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

ENGINE 272.9 (except 272.982) in MODEL 204.0/2 as of model year 2009

ENGINE 272.9 (except 272.983) in MODEL 212

ENGINE 272.9 (except 272.984) in MODEL 207

ENGINE 272.9 in MODEL 204.9

ENGINE 273.9 in MODEL 207, 212

Exhaust gas conversion in the three-way catalytic converter

Fig 1: Fuel Air/Fuel Ratio Graph

Courtesy of MERCEDES-BENZ USA

Function requirements for lambda control, general points

Circuit 87M ON (engine timing ON)
Engine running
Coolant temperature at start (dependent on the particular operating condition)
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:

hot film MAF sensor (B2/5), engine load
coolant temperature sensor (B11/4)
left and right 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 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 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.

Fig 2: Lambda Control Loop Flow Diagram

Courtesy of MERCEDES-BENZ USA

Assuming fuel-air mixture becomes leaner. As a result, the oxygen sensor voltage drops as a result 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 lambda control factor indicated with the Diagnosis Assistance System (DAS) changes 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 caused by the ME-SFI [ME] control unit.

Fig 3: Identifying Lambda Control Factor

Courtesy of MERCEDES-BENZ USA

Additional function requirements for two-sensor control

Operating temperature of firewall catalytic converters is 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 firewall 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 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.

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 IC (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 DAS 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
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, 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 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 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.

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 purging switchover valve (Y58/1)
faulty fuel pressure sensor (B4/7)
wear on engine (e.g. leaky valves)

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 constantly drifts out of the middle controlled range (for example 0 ±18%), the ME-SFI [ME] control unit, under certain operating conditions, shifts the lambda characteristics map sufficiently far until a lambda control factor of approx. 0% is obtained again.

Example: shifting of the lambda characteristics map

Fig 4: Shifting Of Lambda Characteristics

Courtesy of MERCEDES-BENZ USA

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-adjustment has taken place the lambda regulating factor will again find itself in the medium controlled range.

Presentation of self-adjustment value using the DAS

The following can be read out using the DAS:

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 the DAS: 1.1

For determining the injection time (fuel injection quantity) the ME-SFI [ME] control unit uses a calculated value of 1.1).165 kg/h (150 kg/hX1.1).

The maximum correction values are -0.68 to +1.32.

Electrical function schematic for lambda control	MODEL 204	PE07.10-P-2710-97FAB
	MODEL 207	PE07.10-P-2710-97EAB
	MODEL 212	PE07.10-P-2710-97DAB
Overview of system components motor electronics (ME-SFI) fuel injection and ignition system		GF07.61-P-9997MI