Exhaust purification system

Abstract

To provide an exhaust emission control system capable of reducing nitrogen oxides in exhaust gas in a wide temperature range.SOLUTION: The exhaust gas purification system 1 includes an exhaust gas passage 20, a temperature adjustment device 30, a hydrogen SCR device 41, a urea SCR device 61, a hydrogen addition device 71, a urea addition device 72, and a control unit 80, the hydrogen SCR device 41 has a second activation temperature T2 lower than a first activation temperature T1 which is an activation temperature of the urea SCR device 61, when the inlet temperature Ta of the hydrogen SCR device 41 is within the range of the first activation temperature T1 and the inlet temperature Tb of the urea-SCR device 61 is lower than the lower limit value of the first activation temperature T1, the control unit 80 lowers the inlet temperature Ta of the hydrogen SCR device 41 to within the range of the second activation temperature T2 by activating the temperature adjustment device 30 and adds H2 to the exhaust gas on the upstream side of the hydrogen SCR device 41 by the hydrogenation device 71.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an exhaust gas purification system. [Background technology]

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

[0003] [Patent Document 1] Patent No. 3330654 Summary of the Invention [Problem to be solved by the invention]

[0004] In the above technical field, there is known an exhaust purification system that uses a selective catalytic reduction (SCR) device that reduces (purifies) nitrogen oxides (NOx) in exhaust gas using an SCR catalyst and ammonia (NH3) generated from urea water as a reducing agent. The SCR device of this exhaust purification system becomes active when the exhaust gas temperature is approximately 180°C or higher, and becomes highly active when the exhaust gas temperature is 200°C or higher. However, for example, when the engine is operating at a low load or the vehicle is repeatedly moving and stopping (go-stop), the temperature of the exhaust gas upstream of the SCR device may fall below the temperature at which nitrogen oxides can be reduced (e.g., 180°C), and nitrogen oxides may not be sufficiently reduced.

[0005] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an exhaust gas purification system that can reduce nitrogen oxides in exhaust gases over a wide temperature range. [Means for solving the problem]

[0006] The exhaust purification system according to the present invention includes: [1] "an exhaust passage mounted on a vehicle and through which exhaust gas from an internal combustion engine flows; a temperature adjustment device provided in the exhaust passage for lowering the temperature of the exhaust gas; a hydrogen SCR device provided in the exhaust passage downstream of the temperature adjustment device for reducing nitrogen oxides contained in the exhaust gas using hydrogen as a reducing agent; a urea SCR device provided in the exhaust passage downstream of the hydrogen SCR device for reducing nitrogen oxides in the exhaust gas using urea as a reducing agent; a hydrogen addition device provided in the exhaust passage upstream of the hydrogen SCR device for adding hydrogen to the exhaust gas upstream of the hydrogen SCR device; a urea addition device provided in the exhaust passage upstream of the urea SCR device for adding urea to the exhaust gas upstream of the urea SCR device; and a control unit for controlling the temperature adjustment device, the hydrogen addition device, and the urea addition device, and when the temperature of the exhaust gas upstream of the hydrogen SCR device is within the range of the first activation temperature and the temperature of the exhaust gas downstream of the hydrogen SCR device and upstream of the urea SCR device is lower than a lower limit of the first activation temperature, the control unit activates the temperature adjustment device to lower the temperature of the exhaust gas upstream of the hydrogen SCR device to within the range of the second activation temperature of the hydrogen SCR device and adds hydrogen to the exhaust gas upstream of the hydrogen SCR device by the hydrogen addition device, and when the temperature of the exhaust gas upstream of the hydrogen SCR device and the temperature of the exhaust gas downstream of the hydrogen SCR device and upstream of the urea SCR device are both within the range of the first activation temperature, the control unit stops the temperature adjustment device and adds urea to the exhaust gas upstream of the urea SCR device by the urea addition device.

[0007] This exhaust purification system includes a hydrogen SCR device that uses hydrogen as a reducing agent to reduce nitrogen oxides contained in exhaust gas, and a urea SCR device that uses urea as a reducing agent to reduce nitrogen oxides in the exhaust gas. That is, the system includes not only a urea SCR device but also a hydrogen SCR device. As a result, even when the exhaust gas temperature is below a first activation temperature of the urea SCR device and it is difficult to reduce nitrogen oxides using the urea SCR device, the hydrogen SCR device, which has a second activation temperature lower than the first activation temperature, can still reduce nitrogen oxides. Furthermore, when the exhaust gas temperature upstream of the hydrogen SCR device is within the first activation temperature range of the urea SCR device and the exhaust gas temperature downstream of the hydrogen SCR device but upstream of the urea SCR device is outside the first activation temperature range, the control unit activates the temperature adjustment device to lower the exhaust gas temperature upstream of the hydrogen SCR device to within the second activation temperature range of the hydrogen SCR device, and the hydrogen addition device adds hydrogen to the exhaust gas upstream of the hydrogen SCR device. As a result, even if the exhaust gas temperature is outside the first activation temperature range and it is difficult to reduce nitrogen oxides using the urea SCR device, the temperature adjustment device can adjust the exhaust gas temperature to within the second activation temperature range of the hydrogen SCR device, allowing the hydrogen SCR device to reduce nitrogen oxides in the exhaust gas. Furthermore, when the exhaust gas temperature upstream of the hydrogen SCR device and the exhaust gas temperature downstream of the hydrogen SCR device but upstream of the urea SCR device are both within the first activation temperature range, the control unit stops the temperature adjustment device and causes the urea addition device to add urea to the exhaust gas upstream of the urea SCR device. As a result, even if the exhaust gas temperature is outside the second activation temperature range of the hydrogen SCR device and it is difficult to reduce nitrogen oxides using the hydrogen SCR device, it is possible to reduce nitrogen oxides in the exhaust gas using the urea SCR device. As described above, nitrogen oxides in the exhaust gas can be reduced by at least one of the hydrogen SCR device and the urea SCR device depending on the exhaust gas temperature, allowing nitrogen oxides in the exhaust gas to be reduced over a wide temperature range, from low to high.

[0008] The exhaust gas purification system according to the present invention may be [2] "the exhaust gas purification system according to [1], wherein the control unit adds hydrogen using the hydrogen adding device when the temperature of the exhaust gas upstream of the hydrogen SCR device is within the second activation temperature range or is lower than the lower limit of the second activation temperature range." In this case, nitrogen oxides in the exhaust gas can be reduced over a wider temperature range.

[0009] The exhaust gas purification system according to the present invention may be the exhaust gas purification system according to [3], wherein "when the temperature of the exhaust gas upstream of the hydrogen SCR device is lower than the lower limit of the second activation temperature, the control unit adds hydrogen in an amount greater than the amount of hydrogen added by the hydrogen addition device when the temperature of the exhaust gas upstream of the hydrogen SCR device is within the second activation temperature range." In this case, for example, the temperature of the exhaust gas can be raised to within the second activation temperature range of the hydrogen SCR device by burning the hydrogen added by the hydrogen addition device. This allows nitrogen oxides to be effectively reduced by the hydrogen SCR device. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide an exhaust gas purification system that can reduce nitrogen oxides in exhaust gas over a wide temperature range from low to high temperatures. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram showing an exhaust gas purification system according to an embodiment. [Figure 2] FIG. 2(a) is a graph showing the temperature of the exhaust gas when the first control is executed, and FIG. 2(b) is a graph showing the temperature of the exhaust gas when the second control is executed. [Figure 3] FIG. 3(a) is a graph showing the temperature of the exhaust gas when the third control is executed, and FIG. 3(b) is a graph showing the temperature of the exhaust gas when the fourth control is executed. DETAILED DESCRIPTION OF THE INVENTION

[0012] An exhaust gas purification system according to one embodiment will be described below with reference to the drawings. In the drawings, the same or corresponding elements are denoted by the same reference numerals, and redundant description will be omitted. [Embodiment]

[0013] Fig. 1 is a schematic diagram showing an exhaust purification system 1 according to an embodiment. The exhaust purification system 1 shown in Fig. 1 is mounted on a vehicle, such as a large vehicle such as a bus or truck, or an industrial vehicle, for purifying exhaust gas generated by an engine (internal combustion engine) 10. The vehicle on which the exhaust purification system 1 is mounted may be, for example, a so-called series hybrid vehicle that uses the engine 10 as a power source for generating electricity. Here, nitrogen monoxide (NO), nitrogen dioxide (NO2), and the like generated by the engine 10 are collectively referred to as nitrogen oxides (NOx).

[0014] The engine 10 is an internal combustion engine such as a diesel engine, and is driven when the remaining capacity of the battery is low (for example, in the case of a hybrid vehicle). As shown in FIG. 1, the exhaust purification system 1 includes an exhaust passage 20. The exhaust passage 20 is used to circulate exhaust gas from the engine 10 mounted on a vehicle. The exhaust passage 20 extends from the outlet of the engine 10 to the vehicle's tailpipe. Hereinafter, the upstream side of the exhaust passage 20 in the direction of exhaust gas flow (the left side in the example of FIG. 1) may be simply referred to as the "upstream side," and the downstream side of the exhaust gas flow (the right side in the example of FIG. 1) may be simply referred to as the "downstream side." The exhaust purification system 1 includes a temperature adjustment device 30, a first purification unit 40, a second purification unit 50, and a third purification unit 60. The temperature adjustment device 30, the first purification unit 40, the second purification unit 50, and the third purification unit 60 are arranged in this order from the upstream side to the downstream side of the exhaust passage 20.

[0015] The temperature adjustment device 30 is provided in the exhaust passage 20 and serves to lower the temperature of the exhaust gas. The temperature adjustment device 30 is, for example, a waste heat recovery device that recovers heat from the exhaust gas. As an example, a waste heat recovery device can lower the temperature of the exhaust gas by recovering heat from the exhaust gas using a heat medium and converting it into mechanical or electrical energy. The temperature adjustment device 30 is electrically connected to a control unit 80, which will be described later. The temperature adjustment device 30 is configured to be switched between an operating state (ON) and a stopped state (OFF) based on a control signal from the control unit 80. The temperature adjustment device 30 may be configured to relatively greatly reduce the temperature of the exhaust gas in the operating state, while having a certain heat capacity even in the stopped state, thereby relatively slightly reducing the temperature of the exhaust gas. Furthermore, the temperature adjustment device 30 may not only be capable of simply switching between the stopped state and the operating state, but may also be configured to continuously adjust the exhaust gas temperature (e.g., a Rankine cycle device or a thermoelectric element).

[0016] The first purification unit 40 has a hydrogen SCR device 41 (H2-SCR) and a temperature sensor 42. The first purification unit 40 is provided downstream of the temperature adjustment device 30 in the exhaust passage 20. The first purification unit 40 uses hydrogen (H2) as a reducing agent to reduce NOx contained in the exhaust gas.

[0017] The hydrogen 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. The selective reduction catalyst selectively reduces NOx contained in the exhaust gas using a reducing agent. Examples of the selective reduction catalyst that can be used include platinum-based catalysts, palladium-based catalysts, iridium-based catalysts, and rhodium-based catalysts. The hydrogen SCR device 41 reduces NOx contained in the exhaust gas to nitrogen (N2) and water (H2O) using a reducing agent such as H2. Note that the "reducing agent" includes a precursor of the reducing agent.

[0018] In this embodiment, the hydrogen SCR device 41 has a second activation temperature T2 that is lower than a first activation temperature T1, which is the activation temperature of the urea SCR device 61 described below (see FIG. 2). The second activation temperature T2 is the activation temperature of the hydrogen SCR device 41. The activation temperature of the hydrogen SCR device 41 refers to the temperature at which H2 is adsorbed onto the SCR catalyst (selective reduction catalyst), thereby activating the SCR device and enabling it to reduce NOx. In this embodiment, the second activation temperature T2 of the hydrogen SCR device 41 is set to a range of, for example, 100°C or higher and lower than 150°C. The activation temperature of the hydrogen SCR device 41 can be changed as appropriate depending on the configuration of the selective reduction catalyst, etc.

[0019] The temperature sensor 42 is provided upstream of the hydrogen SCR device 41. The temperature sensor 42 acquires the temperature of the exhaust gas upstream of the hydrogen SCR device 41 (the inlet temperature Ta of the hydrogen SCR device 41). The temperature sensor 42 is electrically connected to a control unit 80, which will be described later. The temperature sensor 42 acquires the inlet temperature Ta of the hydrogen SCR device 41 and outputs information related to the acquired temperature to the control unit 80. Hereinafter, the temperature of the exhaust gas acquired by the temperature sensor 42 may be simply referred to as the "temperature of the temperature sensor 42."

[0020] The second purification section 50 has a DOC (Diesel Oxidation Catalyst) device 51 and a DPF (Diesel Particulate Filter) device 52. The DOC device 51 and the DPF device 52 are arranged in this order from the upstream side to the downstream side of the exhaust passage 20. The DOC device 51 includes a carrier made of, for example, ceramic and an oxidation catalyst supported on the carrier, and oxidizes and purifies hydrocarbons (HC), carbon monoxide (CO), and the like contained in the exhaust gas. Examples of the oxidation catalyst included in the DOC device 51 include precious metal catalysts such as platinum, rhodium, and palladium.

[0021] The DPF device 52 is, for example, a ceramic filter having a large number of air holes formed therein, and is used to capture particulate matter (PM) contained in the exhaust gas.

[0022] The third purification section 60 has a urea SCR device 61, an ASC (ammonia slip catalyst) device 62, and a temperature sensor 63. The urea SCR device 61 is provided downstream of the hydrogen SCR device 41 in the exhaust passage 20. The urea SCR device 61 uses urea as a reducing agent to reduce nitrogen oxides in the exhaust gas. The urea SCR device 61 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. The selective reduction catalyst selectively reduces NOx contained in the exhaust gas using a reducing agent. For example, a copper-based catalyst, an iron-based catalyst, or a vanadium-based catalyst is used as the selective reduction catalyst. The urea SCR device 61 uses a reducing agent such as ammonia (NH3) to reduce NOx contained in the exhaust gas into nitrogen (N2) and water (H2O).

[0023] The urea SCR device 61 has a first activation temperature T1, which is the activation temperature of the urea SCR device 61 (see FIG. 2). The activation temperature of the urea SCR device 61 refers to the temperature at which urea is decomposed into NH3 on an SCR catalyst (selective reduction catalyst) and adsorbed thereon, thereby activating the SCR device and enabling it to reduce NOx. In this embodiment, the urea SCR device 61 is activated at 180°C or higher and highly active at 200°C or higher. Therefore, the range of the first activation temperature T1 of the urea SCR device 61 in this embodiment is set to, for example, 200°C or higher. The activation temperature of the urea SCR device 61 can be changed as appropriate depending on the configuration of the selective reduction catalyst, etc.

[0024] The ASC device 62 receives the exhaust gas that has passed through the urea SCR device 61 and purifies the exhaust gas by oxidizing excess NH3 contained in the exhaust gas.

[0025] The temperature sensor 63 is provided downstream of the first purification section 40 (i.e., the hydrogen SCR device 41) and the second purification section 50 and upstream of the urea SCR device 61 (hereinafter may be simply referred to as "upstream of the urea SCR device 61"). The temperature sensor 63 acquires the temperature of the exhaust gas upstream of the urea SCR device 61 (the inlet temperature Tb of the urea SCR device 61). The temperature sensor 63 is electrically connected to a control unit 80, which will be described later. The temperature sensor 63 acquires the inlet temperature Tb of the urea SCR device 61 and outputs information related to the acquired temperature to the control unit 80. Hereinafter, the temperature of the exhaust gas acquired by the temperature sensor 63 may be simply referred to as "the temperature of the temperature sensor 63."

[0026] The exhaust purification system 1 further includes a hydrogen addition device 71 and a urea addition device 72. The hydrogen addition device 71 is provided upstream of the hydrogen SCR device 41 in the exhaust passage 20. The hydrogen addition device 71 is electrically connected to the control unit 80. The hydrogen addition device 71 adds H2 to the exhaust gas upstream of the hydrogen SCR device 41 based on a control signal from the control unit 80. Specifically, the hydrogen addition device 71 adds H2 as a reducing agent to the exhaust passage 20 and the exhaust gas upstream of the hydrogen SCR device 41. In this way, the hydrogen addition device 71 supplies H2 to the hydrogen SCR device 41 for reducing NOx. The H2 is supplied to the hydrogen SCR device 41 together with the exhaust gas and is adsorbed by the hydrogen SCR device 41. The H2 adsorbed by the hydrogen SCR device 41 reacts with NOx contained in the exhaust gas in the hydrogen SCR device 41 to reduce the NOx.

[0027] The urea addition device 72 is provided in the exhaust passage 20 upstream of the urea SCR device 61. The urea addition device 72 is electrically connected to the control unit 80. The urea addition device 72 adds urea to the exhaust gas upstream of the urea SCR device 61 based on a control signal from the control unit 80. Specifically, the urea addition device 72 adds urea water as a reducing agent to the exhaust passage 20 and the exhaust gas upstream of the urea SCR device 61. In this way, the urea addition device 72 supplies urea water (reducing agent) to the urea SCR device 61 for reducing 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 urea SCR device 61 together with the exhaust gas and is adsorbed by the urea SCR device 61. The NH3 adsorbed in the urea SCR device 61 reacts with NOx contained in the exhaust gas in the urea SCR device 61 and is used to reduce NOx.

[0028] As described above, the exhaust purification system 1 includes the control unit 80. Details of the control of the control unit 80 will be described later. The control unit 80 is an electronic control unit having a CPU (Central Processing Unit), ROM (Read Only Memory), RAM (Random Access Memory), a CAN (Controller Area Network) communication circuit, etc. The control unit 80 loads a program stored in the ROM into the RAM, and executes the program loaded into the RAM by the CPU, thereby realizing the functions of each unit described later. The control unit 80 may be made up of multiple electronic control units.

[0029] Next, details of the control of the control unit 80 will be explained with reference to Figures 2 and 3. The control unit 80 controls the amount of H2 added by the hydrogen addition device and the amount of urea added by the urea addition device 72 based on the inlet temperature Ta (temperature of the temperature sensor 42) of the hydrogen SCR device 41 and the inlet temperature Tb (temperature of the temperature sensor 63) of the urea SCR device 61. The control unit 80 executes first control, second control, third control, and fourth control. The first control, second control, third control, and fourth control will be explained in detail below.

[0030] Figure 2(a) is a graph showing the exhaust gas temperature when the first control is executed. Each of the graphs in Figures 2 and 3 shows the exhaust gas temperature at the position of each device in the exhaust passage 20 from the engine outlet to the tailpipe. Therefore, the horizontal axis of each of the graphs in Figures 2 and 3 shows the name of each device as the exhaust gas temperature measurement position in the exhaust passage 20. These names correspond to the upstream temperature measurement position of each device. Furthermore, the vertical axis of each of the graphs in Figures 2 and 3 shows the exhaust gas temperature.

[0031] 2(a), the temperature of the exhaust gas when the first control of this embodiment is being executed is indicated by a solid line E1. The temperature of the exhaust gas when the control unit 80 is not executing the first control is indicated by a dashed-dotted line C1 (however, the temperature drop due to the heat capacity of the temperature adjustment device 30 is not shown). Furthermore, the range of the first activation temperature T1 of the urea SCR device 61 (the high activation region of the urea SCR device 61) is indicated by dotted hatching, and the range of the second activation temperature T2 of the hydrogen SCR device 41 (the high activation region of the hydrogen SCR device 41) is indicated by hatched hatching.

[0032] The first control is executed when the inlet temperature Ta of the hydrogen SCR device 41 (i.e., the temperature of the temperature sensor 42, which is the temperature of the exhaust gas upstream of the hydrogen SCR device 41) is within the range of the first activation temperature T1 (200°C or higher in this example), as shown by the dashed-dotted line C1 in FIG. 2(a), and the inlet temperature Tb of the urea SCR device 61 (i.e., the temperature of the temperature sensor 63, which is the temperature of the exhaust gas upstream of the urea SCR device 61) is lower than the lower limit of the first activation temperature T1 (200°C in this example). In the first control, as shown by the solid line E1 in FIG. 2(a), the control unit 80 first activates the temperature adjustment device 30 to lower the temperature downstream of the temperature adjustment device 30 (arrow M1 in the figure).

[0033] As a result, the inlet temperature Ta of the hydrogen SCR device 41 is lowered to within the range of the second activation temperature T2 of the hydrogen SCR device. At the same time, the control unit 80 causes the hydrogen addition device 71 to add H2 for NOx reduction to the exhaust gas upstream of the hydrogen SCR device 41 (arrow M2 in the figure). This allows the hydrogen SCR device 41 to reduce NOx in the exhaust gas. Note that in the first control, the control unit 80 does not execute the urea addition device 72 to add urea to the exhaust gas. Here, the amount of H2 for NOx reduction necessary to reduce NOx in the exhaust gas is added. Furthermore, as shown in FIG. 2(a), when H2 for NOx reduction is added, the inlet temperature Ta of the hydrogen SCR device 41 rises slightly due to the combustion of H2.

[0034] As described above, in the first control, even if the inlet temperature Tb of the urea SCR device 61 is lower than the lower limit value of the first activation temperature T1 and it is difficult to reduce NOx using the urea SCR device 61, the temperature adjustment device 30 deliberately lowers the inlet temperature Ta of the hydrogen SCR device 41 to within the range of the second activation temperature T2, thereby making it possible to reduce NOx using the hydrogen SCR device 41.

[0035] 2(b) is a graph showing the exhaust gas temperature when the second control is executed. In FIG. 2(b), the exhaust gas temperature when the second control of this embodiment is executed is shown by a solid line E2. The exhaust gas temperature when the control unit 80 is not executing the second control is shown by a dashed-dotted line C2 (however, the temperature drop due to the heat capacity of the temperature adjustment device 30 is not shown).

[0036] The second control is executed when the inlet temperature Ta of the hydrogen SCR device 41 and the inlet temperature Tb of the urea SCR device 61 are both within the range of the first activation temperature T1 (200°C or higher in this example), as shown by the dashed-dotted line C2 in Fig. 2(b). In the second control, the control unit 80 stops the temperature adjustment device 30 (arrow M3 in the figure), as shown by the solid line E2 in Fig. 2(a), and causes the urea addition device 72 to add urea to the exhaust gas upstream of the urea SCR device 61 (arrow M4 in the figure). This allows the urea SCR device 61 to reduce NOx in the exhaust gas.

[0037] That is, even if the inlet temperature Ta of the hydrogen SCR device 41 is outside the range of the second activation temperature T2 and it is difficult to reduce NOx using the hydrogen SCR device 41, it is possible to reduce NOx in the exhaust gas using the urea SCR device 61. Note that in the second control, the control unit 80 does not add H2 to the exhaust gas using the hydrogen addition device 71. Also, in FIG. 2(b), the temperature of the exhaust gas at the temperature adjustment device 30 drops slightly, but this temperature drop is caused by the heat capacity of the temperature adjustment device 30 as described above.

[0038] 3A is a graph showing the exhaust gas temperature when the third control is executed. In FIG. 3A, the exhaust gas temperature when the third control of this embodiment is executed is shown by a solid line E3. The exhaust gas temperature when the control unit 80 is not executing the third control is shown by a dashed-dotted line C3 (however, the temperature drop due to the heat capacity of the temperature adjustment device 30 is not shown).

[0039] The third control is executed when the inlet temperature Ta of the hydrogen SCR device 41 is lower than the lower limit value (100°C in this example) of the range of the second activation temperature T2, as shown by the dashed line C3 in Fig. 3(a), for example. In the third control, as shown by the solid line E3 in Fig. 3(a), for example, the control unit 80 stops the temperature adjustment device 30 (arrow M5 in the figure) and causes the hydrogen addition device 71 to add H2 for raising the temperature and H2 for reducing NOx to the exhaust gas upstream of the hydrogen SCR device 41 (arrow M6 in the figure).

[0040] Here, the H2 for temperature increase refers to H2 that is added to the exhaust gas in addition to the H2 for NOx reduction in order to increase the temperature of the exhaust gas. That is, the control unit 80 adds a larger amount of H2 to the exhaust gas than the amount of H2 added for NOx reduction. In this way, even if the inlet temperature Ta of the hydrogen SCR device 41 is lower than the lower limit of the second activation temperature T2, by adding a larger amount of H2 than the H2 for NOx reduction, the temperature of the exhaust gas can be raised to the range of the second activation temperature T2 by combustion of H2. This allows the hydrogen SCR device 41 to effectively reduce NOx. Note that in the third control, the control unit 80 does not execute the urea addition device 72 to add urea to the exhaust gas.

[0041] 3(b) is a graph showing the exhaust gas temperature when the fourth control is executed. In FIG. 3(b), the exhaust gas temperature when the fourth control of this embodiment is executed is shown by a solid line E4. The exhaust gas temperature when the control unit 80 is not executing the fourth control is shown by a dashed-dotted line C4 (however, the temperature drop due to the heat capacity of the temperature adjustment device 30 is not shown).

[0042] The fourth control is executed when the inlet temperature Ta of the hydrogen SCR device 41 is within the range of the second activation temperature T2 (in this example, 100°C or higher and lower than 150°C), as shown by the dashed-dotted line C4 in FIG. 3B. In the fourth control, the control unit 80 stops the temperature adjustment device 30 (arrow M7 in the figure) and causes the hydrogen addition device 71 to add H2 for NOx reduction to the exhaust gas upstream of the hydrogen SCR device 41 (arrow M8 in the figure), as shown by the solid line E4 in FIG. 3B. This allows the hydrogen SCR device 41 to reduce NOx in the exhaust gas even when the inlet temperature Ta of the hydrogen SCR device 41 is within the range of the second activation temperature T2 (outside the range of the first activation temperature T1) and it is difficult to reduce NOx using the urea SCR device 61. Note that in the fourth control, the control unit 80 does not execute the urea addition device 72 to add urea to the exhaust gas.

[0043] As described above, the control unit 80 executes the third control when the inlet temperature Ta of the hydrogen SCR device 41 is lower than the lower limit of the range of the second activation temperature T2, and executes the fourth control when the inlet temperature Ta of the hydrogen SCR device 41 is within the range of the second activation temperature T2. That is, the control unit 80 executes the third control or the fourth control when the inlet temperature Ta of the hydrogen SCR device 41 is within the range of the second activation temperature T2 or is lower than the lower limit of the range of the second activation temperature T2, and adds H2 (H2 for NOx reduction) in an amount sufficient to reduce at least the NOx in the exhaust gas. As a result, even if the inlet temperature Ta of the hydrogen SCR device 41 is outside the range of the first activation temperature T1 and it is difficult to reduce NOx using the urea SCR device 61, the hydrogen SCR device 41 can reduce NOx in the exhaust gas.

[0044] Furthermore, the amount of H2 added in the third control is greater than the amount of H2 added for NOx reduction in the fourth control. That is, the control unit 80 adds a greater amount of H2 than the amount of H2 added by the hydrogen adding device 71 when the inlet temperature Ta of the hydrogen SCR device 41 is within the range of the second activation temperature T2 (the amount of H2 added in the fourth control). As a result, as described above, even if the inlet temperature Ta of the hydrogen SCR device 41 is lower than the lower limit of the range of the second activation temperature T2, the combustion of H2 can raise the temperature of the exhaust gas to within the range of the second activation temperature T2, and the hydrogen SCR device 41 can effectively reduce NOx. [Action and effect]

[0045] As described above, the exhaust purification system 1 according to this embodiment includes the hydrogen SCR device 41 for reducing NOx contained in exhaust gas using H2 as a reducing agent, and the urea SCR device 61 for reducing NOx in exhaust gas using urea as a reducing agent. That is, the system includes not only the urea SCR device 61 but also the hydrogen SCR device 41. As a result, even if, for example, the temperature of the exhaust gas is below the first activation temperature T1 of the urea SCR device 61 and it is difficult to reduce NOx using the urea SCR device 61, it is possible to reduce NOx using the hydrogen SCR device 41, which has a second activation temperature T2 that is lower than the first activation temperature T1. Furthermore, when the inlet temperature Ta of the hydrogen SCR device 41 is within the range of the first activation temperature T1 of the urea SCR device 61 and the inlet temperature Tb of the urea SCR device 61 is outside the range of the first activation temperature T1, the control unit 80 activates the temperature adjustment device 30 to lower the inlet temperature Ta of the hydrogen SCR device 41 to within the range of the second activation temperature T2 of the hydrogen SCR device 41, and causes the hydrogen addition device 71 to add H2 to the exhaust gas upstream of the hydrogen SCR device 41 (first control). As a result, even if the temperature of the exhaust gas is outside the range of the first activation temperature T1 and it is difficult to reduce NOx using the urea SCR device 61, the temperature adjustment device 30 can adjust the exhaust gas temperature to within the range of the second activation temperature T2 of the hydrogen SCR device 41, allowing the hydrogen SCR device 41 to reduce NOx in the exhaust gas.

[0046] Furthermore, when the inlet temperature Ta of the hydrogen SCR device 41 and the inlet temperature Tb of the urea SCR device 61 are both within the range of the first activation temperature T1, the control unit 80 stops the temperature adjustment device 30 and causes the urea addition device 72 to add urea to the exhaust gas upstream of the urea SCR device 61 (second control). As a result, even if the temperature of the exhaust gas is outside the range of the second activation temperature T2 of the hydrogen SCR device 41 and it is difficult to reduce NOx using the hydrogen SCR device 41, the urea SCR device 61 can reduce NOx in the exhaust gas. As described above, NOx in the exhaust gas can be reduced by at least one of the hydrogen SCR device 41 and the urea SCR device 61 depending on the temperature of the exhaust gas, and therefore NOx in the exhaust gas can be reduced over a wide temperature range from low to high temperatures.

[0047] When the inlet temperature Ta of the hydrogen SCR device 41 is within the range of the second activation temperature T2 or is lower than the lower limit of the range of the second activation temperature T2, the control unit 80 adds H2 using the hydrogen addition device 71 (third control or fourth control). In this case, nitrogen oxides in the exhaust gas can be reduced over a wider temperature range.

[0048] When the inlet temperature Ta of the hydrogen SCR device 41 is lower than the lower limit value of the second activation temperature T2, the control unit 80 adds a larger amount of H2 than the amount of H2 added by the hydrogen addition device 71 when the inlet temperature Ta of the hydrogen SCR device 41 is within the range of the second activation temperature T2 (third control). In this case, for example, by burning the H2 added by the hydrogen addition device 71, the temperature of the exhaust gas can be raised to within the range of the second activation temperature T2 of the hydrogen SCR device 41. This allows the hydrogen SCR device 41 to effectively reduce NOx. [Variations]

[0049] The above embodiment describes one aspect of the present disclosure, and the present disclosure is not limited to the above example and may be modified. For example, the exhaust gas purification system 1 does not need to have the temperature sensor 63. In this case, the inlet temperature Tb of the urea SCR device 61 may be estimated from the inlet temperature Ta of the hydrogen SCR device 41.

[0050] Furthermore, the temperature conditions (threshold values) of the exhaust gas when the control unit 80 switches between the first control, second control, third control, and fourth control may be changed as appropriate. For example, the range of the first activation temperature T1 and the range of the second activation temperature T2 may be changed as appropriate. In the embodiment, the range of the first activation temperature T1 is 200°C or higher, but the range of the first activation temperature T1 may be 150°C or higher.

[0051] In this case, the control unit 80 may execute the first control when the inlet temperature Ta of the hydrogen SCR device 41 is equal to or greater than 150° C. and less than 240° C. Specifically, when the inlet temperature Ta of the hydrogen SCR device 41 is equal to or greater than 150° C. and less than 240° C. (within the range of the first activation temperature), the control unit 80 may execute the first control by estimating that the inlet temperature Tb of the downstream urea SCR device 61 will be lower than the lower limit value of the first activation temperature T1 (150° C. in this example).

[0052] In this case, the control unit 80 may also execute the second control when the inlet temperature Ta of the hydrogen SCR device 41 is 240° C. or higher. Specifically, when the inlet temperature Ta of the hydrogen SCR device 41 is 240° C. or higher, the control unit 80 may also estimate that the inlet temperature Tb of the urea SCR device 61 is also within the range of the first activation temperature T1 (150° C. or higher in this example), and execute the second control.

[0053] Furthermore, when the temperature of the exhaust gas is rising or falling, hysteresis may be applied to the threshold value that serves as the temperature condition for switching the control of the control unit 80. Furthermore, the control unit 80 may add H2 for raising the temperature by using the hydrogen addition device 71 in all control operations. In this case, the temperature of the exhaust gas is raised, and the urea SCR device 61 can be activated early.

[0054] Furthermore, in the first control, when the inlet temperature Tb of the urea SCR device 61 reaches the range of the first activation temperature T1 due to the addition of H2 for NOx reduction, the control unit 80 may add urea using the urea addition device 72 and reduce NOx using the urea SCR device 61. Furthermore, in the first control, the control unit 80 may gradually lower the temperature of the exhaust gas using the temperature adjustment device 30. Similarly, in the first control, the control unit 80 may gradually add H2 to the exhaust gas using the hydrogen addition device 71. The hydrogen SCR device 41 may also have the function (action) of the DOC device 51. In this case, the second purification unit 50 does not need to have the DOC device 51. [Explanation of symbols]

[0055] 1...exhaust gas purification system, 10...engine (internal combustion engine), 20...exhaust passage, 30...temperature adjustment device, 41...hydrogen SCR device, 61...urea SCR device, 71...hydrogen addition device, 72...urea addition device, 80...control unit, T1...first activation temperature, T2...second activation temperature.

Claims

1. an exhaust passage mounted on a vehicle and through which exhaust gas from an internal combustion engine flows; a temperature adjusting device provided in the exhaust passage for reducing the temperature of the exhaust gas; a hydrogen SCR device provided in the exhaust passage downstream of the temperature adjustment device for reducing nitrogen oxides contained in the exhaust gas using hydrogen as a reducing agent; a urea SCR device provided in the exhaust passage downstream of the hydrogen SCR device for reducing nitrogen oxides in the exhaust gas using urea as a reducing agent; a hydrogen addition device provided in the exhaust passage upstream of the hydrogen SCR device and configured to add hydrogen to the exhaust gas upstream of the hydrogen SCR device; a urea addition device that is provided in the exhaust passage upstream of the urea SCR device and that adds urea to the exhaust gas upstream of the urea SCR device; a control unit that controls the temperature adjustment device, the hydrogen addition device, and the urea addition device, the hydrogen SCR device has a second activation temperature that is lower than the first activation temperature, which is the activation temperature of the urea SCR device; The control unit When the temperature of the exhaust gas upstream of the hydrogen-SCR device is within the first activation temperature range, and the temperature of the exhaust gas downstream of the hydrogen-SCR device and upstream of the urea-SCR device is lower than the lower limit of the first activation temperature, the temperature adjustment device is activated to lower the temperature of the exhaust gas upstream of the hydrogen-SCR device to within the second activation temperature range of the hydrogen-SCR device, and the hydrogen addition device adds hydrogen to the exhaust gas upstream of the hydrogen-SCR device, When the temperature of the exhaust gas upstream of the hydrogen SCR device and the temperature of the exhaust gas downstream of the hydrogen SCR device and upstream of the urea SCR device are both within the first activation temperature range, the temperature adjustment device is stopped, and the urea addition device adds the urea to the exhaust gas upstream of the urea SCR device. Exhaust purification system.

2. The control unit When the temperature of the exhaust gas upstream of the hydrogen SCR device is within the second activation temperature range or is lower than the lower limit of the second activation temperature range, the hydrogen is added by the hydrogen addition device. The exhaust gas purification system according to claim 1 .

3. The control unit When the temperature of the exhaust gas upstream of the hydrogen SCR device is lower than the lower limit value of the second activation temperature, an amount of hydrogen that is greater than the amount of hydrogen added by the hydrogen adding device when the temperature of the exhaust gas upstream of the hydrogen SCR device is within the range of the second activation temperature. The exhaust gas purification system according to claim 2 .

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