Before we jump into the science behind a turbocharger and a supercharger, we need to understand what they are and why they are needed. Internal Combustion Engines work by burning a mixture of air and fuel. But not every air fuel ratio can be used in a normal engine. This is because of the fact that a unit quantity of fuel needs certain amount of air to combust properly. So, if more fuel is injected, equal amount of air is needed. In situations where more power is needed, more fuel needs to be burnt and sufficient quantity of air needs to be mixed with it. This is limited by the maximum amount of fuel-air that can the engine can hold and combust. So, to make the engine function in conditions not possible normally, artificial aspirating mechanisms like turbochargers and superchargers are used. These compress the air and thus these systems enable the mixing of more fuel with sufficient air in compressed form to obtain more power.
A supercharger is an artificial aspirating mechanism that compresses air using a turbine powered by the engine. The supercharger is mechanically connected to the engine by means of a belt or chain. The supercharger takes some power from the engine and compresses air. The compression is dependent on the speed of rotation of the flywheel / crankshaft to which the supercharger is connected to. The Turbocharger on the other hand does not consume power from the engine. It utilizes the exhaust gases from the engine to rotate a turbine which in turn powers the compressor. The turbocharger thus adds to the output power without consuming power from the engine.
A supercharger has some disadvantages when compared to the turbocharger. Let us see an example. The supercharger employed in Rolls Royce Merlin engine uses about 110 kW of the engine’s power and generates approximately 300 kW. So, there is a net gain of 190 kW. This brings to light the main problem with superchargers. Not only should the engine withstand its power output but also be able to drive the supercharger. Superchargers have less adiabatic efficiency when compared to turbochargers. Adiabatic efficiency is the ability to compress the air without adding heat to it. Even though naturally compression leads to elevated temperatures, some supercharger varieties like roots type supercharger increases the temperature of the compressed air. Thus, when compared to turbocharger, for a given volume the temperature of the compressed air is high. This difference in temperature affects the power output of the engine. Based on this alone, the turbocharger is 15% to 30% more efficient than a supercharger.
The turbocharger does not consume power from the engine. Rather it makes use of the exhaust gases from the engine. This causes pumping losses in the engine. Even though there are losses, the net effect is very desirable than the superchargers because the energy that usually gets wasted in the form of heat is utilized to run the turbine.
One primary disadvantage of the turbo in comparison to the supercharger is that the turbocharger suffers from throttle lag. It is the time of response that the turbocharger to give the needed power output. This is because the turbocharger works with the exhaust gases. It takes some time for the engine to burn the fuel and produce sufficient volume of exhaust gases.
The Supercharger and the turbocharger have their own advantages and disadvantages. However mostly turbochargers are preferred over superchargers in most of the vehicles. In some applications, the manufacturers have tried a combination of both to combine the advantages of both the mechanisms and get the most out of it.