Exhaust purifying system

Abstract

To provide an exhaust emission control system capable of achieving both control of nitrogen oxides (NOx) and emission suppression of nitrous oxides (N2O).SOLUTION: An exhaust emission control system 1 includes: an exhaust passage 20; a DPF device (filter) 31; a first SCR device (first catalyst part) 41 including an iron-based catalyst 83 for reducing NOx (nitrogen oxides); a second SCR device (second catalyst part) 51 including a copper-based catalyst 92 for reducing NOx; and a control unit 70 for controlling an amount of deNOx in the first SCR device 41 and the second SCR device 51. The control unit 70 calculates a first possible amount (possible amount) that can reduce the NOx by the first SCR device 41. When the first possible amount becomes a predetermined amount or more, the control unit decreases the amount of deNOx in the second SCR device 51, and increases the amount of deNOx in the first SCR device 41.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to an exhaust gas purification system.

Background Art

[0002] Patent Document 1 describes an exhaust gas purification device that purifies nitrogen oxides in exhaust gas. This exhaust gas purification device includes a medium-high temperature active purification catalyst provided in an exhaust passage of exhaust gas, a low temperature active purification catalyst provided downstream of the medium-high temperature active purification catalyst, and a Co ion-exchanged metal-containing silicate catalyst provided upstream of the medium-high temperature active purification catalyst.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Incidentally, in the above technical field, exhaust gas purification systems using SCR (Selective Catalytic Reduction) devices are known, which use ammonia (NH3) produced from urea solution as a reducing agent and purify nitrogen oxides (NOx) in exhaust gas using an SCR catalyst. Nitric oxide (NO) and nitrogen dioxide (NO2) generated from engines are collectively referred to as nitrogen oxides (NOx). The proportion of nitrogen dioxide in exhaust gas from engines is lower than the proportion of nitric oxide. This exhaust gas purification system, as an example, includes a DPF (Diesel Particulate Filter) device for collecting particulate matter such as PM (Particulate Matter), which is a regulated substance similar to nitrogen oxides, and a DOC (Diesel Oxidation Catalyst) device for oxidizing predetermined substances contained in the exhaust gas. For PM regeneration (oxidation) by the DPF device, nitrogen dioxide produced when nitric oxide, a by-product of nitrogen oxides, is oxidized in the DOC device is used. Since NOx purification in SCR catalysts can be efficiently performed when the ratio of NO to NO2 in NOx is close to 1:1, SCR devices are installed downstream of the DOC device and DPF device in the exhaust duct.

[0005] In the DOC, DPF, and SCR systems, the catalyst is supported on a carrier with a large heat capacity, such as cordierite. Therefore, for example, during a cold engine start, the SCR system located downstream of the DOC and DPF systems requires a predetermined amount of time to reach a temperature at which nitrogen oxides can be reduced and become active. During this time, the SCR system cannot reduce nitrogen oxides, which may result in insufficient purification of nitrogen oxides. To address this, it is conceivable to use a copper-based catalyst capable of reducing nitrogen oxides even at low temperatures in the SCR system. However, in this case, copper-based catalysts tend to produce nitrous oxide (N2O), a greenhouse gas, as a by-product when reducing nitrogen oxides, making the reduction of nitrous oxide emissions a challenge.

[0006] This disclosure aims to provide an exhaust gas purification system that can achieve both the purification of nitrogen oxides and the reduction of nitrous oxide emissions. [Means for solving the problem]

[0007] The exhaust gas purification system according to this disclosure comprises: an exhaust passage for circulating exhaust gas from an internal combustion engine mounted on a vehicle; a filter provided in the exhaust passage for collecting particulate matter contained in the exhaust gas; a first catalyst section provided downstream of the filter in the exhaust passage and including an iron-based catalyst and / or a vanadium-based catalyst for reducing nitrogen oxides contained in the exhaust gas; a second catalyst section provided upstream of the filter in the exhaust passage and including a copper-based catalyst for reducing nitrogen oxides contained in the exhaust gas; and a control unit for controlling the amount of nitrogen oxide reduction in the first catalyst section and the second catalyst section. The control unit calculates a possible amount of nitrogen oxides that can be reduced by the first catalyst section, and when the possible amount exceeds a predetermined amount, it reduces the amount of nitrogen oxide reduction in the second catalyst section and increases the amount of nitrogen oxide reduction in the first catalyst section.

[0008] In this exhaust gas purification system, nitrogen oxides contained in the exhaust gas are reduced by a second catalytic section containing a copper-based catalyst, located upstream of the filter in the exhaust passage, and a first catalytic section containing an iron-based catalyst or a vanadium-based catalyst, located downstream of the filter in the exhaust passage. The copper-based catalyst used in the second catalytic section can reduce nitrogen oxides over a relatively wide temperature range compared to, for example, an iron-based catalyst. Furthermore, during cold starts of an internal combustion engine, the second catalytic section located upstream of the filter in the exhaust passage reaches its activation temperature earlier than the first catalytic section located downstream of the filter. Therefore, even when the internal combustion engine is cold-started, nitrogen oxides can be reduced earlier compared to a configuration in which, for example, a catalytic section that reduces nitrogen oxides is located only downstream of the filter in the exhaust passage. Thus, nitrogen oxides can be efficiently purified even at low temperatures. Here, the first catalytic section containing an iron-based catalyst and / or a vanadium-based catalyst can suppress the amount of nitrous oxide produced as a by-product when reducing nitrogen oxides compared to, for example, the second catalytic section containing a copper-based catalyst. Therefore, in this exhaust gas purification system, the control unit calculates the amount of nitrogen oxides that can be reduced by the first catalyst unit, and when this amount exceeds a predetermined amount, it reduces the amount of nitrogen oxides reduced by the second catalyst unit and increases the amount of nitrogen oxides reduced by the first catalyst unit. As a result, for example, after a certain period of time has elapsed since the cold start of the internal combustion engine, nitrogen oxides can be reduced using the first catalyst unit while suppressing the amount of nitrous oxide produced as a by-product. Thus, this exhaust gas purification system can achieve both the purification of nitrogen oxides and the suppression of nitrous oxide emissions. The first catalyst unit may also include a copper-based catalyst. In this case, the weight ratio of the iron-based catalyst (and / or vanadium-based catalyst) to the copper-based catalyst in the first catalyst unit is greater than the weight ratio of the iron-based catalyst (and / or vanadium-based catalyst) to the copper-based catalyst in the second catalyst unit. The weight ratio of the iron-based catalyst (and / or vanadium-based catalyst) to the copper-based catalyst is determined by ((weight of iron-based catalyst) + (weight of vanadium-based catalyst)) / (weight of copper-based catalyst).

[0009] The exhaust gas purification system according to this disclosure further comprises a first supply unit that supplies a reducing agent for reducing nitrogen oxides to a first catalyst unit, and a second supply unit that supplies a reducing agent for reducing nitrogen oxides to a second catalyst unit. The control unit may change the amount of nitrogen oxide reduction in the first and second catalyst units by controlling the amount of reducing agent supplied to the first and second supply units. In this way, when adjusting the amount of nitrogen oxide reduction in the first and second catalyst units, the amount of reducing agent supplied to the first and second catalyst units can be controlled.

[0010] The exhaust gas purification system according to this disclosure further includes a bypass path that branches off from the exhaust passage upstream of the second catalyst section in the exhaust passage and rejoins the exhaust passage downstream of the second catalyst section and upstream of the filter. The control unit may change the amount of nitrogen oxide reduction in the first and second catalyst sections by controlling the amount of exhaust gas flowing through the bypass path. In this way, the amount of exhaust gas flowing through the first and second catalyst sections can be controlled when adjusting the amount of nitrogen oxide reduction in the first and second catalyst sections. [Effects of the Invention]

[0011] According to this disclosure, it is possible to provide an exhaust gas purification system that can achieve both the purification of nitrogen oxides and the reduction of nitrous oxide emissions. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 is a schematic diagram showing an exhaust gas purification system according to the first embodiment. [Figure 2] Figure 2(a) is a schematic diagram showing an example of a part of the first SCR device shown in Figure 1, Figure 2(b) is a schematic diagram showing a part of the second SCR device shown in Figure 1, and Figure 2(c) is a schematic diagram showing another example of a part of the first SCR device shown in Figure 1. [Figure 3]Figure 3(a) is a graph showing an example of the amount of N2O produced during NOx reduction with respect to temperature for each catalyst configuration, and Figure 3(b) is a graph showing an example of the deNOx reduction efficiency with respect to temperature for each catalyst configuration. [Figure 4] Figure 4(a) is a graph showing the temperature over time in the first and second SCR devices, and Figure 4(b) is a graph showing the amount of deNOx over time in the first and second SCR devices. [Figure 5] Figure 5 is a block diagram showing the functional configuration of the control unit shown in Figure 1. [Figure 6] Figure 6 is a flowchart showing the processing of the control unit. [Figure 7] Figure 7 is a schematic diagram showing an exhaust gas purification system according to the second embodiment. [Figure 8] Figure 8 is a schematic diagram showing a part of the first SCR apparatus according to a modified example. [Modes for carrying out the invention]

[0013] The exhaust gas purification system according to one embodiment will be described below with reference to the drawings. In each drawing, the same or corresponding elements are denoted by the same reference numerals, and redundant explanations are omitted. [First Embodiment]

[0014] First, the exhaust gas purification system 1 according to the first embodiment will be described. Figure 1 is a schematic diagram showing the exhaust gas purification system 1 according to the first embodiment. The exhaust gas purification system 1 shown in Figure 1 is installed in vehicles such as large vehicles like buses and trucks, or industrial vehicles, and is used to purify exhaust gases generated by the engine (internal combustion engine) 10. The vehicle on which the exhaust gas purification system 1 is installed may be a so-called series hybrid vehicle that uses the engine 10 as a power source for power generation. Here, nitric oxide (NO) and nitrogen dioxide (NO2) generated from the engine 10 are collectively referred to as nitrogen oxides (NOx).

[0015] The engine 10 is an internal combustion engine such as a diesel engine, and is driven when the remaining capacity of the battery decreases (for example, in the case of a hybrid vehicle). Further, as shown in FIG. 1, the exhaust gas purification system 1 includes an exhaust passage 20. The exhaust passage 20 is for allowing the exhaust gas of the engine 10 mounted on the vehicle to flow. Hereinafter, in the exhaust passage 20, the upstream side in the direction in which the exhaust gas flows (the left side in the example of FIG. 1) may be simply referred to as the "upstream side", and the downstream side in the direction in which the exhaust gas flows (the right side in the example of FIG. 1) may be simply referred to as the "downstream side". The exhaust gas purification system 1 includes a first purification unit 30, a second purification unit 40, and a third purification unit 50. The third purification unit 50, the first purification unit 30, and the second purification unit 40 (hereinafter may be simply referred to as "purification units") are arranged in this order from the upstream side to the downstream side of the exhaust passage 20.

[0016] The first purification unit 30 has a DPF device (filter) 31 disposed in the exhaust passage 20. The DPF device 31 is, for example, a ceramic filter in which a large number of ventilation holes are formed, and is for collecting particulate matter (PM) contained in the exhaust gas.

[0017] Further, the first purification unit 30 has a DOC device 32 provided on the upstream side of the exhaust gas with respect to the DPF device 31. The DOC device 32 includes, for example, a ceramic carrier and an oxidation catalyst supported on the carrier, and oxidizes and purifies hydrocarbons (HC), carbon monoxide (CO), etc. contained in the exhaust gas. Examples of the oxidation catalyst contained in the DOC device 32 include noble metal catalysts such as platinum, rhodium, and palladium.

[0018] Furthermore, the first purification unit 30 has temperature sensors 33, 34, and 35. The temperature sensor 33 is provided upstream of the DOC device 32, the temperature sensor 34 is provided between the DOC device 32 and the DPF device 31, and the temperature sensor 35 is provided downstream of the DPF device 31. The temperature sensor 33 acquires the inlet temperature of the DOC device 32 and outputs information regarding the acquired temperature to a control unit 70 described later. The temperature sensor 34 acquires the outlet temperature of the DOC device 32 (the inlet temperature of the DPF device 31) and outputs information regarding the acquired temperature to the control unit 70. The temperature sensor 35 acquires the outlet temperature of the DPF device 31 and outputs information regarding the acquired temperature to the control unit 70.

[0019] The second purification unit 40 includes a first SCR device (first catalyst unit) 41 and a first ASC (ammonia slip catalyst) 42 arranged in order from the upstream side to the downstream side of the exhaust gas.

[0020] The first SCR device 41 is provided downstream of the DPF device 31 in the exhaust passage 20. The first SCR device 41 includes a selective reduction catalyst for reducing NOx contained in the exhaust gas. The selective reduction catalyst is supported on a carrier such as ceramic, for example. The selective reduction catalyst selectively reduces NOx contained in the exhaust gas using a reducing agent. As the selective reduction catalyst, for example, a copper-based catalyst, an iron-based catalyst, and a vanadium-based catalyst are used. The first SCR device 41 reduces NOx contained in the exhaust gas to nitrogen (N2) and water (H2O) using a reducing agent such as ammonia (NH3).

[0021] The term "reducing agent" includes a precursor of the reducing agent. Thus, the first SCR device 41 is located downstream of the DPF device 31 in the exhaust passage 20 and is a catalyst unit for reducing NOx contained in the exhaust gas. In this embodiment, the activation temperature of the first SCR device 41 is set to approximately 180°C. The activation temperature is the temperature at which the SCR device becomes active and capable of reducing NOx when urea is decomposed into NH3 and adsorbed on the SCR catalyst (selective reduction catalyst). The activation temperature of the first SCR device 41 can be appropriately changed depending on the configuration of the selective reduction catalyst, etc.

[0022] The first ASC42 receives the exhaust gas that has passed through the first SCR device41 and purifies it by oxidizing the excess NH3 contained in the exhaust gas.

[0023] Furthermore, the second purification unit 40 has temperature sensors 43, 44, and 45. Temperature sensor 43 is located upstream of the first SCR device 41, temperature sensor 44 is located between the first SCR device 41 and the first ASC 42, and temperature sensor 45 is located downstream of the first ASC 42. Temperature sensor 43 acquires the inlet temperature of the first SCR device 41 and outputs information regarding the acquired temperature to the control unit 70. Temperature sensor 44 acquires the outlet temperature of the first SCR device 41 and outputs information regarding the acquired temperature to the control unit 70. Temperature sensor 45 acquires the outlet temperature of the first ASC 42 and outputs information regarding the acquired temperature to the control unit 70. Note that temperature sensors 44 and 45 may be omitted depending on the configuration.

[0024] The third purification unit 50 includes a second SCR device (second catalyst section) 51 and a second ASC 52, which are arranged sequentially from the upstream side to the downstream side of the exhaust gas.

[0025] The second SCR device 51 is located upstream of the DPF device 31 in the exhaust passage 20. The second SCR device 51 includes a selective reduction catalyst for reducing NOx contained in the exhaust gas. The selective reduction catalyst is supported on a carrier such as ceramic. For example, a copper-based catalyst is used as the selective reduction catalyst. The second SCR device 51 reduces NOx contained in the exhaust gas to nitrogen (N2) and water (H2O) using a reducing agent such as ammonia (NH3). Thus, the second SCR device 51 is a catalyst unit located upstream of the DPF device 31 in the exhaust passage 20 for reducing NOx contained in the exhaust gas. In this embodiment, the activation temperature of the second SCR device 51 is set to approximately 180°C. The activation temperature of the second SCR device 51 can be appropriately changed depending on the configuration of the selective reduction catalyst, etc.

[0026] The second ASC52 receives the exhaust gas that has passed through the second SCR device 51 and purifies it by oxidizing the excess NH3 contained in the exhaust gas.

[0027] Furthermore, the third purification unit 50 has temperature sensors 53 and 54. Temperature sensor 53 is located upstream of the second SCR device 51, and temperature sensor 54 is located downstream of the second ASC 52. Temperature sensor 53 acquires the inlet temperature of the second SCR device 51 and outputs information about the acquired temperature to the control unit 70. Temperature sensor 54 acquires the outlet temperature of the second ASC 52 and outputs information about the acquired temperature to the control unit 70. Note that temperature sensor 54 may be omitted depending on the configuration.

[0028] In this embodiment, the exhaust gas purification system 1 is equipped with (or may not be equipped with) a DOC device 37. The DOC device 37 is located upstream of the second purification unit 40 and downstream of the engine 10. The DOC device 37 may be the same as the DOC device 32.

[0029] Furthermore, the exhaust gas purification system 1 is equipped with urea addition devices 61 and 62. The urea addition device (first supply unit) 61 is located upstream of the second purification unit 40 (downstream of the first purification unit 30). The urea addition device 61 adds (injects) urea water as a reducing agent to the exhaust passage 20 and exhaust gas upstream of the first SCR device 41. In this way, the urea addition device 61 supplies urea water (reducing agent) to the first SCR device 41 for the reduction of NOx. The urea water injected into the exhaust passage 20 is decomposed into NH3 by the heat of the exhaust gas. The NH3 derived from the urea water is supplied to the first SCR device 41 together with the exhaust gas and is adsorbed by the first SCR device 41. The NH3 adsorbed by the first SCR device 41 reacts with NOx contained in the exhaust gas on the first SCR device 41 to reduce NOx. The urea addition device 61 outputs information regarding the amount of urea solution supplied to the first SCR device 41 to the control unit 70.

[0030] The urea addition device (second supply unit) 62 is located upstream of the third purification unit 50. The urea addition device 62 adds (injects) urea water as a reducing agent to the exhaust passage 20 and exhaust gas upstream of the second SCR device 51. In this way, the urea addition device 62 supplies urea water to the second SCR device 51 for the reduction of NOx. The urea water injected into the exhaust passage 20 is decomposed into NH3 by the heat of the exhaust gas. The NH3 derived from the urea water is supplied to the second SCR device 51 together with the exhaust gas and is adsorbed by the second SCR device 51. The NH3 adsorbed by the second SCR device 51 reacts with NOx contained in the exhaust gas on the second SCR device 51 and is used for the reduction of NOx. The urea addition device 62 outputs information regarding the amount of urea water supplied to the second SCR device 51 to the control unit 70.

[0031] Furthermore, the exhaust gas purification system 1 is equipped with NOx sensors 63, 64, and 65. NOx sensor 63 is located upstream of the second SCR device 51 (in this case, downstream of the DOC device 37), NOx sensor 64 is located downstream of the second ASC 52, and NOx sensor 65 is located downstream of the first ASC 42. Each of the NOx sensors 63, 64, and 65 (hereinafter sometimes simply referred to as NOx sensors) acquires the amount of NOx contained in the exhaust gas and outputs information regarding the acquired amount of NOx to the control unit 70. The NOx sensors may also acquire information regarding the amount of NOx by acquiring the concentration of NOx contained in the exhaust gas. Note that NOx sensor 63 may be located upstream of the DOC device 37 depending on the configuration.

[0032] In this embodiment, the exhaust gas purification system 1 is equipped with a fuel additive valve 66. The fuel additive valve 66 is located between the first purification unit 30 and the second purification unit 40. The fuel additive valve 66 adds (injects) fuel into the exhaust passage 20 at an appropriate position upstream of the DPF device 31. The fuel additive valve 66 may be omitted depending on the configuration.

[0033] As described above, the exhaust gas purification system 1 includes a control unit 70. Details of the control of the control unit 70 will be described later. The control unit 70 is an electronic control unit having a CPU (Central Processing Unit), ROM (Read Only Memory), RAM (Random Access Memory), CAN (Controller Area Network) communication circuit, etc. The control unit 70 loads a program stored in ROM into RAM, and executes the program loaded into RAM with the CPU to realize the functions of each part described later. The control unit 70 may be composed of multiple electronic control units. Furthermore, the control unit 70 may be configured to control the engine 10 as well.

[0034] Figure 2(a) is a schematic diagram showing an example of a part of the first SCR device 41 shown in Figure 1, Figure 2(b) is a schematic diagram showing a part of the second SCR device 51 shown in Figure 1, and Figure 2(c) is a schematic diagram showing another example of a part of the first SCR device 41 shown in Figure 1. As shown in Figure 2(a), the first SCR device 41 includes a carrier 81, a copper-based catalyst 82, and an iron-based catalyst 83. The carrier 81 is for supporting the catalysts, and cordierite can be used as an example. The copper-based catalyst 82 and the iron-based catalyst 83 are supported on the carrier 81. The copper-based catalyst 82 and the iron-based catalyst 83 are arranged sequentially on the carrier 81 from the upstream side to the downstream side of the exhaust gas G. That is, the iron-based catalyst 83 is positioned upstream of the copper-based catalyst 82. Furthermore, as shown in Figure 2(c), the iron-based catalyst 83 may be placed on the surface of the copper-based catalyst 82 (i.e., on the copper-based catalyst 82). More specifically, the copper-based catalyst 82 and the iron-based catalyst 83 may be arranged (stacked) in this order on the carrier 81. In this embodiment, the first SCR apparatus 41 includes the copper-based catalyst 82 and the iron-based catalyst 83, but the iron-based catalyst 83 may be replaced with a vanadium-based catalyst. Also, the first SCR apparatus 41 may include both the iron-based catalyst and the vanadium-based catalyst. Thus, the first SCR apparatus 41 may include the iron-based catalyst 83 and / or the vanadium-based catalyst in addition to the copper-based catalyst 82. However, the first SCR apparatus 41 may include only the iron-based catalyst 83 and / or the vanadium-based catalyst and may not include the copper-based catalyst 82.

[0035] As shown in Figure 2(b), the second SCR apparatus 51 includes a carrier 91 and a copper-based catalyst 92. The carrier 91 is for supporting the catalyst, and as an example, cordierite can be used. The copper-based catalyst 92 is supported on the carrier 91. Unlike the first SCR apparatus 41, the second SCR apparatus 51 does not have an iron-based catalyst (and / or vanadium-based catalyst) on the carrier 91. Thus, the second SCR apparatus 51 may contain only the copper-based catalyst 92 and may not contain an iron-based catalyst and / or a vanadium-based catalyst. However, the second SCR apparatus 51 may also contain an iron-based catalyst and / or a vanadium-based catalyst in addition to the copper-based catalyst 92. In this embodiment, the weight ratio of the iron-based catalyst 83 (and / or vanadium-based catalyst) to the copper-based catalyst 82 in the first SCR apparatus 41 is greater than the weight ratio of the iron-based catalyst (and / or vanadium-based catalyst) to the copper-based catalyst 92 in the second SCR apparatus 51. The weight ratio of the iron-based catalyst (and / or vanadium-based catalyst) to the copper-based catalyst is defined by ((weight of iron-based catalyst) + (weight of vanadium-based catalyst)) / (weight of copper-based catalyst).

[0036] As described above, the exhaust gas purification system 1 is provided with a first SCR device 41 downstream of the first purification unit 30, which has an iron-based catalyst 83 (and / or a vanadium-based catalyst) and a copper-based catalyst 82, and a second SCR device 51 upstream of the first purification unit 30, which has a copper-based catalyst 92 and does not have an iron-based catalyst (and / or a vanadium-based catalyst). In the exhaust gas purification system 1, these first SCR devices 41 and second SCR devices 51 work together to reduce NOx in the exhaust gas G. Next, the characteristics of each catalyst used in the first SCR device 41 and second SCR device 51 will be described.

[0037] Figure 3(a) is a graph showing an example of the amount of N2O produced during NOx reduction with respect to temperature for each catalyst configuration, and Figure 3(b) is a graph showing an example of the NOx reduction efficiency with respect to temperature for each catalyst configuration. In the examples of Figures 3(a) and (b), graph C, shown as a solid line in the figure, shows the amount of N2O produced and the NOx reduction efficiency when only copper-based catalysts are used as the selective reduction catalyst in the SCR system. Graph F, shown as a dashed line in the figure, shows the amount of N2O produced and the NOx reduction efficiency when only iron-based catalysts are used as the selective reduction catalyst in the SCR system. Furthermore, graph CF, shown as a thicker solid line than graph C, shows the amount of N2O produced and the NOx reduction efficiency when both copper-based and iron-based catalysts are used as the selective reduction catalysts in the SCR system.

[0038] As shown in Figure 3(a), the amount of N2O produced by the SCR apparatus using both a copper-based and iron-based catalyst (Graph CF) is lower than that produced by the SCR apparatus using only a copper-based catalyst (Graph F). This indicates that the amount of N2O produced (by-product) can be suppressed by using an iron-based catalyst instead of a copper-based catalyst, or in addition to a copper-based catalyst. When using an iron-based catalyst in addition to a copper-based catalyst, for example, the iron-based catalyst can be placed on the surface or upstream of the copper-based catalyst, as described above.

[0039] Furthermore, as shown in Figure 3(b), the SCR apparatus using only a copper-based catalyst (Graph C) shows a rise in NOx reduction efficiency at lower temperatures compared to the SCR apparatus using only an iron-based catalyst (Graph F) and the SCR apparatus using both copper-based and iron-based catalysts (Graph CF). This indicates that copper-based catalysts become active at lower temperatures than iron-based catalysts.

[0040] Therefore, by primarily using an SCR system with a copper-based catalyst to reduce NOx in the relatively low temperature range, and primarily using an SCR system containing an iron-based catalyst to reduce NOx in the relatively high temperature range, it is possible to achieve both suitable NOx purification and suppression of N2O emissions.

[0041] In particular, in the exhaust gas purification system 1, a second SCR device 51, which has a copper-based catalyst 92 but no iron-based catalyst (and / or vanadium-based catalyst), is installed upstream of the DPF device 31 and DOC device 32, which have large heat capacities in the first purification unit 30. Therefore, as shown in graph FA in Figure 4(a), the temperature rise in the second SCR device 51 occurs earlier compared to the first SCR device 41 shown in graph CFA. In this example, the second SCR device 51 reaches a temperature (here, 180°C) at the first time T1 from the start of the engine 10, at which point urea is decomposed into NH3 on the SCR catalyst (selective reduction catalyst) and adsorbed, enabling the reduction of NOx.

[0042] On the other hand, since the first SCR device 41 is located downstream of the DPF device 31 and DOC device 32, which have large heat capacities in the first purification unit 30, it reaches a similar temperature (180°C) at the second time T2, which is later than the first time T1 from the start of the engine 10, and the amount of reducible NOx gradually increases with further temperature rise. Therefore, in the exhaust gas purification system 1, the control unit 70 reduces the amount of deNOx in the second SCR device 51 and increases the amount of deNOx in the first SCR device 41 as the amount of reducible NOx in the first SCR device 41 increases, thereby enabling both suitable NOx purification and suppression of N2O emissions. The specific configuration of the control unit 70 will be described next.

[0043] Figure 5 is a block diagram showing the functional configuration of the control unit 70 shown in Figure 1. As described above, the control unit 70 changes the amount of deNOx in each of the first SCR device 41 and the second SCR device 51 (hereinafter sometimes simply referred to as "SCR device"). To this end, the control unit 70 includes an information acquisition unit 71, a first possible amount calculation unit 72, a second possible amount calculation unit 73, a first target amount calculation unit 74, a second target amount calculation unit 75, a first indicated amount calculation unit 76, a second indicated amount calculation unit 77, and a urea addition amount calculation unit 78.

[0044] The information acquisition unit 71 acquires various information within the exhaust passage 20. More specifically, the information acquisition unit 71 acquires information regarding the temperature of each part from the temperature sensors 33, 34, and 35 of the first purification unit 30, the temperature sensors 43, 44, and 45 of the second purification unit 40, and the temperature sensors 53 and 54 of the third purification unit 50. The information acquisition unit 71 also acquires information regarding the amount of NOx from the NOx sensors 63, 64, and 65. Furthermore, the information acquisition unit 71 acquires information regarding the amount of urea water supplied from the urea additive devices 61 and 62 to the first SCR device 41 and the second SCR device 51. As a result, the information acquisition unit 71 acquires information regarding the temperature of the exhaust gas G, the flow rate of the exhaust gas G, the amount of NH3 adsorbed, and the deNOx efficiency (NOx reduction efficiency) of the catalyst in each of the first SCR device 41 and the second SCR device 51. Furthermore, the information acquisition unit 71 acquires information regarding the ratio of the amount of NO2 to the amount of NOx contained in the exhaust gas G flowing into the first SCR device 41 and the second SCR device 51 (hereinafter sometimes simply referred to as the "inflow NO2 / NOx ratio"). The information acquired by the information acquisition unit 71 may be changed as appropriate. The flow rate of the exhaust gas G may be acquired by an intake flow sensor (not shown) mounted on the engine 10, or it may be acquired from a map of the engine speed and injection amount of the engine 10.

[0045] The first possible amount calculation unit 72 calculates the first possible amount (possible amount), which is the amount of NOx that can be reduced by the first SCR device 41. The first possible amount calculation unit 72 calculates the first possible amount based on the information acquired by the information acquisition unit 71. As an example, the first possible amount calculation unit 72 calculates the first possible amount based on the temperature and flow rate of the exhaust gas G in the first SCR device 41, the amount of NH3 adsorbed in the first SCR device 41, the deNOx efficiency of the catalyst (copper-based catalyst and iron-based catalyst) in the first SCR device 41, and the inflow NO2 / NOx ratio.

[0046] The second possible amount calculation unit 73 calculates the second possible amount, which is the amount of NOx that can be reduced by the second SCR device 51. The second possible amount calculation unit 73 calculates the second possible amount based on the information acquired by the information acquisition unit 71. As an example, the second possible amount calculation unit 73 calculates the second possible amount based on the temperature and flow rate of the exhaust gas G in the second SCR device 51, the amount of NH3 adsorbed in the second SCR device 51, the deNOx efficiency of the catalyst (copper-based catalyst) in the second SCR device 51, and the inflow NO2 / NOx ratio.

[0047] The first target amount calculation unit 74 calculates the first target amount, which is the target amount of deNOx produced by the first SCR device 41. Here, "deNOx amount" refers to the amount of NOx reduced by the SCR device. As an example, the first target amount calculation unit 74 calculates the first target amount based on the amount of NOx downstream of the DOC device 37 (outlet of the engine 10) acquired by the NOx sensor 63 and the second indicated amount, which will be described later.

[0048] The second target amount calculation unit 75 calculates a second target amount, which is the target amount of deNOx produced by the second SCR device 51. As an example, the second target amount calculation unit 75 calculates the second target amount based on the amount of NOx downstream of the DOC device 37 (outlet of the engine 10) acquired by the NOx sensor 63 and the first possible amount calculated by the first possible amount calculation unit 72.

[0049] The first indicator amount calculation unit 76 calculates a first indicator amount, which is the deNOx amount indicated by the first SCR device 41. As an example, the first indicator amount calculation unit 76 calculates the first indicator amount based on a first possible amount and a first target amount.

[0050] The second indicator calculation unit 77 calculates a second indicator, which is the deNOx amount indicated by the second SCR device 51. As an example, the second indicator calculation unit 77 calculates the second indicator based on the second possible amount and the second target amount.

[0051] The urea addition amount calculation unit 78 calculates the amount of urea solution to be supplied (added) to the SCR device by the urea addition devices 61 and 62. Based on the first indicator amount calculated by the first indicator amount calculation unit 76 and the second indicator amount calculated by the second indicator amount calculation unit 77, the urea addition amount calculation unit 78 calculates the amount of urea solution to be supplied to the SCR device by the urea addition devices 61 and 62.

[0052] In this embodiment, the control unit 70 controls the amount of urea water supplied to the urea addition devices 61 and 62 based on the value calculated by the urea addition amount calculation unit 78, thereby changing the amount of deNOx in the first SCR device 41 and the second SCR device 51.

[0053] Next, a series of processes performed by the control unit 70 to change the amount of deNOx in the first SCR device 41 and the second SCR device 51 will be described. Figure 6 is a flowchart of the processes performed by the control unit 70. As shown in Figure 6, first, when the vehicle engine is started (for example, during a cold start) and while the vehicle is running, the information acquisition unit 71 acquires various pieces of information (step S1). During a cold start of the engine 10, the temperature of the SCR device has not reached the activation temperature of the catalyst in the SCR device, which enables the reduction of NOx by decomposing urea into NH3 and adsorbing it on the SCR catalyst (selective reduction catalyst). For example, the temperature of the SCR device is 180°C or lower.

[0054] Next, the second possible amount calculation unit 73 calculates the deNOx possible amount (second possible amount) of the second SCR device 51 (step S2). As described above, the second possible amount calculation unit 73 calculates the second possible amount based on the information acquired by the information acquisition unit 71.

[0055] Next, the first possible amount calculation unit 72 calculates the deNOx possible amount (first possible amount) of the first SCR device 41 (step S3). As described above, the first possible amount calculation unit 72 calculates the first possible amount based on the information acquired by the information acquisition unit 71.

[0056] Next, the second target amount calculation unit 75 calculates the deNOx target amount (second target amount) for the second SCR device 51 (step S4). As described above, the second target amount calculation unit 75 calculates the second target amount based on the amount of NOx at the outlet of the engine 10 acquired by the NOx sensor 63 and the first possible amount calculated by the first possible amount calculation unit 72 in step S3. More specifically, the second target amount calculation unit 75 calculates the second target amount by subtracting the first possible amount from the amount of NOx at the outlet of the engine 10.

[0057] Therefore, if NOx reduction is not possible in the first SCR device 41 (the first possible amount is 0), the second target amount is calculated so that the entire amount of NOx at the outlet of the engine 10 is reduced by the second SCR device 51. On the other hand, if NOx reduction becomes possible in the first SCR device 41 (the first possible amount becomes greater than 0), the second target amount decreases by the amount of the first possible amount. A lower limit may be set for the second target amount.

[0058] Next, the second indicator amount calculation unit 77 calculates the deNOx indicator amount (second indicator amount) of the second SCR device 51 (step S5). As described above, the second indicator amount calculation unit 77 calculates the second indicator amount based on the second possible amount and the second target amount. More specifically, the second indicator amount calculation unit 77 compares the second possible amount and the second target amount, and if the second possible amount is greater than the second target amount, the second target amount is set as the second indicator amount. On the other hand, the second indicator amount calculation unit 77 also compares the second possible amount and the second target amount, and if the second possible amount is less than the second target amount, the second possible amount is set as the second indicator amount.

[0059] Next, the first target amount calculation unit 74 calculates the deNOx target amount (first target amount) of the first SCR device 41 (step S6). As described above, the first target amount calculation unit 74 calculates the first target amount based on the amount of NOx at the outlet of the engine 10 acquired by the NOx sensor 63 and the second indicated amount. More specifically, the first target amount calculation unit 74 calculates the first target amount by subtracting the second indicated amount from the amount of NOx at the outlet of the engine 10.

[0060] Next, the first indicator amount calculation unit 76 calculates the deNOx indicator amount (first indicator amount) of the first SCR device 41 (step S7). As described above, the first indicator amount calculation unit 76 calculates the first indicator amount based on the first possible amount and the first target amount. More specifically, the first indicator amount calculation unit 76 compares the first possible amount and the first target amount, and if the first possible amount is greater than the first target amount, the first target amount is set as the first indicator amount. On the other hand, the first indicator amount calculation unit 76 also compares the first possible amount and the first target amount, and if the first possible amount is less than the first target amount, the first possible amount is set as the first indicator amount.

[0061] As described above, in process S4, the second target amount is calculated by subtracting the first possible amount calculated in process S3 from the amount of NOx at the outlet of engine 10. Then, in process S5, the second instruction amount is calculated according to the second target amount. On the other hand, in process S6, the first target amount is calculated by subtracting the second instruction amount calculated in process S5 from the amount of NOx at the outlet of engine 10, and in process S7, the first instruction amount is calculated according to the first target amount.

[0062] Therefore, if NOx reduction is not possible in the first SCR device 41 (when the first possible amount is 0), the amount of NOx at the outlet of the engine 10 becomes equal to the second target amount, and the second indicated amount is calculated so that the entire amount of NOx at the outlet of the engine 10 is reduced by the second SCR device 51. In this case, since the amount of NOx at the outlet of the engine 10 is equal to the second indicated amount, the first target amount and the first indicated amount are set to 0.

[0063] As a result, as shown in Figure 4(b), for example, from the first time T1 to the second time T2 when NOx reduction becomes possible in the first SCR device 41, the ratio of the amount of deNOx produced by the first SCR device 41 to the amount of deNOx produced by the second SCR device 51 (graph CFB) is 0.

[0064] On the other hand, when NOx reduction becomes possible in the first SCR device 41 (when the first possible amount becomes greater than a predetermined amount = 0), the second target amount and the second indicator amount decrease by the amount of increase in the first possible amount, and the first target amount and the first indicator amount increase. As a result, for example, from the second time T2 onward, the ratio of the amount of deNOx from the first SCR device 41 to the amount of deNOx D from the second SCR device 51 (graph CFB) gradually increases.

[0065] Thus, in the exhaust gas purification system 1, the control unit 70 reduces the amount of deNOx in the second SCR device 51 and increases the amount of deNOx in the first SCR device 41 as the first possible amount, which is the amount of NOx that can be reduced in the first SCR device 41, increases, thereby enabling both suitable NOx purification and suppression of N2O emissions. In this embodiment, the control unit 70 adjusts the amount of deNOx in the first SCR device 41 and the second SCR device 51 by adjusting the amount of urea water supplied to the first SCR device 41 and the second SCR device 51 based on the first and second indicated amounts calculated as described above.

[0066] In other words, in the subsequent step, the urea addition amount calculation unit 78 calculates the amount of urea solution (urea addition amount) to be supplied to the SCR device by the urea addition devices 61 and 62 (step S8). As described above, the urea addition amount calculation unit 78 calculates the amount of urea solution supplied to the SCR device by the urea addition devices 61 and 62 based on the first indicator amount calculated by the first indicator amount calculation unit 76 and the second indicator amount calculated by the second indicator amount calculation unit 77. In this embodiment, in step S8, the amount of urea solution supplied by the urea addition device 61 to the first SCR device 41 is calculated so that the first indicator amount is the amount of deNOx produced by the first SCR device 41. Also in step S8, the amount of urea solution supplied by the urea addition device 62 to the second SCR device 51 is calculated so that the second indicator amount is the amount of deNOx produced by the second SCR device 51.

[0067] Furthermore, after step S8, the urea addition amount calculation unit 78 outputs information regarding the amount of urea solution calculated to the urea addition device 61 and the urea addition device 62. The urea addition device 61 and the urea addition device 62 each supply urea solution to the SCR device based on the information regarding the amount of urea solution obtained. This adjusts the amount of deNOx in the SCR device.

[0068] As described above, in the exhaust gas purification system 1 according to this embodiment, NOx contained in the exhaust gas G is reduced by a second SCR device 51, which is provided upstream of the DPF device 31 in the exhaust passage 20 and includes a copper-based catalyst 92, and a first SCR device 41, which is provided downstream of the DPF device 31 in the exhaust passage 20 and includes an iron-based catalyst 83. The copper-based catalyst 92 used in the second SCR device 51 can reduce NOx over a relatively wide temperature range compared to, for example, an iron-based catalyst. Furthermore, when the engine 10 is cold-started, the second SCR device 51, which is provided upstream of the DPF device 31 in the exhaust passage 20, reaches its activation temperature earlier than the first SCR device 41, which is provided downstream of the DPF device 31. Therefore, even when the engine 10 is cold-started, NOx can be reduced earlier compared to a configuration in which, for example, a catalyst that reduces NOx is provided only downstream of the DPF device 31 in the exhaust passage 20. Thus, NOx can be efficiently purified even at low temperatures.

[0069] Here, the first SCR device 41, which includes an iron-based catalyst 83, can suppress the amount of N2O produced as a by-product when reducing NOx compared to the second SCR device 51, which includes a copper-based catalyst 92, for example. Therefore, in this exhaust gas purification system 1, the control unit 70 calculates a first possible amount of NOx by the first SCR device 41, and when this first possible amount becomes greater than or equal to a predetermined amount (in this embodiment, when it becomes greater than 0, i.e., when the first SCR device 41 becomes capable of reducing NOx), the amount of deNOx in the second SCR device 51 is reduced and the amount of deNOx in the first SCR device 41 is increased. As a result, for example, after a certain period of time has elapsed since the cold start of the engine 10 (for example, after the second time point T2), NOx can be reduced using the first SCR device 41 while suppressing the amount of N2O produced as a by-product. Thus, with this exhaust gas purification system 1, both NOx purification and N2O emission suppression can be achieved.

[0070] In this embodiment, a lower limit can be set for the second target amount. The effects of this case will now be explained. First, in this embodiment, as described above, the control unit 70 calculates the target amount of NOx reduction in the second SCR device 51 (second target amount) by subtracting the amount of NOx that can be reduced in the first SCR device 41 (first possible amount) from the amount of NOx at the outlet of the engine 10. Therefore, when the first SCR device 41 is in a state where it can sufficiently reduce NOx, the first possible amount becomes large, and as a result, if no lower limit is set for the second target amount, the second target amount becomes extremely small (for example, 0). When the second target amount is extremely small, the second instruction amount that defines the amount of urea water supplied to the second SCR device 51 also becomes small accordingly, and as a result, the amount of urea water supplied to the second SCR device 51 also becomes small, for example, 0.

[0071] Consequently, if no lower limit is set for the second target amount, a situation may occur where urea solution is not supplied to the second SCR device 51. If the exhaust gas purification system 1 continues to operate in this state, the NH3 adsorbed on the second SCR device 51 will be consumed by the reduction of NOx contained in the exhaust gas G discharged from the engine 10, and after the engine 10 is stopped, a sufficient amount of NH3 may not be adsorbed on the second SCR device 51. Therefore, when the engine 10 is cold-started again, the amount of deNOx produced by the second SCR device 51 may be insufficient before the temperature reaches a level where NH3 can be generated from the urea solution. In this regard, by setting a predetermined lower limit for the second target amount, it is possible to avoid the second indicated amount becoming extremely small, and to suppress the occurrence of a situation where urea solution is not supplied to the second SCR device 51. Therefore, after the engine 10 is stopped, NH3 can be pre-adsorbed on the second SCR device 51. Therefore, even when the engine 10 is cold-started next, it is possible to prevent a shortage of deNOx produced by the second SCR device 51. [Second Embodiment]

[0072] Next, the exhaust gas purification system 1A according to the second embodiment will be described. The exhaust gas purification system 1A according to the second embodiment differs from the exhaust gas purification system 1 according to the first embodiment in its configuration and the control performed by the control unit 70. The details of the differences will be described below.

[0073] Figure 7 is a schematic diagram showing an exhaust gas purification system 1A according to the second embodiment. The exhaust gas purification system 1A further includes a bypass passage 20A. The bypass passage 20A branches off from the exhaust passage 20 upstream of the second SCR device 51 in the exhaust passage 20, and rejoins the exhaust passage 20 downstream of the second SCR device 51 and upstream of the DPF device 31 in the exhaust passage 20.

[0074] Furthermore, upstream of the second SCR device 51 in the exhaust passage 20, a control valve 21 is provided at the point where the bypass passage 20A branches off from the exhaust passage 20 to adjust the flow rate of exhaust gas G in the bypass passage 20A. The control valve 21 is controlled by the control unit 70.

[0075] The exhaust gas purification system 1A differs from the exhaust gas purification system 1 in its control for changing the amount of deNOx in the first SCR device 41 and the second SCR device 51. As described above, the bypass passage 20A branches off from the upstream side of the second SCR device 51 in the exhaust passage 20 and merges with the downstream side of the second SCR device 51 in the exhaust passage 20. Therefore, the control unit 70 can increase the amount of exhaust gas G that reaches the first SCR device 41 without passing through the second SCR device 51 by controlling the opening of the control valve 21 to increase the flow rate of exhaust gas G in the bypass passage 20A, thereby increasing the amount of deNOx in the first SCR device 41. Alternatively, the control unit 70 can increase the amount of exhaust gas G that passes through the second SCR device 51 by decreasing the flow rate of exhaust gas G in the bypass passage 20A, thereby increasing the amount of deNOx in the second SCR device 51. By performing this control, the control unit 70 can change the amount of deNOx in the first SCR device 41 and the second SCR device 51.

[0076] Thus, in the exhaust gas purification system 1A, instead of (and in addition to) adjusting the supply amount of urea water in the processing step S8 of the control unit 70 in the exhaust gas purification system 1, the amount of deNOx in the first SCR device 41 and the second SCR device 51 can be changed by adjusting the flow rate of exhaust gas G in the bypass path 20A according to the first and second indicated amounts. In other words, in the exhaust gas purification system 1A, the control unit 70 changes the amount of deNOx in the first SCR device 41 and the second SCR device 51 by controlling the flow rate of exhaust gas G in the bypass path 20A.

[0077] The exhaust gas purification system 1A according to the second embodiment described above can achieve both NOx purification and N2O emission suppression for the same reasons as the exhaust gas purification system 1 according to the first embodiment. Furthermore, according to the exhaust gas purification system 1A according to the second embodiment, when adjusting the amount of deNOx in the first SCR device 41 and the second SCR device 51, the amount of exhaust gas G flowing through the first SCR device 41 and the second SCR device 51 can be controlled. In addition, by configuring the exhaust gas G to bypass the second SCR device 51, the pressure loss due to the resistance of the second SCR device 51 and the second ASC 52 caused by the provision of the second SCR device 51 in the exhaust passage 20 is reduced. As a result, the decrease in the fuel efficiency of the engine 10 can be suppressed.

[0078] In this embodiment, when the first possible amount exceeds a predetermined amount, the control unit 70 controls the adjustment valve 21 to increase the flow rate of exhaust gas G in the bypass passage 20A, thereby reducing the amount of deNOx in the second SCR device 51. In this case, because the flow rate of exhaust gas G to the second SCR device 51 decreases (or exhaust gas G does not flow to the second SCR device 51), even if the supply amount of urea water is reduced to reduce the amount of deNOx in the second SCR device 51 (or even if no urea water is supplied), the NH3 adsorbed on the second SCR device 51 by NOx contained in the exhaust gas G is less likely to be consumed. As a result, after the engine 10 is stopped, the second SCR device 51 can be in a state where NH3 is pre-adsorbed. Therefore, even when the engine 10 is cold-started next, it is possible to suppress insufficient deNOx from the second SCR device 51.

[0079] The embodiments described above illustrate one aspect of the present disclosure, and the present disclosure can be modified without being limited to the above examples.

[0080] Figure 8 is a schematic diagram showing a part of the modified first SCR apparatus 41. The modified first SCR apparatus 41 differs from the first SCR apparatus 41 according to the first embodiment in the arrangement of the catalysts on the carrier 81. As shown in Figure 8, the first SCR apparatus 41 includes a carrier 81, a first copper-based catalyst 82A, a second copper-based catalyst 82B, and an iron-based catalyst 83. The first copper-based catalyst 82A, the second copper-based catalyst 82B, and the iron-based catalyst 83 are supported on the carrier 81. The first copper-based catalyst 82A and the iron-based catalyst 83 are arranged on the carrier 81 in this order from the upstream side to the downstream side of the exhaust gas G. The second copper-based catalyst 82B is arranged between the iron-based catalyst 83 and the carrier 81. Even with the first SCR apparatus 41 in which the catalysts are arranged in this manner, NOx can be reduced using the iron-based catalyst 83 positioned upstream of the carrier 81, and then further reduced using the first copper-based catalyst 82A and the second copper-based catalyst 82B positioned on the carrier 81. This reduces the amount of NOx reduced by the first copper-based catalyst 82A and the second copper-based catalyst 82B, thereby suppressing the amount of N2O produced as a by-product while ensuring a sufficient amount of NOx reduction. The first copper-based catalyst 82A and the second copper-based catalyst 82B may be provided integrally.

[0081] Furthermore, as a modification of the first embodiment, after the vehicle has finished running (for example, when the driver turns off the key), the control unit 70 may perform control to supply heated air and urea water to the second SCR device 51. In this case, for example, the engine stop after turning off the key may be delayed to allow the exhaust gas G and urea to be supplied to the second SCR device 51. As a result, NH3 can be adsorbed onto the second SCR device 51 in advance after the vehicle has finished running. Therefore, even with this configuration, it is possible to suppress the insufficient amount of deNOx produced by the second SCR device 51 when the engine 10 is cold-started. [Explanation of symbols]

[0082] 1... Exhaust gas purification system, 20... Exhaust passage, 20A... Bypass passage, 31... DPF device (filter), 41... First SCR device (first catalyst section), 51... Second SCR device (second catalyst section), 61... Urea additive device (first supply section), 62... Urea additive device (second supply section), 70... Control unit, 83... Iron-based catalyst, 92... Copper-based catalyst, G... Exhaust gas.

Claims

1. An exhaust passage for circulating exhaust gases from the internal combustion engine installed in the vehicle, A filter provided in the exhaust passage for collecting particulate matter contained in the exhaust gas, A first catalyst section provided downstream of the filter in the exhaust passage, comprising an iron-based catalyst and / or a vanadium-based catalyst and a copper-based catalyst for reducing nitrogen oxides contained in the exhaust gas, A second catalyst section is provided upstream of the filter in the exhaust passage and includes a copper-based catalyst for reducing the nitrogen oxides contained in the exhaust gas, but does not include an iron-based catalyst and / or a vanadium-based catalyst. The system comprises a control unit for controlling the amount of nitrogen oxide reduction in the first catalyst section and the second catalyst section, The control unit, The possible amount of nitrogen oxides that can be reduced by the first catalyst is calculated. When the amount of possible reduction exceeds a predetermined amount, the amount of reduction of nitrogen oxides in the second catalyst section is reduced, and the amount of reduction of nitrogen oxides in the first catalyst section is increased. The exhaust passage further comprises a bypass path that branches off from the exhaust passage upstream of the second catalyst in the exhaust passage and merges with the exhaust passage downstream of the second catalyst in the exhaust passage and upstream of the filter, When the possible amount exceeds a predetermined level, the control unit controls the flow rate of the exhaust gas in the bypass path, thereby changing the amount of nitrogen oxide reduction in the first catalyst and the second catalyst. In the first catalyst section, the iron-based catalyst and / or vanadium-based catalyst are arranged upstream of the copper-based catalyst, or on the surface of the copper-based catalyst. Exhaust gas purification system.

2. A first supply unit that supplies a reducing agent for reducing the nitrogen oxides toward the first catalyst unit, The system further comprises a second supply unit that supplies the reducing agent for reducing the nitrogen oxides toward the second catalyst unit, The control unit controls the amount of the reducing agent supplied to the first supply unit and the second supply unit, thereby changing the amount of nitrogen oxide reduction in the first catalyst unit and the second catalyst unit. The exhaust gas purification system according to claim 1.

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