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 (precombustion) chamber. Once in the combustion chamber, the diesel combusts spontaneously from the high pressure and heat. Compression ignition engines do not use spark plugs.