Differences in Pre- and Post-Converter Lambda Sensor Characteristics

The two characteristics of wide-range air/fuel ratio sensors when located in front of and behind a three-way catalytic converter are investigated. Input as well as output gas concentration measurements and sensor readouts are presented. Behind a new converter almost no oxygen can be measured for rich air/fuel ratios.

The wide-range sensor's signal is sensitive to changes in the gas composition when keeping the air/fuel ratio constant at a rich value. Since the gas compositions up- and down-stream of the converter differ, the sensor signals are not identical for the same rich air/fuel ratio before and after the converter.

The various diffusion coefficients of the exhaust gas species flowing through the porous coating of the sensor combinded with the different up- and downstream gas compositions are responsible for the different sensor characteristics.