Abnormality diagnosis system of internal combustion engine

An abnormality diagnosis system of an internal combustion engine comprising an exhaust purification catalyst which is arranged in an exhaust passage of the internal combustion engine and can store oxygen, and a downstream side air-fuel ratio sensor which is arranged at a downstream side of said exhaust purification catalyst in a direction of exhaust flow, wherein said system comprises a catalyst abnormality diagnosis system which uses the output air-fuel ratio of said downstream side air-fuel ratio sensor when making the air-fuel ratio of the exhaust gas flowing into said exhaust purification catalyst change between a rich air-fuel ratio which is richer than the stoichiometric air-fuel ratio and a lean air-fuel ratio which is leaner than the stoichiometric air-fuel ratio as the basis to diagnose said exhaust purification catalyst for abnormality, and

• wherein said catalyst abnormality diagnosis system calculates the maximum storable oxygen amount of said exhaust purification catalyst based on at least one of the amount of oxygen which is stored in said exhaust purification catalyst in an air-fuel ratio reversal time period where the output air-fuel ratio of said downstream side air-fuel ratio sensor changes from a rich judgment air-fuel ratio which is richer than the stoichiometric air-fuel ratio or less to a lean judgment air-fuel ratio which is leaner than the stoichiometric air-fuel ratio or more and the amount of oxygen which is released from said exhaust purification catalyst in an air-fuel ratio reversal time period where the output air-fuel ratio of said downstream side air-fuel ratio sensor changes from said lean judgment air-fuel ratio or more to less than said rich judgment air-fuel ratio or less, and performs first abnormality diagnosis wherein it judges that said exhaust purification catalyst has become abnormal when said calculated maximum storable oxygen amount is smaller than a lower limit storage amount, and
• wherein the system uses one of any of a cumulative oxygen excess/deficiency and a cumulative amount of intake air and time as a judgment parameter, and
• said catalyst abnormality diagnosis system performs second abnormality diagnosis wherein it judges that said exhaust purification catalyst has become abnormal even when said calculated maximum storable oxygen amount is a predetermined lower limit storage amount or more, if the ratio of the value of said judgment parameter in the time period where the stoichiometric air-fuel ratio judgment condition which shows that the output air-fuel ratio of said downstream side air-fuel ratio sensor is within a predetermined range near the stoichiometric air-fuel ratio compared with at least said two judgment air-fuel ratios is satisfied to the value of said judgment parameter in said air-fuel ratio reversal time period is a given reference value or less.