Supercharger

A supercharger is a form of forced induction that increases an engine's power output by compressing the air entering the cylinders, so that each combustion event can burn more fuel and release more energy. What distinguishes it from the more common turbocharger is the source of its drive: a supercharger is mechanically coupled to the engine, usually by a belt from the crankshaft pulley, rather than being spun by exhaust gases. This direct link gives the supercharger its defining characteristic, the delivery of boost in immediate proportion to engine speed with effectively no lag.

Several mechanical designs exist. Positive-displacement types, including the Roots and the more efficient twin-screw, trap and push fixed volumes of air with each revolution and build strong boost from very low engine speeds, making them well suited to producing instant, muscular low-end torque. Centrifugal superchargers, by contrast, use an impeller much like a turbocharger's compressor wheel and tend to build boost more progressively as revs rise. In all cases the compressed, and therefore heated, air is typically routed through an intercooler before reaching the cylinders, since cooling it raises its density still further and reduces the risk of detonation.

For the driver, the great attraction is response. Because the supercharger is turning whenever the engine is, boost arrives the moment the throttle is opened, giving a linear, predictable surge of power without the momentary hesitation associated with turbo spool-up. This makes supercharged engines feel eager and tractable, behaving rather like a larger naturally aspirated unit.

The principal drawback is efficiency. Driving the compressor consumes a portion of the engine's own output, a parasitic loss that does not exist in a turbocharger, which recovers otherwise-wasted exhaust energy. A supercharger therefore tends to be thirstier than a turbo of equivalent boost, which is one reason the turbocharger has come to dominate mainstream engine downsizing where fuel economy is paramount.

This efficiency penalty has gradually confined the supercharger largely to performance applications, where its instant response and characteristic power delivery are valued above outright economy, and to specialist niches such as drag racing. Mercedes-Benz famously badged a generation of supercharged engines as Kompressor, the German word for compressor, and brands such as Jaguar and various American muscle cars have made notable use of the technology. Some manufacturers have even combined a supercharger for low-speed response with a turbocharger for top-end efficiency in a single engine, seeking the best of both methods. As a relative of the turbocharger, the supercharger remains the clearest illustration of how mechanical drive trades efficiency for the elimination of lag.