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 fi lter is fi tted in the oil circuit to fi lter out metal shavings, carbon deposits and dirt. Because the fi lter 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 fi lter at regular intervals.
2.2.3 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 fl ows 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 fi lter 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 the
Key fact
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.141
Engine systems
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Figure 2.118 Pick-up pipe and strainer Figure 2.119 Plunger and spring
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
142
2
Automobile mechanical and electrical systemsconnecting rod may be drilled along its entire length so that oil from the big- end 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 fi ns to transfer heat from the oil to the passing air stream. This cooler is fi tted next
Figure 2.122 Oil drillings for the valve gear
143
Engine systems
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to the cooling-system radiator at the front of the vehicle. Pipes from the fi lter housing carry oil to and from the oil-cooler radiator.
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 fi tted 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.
To warn the driver about low oil pressure, a pressure-sensitive switch is fi tted into the main gallery. It makes an electrical contact when the pressure is below about 0.5 bar (7 psi). The switch may be fi tted 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.