forms of self-propelled torpedoes used high pressure compressed air, although this was superseded by internal or external combustion engines, steam engines (driven by the catalytic decomposition of hydrogen peroxide), or electric motors. Railways Compressed air engines were used in trams and shunters, and eventually found a successful niche in mining locomotives, although in the end they were replaced by electric trains, underground. Over the years designs increased in complexity, resulting in a triple expansion engine with air-to-air reheaters between each stage. For more information see Fireless locomotive and Mekarski system. Flight Water rockets use compressed air to power their water jet and generate thrust, they are used as toys. Air Hogs, a toy brand, also uses compressed air to power piston engines in toy airplanes (and some other toy vehicles). Automotive There is currently some interest in developing air cars. Several engines have been proposed for these, although none have demonstrated the performance and long life needed for personal transport. Energine The Energine Corporation was a South Korean company that claimed to deliver fully assembled cars running on a hybrid compressed air and electric engine. The compressed-air engine is used to activate an alternator, which extends the autonomous operating capacity of the car. The CEO was arrested for fraudulently promoting air motors with false claims. EngineAir EngineAir, an Australian company, is making a rotary engine powered by compressed air, called The Di Pietro motor. The Di Pietro motor concept is based on a rotary piston. Different from existing rotary engines, the Di Pietro motor uses a simple cylindrical rotary piston (shaft driver) which rolls, with little friction, inside the cylindrical stator. It can be used in boat, cars, burden carriers and other vehicles. Only 1 psi (≈ 6,8 kPa) of pressure is needed to overcome the friction. The engine was also featured on the ABC's New Inventors programme in Australia on 24 March 2004. K'Airmobiles K'Airmobiles vehicles were intended to be commercialized from a project developed in France in 2006-2007 by a small group of researchers. However, the project has not been able to gather the necessary funds. People should note that, meantime, the team has recognized the physical impossibility to use on-board stored compressed air due to its poor energy capacity and the thermal losses resulting from the expansion of the gas. These days, using the patent pending 'K'Air Generator', converted to work as a compressed-gas motor, the project