Purification System for Variable Post Injection in LP EGR and Control Method for the Same

A controlling method for a purification system for variable post injection in LP EGR, including a first flap that controls flowing of exhaust gas flowing to a NOx reduction apparatus and a second throttle flap that controls flowing of exhaust gas flowing to the bypass line, the control method comprising:

• determining regeneration timing based on a calculated NOx amount according to engine operation conditions and a measured NOx concentration in the exhaust gas by a NOx sensor;
• determining fuel injection patterns according to the NOx concentration, and performing a post injection according to a determined fuel injection pattern;
• determining whether the engine operation condition is changed during regeneration of the NOx storage catalyst;
• closing the second throttle flap and turning off a post injector if an engine operation speed is decreased during the regeneration of the NOx storage catalyst and increment of oxygen concentration in the exhaust gas is equal or larger than a predetermined value;
• determining whether HC slip amount is equal or larger than a predetermined value, modulating fuel injecting pattern of the post injector if the HC slip amount is equal or larger than the predetermined value and maintaining the regeneration of the NOx storage catalyst if the HC slip amount is less than the predetermined value when the engine operation speed is decreased during the regeneration of the NOx storage catalyst and the increment of the oxygen concentration in the exhaust gas is less than the predetermined value;
• determining the fuel injection pattern according to increment of the NOx when the engine operation speed is increased during the regeneration of the NOx storage catalyst;
• and
• determining whether closing regeneration of the NOx storage catalyst according to concentration of the NOx in the exhaust gas when the engine operation condition is not changed during the regeneration of the NOx storage catalyst.