Page 213 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Mitsubishi Motors Corporation - Mitsubishi Electric Corporation 195

In the method of US5591905 (1997). first it is decided whether or not the catalytic converter is
activated and only then the deterioration diagnosis procedure starts. In this way an erroneous
decision of deterioration of the catalytic converter is avoided. The method comprises the
following steps:
I) deciding whether a catalyst is activated in a catalytic converter
2) determining a steady state operation of the internal combustion engine
3) deciding deterioration of said catalyst only when it is decided that said catalyst is activated
and when it is decided that the engine is in a steady state operation
4) detecting an operating state of the internal combustion engine by means of operating state
detecting means
5) deciding that said catalyst is activated during a period starting after elapse of a first time
count, in which an output value from said operating state detecting means is kept in a
predetermined range
6) ending upon the elapse of a second time count commencing after said output value is out of
the predetermined range
7) assessing the deterioration of the catalytic converter, after determining that the converter is
activated, by comparing the signals of the upstream and downstream sensors by means of
any of the methods described above.

The predetermined range is varied according to the operating state of the internal combustion
engine. The operating state detecting means can be a hot-wire intake air flow sensor, an
ignition coil, an igniter, a water temperature sensor, a vehicle speed sensor or an intake air
flow rate sensor.

Fig. 88a shows the variation of the output voltage VI of the upstream sensor during feedback-
control of the engine airlfuel ratio. Figs. 88b and c show the corresponding variation of the
output voltage signal VZ of the downstream sensor for the cases of a non-deteriorated and a
deteriorated converter respectively. Fig. 88d shows the temperature of the catalytic converter,
where T, corresponds to the activation temperature of the converter at constant operation.
During a period to in which the converter is activated, the aidfuel ratio signal VZ can have a
substantially constant value because of the puritjling action of the converter in case the
converter is normal (fig. 88b). In the case of a deteriorated converter (fig. 88c) the signal V2
oscillates within the time period to in a way similar to that of signal V1 (fig. 88a). The
deterioration of the catalytic converter is then easily recognized from the oscillation of V2.
However, when the temperature of the catalytic converter is lower than the activation
temperature T,, the aidfuel ratio signal VZ shows the same wave form as that at a time of
deterioration of the converter even if the catalytic converter is not deteriorated (figs. 88b-d).
Consequently, when deterioration of the converter is decided depending upon the wave form
of the aidfuel signal V2, the deterioration of the catalytic converter is erroneously detected
though the converter is not deteriorated.