Oil Pans

In a piston engine, the crankcase is the housing for the crankshaft. The enclosure forms the largest cavity in the engine, separated from the cylinders by the reciprocating pistons. more...

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Besides protecting the crankshaft and connecting rods from foreign objects, the crankcase serves other functions, depending on engine type.

Two-stroke engines

In two-stroke engines, the crankcase is sealed and is used as a pressurization chamber for the fuel/air mixture. As the piston rises, it pushes out exhaust fumes and provides a vacuum in the crankcase which sucks in new fuel/air. As the piston lowers, the fresh fuel/air is pushed from the crankcase and sucked into the cylinder.

Unlike four-stroke engines, the crankcase does not hold the engine oil because it holds the fuel/air mixture. Instead, oil is mixed in with the fuel, and the mixture provides lubrication for the cylinder walls, crankshaft and connecting rod bearings.

Four-stroke engines

In a four-stroke engine, the crankcase is filled mainly with air and oil, and is largely sealed off from the fuel/air mixture by the pistons.

Oil circulation

Oil circulation is kept separate from the fuel/air mixture, thereby preserving oil rather than burning it as happens in two-stroke engines. Oil moves from its reservoir, is pressurized by an oil pump, and is pumped through the oil filter to remove grit. The oil is then squirted into the crankshaft and connecting rod bearings and onto the cylinder walls, and eventually drips off into the bottom of the crankcase. In a wet sump system, oil remains in a reservoir at the bottom of the crankcase, referred to as the oil pan. In a dry sump system, oil is instead pumped to an external reservoir.

Even in a wet sump system, the crankshaft has minimal contact with the sump oil. Otherwise, the high-speed rotation of the crankshaft would cause the oil to froth, making it difficult for the oil pump to move the oil, which can starve the engine of lubrication. Small amounts of oil may splash onto the crankshaft during rough driving, referred to as windage.

In a wet sump system, the main dipstick and oil filler cap connect to the crankcase.

Air ventilation

During normal operation, a small amount of unburned fuel and exhaust gases escape around the piston rings and enter the crankcase, referred to as "blow-by". If these gases remained in the crankcase and condensed, the oil would become more diluted over time, decreasing its ability to lubricate. Condensed water would also cause parts of the engine to rust. To counter this, a crankcase ventilation system exists to draw fresh air in from the air filter and expel the gases out the PCV valve into the intake manifold. The intake manifold is at a lower pressure than the crankcase, providing the suction to keep the ventilation system going.

Read more at Wikipedia.org