The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidiser (usually air) in a combustion chamber. In an internal combustion engine the expansion of the high temperature and pressure gases, that are produced by the combustion, directly apply force to a movable component of the engine, such as the pistons or turbine blades and by moving it over a distance, generate useful mechanical energy.[1][2][3][4]

The term internal combustion engine usually refers to an engine in which combustion is intermittent, such as the more familiar four-stroke and two-stroke piston engines, along with variants, such as the Wankel rotary engine. A second class of internal combustion engines use continuous combustion: gas turbines, jet engines and most rocket engines, each of which are internal combustion engines on the same principle as previously described.[1][2][3][4]

The internal combustion engine (or ICE) is quite different from external combustion engines, such as steam or Stirling engines, in which the energy is delivered to a working fluid not consisting of, mixed with or contaminated by combustion products. Working fluids can be air, hot water, pressurised water or even liquid sodium, heated in some kind of boiler by fossil fuel, wood-burning, nuclear, solar etc.

A large number of different designs for ICEs have been developed and built, with a variety of different strengths and weaknesses. While there have been and still are many stationary applications, the real strength of internal combustion engines is in mobile applications and they completely dominate as a power supply for cars, aircraft, and boats, from the smallest to the biggest. Only for hand-held power tools do they share part of the market with battery powered devices. Powered by an energy-dense fuel (nearly always liquid, derived from fossil fuels) the ICE delivers an excellent power-to-weight ratio with few safety or other disadvantages.



A 1906 gasoline engine

Internal combustion engines are most commonly used for mobile propulsion in vehicles and portable machinery. In mobile equipment, internal combustion is advantageous since it can provide high power-to-weight ratios together with excellent fuel energy density. Generally using fossil fuel (mainly petroleum), these engines have appeared in transport in almost all vehicles (automobiles, trucks, motorcycles, boats, and in a wide variety of aircraft and locomotives).

Internal combustion engines appear in the form of gas turbines as well where a very high power is required, such as in jet aircraft, helicopters, and large ships. They are also frequently used for electric generators and by industry.


At one time the word, “Engine” (from Latin, via Old French, ingenium, “ability”) meant any piece of machinery—a sense that persists in expressions such as siege engine. A “motor” (from Latin motor, “mover”) is any machine that produces mechanical power. Traditionally, electric motors are not referred to as, “Engines”; however, combustion engines are often referred to as, “motors.” (An electric engine refers to a locomotive operated by electricity.)

Engines can be classified in many different ways: By the engine cycle used, the layout of the engine, source of energy, the use of the engine, or by the cooling system employed.

Principles of operation


  • Two-stroke cycle
  • Four-stroke cycle
  • Six stroke engine
  • Diesel engine
  • Atkinson cycle


  • Wankel engine

Continuous combustion: Brayton cycle:

  • Gas turbine
  • Jet engine (including turbojet, turbofan, ramjet, Rocket etc.)