A Compressed air engine is one that creates useful work by expanding compressed air. They have existed in many forms over the past two centuries, ranging in size from handheld units to turbines up to several hundred horsepower. Some types rely on pistons and cylinders, others use turbines. Many compressed air engines improve their performance by heating the incoming air, or the engine itself. Some took this a stage further and burnt fuel in the cylinder or turbine, forming a type of internal combustion engine.