Why do some turbos have wastegates?

Applications that require a good response at low engine speeds benefit from the use of a small turbo. However, even though they may be designed to spin at up to 250,000 rpm and withstand exhaust gas temperatures of over 1000°C, there is a danger that a small turbine can overspeed and overboost at higher engine speeds.

In order to prevent this from happening, some turbochargers are fitted with a wastegate or turbine bypass: as the pressure reaches the maximum preset level, a valve opens to allow some of the exhaust gas to bypass the turbine and flow straight into the exhaust system.

The simplest form of wastegate control is a pneumatic actuator. The sensor port on the actuator is connected directly to the compressor outlet and, as pressure rises in the top part of the actuator above the diaphragm, it acts against the pressure of a spring to move a rod, thereby opening the turbine bypass valve (wastegate).

The following illustrations show the means of operation for both a swing valve and vacuum actuator:

Swing Valve Type

The two sections of the chamber are separated by a diaphragm. As turbo speed and boost rise, the pressure rises in the chamber until it overcomes the pressure from the spring on the underside of the piston. For diesel applications, springs must be quite heavy, to balance against the high compressor outlet pressure.

Vacuum Type

A vacuum activator works on a similar principle. As the chamber is evacuated, atmospheric pressure acts on the underside of the diaphragm/piston and overcomes the spring pressure. Lighter springs can be used.

The calibration of the opening point has to be set extremely accurately to ensure that the engine performs to specification. If the calibration is set too low and the turbo does not reach its maximum boost pressure, the engine performance will be low and may cause smoking and excess temperatures on diesels.

If the calibration is set too high the turbo may run too fast, overboosting the engine and causing damage.