Turbocharger is an air compressor which is powered by exhaust gases. Exhaust gases when leaving the engine through the exhaust collector are led to the turbine pit where the rotor is located. The shaft drives the compressor rotor which is located on the other side of the device (often called the cold side), which compresses air that supplies the engine. Because compressed air contains more oxygen in a unit, it gives better conditions for combustion process. The additional oxygen can be used with some more fuel for increasing engine power. Or as it is the case of diesel engines – it enables higher combustion which reduces the emission of exhaust gases and makes the engine more efficient.
When enhancing the air compression, that is the quantity of gas located in the same capacity, its temperature is rising. The higher the temperature, the lower the density, what means, that less oxygen is supplied to the cylinder. To make the combustion process even more efficient an intercooler is used. In more sophisticated systems, used mainly in sports, to reduce the compressed air temperature a water or alcohol injection is used. Water needs 4 times more energy to raise the temperature by 1 °C than air to reduce the temperature.