Page 217 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Nissan Motor Co. Ltd. 199

In the method of JP2310453 (1990), the-response delay time of the downstream sensor output
signal when compared to the upstream sensor output signal is a measure of the deterioration of
the catalytic converter.

In the method of JP4001449 (1992) the criterion of degradation of the catalytic converter is
the ratio of frequency of oscillation of the downstream sensor output signal to the frequency of
oscillation of the upstream sensor output signal during a feedback control of the engine aidfuel
ratio from rich to lean and vice versa.

In the method of JP4116239 (1992) the deterioration diagnosis of the catalytic converter is
inhibited for accuracy reasons when the number of renewals of a learning value based on the
output signal of the downstream sensor is lower than a specified value. The learning value is
the value used to correct the feedback compensation coefficient used during the feedback-
control of the engine air/fbel ratio. The feedback compensation coefficient is calculated based
on the output signal of the upstream sensor.

In the method of US51 19628 (1992) the following steps are considered:
1) detecting the aidfuel ratio of the exhaust gases upstream of the catalytic converter
2) detecting the air/fuel ratio of the exhaust gases downstream of the catalytic converter
3) checking whether the upstream sensor output signal is on the rich or on the lean side when
compared to the stoichiometric value
4) determining a basic engine air/fuel ratio feedback control constant according to the output
signal of the upstream sensor (lean or rich side)
5) checking whether the downstream output signal is inverted to rich or lean side across an
adfuel ratio feedback controlling rich (upper) or lean (lower) slice level respectively (fig.
90). Upper (lower) slice level is a predefined value being lower (higher) than the maximum
(minimum) value of the sensor output signal for each operating condition and it always cuts
the sensor output signal during deterioration detection
6) calculating a correction value of the basic aidfuel ratio feedback control constant according
to the rich or lean side
7) determining an engine aidfuel ratio feedback correction coefficient on the basis of the basic
feedback control constant corrected by the correction value
8) measuring a period of the engine aidfuel ratio feedback correction coefficient
9) measuring an amplitude of the aidfuel ratio feedback correction coefficient
10) setting a rich discriminating catalytic converter diagnosing slice level higher than a rich
discriminating aidtiel ratio feedback controlling slice level and a lean discriminating
catalytic converter diagnosing slice level lower than a lean discriminating air/fbel ratio
1 I) feedback controlling the slice level in such a way that the rich discriminating catalytic
converter diagnosing slice level increases and the lean discriminating catalytic converter
diagnosing slice level decreases, both increasing anyone of the measured period and
amplitude of the aidfuel feedback correction Coefficient (fig. 9 1). wherein the slice levels