CONTROL DEVICE OF INTERNAL COMBUSTION ENGINE

PROBLEM TO BE SOLVED: To avoid that a deteriorated catalyst is erroneously determined to be a non-deteriorated catalyst, in a diagnosis of the catalyst for the purification of exhaust emission which is attached in an exhaust passage of an internal combustion engine.SOLUTION: When determining a deterioration degree of a catalyst on the basis of a magnitude of a difference between a maximum value and a minimum value of an output signal of an air-fuel ratio sensor which is installed at a downstream side of the catalyst when performing control for forcibly vibrating an air-fuel ratio at an upstream side of the catalyst between a lean state and a rich state, a cycle of the vibration of the air-fuel ratio at the upstream side of the catalyst when performing the control is changed according to a length of a duration after the air-fuel ratio at the upstream side of the catalyst is operated to the lean state until the air-fuel ratio is switched to a state that the output signal of the air-fuel ratio sensor indicates the lean state in a state that the output signal of the air-fuel ratio sensor indicates the rich state, or a length of a duration after the air-fuel ratio at the upstream side of the catalyst is operated to the rich state until the air-fuel ratio is switched to a state that the output signal of the air-fuel ratio sensor indicates the rich state in a state that the output signal of the air-fuel ratio sensor indicates the lean state.SELECTED DRAWING: Figure 2 【課題】内燃機関の排気通路に装着される排気浄化用の触媒のダイアグノーシスにおいて、劣化している触媒を劣化していない触媒であると誤判定することを回避する。【解決手段】触媒の上流側の空燃比をリーンとリッチとの間で強制的に振動させる制御を実施したときの、触媒の下流側に設置された空燃比センサの出力信号の極大値と極小値との差分の大きさに基づいて触媒の劣化度合いを判定するにあたり、前記空燃比センサの出力信号がリッチを示している状態で触媒の上流側の空燃比をリーンに操作してから前記空燃比センサの出力信号がリーンを示す状態に切り替わるまでの所要時間の長さ、または、前記空燃比センサの出力信号がリーンを示している状態で触媒の上流側の空燃比をリッチに操作してから前記空燃比センサの出力信号がリッチを示す状態に切り替わるまでの所要時間の長さに応じて、前記制御を実施する際の触媒の上流側の空燃比の振動の周期を変更する。【選択図】図２