Supercharge, power as well as engine torque depend on the amount of supplied air and amount of fuel injected to the cylinder. Compression-ignition engines cannot reach greater power from the same volume of air as the petrol engines. This is because optimum combustion in Diesel engines takes place with surplus of air and in petrol engines with shortage of air. This is why, in order to reach the same power as the petrol engine, the Diesel engine needs to have larger capacity or must be supercharged, which allows mainly for obtaining more power from the engine, but is used also to cool and limit soot emission.
Designers use different systems of rotational compressors where top turbine speed is at least 140,000 – 250,000 r.p.m. In order to prevent from overheating of oil in casing, bearings of turbo compressor of supercharged engines are cooled with the cooling system liquid. Without this, if the engine is stopped too early, oil in bearing and casing on the side of the turbine may reach the temperature even higher than 300 Celsius degrees.
Soot in Diesel engines formed as a result of the combustion process gets to the oil, causing its thickening. If one does not take preventive measures, then the oil viscosity increases proportionally to the amount of soot which has got to the oil. This is why a group of oils suitable for Turbo-diesel engines was developed in which the use of special additives allows to control the contents of soot and keep it dispersed, to seal and cool the compressor elements when maintaining a suitable permanent lubricating film.