PURPOSE: To perform recovery treatment to recover catalytic performance when a catalyst is temporarily deteriorated.
CONSTITUTION: There are provided a catalyst 1 for purifying exhaust gas in which palladium is mainly deposited as catalytic metal installed in an engine exhaust gas system, a deterioration degree detecting means 51 for detecting the degree of deterioration of the catalyst 1, an exhaust gas temp. detecting means 13 for detecting the temp. of exhaust gas flowing in the catalyst 1, a means 52 for judging if the time is ripe for the deterioration recovery treatment of the catalyst 1 according to the catalyst deterioration degree detected, and a means 53 for controlling the air/fuel ratio of exhaust gas to the deterioration recovery air/fuel ratio on the lean side rather than the theoretical air/fuel ratio to perform the deterioration recovery treatment of the catalyst 1 when the judged result is that the time is ripe for the deterioration recovery treatment and the detected exhaust gas temp. is not less than the prescribed one.
OTA TADAKI
TAYAMA AKIRA
ISOBE AKIO
