Patent ID: 8336296

Claim:
An exhaust gas purification system for an internal combustion engine comprising: an NOx storage reduction catalyst provided in an exhaust passage of an internal combustion engine; a reducing agent addition unit provided in the exhaust passage upstream of said NOx storage reduction catalyst for adding reducing agent to exhaust gas; an exhaust gas air-fuel ratio control unit for controlling the air-fuel ratio of exhaust gas discharged from the internal combustion engine; an NOx reduction unit for reducing NOx stored in said NOx storage reduction catalyst by decreasing the air-fuel ratio of ambient atmosphere around said NOx storage reduction catalyst to a target air-fuel ratio; and a reducing agent evaporation determination unit for determining whether or not, if the reducing agent is added through said reducing agent addition unit, the reducing agent adhering to said NOx storage reduction catalyst can evaporate in said NOx storage reduction catalyst, wherein when reducing NOx stored in said NOx storage reduction catalyst, if a negative determination is made by said reducing agent evaporation determination unit, said NOx reduction unit decreases the air fuel ratio of ambient atmosphere around said NOx storage reduction catalyst to said target air-fuel ratio only by decreasing the air-fuel ratio of exhaust gas discharged from the internal combustion engine by said exhaust gas air-fuel ratio control unit, and if an affirmative determination is made by said reducing agent evaporation determination unit, said NOx reduction unit decreases the air fuel ratio of ambient atmosphere around said NOx storage reduction catalyst to said target air-fuel ratio either only by adding the reducing agent through said reducing agent addition unit, or by decreasing the air-fuel ratio of exhaust gas discharged from the internal combustion engine by said exhaust gas air-fuel ratio control unit and adding the reducing agent through said reducing agent addition unit, said reducing agent evaporation determination unit makes a negative determination if the temperature of said NOx storage reduction catalyst is lower than a specific temperature, and makes a positive determination if the temperature of said NOx storage reduction catalyst is equal to or higher than said specific temperature, and in the case where the engine rotational speed of the internal combustion engine at the time when NOx stored in said NOx storage reduction catalyst is to be reduced is high, said specific temperature is set to a value lower than that in the case where the engine rotational speed is low.