An internal combustion engine is an engine in which combustion, or the burning of fuel, occurs on the inside. It is a machine that makes large pressure increases inside a sealed box (cylinder). The pressure increase 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.
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. Rocket and jet engines are combustion engines but they do not turn wheels. The fire in a rocket combustion engine is so strong that it blows a mighty wind out the back, which pushes the rocket forward.
Another type of internal combustion engine is the Wankel engine.
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Internal combustion engine Facts for Kids. Kiddle Encyclopedia.