An internal combustion engine (ICE) is a type of heat engine that converts chemical energy from fuel into mechanical energy through combustion. The combustion of a fuel occurs with an oxidizer, usually air, in a combustion chamber that is an integral part of the working fluid flow circuit. The engine consists of a fixed cylinder and a moving piston. The expanding combustion gases push the piston, which in turn rotates the crankshaft. Ultimately, through a system of gears in the powertrain, this motion drives the vehicle’s wheels.

There are two kinds of internal combustion engines currently in production: the spark ignition gasoline engine and the compression ignition diesel engine. Most of these are four-stroke cycle engines, meaning four piston strokes are needed to complete a cycle. The cycle includes four distinct processes: intake, compression, combustion and power stroke, and exhaust.

The first commercially successful internal combustion engine was created by Étienne Lenoir around 1860, and the first modern internal combustion engine, known as the Otto engine, was created in 1876 by Nicolaus Otto. There are several possible ways to classify internal combustion engines, including by number of strokes, type of ignition, and mechanical/thermodynamic cycle.