Patent ID: 8607545

Claim:
An air-fuel ratio control device for controlling the air-libel 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 for apportioning the exhaust flow between the exhaust passage and the bypass passage; and a control circuit programmed 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 a first air-fuel ratio sensor disposed in the bypass passage; during an open period of the valve mechanism, the air-fuel ratio is controlled based on a signal from a second air-fuel ratio sensor disposed in the exhaust passage downstream of the valve mechanism; and during a predetermined period during which the valve mechanism is in a partially open state before moving to the open state, the air-fuel ratio is controlled based on a signal from the first air-fuel ratio sensor, the predetermined period being the time it would take for moisture in gas lingering upstream of the valve mechanism to wet and crack an element of the second air-fuel ratio sensor after the air-fuel mechanism switches from the closed state to the open state.