What is function of a piston in a petrol engine?

What is function of a piston in a petrol engine?
AnswerThe piston is a key engine part, which is used to compress gases withing the combustion chamber or cylinder of an engine.The piston is sealed with a piston ring, often constructed under freezing temperatures, to allow expansion and therefore complete sealing, to seal the chamber to prevent leaks, or dangerous gases escaping.As the piston moves downwards, a valve opens to allow fuel into the chamber, this fuel is then compressed, combusted and expelled. All of which is transferred through the cam or prop shaft, through a differential and gear box, into the axle and ultimately onto the road via the tire.For more information try http://auto.howstuffworks.com/engine1.htm