A turbocharger is a device used to recapture energy from exhaust gasses and improve the efficiency of any piston engine, including diesel and petrol engines. A turbocharger uses exhaust gasses to spin a turbine, which in turn compresses air entering the combustion chamber. This compressed air results in more complete fuel combustion, which in turn results in increased efficiency and improved gas mileage. Turbochargers are becoming more popular because of improved fuel efficiency, particularly in turbo diesel engines, but have always been popular in some performance cars because the process also creates an increase in horsepower. In a well-tuned turbocharged system, it is possible to double the horsepower of an engine.