Exhaust purification system for internal combustion engine

Abstract

An exhaust purification system is provided that can continuously maintain a NOx purification rate to be high while suppressing the occurrence of ammonia slip. The exhaust purification system includes a slip determination portionthat determines the occurrence of ammonia slip based on an output value NH3of an ammonia sensor. A reference injection amount calculating portioncalculates a reference injection amount Gbased on a parameter correlating to an operating state of an engine. A switching injection amount calculating portiondecreases in amount a urea injection amount Gby setting a switching injection amount Gto a negative value in response to having determined that ammonia slip has occurred, and increases in amount the injection amount Gby setting the switching injection amount Gto a positive value in response to a storage amount estimated value SThaving fallen below a predetermined switch storage amount ST.