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Lambda Control, Function - GF07.10-P-1010MRS

Engine 264.9 in Model 205, 253 

Function requirements, general 

Lambda control 

The mixture composition is maintained within the narrowest possible limits around λ = 1 in order to achieve a high conversion of the exhaust gases (exhaust gas conversion) in the catalytic converter.

The ME-SFI [ME] control unit (N3/10) reads in the following signals for this purpose:

The mixture composition is mainly changed by modifying the injection duration and the injection pressure.

The ME-SFI [ME] control unit distributes the required fuel quantity to each cylinder via the actuation of the following components of the fuel injection system:

Exhaust conversion in three-way catalytic converter (Engine with homogeneous injection operation) 

G14180756Courtesy of MERCEDES-BENZ USA

Closed-loop control circuit 

The sensor element "lambda sensor upstream of the catalytic converter" reacts to the oxygen content in the exhaust gas. The ME-SFI [ME] control unit directly reads in the signals of the sensor element "lambda sensor upstream of the catalytic converter".

The ME-SFI [ME] control unit reduces the injection quantity when the oxygen content in the exhaust is low (A < 1) and modifies the mixture composition in the direction of "lean". The oxygen content in the exhaust increases, and the lambda value is modified in the direction of λ = 1.

The ME-SFI [ME] control unit increases the injection quantity when the oxygen content in the exhaust is high (A > 1) and modifies the mixture composition in the direction of "rich". The oxygen content in the exhaust drops, and the lambda value is modified in the direction of λ = 1.

This process is repeated (control loop). To avoid Engine jolting, the ME-SFI [ME] control unit changes the mixture composition slowly.

IMPORTANT Using a diagnostic tester, the control status can be shown as a lambda control factor that changes in the positive direction in the case of leaning and in the negative direction in the case of enrichment.

Two-sensor control 

The two-sensor control monitors the oxygen sensor upstream of the catalytic converter function and the catalytic converter efficiency.

The ME-SFI [ME] control unit reads in the signals of the following sensors for this purpose:

The ME-SFI [ME] control unit uses the signals from the sensor element for the oxygen sensor downstream of the catalytic converter to determine the lambda mean value. This value is compared with a stored value. If the deviation is too large after several measurements, a correction value is calculated for the lambda control.

Using the correction value (value for new lambda sensor upstream of the catalytic converter = 0), the aging of the lambda sensor upstream of the catalytic converter is offset within certain limits. The correction value is achieved by the ME-SFI [ME] control unit by adapting the injection duration of the fuel injectors. If the correction value exceeds the prescribed limit value and if the following sources of error are excluded for the mixture formation then the oxygen sensor upstream of the catalytic converter must be replaced.

The following faults can, for example, occur during mixture formation:

If the specified limit value is exceeded or if the plausibility check of the oxygen sensor signals (upstream or downstream of catalytic converter) is negative, the instrument cluster (A1) issues a fault message. The ME-SFI [ME] control unit transmits the corresponding request via the drive CAN (CAN C1), powertrain control unit (N127), suspension FlexRay (Flex E), electronic ignition lock control unit (N73) and user interface CAN (CAN HMI) to the instrument cluster.

IMPORTANT Exceeding of the limit value is stored in the fault memory of the ME-SFI [ME] control unit and can be read out and deleted using the diagnostic tester.

Oxygen sensor heater 

The lambda sensors are heated in order to more quickly reach their operating temperature. The heating also prevents ceramic damage in the lambda sensors.

The ME-SFI [ME] control unit directly actuates the following lambda sensor components:

In the case of a very cold exhaust system (condensation present), the lambda sensor heaters are deactivated to prevent damage (thermal shock).

Additional function requirements self-adjustment of the mixture formation 

Self-Adaptation of mixture formation 

For regulated catalytic converters, the lambda control determines the injection period so exactly that a specified air/fuel ratio (A) is maintained under all operating conditions.

Self-adjustment ensures that the mixture composition in the open-loop operation (e.g. warm-up) is neither too rich nor too lean. This also prevents the lambda control from reaching the control limit at high altitudes.

If faults occur, the ME-SFI [ME] control unit corrects the mixture formation independently. In this case, the lambda characteristics map is shifted within the specified control limits so that the lambda control is not at the upper or lower control limit.

If the mixture composition constantly drifts from the center control range, the ME-SFI [ME] control unit shifts - under certain operating conditions - the lambda characteristics map until the lambda control factor is again approx. 0.

Shifting of the lambda characteristics map 

G14180757Courtesy of MERCEDES-BENZ USA

Display of self-adjustment values with diagnostic tester 

The following information can be read out with the diagnostic tester:

Representation takes place in the form of a factor, and means that the measured air mass value must be multiplied by the factor.

Example:

The ME-SFI [ME] control unit uses a calculated air mass value of 165 kg/h (150 kg/h 1.1) to determine the injection duration (fuel injection quantity).

The maximum correction values are -0.68 to 1.32.

Catalytic monitoring 

The law maker requires that the hydrocarbon (HC) emissions do not exceed a certain value. It is therefore necessary to constantly check the catalytic converters for aging. Aging of a catalytic converter is caused by the oxygen storage capacity reducing over time and the reduced HC conversion resulting from this. The catalytic converter stores oxygen during the leaning of the mixture (control loop) and releases it again during enrichment of the mixture for HC conversion.

To check the oxygen storage capacity, the fuel/air mixture is enriched (λ < 1) until the sensor element "lambda sensor downstream of the catalytic converter" has reached a certain maximum value and the majority of the oxygen has been depleted.

The fuel/air mixture is then leaned out (λ > 1) and the time it takes until the sensor element "lambda sensor downstream of the catalytic converter" reaches a certain minimum value and for the catalytic converter to fill up its oxygen storage is measured as part of this.

If the measured time lies below a stored characteristic, the oxygen storage capacity is insufficient and a fault is stored in the ME-SFI [ME] control unit.

  Electrical function schematic for lambda control   PE07.10-P-2710-97FBI 
  Overview of system components for gasoline injection and ignition system with direct injection   GF07.70-P-9998MRS 
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