Patent ID: 8875497

Claim:
An exhaust purification system for an internal combustion engine, the system comprising: a NOx selective reduction catalyst provided in an exhaust channel of the internal combustion engine, and having a function of selectively reducing NOx in exhaust under the presence of a reducing agent, and adsorbing NO 2 in the exhaust; a reducing agent supply device provided on an upstream side of the NOx selective reduction catalyst, and supplying a reducing agent or a precursor thereof into the exhaust channel on the upstream side of the NOx selective reduction catalyst; a NO 2 —NOx ratio adjustment mechanism provided on the upstream side of the NOx selective reduction catalyst, and causing a NO 2 —NOx ratio corresponding to a ratio of NO 2 to NOx in exhaust flowing into the NOx selective reduction catalyst to change; and a perturbation control unit configured to execute NO 2 —NOx ratio perturbation control, which is control that alternately executes NO 2 increase control to cause the NO 2 —NOx ratio to be greater than a reference value in a range of at least 0.4 to less than 0.7 by way of the NO 2 —NOx ratio adjustment mechanism, and NO 2 decrease control to cause the NO 2 —NOx ratio to be less than the reference value by way of the NO 2 —NOx ratio adjustment mechanism, wherein the NO 2 —NOx ratio perturbation control is executed so that a NO 2 balance of the NOx selective reduction catalyst in a predetermined time period, with NO 2 adsorption being positive and NO 2 release being negative, is a target value.