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

Engine 274 in model 447, 448 

except code XM0 (Facelift) 

Function requirements for lambda control, general points 

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

Lambda control, general points 

The mixture composition is maintained within the narrowest possible limits around fuel air/fuel ratio (λ)=1 in order to achieve a high conversion of the exhaust gases (exhaust gas conversion) in the catalytic converter.

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 the sensor element for the oxygen sensor upstream of the catalytic converter sends the "rich mixture" signal 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.

At λ>1 and with a high oxygen content in the exhaust, the sensor element for the oxygen sensor upstream of the catalytic converter sends the "lean mixture" signal 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 [ME] control unit alters the mixture composition with a time delay in order to prevent any risk of jerking.

Indication of the regulation status takes place in Xentry Diagnostics based on a lambda regulating factor which alters in a positive direction for leaning of the mixture and in a negative direction for enriching the mixture.

Additional function requirements for two-sensor control 

• Catalytic converter operating temperature reached
• Lambda control active
• Oxygen 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.

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

The following errors 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
• Damaged fuel pressure and temperature sensor
• Damaged oxygen sensors
• Damaged purging switchover valve (Y58/7)
• 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 (A11e58) in the instrument cluster (A1) via the chassis CAN 1 (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 Xentry Diagnostics.

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 phase) 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 automatically makes 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.

Lambda characteristics map 

Courtesy of MERCEDES-BENZ USA

If the mixture composition is constantly drifting out of the middle 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%.

Shown with self-adjustment value with Xentry diagnostics 

The following can be read out with Xentry diagnostics:

• 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 period (characteristics map): 3.0 ms
• Displayed value in Xentry Diagnostics: +0.3 (ms)

For determining the injection period the ME-SFI [ME] control unit uses a calculated value of 3.3 ms

Idle correction value:

• The maximum correction value is ±1.0 ms.

Partial load correction value:

• The maximum correction value is -0, 68 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 (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 oxygen mixture and releases it again during enrichment of the mixture for HC conversion.

Function sequence for catalytic converter monitoring 

In order to check the oxygen storage capacity a rich mixture is output until the oxygen sensor element downstream of the catalytic converter has reached a certain maximum value and the largest part 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 [ME] control unit.

	Electrical function schematic for lambda control	Engine 274 in model 448 except code XM0 (Facelift)	PE07.10-S-2710-97VDC
		Engine 274 in model 447 except code XM0 (Facelift)	PE07.10-S-2710-97TRC
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