Exhaust gas purifying system

An exhaust gas purifying method for an internal combustion engine having an exhaust gas purifying system, the system comprising:

• a NOx trapping agent which adsorbs nitrogen oxide when an excess air ratio of exhaust gas is more than 1, and releases nitrogen oxide when the excess air ratio is 1 or less;
• a NOx purifying catalyst which reduces nitrogen oxide to nitrogen;
• a HC trapping agent which adsorbs and releases hydrocarbons;
• and a CPU including instructions such that the CPU controls oxygen concentration in the exhaust gas, wherein a HC trapping agent layer containing the HC trapping agent is formed on a monolithic substrate, and a NOx trap-purifying catalyst layer containing the NOx trapping agent and the NOx purifying catalyst is formed on the HC trapping agent layer, the method comprising:
• adsorbing nitrogen oxide to the NOx trapping agent when the excess air ratio of the exhaust gas is more than 1;
• controlling the excess air ratio of the exhaust gas to be between 0.75 and 0.83 and when the excess air ratio of the exhaust gas is between 0.75 and 0.83, controlling the oxygen concentration, using the CPU, of the exhaust gas at an inlet of the NOx purifying catalyst between 1.1 and 1.2% by volume to enhance a partially-oxidized reaction of hydrocarbons in the exhaust gas and generate hydrogen and carbon monoxide from the hydrocarbons in the exhaust gas.