Withinpiston engines, awet sump is part of alubrication system whereby thecrankcasesump is used as an integraloil reservoir. An alternative system is thedry sump, whereby oil is pumped from a shallow sump into an external reservoir.[1]
Piston engines are lubricated by oil which is pumped into variousbearings, and thereafter allowed to drain to the base of the engine under gravity. In most productionautomobiles andmotorcycles, which use a wet sump system, the oil is collected in a 3 to 10litres (0.66 to 2.20 imp gal; 0.79 to 2.64 US gal) capacity pan at the base of the engine, known as thesump or oil pan, where it is pumped back up to the bearings by the internaloil pump.
A wet sump offers the advantage of a simple design, using a single pump and no external reservoir. Since the sump is internal, there is no need for hoses or tubes connecting the engine to an external sump which may leak. An internal oil pump is generally more difficult to replace, but that is dependent on the engine design.
A wet sump design can be problematic in a racing car, as the largeg force pulled by drivers going around corners causes the oil in the pan to slosh, gravitating away from the oil pick-up, briefly starving the system of oil and damaging the engine. However, on a motorcycle this difficulty does not arise, as a bike leans into corners and the oil is not displaced sideways. Nevertheless, racing motorcycles usually benefit fromdry sump lubrication, as the shallow sump allows the engine to be mounted lower in the frame; and a remote oil tank can allow for improved lubricant cooling. On some dry sump motorcycles, such as theYamaha TRX850 and theYamaha TDM, the oil tank is integral to the engine, sitting atop the gearbox.
Earlystationary engines employed a smallscoop on the extremity of thecrankshaft orconnecting rod to assist with thelubrication of thecylinder walls by means of asplashing action. Modern small engines, such as those used in lawnmowers, use a "slinger" (basically a paddle wheel) to perform the same function.
Smalltwo-stroke engines, such as inmotorcycles andlawnmowers, havetotal loss lubrication. Such engines usecrankcase compression to feed thefuel/air mixture through the crankcase. This precludes the use of both wet sump and dry sump systems, as excess oil here would contaminate the mixture, leading to excess oil being burned in the engine and so excessivehydrocarbon emissions. These small engines run on specifically prepared fuel, a mixture of gasoline andtwo-stroke oil in a ratio recommended by the manufacturer. In some engines this mixture may also be independently pre-injected into the engine cylinders and bearings by a pump.
Four-stroke engines and large (non-petrol) two-stroke engines used in locomotives andships may have either wet or dry sumps. Large two-stroke engines do not use crankcase compression; instead they use amechanical blower or aturbocharger to aspirate air.