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

Engine 276.8 in model 172.4 

Function requirements for lambda control, general points 

Lambda control, general points 

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

To do this the ME-SFI control unit (N3/10) reads in the following signals:

Exhaust conversion in the CAT (three-way catalytic converter) 

G13845331Courtesy of MERCEDES-BENZ USA

Function sequence for lambda control 

Lambda control is described in the following points:

Function sequence for closed-loop control circuit 

The oxygen sensor upstream of catalytic converter reacts to the oxygen contents in the exhaust and transmits a corresponding signals to the ME-SFI [ME] control unit.

For a low oxygen content in the exhaust (<D <1) the left and right oxygen sensor sensor elements upstream of the catalytic converter transmit the signal "rich mixture" to the ME-SFI [ME] control unit. This then reduces the injection quantity through regulation of the fuel injectors for cylinders 1 to 6 (Y76/1 to 6) and alters the mixture composition in the direction "lean". The oxygen content in the exhaust increases and the value alters in the direction <D=1.

For a high oxygen content in the exhaust (<D > 1) the left and right oxygen sensor sensor elements upstream of the catalytic converter transmit the signal "lean mixture" to the ME-SFI [ME] control unit. This then increases the injection quantity through regulation of the fuel injectors for cylinders 1 to 6 and alters the mixture composition in the direction "rich". The oxygen content in the exhaust decreases and the value alters in the direction <D=1.

This process is repeated (control loop). The ME-SFI [ME] control unit alters the mixture composition with a time delay in order to prevent any risk of jerking.

IMPORTANT Indication of the regulation status takes place in Xentry Diagnostics based on a lambda regulating factor which alters in a positive direction for leaning of the mixture and in a negative direction for enriching of the mixture.

Additional function requirements for two-sensor control 

Function sequence for two-sensor control 

The two-sensor control monitors the function of the left and right oxygen sensors upstream of the catalytic converter and the effectiveness of the catalytic converters.

To do this the ME-SFI [ME] control unit reads in the following signals:

The ME-SFI [ME] control unit determines the Lambda mean value from signals LH and RH oxygen sensor sensor elements downstream of catalytic converter. This value is compared with a stored value for optimum exhaust emissions.

If the deviation is too large after a number of measurements, a correction value is determined for the lambda control.

Using the correction value (value for new left and right oxygen sensors upstream of catalytic converter is about 0), aging of the oxygen sensors upstream of the catalytic converter is adjusted for within certain limits.

The correction value depends on the performance map and the ME-SFI [ME] control unit applies it by adaptation of the injection period of the fuel injectors. If the correction value exceeds the prescribed limit value and if the following error causes are excluded for the mixture formation then the oxygen sensors upstream of CAT must be replaced.

The following errors 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 CAT) is negative, the ME-SFI [ME] control unit actuates the engine diagnosis indicator lamp (A1e58) in the IC (A1) via the chassis CAN 1 (CAN E1), front SAM control unit with fuse and relay module (N10/1) and chassis CAN 2 (CAN E2).

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

Function sequence for oxygen sensor heater 

The left and right oxygen sensor heaters upstream of the catalytic converter (G3/3r1, G3/4r1) and left and right oxygen sensor heaters downstream of the catalytic converter (G3/5r1, G3/6r1) heat the oxygen sensors up to operating temperature faster. Controlled heating also prevents damage to the oxygen sensor ceramics.

When the exhaust system is very cold (while condensation is present), the oxygen sensor heaters are switched off to prevent damage (due to thermal shock).

Additional function requirements self-adjustment of the mixture formation 

Function sequence for self-adjustment of the 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 phase) is neither too rich nor too lean. It also prevents the lambda control from coming to the end stop at high altitudes.

If a fault occurs, the ME-SFI [ME] control unit automatically makes a correction of the mixture formation. 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 end stop.

Shifting of the lambda characteristics map 

G13845332Courtesy of MERCEDES-BENZ USA

If the mixture composition is constantly drifting out of the central controlled range, the ME-SFI [ME] control unit shifts the lambda characteristics map under certain operating conditions until the lambda control factor is about 0.

Shown with self-adjustment value with Xentry diagnostics 

The following can be read out with Xentry diagnostics:

Presentation takes place in the form of a factor and means that the measured air mass value is multiplied by the factor.

Example:

Measured air mass: 150.0 kg/h

Displayed factor in Xentry diagnostics: 1.1

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

The maximum correction values are -0.68 to +1.32.

Additional function requirements for catalytic converter monitoring function sequence 

Function sequence for catalytic converter monitoring in general 

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 arises from the oxygen storage capacity reducing over time and the subsequent resultant reduced HC conversion.

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.

Function sequence for catalytic converter monitoring 

In order to check the oxygen storage capacity, a rich mixture (<D<1) is output until the LH and RH oxygen sensors sensor elements downstream of catalytic converter have reached a certain maximum value and the largest part of the oxygen is removed.

A lean mixture (<D>1) is then output and the time is measured to see how long it takes until the sensor elements of the left and right oxygen sensors downstream of the catalytic converter have reached a certain minimum value and the catalytic converter has filled up its oxygen storage capacity.

If the time measured lies below a stored characteristic then the oxygen storage capacity is inadequate and an error is stored in the ME-SFI [ME] control unit.

  Electrical function schematic for lambda control   PE07.10-P-2710-97TAH 
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