Page 83 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Ford Motor Co. - Ford France SA - Ford Werke AG - Ford Motor Co. Ltd. 65

In EP0466311 (1992) a forced modulation (from rich to lean and vice versa) of frequency
and/or amplitude of the engine air/fbel ratio takes place. The diagnosis is initiated when the
engine is operating in closed-loop feedback mode and the engine is in steady state speed
condition. The frequency and the amplitude of the modulation must hlfill certain conditions.
During this modulation the catalytic converter is considered as degraded, when it is determined
that there is an absence of substantial change between events in the downstream sensed air/fbel
characteristic. This change can be
I) a change in degree of frequency andlor amplitude of downstream A/F characteristic
or
2) a change in time for the downstream sensor to return to cyclical operation.

In Fig. 30 an example of an artificial amplitude modulation is presented. The left part
corresponds to a good catalyst, the middle part corresponds to a good catalyst with low
oxygen storage while the right part corresponds to a degraded catalyst. At time instant TO, the
CPU imposes a A/F modulation signal and determines whether or not the downstream sensor
output exceeds a particular threshold E during a predetermined time interval AT (timed
window). In the right hand portion of fig. 30 the voltage of the downstream sensor passes over
threshold E before the expiration of AT, so the catalytic converter is considered as degraded.

good catalyst with
high 92 storage
limit +test Tilimit limit I - test I limit limit 1- test +imit
cycle interva cycle cycle I intemi cycle cycle interva cycle
airfluel
modulation 1
signal

engine
airfiue114.7

threshold
detector
output
catalyst catalyst

*Tn-
TO window) To window)

Fig. 30 (from EP04663 11)