Exhaust gas purification system and controlling method thereof

A controlling method of an exhaust gas purification system to which a lean combustion engine is applied and a lean NOx trap (LNT) device, a diesel particulate matter filter (DPF) or an SDPF, and an SCR device are provided, the method comprising:

• detecting vehicle information and determining whether the vehicle information satisfies nitrogen oxide desorption condition of the LNT device;
• desorbing nitrogen oxide until the nitrogen oxide of the LNT reaches a predetermined reference amount through an engine rich combustion when the vehicle information satisfies the nitrogen oxide desorption condition of the LNT device;
• injecting urea to purify the nitrogen oxide after a first period after desorption of the nitrogen oxide of the LNT disposed upstream of the DPF or the SDPF is ended when the nitrogen oxide of the LNT reaches the predetermined reference amount;
• desorbing sulfide of the LNT through the engine rich combustion;
• and
• injecting urea to purify the nitrogen oxide after a second period after ending of desorption of the sulfide rich combustion of the LNT,
• wherein the first period is configured to be determined by engine driving condition, LNT temperature, SDPF catalyst temperature, engine temperature, amount of nitrogen oxide at a rear end portion of the LNT device, ammonia trap amount and relative trap rate of the SDPF, exhaust gas flow rate, and internal temperature of a urea injector,
• wherein the second period is configured to be determined by engine driving condition, LNT temperature, SDPF catalyst temperature, engine temperature, amount of nitrogen oxide at a rear end portion of the LNT device, ammonia trap amount and relative trap rate of the SDPF, exhaust gas flow rate, and internal temperature of a urea injector, and
• wherein the first period and second period are urea injection stop periods, so that abnormal increase of amount of urea supply is prevented to suppress ammonia slip from the SCR device by the urea injection stops.