Turbocharger

A turbocharger is a small turbine-and-compressor pair that recycles energy that would otherwise be wasted: hot exhaust gases spin a turbine wheel, which drives a compressor that crams more air into the engine, letting it burn more fuel and make significantly more power than its size suggests. Because it harnesses free exhaust energy, it improves efficiency too — which is why "downsizing" replaced large engines with small turbocharged ones to cut fuel use and CO2 while keeping performance. Its classic drawback is turbo lag, the brief delay before exhaust flow spins the turbo up, addressed by twin-scroll, variable-geometry and twin-turbo designs. An intercooler cools the compressed air for more density and less knock. Turbocharging is now near-universal on modern petrol and diesel engines.