All four-stroke engines use air, fuel, an engine and a spark to run. The engine uses four piston strokes to complete one operating cycle. The four separate strokes of the engine are the intake stroke, the compression stroke, the power stroke and the exhaust stroke.
In the intake stroke, air and fuel enters the cylinder through a carburettor, which blends the air and fuel to allow for proper combustion. The intake valve between the carburettor and the combustion chamber opens and lets the atmospheric pressure force the air-fuel mixture into the cylinder bore as the piston moves downward.
In the compression stroke, the piston travels from the bottom dead center to the top dead center and compresses the air-fuel mixture in preparation for the ignition. The intake and exhaust valves stay closed during this stage.
The power stroke is the only stroke that is responsible for generating power to turn the crankshaft. During the power stroke, the fuel is ignited by a spark plug in gasoline engines or by the heat generated by compression in diesel engines. At ignition, the air-fuel mixture in the cylinder burns abruptly and expands, forcing the piston to return to the bottom dead center.
During the exhaust stroke, the exhaust valve opens, and the piston returns from the bottom dead center to the top dead center, forcing the used air-fuel mixture out of the cylinder through the exhaust valve.