What is a turbocharger and what does it do?

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Garrett turbocharger. Photo © Garrett/Gale Banks Engineering

What is a turbocharger?

A turbocharger is an exhaust-driven device that boosts an engine's power output.

Normally, the downward motion of the pistons pulls air into the engine; this air is mixed with fuel, which is ignited to make power. Stepping on the accelerator increases the amount of air that can be drawn in. (So you're not really stepping on the gas, you're stepping on the air!)

A turbocharger uses a pair of fan-like castings mounted on a common shaft.

One (called the turbine) is piped to the exhaust, while the other (the compressor) is piped to the intake. The flow of exhaust spins the turbine, which causes the compressor to turn. The compressor blows air into the engine at a greater rate than it can pull in. The greater volume of air can be mixed with a greater volume of fuel, increasing power output.

Turbo lag

In order for the turbocharger to work, there needs to be enough exhaust pressure to spin ("spool up") the turbines. This may not happen until the speed of the engine reaches 2000-3000 revolutions per minute (RPM). This is called turbo lag. Once the turbo spools up, look out -- the result is usually a strong surge of power, sometimes accompanied by a jet-engine-like whistle.

Which cars use turbochargers?

In the past, turbochargers were used to give sports cars an extra kick. Since the government mandated higher fuel economy standards, many automakers are turning to small turbocharged engines to replace larger engines.

A turbocharger allows a small engine to produce big-engine power on demand, but when demands are low (such as cruising down the highway) the small engine uses less fuel. Traditionally, turbocharged engines require high-octane fuel, so many of these fuel-saving turbo engines use direct fuel injection, which allows the use of cheap 87-octane gas.

Keep in mind that your mileage will (literally) vary -- if you have a heavy food, a small turbocharged engine will consume as much fuel as a big engine.

Most diesel engines use turbochargers. Diesel is strong on low-RPM power but lack urge at higher RPMs; turbochargers give them a broad, flat power curve that makes them better suited to cars. Unlike gasoline engines, diesel is generally more fuel-efficient when fitted with a turbocharger.

Turbochargers vs. superchargers

A similar type of device is called a supercharger. Instead of using an exhaust-driven turbine, the supercharger is mechanically driven (usually by a belt, sometimes by gears) by the engine. Superchargers have the advantage of eliminating turbo lag, but they require a good deal of power to turn, so they don't always produce the same net power gains as a turbocharger. Superchargers are often used in drag racers, which need to produce lots of low-end power. Swedish automaker Volvo combines supercharging and turbocharging in their Drive-E engine.