Oxygen sensor emissions control

For complete and perfect combustion, an air/fuel ratio of 1 kilogram of fuel to approximately 14.7 kilograms of air is necessary. The air mass corresponds to around 11 cubic meters. The ratio between the air quantity that is actually being supplied and that required for a stoichiometric ratio is designated as lambda. During normal operation of the vehicle, the Lambda value fluctuates. The engine provides maximum performance operating with low air (lambda roughly 0.9 = rich mixture). The engine attains maximum fuel economy (minimal consumption) running on excess air (lambda of roughly 1.1 = rich mixture). The catalytic converter furnishes its best reduction of pollutant emissions with an air-fuel mixture in the range of lambda = 1. With modern catalytic converters the catalyst's conversion rate, meaning the proportion of pollutants that are actually converted, lies between 98 and almost 100 percent. Optimal composition of the air-fuel mixture is controlled by the DME digital engine electronics system. The oxygen sensors deliver essential information on the composition of the exhaust gas. The pre-catalyst oxygen sensor continuously monitors levels of residual oxygen in the exhaust gas. The fluctuating values for residual oxygen are relayed to the DME digital engine electronics system in the form of a voltage signal. The DME manipulates the fuel-injection system to correct the mixture composition. A second oxygen sensor is installed behind the catalytic converter (monitor sensor).

The catalytic converter has a high oxygen storage capacity. This means there is only a little oxygen behind the catalytic converter. The post-catalyst oxygen sensor transmits a virtually constant (attenuated) voltage. With increasing age, the oxygen storage capacity of the catalytic converter declines. The post-catalyst sensor then responds with progressively more pronounced fluctuations in its lambda readings. This characteristic is exploited by a special diagnostic function employed to monitor the catalytic converter.

A malfunction of the catalytic converter is indicated by the emissions warning light.