ion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is applied typically to pistons, turbine blades, rotor or a nozzle. This force moves the component over a distance, transforming chemical energy into useful mechanical energy. CLassifacation: By number of strokes Two-stroke engine Clerk cycle[7] Day cycle Four-stroke engine (Otto cycle) Six-stroke engine By type of ignition Compression-ignition engine Spark-ignition engine (commonly found as gasoline engines) By mechanical/thermodynamical cycle (these 2 cycles do not encompass all reciprocating engines, and are infrequently used): Atkinson cycle Miller cycle