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).