CATALYST MONITORING SYSTEM AND METHOD

摘要

A total amount of NO_X (NO_XIN) that flows into an NO_X catalyst (18) between the time that air-fuel ratio control ends and the time that the air-fuel ratio control starts the next time, is obtained. A total stored amount (TSA), which is the sum of an oxygen stored amount (OSA) and an NO_X stored amount (NSA) in the NO_X catalyst (18) before the air-fuel ratio control started, is calculated based on the amount of reducing agent that has flowed into the NO_X catalyst (18) during the air-fuel ratio control. The oxygen stored amount (OSA) is calculated by extrapolating a relationship between the total amount of N0_X(NO_XIN) and the total stored amount (TSA). The relationship is established beforehand by executing the air-fuel ratio control with at least two levels.