Metro/Handout An Audi turbocharger.

Your engine runs on gasoline, but before the fuel can be used, it must be mixed with air. That air is compressed by the pistons to produce power, but they can only do so much.

Automakers often add turbochargers or superchargers, which can coax more power out of a smaller engine without having to move up to a larger one.

“The strategy of downsizing and replacing engine displacement by charging led to large increases in torque, weight savings and fuel efficiency,” says Cort Nielsen, public relations manager for Audi Canada.

Although turbochargers and superchargers both do the same thing — pump more air into the engine — they differ in the way they run.

On a turbocharger, the engine’s exhaust is routed through a turbine to make it spin. A supercharger is connected by a belt (or sometimes by gears) to the engine, which powers the unit.

Originally, these systems were all about power, and automakers used them to make their performance models even sportier.

While that’s still the case with some vehicles, turbochargers are increasingly used for fuel efficiency. Although they do use more fuel when they’re working — when they push in more air, the engine must also burn more gasoline — they only provide a boost when extra power is needed, such as during hard acceleration.

In the past, automakers might simply have used a larger engine, which could provide that power when needed, but which would use more fuel the rest of the time. A smaller turbocharged engine handles everyday driving chores while using much less fuel, but can still provide that power boost when the driver demands it.

Because the turbocharger’s turbine has to “spool up,” or  start spinning faster  when the driver accelerates, there can be a slight delay in power.

Known as turbo lag, it was very noticeable in older engines, but newer units provide their power almost instantaneously.

“With modern electronics, the former deficiencies are no longer present,” Nielsen says.

“The use of new combustion methods is another key topic in advanced development of engine charging technology, especially in efforts to increase charge pressure.”

Whether an engine uses a turbocharger or supercharger depends on several factors, including where it will fit into the engine bay, and how easily its heat can be dissipated.

Superchargers also tend to be more expensive and are more likely to be used on higher-priced vehicles.

Did you know?

•    Fact. Turbochargers and superchargers are known as forced induction systems because they force air into the engine. An engine without such a system draws in non-compressed air and is referred to as “naturally aspirated.”

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