Lambda Control, Function - GF07.10-P-1010MND
Engine 276.8 in model 213, 238
up to model year 2021
Function requirements for lambda control, general points
- Circuit 87M (Engine management ON)
- Engine runs
- Coolant temperature > -10°C
- Operating temperature reached in left and right oxygen sensors upstream of catalytic converter (G3/3, G3/4)
- Deceleration fuel shutoff not active
The "Engine running" function requirements include all function sequences of the fuel low pressure and fuel high-pressure circuit, as described in the "Fuel supply" function.
Lambda control, general points
The mixture composition is controlled within narrow limits of approximately λ = 1 in order to achieve high conversion of the exhaust gases (exhaust gas conversion) in the catalytic converters (three-way catalytic converters).
To do this the ME-SFI control unit (N3/10) reads in the following signals:
- Coolant temperature sensor (B11/4), coolant temperature
- Pressure sensor downstream of throttle valve (B28/7), engine load
- Left and right oxygen sensor elements upstream of catalytic converter (G3/3b1, G3/4b1), oxygen- and nitrogen contents
Exhaust gas conversion in three-way catalytic converter
Function sequence for lambda control
Lambda control is described in the following points:
- Function sequence for closed-loop control circuit
- Function sequence for two-sensor control
- Function sequence for oxygen sensor heater
- Function sequence for self-adjustment of the mixture formation
- Function sequence for catalytic converter monitoring
Function sequence for closed-loop control circuit
The left and right oxygen sensor elements upstream of catalytic converter react to the oxygen contents in the exhaust and send corresponding signals to the ME-SFI [ME] control unit.
The sensor elements send a "rich mixture" signal to the ME-SFI [ME] control unit when the oxygen content in the exhaust is low (λ < 1). This then reduces the injection quantity through regulation of the fuel injectors (Y76) 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.
The sensor elements send a "lean mixture" signal to the ME-SFI [ME] control unit when the oxygen content in the exhaust is high (λ > 1).
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.
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
- Operating temperature of catalytic converters reached
- Lambda control active
- Left and right oxygen sensors downstream of catalytic converter (G3/5, G3/6) are error-free
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:
- Left and right oxygen sensors upstream of catalytic converter, oxygen sensor signals
- Left and right oxygen sensors sensor elements downstream of catalytic converter (G3/5b1, G3/6b1), oxygen sensor signals
The ME-SFI [ME] control unit determines the Lambda mean value from signals left and right oxygen sensors 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 = 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:
- Unmetered air due to leaks in the exhaust system upstream of the oxygen sensors
- Wear or carbon deposits on the fuel injectors, Damaged pressure sensor downstream of the throttle valve
- Damaged fuel pressure and temperature sensor (B4/25)
- Damaged oxygen sensors
- Damaged purging switchover valve (Y58/1)
- Wear on the engine (e.g. valve leaky)
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, the ME-SFI [ME] control unit actuates the indicator lamps in the instrument cluster (A1) via the drive train CAN (CAN C1), powertrain control unit (N127), chassis 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 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
- Lambda control active
- Engine at idle or partial load operation
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.
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.
Shifting of the lambda characteristics map
Shown with self-adjustment value with Xentry diagnostics
The following can be read out with Xentry diagnostics:
- Shifting of the lambda characteristics map
- Direction of shift (rich or lean)
- Size of the shift
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 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 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
- Operating temperature of catalytic converters reached
- Lambda control active
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 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 (λ < 1) is output until the signals from the sensor elements of the left and right oxygen sensors downstream of catalytic converter have 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 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-2710-97DBF | ||
| Overview of system components for gasoline injection and ignition system with direct injection | GF07.70-P-9998MND |