Page 223 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Nissan Motor Co. Ltd. 205

The method of US5644912 (1997) comprises the following steps:
1) measuring the oxygen content of the exhaust gases upstream and downstream of a catalytic
converter by means of two oxygen sensors installed in the exhaust system upstream and
downstream of the converter
2) feedback-controlling the air-fuel ratio of mixture fed to the engine in accordance with an
output signal issued from the upstream oxygen sensor
3) detecting the elapsed time (time lag) from the time on which the output signal from the
upstream oxygen sensor carries out a rich-lean reverse to the time on which the output
signal from the downstream oxygen sensor carries out a corresponding rich-lean reverse
4) calculating an average value of time lags which are measured each time the ricMean reverse
takes place in the output signals from the upstream and downstream oxygen sensors
5) causing a step change, upon a given diagnosing mode thereof, in a feedback correction
factor in synchronization with the richllean reverse of the output signal fiom the downstream
oxygen sensor, thereby correcting the air-fuel ratio of mixture to a generally stoichiometric
value
6) deriving a judgment reference value in accordance with the detected number of times for
which the measurement of the time lag is repeated
7) comparing the judgment reference value with the averaged time lag to judge the
deterioration degree of the catalytic converter
8) suspending the judgment of the deterioration of the catalytic converter when the averaged
time lag is within a predefined border zone

In a second embodiment of the invention, instead of calculating an average value of time lags
(step 4), an integrated value of the amount of air fed to the engine within the time lag of step 3
is calculated. An average of integrated values, which are derived each time the richllean reverse
takes place in the output signals of the two oxygen sensors, is then calculated.
A judgment reference value is then derived in accordance with the detected number of times for
which the measurement of the time lag is repeated (step 6). The judgment reference value is
compared with the average of the integrated values to judge the deterioration degree of the
catalytic converter (step 7). The judgment of the deterioration of the catalytic converter is
suspended when the average of integrated values is within a predefined border zone (step 8).

Other methods presented by Nissan are: JP7301115 (1995), JP7301116 (1995).