METHOD AND SYSTEM FOR PURIFYING NOx IN INTERNAL COMBUSTION ENGINE

A method for purifying NOX in an internal combustion engine, comprising:

• dividing operating conditions in an internal combustion engine into an intermediate/high-load operating condition and a low-load operating condition before regeneration of a nitrogen oxide purification catalyst is performed;
• performing unconstrained condition catalyst regeneration in which, when the generation is performed in the intermediate/high-load operating condition, an air-fuel ratio of the engine is changed from a lean state to a rich state and the catalyst is regenerated by exhaust gas having a rich air-fuel ratio;
• and
• performing constraint condition catalyst regeneration in which, when the generation is performed in the low-load operating condition, an air-fuel ratio of exhaust gas discharged from the engine is alternated through injection of fuel from a secondary fuel injection device and the catalyst is regenerated through a chemical reaction between nitrogen oxides and hydrocarbon.