Lambda Control, Function - GF07.10-S-1010TRJ

Engine 274 in model 447.6/7 

with code XM0 (Facelift) 

Function requirements for lambda control, general points 

• Circuit 87M ON (Engine management ON)
• Engine running.
• Operating temperature of oxygen sensor upstream of catalytic converter (G8/6) reached
• Deceleration fuel shutoff not active

Lambda control, general points 

To achieve a high exhaust gas conversion in the catalytic converter, the mixture composition is regulated within narrow limits around the air-fuel ratio (λ) = 1.

To do this the ME-SFI [ME] control unit (N3/34) reads in signals from the sensors:

• Coolant temperature sensor (B11/21)
• Intake manifold pressure sensor (B28/24)
• Oxygen sensor element downstream of catalytic converter (G3/21b1)
• Oxygen sensor element upstream of catalytic converter (G8/6b1)

Exhaust conversion in the CAT (in homogeneous mode) 

Courtesy of MERCEDES-BENZ USA

Lambda control, function 

Lambda control is described in the following points:

• Function sequence for lambda control
• Function sequence for two-sensor control
• Function sequence for oxygen sensor heater
• Function sequence for self-adjustment of the mixture formation
• Function sequence for catalytic converter monitoring

Function sequence for lambda control 

The sensor element for the oxygen sensor upstream of the catalytic converter responds to the oxygen content in the exhaust and sends corresponding signals to the ME-SFI [ME] control unit.

For λ < 1 and a low oxygen content in the exhaust gas, the sensor element "lambda sensor upstream of the catalytic converter" transmits the signal "rich mixture" to the ME-SFI [ME] control unit. This reacts by reducing the injection quantity and thereby 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 gas, the sensor element "lambda sensor upstream of the catalytic converter" transmits the signal "lean mixture" to the ME-SFI [ME] control unit. This reacts by reducing the injection quantity and thereby 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 control unit alters the mixture composition with a time delay in order to prevent any risk of jerking.

In XENTRY Diagnosis, the control status is displayed by means of a lambda control factor that changes in a positive direction in the case of a leaning of the mixture and in the negative direction in the case of an enrichment of the mixture.

Additional function requirements for two-sensor control 

• Catalytic converter operating temperature reached
• Lambda control active
• Lambda sensor downstream of catalytic converter (G3/21) fault-free

Function sequence for two-sensor control 

The two-sensor control monitors the oxygen sensor upstream of the catalytic converter function (G8/6) and the catalytic converter efficiency. For this purpose, the ME-SFI [ME] control unit reads in the signals of the lambda sensor upstream of catalytic converter (G8/6) and the lambda sensor downstream of catalytic converter (G3/21).

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, the aging of the lambda sensor upstream of the catalytic converter in compensated for within certain limits.

The value of a new lambda sensor upstream of the catalytic converter (G8/6) is approx. 0.

The correction value is dependent on a performance map and is reached by the ME-SFI [ME] control unit by adapting the injection time.

The following faults can, for example, occur during mixture formation:

• Unmetered air due to leaks in the exhaust system
• Wear to or carbon deposits on the fuel injectors (Y62),
• Damaged intake manifold pressure sensor
• Defective sensors or actuators
• Damaged lambda sensors
• Damaged purging switchover valve (Y58/7)
• Engine wear (e.g. leaky valves)

If a specified limit value is exceeded or if the plausibility test for the lambda sensor signals (upstream or downstream of the catalytic converter) is negative, the ME-SFI [ME] control unit actuates the engine diagnosis indicator lamp (A1/1e58) in the instrument cluster control unit (IC) (A1/1).

The exceedance of the limit value is stored in the fault memory by the ME-SFI [ME] control unit and can be read out and cleared with XENTRY Diagnosis.

Function sequence for oxygen sensor heater 

The oxygen sensor heater downstream and upstream of the catalytic converter (G3/21r1 G8/6r1) heat the oxygen sensors up to operating temperature faster. With a controlled heater, they also prevent damage to the oxygen sensor ceramics. They are actuated directly by ME-SFI [ME] control unit.

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 composition 

For a regulated catalytic converter, 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) is neither too rich nor too lean. It also prevents the lambda control from coming to the control abutment at high altitudes.

If a fault occurs, the ME-SFI [ME] control unit carries out a correction of the mixture composition. In this case, the lambda characteristics map is shifted within the specified control limits so that the lambda control is not at the lower or upper control abutment.

If the mixture composition constantly drifts out of the center controlled range, the ME-SFI [ME] control unit may under certain operating conditions shift the lambda characteristics map until a lambda control factor of approx. 0 arises again.

Lambda characteristics map 

Courtesy of MERCEDES-BENZ USA

Display of self-adjustment values with XENTRY Diagnosis 

The following can be read out with XENTRY Diagnosis:

• Shifting of the lambda characteristics map
• Direction of shift (rich or lean)
• Size of the shift

Values are shown in "ms", and mean that the value indicated for calculating the injection period is either added to or subtracted from the injection period determined by the characteristics map.

Example:

• Calculated injection duration (characteristics map): 3.0 ms
• Displayed value in XENTRY Diagnosis: "+ 0.3 ms"

For the determination of the injection duration, a calculated value of 3.3 ms is used by the ME-SFI [ME] control unit.

Idle correction value:

• The max. correction value is ±1.0 ms.

Partial load correction value:

• The max. correction value is -0.68 up to 1.32 ms.

Additional function requirements for catalytic converter monitoring 

• Operating temperature of catalytic converters reached
• Lambda control active

Catalytic converter monitoring, general points 

The law maker requires that the hydrocarbon emissions do not exceed a certain value. It is therefore necessary to constantly check the catalytic converter for aging.

The aging of a catalytic converter arises from the oxygen storage capacity that decreases over time and the resulting prevented hydrocarbon conversion.

The catalytic converter stores oxygen during the leaning of the oxygen mixture and releases it again during enrichment of the mixture for HC conversion.

Function sequence for catalytic converter monitoring 

To check the oxygen storage capacity, a rich mixture is output until the sensor element "lambda sensor downstream of catalytic converter" measures a certain maximum value and the majority of the oxygen is removed.

After this a lean mixture is issued and in doing so the time measured until the oxygen sensor element downstream of catalytic converter measures a certain minimum and the CAT fills its oxygen store.

If the time measured lies below a stored characteristic, then the oxygen storage capacity is inadequate and an error is stored in the ME-SFI control unit.

	Function schematic for lambda control		PE07.10-S-2511-97TRL
	Overview of system components for gasoline injection and ignition system with direct injection		GF07.70-S-9998TRJ