GF07.10-P-1010MMC Lambda Control, Function
GF07.10-P-1010MMC Lambda Control, Function
- ENGINES 157.9 in MODEL 216.3, 221.0 /1
- ENGINES 278.9 in MODEL 216.3, 221.0 /1
Exhaust gas conversion in the three way catalytic converter
A Exhaust gas emission
B Controlled range around lambda=1 (catalyst window)
CO Carbon monoxide
HC Hydrocarbon
NOX Nitrogen oxides
lambda Fuel air/fuel ratio
Function requirements for lambda control, general points
- Circuit 87M ON (engine control ON)
- Engine running
- Coolant temperature >-10°C
- Operating temperature reached in LH and RH O2 sensors upstream of CAT (G3/3, G3/4)
- Deceleration fuel shutoff not active.
Lambda control, general points
The mixture composition is controlled within the narrowest limits around lambda = 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 the following sensors:
- Coolant temperature sensor ()B11/4
- Pressure sensor downstream of throttle valve actuator (B28/7), engine load
- LH and RH O2 sensors upstream of CAT
Function sequence for lambda control
Lambda control is described in the following points:
- Function sequence for the lambda closed loop
- 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
The LH and O2 sensors upstream of CAT react to the oxygen content in the exhaust and send corresponding voltage signals to the ME-SFI [ME] control unit. The control unit then varies the mixture composition by adjusting the injection time of the fuel injectors (Y76), so that lambda=1 is reached. This process is repeated constantly (control loop).
The ME-SFI [ME] control unit alters the mixture composition with a time delay in order to prevent any risk of jerking.
Example:
Assuming a leaner fuel-air mixture is produced. As a result, the oxygen sensor voltage drops and the ME-SFI [ME] control unit compensates for the leaner mixture by enriching it accordingly (extending the injection time).
This makes it possible to achieve a fuel-air mixture of approximately lambda= 1.
The lambda control factor indicated with the Diagnosis Assistance System (DAS) changes towards +25%.
The more the lambda control factor moves in the direction +25%, the leaner is the fuel-air mixture and the greater is the enrichment of the mixture on the part of the ME-SFI [ME] control unit.
Additional function requirements for two-sensor control
- Operating temperature of firewall catalytic converters is reached
- Lambda regulation active
- The LH and RH O2 sensors downstream of CAT (G3/5, G3/6) are fault-free
Function sequence for two-sensor control
The two-sensor control monitors the function of the LH and RH O2 sensors upstream of the CAT and the effectiveness of the firewall catalytic converters.
To do this, the ME-SFI [ME] control unit reads the following sensors:
- LH and RH O2 sensors upstream of CAT, oxygen sensor signals
- LH and RH O2 sensors downstream of CAT, oxygen sensor signals
The ME-SFI [ME] control unit determines the lambda mean value from signals from the O2 sensors downstream of CAT. 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.
The correction value (value for new LH and RH O2 upstream of the CAT about 0) is used to compensate for aging of the oxygen sensors upstream of CAT, within certain limits.
If the correction value exceeds the specified limit value, the O2 sensors upstream of the CAT must be replaced. The correction value depends on the performance map and ME-SFI [ME] control unit applies it by adjusting the injection time of the fuel injectors.
If the specified limit value is exceeded or if the plausibility check of the O2 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 (CAN E) and the backbone CAN (CAN F).
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 DAS system.
Function sequence for oxygen sensor heater
The oxygen sensor heaters bring the O2 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
- Voltage-leap oxygen sensor
Wideband oxygen sensor
The LH and RH O2 sensors upstream of the CAT 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 ()
Voltage-leap oxygen sensor
Using a ground signal, the ME-SFI [ME] control unit actuates and pulses the sensor heaters for the LH and RH O2 sensors downstream of CAT.
The ME-SFI [ME] control unit reads in the signal from the coolant temperature sensor to do this.
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 regulation 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 time so exactly that a specified fuel-air ratio (lambda) is maintained under all operating conditions.
Self-adjustment ensures that the mixture composition in the control mode (e.g. warming-up phase) is neither too rich nor too lean. It also prevents the lambda control from coming to the end stop at high altitudes.
The following errors can occur during mixture formation:
- Unmetered air
- Wear or carbon deposits on the fuel injectors,
- Damaged pressure sensor downstream of throttle valve actuator
- Damaged fuel pressure and temperature sensor (B4/25)
- Defective purge control valve (Y58/1)
- Wear on the engine (e.g. valve leakage)
If a fault occurs, the ME-SFI [ME] control unit automatically makes a correction in the mixture formation. In this case, the lambda performance map is shifted within the specified limits so that the lambda control is not at the upper or lower end stop.
Shifting of the lambda performance map
a Controlled range of lambda control factor
b Control factor prior to self-adaptation (lean)
c Shifting of the lambda performance map
d Control factor following self-adaptation
e Position of lambda performance map prior to self-adjustment
If the mixture composition is constantly drifting out of the middle controlled range (e.g. 0±18%), the ME-SFI [ME] control unit in certain operating conditions shifts the lambda performance map until the lambda control factor is about 0%.
This shifting of the lambda performance map is the self-adjustment of the mixture formation process to the existing air/fuel mixture. Once this self-adaptation has taken place the lambda regulating factor will again find itself in the medium range.
Presentation of self-adjustment values using the DAS
The following can be read out using the DAS:
- Shifting of the lambda performance 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: 150.0 kg/hour
Indicated factor in the DAS: 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 kg/h X 1.1).
The maximum correction values are -0.68 to +1.32.
