Page 168 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

150 Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters

The method of EP0478133 (1992) comprises the following steps:
1) controlling the air/hel ratio of the air/fbel mixture supplied to each cylinder of the engine to
the stoichiometric value @=I)
2) changing the air/fuel ratio supplied to the engine from one predetermined rich value to
another predetermined lean value around the stoichiometric one by means of a ;L correction
coefficient (fig. 69b) in such a way that the mean value is leaner than the stoichiometric one
(fig. 69c, left side), then
3) changing the aidfuel ratio supplied to the engine from one predetermined rich value to
another predetermined lean value around the stoichiometric one by means of a 1 correction
coefficient (fig. 69b) in such a way that the mean value is richer than the stoichiometric one
(fig. 69c, right side)
4) measuring the response time At of the downstream sensor from the moment the mean value
of the air/fbel ratio supplied to the engine turns from lean to rich till the moment the signal
of the downstream sensor passes from lean to rich respectively (fig. 69d)
5) deciding that the catalytic converter is deteriorated if the response time At is lower than a
predetermined threshold

In the method of DE4238807 (1993) a cross-correlation function of the output signals of the
upstream and downstream sensors is calculated in a way similar to the method of US5237818
(1993) described above. The correlation coefficient calculated from the two signals is averaged
and from this coefficient an averaged conversion efficiency of the catalytic converter is
estimated. This value of the conversion efficiency is compared to a predetermined region of
values. If the conversion efticiency falls out of this region then the catalytic converter is
considered as deteriorated.

The method of DE4243339 (1993) and US5341642 (1994) comprises the following steps:
I) calculating an auto-correlation fbnction @=of the output signal x from the upstream sensor
to output the maximum values of the auto-correlation function at predetermined intervals,
each maximum value being in each of the predetermined intervals. @= is calculated as:

where the maximum value is determined by changing the phase 7 in an integration region
2) calculating a cross-correlation hnction QAT. between the output signal y from the upstream
and downstream aidfuel sensors to output the maximum values of the mutual correlation
hnction at predetermined intervals, each maximum value being in each of the predetermined
intervals. is calculated as:

where the maximum value is determined by changing the phase 7 in an integration region