What is Wastegating?
A wastegate is simply a valve that allows some of the exhaust gas to pass directly from the exhaust manifold to the exhaust pipe. It is a simple but effective way to control the turbocharger speed and boost pressure.

Why Wastegate?
Wastegate turbochargers are matched to give good performance at low engine speed with the valve closed. This improves transient response and reduces exhaust temperatures and emissions. As engine speed increases, the wastegate valve begins to open at a pre-set boost pressure. This has the effect of increasing the swallowing capacity of the turbine, reducing shaft power and avoiding excess air delivery and rotor overspeed.

Recently, an alternative method has been developed by Cummins Turbo Technologies: The Holset Command Valve. This technology allows the Engine Control Unit (ECU) to control the wastegate using boost air from the compressor. This generates higher boost pressure in the middle of the speed range without exceeding the recommended speed limit.

As well as improving boost at low engine speeds, wastegates are used for other reasons. Gas fueled engines use them to precisely control air flow into the engine and two-stage systems can use wastegates to change the work split between the two turbines.

The Future
As emissions aftertreatment systems become more widespread, being able to vary the turbine characteristics of a turbocharger is necessary. Exhaust systems have variable back pressures during their life, which affect boost pressures.

While technology such as Holset VGT^TM offer more sophisticated ways of varying turbine geometry and controlling boost pressures, wastegates are still valuable for current and future emissions legislation, due to their simple and cost effective approach.