lemon-mirror:
Home >> Mercedes Benz >> 2021 >> GLC300 Base >> Repair and Diagnosis >> Engine Performance >> Engine Control Systems >> Mixture Formation - 253 Chassis 1 Of 3 >> Basic Knowledge >> Lambda Control, Function >> Lambda Control, Function - GF07.10-P-1010MRH

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

Engine 274.9 in model 205.047/053/054/147/247/253 

Engine 274.9 in model 253.353/953 

Engine 274.9 in model 253.354/954 

Function requirements for lambda control, general points 

Lambda control, general points 

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.

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

Exhaust conversion in three-way catalytic converter (for Engine with homogeneous mode) 

G12816683Courtesy 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 sensor element for the oxygen sensor upstream of the catalytic converter responds to the oxygen content in the exhaust and sends corresponding signals to the ME-SFI [ME] control unit.

For a low oxygen portion (λ < 1) in the exhaust the oxygen sensor sensor element upstream of CAT gives out the signal "rich mixture" to the ME-SFI [ME] control unit. This then reduces the injection quantity through regulation of the fuel injectors (Y76/1 to Y76/4) and alters the mixture composition in the direction "lean". The oxygen content in the exhaust increases and the value alters in the direction λ=1.

For a high level of oxygen portion (λ > 1) in the exhaust the oxygen sensor sensor element upstream of CAT gives out the signal "lean mixture" to the ME-SFI [ME] control unit. This then increases the injection quantity through regulation of the fuel injectors and alters the mixture composition in the direction "rich". The oxygen content in the exhaust decreases and the value alters in the direction λ=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 The regulation status is displayed in Xentry Diagnostics based on a lambda control factor which changes 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 oxygen sensor upstream of the catalytic converter function and the catalytic converter efficiency.

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

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 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 a new oxygen sensor upstream of catalytic converter is about 0), the aging of the oxygen sensors upstream of the catalytic converter is adjusted within certain limits.

It depends on the characteristics map and the ME-SFI [ME] control unit applies it through adaptation of the injection period 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 errors can, for example, occur during mixture formation:

If the specified limit value is exceeded or if the plausibility check for the oxygen sensor signals (upstream or downstream of the catalytic converter) is negative, then the ME-SFI [ME] control unit actuates the indicator symbols in the instrument cluster (A1) over the drive train CAN (CAN C1), powertrain control unit (N127), suspension FlexRay (Flex E), electronic ignition lock control unit (N73) and the user interface CAN (CAN HMI).

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 oxygen sensor heaters downstream and upstream of the catalytic converter (G3/1r1, G3/2r1) heat the oxygen sensors up to operating temperature faster. With a controlled heater, they also prevent damage to the oxygen sensor ceramics.

When the exhaust system is very cold (while condensation is present), the oxygen sensors heaters downstream and upstream of the catalytic converter 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 (λ) 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 

G12816684Courtesy 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

For determining the injection period (fuel injection quantity) the ME-SFI [ME] control unit uses a calculated value of 165 kg/h (150 kg/h X 1.1).

The maximum correction values are -0.68 to +1.32.

Additional function requirements for catalytic converter monitoring 

Catalytic converter monitoring, general points 

The law maker requires that the hydrocarbon (HC) emissions do not exceed a certain value. It is therefore necessary to constantly check the catalytic converter 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 (λ<1) is output until the oxygen sensor downstream of the catalytic converter has reached a certain maximum value and the largest part of the oxygen is removed.

A lean mixture (λ>1) is then output and the time is measured to see how it takes until the oxygen sensor downstream of the catalytic converter has 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-2710-97FBH
  Overview of system components for gasoline injection and ignition system with direct injection   GF07.70-P-9998MRH 
· About lemon-mirror