A turbocharger is the actual part: a pair of small fan-wheels on one shaft, sitting beside the engine. Exhaust gas leaving the engine blows through one wheel and spins it very fast; the other wheel, spinning with it, sucks in fresh air and pushes it into the cylinders under pressure. Squeezing in more air lets the engine burn more fuel each time, so you get more power without a bigger engine. It is clever because it runs on waste — the hot gas was going out of the pipe anyway. Mechanics and car people almost always shorten it to 'turbo'. The full word appears in writing, manuals, and adverts. Two things drivers mention: 'turbo lag', the short delay before the wheel spins up fast enough to help, so the car feels sleepy for a moment and then surges; and the whistling or hissing sound a turbo makes, which enthusiasts love. Turbochargers are also standard on diesel trucks and were first used on aircraft engines flying high, where the air is thin.