An operating principle used in most internal combustion engines. The four-stroke cycle commences with the induction stroke, during which the piston moves from the top to the bottom of the cylinder and charge air is drawn or fed under pressure into the cylinder. During the compression stroke, the piston moves to the top of the cylinder, compressing the charge air. In a diesel engine, the fuel is injected and ignition commences fractionally before the piston reaches its top position. Then it follows the power stroke, during which the combustion of fuel takes place and the pressure of the expanding combustion gases forces the piston down the cylinder. Finally, during the exhaust stroke, the piston rises to the top of the cylinder, expelling the exhaust gases.