Overrun Mode Bypass Air, Basic Function - GF09.40-P-1002A
Engine 139, 176, 177, 254, 256, 260, 264, 274, 279, 282
Sample diagram of overrun mode bypass air
Sample diagram of overrun mode bypass air, shown on engine 139
Sample diagram of overrun mode bypass air, shown on engine 256
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.
A turbocharger surge is prevented by the opening of the overrun mode bypass air switchover valve via the rapid pressure reduction in the intake side of the exhaust gas turbocharger (3) via a bypass.
Function requirements
- Engine management ON (circuit 87M)
- Engine in operation
Function
The overrun mode bypass air switchover valve (2) is integrated in the compressor wheel housing and opens the bypass from the compressor side via the energization of the integrated solenoid. The excess boost pressure and corresponding air volume are thus reduced and prevent the exhaust gas turbocharger (3) from reaching the surge limit. If the engine changes from overrun mode to load operation, the overrun mode bypass air switchover valve (2) is no longer actuated and the bypass is closed by the spring load of the overrun mode bypass air switchover valve (2).
The combustion engine control unit actuates the overrun mode bypass air switchover valve (2) in a load-dependent manner.