Patent ID: 8141343

Claim:
An air-fuel ratio control apparatus comprising: an exhaust system including an exhaust channel with a main catalytic converter disposed in the exhaust channel, a bypass channel with a bypass catalytic converter disposed in the bypass channel, and a valve mechanism disposed between a branching section of the bypass channel and a merging section of the bypass channel on the upstream side of the main catalytic converter to close the exhaust channel to switch a pathway for exhaust gas from the exhaust channel to the bypass channel; a first sensor arranged to detect a property indicative of an air-fuel ratio of exhaust flowing in the bypass channel; a second sensor arranged to detect a property indicative of an air-fuel ratio of exhaust flowing in the exhaust channel at a point downstream of the valve mechanism; and a controller including a first air-fuel ratio control section configured to control an engine air-fuel ratio based on an output of the first sensor, a second air-fuel ratio control section configured to control the engine air-fuel ratio based on an output of the second sensor, and a control mode switching section configured to switch from control by the first air-fuel ratio control section to control by the second air-fuel ratio control section after a prescribed interval of time has elapsed from when the valve mechanism is switched from a closed state to an open state, the controller being further configured such that the prescribed interval of time is established based a time required for a temperature of the second sensor reach a prescribed temperature after the second sensor being cooled due to the valve mechanism being switched to the open state.