Page 227 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

General Motors Corp. 209

b) grouping each of the differentiated signals into one of a plurality of test
periods
c) determining a maximum differentiation signal for each of the plurality of test
periods
d) generating an average signal value as a predetermined hnction of the
determined maximum differentiation signals
3) carrying out a first stage of diagnosis by comparing the average signal value to a
predetermined first threshold
4) carrying out a second stage of diagnosis when the average value signal exceeds the
predetermined first threshold
5) indicating a faulty catalytic converter upon failure of the second stage of diagnosis

In DE19651559 (1997) the deterioration of the catalytic converter is determined by one of the
methods described in US5509267 (1996) and US543101 1 (1995). A difference between actual
engine parameters and reference engine parameters provides a measure of the efficiency of the
catalytic converter. Such parameters can be the ignition timing, the engine speed at set
throttling settings and the heating system of the catalytic converter at cold start-up conditions.
The engine is then controlled to take account of the deterioration of the catalytic converter.
The temperature of the catalytic converter is estimated by means of detected operating
parameters of the engine and it is adjusted to maintain the effectiveness of the catalytic
converter.

The method of EP0799984 (1997) comprises the following steps:
I) measuring the oxygen content of the exhaust gas upstream and downstream of the catalytic
converter by means of an upstream and a downstream oxygen sensor respectively
2) biasing during a monitoring period the air/fbel ratio of fluid passing into the engine from the
stoichiometric ratio to a steady rich state while controlling in a closed-loop manner the
performance of the engine on the basis of the upstream sensor signal
3) controlling the amount of rich bias in a manner as to avoid saturation of the sensors
4) generating lean transitions from the fbel rich condition
5) changing the duration of the lean excursions
6) determining the duration in which the signal of the downstream sensor is beyond a
threshold
7) determining the oxygen ion storage capacity of the converter from the upstream and
downstream sensor signals
8) determining the deterioration of the catalytic converter by evaluating the output signals of
the upstream and downstream sensors

The method can be better explained by means of fig. 93. Fig. 93a shows the response of the
upstream sensor to rich bias with long lean excursions. Low oxygen ion content exists in the
exhaust gas and both sensors produce a low oxygen output signal. A lean excursion will
increase the oxygen ion content in the exhaust gas, which will be immediately sensed by the
upstream sensor. The same will not normally apply to the downstream oxygen sensor since the