| 摘要 |
An upstream-side catalyst is arranged in an exhaust passage and a downstream-side catalyst is arranged in the exhaust passage downstream of the upstream-side catalyst. An air-fuel ratio sensor is attached for detecting the air-fuel ratio in the exhaust passage between the upstream-side catalyst and downstream-side catalyst. A representative upstream-side stored oxygen value OXU is calculated based on whether the control state of the engine is the "slight rich" control, "slight lean" control, fuel increase correction, or engine fuel cut and a representative downstream-side stored oxygen value OXD is calculated based on the engine control state and the output of the air-fuel ratio sensor. If the representative upstream-side stored oxygen value OXU or the representative downstream-side stored oxygen value OXD becomes smaller than the corresponding allowable lower limit LLU or LLD, "slight lean" control is performed, while if it becomes larger than the corresponding allowable upper limit ULU or ULD, "slight rich" control is performed. |
