What is a Turbocharger ?
A turbo charger is a device which uses exhaust heat from an Engine to get more air into the Cylinder during Induction stroke. (Air + Fuel for petrol Engine). More air translates to increased ability to burn more fuel which in turn increases the Power output (brake horse power) of the Engine.
Effectively a Turbocharger increases the Power output for the same cubic capacity of the Engine.
Diesel as a fuel is not as combustible as petrol. The air-fuel mixture inside the Cylinder should be at higher pressures for effective combustion of diesel. To withstand higher combustion pressures a diesel engine is built with strong, heavy parts. The heavier components of a diesel engine cannot rotate at higher rpms.
Lesser rpm means lesser air intake into combustion chambers. Hence a turbo charger helps a diesel engine to get more air into the combustion chamber.
Petrol is more combustible than diesel and hence at lower pressures combustion can take pla place.
This means lower in-cylinder pressures and lighter moving parts which can rotate at higher rpm. This explains why RPM bands of petrol engines are much higher than that of their diesel counterparts.
More rpm and hence it is easier to take in more air-fuel mixture during induction stroke.
This explains why Diesel Engines are found with turbochargers more than petrol Engines.
And adding onto this the role of Turbocharger with Petrol Engines is to increase Boost Pressure (ratio of Pressure at outlet of Turbocharger compressor to inlet pressure )
Where as in Diesel Engines the role of turbo is simply to take in more air .