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Applications that require a good response at low engine speeds benefit the use of a small turbo. However, even though they are designed to spin at up to 250,000 rpm and withstand exhaust gas temperatures of over 1000oC, there is a danger that a small turbocharger can over speed and over boost at higher engine speeds.
In order to prevent this from happening, some turbochargers are fitted with a wastegate or turbine bypass. This manage that 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 and some of the exhaust gas flow straight into the exhaust system.
Swing Valve Type
The two sections of the chamber are separated by a diaphragm. As turbocharger 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, the opening pressure of actuator is higher then for petrol applications.
A vacuum activator works on a similar principle likewise a swing valve actuator. The basic difference is actuator guidance by engine central unit with assistance of electro-magnetic valves and vacuum.
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 diesel applications.
If the calibration is set too high the turbo may run too fast, speeds inside the compressor will be higher, causing damage on turbocharger, engine defective, and damage on engine’s air feed system too.