Patent attributes
An air-fuel ratio control system has an upstream catalyst, a downstream catalyst, a first sensor for detecting a first air-fuel ratio present upstream the upstream catalyst, a second sensor for detecting a second air-fuel ratio present between the catalysts and a third sensor for detecting a third air-fuel ratio present downstream the downstream catalyst. An ECU determines a target second air-fuel ratio based on the detected third air-fuel ratio, and a target first air-fuel ratio based on the target second air-fuel ratio and the detected second air-fuel ratio. The ECU feedback controls an air-fuel ratio of mixture based on the target first air-fuel ratio and the detected first air-fuel ratio. The target first air-fuel ratio is corrected based on the detected third air-fuel ratio so that a change in the target first air-fuel ratio becomes smaller in a predetermined range other than a normal range.
