Lambda control, function - GF07.10-P-1010MMA
ENGINE 157.9 in MODEL 212, 218 up to model year 2014
ENGINE 278.9 in MODEL 207, 212, 218 up to model year 2014
Function requirements for lambda control, general points
- Circuit 87M (engine timing ON)
- Engine running
- 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
Lambda control, general points
The mixture composition is controlled within the narrowest limits of approx. λ = 1 in order to achieve high conversion of the exhaust gases (exhaust gas conversion) in the catalytic converters.
For this purpose, the ME-SFI [ME] control unit (N3/10) reads in the following sensors:
- Coolant temperature sensor (B11/4) (model 207, 212), coolant temperature sensor (B11/4) (model 218)
- Pressure sensor downstream of throttle valve (B28/7), engine load
- Left and right oxygen sensors upstream of catalytic converter
Exhaust gas conversion in three-way catalytic converter
Function sequence for lambda control
Lambda control is described in the following points:
- Function sequence for the lambda 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 the lambda closed-loop control circuit
The oxygen sensors upstream of the catalytic converter respond to the oxygen content in the exhaust and send corresponding signals to the ME-SFI [ME] control unit.
For λ<1 and a low oxygen content in the exhaust the oxygen sensors transmit the signal "rich mixture" to the ME-SFI [ME] control unit. 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.
For λ<1 and a high oxygen content in the exhaust the oxygen sensors transmit 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.
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
- 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 control unit reads the following sensors:
- Left and right oxygen sensors upstream of catalytic converter, oxygen sensor signals
- Left and right oxygen sensors downstream of catalytic converter, oxygen sensor signals
The ME-SFI [ME] control unit determines the lambda mean value using the oxygen sensor signals downstream of the 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. It depends on the characteristics map and the ME-SFI [ME] control unit applies it through adaptation of the injection time 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 throttle valve
- Defective fuel pressure and temperature sensor (B4/25)
- Defective oxygen sensors
- Defective purging switchover valve (Y58/1)
- Wear on engine (e.g. leaky valves)
If the specified limit value is exceeded or if the plausibility check on the oxygen sensor signals (upstream or downstream of the catalytic converter) is negative, the ME-SFI [ME] control unit actuates the engine diagnosis indicator lamp (A1e58) in the instrument cluster (A1) via the chassis CAN (CAN E). 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 bring the oxygen sensors up to operating temperature more quickly. With controlled heating, they also prevent damage to the oxygen sensor ceramics.
The oxygen sensor heaters differ according to lambda sensor type:
- Wideband oxygen sensor
- Narrow band oxygen sensor
Wideband oxygen sensor
The left and right oxygen sensors upstream of the catalytic converter are heated continuously when the engine is running, in order to keep them functioning.
Temperature control and temperature measurement (by measuring the internal resistance) is performed by special control electronics in the ME-SFI [ME] control unit.
Narrow band oxygen sensor
Using a ground signal, the ME-SFI [ME] control unit actuates and synchronizes the oxygen sensor heaters for the left and right oxygen sensors downstream of the catalytic converter.
To do this ME-SFI [ME] control unit reads in the signal from coolant temperature sensor (model 207, 212), coolant temperature sensor (model 218).
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 in partial load
Function sequence for self-adjustment of the mixture formation
For regulated catalytic converters, the lambda control determines the injection time 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) 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 middle 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 lambda characteristics map
- Shift direction (rich or lean)
- Size of 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: 150.0 kg/h
Displayed factor in Xentry diagnostics: 1.1
To determine the injection duration (fuel injection quantity), the ME-SFI [ME] control unit uses a mathematical air mass value of 165 kg/h (150 X kg/h1.1).
The maximum correction values are -0.68 to +1.32.
| Electrical function schematic for lambda control | MODEL 207 | PE07.10-P-2710-97EAI | |
| MODEL 212 | PE07.10-P-2710-97DAK | ||
| Model 218 | PE07.10-P-2710-97XAC | ||
| Overview of system components for gasoline injection and ignition system with direct injection | ENGINE 157.9 in MODEL 212, 218 up to model year 2014 ENGINE 278.9 in MODEL 207, 212, 218 up to model year 2014 |
GF07.70-P-9998MM |