Patent ID: 8266896

Claim:
An exhaust purification system for an internal combustion engine comprising: a first exhaust purification device that is provided in an exhaust passage of an internal combustion engine and purifies exhaust gas passing through said exhaust passage; a second exhaust purification device that is provided on a downstream side of said first exhaust purification device in said exhaust passage and purifies exhaust gas passing through the downstream side of said first exhaust purification device in said exhaust passage; a bypass passage that branches from an upstream side of said first exhaust purification device in said exhaust passage and merges with said exhaust passage at a portion on the downstream side of said first exhaust purification device and an upstream side of said second exhaust purification device in said exhaust passage to cause the exhaust gas passing through said bypass passage to bypass said first exhaust purification device; an exhaust control valve that is provided between a branch portion toward said bypass passage and said first exhaust purification device in said exhaust passage or in said bypass passage and that is capable of switching between passing and blocking of the exhaust gas; a reducer injector that supplies a reducer to the exhaust gas passing through said exhaust passage on an upstream side of said branch portion; and an electronic control unit configured to control the exhaust control valve and the reducer injector such that a reducer concentration in the exhaust gas flowing into said first exhaust purification device and a reducer concentration in the exhaust gas passing through said bypass passage are controlled by supplying the reducer intermittently to the exhaust gas from said reducer injector to cause a periodic change in the reducer concentration in the exhaust gas passing through said exhaust passage and opening/closing said exhaust control valve periodically to control an opening/closing timing of said exhaust control valve based on the periodic change in the reducer concentration.