It is two wheels on one shaft. Exhaust gas on its way out of the engine still carries energy, and a turbocharger puts a turbine in its path to catch some of it. That turbine drives a compressor at the other end of the shaft, and the compressor pushes fresh air into the cylinders at above atmospheric pressure. The US Department of Energy's fuel economy service describes the result directly: "Turbochargers are compressors that force additional air into the engine's cylinders, allowing more fuel to be injected as well."
More air plus more fuel is more power from an engine that hasn't got any physically bigger, which is the entire appeal. It's why displacement stopped being a reliable guide to output, and why small turbocharged engines replaced large ones across most of the market. It also lifts torque low down, which is the part a driver actually notices.
The trade is response. The turbine has to be spun up by exhaust flow before boost arrives, so there's a moment between the throttle opening and the extra air showing up. A naturally aspirated engine has nothing to spin up and so has none of that lag, which is a large part of why enthusiasts still argue for them.
Written by Craig Fearn, Petrol & Ink. Last updated .