Exhaust gas purification device for an internal combustion engine

An exhaust gas purification device for an internal combustion engine comprising:

• a three-way catalyst disposed in the exhaust gas passage of an internal combustion engine;
• a NO.sub.X occluding and reducing catalyst disposed in the exhaust gas passage downstream of the three-way catalyst and absorbing NO.sub.X in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the NO.sub.X occluding and reducing catalyst is at a lean air-fuel ratio and releasing and reducing the absorbed NO.sub.X when the air-fuel ratio of the exhaust gas flowing into the NO.sub.X occluding and reducing catalyst is at a stoichiometric or a rich air-fuel ratio;
• a first air-fuel ratio sensor disposed in the exhaust gas passage at the position between the three-way catalyst and the NO.sub.X occluding and reducing catalyst;
• a second air-fuel ratio sensor disposed in the exhaust gas passage downstream of the NO.sub.X occluding and reducing catalyst;
• air-fuel ratio control means for switching the air-fuel ratio of the exhaust gas flowing into the three-way catalyst either from a rich air-fuel ratio to a lean air-fuel ratio or from a lean air-fuel ratio to a rich air-fuel ratio;
• and
• evaluating means for evaluating the abilities of both of the three-way catalyst and the NO.sub.X occluding and reducing catalyst when the air-fuel ratio control means switches the air-fuel ratio of the exhaust gas either from a rich air-fuel ratio to a lean air-fuel ratio or from a lean air-fuel ratio to a rich air-fuel ratio, wherein the evaluating means evaluates the ability of the three-way catalyst based on the output signal of the first air-fuel ratio sensor and evaluates the ability of the NO.sub.X occluding and reducing catalyst based on the output signals of both the first and the second air-fuel ratio sensors.