Page 303 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Toyota Motor Co. Ltd. 285

difference between the inlet exhaust gas and outlet exhaust gas of the catalytic converter is also
detected by means of the upstream and downstream temperature sensors. A degradation extent
of the catalytic converter is then detected based on the difference between the detected current
temperature and the reference temperature difference.

The degradation of the catalytic converter can be also based on the comparison of the actually
measured NO, content of the exhaust gases downstream of the catalytic converter and the
stored NO, value for a certain accumulated running distance of the vehicle.

Fig. 128 shows the NO, purification of the converter vs. the temperature of the converter. The
points TI and Tz of fig. 128 correspond to a lower temperature limit and to an upper
temperature limit respectively of a temperature range, where the lean NO, converter can work
with a high NO, purification rate for a certain accumulated running distance. In fig. 128, TI
and Tz correspond to a lean NO, converter at an initial state (no running distance).

When it is determined, based on the accumulated running distance, that the lean NO, catalytic
converter has been degraded, the HC amount is increased by the means for increasing the HC
amount in accordance with the degradation extent of the lean NO, catalytic converter . The
more degraded the catalytic converter is, the less the NO, purification rate of the catalytic
converter is, and the more the HC amount supplied to the catalytic converter is, the more the
NO, purification rate of the catalytic converter is.

Therefore, even if the NO, purification characteristic shifts from a to b, and from b to c in fig.
128, due to degradation of the catalytic converter, the characteristic line is raised, as shown by
a broken line in fig. 128, by increasing the amount of HC supplied to the catalytic converter.

Further, if the catalytic converter temperature is changed to a higher side, together with the
above-described HC amount increase, the NO, purification rate of the lean NO, catalytic
converter is hrther increased. More particularly, even if the lean NO, catalytic converter is
degraded, accompanied by a shift of the NO, purification rate peak temperature, to a higher
temperature side, the catalytic converter temperature also is changed to the higher side
corresponding to the degradation extent of the catalytic converter, so that the lean NO,
catalytic converter is always used at or near its NO, purification rate peak temperature and the
NO, purification ability of the lean NO, catalytic converter can be extended for a long period
of time.