Page 92 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

74 Methods for Monitoring and Diagnosing the Eficiency of Catalytic Converters

of the catalytic converter are used. The measured inducted flow of air to the engine (hrther
called simply “air flow”) is used as a criterion of the catalytic converter degradation. The
method consists of the following steps:
I) adjusting the engine aidfuel ratio in response to the feedback signal derived from the
upstream EGO output
2) accumulating one of a plurality of air flow values upon each transition in output states of the
downstream EGO sensor output, each of the air flow values being related to one of a
plurality of inducted air flow ranges inducted into the engine and said air flow value which is
accumulated upon the downstream sensor output transition is related to the air flow range
in which the downstream sensor output transition occurred
3) averaging the accumulated air flow values over a test period e.g. by dividing said air flow
values accumulated over said test period by an accumulation of said downstream sensor
transitions over said test period
4) and providing an indication of converter degradation when the average falls below a pre-
selected average.

The test period is completed when the engine has operated for at least a minimum pre-selected
number of upstream sensor output transitions per air flow range. The transitions in the
upstream and downstream EGO sensors are generated by comparing each sensor output to a
reference.

In EP0619420 (1994) a ratio of the transition frequency of the downstream EGO sensor over
the transition frequency of the upstream EGO defines the criterion of efficiency of the catalytic
converter. The method consists of the following steps:
1) defining a number of inducted engine air flow ranges
2) separately counting the number of transitions from lean aidfuel ratio to rich and vice versa
of the upstream EGO sensor in each of the inducted air flow ranges
3) limiting each of the separate upstream sensor counts to one of a corresponding plurality of
pre-selected maximum values for each of the air flow ranges
4) generating a separate test period for each of the flow ranges when the upstream sensor
reaches the corresponding maximum value of transitions
5) counting the corresponding transitions of the downstream sensor output for each separate
test period
6) summing all transition counts of the downstream sensor output for air flow ranges for at
least the test period corresponding to each of the flow ranges
7) summing all transition counts of the upstream sensor output for air flow ranges for at least
the test period corresponding to each of the flow ranges
8) forming the ratio of the sum of step 6 over the sum of step 7
9) concluding that the catalytic converter is degraded when this ratio exceeds a predetermined
ratio

The method also includes the steps of controlling fuel delivered to the engine in response to a
feedback of the upstream sensor output and trimming this output in response to a trim signal
derived from the downstream sensor output.