Capable of using either petrol or natural gas for power, the EcoFuel vehicles are powered by a 1.4L TSI engine with around 150hp (112kW). The engine features both a supercharger and a turbocharger operating sequentially to provide relatively high power outputs from its small capacity.
In order to allow the engine to cope with the additional loading encountered through the burning of natural gas rather than gasoline, significant changes have been made. The valves, piston rings and the pistons themselves are all uprated to cope, while the turbocharger has also been replaced with a smaller unit. Controlling the switch between conventional fuel and natural gas is a new engine management computer.
Source: Motor Authority