An internal combustion engine is an engine in which the combustion of a fuel occurs in a combustion chamber inside and integral to the engine. In an internal combustion engine it is always the expansion of the high temperature and pressure gases that are produced which apply force to the movable component of the engine, such as the pistons or turbine blades.
The term internal combustion engine most usually refers to an engine in which combustion is intermittent, such as the familiar four-stroke and two-stroke engines, along with a very few more exotic variants, such as the Wankel engine. These engines almost invariably use reciprocating pistons, with crankshafts, connecting rods and most of them now use camshafts with cams. A second class of internal combustion engines use continuous combustion - Jet engines (including gas turbines) and most rockets, each of which are internal combustion engines on the same principle as previously described.
The internal combustion engine contrasts with the external combustion engine, such as a steam or Stirling engine in which the energy is delivered within a working fluid heated in a 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 very few safety or other disadvantages.