A supercharger is simply a mechanical device that forces more air and fuel into an internal combustion engine. The more air and fuel that can be forced into an engine, the more power it can produce. It sounds a lot like a turbocharger and it is very similar to the turbocharger, except that a supercharger is driven off the engine rather than the engine's exhaust system. This can rob the engine of as much as 20% of engine power. But this is more than compensated for by the increase in power that the supercharger produces. Despite this difference, a centrifugal supercharger can be described as a belt driven turbocharger. A positive-displacement supercharger is somewhat different in design, but we'll get to that in a little while.
Increasing pressure with a supercharger, as with a turbocharger, will result in increased intake air (or intake charge) temperatures. The increase in air temperature is a result of the thermodynamic effect of compressing air. A term called adiabatic efficiency is a measure of how much more the supercharger heats the air than the thermodynamic effect. Thus a more efficient supercharger will result in a cooler intake charge. The cooler the intake charge, denser it is; and denser air makes more power. A cooler intake charge is also less prone to detonation. Like a turbocharger, a supercharger will benefit from the fitting of an intercooler.
Obviously, the higher the pressure, the higher the temperature and the lower the density. This means that at some point, increased boost will result in a temperature rise that offsets the pressure increase. At that point, the additional horsepower created by the supercharger is offset by the power required to spin the supercharger.
There are two basic types of superchargers: positive-displacement superchargers and centrifugal superchargers.
The superchargers fitted to classic hot rods are positive-displacement superchargers of which the Roots supercharger is an example. These superchargers consist of two lobe rotors or two screws that spin inside an aluminum housing. The Roots supercharger uses two lobe rotors. They are also widely used and usually the most cost efficient supercharger.
However, the required internal clearance between the lobes and the housing means that the positive-displacement supercharger is usually considered to be the least efficient supercharger. However, recent engineering developments have resulted in a more efficient Roots supercharger with an adiabatic efficiency of 50-60%.
The screw-type supercharger does not suffer from the same internal leakage and has an adiabatic efficiency in the region of 70%.
Positive displacement superchargers work by pumping air into the intake manifold at a faster rate that the engine would normally ingest. These superchargers are referred to as 'positive-displacement' superchargers because they pump air at a fixed rate in relation to engine speed and supercharger size. Therefore, there is not threat of over boost. Positive displacement superchargers are also more effective at producing compression at low engine speeds than centrifugal superchargers as they do not need to spool up.
The major disadvantage of positive-displacement superchargers is their size. To create more boost you need a bigger supercharger. This usually means that there is no place for the supercharger inside the engine bay. They also generate lots of heat but this can be tempered by using an intercooler.
The centrifugal supercharger has become quite popular in recent times, mainly because it is much smaller than a positive-displacement supercharger. The centrifugal supercharger is a completely different design to the positive-displacement superchargers and has more in common with a turbocharger. Indeed, the centrifugal supercharger housing is similar to that of a turbocharger and it uses an impeller wheel rather than lobes or screws to move air. The centrifugal supercharger uses step-up gears to spin the impeller wheel much faster than the rotors or screws in positive-displacement superchargers. But this means that the supercharger must 'spool' up before it creates boost.
These superchargers are also true compressors rather than air pumps. The air is drawn in by centrifugal force created by the impeller wheel and passes through a vaneless diffuser. The air is then sent into a scroll where it is compressed. Obviously, the size of the impeller wheel and the step-up gear ratio will determine the boost pressure produced by the centrifugal supercharger.
Because the centrifugal supercharger uses step-up gearing, its displacement is not proportional to its size and a smaller supercharger can be used to create greater boost. Because they are much smaller, centrifugal superchargers can be mounted at the front of the engine rather than on top of it.
Despite being described as a supercharger, the centrifugal supercharger could fall into the turbocharger category in the great supercharger vs turbocharger debate.