Lambda Control Diagnosis; Overrun Fuel Cut-Off (Sas); Switch-On Conditions For Gas Injection - Mercedes-Benz M 447 hLAG Service Manual

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2.5.4

Lambda control diagnosis

When the entire natural-gas engine system is operating properly, the lambda correction factor should be
approx. ± 5 % in stationary operating points. A clear deviation from these values can have serious
consequences for the engine and catalyst, and therefore this value is monitored by the EGM. If the value
of the lambda correction factor is above 0.20 or below -0.25 for more than 20 seconds, then the result
"Lambda controller: control deviation too great" is detected and the fault code 0775 is set. As a reaction
to this fault, the Engine protection: lambda lean-controller probe becomes active and the engine is only
operated further controlled (lambda control OFF). A substitute value of 1.2 is selected for the target
lambda.
2.5.5

Overrun fuel cut-off (SAS)

The overrun fuel cut-off (SAS) of the M 447 hLAG is realised in the EGM. The overrun fuel cut-off is a
function of the target engine torque (FR), engine speed and vehicle speed.
The overrun fuel cut-off process is divided into the sub-processes SAS_soft (soft overrun fuel cut-off) and
SAS_hard (hard overrun fuel cut-off). While the overrun fuel cut-off is active, the throttle valve is moved
into a throttle valve position dependent on the engine speed and gas injection is prevented.
2.5.6

Switch-on conditions for gas injection

So that the previously calculated gas injection mass may also actually be injected by the control unit,
several conditions must be met beforehand. In addition, the gas cut-off valve must be actuated to release
gas for gas injection (open gas cut-off valve).
Status September 2003 (EvoBus-Service / AFT)
Functional Description of EGM
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