Four-stroke cycle or (Otto cycle)

The air-fuel mixture is then ignited, usually by a spark for a gasoline or Otto cycle engine or by the heat and pressure of compression for a Diesel cycle or compression ignition engine, at approximately the top of the compression stroke. The resulting expansion of burning gases pushes the piston downward for the third stroke (power) and in the fourth stroke (exhaust) the piston pushes the products of combustion from the cylinder through an exhaust valve or valves.