Page 181 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Mazda Motor Corporation 163

applied to the engine aidfuel ratio to converge it to a stoichiometric one. The degradation of
the catalytic converter is then judged according to the output signal of the downstream sensor.

In the method of JP5312024 (1993) both temperature and oxygen content ofthe exhaust gases
are measured upstream and downstream of a lean NO, catalytic converter. The measurement
takes place within specified time intervals immediately after starting a lean stationary operation
of the engine. Multiple values of exhaust gas temperature and oxygen content differences are
calculated upstream and downstream of the converter. In case that the temperature difference
of the exhaust gases is lower than a threshold or the oxygen content difference is higher than a
specified value, it is judged that the lean NOx converter is deteriorated.

The methods of US5337555 (1994) and US5414995 (1995) first judge the deterioration
condition of the upstream sensor by forming the ratio of the reversal frequency of the output
signal of the upstream sensor (from rich to lean state and the opposite) to the reversal
frequency of the output signal of the downstream sensor. The feedback control is based on the
output of the upstream sensor in order to maintain a desired engine air/hel ratio. Further, the
feedback control parameter is corrected according to the deterioration of the upstream sensor.
It is then concluded that the catalytic converter is deteriorated when the ratio of reversal
frequencies is not equal to one during off-idling and when the ratio is smaller than a
predetermined threshold frequency ratio during on-idling, while the ratio is equal to one during
off-idling. Failure judgment of the upstream sensor is made when the feedback control
parameter is below a predetermined threshold value, which may be changed according to levels
of degradation of the catalytic converter.

The method of US5337558 (1994) first checks whether or not the engine runs with a feedback
control, whether or not the vehicle speed is within certain limits and whether or not a change
of rate of the vehicle speed is less than a predetermined rate. If all conditions are satisfied, then
the catalytic converter deterioration detection starts by counting the frequency of the signal
outputs of the upstream and downstream oxygen sensors. The output ratio of the reversal
frequency of the output signal of the upstream sensor (from lean aidfuel ratio to rich airlfuel
ratio and vice-versa) to the reversal frequency of the output signal of the downstream sensor is
calculated and it is compared to a predetermined value. If this ratio is lower than the
predetermined value, then the catalytic converter is considered as deteriorated.

The method takes into account possible deterioration of the oxygen sensors. Practically as
output ratio is used a frequency ratio of reversal of an output from the downstream oxygen
sensor with respect to a threshold value to an output from the upstream oxygen sensor with
respect to a threshold value. If at least one of the sensors has been deteriorated, the threshold
value of the deteriorated oxygen sensor is varied so as to correctly change the frequency ratio
of reversal, thereby avoiding an erroneous deterioration detection of the catalytic converter
due to deterioration of the oxygen sensor.