The internal combustion engine is a heat engine that uses the byproducts of combustion as the working fluid instead of using a heat transfer medium to transfer heat. The combustion is done in a way that produces high-pressure combustion products that can be expanded through a turbine or piston to create work. Internal combustion engines can again be categorized into three kinds: (1) spark ignition gasoline engines, most commonly used in automobiles, (2) compression ignition diesel engines that are used in heavy vehicles and industrial machinery, and (3) gas turbines, which are used in aircraft.

The working of the internal combustion engine can be explained using spark ignition gasoline engines. Most of these are four-stroke cycle engines requiring four piston strokes to complete a cycle. The four strokes are the intake stroke, the compression stroke, the ignition and power stroke, and the exhaust stroke.

During the intake stroke, the intake valve is open, and the exhaust valve is closed. The descending piston draws the air-fuel mixture through the intake valve. At the end of this stroke, the intake valve is closed, and the piston compresses the mixture approximately adiabatically. This stroke is called the compression stroke. In the power stroke, the spark plug ignites the gas. The heated gas expands almost adiabatically to its maximum volume. Work is done on the piston by pushing it down. Finally, the exhaust valve opens, and the piston moves up, expelling the burned gas from the cylinder and preparing the cylinder for the next intake stroke.

The efficiency of a combustion engine is expressed in terms of the compression ratio. It is the ratio of the maximum to the minimum volume of the cylinder. For automobile engines, it ranges from 8 to 10.