Page 138 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

120 Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters

In JP3246456 (1991) the average frequencyfo of the aidhe1 ratio oscillation is measured
downstream of the catalytic converter during a time interval T,, when certain operating
conditions hold. During the same diagnostic period, the exhaust gases bypass the catalytic
converter and pass through a fresh reference catalytic converter. The average frequency f, of
the air/hel ratio oscillation is then measured downstream of the reference catalytic converter
during a time interval 7;. The catalytic converter is considered as deteriorated when

where 6 is a preset threshold.

In JP4066748 (1992) the air/lhel ratio of the engines starts oscillating around the
stoichiometric value. The time elapsed between a maximum and a minimum value as well as
the number of turnover cycles of the variation of the output of the downstream sensor in a
specific period are detected. When the number of cycles is higher than a predetermined number
and the elapsed time is lower than a predetermined time, then the catalytic converter is
considered as degraded.

In JP4081540 (1992) the aidfuel ratio of the engines starts oscillating around the
stoichiometric value. The upstream and downstream oxygen sensors detect the phase
difference of the two signals upstream and downstream of the catalytic converter and an
average phase difference is calculated within a specified time. When the average value is
judged to be less than a predetermined value the catalytic converter is considered as degraded.

The method of JP4321744 (1992) detects deterioration of catalytic converters even in the case
of a deteriorated upstream oxygen sensor. The engine air/hel ratio is corrected according to
the output signal of the upstream sensor in order to achieve the theoretical engine aidfuel ratio.
The deterioration of the catalytic converter is based on the delay time length between a change
in the corrected air/fuel ratio of the engine and the corresponding change in the output signal
of the downstream sensor. The catalytic converter is considered as deteriorated when the delay
time length becomes shorter than a predetermined threshold.

The method of EP0475177 (1992) and US5220788 (1993) is based on a change of the
feedback control of the engine aidhe1 ratio period to a value different from the feedback
period (responsive to the upstream sensor) that occurs during normal aidfuel ratio control
operation. Two methods are presented for changing the feedback period:
1) the period is set to a predetermined (target) value such so to ensure optimum accuracy of
detecting the purification factor, i.e. accuracy of determining whether the purification factor
is above a certain reference level or accuracy of measuring the value of the purification