Fuel Delivery and Air Induction: Adjustments

GF07.10-P-1029V Self-Adjustment Of Mixture Formation, Function







On vehicles fitted with a closed-loop controlled catalytic converter, the lambda control determines the injection time so exactly that a certain fuel-air ratio (Lambda) is maintained in all operating states.

If faults occur, the ME-SFI [ME] control unit automatically makes a correction in the mixture formation. Here the lambda performance map is shifted within certain limits in such a way that the lambda control is not at the upper or lower regulation limit (d).

The following faults require a mixture adjustment:
- Wear or carbon deposits on fuel injection valves
- Fault on mass air flow sensor
- Incorrect injection pressure (e.g. through blocked fuel filter)
- Defective purge control valve
- Engine wear (e.g. leaky valves)

If the mixture composition constantly drifts out of the middle control range (e.g. 0 ± 10%) the ME-SFI [ME] control unit under certain operating conditions shifts the lambda map sufficiently far until a lambda control factor of approx. 0% (g) is again achieved.

Self-adjustment ensures that the mixture composition in the control mode (e.g. warming-up phase) is neither too rich nor too lean. This also prevents the lambda control from moving up to the control limit at high altitudes.

Example:
As the result of a lean fuel/air mixture the readout of the lambda control factor changes to 18% (e). Under certain operating conditions the lambda performance map is shifted (arrow) by the ME-SFI [ME] control unit.

This shifting of the lambda performance map is the self-adjustment of the mixture formation process to the existing air/fuel mixture. After this self-adjustment the lambda control factor is again in the central control range (g).

This self-adjustment of the mixture formation can be carried out in the following operating states:
- Idle speed
- Lower part load
- Upper partial load.

The following actual values can be read out using STAR DIAGNOSIS
- Lambda map shift active
- Direction of the shift (tendency towards rich or lean)
- Extent of the shift

Representation of self-adjustment values with STAR DIAGNOSIS in idle
The representation in milliseconds means that the value displayed is added to, or subtracted from, the air mass inducted by the engine in order to determine the injection time.
The maximum correction value is ± 1 ms.

Example:
Calculated injection time (performance map): 3.0 ms
Displayed correction value: +0,3 ms
For determining the injection time (fuel injection quantity) the ME-SFI [ME] control unit uses a calculated value of 3.3 ms
(3.0 ms+0.3 ms).

Representation of self-adjustment values with STAR DIAGNOSIS for partial load
The representation of the factor means that the indicated value for determining the injection time is multiplied by the air mass inducted by the engine.

The correction factor lies between 0.68 and 1.32.

Example:
Calculated air mass: 150 kg/hour
Displayed correction value: 1,10

A calculated air mass value of the of 165 kg/h is used by the engine control unit to determine the injection time (fuel injection quantity).