Exhaust emission control device of internal combustion engine

Abstract

An exhaust emission control device of an internal combustion engine, which purifies exhaust gas exhausted from the internal combustion engine, comprises: a catalyst device composed of a three way catalyst for purifying harmful substance in the exhaust gas when an exhaust air-fuel ratio is substantially stoichiometrical and an NOx catalyst having a function of absorbing NOx in the exhaust gas when the air-fuel ratio is closer to a lean air-fuel ratio than to the stoichiometrical air-fuel ratio, the catalyst device being provided in an exhaust passage of the internal combustion engine; catalyst deterioration determination means for determining deterioration state of the catalyst device, the deterioration resulting from at least temperature; and control means for deteriorating exhaust gas components flowing into the catalyst device much more than exhaust gas components flowing into the catalyst device when an air-fuel ratio of a mixture supplied to the internal combustion engine is substantially stoichiometrical, if the catalyst deterioration determination means determines that the catalyst device is in a predetermined deterioration state. The deterioration of an NOx absorbing function of the NOx catalyst results from at least the destabilization of absorbing material in the NOx catalyst, which is caused by the rise in the temperature. Thus, if the catalyst deterioration determination means determines that the catalyst device is in the predetermined deterioration state, the exhaust gas components flowing into the catalyst device are deteriorated much more than the exhaust gas components flowing into the catalyst device when the air-fuel ratio of the mixture supplied to the internal combustion engine is substantially stoichiometrical. Therefore, CO, H2, NOx, O2, THC and the like are supplied to the absorbing material to thereby stabilize the absorbing material as carbonate, nitrite or acid oxide. This prevents the heat deterioration. This extends a heat-resisting life of the catalyst device, and prevents the deterioration of an exhaust gas characteristic and the increase in cost.