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. 