CATALYST DEGRADATION DETERMINATION SYSTEM

摘要

A catalyst deterioration determination system for determining deterioration of an exhaust gas purification catalyst having a capability of storing oxygen determines deterioration of the exhaust gas purification catalyst on the basis of its oxygen storage capacity. The system includes an oxygen concentration sensor provided downstream of the catalyst and having characteristics by which as rich gas components in exhaust gas increase, the oxygen concentration sensor outputs a measurement value of the oxygen concentration corresponding to a richer air-fuel ratio. When determining deterioration, the system performs a rich shift mode in which the exhaust gas air-fuel ratio is shifted from a lean air-fuel ratio to a rich air-fuel ratio and a lean shift mode in which the exhaust gas air-fuel ratio is shifted from a rich air-fuel ratio to a lean air-fuel ratio by exhaust gas air-fuel ratio control means, based on the measurement value of the oxygen concentration sensor. The rate of change of the exhaust gas air-fuel ratio in at least the lean shift mode among the rich shift mode and the lean shift mode is limited to a predetermined rate of change or lower, and the rate of change of the exhaust gas air-fuel ratio in the rich shift mode is set higher than the rate of change of the exhaust gas air furl ratio in the lean shift mode.