The turbine wheel is connected by a shaft to a compressor wheel and the two wheels turn together to suck in and compress large amounts of ambient air. This air is very dense and very hot, so it is passed through a charge-air cooler, where it cools and gains even higher density before entering the engine. The presence of this compressed air makes the fuel burn more efficiently, delivering greater power while consuming less energy.
As a result, more power can be generated from smaller displacement engines – and ultimately this means better fuel efficiency. Increasingly, turbos are coupled with high pressure fuel injection systems, which makes for even more thorough, efficient and cleaner combustion.
Although the underlying concept of turbocharging is simple, its application is extremely complex.
In a wastegate turbo, an actuator is used to open and close a by-pass valve to divert exhaust gas. This ability to restrict the amount of gas reaching the turbine makes it possible to regulate boost by controlling the rotating speed of the compressor.
In a Garrett® VNT™ turbo, a row of moveable vanes positioned around the inlet of the turbine wheel can be adjusted instantaneously to vary the exhaust gas flow through the turbine wheel. As a result, it is possible to regulate the flow of gas to make a VNT™ turbo act like a small turbo at low speeds – thereby supplying greater levels of engine boost – while at higher speeds, the turbo configures itself automatically to deliver the performance of a larger turbo.
In a Garrett® parallel sequential two-stage system, two small turbos work side-by-side – one turbo delivers boost at low rpm and both work together at high rpm.