Page 228 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

210 Methods for Monitoring and Diagnosing the EWciency of Catalytic Converters

catalytic converter will absorb some or all of the excess oxygen ions, depending upon the
oxygen capacity of the converter and the duration of the lean excursions.

Fig. 93b shows the response of the upstream sensor to a rich bias with short excursions. The
troughs in the figure are much narrower than with long excursions since there will be less time
during which there is an excess of oxygen ions in the exhaust gas. This will in turn lead to
narrower troughs in the downstream sensor signal, so long as there is still breakthrough of
oxygen ions. Short excursions are preferred in this embodiment to reduce or eliminate
noticeable changes in engine performance.
Fig. 93c shows the response of the output signal of the downstream oxygen sensor for the case
of an efficient catalytic converter. The signal is uneven compared to the upstream sensor
signal, caused by oxygen ion absorption by the catalytic converter, and is also delayed relative
to the upstream sensor signal.
Fig. 93d shows the response of the output signal of the downstream oxygen sensor for the case
of a deteriorated catalytic converter. The signal is similar to the one of the upstream sensor
(fig. 93b) and is little delayed relative thereto. This is a result of low oxygen ion absorption by
the converter.
The time during which the downstream oxygen sensor passes through a threshold signal level
is detected. This time, when compared to the equivalent time for the upstream sensor is
directly related to the oxygen storage capacity of the converter.
The method is substantially unrelated to sensor response times, which can vary to give
inaccurate results.