Abstract: A method is provided for purifying exhaust gas from an engine having an exhaust passage with a selective reducing catalyst and an oxidation catalyst upstream of the selective reducing catalyst. The method calculates an amount of NOx that flows into the selective reducing catalyst; sets an amount of an additive agent to be supplied to the selective reducing catalyst based on the calculated amount of NOx; determines a degradation degree of the oxidation catalyst; determines whether a correction to the amount of the additive agent is necessary based on the determined degradation degree of the oxidation catalyst; corrects the amount of the additive agent to be supplied when the correction is necessary; and supplies the additive agent to the selective reducing catalyst in the corrected amount if the amount is determined to be corrected and in the uncorrected amount if the amount is determined not to be corrected.

Abstract: A method of controlling a system having an internal combustion engine, a filter in an exhaust passage, a reduction catalyst downstream of said filter capable of reducing nitrate oxide contained in the exhaust gas with reductant supplied thereto is provided. The method may include, in a first mode, combusting a first amount of particulate matter trapped in said filter and supplying a first amount of reductant to said reduction catalyst when an amount of particulate matter trapped in said filter is less than a predetermined trapping amount (?). The method may further include, in a second mode, combusting a second amount of particulate matter that is greater than said first amount of particulate matter and supplying a second amount of reductant to said reduction catalyst when an amount of particulate matter trapped in said filter is equal to or greater than said predetermined trapping amount (?).

Abstract: A method is provided for purifying exhaust gas from an engine having an exhaust passage with a selective reducing catalyst and an oxidation catalyst upstream of the selective reducing catalyst. The method calculates an amount of NOx that flows into the selective reducing catalyst; sets an amount of an additive agent to be supplied to the selective reducing catalyst based on the calculated amount of NOx; determines a degradation degree of the oxidation catalyst; determines whether a correction to the amount of the additive agent is necessary based on the determined degradation degree of the oxidation catalyst; corrects the amount of the additive agent to be supplied when the correction is necessary; and supplies the additive agent to the selective reducing catalyst in the corrected amount if the amount is determined to be corrected and in the uncorrected amount if the amount is determined not to be corrected.

Abstract: A method of controlling a system having an internal combustion engine, a filter in an exhaust passage, a reduction catalyst downstream of said filter capable of reducing nitrate oxide contained in the exhaust gas with reductant supplied thereto is provided. The method may include, in a first mode, combusting a first amount of particulate matter trapped in said filter and supplying a first amount of reductant to said reduction catalyst when an amount of particulate matter trapped in said filter is less than a predetermined trapping amount (?). The method may further include, in a second mode, combusting a second amount of particulate matter that is greater than said first amount of particulate matter and supplying a second amount of reductant to said reduction catalyst when an amount of particulate matter trapped in said filter is equal to or greater than said predetermined trapping amount (?).