Patent ID: 6925797

Claim:
An apparatus for purifying an exhaust gas of an engine, said apparatus comprising: a catalyst for a NOx absorbing-and-reducing type disposed in an exhaust passage, for absorbing a NOx component in the exhaust gas under an over-oxygen atmosphere and for reductively emitting the absorbed NOx component according to a reduction of an oxygen concentration; a momentary NOx absorption amount estimation section for estimating a momentary amount of the NOx component absorbed in said catalyst on the basis of a unit time; an integrated NOx absorption amount estimation section of estimating an integrated amount of the NOx component absorbed in said catalyst according to the integration of momentary amounts estimated by the estimation section; a NOx emitting section for letting said catalyst to emit the NOx component when the integrated amount estimated by the estimation section is equal to or greater than a predetermined absorption amount; a momentary NOx absorption amount correcting section for correcting the momentary amount estimated by said momentary NOx absorption amount estimation section to a value smaller in adverse proportion to the increase in the integrated amount estimated by said integrated NOx absorption amount estimation section; a momentary NOx supply amount setting section for setting a momentary amount of the NOx component supplied on the basis of a unit time to the catalyst; and a momentary NOx absorbable amount setting section for setting a momentary amount of the NOx component that can be absorbed in the catalyst on the basis of a unit time, wherein said momentary NOx absorption amount estimation section determines smaller one of the values of the momentary amounts, which have been set by said setting section, to be a momentary NOx absorption amount.