It’s 1816, and boilers are blowing up left and right, in steps Rev. Robert Stirling, who’s engine doesn’t use combustion and is easier to make. The key to his engine is the natural reaction of a fixed amount gas, to heat, inside a a sealed container. When heated the gas would expand propelling a piston, which turns a shaft that cycles compressed cold air in using another piston. Simply put, the gas is being heated then cooled, and in turn expanding and compressing, driving the pistons. The reason they are not mainly used today is because it needs an external heat source to operate, takes time to start, and does not generate enough power for its size. Stirling engines can be found on submarines, since there is no combustion they do not make noise, allowing a stealthy provider of energy. Also they are used to generate electricity utilizing the thermal heat captured by solar panels. Imagine all the other things it could be used for.
http://www.mpoweruk.com/stirling_engine.htm
http://auto.howstuffworks.com/stirling-engine1.htm
http://www.robertstirlingengine.com/applications1_uk.php