Patent ID: 7716918

Claim:
In a regeneration control of purification system for nitrogen oxides in exhaust gas, having a catalyst unit carrying a nitrogen oxides occlusion-reduction type catalyst, which occludes nitrogen oxides when an air/fuel ratio in exhaust gas is in a lean condition, and releases and reduces the occluded nitrogen oxides when an air/fuel ratio in exhaust gas is in a rich condition, and performing the regeneration control to restore a capacity of the catalyst unit for occluding nitrogen oxides when it is determined that an estimated amount of nitrogen oxides occluded in the catalyst unit reaches a predetermined determination value: a method of exhaust gas purification comprising: performing a first rich control by adding an amount of reductant meeting an amount of oxygen released from the catalyst unit in an initial stage of the regeneration control to an amount of reductant supplied in order to reduce nitrogen oxides released from the catalyst unit so that an air/fuel ratio in the exhaust gas becomes a first target air/fuel ratio determined on the basis of the added amount of the reductant; determining completion of the oxygen release on the basis of an oxygen concentration in a downstream of the catalyst unit in said first rich control; and performing a second rich control such that an amount of reductant supplied in order to reduce the nitrogen oxides released from a nitrogen oxides occlusion-reduction type catalyst is calculated and an air/fuel ratio in the exhaust gas becomes a second target air/fuel ratio determined on the basis of the calculated amount of the reductant, when the oxygen release is determined to have been completed.