Schematic diagram of overrun mode bypass air |
1 | Charge air cooler |
2 | Divert air switchover valve |
3 | Turbocharger |
4 | Crankcase |
5 | Charge air distributor |
A | Charge air (cooled) |
B | Exhaust |
C | Charge air (uncooled) |
D | Intake air |
P09.40-2671-75
P09.40-A057-79
Example illustration of engine 139 |
2 | Divert air switchover valve |
P09.40-A042-79
Example illustration of engine 256 |
2 | Divert air switchover valve |
Overview
This document contains information on:
General
Function requirements
Function
General
At a high compressor wheel rotational speed, the compressor and turbine wheels continue to turn due to inertia after the throttle valve is closed.
A high back pressure arises between the compressor wheel and the throttle valve. The back pressure causes a breakaway of flow at the compressor blades. A breakaway of flow results in compressor pumping. In the case of compressor pumping, there is a backlog of air between the throttle valve and the compressor wheel, and the compressor wheel blocks itself.
Compressor pumping is prevented by the opening of the overrun mode bypass air switchover valve due to the rapid pressure reduction in the intake section of the exhaust gas turbocharger via a bypass.
Function requirements
Engine management ON (circuit 87M)
Engine in operation
Function
The overrun mode bypass air switchover valve is integrated in the compressor wheel housing and opens the bypass on the compressor side through energization of the integrated solenoid. The excess boost pressure and corresponding air volume are relieved as a result and the exhaust gas turbocharger is prevented from reaching the surge limit. If the engine changes from overrun mode to load mode, the overrun mode bypass air switchover valve is no longer actuated and the bypass is closed by spring load of the overrun mode bypass air switchover valve.
The combustion engine control unit actuates the overrun mode bypass air switchover valve depending on the load.