A wet sump is a lubricating oil management design for piston engines which uses the crankcase as a built-in reservoir for oil, as opposed to an external or secondary reservoir used in a dry sump design.
Piston engines are lubricated by oil which is pumped into various bearings, and thereafter allowed to drain to the base of the engine under gravity. In most production automobiles and motorcycles, which use a wet sump system, the oil is collected in a 3 to 10 litres (0.66 to 2.20 imp gal; 0.79 to 2.64 US gal) capacity pan at the base of the engine, known as the sump or oil pan, where it is pumped back up to the bearings by the oil pump, internal to the engine.
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 large g 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 from dry sump lubrication, as this allows the engine to be mounted lower in the frame; and a remote oil tank can permit better lubricant cooling.
Early stationary engines employed a small scoop on the extremity of the crankshaft or connecting rod to assist with the lubrication of the cylinder walls by means of a splashing action. Modern small engines, such as those used in lawnmowers, use a "slinger" (basically a paddle wheel) to perform the same function.
Small two-stroke engines, as for motorcycles and lawnmowers, use crankcase compression: the fuel mixture passes through the sump space in 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 excessive hydrocarbon emissions. These engines are instead lubricated by petroil mixtures, where a carefully measured proportion of oil is added to the fuel tank (between 1:16 and 1:50 ratios). This oil is consumed immediately and entirely, so there is no need for a sump to collect and re-use it.
Four-stroke engines, as for almost all cars, and large two-stroke engines used in locomotives and ships can both use either wet or dry sumps. Large two-stroke engines do not use crankcase compression; instead they use a separate blower or supercharger, either a mechanical blower such as a Roots blower or else a turbocharger.
Types of Wet Sump
- The Splash System
- The Splash and Pressure System
- The full Pressure Feed System