Page 209 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Mitsubishi Motors Corporation - Mitsubishi Electric Corporation 191

where the catalytic converter is considered as deteriorated for C>C,im , where Ctim is a
threshold (limit) value of C

upstream or
downstream
sensor output
(V)

I
time

Fig. 85 (from US5417061)

The method of US5363647 (1 994) comprises the following steps:
1) detecting concentration of a particular component of the exhaust gas upstream of the
catalytic converter to output an air/fbel ratio signal indicating said concentration
2) detecting concentration of the particular component of the exhaust gas downstream of the
catalytic converter to thereby output a downstream aidfuel ratio signal indicating the
concentration
3) controlling an aimed aidfuel ratio of a mixture supplied to the engine by arithmetically
determining an aidfuel ratio control quantity on the basis of the upstream and downstream
aidfuel ratio signals
4) detecting the operation state of said internal combustion engine and when the operation
state of the engine lies within a predetermined range:
5) filtering the downstream aidfuel ratio signal to thereby derive a filtered aidfuel ratio signal
6) comparing the downstream aidfuel ratio signal with the filtered aidfuel ratio signal
7) comparing a deterioration parameter value derived from the result of the comparison with a
Predetermined value (which varies in dependence on change in the operation state of the
engine) corresponding to the detected operation state of the engine to thereby determine
deterioration of the catalytic converter when the deterioration parameter value exceeds the
predetermined value