Patent ID: 8176728

Claim:
An air-fuel ratio control device for controlling an air-fuel ratio of an engine, comprising: an exhaust passage for conveying exhaust flow in a downstream direction away from the engine; a bypass passage diverging from the exhaust passage at an upstream junction and rejoining the exhaust passage at a downstream junction located downstream from the upstream junction; a bypass catalytic converter disposed in the bypass passage; a main catalytic converter disposed in the exhaust passage downstream from the downstream junction; a valve mechanism disposed in the exhaust passage between the upstream junction and the downstream junction, the valve mechanism moving between a closed state and an open state; and a first air-fuel ratio sensor disposed in the bypass passage; a second air-fuel ratio sensor disposed in the exhaust passage downstream of the valve mechanism; a controller configured to control the air-fuel ratio such that: during a closed period of the valve mechanism, the air-fuel ratio is controlled based on a signal from the first air-fuel ratio sensor; during an open period of the valve mechanism, the air-fuel ratio is controlled based on one of a signal from the first air-fuel ratio sensor and a signal from the second air-fuel ratio sensor; and when the air-fuel ratio is controlled based on a signal from the first air-fuel ratio sensor during the open period of the valve mechanism, the controller makes an amount of adjustment less than would be made based on the same signal from the first air-fuel ratio sensor during the closed period of the valve mechanism.