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

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

Fig 1: Fuel Air/Fuel Ratio Graph

Courtesy of MERCEDES-BENZ USA

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 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:

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 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 Chart

Courtesy of MERCEDES-BENZ USA

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 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.

Fig 3: Identifying Lambda Control Factor Range

Courtesy of MERCEDES-BENZ USA

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 (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 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

Fig 4: Lambda Performance Map Shifting Graph

Courtesy of MERCEDES-BENZ USA

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/hX1.1).

The maximum correction values are -0.68 to +1.32.

Electrical function schematic for lambda control	Model 216	PE07.10-P-2710-97CAH
Electrical function schematic for lambda control	Model 221	PE07.10-P-2710-97SAH
Overview of system components for gasoline injection and ignition system with direct injection		GF07.70-P-9998MMB