Page 165 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Hitachi Ltd. I47

converter therein, and a high pass filter. The microcomputer is operated in accordance with
stored software for achieving functions of the above mentioned means such as auto-correlation
tinction calculating means and the catalytic converter deterioration determining means.

The auto-correlation bnction of the output signal from the upstream oxygen sensor is
calculated by an auto-correlation function calculating means. The cross-correlation function
between the output signal x of the upstream oxygen sensor and an output signal y of the
downstream oxygen sensor is calculated by the cross-correlation tinction calculating means.

Determination of whether or not the catalytic converter has deteriorated is carried out by
catalytic converter deterioration determining means, which calculates a successive
deterioration index and compares the calculated index with a predetermined reference value.
Determination as to whether or not the upstream oxygen sensor deteriorates is made by aidtiel
ratio sensor deterioration determining means.

In the method of DE4209136 (1992) and US5237818 (1993) the catalytic converter
conversion efficiency is estimated by comparing a correlation function @(T) computed from the
output signals of the upstream and downstream sensors with a reference value.

Figs. 68a,d show the variation in time of the output signals of the upstream and downstream
sensors respectively, when the aidfuel feedback control of fig. 68c is imposed on the engine.
Figs. 68b,e show the corresponding binary code signals (rectangular waves) generated by
comparing the output signals of figs. 68a,d to the comparison levels EI and EZ respectively.
Level equals a mean value of the maximum and minimum values of the wave of the upstream
airltiel ratio. The value of the comparison level is based on a characteristic of the upstream
aidtiel sensor. Symbols y and z (figs. 68b.d) denote an interval in which the correlation
function is calculated. The region I containing the space where a rectangular wave is not
generated is put equal to a fixed interval of the feedback coefficient of the air/fhel ratio.

The correlation function is then calculated byy and z as follows:

where,
.f, , .f2: functions of time, such as the input and output of a communication system,
r : a time delay parameter
1' : is a time interval

The correlation function @, is calculated as