Catalyst abnormality diagnosis apparatus

A catalyst abnormality diagnosis apparatus that diagnoses abnormality of a catalyst arranged in an exhaust passage in an internal combustion engine, comprising:

• a post-catalyst sensor that detects an exhaust air-fuel ratio in an area located downstream of the catalyst;
• active air-fuel ratio control unit for alternately controlling an air-fuel ratio in an area located upstream of the catalyst between a lean side and a rich side;
• measurement unit for measuring an amount of oxygen released by the catalyst during rich control of the air-fuel ratio;
• and
• determination unit for determining whether the catalyst is normal or abnormal, and
• in that when an output from the post-catalyst sensor reaches a predetermined rich threshold during change to a rich side, the active air-fuel ratio control unit simultaneously switches the air-fuel ratio control from the rich control to the lean control, and the measurement unit ends measuring the amount of oxygen,
• the rich threshold is set to a value leaner than a reference rich determination value set richer than the stoichiometry equivalent value, and is set to a value leaner than the stoichiometry equivalent value, and
• the determination unit determines whether the catalyst is normal or abnormal based on the amount of oxygen measured until the output from the post-catalyst sensor reaches the rich threshold and behavior of the output from the post-catalyst sensor observed after the output from the post-catalyst sensor reaches the rich threshold.

A catalyst abnormality diagnosis apparatus that diagnoses abnormality of a catalyst arranged in an exhaust passage in an internal combustion engine, by comprising:

• a post-catalyst sensor that detects an exhaust air-fuel ratio in an area located downstream of the catalyst;
• active air-fuel ratio control unit for alternately controlling an air-fuel ratio in an area located upstream of the catalyst between a lean side and a rich side;
• measurement unit for measuring an amount of oxygen absorbed by the catalyst during lean control of the air-fuel ratio;
• and
• determination unit for determining whether the catalyst is normal or abnormal, and
• in that when an output from the post-catalyst sensor reaches a predetermined lean threshold during change to a lean side, the active air-fuel ratio control unit simultaneously switches the air-fuel ratio control from the lean control to the rich control, and the measurement unit ends measuring the amount of oxygen,
• the lean threshold is set to a value richer than a reference lean determination value set leaner than a stoichiometry equivalent value for the output from the post-catalyst sensor, and is set to a value richer than the stoichiometry equivalent value, and
• the determination unit determines whether the catalyst is normal or abnormal based on the amount of oxygen measured until the output from the post-catalyst sensor reaches the lean threshold and behavior of the output from the post-catalyst sensor observed after the output from the post-catalyst sensor reaches the lean threshold.