Patent ID: 8263031

Claim:
A method for purifying exhaust gas from an engine that arranges in an exhaust passage of the engine a selective reducing catalyst and an oxidation catalyst upstream of the selective reducing catalyst, comprising: calculating an amount of NOx that flows into the selective reducing catalyst; setting an amount of an additive agent to be supplied to the selective reducing catalyst based on the calculated amount of the NOx that flows into the selective reducing catalyst; determining a degradation degree of the oxidation catalyst; determining whether a correction is necessary to be made to the amount of the additive agent to be supplied based on the determined degradation degree of the oxidation catalyst; correcting the amount of the additive agent to be supplied when the correction is determined to be necessary; and supplying the additive agent to the selective reducing catalyst in the corrected amount of the additive agent if the amount is determined to be corrected and in an uncorrected amount of the additive if the amount of the additive agent is determined not to be corrected.