All About Superchargers

A supercharger, which is also known as a blower, is a mechanical device that is mechanically driven by belt, gear, or chain-drive of the engine's crankshaft. It is used to force more air into an engine but it is not necessarily a compressor. Today you get two types of superchargers; positive displacement devices that are blowers in the true sense of the word and are installed on top of the engine cover, and the more modern centrifugal devices that is more of a mechanically driven turbocharger that compresses air rather than just blowing air.

The classic positive displacement blower falls into two types: the Roots type and the Lysholm or twin screw. The Roots type operates by pulling air through a pair of meshing lobes by trapping the air in pockets surrounding the lobes and caring it from the intake side to the outlet side of the device. These type of superchargers operate a fixed speed in relation to the engine speed so that there is always some boost, even at low rpm and hence these devices do not cause lag. Boots is also constant right through the rev range and there's no threat of over boost.

Another positive displacement blower is the Lysholm or twin screw supercharger that is similar to the Roots type but it has one major difference; it is not just an air blower but an air compressor that builds boost pressure internally. This internal compression is created by the profile and shape of the counter-rotating lobes. The two lobes do not overlap completely, leaving a small air pocket between them, which become gradually smaller as the air pocket moves through the device, the result in air compression and an increase in air pressure. Unlike the lobes in a Roots supercharger, the lobes in twin-screw superchargers are manufactured to a high precision to ensure that internal leakage does not occur. This makes these devices more efficient but also expensive than the Roots type.

The last type of supercharger is the more modern centrifugal supercharger that has a lot more in common with a turbocharger than a blower in both design and function. Like turbochargers, centrifugal superchargers have a narrowing circular scroll through which air is forced to create internal boost pressure. Unlike other types of blowers, these devices use a step-up gears to increase the rate of air flow exponentially to the increase in engine speed making it more like a turbocharger. But the major difference between a turbocharger and centrifugal supercharger is that the latter is mechanically driven while the former is driven by exhaust energy. Another difference is that because it is mechanically driven, a centrifugal supercharger has a very slight amount of boost pressure at low rpm.

Of late the centrifugal supercharger has become more popular as it is smaller and easier to install than the positive displacement blowers.

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