An internal combustion engine is an engine in which the combustion of fuel and an oxidizer (typically air) occurs in a confined space, referred to as a combustion chamber, that is integral to the engine. This is in contrast to an external combustion engine, such as a steam engine or Stirling engine, in which combustion does not occur within the engine itself. Instead an external device (for example, a boiler) heats a separate working fluid that is delivered to the engine to perform useful work.

Within an internal combustion engine an exothermic reaction creates gases at high temperature and pressure, which are permitted to expand and apply force to a movable component of the engine, such as a piston or turbine.

The term Internal Combustion Engine (ICE) is often used to refer to an engine in which combustion is intermittent, such as a Wankel engine or a reciprocating piston engine in which there is controlled movement of pistons, cranks, cams, or rods. However, continuous combustion engines such as jet engines, most rockets, and many gas turbines are also classified as types of internal combustion engines.

A huge number of different designs for internal combustion engines exist, each with different strengths and weaknesses. Although they're used for many different purposes, internal combustion engines particularly see use in mobile applications such as cars, aircraft, and even handheld applications: all where their ability to use an energy-dense fuel (especially fossil fuels) to deliver a high power-to-weight ratio is particularly advantageous.

A Petrol engine or Gasoline engine is an internal combustion engine with spark-ignition designed to run on petrol (gasoline) and similar volatile fuels. It differs from a diesel engine in the method of mixing the fuel and air, and in the fact that it uses spark plugs. In a diesel engine, merely the air is compressed, and the fuel is injected at the end of the compression stroke. In a petrol engine, the fuel and air are pre-mixed before compression injection. Pre-mixing of fuel and air allows a petrol engine to run at a much higher speed than a diesel, but severely limits their compression, and thus efficiency .

Concerns about global warming and air pollution have put a question mark over the future of the petrol engine. Much has been done to improve its fuel efficiency and reduce emissions and this has bought it more time.

A diesel engine is an internal combustion engine which operates using the diesel cycle (named after Dr. Rudolph Diesel). The defining feature of the diesel engine is the use of compression ignition to burn the fuel, which is injected into the combustion chamber during the final stage of compression. This is in contrast to a petrol (gasoline) engine, which uses the Otto cycle, in which a fuel/air mixture is ignited by a spark plug .

Diesel engines are manufactured in two stroke and four stroke versions. They were originally used as a more efficient replacement for stationary steam engines. Since the 1910s they have been used in submarines and ships. Use in locomotives, large trucks and electric generating plants followed later. In the 1930s, they slowly began to be used in a few automobiles. Since the 1970s, the use of diesel engines in larger on-road and off-road vehicles in the USA increased. As of 2007, about 50 percent of all new car sales in Europe are diesel.

The diesel internal combustion engine differs from the gasoline powered Otto cycle by using a higher compression of the air to ignite the fuel rather than using a spark plug ("compression ignition" rather than "spark ignition").

Advantages of the diesel engine are numerous. It burns considerably less fuel than a gasoline engine performing the same work. It has no ignition system to attend to. It can deliver much more of its rated power on a continuous basis than can a gasoline engine. The life of a diesel engine is generally about twice as long than a gasoline engine. Although diesel fuel will burn in open air using a wick, it will not explode. With partial load the fuel consumption lbs/Kwh is the same in opposition to gasoline and turbine engines.

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