To realize an active silencer system, the exhaust system was given a controlled electro-pneumatic exhaust flap at the tail pipe of the main silencer. The exhaust flap is open (spring force) in idle setting, the vacuum unit is aerated.
The DME control module actuates an electronic switch-over valve which supplies vacuum to a vacuum unit. If the DME control module requires that the exhaust flap is closed, the control module outputs ”ground” and the electric changeover valve is activated. Vacuum from the vacuum accumulator is applied to the vacuum unit and the exhaust flap is closed.
A minimum vacuum of approx. 200 mbar is required to close the exhaust flap. The angle of flap movement is approx. 70°. Electrical activation takes place by means of the DME engine control module after evaluating various input variables:
Road speed signal > 3 km/h
Engine speed signal
Engine load signal
The vacuum reservoir is necessary as the M52 B28, M54 B30 and M56 B25 engines only have a very low vacuum in the intake pipe at full load. The volume of the vacuum reservoir is set in such a way that 5 switching operations of the exhaust flap can be carried out in succession with the throttle valve in full load position. This is sufficient for normal engine operation as the vacuum level is built up in the vacuum reservoir in each temporary partial load setting of the throttle valve.