Test Group YBMXV04.4LEV, YBMXT04.4E53 & YBMXT04.453M
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- Catalyst Monitoring General Description -
Catalyst monitoring is based on monitoring its 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, oscillation of the upstream sensor is need to calculate "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, a defective catalyst is recognized.
- Monitoring Structure
For catalyst monitoring structure flow chart, see Fig 1.- Computation Of Efficiency Factor - First lambda controller signal 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 (OKB) into the catalyst. The standard amplitude (NSA) of the downstream sensor is computed by averaging the measured signals. The difference between NSA and OKB is integrated and divided continuously by a time range during which catalyst monitoring is active. This factor (GW) is an indicator for the catalyst efficiency and it is determined continuously in a certain engine speed and engine load range within the time of monitoring.
- Fault Evaluation - After time range of monitoring has elapsed, the efficiency factor (GW) is compared with the threshold value. If GW is greater than the threshold value, a fault is detected and the MIL is illuminated after the next driving cycle.
- 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. See Fig 2. Thus, a distortion of the monitoring information is avoided.