Stirling engine is using fire to heat up gas in special cylinder. As gas heats up it expands moving the piston and thus producing mechanical energy. Really big downside of this engine is having the gas in cylinder cool down and repeat the process. Other then that very similar to petroleum engine. Moving piston and producing energy. I see why this type of energy production never got as much action as coil or oil did. It is not efficient when compared to them at all. and efficiency is all that matters, it makes all the difference in the world. Today it does have small usage today for single reason, it is more quite then other alternatives. So machines such as military Submarines or small yachts come with built in Stirling engine. But as far as big industry usage is concerned, it gets overrun in competition by alternative engines.