Turbochargers are the most usual means of forced aspiration used on motor vehicles. They work as follows: a turbine wheel uses the energy in the exhaust to drive a compressor wheel on the same shaft, (but in the intake air flow). It compresses the intake air and forces it into the cylinders. This supplies the engine with more oxygen for the combustion process. Compared with a naturally aspirated engine with the same displacement, the turbocharger increases power and torque as well as efficiency. Exceptionally high-performance engines in certain Audi models even have two turbochargers (“biturbo”).
In the past, increased power was the main reason for using a turbocharger, but today energy conservation and reduced emissions are the primary reasons, together with good performance. Many modern diesel engines have turbochargers with Variable Turbine Geometry (VTG). The angle of the blades changes according to engine load.