Catalyst Monitoring Description

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

The engine closed loop feedback control results in regular lambda (normalized air/fuel ratio) oscillations in the exhaust gas. These oscillations are damped by the oxygen storage activity of the catalyst. The amplitude of the remaining lambda oscillations downstream of the catalyst indicates the storage capability.

The monitoring function compares the signal amplitudes obtained from the downstream oxygen sensors with modeled signal amplitudes. The modeled signal amplitudes are derived from a model of a borderline catalyst. If the measured amplitudes exceed those of the model, then the catalyst is considered to be defective.

Unlike the previous catalyst monitoring function, this operates over a single range of engine speed and load.

The monitoring function can be broken down into the following sub-sections:

• Computation of the downstream oxygen sensor signal amplitudes.
• Modeling of a borderline catalyst and the downstream oxygen sensor signal amplitudes.
• Signal evaluation.
• Fault processing.
• Function enable criteria.