Page 139 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Nippon Denso Co. 121

factor. This can be achieved by adding a constant delay time interval to the normal feedback
control period. The purification factor can then be judged by detecting
a) the amplitude of the downstream sensor and checking whether it
exceeds a predetermined threshold value or
b) the phase difference between the output signals of the upstream and
downstream sensor
2) the period is changed by successive small amounts, and it is detected when the amplitude
AVz of the output signal of the downstream oxygen sensor reaches a predetermined
threshold voltage, The value of feedback period at which that threshold voltage is reached is
then measured, and the purification factor of the catalytic converter can then be obtained
based on a known relationship between values of feedback period at which the threshold
voltage is reached.

Fig. 58 shows said relationship between the feedback period and the output signal amplitude
AVZ of the downstream sensor, with the purification factor of the catalytic converter as a
parameter. By using that characteristic, it becomes possible to measure the purification factor
of the catalytic converter, based on the specific feedback period at which the amplitude AV2 is
found to exceed the threshold value, which in this example is 0.7 Volts. The relationship
between values of feedback period and values of the purification factor is stored beforehand, in
a memory map. If the detected purification factor is lower than a reference value the catalytic
converter is considered as degraded.

when purification when purification
factor is medium

feedback period

Fig. 58 (from EP0475 177)