Nitrogen oxide purifying system and method an for internal combustion engine

摘要

An exhaust gas purifying apparatus for an internal combustion engine having an exhaust system. The exhaust gas purifying apparatus includes a NOx purifying device provided in the exhaust system for purifying NOx in exhaust gases, and a temperature sensor for detecting a temperature of the NOx purifying device. The NOx purifying device has NOx absorbing capacity and generates ammonia and retains the generated ammonia when the air-fuel ratio is set to a value on the rich side with respect to the stoichiometric ratio. The NOx purifying device purifies NOx with the retained ammonia when the air-fuel ratio is set to a value on a lean side with respect to the stoichiometric ratio. The air-fuel ratio is enriched to a value on the rich side with respect to the stoichiometric ratio so as to increase an amount of reducing components in the exhaust gases flowing into the NOx purifying device.