Exhaust emission control system for multiple cylinder internal combustion engine

摘要

An exhaust emission control system for an internal combustion engine includes an exhaust air-fuel ratio changing device switching the exhaust air-fuel ratio of exhaust gas flowing into one of an upstream exhaust passages and the exhaust air-fuel ratio of exhaust gas flowing into the other one of the upstream exhaust passages to a rich air-fuel ratio and to a lean air-fuel ratio alternately on a predetermined cycle longer than each of combustion cycles of the internal combustion engine, to thereby determine whether upstream catalytic converters have deteriorated or not according to a change in the exhaust air-fuel ratio outputted by upstream exhaust sensors provided downstream of the upstream catalytic converters. Therefore, it is possible to surely determine whether catalytic converters have deteriorated even slightly or not while ensuring a high exhaust emission control performance.