Patent ID: 8240139

Claim:
A method for purifying nitrogen oxide which purifies nitrogen oxide contained in an exhaust gas, the method comprising: providing an exhaust system mounted at an exhaust pipe, wherein the exhaust system comprises: an injector mounted at the exhaust pipe or an engine and additionally injecting fuel; a diesel fuel cracking catalyst mounted at the exhaust pipe downstream of the injector and converting additionally injected fuel into a high-reactivity reducing agent through thermal cracking; a DE-NOx catalyst mounted at the exhaust pipe downstream of the diesel fuel cracking catalyst, storing the nitrogen oxide contained in the exhaust gas, and releasing the stored nitrogen oxide by the additionally injected fuel so as to reduce the nitrogen oxide through oxidation-reduction reaction with the high-reactivity reducing agent; and a control portion controlling an additional injection of fuel according to driving condition of the engine; and regenerating the DE-NOx catalyst by additionally injecting the fuel according to a predetermined injection pattern in a case that the driving condition of the engine satisfies an additional injection condition of the fuel and an additional injection timing condition of the fuel; wherein the additional injection condition of the fuel is satisfied when the engine operates normally and is not an idle state, shift does not occur, a current shift speed is higher than or equal to a first forward speed, an inlet temperature of the DE-NOx catalyst is within a catalyst activation temperature range, an engine rotation speed is within a predetermined rotation speed range, and regeneration does not occur successively.