Turbocharging:

Turbochargers have long been used on large diesel engines, and are becoming common on small ones. The turbocharger uses the energy of the escaping exhaust gas to drive a turbine which pressurises the air in the inlet manifold allowing more air to be present. More air means more fuel can be burnt and more power can be developed from the same size engine.

Additional benefit can be gained from turbocharging if the pressurised air is cooled before it enters the engine. This is done using an air-to-air or air to water to air heat exchanger called an intercooler. An intercooler lowers the air between the turbo and the engine. The cooled air is denser and contains more oxygen in a given volume than warm air straight from the turbocharger.

On the increase of late is the use of a variable geometry turbocharger (VNT). With this design, the exhaust gasses entering the turbocharger pass through a variable size venturi. When the engine speed is low and the exhaust gas speed is slow, the venturi diameter is reduced. This has the effect of speeding up the gasses just before they meet the turbocharger wheel. This maintains a high turbine speed, which improves turbocharger performance at low engine speeds. At higher engine speeds when the gasses are moving much faster, the venturi is enlarged. Using this system allows the turbocharger to operate at close to maximum efficiency at a greater range of engine speeds than a standard waste gate controlled turbo.