Nitrogen oxide purification system and control method of the same

Abstract

A nitrogen oxide purification system includes an exhaust line exhausting exhaust gas from a combustion chamber. A first nitrogen oxide purification device is mounted at an upstream side of the exhaust line to primarily purify nitrogen oxides (NOx) included in the exhaust gas, and a second nitrogen oxide purification device is mounted to a downstream side of the first nitrogen oxide purification device to secondarily purify the NOx. A second injector is disposed between the first nitrogen oxide purification device and the first injector to additionally inject fuel for controlling an air/fuel ratio of the exhaust gas.

What is claimed is: 
     
         1 . A nitrogen oxide purification system comprising:
 a first injector injecting fuel into a combustion chamber;   an exhaust line exhausting exhaust gas combusted in the combustion chamber;   a first nitrogen oxide purification device mounted at an upstream side of the exhaust line to primarily purify nitrogen oxides (NOx) included in the exhaust gas;   a second nitrogen oxide purification device mounted at a downstream side of the first nitrogen oxide purification device to secondarily purify the NOx included in the exhaust gas; and   a second injector disposed between the first nitrogen oxide purification device and the first injector to additionally inject fuel for controlling an air/fuel ratio of the exhaust gas,   wherein the first nitrogen oxide purification device and the second nitrogen oxide purification device enrich an air/fuel ratio of the exhaust gas passing through the exhaust line in a nitrogen oxide purification mode when a driving condition of an engine is in a normal or high load driving section, and vary the air/fuel ratio of the exhaust gas passing through the exhaust line for a predetermined period when the driving condition of the engine is in a low load driving section.   
     
     
         2 . The system of  claim 1 , wherein the first nitrogen oxide purification device includes a noble metal, an alkali metal, an alkaline-earth metal, or a rare earth metal. 
     
     
         3 . The system of  claim 1 , wherein the second nitrogen oxide purification device includes a metal oxide or a zeolite. 
     
     
         4 . The system of  claim 1 , wherein the driving condition includes a normal load, a high load, and a low load driving sections depending on any combination of a rotational speed, an output torque, or a fuel amount of the engine. 
     
     
         5 . The system of  claim 1 , wherein the predetermined period for varying the air/fuel ratio is 0.1-10 seconds and a duty cycle is 0-95% in the nitrogen oxide purification mode under the low load driving section, and
 the predetermined period and the duty cycle vary depending on the driving condition.   
     
     
         6 . The system of  claim 1 , wherein in the nitrogen oxide purification mode under the low load driving section, the air/fuel ratio varies depending on the driving condition. 
     
     
         7 . The system of  claim 1 , wherein the air/fuel ratio is varied according to the nitrogen oxide purification mode in the low load driving section by varying a duty cycle of the second injector. 
     
     
         8 . The system of  claim 1 , further comprising:
 an oxygen sensor mounted at the exhaust line to detect a concentration of oxygen included in the exhaust gas; and   a NOx sensor mounted at the exhaust line to detect a concentration of the NOx included in the exhaust gas.   
     
     
         9 . The system of  claim 8 , further comprising:
 a controller configured to receive and process a detection signal of the oxygen sensor and a detection signal of the NOx sensor, and to control the first injector or the second injector to control the air/fuel ratio of the exhaust gas, which passes through the exhaust line, such that the NOx is adsorbed and purified in the first nitrogen oxide purification device and the second nitrogen oxide purification device.   
     
     
         10 . The system of  claim 1 , wherein the driving condition includes a driving condition of a vehicle. 
     
     
         11 . A control method of a nitrogen oxide purification system for removing NOx, the method comprising:
 determining, by a controller, a nitrogen oxide purification mode of a first nitrogen oxide purification device and a second nitrogen oxide purification device;   determining, by the controller, a driving condition of an engine; and   completing, by the controller, the nitrogen oxide purification mode.   
     
     
         12 . The control method of  claim 11 , wherein the driving condition includes a low load driving condition, a normal load driving condition, and a high load driving condition. 
     
     
         13 . The control method of  claim 11 , wherein the driving condition is determined depending on any combination of a rotational speed, an output torque, or a fuel amount of the engine. 
     
     
         14 . The control method of  claim 11 , further comprising, before the step of completing:
 maintaining an air/fuel ratio of exhaust gas which flows into the first nitrogen oxide purification device when it is determined that the driving condition is a normal or high load driving condition.   
     
     
         15 . The control method of  claim 11 , further comprising, before the step of completing:
 varying an air/fuel ratio of exhaust gas which flows into the first nitrogen oxide purification device when it is determined that the driving condition is a low load driving condition.   
     
     
         16 . The control method of  claim 11 , wherein the nitrogen oxide purification system comprises:
 a first injector injecting fuel into a combustion chamber;   an exhaust line exhausting the exhaust gas combusted in the combustion chamber;   the first nitrogen oxide purification device mounted at an upstream side of the exhaust line to primarily purify nitrogen oxides (NOx) included in the exhaust gas;   the second nitrogen oxide purification device mounted at a downstream side of the first nitrogen oxide purification device to secondarily purify the NOx included in the exhaust gas; and   a second injector disposed between the first nitrogen oxide purification device and the first injector to additionally inject fuel for controlling an air/fuel ratio of the exhaust gas.