Supercharger Basics
Supercharger Basics
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.
POSITIVE-DISPLACEMENT SUPERCHARGERS
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.
CENTRIFUGAL SUPERCHARGERS
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.
The Roots Supercharger
The Roots supercharger, or the Eaton supercharger as its also called, is the oldest type of supercharger around, having been designed by the Roots brothers in 1859 as an air pump for use in the mining industry. The Roots or Eaton supercharger is a positive-displacement supercharger that consists of at least two lobed rotors housed in an aluminum casing. The rotors are meshed together and geared to rotate in the opposite directions. As the lobes turn, air trapped in the space around the lobes and is forced along the inside of the casing until it is discharged into the intake manifold. Being a positive-displacement supercharger, it moves air at a fixed rate in relation to engine RPM; hence a larger capacity Roots supercharger is required if you want to achieve higher boost levels.
Sincethe 1950′s, engineers at the Eaton Corporation have been redeveloping the Roots-type supercharger. In the mid 1970′s they made significant breakthroughs when they developed the twisted rotor for the Roots supercharger to improve its thermal efficiency to 50-60% and reconfigured the Roots supercharger’s outlet port to reduce noise levels. The Eaton Corporation has also improved the fuel efficiency of the Roots-type supercharger. This resulted in the Roots supercharger also being referred to as the Eaton supercharger and it made the Roots supercharger an attractive option for both car manufacturers and hot rodders. They are also popular with muscle cars and hot rods as the Roots-type supercharger is usually installed on top of the engine and sticks out of the hood of the car.
However, the Eaton supercharger and the Roots Supercharger does not compress air but simply moves air at a fixed rate in relation to engine RPM. In this sense it is nothing more than an air blower with air compression taking place externally. In other words, the Roots supercharger produces boost pressure by stacking more and more air into the intake runners and into the intake manifold. Thus, with the Roots and Eaton supercharger, boost pressure is the result of more air being forced in to the intake runners and into the intake manifold and boost pressure only increases after the air is discharged from the supercharger. Hence the Roots supercharger is also called an external compression supercharger. This external compression is also a major contributor to the relatively poor thermal efficiency of the Roots supercharger.
THE PROS & CONS OF ROOTS-TYPE SUPERCHARGERS
If you’re looking for a cost effective, low boost supercharger with excellent boost at low RPM then the Roots-type supercharger may just be your best option. However, these types of superchargers do suffer from internal leakage which reduces it efficiency at low RPM. Furthermore, the Roots supercharger draws the most engine power of all types of superchargers and also has the least thermal efficiency of all superchargers. However, its simple construction with few moving parts makes the Roots supercharger one of the most reliable types of superchargers you can find. The Roots supercharger also doesn’t suffer from surge as it is a positive displacement supercharger, which means that it moves air at a fixed rate in relation to engine RPM. This also means that a large capacity Roots supercharger is required if you want to achieve higher boost pressures. However, if you’re looking for boost pressure of over 12 psi, you’d have to look elsewhere as the poor thermal efficiency of the Roots supercharger becomes a major problem at higher boost pressures. Although, they work wonders on exotic fuel dragsters that are used for short bursts of around 15 to 20 seconds at a time and were boost from low RPM is more important than thermal efficiency, they are not ideal for constant high boost applications.
THROTTLE PLACEMENT
When implementing a Roots supercharger, you must install a bypass valve and relocate the throttle body ahead of the supercharger’s inlet port. If you don’t move the throttle body, the supercharger will build up pressure between the supercharger and the throttle whenever your foot is off the accelerator, such as when you’re idling, decelerating, or changing gears. When the pressure between the supercharger and the closed throttle exceeds the boost pressure being supplied by the supercharger, the air will be forced back through the supercharger. However, air can only move in one direction through a Roots-type supercharger. If the air tries to flow back through the supercharger, the supercharger will cease and will destroy the drive belt. This can also cause the throttle plate to buckle and get jammed in the throttle bore as the pressure will not be released. Of course, moving the throttle body further away from the intake valves will make the engine less responsivto throttle input, but that’s just the cost of running a positive-displacement supercharger.
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