side of the piston, there is every possibility of scavenge fire if the blowpast of the hot exhaust gas takes place due to worn out cylinder liner or broken piston rings or piston ring cut off in the groove. In such circumstances, the hot gases or the spark, entering the scavenge space, will start burning the sludge and accumulated oil inside the scavenge space. As plenty amount of air is available this scavenge fire may continue for a longer period and it may also go beyond control. Hence it is necessary that a permanent arrangement must be provided so that if all other means, such as stopping of the engine and covering the exhaust and the turbocharger suction filter with a cloth, fail. In such circumstances, it should be possible to extinguish the fire by the use of a permanent extinguishing medium which is connected to the underside of the piston scavenge space mostly CO2 or steam is utilized. The requirement for this is very small and it should be independent of the engine room system because if it is connected to the engine room fire fighting system, it may possible that you may discharge the complete safe battery of the cylinder which are actually kept for the engine room fire. In such condition, engine room fire fighting equipment capacity will be reduced. Hence the scavenge space fire fighting installation should be entirely independent of the engine room fire fighting installation. Q. 174 Which of the following statement is an accurate description of the fuel injection piping used in a diesel engine with a cylinder bore of 250mm and above? Ans. (b) The piping is to be effectively shielded and secured to prevent fuel or the fuel mist from reaching the source of ignition on the engine or the surrounding. In case of engines of high capacity or those utilized in UMS ships, it is necessary that if the high-pressure pipe do burst, the oil or the oil mist should not come in contact with hot surfaces such as exhaust manifold so as to prevent the fire. For this purpose, it is necessary to double-walled the high pressure injection piping between the fuel pump and the fuel valve. Hence in case of any rupture in the pipe, the outcoming high-pressure oil will not get injected directly over the hot surfaces and will not cause fire in the engine room. Also in the doublewalled pipes, the outer portion of the pipes is connected to a leak-off tank whereby if any high pressure pipe does crack, the leak-off will led to the leak-off tank where it will give an alarm which indicates that there is a crack in the fuel piping
Q.501 A large slow speed main propulsion diesel engine may become overloaded by (a) heavily fouled hull. (b) strong head wind and heavy seas. Ans. (c) both of the above. When the hull is fouled, the resistance offered by the ship for the movement increases. Hence for the same rpm, the engine will be overloaded as it will have to produce more power to overcome the resistance. Similarly, when there is strong head wind, the ship will have to work against the head wind and overcome the force created by the headwind. In this condition also, the engine will be overloaded to maintain the same revolution on the engine shaft. Q.502 Immediately after any diesel engine is started, the engineer should check the Ans. (b) lubricating oil pressure. Lubricating oil is the most important component of the engine. If the lubricating oil pressure is lost, even for a fraction of a second, there will be metalto-metal contact between the bearing and the shaft, and they will get damaged. Hence the lubricating oil pressure should be checked as soon as the engine is started. Q.503 In a diesel engine lubricating oil system, which of the following part should be lubricated first? Ans. (b) Main bearings. In case of the four stroke engines, the oil is first supplied to the main bearings. From the main bearings, through the holes of the crankshaft, it comes to the bottom end bearings. From the bottom end bearings, through an oil passage, it passes to the top end bearing. This passage shows that the main bearing is lubricated first. In case of large two-stroke engines, where separate lubrication is given to the crosshead bearing, both the main bearing and the crosshead bearing get the lubrication together. Q.504 Poor combustion in a diesel engine can be caused by Ans. (b) low intake air temperature. If the intake air temperature is low, then the temperature of the air at the end of compression stroke will also be low. In such condition, if the fuel is injected it will have improper combustion because it will take more time for the oil to evaporate and form a combustible mixture. Due to this, there will be more amount of fuel combustion and loss of thermal efficiency.
Q.505 Diesel engine exhaust gas temperature can be used to determine individual cylinder Ans. (a) performance. The exhaust gas temperature gives the approximate power developed by the engine. Hence if the exhaust temperature is higher, we can consider that the power developed by that unit is more than the other units. This may not true always because if there is any discrepancy developed during the fuel injection, the exhaust temperature will be more but the power will be less. Hence the combination of exhaust temperature and the indicator diagram will indicate the exact problem. So exhaust temperature is a rough indication of the performance of the unit. Q.506 Which of the following problem may occur if the opening pressure of a fuel injection nozzle is greater than specified by the fuel injection manufacturer? If the nozzle opening pressure is higher than specified, the nozzle will open late because the fuel pump has to develop higher pressure to lift the nozzle and open the fuel injection valve. In such circumstances, the injection will be delayed. Q.507 All the diesel engine firing cylinder pressures are normal; yet all exhaust temperatures are low. Which of the following situation is responsible for this condition? Ans. (d) Light load. When the temperature of all the units are low and the peak pressures are equal, it means that all units are sharing equal power. If the exhaust temperature is low, it also indicates that all units are producing less power. When all units are producing less power, then the total load on the engine is reduced. Hence low exhaust temperature is a cause of low loading of the engine. Q.508 Insufficient piston cooling of a large low-speed main propulsion diesel engine burning heavy fuel may result in Ans. (c) high temperature corrosion and burning of piston crown metal. If insufficient cooling takes place, the piston crown temperature will increase. When the crown temperature increases above 470°C, the vanadium present in the fuel, along with calcium, will be melted. This molten vanadium will form vanadium pentoxide, which is a highly corrosive compound. This will get coated over the piston crown and cause excessive high temperature corrosion of the crown. Q.509 Which of the following factors tends to increase scale formation on the salt-water side of a heat exchanger used in a diesel engine cooling water system? Ans. (c) Operating the engine while maintaining a high sea-water outlet temperature. The sea water outlet temperature of a cooler can be high because of restricted sea-water inlet or reduced quantity of the sea water inlet due to some reason. Under such circumstances, the flow of water through the heat exchanger will get reduced and will get overheated resulting in high sea-water outlet temperature. This high outlet temperature will cause the calcium and magnesium salts to get deposited inside the tube of the heat exchanger causing the more amount scale formation. Q.510 High exhaust backpressure will result in increase in Ans. (c) carbon deposits on injector. Q.511 Any increase in exhaust gas backpressure in a four-stroke cycle diesel engine will Ans. (a) reduce diesel engine power output. High backpressure will cause the piston to push the exhaust gas out of the cylinder with the backpressure acting over it from the exhaust manifold. This will cause loss of power because the power required to drive out the exhaust gas from the cylinder will be taken from other units which are firing. Thus the total power output of the engine will be reduced because of the pumping effect of the trapped exhaust gas by the use of the piston. Q.512 In diesel engine, blowby Ans. (b) causes excessive crankcase pressure. Especially, in case of four-stroke engine, if there is blowpast, high-pressure gases will enter the crankcase by blowing through across the piston rings. This will cause increase in the crankcase pressure. Q.513 A crack in the cylinder liner can be caused by Ans. (b) restricted cooling water passage. Due to the restricted cooling water passages, the cooling of the liner flange on top will get affected. The liner will run at the higher temperature and because of that the material of the liner will lose its strength. Hence the liner will crack under the load. Q.514 A cracked cylinder head may be indicated by Ans. (c) combustion gases vented in expansion tank. When the cylinder head is cracked and, under the compression stroke, when the fuel is injected at high pressure, the combustion gases produced will find its way into the cooling space through the cracks (in the cylinder head). Those gases which are lighter will come out of the cooling water from the expansion tank vent. Q.515 A direct cause of crankcase explosion can be attributed to Ans. (c) an overheated bearing. Crankcase explosion will take place only if there is hotspot present inside the crankcase. If the bearing gets heated, due to various reasons, it will act as a hotspot and crankcase explosion may take place. Q.516 Crankcase explosion in propulsion diesel engine results from Ans. (d) the ignition of unburnt fuel and air in the crankcase.
If there is unburnt fuel oil in the crankcase due to contamination of lubricating oil with fuel oil, the crankcase will consist of lubricating oil as well as fuel oil vapors. This fuel oil will ignite in presence of large amount of air present in the crankcase and thus cause crankcase explosion. Q.517 Combustion knock occurring in a diesel engine can be caused by Ans. (a) low coolant temperature. If the coolant temperature is low, it will take large amount of heat from the compressed air at the end of the compression stroke. As a result, the temperature of the air will drop and ignition will be delayed. When the ignition gets delayed, fuel will get accumulated and get ignited resulting in knocking in the unit. Q.518 Persistent knocking of one cylinder of a diesel engine ceases when the fuel supply of that cylinder is secured. This problem may be a result of Ans. (b) faulty combustion in that cylinder. When the engine cylinder is knocking, and after shutting of the fuel if the knocking stops, it is a clear indication that the knocking was a result of problem related to fuel injection or quality. If the knocking persisted even after the fuel injection was cut-off, it indicates that the knocking is a mechanical knocking which is a serious problem. Q.519 Which of the condition will occur when in an operating diesel engine if air pockets are formed within a cylinder head circulating water passage? Ans. Hotspots will be developed. If the air pocket is formed, as air is a low conductor of heat, the heat will not be conducted form the hot surface and it will not be carried away by water. Hence that particular spot will get overheated and hotspot will be developed. Q.520 If a diesel engine exhaust temperature is abnormally higher, the cause could be Ans. overload of the engine or the compression pressure would be too low. If the engine is overloaded, the exhaust temperature of all units will be higher. If the temperature of only one unit goes up, it is an indication that there is some thing wrong with that particular unit i.e. either incomplete fuel combustion or improper timing adjustment of that unit or too low compression pressure of that unit due to worn out cylinder liner or leaky piston ring. Q.521 Heat damage to the fuel injection nozzle can be prevented by avoiding (a) excessive fuel oil temperature. (b) long period of engine overload. (c) hard carbon deposits and varnish on the nozzles. (d) All of the above Ans. (d) All of the above. If excessive fuel oil temperature is permitted and if there is no separate cooling is given to the injector and the circulating fuel itself is part of cooling system, in such conditions the hot fuel oil will keep the nozzle at high temperature. This will cause damage to the fuel injector because at higher temperature the fuel injector nozzle and the valve seats will lose its strength and it will fail. Secondly, if the engine is working at high load for long time, the fuel injector nozzle will be subjected to high temperature from the combustion space due to which the fuel injector nozzle may get damaged. Thirdly, if the quality of the fuel is bad, there is a possibility that hard carbon deposits may be formed. Also the needle guide may get coated with hard varnish and because of this the fuel oil nozzle operation may get affected and it would ultimately cause to seep in place or have excessive weardown and it will get damaged. Q.522 The main propulsion diesel engine is normally shutdown by Ans. (c) securing the fuel supply. Under normal circumstances, the fuel oil supply supplied to the engine is stopped. Q.523 Excessive diesel engine backpressure may be indication of Ans. (a) carbon built up in exhaust manifold. When the carbon is built up in exhaust manifold the exhaust gas will have to overcome resistance of the built up carbon to pass through the exhaust manifold. Due to this there will be excessive backpressure. In order to overcome this backpressure, the piston has to work against the exhaust gas and hence there will be loss of power. Q.524 White smoke from the exhaust of diesel engine can result from Ans. (b) low cooling water temperature. Due to low cooling water temperature, the entire atmosphere inside the cylinder liner will be at lower temperature because of which the exhaust gas temperature will drop down. When the exhaust gas temperature will drop down, the water vapor in the exhaust gas will get condensed to form white smoke. Q.525 What is the best way of stopping an overspeeding diesel engine? Ans. (d) secure the fuel supply and block the air intake. The best way to stop an overspeeding engine is to cut-off the fuel to all the units simultaneously and immediately. If the fuel cannot be cut-off, then even if the air intake suction filter or the turbocharger suction filter is blanketed, it will prevent the air entry into the engine. Hence due to lack of availability of air fuel, combustion will not take place and the speed of the engine will drop and ultimately it will stop. Q526 By comparing the exhaust gas outlet of each cylinder in a diesel engine, one method of determining if the engine load is balanced is by the use of
Ans. (d) pyrometer. Pyrometer will show the exhaust gas temperature of each unit. Generally and roughly, it is accepted that the exhaust gas temperature is the measure of power developed by individual unit. Hence higher the exhaust temperature, higher will be the power developed and lower the exhaust temperature lower is the power developed. This is a rough estimation and cannot be utilized unless it is backed up by other parameters such as * compression and * maximum. Q.527 One cylinder of a diesel engine is persistently knocking and does not ceases when the fuel oil supplied to the cylinder is secured. What is the following problem which can cause this? Ans. (d) A mechanical defect in working part. If the diesel engine one cylinder is knocking the knocking may fuel related or mechanical In order to know the exact cause of knocking first the fuel is cut-off. If after cutting of the fuel the knocking stops, it indicates that the knocking is due to the fuel timing or the fuel quality. But if the knocking continues even after the fuel is cut-off, it is a clear indication that somewhere metal-to-metal contact is taking place and there is mechanical defect in the working parts. Q.528 A diesel engine is warmed up and white vapor is noted in the exhaust. What does it indicate? Ans. leaky cylinder liner. If engine is properly warmed up and is working at proper load and under such circumstances if there white vapour is indicated in the exhaust, it is an indication that at some point water is finding its way into the combustion space. This may be due to a liner or cylinder head crack. Q.529 A smoky exhaust from an operating diesel engine could be caused by Ans. (c) late fuel injection. In case the fuel injection is too late, there will be afterburning. When afterburning takes place the efficiency of combustion reduces and excessive black smoke may occur. Q.530 Oil accumulating in exhaust gas piping or manifold of a diesel engine can be caused by Ans. (b) worn valve guide. If the valve guide is worn out, the clearance between the valve guide and valve stem will increase and the valve guide will be lubricated by the use of rocker lubrication oil. If the clearance will be more, then the finer particles of oil will, along the stem of the valve, enter into the exhaust manifold. Q.531 Diesel engine operating conditions are indicated by color of exhaust smoke. Blue smoke can indicate Ans. (c) clogged drain holes in the oil control ring. If the drain holes in the oil control ring get clogged, the oil which is trapped in the oil control link will not get drained into the crankcase but it will remain on the cylinder liner and it will get burnt with the exhaust gas. This burnt oil vapor will find its way along with the exhaust gas and hence the exhaust gas will have a blue tinge. Q.532 If the diesel engine is smoking excessively under load, the cause could be Ans. (d) plugged injection holes or low compression pressure. If the compression pressure in one of the cylinder is low, the fuel will not burn efficiently and black smoke will be emitted by the engine. Also if the injector holes are plugged or are choked, there will not be proper penetration and the engine will have late ignition and because of this, there will be excessive black smoke. Q.533 Sticking of diesel engine compression rings may be caused by Ans. (c) excessive cylinder lubrication. If the cylinder lubrication is excessive and the excessive oil is not been scrapped by the oil control rings, the excessive lubricating oil will get burnt and form carbon or calcium carbonate due to which the piston ring will get stuck up in the grooves. Also the improper rotation of the rings may cause them to get stuck up in the grooves because rotation of the piston rings give the self-cleaning effect for the piton rings. If they are not allowed to rotate, the carbon may get deposited and get choked up or stick the piston ring in the groove Q.534 Which of the following condition can cause excessive lubricating oil consumption in a diesel engine? Ans. (d) High lubricating oil temperature. If the lubricating oil temperature is high, excessive vaporization of the lubricating oil will take place and ultimately it will cause excessive lubricating oil consumption. Q.535 Late fuel oil injection in a diesel engine may result in Ans. (d) high exhaust temperature. When the fuel oil injection is late, there is little time for fuel and air to mix properly. Hence there is improper combustion will take place. Also there will be afterburning and hence the exhaust temperature will be on the higher side. Q.536 Which of following condition is likely to develop if the thermocouple element of a pyrometer becomes coated with excessive amount of combustion byproducts? Ans. (b) Pyrometer response will be retarded. The carbon deposits around the pyrometer sensor will retard the heat transfer because that coating will form an insulating material and hence heat transfer will be affected. Also the heat transfer will take more time. Due to this, the response in change in temperature will be retarded and the change in temperature will be registered not immediately but comparatively late.
Q.537 When the opening pressure injector is greater than specified by the engine manufacturer which of the following problem can be expected? Ans. (a) quantity of fuel to be injected tends to decrease. The effective delivery stroke of the pump plunger will reduce because it will take more time for the plunger to travel before the sufficient pressure is developed. Hence as the effective delivery stroke is reduced, it will automatically reduce the quantity of the fuel injected. Q.538 Late fuel injection occurring, at or after TDC, is indicated by excessive exhaust smoke and Ans. low firing pressure. When the injection is late, the fuel will get ignited late when the piston is already on its downward travel and because of this the firing pressure will reduce and afterburning will take place. Q.539 In a diesel engine, late fuel injection is indicated by black or gray exhaust smoke with Ans. (a) low firing pressure. As the firing is late, the combustion of the fuel is late when the piston is moving downwards and because of that the volume will increase. Hence the firing pressure will reduce. Q.540 If a crankcase explosion occurs in a diesel engine you should stop the engine and Ans. (b) allow the engine to cool naturally . The crankcase doors should not be opened immediately as if they are opened there is a possibility that outside air may enter the crankcase and result in secondary explosion. In order to avoid the secondary explosion, which is of higher intensity and it will cause large scale destruction, it is necessary to allow the engine to cool down naturally and not by forced ventilation. Q.541A diesel engine will lose power if the injection occurs too late in a cycle because of Ans. (c) maximum expansion of burnt fuel cannot take place in the cylinder. If the fuel injection occurs too late then in such conditions the maximum pressure will be reduced. At the same time, when the actual burning of the fuel takes place, the piston will be on its downward movement under which the volume will increase and the pressure cannot be increased. Ultimately, the power developed by the engine will reduce because the power developed by the engine will be given by Power developed = PLAN In the above formula as P reduces the power developed is reduced Q.542 Which of the following condition will be developed when unburnt fuel contaminates the crankcase of a diesel engine? Ans. When the unburnt fuel oil finds its way into the crankcase, it will mix with the lubricating oil from the crankcase and bring down the viscosity of the lubricating oil. Also it will bring down the flash point and fire point of the lubricating oil Q.543 Same as 539. Q.544 The ignition quality of diesel oil becomes less critical as Ans. (d) the engine speed decreases. As the engine speed decreases, there is ample amount of time available for the fuel and air to mix and form a good combustion mixture. Hence the quality of the fuel does not matter much under the low speed engine as compared to medium or high-speed engines. Q.545 Higher than normal temperature air passing through the intake of a diesel engine will result in Ans. (c) low horse power . If the air entering the cylinder is at high temperature, the volumetric efficiency of the cylinder will reduce because the quantity of the air which enters the cylinder reduces. When this happens, automatically the fuel injected will not burn properly. Hence the thermal efficiency will drop down and will cause black smoke and higher exhaust temperature and low power is developed. Q.546 When the fuel injector nozzle overheats which of the following problems may developed? Ans. (a) The fuel metering will vary. When the nozzle gets overheated, the clearance between nozzle and needle guide will change due to which there will excessive leakage pass the needle. Also the needle will not seat properly on the seat and there will leakage pass the seat. Due to all this the total quantity of fuel which is injected will vary. Q.547.An increase in fuel injection pump discharge pressure can be caused by Ans. (c) plugged fuel injector spray holes. When the fuel injector spray holes are choked, it will develop excessive backpressure on the fuel pump. This will cause excessive fuel injection discharge pressure. Q.548 If fuel injection in a diesel engine begins earlier than designed start of injection, ignition may be delayed because of Ans. If there is excessive early fuel injection, as the compression pressure in the cylinder has not sufficiently reached to cause the ignition of the fuel and because of this the fuel will get accumulated and burn with a heavy knock. This is the reason for not advising early injection of fuel. Q.549 Prior to starting, the purpose of turning over the main propulsion diesel engine with a cylinder test cock open is to Ans. (b) remove the condensation from the cylinder.
When condensation takes place of the moisture in the air on the top of the piston or if there is any water or fuel accumulation and an attempt is made to start the engine, as the liquid is incompressible, there will be hydraulic pressure developed which will act over the piston, cylinder head, connecting rod and crankshaft thus causing them to bend, break or fracture. Hence it is advisable everytime to blow through the engine before starting after a long stoppage. Q.550 A fuel injection valve opening at pressure below than normal will result in Ans. (b) early fuel injection. If the fuel injection valve opening pressure is low, the pressure required to lift open the needle valve will be less. This low pressure can be reached with a very little upward movement of the plunger and because of this fuel may enter the chamber too early and this may also cause knocking. Q.551 Faulty operation of diesel engine fuel injection nozzle can be directly caused by Ans. water in the fuel oil supply or excessive fuel nozzle holder cooling or distorted fuel pattern or leakage past the plunger into the oil drain. (All the above noted things are possible for the faulty operation of diesel engine fuel nozzle.) Water will cause sticking of the needle in the guide and excessive weardown of between nozzle and guide and missing of the firing in between and so it will cause the improper combustion and surging of the engine. Excessive cooling of the nozzle will cause the formation of sulfuric acid and it will get corroded. Due to the corrosion of nozzle, the spray pattern of fuel which is injected will change. If the plunger and barrel clearance is more then also there will be excessive leakage between the plunger and barrel and this will also cause the late opening of the fuel injector. At the same time, because of the drop in pressure, the spray will not be proper and atomization and penetration will not be good. Q.552 Water in the fuel can prevent the engine from starting, preventing it to develop full power or run at full speed. Ans. If there is water in the fuel the power developed by the engine will be reduced and it will miss firing. When missing of firing takes place, the rpm will go on fluctuating continuously and smooth running of the engine will get hampered. Q.553 Air in fuel lines of a diesel engine can cause Ans. ignition failure. As air is compressible fluid, it will get compressed and will not transfer the pressure to the fuel injector valve. Hence it will not get lifted up and there will be loss of power and ignition failure. Q.554 Before starting a diesel engine you should always check Ans. (b) crankcase oil level. As oil is the most important part of the engine, it is necessary to check the oil level. If the oil level is less and the engine is started, the oil level will go down and lubricating oil pump may lose suction. If the lubricating oil pump loses suction, even for a fraction of a second, the bearing will get damaged. Q.555 Dirt in a fuel oil system of a diesel engine may cause Ans. (d) to the fuel oil injector damage. Due to the dirt, there will excessive weardown between the needle and guide. At the same time, there will be scratching on the needle and the guide. The fuel injector may get worn out faster or the dirt may also cause the needle to stuck up. All this will lead to damage to the fuel injector. Q.556 If cooling water flow through after cooler is interrupted the power output of a turbocharged diesel engine will drop because the Ans. (b) density of the air will decrease. When the water flow through the after cooler is interrupted, the air temperature increases. Automatically, the volume of the air increases and the density decreases. Hence for the same said volume, the quantity of the air from the point of view of weight, as concerned, will be always less than what it would be if the temperature was lower. Higher the temperature, the weight of the air reduces. When the weight of the air reduces, the fuel injected will not burn efficiently. Hence, there is automatically a drop in the power output of the engine. Q.557 Operating a propulsion diesel engine at less than 30% of designed normal load for prolonged periods will result in Ans. (d) carbon formation on the combustion chamber surfaces. When an engine is operated at such low loads, the turbocharger efficiency drops. When the turbocharger efficiency drops, less quantity of air is supplied to the cylinder. Hence the quantity of air available in the cylinder for combustion is reduced. This will automatically reduce the quantity of fuel which is burnt. At the same time, the fuel will not burn efficiently. Thus unburnt fuel will lead to carbon formation and the combustion chamber surfaces will get coated with the carbon deposits. Q.558 Diesel engine fuel oil contamination often results in Ans. (b) fuel injection system malfunction. The fuel oil of diesel engine will get contaminated with water or with solid particles such as silica, rust, or allumina. All these three components will result in improper functioning of the fuel combustion injection equipments. Because of the water, there will be rusting and lots of misfiring in the combustion stroke. Also if there is a hard particle, silica or allumina, present then it will give abrasion on the needle and guide and cause the leakage of fuel. Any catalysts and fines entering the clearance spaces of part of fuel injection equipments can lead to accelerated wear of these parts. The wear may be extensive with few days of operation with contaminated fuel. Also it will block the injection holes on the nozzle and cause complete malfunctioning of the fuel injection system. Q.559 Operating a diesel engine under light load and at lower temperatures for extended period can result in Ans. (a) formation of carbon on the inlet and exhaust valves. When the engine is operated at low load and low temperature for a longer time, the turbocharger efficiency drops. When the turbocharger efficiency drops, the quantity of air supplied to the engine reduces. This results in improper combustion and hence large amount of carbon formation takes place in the combustion chamber. When running at low load, the exhaust gas pressure and exhaust gas energy are not sufficient. Hence there will be backpressure
exerted on the exhaust. If the same overlap is maintained between exhaust and inlet valve is maintained during that time, then when the inlet valve opens the exhaust pressure is not sufficiently dropped. Under such circumstances, momentarily there will be back-flow of exhaust gas from the cylinder into the air inlet valve before the actual flow of the inlet air to the unit starts. During this time, the formation of carbon takes place in the air inlet valve. Q.560 Which of the conditions listed occurring in the diesel engine would cause carbon deposits to develop in the piston ring belt. Ans. (d) all above i.e. faulty combustion, excessive ring temperature, over-lubrication. The carbon deposits develops in the piston ring belt because of 1. Improper combustion: The improper combustion of fuel results in carbon being deposited. 2. Excessive ring temperature: If ring temperatures are excessive, then lubricating oil will get burnt which also leads to the formation of carbon. 3. Over lubrication: The excessive lubricating oil that is supplied to the piston ring will get burnt and convert into ash or carbon thus forming the excessive coating of the carbon on the ring. Q.561 If a crankcase explosion occurs in a diesel engine which of the listed action should be taken. Ans. (c) Allow the engine to cool before opening the crankcase. There will be two kinds of crankcase explosion, one a primary explosion and other the secondary explosion. When the primary explosion takes place because of the oil mist and the appearance of hot spot in the crankcase, there is a high-pressure rise in the crankcase which is released into the engine room atmosphere with the help of crankcase relief valves. At the same time, the non-return valve will shut and not allow the new air to enter into the crankcase. The crankcase atmosphere will be highly rich and hence there is a less possibility of exposure. Under such circumstances if by mistake the crankcase door is opened, fresh air will enter the crankcase and bring the crankcase atmosphere in the range of explosive limit. When this happens, there will be heavy explosion and a secondary explosion may take place. So the best thing to do is to allow the engine to cool down before opening the crankcase doors. Q.562 The burning of fuel oil in a diesel engine having a high sodium content will cause Ans. corrosion and grooving of exhaust valves. In general, fuels leaving the refinery have sodium content well below 50mg/kg. If the sodium content increases, which is normally caused due to seawater contamination. A 1% seawater contamination represents potentially a 100mg/kg increase. Vanadium is also present in the fuel oil, which combines with oxygen to form V2O5 (vanadium pentoxide), which combines with sodium to form sodium/vanadium complexes. It is well known that there are low melting temperatures of sodium/vanadium complexes of certain critical ratios. The most critical sodium/vanadium ratio is about 1:3. This will form a sodium/vanadium complex with a low melting point which will flow with the exhaust gases. It will get deposited as a hard and brittle layer on the cold surfaces such as exhaust valve spindles, turbocharger nozzles and turbine blades. This layer is highly corrosive and corrodes the metal. It is also brittle and breaks away exposing the metal for fresh attack especially when they get deposited on exhaust valve seats. The hard layer breaks and gives a cutting effect on the seat. Preventive measures can be taken such as keeping the temperature of the exhaust below the melting point of V2O5 and removal of sodium by proper purification and proper draining of the settling tanks. Therefore high sodium content in the fuel oil will result in corrosion and grooving of exhaust valves.
Q.563 The longer the ignition delay period resulting from improper use of low cetane fuel, the Ans. more rapid the rise in the combustion pressure. Longer the period of ignition delay, more quantity of fuel will accumulate before the actual firing will takes place. As a result, there will be sudden increase in the combustion pressure. When the large quantity of accumulated fuel ignites, there is a sudden development of the high-pressure wave. At the same time if the ignition delay is too much, there will afterburning. When afterburning takes place, there will be high enforced temperature. Q.564 If the relief valve on the diesel engine cylinder lifts, the cause could be Ans. (a) liquid in the cylinder. When there is a liquid in the cylinder, the piston is moves upward. Hydraulic pressure will develop because of the presence of liquid. As liquid is incompressible, large amount of hydraulic pressure is developed. In order to save the engine components from damage, the relief valve will lift and release the pressure. Q.565 The substantial increase in the crankcase pressure could be an indication of Ans. (c) a worn cylinder liner. When the pressure in the crankcase of a diesel engine increases, it could indicate blow-past by the piston. This could be due to worn out cylinder liner or the piston rings are leaking or piston rings are broken. In that case large amount of high-pressure gas will enter into the crankcase and produce high pressure in a crankcase. Hence the substantial increase in the crankcase pressure could be an indication of a worn out cylinder liner. Q.566 An indication of an overloaded main propulsion engine. Ans. (b) High exhaust pyrometer readings. As a general rule the exhaust temperature is related with the load of the engine. Higher the load, higher the exhaust temperature of the engine. Hence it is the direct indication of the load on the engine. Hence if the exhaust temperature is higher, then the engine is overloaded. Q.567 The dry-type exhaust silencer clogged with soot will cause Ans. (b) loss of engine power. When the dry-type exhaust gas silencer gets clogged with soot, there will be large amount of exhaust gas to flow and because of that the exhaust gas pressure will be higher. Hence the complete removal of exhaust gas will not be possible which will cause the mixing of the exhaust gas with the incoming air. Thus it dilutes the incoming air with the exhaust gas. This will cause improper combustion because of the starvation of the oxygen for the combustion of the fuel. The exhaust temperatures will increase and consequently there will be loss of power in the engine.
Q.570 The color of the exhaust from the diesel engine should be Ans. (a) clear. The clear color of the exhaust gas from the engine exhaust will indicate the best possible combustion without any carry over of the fuel, lubricating oil, water in the fuel or improper combustion. So it should be noted that the colour of the exhaust indicates the health of the engine or any defects in the combustion. If the colour of the exhaust is black, it indicates improper combustion that may be due to some fuel system malfunctions or an overloaded engine. If the colour of the exhaust is white, it is consists of water. If the colour of the exhaust is bluish, then it indicates the burning of lubricating oil in the engine. So clear exhaust is an indication that the combustion is perfect. Q.571 Engine operating conditions maybe indicated by the colour of the exhaust smoke. Black smoke could indicate. Ans. (b) an overloaded engine. If the engine is overloaded beyond particular limit then, the quantity of the fuel will be more but comparatively the quantity of the air will be less. Hence larger quantity of fuel, which is injected into the unit, will be difficult to burn in allotted time and so after-burning will occur. This will increase the exhaust temperature and black smoke will be formed.
Q.572 Restricted air intake to a diesel engine may result in Ans. (a) engine failing to reach the rated speed. When the air intake to the engine is restricted, automatically the fuel will not burn properly. Hence power generated by the engine will be less. When power generated by the engine is less, it may be possible that it will fail to reach the rated speed at a lower condition.
Q.573 Color of engine exhaust form diesel engine should be Ans. (a) clear. (Refer to Q.570) Q.574 With regards to diesel engine crankcase explosion, the most violent is (i) secondary explosion. (ii) primary explosion. Ans. (ii) only. Q.576 Some diesel engines are fitted with thermometer in cooling water outlet of each cylinder. The cooling water temperature from all the cylinder being risen above the normal, you should suspect Ans. overloading of all the cylinders i.e. overloading of the engine. If overloading of engine takes place, then the quantity of heat taken out by the cooling water will be very high and in that case jacket cooling water temperature will increase. Also, it may increase due to the malfunctioning of the control valve fitted on the water cooling system. Q.577 Some diesel engines are fitted with thermometers in the cylinder. If the cooling water temperature from an individual cylinder begins to rise you should suspect Ans. (b) overloading of that cylinder. If the heat generated in that cylinder is excessively high, heat carried out by water from that unit will be very high and hence the jacket cooling temperature of that unit will increase. It is an indication that there is a overloading of that particular unit. This may be because of leaky fuel valve, or fuel pump rack stuck up in the high fuel condition. Q.578 A diesel engine using lubricating oil having too high viscosity will exhibit Ans. increase in starting difficulty in cold weather. using high viscosity oil and having low viscosity index, because of the excessively high viscosity, there will be too much of drag and excessive friction between the bearing. The loss of power to overcome this drag will be so high that it will be difficult to start the engine from the cold and most of the power, which is used for starting and turning the engine, will be wasted to overcome this friction caused by the high viscosity. Q.579 One result of operating a diesel engine at light load with excessively low cooling water temperature is Ans. (b) increase in the cylinder misfiring. If the engine is operating at low load and with low cooling water temperature, the compression temperature and compression pressure will be very low. If the fuel is injected, then it may stop igniting and may cause misfiring of the engine. Q.580 Which of the following condition may tend to increase the ignition delay period of combustion in a compression ignition engine? Ans. (b) Decreasing the charge air temperature. If the charge air temperature is decreased, automatically the temperature at the end of compression stroke will also decrease. When the temperature at the end of compression decreases beyond certain limit, it will be difficult for the fuel which is injected to convert into combustible mixture and large amount of time will be required to achieve this process. In this situation, the time delay will increase, especially in compression ignition engines
Q.581 In a diesel engine excessive cylinder liner wear will cause (i) increase blow-by.
(ii) wear between the piston ring and the groove. Ans. Both answers are correct. If there is excessive cylinder liner wear, there will be blowpast because piston ring will not seal the liner properly. When the piston rings will be sliding over the worn surface of the liner, which are not evenly worn-out, in order to keep the contact with the liner, the piston ring will move in radial direction in and out continuously. Due to this, piston rings will slide over bottom surface of the ring groove thus causing the excessive wear of the bottom of the ring groove. Q.582 The change in the degree of fuel atomization in a fuel diesel engine will mostly affect Ans. (b) fuel penetration. Penetration and atomization are inversely proportional to each other. Hence if there is excessive penetration there will be less atomization and if there is excessive atomization there will be less penetration. When atomization takes place, the fuel particles are divided into very small minute microscopic particles which will have less amount of energy to overcome the dense atmosphere in the combustion chamber and hence it will not penetrate deep into the combustion space. On the other hand, if the atomization is less the particle size will be bigger and their mass will be higher and hence they will have high amount of kinetic energy to penetrate into the dense atmosphere air deep into the combustion chamber. Q.583 The longer the ignition period occur in a diesel engine the more rapid rise in the combustion pressure. More delay will give more accumulation of the fuel in the combustion space before the actual firing of the fuel takes place. As a result, the accumulated fuel will get ignited. When the accumulated fuel will get ignited suddenly or instantaneously, it will cause excessive amount of pressure rise in the cylinder. Q.584 Diesel engine exhaust temperature may be used to indicate (a) leaky exhaust valve. (b) overloaded cylinder. (c) clogged injector nozzle. (d) All of the above. Ans. All of the above. The leaky exhaust valve will indicate increase in temperature because of the high-pressure gases leaking past the exhaust valve. An overloaded cylinder will be indicated by excessive exhaust temperature as higher the load, more will be the quantity of fuel which is injected. When the quantity of fuel injected increases, automatically the temperature of the exhaust gas increases. Thirdly, as the injector nozzle is clogged automatically the quantity of the fuel injected will be decreased. In that case, the temperature may drop down for that particular unit but it may increase for the other units. Also, if one or two holes of the injector are clogged, there is a possibility that more fuel will be injected through the other holes resulting in unbalanced distribution of the fuel in the combustion chamber which may cause the afterburning effect and increase the temperature. Q.585 Maintaining proper fuel oil temperature will result in Ans. (b) improved atomization. Maintaining proper fuel oil temperature will maintain proper viscosity and viscosity is important from point of view of atomization. If viscosity is properly maintained, there will be proper atomization and proper penetration. Hence there will be proper balance between penetration and atomization, and there will be better combustion. Q.586 Combustion knock can occur in the cylinder of a diesel engine under any condition permitting Ans. (c) excess fuel in the combustion chamber. Excessive fuel in the combustion chamber may be due to ignition delay and cause sudden increase in the pressure. This excessive pressure will cause excessive knocking effect in the engine. Q.587 Persistent knocking in one cylinder of a 8-cylinder diesel engine is most likely to be caused by Whether the knocking in the cylinder is by fuel oil or not, it can be checked by cutting of the fuel oil and checking whether the knock persists or not. Even if the knock persist after cutting the fuel oil it indicates that there is some excessive clearance in some of the running parts. Most possible place is badly worn piston pin and bush. Hence every time when the piston is changing its direction from upwards to downwards it will give the knocking effect. Q.588 A crack cylinder head in an operating engine may be indicated by Ans. (b) combustion gases venting from expansion tank. Due to the crack, high-pressure gases will enter through the cracks into the water space during the exhaust stroke and these gases may come out of from the expansion tank or the expansion tank may overflow. At the same time, this will be indicated by high jacket cooling temperature of that particular unit. Q.589 Coolant can be lost from jacket cooling water system by leakage from (a) cracked cylinder head. (b) piping joints. (c) pump seal leaking . (d) All of the above. Ans. (d) All of the above. Water can be lost if there is crack in the cylinder liner or head during the suction stroke when the pressure inside the cylinder is low. When the pressure in the cylinder is low, water will leak into the combustion space. When during the compression and ignition stroke there is high pressure gas in the cylinder it will enter the cooling space. Thus there will be loss of water. The water loss can also be due to leaking piping joints that can be seen externally. The pump sealing leakage will also result in loss of water and it may enter the crankcase or it may come out of the * hole fitted between the crankcase and the pump. Q.590 When oil vapors, oxygen and hotspots are present at the same time with in a crankcase which of the following hazards could develop? Ans. (a) Explosion.
When oil vapor, hotspot and oxygen is present the triangle is completed for fire resulting in explosion. This is called primary explosion which will be followed by secondary explosion. Q.591A possible cause for an individual piston to knock when at TDC on a slow speed two-stroke diesel engine could be due to (a) early fuel injection. (b) excessive bearing play between the gears. (c) overloading of the cylinder. (d) All of the above. Ans. (d) all of the above. The main reason is that if there is excessive early fuel injection, fuel will get accumulated and there will be sudden firing of the cylinder and cause the knock. Excessive bearing clearance will cause the mechanical knocking. Also excessive bearing clearance between the running gears will cause low compression pressure. Due to low compression pressure automatically the ignition will get delayed and knocking will take place. Also because of overloading of engine, excessive fuel will be loaded in the engine and the time available to burn will be less. Hence more fuel will accumulate and that accumulated fuel will ignite violently to give knocking. In other words, knocking will generally be because of increase in the ignition lag. Q.592 An automated diesel engine should normally shut down due to Ans. (c) low lubricating oil pressure. Lubricating oil is the most important element of the engine. If there is low lubrication oil pressure, the engine will not run even for short time and may cause excessive damage. Hence the first and foremost important thing for the engine is low lubrication oil pressure cut-off. Q.593 Routine monitoring of a diesel engine should include (i) checking for leak. (ii) checking for temperature and pressure. (iii) listening for abnormal noise. (iv) All of the above. Ans. (d) All of the above. All of the above aspects will indicate proper running of the engine and to ensure proper running of the engine, all the parameters should be continuously monitored and checked, and noted in the log book. Q.594 One remedy for a high firing pressure, in addition to high exhaust temperature, in one cylinder of a diesel engine is to Ans. (c) adjust the fuel rack. If both the exhaust temperature and peak pressure is high, it indicates that the quantity of fuel injected is more. Hence it is necessary to reduce the quantity of fuel in the cylinder. This is done by adjusting the position of fuel rack. Due to this the quantity of fuel injected will automatically get reduced and knocking will be stopped. Q.595 If a diesel engine is difficult to start and operates at engine temperature lower than normal you should suspect that Ans. (a) compression pressures are below normal. If the compression pressure is below normal, it will be difficult to start the engine immediately as the fuel does not burn immediately. As the engine is cold and compression pressure is low, it will be difficult to start the engine. Q.596 A distorted spray pattern of a fuel injector can cause Ans. (d) loss of power. When there is distorted pattern of fuel injector, there will not be a proper distribution in the combustion space. Hence fuel and air will not mix properly and complete air will not be available for combustion. This will cause the improper combustion of fuel and because of this there will be loss of power and high exhaust temperature. Q.597 White smoke issuing from exhaust of an auxilary diesel engine would mean Ans. (b) engine is cold. If the engine is cold, the exhaust will have condensation of the moisture. At the same time, the exhaust gas generated will cool down and the moisture in the exhaust gas will also get condensed. This will result in white smoke from the exhaust gas outlet. Q.598 White smoke in the exhaust of a diesel engine can be caused by Ans. low combustion temperature. Same as 597 Q.599 Bluish smoke in the exhaust of an operating diesel engine can be caused by Ans. (b) cold cylinder liner When the cylinder liner is cold, lubricating oil will not get scrapped out of the liner into the crankcase but it will get burnt along with the fuel into exhaust gas. This will result in blue smoke emitting from the exhaust manifold Q.600 One of the factors limiting the amount of load which can be put on a modern diesel engine is Ans. (b) exhaust temperature. Exhaust temperature is a criteria for loading the engine because higher the exhaust temperature, the higher the temperature to which the components of machinery in combustion chamber are subjected. Higher the temperature the components are subjected to, they lose their strength much earlier and hence their load carrying capacity decreases. Hence the components fail under the influence of higher temperature and pressure.
Q.601 White smoke exhausting from a diesel engine can be caused by Ans. (c) cracked cylinder liner. If the cylinder liner is cracked, especially at the lower end, water will enter the cylinder. This water will be evaporated during the burning of the fuel and this evaporation will cause the steam which will give white smoke in exhaust gas. Q.602 If a diesel engine runs roughly, which one of the system listed is likely to be at fault? Ans. (a) Fuel system. If the fuel is of bad quality, it may have misfiring. When misfiring takes place, there will be fluctuation in the load and rpm of the engine. Q.603 One cause of diesel engine surging can be result of Ans. (a) injection pump plunger stuck or worn. The surging, in question, is that of turbocharger. When one unit fuel pump plunger is stuck or worn out, there will be no injection in that unit. Due to this, there will be unbalance of the engine power sharing. At the same time, there will be excessive fluctuation in the rpm and this will cause the turbocharger to surge. Q.604 Which of the listed prestart procedures should be carried before starting a crosshead-type diesel engine after an overhaul? (a) pre-lubrication of the cylinder with hand crank. (b) open all air space drain cock. (c) open all indicator drain cocks. (d) All of the above. Ans. (d) All of the above. Before starting engine, it should be necessary that there should not be any accumulation of water or fuel on top of the piston. Otherwise, there will be hydraulic pressure developed and cause damage to the engine. It is therefore necessary to open the indicator cock and turn the engine on turning gear. Also, the indicator cock should be kept open before starting and do blow-through with air. Secondly, when the engine is in standstill condition, the cylinder liner and the valve seats are dry. Hence it is necessary to supplement it with the lubricating oil so that initially during few strokes, when the cylinder lubricator is not injecting, sufficient amount of lubrication oil is available on the cylinder liner wall. This is carried out by the lubricator turning by hand. Also, it is necessary in order to avoid the entry of water through the air which may be because of condensation of moisture in the air, it is necessary to drain the air manifold. Q.605 Plugged spray hole in a diesel engine fuel injector will Ans. cause excessive smoking at idling speed or when the engine is on load. When the engine is at idling speed or on load, due to the plugged injector spray holes there will be uneven distribution of fuel inside the combustion space. When there is uneven distribution of fuel in the cylinder, the complete combustion of fuel will not take place because of unavailability of air to the fuel. As a result, air pockets will be created and because of lack of oxygen, the combustion will deteriorate and will form carbon formation and smoking. Q.606 Late fuel injection in a diesel engine is indicated by low firing pressure with Ans. (c) black or grey exhaust smoke. When the fuel injection firing is late there will be afterburning. When afterburning takes place, the exhaust temperature will be high and as the burning takes place under increase in volume condition, the pressure will drop. At the same time, there will be large amount of black smoke. Q.607 Distortion of spring pattern of nozzle may be indicated by Ans. (c) smoky exhaust. Refer to above. Q.608 An incorrect spray pattern of a diesel injector nozzle can be directly caused by Ans. overheating or overcooling of the nozzle or carbon blockage of spray holes. Under all the three condition, there will be distortion of fuel pattern and loss of power and high exhaust temperature. If there is overheating, the needle will not sit properly on the seat. At the same time, the clearance between the needle and guide will change which will cause the leaking and because of which the spray pattern will change because of drop in pressure and leaking of the needle valve. In case of overcooling, the nozzle will get distorted because of corrosion and because of which the spray pattern will change. Thirdly, carbon blockage of spray holes will cause the distortion of the spray as the fuel will hit the carbon deposits and it will be distorted. Q.609 High cylinder pressure accompanied by low exhaust temperature can result from Ans. (d) excessively injection timing. If the fuel is injected early, there will high firing pressures but low exhaust temperature because enough amount of time will be available for the fuel to mix properly and form a combustible mixture. Hence the combustion will be perfect. When combustion is perfect, it will produce more amount of power and there will not be any afterburning. Hence the exhaust temperature will be low but the firing pressure will be high. Q.610 A leaky diesel engine fuel injector will cause Ans. (d) incomplete combustion. Leaky fuel injector will cause alteration in the fuel injection timing because there will be loss of fuel from the system. At the same time, the fuel that leaks in the combustion chamber will be in the form of droplets and not in atomized condition and hence it will not burn properly and it will form a * and it will cause incomplete combustion which will give black smoke. Q.611 Diesel engine piston seizure can be caused by (a) poor cooling of the cylinder wall. (b) improper cooling of piston.
(c) insufficient piston lubrication. (d) All of the above. Ans. (d) All of the above. If there is insufficient cooling of the cylinder liner, the cylinder liner wall temperature will be very high. Under such circumstances, the lubricating oil over the wall will get evaporated faster. Hence excessive friction will be developed between the piston and the cylinder wall causing the high temperature which may cause the piston to get welded and cause the seizure of the piston. Because of improper cooling of the piston, it will expand in radial direction and that will cause the seizure of the liner. Insufficient lubrication will also cause excessive friction between the piston rings and the liner, resulting in large amount of heat being developed. This will cause the piston to get welded resulting in the seizure of the piston. Q.612 A diesel engine will lose power if the fuel injection occurs too early because Ans. (b) the ignition will be delayed due to low ignition temperature. If the injection is too early, the compression temperature and compression pressure will not be sufficient to ignite the fuel. Hence fuel will get accumulated and the fuel will burn with explosive pressure that will give the knocking effect. At the same time, there will be afterburning and this would cause excessive loss of power. Q.613 When the fuel is injected in a diesel engine cylinder too early Same as 612. The ignition will be delayed because the temperature of air will not rich the self-ignition temperature of the fuel. Also, there will be excess of fuel in the cylinder resulting in too rich mixture and hence improper combustion will take place and hence there will be loss of power. Q.614 Which of the following should always be checked prior to starting a diesel engine? Ans. (c) crankcase oil level. Q.615 When high firing pressure and low exhaust temperature occurs simultaneously in a diesel engine it may be because of Ans. early timing of the fuel injection. If the injection is not too early, there will be sufficient time for fuel and air to mix properly. At the same time, there will be sufficient ignition temperature so that the fuel will burn completely and efficiently, and produce more power. As there is no afterburning, the exhaust temperature will drop and the firing pressure will be high. Q.616 If the firing pressure in a diesel engine is high, although the exhaust temperature is normal, the cause may be Ans. (b) high compression pressure and high scavenge air pressure. If the scavenge air pressure and compression pressure is high, the firing pressure will be high. As good quantity of the air is available, at slightly low temperature, it will not effect any rise in the temperature but the firing pressure will be definitely high. Q.618 Which of the following statement represent a rolling of a diesel engine with a cylinder indicator cock open prior to starting? Ans. (b) To ensure foreign material, such as water or lubricating oil or fuel, is not present in the cylinder. If liquid, such as water, lubricating oil, or fuel, is present on top of the piston due to any reason, as these liquids are incompressible, they may produce large amount of pressure which will cause breaking of the piston rings, liner, cylinder head and in worst case it may damage the crankcase. In order to avoid this, the engine must be turned by turning gear with the indicator cock open so that if there is any pressure developed or liquid on the top of the piston, it would be released from the cylinder and would not be able to develop any excessive pressure in the cylinder. Q.619 In order to start a large slow speed diesel engine, on high viscosity fuel oil after an extended shutdown, the Ans. the fuel must be preheated. Fuel has to be preheated so as to bring its viscosity down so that it can be injected. This is done by preheating of fuel. When the viscosity is excessively high, there will be excessive drag and the atomization will suffer. When atomization suffers, the combustion will not be proper and sometimes, the fuel may not get fired at all. Under such circumstances, it is necessary to preheat the fuel before starting the engine after prolonged shutdown Q.620 After starting a diesel engine which of the listed operating condition should be checked first? Ans. lubricating oil temperature. Q.621 What harmful condition could result in if a diesel engine is operated at very light load for a long time? Ans. (a) increase carbon built-up. If a diesel engine is operated at very light load for a long period of time, the combustion is inefficient because of low compression temperature and pressure. Also the quantity of air will be less and under such circumstances, there will be improper combustion of fuel and improper combustion of fuel will cause the formation of carbon deposits in the combustion chamber. Q.622 A diesel engine should not be operated at low load for a long period of time because (a) heavy carbon deposit will be built up on the exhaust valve and exhaust manifold. (b) fuel dilation is increased at low load. (c) exhaust valve may get damaged. (d) All of the above. Ans. (iv) All of the above. Due to lack of air available, the combustion will be affected and this will cause the improper combustion and carbon deposition will take place. Secondly, as sufficient amount of air is not available for combustion, some amount of fuel will remain unburnt which will get coated over the cylinder walls and will be scrapped downwards by the oil scrapper rings and it may finds its way to the crankcase. The excessive fuel along the cylinder wall may get scrapped along with the excessive lubricator oil and finds its way in the crankcase and the crankcase oil will get contaminated. At the same time, the carbon deposits formed due to improper burning
may get logged on the exhaust valve seat and because of that local burning of the seat and the valve will take place which may damage the valve. At the same time, when the exhaust valve does not close properly, there will be wiredrawing effect and the cutting effect on the exhaust valve seat and this will cause the exhaust temperature to rise. Q.623 The possibility of diesel engine crankcase explosion will increase by operating the engine Ans. (b) by a leaking crankcase inspection cover gasket. If the crankcase inspection cover gasket is leaking, there is a possibility of air entering into the crankcase thus causing more amount of air available for the explosion to take place. This will be primary explosion. When primary explosion takes place, the crankcase relief door will be opened and it will release the excess pressure. As soon as the pressure is relieved, due to the leaky gasket, fresh air will enter into it and cause the secondary explosion. Hence these chances of severe explosions are more in case of leaky crankcase inspection cover gasket. Q.624 Main propulsion diesel engine continuous running after shutdown, you should now attempt to Ans. (a) stop the air supply. There is may be a possibility that if the fuel pump racks are stuck-up in one position, then even if the stop signals are given by the governor to the engine, some of the fuel pump rack may not be brought back to the stop condition and the engine will continue running. For this purpose, puncture valves are provided to avoid this situation. Under such condition, if the engine keeps running and it is necessary to bring it to stop condition, the blanketing of the suction filter of the turbocharger is the best method. By this method, the blanketing will deprive the engine of the air. When the engine is deprived of the air the fuel will not ignite and the engine will stop. Q.625 Low compression pressure in the diesel engine can be caused by Ans. (d) carbon deposits on the piston and piston rings. Due to this, the piston will get seized inside the liner and they will not exert sufficient pressure on the liner walls. As a result, there will be excessive blowpast and the compression pressure will automatically drop because of lack of proper sealing between the piston and the liner. Q.626 If the diesel engine has been stopped due to piston seizure due to severe overheating, the crankcase inspection cover should not be opened Ans. until the engine is cooled Under such circumstances as the piston is hot, there will be hotspot present inside the crankcase and there is already possibility of high amount of fuel mist. If at this moment the crankcase door is opened for cooling purpose, the fresh air will rush inside the crankcase. When sufficient amount of air is available along with the mist and the hotspot is present, there will be severe explosion and severe damage. Q.627 Which of the following condition indicate dilution of diesel engine lubricating oil by fuel? Ans. Drop of viscosity. The dilution of diesel engine lubricating oil by fuel can be indicated by the drop in viscosity because fuel acts like solvent. When the fuel acts like solvent, then automatically the lubricating will be become thinner and hence its viscosity will drop. Also, its flash point will drop. Q.628 Already done. Q.629 A large slow speed diesel marine engine is operating at rated load and speed while the vessel is in calm seas. As the intensity of the sea increases, the engine speed governor indicates same rpm, although the load indicator indicates an Ans. decrease in the load limit setting. If the governor is rpm-dependent governor, it will try to maintain constant rpm irrespective of the load change. In order to avoid the excessive loading of the engine and to keep the rpm constant, it is necessary to set low load limit. Under such circumstances, the fuel will be given only to maintain the rpm and the load will be reduced. (To keep the rpm constant at higher load we have to give more fuel. Hence main engines which are directly coupled to the propeller, the governors are mainly constant load governors where they try to keep the load constant and avoid loading resulting in drop of rpm.) Q.630 A drop in compression pressure of a diesel engine can caused by Ans. (d) burnt valves. If the compression pressure of only a particular unit is dropping, the problem is only of that particular unit and not of the whole engine. Under such circumstances, it is necessary to investigate the possible causes for that unit. In this case, the possible causes may be: the piston ring may be sticky or the piston rings may be worn out or liner may worn out or there is a possibility that valves (exhaust or inlet) are leaking. Q.631 Which of condition listed could cause the cylinder relief valve on a large low speed propulsion diesel engine to lift? Ans. (b) Excessive fuel injection. When there is excessive fuel injection, there will be excessive rise in the firing pressure. When the firing pressure reaches above the setting of the relief valve, the relief valve lifts. Q.632 An increase in crankcase pressure generally indicates Ans. (b) worn cylinder liner. If the cylinder liner is worn out or piston rings are worn out or stuck up, in all these conditions, the high pressure gases will find its way between the liner and piston rings, into the crankcase and the crankcase pressure will increase in case of the trunk type of engines. Q.633 An increase in diesel engine crankcase pressure indicates generally the excessive Ans. d excessive piston ring blowby. (Already explained) Q.634 If the diesel engine was running at 20% overload with a smoky exhaust, you should Ans. (c) slow the engine, allowing it to cool gradually.
You have to reduce the load on the engine so that it will be brought slowly into the normal working temperature and hence there will no increase in the temperature. Hence sudden stoppage should not be done; otherwise, there is a possibility that the liner or the piston may crack due to sudden cooling. Q.635 The starting air rotates a diesel engine at a proper speed but the engine fails to start, you should check (a) overspeed trip (b) for an obstruction in an air filter (c) air in the fuel line (d) All of the above. Ans. (d) All of the above. If the overspeed trip has taken previously, fuel lever has gone in blocking condition and hence it will not be released unless you reset it. In such condition, even if the engine starts up on air, it will not pick up on fuel. Secondly, if there is air in the fuel system, the fuel pump will not inject the fuel into the cylinder and hence the engine will not start on fuel. If there is obstruction in the air filter, engine will not change over from air to fuel because the fuel will not burn due to lack of air. Q.636 If the diesel engine turns over at normal crank * speed but fail to start the cause could be late fuel injection or Ans.(c) inadequate fuel injection. If the fuel injection is inadequate, during starting the engine has to overcome the inertia of rotating parts and from standstill it has to speedup the engine to the full speed. For this purpose, you require excessive amount of power. Hence it is necessary that initially while starting the amount of fuel injected should be more than the normal running fuel. If the fuel injected is less, the engine will not pickup on the fuel. Q.637 An increase in power output of a turbocharged diesel engine operating at constant engine speed results in (a) high exhaust temperature. (b) increased turbocharged speed. (c) high air box pressure. (d) All of the above. Ans. (d) All of the above. As the power output of the engine increases, more amount of thermal energy is available in the exhaust gas which can be utilized by the turbocharger. Hence the exhaust temperature will increase and more heat is available at the turbocharger hence the turbocharger speed will increase. When the turbocharger speed increases, it will increase provide more quantity of air and hence the scavenge air pressure will increase. Q.638 An accumulation of carbon on one of the thermocouple of an exhaust gas pyrometer will Ans. read low temperature due to the insulation effect of the carbon deposits. The carbon deposited on the thermocouple will act as an insulator and the actual temperature will not be read. At the same time, the response to the temperature will also be low. Q.639 Which of the listed state of condition indicate early fuel injection timing? Ans. (b) High firing pressure and low exhaust temperature. Early injection will induce a high firing pressure but low exhaust temperature (DISCUSSED EARLIER) Q.640 Black smoke exhaust in a diesel engine indicates Ans. (c) incomplete combustion. Incomplete combustion will form unburnt carbon in the exhaust which will be black in color. (DISCUSSED EARLIER) Q.641 Clogged or partially obstructed exhaust port on a diesel engine can cause Ans. high exhaust temperature. As the exhaust ports are clogged or partially obstructed, the exhausting will be affected and hence complete removal of exhaust gas from the cylinder will not take place. Hence the sweeping complete action of the exhaust gas by the incoming scavenge air will not take place. The incoming scavenge will get mixed with the exhaust gas and there will be dilution of the air charge with the remaining exhaust gas of the cylinder liner. Due to this, improper combustion will take place which will cause the black smoke and loss of power will take place and high exhaust temperature because of the afterburning effect. Q.642 Combustion knock is likely to appear as a result of fuel with Ans. low ignition quality. If the fuel with low ignition quality is utilized there will be increase in ignition lag. When ignition lag is higher then more amount fuel is accumulated before actual combustion ignition begins. This high amount of fuel accumulated when catches fire, it will instantaneously produce heavy knocking effect. Q.643 Black smoke exhausting from a diesel engine may be caused by Ans. (d) clogged air cleaner. If the air cleaner is clogged, less quantity of air will enter the cylinder. When less quantity of air enters the cylinder because of that the fuel will not burn properly. Due to this, unburnt carbon deposits will remain in the exhaust which will give the black smoke. Q.644 If two-stroke diesel engine is overspeeding due to lubricating oil leakage of the cylinder what should you do to stop the engine? Ans. (c) Shut of the fuel injector and block the flow of intake air. By shutting of the supply of intake air the speed will be reduced. But if the lubricating oil is igniting, which is causing the running of engine even after shutting the air supply, then under such circumstances, the only possible way of stopping the engine is to stuck off the air supply by covering the suction filter with blanket. Q.645 Two stroke diesel engine operates electrically, overspeeds and fails to restart when the crankcase at normal speeds Which of the following problems is likely the cause for the engine failing to restart?
Ans. (c) Failure to reset overspeed trip. Overspeed trip is a type of switch. This is provided for a safety purpose so that the engine speed will reduce after the overspeed trip operates. It should not restart unless it is resetted manually. To reset the overspeed trip manually, we have to bring the fuel lever to stop condition and we have to restart. If this is done properly, then even if we try to start the engine on air, it will turn but it will not come on fuel because the fuel is blocked because the overspeed is already operated. Hence it is necessary to reset the overspeed trip. Q.646 When running a large slow speed main propulsion diesel engine on heavy fuel which of the following precautions should be observed when switching back to diesel? Ans. (b) the temperature of fuel from the preheater should be gradually retarded after switching over to the three way valve. Whenever the changing from diesel oil to heavy oil or vice-versa is to be taken place, it is necessary that the temperature of oil heated or lowered should be carried out gradually so that there is no sudden shock thermal loading of the components of the fuel injection system. At the same, it will allow proper expansion and contraction of different parts thus maintaining the proper clearance between the running parts of the system. Otherwise there is a possibility that one part will cool at a faster rate than other. In that case, there is seizure of the plunger in the fuel pump or needle in the guide and there will be heavy damage. Q.647 If it becomes necessary to cut-out an individual cylinder of a large low speed main propulsion diesel engine Ans. the fuel oil to the cylinder should be secured and its fuel oil rate should be reduced. As the unit is not firing, there is no need of cylinder lubrication. Otherwise, the excessive lubrication will go along with the exhaust and damage the turbocharger and it may catch fire in the exhaust manifold causing the overspeeding of the turbocharger. Q.648 Late fuel injection is indicated by Ans. (b) lower normal cylinder pressure and high exhaust temperature This will happen because of afterburning not due to burning the fuel at right time. Due to burning during the expansion stroke there will loss of pressure and increase of temperature MOTOR MAINTAINENCE Q.649 Bearing wear on sleeve type bearing is measured with (a) ball anvil micrometer. (b) pair of outside caliper and dial indicator. (c) pair of inside caliper. (d) depth micrometer. Ans. (a) ball anvil micrometer. The ball anvil micrometer has got a ball end, instead of flat, end so that it can measure the curved surface thickness. Bearing being curved surface, ball anvil micrometer can be used for measuring the thickness and after measuring the thickness of the bearing it is possible to measure the weardown of the bearing. Q.650 Wear occurring at the tip of the reduction gear teeth is usually the result of Ans. (d) gear misalignment. The tip of the gear teeth gets worn out entirely because of gear misalignment. When there is misalignment between gear, the tip of the gear will rub against the bottom of the gear teeth and cause excessive weardown. This also gives the excessive axial thrust and radial thrust which will cause the weardown of the gear teeth.
Q.651 While inspecting the main bearings on a diesel engines you find impregnated dirt and scratches. You would therefore suspect that Ans. (c) the lubricating oil was not properly filtered. If the lubricating oil is not properly filtered, dirt and hard particles will find its way between the bearing and the shafting. Due to this, there will be scratching marks on the white metal of the bearing. At the same time, the hard dirt will get impregnated inside the soft surface of the bearing. Q.652 When nozzle tester is being used to test a closed type fuel injection nozzle a clogged nozzle orifice will be indicated by a Ans. (a) distorted spray pattern. When the nozzle hole is clogged the spray pattern will be distorted because that particular hole will not have any injection but same quantity of fuel will be injected by remaining holes. Hence complete pattern of fuel will be distorted and that can be checked by the use of a plate which is held under the nozzle and the nozzle is tested. Q.653 The spray holes in the diesel engine fuel injection valve should be cleaned using carbon solvent and Ans. (b) a special cleaning wire. The carbon solvent will dissolve only carbon but hard particles such as silica or alumina cannot be dissolved in the solvent. Hence it is necessary to remove them with special cleaning wire which is provided. This cleaning wire will clean the hole but it will not damage the holes. Hence cleaning should be carried out by special cleaning wires. Q.654 When dissembling or assembling an injection pump plunger and barrel, you must (a) keep the parts immersed in diesel oil. (b) always keep the plunger and barrel together. (c) work over a linoleum type surface. (d) All of the above. Ans. (d) All of the above.
When dismantling pump plunger and barrel, if we keep them in the diesel oil there is less possibility that of any rusting taking place on the surface and thus surface finish will be maintained. Secondly, they should always be kept together because they are lapped surface so that no dirt will enter in between them. At the same time, because of their better sealing effect, they should always be replaced as a pair and should always assembled as a pair. Also, it should be worked over a linoleum type surface as it is smooth and dirt-free surface. Due to this, there is a less possibility this surface will attract any foreign dirt or any hard particles in the surface. So linoleum type surface can be kept clean by just wiping it out and the fuel injector can be kept clean without getting the scratching due to the hard particles.
Q.655 Visual inspection of fuel injector valve removed during overhaul shows heat discoloration of lower end of the valve. This is indicative of Ans. (a) valve leakage. Fuel injector valve is a non-return valve and it seals the nozzle against the high temperature and high-pressure gases. When the fuel injector needle valve is leaking, high temperature gases from the cylinder will enter the nozzle because of which the nozzle will get overheated and it will get discolored. Q.656 A visual examination of chrome plated piston compression ring at the liner port reveals vertical brown strip on the face of the rings. This condition indicates Ans. (c) ring blowby. When ring blowby takes place the lubrication oil film between the piston and the cylinder liner gets distorted resulting in the piston rings being coated with the brown tinge of the exhaust gas. At the same time, when this happens because of lack of lubrication between the piston ring and the liner the excessive heat is generated. This will also give discoloration of the liner and the piston rings, and vertical lining on the surface of the piston ring called scuffing Q.657 Uneven bolt tightening during the installation of fuel injection pump can result in Ans. variation of clearance between the plunger and the barrel. Due to the uneven tightening of the bolts during the installation, it will result in variation of clearance between the plunger and the barrel. Hence the clearance between them will not be uniform all around the circumference. There is a possibility that seizure may take place between the moving parts of the pump where there is less clearance; therefore it is necessary to have uniform tightening. Q.658 In a diesel engine a cylinder liner should be replaced if it is Ans. (b) scoured. Actually speaking, the liner should be replaced if it is badly scoured or scuffed because scuffing will also give the distortion in the geometry of piston and the liner. At the same time, the scratches will prevent good sealing between the liner and the piston rings. Hence it should be replaced if it is badly scuffed or scoured. Q.659 If the plunger or barrel of a fuel injection jerk pump gets damaged Ans. (a) the entire pump must be replaced. As the pump plunger and barrel is manufactured as an assembly, there is good sealing between the plunger and barrel and both surfaces are lapped together to form air-metallic seal. Hence it should be replaced as an assembly and because good bearing between the two surfaces is possible if they are replaced together and not replaced individually. Q.660 Which of the listed devices could be used as a substitute for a ring-grooving tool? Ans. (c) Section of removed compression ring. Q.661 Cracking of diesel piston crown can result from Ans. (b) the under side piston crown being excessively dirty lowering the rate of heat transfer. If the cooling space of the piston crown becomes excessively dirty due to the deposition of the carbon or scale, proper transfer of heat will not take place from the crown to the cooling space. Hence the piston crown will become excessively hot and it will loose its strength and it will crack. Q.662 It is easier to replace a dry cylinder liner than wet liner because Ans. (c) water seals are not required. In case of dry cylinder liner as there are no water seals or o-rings to seal the water spaces and hence replacement of the liner becomes very easy. In case of water cooled liner the sealing rings has to be properly fitted. At the same time, the surface on which the sealing rings are to rest should be properly cleaned; otherwise the leakage possibilities are more which is not applicable in case of dry liner. Q.663 To facilitate early ring seating of newly installed piston rings while still providing extended rings wear Ans. (c) a tapered faced ring can be used. A tapered face ring can be utilized for faster running in of the cylinder liner and the rings due to which sealing between the cylinder liner surface and the piston can be obtained in a shortest possible time. Due to tapered face, the surface contact area is reduced and because of this the pressure between the ring and cylinder liner increases which causes the faster weardown of the piston rings and cylinder liner, which ultimately results in better sealing between them in a shortest possible time. Q.654 Insufficient end clearance on newly fitted piston rings on a diesel engine will cause the rings to Ans. (a) jam in the least worn part of the cylinder liner when the ring expands. The end clearance, also called the butt clearance, is less when the piston ring expands, the two ends of the piston rings will butt against each other. As there is no further expansion possible in the circumferential direction, they will have to expand in the radial direction. In doing so, it will give excessive pressure over the liner wall and cause the seizure of the piston rings into the cylinder liner.
Q.665 Heavy fuel oil will generally have an upper average ash content of .1% by weight. Which of the following condition will occur if the ash content exceeds the above amount? Ans. (b) Increase valve wear. The ash which is present in the fuel oil is mostly in the form of heavy metals. Hence when the percentage of ash increases, this ash will get deposited on the valve seat. Due to this, the valve will not get properly seated on the valve seat and there will be increase in the weardown. At the same time, there will be burning of valve and valve slip may occur. Q.666 The face surface appearance of shallow groove stainless steel chrome plated compression ring should exhibit through its operating life Ans. (a) smooth shallow groove chrome surface.
Q.667 To determine the main bearing clearance of a propulsion diesel engine, you should measure the shell by using ball anvil outside micrometer and measure the crankshaft journal using the Ans. (a) outside micrometer. The weardown of the bearing can be measured by using a ball anvil outside micrometer so that it can the thickness of curved surface of the bearings. By determining this, the total weardown of the bearing can be found out which can be utilized to find the inner diameter of the bearing. By measuring the pin diameter, by using an outside micrometer; it is possible to measure the clearance of the bearing by knowing the inner diameter of the bearing and the pin diameter. Q.668 Visual inspection of chrome plated piston compression rings reveals a black ring face at the position of the cylinder ports. This condition indicates Ans. the ring which has excessive blowby. When excessive blowby takes place the ring surface which is in contact with the liner will get discolored. It will become blackish and dry as the lubricating oil will get burnt. This is especially present in case of first piston ring because first ring is subjected to high temperature and pressure and so blowpast is maximum on the first piston ring which will make the ring surface into black. Q.669 The consistent burning of fuel oil in a diesel engine with high sulphur content will result in Ans. (d) increased cylinder liner wear. The excessive sulphur in the fuel which is burnt will form sulphuric acid in the cylinder. When this acid gets condensed on the liner wall excessive corrosion of the liner will take place. In order to avoid this, it is necessary to use cylinder oil with a high TBN value or to keep the liner wall temperature high by using the jacket cooling water temperature at a higher range. Q.670 Burning fuel with high sulphur content in a diesel engine will Ans. (d) produce corrosion in the cylinder and the exhaust system at low load. Burning fuel with high sulphur content will produce sulphuric acid and at low load the temperature of the exhaust gas and inside the cylinder is low. During this low load operation the sulphuric acid will get condensed over the liner wall. Hence excessive corrosion wear will take place when fuel with high sulphur content is used at low load operation. Q.671 Which of the following statement concerning cylinder liner wear is true? Ans. (b) Excessive liner wear causes the wear the between the piston ring and the grooves. Due of excessive and non-uniform liner wear, the piston rings will try to maintain the contact with the liner wall. As a result, there is continuous radial movement of the piston rings over the groove in the piston because of which there will be friction between the piston rings and the groove. Hence the bottom of the groove will get worn out faster if the cylinder liner is worn out excessively and non-uniformly. Q.672 Generally where you should find the greatest amount of wear on the cylinder liner? Ans. (c) Opposite the top ring shortly after the piston travel has ended the compression stroke. The liner wear is maximum when the piston is at TDC. It is time when the piston is subjected to high temperature and low lubrication. As a result, the cylinder liner is worn-out excessively along with the piston rings because of the excessive pressure between the liner and the piston rings and lack of lubrication due to high temperature. Q.673 Worn main bearing will cause the compression ratio of an auxillary diesel engine Ans. (b) to decrease. When the main bearings are worn out, automatically the crankshaft inorder to keep contact with the bottom half of the main bearing will try to sink downwards. Due to this, the clearance volume between the piston and cylinder head, when the piston is at TDC, will increase. This will cause the loss of compression pressure. Q.674 Insertion shims in the main marine diesel engine between the connecting and the bearing box would result in Ans. (a) increase in compression ratio. The marine type of connecting rod has a * and if that * is fitted over the bottom end bearing housing and by inserting the shims between the palm and the bearing housing automatically the piston is raised up and when the piston is raised up the compression ratio increases Q.675 Diesel engine crankshaft deflections are taken at four crankshaft positions. Good engineering practice require the deflection gauges are indicated to be Ans. left in place for all four readings.
When the crankshaft deflection is taken it is done at four position of the piston. As a matter of fact actually five positions of piston. During this, we are checking the distance between the two webs. Inorder to check the distance between the two webs, it is necessary that the dial gauge fitted between the two webs should be continuously kept in position to get the correct reading all around the cycle. Q.676 An acceptable method of measuring the correct rotational force applied to the connecting rod and main bearing bush is to use a Ans. torque range. The tightening of the bottom end bearing bush can be done safely and properly by use of torque range. It has got silicon clutch and hence it will never get overtightened. When a particular torque is reached it will give a sound which will indicate proper tightening without over stressing or over tightening of the bolts. Hence torque range is the better method of tightening the bottom end bearing bolts. At present, there are other methods such as angle and hydraulic methods which are much better than the torque method. Q.677 Small cracks in the surface of diesel engine crankshaft bearings are an indication of Ans. fatigue failure. When the crankshaft bearing is subjected to fatigue, the strength at which it will crack will lower down and hence the cracks appear. These cracks are quiet significant and are not because of corrosion or insufficient lubrication or abnormal wear. This will not give the cracking. Cracking is due to the material which is under stress due to fatigue. Q.678 If water is found in the crankcase of a diesel engine the cause may be Ans. due to cracked cylinder head, cracked cylinder liner and leaky cylinder gasket. In case of four-stroke engine if the cylinder head is cracked, there is a possibility that water may enter the crankcase when the engine is in stopped condition. If the cylinder liner is cracked water may enter from the cylinder head through the gap between piston rings and it will enter into the crankcase. Q.679 Diesel engine piston ring blowby is usually caused by Ans. excessive ring clearance at the ring gap Ring gap clearance is also called as butt clearance. If this butt clearance is excessive, it finds an easy assess for the hot and high pressure gases to pass through and it will give more amount of blowpast Q.680 Diesel engine blowby into crankcase is caused by Ans. excessive ring gap clearance. Q.681 The wear is usually greatest at the top of the cylinder in a diesel engine due to Ans. the highest pressure being exerted. At the TDC, when the fuel is injected, high pressure is developed. This high pressure finds its way behind the piston rings and they are forced against the liner walls with great contact pressure and thus more amount of weardown will take place. At the same time, near TDC because of the high temperature, the lubricating effect is reduced and hence there is more amount of wear of the liner and the piston rings. Q.682 Which of the following operation will have a direct impact on the rate of wear of cylinder liner? Ans. (b) Quality of the fuel injected. If the quality of the fuel injected is bad i.e. it has more amount of sulphur or vanadium or sodium or abrasive particles, excessive wear down of the liner will take place due to corrosive effect. At the same time, the wear down may also take place due to abrasive effect. Q.683 Failure to remove the carbon ridge from the top of the cylinder when replacing the piston ring will result in Ans. damaged upper piston ring and the ring landing surface. If the carbon ridge is not removed and new piston rings are replaced, the top most ring will hit at the ridge and may get damaged. At the same time, it will damage the ring landing surface. Hence it is necessary to grind off the ridge when the piston is removed and the rings are replaced (repressed). Q.684 Scuffed cylinder liner bearing surface in a diesel engine can result from Ans. (c) operating the engine on overheating. When the engine is operating on overheating condition (which can exist due to blowpast), the lubrication between the piston and the liner is not effective because the lubricating oil will evaporate faster. Under such circumstances, because of loss of lubrication oil, the sliding friction between the piston and liner walls will cause great amount of heat which will ultimately cause the scuffing of the liner walls. Q.685 Water accumulation in the cylinder of a secured engine will indicate Ans. cracked cylinder liner. When the engine is secured, i.e. it is standstill, if there is water accumulation on the top of the piston it is an indication that the liner is cracked or the cylinder head may be cracked. Hence there is a possibility that water may get accumulated on the top of the piston through these cracks. Q.686 The formation of carbonmonoxide in diesel exhaust gas can be reduced by Ans. (c) maintaining proper combustion and scavenging. Carbonmonoxide gas is produced because of lack of oxygen during the burning process. Hence if proper combustion and scavenging process is maintained, fuel will burn efficiently and because of proper scavenging the amount of air will be sufficient and hence better combustion and no starvation and no formation of carbonmonoxide.
Q.687 The amount of wear on a speed precision main bearing can be accurately determined by comparing the data from previous reading to the present reading taken with ball anvil outside micrometer. Ball anvil outside micrometer can be utilized to measure the thickness of curved surface. So by comparing the thickness (previous and present) of the shell bearing, we will be able to find out the degree of wear of the bearing. Q.688 On diesel engine equipped with individual jerk type fuel pumps, adjustments should be made to the tappet push rods of the pump to Ans. equalize effective delivery stroke. The effective delivery stroke of all units can be equalized when the upward moving plunger will close the suction ports of all the fuel pumps at the same crank angle. This can be achieved by adjusting the tappet of the plunger under the jerk pump so that the effective delivery stroke can be adjusted equally. Q.689 The amount of wear on a split fixed sleeve type main diesel bearing can be accurately determined with a Ans. (c) ball anvil outside micrometer. (ALREADY DISCUSSED) Q.690 Friction, engine wear, and oil consuming in a diesel engine are directly related to the Ans. viscosity of the oil. In case of lubricating oil, viscosity of the oil place an important role in reducing the friction wear and wear down of different components of the engine because the formation of film lubrication depends on the viscosity of the oil. The viscosity of oil has to maintained in a preset condition so as to have proper film thickness and to reduce the friction between the rotating parts and to take out the heat generated due to friction. Hence viscosity plays an important part in determining the engine wear and friction. At the same time, if the viscosity is too low there is possibility that excessive consumption will take place because of leakage of oil and more amount of evaporation will take place. Q.691 Improper cooling of diesel engine cylinder liner due to the accumulation of scale deposits may cause Ans. increased piston ring wear. When the liner wall temperature is high, the lubrication between the liner and piston will get evaporated. Hence less amount of lubrication oil film will be there between the liner and piston rings thus causing excessive friction between liner and piston rings causing excessive weardown of the piston rings. Q.692 The nuts of main bearing, connecting rod bolts and all other moving parts are to be secured by Ans. split pins or other effective means. All the nuts of these parts after tightening should be locked by means of locking washers, star washers or split pins so that they will not tend to loosen out. This is a safety measure in addition to hydraulic tightening. Q.693 The easiest way to locate a defective diesel engine exhaust valve is Ans. by comparing the exhaust pyrometer readings. By comparing the exhaust pyrometer readings, leaky exhaust valves can be easily detected because exhaust gas temperature will be high. It will be on the continuous rise and it will indicate that the exhaust valve is leaking. This could be found out by taking a compression and peak pressure curves where we find that both the compression and peak pressure curves for that particular unit is reduced. Q.694 Engine coolant accumulating in diesel engine lubricating oil can result from a Ans. cracked cylinder head or liner. The cooling water finding its way into the engine lubricating oil in the crankcase is only possible because of the cylinder or liner crack or the O-ring which seals the space between the liner and cylinder block are leaking and hence water is finding its way into the crankcase. Q.695 If it becomes necessary to clean the spray holes in a diesel engine fuel injector Ans. you should use a suitable sized * wire and carbon solvent. The carbon solvent will dissolve the hard particles around the holes. Hence the hard particles because of which the holes are clogged get dissolved and it would be easy to remove the dissolved carbon particles by use of wire and hence carbon solvent is necessary to clean the clogged spray holes. Q.696 While over-oiling a jerk type fuel pump it is necessary to replace pump plunger which of the part listed below must also be replaced? Ans. pump barrel The pump barrel and plunger are lapped together and they form a single unit. They cannot be replaced individually because perfect surface finish can be achieved between the two surfaces. Hence it is not possible to replace them individually. If it is necessary to replace the plunger the barrel also has to be replaced. Q.697 An acceptable means of tightening connecting rods and main bearing cap bolts is (a) to measure the torque applied to each nut and bolt assembly (b) stretch up each nut before and after tightening Ans. (a) to measure the torque applied to each nut and bolt assembly. The torque should be measured so that they are tightened to the required torque and they will not be overtightened. Hence it is necessary to measure the torque applied to each bolt at the time of tightening. At the same time, the elongation can also be checked to countermeasure to confirm the proper tightening. Q.698 In a diesel engine when refitting piston rings you should (a) change the ring gap at the smallest diameter of the cylinder liner (b) remove carbon from ring grooves
Ans. both (a) and (b). When fitting new piston rings it is necessary to check the butt clearance and that is possibly checked by fitting the rings at the least worn out position of the liner and checking the gap at the end of the piston rings. At the same time it is necessary to make sure that the ring groove is properly cleaned and there is no dirt or carbon deposits in the groove otherwise the rings will collapse and blowpast will take place. Hence it is necessary to check both i.e. the ring gap and carbon deposits. Q.699 Problems with the diesel engine fuel injection pumps are usually caused by Ans. (b) contaminated fuel. When the fuel is contaminated with water or insoluble hard particles under, the fuel pump plunger and barrel will get excessively wornout because of the abrasive wear and the corrosive wear and all the problems related to the jerk type pump will start from here onwards. Q.700 Dirt lodged on the nozzle valve seat of a fuel injector nozzle will cause Ans. (c) fuel leakage after and before injection. If the dirt will be lodged at the seat, the seat will not be leak-proof. Hence whenever there is low pressure inside the cylinder liner or in combustion space, oil will leak from the seat and it will drip in the combustion space thus causing the high exhaust temperature and loss of power. Q.701 When a diesel engine is operated having a piston ring cold gap clearance less than that recommended by that manufacturer, the ring will Ans. seize and buckle. If the ring gap clearance is less, the ring will expand. Thus ends of the rings will butt against each other hard. As there is no way to expand, the rings will expand radially and ultimately the rings will crack and it will collapse and break. At the same time, due to the excessive heat, which is generated due to pressure between the piston and the liner, the piston ring will get seized inside the liner. Q.702 Due to excessive water in the fuel diesel engine fails to start. Before engine is started the water must be removed from the fuel pump fuel stainer. If excessive water is developed in the fuel in that case the engine will not start. In that case, it is necessary to remove the water from the fuel oil at settling tanks, service tanks through the filter so that when fuel injection starts the engine will pick up on fuel immediately. Q.703 Failure to remove the carbon ridge at the top of the piston while removing the piston may result in Ans. seizure of piston and rings inside the liner. It will be difficult to remove the liner due to the carbon ridge at the top which results in the seizure of the piston and rings. Hence it is necessary to grind the carbon ridge in order to facilitate the removal of the liner. Otherwise, the top most ring will get jammed up between the ridge and hence the liner may come out with the liner or the piston may get jammed up in the liner. Hence it will not be possible to push it either upwards or downwards and this is the most critical condition we can reach to. Hence it is necessary to grind it before attempting removal of piston. Q.704 If the piston ring gap remains insufficient when installing new rings, you should Ans. file the ends of the rings. Ends of the rings are to filed to have a proper butt clearance if required. As far as possible this should not be done because it may damage the profile of the piston rings or it may cause the excessive weardown of the liner or vertical scratching of the liner may take place because of the alteration of the profile of the piston ring. Hence as far as possible when new rings are to fitted the original methods must be utilized where the end gap clearance is always maintained. Q.705 When inspecting piston rings through the ports of two stroke diesel engines black areas on the sealing surface are Ans. result of blowpast Black areas are because of the blowpast where the lubricating oil get washed away. Due to blowpast and heat and the carbon deposit this particular portion of the ring as well as liner wall there will be a new patch of black surface. If this is happening, cylinder lubricating oil has to be increased or checked whether the piston rings are free in the grooves. Q.706 An examination of the piston compression rings of an efficiently operated diesel engine should appear with a Ans. (a) shiny face end bottom and black end at the top. If the engine is efficiently operated when the piston rings are in very good condition, then the sliding surface of the piston rings over the liner should be shining without any black markings or scratching. At the same time the piston ring is rubbing at the bottom end of the groove so that the bottom end of the groove should also be shinning. As the gas goes behind the piston rings into the backside some carbon get deposited on the top and hence the top end of the piston ring may become black.
Q.707 Excessive side clearance between the side and the groove will cause Ans. the ring to hammer the piston land below the ring. Excessive groove clearance will cause the ring to hammer on the piston ring groove bottom surface. This will especially happen when the piston is changing its direction, i.e. when the piston is at TDC and BDC, and the piston ring may break due to this. At the same time, the piston ring groove will wear faster. Q.708 In certain cases the cylinder liner can be refinished rather than replaced when the properly honned Ans. liner surface should have a crosshatched pattern. When the liner surface have excessive scuffing or polishing effect and is not possible to retain the lubricating oil film on the liner surface then in that case it is necessary artificially to make it roughened out under the controlled condition. This is done by honing which is a method of cross pattern which
will have the roughening carried out in two direction so that there will be tiny cavities on the liner surface which will retain the lubricating oil on the liner surface thus reducing the liner and the ring wear. Q.709 One method of determining crankshaft misalignment is by Ans. (d) taking micrometer readings between the crank webs opposite the crank pin every 90 degrees or the crank angle rotation. This means that we are actually measuring the distance between the two crank webs at every 90 degrees position of the crank. The distance between the webs will change according to the misalignment of the crankshaft centerline. If the crankshaft centerline is misaligned because of weardown of the main bearings, the gap between the webs may increase at particular crank position and it may decrease excessively at particular crank position. Hence by comparing the web reading between these two positions, it is possible to find out the misalignment of the crankshaft. Q.710 Which of the following statements concerning the cylinder liner wear in a single acting diesel engine is correct? Ans. (c) Excessive liner wear causes wear between the piston rings and the groove. Excessive liner wear will cause the radial movement of the piston rings inorder to maintain the contact between the liner wall and piston ring surface. This continuous radial movement of the piston ring over the entire stroke of the piston will cause the piston ring to slide on the bottom of the piston groove. Hence there will be excessive weardown of the piston ring groove as well as the piston ring landing surface if the liner wornout is excessive. Q.711 On large slow speed main propulsion diesel engine lower main bearing wear is usually measured by using Ans. bridge gauge. Considering that the crankshaft is sufficiently flexible and heavy enough, it will always keep the contact with the bottom half of the main bearing. If the bottom half main bearing wears down, the centerline of the crankshaft moves downward inorder to keep the contact with the bearing. Hence automatically the bridge gauge reading which measures the position of the crankshaft centerline with the fixed machine surface transverse girder. If this increases it indicates that weardown has taken place. This reading has to be compared with the previous reading so as to measure the weardown of the main bearing bottom half. Q.712 The rate of wear on a cylinder liner depends on the (a) quality of air filtration. (b) effectiveness of lubrication. (c) type of fuel used. (d) All of the above. Ans. (d) All of the above. If the air is not properly filtered, along with air, sand and hard particles from the air will be drawn into the cylinder liner which will give the abrasive wear. If the effectiveness of lubrication is not proper, i.e. the quantity of lubricating oil is less or the lubricating oil is not injected at proper time, the effectiveness of lubricating oil is reduced because of which friction will increase and more weardown will take place. If the fuel contains more amount of sulphur, the liner will weardown because of corrosive effect. If the fuel contains more amount of abrasive particles such as silica, alumina then it will cause the abrasive wear and if the fuel contains more amount of vanadium, sodium then it will cause excessive amount of weardown because of hot corrosion.
Q.713 Abnormal crankpin bearing and piston skirt surface wear indicates Ans. incorrect connecting rod alignment. The connecting rod is to be aligned perfectly with the center of the liner and piston. If this alignment is not carried out properly, the centerline of the piston with respect to the liner will get altered. Under such circumstances, the piston will start rubbing against the liner wall thus giving excessive weardown of piston skirt. At the same time, it will cause alteration in the lubricating oil film produced in the bottom end bearing and hence bottom end bearing weardown will be non-uniform and excessive. Q.714 To determine the main bearing clearances in a diesel engine, you should use Ans. (c) plastic gauge. Plastic gauges are like lead wire in the form of stripes. These stripes are to flexed between the top half of the main bearing and the journal and the main bearing is to be tightened. When this is done the plastic will flow and the thickness of the plastic will get reduced and the width of the plastic will increase. This increase in the width of plastic is directly calibrated with the clearance in the bearing and hence the bearing clearance can be checked with the help of plastic gauges. These are more accurate and safe than any other method of checking. Q.715 The ring land on a large low speed main propulsion diesel engine piston may crack due to Ans. insufficient ring groove clearance. If the ring groove clearance is insufficient in that case the ring will expand and get jammed up into the ring groove. As there is no further expansion possible it will exert excessive pressure over the land of the piston ring groove and because of this the stress concentration on the piston ring groove will exceed and cracks will originate on piston ring groove. Q.718 A condition contributing to diesel engine piston ring sticking into the ring grooves is Ans. insufficient ring clearance at the ring sides i.e. the groove clearance. If the groove clearance is insufficient, the piston ring will expand because of heat and it will occupy the complete clearance which is available. As there is no clearance left, it will get jammed up in the piston ring grooves and hence it will get seized up in the piston ring grooves. Q.719 Low compression in diesel engines could be caused by Ans. (c) worn or broken piston rings.
When the piston rings are worn out or broken the sealing effect of rings with liner will get affected considerably. Hence during compression compressed air will get leaked out past the piston ring and it will blowback into the crank space or under scavenge space. This will result in loss of compression pressure. Q.722 Which of the listed problems can be caused by low compression pressure in diesel engine? Ans. Combustion will not be proper. Actually due to low compression pressure, combustion will not be proper and when combustion is not proper there is a possibility that more carbon will be formed on the exhaust valve. The exhaust valve may start leaking, black smoke and turbocharger may get fouled and because of that the scavenge pressure may drop. All this things may happen because of the low compression pressure. Q.723 Low compression pressure in a diesel engine may be a result of Ans. (b) excessive mechanical clearance between the piston crown and the cylinder head. When the clearance between the piston crown and cylinder head increases the clearance volume increases. Automatically the compression pressure at the end of stroke reduces due of which the temperature and pressure of air will reduce which will cause improper combustion of fuel and may cause afterburning and high exhaust temperature. Q.724 When a nozzle tester is used to check the spray pattern of an injector nozzle Ans. (d) the oil spray could cause blood poisoning if the spray penetrates the skin of the operator. It is well-known fact that with the high-pressure injection of the fuel, the fuel in highly atomized condition will penetrate the skin and the diesel will go into the blood and it will cause the blood poisoning. Hence during checking of a fuel injector valve on a test bed, it is not advisable to put your palm under the nozzle. Q.725 When the piston is removed from a diesel engine for maintenance the piston should be examined for Ans. scouring, crack, burned spots, gummy deposits and sticky rings. The piston should be checked for whether the rings are free. If the rings are free then the ring grooves should be checked for formation of carbon and ash. At the same time, the piston rings and the landing site of the piston rings should be checked for cracks and any gummy deposits which are there on the piston rings and the grooves. This all will indicate if the lubrication is proper and quantity and quality of the oil is in good indication. Q.726 To reduce the load during jacking operations which of the listed device should be operated? Ans. Indicator valve or cylinder test valve. During jacking operations i.e. turning the engine with turning gear it is necessary to open the indicator valve. Q.727 After removing the old set of rings which of the following conditions is indicated if a bright spot is found on each end of broken rings? Ans. Butt clearance was not sufficient. If broken piston rings are to be examined and the broken ends are found out to be polished and shining, it indicates that the butt clearance was not sufficient. The main cause of piston ring breakage is the lack of butt clearance. Once the crack takes place, some amount of material will get removed and the clearance will automatically come into existence. This will cause the two ends of the rings to move over each other and hence the broken ends will get polished and automatically it will become shining. Q.728 After an engine has been started using a * drive unit the drive gear or pinion disengage from the flywheel due to Ans. the high rotational speed of the flywheel. In the * drive when the engine starts because of high speed of the flywheel, the pinion speed becomes excessively high. As a result, the disengagement of the pinion from the flywheel takes place and which avoids the damage caused to the pinion. Q.729 Bearing crush as applied to diesel engine main bearing will result in Ans. (a) positive sitting of the bearing in the housing. Due to the crush there is interference fit between the housing and the bearing. The back side of the bearing will have frictional contact or interference contact or interference fit with the housing which will prevent the rotation of the bearing in the housing when the shaft is rotating. Thus it will give positive sitting of the bearing in the housing. Q.730 Crank web deflection readings will give a positive indication of Ans. (a) worn main bearing journal. Whether the main bearing journal is worn out or the main bearing itself is wornout because of the wear down, the shaft will deflect downwards to maintain the contact with the bearing surface. Due to this the centerline of crankshaft will get sagged. Hence where the deflection is taken it will indicate an excessive crankshaft deflection. This will indicate that the bearing or the journal is wornout or the bearing housing has sagged because of the transverse girder of bedplate is sagged or distorted. Q.731 When accumulated carbon at the air inlet port of two-stroke cycle diesel engine is removed you should take care Ans. to avoid carbon entering the cylinder. If carbon enters the cylinder, it may get lodged between the piston and the cylinder and cause the sticking of the piston rings. Ultimately it will cause the breaking of the piston ring and blowpast. Hence it is necessary that carbon should not enter the cylinder especially between the cylinder and the piston. Q.732 Replacement piping for the diesel engine high pressure fuel system must be of same length and diameter as the original piping to Ans. avoid excessive amount of ignition delay.
Q.733 The burnt exhaust gas valve may be burnt by Ans. higher than normal exhaust gas temperature from a particular cylinder. If the exhaust valve is burnt out, the high temperature and pressure gases may leak past between the valve and the seat and which will ultimately give rise to the temperature of the exhaust gas after the valve. At the same time, the exhaust valve will start burning and hence there will be continuous rise in the exhaust gas temperature. Also there will be drop in the compression pressure which will cause afterburning and high exhaust temperature. Q.734 In diesel engine a leaky exhaust valve can cause Ans. misfiring. Leaky exhaust valve will give rise to the drop in the compression pressure which will result in decrease in compression temperature due to which fuel may not ignite and which will cause misfiring of the fuel. Q.735 The most severe cavitation corrosion occurring on the water side of a diesel engine wet cylinder liner normally occurs Ans. (d) near the middle of the thrust side of the liner. Thrust side of the liner is the side which has got the maximum thrust because thrust side of the liner, in case of two stroke engine, is on both side of the thrust side. In case of four stroke engine, the thrust is taken by the liner depending upon the rotation one side will be thrust side which has got excessive amount of thrust transmitted to the liner wall by the piston skirt. This excessive thrust will cause the variation of the pressure and because of the variation of the pressure, the water pressure will fluctuate and because of fluctuation of water pressure, bubbling will start. At high temperature, bubble formation will take place which will cause the cavitation. Q.736 Exhaust valve grooving and corrosion is caused by certain component of the residual fuel oil. These components are Ans. sodium, sulphur and vanadium. Vanadium, in presence of sodium, will give the hot corrosion because in presence of sodium, vanadium pentoxide which is formed because of burning of the vanadium will melt at much lower temperature in the range of about 500°C. This molten vanadium pentoxide is highly corrosive and it will give the corrosive effect. Hence the combination of sodium and vanadium in the ratio of 1:3 is the most dangerous combination which will cause the grooving. Both sodium and vanadium are present in the residual fuel. Q.737 Which of the listed cylinder liner surface condition indicates proper lubrication? Ans. Bright appearance. The liner surface should be dull white without excessive brightness or black spots or bright spots. Black spots indicate the blowpast and excessive bright spots indicates scuffing. Hence well runned and properly lubricated liner will have a dull silvery finish on the liner surface. Q.738 Well lubricated bearing surface always appears Ans. dull white. Dull white is the colour of the overlay of tin which the topmost layer of bearing surface. Below this is the layer of nickle barrier which is hard and shining layer. Hence if the bearing is properly running and there is no wiping then the bearing surface must be dull silvery white. Q.741 You are inspecting a lower main precision bearing on a diesel engine You observe that about half the thin babbit lining are of milky white color The condition is caused by Ans. wiping out. If wiping out takes place, the top layer of tin i.e. overlay is wiped out. When overlay is wiped out there is bright nickel barrier will be seen as shiny spots. Hence half the babbit is already visible in a milky white color which indicates that wiping has taken place because of lack of lubrication or lubricating oil having water content or excessive loading of the bearing. Q.742 You are testing a non-popping closed fuel injector diesel nozzle using a nozzle tester. A pressure less than the designed valve opening pressure is applied. If no fuel appear in the spray tip this indicates Ans. the needle valve is sitting properly and is not leaking. The needle valve must only allow the fuel to inject when the injection pressure is reached and not below it. If the pressure is raised to less than injection pressure and if there is no leakage, it means that the injector is in very good condition and it is leak proof. Q.743 Because of closed tolerance used in diesel engines fuel oil pump a worn plunger requires Ans. (b) replacement of pump plunger and barrel. The plunger and barrel are lapped to form a best sealing surface between them and hence it manufactured and lapped as a pair and it should be replaced as a pair. Replacing a single unit will cause excessive leakage or may cause seizure of plunger in the barrel. Hence it should be avoided and complete unit should be replaced, i.e. both plunger and barrel, should be replaced. Q.744 Which of fuel nozzle listed requires the least maintenance? Ans. Pintle type nozzle. Pintle type nozzle will have a center hole and the hole is comparatively bigger one and it is completely covered by the pin at the tip of the needle. The needle sits squarely on the pin and hence there is less possibility of any distortion taking place between the needle seat and the valve seat and hence least amount of maintenance is required. Q.745 When inspecting pistons, liner and rings of a large two-stroke diesel engine through cylinder ports a wet piston crown could indicate Ans. (a) leaky fuel injector.
If the oil is accumulated on the top of the piston crown there is no other way that the oil can come on top of the piston other than a leaky fuel injector. When engine is in stopped condition, because of leaky fuel injector, the oil is accumulated on top of the piston. Under such circumstances, immediate action of replacing the fuel injector is most appreciated. Q.746 Which of the listed devices could be used as a substitute for a ring grooving tools used to remove hard carbon deposits from the groove in preparation for installing new rings? Ans. (a) Section of removed compression ring. A section of a removed compression ring can be wrapped all around by a thin Emery paper and the ring section to be moved in the groove so that all around cleaning of the groove can be possible with the help of Emery paper which is wrapped around the section of the compression ring just removed. Q.747 If the plunger or barrel of a fuel injection jerk pump becomes damaged Ans. The pump and barrel of the pump must be replaced as a unit. Q.748 Surface irregularities such as erosion and pitting on the injection pump plunger will Ans. effect the fuel oil metering. If there is excessive erosion or pitting on the plunger then the integrity of the sealing between the plunger and barrel will be affected. As a result, the high-pressure oil will leak past the plunger. Hence the quantity of oil injected into the engine will reduce and which will affect the metering of fuel to the engine. Q.749 Side clearance (groove clearance) of the compression rings on a diesel engine piston is necessary to Ans. for the gas to have easy access to the back of the piston rings. Due to the easy access, it will force the piston rings against the liner walls. Hence this clearance is very important. If this clearance is less or the space between the ring and groove gets clogged or deposited with carbon, sufficient amount of gas will not find its way behind the piston rings. Hence the contact pressure between the piston rings and liner will reduce which will cause the blowpast and weardown of liner and breakage of liner and piston rings. Q.750 If lost motion is present in a individual fuel injection pump which of the following problem will occur? Ans. The fuel injection will occur late. Due to the lost motion, the exact motion of cam will not be transferred to the plunger movement and hence there will be late injection because the position of plunger with respect to the cam will be retarded. Q.751 In a large low speed propulsion diesel engine if the injectors have formed carbon around the nozzle holes which of the following would help avoid further building up? Ans. Decrease the injection cooling water temperature. This may be happening because of the excessively hot fuel injector nozzle due to which the sealing between the nozzle and the seat is affected and nozzle will start leaking. If the nozzle starts leaking, formation of carbon deposit takes place. Q.752 A condition contributing to diesel engine piston ring sticking in the ring grooves is Ans. insufficient ring clearance at the ring gap or side clearance. If the groove clearance of the piston ring i.e. axial clearance between the piston rings and the ring groove if it is less then in that case the piston ring will expand. At the same time some carbon deposit may take place between the groove and the piston ring will cause the piston ring to get seized or stick up in the groove. Hence sufficient clearance has to be maintained between the groove and the ring. Q.753 In diesel engine when installing new piston rings it is important to check (a) ring gap clearance. (b) side clearance . Ans. Both (a) and (b). It is necessary to check both the butt clearance and groove clearance. If any of the clearance is less, it will cause the breaking of the piston ring or seizing of the piston in the groove which will ultimately cause the malfunctioning or improper combustion of the fuel and which may cause liner breakage, scuffing, piston ring breakage or loss of power. Q.754 The purpose of interference angle in a diesel engine exhaust valve is to (a) work in conjunction with valve to rotate the valve. (b) break up the seat deposits. Ans. (b) break up the seat deposits. Actually interference angle of about ½ degree is given between the seat and the valve so that when the actual operating temperature reached it will expand. It will have a proper contact with the seat. At the same time, when this action is taking place, any carbon deposition that is present between the seat and valve will get crushed and removed. Hence it acts like a self cleaning method of the seat. Q.755 When accumulated carbon at air inlet port of a two stroke diesel engine is being removed you should take care to avoid carbon particles Ans. entering the cylinder. (ALREADY DISCUSSED) Q.756 You are installing new piston rings on a single acting diesel engine piston. To check the ring gap clearance, the ring should be placed Ans. at the point of minimum cylinder wear.
At the point of minimum cylinder wear you will have the minimum gap clearance which will be your yardstick for the proper operation of the engine. If the clearance is satisfactory at the position of minimum weardown then it can be surely assured that when the piston is at top position where the weardown of the liner is maximum there will be sufficient clearance between the ends of the piston. Hence during entire stroke of piston there will not be a single position where the piston ring ends will butt against each other and cause cracking of the piston rings, seizure of the piston ring or cracking of the liner. Hence as a safety measure, butt clearance is always measured where there is minimum weardown. Q.757 A defective injector nozzle in a propulsion diesel engine can cause (a) engine power loss. (b) smoking due to unburnt fuel. (c) high exhaust temperature reading. (d) All of the above. Ans. (d) All of the above. If the fuel injector nozzle is defective i.e. it is not spraying properly or if it is not penetrating properly or if the nozzle is leaking or if the holes are choked, the fuel will not be atomized properly and penetrated which will result in improper combustion. Because of improper combustion, there will be loss of power, afterburnining, and hence unburnt fuel and smoke in the exhaust and the exhaust temperature will rise. Q.758 Which of the following problems is the main source of fuel pump and injection system malfunctioning? Ans. Improper adjustment. Improper timing adjustment of fuel pump will ultimately lead to improper functioning of the fuel injector and fuel pump which will ultimately lead to loss of power, excessive weardown of the piston rings, black smoke, excessive carbon deposits, turbocharger surging. All these problems will be because of improper adjustment of the fuel pump which will ultimately cause the improper adjustment or improper spraying of nozzle holes. Q.760 If you have determined that entrained air in fuel nozzle system is causing a diesel engine to stall you should first check for Ans. a loose fitting in the booster fuel pump suction piping. If there is loose fitting on the fuel booster suction piping, the fuel booster pump will draw the air. This air will cause the problem of air traps in the fuel injection system because booster pump suction side is the only low pressure side in the entire fuel injection system and hence possibility of air leakage is more. Q.761 In a large low speed diesel engine excessive piston clearance can be restored by Ans. inserting shims between crankpin bearing box and connecting rod foot. By inserting shims between the bearing box and connecting rod foot the piston position can be raised in the liner. When the piston position is raised in the liner automatically the clearance volume reduces and the compression pressure increases. It can be restored back to the original. Q.762 Which of the following procedures should be carried out to permit the use of crosshead engines with an inoperable aftercooler? Ans. Run the engine at reduced speed until the cooler can be repaired or renewed If the cooler is not working then the temperature of the air which is admitted to the engine will be very high. When the admitted air is high the density reduces and hence the quantity of air admitted will be less and the quantity of fuel that can be burnt will have to be reduced. At the same time, as the air temperature increases the exhaust temperature will also increases. Hence complete thermal loading of the engine will take place and at the same time because of higher temperature in the cylinder lubrication will also get affected. Hence in order to avoid all these things it is better to run the engine at lower load. Q.763 A faulty injector in one cylinder of an operating diesel engine can be located by Ans. (a) cutting out the individual injector and noting the engine performance. Faulty injector in general can be detected by exhaust temperature of the unit but it is not a reliable method. Hence one has to take the draw card. By draw card you can find out the compression pressure and peak pressure. If the compression pressure is satisfactory and peak pressure is varying, it clearly indicates that the injector is bad. Q.764 Which of the following conditions will cause carbon deposit to form in the piston ring belt of a diesel engine? Carbon deposit on the piston ring belt can be caused because of faulty combustion, excessive ring temperature or over lubrication. All this will lead to carbon formation. If the combustion is not proper, more amount of carbon deposits will be formed on the piston rings. If excessive lubrication is given the excess oil will get burnt, it will form carbon and ash and they will also lead to carbon deposits on the piston ring. Also if the ring temperature is excessively high i.e. the cooling of the piston is not properly done in that case the lubricating will burn which will form carbon deposits on the piston rings.
Q.767 A change in the degree of fuel atomization will affect Ans. (b) fuel penetration. Penetration and atomization both depends on the hole size of the nozzle as well as the injection pressure. But as they are opposite to each other if excessive penetration takes place which occurs because of the increase in diameter of the injection holes the atomization reduces. On the other hand, if the holes are of very small size which will cause more amount of atomization. In this case, because of the pressure drop during atomization, the penetration will reduce. Hence as they are opposite to each other as atomization increases penetration decreases and vice-versa. Q.768 Which of the following problems may occur if the clearance between the piston and the cylinder is insufficient? Ans. Excessive wear, scuffing of the liner and piston seizure. There will be excessive wear of the liner as well as the piston as excessive pressure is developed by the rings over the liner walls. At the same time, the skirt of the piston will come in contact with the liner wall and wiping out of the lubrication oil will take place which will also cause excessive wear of the
liner. At the same time, as the expanded piston will come in contact of the liner wall it will wipe out the lubricating oil film. When the lubricating film is wiped out because of metal-to-metal contact between the piston and liner wall there will be scuffing of the liner and blowpast. If the clearance is excessively low and the piston expands beyond that clearance, the piston will get seized in the liner. Q.769 Black areas on the sealing surface of the piston rings indicates Ans. passage of hot gases or blowpast. Black areas is an indication of blowpast and the lubricating oil is wiped out between the ring and liner and the areas get coated with carbon deposits from the exhaust gas or high pressure gas. Q.770 A bright side on the compression ring of a diesel engine indicates Ans. properly functioning of the rings. The job of the rings is to seal the surface between the liner and piston against the high pressure and high temperature gases. When the piston and the liner surfaces are rubbing against each other it will give a polishing effect as long as there is no blowpast and the surface remains polished. This indicates that the sealing between the piston rings and the liner is proper and the piston rings are functioning properly and the cylinder lubrication is also proper to maintain the seal between the liner and piston rings. Q.771 The best tool to remove the carbon ridge at the top of engine cylinder prior to removing the piston is to use Ans. metal scrapper. Metal scrapper is used to remove to ridge formed because of carbon. If the ridge is formed because of the weardown of liner and it is a metal ridge, it is to be removed by use of hand grinder as it cannot be removed by metal scrapper. Q.772 Which of the following devices is commonly used to measure the clearance between the main engine bearing and crankshaft? Ans. (a) Plastic gauge. Plastic gauge are utilized like lead wire and they are made to compress between the bearing and the journal and because of the compression the thickness will reduce and the width will be increase and the increase in width can be directly calibrated in terms of clearance. Q.773 Which of the following condition can result in cracking of the piston land? Ans. Insufficient ring groove clearance. If the ring groove clearance is insufficient, the ring will expand and it will occupy the complete space in the ring groove. Further expansion of the ring in the groove will cause excessive stresses induced on the piston ring groove which will cause cracking of piston land. Q.774 Which of the following conditions may occur while using fuel with high sulphur content? Ans. (d) Corrosion If fuel with high sulphur content is used, sulphur will burn and it will convert into sulphur trioxide and in presence of water and moisture it will convert into sulphuric acid which is highly corrosive. This will give the corrosive wear on the liner. Q.775 If the back clearance of a piston ring is excessive There will be excessive amount of carbon deposits behind the piston rings and there will not be enough support for the piston rings. Under such circumstances the piston rings will protrude more from the ring groove and it will cause the piston ring to collapse or twist and break. Q.776 Which of the following conditions could contribute to the cracking of diesel engine cylinder head? Ans. Blocked cooling water passages to the head. If the cooling water passages to the head are blocked or scale formation takes place in the cooling water spaces, the cooling of the cylinder head is reduced and it is subjected to very high temperature and pressure. Because of this high temperature and pressure, in order to maintain the strength of the cylinder head, it is necessary to cool the cylinder head and maintain the temperature of cylinder head. If the cooling is not carried properly, the strength of cylinder head will reduce which will ultimately cause the cracking of the cylinder head. Q.777 A properly honed diesel engine cylinder liner will Ans. shorten the breaking in period. A cross honing is carried out so as to give controlled roughness on the cylinder liner surface. At the same time to have the retention capacity of the lubricating oil on the cylinder head surface. Both of this will contribute to the faster running and faster breaking in period so as to prevent scuffing or blowpast. Hence honing is done at the initial stages on new liner or if the liner is excessively glazed and the surface is too smooth to retain the lubricating oil. Q.778 A scoured diesel engine cylinder liner will cause Ans. rapid wear of the piston rings. If excessive scouring is present on the cylinder liner blowpast will take place. When blowpast takes place the lubricating oil film between the liner and piston will get distorted and there will be excessive metal-to-metal friction between the liner and the piston rings due to which excessive weardown of the piston ring will take place. Q.779 Which of the following operating conditions can occur when shims are removed from the joint between marine type diesel engine gearing box and the foot of connecting rod? Ans. The piston position will move downwards with respect to the cylinder head.
When the shims are removed, the piston will move downwards with respect to the cylinder head. Hence the compression pressure and clearance volume will increase and compression ratio will reduce. Q.780 After long period of operation a wear ridge caused by piston ring action will developed near the top of the cylinder liner This ridge must be removed when piston rings are removed inorder to Ans. prevent breaking of the top ring and land or both. If the ridge is not removed while removing the piston it may happen that the rings may get jammed up in the ridge and it may cause the seizure of the piston in the liner. When new rings are fitted and the piston is lowered and the ridge is not removed, the piston rings will hammer against the ridge which will cause the breaking of the piston rings and damage the landing face of the piston ring groove. Q.781 Crank lead can be adjusted to change which of the listed operating conditions? Ans. Longer combustion event. When the crank lead is adjusted the injection timing gets adjusted. When the injection timing gets adjusted, the combustion can be delayed or advanced. Q.782 When hydraulic valve lifter is on the base circle of the cam, zero tappet clearance is maintained by Ans. plunger spring. When the hydraulic valve lifter is on the base circle the plunger springs will lift it against the base circle and hence the tappet clearance is maintained. During this time, due to the spring action on the valve plunger, the plunger will be pushed downwards and the roller will keep contact with the base circle of the cam. Hence zero tappet clearance will be maintained. Q.783 In a diesel engine blowby is generally a result of Ans. worn compression ring. Blowby or blowpast is because of improper sealing between the cylinder liner walls and the piston rings. This is because of the excessive weardown of the liner wall and piston rings or improper or uneven weardown of liner wall and piston rings so that the sealing gets affected and blowpast will take place. Q.784 Which of the listed condition can cause lacquer to be deposited on the piston skirt? Ans. High lubricating oil temperature. Because of high lubricating oil temperature, the lubricating oil will burn over the piston skirt. When the lubricating oil burns a lacquer like formation will take place and deposition will take place on the skirt. Q.785 Scuffed diesel engine cylinder liner wearing surface in a diesel engine can result from Ans. applying load to cold diesel engine. When the engine is cold it will take some time to form a good sealing between the piston rings and the liner. Hence the load should be kept low initially and increased gradually. If the load is suddenly increased, blowpast will take place between the piston ring and the cylinder liner wall. When blowpast takes place it will wipe out the lubricating oil between the liner and piston ring and which will cause the excessive friction between the liner and piston ring which will ultimately cause excessive scuffing of the liner. Q.786 If a diesel engine driving a generator turns over freely but fails to fire properly, the cause could be Ans. air in the fuel. If the engine can accelerate sufficiently to the firing speed and the fuel is released by the governor but the engine does not pick up on fuel, this indicates that the fuel is of bad quality and one of the reason is that the fuel contains air or water. Q.787 When diesel engine compression pressure is checked Ans. the indicator cork is kept open. When the indicator cork is connected to the cylinder head, the indicator cork is kept open so whatever the change of pressure is taking place inside the cylinder at different positions or movements of the crank or the main piston will be directly indicated by the stylus movement of the indicator piston. Q.788 In a diesel engine exhaust system the cooling of the exhaust gases below their dew point will result in Ans. sulphuric acid corrosion. If the fuel contains sulphur it will form sulphuric acid and if this sulphuric acid is in the form of vapour but if this vapours are allowed to cool below the dew point then it will convert into liquid form and it will cause excessive corrosion of the liner. Q.789 Which of the following statements is true regarding the installation of piston rings on two-stroke diesel engine as compared to four stroke diesel engines? Ans. Some provisions must be made in two stroke diesel engines to prevent the rings from bending in the ports. In two-stroke engines, there will be ports in the liner walls over which the piston rings will slide. Hence it is necessary to make sure that the ends of the piston rings will not get entangled with the ports and the piston rings will not break. In order to avoid such things the ends of the piston rings, in twostroke engines, are slightly curved inside so that during the operating temperature they will not get entangled with the ports and get broken. Q.790 In a large low speed diesel engine the clearance between the piston crown and cylinder head is found to be excessive. Inorder to correct this Ans. shims should be inserted between the crankpin bearing box and connecting rod foot.
In case of marine type of connecting rod, where connecting rod is fitted on top of bottom end bearing, it is possible to insert the shims between the crankpin bearing box and connecting rod foot so that the position of the piston in the cylinder liner with respect to the cylinder head can be altered. Hence the compression pressure and clearance volume can be changed. Q.791 Bouncing of valve gear in a diesel engine can be caused by Ans. spring surge. If the spring is not getting released or if it is not following the action of rockerarm when the rockerarm moves upward, the spring will not push the valve upward. After sometime, it will cause the valve to push upwards in the gap between the valve and rockerarm. During this time, the movement of the spring will be abrupt due the stored compressed energy in the spring and this will cause the surging effect of the spring and the valve. This will cause the valve and the spring to get damaged. This can also be caused because of sticky valve in the guide. 9nppffTill that time there is overflow of oil to the suction side. By moving upwards, depending upon the load of engine, there is relative position of plunger helix with respect to the suction. This will decide as to when the discharge side of the plunger will be in connection with the suction side, hence ending the delivery. Also this will decide quantity of fuel injected depending upon the load. This system has constant beginning but the end and quantity of injection is controlled by the helix position.
Q.31 Same as Q.30 Q.32 An individual pump is designed for variable beginning and constant end of injection. For diesel engine operating at constant speed, the start of injection will Ans. advance as the load increases. As the load increases, engine requires more fuel to inject and burn. As the speed is constant, more fuel is to be injected in same time frame available, it is necessary to begin the injection earlier, i.e. to advance the beginning of injection, so that the fuel can be completely burnt, effectively and efficiently, within the time frame available thus giving better thermal efficiency of the engine. Q.33 Crankshaft whose center of gravity coincides with the centerline is said to be Ans. (b) statically balanced. B012 Ans. (b) increase the distance of heat travel from the crown to top ring groove. As the first ring or the top ring is subjected to maximum amount of temperature and pressure, it is difficult to maintain the film of lubrication between the top ring and liner. Also there is every possibility that due to heat and high pressure, the top ring will get broken or seized in the piston groove. Due to lack of lubricating oil film, there is maximum weardown for the top ring and top end of the liner. In order to avoid this, it is necessary that the piston ring should be maintained as far as away for the firing temperature so that a good lubricating oil film can be maintained between the liner and piston rings. In order to achieve this, a seat damp is utilized in the engine. Q.40 Diesel engines are classifieds as internal reciprocating engines because they Ans. (c) burn the fuel in chamber and their energy moves the piston back and forth. As the fuel is burning in confined space which is sealed where fuel burning takes place because of high temperature of compressed air. All the burning process is carried out inside the cylinder liner between the top of the piston and bottom of the cylinder head. The heat released is converted into mechanical energy by the movement of the piston which is transmitted to the crankshaft. As the combustion takes place inside the cylinder, it is called internal combustion engine. B015 Q.53 Turbulence in diesel engine cylinder is of major importance in providing Ans. (c) complete fuel air mixing. Turbulence is created due to the circular movement of air in the cylinder. This turbulence is created by the geometry of entry ports, scavenge air as well as the geometry of the cylinder head and the piston crown. Due to turbulence there is thorough mixing and homogenize mixture is produced between fuel and air. This mixture will burn efficiently and hence the full quantity of air is available for the combustion of fuel. This gives higher thermal efficiency of the engine and reduce the unburnt fuel going into the exhaust thus reducing the specific fuel oil consumption and less smoke is formed in the exhaust. At the same time, this will give the uniform distribution of heat over the combustion space.
B016 Q.54 Load is always placed on lower half of main bearing in Ans. (a) two-stroke cycle engine. In two-stroke engine, as there are two strokes for the complete cycle, the piston will always be loaded downwards whether it is expansion or compression stroke. Hence for first half revolution, the piston is loaded downwards due to expansion gases which is transmitting load force to the crankshaft. When the piston is moving upwards it is compressing the air to perform compression stroke. Due to this unidirectional downward loading of the piston which is transmitted to the crankshaft, the main bearing of the two-stroke cycle engine is always loaded at bottom half. Also the pressure acting is varying and hence difficulties are encountered in lubrication of bearings in case of two-stroke engines rather than four-stroke engines.
B017 Q.55 Cavitation and corrosion in cooling water system of diesel engine usually occurs at the pump impeller and the waterside of the Ans. (c) cylinder liner. When the water enters into the cylinder liner, it gets heated up. At the same time when water passes through the cylinder liner, the velocity of water increases due to restricted passages. As the velocity increases, as per Bernoulli’s theorem, pressure drop takes place. As the pressure drop takes place, water tends to boil off at lower temperature due to which steam bubbles are formed. These bubbles get bursted creating cavitation. Hence if the liner is opened for inspection any time, especially at the places of restricted entries and passages, cavitation is easily visible. B018 Q.56 The main operating characteristics that distinguishes it from other internal combustion engines is Ans. (c) method of igniting the fuel. In petrol engines, the ignition is carried out with the help of igniter or spark plug. Diesel engine fully depends on compression ignition for the purpose of igniting the fuel. In diesel engine the air is compressed. As the air is compressed its temperature increases to a temperature which is more than the selfignition temperature of the fuel. When fuel is injected at this temperature, it vapourizes and mixes with air to form combustible mixture. As the temperature is above the self-ignition temperature this mixture ignites and produces a high pressure wave. This pressure wave and the heat is transmitted to the crankshaft by the downcoming piston.
B019 Q.57 Which term describes piston pin having bearing surface in both piston bosses and connecting rod eye? Ans. (b) Full floating. If the gudgeon pin is having a press-fit in the bosses of the piston and is free to move over the top end bearing of the connecting rod, there is a possibility that misalignment cannot be taken care off. Hence the pin is made to float or have bearing surface in the piston boss and the connecting rod eye so that it can take care of misalignment. At the same time the pin can have a rotating effect. When the pin has rotating effect, it will not have continuous loading on one surface and the weardown of the bearing will be uniform thus preventing the ovality of the bearing due to wear in only one place. In order to prevent this pin from coming out when the piston is moving up and down, at the end of bosses, circlips are provided which prevents the gudgeon pin from coming out and touching the liner.
B0021 Q.58 Compared to other fuel injection systems, unit injection system operates with virtually no Ans. (a) injection lag. In unit injection system, the fuel pump and the fuel valve consists of a single block. Hence there is no fuel piping the valve and the pump. As there is no piping, the time required for injection from the time the fuel pump plunger starts moving up is reduced resulting in reduction of injection lag. In case of other types of fuel pumps, where the fuel pump is connected to the injector by long high pressure pipes, some amount of pressure drop takes place inside the fuel pump piping. Also due to the pressure wave created, the fuel piping gets slightly expanded which can cause fuel injection delay. Hence in order to avoid fuel injection delay, the best possible way is to fit the fuel pump very near to the injector. In unit fuel injection system, this delay is nearly reduced to zero. B0022 Q.59 Piston cooling fins are located Ans. (b) underneath the piston crowns. Piston cooling fins act like cooler. When the fins are provided, better heat transfer takes place from the hot surface towards the pointed tips of the fins due to which the hot crown remains comparatively cool and better heat transfer takes place across the cross-section crown. Hence the piston crown induced thermal stresses are reduced to minimum. This has resulted in less cracking of the piston crowns. B0023 Q.60 Properly honed diesel engine cylinder liner will Ans. (b) shorten the ring breaking in period. When the liner and the piston rings are new, microscopic * and roughness is always present in the liner wall and piston ring surface. During the breaking in period, these * are grounded or broken up so that they will be smoothened. Running in can be done afterwards to achieve complete bedding between the liner and piston rings. Hence breaking in is importance process. If it is not done properly, there is a possibility of scuffing and blowpast. The process of breaking in should be carried out as fast as possible so that the engine load can be increased. In order to get fastest breaking in, controlled fast weardown should be achieved. For this purpose, honing should be carried. During honing process, the surface of the liner is made artificially rough. Thus lubricating oil is retained at the surface of the liner. Also due to roughness, both piston rings and liner will get worn out faster so that breaking in can be carried out fastly, without causing blowpast or scuffing. Hence honing is important from the point of view of breaking in. B0025
Q.61 Besides the use of piston cooling, fins provides extra strength for the Ans. (b) piston crowns. As the fins are provided on the underside of the piston crown, the top portion of the piston crown which is subjected to higher temperature and pressure is supported by the fins from underside. As the support is provided by the fins, they give extra strength to the piston crown. Hence it is possible to use the piston crown of thinner cross-section without causing it to buckle under the influence of high temperature and pressure.
B0026 Q.62 Minimum fuel oil delivery pressure required for efficient fuel injection depends primarily on Ans. (b) maximum pressure in the engine cylinder during injection. When the piston is approaching TDC on compression stroke, the fuel oil injected into the cylinder. The fuel oil injected has to overcome the resistance or backpressure offered by the high pressure compressed air. Hence the pressure of the fuel oil injected has to be more than the pressure inside the cylinder head; otherwise it will not be possible for the fuel to penetrate the mass of dense air available. This will result in improper combustion of the fuel. Hence it is necessary to maintain minimum injection pressure higher than the compression pressure existing at the time of injection. B0027 Q.63 A thrust bearing is used in diesel propulsion engine to Ans. (a) control axial movement of the crankshaft. When the ship moves forward or aft, the propeller produces a thrust which is applied to ship’s hull. At the same time this thrust is exerted on the propeller blades. As the propeller is connected to the propeller shaft which is connected to the engine shaft, due to the axial thrust created, there is a tendency for the crankshaft to move inside the engine i.e. the crankshaft will have axial movement. If this axial movement is not prevented or taken care off, there will be misalignment between the piston connecting rod and bottom end bearing. Also the crankshaft webs may touch the A-frame and the main bearings which may damage to the engine structure. Also the rest of the crankshaft will be subjected to axial thrust and hence it may bend or break. Hence it is necessary to take care of this axial thrust before it enters the engine and affects the moving parts of the engine. For this purpose, thrust block is provided at the end of the last unit so that all other units and crankshaft is protected from the axial thrust and the crankshaft alignment will not be affected. B0028 Q.64 The two strokes of the two-stroke cycle diesel engines are Ans. (d) compression and power. As any diesel cycle, the two-stroke cycle will also consist of all four major processes i.e. suction, compression, power and exhaustion. In case of twostroke engines, the process of compression and power are distinct but air intake and exhaustion are combined together and they form a part of the distinct compression and power stroke. Hence there is no separate distinct stroke for intake and exhaustion as exhausting is carried out by intake of air. B0030 Q.65 Which of the following statements pertains to propulsion engines with bore exceeding 200mm? Ans. (c) The units will be fitted with means to display the exhaust temperature of each cylinder. When the piston diameter exceeds 200mm, the engines are considered as highly rated engines. In such engines, the exhaust temperature of individual units will be on higher side and sufficient amount of power is developed by the units. In order to have proper running of the engine, it is necessary to have a balanced engine. The balanced engine can be roughly described by comparing the individual unit temperature. If the exhaust temperatures of the units are within prescribed range, one can consider that the engine is properly balanced. It should also be noted that exhaust temperature is not the only indication of power balance. For this purpose, the indicator card is taken and the power developed by the individual unit is calculated the best method. It can be a rough estimation of power balance. B0031 Q.66 When engine bore exceed 230mm, a berthing disc or flame arrestor is fitted Ans. (a) to the engine air supply inlet to the starting air supply manifold for non-reversing engines. When the bore of the engine exceeds 230mm it is considered as highly rated engine as far as power is concerned. These engines are mostly started with compressed air. If air inlet valve or starting air valve on the cylinder leaks, high pressure exhaust gas will enter the air manifold. As the pressure and the temperature is high, it will cause a air starting line explosion. This will result in heavy damage to the engine and the personnel working. Also the engine will be completely immobilized. B0033 Q.1 The precision engineering bearing inserts are manufactured with small portion of bearing ends extending beyond the bearing housing or caps. The installation process of these bearings requires sufficient Ans. (b) crush. The bearing shell itself is in two halves. The bearing shell halves are extended beyond the bearing housing by approximately 0.3mm. This is applicable in modern thin-shelled bearings. As these bearings are thin-shelled, they cannot support shaft on their own and hence need backing-up support of the bearing housing. Hence it is necessary to have full contact with the back of the bearing and the bearing keep. Hence crush is provided to achieve this. Also, it prevents the rotation of the shelf in the housing due to the rotation of the shaft.
Q.2 The method of piston cooling in which oil is delivered through the connecting rod to a compartment within the piston, then distributed by the motion of the piston and allowed to drain to the crankcase via one or more pipes or holes is termed as Ans. shaker method of cooling of the piston. In shaker method of piston cooling, it is actual movement of the piston which causes the movement of oil in the piston. The oil outlet from the piston crown cooling space is kept at higher level than that of inlet. Hence some oil is always retained in cooling spaces. This oil is splashed against the underside of the hot crown as the piston moves up and down so as to have better cooling of the piston crown. This spraying action is done due to the movement of the piston. This is very efficient piston cooling method. Nowadays, new Sulzer engine is employing the combination of shaker and nozzle cooling method and has reported to have reduction in piston crown temperature.
Q.3 The piston rod scrapper box incorporated in two-strokes cycle crosshead diesel engine serves to Ans. (c) prevent the sludge and dirty oil from entering the crankcase. In two-stroke crosshead engines, under-piston scavenge space are separated from crankcase by use of diaphragm. Under-piston scavenge space gets accumulated with unburnt carbon, unburnt fuel and mixture oil in the form of sludge. If this finds its way into crankcase, then crankcase oil will get deteriorated. Hence stuffing box with scrapper and sealing rings is mounted on the diaphragm through which reciprocating piston rod passes thus scrapping off and preventing the entry of sludge into the crankcase. Also it prevents the crankcase oil into the under-piston space hence preventing the loss of crankcase oil. Q.4 In large slow speed main propulsion diesel engines which of the parts listed is under tension when the engine is running? Ans. Tierods. The job of the tierods is to hold bedplate column, entablature under compression so as to prevent the relative movement between these parts. When the engine is running hence these parts are tied together and are kept under compression by use of tierods. While doing so, the tierods itself gets into tension. Hence the firing load is transferred to the bedplate and ship structure. Q.5 The main advantage of unit injector over other fuel injection system is Ans. lack of high pressure fuel lines. In case of unit injectors, which is a combination of fuel pump and fuel valve, as the fuel valve is not separately mounted away from the fuel pump, there is no need to provide a connecting high pressure oil carrying fuel between the fuel pump and fuel injector. This combination improves safety and chances of high-pressure fuel leakages also reduces. Also the possibility of fire due to leaky high-pressure pipes is reduced. Q.6 The passages are drilled in the crankshaft of diesel engines to provide lubricating oil to Ans. the main bearings, connecting rod bearings and piston pin bushings. Crankshaft moves in the main bearings. At the same time the connecting rod big end bearings moves over the crankpin. Also the connecting rod top end bearing moves over the piston pin bushing. As there is relative movement between these parts, friction always exists between them. In order to reduce the friction and remove the heat generated due to small amounts of film friction that may exist, it is necessary to supply sufficient quantity of lubricating oil to all these three parts so as to have a film lubrication which will separate the moving parts from the stationary bearings. At the same time, it will provide sufficient amount of lubricating oil for the purpose of cooling. This arrangement is utilized only in case of small engines where the drilling of the crankshaft is permitted. In case of big engines, there is possibility of stress concentration due to the drilling in the crankshaft and there is a possibility of crankshaft breaking. Therefore this type of system is not utilized in big engines. In big engines, main bearing lubricating oil is supplied separately through the crosshead bearing which will be branched to the piston cooling as well as crosshead lubrication and through the connecting rods drilling it will go to the bottom end bearing lubrication. Q.7 When can the main crankcase ventilation pipes or oil drain pipes of two or more engines be connected? Ans. As far as possible no connection should be provided between the crankcase ventilation of different engines. Crankcase ventilation pipes or drain pipes of two or more engines should not be connected as in case of crankcase explosion, the heat wave may enter other healthy engine crankcase causing explosion in that engine also. Also if one engine crankcase oil is contaminated, other engine oil may also get contaminated if the drains are connected. Also this may occur due to the oil vapour getting mixed up. Q.8 The main function of tierod in the construction of a large low speed diesel engine is to Ans. absorb most of the tensile load that results from the firing forces during the operation. The main function of the tierods is to keep the cylinder block, A-frame and bedplate tied together under compression. At the same time it itself goes into tension. Also tierods accept most of the tensile loading that results from the firing forces developed during operation and relieve the bedplate, entablature and columns off these firing forces. Hence it can be seen that the tierods are always under tension so as to maintain all other parts of the engine structure under compression and to prevent the relative movement between the parts when the engine is in operation. Q.9 On most modern diesel engines, the main and connecting rod bearings receive their lubricating oil by Ans. (d) pressure feed. On most modern diesel engines, where the load is comparatively high, it is necessary that the bearings should be lubricated by lubricating oil. This lubricating oil will form a wedge type film between the stationary bearing and the rotating shaft so as to prevent the metallic contact and the friction between the bearings and the journal. For this purpose, it is necessary to supply large quantity of oil so as to have a continuous and uninterrupted supply of lubricating oil which is required to form the film as well as cooling purpose. In order to achieve this, it is necessary to continuously supply lubricating oil under pressure. This will overcome the piping friction and also it will maintain uninterrupted and sufficiently large quantity of lubricating oil to the bearings.
Q.10 Fuel injection pump using the port and helix metering principle requires the use of a Ans. (b) lapped plunger and barrel. Due to lapped plunger and barrel, there is a good contact between the plunger and barrel. As the pump develops very high pressure, there will be least amount of leakage between the plunger and barrel. Also due to the cutting of helix on the sliding surface of the plunger, the sealing area between the plunger and barrel reduces. As the area reduces, the possibilities of leakages increases. Hence in order to reduce the leakages at high pressure between the plunger and barrel, it is necessary to lap plunger and barrel together so as to have a perfect bearing between the plunger and barrel and to reduce the leakages. The plunger and barrel are to be replaced together as a set because they are lapped individually with each other and they should not be replaced individually as single unit. Q.11 Lubricating oil is supplied to the crankshaft main bearing in a marine diesel engine by Ans. internal crankshaft passages. In small diesel engines where the crankshaft size is small and the engine pistons are trunk-type, oil is supplied to the main bearing by a pipe. From there through the passage in the crankshaft it is fed to the crankpin bearing thus avoiding complicated piping to supply lubricating oil to crankpin bearing. From there, oil is led upward through the connecting rod holes. Q.11 The lubricating oil supplied to the crankpin bearing in a marine diesel engine Ans. by internal crankshaft passages. The internal passages in the crankshaft are made in such a manner, in case of small diesel engines where crankshaft size is comparatively small, that oil is supplied to the main bearing. From there to the passage in the crankshaft it is feed to the crank pin bearing. Thus avoiding the complicated piping to supply oil to the crankpin bearing and gudgeon pin bearing. This is applicable only in case of small crankshafts. Bigger crankshafts may fail under fatigue if the holes are drilled out for the passage of the oil. Hence in case of the bigger crankshafts, no drilling of the holes is done on the crankshaft and hence this method of lubricating the bottom end bearing and gudgeon pin bearing is not applicable. In that case, there is a separate supply of lubricating oil to main bearing and as well as to the crosshead bearing, which will be further bifurcated into two: the crosshead bearing and piston cooling, and the bottom end bearing, for lubricating. Q.12 Gudgeon pin bearings are difficult to lubricate because of their oscillating motion and Ans. (d) their position in the lubricating system. As a gudgeon gearing is fitted between the piston and the connecting rods. Q.12 Gudgeon pin bearing are difficult to lubricate because of their oscillating motion and Ans. (d) their position in the lubricating system. As the gudgeon pin is fitted between the piston and connecting rod, it is difficult to convey oil to it. Also by the time oil reaches this bearing after passing through the main bearing and bottom end bearing, there is a large amount of pressure drop as well as loss of quantity of the lubricating oil. Thus this bearing is starved of the oil. Also load on this bearing is acting downwards. Also the area of the bearing is less. All these factors contributes to the difficulty in the lubricating the gudgeon pin bearing.
Q.13 The camshaft drive is designed to maintain the proper camshaft speed relative to the crankshaft speed. In maintaining this relationship the camshaft drive causes camshaft to rotate at Ans. (b) crankshaft speed in a two-stroke diesel engine i.e. camshaft and crankshaft speeds for two stroke engines are same. In a two stroke engines for every revolution there is one power stroke. For every power stroke, there is one fuel injection movement and one exhaust valve opening movement. This fuel injection and exhaust valve opening is controlled by the camshaft rotation. Hence it is necessary for camshaft to rotate at the same speed as the crankshaft, so as to have fuel injection and exhaust to be carried out for every revolution of the crankshaft. Q.14 Engine having bore exceeding 250 mm, but not exceeding 300 mm, are to have at least Ans. (c) one explosion relief door or one explosion relief valve in the way of each alternate crank throw with a minimum of two valves. This is a minimum requirement under the classification society rules. When arriving to this requirement figures, the calculation of the crankcase volume and the rate at which the explosive gases are released, in order to avoid the building of pressure and explosion of the crankcase, atleast two safety valves are needed; in case one valve fails to open atleast the second one will respond.
Q.15 The opening of an exhaust valve on a modern large two-stroke slow-speed main propulsion diesel engine may be actuated by Ans. (c) hydraulic “pushrods”. The opening of exhaust valve on a modern engine of a large slow-speed main propulsion diesel engine may be actuated by hydraulic pushrods as it eliminates the need of mechanical pushrods and the rocker arms. Also it eliminates the problem encountered due to the thermal expansion of the valve spindle. There is no need for checking and adjustment of the tappet clearance. Also, closing and opening timing of the valve can be adjusted depending
upon the load, and the closing and opening action can be made smooth with less hammering of the valve on the seat and hence less noisy. Thus hydraulic pushrod is most convenient method for opening of exhaust valve. In modern engines, closing is carried out by mechanical or air springs. Q.16 Shaker/circulation and spray are the three general methods used in Ans. (d) piston cooling.
Q.16 The shaker, circulation, and the spray are the three general methods used in piston cooling. As these three methods are the most effective and easier to be applied to the rapidly reciprocating pistons without providing any external methods and this the most convenient method of supplying lubricating oil or any other cooling medium to the piston cooling. At the same time, movement of the piston is utilized for the splash-type shaker method of cooling for the under side of the piston crown. This method is highly effective method. Q.17 Speed of the camshaft of a two-stroke cycle diesel engine running at 950 rpm is Ans. 950 rpm. In two-stroke engine, it is necessary to run the camshaft at the same speed as that of the crankshaft. As for every revolution there is one power stroke and for every power stroke fuel has to be injected. Hence the speed of the camshaft and the crankshaft should be equal. Q.18 Explosion relief valve on diesel engine crankcase should relieve the pressure at not more than Ans. 0.2 bar. This rule is governed by the classification society and SOLAS regulations. When arriving to this figure, all necessary calculations are done to reduce the severity of explosion. If the pressure is allowed to reach more than this value, the crankcase temperature will be higher and the chances of explosion and severity of explosion is on higher side. Hence it is necessary to relieve the pressure from the crankcase if it exceeds more than 0.2 bar Q.19 Two-stroke diesel engine exhaust temperature will be lower than a four-stroke diesel engine of same displacement because (i) scavenging air cooling the exhaust gases. (ii) exhaust cycle time is longer. Ans. (i) scavenging air is cooling the exhaust gases. In two stroke diesel engines, exhaust temperature is lower than the four stroke diesel engine of the same power because the scavenging air is cooling the exhaust gas while the incoming scavenge air is driving out the exhaust gas. During this process, some cool scavenge air mixes with the exhaust gases and the thermometer fitted on the exhaust gas outlet measures the mean exhaust gas temperature. Hence it is always lower than the four-stroke engines where the exhaust gas is expelled due to the physical movement of the piston and not due to the incoming air. Q.20 The sealing surface of a diesel engine piston rings which faces are considered to be in contact with cylinder walls in addition to the ring grooves? Ans. (a) Bottom. The high pressure combustion gases goes behind the piston rings to push them hard against the liner to have a good sealing effect between the liner and the piston rings. But the gas pressure behind the piston rings should always be maintained and there should not be any leakage from the landing faces of the rings on the grooves of the piston; otherwise blowpast may take place. Hence the sealing, between the contact faces of the rings and the bottom surface of the grooves on which the ring rests, is important to maintain gas pressure behind the piston ring to give better sealing effect between the piston rings and the liner. Q.21 Where is the fuel delivery check valve located in jerk type fuel injection system? Ans. (c) In the injection pump housing discharge side. As the valve job is to prevent the backflow of the fuel from the high-pressure piping and fuel valves into the pump during the suction stroke of the pump and also at the end of delivery stroke when suction and discharge gets short-circuited to terminate the delivery stroke. Otherwise the backflow of the oil from the fuel piping will take place. In that case, every time at the end of delivery the high-pressure side of the piping will get partially empty and this will cause late beginning of injection for the next stroke. Hence it is necessary to maintain the high-pressure pipe between the fuel pump and fuel valve to be always filled with the oil. At the same time, they should be maintained at certain high pressure. Q.22 How many crankcase relief valves are required for a 325mm bore, 8-cylinder in-line engine? Ans. (d) 8 As the bore is 325mm as per classification society rules, each crankcase should have atleast one relief valve fitted. Hence in case of 8-cylinder engine, it is necessary to have 8 crankcase relief valves. Q.23 Which of the air-intake system listed will result in lowest specific fuel oil consumption? Ans. (b) Turbocharged. The turbocharger will provide the extra air under pressure so that more quantity of air can be admitted into the cylinder so that more fuel can be burnt efficiently. For this, turbochargers are provided. As these turbochargers are driven by waste exhaust gas without any power from the engine, it is the most fuel efficient method of improving the performance of the engine. In all other methods of supercharging, the power for driving the supercharger is taken from the engine crankshaft. Hence there is a loss of power and from the point of view of available power at the engine output, the specific fuel oil consumption will be on the higher side as compared to the turbocharged engine. Q.24 In which of the area of the crosshead engine is a permanently connected fire extinguishing system required? Ans. (d) Scavenging space In open connection to the cylinders i.e. it is required for the under piston scavenge space.
Under piston scavenge spaces are connected on the cylinder underside. As under side of the piston is having fuel unburnt loops and carbon mixture. In case of blowpast in presence of plenty of availability of air, this sludge may catch fire. This type of fire is called crankcase fire. In order to extinguish such type of fire, permanently connected fire extinguishing system is required. This is possible by providing a permanent connection of steam smothering or connection to extinguish scavenge fire because there are circumstance under which most of the other fire fighting processes will fail. In such circumstances, there is no other alternative but to flood the under piston scavenge space weight smothering element such as carbondioxide or steam.
Q.25 Camshafts are usually driven by timing gears or Ans. (b) chain drive. Chain drives is one of the reliable method of driving the camshaft with consumption of minimum energy. It is simple in construction and easy to maintain. By using the chain drive, it is easier to position the camshaft very near to the cylinder head along with the fuel pump placed near the head which reduces the fuel injection delay and better combustion of fuel because it is injected exactly at time which is having the best combustion efficiency. Q.26 The valve cam slope angle determines the Ans. (b) opening and closing rate of the valve. As the roller is always in contact with the camshaft, the profile of the camshaft plays an important role in deciding the time of opening and closing and the time for which the valve will remain open and the rate of opening and closing of the valve. The rate of opening and closing of the valve will depend upon the cam-slope. As more steeper the cam profile, faster the rate of opening and smarter the closing. This is required for the valves in order to avoid the throttling effect when the valve is opening very very slowly or when it is closing very slowly. If the valve opens or closes slowly, in that case, there is always a possibility of improper scavenging taking place. In such circumstances, the efficiency of the combustion is poor. This is mostly applicable to the high-speed engines where the time available for the process of exhaustion and suction of air is very very limited. Q.27 Arrangement and shape of the cams on diesel engine camshaft directly controls which of the listed group of operating condition? Ans. (b) Firing order, valve timing and valve lift. Justification: The position of the individual cam on the camshaft will decide the firing order of the engine. At the same time exhaust and intake valve opening will be also decided by the positioning of the respective cams on the camshaft. The position of the cams on the camshaft with respective crankshaft will decide the opening and closing timing for the fuel valve, exhaust valve and intake valve. The profile of the cam will also decide the rate of opening and the rate of closing. Also the valve lift will depends upon the roller lift which will be decide by the total cam lift. Q.28 Carbon deposits building up in and around injection nozzles are least likely to occur when using which of the listed type of the fuel injection nozzles? Ans. (c) Pintle type of nozzles. Justification: Pintle type of nozzles are mostly used in the low powered engines where the temperature inside the cylinder is much lower. At the same time, because of its single whole structure, there is least amount of leakages or dripping of the nozzle. Also it has very very low sac volume and hence its dripping possibility reduces. All this will contribute to the minimum amount of carbon deposit around the fuel nozzle. Q.29 A port and helix fuel injection pump having upper and lower plunger helix is designed to Ans. (b) vary the beginning and end of injection timing. Justification: When there are two helixes cut on the plunger one helix controls the beginning of injection i.e. lower helix controls beginning of injection and upper helix controls the end of injection. Thus it is a good old method of variable control which were fitted on the previous generation of engines. Q.30 Which of the following statements describes the fuel injection pump mark timed for port closing. Ans. (a) Injection has a constant beginning and variable ending. The helix cuts on the plunger surface and inlet port on the barrel. The time at which the upward moving pump covers the suction port on the barrel is the beginning of injection as the fuel oil gives compressed and it lift opens the fuel injection valve.
Q. 223 The rate of pressure rise during the period following fuel ignition in a diesel engine is influenced by the length of ignition delay period due to the Ans. (c) turbulence of the air charge The turbulence of air charge is of most important as it results in thorough mixing of fuel and air due to which the fuel oil will evaporate faster and form a combustible mixture in a shortest possible time. Hence the ignition delay will be reduced as the turbulence increases. Q.224 Lower water seal on a diesel engine wet cylinder must allow for liner axial movement. This seal is most commonly a Ans. (a) neoprene o-ring. Neoprene o-ring is fitted at the bottom of the cylinder jacket which seal the cooling spaces from the crankcase. Hence prevents the cooling water which is circulating around the liner from entering the crankcase. At the same time, because of the heat generated due to running of the engine there is always a linear expansion of the liner; so this should also be taken careof. So neoprene is the right rubber material which will allow the rings to expand in the axial direction. At the same time, it will do the sealing job so as to prevent the water from the jacket enter the crankcase. Hence it forms a flexible and movable seal between the jacket and the liner.
Q.225 Which of the device listed is installed in a diesel engine to isolate some of the crankshaft vibration created by rotational and reciprocating process? Ans. (b) Torsional vibration dampener. Torsional vibration dampener is fitted either on the engine crankshaft and driven by it or it separately fitted in the engine room platform or in the steering room are controlled by electric motors. These torsional vibration dampeners consists of rotating masses. The frequency of the vibration caused by these masses will be out-of-phase with the original frequency of vibration. Hence it will nullify the frequency caused by the rotation of the crankshaft thus total frequency of vibration is reduced. Q.226 Differential type fuel oil nozzle in diesel engine are closed directly by (a) spring pressure (b) fuel oil pressure Ans. (a) spring pressure only In a fuel injection nozzle, which operates on hydraulic pressure of the fuel oil, the opening of the nozzle is carried out by fuel pressure which is developed by the fuel pump. This pressure will lift the nozzle against the spring pressure and thus injection will take place. As soon as injection pressure drops in order to have the efficient and positive sealing between the needle and landing face. it is necessary to close the needle by use of spring force against the seat so it will prevent the dripping. At the same time, the hot gases will be prevented from entering the fuel valve nozzle from the combustion space. Q.227 Modern marine diesel engines equipped with mechanical injection operates on a combustion cycle which is Ans. (c) a combination of constant volume and constant pressure. In diesel engines when the fuel is injected near to the TDC the piston is nearly in a standstill condition and change of volume is very low. During that time, when the fuel is injected it burns under the constant volume condition. This results in sudden increase of pressure near the TDC. Hence the volume is kept constant but the pressure increases. When the fuel starts burning and piston starts to move towards the BDC position, during this period the fuel burns and the pressure rises. Even though the volume increases due to the movement of the piston, the pressure remains constant though the volume changes. So the fuel injection and burning process is divided in constant volume and constant pressure phase. Q.228 An efficient seal between cylinder block and cylinder head on many diesel engines is obtained with Ans. (d) gasket. It is necessary to form a gas-tight combustion space between the top side of the piston and the underside of the cylinder head. This space has to seal high-pressure gases and this is possible only if the seal is perfect and without any leakage. For that purpose, it is necessary to have some sealing arrangement between the cylinder head and liner. In earlier days this used to be a metal-to-metal contact but to maintain a high-pressure seal with this is a difficult process. Also, there is a possibility that some leakage may take place and the liner and the cylinder head landing face may get damaged. In order to avoid this, a soft metal gasket is utilized which is fitted between the cylinder head and liner. When the cylinder head is tightened against the liner flange, the softness of the gasket will allow it to flow into the small deformities which will be formed between the cylinder head and liner, thus forming a perfect seal. Q.229 Oil for piston cooling is delivered through the connecting rod to a compartment in the piston head. Then, it is distributed as a result of piston motion and finally drained to the crankcase through one or more holes or pipe. This process is known as Ans. (c) shaker method. In the piston cooling, the cooling medium, the oil or water, is conveyed to the underside of the piston crown by the pipes. There are two pipes for inlet and outlet. The outlet pipe is kept slightly higher so that the piston cooling face will always be having some amount of cooling medium places. This cooling medium gathers the momentum of the piston when the piston moves up and down. Due to this, the medium gets the agitating movement and because of this it splashes against the hot piston crown underside and gives better cooling effect rather than only jet or any other method. At present, most of the engines are using a combination of jet and agitation method, also called the shaker method, for piston cooling. This combination gives better cooling efficiency and better running. Q.232 The breaking up of fuel oil as it enters the diesel engine cylinder is known as Ans. (c) atomization. When the fuel enters the cylinder, it is at high pressure and it passes through the small holes of the nozzle tip. Due to this, when it is injected through the nozzle tip, it gets broken up into tiny particles which are useful in efficient burning of the fuel. This is known as atomization. Q.233 The exhaust ports of a diesel engine using the crossflow scavenging method are closed by Ans. (c) the reciprocating motion of the piston. Scavenge and exhaust ports, on a cross flow scavenging system, are cut on the wall of the liner at different level. When they are cut on the liner wall at different level, the opening and closing of these ports are carried out with the piston movement. As the piston moves downward towards BDC, the piston top will uncover the exhaust ports first and exhausting takes place. When it moves further it will open the scavenge ports and scavenging will take place and scavenge air will enter the units. This is called unit injection. Q.234 Which of the bearing listed below is most widely used for main and connecting rod bearing of a modern high-speed diesel engine? Ans. (b) Precision insert. The precision insert bearings are factory made bearing which are just to be inserted into the bearing keep and no scrapping or * of the bearing is required. These bearings are mostly thin shelled white metal bearing and they have got better fatigue resistance and high load carrying capacity. Hence they are called precision insert bearings. Most of the engines and bearings, where there is fluctuation of load and heavy loading such as crosshead bearings and the bottom end bearing, the thin shelled aluminium bearings are utilized. Even on many engines, main bearings are also made up of thin shelled bearings.
Q.235 Valve rotors are commonly used on which of the listed diesel engine cylinder head valves? Ans. (c) Exhaust valve. When the exhaust gas pass through the exhaust valves it is having some unburnt carbon particles. This particles will get lodged or get in between. So when the exhaust valve closes there is a possibility that the carbon particles may get lodged between the valve and the valve seat. If the carbon particles remains continuously in that same position, there is a possibility that carbon will burn on the valve seat and valve. Due to this, the valve and the valve seat will get damaged and dents will be formed on the valve seat and life of the valve will be reduced. In order to avoid this, it is necessary that the valve should be rotated so that it will have a self cleaning effect and whatever carbon is deposited between the valve and the valve seat, it will get removed due to the rotation of the valve. This will increase the life of the valve. Hence it is necessary to fit a valve rotor on the exhaust valve so as to keep the valve rotating while it is opening and closing. Q.236 Exhaust valve opening in a diesel engine cylinder head are made as large as practical to Ans. (c) reduce the pumping loses associated with scavenging. When the exhaust valve is opened and the piston is moving in upwards on exhaustion stroke, there should not be any backpressure on the piston. When there is backpressure on the piston, it will consume the energy from the engine for exhausting the gas from the engine through the exhaust valve. Hence in order to reduce these pumping losses, it is necessary that there should be no restriction for the exhaust gas to flow through and there should not be any backpressure on the piston because of the exhaust gases. This is possible only by providing sufficiently large exhaust valve area so as to avoid backpressure and loss of power because of pumping effect. Q.237 Piston cooling fins are located Ans. (c) beneath the piston crown. The underside of the piston acts like a cooling area for the piston crown. In this area the cooling medium is circulated and the heat transfer takes place at the underside of the piston crown. If the underside of the piston crown is provided with the fins, as the fins have better cooling effect with sufficiently thickened crown, we can have better cooling of the crown the thickened crown have more strength. At the same time the fin effect will be utilized for better cooling. Hence both, more strength and better cooling effect, is achieved by having a thicker crown and fins. Q.238 Diesel engine muffler reduces the noise by Ans. (d) changing exhaust gas direction. In the muffler, there are plates which are fitted so that gas passes in a zigzag manner when the gas passes in such way there is continuous change in the direction of exhaust gas. Due to this, the dampening effect is imparted to the waves which are created by the exhaust gas. This causes the reduction in the noise as well as vibration caused by the exhaust gas. Q.239 In the cylinder head of two-stroke diesel engine valves are used for Ans. (d) exhausting combustion gas. The two-stroke engine mostly works on uniflow scavenging system. In uniflow scavenging system, the exhaust valve is fitted on the cylinder head and the inlet ports are cut on the cylinder liner at much lower position. The cylinder head valves are only used to take out the exhaust gas. The inlet of air is from the bottom position so that there will be complete sweeping action of cylinder liner will take place and there is a less possibility of exhaust gas mixing with the incoming air. Hence unidirectional scavenging will give better efficiency. Therefore exhaust valves are fitted on the cylinder head and the inlet ports are cut in the liner walls. Q.241 The intake ports of two-stroke diesel engines are opened and closed by the action of the Ans. (b) piston movement. On a two-stroke cycle diesel engine, the exhaust and inlet or only inlet ports are cut in the walls of the cylinder liner at much lower height from the cylinder top. Hence the opening and closing of these ports are controlled by the vertical movement of the piston. When the piston moves down and uncovers the ports, scavenge air enters the cylinder. At the same time, when the piston is moving upwards and closes the scavenge ports, the scavenging process stops. If the exhaust valve is also closed, compression stroke starts. Q.242 Compared to four-stroke diesel engines two-stroke diesel engines have the disadvantage of Ans. (b) high cylinder head temperature. In two-stroke engine, for every revolution there is one power stroke i.e. for every revolution there is one fuel firing. This means that the total power developed in individual cylinder head is more. This contributes to more heat being developed by the engine as compared to the four-stroke engine. So the cylinder head temperature in two-stroke engine is more than that of four-stroke engine. In reality, the exhaust temperature of a two-stroke engine is lower than the four-stroke engine because during the scavenging process the air is driving the exhaust gas. Along with exhaust gas, some amount of cold air is expelled from the cylinder due to which the mean temperature recorded by the exhaust gas thermometer will always be lower, in case of two stroke engines as compared with four stroke engines. Theoretically the exhaust temperature will be on the higher side in two-stroke engines than four-stroke engines. Q.243 Which of the listed conditions will affect the mean effective pressure the most in a diesel engine cylinder? Ans. (c) Completeness in the mixing of fuel and air. When the fuel and air is completely mixed and fuel is fully vaporized, the combustion efficiency will always be better because the fuel will burn efficiently in the allocated time and resulting in high peak pressure. When the peak pressure increases automatically, the mean indicated pressure also increases and more power will be developed by the engine. Q.244 In comparing engines of equal horse power higher exhaust gas temperature occurs in a Ans. (d) four-stroke cycle diesel engine. In four-stroke cycle diesel engines, the mixing of the incoming air and outgoing exhaust gas through the exhaust valve is less because the expelling of the exhaust gas is carried out by the movement of the piston and not by the incoming air in case of two-stroke cycle engine. In two-stroke cycle engine,
the expelling of the exhaust gas is carried out by the incoming air. Hence, there is mixing of fresh air with the exhaust gas and some amount of incoming air is being exhausted along with the exhaust gas through the opened exhaust valve. Therefore exhaust temperature registered by the thermometer always drops. But, in case of four stroke engines, the exhausting is carried out by the upward movement of the piston and it is not much assisted by the incoming air. So in the exhaust manifold less amount of incoming air is exhausted. Hence the exhaust temperature of four-stroke cycle diesel engine is more than two-stroke cycle diesel engine for the same amount of power developed. Q.246 Which of the bearing listed are most widely used for the connecting rod and main bearing of a diesel engine? Ans. (c) Precision insert. Q.247 Which of the following is an example of a solid bearing? Ans. Piston pin bushing. Piston pin bushing, i.e. the gudgeon pin bearing, is a solid bearing as it is not in two halves. It is bush which is shrunk-fit into the boss of the connecting rod top end thus forming a solid bearing. Q.248 The upper section of piston is called the Ans. (c) crown. It is the portion of the piston which takes the full load of the combustion. It is subjected to highest temperature and pressure, and wear down. Q.249 In a diesel engine, the main bearings are used between Ans. the crankshaft and the engine block. The main bearings are used for supporting the crankshaft on the engine blocks. Thus it is necessary to use the main bearings in between the crankpin bearings so that the crankshaft will be properly supported over the entire length by the bearings at equal distance. This will prevent the bending and sagging of the crankshaft and it will transfer the power to the bedplate. This will help in having a rigid bedplate which will support the crankshaft and have a proper alignment of the crankshaft. Q.250 Which of the listed bearing installation is subjected to swinging motion? Ans. (c) Gudgeon pin bearings. In the gudgeon pin bearing, there is no relative movement between the pin and the bearing and there is only swinging motion which takes place. The swinging is on one side when the piston is moving downwards. The swinging comes to zero at TDC and BDC and the swinging action turns over the other side as the piston turns over the dead center and starts moving upwards. Thus there is a swinging action between the two extremities which will cause the oscillating swinging of the connecting rod which ultimately gives the oscillating movement to the gudgeon pin bearing. Q.251 Which of the bearing listed is widely used as connecting rod and main bearing on a modern diesel engine? Ans. (d) Precision bearing. Precision bearing are thin shelled bearings having more load bearing capacity and resistance to fatigue. These are factory manufactured with precision clearance maintained having thin layer of bearing materials. Q.252 The proper location for journal bearing oil groove is Ans. (b) at the side relief where the two shelves meet. If oil is supplied to the grooves, it can be conveyed very easily and without any pressure drop on the loaded bearing, i.e. bottom half of main bearing. Hence for the pumping action of the rotating journal, it is necessary to supply oil at the side of the journal, at the junction of the bottom and top half, so that the rotating movement of the journal the oil will be drawn and pressure will be created. This pressure will cause the wedge type lubricating oil film due to which the hydrodynamic lubrication will be possible. At the same time, as the grooves are not in the loaded area, it will not reduce the load carrying capacity of the bearings. Q.253 Fuel is admitted to the diesel engine cylinder through the Ans. fuel injector valve. The fuel injector valve injects the fuel at high pressure against the compression pressure developed inside the cylinder so as to form a combustible mixture and ignite the fuel. Q.254 Directional intake ports in diesel engines are used to Ans. (b) induce air swirl. The intake ports are cut at an angle to the periphery on the cylinder wall so that when air enters through these ports into the cylinder they get a swirling action. Due to this, the complete mass of air, which enters the cylinder, will go on rotating. This rotational air mass will be useful for complete sweeping action of the liner. At the same time, it will aid in thorough mixing of air and fuel in the cylinder thus forming a better combustion mixture and improving the thermal efficiency of the engine. Q.255 Telescopic pipes to the piston of a large slow speed main propulsion diesel engine are designed to prevent Ans. (c) contamination of lubricating oil by water. When the piston is water-cooled and water is conveyed to the underside of the piston crown, it has to be conveyed through the telescopic pipes because there is a continuous vertical movement of the piston. To achieve this, telescopic pipes are utilized which will give the sliding sealing between the stationary pipes and reciprocating piston. For this, there is telescopic and gland packing is utilized in order to prevent water from leaking into the crankcase; otherwise crankcase contamination will take place in crankcase. Telescopic pipes, arrangement along with the carbon seal provided, will prevent water from contaminating the crankcase oil.
Q.257 Large slow speed main propulsion diesel engine uses sea water to directly cool the Ans. (c) scavenging air. Scavenging air temperature before cooling is in the range of more than 100°C which is to be cooled to about 40°C. At the same time, the quantity of air available is also very large and air is having a low thermal conductivity. Hence to cool such large quantity of air through such a large temperature difference it is necessary that cooling medium should have a good cooling efficiency. Otherwise if it is cooled by any other medium in such cases the cooler has to be very big. Also, seawater being a convenient medium can be thrown overboard after cooling and new abundant cooling medium is available. Hence cooling is carried out by seawater directly rather than indirect cooling or by using any other medium. Seawater cooling is most efficient method of cooling scavenge air. Q.258 The purpose of mist detector in a main propulsion diesel engine is to warn of Ans. (a) excessive mist density in the crankcase. Oil mist detector continuously samples the atmosphere of the crankcase and compares it with the average atmosphere or the atmosphere in the engine room. Hence any change in the density of oil mist will be immediately revealed or brought to notice by means of audio and visual indication. The high density in the crankcase is a direct indication that there is hotspot present in the crankcase because of which the oil is getting evaporated forming oil mist which may lead to crankcase explosion if ignored. Hence high oil mist density alarm which is given by oil mist detector is a safety alarm and immediate action should be taken inorder to avoid future destruction to the engine as well as the danger to personnel working in the engine room. Q.259 One method of constructing large marine diesel engine and reducing the frame weight is through Ans. (b) welding plates to form the section for assembly. In earlier days the frame and the bed plate assembly were made up of casting. Compared to the casting the plates which are rolled has got more strength and by assembling it properly and proper welding, it is possible to construct a structure which has got more strength for the same weight. Hence it is possible to use a welded structure so that the weight of the structure is reduced and its strength is increased. Hence in present generation of engines, the longitudinal girders are made up of steel plates. They are welded in between with the transverse girders which are made from the casting because their intrinsic structure is difficult to fabricate. The total welding of these two different parts of bedplate has been generated into a box type bedplate which has high strength and low weight. Q.260 Which of the fuel injection system listed below uses a spring-loaded differential spray needle valve and an individual pump for each cylinder? Ans. (c) Jerk pump fuel injection. In the jerk pump fuel injection, a sudden pressure rise of fuel is achieved. This rise is conveyed to the needle valve where the high-pressure wave acts under the conical surface of the needle because of which the needle gets lifted against the spring compression. Hence the oil is conveyed through the spray holes and it is injected inside the cylinder. As soon as the suction and delivery of the pump are short-circuited the sudden drop in pressure of fuel in the pump takes place and hence the pressure in the fuel valve drops. Under such circumstances the spring pressure is more than the oil pressure and because of the spring pressure the valve snap closes over the seat and prevents further injection of fuel or leakage of the fuel. This is one of the best methods of injecting the fuel at right time and without unnecessary dripping of the fuel from the engine. Q.261 Concerning diesel propelled vessel the astern power is provided for continuous operation astern Ans. (b) at 70% of ahead rpm at the rated speed. Most of the diesel engines are designed from the point of view of running ahead. The fuel pump timing, the exhaust valve timing and the distribution of the weight of the rotating parts is made in such a way that there is a balance in ahead direction. Hence the best possible efficiency of the engine as far as fuel is concerned is in the ahead direction due to proper timing for the exhaust and fuel valve. At the same time, there is minimum torsional vibration because of dynamic balancing which is taken care off at the design stage from the point of view of ahead running. If the engine is runned at astern direction, the combustion efficiency reduces. At the same time, because of unbalanced forces, there will be more amount of vibration. Due to profile of the ship and the efficiency of the propeller, for the same rpm, the power required for running astern is always more than that of what is required for running ahead. If the engine is runned at 100% rated rpm in the astern direction it may be excessively overloaded. Hence it is always provided to give astern power at 70% of ahead rpm at the rated speed Q.262 The crankcase of many diesel engine are kept at slightly vacuum by Ans. (b) crankcase exhaust fans. (In the olden days, this was very practice but not utilized presently.) By keeping the crankcase ventilated by use of exhaust fans fitted on the crankcase, the atmosphere in the crankcase is kept slightly under vacuum. By this, we are depriving the oil mist of air. If even little oil mist is present it will not cause any major explosion as it may be excessively rich and it will lack of air and hence possibility of explosion will be reduced because of excessively rich mixture which will be above high explosive limit. But this method is no more used as if there is any leakage there is every possibility that air may enter through the leaky crankcase doors. At the same time, because of the vacuum in the crankcase, more amount of air may leak through the underpiston scavenge through the stuffing box into the crankcase or excessive blowpast may take place in case of four stroke engines. Q.263 Which of the fuel injection system combines the fuel pump and nozzle in one housing? Ans. (b) unit injector. Unit injector is a combination of fuel pump and injector without any high-pressure pipe connected between fuel injector and fuel valve. Hence some advantages such as elimination of pipes and reducing injection delay are achieved. Q.264 In unit injector upper and lower helix are machined in the lower part of plunger for Ans. (c) accurate metering of fuel oil.
By having the helix cut on the plunger and suction hole on the barrel, and positioning the suction hole with respect to helix by means of rotating the plunger with the help of fuel pump rack controlled by the governor, it is possible to control the fuel injection timings. Due to the double helix cut on the plunger surface, it is possible to control the beginning of injection and end of injection. Thus both the beginning and end of injection is controlled thus controlling the quantity of fuel injected. Thus proper metering of quantity of fuel depending upon the load and changing the fuel injection timing with respect to load is achieved by the double helix cut over each other in the fuel pump plunger surface of unit injector. Q.265 Regarding the diesel engine crankcase the general arrangement and installation should preclude the possibility of Ans. (a) free entry of air to the crankcase. The crankcase atmosphere of a diesel engine is full of large amount of small oil droplets floating in the atmosphere. At the same time, if any hotspot is created in the crankcase oil mist may also be formed which is susceptible to burning, if sufficient amount of air is available. As hotspot and oil mist is present, only air is required so that explosion can occur. If the entire atmosphere of crankcase is entirely filled and because of oil mist, formed due to evaporation of oil, is rich in concentration and is above the upper explosion limit and the chances of explosion are less. But if air is allowed to enter the crankcase, through the crankcase doors or by any other means, then dilution of the oil mist will take place and it passes through the upper and lower explosive limit. When it passes through this range, in presence of hotspot, this oil mist may explode and chances of explosion are very high. This is the main reason that notices are put on the crankcase doors that the door should not be opened for atleast half hour after the engine has been stopped. Q.266 In a diesel engine, internal combustion causes the piston to be moved by Ans. (a) the pressure of the gases developed. When the piston is near to TDC fuel is injected. This fuel injected will burn and produce the heat and pressure wave. This pressure wave will act over the piston. By the time this pressure wave is created, the piston has already passed the TDC position. As it is turned over the TDC position, these highpressure gas acting over the piston forces it downwards. This phase of the cycle is called power or expansion stroke. Under this stroke the piston is forced downwards. When the pressure acts over the area of the piston it produces the force. This force when is multiplied by the distance traveled by the piston, i.e. the stroke, will give the actual work done by the engine. This work done is taken from the fuel and is transferred after burning, due to the high-pressure gases produced, by acting on the cross section of the piston and forcing it downwards. Thus producing the work or the power developed by the engine inside the cylinder which is termed as indicated horsepower. Q.267 What function is provided by the crankcase ventilation system on some diesel engines? Ans. To reduce the pressure in the crankcase. Crankcase ventilation system, whether it is forced or ordinary ventilator with a flame arrestor fitted, its job is to produce pressure inside the crankcase because in the crankcase oil in the form of droplets and numerous hot places, such as bearings and pistons, are always present. Due to this, continuous evaporation of oil is taking place. If these vapors are not removed from the crankcase, then the crankcase starts getting pressurised. In that case, oil starts leaking from the crankcase doors. At the same time, consumption of crankcase oil will increase. If oil mist increases continuously there is a possibility that it may pass through the explosive range and cause explosion. Hence to avoid this things it is necessary that the crankcase of the internal combustion engine should always be ventilated so that whatever gases or oil mist are accumulated will be reduced to safe limit. Q.268 For diesel engine piston cooling lubricating oil can be supplied to the piston by Ans. (a) oil spray. For cooling of the piston, lubricating oil is the best and convenient medium. Oil is conveyed to the underside of the piston by different methods: In case of four stroke engines, it passes from the main bearing to the bottom end bearing; from bottom end bearing through the drilled passages in the connecting rod, it will be supplied to the top end bearing where the nozzle is fitted which will inject the oil to the underside of the piston. In case of two stroke engines, the oil may be supplied through the crosshead and the telescopic pipe, which are fitted inside the piston rods, the oil is supplied to the underside of the piston where it will be injected through the drill passages or nozzle so that efficient cooling can be done rather than any other method. In present engines, combination of nozzle and shaker method of cooling rather than normal single nozzle or shaker method is used which gives efficient cooling of the piston. (At present *RTI engine is using the combination of spray and shaker method.) Q.269 The purpose of delivery check valve used in a diesel fuel injection jerk pump is Ans. to prevent the backflow of oil from the pipe to the pump and also to prevent the emptying of the delivery. This pipe has to be filled every time so that pressure is built up resulting in delayed fuel injection and the injection timing will not be proper. This valve shuts at the end of injection stroke thus preventing the emptying of injection pipe and backflow of fuel to the pump. At the same time, the valve is designed in such a manner that the closing action of the valve under the spring and returning oil is delayed slightly. Due to sudden closing of the valve, backpressure and the pressure wave created is allowed to get released and prevents the secondary injection. Also this valve prevents the fuel from entering the fuel valve till sufficient pressure is developed. Thus in general the valve job is to avoid the backflow and secondary injection and delivery of fuel to the fuel injector before the pressure is sufficiently high. Q.270 In a two-stroke diesel engine the cam shaft rotates at Ans. (c) the same speed as the crankshaft. In two-stroke engine for every stroke there is one power and one exhaust and inlet stroke. This power stroke is controlled by the fuel injection and the opening of exhaust valve is controlled by camshaft. Hence for every revolution fuel has to be injected once and this is possible only if the camshaft rotates at the same speed as that of crankshaft. Q.271 The primary function of a delivery check valve assembly is to Ans. 269 Q.272 For any piston ring to operate smoothly without scuffing, the ring must be Ans. (b) properly lubricated.
In a diesel engine, when in operation the piston moves up and down vertically. During this motion the piston rings are forced radially outward to have very high contact pressure between liner and piston rings thus forming a sealed combustion space. Under this extreme pressure between the piston rings and liner, it is necessary to have a thin film of lubricating oil so as to reduce the coefficient of friction between them. If the lubricating oil film is not present the coefficient of friction will become excessively high because of which, under extreme pressure, excessive heat will be generated and due to this heat, instantaneous welding between the liner and piston will take place. At the same time, due to the vertical movement of the piston ring, this weld will get worn out thus causing the scuffing. In order to avoid this scuffing, it is necessary that the coefficient of friction should be reduced. This is only possible if sufficient amount of lubricating oil is spread over the liner. Q.273 The upper piston ring in large two stroke cycle main propulsion diesel engine are most efficiently lubricated by oil Ans. (a) fed from a mechanical lubricator. In a large slow speed engine, the stroke of the piston is large. At the same time the piston is separated from the crankcase by a diaphragm and a stuffing box. Also the oil required for the cylinder lubrication is of different characteristics, i.e. high TBN, as compared to crankcase lubricating oil. Hence taking into consideration all these aspect it is not possible to have the flash lubrication as it is carried out in the small (four stroke) diesel engines. Also as it is slow speed, it is difficult to splash oil to such a height. In case of two stroke engines, the oil has to be injected into the cylinder at appropriate time so that it can be spread over the liner wall and it will be burnt and go out as exhaust. For this process the necessary timed lubrication is possible only with the help of mechanical lubricator which are controlled by the camshaft and crankshaft. Thus it is best method of lubricating the piston ring and in latest engines, like Sulzer RTA, the mechanical lubricator are in conjunction with the accumulator principle. Some of the modern engines have electronically controlled cylinder lubrication to give a minimum quantity of lubricating oil with best possible effect and prevent excessive burning of oil. At the same time, it avoids weardown because of lack of lubrication. All this is possible because of mechanical or electronic controlled cylinder lubricating system. Q.274 Fuel injector nozzles are usually of the multi-orifice type with the number and placement of the holes arranged according to the Ans. (d) design of the combustion chamber. Design and geometry of the combustion chamber and the placement of the fuel injector in the combustion chamber is so that oil can be sprayed evenly in the combustion chamber and it will be injected deep into the combustion chamber so that it can completely mix with the amount of air available in the combustion chamber. At the same time, it should not be injected so deep that it will not burn over the wall of the liner or on the piston top. This can be achieved only be achieved by taking into consideration the geometry of the combustion chamber and the number of holes in the fuel injector and the angle at which oil is injected so that the best thermal and fuel combustion efficiency is achieved without harming the engine. For this, it is necessary to take into consideration the geometry of the combustion chamber and design before deciding the number and diameter of holes and the angle at which the holes are drilled in the fuel injector nozzles. Q.275 The delivery valve installed in the port and helix fuel injection pump is designed to Ans. (b) maintain the column of fuel in the line. The delivery valve is fitted at the delivery side of the fuel pump and it opens against the spring force when the plunger is moving upwards in the delivery stroke. As soon as the delivery stroke ends i.e. the short-circuiting takes place between the suction and delivery side of the fuel pump, sudden pressure drop takes place on the top of the plunger. As a result, the fuel from the pipe connecting the pump to the injector will return back to the suction side. Hence it may be possible that this pipe becomes empty and everytime, for the next stroke again, you will have to fill this pipe before the fuel injection starts which will cause the excessive delay in fuel injection. Hence inorder to avoid this, it is necessary to maintain a column of oil at a slight pressure inside the pipe connecting the fuel pump to the injector. This is carried out by the shutting of the delivery valve fitted on the pump so as to avoid the back-flow of fuel from the injector to the pump. Also, this valve is made in such a manner that there is slight delay in closing time that it will prevent the excessive development of the secondary pressure wave and secondary injection is also avoided.
Q.276 The shape of a diesel engine cam determines the (a) point of opening and closing of valves. (b) velocity of opening and closing of valve. (c). amount of valve lift from its seat. (d). All of the above. Ans. (d) All of the above. By taking into consideration the profile of the cam and drawing the figure of the cam, we can see that the cam is divided into base circle, riding flange, following flange and the dwell of the cam. The height difference between the base circle and the dwell is the total lift of the cam. Thus when the cam is rotating, depending upon the profile of the cam, when the roller comes in contact with the rising side of the flange, this is time when the roller starts to rise upwards and push the pushrod upwards and it will open the valve. The lift of the valve will depend upon the movement of the roller and this is decided by the total lift of the cam. It will remain open till the roller is on the dwell of the cam. Due to the movement of the cam, when the roller comes at the end of the dwell i.e. it will come on the following flange or receding flange, this is the time when the roller will start descending and it will start riding over the receding flange of the cam. This will be the beginning of the closing of valve. When the roller completely crosses the receding flange and comes in contact with the base circle, this will indicate the complete closure of the valve. Thus the profile of the cam decides the opening and closure of valve and the lift of the valve from its seat. Also, the angle between the base circle and receding or rising flange will decide the rate of opening and closure of valve. The best possible effect is achieved when the rising and receding flange will be at tangent to the base circle. Q.277 Additional explosion relief valve are fitted in separate spaces in the crankcase such as gear or chain case for camshaft or similar drives when the Ans. the gross volume of such place exceeds 0.6d x .340m3
Q.278 If all the conditions such as bore, stroke, speed and mean effective pressures are equal, a two-stroke cycle diesel engine will develop approximately
Ans. (c) twice the indicated horsepower as developed by four-stroke engine. This is because in two-stroke diesel engine, for every revolution of the engine there will be a power stroke. The power stroke is responsible for the development of the horsepower. Hence if all the parameters are equal then it is the number of power strokes which governs the power developed by the engine. For every power stroke of a four-stroke engine, it will have two revolutions of the crankshaft whereas a two-stroke engine produces power strokes for each of its revolution. Hence two-stroke engine with all equal parameters will develop twice the power developed by four-stroke engine. Q.279 In order to reverse the rotation of the two-stroke cycle loop scavenging direct reversing propulsion diesel engine the cam position must be changed for Ans. (a) starting air fuel pumps. For reversing the engines, it is necessary that it must be started in reverse direction. For that purpose it is necessary to admit the starting air on the piston before TDC rather than after TDC. Once the air is admitted before TDC, the engine will run in the reverse direction. This is possible by changing the air starting distributor timing so that the air starting valve will open much early TDC and hence the engine will start in reverse direction. In order to keep the engine running in reverse direction, it is necessary to alter the fuel injection timing also because fuel pump injection timing is not symmetrical over the dead center position. It is also necessary to adjust the timing of inlet and exhaust valve but being a loop scavenging or cross scavenging engine, where the inlet and exhaust valves are absent and the opening and closing of the ports is controlled by the movement of the piston, this alteration is not required. Q.280 Viscous damper as used on a marine diesel engine is a sealed precision built device which dampens the torsional vibration in the Ans. (c) crankshaft. By providing the viscous fluid inside the sealed chamber, the vibration which are created by the firing frequency of the units on the crankshaft is dampened by providing an equal and opposite dampening frequency so that total torsional vibrations are reduced. In order to reduce the torsional vibration, the fluid gives an opposite torque to the vibrations, which has the dampening effect on the vibrations. Q.281 The rotary motion of the crankshaft is obtained from the up and down motion of the piston via the Ans. connecting rod or crosshead and piston rod assembly. The vertical movement of the piston has to be converted into rotational movement of crankshaft. This is achieved by providing a connecting rod which is connected at one end to the gudgeon or crosshead pin and at the other end to the crank pin which is eccentric with respect to the arrangement of the crankshaft. Because of this arrangement whenever the piston moves up and down the crankpin makes a complete rotation and thus converts the vertical movement of the piston to the rotary movement of the crankshaft. Q.282 Indirect cooling of a fuel injector nozzle holder of a diesel engine is accomplished primarily by Ans. (b) heat conducted into the water jacket wall. Most of the system cylinder nowadays are made to cool by bore cooling systems. In the bore cooling system, the water for cooling is brought very near to the combustion space. Due to this water comes very near to the bore in the cylinder where the injector is fitted and it is circulated very near the to the seat in the cylinder head where the injector is fitted. Thus this water is brought so close to the tip of the nozzle and with sufficient amount of water circulation, heat can be taken out indirectly from the injector through the cylinder cooling water and thus cooling of the injector is carried out. Also, the circulating fuel oil helps in carrying out the cooling of injector. Q.283 Diesel engine main and connecting rod precision bearing are made in two half. Each half exceeds one-half bearing circumference by a small amount. The small amount is termed Ans. (b) the crush. This crush is highly essential so that when two halves of the bearing are fitted over each other and has been tightened by bolts the bearing halves, they will have easy flow into the bearing housing and will have a good first contact with the back side of the bearing housing thus preventing rotation of bearing in the bearing housing. At the same time, the load of the shaft is transmitted on the bearing because thin shelled bearing due to its thinness they are unable to carry out load on their own and so they require some backing and this backing is possible through the crush. Also due to this crush, there is a perfect contact between the back of the bearing and the bearing housing and this provides thermal conductivity between the bearing and the housing. Thus it keeps the bearing cool. This crush provides the interference fit between the bearing and that of the housing. Q.284 The port and helix metering pump used in diesel injection systems are usually designed to produce a constant beginning and variable ending of injection. These pumps are usually Ans. (b) timed for port closing and are controlled by the rack movement. In helix controlled pump, when the plunger is moving upwards and the plunger top closes the suction port in the barrel, the further movement of plunger upwards will create a hydraulic pressure on top of the plunger which will cause the fuel valve to open and fuel injection starts. Thus the fuel injection starts when the plunger closes the ports. At the end of injection even the port is covered by the plunger which is connected with the discharge side of plunger by means of helix and that terminates the delivery. The quantity of the fuel depending upon the load of the engine will depend upon the variable end of the delivery. The later the end of delivery, more is the fuel injected. This is controlled by the rack which controls the position of the plunger helix with respect to the suction port on the barrel thus it controls the quantity of the fuel injected. Q.285 The end of injection in a port and helix metering pump is controlled by Ans. short cycling or connecting the delivery ports with the suction ports in the barrel. When the piston is upwards and it has already covered the suction port the fuel is being injected. As it is moving upward, a time comes because of the helix cut on the plunger surface which short-circuits the discharge side of the pump with that of suction side and it will cause the end of the delivery. Hence the combination of helix, which is cut, and the suction port will decide quantity and point of termination of fuel injection. The position of helix with the port cut in the barrel will be decided by the rack position which will rotate the fuel pump plunger with that of the barrel. Q.286 The main reason counterweights are added to crankshaft is to
Ans. (c) provide uniform loading and wear of the main bearing. Counterweights are provided so as to properly balance the engine statically. At the same time, an effort is made to balance it dynamically. Though it is not possible to completely balance the engine dynamically, some efforts are made by counterweights to produce proper dynamic balance of the engine. When the balancing of engine is proper the load is equally distributed on all bearings. At the same time vibration is also reduced. This two automatically reduces the weardown of the bearings and cause uniform distribution of load on the bearing. Q.287 For a given size, the two-stroke diesel engine will deliver more power than a four-stroke diesel engine because Ans. (c) it develops more power stroke at the same speed. It is wellknown fact that in case of two-stroke engines for every revolution there is one power stroke whereas in four stroke engines for every two revolution there is one power stroke. Hence it is natural that, with same size of the engine and same parameters, it is only the number of strokes which will be responsible for the quantity of the power developed. As two-stroke engines have more power strokes compared to the four-stroke engines, it will atleast theoretically produce more power than a four-stroke engine.
Q.288 In a jerk pump, the amount of fuel that is forced through the spray nozzle on each upward stroke depends on Ans. (c) how the plunger is rotated. In a jerk type fuel injection pump, the quantity of fuel delivered by the injector into the combustion chamber will depend on the effective delivery stroke i.e. the beginning of the delivery stroke is mostly constant and the end of the delivery stroke is responsible for the quantity of the fuel which is delivered. The quantity of fuel delivered will depend on the load of the engine. Hence more the load more fuel is required and vice-versa. Under such circumstances, it is the plunger which is rotated by means of rack connected to the governor. Due to the position of governor, the rack position changes depending upon the load on the engine. As the rack position changes the rack will make the plunger to rotate with respect to the barrel and under such circumstances the helix which is cut on the plunger surface will change its position with respect to the suction port on the barrel. This relative position of the helix to the plunger surface with respect the suction hole on the barrel will decide quantity of fuel injected which decides by the termination of the delivery stroke of plunger of fuel pump. Q.289 Modern marine engines using mechanical fuel injectors operates on a combustion cycle which is Ans. (a) combination of constant volume and constant pressure. When mechanical fuel injectors are fitted on diesel engines, as soon as the fuel injection starts, i.e. when the main piston is near to TDC, the volume is nearly constant as there is no much vertical movement of the piston. When the fuel is ignited, it catches fire and as the volume constant it develops high pressure. In this condition, the pressure is kept constant as the volume increases. So part of fuel ignition takes place at constant volume. As the piston turns over the TDC and start moving downwards, some amount of fuel is still injected. During this time, the fuel is injected and is burnt, the pressure rises which pushes the piston downwards. As the piston moves from TDC to BDC, though the volume increases, the pressure is kept constant. Hence the ignition of fuel is carried out partly at constant volume and partly at constant pressure. This combination method gives better thermal efficiency. Q.290 The amount of fuel oil delivered by the fuel injector is controlled by the Ans. (c) rack position. Unit type of fuel injector pump is nothing but a jerk type of fuel pump and a fuel injector together. Hence whatever is applicable to the jerk type of fuel pump is applicable to the unit type of system. In unit injector, it is the rack position which will control the position of helix on the plunger with respect to the suction hole on the barrel. Hence the quantity of the fuel decided by the position of the rack which will decide the position of helix on plunger surface with that of suction port on the barrel. Q.291 The diesel engine valve which is subjected to most severe condition of service is Ans. (a) the cylinder exhaust valve. It is wellknown fact that the exhaust valve is subjected to very high temperature and pressure of the exhaust gas. At the same time these exhaust gas which passes over the exhaust valve will consist of some unburnt particles of carbon, ash, silica, or alumina from the fuel. Under such circumstances, when these gases pass over the exhaust valve, they try to settle over the valve because of which it will cause the valve to burn or severe dent may be formed on the valve seat. At the same time, the heat to which this valve is subjected the strength of the material will reduce and hence there is a possibility of deformation of valve and valve seat. Hence taking into consideration all these aspects, it is necessary that proper care of the valve should be taken. If the engine is burning heavy fuel oil where the concentration of contamination such as ash, alumina, silica, unburnt carbon is bound to be more, for self cleaning purpose and uniform thermal distribution on the seat of the valve, it is necessary to make the valve rotate on the seat. Q.292 For a given fuel, a change in compression ratio will affect the ignition lag by which of the listed means? Ans. (b) An increase in compression ratio will decrease the ignition lag. As the compression ration increases, the quantity of air available for combustion also increases. At the same time, the temperature of air at the end of compression stroke will also increase. Due to this the fuel which is injected will vaporize faster and mix properly with the air and thus it will ignite instantaneously and thus reduce the ignition lag. Q.293 The lower section of the piston is called the skirt of the piston. This skirt of the piston is utilized in case of four stroke engine to transfer the Ans. athwart ship thrust to the liner wall and also guides the piston in the liner. In case of two-stroke engine, the skirt is very small and it is only in the initial stages that this skirt is utilized as a rubbing guide which are used only when the piston and liner are new. Q.294 What is the function of stationary parts of the engine? Ans. (c) To keep the moving parts of the engine in their proper relative position.
Engine moving parts are supported by the stationary parts. As long as the stationary parts of engines, such as bedplate A-frame and cylinder block, are rigid and firm, and they do not have movement relative to each other, in such circumstances it is possible to give a support to the rotating parts and to maintain the alignment between the different rotating parts so as to prevent the malfunctioning and breakdown of the engine. Hence the strength of the stationary parts is the most important aspect of engine performance. Q.295 Fuel oil discharged to the diesel engine cylinder is atomized at the Ans. (b) fuel injector nozzle tips. As the high-pressure oil passes through the fuel injector nozzle tips, which are having the very small holes, they are broken up into tiny particles. This process is called atomization. This is possible by passing the fuel through the holes of the injector under high pressure. Thus converting the pressure energy of the fuel into kinetic energy partly and thus breaking the fuel into tiny particles which is responsible for rapid evaporation and mixing with air in the cylinder and better thermal efficiency of the engine. Q.296 Proper dispersion of fuel in diesel engine cylinder is dependent on Ans. injection pressure, shape of combustion space and turbulence pattern in the combustion space. Proper distribution and mixing of the fuel with the air in combustion space will ultimately depend upon how finely the fuel is atomized which is carried out by injection pressure. Also the depth to which the fuel is distributed in the cylinder volume is also important so that it can be fully mixed with volume of air which is available and no pockets will be created. This is possible with the help of the pressure. At the same time, shape and geometry of the combustion space will also important part in the formation of sphere on top of the piston so that fuel can be mixed properly with the air. Also turbulence of the cylinder is also significant for proper combustion of fuel as the motion of air helps in proper mixing of the air and fuel. Hence for increasing the thermal efficiency of the engine all the listed factors play an important role. Q.297 In a diesel engine, crosshatch on the liner surface aids in (a) rapid ring sealing. (b) retention of lubricating oil. Ans. both (a) and (b). Cross hatch on the liner surface is honing. In this, there is cross honing so that it will form microscopic cells over the liner wall. These microscopic cells will help in retaining oil in the cell as well as it will use this oil for rapid sealing and spreading of the oil. The unevenness which is intentionally caused because of the honing will also be useful in the rapid weardown of the liner in running-in process. Due to honing the running and breaking in process of the liner will be shortened and hence better sealing between liner and piston ring will be achieved in the shortest possible time. At the same time, during this process of breaking and running, this tiny cells which are formed on the surface of liner due to honing will act like a oil retaining cells. Q.298 Why is it necessary to compress the air charge in the cylinder of a diesel engine? Ans. (a) To ignite the fuel. When the air is compressed in the cylinder of a diesel engine, it is compressed adiabetically. Due to this the temperature at the end of compression increases to the self-ignition temperature of the fuel. Under such circumstances, if the fuel is injected in a highly atomized condition inside the combustion space, the fuel will ignite immediately and convert the chemical energy of fuel into mechanical energy of the engine. Q.299 Which of the following statement concerning a closed type fuel injection nozzle is true? Ans. (c) The most closed nozzle open inwards under the pressure acting on the differential area of the needle valve. In the fuel injector, the needle sits over the seat by the spring pressure. When high-pressure fuel oil enters the fuel nozzle, it acts over the annular space and the differential area of the needle valve. As this fuel pressure acts over a greater area, an upward force is created which overcomes the downward force of the spring thus lifting the needle upwards. As the needle is lifted, the fuel will pass between the needle and the seat. It will further act over the needle area thus increasing the area of pressure acting and lifting it further up and because of this, the full lift of the needle is possible. Under such circumstances, the fuel under high pressure will pass between the needle and seat and inject through the nozzle holes. As soon as at the end of injection when the pressure drops, the spring force will be greater than the hydraulic pressure acting on the differential area of the needle and the needle will snap close because of the action of the spring. Q.300 In many diesel engines exhaust valves are being constructed with hollow stem filled with sodium in order to Ans. (c) assist in dissipating heat due to the extreme operating temperature. Sodium is a good conductor of heat and as it does not evaporates easily there is less possibility of gas or vapour pocket formation taking place and obstructing the cooling process. Thus when the stem is filled up with sodium the liquid sodium will transfer the heat in a better way and keep the valve cool. Hence in the older engine this method was utilized but at presently it is not utilized due to the development taken place in the material of the exhaust valve and the cooling of the exhaust valve seat with water. Q.301 Turbulence of the compressed air charge in the diesel engine cylinder increases Ans. (c) the efficiency of the engine. When the turbulence of the compressed air increases, it increases the movement of the air in the combustion. Due to this, the air sweeps the complete volume of the cylinder with that of the exhaust gas and hence all the exhaust gas from the cylinder will be removed and there will not be any gas in the cylinder. Thus the cylinder will be charged with pure quality of air. Secondly, this motion of air will aide in thorough mixing of air and fuel. Thus creating a homogenous mixture throughout the combustion space. Hence having a better combustion of the fuel and increase in the thermal efficiency of the engine. Q.302 Thin brass rings are inserted in the face of some chromium-plated piston rings to Ans. (a) promote the piston ring sealing in the cylinder. Piston rings are coated in order to prevent corrosion. At the same time, because of chromium coating the surface of the piston rings become hard and it takes long time to weardown and form a proper sealing between the cylinder wall and piston ring sliding surface. Hence there is a possibility of improper
sealing between the piston and liner surface and because of this blowpast of hot gases will take place. Also the chromium coating will increase the time for breaking-in and running-in period. Hence in order to have better sealing between piston rings and liner and give corrosion resistance to the piston rings, they are coated with chromium and an insert of brass is put inside the sliding surface so that the brass insert will wear down fast and have a proper bearing with the liner and provide a good sealing. Q.303 In the Otto cycle the oil fuel is ignited at what time and by what means? Ans. (c) Just before TDC by spark ignition. In Otto cycle, the compression pressure is very low and hence it is not possible to achieve the air temperature to that of the self-ignition temperature of the fuel. Hence it is necessary to ignite the fuel by means of spark ignition just before TDC. Q.304 The critical speed in diesel engine occurs when engine torque pulsation becomes Ans. (b) resonant with the crankshaft natural frequency. Engine torque pulsation takes place due to the units firing at different positions of the crankshaft. Lesser the number of units, less pulsation will take place and higher the number of units higher the pulsation will take place because the angular difference between the two firing strokes will be quite large. Also it will depend on the size of the flywheel. Bigger the flywheel, lesser will be the pulsation on the engine torque. In addition to this, there is a natural frequency of vibration in the crankshaft. If the natural frequency of vibration of crankshaft and torsional vibration coincides, these two frequencies will resonate and total frequency will be excessively high causing excessive vibration and damage to the engine. Hence the speed at which this happens is called critical speed. Q.305 A diesel engine piston ring pitting face is in direct contact with Ans. (b) cylinder liner oil film and bottom of ring groove. The sliding face of the piston ring will be in direct contact with the vertical face of the cylinder liner wall. In order to reduce the coefficient of friction between these surfaces and to avoid the scuffing and blowpast, it is necessary to maintain the lubrication between the surfaces which is carried out by cylinder lubrication or splash lubrication. Also when the cylinder is moving downwards, the pressure acts over the piston ring and it forces the piston ring radially outward against the liner walls. At the same time, it forces the bottom of the piston ring against the bottom portion of the ring groove. This sealing is also important to maintain the pressure behind the piston rings. If this sealing is not maintained, the piston ring will not seal properly and blowpast will take place. Q.306 The secondary function of the diesel engine piston ring is to Ans. (d) prevent excess lubrication from reaching the combustion space. The primary function of the piston ring is to forms a sealing between piston and the liner surface. One of its secondary function is to transfer the heat from the piston to the cylinder wall and thus keeping the piston cool. Also, it has to spread oil over the cylinder wall so as to spread oil uniformly over the wall and thus reduce the friction between the piston and the liner. Also due to this, the piston ring and liner wear is reduced. Also, if excess lubricating oil is given, it will be scrapped downwards by means of scrapper rings rather than by piston rings. Hence scrapper rings are more useful for scrapping the excess oil than piston rings and send it back to the crankcase. In case of two stroke engines, there are no scrapper. In such condition, the quantity of lubrication injected oil is so less that is only necessary to spread over the liner walls than scrapping. Q.307 The fuel is ignited in diesel engine cylinder by Ans. (c) heat of compression. In a diesel engine, air is compressed adiabetically so that by the end of compression the temperature and pressure of air increases which reaches about 250°C which is more than the self-ignition temperature of the fuel oil. Hence at this high temperature when the fuel injected being at highly atomized condition will ignite and produce power in the cylinder
Q.308 Barrel faced, tapper faced grooved and chromium platted are all types of diesel engine Ans. (b) piston rings These are different shape and coatings of piston rings. They are provided to spread the lubricating oil over the liner walls, to have a scrapping effect of excess lubricating oil or to reduce the corrosion effect of fuel on the piston rings. They provide sealing between the piston and liner and accelerate the process of sealing between the liner and piston rings when they in new condition i.e. to reduce the breaking and running in time. Q.309 Injectors that use heavy fuel oil must be cooled by water or light oil to Ans. (d) avoid carbonization of nozzle tips. When engine is burning heavy fuel oil it is necessary to cool the piston and nozzle tips. As the nozzle tips becomes hot the seat and needle looses its strength and because of this the proper sealing between seat and nozzle is not achieved and the fuel injector will start leaking. Under such circumstances even after the completion of injection, when the needle is sitted on its seat, some amount of minute drops of fuel will leak through nozzle holes and it will get accumulated on the nozzle tips. This accumulated oil will burn at high temperature and, due the local burning, form a carbon formation in the form of like a flower near the tip of the injector thus damaging the injector and fuel injection pattern. Hence it is necessary to maintain the temperature of the fuel injection nozzle tips within certain limit so as to avoid loss of strength and leakage of fuel through the tips. Hence cooling is an important factor of fuel injector nozzle burning heavy fuel oil. Also as the fuel oil itself is heated to high temperature and hence the cooling effect of the fuel oil is comparatively low than in case of diesel engine fuel injector burning diesel oil where the fuel oil injected during every stroke does the cooling job. Q.310 Which of the equal power diesel engine listed running at the same speed is least affected by the exhaust gas backpressure? Ans. (a) Four-stroke cycle V-type of engine.
In case of four-stroke engine, for every two revolution there is a power stroke so that the time between two power stroke is more so that the exhaust gas, which is discharged into the exhaust manifold, will get dissipated through the turbocharger before the next unit exhaust valve opens. Hence there is a less possibility of backpressure. In addition, if we are using V-type of engines, where there are two banks of cylinders, they are exhausting alternatively into the two banks of exhaust manifold. Hence there is a further delay in the exhausting of two cylinders in the same exhaust manifold. Hence less amount of backpressure is created in four-stroke V-type engines as compared to two-stroke engine with any configuration (in-line, or V-type). Q.311 In a diesel engine, the time taken to heat the fuel particles turned into vapors and bring about combustion is called Ans. ignition delay. The time from which the first particle of the fuel enters the combustion space through the fuel injector till the time the first ignition starts is called the ignition delay. During this process, the fuel is atomized, vaporized, mixed with the available air so as to forms a combustible mixture is called ignition delay. It will depend on the compression pressure and the quality of the fuel. If the compression pressure is low or the fuel quantity is bad, the ignition delay will be more. Higher the ignition delay there will be more amount of power loss due to afterburning effect. Q.312 In a diesel engine the contact surface of a piston compression ring are those in contact with (a) back of the ring groove. (b) bottom of the ring groove. Ans. (b) bottom of the ring groove. For sealing between piston ring and liner, the high pressure gases has to find its way behind the piston rings and has to force the piston rings against the cylinder wall. This pressure behind the piston rings is to be maintained in order to get the proper sealing between them. This is possible if there is proper sealing between the bottom of piston ring and bottom of the ring groove. If the sealing between them is not properly maintained, the gas which is acting from behind the piston ring will get leaked off. As this pressure is lost, there will not be sufficient pressure between the piston ring and the liner wall and the high pressure gasses will force the rings inwards will cause the blowpass and ring to collapse and break. Hence sealing between the between the bottom of groove and bottom of piston rings is also of prime importance. Q.313 The thermal energy produced by internal combustion engine is transformed into Ans. (d) mechanical energy. When the fuel burns on top of the piston inside the cylinder, it is the chemical energy of the fuel which is converted into heat energy. This heat energy and the pressure energy together is acting over the piston thus forcing the piston downwards, thus converting this energy into mechanical energy to the crankshaft. Thus it is the thermal energy which is converted into mechanical energy by the engine. Q.314 The reason that some two-stroke diesel engine piston rings are pinned to prevent rotation is Ans. (c) to keep the ring ends from catching in the scavenge ports. When the rings are pinned in the ring grooves, they are prevented from rotation. The rings can be initially set up in such a manner so that the ends of piston rings will not come in way of scavenge and exhaust ports and thus it will be prevented from being damaged because of getting entangled with the scavenge and exhaust ports and thus avoid their breaking of the piston rings. Hence in older engines the pinning was done to prevent the rotation of the rings and to keep the rings in their original position (at which they are fitted) when they are removed for overhauling and the rings are in new condition. In modern engines, the ends of the rings are turned slightly inwards so that when they expand due to heat, they will not get entangled with the scavenge and exhaust ports and will not break. At the same time, the rings are allowed to rotate in the groove to give the self-cleaning effect of the groove and to have the even distribution of heat over the entire circumference of the groove. Hence the pinning arrangement is no longer in use. Q.315 Oil control rings are designed with slotted holes to Ans. (d) permit excess oil to drain in the sump. The job of the oil control rings is to scrap the excess oil that has been splashed over the liner walls to prevent the excess lubrication oil consumption and to prevent the excessive burning of the lubricating oil into the combustion space. Hence it is necessary to scrap this oil. For this, the scrapping edges are provided onto the oil control rings. As the oil is getting scrapped, it should get drained into crankcase * and that is carried out by providing the oil control rings and the small holes in the grooves of the piston rings. Thus the oil being scrapped will find its way through these slotted holes in the grooves and into the crankcase sump. Q.316 Precombustion chamber differs from turbulence chambers as the precombustion Ans. (d) contains a small portion of the total clearance volume. Precombustion chamber is an extension of combustion chamber and fuel is injected into it. Precombustion chamber is very small chamber in which the fuel is injected. When firing takes place in the precombustion chamber, due to restricted area, a high-pressure wave is developed. This forces the remaining fuel and air mixture in the combustion chamber thus having a proper mixing of fuel and air in main combustion chamber and have better burning of fuel in the main combustion chamber. Presently, the precombustion chamber is utilized only in very small engines where the compression pressure is not sufficient to ignite the fuel efficiently. Q.317 The primary purpose of oil control rings on a diesel engine piston is to Ans. (c) prevent excessive lubricating oil consumption. In a four stroke trunk piston diesel engine the liner lubrication is carried out with the help of splash lubrication and large quantities of lubricating oil is splashed by the waves of the crankshaft onto the liner walls. This oil is necessary to spread properly on the cylinder liner and farther lubrication between the piston rings and liner wall. At the same time, excess oil is returned back to the crankcase otherwise it will get burnt along with the exhaust and lubricating oil consumption will increase. At the same time, fouling of turbocharger and manifold will also take place. Inorder to avoid this things, oil control rings are provided the job of which is to scrap the oil and send it back to the crankcase and to avoid excess consumption of the crankcase oil. Q.318 The compression ratio of a diesel engine refers to the ratio between the Ans. (b) cylinder volume at top dead center and that of the bottom dead center.
At the bottom dead center, there is large amount of volume available in the cylinder and air is charged into the cylinder at this volume. If the piston moves upward, due to the proper sealing of the compression space, the air will get compressed. At near TDC, the volume of air will be reduced but its pressure will increase. The ratio between the cylinder volume at TDC and BDC is called compression ratio. Q.319 In describing basic diesel engine operation the term events refers to (a) production of high pressure gases . (b) removal of expanded combustion gases. (c) admission of air to the cylinder. (d) All of the above. Ans. (d) All of the above. In any diesel cycle, there are four events: fuel firing and high pressure gas is produced which will drive the piston downwards thus producing the power stroke. At the end of power stroke, the exhaust valve will open and exhausting will take place by which the exhaust gas will be removed which is the exhaust stroke. After completion of exhaust stroke, when the piston is moving downwards the cylinder will be filled with fresh air charge which is called the suction or induction stroke. This continues till the piston reaches TDC. When the piston is moving upwards all the valves are in closed condition the air which is available on top of the piston will get compressed to produce high pressure and high temperature air in which the fuel will be burnt. This is the fourth event or the compression stroke. All these events together will form one cycle. This cycle can be completed in one revolution (two-stroke) or two revolutions (four-stroke) of the engine. Q.320 The ratio brake horsepower to the indicated horsepower is its Ans. (b) mechanical efficiency. Mechanical movement of the piston and the mechanical movement taking place inside the cylinder and the bearings will result in mechanical loses. The power which the output of the engine will be the difference between the power which is generated inside the engine i.e. indicated horsepower minus the mechanical losses. Hence it will be ratio of losses and indicated loss power. The output of the engine is called the brake horsepower. Hence the ratio of brake horsepower and indicated horsepower will determine the mechanical efficiency. More the brake horsepower more the engine will be efficient from the mechanical point of view. Q.321 The slots are provided in the diesel engine piston oil scrapper rings to remove excess oil from the cylinder liner and to scrap and send it back to the crankcase.317 Q.322 The function of piston compression rings used in diesel engines is (a) to seal the space between the piston and liner. (b) transmit the heat from the piston to the liner. (c) reduce the amount of combustion gases blowby. (d) All of the above. Ans. (d) All of the above. The primary function of the piston rings is to form a sealing space between the piston and liner. This is carried out by the seal between the piston ring sliding surface and the liner. This is achieved by the gas pressure finding its way behind the piston and forcing the piston rings against the cylinder liner. Thus forming the gas-tight combustion space. At the same time, when the piston is in hard contact with the liner and the piston. The piston is always running at higher temperature than the liner, hence the piston rings does the job of transmitting heat from the piston to the liner. Also because of the sealing between the liner and piston, gas-tight combustion space is formed which prevents the blowing out of the high-pressure gas through the gap between the liner and the piston rings. All these are carried out efficiently by the piston rings. Q.323 How are the temperature and pressure affected in a diesel engine cylinder during compression? Ans. In the diesel engine adiabatic compression takes place because of which the pressure of air inside the cylinder increases. Theoretically as there is no heat transfer the temperature also increases to the surrounding hence they both increase during compression. Q.324 Modern diesel engine using mechanical fuel injection operates on combustion cycle which is Ans. a combination of constant volume and constant pressure. Q.325 If the compression ratio is increased on any diesel engine Ans. (b) the thermal efficiency will increase. Thermal efficiency will depend upon how efficiently the fuel is burnt and efficient burning of fuel will depend on the compression ratio. If the compression ratio is high, more amount of air will be available which will be higher temperature. Hence more amount of air at higher temperature will give better combustion and minimum ignition delay. Thus maximum pressure will be developed in the cylinder near the TDC and because of this, automatically the thermal efficiency will increase. Q.326 The piston compression rings used in diesel engine functions to Ans. (c) seal the combustion space from the crankcase. In case of trunk piston engine, the bottom of the piston is directly connected to the crankcase and top of the piston is connected to the combustion space. Hence the piston ring does the sealing between the liner and the ring surface and thus forming a gas-tight combustion space above piston ring which if properly sealed will prevent the blowing of the hot gases from the combustion space into the crankcase. Q.327 At bottom dead center the centerline of the connecting rod usually coincides with the Ans. (c) centerline of the cylinder. At the bottom dead center position the connecting rod will be exactly vertical and the piston will be at the lowest position of the cylinder liner. Under this position, the centerline of the connecting rod and the cylinder must coincide to get the proper alignment between the liner and the piston. If this does
not take place, there is a possibility that the liner will weardown on one side and the piston rings will weardown on one side and the piston may get seized in the liner. Hence the most important part is to have coincide the liner and connecting centerline when at TDC of BDC. Q.328 A diesel engine exhaust valve spring is under compression when (a) the valve is open (b) the piston is at TDC (c) the valve is closed (d) All of the above. Ans. (d) all of the above. When the exhaust valve is opened the spring is forced downwards by the rocker arm and the spring is under compression. When the piston is at TDC, the exhaust valve is opened and the spring is under compression. When the valve is closed, due to the initial compression which is given to the spring it gives equal upward force to the valve and keeps the valve in closed condition. Hence even under closed condition, the spring is under initial compression given during the boxing up of the valve and to make sure that the valve sits on the valve seat properly and without any leakage. Q.329 The piston is said to be at TDC when it is Ans. (d) nearest to the cylinder head. When the piston is moving upwards and the crankpin is at topmost position, at this point it will start taking turn and start moving downwards. Hence the position where it momentarily stops and changes its direction is the position of TDC. As there is no possibility of piston moving upwards further than this position, there will be minimum space between the cylinder head and the piston top. Q.330 The purpose of flywheel is to Ans. (a) provide energy to operate the engine between the power pulses. In a diesel engine, the firing is not continuous. The firing of the fuel in different units depends upon the crank position and it is in the form of pulse. Hence depending upon the number of units, and whether the engine is two-stroke or four-stroke, there is time difference between the firing of fuel in the two consecutive units in the firing order. In between two firing strokes, there is a time delay. During this time, the engine will try to slow down because lack of power available on crankshaft. Hence there will be fluctuation in the rpm of the engine. In order to avoid, this a heavy flywheel is fitted on the crankshaft. This flywheel will store the energy during the firing stroke when the energy level is above the average level and it will give back the energy to the crankshaft when the energy level is below the average when no unit is firing thus maintaining a constant torque and constant rpm over the entire load range of the engine. The size and weight of the flywheel will depend upon the number of units and whether the engine is four-stroke or two-stroke. If the number of units is more, the flywheel will be smaller size as compared to engine with less number of units. Q.331 A 7-cylinder two-stroke cycle single acting diesel engine with cylinder indicated horsepower calculated to 1350kW and the brake horse power measured at 7466kW has a mechanical efficiency of
Q.332
Q.333 In diesel engine pistons are attached to the crankshaft by Ans. (c) connecting rods. The connecting rod or the crosshead and connecting rod assembly will convert the reciprocating movement of the piston into rotary movement of the crankshaft with the help of eccentrically situated crankpin on the crankshaft. Q.334 One advantage dry cylinder liner used in a diesel engine is Ans. (d) procedure to replace it is simpler than the wet liner. As in case of dry liner, there is no cooling provided and hence there is no provision for o-rings at the bottom of the liner and the jacket hence it is easier to replace it than wet liner Q.335 Vibration from the diesel engine and engine driven equipments are isolated from the hull structure by Ans. (d) flexible engine mountings. Flexible engine mountings consist of shock absorbing material, such as rubberized cork, so that it will absorb the vibration created by the machines and those vibrations will not be transferred to the ship structures or vice-versa. They are known as elastomount and they have got the property of dampening the vibrations. Only care should be taken while tightening them that they should not be overtightened; otherwise they will burst. Hence the tightening of the bolts should be carried out with the help of torque
Q.336 Which of the following statements in a fuel atomization in a diesel engine cylinder is correct? Ans. (b) Greater the atomization lesser the penetration. For greater atomization, it is necessary to break the fuel droplets into microscopic particles. When they are reduced in such manner their weight reduces and hence their inertia also reduces thus reducing the force at which it will drill through the dense atmosphere of air reduces. Thus the potential energy of the particle reduces and it cannot penetrate the dense atmosphere of the air in the combustion chamber. Hence as the atomization increases the penetration decreases and vice-versa. Q337 The most rapid period of fuel injection occurring in a diesel engine cylinder should begin just before the piston reaches the TDC position Ans. (d) should be completed after the TDC.
In diesel engine, the best possible efficiency is obtained if the fuel is burnt partially at constant volume and constant pressure. For achieving this, the fuel injection and ignition must start just before TDC and it should completed after the TDC. During this time, the quantity required as per the load should be injected and burnt efficiently. If the complete combustion takes place before or after TDC, the thermal efficiency will reduce. Q.338 When comparing different fuel for different engines the ignition quality of diesel fuel oil becomes a less critical consideration as Ans. (d) the engine speed decreases. As the engine speed decrease the time for the fuel to atomize evaporate and mix with air to form a combustible mixture is ample. When ample time is available, it is possible to burn lower grade of the fuel with low speed engine. Hence when low grade of fuel is utilized, less critical consideration is given to the speed of the engine because lesser the speed of the engine inferior is the quality of fuel which can be burnt efficiently. Q. 359 The power developed by slow speed main propulsion diesel engine depends upon the: (a) quantity of air takes in and retains in the cylinder during a given time period. (b) proportion of trapped air that is utilized in the combustion process. (c) thermodynamic efficiency of the engine cycle. (d) All of the above. Ans. (d) All of the above. The power developed by diesel engine depends upon the how efficiently the fuel is burnt. The quantity of the fuel that burns will depend upon the quantity of the air which is available at the end of operation stroke. This depends upon the how much quantity of air taken inside during the suction stroke. At the same time, it will depend upon the efficiency. Secondly when air is compressed whatever the quantity of air that is retained at the end of the operation is also important factor. Otherwise if the piston rings are leaking or broken or stuck up, the quantity of air is taken during the induction stroke will be higher. The retention of the air at the end of compression stroke will be low. Under these circumstances, the power developed will be reduced. Hence the power develop will depend upon the quantity of air taken during the induction stroke and quantity of air which is available at the end of compression stroke in the combustion chamber. Lastly, it depends upon the thermodynamic efficiency of the engine because higher the thermodynamic efficiency of the engine, the more efficient burning of the fuel and more power develops for the same quantity of fuel. Hence it can very well be said that all three are most important factors which will decide the power development. Q.340 An indicator card or pressure volume diagram shows graphically the: (a) relation between pressure and the volume during the stroke of the engine. (b) relation between pressure and volume during one cycle of the engine . (c) volume of the engine. (d) the compression ratio of the engine. Ans. (b) relation between pressure and the volume during one cycle of the engine. It shows the relationship between the pressure and volume during one cycle of the engine. When indicator card is taken, it is drawn with x-axis as volume and y-axis as pressure. It is drawn for the complete one revolution of the engine. So in case of a two-stroke engine, it is a two stroke it will represent on the indicator card. Hence it is a relationship between a volume and pressure for a complete cycle of a engine because by measuring the area of the diagram of the indicator card, you will be in a position to estimate the power development in the engine. Power development of the engine is taken into consideration for the first cycle and not for the first stroke because for every cycle there may be two strokes or four strokes. The power developed will be considered for entire cycle, not for particular stroke as such. Q.341 The cylinder lining constructed as an integral part of a block are characterized by which of the following disadvantages. (a) They conduct heat poorly (b) They are expensive (c) They cannot be replaced and (d) They require special tools for removal. Ans. (d) They require special tools for removal Now when the cylinder liner becomes an integral part of a cylinder block, it is not possible to replace the liner in case it is worn out. It is a complete part of the cylinder block and in such condition, you may have to replace a complete cylinder block or recondition the complete cylinder block in the way of liner bore. Q.342 One end of diesel engine cylinder is sealed with the hot air and the other end by the Ans. (b) piston. Air is present on top of the piston and this air is compressed between the cylinder head and top of the piston thus forming a combustion space holding high pressure and high temperature air where the fuel injected will burn and produce the power. Hence the sealing for the compressed air is possible because of the heat which is existing between the cylinder head and the piston. Q.343 Where is a charge for an air starting system is stored. Ans. (b) Presurise tank. For starting the engine, it is necessary to have large quantity of air at a different minimum pressure in order to get a large quantity of air at different pressure. It is therefore necessary that the compressor should be in a position to store the quantity of air and hence it will be easily available. Also, as per the regulation it is necessary to have 12 consecutive starts, 6 ahead followed by 6 astern alternatively. Under such circumstances, unless storage of air is not available, it is not possible to the starting to many starts for the engine. Hence starting air is highly essential from the of manually and safety point of
view of the engine so that it is possible to maneuver the ship continuously without even if it has designed very moderate size of the compressor. Thus the air bottle acts as source of stored energy for the engine to turn during starting.
Q.344 Maximum power of the diesel engine is attended Ans. (b) at rated engine speed. When the engine is designed, all parameters are considered and calculated. During this time, the rpm is also taken into consideration for calculating a maximum power which can be developed and given by Power developed = PLAS. In this, rpm is most the important part. If rpm is low and the power developed is high, it may possible that the engine may get thermally overloaded and maximum pressure may exceed the safe limit. It may also give the cracking to the engine part. Hence it is necessary that the engine maximum power output can be calculated and attended only at the rated rpm and not at the lower rpm. Q.346 During fuel injection period the fuel pressure must exceed cylinder gas pressure to Ans. (a) insure penetration and distribution of fuel in the combustion chamber. When the fuel is injected by opening the nozzle of the fuel injector, the flow of the fuel is into the high-pressure air. This high pressure air will try to oppose the entry of the fuel. Hence the fuel, which is entering, has to drill through the dense atmosphere to move in further away from the tip and mix up with the complete quantity of the air available in the combustion space. For that purpose, the pressure at which it is injected should be always more than the compression pressure in the cylinder so that penetration deep into the dense atmosphere is possible. Q.348 Telescopic pipe through the piston of a large of a slow speed main propulsion diesel engine are designed to prevent Ans. (c) contamination of the lubricating oil by water. As the telescopic pipe has to convey the water to the piston, it has to pass through the crankcase or scavenge space. By having the telescopic pipe, it is possible to convey between the stationery part of the pipe to the reciprocating part of the compressor. This is possible with the help of telescopic pipe and the gland. Q.349 The exhaust system for a turbocharged two-stroke diesel engine functions to (a) discharge exhaust gases and smoke. (b) permit energy to turbocharger. (c) reduce the engine room noise. (d) All the above. Ans. (d) All of the above. As the exhaust valve opens, the high pressure exhaust gas, which is slightly higher than the atmospheric pressure, will get release from the exhaust from the cylinder. It will pass through the exhaust valve towards the turbocharger. Thus it does the function of removing the exhaust gas and the smoke from the unit. Secondly, this energy which is available in the exhaust gas can be utilized which may be combination of pressure and heat energy can be utilized for driving the turbocharger so as to supply the pressure as per the engine. Thirdly, when the exhaust gas is released from the exhaust of the engine unit it enters the exhaust manifold, which is also bigger size, dampening of the exhaust gas will take place in the manifold and because of this it will reduce the noise created by the engine. Q. 350 Which of the diesel engine exhaust muffler listed is usually equipped with spark arrester? Ans. (c) A dry-type exhaust muffler. Dry-type exhaust muffler has been fitted with a flame or spark arrestor in the form of a copper. This will cool down the exhaust gas coming out and prevent any flame or spark from the engine to get distributed on the desk. Q.351 When monitoring diesel engine performance the most useful instrument to be used is Ans. (b) exhaust gas pyrometer. Exhaust gas pyrometer gives rough estimation of the power developed by the engine. Higher the power developed by the engine more the exhaust temperature. At the same time, it can also monitor any discrepancies or problems encountered by the engine. If the fuel valve is dripping or the fuel nozzle is not spraying properly or the fuel ignition timing is not proper, even then large variation in temperature will take place. If the pistol rings are leaking, immediately compression pressure and the peak pressure will reduce but the exhaust temperature will be high. The exhaust temperature can be considered as a most important tool to determine the performance of the engine and diagnose the possible cause of the malfunctioning of the engine or dropping of the performance of the engine. Q.352 The exhaust system of the diesel engine usually designed to remove exhaust gases and to Ans. (d) muffer the exhaust gas noise. The pressure waves when the exhaust gas is released from the cylinder has been dampened by the use of muffler where in the direction of the exhaust gas changes. This causes the dampening of the noise wave and causing the total dampening of the exhaust gas noise. Q.353 The water jacket is placed around the exhaust manifold of propulsion diesel engine to Ans. (a) reduce the radiation of the engine room. When the exhaust gas manufold is water cooled, heat is carried out by water and less amount of heat is radiated from the exhaust manifold into the engine room. Thus keeping the engine room cool. At the same time it protects the engine from exhaust manifold fire because the temperature of the exhaust manifold here is less than 240º C thus the chances of fire is reduced and hence it acts like a safety device. Q.354 In a two-stroke cycle diesel engine the exhaust gases are expelled from the cylinder by
Ans. (c) pressure of the fresh air charge. In two stroke cycle engine the exhausting is carried out with the help of incoming air and not by the movement of the piston as in case of four stroke engine. When the piston is moving down and when the exhaust valve opens, after certain lapse of time the scavenge ports open. By time the scavenge ports open, the gas pressure inside the cylinder has dropped sufficiently. When the scavenge air under the pressure higher than the exhaust enters the cylinder through the scavenge ports, at the same time, the exhaust valve is also opened. This incoming air from the exhaust manifold will drive the exhaust gas out through the already opened up exhaust valve.
Q.355 Diesel engine muffler or silencers reduce the engine exhaust noise by Ans. (d) reducing the exhaust gas velocity. When the exhaust gas is passed through the muffler it is made to pass in the zigzag manner. When passing in this manner, everytime it changes the direction and it’s velocity drops. As a result, vibration caused by the exhaust gas also reduces and hence the noise is dampened and the muffler effect is carried out. Q.356 is a repetition of Q.354. Q.357 Exhaust gases in two-stroke cycle diesel engine are discharged through Ans. (c) exhaust ports or valve. When the piston is moving downward and the exhaust ports are cut on the liner. As soon as the downward moving piston uncovers the exhaust ports, the exhausting takes place. This will continue till the time the piston when it is moving up again closes the exhaust ports. This process is the combination of exhausting and scavenging. In case of the engines in which valves are fitted in the cylinder head, the exhausting is carried out by opening the valve when the piston is moving down. As soon as the valve opens the pressure in the cylinder drops and further movement of the piston downward opens the scavenge ports through which the scavenge air enters the cylinder liner and forces the exhaust gas upward through the already opened up exhaust valve.
Q.358 The disadvantage of a two stroke cycle diesel engine is a Ans. (b) high working temperature of a piston and cylinder. In two stroke cycle diesel engine, for every revolution there is one power stroke. The amount of fuel engine burnt is more and amount of power develop is more and because of this the liner and the temperature is more. In general, the exhaust temperature is lower than the four stroke engine because of the mixing of the incoming scavenge air thus giving a cooling effect on to the outgoing exhaust gas. The main temperature is always less than the maximum temperature which will be encountered. Q.359 Many diesel engines have piston with concave heads to Ans. (b) increase the air turbulence and improve the fuel mixing. The formation of concave top of the piston, in combination with the cylinder head, it forms a spherical combustion space between the piston and cylinder head. Because of this spherical geometry of the combustion space, good turbulence is achieved due to which there is good mixing of fuel and air, resulting efficient burning of the fuel and the thermal efficiency increases and the fuel consumption can be reduced. At the same time, concave surface of the piston top acts as a self-supporting surface even with the thinner wall of the piston crown and without any support from the cooling water side, it can withstand high temperature and high pressure without any external supports. Q.360 In a single acting diesel engine, the cylinder liner area that is most difficulty to lubricate is Ans. (c) the top circumference Combustion of fuel takes place near the TDC. Hence this portion of liner is very hot. At the same time, it is subjected to large amount of pressures from the piston rings because of high-pressure gases acting behind the piston. Also whatever lubricating oil reaches this place gets evaporated quickly because of temperature and the contact pressure between the piston rings and cylinder liner. Hence taking into consideration all these aspects it is very difficult to lubricate this top portion. On the contrary, there is a possibility of loss of lubricating oil and scuffing may takes place.
Q.361 repeated Q.362 Which of the following design features will reduce the possibility of overheating the top compression ring of cast iron piston? Ans. (d) Heat damp design is sometimes used in the piston head. Due to the heat damp design which is incorporated in the piston, the piston rings are not directly subjected to heat from combustion. As a result, the life of the piston rings increases and lubrication between the liner and piston rings increases and the weardown of the piston ring and the liner reduces. This is used to prolong the life and overheating of the top piston ring. Q.363 Which of the following statements concerning the factors affecting ignition delay is correct? Ans. (b) An increase in coolant temperature will decrease the ignition delay. The coolant surrounds the liner and it takes out all the heat from the compressed air at the end of compression stroke through the liner wall. If the jacket cooling water temperature is maintained at higher level, the heat transfer from the compressed air to the cooling medium reduces automatically. When
this happens, the temperature of the air at the end of compression stroke remains at higher level. If the air temperature remains at higher level, it helps in fast evaporation of fuel and good mixing of fuel and air instantaneously and ignite it. Hence the delay period is reduced. Q.364 Which of the following statement is a description of combustion cycle? Ans. (c) The heat of process which produces the force to initiate the movement of engine parts. Internal combustion engine is a heat engine. In this, the heat of combustion is converted into mechanical energy. For this conversion, the increase of heat energy will increase the pressure and the heat energy is converted into pressure energy. This pressure energy acts on the piston and moves the piston downwards and converts into mechanical energy. During combustion cycle, the heat generated is converted into mechanical energy on the crankshaft. Q.365 In a large slow speed propulsion diesel engine the force applied to the engine piston is Ans. (c) against the crosshead during the power and compression stroke. During two-stroke cycle engine compression for every revolution there is one compression and power stroke. During the power stroke the piston is moving downwards under the force of high-pressure gases produced on the top of the piston due to the combustion of fuel. Hence the force acts downwards and it is taken by the bottom half of the crosshead bearing. When the piston is moving upwards, during compression stroke, there is another half revolution of the engine and the piston is moving towards TDC. During this time, the exhaust valve is closed scavenge ports are closed and the air on top of the piston gets compressed so that next power stroke can commence. Hence during the compression stroke, because of compression of the air, the force acts downwards on the crosshead bearings. As both, during compression as well as power stroke, the force acts downwards, it is difficult to lubricate the crosshead bearing Q.366 One of the advantage in the use of dry liner over wet liner Ans. (c) there is likely leakage of water into the combustion space, In case of dry liner, there is no cooling medium involved around the liner or in between the liner and cylinder block. As there is no cooling medium involved, there is no possibility of water leaking into crankcase and contaminating the lubricating oil, as in case of wet liner because in wet liner there are O-rings which seals the water space around the liner with that of the crankcase. Q.367 Which of the construction technique listed is used on new piston rings to facilitate running or seating? Ans. (a) Special ring facing such as thin bearing surface of anti-friction metal. When the piston rings are new, their sliding surfaces are not even which may cause long time to get properly worn out and having a proper bedding between the liner and the piston rings. Hence there is a possibility of blowpast and scuffing may take place. For this purpose, it is necessary to increase the cylinder lubrication when new piston rings are used and running in is carried out. Inorder to accelerate the running-in process and to have better sealing between the piston rings and liner, the piston ring sliding surface is coated with anti-friction material such as tin-aluminium, aluminium-bronze and nickel-graphite. These materials being soft will weardown faster and will form a good seal between the piston ring sliding surface and liner surface. This coating will get worn out over the number of hours slowly, and in the meantime, there will be proper bedding of liner and piston ring because the liner will worn out to form a good surface sealing between the liner and the piston ring. Hence by providing the anti-friction coating on the sliding surface, the running in time will be reduced. At the same time the requirement of cylinder lubrication is reduced. Q.368 In diesel engines the four basic events: suction, compression, power and exhaust are formed once in Ans. (a) one complete revolution in two-stroke engines and two revolutions in four-stroke engines. In two-stroke cycle engine, the complete cycle is completed in two strokes or one revolution of the engine. For this purpose, the four basic events are completed in one revolution or two stroke so that, for every revolution there will be one power stroke. Q.369 The intake air ducting in diesel engine should not drop from an area on deck where flammable vapours released from tanks are present because Ans. (b) the engine may overspeed and normal governor or overspeed trips will not be able to stop or secure the engine. When engine is drawing air rich in hydrocarbons, it acts like additional fuel to the engine. Thus whatever the governor has allowed the fuel pump to admit, in addition to the fuel vapours present in air suction, will also act as additional fuel. This fuel will cause the overspeeding of the engine. Under such circumstances, even though the governor will try to reduce the engine speed by reducing the fuel, the fuel coming along with air intake, on which we don’t have any control, will continue to burn and the engine will rotate at the overspeed irrespective of the governor cutting off the fuel supply. Hence the governor will not be able to control the speed. Even if the overspeed trip operates and cuts off the fuel supply, as the engine is still rotating, it will draw air from the atmosphere which is rich in hydrocarbons and continue burning inside the cylinder and cause the continuation of running of engine irrespective of overspeed trip. Q.371 The purpose of delivery check valve used in the diesel engine fuel injection jerk pump is to (a) assist in quick cut-off of fuel injection . (b) prevent fuel oil backflow from the injection pump. Ans. (b) prevent fuel oil flow from the injection pump. Quick cutting off of the fuel injection is carried out with the help of seal valve or the helix cut on the side of the fuel injector pump. Hence the delivery valve will not come into picture. Delivery valve will only prevent the backflow of the fuel from the fuel pump and fuel valve into the suction side of the fuel injection pump. Q.372 In a diesel engine, the advantage of precombustion chamber over the open-type combustion chamber is that (a) it permits coarser fuel atomization (b) it allows low fuel injection pressure Ans. Both (a) and (b) possible. As the precombustion chamber is a small chamber hence the atomization is not proper. The quantity of air entered with which the amount of fuel has to mix up is also very small and proper mixing is not done. When precombustion takes place, the ignition wave, formed because of precombustion, will
form a turbulence effect in the combustion chamber that will allow the proper mixing of air and fuel. Hence even though the atomization in the initial stage is not proper or it is coarse, it will not have much affect on the performance of the engine. As the precombustion chamber is a small chamber, there is no necessity of penetrating the air deep into the chamber and hence requires low injection pressure as compared to open type combustion chamber as the depth of penetration is low. Q.373 A large slow speed main propulsion diesel engine must be operated with one cylinder secured when the engine stops the affected cylinder stranded in the position preventing the engine from being started. Which of the following actions must be taken? Ans. (c) Admit the starting air in the position opposite to the desired direction. There is a possibility that when one unit is cut-off the maneuverability of the engine reduces and when the engine stops it stops in such a position that the affected unit may come under the starting position. Under such circumstances, air will not be admitting to that unit and it is not possible to start the engine. Under such circumstances, it is better to reverse the engine i.e. to start the engine in reverse direction, some other unit may come under starting position and air will be admitted to the unit and the engine can be started. When the engine starts, it must be stopped and restarted in the desired direction. There is a possibility that the engine might be stopped in such a position that air will not be admitted to the affected unit but air can be admitted to other units and the engine can be started in the desired direction. Q.374 The average exhaust temperature of a two-stroke cycle diesel engine with a turbine-driven turbocharger is lower than a similar fourstroke diesel engine running at equal load because Ans. (d) the opening of two stroke cycle diesel exhaust ports or the valves occurs much later than in case of four stroke cycle diesel engines. In two stroke cycle diesel engines, the exhaust opens much later than the four-stroke cycle engine. Hence the speed of the engine is low, and hence to get more amount of power, more amount of heat energy is released from the exhaust gas on top of the piston to convert into power. Hence when the exhaust valve opens late, as in case of long and superlong stroke engines, the temperature has been sufficiently dropped down so as to have lower temperature at the exhaust valve. Q.375 Oil control rings used in two-stroke diesel cycle engines are located near the bottom of the piston skirt inorder to Ans. (c) keep the excess oil away from the intake and exhaust ports. In two-stroke engines, the oil rings or scrapper rings are provided so as to distribute the oil over the surface of the liner rather than scrapping effect which is required in case of four-stroke engines. Hence the oil should be spread upwards over the liner walls and it should not get washed away into the exhaust or scavenge ports. Hence the oil control ring is provided. They increase the liner area covered by the oil film because the lower scrapper rings, which are provided, will spread the oil over the entire length of the liner and hence it will cover the entire length of liner. These rings were provided in the older MAN engines where there was long skirt was present and because of proper spreading of lubricating oil over the liner, it was possible to have some amount of lubricating oil between skirt and liner. Thus when the skirt and liner have a mechanical contact the friction is reduced because of the presence of the oil film but in modern engines these rings are no more utilized. Q.376 Which of the following statement represent the function of the compression rings installed in the top of the diesel engine piston? Ans. (a) Transmit the heat from the top of the piston to the cylinder liner. The piston is always running hotter than the cylinder liner and the cooling of piston is a difficult task. Hence when the piston rings are keeping sliding contact over the liner surface, some amount of heat is also transferred from the piston rings to the liner. While doing so, the piston gets itself cooled and is prevented from overheating and this reduces the piston ring weardown. Q.377 In a diesel engine the time period between the fuel injection and fuel ignition is usually defined as Ans. (d) ignition delay. It is the time difference from the time the fuel is injected into the combustion space till the time the actual fuel firing takes place. This will depend upon quality of fuel, compression pressure and the atomization and penetration of the fuel.
MOTOR OPERATION Q.378 A loose crosshead plunger assembly in a metering or proportionating pump will cause Ans. (c) rapid wear on the plunger packing. If there is excessive clearance between the plunger and barrel, there will be radial movement of the plunger in the barrel due to which the packing between the plunger and barrel will get worn out on one side, thus causing the damage and rapid weardown of the packing. Hence this plunger clearance has to be kept as low as possible and the plunger and barrel assembly should be replaced when the clearance exceeds the limit. Q.379 In a naturally aspirated diesel engine the volume of air intake is directly related to the engine Ans. (b) valve size. Valve size will govern the cross sectional area of the intake port. If the valve assembly is of bigger size more area will be available for the air to enter and less amount of resistance will be available for air to enter because of which the quantity of air drawn will be more. Hence volumetric efficiency of the engine will increase. Q.380 A naturally aspirated diesel engine running at full throttle will have an intake manifold pressure Ans. (a) slightly less than the atmospheric pressure. When the piston is moving downwards in a naturally aspirated engine and the air intake valve is open, air enters with a velocity inside the cylinder. As a result, there will be movement of air which will increase the velocity of air which passes through the air intake manifold to the cylinder. When the velocity increases, as per Bernoulli’s theorem, there is a pressure drop. As the pressure in the intake manifold is lower than the atmospheric pressure, the atmospheric pressure will drive the outside air into the intake manifold and hence to the piston top through the opened inlet valve.
Q.381 An increase in air inlet manifold pressure of a diesel engine will result in a Ans. (c) decrease in fuel consumption per kW. When large amount of air is admitted into the cylinder of a engine under pressure the large quantity of air can burn large quantity of fuel in more efficient manner. Hence complete and efficient combustion of fuel is possible because of the availability of larger quantity of air. Due to this, the thermal efficiency of the engine increases and thus quantity of fuel consumed per kW per hour reduces i.e. the specific fuel oil consumption of oil reduces. Q.382 On a large diesel engine installation, crankshaft axial alignment is maintained by the Ans. (b) engine thrust bearings. In a large diesel engines, which is used for propulsion purpose, because of the axial thrust created by the propeller the entire engine shafting will get pushed inwards inside the engine. Due to this there will be misalignment with the piston, piston rod, connecting rod and bottom-end bearing positions. At the same time, there is a possibility that due to the axial movement of the crankshaft, the crankshaft web will move and touch the transverse girder of the bedplate thus causing damage to the bedplate. Q.383 The crankcase of many diesel engines are kept under slight vacuum Ans. (c) reduce the risk the explosion. If the crankcase is kept slightly under vacuum the quantity of air in the crankcase reduces. Under such circumstances, if the fuel mist is formed it will be rich and it will not fall within the lower or higher explosive limit. On the contrary, it will be on the higher side of the explosive limit and hence there is less possibility of it getting ignited. Q.384 Maintaining a lowest scavenge air temperature at all times is not recommended due to the possibility of Ans. (c) the formation of excessive quantity of condensate. If the air temperature is dropped too much then excessive condensate will be formed which will interfere with proper burning of the fuel. At the same time, it will wash out the cylinder lubrication. This condensate, along with the sulphur present, will burn and form sulphuric acid. Hence it will give more amount of wear because of corrosion. At the same time, the scavenge spaces and the lower part of the piston may get rusted. Q.385 On a turbocharged medium speed diesel engine which of the following problems is an indication of restricted air inlet passage? Ans. (a) Engine is hard to start. In case of turbocharged engine, if the inlet passage is choked or the turbocharger air suction filter is choked, at the time of starting or during running, sufficient amount of air will not reach the engine cylinder and a point may reach that the combustion will not be possible and thus the engine will have difficulty in starting on fuel. It will turn on air, but as soon as it changes on to fuel, because of lack of availability of air it will stop. Q.386 An indication of a diesel engine air intake being partially clogged is Ans. low firing pressure and high exhaust temperature. As the air intake is partially choked the quantity of air admitted during the suction will be restricted. If the quantity of fuel remains same, the fuel will be starved off the air and in such condition the fuel will not burn fully. If the fuel does not burn in proper time, the peak pressure will drop but this fuel burn during the exhausting time and there is a possibility that the exhaust temperature will go up but the peak pressure will decrease. If the air inlet filter is excessively choked, peak pressure and the exhaust temperature will also drop down because the available air will be so low that it will not be possible for the fuel to burn during exhaustion stroke also. Q.387 The lower control on a diesel engine is accomplished by Ans. (b) rotating the fuel injector pump plunger. By rotating the fuel injector pump plunger, the effective delivery of the pump is changed. This means the total time at which the fuel is injected and the total quantity of fuel injected will automatically depend upon the helix position on the fuel pump plunger surface with that of the fuel pump suction port. This position can be changed by rotating the plunger by means of rack and pinion arrangement. This rack is controlled by the governor which is controlled by the load of the engine. Q.388 Operating a diesel engine for a prolonged period with a closed fresh water cooling system at temperature lower than the normal designed temperature can cause Ans. (a) formation of sulphuric acid. If the engine is burning heavy fuel oil which has high amount of sulphur content and the cylinder jacket cooling water temperature is low, then because of the low temperature of cylinder wall of the liner the sulphuric acid, which is formed due to burning of sulphur from the fuel, will get condensed. It will get deposited over the liner walls and the liner will have corrosive effect of sulphuric acid. Hence it is necessary to maintain the jacket water temperature at a prescribed higher limit rather than running the engine at low jacket water temperature for a longer period. Q.389 The air supplied to the diesel engine is compressed to provide heat for ignition of the fuel or to decrease injection delay Ans. (a) ignition of the fuel. The air supplied to the engine during the induction or suction stroke is compressed by piston moving upwards with the inlet and exhaust valves closed so as to compress the air and increase the temperature. This compression is done adiabetically so that at the end of the compression the pressure and the temperature will be high so that when the fuel is injected, because of the air temperature being higher than the self-ignition temperature of the fuel, the fuel will ignite instantaneously and convert the chemical energy of fuel into pressure and heat energy to be converted into mechanical energy. Q.390 Fuel droplets injected into a diesel engine cylinder must have adequate penetration to Ans. (c) thoroughly utilize the air charge. The fuel injected should mix up properly with the available air in the combustion space. For this purpose, it is necessary that the fuel should be atomized properly. At the same time, it should penetrate deep into the dense atmosphere of the compressed air in the combustion space. Otherwise, if the
penetration is not sufficient, local burning will be caused and the high density air which is available away from the nozzle tip will not be utilized efficiently. Hence under such circumstances fuel will not burn properly which will result in afterburning, loss of power and high exhaust temperature. Q.391 Loss of lubricating oil pressure to a main propulsion diesel engine will actuate a audible/visual alarm. Loss of lubricating oil pressure is a most dangerous thing which can happen to the engine as it may damage the bearings and cause severe damage to the engine and the crankshaft. Inorder to avoid this thing, a prewarning should be given as the drop of the pressure takes place and action has to initiated immediately by getting audible/visual alarm. If the pressure drops still further, there is immediate danger to the engine. In addition to the audible visual alarm, there will be tripping of the engine and the engine will stop. Q.392 The amount of fuel injected into cylinder by unit injector is controlled by Ans. (b) metering helix. Metering helix will control the end of fuel injection and thus it will determine the effective delivery stroke of the plunger. This is possible by having a proper angular relation between the helix on the plunger surface and the suction ports. This is possible by rotating the plunger with respect to the barrel by use of rack and pinion arrangement. The position of rack is controlled by the governor. Q.393 The dripping of fuel from an injector nozzle after injection terminates often results in Ans. (c) choking and blocking of the injection nozzles. Once the injection terminates the fuel injection pump pressure drops. At the same time the fuel injector needle valve closes. If the fuel injector needle valve still leaks, some amount of oil ooze out between the nozzle and seat this oil will get accumulated over the tip of the nozzle. As it in not in the atomized condition or it has not penetrated inside the combustion space, this oil as a droplet will burn on the hot nozzle during the combustion process. Due to improper combustion of the dripping oil it will form a * and this * is mostly because of asphalt present in the fuel because asphalt being a heavier hydrocarbon requires more time for burning and it requires more quantity and thorough mixing of fuel with the air. All this is not possible when the dripping droplets accumulate on the nozzle tip thus causing the * on the nozzle tip. Q.394 The amount of fuel delivered in each cycle must be in accordance with engine load and the same quantity of fuel must be delivered to each cylinder for each power stroke as the load. Which of the following statements describes the requirements? Ans. (b) Accurate metering The complete fuel delivered to the engine will depend on the load. Hence for particular load particular amount of fuel has to be burnt in the engine. At the same time, the total fuel given to the engine must be distributed equally to the units so that all cylinders will produce equal power for the balancing of the engine. This two process requires a proper quantity of fuel to be measured and delivered equally. Hence this process is called metering. Q.395 The knock occurring when a cold diesel engine is started and continues while running at low speed but stops when the engine reaches the normal operating speed and temperature is Ans. (d) normal for this condition. When the engine is started from cold, the temperature at the end of compression stroke is not sufficiently high. When the fuel is injected in combustion space, it takes some time for it to ignite i.e. the ignition delay is more. When the ignition delay is more, the total quantity of fuel accumulated inside the cylinder before the ignition actually starts will go on increasing. When the accumulated fuel gets ignited, it produces a knocking sound and knock wave. As the temperature goes on increasing, the temperature of the air at the end of compression stroke also goes on increasing. When this happens, the fuel injected will reach the self-ignition temperature much faster and hence there will not be much accumulation of fuel before it starts to ignite. Automatically the knocking sound and the wave will reduce as the temperature reaches the normal operating temperature. Q.396 The most critical time for any bearing with regards to lubrication is Ans. (c) during starting. Most of the bearings used on the engines work on the principle of hydrodynamic lubrication. During hydrodynamic lubrication, there is no metal-tometal contact between the bearing and the shaft. The bearing and shaft is separated by lubricating oil film. This oil film is formed due to the rotation of the shaft inside the bearing. As the speed of the shaft increases, more oil is drawn between the shaft and the bearing and thicker lubricating oil film is formed. During starting of the engine, the speed is very low. Hence it is difficult to form a hydrodynamic lubrication between the shaft and the bearing. During this time, for a short period, there is metal-to-metal contact between the shaft and the bearing surface. Hence this is the most crucial period as the characteristics of the lubricating oil (oiliness) comes into use and this is time when more wiping out of the bearing takes place. Hence it is necessary to coat the bearings with anti-friction material to reduce the friction between the bearing and the shaft at the time of starting because once the engine is started and the engine has achieved the speed, the white metal does not play an important role because there is no metal-to-metal contact and it is pure hydrodynamic lubrication. Hence white soft coating is useful for the boundary lubrication which is existing at the time of starting the engine. Q.397 Which of the listed diesel engine operating conditions should be checked immediately after any diesel engine is started? Ans. (c) Lubricating oil pressure. Lubricating oil system is one of the most important system and if malfunctioning of lubricating oil takes place the failure of engine is immediate and severe. Hence it is necessary to check the lubricating oil pressure and the lubricating oil sump as soon as the engine is started. Otherwise, it will be too late and there will be irrecoverable and irreparable damage to the engine. Nozzle tip will get damaged because of sulphuric acid corrosion. That is the main reason most of the engines cooling water temperature of the nozzle cooling is kept at 85°C and in modern engines the cooling of the nozzle by separate cooling system has been abandoned. Q.399 What occurs in the combustion space of a diesel engine cylinder shortly after ignition and before piston reaches TDC? Ans. (c) Rapid increase in the pressure and temperature. When the piston is nearing the TDC and the fuel is injected, the temperature inside the combustion space is sufficiently high near the self-ignition temperature of the fuel. Hence the fuel catches fire and because of this, sudden increase in the pressure and temperature takes place. The initial stage of
ignition takes place under constant volume therefore the temperature and the pressure increases. In the later stage, the combustion takes place under constant pressure because the piston is moving downwards and the volume is increasing. Q.400 The effective pump stroke of a port and helix fuel injection pump is determined by Ans. (c) plunger control rack position. The rack position will determine the position of the plunger helix with respect to the suction port of the barrel. This angular position will determine the quantity and time for which the fuel is injected. Hence more the quantity of fuel more the load and lesser the fuel lower the load. The time for which the fuel is actually injected into the cylinder is called effective delivery stroke of the plunger which is decided by the helix position and the position of plunger with respect to the barrel. Q.401 The amount of fuel delivered by helical plunger type fuel injection pump is controlled by Ans. (a) rotation of the pump plunger. Rotation of pump plunger by means of rack and pinion arrangement will cause variation of angular position of helix with respect to the suction port on the fuel pump barrel. Due to this, the effective delivery stroke of the fuel pump will change which means that it will change the time for which the fuel is injected. Hence it will control the quantity of fuel delivered in the unit because the position of plunger helix whether the termination of the fuel injection is carried out earlier or later part of the injection stroke. Q.402 Which of the following conditions will develop if the flow of raw cooling water to diesel engine is obstructed? Ans. (c) Jacket water temperature will increase. The jacket water-cooling is a closed circuit and it uses the fresh water. The jacket water is cooled by raw water by the use of heat exchanger. If the raw cooling water flow is obstructed, the quantity of heat removed from the jacket cooling water is reduced. At the same time, the jacket cooling water is receiving heat from the engine which is not being removed from the cooling water. Hence the jacket cooling water will go on rising continuously. Q.403 The effect stroke of a constant stroke individual fuel injection pump is varied by the Ans. (a) controlled rack. The controlled rack is engaged is with the pinion which in turn is engaged with the controlled sleeve of the fuel pump. The controlled sleeve, when it rotates, will rotate the fuel pump plunger with respect to the barrel. This rotation will change the angular position of helix with that of the suction port on the barrel. Hence ultimately the effective delivery stroke of the pump will change and hence it will change the quantity of fuel injected. Q.404 Which of the listed conditions occur if a diesel engine cylinder exhaust valve is leaking? (a) loss of compression for the cylinder. (b) misfiring or rough engine running. (c) damaged valve. (d) All of the above. Ans. (d) All of the above. When the exhaust valve is leaking automatically, the compression pressure drops. When the compression pressure drops the fuel will not ignite properly. Hence there is a possibility that misfiring takes place. At the same time, due to low compression pressure the fuel will not burn properly which may cause high exhaust temperature and loss of the peak pressure. When the valve starts to leak, the high-pressure gas at high temperature will start to pass through the leaky portion of the valve. This will result in wear or wire-drawing effect on the valve and valve seat. Thus causing excessive damage to the valve and valve seat and excessive high exhaust temperature which increases fast. Q.405 High firing pressure and low exhaust temperature in a diesel engine may result from Ans. (c) early fuel injection timing. In case of v.i.t. controlled injection, the early injection cause the fuel to mix up better with the quantity of air available it will have more time to mix up and form combustible mixture. Due to this, fuel will burn efficiently and very near to the TDC which will cause high peak pressure and low exhaust temperature. Q.406 Exhaust pyrometer reading provides an indication of Ans. (b) distribution of the load between the engine cylinders. Exhaust gas pyrometer reading which gives the exhaust temperature of individual unit is a rough estimation of power developed by the individual unit. As a thumb rule, higher the exhaust temperature more is the power developed and lesser the temperature less is the power developed but this may not be always true. If the exhaust temperatures are nearly equal, it indicates that all units are developing equal powers and hence the engine is properly balanced as far as power is concerned. Q.407 A change in the degree of fuel atomization in a diesel engine cylinder has the greatest effect on the Ans. (d) combustion of fuel in the cylinder. The more degree of atomization, with proper penetration, will cause better fuel efficiency because the fuel will be divided into very minute particles which will have more surface area available and the evaporation of the fuel will be faster so as to form a combustible mixture. Hence the mixture will burn efficiently and promptly due to which the combustion inside the cylinder will be efficient. Q.408 Diesel engine oil having low cetane number can result in Ans. (c) combustion knock. Lower cetane number of the fuel low is the grade of the fuel and because of that ignition delay will increase. Hence more fuel will get accumulated inside the cylinder before it catches fire. When it catches fire it will give knocking effect because of sudden formation of pressure wave. This knocking effect is called diesel knock which directly depends on the quality of the fuel.
Q.409 To successfully reduce an excessively high diesel engine exhaust gas temperature Ans. (d) you should reduce the load on the engine. As a general rule more the load on the engine more will be the exhaust temperature. Hence the first action to be taken if high exhaust temperature is noted is to reduce the quantity of fuel which is burnt. This is possible by reducing the load on the engine which reduces the quantity of fuel burnt and the exhaust temperature reduces immediately. Q.410 High exhaust temperature and black smoke exhausting from an engine can be caused by Ans. (a) engine overload. When engine overload takes place, the amount of fuel burnt is always higher than the quantity of the air supplied. At the same time, high quantity of fuel is injected. This high quantity of fuel injected cannot burn in the available time. Under such circumstances, afterburning takes and along with lack of proper quantity of air it will have higher exhaust temperature and black smoke. Q.411 Combustion gas venting through the expansion tank can be caused by a Ans. (b) cracked cylinder head. When the cylinder head is cracked, during compression stroke when high peak pressure and high exhaust temperature takes place, these high pressure combustion gas will enter through the cracked cylinder head valve into the water space. This gas being lighter it will try to rise upwards and get accumulated inside the expansion tank and it will come out through the expansion tank weld. The gas coming out through the expansion tank weld will have exhaust gas smell and water in the expansion tank will get discoloured. Also if the crack is big, more quantity of exhaust gas will find its way into the expansion tank, the expansion tank may get bubbled out and may get empty due to trapped exhaust gas. Also, the jacket cooling outlet temperature of that particular unit will also be high. Q.412 Which of the following problems can cause fluctuating pressures in the closed cooling system of a main propulsion diesel engine? Ans. (b) Cavitation in the cooling water pump. If the inlet in the cooling water pump is restricted or the water is having high temperature, cavitation will take place in the form of bubble at the suction of pump. Due to this, there will be partial loss of suction. This will cause the variation in discharge pressure which will result in fluctuations in the pressures of closed cooling water system. Q.413 A fuel leak occurs in a high pressure piping between the fuel pump and fuel nozzle. This requires immediate repairs because of the Ans. (b) serious fire hazard. The high pressure pipe leakage will cause the high pressure fuel to leak through the cracked portion of the fuel pipe. Due to this, the oil coming out through the cracked portion will be in the form of a fine mist. This mist when it comes in contact with the exhaust manifold or hot surface of the engine, as there is sufficient amount of air in the engine, it will result in fire in the engine room. Hence immediate repair is necessary to avoid this fire. Q.414 Individual cylinder performance in the diesel engine is routinely determined by the exhaust gas Ans. (c) pyrometer readings. As a general rule, the exhaust temperature will give a fairly good idea of power developed by the individual units. The exhaust gas temperature from individual unit is taken into consideration as a measure to determine the power developed by the individual unit. Hence as long as all units’s temperatures are equal or they are within a prescribed limit, this indicates that all units are developing nearly same power and the engine is properly balanced. Q.415 Excessive diesel engine cylinder exhaust backpressure will be caused by Ans. (c) an obstruction in the exhaust silencer. When the exhaust valve opens and exhausting takes place through the exhaust valve, if the exhaust gas silencer valve is blocked or choked, there will not be easy flow of exhaust gas and complete emptying of the exhaust gas from the cylinder will not possible. Under such circumstances, when the piston is on the upward stroke it will give a backpressure over the piston and there will be loss of power in overcoming this backpressure. Q.416 The injection pressure of a hydraulic fuel injection nozzle can be increased by Ans. (d) increasing compression on the pressure spring. The fuel is injected when the pressure developed by the fuel injection pump will act on the annular space around the fuel injection nozzle will overcome the spring compression from top on the fuel injection nozzle. As the compression pressure increases, more pressure will be required by the fuel injector nozzle to lift up against the spring compression. Hence more the compression pressure more will be pressure required for the fuel injector to open. Hence the injection pressure will increase as the compression pressure of the spring increases. Q.417 The combustion knock occurring in a diesel engine can be caused by Ans. (d) prolonged ignition lag. Due to prolonged ignition lag the time between the fuel injection and fuel ignition will be large. During this time, large amount of fuel will get accumulated into the combustion space. When this accumulated fuel suddenly gets ignited it will cause an explosive pressure wave which will create the knocking effect. Q.418 Which of the following problems could develop due to the accumulation of oil vapours into the crankcase of diesel engine? Ans. Crankcase explosion. Accumulation of oil vapours in the crankcase and presence of hot spot will cause the vapour to convert into oil mist. When the oil mist comes in contact with the hot spot in presence of large quantity of air, which is available in the crankcase, it will form a lethal combustible mixture and it will cause a crankcase explosion. Q.419 One problem resulting in a diesel engine fuel injector opening pressure being lower than specified by the engine manufacturer is
Ans. (d) quantity of fuel injected tends to increase. As the fuel injection pressure is lower than recommended by the engine manufacturer, the fuel injector valve will open much earlier but the closing will not alter. Due to this, the quantity of fuel injected will be more than what is required. Q.420 Which of the listed reasons is the most likely cause of a sudden drop in compression pressure in one diesel engine cylinder? Ans. (b) Malfunctioning of the valves. Malfunctioning of the valves may be because of improper adjustment of the tappet clearance or the valves may be sticky or leaky. Under all these conditions, during compression, the compressed air will leak through the leaky valve and automatically the compression pressure will drop.
Q.421 repeated in Q.420 Q.422 Failure to open the diesel engine test cock after a long period of shutdown prior to starting may result in Ans. (b) damage to cylinder head and piston. If the test cock or the indicator cock is not opened and the engine is started and due to any reason there is accumulation of fluid on top of the piston, this fluid will get compressed. As the fluid is incompressible, excessive pressure will be developed between the piston top and cylinder head bottom portion. This will cause the cracking of the cylinder head, damage to the piston, bending of the connecting rod or in worse case it may cause damage to the crankshaft in the way of slippage. Q.423 Which of the listed conditions is most likely to cause a crankcase explosion? Ans. (b) Fuel dilation of lubricating oil. When fuel dilation of lubricating oil takes place, automatically the flash point of the lubricating oil reduces. Under such circumstances, if there is any hot spot present in the crankcase and the oil mist is formed along with the oil mist there will mist of the fuel present. This mist of the fuel will be more prone to explosion and hence explosion will be caused much early.
Q.424 repeated Q.425 A diesel engine is operating with excessively high exhaust temperature in all cylinders. In order to correct this condition, you should first Ans. (a) reduce the engine load. As a general rule if the load is more on the entire engine, the entire engine will be using more quantity of fuel. When more quantity of fuel is burnt, automatically more heat will be generated. This will give higher exhaust temperature for entire engine. Hence to reduce this exhaust temperature the quantity of fuel injected is reduced and to reduce to quantity of fuel injected, you have to reduce the load on the engine. Q.426 The overspeed trip installed on most diesel engines will stop the engine by shutting off the Ans. (b) fuel oil supply. When overspeed takes place in order to prevent the damage to the engine it is necessary to stop the engine. For instantaneous stopping of the engine. the best possible way is to shut off the fuel immediately. This is done by the overspeed trip which will bring the position of the fuel rack of all the fuel pump to zero position and no fuel will be injected and the engine will be immediately stopped. Q.427 White smoke exhausting from a operating diesel engine may be due to Ans. (a) cracked liner. Especially if the liner is cracked at the lower position, the water will enter into the combustion space. This water when it gets evaporated, during the combustion stroke because of combustion of fuel, will form steam. This steam will come along with the exhaust gas into the atmosphere through the funnel of the ship. When it comes out it will get cooled down by the atmospheric temperature. This steam will be visible due to water present in it and it will form white mist when it comes out of the funnel. Q.428 Black smoke exhausting from an operating diesel engine can be caused by Ans. (a) fuel dribbling from leaky fuel injector. When the fuel is dribbling from leaky fuel injector it is actually coming into the combustion space when it is not required. At the same time, it is not in atomized condition. Under such conditions, afterburning will take place and fuel which is dribbling will not mix properly with the air. Hence the improper combustion of this fuel will take place during the exhausting stroke of the engine and hence black smoke and higher exhaust temperature will be indicated. Q.429 A device which functions to bring a diesel engine to fullstop to protect it from damage is known as Ans. (b) overspeed trip. If the due to any reason the engine gets into overspeed, due to the centrifugal force caused by rotation, there will be excessive damage may take place to the engine to the extend that the piston may come out from the cylinder or from the crankcase or the bottom end bearings may break. In order to prevent such heavy damage, it is necessary that the engine should stop immediately. This is possible by installing the overspeed trip which will sense the overspeed and immediately shut off the fuel. Q.430 Which of the following conditions can cause oil to accumulate in the cooling system of a diesel engine? Ans. (b) Defective oil cooler core.
If the lubricating oil cooler, in which the cooling is carried out by jacket cooling water especially in case of small engines, is leaking the lubricating oil will find its way into the jacket cooling water. This system is utilized in the engines working in the cold weather where the lubricating oil is cooled by jacket cooling water in a cooler. Q.431 Lubricating oil accumulating in the cooling water system of a diesel engine will result in Ans. (b) poor heat transfer. When the lubricating oil find its way into the jacket cooling system it will coat over the heat transfer surface. As lubricating oil is poor conductor of heat, it will effect the efficient heat transfer and will cause the overheating of the liner and the cylinder head due to the overheat transfer. Q.432 When the opening pressure of a diesel engine fuel injector is greater than that specified by the engine manufacturer Ans. (a) the quantity of the fuel injected decreases. If the opening pressure of fuel injector is more than that specified by the engine manufacturer, the fuel injector will open late and it will close at particular time depending upon the load. Hence the automatically the quantity of the fuel injected will be reduced because of the opening of the fuel injector is late. Q.433 Black smoke exhausting from an operating diesel engine is an indication of poor combustion which may be caused by Ans. (c) clogged air intake passage. If the air intake passage is clogged, automatically the engine will be starved off the air. The quantity of fuel injected will not burn efficiently because of lack of sufficient amount of air. If the fuel does not burn efficiently, it will be indicated by black smoke and high exhaust temperature. Q.434 The highest pressure in a diesel engine cylinder normally occurs Ans. (c) after TDC. As is indicated by the draw card the fuel is injected slightly before TDC. Depending upon the quality of the fuel, the fuel will mix with the hot air which is available to convert itself into a combustible mixture. This combustible mixture will ignite to give a sudden maximum pressure denoted by Pmax. For this process to take place, there is some time lag between fuel injection and fuel ignition. Hence by the time complete fuel burning starts, the piston has already passed the TDC and it is slightly on its downward stroke. This is the time when complete fuel will burn and high pressure wave will be formed. Thus the maximum pressure in a diesel engine will be always after TDC. Q.435 Diesel engine lubricating oil dilation with fuel oil is indicated by Ans. (a) decreased viscosity. If the lubricating oil is getting contaminated due to fuel oil, the viscosity of the lubricating oil will drop down because the viscosity of the fuel oil is much lesser than the viscosity of lubricating oil. Hence when they get mixed together, the viscosity of lubricating oil will drop down depending upon the quantity of fuel which is contaminating the lubricating oil. This is easily checked by the flow stick which are supplied for the on-board testing of the lubricating oil. Also, it will drop the flash point of the lubricating oil. This is the most dangerous situation and action should be taken immediately otherwise bearing damage will also take place. Q.436 A broken pintle in a fuel injector usually causes Ans. (c) distortion of the spray pattern. When the pintle breaks or gets eroded, as it is controlling the single hole in the center of the nozzle, when the needle gets lifted up under the high pressure of the fuel oil. When the fuel oil passes around the pintle the distribution pattern of the fuel oil changes. Q.437 Diesel engine cylinder head test cocks are used Ans. to remove the accumulated oil or moisture from the top of the piston. In order to avoid the hydraulic pressure developed during starting of the engine which may otherwise cause damage to the cylinder head, liner, piston, connecting rod and the crankshaft, it is necessary to blow through the engine just before starting it on air and fuel. Q.438 A classical way of checking for excessive fuel injection in one cylinder of an operating diesel engine is Ans. (c) frequently checking the cylinder exhaust temperature. As a general rule, the cylinder exhaust temperature is an indication of the quantity of fuel burnt in that unit. Hence higher the fuel burnt higher is the exhaust temperature. Hence by checking the exhaust temperature we can get a rough idea as how the distribution of fuel is taking place for individual cylinder. If the temperature is at higher level by rough estimation we can assume that the quantity of fuel delivered to that unit is higher than as compared to any other units. Q.439 A diesel engine emits a blue exhaust smoke as a result of Ans. (c) excessive cylinder lubrication. If the cylinder lubrication is excessive in that case the cylinder lubricating oil will burn along with the fuel. When the lubrication oil burns it gives a blue tinge to the exhaust gas from the funnel. Q.440 Which of the following problems could cause misalignment between the needle valve and fuel injection nozzle? (a) A bad fuel needle. (b) A defective nozzle tip seat. (c) Distorted valve body. (d) All of the above. Ans. All of the above.
If the needle is bad it will not sit squarely on the valve seat. If the valve seat itself is distorted then the needle will not sit properly on the valve. At the same time, if the complete body is distorted, then along with the body, the cylinder in which the needle moves will also get distorted and hence needle will not sit squarely onto the seat. This will also cause the distortion and misalignment of the needle and valve seat. Q. 443 The highest load applied to the diesel engine crankshaft main bearings are Ans. (c) firing loads. During the firing of the fuel, the piston is nearly at TDC position and the piston rod, connecting rod and the connecting rod bearings are nearly in the same vertical plane because of which full load of firing is transmitted to the crankshaft. When the load is transmitted to the crankshaft, the crankshaft is supported by the main bearings. Thus the load is transmitted to the main bearings and ultimately to the bed plate and transverse girder. Q.444 If the diesel engine fuel injection timing is changed to delay the start of injection until the pistons are at TDC, the engine will Ans. (b) develop less power under load. If the fuel is injected when the piston is at TDC, by the time it mixes up with air and forms a combustible mixture, the piston has already passed TDC and is moving downwards. Under such circumstances, the firing of the engine will take place when the piston is moving towards BDC. In this condition, the firing will take place under increasing volume instead of constant volume so the pressure cannot be developed onto the piston. As the pressure on the piston top is most important from the point of view of power developed considering the power developed is given by Power developed = PLAN In this condition, power developed by the engine will reduce and whatever fuel is burnt will be wasted due to afterburning resulting in high exhaust temperature but low power. Q.445 Increase in compression ratio of a diesel engine while maintaining designated rate of fuel flow will result in Ans. (a) increase in the horsepower. If the compression ratio increases, the air in the cylinder will be more dense and the quantity of air increases. At the same time, the temperature of the air at the end of compression increases. As a result, the fuel injected will burn efficiently and promptly. All this will result in more power being developed by the engine because more heat can be released from the fuel which is burnt. Q.446 If the jacket water temperature in a diesel engine rises rapidly above normal, you should Ans. (b) first reduce engine load. Jacket cooling water temperature will also roughly estimate the engine load. Higher the engine load, higher will be the heat absorbed by the jacket water and hence the temperature will increase. Hence jacket water temperature is a rough estimation of power or load on the engine. If the jacket water temperature is excessively high and it is not because of any other reason such as malfunctioning of jacket water cooler, then it is necessary to reduce the load and carry out the further investigation and rectify the fault. Q.447 During which of the listed piston stroke of a four-stroke cycle diesel engine is the piston moving downwards? Ans. (a) Intake stroke. In a four-stroke cycle diesel engines, the piston will move downwards two times: one during the expansion stroke and one during which air will be drawn inside the cylinder called the intake stroke. During the intake stroke, the intake valve is open and the piston is moving downwards so whatever increase in volume is there, due to downward movement of the piston, will be filled up by the incoming air under intake stroke. Q.448 In operating diesel engine, preignition can be caused by Ans. (b) oil in the air charge. Preignition is caused because ignition takes place before the fuel is injected. This is possible only if the fuel vapours are present in the air suction. When this air is compressed these vapours will get ignite before the fuel is injected and will cause the preignition. Q.449 Significant retardation of diesel engine fuel injection timing will result in Ans. (d) reduced engine power. Retardation of fuel injection timing will cause the fuel injection to occur late during the compression stroke. As the fuel injection is late, by the time it forms a combustible mixture with the compressed air, the piston will be near to TDC or it might have already passed the TDC. If it has already passed the TDC and if fuel ignition takes place, the pressure is increasing; also the volume is increasing as the piston is moving downwards, hence the maximum peak pressure reduces which will cause loss of power. Also, afterburning will take place and the exhaust temperature will be high but the power developed by the engine will be low. Q.450 When fuel is injected late into diesel engine cylinder Ans. (d) fuel consumption will be high. When the fuel is injected late into the diesel engine cylinder the combustion will take place late and afterburning effect will take place. The maximum pressure will reduce due to which the mean effective pressure will be reduced. Ultimately the power developed by the engine will be less but the quantity of the fuel burnt will be same. Hence the specific fuel oil consumption of the engine will increase and the thermal efficiency of the engine will drop. Q.452 If a single cylinder relief valve on a diesel engine lifts frequently while the engine is running, the cause can be Ans. (c) an incorrectly adjusted fuel injector. If the fuel injector is incorrectly adjusted it means that the fuel injection pressure is low or if the fuel injector is leaking, excessive amount of fuel will find its way inside the unit and excessive amount of fuel burns inside the unit. Automatically, the peak pressure will increase and it may increase pressure and lift the relief valve. At the same time, relief valve may lift if the rack position is stuck up in full load condition. The valve may also lift if the injection timing is advanced too much. If excessive amount of injection timing is advanced, the peak pressure increases Also if the v.i.t. (variable injection timing) which is suppose to start reducing too much after 85% to 100% is stuck up in full position and it is not reducing as it is required to do because of this the pressure will be excessively high.
Q.453 When one cylinder has low compression pressure and high exhaust temperature than any of the engine cylinders which of the conditions listed will be indicated? Ans. (c) Leaky exhaust valve. If the exhaust valve is leaking, the compression pressure will drop because the compressed air will get leaked out from the leaky exhaust valve. At the same time, because of low compression pressure, the fuel which is injected will not burn properly which will give high exhaust temperature. Also when compression is taking place and the fuel is injected and as the fuel starts burning, as the fuel valve is leaky, high temperature and high pressure gas will leak through the leaky exhaust valve and the exhaust temperature will rise. Hence low compression pressure, high exhaust temperature, low peak pressure and loss of power all of them will be indicated by leaky exhaust valve. Q.454 A distorted spray pattern from a fuel injector can cause a diesel engine to have Ans. (d) less power output. The fuel should be distributed equally and evenly around the entire combustion space available. If the spray pattern of the injector has been distorted due to any reason, the fuel will not mix properly with the air and the complete use of the air available inside the combustion space will not be possible by the fuel. At the same time, concentration of fuel at one particular point in the combustion space will be more than the other. Due to this, the availability of air will not be uniform for the fuel to burn in the entire combustion space and because of this there is a possibility that the exhaust temperature will be high and power loss will take place. Q.455 If a few injector spray holes becomes plugged the reason could be (a) excessive surging under governor speed. (b)combustion knock under load. (c) poor fuel combustion. (d) All of the above. Ans. (d) All of the above. When few holes of the injector are plugged, the fuel distribution for the units in which the holes are plugged will get disturbed. Hence the power balance of engine will change. As there is change in power balance, the governor will try to continuously try to maintain the speed and hence it will give surging effect to the engine speed. So engine speed will continuously fluctuating. Secondly, as the distribution of fuel is not proper and penetration and atomization will also get affected the fuel will not burn properly and hence it may cause the fuel knock and poor fuel combustion resulting in knocking sound, black smoke, and loss of power. Q.456 A diesel engine fails to start because of water in the fuel. Inorder to start the engine we should Ans. (d) drain the filter and stainer and bled off the water at each of the injection pump. All methods should be taken to remove the water content from the entire fuel injection system upto the nozzle tip. This is carried out by bleeding off the water content at different position including the fuel pump and injector so that water will not be injected during the injection stroke, resulting in stopping of the engine or incomplete combustion or fluctuation in the rpm. In such cases, if water is present engine may fail to start also. Q.457 A diesel engine piston crown can crack from (a) excessive piston to liner clearance. (b) excessive dirt beneath the piston crown that reduces the heat transfer. (c) faulty nozzle springs (d) All of the above. Ans. (d) All of the above If there is excessive clearance between piston and liner because of liner wear, the piston ring may get overhanged as the radial clearance increases. Hence there is a possibility that the piston rings may get broken or twisted. Under such circumstances, blowpast will take place and the liner and the piston will get damaged. Also in four-stroke engine the piston will snap against the liner if the clearance is excessively high and in that case the piston crown will hit against the liner and it may crack. The piston crown may also crack as a result of formation of carbon deposit or scale in the cooling space of the piston. Due to faulty fuel injector nozzle, the fuel will get penetrated too much and it may directly burn over the piston crown. When this happens, the piston crown may also crack. Q.458 Permitting a daytime fuel tank to run dry can cause Ans. (d) air in the fuel. It is natural that if the engine is running and the fuel tank is allowed to run dry then it will suck air in the system and the engine will stop. Q.459 Air in the fuel line through a fuel injection nozzle of a diesel engine will cause the engine to Ans. (c) operate with reduce power or stop. If there is air in the fuel injection system, there is a possibility that the quantity of fuel injected will get reduced or there is intermittent firing. If the air ingress in the system is excessively high, air being a compressible medium, the fuel injection nozzle will not open and no fuel is injected into the cylinder and hence the engine may stop. Q.460 Which of the following problems will occur if needle valve of an injector stick in open position? Ans. (a) Fuel injection timing may change. If the fuel injection valve get stuck up in open position, during the suction stroke of the engine, the pressure inside the cylinder drops. At that time, the fuel will drip through the open needle valve into the combustion space and some part of fuel will be emptied off from the system. Hence for the next injection stroke it is necessary to fill up the space and because of that the fuel injection timing will change. At the same time, because the valve is staying in the open condition, the fuel will be injected as soon as the plunger covers the suction port. Hence too much fuel will be injected which will not be
under sufficient pressure resulting in the dripping of the fuel and not spraying properly. All this will cause excessively high exhaust temperature or the relief valve will lift resulting in loss of power and black smoke. Q.461 A dry type exhaust muffler clogged with soot will cause Ans. (c) loss of engine power. When the exhaust gas muffler gets clogged, it will cause excessive backpressure on the exhausting system. In case of turbocharged engine, the turbocharger will not attain sufficient speed and it will not supply sufficient air for the combustion. Even in case of naturally aspirated engine because of the backpressure when the piston is moving upwards on the induction stroke it is to force the exhaust gas out of the unit. This will cause the loss of power developed by the engine and hence the power developed will be less. Q.463 In a diesel engine cylinder head, test cocks are used Ans. (b) to connect the pressure indicator. Pressure indicators are connected to the exhaust gas so that the indicator device piston will have the exact duplicate movement as that of the main piston. Hence the indicator device is connected to the indicator cock. Q.463 Air may be bled from fuel system by Ans. (b) loosening the compression nut at the injector Q 464 Which of the terms listed below represent the operational speed at which excessive engine vibration is created? Ans. Critical speed. Critical speed is that speed at which the natural frequency of vibration of the shafting will coincide with the frequency of vibration caused because of the firing of the fuel in the cylinder i.e. firing frequency. If they coincide, there will be resonance frequency and excessive vibration of engine will take place. The condition when this occurs is called critical condition. If this persists for longer period, there is a possibility that the crankshaft and the connecting rod may get cracked. Also the tierods may get cracked or the foundation bolts may get loose. Q.465 If the needle valve in a fuel injection nozzle sticks open Ans. (c) the valve will get overheated. If the fuel valve will get stuck open then during the firing stroke the pressure inside the cylinder will be high enough for the high temperature and high pressure gas to find its way to the nozzle and the nozzle may get overheated and it might be damaged. Q.466 Which of the condition listed below would cause simultaneous high cylinder pressure and low exhaust temperature? Ans. (c) Excessively early injection timing Earlier the injection more is the pressure developed. If the injection is too early the pressure developed will be excessively high and there is a possibility that the relief valve on the cylinder head will get lifted up to relieve the pressure. Also early fuel injection will result in high maximum cylinder pressure but the exhaust temperature will be low. Q.467 Early injection timing is indicated by cylinder pressure being Ans. (a) above normal firing pressure and below normal exhaust temperatures. Due to early injection, there is more time for the fuel and air to mix properly resulting in better combustion which will produce high firing pressure and low exhaust temperature. Q.468 If fuel injection occurs too early the diesel engine will lose power because of Ans. (b) ignition will be delayed due to low compression pressure Early fuel injection is better for the proper combustion of fuel as we derive more power, high peak pressures, and low exhaust temperature. If the fuel is injected too early, the compression pressure and the temperature of the air is not sufficiently high enough. The fuel will only get accumulated and it will not catch fire immediately due to which knocking will take place and since the fuel will not burn properly there will be loss of power. Hence the timing should be properly adjusted it should not be too early or too late. Q.469 Diesel engine air starting valve timing is controlled by Ans. air distributor. The diesel engine air starting valve timing is controlled by air distributor which has got sets of cam; one set is used for ahead and other is used for astern These cams are rotated by shaft which is connected to the main camshaft. Also individual air distributor valve will have a spring loaded double acting valve and because of this the pilot air will be conveyed to the main air starting valve fitted on the cylinder depending upon the position of the piston in the starting condition. Q.470 A diesel engine fails to start due to water in the injection system. Before engine is started water should be removed from the Ans. (a) fuel line. If water is present it cannot ignite due to which the engine will not start. Q.471 If you are suspecting misfiring due to air leakage into the air system, Ans. (d) you should be looking for a leak on the suction side of the fuel transfer pump or booster pump.
As the fuel in the suction side of booster pump will always be lower as compared to its delivery side, it is necessary to check at the suction side of pump for any ingress of air. In order to avoid this, there should be sufficient height of fuel in the service tank so that possibility of ingress of air through the suction glands is minimized. Q.472 An increase in the load of a turbocharged diesel engine operating at constant speed will result in an increase in (a) exhaust temperature (b) air box pressure (c) brake mean effective pressure (d) All of the above Ans. (d) All of the above. If the load on the turbocharger is increased the speed of the turbocharger increases. As the load on the turbocharger increases more amount of fuel burnt increases. Because more quantity of fuel is burnt the exhaust temperature increases. At the same time, more amount of exhaust gas is produced. This high quantity and high temperature gas will drive the turbocharger at higher speed due to which more amount of air will enter the inlet manifold through the turbocharger and hence scavenge air pressure will increase. As the scavenge air pressure increases, more amount of air will enter the cylinder. As a result, compression pressure will increase and peak pressure will increase because of firing of more quantity of fuel and burning the fuel efficiently. Q.473 When a diesel engine is operated at partial load as compared to full load, a decrease will occur average (a) in air quantity aspirated. (b) fuel injection pressure (c) combustion pressure on power stroke (d) compression pressure on the compression stroke Ans. (a) in air quantity aspirated. If it is naturally aspirated engines the quantity of air aspirated will reduce. In case of turbocharged engines, the turbocharger at partial load will supply less quantity of air. In this respect the quantity of aspirated air will reduce. As the quantity of air admitted into the cylinder reduces, the compression pressure during the compression stroke will also reduce. When the compression pressure reduces at part load, the quantity of fuel burnt will also reduce ultimately the firing pressure and mean indicated pressure, which is acting over the piston when it is moving downwards on expansion stroke, will also reduce. Q.474 Air in the fuel can cause Ans. the engine to stop. If there is air in the fuel there will be discontinuity of fuel injection will take place. If there is discontinuation of fuel injection the engine will stop. Q.475 Early injection timing is indicated Ans. (b) low exhaust temperature. Q.476 Which of the following operating procedures should be carried out immediately before any diesel engine is started? Ans. (c) Verify proper lubricating oil pressure. Verification of proper lubricating oil pressure is most important part when the engine is started because if there is insufficient lubricating oil pressure and if the engine is allowed to run even for smallest possible time, severe damage will occur to the crankshaft and the complete engine will be damaged beyond repair. Q.477 The most common diesel engine fuel injection system problems are caused by Ans. (b) dirty fuel. If the fuel is dirty the fuel filter will get choked. The fuel injector nozzle will get choked. There will be excessive weardown of fuel pump barrel and plunger. All this will lead to deterioration of fuel injection system. Q.478 If sludge accumulation on the underside of the diesel engine piston it will Ans. affect the heat transfer. If there is sludge accumulation on the underside of the diesel engine piston in the cooling space it will affect the heat transfer. When the heat transfer is affected ,there will less amount of heat transfer which will cause the piston crown to overheated and ultimately it may crack. Q.479 Faulty operation of diesel engine fuel injection nozzle can be a direct cause of Ans. sedimentation in the fuel supply. If the fuel supply is having sedimentation, sludge or hard particles the injection nozzle will get seized or there will excessive wear of injection nozzle and needle and guide and because of this the spray pattern will change and there will be faulty operation of diesel engine. Also the sludge may enter the combustion space and cause the weardown of the liner and piston rings. Q.480 Low cylinder compression pressure and high exhaust temperature may indicate Ans. (b) leaky valve. When the valves are leaking during compression the air will leak out and because of this the compression pressure will reduce. When this happens sufficient amount of air will not be available for the fuel to burn and under such circumstances there will loss of power and high exhaust temperature and black smoke. All this is result of leaky valve. Q.481 The loss of diesel engine cylinder air charge through leaky valves, piston rings, worned or scorned liner will be indicated by which of the following sets of conditions?
Ans. Low compression pressure and high exhaust temperature Due to loss of air during compression stroke, the compression pressure will drop. As the quantity of air available for combustion is less, incomplete combustion will take place which will cause high exhaust temperature, afterburning and loss of power and black smoke. As the compression pressure is low and the power is lost, the firing pressure will also drop with increase in the exhaust temperature. Q.482 During diesel engine warm-up which type of tappet clearance adjuster listed below for a change in length of exhaust valve? Ans. (c) Hydraulic tappet clearance adjuster. In hydraulic tappet clearance adjuster, there is no metal-to-metal contact between the pushrod and the stem of the valve. The opening of the valve is carried out by the hydraulic piston and hence it replaces the pushrod and rocker-arm combination. Hence whatever expansion of the valve and spindle takes place because of temperature, it will be taken care off by the hydraulic oil and hydraulic cylinder that is on the top of the valve. This cylinder is connected to the lubricating oil system through a throttle valve which will also allow for the excess oil to be drained off and the valve will close positively. Q.484 Worn cylinder head valve seat in a diesel engine will cause Ans. (a) less cold tappet clearance. As the valve will sit over the worn valve seat there will be more amount of vertical displacement of valve spindle. As a result, the clearance between the spindle and pushrod or rocker-arm will reduce and therefore the cold tappet clearance will reduce. Q.485 Which operating condition of a diesel engine is indicated by excessive firing pressure? Ans. (b) Overload. If the engine is overloaded and if the fuel pump timings are proper, more fuel will be burnt in the engine to accommodate for the increased load. When more fuel is burnt, automatically the firing pressure will be on the higher side. Q.486 If the jacket water temperature rises rapidly above normal in a diesel engine you should first Ans. (b) reduce the engine load. As a thumb rule, it is essential that for any rise in temperature of jacket cooling or exhaust the main reason may be either overloading of the engine or lack of cooling. Before diagnosing or rectifying the cause it is better to reduce the load and then carry out the necessary investigation and rectify the fault. Otherwise, time will be wasted in diagnosing and rectifying the cause and during that time the engine may damaged. As soon as we reduce the load, the heat generated inside the engine is released and this will help in reducing the jacket cooling temperature or atleast maintaining it and not allowing it to reach more dangerous limit. Also if the temperature is excessively high and if immediate action is taken to reduce the heat by increasing the flow of water, there is a possibility that the liner may crack due to sudden change in temperature. Inorder to avoid this, it is advisable to reduce the load and then carry out investigation. Q.487 A substantial increase in crankcase pressure could be an indication of Ans. worn cylinder liner. Especially in case of four stroke engine with trunk-type piston when liner worn out is excessive there is improper sealing between the liner and piston rings resulting in blowpast which will cause the high pressure gas to enter into the crankcase. This high pressure gas will cause the higher pressure in the crankcase. Q.488 Which of the following procedures should be carried out when large slow speed diesel engine is operated with one cylinder secured i.e. one cylinder is cut-off from the fuel? Ans. (d) The engine speed should be reduced. When the engine is operated with one unit cut-off the other unit may get over loaded and because of this the other units may get damaged and there will be severe damage to the entire engine. At the same time, there will be excessive vibration. In order to avoid all these things, it is necessary that when one unit is cut-off the load on engine be reduced so that the other units will not get overloaded and the engine will work moderately over a period of time. Q.489 The most practical way of detecting the overload in one cylinder of an operating large low speed main propulsion diesel engine is to Ans. (d) check the cylinder exhaust frequently. The exhaust temperature of the cylinder is a rough indication of power developed by the units. If the exhaust temperature is excessively high for a particular unit and other operating conditions are normal, it can be easily determined that the unit is overloaded. Hence the continuous observation of exhaust temperature of the units will indicate whether the units are properly loaded or they are overloaded and whether they are equally sharing the power or not Q.490 A diesel engine can fail to start because of Ans. incorrect fuel injection timing. If the fuel injection timing is too much out of the way, i.e. if the injection timing is too much earlier, the fuel will be injected so early without sufficient compression pressure that it will fail to ignite. If the fuel injection timing is excessively late then when the fuel is injected the compression pressure is sufficiently dropped down so that the fuel will not be properly ignited and the engine will fail to start. Q.491 One cause of diesel engine piston ring blowby is Ans. excessive lubrication Excessive lubrication will cause the lubrication oil to burn in the combustion space and top of the piston and excessive carbon deposits will be formed. They will get deposited on the piston rings which will cause the rings to seize inside the groove and thus the blowpast will take place. If the lubricating
oil supply is too low there will be metal-to-metal contact between the piston rings an the liner causing scuffing between the liner and the piston rings and cause excessive weardown of the liner and the blowpast. Q.492 If you notice smoke coming out from crankcase exhaust fans outlet of an operating diesel engines you should suspect Ans. broken piston rings. In case of trunk-type piston if the piston rings are broken, high temperature and high pressure combustion gases will enter into the crankcase because of blowpast. Due to this, the crankcase atmosphere will be filled with high temperature, high pressure exhaust gases. This can be seen from the exhaust of the crankcase when gas comes out of the exhaust fan outlet.
Q.493 Which of the following conditions may need to be reduced when operating a large low speed main propulsion diesel engine at low load? Ans. (c) Cooling water flow through aftercooler. The cooling water flow through the aftercooler has to be reduced in order to maintain the scavenge air temperature above the dew point; otherwise condensation of moisture will take place which will damage the lubricating oil film which will cause the excessive weardown of the liner and the piston rings. Q.494 A restricted diesel engine exhaust manifold operating under a normal load is indicated by Ans. (b) low firing pressure and high exhaust temperature. Due to the restriction in the exhaust manifold, the turbocharger rpm will be reduced and because of that less amount of air will be pumped by the turbocharger into the engine and hence it will cause lower compression pressure. As the complete removal of exhaust gases from the cylinder does not takes place, there will be mixing of the exhaust gas with the incoming fresh air thus diluting it which will cause the improper combustion of the fuel and hence low firing pressure and high exhaust temperature. Q.495 Excessive exhaust temperature in two-stroke diesel engine can be caused by Ans. (b) carbon built up in the exhaust ports. If carbon is built-up in the exhaust ports, the exhausting process is restricted. Due to this complete removal of exhaust gas from the cylinder is not achieved. As a result, the incoming fresh air will get mixed up with the cylinder thus diluting or contaminating the fresh air which will cause the improper combustion of the fuel. Hence exhaust temperature will increase and the peak pressure will drop and cause afterburning and black smoke. Q.496 What colour exhaust will be exhibited when a low speed two stroke main propulsion diesel engine designated to operate on light and heavy fuel oil is operated on insufficiently preheated heavy fuel oil? Ans. (b) Black When the heavy fuel oil is insufficiently heated up viscosity of the fuel oil will be high because of which when it is injected it will not properly atomize. At the same time the penetration will be high and because of all this reasons the fuel will not properly mix with the air. It will not evaporate neatly in the combustion space to form a combustion mixture which will ultimately result in high exhaust temperature, black smoke and loss of power. Q.497 Immediately after starting a diesel engine normal raw water and jacket water pressures are indicated, however jacket water temperature continues to rise. If there is no change in the seawater temperature, you should suspect Ans. jammed up three-way thermostatic valve. If the engine load and sea-water temperature is not changing still if the jacket water temperature is rising, it definitely indicates that the three-way valve which controls the flow of water through the coolers so as to maintain the jacket water temperature may be stuck up in such a manner that the jacket cooling water is getting by-passed and is not passing through the cooler. Thus the temperature will continuously go on increasing. Q.498 Before starting a diesel engine which has an engine-driven lubricating oil pump the engine should Ans. (c) pressurize the lubricating oil system. When the lubricating oil for the engine is supplied by an engine-driven pump, there will be some time delay till the attached lubricating oil pump will take the suction and develop sufficient pressure to provide the lubricating oil. During this transient time there is a possibility that engine bearings may get damaged. Hence it is necessary that an externally electrically driven lubricating oil priming pump has to be started earlier and proper lubrication of the bearings should be carried out and to keep the lubricating system under pressure. When the engine starts, the attached lubricating oil pump will have achieved high enough pressure and the electrically driven lubricating oil pump can be cut-off safely without causing any damage to the engine. Q.499 Collapsed hydraulic valve lifter in a diesel engine will result in Ans. (c) excessive valve clearance. Due to the collapse of the valve lifter the valve will not be lifted sufficiently high enough and it will not seat properly on the seat and it will leak. At the same time, the tappet clearance will also increase. Q.500 Prolonged operation of a diesel engine with a closed cooling water system at low operating temperature than designed will result in Ans. (d) sulphuric acid formation. If the engine is operated with the fuel having high sulphur content and low jacket cooling water temperature, the sulphuric acid formed because of burning of sulphur will reach its dew point. As a result the sulphuric acid will coat over the liner wall and this will cause corrosive wear to the liner and hence total liner weardown will increase. Hence it is necessary to maintain a sufficiently high temperature if the fuel is expected to have more amount of sulphur content. Q.67 What is the function of the main thrust bearing? Ans. (b) Transmitting propeller thrust to the hull.
In case of thrust bearing, there is a thrust collar and a pad. Due to the power produced by the engine, axial thrust is produced because the power is transmitted to the propeller shaft and the load is taken by the propeller blades. This loading of the propeller blades will try to push the propeller inwards and along with that the thrust will be transmitted in the form of axial thrust to the propeller shaft and then to the crankshaft. This thrust will be transmitted to the ship side through the collar which is an integrated part of the propeller shaft or the crankshaft and the pads which are fitted on the forward side during the forward running and to the aft side during the astern running. The thrust is transmitted to the ship’s hull as this is useful which helps in moving the ship in the desired direction. Q.68 The main function of compression ring is to Ans. (c) seal the space between the piston and the liner. High pressure gases due to combustion enters behind the piston rings. This pushes the piston rings in the outward direction to have a good contact between liner and the piston rings. At the same time, piston rings are also pushed downwards on the piston ring landing grooves. Thus it forms a complete sealed space above the piston rings in between the liner and the piston. Hence the force of gas behind the piston rings which forces the piston rings outwards is of prime importance. If the piston rings are seized or the grooves are not clear i.e. it is obstructed with carbon, there is a possibility the piston rings will not have proper contact with the liner and blowpast will take place which will cause breaking of piston rings and scuffing. Q.69 The function of diesel engine piston compression ring is to Ans. (c) to transmit heat from the piston to the cylinder liner. As the cylinder liner is better cooled off than the piston and in order to keep the piston and the piston rings in cooled condition to have better lubrication between piston rings and the liner, it is necessary that the piston rings should be kept as cool as possible. When the piston ring is in contact with the liner, some amount of heat from the piston is transmitted to the liner via the piston rings thus keeping the piston crown and the rings in cool condition, thus promoting the lubrication between the liner and the piston rings. Q.70 Diesel engine piston ring gaps can be straight-cut or angle-cut. In compression rings the angle cutting Ans. (d) decreases the combustion gas leakages. Due to the angle-cut rings and if they are left and right cut, if they are fitted alternatively, the resistance offered for the gas to flow through the gap between piston rings end i.e. butt clearance, is more. Hence less leakage takes place from the top of the piston rings to the bottom of the piston rings through the gap between the piston rings. Hence angle cutting is more preferred than straight cut. But in angle-cut rings, the joint at the cut position will be slightly weaker than the straight-cut rings. Q.71 Modern marine diesel engines equipped with mechanical fuel injection operates on combustion space within the cycle which is Ans. (c) combination of constant volume and constant pressure. In the mechanical fuel injection system, when fuel is injected the piston is approaching TDC, but the fuel is not injected exactly at TDC but much earlier than TDC. During this time, the volume is changing. At the same time, when the piston is at TDC when firing takes place though the pressure is increasing but as the piston is moving downwards the volume increases and the pressure remains constant. Thus there is a combination of constant pressure and constant volume which helps to have a better thermal efficiency as compared to other fuel injection systems. Q.72 Which of listed design is successfully used to provide the turbulence necessary for proper combustion in the diesel engine cylinder? Ans. (c) Precombustion chamber. The precombustion chamber is a part of the cylinder head where fuel is injected and combustion takes place in this chamber. This chamber is made up of spherical shape and because of the geometry of the chamber better turbulence is achieved. If the fuel is injected in this condition, it is burnt effectively and thus causing better combustion and thermal efficiency increases. These engines were used in earlier days but at presently they are not utilized because the geometry of the combustion space is made in such a manner including the piston top so as to have a turbulence created inside the main cylinder head rather than having precombustion chamber. Q.73 The purpose of counter-boring the top of the cylinder liner extending to the top of point of travel of the top compression ring is to Ans. (c) prevent wear of the liner to form ridge at the upper level of the ring crown. Counter-boring is the word utilized to give the tapered shape to the top of the cylinder liner. Because of the tapered boring the formation of ridge is reduced or it will not take place as there is gradual change in the diameter of the liner. Hence if there is any ridge formation it will be removed and any metal will not be allowed to form a ridge. If ridge formation takes place there is a possibility that the piston rings will hit the ridge which may cause the rings to crack and the liner to break. Q.74 The most rapid period of fuel combustion in a diesel engine cycle should begin just before the piston reaches the TDC and Ans. (d) should be completed just after the TDC. The maximum possible efficiency of any diesel can be achieved if the most of fuel is burnt near the TDC. In order to burn it near TDC, it is necessary that it should be injected before the TDC so that combustion mixture can be formed by the TDC. At the same time, complete combustion should be carried out before the expansion stroke begins so that no power is lost. If the combustion continues in the expansion stroke, it is called afterburning, in which there is loss of power and high exhaust temperature which gives lot of problems to the engine and hence result in loss of thermal efficiency. Taking into consideration all these things, it is necessary that the completion of the fuel burning should be shortly after the piston crosses the TDC position and should not be continued longer than this. Q.75 Exhaust gas pyrometers are useful for Ans. (a) detecting faulty combustion in an individual cylinder. Pyrometers are fitted on the individual cylinders. The exhaust gas temperature is a direct indication of any fault in the combustion process. If the combustion is late, we have high exhaust temperature and low power developed. If the fuel injectors are dripping, it will be revealed by high exhaust temperature. Also, any exhaust valve is leaking it will be also indicated by high exhaust temperature. This all indicates that the exhaust gas pyrometers fitted on individual cylinders will give the complete picture of health of combustion process. Also if the fuel nozzles are choked or if the rack position is
not proper or low in position, it will also indicated on the pyrometer by low exhaust temperature. Also in case of liner crack near the top dead center position, because of water ingress the exhaust temperature will drop. Hence considering all these problems which can be indicated by the use of exhaust temperatures it is necessary to installed an individual exhaust gas pyrometer on each cylinder. This can be utilized as a diagnostic tool. Q.78 Oil control rings functions to (a) allow proper lubrication of cylinder and compression rings. (b) reduce the amount of lubrication oil burnt in the combustion chamber. (c) scrap the excess oil from the liner on the down stroke. (d) All of the above. Ans. (d) All of the above. Oil scrapper or control ring’s job, in case of four stroke engines, as their edges are in downward direction is to scrap down the excess oil splashed onto the liner so as to prevent the excess lubricating oil consumption and burning of the excess lubricating oil along in the combustion process. Hence excess cylinder lubrication oil is scrapped. In case the oil control rings are fitted wrongly, the lubrication oil consumption will increase and the turbocharger and the exhaust manifold will get fouled. Q.78 The main function of the oil control rings is to Ans. (b) reduce the amount of lubrication oil burnt in the combustion chamber. In case of trunk type four-stroke engines the cylinder lubrication is carried out by splash lubrication. Hence large amount of excessive lubrication oil is carried to the liner. This excess lubricating oil from the crankcase which is coated onto the cylinder liner should be scrapped off so that only required amount of lubricating oil should be available for lubrication and sealing between the piston and liner. If the excess oil is not scrapped, it will get burnt in combustion space resulting in turbocharger and exhaust passage fouling and piston ring getting seized in the grooves due to excess carbon formation. In order to avoid all these problems, it is necessary to scrap the excess oil. This is done by the oil scrapper rings with sharp edge pointing downwards so that when piston comes downwards the oil will be scrapped from the liner and the scrapped oil will be dropped in the crankcase. Q.80 What is commonly used to create turbulence in a diesel engine combustion system? Ans. (a) Shape of the piston crown. As the air inlet valve opens, the piston is near TDC and exhaust valve also opens during the overlap period. The air entering from the air manifold through the air inlet valve hits over the crown and drives the exhaust gas from the cylinder through the already opened exhaust valve. As the incoming air is hitting the piston crown, the shape of the piston crown very important to create the flow patterns of the air which will be ultimately useful in creating turbulence for the better combustion of the fuel. Thus the shape of the piston crown is of prime importance. This shape will form a spherical combustion space which have better combustion efficiency and better turbulence. Hence thermal efficiency will increase. In case of two-stroke engines, the liner scavenge port configuration and angle at which the scavenge ports are cut is also of importance to create the turbulence. Q.80 How many power strokes per crankshaft revolution are there in an 8-cylinder two-stroke cycle diesel engine? Ans. (d) 8. As it is two-stroke engine, for every revolution there is a power stroke per cylinder. As there are 8 cylinders there will be 8 power strokes per revolution. Hence there will 8 strokes of power. Q.81 To shutdown a diesel engine after it exceeds the set maximum speed which type of device listed below is used? Ans. (c) Overspeed trip. It is necessary for the engine to trip if the speed exceeds beyond the set speed as the inertia and the centrifugal forces created because of high speed of the engine will give large amount of stresses onto the moving parts which may cause the failure of the moving parts and bottom end bearing bolts. Hence to avoid these damages, it is necessary that as soon as the speed increases beyond the preset limit the fuel should be cut-off and the engine should be made to stop. This is achieved either mechanically or electrically depending upon the engine and the type of injection system fitted. Mostly it is at 10% above the normal running rpm. If the engine trips because of overspeed, the restarting of the engine is possible only after resetting the fuel lever to the start position and then starting the engine. This is provided so that continuous stopping and starting of engine is prevented in case if the engine trips and overspeeds. Q.82 Turbulence is created in the cylinder of diesel engines to Ans. (b) help mixing of fuel and air. Turbulence is the swirling movement of the air due to which the air sweeps the complete volume of the liner and the combustion space. This causes the complete and efficient sweeping action thus leaving behind no exhaust gases from the previous stroke. Also because of the sweeping action, the fuel will be properly mixed with the air and complete air in the combustion chamber will be available for combustion rather than forming isolated pockets of combustion. This helps in better mixing between fuel and air and hence better combustion and hence thermal efficiency increases and fuel consumption decreases. Q.83 To function properly oil control rings used on diesel engine must distribute sufficient oil to all cylinder parts and also Ans. (b) prevent excessive lubrication from reaching the combustion space. The oil control rings or scrapper rings are used in the trunk type of diesel engines so that excessive lubricating oil which is splashed on the liner walls should be removed and drained back into the crankcase so that excessive lubrication will not get carried away by the exhaust gas into the exhaust manifold and turbocharger which may otherwise get fouled because of the lubricating oil burning in the exhaust piping and the combustion space. As this oil cannot withstand high temperature, it will get burnt and form large amount of carbon deposits. These carbon deposits will cause the piston rings to seize or turbocharger efficiency to drop.
Q.84 An important design characteristics of an explosion relief valve of a diesel engine is its ability to Ans. (a) close quickly inorder to prevent inrush of air. Explosion relief valves or crankcase relief valves are non-return flap valves which opens against the spring pressure due to the formation of excess pressure inside the crankcase. When the pressure reaches in the region of about maximum upto 0.2bars, these valves must open and let the crankcase gases to get relieved into the engine room atmosphere. At the same time as soon as this release is carried out, the spring should close this relief valve onto the seat. In case of crankcase explosion, viz. the primary explosion and secondary explosion, if the primary pressure wave created is not allowed to relieve, it will damage the crankcase and the personnel. This primary explosion leads to the secondary explosion which takes place due to ingress of fresh air into the crankcase from the atmosphere. If the ingress of air in the crankcase is prevented after the primary explosion in such condition a major secondary explosion can be avoided. Hence inorder to relieve the excess pressure generated into the crankcase and to prevent the ingress of fresh air into the crankcase to avoid secondary explosion, the crankcase relief doors are made up of non-return type. Q.85 In modern diesel engines which of the following is used as a support for main bearings? Ans. (a) Bed plate. In most of the modern engines, in case of conventional crankshaft, the crankshaft rests on the bottom half of the main bearing. The bottom half of the main bearing is resting over the saddle created in the bedplate. Thus the complete load of the engine will be transmitted via the main bearings to the bedplate and from there to the ship structure. But in case of undershrunk crankshafts, the crankshaft will not be resting over the bedplate but it is overhanged or undershrunked from the bedplate. This is useful for the easy removal of the main bearing for inspection and replacement. Q.86 Turbulence in the cylinder of two-stroke main propulsion diesel engine is mainly created by Ans. (d) intake port design. In case of two stroke engines, having the scavenge ports cut in the cylinder liner, the air enters through the scavenge ports. This air is responsible for sweeping clean the cylinder liner of any remaining exhaust gases and creating turbulence in the liner and combustion chamber. The entry of air through the scavenge port is made at some angle with respect to the circumference of the cylinder liner. In such condition, turbulence can be created in the cylinder liner and combustion space. Hence to have a tangential entry into the cylinder and to have the proper turbulence created by the scavenge air entering the cylinder liner, it is necessary to cut the cylinder liner intake ports at a specific design which will help in promoting the turbulence in the cylinder of two-stroke engines. As sweeping action by the scavenge air is very important in case of two-stroke engine, the exhaust removal is carried out by the incoming air rather than the upward movement of the piston as in case of four-stroke engines. Q.87 In internal combustion engine, which of the device listed will force the compression rings to seal the compression gases in the space above the piston? Ans. The gas pressure acting against the back of the rings The gas pressure due to combustion of fuel and compression of gas will find its way through the piston ring groove clearance i.e. the vertical clearance between the piston ring and the groove. This high pressure gases will act on the piston rings from behind because of which the piston rings will be forced radially outwards and it will exert a pressure onto the liner thus forming a perfect seal between ring rubbing surface and the liner. At the same time, this piston ring will be forced downwards against the piston ring groove by the gas pressure thus preventing the leakage of gas between the landing face of the piston ring and the groove. This clearance is of very importance and for this the piston ring should be free in the groove so that gas can find its way behind the piston rings to form a best possible seal. If this seal is not maintained due to any reason, the piston ring will not seal effectively against the liner which will cause the blowpast and ultimately lead to scuffing due to the wiping out of lubricating oil film between the liner and piston ring. The gap between the groove and the piston ring is maximum for the top ring and it reduces as the piston rings are fitted away from the combustion space i.e. the topmost ring will have maximum clearance and the lowermost ring will have the least clearance. Q.89 The size of the exhaust valve opening is Ans. (b) most critical in two-stroke cycle diesel engines. In case of two-stroke engines the process of exhausting is carried out with the help of incoming scavenge air and not by physical movement of piston as in case of four-stroke engines. Hence in two-stroke engines the exhausting process is not effective as that in case of four-stroke engines. Hence in order to improve the efficiency of exhausting it is necessary that there is no backpressure created onto the exhausting system. In such cases, it is necessary to have a maximum exhaust opening possible so that there is no backpressure on the exhaust gas to flow. Hence the scavenging and sweeping action in the cylinder liner of the exhaust gas is carried out efficiently which ultimately improves the fuel oil combustion characteristics and also it improves the thermal efficiency of the engine. Q.90 Provision is to be made for ventilation of an enclosed diesel engine crankcase by means of a Ans. (d) small breather or slight suction not exceeding 25mm of water. By ventilating the enclosed crankcase of a diesel engine, the pressure inside the crankcase can be reduced. Thus preventing the excess pressure developed which may cause the leakage in the crankcase doors and crankcase oil may finds its way out. At the same time, it will prevent the development of high concentration of oil mist in the crankcase. Thus reducing the possibility of crankcase explosion. In modern engines mostly, the crankcase external fans are not provided but crankcase weld is provided which is led to a remote plate which prevents the excessive built up of pressure inside the crankcase. This pressure should not be above 25mm of water gauge otherwise the leakage from the crankcase and the loss of oil will be very much. Q.91 Which of the general advantages listed does electrical pyrometers have over mechanical pyrometers? Ans. (d) It can indicate temperature at a distant point from the source of heat. In case of electrical pyrometer which works on the principle of thermocouple and the hot and cold junction, due to the temperature difference between the hot and cold junctions, an electric current is produced in the circuit. This current is directly proportional to the temperature difference between the cold and hot junction. This electric current which is produced due to the temperature difference can be amplified by use of amplifier and can be relayed through the wire to a remote place from where it can be indicated in the form of temperature. Hence with electric pyrometer, it is possible to have a remote indication rather than in case of ordinary pyrometer. In ordinary pyrometer, it is not possible to have remote indication of temperature.
Q.92 In large slow speed diesel engines the force applied to the crosshead is Ans. (c) against the crosshead during the power and compression stroke. In a crosshead engine, there is a crosshead shoe with two rubbing surfaces. At the same time, there are two crosshead guides one each at port side and starboard side. During the half revolution of the engine one face of the crosshead shoe rests against one side of the crosshead guides. This will continue upto BDC when the crank turns around the dead center and the piston starts moving upwards. In such condition, the directional forces of the crosshead changes and other face of the crosshead shoe will start resting against the other crosshead guide. Hence for half the revolution one face of shoe and guides are coming in contact and for the remaining half revolution other face of the shoe and guide are coming in contact. Hence both crosshead guides and shoes are required. Sometimes they are called ahead or astern shoes and guides. Hence even the engine may be unidirectional, it is necessary to have guides and shoes at both, port and starboard sides. Q.93 A diesel engine is driving an alternator required to run at 1800rpm. The overspeed governor is required to set within the range of Ans. (a) 1980rpm to 2070rpm. The overspeed governor is to be set at 10% of the rated rpm of the engine As the rated rpm of engine is 1800rpm the governor is to be set at 1980. If the rpm goes above this there is a possibility that excessive damage can be caused to the running gear of the engine as well as the alternator due to the centrifugal force which is created. At the same time, the bottom end bearings may be sheered off. Also the alternator gap between the stator and rotor. If the speed exceeds beyond particular limit because of centrifugal force, the mass of rotating poles on the rotor will try to move away from the center and reduce the air gap between the stator and rotor and the stator poles may touch the rotor and may cause severe damage. Hence it is necessary to control the speed within 1980rpm to 2070rpm. Q.94 A crankshaft whose center of gravity coincides with its centerline is said to be (a) statically balanced (b) dynamically balanced Ans. (a) statically balanced only. In case of statically balanced crankshaft the center of gravity through which all the load will pass will coincide with the center line of the crankshaft. Hence it will not come to the standstill position at the same point again and again. In such condition, the entire weight of the crankshaft is evenly distributed but this does not mean that when it starts rotating, the forces which are created at different planes are balanced. The vectors of forces created because of rotation of crankshaft may be in different planes and they may not balanced though the crankshaft is statically balanced. For dynamic balance we have to carry out the special procedure of dynamic balancing so that the vectors created because of the rotation of masses will be balanced with respect to each other and hence there is no unbalanced vector which is affecting the crankshaft.
Q.95 & Q.96 MATHEMATICAL Q.97 Diesel engine valve springs functions to Ans. (c) close the valves. Valves of a diesel engine always opens inside the cylinder and closes against the pressure inside the cylinder. So cylinder pressure acts over the valve lid which helps in keeping the valve closed. In addition to this, springs are also utilized which keeps the valve closed because of its upward pressure on the valve. These springs are of high importance to keep the valves closed onto the seat. The tension in the spring is also of high importance. If the tension in the spring is too much, the valve will be held against the seat with great pressure which will prevent the leakage but it may damage the valve seat. At the same time, for opening the valve excessive pressure will be required which will cause more amount of weardown to roller and cams. If the spring tension is not sufficient, the valve will not close properly on the seat and due to its own weight it may fall down and also cause leakage of the valve seat. Hence the spring is the most important part of valve gear and must be used for properly closing the valve on the valve seat. But in modern diesel engines, instead of springs of mechanical pressure, nowadays air springs are utilized due to various advantages it has over mechanical springs. Also it gives better rotation to the valve which will keep the valve seat clean and prevent the burning of the valve and extend the valve life. Q.98 Built up exhaust valve is one thing in which Ans. (a) stem and head are made of different material In earlier days these types of valves were utilized. In these valves the valve spindle and valve head material can be selected properly depending on working conditions to which they are subjected. As the valve seat and the valve head are subjected to high temperature and corrosion they are made up of special material and coated with stelite. In case of valve stem, which is subjected to more amount of wear because of its reciprocating movement in the valve spindle guide, valve spindle has to be wear-resistant. This is achieved by providing a wear-resistant material for the valve spindle. Hence in order to achieve best result from all aspect of the valve operating condition, it is necessary to make the valve part from the material which is suitable for the working condition of that part. Q.99 & Q100 MATHEMATICAL Q.101 Between the periods of injection and ignition of a fuel a diesel engine crankshaft rotates through the Ans. c delay period. The time between the injection and ignition is called delay period. This delay period is caused because as the fuel enters the combustion space, it takes some time for it to mix up with the hot compressed air and evaporate and to form a combustible mixture. Hence it is not possible to ignite the fuel at the instant it enters the cylinder, but it takes some time and by the time this delay period is lapsed, the combustion space is filled with thorough mixture of air and fuel so that the combustion can take place efficiently. This delay period depends on the quality of the fuel. If the fuel is of inferior quality the ignition delay increases. If the fuel is of superior quality the ignition delay is reduced. If the delay period is excessively high there is a possibility of knocking as well as afterburning and loss of power.
Q.102 The cylinder liner forming the cylinder wall and the inside of the water jacket is called a Ans. (b) wet liner Wet liner is a liner which is cooled by water and, for cooling by water, it is necessary to form an annular space between the jacket and outside of the liner so that water can be circulated through this space. For this purpose, annular space is provided between the liner outer wall and jacket so water can be circulated. This is called wet liner.
