Page 219 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Nissan Motor Co. Ltd. 20 1

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catalyst
diagnosing
slice level

amplitude
period

Fig. 91 (from US5 I 19628)

In the method of JP6074025 (1994) the feedback-control of the engine air/hel ratio during
testing is based on the output signal of the downstream oxygen sensor, so as to largely swing
the airlfbel ratio to rich and lean sides. When the catalytic converter is deteriorated, the
inversion delay time between the output signals due to oxygen storage in the converter
becomes shorter than a predetermined threshold.

In the method of JP6254726 (1994) the testing takes place when certain operating conditions
are fulfilled. Then the exhaust gas temperature is raised by delaying the injection of fuel in the
engine cylinders and by delaying the ignition at a time point later than the optimum ignition
time. The temperature of the catalytic converter is thus kept high and the reliability of the
diagnosis procedure by means of the oxygen sensors increases.

In the method of JP7180534 (1995) two HEGO sensors are instailed upstream and
downstream of the converter. When the tfequency ratio of the output signals of the two
sensors indicates a deteriorated converter, the heater of the downstream sensor is disconnected
and the temperature of the downstream sensor becomes equal to the one of the catalytic
converter. Then it is checked whether or not the measured output signal of the downstream
sensor is within a certain range extracted from an output signal state available when the
temperature of the downstream sensor is approximately equal to the activation temperature of
the converter. If the measured output signal of the downstream sensor is out of this certain