The document describes the four stages of the combustion cycle in a four-stroke engine: intake, compression, power, and exhaust. During intake, the piston moves downward and draws in the fuel-air mixture. In compression, the piston rises and compresses the mixture. At top of compression, the spark plug ignites the fuel. During power, the burning fuel expands and drives the piston downward. In exhaust, the exhaust valve opens and the spent gases are driven out.