Page 156 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

138 Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters

8) determining whether the catalytic converter is deteriorated, based on the calculated average
value and output fluctuation width

In JP6193436 (1994) the engine air/fkel ratio forcibly varies from rich to lean and the opposite
with a specific amplitude and frequency. The average engine air/hel ratio is set leaner than the
stoichiometric value during execution of the perturbation. The deterioration judgment of the
three-way catalytic converter is then based on the average output signals of the upstream and
downstream sensor.

airlfuel correction

e

Fig. 66 (from US5381657)

The method of US5381657 (1995) comprises the following steps:
1) sensing the concentration of oxygen contained in the exhaust gases downstream of the
catalytic converter by means of the downstream sensor (fig. 66 lower part)
2) controlling an airhe1 ratio of a mixture supplied to the engine in relation with the output of
the downstream sensor (feedback control) (fig. 66 upper part)
3) comparing the output from the downstream sensor with a predetermined reference value to
thereby determine whether the aidfuel ratio of the mixture supplied to the engine is on a rich
side or a lean side with respect to a stoichiometric aidfuel ratio
4) changing the air/fkel ratio of the mixture supplied to the engine across the stoichiometric
aidfuel ratio upon the lapse of a delay time period AtIan or after the time the inversion
of the aidfuel ratio (from rich to lean and vice versa) is detected from the downstream
sensor (fig. 66)