An animation showing a four-stroke engine running.
An internal combustion engine is an engine in which combustion, or the burning of fuel, occurs on the inside. There are many kinds but the term often means the machine that Niklaus Otto invented. In this kind, fire makes pressure increase inside a sealed box (cylinder). The pressure pushes a rod which is attached to a wheel. The rod pushes the wheel and makes it spin around. The spinning wheel is attached to other wheels, such as four car wheels, with a belt or a chain. The engine is very strong and can make all the wheels move.
Engines need oil to make them slippery or the moving parts would grind together and stick. Parts of a car engine are measured to 0.01 of a millimeter and some engine parts fit together very tightly.
Internal differs from external combustion where the fire is outside the engine, such as a steam engine.
Most road vehicles use the internal combustion engine today, and most of those use the four-stroke engine. Another type of internal combustion engine is the Wankel engine.
Gas turbines are internal combustion engines that work continuously, not by strokes. Rocket engines and guns are internal combustion engines but they do not turn wheels.
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