An internal combustion engine (ICE) is a heat engine where 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. In an internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is applied typically to pistons, turbine blades, rotor or a nozzle. This force moves the component over a distance, transforming chemical energy into useful mechanical energy.
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 six-stroke piston engine and the Wankel rotary engine.
The two major kinds of internal combustion engines currently in production are the spark ignition gasoline engine and the compression ignition diesel engine. Spark ignition gasoline and compression ignition diesel engines differ in how they supply and ignite the fuel. 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.
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. Firearms are also a form of internal combustion engine.
Typically an ICE is fed with fossil fuels like natural gas or petroleum products such as gasoline, diesel fuel or fuel oil. There is growing usage of renewable fuels like bio-diesel for compression ignition engines and bioethanol or methanol for spark ignition engines. Hydrogen is also used sometimes, and can be obtained from either fossil fuels or renewable energy.
Over the last 30 years, research and development has helped manufacturers reduce ICE emissions of criteria pollutants, such as nitrogen oxides (NOx) and particulate matter (PM) by more than 99%. Research has also led to improvements in ICE performance (horsepower and 0-60 mph acceleration time) and efficiency, helping manufacturers maintain or increase fuel economy.