CATALYST DETERIORATION DIAGNOSIS DEVICE FOR ENGINE

摘要

PROBLEM TO BE SOLVED: To accurately diagnose deterioration in NOx absorbing performance by diagnosing deterioration of NOx absorbing performance from correlation between responsive delay of change from lean to rich across NOx absorbing catalyst and a NOx absorbed quantity up to the change to be rich. SOLUTION: At judging realization of a lean combustion condition, a NOx absorbing quantity detecting means B estimatedly computes the NOx absorbed quantity of NOx absorbing catalyst from the operating condition of an engine. An air-fuel ratio increasing means C inverts the air-fuel ratio from lean to rich, when the estimatedly computed NOx absorbed quantity reaches the saturated quantity, and a fuel injection quantity is corrected to increase for performing separation and reduction of the absorbed NOx. Hereat a delay time measuring means D measures a time since the exhaust air-fuel ratio before the catalyst is detected to be inverted to rich by an oxygen sensor on the upper stream side, until delayed inversion to rich after the catalyst is detected by an oxygen sensor on the down stream side. A catalyst deterioration detecting means E judges deterioration of NOx absorbing performance, when delay of a time corresponding to the estimated NOx absorbed quantity is not in the measured delay time.