What is "dry sump"

n. (context automotive English) A type of engine sump where oil that flows down from the crankshaft etc is sucked up into an external reservoir (as opposed to a wet sump where it simply pools in a pan at the bottom of the engine).

A dry sump is a lubricating motor oil management method for four-stroke and large two-stroke piston internal combustion engines that uses additional pumps and a secondary reservoir for oil, as compared to a conventional wet sump system, which uses only the main sump (U.S.: oil pan) below the engine and a single pump. A dry sump can also be implemented with a single vacuum pump that creates positive and negative pressures to pull and push the oil out and into the engine. A dry sump engine requires a pressure relief valve to regulate negative pressure inside the engine, so internal seals are not inverted.

Engines are both lubricated and cooled by oil that circulates throughout the engine, feeding various bearings and other moving parts and then draining, via gravity, into the sump at the base of the engine. In the wet sump system of most production automobile engines, an oil pump collects this oil from the sump and circulates it back through the engine.

In a dry sump, the oil still falls to the base of the engine, but rather than collecting in a reservoir-style oil sump, it falls into a much shallower sump, where one or more scavenge pumps draw it off and transfer it to a (usually external) reservoir, where it is both cooled and deaerated. A pressure pump then draws this oil and circulates it through the engine. Often, dry sump designs mount the pressure pump and scavenge pumps on a common shaft, so that one pulley at the front of the system can run as many pumps as the engine design requires. It is common practice to have one scavenge pump per crankcase section and in the case of a V-type engine an additional scavenge pump to remove oil fed to the valve gear. Therefore, a V8 engine would have four scavenge pumps and a pressure pump in the pump stack.