Patent ID: 8402753

Claim:
An exhaust gas purification system for an internal combustion engine comprising: an exhaust passage; an NOx storage reduction catalyst that is arranged in the exhaust passage of the internal combustion engine; an O 2 concentration lowering device that lowers an O 2 concentration of an exhaust gas by lowering an O 2 concentration of a mixture in said internal combustion engine; a fuel addition valve that supplies a reducing agent to said NOx storage reduction catalyst; and an electronic control unit that includes program logic, which when executed: estimates an amount of NOx occluded in said NOx storage reduction catalyst; determines whether an execution condition for NOx reduction control to cause the NOx occluded in said NOx storage reduction catalyst to be released and at the same time reduced holds; and executes NOx reduction control when it is determined that the execution condition for NOx reduction control holds; wherein in case where the amount of NOx occluded in said NOx storage reduction catalyst at the time when the execution condition for NOx reduction control holds is more than a predetermined amount of occlusion, said electronic control unit executes NOx reduction control by lowering the O 2 concentration of the exhaust gas by means of said O 2 concentration lowering device and at the same time supplying the reducing agent to said NOx storage reduction catalyst by means of said fuel addition valve, and in case where the amount of NOx occluded in said NOx storage reduction catalyst at the time when the execution condition for NOx reduction control holds is equal to or less than said predetermined amount of occlusion, said electronic control unit executes NOx reduction control by supplying the reducing agent to said NOx storage reduction catalyst by means of said fuel addition valve without lowering the O 2 concentration of the exhaust gas by means of said O 2 concentration lowering device.