If you enlarge the above picture, it should be easier to recognize that it is the fresh-air side of a turbo-charged system. Whether a Diesel- or a petrol engine, the valve shown here is only needed by a turbo-charger. One could also call it a pressure-relief valve. However, as a pressure-relief valve, no fixed value can be assigned to it.
In the sectional view, a valve spring can be seen in the valve, this is lifted through vacuum, in a separate area, against a spring. Activated in this way, the valve releases the pressure outwards between the compressor-turbine and the intake valves.
function is used to maintain the RPM of the turbine when the driver takes his/her foot off the gas pedal. In this case, either a flap in the air-intake would be closed or the inlet valve-lift would be limited, which would cause a congestion and would slow down the turbine.
In a number of vehicles one can also hear the switch-over when the gas pedal is abruptly released. Apart from this, it must be ensured, that the valve properly seals to the outside, because pressure loss has a direct negative influence on the performance and the efficiency. 10/12