An internal-combustion engine is any engine that operates by burning its fuel inside the engine. This can be contrasted with external combustion engines such as steam engines and Stirling engines, which burn their fuel outside the engine. Jet engines and gas turbines use internal combustion, but the term 'internal-combustion engine' is normally used only to refer to engines in which combustion is intermittent (and usually featuring reciprocating machinery).

The internal-combustion engine was invented by Nikolaus August Otto.

Nikola Tesla devised an electric igniter for gasoline engines. His designs are nearly identical to ideas which deal with the same process which modern internal combustion engines use.

The de Havilland Gypsy Queen engine, powering Dove and Heron propeller aircraft.

The most common internal combustion engines are the gasoline powered engine and the diesel engine. Others include those fueled by hydrogen, methane, propane, etc. Engines typically can only run on one type of fuel and require adaptations to adjust the air/fuel ratio or mix to use other fuels.

In a gasoline engine, a mixture of gasoline and air, controlled by a throttle, is inducted into a cylinder. Some engines use aftermarket cold air intakes to gain horsepower, torque, and throttle response.

This is compressed by a piston and at optimal point in the compression stroke, a spark plug creates an electrical spark that ignites the fuel.

The combustion of the fuel results in the generation of heat, and the hot gases that are in the cylinder are then at a higher pressure than the fuel-air mixture and so drive the piston back down.

These combustion gases are vented and the fuel-air mixture reintroduced to run a second stroke. The outward linear motion of the piston is ordinarily harnessed by a crankshaft to produce circular motion. Valves control the intake of air-fuel mixture and allow exhaust gases to exit at the appropriate times.

A critically important portion of any internal-combustion engine is its ignition system, which controls the timing of the burning of the fuel mixture. If this burn begins either too early or too late the engine performance will be reduced, sometimes seriously, and in extreme cases can even damage the engine.

Some types of ignition systems that have been or are used in internal-combustion engines are:

;Outside flame ignitors

Early mechanical ignition system, soon replaced by the:
;Hot-tube ignitors
Early ignition system, these were used on some engines until the time of the First World War.
;Electrical ignitorss
The precise control these provide soon made all earlier devices obsolete. All modern internal-combustion engines (except the diesel) use them.
;Compression heating ignition
Only used in diesel engines. Actual timing controlled by fuel injection system.

See also: two stroke cycle, four stroke cycle, straight engine, V engine, diesel cycle, rotary engine, Wankel engine, Miller cycle, stratified charge engine, Engine tuning and Gas Laws, Samuel Morey