Page 170 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

152 Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters

Diagnostic area AF: airlfuel
AIR-FUEL RATIO SHIFT
WRING CATALYST

b)
-
UPSTREAM 02
SENSOR OWPUT

c) RICH
DOWNSTREAMOz
SENSOR OUTPUT
I 2
11 t2

Mais
time

Fig. 70 (from DE4404449)

As shown in fig. 70a, a signal is supplied to the air/hel ratio feedback calculating means (fig.
67) which causes the output signal of the downstream 02 sensor to move into proximity with
the stoichiometric line by shifting it to the lean side when the air/hel ratio is on the rich side,
and shifting it to the rich side when it is on the lean side in the diagnostic area (from tl up to
t2). The correlation function of the output signal of the upstream 02 sensor shown in fig. 70b
with the downstream 02 sensor shown in fig. 70c can then be calculated accurately.
In a preferred embodiment of the invention, instead of simply taking the mean value of N
instantaneous degradation indices as described above, the final deterioration index I calculation
is performed by taking the mean of the M largest values of @i, according to the following
formula:

I
I = -Xmax,(Qi)
M