Turbocharger — How It Works

A turbocharger is an exhaust-driven air compressor. Hot exhaust gas spins a turbine wheel at speeds up to 150,000 RPM or more. That turbine is connected by a shaft to a compressor wheel on the intake side. The compressor wheel forces more air into the engine than it could inhale naturally. More air means more fuel can be burned per combustion cycle, which means more power from a smaller engine. This is why modern four cylinder turbocharged engines produce the same power as the naturally aspirated six and eight cylinder engines they replaced — while using less fuel.

The turbine and compressor wheels are connected by a shaft that rides on either journal bearings lubricated by engine oil under pressure, or ball bearings in higher performance applications. Oil supply to the turbo bearings is critical — any interruption or contamination in the oil supply destroys the bearings in seconds at those rotational speeds. This is why proper oil change intervals and using the correct oil specification matter even more on turbocharged engines than naturally aspirated ones.

Boost pressure is the pressure above atmospheric that the turbo forces into the intake. Typical boost on a modern gasoline turbo engine ranges from 8 to 22 PSI depending on the application. The wastegate is a valve that bypasses exhaust around the turbine wheel to limit maximum boost pressure. On most modern vehicles the wastegate is electronically controlled by the PCM. A stuck-closed wastegate causes overboost — the engine sees more pressure than designed, which can cause detonation and engine damage. A stuck-open wastegate causes underboost — the turbo cannot build enough pressure and the engine feels sluggish.

Compressing air heats it. Hot air is less dense. Less dense air contains fewer oxygen molecules per cubic foot, which reduces the power benefit of turbocharging. An intercooler is a heat exchanger — either air-to-air or liquid-to-air — that cools the compressed intake air before it enters the engine. Cooler air is denser. Denser air makes more power. A damaged or clogged intercooler reduces power output and can cause detonation from elevated intake temperatures.