Just like humans, engines breathe: The motion of the pistons inside the engine creates a vacuum that sucks in air, which is mixed with fuel and burned to produce power. The ratio of air to fuel is relatively constant, so the more air an engine can “inhale,” the more fuel it can burn, and the more power it can produce.
One way to increase an engine’s power is by forced induction — force-feeding an engine more air than it could breathe in on its own. Many cars use one of two methods of forced induction: turbocharging or supercharging.
A turbocharger is a type of air pump that’s driven by the engine’s exhaust. It consists of two turbines (fans not entirely unlike those used in jet engines) on a common shaft. One fan is plumbed into the exhaust, the other into the intake. When the engine develops power, the exhaust spins one turbine, which causes the other to turn as well. This other turbine, called the compressor, pumps air into the engine.
The advantage of a turbocharger is that it works on demand: When power demands are high, the turbo pumps more air into the engine, allowing it to burn more fuel and create more power. When power demands are low, the turbo doesn’t do very much, so the engine takes in less air and burns less fuel. A turbocharger allows a small engine to produce big-engine power when the driver demands it but still return small-engine fuel economy when demands are low. Turbocharged 4-cylinder engines have replaced V6s in many vehicles, and some vehicles use small turbo engines in place of larger, non-turbo 4-cylinders.
One problem with a turbocharger is turbo lag: It takes time for the exhaust pressure to spin up the turbo, so if the driver steps hard on the accelerator (especially from a stop), there’s a brief delay before the engine delivers full power.
Superchargers eliminate the problem of lag. A supercharger is a different type of pump that’s driven directly by the engine. Because it’s engine-driven, boost pressure is always available, and power is immediate. The chief disadvantage to superchargers is that they take a lot of engine power to drive (as opposed to turbochargers, which are powered by “free” power from the exhaust). Generally, supercharged engines don’t deliver the same fuel-economy benefits as turbocharged engines. Superchargers are popular in drag racing, and Audi uses superchargers on the V6 engines fitted to several of their performance models.