Heat engine that uses two or more reciprocating pistons to convert pressure created by fuel combustion into rotating motion. Its most common use is in motor vehicles. The linear motion of the pistons is converted through the connecting rod and crankshaft to the rotation of the common shaft. During piston movement, fuel is injected with an oxidizer into the combustion chamber. Here, the piston movement compresses it. The compressed fuel is ignited, and the expanding gases compress the piston and do the work. The combustion gases are expelled from the combustion chamber, and new fuel is injected.