A four-stroke engine is an internal combustion engine in which the operation of each piston is in four stages. This kind of operation is called the four-stroke cycle or the Otto cycle, after its inventor, Nickolaus Otto. Most internal combustion engines of the type found in cars, including diesel engines, use the four-stroke principle. Each of the four stages corresponds to one movement of a reciprocating piston along a cylinder. First, the piston descends to admit the fuel-air mixture (intake stroke). Second, the piston rises, compressing the mixture (compression stroke). Third, the mixture is ignited by a spark or compression, and expanding gases from the explosion force the piston downward (ignition or power stroke). Fourth, the piston again rises to expel the spent gases from the cylinder (exhaust stroke).
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