Oxygen-sensor control

For complete and perfect combustion, a fuel-air ratio of 1 kilogram of fuel and approx. 14.7 kilograms of air is necessary. The air volume corresponds to around 11 cubic meters. The ratio of the air volume actually delivered to stoichiometric air volume is referred to as the Lambda. During normal operation of the vehicle, the Lambda value fluctuates. The engine has its best performance with a lack of air (Lambda approx. 0.9 = rich mixture). The engine has its lowest consumption with an air surplus (Lambda approx. 1.1 = lean mixture). The catalytic converter achieves the best reduction in the pollutant emissions when the mixture is in the range of Lambda = 1. The conversion rate, i.e. the proportion of converted pollutants, is 98 % to virtually 100 % in the case of modern catalytic converters. The Digital Engine Electronics (DME) control the optimized composition of the fuel-air mixture. The oxygen sensors deliver essential information on the composition of the emissions.

The front oxygen sensor continuously measures the residual oxygen in the exhaust gas. The fluctuation values of the residual oxygen are forwarded to the DME control module as a voltage signal. The DME corrects the fuel trim composition by means of fuel injection. A second oxygen sensor (monitoring sensor) is built in behind the catalytic converter. The catalytic converter has a high oxygen accumulation capability. This means there is only a little air behind the catalytic converter. The monitoring sensor supplies a virtually constant (attenuated) voltage. With increasing age, the oxygen accumulation capability of the catalytic converter declines. The control sensor then reacts increasingly to lambda deviation with voltage fluctuations. These characteristics are used by a special diagnosis function for catalytic converter monitoring. A malfunction of the catalytic converter is indicated by the emissions warning lamp.