Method for detection of performance reduction of exhaust gas purification catalyst

#1. Field of the Invention

#The present invention relates to a method for detection of the performance reduction of an exhaust gas purification catalyst used in an automobile or the like.

#2. Description of Related Art

#FIG. 1 is a graph showing a decrease in generated heat amount E.sub.g, caused by the performance reduction of an exhaust gas purification catalyst.

#FIG. 2 is a drawing showing the heat balance seen in an exhaust gas purification catalyst.

#FIG. 3 is a sectional drawing showing an example of the temperature sensor used in the method of the present invention.

#In the present invention, the performance reduction of an exhaust gas purification catalyst can be detected by detecting a decrease in the heat amount E.sub.g generated from a unit volume of an exhaust gas in a unit time by the reaction of the exhaust gas catalyzed by the catalyst. Since the oxidation reaction of the combustible components present in an exhaust gas, catalyzed by an exhaust gas purification catalyst is an exothermic reaction, the above heat amount E.sub.g generated decreases when the catalyst shows a reduction in performance because the above reaction does not take place efficiently. Hence, by detecting the decrease in generated heat amount E.sub.g, the performance reduction of an exhaust gas purification catalyst can be detected.

#Decrease in generated heat amount E.sub.g, caused by the performance reduction of catalyst is shown in FIG. 1. In FIG. 1, the upper curve is for a fresh catalyst and the lower curve is for a used catalyst having a reduced performance. The ordinate and the abscissa each have a larger value as the former proceeds toward the right and the latter proceeds upward. As is clear from FIG. 1, the decrease in generated heat amount E.sub.g, caused by the performance reduction of catalyst is larger when the catalyst temperature is lower. Therefore, the performance reduction of catalyst can be detected more accurately by dividing, into appropriate temperature zones, a predetermined temperature range taken by an exhaust gas purification catalyst during the catalyst temperature increase after engine start, measuring a heat amount E.sub.g generated from a unit volume of an exhaust gas in a unit time by the reaction of the exhaust gas catalyzed by the catalyst, in each temperature zone, cumulating the heat amounts E.sub.g, and calculating a reduction in the cumulative heat amount obtained.

#It is preferable that the lower limit of the above predetermined temperature range is 100.degree.-250.degree. C., preferably 150.degree.-250.degree. C., more preferably 150.degree.-200.degree. C. and that the upper limit is 200.degree.-400.degree. C., preferably 250.degree.-400.degree. C., more preferably 250.degree.-350.degree. C. The reasons are as follows. When the catalyst temperature is lower than 200.degree. C., the catalyst is not activated sufficiently and the accurate detection of the performance reduction of catalyst is impossible. When the catalyst temperature is higher than 300.degree. C., there is no big difference in generated heat amount E.sub.g between catalyst of reduced performance and fresh catalyst. Further, the hydrocarbon concentration in exhaust gas is high soon after engine start, that is, when the catalyst temperature is in the above range; therefore, the catalyst efficiency in this temperature range is most important.