The abnormality diagnosis system comprises an air-fuel ratio control means which sets the target air-fuel ratio of exhaust gas to a first set air-fuel ratio set to a first side of a rich side or a lean side, then, when a downstream side output air-fuel ratio is at the first side, switches the target air-fuel ratio to a second set air-fuel ratio set to a second side at the opposite side from the first side. The abnormality diagnosis system calculates the time from when the target air-fuel ratio is switched to when the downstream side output air-fuel ratio starts to change toward the stoichiometric air-fuel ratio based on a differential value of the downstream side output air-fuel ratio and, when the calculated time is a predetermined time or more, judges that a dead time at the downstream side air-fuel ratio sensor is abnormal.

The internal combustion engine comprises an exhaust purification catalyst able to store oxygen, and a downstream side air-fuel ratio sensor arranged at a downstream side of the exhaust purification catalyst in a direction of exhaust flow. The control system performs feedback control of an amount of fuel fed to a combustion chamber of the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst becomes a target air-fuel ratio and performs learning control to correct a parameter relating to the feedback control based on an air-fuel ratio of exhaust gas detected by the downstream side air-fuel ratio sensor. The target air-fuel ratio is alternately switched between a rich set air-fuel ratio and a lean set air-fuel ratio leaner. When a condition for learning acceleration, which is satisfied when it is necessary to accelerate correction of the parameter by the learning control, is satisfied, a rich degree of the rich set air-fuel ratio is increased. Therefore, there is provided an internal combustion engine able to suitably change the speed of updating the learning value.

The internal combustion engine comprises an exhaust purification catalyst able to store oxygen, and a downstream side air-fuel ratio sensor arranged at a downstream side of the exhaust purification catalyst in a direction of exhaust flow. The control system performs feedback control of an amount of fuel fed to a combustion chamber of the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst becomes a target air-fuel ratio and performs learning control to correct a parameter relating to the feedback control based on an air-fuel ratio of exhaust gas detected by the downstream side air-fuel ratio sensor. The target air-fuel ratio is alternately switched between a rich set air-fuel ratio and a lean set air-fuel ratio leaner. When a condition for learning acceleration, which is satisfied when it is necessary to accelerate correction of the parameter by the learning control, is satisfied, a rich degree of the rich set air-fuel ratio is increased. Therefore, there is provided an internal combustion engine able to suitably change the speed of updating the learning value.

The abnormality diagnosis system comprises an air-fuel ratio control means which sets the target air-fuel ratio of exhaust gas to a first set air-fuel ratio set to a first side of a rich side or a lean side, then, when a downstream side output air-fuel ratio is at the first side, switches the target air-fuel ratio to a second set air-fuel ratio set to a second side at the opposite side from the first side. The abnormality diagnosis system calculates the time from when the target air-fuel ratio is switched to when the downstream side output air-fuel ratio starts to change toward the stoichiometric air-fuel ratio based on a differential value of the downstream side output air-fuel ratio and, when the calculated time is a predetermined time or more, judges that a dead time at the downstream side air-fuel ratio sensor is abnormal.