Abnormality diagnosis apparatus for exhaust gas purification apparatus

An abnormality diagnosis apparatus for an exhaust gas purification apparatus of an internal combustion engine that operates in a lean burn mode, said exhaust gas purification apparatus including an NOx storage reduction catalyst provided in an exhaust passage of said internal combustion engine and a fuel addition valve provided in the exhaust passage upstream of said NOx storage reduction catalyst to add fuel to exhaust gas, comprising a controller comprising at least one processor configured to:

• determine an outflowing NOx quantity defined as the quantity of NOx flowing out of said NOx storage reduction catalyst;
• determine an NOx storage amount defined as the amount of NOx stored in said NOx storage reduction catalyst on the basis of said outflowing NOx quantity;
• perform a specific fuel addition process of adding fuel by said fuel addition valve repeatedly at a specific cycle to raise the temperature of said NOx storage reduction catalyst, the quantity of fuel added by each fuel addition in said specific fuel addition process and said specific cycle being set in such a way that NOx stored in said NOx storage reduction catalyst is desorbed and the average air-fuel ratio of the exhaust gas flowing into said NOx storage reduction catalyst is kept at a lean air-fuel ratio higher than the stoichiometric air-fuel ratio during the time in which said specific fuel addition process is being performed;
• and
• make a diagnosis as to whether said NOx storage reduction catalyst is normal or abnormal,
• wherein said controller is further configured to start said specific fuel addition process when said NOx storage reduction catalyst is in a specific start condition in which said NOx storage amount determined by the controller falls within a specific range and the temperature of said NOx storage reduction catalyst is equal to or lower than a specific temperature that is lower than a thermal desorption temperature defined as a temperature above which NOx can flow out of said NOx storage reduction catalyst when said specific fuel addition process is performed even if said NOx storage reduction catalyst is normal, and equal to or higher than the activation temperature of said NOx storage reduction catalyst, and
• diagnose said NOx storage reduction catalyst on the basis of said outflowing NOx quantity over a specific period in a period from when said specific fuel addition process is started to when the temperature of said NOx storage reduction catalyst reaches said thermal desorption temperature.