The bottom half of the crankcase is called the oil pan or sump. It is bolted or screwed to the lower flange of the main casting of IC engine and usually is made of pressed steel or aluminium. Oil pan serves as the reservoir for the storage, cooling and ventilation of engine lubricating oil.

The plane of the joint between the crankcase and the oil pan may be either on the level of the crankshaft axis or it may be lower. If it is on the level of the crankshaft axis, it will increase the bottom oil pan portion. If it is lower than this axis, it will increase upper portion of the crankcase thus increasing rigidity.

Oil pan or sump (image credit: autorepair.about.com)

The oil pump in the lubricating system draws oil from the oil pan and sends it to all working parts in the engine. The oil drains off and runs down into the pan. Thus there is a constant circulation of oil between the pan and the working parts of the engine.