Some of it is dispersed throughout the engine-in the crankcase, cylinder heads, and oil lines and filter. Most of it, however, is held in reserve in the oil sump, which is nothing more than a catch basin attached to the bottom of the engine.
An engine that has an integral oil sump is referred to as a wet sump system. If the oil sump is mounted remotely, such as in a wheel well or in the aft portion of the engine nacelle, it is known as a dry sump system. A dry sump system is used on aero-batic engines to keep the oil from leaking out through a breather tube when the airplane goes upside down.
As the accompanying photo shows, a wet sump may hide a surprise. The large tubes that run through the sump basin are part of the engine induction system. After fuel is mixed with air in the carburetor, it is pumped via the induction tubes through the hot oil in the sump and up and into the cylinder intake ports. Doing this serves a couple of purposes. First, it ensures more uniform vaporization of the fuel/air mixture, which in turn means more complete combustion. Second, it wicks off some heat that has been absorbed by the oil. (Engine oil is an important part of the engine cooling system.)
A small mechanical pressure pump draws oil from the sump and sends it on its lubricating way throughout the engine. This is what the oil pressure gauge monitors. A loss of oil pressure means oil is not being picked up from the sump, which in turn means that moving parts are not receiving proper lubrication. And that can ruin your day.
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