Patent Publication Number: US-2017362989-A1

Title: Nitrogen oxide purification system and control method of the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional patent application of U.S. patent application Ser. No. 14/835,565, filed on Aug. 25, 2015 which claims the benefit of priority to Korean Patent Application No. 10-2014-0169905 filed in the Korean Intellectual Property Office on Dec. 1, 2014, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a nitrogen oxide purification system for removing nitrogen oxides (NOx) and a control method of the same, which are noxious substances included in exhaust gas, by varying an air/fuel ratio depending on a driving condition of an engine or vehicle, and a control method of the same. 
     BACKGROUND 
     Due to recent fortified emission regulations, a complex exhaust gas purification system combining an oxidation catalyst, a particulate matter filter device, a nitrogen oxide purification catalyst (LNT: lean NOx trap, or SCR: selective catalytic reduction), and so on has been used to limit pollutant emission. 
     Particularly, in the EURO emission regulations, a world-harmonized light vehicles test cycle (WLTC) mode, which represents an actual driving condition, has been applied instead of a new European driving cycle (NEDC) mode. 
     Due to the fortified emission regulations, an additional NOx reducing apparatus is required since a post-processing system, which comprises a combination of an oxidation catalyst and a particulate matter filter device, exhausts a large amount of NOx in a high load driving condition. 
     When a vehicle is driven in the high load driving condition, the amount of NOx increases and the temperature of exhaust gas passing through a nitrogen oxide purification catalyst also increases, and as a result, a purification rate of NOx is significantly deteriorated. 
     Further, since the NOx increased in the high load driving condition cannot be purified with the combination of the oxidation catalyst and the particulate matter filter device, and a purification rate may be significantly deteriorated as a temperature of the LNT increases to 450° C. or more in the high load driving condition when the LNT and the particulate matter filter device are used. 
     In addition, in a low load driving condition, it is difficult to enrich an air/fuel ratio of exhaust gas, and thus purification efficiency of NOx may be deteriorated. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY 
     The present disclosure has been made in an effort to provide a nitrogen oxide purification system and a control method thereof having advantages of separating a driving condition of an engine or a vehicle to a normal or high load condition and a low load condition, and stably or effectively purifying NOx in a normal or high load condition and a low load condition by differently applying air/fuel ratio of exhaust gas depending on a load condition of the engine or a vehicle in a case of purifying the NOx using a nitrogen oxide purification catalyst. 
     A nitrogen oxide purification system according to an exemplary embodiment of the present inventive concept includes an exhaust line exhausting exhaust gas combusted in 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. A second nitrogen oxide purification device is mounted at a downstream side of the first nitrogen oxide purification device to secondarily purify NOx included in the exhaust gas. 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. The first nitrogen oxide purification device and the second nitrogen oxide purification device may enrich the air/fuel ratio of the exhaust gas which passes through the exhaust line in a nitrogen oxide purification mode when a driving condition of an engine or a vehicle is in a normal or high load driving section, and vary the air/fuel ratio of the exhaust gas which passes through the exhaust line for a predetermined period when a driving condition of the engine or the vehicle is in a low load driving section. 
     The first nitrogen oxide purification device may include a noble metal, an alkali metal, an alkaline-earth metal, or a rare earth metal. 
     The second nitrogen oxide purification device may include a metal oxide or a zeolite. 
     The driving condition of the vehicle may include 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 an engine. 
     The predetermined period for varying the air/fuel ratio may be 0.1-10 seconds and a duty cycle may be 0-95% in the nitrogen oxide purification mode under the low load driving section. The predetermined period and duty cycle may vary depending on the driving condition. 
     In the nitrogen oxide purification mode under the low load driving section, the air/fuel ratio varies depending on the driving condition. 
     The system may further include an oxygen sensor mounted at the exhaust line to detect a concentration of oxygen included in the exhaust gas. A NOx sensor is mounted at the exhaust line to detect a concentration of NOx included in the exhaust gas. 
     The system may further include a controller configured to receive and process a detection signal of the oxygen sensor and a detection signal of the NOx sensor. The controller may further control the first injector or the second injector to control the air/fuel ratio such that NOx are adsorbed and purified in the first nitrogen oxide purification device and the second nitrogen oxide purification device. 
     A control method of a nitrogen oxide purification system for removing NOx according to another exemplary embodiment of the present inventive concept includes: determining, by a controller, a nitrogen oxide purification mode of a first nitrogen oxide purification device and a second nitrogen oxide purification device; determining a driving condition of an engine; and completing the nitrogen oxide purification mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a nitrogen oxide purification system according to an exemplary embodiment of the present inventive concept. 
         FIG. 2  is a flowchart showing control flow of a nitrogen oxide purification system according to an exemplary embodiment of the present inventive concept. 
         FIG. 3  is a table showing material of a nitrogen oxide purification device used for a nitrogen oxide purification system according to an exemplary embodiment of the present inventive concept. 
         FIG. 4  is a graph showing a driving condition of a nitrogen oxide purification system according to an exemplary embodiment of the present inventive concept. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the present inventive concept will hereinafter be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a schematic diagram of a nitrogen oxide purification system according to an exemplary embodiment of the present inventive concept. 
     Referring to  FIG. 1 , a nitrogen oxide purification system may include an intake line  100 , an engine  110 , a first injector  120 , an exhaust line  130 , a second injector  140 , an oxygen sensor  145 , a first nitrogen oxide purification device  160 , a nitrogen oxides (NOx) sensor  150 , a second nitrogen oxide purification device  170 , and a controller  180 . 
     The intake line  100  supplies outside air into a combustion chamber, and the first injector  120  of the engine  110  injects a fuel into the combustion chamber to generate combustion of the air and the fuel in the combustion chamber. This combustion gas is exhausted out through the exhaust line  130 . 
     The second injector  140  is mounted at the exhaust line  130  and injects fuel or ethanol from a fuel tank, e.g., a fuel tank connected with the first injector  120  or an extra fuel tank, (not shown) to an upstream side of the exhaust line  130 . 
     The oxygen sensor  145  detects oxygen concentration of the exhaust gas flowing into the first nitrogen oxide purification device  160  via an inlet of the exhaust line  130  and generates a signal. The controller  180  detects an air/fuel ratio of the exhaust gas depending on the signal generated from the oxygen sensor  145  and controls the air/fuel ratio of the exhaust gas. 
     The NOx sensor  150  is mounted to a downstream side of the first nitrogen oxide purification device  160  or the second nitrogen oxide purification device  170  so as to detect concentration of the NOx and to transmit the detected information of the nitrogen oxides concentration to the controller  180 . 
     The controller  180  may have at least one microprocessor which is operated by a predetermined program, and the predetermined program may include a series of commands for performing a method according to an exemplary embodiment of the present inventive concept to be described later. 
     In the present disclosure, a nitrogen oxide purification method effectively NOx which is a noxious gas being exhausted from the engine  110 . 
     That is, the air/fuel ratio of the exhaust gas flowing into the first nitrogen oxide purification device  160  is controlled to be an enriched condition (A&lt;1) in a normal or high load driving condition. 
     Further, the air/fuel ratio of the exhaust gas flowing into the first nitrogen oxide purification device  160  varies for a predetermined period in a low load driving condition. Herein, controlling the air/fuel ratio to vary for the predetermined period means controlling the air/fuel ratio to vary while periodically repeating a current air/fuel ratio (λ current ) and a varied air/fuel ratio (λ current −Δλ). 
     As described above, the nitrogen oxide is purified in the first nitrogen oxide purification device  160  by controlling the air/fuel ratio of the exhaust gas which flows into the first nitrogen oxide purification device  160  to be enriched in the normal or high load driving condition, and a reducing agent is generated in the first nitrogen oxide purification device  160  at this step. 
     The second nitrogen oxide purification device  170  additionally purifies the NOx included in the exhaust gas using the reducing agent generated from the first nitrogen oxide purification device  160 . 
     Further, the second nitrogen oxide purification device  170  varies the air/fuel ratio of the exhaust gas flowing into the first nitrogen oxide purification device  160  for the predetermined period in the low load driving condition. 
     The first nitrogen oxide purification device  160  occludes or adsorbs NOx in a lean state of the exhaust gas, and desorbs and purifies the adsorbed nitrogen oxides in an enriched state of the exhaust gas. 
     In addition, in the normal or high load driving condition, the first nitrogen oxide purification device  160  purifies the NOx and simultaneously generates the reducing agent. The second nitrogen oxide purification device  170  adsorbs the reducing agent generated from the first nitrogen oxide purification device  160  and reacts the adsorbed reducing agent with the supplied nitrogen oxide so as to purify the NOx. 
     When the first nitrogen oxide purification device  160  is provided and the second nitrogen oxide purification device  170  is not provided, NOx, which is not purified in the first nitrogen oxide purification device  160 , and the reducing agent may be released into the atmosphere in the normal or high load condition. 
     According to the present disclosure, NOx may be additionally purified and the reducing agent released into the atmosphere may be reduced. 
       FIG. 2  is a flowchart showing control flow of a nitrogen oxide purification system according to an exemplary embodiment of the present inventive concept. 
     Referring to  FIG. 2 , a purification mode of a nitrogen oxide purification device is determined at step S 200 . The purification mode is performed by the NOx sensor  150 , and is performed depending on a driving condition. 
     A driving condition of the engine  110  is determined at step S 210 . Herein, a driving condition may include low load, normal load, and high load driving conditions. 
     In the present disclosure, the driving condition is determined depending on any combination of a rotational speed, an output torque, or a fuel amount of the engine  110 , and the driving condition is the normal or high load driving condition when the determining value is the same as or more than a predetermined value at BMEP 3-10 bar, and is a low load driving condition when the determining value is less than the predetermined value with at BMEP 3-10 bar. Herein, partial sections may overlap. 
     Step S 230  is performed if the driving condition is determined to be a first driving condition, which is the normal or high load driving condition, at step S 220 , and step S 232  is performed if the driving condition is determined to be a second driving condition, which is the low load driving condition, at step S 222 . 
     At step S 230 , the controller  180  controls the first injector  120 , the second injector  140 , or an intake system so as to maintain the air/fuel ratio of the exhaust gas flowing into the first nitrogen oxide purification device  160  to be enriched (λ&lt;1). 
     On the other hand, at step S 232 , the controller  180  controls the first injector  120 , the second injector  140 , or the intake system so as to vary the air/fuel ratio of the exhaust gas flowing into the first nitrogen oxide purification device  160  to periodically repeat the current air/fuel ratio (λ current ) and the varied air/fuel ratio (λ current −Δλ). The purification mode of nitrogen oxide is completed at step S 240 . 
       FIG. 3  is a table showing material of a nitrogen oxide purification device being used in a nitrogen oxide purification system according to an exemplary embodiment of the present inventive concept. 
     Referring to  FIG. 3 , the first nitrogen oxide purification device  160  may include at least one of a noble metal, an alkali metal, an alkaline-earth metal, and a rare earth metal, and the second nitrogen oxide purification device  170  may include at least one of a metal oxide and a zeolite. 
       FIG. 4  is a graph showing a driving condition of a nitrogen oxide purification system according to an exemplary embodiment of the present inventive concept. 
     Referring to  FIG. 4 , a horizontal axis represents rotational speed of the engine  110 , a vertical axis represents output torque of the engine  110 , the first driving condition includes sections where the rotational speed and the output torque are relatively high, and the second driving condition includes sections where the rotational speed and the output torque are relatively low. The first driving condition and the second driving condition are partially overlapped with each other. 
     In the present disclosure, the first injector  120  or the second injector  140  inject a fuel or a reducing agent for varying the air/fuel ratio of the second injector  140  in a nitrogen oxide purification mode of the low load driving section for 0.1-10 seconds. A duty cycle of the second injector  140  used in a nitrogen oxide purification mode of the low load driving section may be within 0-95%. The period and the duty cycle may vary depending on an engine driving condition. 
     In addition, the nitrogen oxide purification mode starts according to one of the amount of nitrogen oxides occluded/adsorbed in the first nitrogen oxide purification device  160  or the driving condition, and the nitrogen oxide purification is performed within 5-80% of the maximum occluded/adsorbed amount of nitrogen oxides in the first nitrogen oxide purification device  160 . 
     Further, the present disclosure, the air/fuel ratio of the exhaust gas passing through the exhaust line  130  may be controlled by the amount and time of the reducing agent injected from the first injector  120  or the second injector  140 . Furthermore, the air/fuel ratio of the exhaust gas passing through the exhaust line  130  may be controlled by the amount of the recirculation exhaust gas recirculated from the exhaust line  130  to the intake line  100 , an angle of a turbine vane of a turbocharger, and opening of a throttle valve mounted to the intake line  100 . 
     According to the present disclosure, the second nitrogen oxide purification device is mounted to a downstream side of the first nitrogen oxide purification device, the air/fuel ratio of the exhaust gas is enriched in the normal or high load condition so as to purify nitrogen oxide in the first nitrogen oxide purification device, and the nitrogen oxide are purified again by the reducing agent generated therein in the second nitrogen oxide purification device such that purification performance of nitrogen oxide may be improved and fuel consumption may be better. 
     In addition, in the low load condition, purification performance of the nitrogen oxide may be improved in all engine or vehicle driving conditions by periodically varying the air/fuel ratio of the exhaust gas. 
     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.