Exhaust system and control method of nitrogen oxide desorption

Abstract

Exhaust system including: a first cleaning device (15) mounted on the rear end part of an exhaust manifold of a diesel engine and containing a Lean NOx Trap (LNT) (18); a second cleaning device (20) mounted on the rear end portion of the first cleaning device (15) and including a diesel particulate filter (DPF) (21); and a third purification device (30) mounted on the rear end part of the second purification device (20) and containing a selective catalytic reduction (SCR) (32), wherein the DPF (21) of the second purification device (20) includes at least one inflow channel (20a) through which a fluid flows in, at least one outflow channel (20b) through which a fluid flows out, at least one wall disposed between at least one inflow channel (20a) and the at least one outflow channel (20b) and extending in the longitudinal direction, and a carrier (40a, 40b) disposed in at least one of the at least one inflow channel (20a) and the at least one outflow channel (20b), and at least one catalyst coated on an inner wall of the inflow channel (20a), an inner wall of at least one outflow channel (20b), or the carrier (40a, 40b), wherein at least one of a diesel oxidation catalyst (DOC), the LNT catalyst (18), or the SCR catalyst (32) is coated on the inner wall of the inflow channel (20a), the inner wall of at least one outflow channel (20b), or the carrier (40a, 40b); and wherein the SCR catalyst (32) is coated on the inner wall of the inflow channel (20a), and the LNT catalyst (18) is coated on the inner wall of at least one outflow channel (20b).

Description

BACKGROUND OF THE INVENTIONField of the InventionThe present invention relates to an exhaust system and control method of nitrogen oxide desorption. More specifically, the present invention relates to an exhaust system and control method of nitrogen oxide desorption including a diesel particulate filter simultaneously with functioning as selective catalytic reduction and a nitrogen oxide storage catalyst.Description of the Prior ArtGenerally, an engine includes an exhaust treatment device such as a diesel oxidation catalyst (DOC), a diesel particulate filter (DPF), a selective catalytic reduction (SCR) device, and a lean NOxtrap (LNT catalytic) device, etc. to reduce carbon monoxide (CO), hydrocarbons (HC), particulates (PM), nitrogen oxides (NOx), etc. included as pollution in exhaust gases.The LNT catalyst is simple and favorable compared to the SCR catalyst, but the nitrogen oxide purification efficiency is very low at high temperature and high load. Accordingly, the LNT catalyst is applied to a small vehicle whose nitrogen oxide purification load is low and whose exhaust gas temperature is low. A high purification performance of the LNT catalyst is required to react with the LNT catalyst to real driving emission (RDE) exhaust regulators without an SCR catalyst.Specifically, the catalyst temperature may be increased to over 400 degrees in RDE driving ranges of high speed and high load to reach a temperature at nitrogen oxide storage, and a plurality of nitrogen oxides flow abruptly and at a high exhaust gas flow rate into the catalyst at high speeds, so that the LNT catalyst cannot store enough nitrogen oxide.Meanwhile, the DPF filters and burns particulate matter (PM), and the DPF has a number of passages in an exhaust gas flow and at least one closed part at the inlet or outlet of the passages, or the passages are alternatively attached.Recently, the function that particulate matter of coating catalysts on the DPF is eliminated is improved or exhaust gas is additionally eliminated. The cleaning function may be improved by adding a coating amount of catalyst to the DPF or by increasing the number of passages, but the backpressure increases and decreases the particulate filter efficiency, the performance of the vehicle, and the fuel efficiency. In the field of the purification and post-treatment of exhaust gases, for example, DE 10 2014 105 210 A1, DE 10 2008 038 736 A1 and DE 10 2015 209 269 A1 are known.The information in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art known to a person skilled in the art.BRIEF OVERVIEWIt is therefore an object of the present invention to provide a DPF which improves the performance of exhaust gas purification by increasing the catalyst function without lowering the particulate filter efficiency, and particularly to provide a DPF structure which simultaneously has SCR catalyst and LNT catalyst function to improve the rate of nitrogen oxide purification at high speed and high load.The object is achieved by an exhaust system having the features of claim 1 and a method having the features of claim 10.An exhaust system according to an exemplary embodiment of the present invention includes a first purification device mounted on the rear end part of an exhaust manifold of a diesel engine and including a lean NOxtrap (LNT); a second purification device mounted on the rear end part of the first purification device and including a diesel particulate filter (DPF); and a third purification device mounted on the rear end part of the second purification device and including a selective catalytic reduction (SCR), wherein the DPF of the second purification device includes at least one inflow passage through which a fluid flows, at least one outflow passage through which a fluid flows, at least one wall mounted between at least one inflow passage and at least one outflow passage and extending in the longitudinal direction, and a support mounted in at least one of the at least one inflow passage and the at least one outflow passage, and at least one catalyst coated on an inner wall of the inflow channel, an inner wall of at least one outflow channel, or the support, wherein on the inner wall of the inflow channel, the inner wall of at least one outflow channel, or the support, at least one of a diesel oxidation catalyst (DOC), the LNT catalyst, or the SCR catalyst is coated on the inner wall of the inflow channel, and the LNT catalyst is coated on the inner wall of at least one outflow channel.The SCR catalyst of the second purification device can be an ion-exchanging zeolite catalyst, in particular a Cu-CHA catalyst.The SCR catalyst may be coated on the support.The amount of platinum (Pt) in the LNT catalyst coated on the DPF of the second purification device may be 1.1 to 1.5 times as high as that of the LNT catalyst of the first purification device.The DPF of the second purification device may have a porosity equal to or greater than 55%.The coating amount of the LNT catalyst and the SCR catalyst of the DPF of the second purification device may be 100 g / L to 200 g / L.The distance between the outlet of the first cleaning device and the inlet of the second cleaning device may be 550 mm to 600 mm.The exhaust system according to an exemplary embodiment of the present invention may further include a first oxygen sensor disposed between the rear end part of the exhaust manifold and the first purification device; a second oxygen sensor disposed between the first purification device and the second purification device; and a third oxygen sensor disposed at the rear end part of the second purification device.The exhaust system according to an exemplary embodiment of the present invention may further include a controller that synchronizes the first oxygen sensor, the second oxygen sensor, or the third oxygen sensor, and controls regeneration of the LNT catalyst.Meanwhile, a control method of nitrogen oxide desorption of a foregoing exhaust system according to an exemplary embodiment of the present invention includes measuring the nitrogen oxide load of the first purification device after completion of the nitrogen oxide desorption; measuring the passage value of nitrogen oxide of the first purification device; measuring the temperature of the first purification device and the second purification device; measuring the nitrogen oxide load of the second purification device; comparing the nitrogen oxide load of the first purification device with a threshold load; comparing the temperature of the first purification device with a threshold temperature when the nitrogen oxide load of the first purification device is greater than the threshold load; comparing the temperature of the second purification device with a threshold temperature when the temperature of the first purification device is greater than the threshold temperature; Desorption of nitrogen oxide from the first cleaning device and from the second cleaning device at the same time if the temperature of the second cleaning device is greater than the limit temperature.The control method of nitrogen oxide desorption according to an exemplary embodiment of the present invention may further include desorption of nitrogen oxide from the first purification device when the temperature of the second purification device is not higher than the threshold temperature.The control method of nitrogen oxide desorption according to an exemplary embodiment of the present invention may further include comparing the temperature of the second purification device with the threshold temperature when the temperature of the first purification device is not greater than the threshold temperature; and desorption of the nitrogen oxide only from the second purification device when the temperature of the second purification device is greater than the threshold temperature.The control method of nitrogen oxide desorption according to an exemplary embodiment of the present invention may further include stopping desorption of nitrogen oxide from the first and second cleaning devices when the temperature of the second cleaning device is not greater than the threshold temperature.According to an exemplary embodiment of the present invention, the contact time of the catalyst through the DPF structure with an additional conventional wall support may be increased to increase the response time of the catalyst and increase the coating amount of the catalyst on a filter support, so that the performance of exhaust gas purification may be improved.Also, the exhaust gas cannot pass through the carrier, so that a backpressure increase does not exist, and the length of the additional carrier may be optimized to prevent catalyst degradation during DPF regeneration.Also, removal of ammonia between the front end of the LNT catalyst and the rear end part of the LNT catalyst can be achieved in an SCR catalyst by the structure of the DPF structure having the SCR catalyst and the LNT catalyst function, or by the structure of the SCR catalyst and the LNT catalyst which are sequentially mounted.The methods and apparatuses of the present invention having other features and advantages will be apparent from or are described in further detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a schematic drawing of an exhaust system according to an exemplary embodiment of the present invention. FIG. 2 is a cross-sectional view illustrating a filter of a particulate filter. FIG. 3 is a front view partially illustrating an inflow passage and an outflow passage of a filter for a diesel particulate filter according to an exemplary embodiment of the present invention. FIG. 4 is a schematic view of an exhaust system according to an exemplary embodiment of the present invention. FIGS. 5A and 5B are flowcharts illustrating an exhaust gas purification process in an exhaust system according to an exemplary embodiment of the present invention.It is to be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features of the basic principles of the present invention. The specific design features of the present invention as set forth herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended use and application environment.In the figures, reference numerals refer to the same or equivalent parts of the present invention throughout different figures of the drawing.DETAILED DESCRIPTIONReference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings and described below. Although the invention will be described in conjunction with the exemplary embodiments, it is to be understood that the present description is not intended to be limited by these exemplary embodiments. On the contrary, the invention is intended to cover not only the embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined in the appended claims.Further, various exemplary embodiments are representatively described because reference numerals denote similar elements having the same configuration, and in other exemplary embodiments, only configurations different from the various exemplary embodiments will be described.The drawings are shown schematically and to scale. Relative dimensions and ratios of parts in the drawings are exaggerated or reduced for clarity and simplicity, and the dimensions are exemplary only and are not limiting. In addition, similar structures, elements, or parts shown in two or more drawings use the same reference numerals to show the same features. It will be understood that when a layer, film, surface, or substrate is referred to as being "on" one element, it may be directly on the other element or an intervening element may also be present.The exemplary embodiments of the present invention show exemplary embodiments of the present invention in detail. As a result, various modifications of the drawings are expected. Therefore, the exemplary embodiment of the present invention is not limited to specific aspects of the illustrated parts, and includes, for example, modification of an aspect by the manufacturing.An exemplary embodiment of the present invention will now be described with reference to FIGS. 1 to 3.FIG. 1 is a schematic drawing of an exhaust system according to an exemplary embodiment of the present invention. The exhaust system shows only a schematic configuration necessary to describe the exemplary embodiment of the present invention, and is not limited to such a configuration.Referring to FIG. 1, an exhaust system according to an exemplary embodiment of the present invention includes a first cleaner 15, a second cleaner 20, and a third cleaner 30.The first purification device 15 is mounted on the rear end portion of the exhaust manifold into which exhaust gas G flows from the engine 10, and includes a Lean NOx Trap (LNT) 18 Here, the engine 10 includes a diesel engine for diesel vehicles as an internal combustion engine that provides lean combustion. Engine 10 may generate power by combusting fuel with air at a constant mixing rate after controlling controller 50.In addition, the second cleaner 20 is mounted on the rear end part of the first cleaner 15, and includes a diesel particulate filter (DPF) 21.In addition, the third purification device 30 is attached to the rear end part of the second purification device 20, and includes a selective catalytic reduction (SCR) 32. Here, the SCR catalyst 32 may include passive SCR (pSCR) of an SCR catalyst type that discharges ammonia (NH 3) from the LNT catalyst 18 without urea injection.Meanwhile, the exhaust system according to an exemplary embodiment of the present invention may further include a first oxygen sensor 42, a second oxygen sensor 44, and a third oxygen sensor 46. Further, the exhaust system may include a controller 50 that synchronizes the first oxygen sensor 42, the second oxygen sensor 44, and the third oxygen sensor 46 and controls regeneration of the LNT catalyst 18.The first oxygen sensor 42 may be disposed between a rear end portion of the exhaust manifold and the first purification device 15, the second oxygen sensor 44 may be disposed between the first purification device 15 and the second purification device 20, and the third oxygen sensor 46 may be disposed at a rear end portion of the second purification device 20.The first oxygen sensor 42 to the third oxygen sensor 46 are attached to the front end of the LNT catalyst 18 and the front / rear end part of the DPF 21 to acquire information as to whether the engine 10 operates under lean or rich conditions.The controller 50 recognizes running information including inflow air amount, engine speed, vehicle speed, and shift speed, etc., to control the running of the engine under standard running conditions. In addition, the controller 50 synchronizes the first oxygen sensor 42 to the third oxygen sensor 46, and controls regeneration of the LNT catalyst 18.Meanwhile, the distance between the outlet of the first cleaner 15 and the inlet of the second cleaner 20 is 550 mm to 600 mm. This is for determining the temperature of the DPF 21.In addition, the amount of platinum (Pt) in the LNT catalyst coated on the DPF 21 of the second purification device 20 may be 1.1 to 1.5 times as high as that of the LNT catalyst of the first purification device 15, and the DPF 21 of the second purification device 20 may have a porosity equal to or greater than 55%. In addition, the coating amount of the LNT catalyst and the SCR catalyst of the DPF 21 of the second purification device 20 may be 100 g / L to 200 g / L.FIG. 2 is a cross-sectional view illustrating a filter of a particulate filter, and FIG. 3 is a front view partially illustrating an inflow passage and an outflow passage of a filter for a diesel particulate filter according to an exemplary embodiment of the present invention.Referring to FIGS. 2 and 3, the DPF 21 of the second cleaning device 20 includes at least one inflow channel 20 aand at least one outflow channel in a housing. A plurality of inflow channels 20a and outflow channels 20b are separated by wall 40. In addition, carriers 40 aand 40 bmay be mounted in at least one inflow channel 20 aand in at least one outflow channel 20 b.Referring to FIGS. 2 and 3, the outflow passage 20 bformed at an end part of the DPF 21 extends along the exhaust gas flow, and may be disposed in parallel with the inflow passage 20 a. At least one inflow channel 20a is placed around the outflow channel 20b. The front end of the drain passage 20b is blocked by a plug 14, so that exhaust gas cannot flow into the particulate filter through the drain passage 20b. The rear end portion of the drain passage 20b is opened so that exhaust gas in the DPF 21 drains from the DPF 21 through the drain passage 20b.The wall 40 is disposed between the inflow channel 20 aand the outflow channel 20 bto define a boundary. The wall 40 may be a porous wall in which at least one micropore is formed. The porous wall 40 hydrodynamically connects the adjacent inflow channel 20 ato the outflow channel 20 b. Accordingly, the exhaust gas flowing in through the inflow passage 20 acan move to the outflow passage 20 bthrough the porous wall 40. When the exhaust gas moves from the inflow passage 20 ato the outflow passage 20 bthrough the porous wall 40, particulates contained in the exhaust gas are extracted through the porous wall 40. The porous wall 40 may include aluminum titanate, cordierite, and silicon carbide, etc.The supports 40 aand 40 bmay be mounted in at least one of the inflow channel 20 aand the outflow channel 20 b. The supports 40 aand 40 may be attached only to the inflow channel 20 aor only to the outflow channel 20 b. FIGS. 2 and 3 show how the beams 40 aand 40 bextend in parallel with the direction of the inflow channel 20 aand the outflow channel 20 b, but is not limited to the disclosed embodiments.In other words, the beams 40 aand 40 bmay extend vertically or obliquely to the direction of the inflow channel 20 aand the outflow channel 20 b. When the supports 40 aand 40 bextend vertically or obliquely to the direction of the inflow channel 20 aand the outflow channel 20 b, at least one of both end parts of the supports 40 aand 40 bmay not be in contact with the porous wall 40.Meanwhile, the supports 40 aand 40 bare not installed to play a role in the filter, but to support the catalyst, therefore they are not necessarily made of a porous material. In other words, the supports 40 aand 40 bmay include the same materials or different materials than the porous wall. Although the carriers 40a and 40b contain porous material, there hardly exist pressure differences between the channels 20a and 20b divided by carriers 40a and 40b, therefore, the exhaust gas hardly passes through the carriers 40a and 40b and moves along the carriers 40a and 40b and the wall 40. In other words, the thickness of the beams 40 aand 40 bmay be thinner than that of the wall 40, and this minimizes an increase in the back pressure.On one of the inner wall of the inflow channel, the inner wall of the outflow channel, or the carrier, at least one of the diesel oxidation catalyst (DOC), the LNT catalyst, or the SCR catalyst may be coated. When the supports 40a and 40b contain a porous material, the catalysts 60, 70 and 80 are coated on the surface of support 40 and the micropores in supports 40a and 40b. In contrast, when the supports 40a and 40b contain nonporous material, the catalysts 60, 70 and 80 are coated on the surface of the supports 40a and 40b.Further, the carriers 40 aand 40 bmay be separated by a first carrier 40 aprovided in the inflow channel 20 aand a second carrier 40 bprovided in the outflow channel 20 b.Meanwhile, the SCR catalyst may be coated on the inner wall of the inflow channel 20 a, and the LNT catalyst may be coated on the inner wall of the outflow channel 20 b. The SCR catalyst of the second purification device may be an ion-exchanging zeolite catalyst, particularly a Cu-CHA catalyst, and the SCR catalyst may be coated on the support.FIG. 4 is a schematic view of an exhaust system according to an exemplary embodiment of the present invention.Referring to FIG. 4, the exhaust system includes a first purification device 15 attached to the rear end part of the exhaust manifold of the diesel engine and including a first LNT catalyst 18, a second purification device 20 attached to the rear end part of the first purification device 15 and including a second SCR catalyst 22 and a second LNT catalyst 24 attached to the rear end part of the second SCR catalyst 22, and a third purification device 30 attached to the rear end part of the second purification device 20 and including a first SCR catalyst 32.The second purification device 20 may include the second SCR catalyst 22 and the second LNT catalyst 24 attached to the rear end part of the second SCR catalyst 22. By continuing the rich state, a large amount of ammonia exists between the first LNT catalyst 18 and the second LNT catalyst 24 during the nitrogen oxide reduction, but about over 30% of the ammonia is oxidized and eliminated during passing through the second LNT catalyst. The eliminated ammonia may be utilized by the second SCR catalyst 22 at the front end of the second LNT catalyst 24. At this time, the second SCR catalyst 22 of the second purification device 20 may be an ion-exchanging zeolite catalyst, particularly a Cu-CHA catalyst.Also, the third purification device 30 is attached to the rear end part of the second purification device 20 and includes a first SCR catalyst 32. here, the first SCR catalyst 32 may include a passive SCR (pSCR) of an SCR catalytic type using ammonia (NH3) discharged from LNT catalysts 18 and 24 without urea injection.The exhaust system may further include a first oxygen sensor 42, a second oxygen sensor 44, and a third oxygen sensor 46, according to an exemplary embodiment of the present invention. Further, the exhaust system may include a controller 50 that synchronizes the first oxygen sensor 42, the second oxygen sensor 44, or the third oxygen sensor 46 and controls regeneration of the first LNT catalyst 18 or the second LNT catalyst 24.The first oxygen sensor 42 may be disposed between the rear end portion of the exhaust manifold and the first purification device 15, the second oxygen sensor 44 may be disposed between the first purification device 15 and the second purification device 20, and the third oxygen sensor 46 may be disposed at the rear end portion of the second purification device 20.The first oxygen sensor 42 to the third oxygen sensor 46 are attached to the front end of the first LNT catalyst 18 and the front end / rear end portion of the second LNT catalyst 24 to provide information as to whether the engine 10 is operating in lean or rich conditions.The controller 50 recognizes running information including inflow air amount, engine speed, vehicle speed, and shift speed, etc., to control the running of the engine under standard running conditions. In addition, the controller 50 synchronizes the first oxygen sensor 42 to the third oxygen sensor 46, and controls regeneration of the LNT catalyst 18.In addition, the controller 50 measures the passage rate of nitrogen oxides of the first LNT catalyst 18 and the charge value of nitrogen oxides of the second LNT catalyst 24, and the temperature of the second LNT catalyst 24 together with the temperature of the first LNT catalyst 18.At this time, the controller 50 may measure the passage rate of nitrogen oxides of the first LNT catalyst 18 using the charge value of nitrogen oxides of the first LNT catalyst 18, the temperature and exhaust gas flow rate, etc. In addition, the controller 50 may measure the loading value of nitrogen oxides of the second LNT catalyst 24 using the passage rate of nitrogen oxides of the first LNT catalyst 18, the temperature of the second LNT catalyst 24, and the inflow flow rate to the second LNT catalyst 24.FIGS. 5A and 5B are flowcharts illustrating an exhaust gas purification process in an exhaust system according to an exemplary embodiment of the present invention.Referring to FIGS. 5A and 5B, a measurement method of nitrogen oxide desorption according to an exemplary embodiment of the present invention is first measured a charge value of nitrogen oxides of the first purification device after completion of nitrogen oxide desorption S 401. The passage rate of nitrogen oxides of the first purification device is measured using the charge value of nitrogen oxides of the first LNT catalyst, the temperature and the exhaust gas flow rate, etc. S 402.Then, the temperature of the first cleaning device and the second cleaning device is measured using temperature sensor S 403. Then, the charge value of nitrogen oxides of the second cleaning device is measured S404.The loading value of nitrogen oxides of the first cleaning device is compared with the limit loading S 405 and the temperature of the first cleaning device is compared with the limit temperature if the loading value of nitrogen oxides of the first cleaning device is greater than the limit loading S 406. Then, the temperature of the second cleaning device is compared with the limit temperature when the temperature of the first cleaning device is higher than the limit temperature S 407.Nitrogen oxides of the first cleaning device and the second cleaning device are desorbed at the same time when the temperature of the second cleaning device is higher than the limit temperature S408.Nitrogen oxides are desorbed from the first cleaning device only when the temperature of the second cleaning device is not higher than the limit temperature S409.Further, the temperature of the second cleaning device is compared with the threshold temperature when the temperature of the first cleaning device is not greater than the threshold temperature S410, and nitrogen oxides are desorbed from the second cleaning device only when the temperature of the second cleaning device is greater than the threshold temperature S411.Meanwhile, desorption of nitrogen oxides from the first and second cleaning devices is stopped when the temperature of the second cleaning device is not greater than the limit temperature S 412.Thereby, according to an exemplary embodiment of the present invention, the contact time of the catalyst through the DPF structure with an additional conventional wall support can be increased to increase the response time of the catalyst and increase the coating amount of the catalyst on a filter support, so that the performance of exhaust gas purification can be improved.Also, the exhaust gas cannot pass through the carrier, so that a backpressure increase does not exist, and the length of the additional carrier can be optimized to prevent catalyst degradation during DPF regeneration.Also, removal of ammonia between the front end of the LNT catalyst and the rear end part of the LNT catalyst can be achieved in an SCR catalyst by the structure of the DPF structure having the SCR catalyst and the LNT catalyst function, or by the structure of the SCR catalyst and the LNT catalyst which are sequentially mounted.To facilitate the explanations and to define the appended claims in detail, the terms "upper", "lower", "inner", "outer", "upper", "lower", "upper", "lower", "upward", "downward", "front", "rear", "inner", "outer", "inner", "outer", "inner", "outer", "forward", and "rearward" are used to refer to features of the exemplary embodiments with respect to the positions of such features as shown in the figures.

Claims

An exhaust system comprising: a first purification device (15) mounted on the rear end part of an exhaust manifold of a diesel engine and including a lean NOx trap (LNT) (18); a second purification device (20) mounted on the rear end part of the first purification device (15) and including a diesel particulate filter (DPF) (21); and a third purification device (30) mounted on the rear end part of the second purification device (20) and including a selective catalytic reduction (SCR) (32), wherein the DPF (21) of the second purification device (20) includes at least one inflow channel (20a) through which a fluid flows, at least one outflow channel (20b) through which a fluid flows out, at least one wall disposed between at least one inflow channel (20a) and the at least one outflow channel (20b) and extending in the longitudinal direction, and a support (40a, 40b) disposed in at least one of the at least one inflow channel (20a) and the at least one outflow channel (20b), and at least one catalyst coated on an inner wall of the inflow channel (20a), an inner wall of at least one outflow channel (20b), or the support (40a, 40b), wherein on the inner wall of the inflow channel (20a), the inner wall of at least one outflow channel (20b), or the support (40a, 40b), at least one of a diesel oxidation catalyst (DOC), the LNT catalyst (18), or the SCR catalyst (32); and wherein the SCR catalyst (32) is coated on the inner wall of the inflow channel (20a), and the LNT catalyst (18) is coated on the inner wall of at least one outflow channel (20b).The system according to claim 1, wherein an SCR catalyst (22) of the second purification device (20) is an ion-exchanging zeolite catalyst, in particular a Cu-CHA catalyst.The system of claim 2, wherein the SCR catalyst (22) is coated on the support (40a, 40b).The system of claim 3, wherein the amount of platinum (Pt) in the LNT catalyst (18) coated on the DPF (21) of the second purification device (20). 1.1 to 1.5 times as high as that of the LNT catalyst (18) of the first purification device (15).The system of claim 1, wherein the DPF (21) of the second cleaning device (20) has a porosity equal to or greater than 55%.The system according to claim 1, wherein the coating amount of the LNT catalyst (18) and the SCR catalyst (22) of the DPF (21) of the second purification device (20) is 100 g / L to 200 g / L.The system of claim 1, wherein the distance between the outlet of the first cleaning device (15) and the inlet of the second cleaning device (20) is 550 mm to 600 mm.The system of claim 1, further comprising: a first oxygen sensor (42) mounted between the rear end portion of the exhaust manifold and the first purification device (15); a second oxygen sensor (44) mounted between the first purification device (15) and the second purification device (20); and a third oxygen sensor (46) mounted on the rear end portion of the second purification device (20).The system of claim 8, further comprising: a controller (50) that synchronizes the first oxygen sensor (42), the second oxygen sensor (44), or the third oxygen sensor (46), and that controls regeneration of the LNT catalyst (18).The control method of nitrogen oxide desorption by an exhaust system according to claim 1, comprising: measuring the nitrogen oxide load of the first cleaning device (15) after completion of the nitrogen oxide desorption; measuring the transmission value of nitrogen oxide of the first cleaning device (15); measuring the temperature of the first cleaning device (15) and the second cleaning device (20); measuring the nitrogen oxide load of the second cleaning device (20); comparing the nitrogen oxide load of the first cleaning device (15) with a threshold load; comparing the temperature of the first cleaning device (15) with a threshold temperature when the nitrogen oxide load of the first cleaning device (15) is greater than the threshold load; comparing the temperature of the second cleaning device (20) with a threshold temperature when the temperature of the first cleaning device (15) is greater than the threshold temperature; Desorption of nitrogen oxide from the first cleaning device (15) and from the second cleaning device (20) at the same time if the temperature of the second cleaning device (20) is greater than the limit temperature.The method of claim 10, further comprising: desorption of nitrogen oxide from the first cleaning device (15) when the temperature of the second cleaning device (20) is not higher than the threshold temperature.The method of claim 10, further comprising: comparing the temperature of the second cleaning device (20) with the threshold temperature when the temperature of the first cleaning device (15) is not greater than the threshold temperature; and desorption of the nitrogen oxide from the second cleaning device (20) when the temperature of the second cleaning device (20) is greater than the threshold temperature.The method of claim 12, further comprising: stopping desorption of nitrogen oxide from the first and second cleaning devices (15, 20) when the temperature of the second cleaning device (20) is not greater than the threshold temperature.

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