four-stroke engine Engine in which the operation of each piston is in four stages, each stage corresponding to one movement of a piston along a cylinder. The stages are: induction, in which the fuel-air mixture enters the cylinder; compression; expansion, in which the exploding mixture forces the piston along the cylinder; and exhaust. This system is called the four-stroke cycle, or the Otto cycle, after its inventor, Nickolaus Otto (1832–91), and is used by many internal combustion engines, including diesel engines.
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