The four-strokes of the spark ignition engine and the compression ignition engine are intake, compression, power (combustion), and exhaust. To complete the four strokes, the crankshaft makes two revolutions. The four-strokes of the compression ignition engine are similar to the spark ignition engine, except fuel is not mixed with air in the intake system. Instead diesel is injected directly into the combustion chamber or indirectly into a swirl (before combustion) chamber. Once in the combustion chamber, the diesel combusted spontaneously from the high pressure and heat. Compression ignition engines do not use spark plugs.
The four strokes of the cycle are intake, compression, power, and exhaust. Each corresponds to one full stroke of the piston; therefore, the complete cycle requires two revolutions of the crankshaft to complete.
During the intake stroke, the piston moves downward, drawing a fresh charge of vaporized fuel/air mixture. The illustrated engine features a poppet intake valve which is drawn open by the vacuum produced by the intake stroke. Some early engines worked this way; however, most modern engines incorporate an extra cam/lifter arrangement as seen on the exhaust valve. The exhaust valve is held shut by a spring.
As the piston rises, the poppet valve is forced shut by the increased cylinder pressure. Flywheel momentum drives the piston upward, compressing the fuel/air mixture.
At the top of the compression stroke, the spark plug fires, ignited the compressed fuel. As the fuel burns it expands, driving the piston downward.
At the bottom of the power stroke, the exhaust valve is opened by the cam/lifter mechanism. The upward stroke of the piston drives the exhausted fuel out of the cylinder.