How does a turbocharger work?

The principle

Turbocharger. An engine is an air pump: the more air it can swallow, the more fuel it can burn and the more power it makes. A turbo uses the engine’s own exhaust to stuff more air in.

Step 1: Exhaust spins the turbine. The exhaust leaves the engine hot (up to about 950 °C in a petrol engine) and fast. It is led round the edge of a small turbine wheel and pushes on its blades, just like wind spinning a pinwheel, then leaves from the middle.

Step 2: A shaft turns the compressor. The turbine sits on one end of a short shaft, and a fan-like compressor wheel sits on the other end, so they always spin together, the same way. They spin very fast: about 100,000 to 200,000 rpm, so the shaft floats on a thin film of engine oil.

Step 3: More air, more power. The compressor sucks in fresh air in the middle and flings it out at the edge into the intake. Instead of the piston just sucking air in, about 1 bar of extra pressure (boost) pushes it in, about twice as much air as normal. With more air the engine can burn more fuel, and the same engine often makes about 40% more power. It’s like blowing into a campfire: more air, bigger fire.

Step 4: Intercooler and wastegate. Squeezing air heats it, to about 100 °C or more. Hot air is thin, so the intercooler, a small radiator, cools it down to about 40 °C before it goes into the engine, and even more air fits in. The wastegate is a little flap in the exhaust: when the boost is high enough, it opens and lets some exhaust go round the turbine, so the turbo doesn’t spin too fast.

Step 5: Turbo lag. When you press the pedal, more exhaust comes straight away, but the turbo wheels need a moment to spin up before the extra air arrives, often about a second. That short wait is turbo lag. Small turbos spin up faster; big ones give more boost but more lag.

Naturally aspirated or turbo?

• Naturally aspirated (no turbo): Upside: instant, smooth response, simpler, fewer hot parts to wear. Downside: to get more power you need a bigger, heavier engine that uses more fuel.
• Turbo: Upside: a small engine can make the power of a much bigger one, often with lower fuel use when driven gently. Downside: turbo lag, more heat and parts (oil lines, intercooler, wastegate) and more that can go wrong.

Try it yourself: hold a paper pinwheel above a hairdryer: the air stream spins it. A turbo does the same with exhaust, and uses that spin to blow a second fan.