Design And Function: Check
To check for any wear on the clutch, there is an inspection hole in the bottom of the compressor housing. It is possible to check the clearance between the friction linings and clutch surface using a feeler gauge through the hole. If the check shows that there is no or not enough play, this means that the compressor is likely to engage continually, which can damage the compressor. In the event of a defective clutch, the compressor is replaced as a complete replacement part.
Bypass
When the compressor has reached its maximum speed (at 3500 engine revs), it shall be disengaged at the same time as the turbocharger takes over the task of boosting the engine. At the handover, the compressor's maximum boost pressure is approx. 0.5 bar (relative pressure). While the compressor is working, the turbo comes up to speed and, when engagement is transferred, provides approximately the same boost that came from the compressor. While the magnetic clutch mechanically disconnects from the compressor, the bypass damper is operated to open. Fresh air is then controlled directly to the turbocharger past the compressor.
Control
The ECM controls the by-pass valve's (throttle's) position via a brushless electric motor with integrated position sensor (Hall sensor). The throttle position is controlled via a 12V PWM signal. Depending on whether the throttle is to open or close, the ECM switches the polarity using an H-bridge.
In no power state, a spring keeps the throttle open. The ECM controls the throttle position continuously based on the calculation models where the air pressure upstream of the turbo must have a desired set point value.
When the throttle is completely closed, the air is only controlled to the compressor and when the throttle is fully open the air is controlled to the turbo without passing the compressor. When the throttle is partially open, the air is distributed to both compressor and turbo, so that the desired pressure upstream of the turbo is achieved. Note that in certain driving conditions, the air goes the back way back to the compressor.