Page 212 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

194 Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters

downstream t

Time t

Fig. 87 (from DE19605103)

The method of DE19605103 (1996) comprises the following steps:
1) calculating an average value <).;,2 the output signal of the downstream sensor by means of
of
the formula:

where
(n - 1) corresponds to calculated values in a previous cycle,
V,,,,, is the present value of the output signal of the downstream sensor and
a is a filtering constant
2) variably setting reference fluctuation values (TH",THr.) according to the average value
ro20f the output signal of the downstream oxygen sensor (fig. 87). The following formulas
are used

TH, = c2 - AV

3) measuring the number of times that the output signal of the downstream sensor crosses the
reference fluctuation values (TY,,THr.) in a predetermined time interval At, and calculating
a fluctuation frequency f2 by dividing this number of crossing times with the predetermined
time interval At
4) determining the ratio of the fluctuation fiequency of the downstream sensor fi to the
fluctuation frequency f, of the upstream oxygen sensor
5) determining deterioration of the catalytic converter when the ratio fdf, becomes greater
than a predetermined threshold