Monitoring Structure

Monitoring Structure







Monitoring Cycle

Computation of the Downstream Oxygen Sensor Signal Amplitudes
The first step is the calculation of the amplitude of the signal oscillations of the oxygen sensor downstream of the catalyst. This is accomplished by extracting the oscillating signal component, computing the absolute value and averaging over time.

Modelling of a Borderline Catalyst and the Downstream Oxygen Sensor Signal Amplitudes
The model simulates the oxygen storage capability of a borderline catalyst. The signal of the downstream oxygen sensor is simulated in the catalyst model; this is based on real-time engine operating data (air fuel ratio and airflow rate). The amplitude of the modelled signal oscillations is then calculated.

Signal Evaluation
The signal amplitude of the downstream sensors is compared with the model over a given time. If these signal amplitudes exceed the modelled amplitudes, then the oxygen storage capability of the catalyst is less than that of a borderline catalyst.

Fault Processing
If the test result for the catalyst in the vehicle shows a lower oxygen storage capability than the model, then a fault is detected and an internal flag will be set. If the fault is detected again during the next drive cycle, then the MIL will be illuminated and a diagnostic trouble code (DTC) stored.

Since the monitored engine has a catalyst for each of two cylinder banks, two evaluations are made with differing fault thresholds, one test is for deterioration in one of the catalysts and the second is at a reduced threshold to check for deterioration in both catalysts.

Function Enable Criteria
The monitoring principle is based on the detection of relevant oscillations of the downstream oxygen sensor signal during regular lambda control. It is necessary to check the driving conditions to ensure that regular lambda control is possible, e.g. fuel cut-off not present. During these conditions and for a certain time afterwards, the computation of the amplitude values and their post-processing is suspended, so that a distortion of the monitoring information is avoided.