Lambda Control, Function - GF07.10-P-1010KJ
ENGINE 281.910 in MODEL 453.0/3/4
Function requirements for lambda control, general points
- Circuit 87 (Engine management ON)
- Engine runs
- Coolant temperature (dependent on each operating conditions)
- Operating temperature of oxygen sensor upstream of catalytic converter (G3/2) reached
- Deceleration fuel shutoff not active
Lambda control, general points
In order to achieve a high exhaust conversion (exhaust conversion) in catalytic converter (three-way catalytic converter, the mixture composition is regulated within tight limits around λ = 1.
To do this the ME-SFI control unit (N3/10) reads in the following signals:
- Coolant temperature sensor (B11/4)
- Pressure sensor downstream of throttle valve (B28/7), engine load
- Oxygen sensor downstream of catalytic converter (G3/1)
- Oxygen sensor upstream of catalytic converter (G3/2)
Exhaust conversion in the CAT
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 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 λ < 1 (low level of oxygen in exhaust) the oxygen sensor upstream of the catalytic converter sends a "rich mixture" signal to the ME-SFI [ME] control unit. This then reduces the injection quantity through regulation of the cylinder 1 injection valve (Y62/1) to cylinder 3 injection valve (Y62/3) 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 (high level of oxygen in exhaust) the sensor element for the oxygen sensor upstream of the catalytic converter sends a "lean mixture" signal to the ME-SFI [ME] control unit. This then enlarges the injection quantity through regulation of the cylinder 1 injection valve to cylinder 3 injection valve 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.
Additional function requirements for two-sensor control
- Catalytic converter operating temperature reached
- Lambda control active
- Oxygen sensor downstream of catalytic converter fault-free
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:
- Oxygen sensor upstream of catalytic converter
- Oxygen sensor downstream of catalytic converter
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 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.
The correction value depends on the characteristics map and the ME-SFI [ME] control unit applies it by adaptation of the injection period of the cylinder 1 injection valve to cylinder 3 injection valve. 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:
- Unmetered air due to leaks in the exhaust system upstream of the oxygen sensor
- Wear or carbonization of the cylinder 1 injection valve to cylinder 3 injection valve
- Damaged pressure sensor downstream of throttle valve (B28/7)
- Damaged oxygen sensors
- Wear on the engine (e.g. valve leaky)
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 interior CAN (CAN).
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 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
- Lambda control active
- Engine at idle or on partial load
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
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.
Additional function requirements for catalytic converter monitoring
- Catalytic converter operating temperature 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 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.
Catalytic monitoring
Monitoring of the catalytic converter by the oxygen sensor downstream of catalytic converter functions after 90 seconds for a warm engine in partial and wide open throttle operation.
| Electrical function schematic for lambda control | PE07.10-P-2710-97BBA | ||
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