Exhaust gas purification system for an internal combustion engine

An exhaust gas purification system for an internal combustion engine characterized by comprising:

• a particulate filter that is arranged on an exhaust passage of said internal combustion engine for trapping particulate matter in an exhaust gas;
• a catalyst having an oxidation function that is arranged on said exhaust passage at a location upstream of said particulate filter or supported on said particulate filter;
• a reducing agent supplying means for supplying a reducing agent to said catalyst;
• an exhaust gas throttle valve that is arranged on said exhaust passage;
• a filter regeneration control execution means that executes filter regeneration control for oxidizing and removing said particulate matter deposited in said particulate filter by raising the temperature of said particulate filter to a target temperature;
• and
• an oxygen concentration decreasing means that decreases the oxygen concentration of an ambient atmosphere around said particulate filter;
• wherein when the amount of intake air in said internal combustion engine decreases to a value within a range that is less than or equal to a first specified amount of intake air and greater than a second specified amount of intake air at the time of the execution of filter regeneration control by said filter regeneration control execution means, said oxygen concentration decreasing means decreases the oxygen concentration of the ambient atmosphere around said particulate filter to a target oxygen concentration by supplying an upper limit amount of reducing agent, which can be supplied within a range where the temperature of said particulate filter does not exceed a specified temperature, to said catalyst by said reducing agent supply means, and at the same time by throttling the flow rate of the exhaust gas by means of said exhaust gas throttle valve;
• whereas when the amount of intake air in said internal combustion engine decreases to a value less than or equal to said second specified amount of intake air during the execution of filter regeneration control by said filter regeneration control execution means, said oxygen concentration decreasing means decreases the oxygen concentration of the ambient atmosphere around said particulate filter to said target oxygen concentration by throttling the flow rate of the exhaust gas to a lower limit of the flow rate, which can be throttled within a range where pressure in said exhaust passage upstream of said exhaust gas throttle valve does not exceed a specified pressure, by means of said exhaust gas throttle valve, and at the same time by supplying the reducing agent to said catalyst by means of said reducing agent supply means.