Simply explained: Recirculation valve | Register charging | Bi-turbo mode

The recirculation valve helps to comply with the limit values of the exhaust gas legislation. If required, it opens a connecting duct between two turbochargers. It is also referred to as a "module for charge air recirculation" or a "regulating throttle".
00:00 The recirculation valve
00:56 Register charging and mono-turbo mode
02:48 Bi-turbo mode
In the low engine speed range, the electrical compressor supports starting. When accelerating in the middle engine speed range, the recirculation valve opens a connecting channel between the two turbochargers and conveys charge air out of the passive turbocharger in front of the compressor impeller of the active turbocharger. At higher engine speeds, the recirculation valve closes again and the engine is operated in bi-turbo mode.
The full designation in Audi catalogues is:
"GX37 charge air recirculation module (recirculation valve)"

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Interessant mal den Aufbau zu sehen und erklärt zu bekommen wie die Bauteile zusammenarbeiten. Ich selber bin kein Fan von unnötig komplizierten Sachen. Irgendwelche Gründe wird es schon geben so ein System statt einfach einen VTG Lader zu verwenden.