Patent ID: 7395658

Claim:
A method of estimating an amount of NOx adsorption contained in a NOx trap catalyst disposed in an exhaust passage of an engine for adsorbing NOx in exhaust gases during a lean operation mode and releasing and reducing the adsorbed NOx during a rich operation mode in which unburned fuel is delivered to the NOx trap catalyst, comprising: a time measuring step of measuring a period in which a rich spike is occurring during the rich operation mode; an operation step of computing an amount of reference emission from the NOx trap catalyst during the measured period; and an estimating step of estimating an amount of adsorption in the NOx trap catalyst based on the computed reference emission amount, a determining step of determining whether the rich spike is occurring based on the excess-air factor downstream of the NOx trap catalyst, wherein in said operation step, the reference emission amount from the catalyst is computed using a map or a polynomial equation that reflects emissions from the NOx trap catalyst based on an exhaust gas temperature and an exhaust gas flow rate of the exhaust passage and an excess-air factor upstream of the NOx trap catalyst, and wherein the operating step corrects the reference emission amount based on the map or the polynomial equation according to the measured time.