The twin-screw type supercharger is a positive displacement type device that operates by pulling air through a pair of meshing lobes not dissimilar to a set of stretched gears. Air is trapped in pockets between the lobes and carried between the intake side to the exhaust. The supercharger is typically driven directly from the engine's crankshaft via a belt. Unlike the Roots type supercharger, however, the twin-screw exhibits internal compression. As the air passes through the supercharger the air pocket decreases in size towards the outlet. This increases the thermal efficiency of the supercharger beyond that of the more available Roots style supercharger. In order for the supercharger to deliver air at greater pressure than atmospheric, the Roots supercharger must be geared so that it turns faster than the engine. Although the twin-screw type supercharger has more potential than the Roots type supercharger, its production costs due to close tolerances put it at a cost disadvantage. Also due to the internal compression the air exiting the supercharger exhaust "pops", this can lead to a whine noise or whistle (or scream!) that has to be tolerated or dealt with via noise suppression techniques. Although not as ultimately efficient as the centrifugal type supercharger, it does not suffer from the latter's lack of low-rpm boost.
All supercharger types benefit from the use of an intercooler to reduce heat produced during compression.
