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 main shaft 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.