A form of reciprocating engine which has an intake and an exhaust valve in each cylinder. Four strokes of the piston (two in and two out) are needed to change chemical energy in the fuel into mechanical energy at the rotating crankshaft.

During the intake stroke, the piston moves inward, with the intake valve open and the exhaust valve closed. A mixture of fuel and air is pulled into the cylinder through the carburetor.

On the compression stroke, the piston moves outward with both valves closed. This compresses the fuel-air mixture. Near the end of the compression stroke, the spark plug fires and ignites the compressed fuel-air mixture.

The burning fuel heats and expands the air inside the cylinder, and the expanding air forces the piston inward on the power stroke. Near the end of the power stroke, the exhaust valve opens.

During the exhaust stroke, the piston moves outward with the exhaust valve open. As the piston moves outward, it forces the burned gases from the cylinder, and the cylinder is prepared for another cycle of operation.