Lambda control, function - GF07.10-P-1010MI

ENGINE 272.9 (except 272.98) in MODEL 204.0 /2 as of model year 2009 /YoM 08 

ENGINE 272.9 (except 272.98) in MODEL 204.9, 207.3 /4, 212.0 /2 

ENGINE 273.9 in MODEL 207.3 /4, 212.0 /2 

Exhaust gas conversion in the three way catalytic converter 

Function requirements for lambda control, general points 

• Circuit 87M ON (engine control ON)
• Engine running
• Coolant temperature at start (dependent on the particular operating condition)
• Operating temperature reached in LH and RH 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 around 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 the following sensors:

• Hot film MAF sensor (B2/5), engine load
• Coolant temperature sensor (B11/4)
• LH and RH oxygen sensors upstream of catalytic converter

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 oxygen sensors upstream of the catalytic converter respond to the oxygen content in the exhaust, and send the 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 injection valves (Y62), so that λ=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 λ = 1.

The readout of the lambda control factor with the Xentry diagnostics alters in the direction +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
• 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 firewall catalytic converters.

To do this, the ME-SFI [ME] control unit reads the following sensors:

• LH and RH oxygen sensors upstream of catalytic converter, oxygen sensor signals
• LH and RH 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. 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), the aging of the oxygen sensors upstream of the catalytic converter is adjusted within certain limits.

If the correction value exceeds the specified limit value, the oxygen sensors upstream of the catalytic converter 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 injection 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).

The exceeded 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 diagnostic system.

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 
• Voltage-leap oxygen sensor 

Wideband oxygen sensor 

The left and right lambda 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 ()

Voltage-leap oxygen sensor 

Using a ground signal, the ME-SFI [ME] control unit actuates and synchronizes the sensor heaters for the left and right oxygen sensors downstream of the catalytic converter.

To do this, the ME-SFI [ME] control unit reads the signal from the coolant temperature sensor ().

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 on 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 fuel-air ratio (λ) 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 carbonization of the injection valves
• Faulty pressure sensor (B28) (intake manifold air pressure)
• Transition resistance in the hot film mass air flow sensor
• Defective purge control valve (Y58/1)
• Faulty fuel pressure sensor (B4/7)
• Wear to 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 

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.

Shown with self-adjustment value with the Xentry diagnostics 

The following can be read out with the Xentry diagnostics:

• 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

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 kg/h x 1.1).

The maximum correction values are -0.68 to +1.32.