Page 141 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Nippon Denso Co. 123

4) performing control of the internal combustion engine by using the air/hel ratio correction
coefficient in such a manner that the air/hel ratio is made to be equal to the theoretical
aidfuel ratio
5) calculating an average aidfuel ratio correction coefficient of the air/hel ratio correction
coeficient when the aidfuel ratio is changed from a rich side to a lean side and the air/hel
ratio correction coefficient when the air/fLel ratio is changed from the lean side to the rich
side
6) discriminating the deterioration of the catalytic converter in accordance with a response
delay time from a moment an output signal from the upstream oxygen sensor has been
changed to a moment an output signal from the downstream oxygen sensor has been
changed in the same direction, and the catalytic converter is considered as deteriorated
when the response delay time is smaller than a predetermined value
7) permitting the catalyst deterioration detection when the internal combustion engine is in
idling state and when the air/hel ratio has been converged to the neighborhood of the
theoretical aidfuel ratio in accordance with the value of the average air/fuel ratio correction
coefficient.

The method of US5412942 (1995) comprises the following steps:
1) deriving a main air/hel ratio correction coefficient based on an output of the upstream-side
oxygen sensor. The main air/hel ratio correction coefficient is derived for correcting an
aidfuel ratio of an aidfuel mixture to be fed to the engine so as to be near the
stoichiometric air/hel ratio
2) controlling the aidfuel ratio to be fed to the engine so as to be near the stoichiometric
air/fuel ratio, using said main aidfuel ratio correction coeficient
3) deriving a high-frequency amplitude of an output of the downstream-side oxygen sensor by
calculating a difference between maximum and minimum values of the output of said
downstream-side oxygen sensor. The maximum and minimum values are derived at every
period of the main air/fuel ratio correction coeficient
4) deriving a low-frequency amplitude of the output of the downstream-side oxygen sensor.
The mean value of maximum and minimum values of the output of the downstream oxygen
sensor at every period of the main aidfuel ratio correction coefficient is derived and the
amplitude of the low-frequency component is calculated by the difference between
maximum and minimum values of said mean values during a preset time period
5) determining that the catalytic converter is deteriorated when said high-frequency amplitude
is greater than a preset value and said low-frequency amplitude is smaller than a preset
value.

The method can be better explained with the help of figs. 59a,b,c, where the voltage variation
of the upstream sensor VI and the downstream sensor Vz are shown for the cases of a high
purification rate, a medium purification rate and a low purification rate of a catalytic converter
respectively. The dashed lines represent the low frequency variation of the output signal of the
downstream sensor, which is generated due to the aidfuel ratio feedback control. As can be
noticed from fig. 59, as the purification factor of the catalytic converter decreases, the high
frequency amplitude becomes larger and the low frequency amplitude becomes smaller. The