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Internal-combustion engine

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An internal-combustion engine is any engine that operates by burning its fuel inside the engine. In contrast an external combustion engine burns its fuel outside the engine, for example a steam engine. In general, the term 'internal combustion engine' is used only to refer to engines in which fuel is burned intermittently, thus excluding jet engines and gas turbines which burn fuel continuously.

The de Havilland Gypsy Queen engine, powering Dove and Heron propeller aircraft.
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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 is sprayed into a cylinder. 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:

Early mechanical ignition system. Already obsolete before 1911.
Early ignition system. Was still common in 1911.
The precise control these provide soon made all earlier devices obsolete. All modern internal-combustion engines (except the diesel) use them.
Only used in diesel engines. Actual timing controlled by fuel injection system.

See also: two stroke cycle, four stroke cycle, diesel cycle, Otto cycle, rotary engine (Wankel), Miller cycle, stratified charge engine and Gas Laws, Samuel Morey

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