| 摘要 |
PROBLEM TO BE SOLVED: To correctly diagnose degradation of a catalyst by providing with means for detecting a purifying rate in the catalyst arranged in an exhaust passage, and diagnosing degradation of the catalyst on the basis of a purifying rate judging value according to a catalyst temperature detected by catalyst temperature detecting means and a detected purifying rate. SOLUTION: In the case where degradation of a Nox catalyst 12 arranged in an exhaust passage 20 is diagnosed, a catalyst temperature representative value is calculated by averaging an upstream end catalyst temperature and a downstream end catalyst temperature detected by upstream and downstream side temperature sensors 13, 14. A Nox purifying rate is calculated from upstream side NOx concentration and downstream side NOx concentration detected by upstream and downstream side NOx concentration sensors 21, 22. A referential NOx purifying rate corresponding to the catalyst temperature representative value is read in. A purifying rate judging value is calculated by multiplying a purifying rate correcting coefficient of 1 or less by the referential NOx purifying rate, and the purifying rate judging value and NOx purifying rate are compared with each other. And when the value of the NOx purifying rate is lower than that of the purifying rate judging value, degradation of the NOx catalyst 12 is diagnosed. |
