Functions of Lubricating Oil in an Engine Lubricating oil with the necessary properties and characteristics will: 1. Provide a film of of proper proper thickness thickness between between the bearing bearing surfaces surfaces under under all conditions conditions of operation. 2. Remain Remain stable stable under under changing changing temper temperatu ature re conditio conditions ns 3. Not Not corrod corrodee the met metal al surf surfac aces es.. If the lubricating oil is to meet these requirements, the engine operating temperature must not exceed a specified limit. In internal-combustion engines, lubricating oil serves six functions: 1. Controls friction between load-bearing surfaces 2. Reduces wear by preventing metal-to-metal contact between moving parts 3. Limits the the temperature temperature by carrying away heat heat from fluid friction friction and combustion combustion of fuel 4. Reduces corrosion by by coating metal parts and by flushing debris from between between moving parts 5. Dampers mechanical shock in gears 6. Forms a seal on the walls of the cylinders Some of these functions and characteristics are discussed in the sections that follow.
Protective Film Direct metal-to-me metal-to-metal tal contact of load-bearing surfaces surfaces is similar similar to the action of a file as it wears away metal. The filing action is a result of very small irregularities in the metal surfaces. The severity of the filing action depends on the finish of the surfaces, the force with which the surfaces are brought into contact, and the relative hardness of the materials. Lubricating oil fills the tiny cavities in bearing surfaces and forms a film between the sliding surfaces to prevent high friction losses and rapid wear of engine parts. The lack of a proper oil film will result result in a seized seized (frozen) (frozen) piston, wiped bearings, bearings, and stuck piston rings.
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Cooling Lubricating oil assists in cooling the engine because the constant flow of oil carries heat away from localized “hot spots.” The principal parts from which oil absorbs heat are the bearings, the journal surfaces, and the pistons. In some engines, the oil carries the heat to the sump where the heat dissipates in the mass of oil. However, most modern internalcombustion engines use a centralized pressure-feed lubrication system. This type of system has an oil cooler (heat exchanger) where the heat in the oil is transferred to the water circulating in the jacket-water cooling system.
Sludge Control Gummy or carbonaceous material that accumulates in lubricating oil is called SLUDGE. Most engine lubricating oils have some natural ability for preventing conditions that may cause sludge to form and for carrying sludge that does form in a finely suspended state until it is removed by filtering equipment. Chemicals are added to some oils to improve the ability of the oils to prevent the formation of sludge. The formation of sludge is greatly reduced when the lubricating oil has the proper stability. STABILITY is defined as the ability of the oil to resist oxidation and deterioration for long periods. Proper stability is essential for a strong oil film to be maintained throughout the normal temperature ranges of an operating engine. A strong oil film provides the required oiliness, or film strength, to form a seal between the piston and the cylinder wall. With the oil seal in place, burned fuel and exhaust gases cannot get by the piston rings to form sludge.
Components of a Lubricating Oil System The lubricating system of an internal-combustion engine consists of two main divisions: • •
One that is inside the engine One that is outside the engine
The internal system consists mainly of passages and piping. The external system includes several components which aid in supplying the oil in the proper quantity, at the proper temperature, and free of impurities. In the majority of lubricating oil systems for internal-combustion engines the external system includes such parts as tanks and sumps, pumps, coolers, strainers, and filters. Tanks and Sumps
The lubricating systems of propulsion installations use tanks to collect, store, and recirculate oil after it has been used for lubrication and cooling. Some installations have a sump or drain tank under the engine to collect the oil as it drains from the engine crankcase. Separate storage and sump tanks are not common in auxiliary engines; these engines generally contain the oil supply directly within the engine oil pan.
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Pumps
Positive displacement, rotary gear pumps deliver oil under pressure to the various parts of the engine. Since the pumps are gear driven by the engine camshaft or, in some engines, directly by the crankshaft, the oil is supplied at flow rates adjusted to the needs of the engine. Changes in engine speed will cause corresponding changes in pump output. The operating pressure is normally controlled by one or more pressure regulating valves, which open or close as necessary to maintain the specified flow rate to various load bearing parts of the engine. These spring-actuated devices divert excess oil directly to the engine sump or back to the inlet of the lubricating oil pump. Detached lubricating pumps on large diesel engines fill the sump tanks from the storage tanks and flush and prime the lubricating oil system. Coolers
The lubricating oil systems of most engines use coolers (heat exchangers) to maintain the oil temperature within the most efficient operating range. Oil, passing through the operating engine, absorbs heat from the metal parts. Since engine oil is re-circulated and used over and over, it is continually absorbing additional heat. Unless the heat is removed, the oil temperature will rise to excessive values. At extremely high temperatures, oil tends to oxidize rapidly and form carbon deposits. Excessive engine operating temperatures also cause an increase in the rate of oil consumption. Consequently, oil coolers are required to remove excess heat from the oil so that the oil will retain its lubricating qualities. Filtering Devices
Oil must be clean before it goes into the lubricating system of an engine. Oil must also be cleaned regularly while it is being re-circulated through the engine. Dust and dirt particles from the intake air get into the oil system. Flakes of metal from the engine parts are also picked up and carried in the oil. Carbon particles from incomplete combustion in the cylinders work into the oil. Heat causes the oil itself to deteriorate and form sludge and gummy material which may coat load-bearing or heat-transfer surfaces, or circulate through the oil system. Some water will get into the oil, even when precautions are taken. The lubricating oil system of an engine uses strainers and filters to remove abrasives and foreign materials which tend to increase wear of engine parts and cause the lubricating oil to deteriorate. Strainers
Lubricating oil strainers may be either simplex or duplex. A duplex strainer is two strainer elements in one assembly. A manual valve directs the flow of oil through either of the elements. When duplex strainers are used, one element can be bypassed, and the element can be removed and cleaned without disturbing the flow of oil through the other element to the engine.
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Filters
Filters possess absorbent material composed of such substances as cellulose, cotton yarn, and paper disks. Filters may be located directly in the pressure-lubricating oil system, or they may be installed as bypass filters. When installed in the pressure system, a filter must contain a built-in, spring-loaded, pressure-relief valve. The valve must be large enough to bypass all oil to the engine in case the filter element becomes restricted. Spin-On Filter
The spin-on filter is not unlike the filter you would find on an automobile engine. The spinon filter consists of a shell, an element, and a gasket combined into a unitized assembly. An oil filter adapter that contains a bypass valve assembly provides the support for each pair of oil filters. Canister-Type Filter
The canister-type filter assembly consists of a replaceable element enclosed within a shell which is mounted in an adapter or base. When the filter shell is in place, the element is restrained from movement by a coil spring. Full-Flow Lubricating Oil System
In the full-flow filtering system, all the oil supplied to the engine by the oil pump normally passes through the filter elements, which remove impurities of 25 microns and larger. There are only two conditions where unfiltered lubricating oil is supplied to the engine: • •
when the lubricating oil is cold (high viscosity) when the filter element is clogged
When one of these conditions exists, a bypass valve opens and a portion of the oil is bypassed around the filter element. The action of the bypass valve results from the resistance of the filter element to allow the oil to pass through it. The resistance creates enough back pressure for the spring-loaded bypass valve to open.
Example of a Lubricating Oil System Figure 1 illustrates a typical lubricating oil system in a General Motors series 71 in-line diesel engine. Refer to the figure as you trace the flow of oil through this type of lubricating oil system. (Note the directional arrows).
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Fig.1
Schematic diagram of a typical lubricating oil system in a General Motors series 71 in line diesel engine
Flow of Lubricating Oil
The lubricating oil is circulated by a gear-type pump mounted on the No. 1 and No. 2 main bearing caps. The pump is gear driven by the crankshaft. All the oil leaving the pump is forced through the full-flow oil filter to the cooler and then into the oil gallery in the cylinder block. From the oil gallery, the oil is distributed to the various engine bearings. Note that the drains from the cylinder head and other engine parts lead back to the oil pan. When the pressure in the engine oil gallery exceeds approximately 6 bars, a spring-loaded integral plunger relief valve (A), located in the oil pump body, bypasses excess oil from the discharge to the inlet side of the pump. If the oil cooler should become clogged, the oil will flow from the pump through a spring-loaded bypass valve (B) directly into the oil gallery.
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Clean engine oil is assured at all times by the use of a replaceable element type of full-flow oil filter that is incorporated in the lubricating oil system. This filter is installed between the pump and the cooler, and it filters all the lubricating oil before it enters the engine. A regulator (relief) valve (D) that is located between the pump outlet and the inlet to the cylinder block works to maintain stabilized lubricating oil pressure within the engine when the engine is operating at all speeds. This valve stabilizes the lubricating oil flow rate regardless of the temperature of the oil. When the oil pressure at the regulator valve exceeds 3 bars, the valve opens and remains open until the pressure falls below the operating pressure. As an optional feature, a bypass type of filter (E) with a replaceable element may also be used. In this design, a portion of the lubricating oil is continually bypassed through the filter. The filtered oil is then returned to the engine oil pan. Distribution of Lubricating Oil
Oil from the cooler is directed by a vertical passage to the main gallery of the cylinder block. As shown in figure 1, this gallery distributes the oil to the main bearings and to a horizontal passage at each end of the cylinder block. From each of these two horizontal passages, oil flows to the end bearings of the camshaft and balancer shaft. In addition, oil is forced through an oil passage in the camshaft which lubricates the camshaft intermediate bearings. Oil to lubricate the connecting rod bearings and piston pins, and to cool the piston head, is provided through the drilled crankshaft from the adjacent main bearings. The gear train is lubricated by the overflow of oil from the camshaft pocket. The blower drive gear bearing is lubricated through an external pipe from the rear horizontal oil passage of the cylinder block. Valve and injector operating mechanisms are lubricated from a longitudinal oil passage on the camshaft side of the cylinder block which connects to the main oil gallery. Oil from this passage enters the drilled rocker arm shafts through the lower end of the rocker shaft bolts and rocker shaft brackets. Excess oil from the rocker arms lubricates the exhaust valves and cam followers.
Ventilation of Internal Spaces
Most engines have some means to ventilate the internal cavities, or spaces, which are related to the lubricating oil system. Systems may be vented directly to the atmosphere or through the engine intake air system. During normal operating conditions, the mixture of oil vapor and air within an engine crankcase is not readily explosive. However, if a working part, such as a bearing or a piston, becomes overheated as a result of inadequate lubrication or clearances, additional oil will vaporize and an explosive mixture will be created. In addition to the vapor created
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when lubricating oil contacts extremely hot surfaces, vapor may accumulate in the crankcase as a result of blow-by past the pistons. Blow-by occurs when the piston is compressing the air and during the power event. The result will be a phenomenon called crankcase explosion. There is little danger of a crankcase explosion or other troubles caused by vapors within the engine if the engine is kept in condition according to the prescribed maintenance program.
Overheated Lubricating Oil
The formation of explosive vapor from lubricating oil is greatly accelerated by a rise in the temperature of the lubricating oil. A rise in temperature may be due to such factors as: • • • • •
insufficient circulation of the oil inadequate cooling of the oil a faulty temperature-regulating valve overloading of the engine damaged or excessively worn parts
In addition to creating explosive vapors, overheated lubricating oil can have other serious effects. The viscosity of the lubricating oil will be greatly reduced, and the tendency to form acids will be increased. Diluted Lubricating Oil
Dilution of engine lubricating oil with diesel fuel increases the tendency toward vapor formation in the crankcase because fuel has lower flash point than lubricating oil. Dilution of engine lubricating oil may be caused by a variety of troubles. In general, dilution of the lubricating oil in diesel engines may result from worn or stuck rings, worn liners or pistons, fuel leaks, or leaky nozzles or injectors.
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