Lambda control, function

S07.10-0009-79

Function requirements for lambda control, general points

 Circuit 87 ON (engine management ON)

 Engine running.

 Operating temperature of lambda sensor upstream of catalytic converter reached

 Deceleration fuel shutoff not active

Lambda control, general points

The mixture composition is controlled to achieve high exhaust gas conversion in the catalytic converter (three-way catalytic converter). To this end, the fuel-air ratio is kept within tight limits around lambda (λ) = 1.

The ME-SFI [ME] control unit performs this task by reading in the signals of the following sensors:

 Coolant temperature sensor

 Oxygen sensor upstream of catalytic converter

 Oxygen sensor downstream of catalytic converter

S07.10-0018-01

Function sequence for lambda control

Lambda control is described in the following points:

 Function sequence for 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 closed-loop control circuit

The lambda sensor upstream of catalytic converter reacts to the oxygen content in the exhaust gas and transmits corresponding signals to the ME-SFI [ME] control unit.

If λ < 1 (low oxygen content in exhaust gas), the lambda sensor upstream of catalytic converter sends the 'rich mixture' signal to the ME-SFI [ME] control unit. This will then decrease the injection quantity by adjusting the fuel injectors and change the mixture composition towards 'lean'. The oxygen content in the exhaust increases and the value alters in the direction λ = 1.

If λ > 1 (high oxygen content in exhaust gas), the lambda sensor upstream of catalytic converter sends the 'lean mixture' signal to the ME-SFI [ME] control unit. This will then increase the injection quantity by adjusting the fuel injectors and change the mixture composition towards '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.

Additional function requirements for two-sensor control

 Catalytic converter operating temperature reached

 Lambda control active

 Oxygen sensor downstream of catalytic converter fault-free

Function sequence for two-sensor control

The two-sensor control monitors the oxygen sensor upstream of the catalytic converter function and the catalytic converter efficiency.

For this, the ME-SFI [ME] control unit compares the signals from the lambda sensor upstream of catalytic converter with those from the lambda sensor downstream of catalytic converter.

The ME-SFI [ME] control unit uses the signals from the lambda sensor downstream of catalytic converter to determine the average λ value and then compares it with the specified value.

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 depends on the performance map and the ME-SFI [ME] control unit applies it by adaptation of the injection period of the fuel injectors.

Function sequence for oxygen sensor heater

The lambda sensor heaters serve to attain the operating temperature more quickly.

When the exhaust system is very cold, the heaters are off to prevent damage (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

In regulated catalytic converters, the injection duration is calculated by the lambda control with such precision that a specified air/fuel ratio is maintained under all operating statuses.

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.

If a fault occurs, the ME-SFI [ME] control unit automatically makes a correction of the mixture formation. In this case, the lambda map is shifted within certain limits so that the lambda control is not positioned at the lower or upper control stop.

P07.61-3786-76

If the mixture composition is constantly drifting out of the central controlled range of the lambda control factor (a), the ME-SFI [ME] control unit will under certain operating conditions shift the lambda characteristics map. It is shifted until the lambda control factor returns to around 0.

Additional function requirements for catalytic converter monitoring

 Catalytic converter operating temperature 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.

Aging of a catalytic converter is caused by the reduction of oxygen storage capacity over time and the resulting decrease in hydrocarbon conversion. The catalytic converter stores oxygen during mixture leaning (closed-loop control circuit) and releases it again during enrichment of the mixture for hydrocarbon conversion.