Catalyst Monitoring

Catalyst Monitoring

General Description







Sensor methods
- The two-sensor method makes use of one upstream and one downstream oxygen sensor, one sensor (UHEGO) before the catalytic converter and one sensor (HEGO) inserted into the catalytic converter, monitoring the front part of the catalyst.
- The two-sensor method stereo makes use of one upstream and one downstream oxygen sensor for each cylinder bank, each bank has one sensor (UHEGO) before the catalytic converter and one sensor (HEGO) after.
- The three-sensor method makes use of two upstream and one downstream sensor, two sensors (UHEGO) before the catalytic converter and one sensor (HEGO) inserted into the catalytic converter, monitoring the front part of the catalyst.

Catalyst monitoring is based on monitoring the oxygen storage capability. The (nonlinear) correlation between conversion efficiency and storage capability has been shown in various investigations.

The engine mixture control results in regular Lambda oscillations of the exhaust gas. When using a linear sensor Lambda control, Lambda oscillations are artificially created during catalyst monitoring.

These oscillations are dampened by the storage activity of the catalyst. The amplitude of the remaining Lambda oscillations downstream the catalyst indicates the storage capability. This information is evaluated during one single engine load and speed range. According to the described operating principle the following main parts can be distinguished:
- Computation of the amplitude of the downstream Lambda sensor
- Modeling of a borderline catalyst and of the signal amplitudes of the downstream Lambda sensor
- In the 3-sensor case, an additional modeling of a single air fuel mixture corresponding to the two front sensors, before the catalytic converter.
- Signal evaluation
- Fault processing
- Check of monitoring conditions