A small compressor driven by the engine’s exhaust gases, also known as a turbo,
working on the principle of the waterwheel.
It is used to force more air into the engine and increase power, much like a
supercharger (which is driven by a belt off the engine).
Turbos were once only for high-performance, but are now often used to improve
fuel consumption (by enabling the use of a smaller engine) and to provide
acceptable performance from diesel engines.
Many cars have two turbochargers – a small one that spins up to speed quickly
and therefore aids low-speed response, and a large one that generates top-end
power but spools-up slowly (called ‘lag’) and is therefore not available the
moment the driver hits the throttle.
Turbos can also be used in tandem so the small one simply gets the larger one
spinning more quickly. Many ‘twin-turbo’ engines simply have units each
operating half the cylinders of the engine (typically in a V6 or V8).
Another variation is the VW Group’s award-winning “TwinCharge” engine, which
uses a small supercharger to spool-up a larger turbocharger.