Page 169 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Hitachi Ltd. 151

3) calculating the ratios between the maximum values of the cross-correlation function mrV and
auto-correlation function (successive deterioration index mi)

4) determining the deterioration condition of the catalytic converter by comparing the ratio
with a predetermined value. If the catalytic converter deteriorates, the successive
deterioration index @, increases since there is an increase in similarity between the
variations of the aidhe1 ratio upstream and downstream of the catalytic converter.

The methods of DE4404449 (1994) and US5400592 (1995) comprise the following steps:
I) sensing at least one operating parameter of the engine and providing at least one output
signal indicative thereof
2) detecting when the at least one output signal falls within a predetermined diagnostic area
and generating an enabling signal indicative thereof In particular the engine must be driven
at no less than a predetermined speed, and the temperature of the catalytic converter must
be at least 500 "C
3) detecting when the output signal from the downstream aidfuel ratio sensor varies within
predetermined limits proximate to a stoichiometric mixture; when the output signal from the
downstream aidfuel ratio sensor is not within the predetermined range, determining whether
it varies on a rich side or a lean side of the predetermined limits; and generating a fuel ratio
adjustment signal which causes said fuel input control system to adjust an amount of fuel
input to the engine to shift the output signal from the downstream aidfuel ratio sensor
within the predetermined limits
4) calculating a cross correlation function of the output signals x, y of the upstream and
downstream air/fbel ratio sensors (see method of US5341642 (1994) described above)
5) calculating an auto-correlation function mXr of the output x of the upstream aidfuel ratio
sensor (see method of US5341642 (1994) described above)
6) periodically calculating an instantaneous deterioration index @, based on the ratio of a
maximum of the cross correlation hnction and the calculated auto-correlation function

max @,,
@, =-
?)SI
7) periodically calculating a mean value of a predetermined number N of instantaneous
deterioration index values having greatest magnitude to define a final deterioration index Z
8) comparing the final deterioration index I with a predetermined deterioration reference level,
and activating an output indication when the deterioration index is greater than a
deterioration reference level