How a Turbocharger Works on an Engine

Garrett turbocharger. Photo © Garrett/Gale Banks Engineering

When you see an automobile advertised as being "turbocharged," everyone has the general sense that is somehow a more powerful engine that is capable of extra performance, but you may not know exactly how it accomplished this magic. 

How a Turbocharger Works

In a standard internal combustion engine, it is actually the flow of air that is most critical to the engine's performance. Normally, in a running engine it is the downward motion of the pistons that draws air into the engine cylinders. The air is mixed with​ fuel, and the combined vapor is ignited to create power. When you step on the accelerator, you are not really pumping liquid fuel into the engine, but rather drawing in more air, which in turn draws in vaporized fuel to create power. 

A turbocharger is an exhaust-driven mechanical device that boost engine power by pumping more air into the engine. 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 engine intake. The flow of exhaust spins the turbine, which causes the compressor to turn. The compressor serves to blow air into the engine at a greater rate than it can pull it in on its own. The greater volume of air can be mixed with a greater volume of fuel, which increases the power output.

Turbo lag

In order for the turbocharger to work properly, 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 gap in time while the engine reaches the necessary RPM 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 only on sports cars to give them an extra kick. But since the government mandated higher fuel economy standards, many automakers are turning to small turbocharged engines to replace larger, less fuel-efficient 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 smaller 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 vary according to your driving habits—if you have a heavy foot, 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 lacks power at higher RPMs; turbochargers give diesel engines a broad, flat power curve that makes them better suited to passenger 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 by the engine— usually by a belt, sometimes by gears. 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.