Index Ratio Method Using A Wide Range Ho2S Sensor (UEGO)

The switching HO2S control system compares the HO2S signals before and after the catalyst to assess catalyst oxygen storage. The front HO2S signal from UEGO control system is used to control to a target A/F ratio and does not have "switches" As a result, a new method of catalyst monitor is utilized.

The UEGO catalyst monitor is an active/intrusive monitor. The monitor performs a calibratable 10-20 second test during steady state rpm, load and engine air mass operating conditions at normal vehicle speeds. During the test, the fuel control system remains in closed loop, UEGO control with fixed system gains. In order to assess catalyst oxygen storage, the UEGO catalyst monitor is enabled during part-throttle, closed-loop fuel conditions after the engine is warmed-up and inferred catalyst temperature is within limits. While the catalyst monitoring entry conditions are being met, the rear HO2S signal length is continually being calculated. When the required total calibrated time has been accumulated, the total voltage signal length of the rear HO2S is divided by a calibrated threshold rear HO2S signal length to compute a catalyst index ratio. The threshold rear HO2S signal is calibrated as a function of air mass using a with a catalyst with no precious metal. This catalyst defines the worst case signal length because it has no oxygen storage. If the monitored catalyst has sufficient oxygen storage, little activity is observed on the rear HO2S voltage signal. An index ratio near 0.0 indicates high oxygen storage capacity, hence high HC/NOx efficiency. As catalyst oxygen storage degrades, the rear HO2S voltage signal activity increases. An index ratio near, 1.0 indicates low oxygen storage capacity, hence low HC/NOx efficiency. If the actual index ratio exceeds the calibrated threshold ratio, the catalyst is considered failed.