Catalyst Monitoring

GENERAL DESCRIPTION

Catalyst monitoring is based on monitoring it's oxygen storage capability.

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

In order to determine catalyst efficiency the oscillation of the upstream sensor is need to calculate the "O2-in and-output (catalyst)" by engine air mass and lambda-deviation. The downstream sensor signal for a "Threshold catalyst" then is derived (model) from this basic value.

Any time the real sensor signal oscillation (downstream) corresponds to the model an defective catalyst is recognized.

MONITORING STRUCTURE





Catalyst monitoring structure

1. Computation of the efficiency factor

First the signal of the lambda-controller is filtered and multiplied by the engine air mass. A value is now added which considers the downstream sensor layer. This result is representative to O2-load (0KB) into the catalyst.

The standard amplitude (NSA) of the downstream sensor is computed by averaging the measured signals.

The difference between NSA and 0KB is integrated and divided continuous by a time range during catalyst monitoring is active.

This factor (GW) is an indicator for the catalyst efficiency and it is determined continuous in a certain engine speed and engine load range within the time of monitoring.

2. Fault evaluation

After time range of monitoring has elapsed the efficiency factor (GW) is compared with the threshold value. If GW > than the threshold value, a fault is detected and the MIL is illuminated after the next driving cycle.

3. Check of monitoring conditions

The monitoring principle is based on the detection of relevant oscillations of the downstream sensor signal during regular lambda control. It is necessary to check the driving conditions for exceptions where no regular lambda control is possible, e.g. fuel cut-off. During such periods, and for a certain time afterwards, the computations of the amplitude values and the postprocessing is halted. Thus, a distortion of the monitoring information is avoided.





Block diagram of system operation