Oxygen sensor emissions control

For complete and perfect combustion, a air/fuel ratio of 1 kilogram of fuel and approx. 14.7 kilograms of air is necessary. The air mass corresponds to around 11 cubic meters. Lambda refers to the ratio between the amount of air actually supplied and the stoichiometric amount of air. 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 excess air (Lambda approx. 1.1 = lean mixture). The catalytic converter can most effectively reduce pollutant emissions if the fuel-air mixture is in the region 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 exhaust gas.

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