types of lubrication in engines
lubrication systems in engines
Bath lubrication or splash lubrication (used in gearboxes and rear axles) may be used for gears, chains, bearings and other moving parts that can be partly submerged in an oil reservoir. In the bath system the gear simply picks up oil as it dips into the reservoir and sprays or carries it to other parts along its path. The splash system increases the efficiency by attaching a special splash ring to a moving part so that the oil is splashed against other parts that need to be lubricated. This is similar to oil-mist lubrication, created by the oil escaping from the engine’s rotating crankshaft, in which the oil is atomized in a stream of air.
Force-feed lubrication uses an oil pump to force the oil under pressure to the parts to be lubricated, normally the engine crankshaft and camshaft. On some high-performance vehicles the mainshaft in the gearbox is pressure fed. Some parts are self-lubricating and require no external lubrication; the lubricant may be sealed in against loss as in sealed ball bearings, or a porous material such as porous bronze can be used so that oil impregnated in the material can penetrate to the point of contact of the moving parts through pores in the material. In small two-stroke gasoline engines the oil is mixed in with the fuel to bring it to the moving parts inside the engine.
Although lubricating oil is used elsewhere in a car, the lubrication of the engine is of greatest importance because it reduces the friction and wear between moving metal parts and also removes heat from the engine. A supply of oil is kept in the engine crankcase. An oil pump, which is powered by the engine, forces oil from the crankcase under pressure to the cylinder block main oil gallery. Passages in the engine block channel the oil to various moving parts, such as the crankshaft and camshaft, and the oil eventually drains back down in the crankcase. An oil filter is fitted in the oil circuit to filter out metal shavings, carbon deposits and dirt. Because the filter is not completely effective, and because of prolonged exposure to high temperatures, the oil eventually becomes contaminated, decomposes and loses its lubricating qualities. This is why routine maintenance calls for changing the oil and oil filter at regular intervals.
Lubrication system
From the sump reservoir under the crankshaft oil is drawn through a strainer into the pump ( Fig. 2.118 ).
Oil pumps have an output of tens of litres per minute and operating pressures of over 5 kg/cm 2 at high speeds. A pressure relief valve limits the pressure of the lubrication system to between 2.5 and 4 kg/cm 2 . The pressure relief valve is a spring-loaded conical, or ball, valve that opens when the pressure in the oil exceeds the spring force acting on the valve seat ( Fig. 2.119 ). When the valve opens, a return drilling is uncovered and the excess oil flows through this to return to the sump. This control is needed because the pump would produce excessive pressure at high speeds. After leaving the pump, oil passes into a filter and then into a main oil gallery in the engine block or crankcase ( Fig. 2.120 ).
Drillings connect the gallery to the crankshaft bearing housings and when the engine is running, oil is forced under pressure between the rotating crank journals and the main bearings. The crankshaft is drilled so that the oil supply from the main bearings is also to the big-end bearing bases of the connecting rods.
The connecting rods are often drilled near the base so that a jet of oil sprays the cylinder walls and the underside of the pistons ( Fig. 2.121 ). In some cases theconnecting rod may be drilled along its entire length so that oil from the bigend bearing is taken directly to the gudgeon pin (small end). The surplus then splashes out to cool the underside of the piston and cylinder. The camshaft operates at half crankshaft speed, but it still needs good lubrication because of the high-pressure loads on the cams ( Fig. 2.122 ).
It is usual to supply pressurized oil to the camshaft bearings and splash or spray oil on the cam lobes. On overhead camshaft engines, two systems are used. In the simplest system the rotating cam lobes dip into a trough of oil. Another method is to spray the cam lobes with oil. This is usually done by an oil pipe with small holes in it alongside the camshaft. The small holes in the side of the pipe aim a jet of oil at each rotating cam lobe. The surplus splashes over the valve assembly and then falls back into the sump.
On cars where a chain drives the cam, a small tapping from the main oil gallery sprays oil on the chain as it moves past, or the chain may simply dip in the sump oil.
Some specialized vehicles use an oil cooler ( Fig. 2.123 ). The oil cooler commonly used is an air radiator similar to an engine-cooling radiator, with tubes and fins to transfer heat from the oil to the passing air stream. This cooler is fitted next to the cooling-system radiator at the front of the vehicle. Pipes from the filter housing carry oil to and from the oil-cooler radiator.
Figure 2.118 Pick-up pipe and strainer
A key component of the lubrication system is the dipstick. No matter how clever the system is it will not work if the oil level is low. The dipstick is marked to show the maximum and minimum acceptable levels.
Many modern engines are now also fitted with an electronic sensor that supplies information to the driver on the level of oil in the engine (low oil pressure indicating low oil level). A warning light, or a gauge in the instrument panel, indicates whether the oil level is within acceptable levels ( Fig. 2.124 ). The sensor is fi tted into the sump or the engine block. Some engines now have oil quality sensors to indicate when the oil should be changed.
Figure 2.120 Oil fl ow: 1, oil to rocker arms; 2, hydraulic tappets; 3, fi lter; 4, crank main
bearings; 5, big end bearings; 6, crank driven oil pump; 7, oil under pressure; 8, camshaft
Figure 2.119 Plunger and spring
Figure 2.121 Drillings in the block and crank
Figure 2.122 Oil drillings for the valve gear
To warn the driver about low oil pressure, a pressure-sensitive switch is fitted into the main gallery. It makes an electrical contact when the pressure is below about 0.5 bar (7 psi). The switch may be fitted in the same circuit as the oil level warning lamp. When the switch contacts make a connection, the lamp lights, and this should occur before the engine is started. Once the engine is running, oil pressure builds up and the switch contacts separate and the warning lamp will go out. This indicates that a minimum oil pressure is being maintained in the system. Oil pressure gauges are also used and employ a piezoelectric pressure sensor fi tted into the main gallery and a gauge unit.
Figure 2.123 Oil cooler for a racing car. (Source: www.prcracing.com Media)
Figure 2.124 Gauge and circuit: 1, voltage stabilizer; 2, gauge; 3, sender unit
Oil filters
Even new engines can contain very small particles of metal left over from the manufacturing process or grains of sand that have not been removed from the crankcase after casting. Old engines continually deposit tiny bits of metal worn from highly loaded components such as the piston rings. To prevent any of these lodging in bearings or blocking oil ways, the oil is fi ltered ( Fig. 2.125 ).
Figure 2.125 Oil filters
Figure 2.126 Oil circuits: 1, full fl ow (bottom left bypass fl ow); 2, sump; 3, pump; 4, filter; 5, main gallery; 6, main bearings; 7, big ends; 8, camshaft
The primary filter is a wire mesh strainer that stops particles of dirt or swarf from entering the oil pump. This is normally on the end of the oil pick-up pipe. An extra filter is also used that stops very fine particles. The most common type has a folded, resin-impregnated paper element. Pumping oil through it removes all but smallest solids from the oil.
Most engines use a full-fl ow system to filter all of the oil after it leaves the pump ( Fig. 2.126 ). The most popular method is to pump the oil into a canister containing a cylindrical filter. From the inner walls of the canister, the oil flows through the filter and out from the centre to the main oil gallery. Full-fl ow filtration works well provided the filter is renewed at regular intervals. If it is left in service too long it may become blocked. When this happens the build-up of pressure inside the filter forces open a spring-loaded relief valve in the housing and the oil bypasses the filter. This valve prevents engine failure, but the engine will be lubricated with dirty oil until the filter is renewed. This is better than no oil!
Figure 2.127 Dry sump: 1, small collection area; 2, not shown; 3, pump; 4, filter; 5, main gallery; 6, main bearings; 7, big ends; 8, camshaft; 9, return pump; 10, remote tank
A bypass filtration system was used on some vehicles ( Fig. 2.126 , bottom left).
This system only filters a proportion of the oil pump output. The remainder is fed
directly to the oil gallery. At first view this seems a strange idea but all of the oil
does eventually get filtered. The smaller amount through the filter allows a higher
degree of filtration.
For many high-performance applications, a larger oil supply is needed so that engine heat can be removed by the engine oil as well as by the engine-cooling system ( Fig. 2.127 ). A separate reservoir of oil is held in a remote tank and drawn into the main oil pump for distribution throughout the engine in the same way as a wet-sump system. The oil returns to a small sump below the engine.
A scavenge pump, with a pick-up pipe in the sump, draws oil out of the sump and delivers it back to the reservoir. An oil cooler is usually fitted in this return circuit.
Oil pumps
The oil pump is the heart of the system. It pumps oil from the sump into the engine. The main types of oil pump are gear, rotor, gerotor, vane and crescent. The gear type uses two gears in mesh with each other ( Fig. 2.128 ). Drive is made to one gear which, in turn, drives the other. The housing has a figure-of-eight internal shape, with one gear in each end. Ports are machined in the housing and align with the areas where the teeth move into, and out of, mesh. As the teeth separate, the volume in the inlet side of the housing increases and atmospheric pressure in the sump is able to force oil into the pump. The oil is carried around inside the pump in between the teeth and the side of the housing. When the teeth move back into mesh, the volume in the outlet side of the housing is reduced, the pressure rises and this forces the oil out into the engine.
The rotor-type pump uses the same principle of meshing but with an inner rotor with externally formed lobes that mesh with corresponding internal profiles on the inside of an external rotor ( Fig. 2.129 ). The inner rotor is offset from the centre of the pump and the outer rotor is circular and concentric with the pump body.
Figure 2.128 Gear pump
Figure 2.130 Gerotor pump driven by the crankshaft
As the rotors rotate, the lobes mesh to give the outlet pressure of the oil supply, or move out of mesh for the intake of oil from the sump.
The gerotor (gear rotor pump) is a variation on the smaller rotor pump ( Fig. 2.130 ). The gerotor pump is usually fitted around, and driven by, the crankshaft. There are inner and outer rotors, with the inner rotor externally lobed and offset from the internally lobed outer rotor. During rotation, the pumping and carrying chambers are formed by the relative positions of the lobes. The crescent pump is named after the solid block in the gear body. This pump is a variation on the gear pump, and also uses gear teeth to create the pumping chambers and to carry oil from the inlet port to the outlet port of the pump.
The operation of this pump is based on the meshing of the gear teeth, the positioning of the ports in the housing and alignment at each end of the crescent where the teeth move in and out of mesh. Oil is carried from the inlet port to the outlet port in the spaces between the teeth and the crescent. This type of pump is used for engine lubrication and for automatic transmissions.
The vane-type pump uses an eccentric rotor with vane plates set at right angles to the axis of the rotor and sitting in slots in the rotor ( Fig. 2.131 ). As the rotor rotates, the vanes sweep around inside the pump housing. The pump chambers increase in volume as the vanes move away from the housing walls, and reduce in volume as the vanes approach the walls. Oil is carried between the vanes and the pump housing from the inlet port to the outlet port.
Most modern engines now use the crankshaft to give a direct drive to the oil pump. These pumps are of the gerotor or crescent design, and are fitted around the front of the crankshaft. This arrangement is used on many overhead camshaft engines because it provides a low position for the pump.
Standards
SAE
The Society of Automotive Engineers (SAE) ( Fig. 2.132 ) has established a numerical code system for grading motor oils according to their viscosity characteristics. SAE viscosity grades include the following, from low to high viscosity: 0, 5, 10, 15, 20, 25, 30, 40, 50 or 60. The numbers 0, 5, 10, 15 and 25 are suffixed with the letter W, designating their ‘winter’ or cold-start viscosity, at lower temperature. The number 20 comes with or without a W, depending on whether it is being used to denote a cold or hot viscosity grade. Viscosity is graded by measuring the time it takes for a standard amount of oil to flow through a standard orifice, at standard temperatures. The longer it takes, the higher the viscosity and thus the higher the SAE code.
Note that the SAE has a separate viscosity rating system for gear, axle and manual transmission oils, which should not be confused with engine oil viscosity. The higher numbers of a gear oil (e.g. 75 W–140) do not mean that it has higher viscosity than an engine oil.
A single-grade engine oil does not use a polymeric viscosity index improver additive (also described as a viscosity modifier). For some applications, such as when the temperature ranges in use are not very wide, single-grade motor oil is satisfactory; for example, lawn mower engines, industrial applications, and vintage or classic cars. However, multigrade oil is far more common. The temperature range the oil is exposed to in most vehicles can be wide, ranging from cold temperatures in the winter before the vehicle is started up, to hot operating temperatures when the vehicle is fully warmed up in hot summer weather.
A specifi c oil will have high viscosity when cold and a lower viscosity at the engine’s operating temperature. To bring the difference in viscosities closer together, special polymer additives called viscosity index improvers (VIIs) are added to the oil. These additives are used to make the oil a multigrade motor oil ( Fig. 2.133 ). The idea is to cause the multigrade oil to have the viscosity of the base grade when cold and the viscosity of the second grade when hot. This enables one type of oil to be generally used all year. The SAE designation for multigrade oils includes two viscosity grades; for example, 10 W–30 is a common multigrade oil. The two numbers used are individually defined by SAE for singlegrade oils. Therefore, an oil labelled as 10 W–30 must pass the viscosity grade requirement for both 10 W and 30 grades.
The American Petroleum Institute API
The American Petroleum Institute (API) sets a minimum for performance standards for lubricants. Lubricant base stocks are categorized into five groups by the API. Group I base stocks, for example, are composed of fractionally distilled petroleum which is further refined with solvent extraction processes to improve certain properties such as oxidation resistance and to remove wax. Other groups describe further refinements and bases. Group III relates to synthetic oils and group V is used for anything that does not fit into the other groups.
The API service classes use two general classifi cations: S for ‘service’ (originating from spark ignition) and C for ‘commercial’ (originating from compression ignition). Engine oil that has been tested and meets the API standards may display the API Service Symbol, also known as the ‘donut’
The latest API service standard designation is SN for gasoline automobile and light truck engines. The SN standard refers to a group of laboratory and engine tests, including the latest series for control of high-temperature deposits. Current API service categories include SN, SM, SL and SJ for petrol/gasoline engines.
There are six current diesel engine service designations: CJ-4, CI-4, CH-4, CG-4, CF-2 and CF. In addition, API created a separated CI-4 PLUS designation in conjunction with CJ-4 and CI-4 for oils that meet certain extra requirements, and this marking is located in the lower portion of the API service symbol. Some oils conform to both the petrol/gasoline and diesel standards. It is the norm for all diesel-rated engine oils to carry the corresponding gasoline specifi cation.
