| 摘要 |
A plurality of catalysts are installed in an exhaust pipe, air-fuel ratio sensors or oxygen sensors are installed upstream and downstream of each catalyst, and the air-fuel ratio of the exhaust gas is feedback controlled to a target air-fuel ratio based on the output of the air-fuel ratio sensor located upstream of the upstream catalyst. In this the exhaust gas is sufficiently purified with the upstream catalyst alone when the exhaust gas flow rate is small, the oxygen sensor located downstream of the upstream catalyst is used as the downstream sensor for setting a target air-fuel ratio. Furthermore, when the exhaust gas flow rate increases, the amount of exhaust gas components passing through without purification in the upstream catalyst is increased. Therefore, the downstream sensor used for setting the air-fuel ratio is switched to the oxygen sensor located downstream of the downstream catalyst. |
