Crankcase
From Wikipedia, the free encyclopedia
In a piston engine, the crankcase is the housing for the crankshaft. It is part of the main cavity in the engine, separated from the cylinders by the reciprocating pistons.
Besides providing room for the crankshaft and connecting rods to freely move, the crankcase serves other functions, depending on engine type.
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[edit] Two-stroke engines
In a two-stroke engine, the crankcase is sealed and is used as a pressurization chamber for the fuel/air mixture. As the piston rises, it provides a vacuum which sucks in new fuel/air. As the piston lowers, the fresh fuel/air is pushed from the crankcase into the cylinder. <ref>The Compression Stroke in Two-stroke Engines at HowStuffWorks</ref>
Unlike four-stroke engines, the crankcase can't hold the engine oil because it holds the fuel/air mixture. Instead, oil is mixed in with the fuel and air, and the mixture provides lubrication for the cylinder walls, and crankshaft and connecting rod bearings.
[edit] Four-stroke engines
Image:4-Stroke-Engine 1.gif 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.
[edit] 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. <ref>How Car Engines Work at HowStuffWorks</ref> 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. <ref>Why do some engines use a dry sump oil system? at HowStuffWorks</ref>
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. <ref>October 1996 question on Car Talk</ref> Small amounts of oil may splash onto the crankshaft during rough driving, referred to as windage. <ref>Jeff Huneycutt. Oil Pans For Power. Circle Track magazine. Retrieved on 2006-11-16.</ref>
In a wet sump system, the main dipstick and oil filler cap connect to the crankcase.
[edit] 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". <ref name="cartalk_sep99">September 1999 question on Car Talk</ref> 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. <ref>January 2001 question on Car Talk</ref> 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.
If an engine is damaged or enters old age, gaps can form between the cylinder walls and pistons, resulting in larger amounts of blow-by than the crankcase ventilation system can handle. The gaps cause power loss, and ultimately mean that the engine needs to be rebuilt or replaced. <ref name="cartalk_sep99"/> Symptoms of excessive blow-by include oil being pushed up into the air filter, out the dipstick<ref>August 2002 question on Car Talk</ref>, or out the PCV valve.
[edit] References
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