From our insanely detailed guide:
Oil pan / Sump
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The oil pan , or sump , is a metal dish which covers the bottom of the engine block, and holds the engine oil when it is not circulating around the engine.
The oil pump has a pickup tube that dangles into the sump and sucks up oil, once used, the engine oil drops back into the sump.
As well as storing the oil, the oil pan has an important role in cooling it. As the car moves forwards, air will flow under the sump, carrying away heat from the hot oil - this airflow may be boosted by using bodywork to direct airflow around the sump. Cooling fins may also be found on the sump to increase its surface area.
This is known as a wet sump system , and is almost universal in standard production vehicles. It is a wet sump because the sump always contains oil - its natural state is to be wet! Another system is a dry-sump system in which the oil is pumped out of the sump into a remote container. We discuss dry sump systems in another article.
Oil sitting in the pan will slosh around as the car accelerates and corners. If this happens a lot then the oil can move away from the pickup pipe and cause oil to temporarily stop flowing through the engine. Given that the engine is likely to be running fast at the same time as hard cornering and acceleration - it’s a doubly bad situation if it is starved of oil.
Sumps, particularly those in sports-type vehicles, will likely have baffles to prevent oil moving around. Baffles can take various forms, but their purpose is to impede the flow of a huge body of liquid, while allowing ready flow of smaller amounts.
Similar to baffles, a windage tray is a piece of sheet metal which prevents oil being splashed up onto the spinning crankshaft, causing it to slow down. Windage trays may also have some form of crankshaft scraper, which removes excess oil clinging to the crankshaft counterweights.
As the lowest point of the lubrication system, the sump is where the oil drain plug is located. Oil plugs are threaded bolts which are removed to allow dirty oil to drain. The drain plug will have a washer - which should ideally be replaced whenever it is removed. Oil is more easily drained when warm.
Engines use a dipstick as a means of determining the amount of oil in the engine. The dipstick will dip down into the oil pan and be marked to show the level of oil - with minimum and maximum markings. Recent cars will include an electronic oil level sensor which shows the oil level on a dashboard indicator.