Blow-off valve
The compressor bypass valve opens to prevent intense oscillations in the impeller when the throttle valve is suddenly closed (for instance, during gearchanges). The sets up a flow around the twin-scroll turbocharger. The compressor bypass valve prevents air from pulsating against the closed throttle valve.
The compressor bypass valve serves the following purposes:
- Improved engine acoustic properties
- Protection for the twin-scroll turbocharger.
Additional effect: The twin-scroll turbocharger responds rapidly to renewed opening of the throttle valve. If the compressor bypass valve were not present the twin scroll turbocharger would be forced to overcome the backpressure from the closed throttle valve, which would reduce its speed. The twin-scroll turbocharger would respond more slowly when the throttle valve reopens.
The following illustration shows how the compressor bypass valve operates.
| Item | Explanation |
|---|---|
| 1 | Compressor bypass valve closed |
| 2 | Compressor bypass valve open |
The blow-off valve is attached to the exhaust turbocharger.
In contrast to the N54 engine, the blow-off valve is not operated pneumatically. The compressor bypass valve on the N55 assumes the form of an electric actuator controlled directly by the DME digital engine electronics system. Placing the compressor bypass valve on the twin-scroll turbocharger has made it possible to substantially reduce the number of components.
The compressor bypass valve is either open or closed, with no intermediate positions.