IC Engine, its components, fuel injection and transmission system…
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1. INTRODUCTION 1.1 Introduction to IC engines 1.2 History 1.3 Engine classification 1.4 Engine components 1.5 Operations 1.6 How engines produce power 1.7 Two stroke engines 1.8 Four stroke engines 2. Construction and details of the engine 2.1 Piston and piston rings 2.2 Connecting rods 2.3 Crank shaft 2.4 Cylinder Block 2.5 Cylinder head and Gasket 2.6 Engine valves 2.7 Valve springs 2.8 Cam shaft 3. Fuel Injection 3.1. Objectives 3.2. Benefits of FI 4. Transmission – How the engine turns the wheels 4.1. Types of transmission
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1-1 INTRODUCTION: The internal combustion engine (I.e.) is a heat engine that converts chemical energy in a fuel into mechanical energy, usually made available on a rotating output shaft. Chemical energy of the fuel is first converted to thermal energy by means of combustion or oxidation with air inside the engine. This thermal energy raises the temperature and pressure of the gases within the engine and the high-pressure gas then expands against the mechanical mechanisms of the engine. The mechanical linkages of the engine to a rotating crankshaft convert this expansion, which is the output of the engine. The crankshaft, in turn, is connected to a transmission and/or power train to transmit the rotating mechanical energy to the desired final use. For engines this will often be the propulsion of a vehicle (i.e., automobile, truck, locomotive, marine vessel, or airplane). Most internal combustion engines are reciprocating engines having pistons that reciprocate back and forth in cylinders internally within the engine. This report concentrates on the study of this type of engine. 1-2 HISTORY Early development of modern internal combustion engines occurred in the latter half of the 1800s and coincided with the development of the automobile. History records earlier examples of crude internal combustion engines and self-propelled road vehicles dating back as far as the 1600s. Most of these early vehicles were steam-driven prototypes, which never became practical operating vehicles. Very early examples of heat engines, including both internal combustion and external combustion, used gunpowder and other solid, liquid, and gaseous fuels. Major development of the modern steam engine and, consequently, the railroad locomotive occurred in the latter half of the 1700s and early 1800s. Most of the very earliest internal combustion engines of the 17thand 18th centuries can be classified as atmospheric engines. These were large engines with a single piston and cylinder, the cylinder being open on the end. Combustion was initiated in the open cylinder using any of the various fuels, which were available.
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Fig 1-1. The Charter Engine made in 1893 at the Beloit works of Fairbanks, Morse & Company was one of the first successful gasoline engine offered for sale in the United States. During the early years of the automobile, the internal combustion engine competed with electricity and steam engines as the basic means of propulsion. Early in the 20th century, electricity and steam faded from the automobile picture-electricity because of the limited range it provided, and steam because of the long start-up time needed. Thus, the 20th century is the period of the internal combustion engine and the automobile powered by the internal combustion engine. Now, at the end of the century, electricity and other forms of propulsion systems for automobiles and other applications are again challenging the internal combustion engine. What goes around comes around. The first fairly practical engine was invented by J.J.E. Lenoir (1822-1900) and appeared on the scene about 1860. These engines were built with power up to about 4.5 kW (6 hp) and mechanical efficiency up to 5%.In 1867 the Otto-Langen engine, with efficiency improved to about 11%, was first introduced. This was a type of atmospheric engine with the power stroke propelled by atmospheric pressure acting against a vacuum. In the 1880s the internal combustion engine first appeared in automobiles. By 1892, Rudolf Diesel (1858-1913) had perfected his compression ignition engine into basically the same diesel engine known today. It wasn’t until the 1920s that multicylinder compression ignition engines were made small enough to be used with automobiles and trucks.
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1-3 ENGINE CLASSIFICATIONS Internal combustion engines can be classified in a number of different ways: 1. Types of Ignition (a) Spark Ignition (SI) (b) Compression Ignition (CI) 2. Engine Cycle (a) Four-Stroke Cycle. (b) Two-Stroke Cycle. 3. Valve Location (a) Valves in block (flat head), L head engine (b) Valves in head (overhead valve), also called I Head engine. (c) One valve in head (usually intake) and one in block, also called F Head
Fig 1-2 various valve locations in engine head 4. Basic Design (a) Reciprocating (b) Rotary 5. Position and Number of Cylinders of Reciprocating Engines (a) Single Cylinder v
(b) In-line (c) V engine (d) Opposed cylinder engine (e) W engine (f) Opposed piston engine (g) Radial engine
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Fig 1-3. Engine classification by cylinder arrangement 6. Air Intake Process (a) Naturally Aspirated (b) Supercharged (c) Turbocharged 7. Method of Fuel Input for SI Engines (a) Carbureted. (b) Multipoint Port Fuel Injection. (c) Throttle Body Fuel Injection. 8. Fuel Used (a) Gasoline. (b) Diesel Oil or Fuel Oil. (c) Gas, Natural Gas, Methane. (d) LPG. (e) Alcohol-Ethyl, Methyl. (f) Gasohol 9. Application (a) Automobile, Truck, Bus. (b) Locomotive. (c) Stationary. (d) Marine. (e) Aircraft. (f) Small Portable, Chain Saw, Model Airplane. vii
10. Type of Cooling (a) Air Cooled. (b) Liquid Cooled, Water Cooled. 1-4 ENGINE COMPONENTS Block: Body of engine containing the cylinders, made of cast iron or aluminum. Camshaft: Rotating shaft used to push open valves at the proper time in the engine cycle, either directly or through mechanical or hydraulic linkage (push rods, rocker arms, tappets). Camshafts are generally made of forged steel or cast iron and are driven off the crankshaft by means of a belt or chain (timing chain). In four-stroke cycle engines, the camshaft rotates at half engine speed.
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Fig 1-4. Cross-section of four-stroke cycle S1 engine showing engine components; (A) block, (B) camshaft, (C) combustion chamber, (D) connecting rod, (E)crankcase, (F) crankshaft, (G) cylinder, (H) exhaust manifold, (I) head, (J) intake manifold, (K) oil pan, (L) piston, (M) piston rings, (N) push rod, (0) spark plug, (P)valve, (Q) water jacket.
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Carburetor: Venturi flow device, which meters the proper amount of fuel into the airflow by means of a pressure differential. Catalytic converter: Chamber mounted in exhaust flow containing catalytic material that promotes reduction of emissions by chemical reaction. Combustion chamber: The end of the cylinder between the head and the piston face where combustion occurs. Connecting rod: Rod connecting the piston with the rotating crankshaft, usually made of steel or alloy forging in most engines but may be aluminum in some small engines. Cooling fins: Metal fins on the outside surfaces of cylinders and head of an air-cooled engine. These extended surfaces cool the cylinders by conduction and convection. Crankcase: Part of the engine block surrounding the rotating crankshaft. In many engines, the oil pan makes up part of the crankcase housing. Crankshaft: Rotating shaft through which engine work output is supplied to external systems. The crankshaft is connected to the engine block with the main bearings. It is rotated by the reciprocating pistons through connecting rods connected to the crankshaft, offset from the axis of rotation. This offset is sometimes called crank throw or crank radius. Most crankshafts are made of forged steel, while some are made of cast iron. Cylinders: The circular cylinders in the engine block in which the pistons reciprocate back and forth. The walls of the cylinder have highly polished hard surfaces. Exhaust manifold: Piping system, which carries exhaust gases away from the engine cylinders, usually made of cast iron. Exhaust: system Flow system for removing exhaust gases from the cylinders, treating them, and exhausting them to the surroundings. Flywheel: Rotating mass with a large moment of inertia connected to the crankshaft of the engine. The purpose of the flywheel is to store energy and furnish large angular momentum that keeps the engine rotating between power strokes and smoothes out engine operation. Fuel injector: A pressurized nozzle that sprays fuel into the incoming air on SI engines or into the cylinder on CI engines. Fuel pump: Electrically or mechanically driven pump to supply fuel from the fuel tank (reservoir) to the engine. Head: The piece which closes the end of the cylinders, usually containing part of the clearance volume of the combustion chamber. The head is usually cast ironer aluminum, and bolts to the engine block. x
Head gasket: Gasket which serves as a sealant between the engine block and head where they bolt together. They are usually made in sandwich construction of metal and composite materials. Some engines use liquid head gaskets. Intake manifold: Piping system which delivers incoming air to the cylinders usually made of cast metal, plastic, or composite material. In most SI engines, fuel is added to the air in the intake manifold system either by fuel injectors or with carburetor. Some intake manifolds are heated to enhance fuel evaporation. Main bearing: The bearings connected to the engine block in which the crankshaft rotates. The maximum number of main bearings would be equal to the number of pistons plus one, or one between each set of pistons plus the two ends. Oil pan: Oil reservoir usually bolted to the bottom of the engine block, making up part of the crankcase. Acts as the oil sump for most engines. Oil pump: Pump used to distribute oil from the oil sump to required lubrication points. The oil pump can be electrically driven, but is most commonly mechanically driven by the engine. Oil sump: Reservoir for the oil system of the engine, commonly part of the crankcase. Piston: The cylindrical-shaped mass that reciprocates back and forth in the cylinder, transmitting the pressure forces in the combustion chamber to the rotating crankshaft. The top of the piston is called the crown and the sides are called the skirt. The face on the crown makes up one wall of the combustion chamber and may be a flat or highly contoured surface. Piston rings: Metal rings that fit into circumferential grooves around the piston and form a sliding surface against the cylinder walls. Near the top of the piston are usually two or more compression rings made of highly polished hard chrome steel. The purpose of these is to form a seal between the piston and cylinder walls and to restrict the high-pressure gases in the combustion chamber from leaking past the piston into the crankcase. Push rods: Mechanical linkage between the camshaft and valves on overhead valve engines with the camshaft in the crankcase. Many push rods have oil passages through their length as part of a pressurized lubrication system. Radiator Liquid-to-air heat exchanger of honeycomb construction used to remove heat from the engine coolant after the engine has been cooled. The radiator is usually mounted in front of the engine in the flow of air as the automobile moves forward. xi
Spark plug: Electrical device used to initiate combustion in an SI engine by creating high-voltage discharge across an electrode gap. Spark plugs are usually made of metal surrounded with ceramic insulation. Throttle: Butterfly valve mounted at the upstream end of the intake system, used to control the amount of air flow into an SI engine. Some small engines and stationary constant-speed engines have no throttle. Valves: Used to allow flow into and out of the cylinder at the proper time in the cycle. Most engines use poppet valves, which are spring loaded closed and pushed open by camshaft action. Valves are mostly made of forged steel. Water jacket: System of liquid flow passages surrounding the cylinders, usually constructed as part of the engine block and head. Engine coolant flows through the water jacket and keeps the cylinder walls from overheating. The coolant is usually a water-ethylene glycol mixture.
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1-5: OPERATION If you put a tiny amount of high-energy fuel (like gasoline) in a small, enclosed space and ignite it, an incredible amount of energy is released in the form of expanding gas. You can use that energy to propel a potato 500 feet. In this case, the energy is translated into potato motion. You can also use it for more interesting purposes. For example, if you can create a cycle that allows you to set off explosions like this hundreds of times per minute, and if you can harness that energy in a useful way, what you have is the core of a car engine!
Fig 1-5. Front section of a 4 cylinder IC engine All internal combustion engines depend on the exothermic chemical process of combustion: the reaction of a fuel, typically with air, although other oxidizers such as nitrous oxide may be employed. xiii
The most common fuels in use today are made up of hydrocarbons and are derived from petroleum. These include the fuels known as diesel, gasoline and liquefied petroleum gas. Most internal combustion engines designed for gasoline can run on natural gas or liquefied petroleum gases without modifications except for the fuel delivery components. All internal combustion engines must have a means of ignition to promote combustion. Most engines use either an electrical or a compression heating ignition system. Electrical ignition systems generally rely on a lead-acid battery and an induction coil to provide a high voltage electrical spark to ignite the air-fuel mix in the engine's cylinders. This battery can be recharged during operation using an alternator driven by the engine. Compression heating ignition systems, such as diesel engine, rely on the heat created in the air by compression in the engine's cylinders to ignite the fuel. Once successfully ignited and burnt, the combustion products, hot gases, have more available energy than the original compressed fuel/air mixture (which had higher chemical energy). The available energy is manifested as high temperature and pressure which can be translated into work by the engine. In a reciprocating engine, the high pressure product gases inside the cylinders drive the engine's pistons. Once the available energy has been removed the remaining hot gases are vented (often by opening a valve or exposing the exhaust outlet) and this allows the piston to return to its previous position (Top Dead Center - TDC). The piston can then proceed to the next phase of its cycle, which varies between engines. Any heat not translated into work is a waste product and is removed from the engine either by an air or liquid cooling system.
1- 6 HOW THE ENGINE PRODUCES POWER: Petrol and air are mixed in the carburetor and drawn into a combustion chamber at the top of each cylinder. Pistons, inside the cylinders, compress the mixture, which is then ignited by a spark plug. As the mixture burns it expands, forcing the piston down on its power stroke. The up and down movement of the piston is transformed into the rotary movement to drive the crankshaft, which in turn transmits power to the wheels through the clutch, gearbox and final drive. Connecting rods link the piston to the crankshaft. xiv
A camshaft, driven by the crankshaft, controls the inlet and exhaust valves at the top of each cylinder. Initial impetus to set the engine in motion comes from the starter motor. This is connected to the starter ring, which is fitted around the edge of the flywheel-a heavy disc bolted to the end of the crank shaft. The starter motor, which is geared to the flywheel, is operated electrically and turns the flywheel and crankshaft, which starts the pistons and the connecting rods moving up and down. The flywheel smoothes out the power impulses of the pistons and gives a relatively smooth rotation on the crankshaft. Because of the heat produced by an internal combustion engine, the metal parts would seize without a cooling system. In most cars, water is circulated through channels in the engine called water-jacket. The hot water then passes through a radiator where the heat is dispersed to the atmosphere. The dispersal of heat is speeded by a fan, which draws cooling air through the radiator. To prevent wear and overheating, the engine has a lubrication system. Oil, kept in a sump underneath the cylinder block, is pumped around the engine.
1-7 Two-stroke Engines based on the two-stroke cycle use two strokes (one up, one down) for every power stroke. Since there are no dedicated intake or exhaust strokes, alternative methods must be used to scavenge the cylinders. The most common method in spark-ignition two-strokes is to use the downward motion of the piston to pressurize fresh charge in the crankcase, which is then blown through the cylinder through ports in the cylinder walls. Spark-ignition two-strokes are small and light (for their power output), and mechanically very simple. Common applications include snowmobiles, lawnmowers, chain saws, jet skis, mopeds, outboard motors and some motorcycles. Unfortunately, they are also generally louder, less efficient, and far more polluting than their four-stroke counterparts, and they do not scale well to larger sizes. Interestingly, the largest compression-ignition engines are two-strokes, and are used in some locomotives and large ships. These engines use forced induction to scavenge the cylinders.
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Fig 1-6. Cross section of a 2 stroke petrol engine A 2-stroke engine is different from a 4-stroke engine in two basic ways. First, the combustion cycle is completed within a single piston stroke as oppose to two piston strokes, and second, the lubricating oil for the engine is mixed in with the petrol or fuel. In some cases, such as lawnmowers, you are expected to pre-mix the oil and petrol yourself in a container, and then pour it into the fuel tank. In other cases, such as small motorbikes, the bike has a secondary oil tank that you fill with 2-stroke oil and then the engine has a small pump which mixes the oil and petrol together for you.
Fig 1-7. Two stroke cycle xvi
1-7-1 Advantages of the two stroke: •
Has more get-up-and-go because it fires once every revolution, giving it twice the power of a four stroke, which only fires once every other revolution.
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Packs a higher weight-to-power ratio because it is much lighter.
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Is less expensive because of its simpler design.
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Can be operated in any orientation because it lacks the oil sump of a four stroke engine, which has limited orientation if oil is to be retained in the sump.
These attributes make two stroke engines very popular for a variety of uses from dirt bikes, mopeds, jet skis, and small outboard motors, to lawn and garden equipment such as mowers, edgers, leaf blowers, chain saws and hedge trimmers. But there are other differences between the two stroke and four stroke engines that aren't so favorable, which is why you won't see two stroke engines in cars.
1-7-2 Disadvantages of the two stroke: •
Faster wear and shorter engine life than a four stroke due to the lack of a dedicated lubricating system.
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Requires special two stroke oil ("premix") with every tank of gas, adding expense and at least a minimal amount of hassle.
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Heavily pollutes because of the simpler design and the gas/oil mixture that is released prior to, and in the exhaust (also creates an unpleasant smell).
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Is fuel-inefficient because of the simpler design, resulting in poorer mileage than a four stroke engine.
1-8 Four-stroke Engines based on the four-stroke cycle or Otto cycle have one power stroke for every four strokes (updown-up-down) and are used in cars, larger boats and many light aircraft. They are generally quieter, more efficient and larger than their two-stroke counterparts. There are a number of variations of these cycles, most notably the Atkinson and Miller cycles. Most truck and automotive Diesel engines use a xvii
four-stroke cycle, but with a compression heating ignition system. This variation is called the diesel cycle. 4-stroke engines are typically much larger capacity than 2-stroke ones, and have a lot more complexity to them. Rather than relying on the simple mechanical concept of reed valves, 4-stroke engines typically have valves at the top of the combustion chamber. The simplest type has one intake and one exhaust valve. More complex engines have two of one and one of the other, or two of each. The valves are opened and closed by a rotating camshaft at the top of the engine. The camshaft is driven by either gears directly from the crank, or more commonly by a timing belt. Because of the nature of 4-stroke engines, you won't often find a single-cylinder 4-stroke engine. They do exist in some off-road motorbikes but they have such a thump-thump-thump motion to them that they require some large balancing shafts or counterweights on the crank to try to make the ride smoother. They also take a little longer to start from cold because you need to crank the single piston at least twice before a combustion cycle can start. . Apart from the increased capacity, more cylinders typically mean a smoother engine because it will be more in balance.
Fig 1-8. Cross section of a modern 4 stroke petrol engine (jaguar) xviii
Fig 1-9. The 4 stroke cycle
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2. Constructional details of the engine 2-1 The piston and piston rings The piston performs the following functions— (1) Forms a movable gas-tight plug to confine the charge in the cylinder (2) Transmits to the connecting rod the forces generated by combustion of the charge (3) Forms a guide and a bearing for the small end of the connecting rod, and takes the lateral thrust due to the obliquity of that rod. Almost all modern engines have aluminium alloy pistons. Because the aluminium alloy is of lower strength than cast iron, thicker sections have to be used so not all the advantage of the light weight of this material is realized. Moreover, because of its higher coefficient of thermal expansion, larger running clearances have to be allowed. On the other hand, the thermal conductivity of aluminium is about three times that of iron. This, together with the greater thicknesses of the sections used, enables aluminium pistons to run at temperatures about 200¢XC lower than cast-iron ones. Consequently, there is little or no tendency for deposition of carbon – due to thermal breakdown of lubricant – beneath the piston crown or in ring grooves. So important is this that sections thicker than necessary for carrying the mechanical loads are in many instances used to obtain a good rate of cooling by heat transfer.
Fig 1-10. Piston and connecting rod xx
The main functions of piston rings are— (1) To form a pressure seal, preventing blow-by of the gases, including combustion products at high temperatures. (2) The transfer of heat from the piston to the cylinder walls. (3) Control of the flow of oil in adequate quantity to the skirt and to the rings themselves, while preventing excessive amounts from entering the combustion chamber. Fine-grain alloy cast iron has proved superior to any other material for this purpose. Its merits arise from its excellent heat- and wear-resistance inherent in its graphic structure. Piston rings are mostly cast in the open condition and then cam-turned to a profile such that, when they are closed to fit into the cylinder, their peripheries are a true circle. Final machining is done with the ring in the closed condition.
Fig 1-11Piston rings In the early days of the internal combustion engine, only one compression ring was used. Subsequently, two were found to seal better, and later even three were used. As speeds increased, greater quantities of lubricant had to be supplied and the problem of controlling it had to be faced. In the first instance, simply using a lower ring having a narrow face width, to increase its contact pressure, did this. Then stepped, or beveled or taper faced rings were employed and, ultimately, grooved and slotted rings were adopted.
2-2 Connecting rods Most connecting rods are medium carbon steel stampings. For special applications, however, they may be forgings, and aluminium alloys or even titanium may be used. Very highly stressed rods – for example in racing car engines – are sometimes machined all over to improve fatigue strength and to reduce weight to a minimum. Power is transmitted from the pistons to the crankshaft- and at the same time converted to rotary motion- by the connecting rods and crankshaft. The connecting rods are usually steel forgings. xxi
The top end of each connecting rod, called the small-end, is fitted inside the piston to the gudgeon pin, which allows the rod to pivot as it moves up and down with the piston. The gudgeon pin, also known as the wrist pin, is usually hollow to save weight and is often located in the piston by two spring clips called circlips. The bottom end of the connecting rod, called the big end, is bolted to the crankshaft and follows a circular orbit, while the small-end follows the up-and-down movement of the piston. A big end maybe split horizontally or obliquely. An obliquely split, by reducing the width of the rod at its widest point, allows a larger connecting rod to be withdrawn upwards through the cylinder bore.
Fig 1-12. Connecting rod assembly
2-3 The crankshaft The crankshaft represents the final link in the conversion of reciprocating motion at the piston to one of rotation at the flywheel. In the case of the multicylinder engine, the crankshaft has to control the relative motions of the pistons, while simultaneously receiving their power impulses. The timing drive for the engine valve mechanism is taken from the front end of the crankshaft, as is the pulley and belt drive for the engine auxiliaries, such as the cooling fan and the alternator for the electrical system. Attached to the rear end of the crankshaft is the engine flywheel.
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The crankshaft is supported radially in the crankcase by a series of bearings, known as the engine main bearings. Each main bearing is divided into two half-liners, similar to the big end bearings, and again to allow assembly around the journals of the one-piece construction crankshaft. Crankpins and are shaped so that they serve as balance weights for smoother running. The fly-wheel, a heavy and carefully balanced disc fitted at the gearbox end of the crankshaft, helps smooth running by maintaining a steady rate of turning. With the downward thrust of the pistons giving repeated sudden thrusts to the crankshaft as
Fig.1-13 crankshaft
the fly-wheel maintains its momentum; the shaft is subject to slight twisting and untwisting, known as torsional vibration. A damper- a metal disc incorporating a ring of rubber- is fitted to the front end of the crankshaft to help control torsional vibration. The firing order of the cylinders is also important in making the crankshaft rotate smoothly. Counting the cylinder nearest to the fan as no.1, the firing order in a 4-cylinder engine is usually 1,3,4,2 or 1,2,4,3, so as to give reasonably even turning of the crankshaft. On their firing strokes, the pistons push the crankshaft down; and on their other three strokes they are forced up or pulled down by the continuing rotation of the crankshaft. The crankpins are set at different angles to the shaft so as to give a uniform spacing of the firing impulses. Crankshafts are generally steel forgings, though high carbon, high copper, chromium silicon iron has been used and nodular, or spheroidal graphite (SG), cast iron is becoming increasingly popular.
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Fig 1-14. Basic crankthrow arrangement
2-4 CYLINDER BLOCK The cylinder block, the main shell of the engine, is usually combined with the crankcase in one casting. Most blocks are made of cast iron because it is fairly strong, cheap and easy to machine for mass production. The block’s strength can be improved by alloying cast iron with other metals. Some cylinder blocks are made of light alloys, which make them lighter and better for conducting heat; but they are more costly. They are also too soft to provide a working surface for the cylinder bores and separate cast iron liners or sleeves must be inserted into the bores. The water-jacket, passages through which the water circulates to cool the cylinders, is usually
Fig 1-15 cylinder block
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cast as an integral part of the cylinder block. It is linked with the corresponding water jacket of the cylinder head through opening at the top. A block can be cracked by water expanding as it freezes in the jacket. Sometimes the expansion will dislodge the core plugs which seal holes required in the process of casting the cylinder block; but core plugs cannot be relied on, to act as ‘safety valves’ in this way. Cylinders can be arranged in one row (in-line), in two rows set at an angle (V-engine) or set sideways in two rows, on either side of the crankshaft (flat or horizontally opposed). Most 4 and 6 cylinder engines are inline.
2-5 Cylinder head and gasket The functions of the cylinder head may be listed as follows: 1. It must provide a closure or chamber for the upper part of each cylinder, so that the gas pressure created by the combustion process is constrained to act against the piston. 2 .Associated with function 1 is the need to incorporate a gas porting system with inlet and exhaust valves, as well as a platform upon which to mount their operating mechanism. Provision must also be made for a screwed boss to retain the sparking plug. 3 .Similar to the cylinder block, the head must form a jacket that allows liquid coolant to circulate over the high temperature metal surfaces. 4 .It is required to contribute to the overall rigidity of the engine structure and maintain a uniform clamping pressure on its sealing gasket with the cylinder block. The sealing gasket is generally known as the cylinder head gasket or simply ‘head gasket’. In liquidcooled engines thefunction of the cylinder head gasket is to seal the combustion chambers and coolant and oil passages at the joint facesof the cylinder block and head. The gasket is therefore specially shaped to conform to these openings, and is also provided with numerous holes through which pass either the studs or the set bolts for attaching the cylinder head to the block. The material most commonly used for the cylinder head of an overhead-valve engine is cast iron. An alternative used in many engines is aluminium. Aluminium is also used on a no. of high-performance engines, particularly in sports cars, because it weighs less and is a better conductor of heat. But an aluminum cylinder head needs inserts to strengthen the valve seats and guides, and may present difficulties in making an effective joint with a cast-iron cylinder block, as the two metals expand at different rates. xxv
Fig 1-16 Operation of four-cylinder engine harmonic balancer
2-6 Engine valves It has long been established practice to employ what are termed poppet valves, which comprise a discshaped head with a conical seating and a stem that acts as a guiding surface. Their main advantages over other possible alternative valve forms are as follows: 1. They are self-centering as they close on to the cylinder head seating. 2. They possess freedom to rotate to a new position. 3. It is relatively easy to restore their sealing efficiency in service. For mechanical strength and to assist gas flow, the valve stem is blended into the head portion to form a neck of fairly generous radius, but a certain amount of relative flexibility between them is generally permitted. Under the influence of cylinder pressure, the valve head may therefore better conform to its seating should there be any distortion present. The valve stem is provided with a few hundredths of a xxvi
millimeter working clearance in its guide; this is usually increased slightly for the hotter exhaust valves to allow for their greater expansion. It is usual for the head diameter of the exhaust valve to be made less than that of the inlet valve, since cylinder gases may be more easily evacuated at exhaust pressure than admitted at induction depression – truly, as Spinoza observed, ‘Nature abhors a vacuum’! Another reason for favoring a smaller-diameter exhaust valve is generally to reduce its thermal loading by virtue of the shorter path of heat flow. The conical facing of the valve makes an angle of either 45° or 30° with the plane of the head. Although the former angle provides a higher seating pressure for a given valve spring load, the latter angle permits a greater gas flow for a given valve lift. Hence in some engines the face angles of the exhaust and the inlet valves may be 45° and 30° respectively.
Fig 1-17 poppet valve terminology and valve seating angles Exhaust valves in particular have to withstand mean operating temperatures that may approach 900°C, with about75 per cent of this heat being transferred to the cooling system through the valve seat and the remainder through the valve guide. Since the thermal loading on the inlet valves is less severe, they are generally produced from low-alloy silicon-chromiumsteel, which is also known as Silchrome. The most widely used material for the exhaust valves is chromium-manganese-nickel steel. This high-alloy material has a combination of hot strength and corrosion resistance that meets the requirements of most engines at temperatures upto over 800°C. The durability of the exhaust valves has sometimes been enhanced by the application of an aluminium coating to their heads.
2-7 Valve springs Valve springs are required to ensure that the motion of the valves and their operating mechanism follows faithfully that of the cams. Since this motion is a constantly changing one, inertia forces are created that may be either positive or negative. The former tend to maintain the component parts of the xxvii
valve train in contact with one another, whilst the latter act to separate them. Hence, the valve springs serve to counteract the unwanted negative inertia forces. The valve springs must also maintain adequate sealing pressure for the valves during their intended closed period, in which respect they are assisted by the cylinder gas pressure acting upon the valve heads during the compression and power strokes. In conventional practice, the valve springs consist of wire wound in the form of a helix or coil. The valve closing load is conveyed axially along the spring, which stresses the material principally in torsion. For the valve open and the valve closed conditions, the ratio of spring loads is usually in the region of 2:1. Since the valve spring is compressed between parallel abutments, its end coils are ground flat and square with the spring axis. Because they are subjected to severe service, valve springs are produced from high-duty materials, these generally being either hard-drawn carbon steel or chrome-vanadium steel. To reduce stress concentration in the spring wire and thus make it less liable to fail by fatigue, the valve spring may be shot peened. In this process the wire surface is bombarded at high velocity with metal shot, which induces a residual compressive stress that discourages crack propagation in the material.
Fig 1-18. Valve operation
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(a)
(b)
Fig 1-19 (a) valve spring position (b) valve train (c) valve timing diagram for commercial vehicle (a) and passenger car (b) and last is the push rod valve assembly.
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2-8 CAMSHAFT This shaft is of either forged steel or cast iron, machined and hardened to give maximum resistance to wear of the flanks of the cams. The cams are spaced at intervals to match the firing order. As the ignition system must provide a spark at each spark-plug at the right time in relation to the valve operation, the distributor, which supplies high voltage current to the plugs, is usually driven by gears from the camshaft or by the crankshaft. The camshaft is carried on 3 or 5 bearings mounted in the block. The cams are spaced around the camshaft at intervals to match the firing order. The profiles and timings of the cams have a major effect on the power output and fuel economy of the engine.
3. Fuel injection The combustion process in the diesel engine cylinder is initiated by spontaneous ignition of the fuel, which is injected into an agitated, highly compressed and therefore exceedingly hot charge of air. To accomplish this demanding task, diesel fuel injection systems may be classified as follows: 1. In-line fuel injection pump 2. Distributor fuel injection pump 3. Unit fuel injection 4. Common rail fuel injection Fuel injection is a method/system for metering fuel into an internal combustion engine, where the fuel is burned in air to release energy in the form of heat, which is then converted to mechanical work by the engine based on the gas laws. In modern automotive applications, the fuel metering task is only one of several functions performed by an engine management system. For gasoline engines, carburetors were the predominant method to meter fuel prior to the widespread use of electronic fuel injection (EFI). However, a wide variety of injection schemes have existed since the earliest usage of the internal combustion engine. One major distinction between carburetors and fuel injection is that fuel injection atomizes the fuel by forcibly pumping it through a small nozzle under high pressure, whereas a carburetor relies on the modest air pressure created by intake air rushing through it to add the fuel to the airstream.
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Another notable difference is that a carburetor performs several important functions in one single component: it measures engine load, calculates the amount of fuel needed, and adds the required fuel to the airstream. With fuel injection, these functions are performed by separate subsystems and components. This means that each subsystem can be specialized and optimized for its particular role, which brings a number of important performance benefits compared to the compromise solution offered by carburetors. Carburetors do have some advantages over fuel injection: Lower complexity, and lower cost. Prior to 1980, nearly all automotive gasoline engines used carburetors. However, carburetors are simply not accurate enough to deliver the performance (particularly with respect to emissions) that is expected today. Since 1990, almost all gasoline passenger cars sold in developed markets like United States, Europe, and Japan use electronic fuel injection (EFI).
3-1 Objectives The functional objectives for fuel injection systems can vary. All share the central task of supplying fuel to the combustion process, but it is a design decision how a particular system will be optimized. There are several competing objectives such as: •
power output
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fuel efficiency
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emissions performance
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ability to accommodate alternative fuels
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durability
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reliability
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drivability and smooth operation
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initial cost
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maintenance cost
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diagnostic capability
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range of environmental operation xxxi
Certain combinations of these goals are conflicting, and it is impractical for a single engine control system to fully optimize all criteria simultaneously. In practice, automotive engineers strive to best satisfy a customer's needs in a competitive manner. The modern digital EFI system is far more capable at optimizing these competing objectives than a carburetor.
3-2 Benefits An engine's air/fuel ratio must be accurately controlled under all operating conditions to achieve the desired engine performance, emissions, drivability and fuel economy. Modern FI systems meter fuel with great precision. The two fundamental improvements are: 1. Reduced response time to rapidly changing inputs, e.g., rapid throttle movements. 2. Deliver an accurate and equal mass of fuel to each cylinder of the engine, dramatically improving the cylinder-to-cylinder distribution of the engine. These two features result in the following performance benefits: •
Exhaust Emissions o Significantly reduced "engine out" or "feed gas" emissions (the chemical products of engine combustion). o A reduction in the final tailpipe emissions (≈ 99.9%) resulting from the ability to accurately condition the "feed gas" in a manner that maximizes the effectiveness of the catalytic converter.
•
General Engine Operation o Smoother function during quick throttle transitions. o Engine starting. o Extreme weather operation. o Reduced maintenance interval. o A slight increase in fuel economy.
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•
Power Output o Fuel injection often produces more power than an equivalent carbureted engine. However, fuel injection alone does not increase maximum engine output. Increased airflow is necessary to permit oxidizing more fuel, which generates more heat, which in turn generates more output. The combustion process converts the fuel's chemical energy into heat energy, whether the fuel arrived via FI or a carburetor is not significant. Airflow is often improved with fuel injectors, which are much smaller than a carburetor. Their smaller size permits more design freedom to improve the air's path into the engine. In contrast, a carburetor's mounting options are limited because it is larger, it must be carefully oriented with respect to gravity, and it must be approximately equal distance from each of the engine's cylinders. These design constraints generally compromise airflow into the engine. o A carburetor relies on a drag inducing venturi in order to create a local air pressure difference, which forces the fuel into the air stream. The flow loss caused by the venturi is small in comparison to other flow losses in the induction system. In a well-designed carbureted induction system, the venturi in and of itself is not a significant airflow restriction. o Fuel injection is more likely to increase efficiency than power. When cylinder-tocylinder fuel distribution is improved (common with EFI), less fuel is required to generate the same power output. Engine efficiency is known as the BSFC, or brake specific fuel consumption. When cylinder-to-cylinder distribution is less than ideal (and it always is under one condition or another, and worse on carburetor systems), more fuel than necessary is metered to the rich cylinders in order to provide sufficient fuel to the lean cylinders. Power output is asymmetrical with respect to air/fuel ratio. In other words, burning extra fuel in the rich cylinders does not reduce power nearly as quickly as burning too little fuel in the lean cylinders. The standard fuel metering compromise is to run the rich cylinders "even richer" of the optimal air/fuel ratio, in order to provide enough fuel to the leaner cylinders. The net power output improves with all the cylinders making maximum power. An analogy is that of painting a wall. One coat of paint may not cover very well. The second coat dramatically improves the appearance of xxxiii
the poorly covered areas, but some extra paint is consumed on areas that were already well covered. There are other benefits associated with fuel injection, such as better atomization of the fuel in the intake (constant-choke carburetors have poor atomization at low air speeds, necessitating modifications such as sequential twin-barrel designs) and better breathing due to the elimination of the carburetor's venturi. FI is becoming more reliable and less expensive through widespread usage. At the same time, carburetors are becoming less available, and more expensive. Even marine applications are adopting FIs reliability improves. If this trend continues, it is conceivable that virtually all internal combustion engines, including garden equipment and snow throwers, will eventually use FI.
Fig 1-20 simple representation of diesel engine fuel injection system (above), CRDI system (below)
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Fig 1-21 schematic representation of Bosch K-jetronic injection system xxxvi
4. TRANSMISSION- HOW THE ENGINE TURNS THE WHEELS The transmission channels power from the engine to the road wheels. In a conventional car with a front-mounted engine, transmission starts at the fly-wheel and continues through the clutch, gearbox and final drive to the rear wheels. The clutch, set between the flywheel and the gearbox, allows the engine power to be disconnected from the transmission to free it from torque (turning effort) when gears are engaged or changed. A greater torque is required from the engine when the car is starting to move or climbing a hill than when it is running at a constant speed along a level road. The final drive, or the rear axle assembly, includes a differential gear which enables the road wheels to rotate at different speeds. When a car turns a corner the outer wheel has to travel further than the inner wheel. The wheels that are not under power rotate at the speeds demanded of them without any mechanical assistance. Power finally reaches the wheels from the final drive through a half-shaft on each side of the differential. WHY CAR HAS A GEARBOX: A gearbox is needed to enable the engine to cope with the wide variations in the power and torque required to drive a car under such different conditions. In effect, the faster the crankshaft revolves in relation to the road wheels, the greater is the force available to drive the car; but the speed of the car is proportionately reduced. Several gears are used, giving a wide range of speed ratios between the engine and the wheels. The top speed of a car depends upon the maximum power of its engine, and this is developed near the engine’s maximum speed. A typical car engine may run at 4000 r.p.m for a top speed of 110 km/hr. But road wheels of average size turn at only about 1000 r.p.m to cover 110 km in an hour, so they cannot be connected directly to the engine. There must be a system which allows the road wheels to make one revolution for every four of the engine. This is done by a reduction gear in the final drive.
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The relationship between the rotation speeds of the engine and the wheel is the axle ratio; 4:1 is common. As long as the car is driven at a steady speed on the level, this gearing will suffice; but when the car meets a hill its speed will drop and the engine falter and stall. A slow running engine cannot provide enough torque for climbing hills or starting from rest. Selecting a lower gear enables the engine to run faster in relation to the road wheels and also multiplies the torque.
4-1 TYPES OF TRANSMISSION The main types of basic transmission used in the automobiles are: •
MANUAL TRANSMISSION
•
SEMI – AUTOMATIC TRANSMISSION
•
AUTOMATIC TRANSMISSION
We would confine ourselves to the study of MANUAL TRANSMISSION system only.
Manual transmission: A manual transmission (also known as a stick shift, straight drive, or standard transmission) is a type of transmission used in automotive applications. Manual transmissions often feature a driveroperated clutch and a movable gear selector, although some do not. Most automobile manual transmissions allow the driver to select any gear at any time, but some, such as those commonly mounted on motorcycles and some types of race cars, only allow the driver to select the next-highest or next-lowest gear ratio. This second type of transmission is sometimes called a sequential manual transmission. Manual transmissions are characterized by gear ratios that are selectable by engaging pairs of gears inside the transmission. Manual transmissions come in two basic types: simple unsynchronized systems, where gears are spinning freely and their relative speeds must be synchronized by the operator to avoid noisy and damaging "clashing" and "grinding" when trying to mesh the rotating teeth; and synchronized systems, which eliminate this necessity while changing gears. xxxviii
Unsynchronized transmission The earliest automotive transmissions were entirely mechanical unsynchronized gearing systems. They could be shifted, with multiple gear ratios available to the operator, and even had reverse; but the gears were engaged by sliding mechanisms or simple clutches, which required skills of timing and careful throttle manipulation when shifting, so that the gears would be spinning at roughly the same speed when engaged; otherwise the teeth would refuse to mesh. When up shifting, the speed of the gear driven by the engine had to drop to match the speed of the next gear; as this happened naturally when the clutch was depressed, it was just a matter of skill and experience to hear and feel when the gears managed to mesh. However, when downshifting, the gear driven by the engine had to be sped up to mesh with the output gear, requiring engagement of the clutch for the engine to speed up the gears. Double declutching, that is, shifting once to neutral to speed up the gears and again to the lower gear, is sometimes needed. In fact, such transmissions are often easier to shift from without using the clutch at all. The clutch, in these cases, is only used for starting from a standstill. This procedure is common in racing vehicles and most production motorcycles. Even though automotive transmissions are now almost universally synchronised, heavy trucks and machinery as well as dedicated racing transmissions are still usually nonsynchromesh transmissions, known colloquially as "crashboxes", for several reasons. Being made of brass, synchronizers are prone to wear and breakage more than the actual gears, which are cast iron, and the rotation of all the sets of gears at once results in higher frictional losses. In addition, the process of shifting a synchromesh transmission is slower than that of shifting a nonsynchromesh transmission. For racing of production based transmissions, sometimes half the dogs on the synchros are removed to speed the shifting process, at the expense of much more wear. Similarly, most modern motorcycles still utilise unsynchronised transmissions. Synchronisers are generally not necessary or desirable in motorcycle transmissions. The low gear inertias and higher strengths mean that 'forcing' the gears to alter speed is not damaging, and the selector method on modern motorcycles (pedal operated) is not conducive to having the long shift time of a synchronised gearbox. Because of this, it is still necessary to synchronise gear speeds by 'blipping-the-throttle' when shifting into a lower gear on a motorcycle.
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Synchronized transmission A modern gearbox is of the constant mesh type, in which all gears are always in mesh but only one of these meshed pairs of gears is locked to the shaft on which it is mounted at any one time, the others being allowed to rotate freely; thus greatly reducing the skill required to shift gears. Most modern cars are fitted with a synchronized gearbox, although it is entirely possible to construct a constant mesh gearbox without synchromesh, as found in motorcycle for example. In a constant mesh gearbox, the gears of the different transmission speeds are always in mesh and rotating, but the gears are not directly rotationally connected to the shafts on which they rotate. Instead, the gears can freely rotate or be locked to the shaft on which they are carried. The locking mechanism for any individual gear consists of a collar on the shaft which is able to slide sideways so that teeth or "dogs" on its inner surface bridge two circular rings with teeth on their outer circumference; one attached to the gear, one to the shaft. (One collar typically serves for two gears; sliding in one direction selects one transmission speed, in the other direction selects the other) When the rings are bridged by the collar, that particular gear is rotationally locked to the shaft and determines the output speed of the transmission. In a synchromesh gearbox, to correctly match the speed of the gear to that of the shaft as the gear is engaged, the collar initially applies a force to a cone-shaped brass clutch which is attached to the gear, which brings the speeds to match prior to the collar locking into place. The collar is prevented from bridging the locking rings when the speeds are mismatched by synchro rings (also called blocker rings or balk rings, the latter being spelled "baulk" in the UK). The gearshift lever manipulates the collars using a set of linkages, so arranged so that only one collar may be permitted to lock only one gear at any one time; when "shifting gears", the locking collar from one gear is disengaged and that of another engaged. In a modern gearbox, the action of all of these components is so smooth and fast it is hardly noticed. Reverse gear, however, is usually not synchromesh, as there is only one reverse gear in the normal automotive transmission and changing gears in reverse is not required.
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Fig 1-21 five-speed all-synchromesh gearbox
Like other transmissions, a manual transmission has several shafts with various gears and other components attached to them. Typically, there are three shafts: an input shaft, a countershaft and an output shaft. The countershaft is sometimes called a layshaft. The input and output shaft lie along the same line, and may in fact be combined into a single shaft within the transmission. This single shaft is called a mainshaft. The input and output ends of this combined shaft rotate independently, at different speeds, which is possible because one piece slides into a hollow bore in the other piece, where it is supported by a bearing. Sometimes the term mainshaft refers to just the input shaft or just the output shaft, rather than the entire assembly. In some transmissions, it's possible for the input and output components of the mainshaft to be locked together to create a 1:1 gear ratio, causing the power flow to bypass the countershaft. The mainshaft then behaves like a single, solid shaft, a situation referred to as direct drive. Fig 1-22 mainshaft
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Even in transmissions that do not feature direct drive, it's an advantage for the input and output to lie along the same line, because this reduces the amount of torsion that the transmission case has to bear. Under one possible design, the transmission's input shaft has just one pinion gear, which drives the countershaft. Along the countershaft are mounted gears of various sizes, which rotate when the input shaft rotates. These gears correspond to the forward speeds and reverse. Each of the forward gears on the countershaft is permanently meshed with a corresponding gear on the output shaft. However, these driven gears are not rigidly attached to the output shaft: although the shaft runs through them, they spin independently of it, which is made possible by bearings in their hubs. Reverse is typically implemented differently, see the section on Reverse. When the transmission is in neutral, and the clutch is disengaged, the input shaft, clutch disk and countershaft can continue to rotate under their own inertia. In this state, the engine, the input shaft and clutch, and the output shaft, all rotate independently.
Synchromesh If the teeth, the so-called dog teeth, make contact with the gear, but the two parts are spinning at different speeds, the teeth will fail to engage and a loud grinding sound will be heard as they clatter together. For this reason, a modern dog clutch in an automobile has a synchronizer mechanism or synchromesh. Thanks to this mechanism, before the teeth can engage, a frictional contact is made which brings the selector and gear to two parts to rotate at the same speed. Moreover, until synchronization occurs, the teeth are prevented from making contact, because further motion of the selector is prevented by a blocker ring. When synchronization occurs, friction on the blocker ring is relieved and it twists slightly, bringing into alignment certain grooves and notches that allow further passage of the selector which brings the teeth together. Of course, the exact design of the synchronizer varies from manufacturer to manufacturer.
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Fig 1-23 a typical synchromesh system
The synchronizer has to change the momentum of the entire input shaft and clutch disk. Additionally, it can be abused by exposure to the momentum and power of the engine itself, which is what happens when attempts are made to select a gear without fully disengaging the clutch. This causes extra wear on the rings and sleeves, reducing their service life. When an experimenting driver tries to "match the revs" on a synchronized transmission and force it into gear without using the clutch, it is actually the synchronizer that makes up for any discrepancy in RPM, deceiving the driver into an exaggerated sense of how much human skill was involved.
Reverse The previous discussion applies to the forward gears. The implementation of the reverse gear is usually different, implemented in the following way to reduce the cost of the transmission. Reverse is also a pair of gears: one gear on the countershaft and one on the output shaft. However, whereas all the forward gears are always meshed together, there is a gap between the reverse gears. Moreover, they are both attached to their shafts: neither one rotates freely about the shaft. What happens when reverse is selected is that a small gear, called an idler gear or reverse idler, is slid between them. The idler has xliii
teeth which mesh with both gears, and thus it couples these gears together and reverses the direction of rotation without changing the gear ratio. Thus, in other words, when reverse gear is selected, in fact it is actual gear teeth that are being meshed, with no aid from a synchronization mechanism. For this reason, the output shaft must not be rotating when reverse is selected: the car must be stopped. In order that reverse can be selected without grinding even if the input shaft is spinning inertially, there may be a mechanism to stop the input shaft from spinning. The driver brings the vehicle to a stop, and selects reverse. As that selection is made, some mechanism in the transmission stops the input shaft. Both gears are stopped and the idler can be inserted between them. Whenever the clutch pedal is depressed to shift into reverse, the mainshaft continues to rotate because of its inertia. The resulting speed difference between mainshaft and reverse idler gear produces gear noise [grinding]. The reverse gear noise reduction system employs a cam plate which was added to the reverse shift holder. When shifting into reverse, the 5th/reverse shift piece, connected to the shift lever, rotates the cam plate. This causes the 5th synchro set to stop the rotating mainshaft. A reverse gear implemented this way makes a loud whining sound, which is not heard in the forward gears. The teeth on the forward gears of consumer automobiles are helically cut. When helical gears rotate, their teeth slide together, which results in quiet operation? In spite of all forward gears being always meshed, they do not make a sound that can be easily heard above the engine noise. By contrast, reverse gears are spur gears, meaning that they have straight teeth, in order to allow for the sliding engagement of the idler, which would not be possible with helical gears. The teeth of spur gears clatter together when the gears spin, generating a characteristic whine. It is clear that the spur gear design of reverse gear represents some compromises—less robust, unsynchronized engagement and loud noise—which are acceptable due to the small volume of driving that takes place in reverse.
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5. Clutch In all vehicles using a transmission (virtually all modern vehicles), a coupling device is used to separate the engine and transmission when necessary. The clutch accomplishes this in manual transmissions. Without it, the engine and tires would at all times be inextricably linked, and anytime the vehicle was at a stop, so would be the engine.
Fig 1-24 multi plate clutch Moreover, without the clutch, changing gears would be very difficult, even with the vehicle moving already: deselecting a gear while the transmission is under load requires considerable force, and selecting a gear requires the revolution speed of the engine to be held at a very precise value which depends on the vehicle speed and desired gear. In a car the clutch is usually operated by a pedal; on a motorcycle, a lever on the left handlebar serves the purpose. •
When the clutch pedal is fully depressed, the clutch is fully disengaged, and no torque is transferred from the engine to the transmission and by extension to the drive wheels. In this state, it's possible to select gears or stop the car.
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Disengaging the clutch separates three parts of the clutch assembly – the fly wheel, the driven plate (also known as the centre plate or the clutch plate) and the pressure plate. The flywheel is bolted to the end of the crankshaft and rotates with it; the driven plate is splined to the gearbox input shaft so that they rotate together; and the pressure plate clamps the driven plate to the flywheel. •
When the clutch pedal is fully released, the clutch is fully engaged, and essentially all of the engine's torque is transferred. In this state, the clutch does not slip, but rather behaves like a rigid coupling. Power is transmitted to the wheels with minimal loss.
Thus when the pressure is released, by depressing the clutch pedal, the crankshaft and gear input shaft rotate independently; when the driver takes his foot off the pedal, they rotate as one.
Fig 1-25 direct release clutch
WHAT HAPPENS WHEN GEARS ARE ENGAGED The gearwheels in a constant-mesh gearbox cannot be all fixed to the gear shafts or no movement would be possible; so there has to be a system which permits all the gear wheels except those required for a particular ratio to run freely. Usually all the gear wheels on one shaft are fixed to it and the wheels xlvi
on the other shafts can revolve around their shaft until a ratio is selected. Then one of the free running wheels is locked to the shaft, and that pair of wheels can transmit power. The locking of the gear wheels to a shaft is done by collars, which are splined to the shaft. This method of fixing means that the collar must revolve with the shaft but it can slide along to lock onto the gear wheels on either side, or remain between them, allowing both to spin freely. Around each collar is a groove engaged by a two-pronged fork which is fixed to a sliding rod mounted in the gearbox housing. One, two or three of these selector rods are linked to the gear lever. Moving the gear lever causes a selector rod to slide to or fro. As it slides, the collar gripped by the selector fork is slid along the shaft to engage with, or move away from, a gear.
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