Closed-loop Lambda control system
Optimum efficiency of the catalytic converter can only be achieved if an ideal fuel/air ratio is used for combustion. To this end, oxygen sensors are used upstream and downstream of the catalytic converter.
The oxygen sensors upstream of the catalytic converter have a steady characteristic output curve (measure oxygen content in rich and lean ranges.)
The measurement method employed by this oxygen sensor is different to an oxygen sensor with an erratic characteristic output curve. The oxygen sensor is therefore connected using 6 pins instead of 4.
- Oxygen sensors upstream of catalytic converter
The oxygen sensors upstream of the catalytic converter (control sensors) are used to assess the composition of the exhaust gas.
The control sensors are screwed into the exhaust manifold.
The oxygen sensors measure the residual oxygen content in the exhaust fumes. The voltage values determined are relayed to the DME control unit. The DME control unit corrects the mixture composition by adjusting the injection period.
- Oxygen sensors downstream of catalytic converter
The oxygen sensors downstream of the catalytic converter (monitoring sensors) are used to monitor the control sensors. In addition to this, the function of the catalytic converter is monitored.
To ensure full operational reliability of the oxygen sensors upstream of the catalytic converter, a temperature of approximately 750°C is required (350°C for oxygen sensors downstream of the catalytic converter). For this reason, all oxygen sensors are heated.
The oxygen sensor heating is controlled by the DME control unit. When the engine is cold, the oxygen sensor heating remains switched off, as condensation which is present would otherwise destroy a hot oxygen sensor due to thermal strain.
This means that the closed-loop Lambda control only becomes active a short time after the engine has started, once the catalytic converters have warmed up. The oxygen sensor is initially warmed up using a reduced heating power, to avoid imposing unnecessary loads on it due to thermal strain.