Exhaust emission control device and engine system

Abstract

To provide a technique capable of eliminating both of nitrogen oxide and ammonia with a high elimination ratio without fluctuating a target air excess ratio in accordance with a hydrogen concentration in exhaust gas.SOLUTION: An exhaust emission control device for purifying exhaust gas discharged from an internal combustion engine that obtains driving force by using at least one of ammonia and hydrogen as fuel includes: a first catalyst that is an oxidation catalyst provided in an exhaust pipe in which exhaust gas flows; a second catalyst that is a three-way catalyst provided downstream of a position where the first catalyst is installed in the exhaust pipe; a first air-fuel ratio sensor provided between the position where the first catalyst is installed and a position where the second catalyst is installed in the exhaust pipe and detecting an air-fuel ratio of the exhaust gas; and a control section that controls a supply amount of fuel to the internal combustion engine in accordance with a first air excess ratio that is an air excess ratio calculated from a first detection air-fuel ratio that is an air-fuel ratio detected by the first air-fuel ratio sensor, so that the first air excess ratio becomes the target air excess ratio.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] There is known an internal combustion engine that obtains driving force by burning at least one of ammonia and hydrogen. Exhaust gas discharged from such an internal combustion engine contains nitrogen oxides and unburned ammonia. Patent Document 1 discloses an internal combustion engine that passes exhaust gas through a catalyst to purify nitrogen oxides and ammonia from the exhaust gas and discharges the purified exhaust gas to the outside. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2021-139344 A Summary of the Invention [Problem to be solved by the invention]

[0004] The purification rate of nitrogen oxides and ammonia by a catalyst is determined according to the excess air ratio of the exhaust gas. In order to adjust the excess air ratio of the exhaust gas, the amount of ammonia and hydrogen supplied to the internal combustion engine is controlled so that the excess air ratio calculated from the air-fuel ratio output by an air-fuel ratio sensor provided downstream of the internal combustion engine becomes a target excess air ratio, which is a target value of the excess air ratio. In general, the higher the hydrogen concentration in the exhaust gas, the more likely the excess air ratio calculated from the air-fuel ratio output by the air-fuel ratio sensor becomes an excess air ratio that is closer to the rich side than the true excess air ratio of the exhaust gas. In other words, if the hydrogen concentration in the exhaust gas is different, the excess air ratio calculated from the air-fuel ratio output by the air-fuel ratio sensor will be a different value even if the true excess air ratio of the exhaust gas is the same. In consideration of this tendency, it is possible to change the target excess air ratio according to the hydrogen concentration in the exhaust gas, but there is a concern that the control will become complicated. Therefore, a technology that can purify both nitrogen oxides and ammonia at a high purification rate without changing the target excess air ratio according to the hydrogen concentration in the exhaust gas has been desired.

[0005] The present invention has been made to solve at least part of the above-mentioned problems, and has an object to provide a technique capable of purifying both nitrogen oxides and ammonia at high purification rates without varying a target excess air ratio in accordance with the hydrogen concentration in exhaust gas. [Means for solving the problem]

[0006] The present invention has been made to solve at least part of the above-mentioned problems, and can be realized in the following forms.

[0007] (1) According to one aspect of the present invention, there is provided an exhaust purification device that purifies exhaust gas discharged from an internal combustion engine that obtains driving force using at least one of ammonia and hydrogen as a fuel, and includes a first catalyst that is an oxidation catalyst provided in an exhaust pipe through which the exhaust gas flows, a second catalyst that is a three-way catalyst provided in the exhaust pipe downstream of a position where the first catalyst is provided, a first air-fuel ratio sensor that is provided in the exhaust pipe between a position where the first catalyst is provided and a position where the second catalyst is provided and detects an air-fuel ratio of the exhaust gas, and a control unit that controls an amount of the fuel supplied to the internal combustion engine in accordance with a first air excess ratio that is an air excess ratio calculated using a first detected air-fuel ratio that is the air-fuel ratio detected by the first air-fuel ratio sensor, so that the first air excess ratio becomes a target air excess ratio.

[0008] According to this configuration, the first catalyst, which is an oxidation catalyst, is provided upstream of the first air-fuel ratio sensor. Therefore, when the true excess air ratio of the exhaust gas flowing into the first catalyst is leaner than the target excess air ratio, the exhaust gas contains a relatively large amount of oxygen, and the amount of hydrogen in the exhaust gas can be oxidized and reduced by that amount, so that the first excess air ratio can be suppressed from becoming richer than the true excess air ratio of the exhaust gas due to the influence of hydrogen in the exhaust gas. In other words, when the true excess air ratio of the exhaust gas flowing into the first catalyst is leaner than the target excess air ratio, the control unit increases the amount of fuel supply so that the first excess air ratio becomes the target excess air ratio, which is richer than the first excess air ratio, without considering the hydrogen concentration in the exhaust gas, thereby making it possible to purify both nitrogen oxides and ammonia with a high purification rate. On the other hand, when the true excess air ratio of the exhaust gas flowing into the first catalyst is richer than the target excess air ratio, the exhaust gas contains only a relatively small amount of oxygen, and only that amount of oxygen can be oxidized to hydrogen in the exhaust gas. However, even if the first excess air ratio becomes richer than the true excess air ratio of the exhaust gas due to the influence of hydrogen remaining in the exhaust gas that has not been oxidized, the true excess air ratio of the exhaust gas is also originally rich. Therefore, when the true excess air ratio of the exhaust gas flowing into the first catalyst is richer than the target excess air ratio, the control unit still reduces the amount of fuel supplied so that the first excess air ratio becomes the target excess air ratio that is leaner than the first excess air ratio, regardless of the amount of hydrogen remaining in the exhaust gas. As a result, both nitrogen oxides and ammonia can be purified at a high purification rate. Therefore, according to this configuration, in either case where the true excess air ratio of the exhaust gas flowing into the first catalyst is leaner than the target excess air ratio or is richer than the target excess air ratio, both nitrogen oxides and ammonia can be purified at a high purification rate without varying the target excess air ratio in accordance with the hydrogen concentration in the exhaust gas.

[0009] (2) In the exhaust gas purification device of the above aspect, the first catalyst does not need to have an ammonia decomposition ability. According to this configuration, hydrogen is not produced by decomposition of ammonia in the exhaust gas by the first catalyst, so that the first air excess ratio can be further prevented from becoming richer than the true air excess ratio of the exhaust gas due to the influence of hydrogen in the exhaust gas.

[0010] (3) In the exhaust gas purification device of the above aspect, the first catalyst does not need to have an oxygen storage capacity. If the first catalyst has an oxygen storage capacity, the flow of oxygen in and out of the first catalyst may cause a deviation in the first detected air-fuel ratio detected by the first air-fuel ratio sensor or a response delay, which leads to a decrease in accuracy of the excess air ratio control by the control unit. On the other hand, with this configuration, since the first catalyst does not have an oxygen storage capacity, no problems occur due to the flow of oxygen in and out of the first catalyst, and the control unit can accurately control the excess air ratio.

[0011] (4) In the exhaust gas purification device of the above aspect, the second catalyst may have an oxygen storage capacity. According to this configuration, when the exhaust gas flowing into the second catalyst has a lean excess air ratio, the oxygen contained in the exhaust gas in a relatively large amount is stored, and when the exhaust gas flowing into the second catalyst has a rich excess air ratio, the stored oxygen is released, thereby purifying the exhaust gas. Therefore, the robustness of the excess air ratio control in the exhaust purification device can be improved.

[0012] (5) In the exhaust purification device of the above aspect, a third catalyst which is a selective reduction catalyst is provided in the exhaust pipe downstream of a position where the second catalyst is provided, and the third catalyst may have an ability to adsorb ammonia and an ability to selectively reduce nitrogen oxides. According to this configuration, the ammonia that has passed through the second catalyst can be adsorbed by the third catalyst, and the nitrogen oxides that have passed through the second catalyst can be purified by the third catalyst, thereby improving the robustness of the excess air ratio control in the exhaust purification device.

[0013] (6) The exhaust purification device of the above aspect may further include a second air-fuel ratio sensor provided in the exhaust pipe downstream of a position where the second catalyst is provided and detecting an air-fuel ratio of the exhaust gas, and the control unit may correct the target air excess ratio to a rich side air excess ratio when a second air excess ratio, which is an air excess ratio calculated using a second detected air-fuel ratio, which is an air-fuel ratio detected by the second air-fuel ratio sensor, indicates that the second air excess ratio is leaner than a lean side set air excess ratio preset as a reference for the lean side air excess ratio, and may correct the target air excess ratio to the lean side air excess ratio when the second air excess ratio indicates that the second air excess ratio is richer than a rich side set air excess ratio preset as a reference for the rich side air excess ratio. According to this configuration, the target excess air ratio can be appropriately corrected according to the second excess air ratio based on the second detected air-fuel ratio detected by the second air-fuel ratio sensor, and as a result, the target excess air ratio can be maintained at an excess air ratio that can purify both nitrogen oxides and ammonia at a high purification rate.

[0014] (7) According to another aspect of the present invention, there is provided an engine system comprising: the exhaust gas purification device according to the above aspect; a reformer that reforms ammonia into hydrogen; the internal combustion engine; and an ammonia supply unit that supplies ammonia used for combustion in the internal combustion engine, and the internal combustion engine also combusts hydrogen when combusting ammonia. According to this configuration, in the internal combustion engine, hydrogen produced in the reformer is combusted together with ammonia, making it easier to combust the ammonia. Also, in the exhaust purification device, the first catalyst, which is an oxidation catalyst, is provided upstream of the first air-fuel ratio sensor, so that both nitrogen oxides and ammonia can be purified with high purification rates without varying the target excess air ratio in response to the hydrogen concentration in the exhaust gas, regardless of whether the true excess air ratio of the exhaust gas flowing into the first catalyst is leaner or richer than the target excess air ratio.

[0015] The present invention can be realized in various forms, for example, in the form of an exhaust purification device and an exhaust purification method, an engine system equipped with an exhaust purification device, a computer program for executing these devices, systems and methods, a server device for distributing this computer program, a non-transitory storage medium on which a computer program is stored, etc. [Brief description of the drawings]

[0016] [Figure 1] 1 is an explanatory diagram illustrating the configuration of an engine system according to a first embodiment. [Diagram 2] FIG. 4 is an explanatory diagram showing the purification rates of nitrogen oxides and ammonia in a second catalyst. [Diagram 3] FIG. 1 is an explanatory diagram illustrating a configuration of an engine system of a comparative example. [Figure 4] FIG. 4 is an explanatory diagram of fluctuations in a first excess air ratio when combustion conditions are different. [Diagram 5] FIG. 4 is an explanatory diagram showing the relationship between the hydrogen concentration in the exhaust gas and a first excess air ratio. [Figure 6] FIG. 11 is an explanatory diagram illustrating the configuration of an engine system according to a second embodiment. [Figure 7] 13 is a flowchart showing a procedure for a variation process. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] First Embodiment 1 is an explanatory diagram illustrating the configuration of an engine system 1 equipped with an exhaust purification device 40 according to a first embodiment of the present invention. The engine system 1 is a system that is mounted on, for example, a vehicle and generates driving force for driving the vehicle using ammonia and hydrogen as fuel. The engine system 1 includes an internal combustion engine 20 that obtains driving force by combusting ammonia. In this embodiment, the internal combustion engine 20 also combusts hydrogen when combusting ammonia.

[0018] Furthermore, the engine system 1 includes a tank 11, a first injector 11J, a reformer 13, a second injector 13J, an intake pipe 15, and an intake valve 17 upstream of the internal combustion engine 20. Note that, hereinafter, the upstream side and the downstream side refer to the upstream side and the downstream side in the flow direction in which air (Air) flows from the outside of the engine system 1 into the intake pipe 15 and is then discharged from the exhaust pipe 35 to the outside of the engine system 1 as purified exhaust gas PG, which will be described later.

[0019] The tank 11 pressurizes and stores ammonia. The first injection valve 11J is connected to the tank 11 and injects the ammonia supplied from the tank 11 into the intake pipe 15. The tank 11 and the first injection valve 11J correspond to an ammonia supply unit that supplies the ammonia used for combustion in the internal combustion engine 20. The reformer 13 is connected to the tank 11 and reforms the ammonia supplied from the tank 11 into hydrogen by a built-in catalyst (not shown). The second injection valve 13J is connected to the reformer 13 and injects the hydrogen supplied from the reformer 13 into the intake pipe 15.

[0020] The intake pipe 15 is connected to the combustion chamber 21 of the internal combustion engine 20, and causes the air taken in from the outside to flow into the combustion chamber 21 of the internal combustion engine 20. The intake valve 17 opens and closes an intake flow path that is a flow path from the intake pipe 15 to the combustion chamber 21. When the intake valve 17 opens the intake flow path, a mixed gas of air, ammonia, and hydrogen flows into the combustion chamber 21.

[0021] The internal combustion engine 20 has a combustion chamber 21, an ignition plug 23, and a piston 25. The internal combustion engine 20 obtains driving force by the reciprocating motion of the piston 25 that occurs when the ignition plug 23 ignites the mixed gas that has flowed into the combustion chamber 21. At this time, the combustion of ammonia is assisted by the fact that the mixed gas contains hydrogen.

[0022] The engine system 1 also includes an exhaust pipe 35, an exhaust valve 37, and an exhaust purification device 40 downstream of the internal combustion engine 20. The exhaust pipe 35 is connected to the combustion chamber 21, and exhausts exhaust gas from the combustion chamber 21 after combustion. The exhaust valve 37 opens and closes an exhaust path, which is a flow path from the combustion chamber 21 to the exhaust pipe 35. When the exhaust valve 37 opens the exhaust flow path, exhaust gas flows into the exhaust pipe 35.

[0023] The exhaust purification device 40 is a device that purifies the exhaust gas of the internal combustion engine 20. The exhaust purification device 40 includes a first catalyst 41, a second catalyst 42, a first air-fuel ratio sensor S1, and a control unit 45. The first catalyst 41 is an oxidation catalyst provided in the exhaust pipe 35. In this embodiment, the first catalyst 41 oxidizes unburned ammonia and hydrogen, but does not have the ability to decompose ammonia. In addition, the first catalyst 41 does not have an oxygen storage capacity. Examples of the first catalyst 41 include platinum and palladium. The second catalyst 42 is a three-way catalyst provided downstream of the position where the first catalyst 41 is provided in the exhaust pipe 35. In addition, the second catalyst 42 has an oxygen storage capacity. The second catalyst 42 purifies ammonia, nitrogen oxides, and hydrogen in the exhaust gas by an oxidation-reduction reaction. The exhaust gas purified by the second catalyst 42 is discharged to the outside of the engine system 1 as purified exhaust gas PG. The second catalyst 42 may be one that uses ceramics, titanium oxide, or the like as a support and supports a precious metal such as platinum, rhodium, or palladium as an active catalytic component, as well as a rare earth metal oxide such as cerium oxide or zirconium oxide as a promoter component.

[0024] The first air-fuel ratio sensor S1 is provided in the exhaust pipe 35 between the position where the first catalyst 41 is provided and the position where the second catalyst 42 is provided, and detects the air-fuel ratio of the exhaust gas. The first air-fuel ratio sensor S1 is a λ sensor. The control unit 45 calculates a first excess air ratio using a first detected air-fuel ratio, which is the air-fuel ratio detected by the first air-fuel ratio sensor S1. Here, the excess air ratio is the ratio between the air-fuel ratio of the exhaust gas and the stoichiometric air-fuel ratio (air-fuel ratio of the exhaust gas / stoichiometric air-fuel ratio), and the first excess air ratio is the excess air ratio (first detected air-fuel ratio / stoichiometric air-fuel ratio) calculated by the control unit 45 using the first detected air-fuel ratio. The control unit 45 controls the amount of fuel supplied to the internal combustion engine 20 according to the calculated first excess air ratio so that the first excess air ratio becomes a target excess air ratio. The target excess air ratio is a target value of the excess air ratio. The control unit 45 controls the amount of ammonia supplied to the internal combustion engine 20 by adjusting the injection period during which the first injector 11J injects ammonia so that the first excess air ratio becomes the target excess air ratio. Also, the control unit 45 controls the amount of hydrogen supplied to the internal combustion engine 20 by adjusting the injection period during which the second injector 13J injects hydrogen in accordance with the amount of ammonia injected from the first injector 11J.

[0025] FIG. 2 is an explanatory diagram showing the purification rate of nitrogen oxides and ammonia in the second catalyst 42 according to the excess air ratio of the exhaust gas. The horizontal axis of FIG. 2 shows the excess air ratio of the exhaust gas, and the vertical axis of FIG. 2 shows the purification rate of nitrogen oxides or the purification rate of ammonia. In FIG. 2, the purification rate of nitrogen oxides is shown by a solid line, and the purification rate of ammonia is shown by a broken line. As shown in FIG. 2, when the excess air ratio of the exhaust gas is λst, the purification rate of nitrogen oxides and ammonia is almost 100 percent (%). On the other hand, when the excess air ratio of the exhaust gas is leaner than λst, the purification rate of ammonia is almost 100%, but the purification rate of nitrogen oxides decreases as it becomes leaner. When the excess air ratio of the exhaust gas is richer than λst, the purification rate of nitrogen oxides is almost 100%, but the purification rate of ammonia decreases as it becomes richer. Therefore, in order to purify both nitrogen oxides and ammonia at a high purification rate in the second catalyst 42, it is necessary to control the excess air ratio of the exhaust gas to λst. Therefore, the target excess air ratio used in the control of the control unit 45 is set to an excess air ratio such as λst shown in FIG.

[0026] 3 is an explanatory diagram illustrating the configuration of an engine system 1c equipped with an exhaust purification device 40c of a comparative example. The engine system 1c of the comparative example is the same as the engine system 1 of the first embodiment, except that, compared to the engine system 1 of the first embodiment, the engine system 1c of the comparative example is equipped with an exhaust purification device 40c that is different from the exhaust purification device 40. Moreover, the exhaust purification device 40c of the comparative example is the same as the exhaust purification device 40 of the first embodiment, except that, compared to the exhaust purification device 40 of the first embodiment, the exhaust purification device 40c of the comparative example does not include a first catalyst 41.

[0027] Fig. 4 is an explanatory diagram for explaining that the first excess air ratio (first detected air-fuel ratio / stoichiometric air-fuel ratio) fluctuates when the combustion conditions are different in the exhaust purification device 40c of the comparative example. The horizontal axis of Fig. 4 indicates the first excess air ratio, and the vertical axis of Fig. 4 indicates the amount of ammonia in the exhaust gas when the first excess air ratio of the exhaust gas is richer than λst1 and λst2 at each of the lines L1 and L2, and indicates the amount of nitrogen oxides in the exhaust gas when the first excess air ratio of the exhaust gas is leaner than λst1 and λst2 at each of the lines L1 and L2. That is, when the first excess air ratio of the exhaust gas is λst1 and λst2 at each of the lines L1 and L2, the amount of nitrogen oxides and the amount of ammonia in the exhaust gas are both 0.

[0028] The lines L1 and L2 in FIG. 4 show the amount of nitrogen oxides and the amount of ammonia in the exhaust gas after passing through the second catalyst 42 when the mixed gas (air, ammonia, and hydrogen) is burned in the combustion chamber 21 under different combustion conditions in the engine system 1c of the comparative example. The difference in the combustion conditions in the lines L1 and L2 is whether or not the injection timing of ammonia from the first injection valve 11J is synchronized with the opening timing of the intake valve 17. In each of the lines L1 and L2, λst1 and λst2 are the excess air ratios at which the amount of nitrogen oxides and the amount of ammonia in the exhaust gas are both zero, and although λst1 and λst2 should be the same value, they are different values ​​here. This is because the hydrogen concentration in the exhaust gas discharged from the internal combustion engine 20 is different under different combustion conditions, and the first excess air ratio is affected depending on the hydrogen concentration.

[0029] FIG. 5 is an explanatory diagram showing the relationship between the hydrogen concentration in the exhaust gas and the first excess air ratio. The horizontal axis of FIG. 5 indicates the hydrogen concentration in the exhaust gas, and the hydrogen concentration increases toward the right side of the drawing. The vertical axis of FIG. 5 indicates the first excess air ratio (corresponding to λst1 and λst2 in FIG. 4) when the amount of nitrogen oxides and the amount of ammonia in the exhaust gas are both zero, and the first excess air ratio decreases toward the bottom of the drawing. As shown in FIG. 5, the higher the hydrogen concentration in the exhaust gas, the smaller the first excess air ratio tends to be. In other words, the higher the hydrogen concentration in the exhaust gas, the more the first air-fuel ratio sensor S1 tends to output a first detected air-fuel ratio that is closer to the rich side. For this reason, in order to purify both nitrogen oxides and ammonia at a high purification rate in the engine system 1c of the comparative example, the target excess air ratio must be changed according to the hydrogen concentration in the exhaust gas. In this regard, in the engine system 1c in which hydrogen generated in the reformer 13 is combusted together with ammonia in the internal combustion engine 20, the amount of ammonia and the amount of hydrogen in the mixed gas flowing into the combustion chamber 21 of the internal combustion engine 20 are unlikely to be constant, and therefore varying the target excess air ratio in accordance with the hydrogen concentration in the exhaust gas leads to complicated control by the control unit 45.

[0030] On the other hand, in the engine system 1 equipped with the exhaust purification device 40 shown in FIG. 1, by providing the first catalyst 41, both nitrogen oxides and ammonia can be purified at a high purification rate without varying the target excess air ratio according to the hydrogen concentration in the exhaust gas. For example, when the true excess air ratio of the exhaust gas flowing into the first catalyst 41 is leaner than the target excess air ratio, the exhaust gas contains a relatively large amount of oxygen, and the hydrogen in the exhaust gas can be oxidized and reduced by the amount of oxygen, so that it is possible to suppress the first excess air ratio from becoming richer than the true excess air ratio of the exhaust gas due to the influence of hydrogen in the exhaust gas. In other words, when the true excess air ratio of the exhaust gas flowing into the first catalyst 41 is leaner than the target excess air ratio, the control unit 45 increases the amount of fuel supply so that the first excess air ratio becomes the target excess air ratio that is richer than the first excess air ratio, without considering the hydrogen concentration in the exhaust gas, thereby making it possible to purify both nitrogen oxides and ammonia at a high purification rate.

[0031] On the other hand, when the true excess air ratio of the exhaust gas flowing into the first catalyst 41 is richer than the target excess air ratio, the exhaust gas contains only a relatively small amount of oxygen, and therefore only that amount of oxygen can be used to oxidize hydrogen in the exhaust gas. However, even if the first excess air ratio is richer than the true excess air ratio of the exhaust gas due to the influence of hydrogen remaining in the exhaust gas that has not been oxidized, the true excess air ratio of the exhaust gas is also originally rich. Therefore, when the true excess air ratio of the exhaust gas flowing into the first catalyst 41 is richer than the target excess air ratio, the control unit 45 still reduces the amount of fuel supplied so that the first excess air ratio becomes the target excess air ratio that is leaner than the first excess air ratio, regardless of the amount of hydrogen remaining in the exhaust gas. As a result, both nitrogen oxides and ammonia can be purified at a high purification rate.

[0032] Therefore, according to the exhaust purification device 40 of the first embodiment described above, in either case where the true excess air ratio of the exhaust gas flowing into the first catalyst 41 is leaner than the target excess air ratio or where the true excess air ratio is richer than the target excess air ratio, it is possible to purify both nitrogen oxides and ammonia at a high purification rate without varying the target excess air ratio in accordance with the hydrogen concentration in the exhaust gas.

[0033] Furthermore, in the exhaust purification device 40 of the first embodiment, the first catalyst 41 does not have the ability to decompose ammonia. Therefore, hydrogen is not generated by the decomposition of ammonia in the exhaust gas by the first catalyst 41, and therefore it is possible to further suppress the first excess air ratio from becoming richer than the true excess air ratio of the exhaust gas due to the influence of hydrogen in the exhaust gas.

[0034] Moreover, in the exhaust purification device 40 of the first embodiment, the first catalyst 41 does not have an oxygen storage capacity. For example, if the first catalyst 41 has an oxygen storage capacity, there is a possibility that the first detected air-fuel ratio detected by the first air-fuel ratio sensor S1 may have a value deviation or a response delay due to oxygen entering and leaving the first catalyst 41, which leads to a decrease in accuracy of the air excess ratio control by the control unit 45. In this regard, in the exhaust purification device 40 of the first embodiment, since the first catalyst 41 does not have an oxygen storage capacity, no malfunction due to oxygen entering and leaving the first catalyst 41 occurs, and the control unit 45 can accurately execute the air excess ratio control. The air excess ratio control here means that the control unit 45 controls the amount of fuel supplied to the internal combustion engine 20 so that the first air excess ratio becomes the target air excess ratio.

[0035] Moreover, in the exhaust purification device 40 of the first embodiment, the second catalyst 42 has an oxygen storage capacity. Therefore, when the excess air ratio of the exhaust gas flowing into the second catalyst 42 is lean, the second catalyst 42 stores a relatively large amount of oxygen contained in the exhaust gas, and when the excess air ratio of the exhaust gas flowing into the second catalyst 42 is rich, the second catalyst 42 releases the stored oxygen, thereby purifying the exhaust gas. Therefore, the robustness of the excess air ratio control in the exhaust purification device 40 can be improved.

[0036] Furthermore, according to the engine system 1 of the first embodiment, in the internal combustion engine 20, the hydrogen produced in the reformer 13 is combusted together with ammonia, making it possible to make the ammonia more easily combustible.

[0037] <Second embodiment> 6 is an explanatory diagram illustrating the configuration of an engine system 1a equipped with an exhaust purification device 40a according to a second embodiment of the present invention. The engine system 1a of the second embodiment is the same as the engine system 1 of the first embodiment, except that, compared to the engine system 1 of the first embodiment, the engine system 1a of the second embodiment is equipped with an exhaust purification device 40a different from the exhaust purification device 40. Moreover, the exhaust purification device 40a of the second embodiment is the same as the exhaust purification device 40 of the first embodiment, except that, compared to the exhaust purification device 40 of the first embodiment, the exhaust purification device 40a of the second embodiment is equipped with a third catalyst 43 and a second air-fuel ratio sensor S2.

[0038] The third catalyst 43 is a selective reduction catalyst provided downstream of the position where the second catalyst 42 is provided in the exhaust pipe 35. The third catalyst 43 has the ability to adsorb ammonia and the ability to selectively reduce nitrogen oxides, and therefore can adsorb ammonia, and purifies the nitrogen oxides in the exhaust gas by using the ammonia as a reducing agent. That is, when nitrogen oxides or ammonia remain in the exhaust gas that has passed through the second catalyst 42, the third catalyst 43 can remove the nitrogen oxides or ammonia from the exhaust gas. In detail, when the air excess ratio of the exhaust gas is rich, the third catalyst 43 adsorbs the ammonia remaining in the exhaust gas that has passed through the second catalyst 42. On the other hand, when the air excess ratio of the exhaust gas is lean, the third catalyst 43 purifies the nitrogen oxides remaining in the exhaust gas that has passed through the second catalyst 42 by using the adsorbed ammonia as a reducing agent. As the third catalyst 43, a catalyst using ceramics, titanium oxide, or the like as a carrier, supporting zeolite as an ammonia adsorption component and copper or iron as an active catalyst component can be mentioned.

[0039] The second air-fuel ratio sensor S2 is provided downstream of the position where the second catalyst 42 is provided in the exhaust pipe 35, and detects the air-fuel ratio in the exhaust gas. In this embodiment, the second air-fuel ratio sensor S2 is provided between the position where the second catalyst 42 is provided in the exhaust pipe 35 and the position where the third catalyst 43 is provided. The second air-fuel ratio sensor S2 is an O2 sensor.

[0040] In the second embodiment, similarly to the first embodiment, the control unit 45 does not vary the target air excess ratio in accordance with the hydrogen concentration in the exhaust gas, but varies the target air excess ratio in accordance with a second air excess ratio (second detected air-fuel ratio / stoichiometric air-fuel ratio), which is an air excess ratio calculated using a second detected air-fuel ratio, which is a detection value of the air-fuel ratio of the exhaust gas by the second air-fuel ratio sensor S2.

[0041] 7 is a flowchart showing the procedure of the variation process. As described above, the variation process is a process for varying the target excess air ratio according to the second excess air ratio. The variation process is repeatedly executed while combustion is being performed in the internal combustion engine 20. Therefore, there may be cases where the target excess air ratio at the time when the variation process is started has already been varied by the previous variation process. However, here, the description will be given on the assumption that the target excess air ratio at the time when the variation process is started is the initially set target excess air ratio λin that has not been varied.

[0042] When the variation process is started, the control unit 45 judges whether the second excess air ratio is larger than the lean side set excess air ratio (step S11). The lean side set excess air ratio is an air excess ratio that is preset as a reference for the lean side excess air ratio, and is an air excess ratio that is a reference for whether to vary the target excess air ratio. When the second excess air ratio is larger than the lean side set excess air ratio, that is, when it indicates that the second excess air ratio is leaner than the lean side set excess air ratio (step S11: YES), the control unit 45 varies the target excess air ratio by correcting the target excess air ratio to the rich side excess air ratio (step S13). Here, the control unit 45 corrects the target excess air ratio to the rich side excess air ratio by subtracting Δλ from the initially set target excess air ratio λin (λin-Δλ). After correcting the target excess air ratio to the rich side excess air ratio (step S13), the control unit 45 ends the variation process.

[0043] On the other hand, when the second excess air ratio is smaller than the lean side set excess air ratio, that is, when the second excess air ratio is richer than the lean side set excess air ratio (step S11: NO), the control unit 45 judges whether or not the second excess air ratio is smaller than the rich side set excess air ratio (step S15). The rich side set excess air ratio is an air excess ratio that is preset as a reference for the rich side excess air ratio, and is an air excess ratio that is a reference for whether or not to vary the target excess air ratio. When the second excess air ratio is smaller than the rich side set excess air ratio, that is, when the second excess air ratio is richer than the rich side set excess air ratio (step S15: YES), the control unit 45 varies the target excess air ratio by correcting the target excess air ratio to the lean side excess air ratio (step S17). Here, the control unit 45 corrects the target excess air ratio to the lean side by adding Δλ to the initially set target excess air ratio λin (λin+Δλ). After correcting the target excess air ratio to the lean side (step S17), the control unit 45 ends the variation process.

[0044] On the other hand, if the second excess air ratio is greater than the rich side set excess air ratio, that is, if it indicates that the second excess air ratio is leaner than the rich side set excess air ratio (step S15: NO), the control unit 45 sets the target excess air ratio to the initial setting target excess air ratio λin (step S19). If the target excess air ratio is already the initial setting target excess air ratio λin when the processing of step S19 is performed, the control unit 45 maintains the target excess air ratio at the initial setting target excess air ratio λin. Thereafter, the control unit 45 ends the variation processing.

[0045] In the above explanation, it is assumed that the target excess air ratio at the time when the fluctuation processing is started is the initial set target excess air ratio λin that has not been changed. However, because step S13 (correcting the target excess air ratio to the rich side air ratio) or step S17 (correcting the target excess air ratio to the lean side air ratio) was executed during the immediately previous fluctuation processing, it is possible that the target excess air ratio at the time when the next fluctuation processing is started has been changed to an air excess ratio corrected to the rich side (λin-Δλ) or an air excess ratio corrected to the lean side (λin+Δλ). In such a case, for example, when the target excess air ratio at the time when the fluctuation process is started is an air excess ratio (λin-Δλ) corrected to the rich side, and step S13 (correcting the target excess air ratio to the rich side air excess ratio) is executed again, the target excess air ratio has already become the air excess ratio (λin-Δλ) corrected to the rich side at the time when the processing of step S13 is executed again, so the control unit 45 maintains the target excess air ratio as it is (λin-Δλ) in the second step S13. Similarly, when the target excess air ratio at the time when the fluctuation process is started is an air excess ratio (λin+Δλ) corrected to the lean side, and step S17 (correcting the target excess air ratio to the lean side air excess ratio) is executed again, the target excess air ratio has already become the air excess ratio (λin+Δλ) corrected to the lean side at the time when the processing of step S17 is executed again, so the control unit 45 maintains the target excess air ratio as it is (λin+Δλ) in the second step S17.

[0046] According to the exhaust purification device 40a of the second embodiment described above, similarly to the first embodiment, by being provided with the first catalyst 41, it is possible to purify both nitrogen oxides and ammonia at a high purification rate without varying the target excess air ratio in accordance with the hydrogen concentration in the exhaust gas.

[0047] Moreover, the exhaust purification device 40a of the second embodiment is provided with a third catalyst 43 that is capable of adsorbing ammonia and purifies nitrogen oxides in the exhaust gas by using the ammonia as a reducing agent. Therefore, the ammonia that has passed through the second catalyst 42 can be adsorbed by the third catalyst 43, and the nitrogen oxides that have passed through the second catalyst 42 can be purified by the third catalyst 43. This makes it possible to improve the robustness of the air excess ratio control in the exhaust purification device 40a.

[0048] Moreover, the exhaust purification device 40a of the second embodiment is provided with a second air-fuel ratio sensor S2, and a variation process is executed using a second excess air ratio based on a second detected air-fuel ratio detected by the second air-fuel ratio sensor S2. Therefore, the target excess air ratio can be appropriately corrected according to the second excess air ratio. As a result, the target excess air ratio can be maintained at an excess air ratio that can purify both nitrogen oxides and ammonia with a high purification rate.

[0049] <Modifications of this embodiment> The present invention is not limited to the above-described embodiment, and can be embodied in various forms without departing from the spirit and scope of the invention. For example, the following modifications are also possible.

[0050] [Variation 1] In the above embodiment, the reformer 13 is provided, and hydrogen generated in the reformer 13 is burned together with ammonia as fuel in the internal combustion engine 20, but this is not limited. For example, the engine system 1, 1a may not be provided with the reformer 13. In such a case, ammonia is used as fuel for combustion in the internal combustion engine 20. At this time, it is assumed that the exhaust gas discharged from the internal combustion engine 20 becomes heterogeneous, and in particular, when a mixed gas (a mixed gas of air and ammonia) showing an excessively rich air excess ratio is burned, hydrogen is generated and contained in the exhaust gas, which may affect the first air excess ratio. However, as described in the above embodiment, the embodiment of the present invention includes the first catalyst 41. With this configuration, hydrogen in the exhaust gas is oxidized and reduced, so that it is possible to suppress the hydrogen in the exhaust gas from affecting the first air excess ratio. Therefore, even when a mixed gas of air and ammonia is used for combustion in the internal combustion engine 20, both nitrogen oxides and ammonia can be purified at a high purification rate without varying the target air excess ratio according to the hydrogen concentration in the exhaust gas, as in the above embodiment. The same applies even when only hydrogen is used as fuel and burned in the internal combustion engine 20, so long as the first catalyst 41 is provided.

[0051] [Variation 2] In the above embodiment, the first catalyst 41 does not have the ability to decompose ammonia, but this is not limited to the above. For example, the first catalyst 41 may be a three-way catalyst that has the ability to decompose ammonia. Of course, it is preferable that the first catalyst 41 is a three-way catalyst that does not have the ability to decompose ammonia.

[0052] [Variation 3] In the above embodiment, the third catalyst 43 is a selective reduction catalyst that can adsorb ammonia and purifies nitrogen oxides in the exhaust gas using the ammonia as a reducing agent, but is not limited to this. The third catalyst 43 may be a three-way catalyst having an oxygen storage capacity. In addition, when the third catalyst 43 is a three-way catalyst having an oxygen storage capacity, a selective reduction catalyst may be provided as a fourth catalyst downstream of the position where the third catalyst 43 is provided in the exhaust pipe 35.

[0053] [Variation 4] In the above embodiment, the second air-fuel ratio sensor S2 is an O2 sensor, but is not limited to this. The second air-fuel ratio sensor S2 may be a λ sensor, similar to the first air-fuel ratio sensor S1.

[0054] [Variation 5] In the above embodiment, in step S13 (correcting the target excess air ratio to the rich side air ratio) and step S17 (correcting the target excess air ratio to the lean side air ratio) of the variation processing, Δλ is added to or subtracted from the initially set target excess air ratio λin, but this is not limited to this. In step S13 (correcting the target excess air ratio to the rich side air ratio) and step S17 (correcting the target excess air ratio to the lean side air ratio), the value added to and the value subtracted from the initially set target excess air ratio λin may be different values.

[0055] [Variation 6] In the above embodiment, the target excess air ratio is corrected to the rich excess air ratio (step S13) or the target excess air ratio is corrected to the lean excess air ratio (step S17) in the variation process, but the present invention is not limited to this. In the variation process, only the correction of the target excess air ratio to the rich excess air ratio (step S13) may be executed. In such a case, an air excess ratio leaner than a theoretical air excess ratio capable of purifying both nitrogen oxides and ammonia at a high purification rate may be initially set as the target excess air ratio, and when it is indicated that the second excess air ratio is leaner than the lean set excess air ratio (step S11: YES), the target excess air ratio may be corrected to the rich excess air ratio (step S13). Similarly, in the variation process, only the correction of the target excess air ratio to the lean excess air ratio (step S17) may be executed. In such a case, an air excess ratio that is richer than a theoretical air excess ratio that can purify both nitrogen oxides and ammonia with a high purification rate may be initially set as the target excess air ratio, and when it is shown that the second excess air ratio is richer than the rich-side set excess air ratio (step S15: YES), the target excess air ratio may be corrected to an air excess ratio on the lean side (step S17).

[0056] As described above, when the true excess air ratio of the exhaust gas flowing into the first catalyst 41 is richer than the target excess air ratio, the exhaust gas contains only a relatively small amount of oxygen, and hydrogen in the exhaust gas is oxidized by the amount of oxygen. At this time, the amount of oxygen contained in the exhaust gas is determined for each value of the true excess air ratio at the true excess air ratio of the exhaust gas (richer than the target excess air ratio), and therefore the amount of hydrogen remaining in the exhaust gas without being oxidized is also determined for each value of the true excess air ratio. Therefore, the degree to which the value of the first excess air ratio is leaning toward the rich side relative to the value of the true excess air ratio can be estimated for each value of the true excess air ratio. Therefore, for example, if a map that associates the flow rate of the air Air flowing into the intake pipe 15, the fuel injection period, the combustion conditions, and the like with the true excess air ratio of the exhaust gas is created in advance and stored in the control unit 45, the degree to which the first excess air ratio is leaning toward the rich side due to the influence of hydrogen remaining in the exhaust gas can be estimated from the true excess air ratio of the exhaust gas estimated with reference to the map. Furthermore, by setting a correction value to be added to the target excess air ratio in step S17 (correcting the target excess air ratio to the lean side) depending on the degree of the shift, the target excess air ratio can be appropriately corrected to the lean side excess air ratio.

[0057] Although the present aspect has been described above based on the embodiment and modified examples, the above-mentioned embodiment of the aspect is intended to facilitate understanding of the present aspect and does not limit the present aspect. The present aspect may be modified or improved without departing from the spirit and scope of the claims, and equivalents are included in the present aspect. Furthermore, if a technical feature is not described as essential in this specification, it may be deleted as appropriate.

[0058] The present invention can also be realized in the following forms. [Application example 1] An exhaust gas purification device that purifies exhaust gas discharged from an internal combustion engine that obtains driving force using at least one of ammonia and hydrogen as fuel, A first catalyst which is an oxidation catalyst provided in an exhaust pipe through which the exhaust gas flows; a second catalyst which is a three-way catalyst provided in the exhaust pipe downstream of a position where the first catalyst is provided; a first air-fuel ratio sensor that is provided in the exhaust pipe between a position where the first catalyst is provided and a position where the second catalyst is provided, and that detects an air-fuel ratio of the exhaust gas; a control unit that controls the amount of fuel supplied to the internal combustion engine in accordance with a first air excess ratio, which is an air excess ratio calculated using a first detected air-fuel ratio, which is an air-fuel ratio detected by the first air-fuel ratio sensor, so that the first air excess ratio becomes a target air excess ratio. [Application example 2] The exhaust gas purification device according to Application Example 1, The exhaust gas purification device, wherein the first catalyst does not have an ability to decompose ammonia. [Application example 3] The exhaust gas purification device according to Application Example 1 or Application Example 2, The first catalyst is an exhaust gas purification device that does not have an oxygen storage capacity. [Application example 4] The exhaust gas purification device according to any one of Application Examples 1 to 3, The second catalyst is an exhaust gas purification device having an oxygen storage capacity. [Application example 5] The exhaust gas purification device according to any one of Application Examples 1 to 4, further comprising: a third catalyst which is a selective reduction catalyst provided downstream of a position where the second catalyst is provided in the exhaust pipe; The third catalyst is an exhaust gas purification device having an ability to adsorb ammonia and an ability to selectively reduce nitrogen oxides. [Application Example 6] The exhaust gas purification device according to any one of Application Examples 1 to 5, further comprising: a second air-fuel ratio sensor that is provided in the exhaust pipe downstream of a position where the second catalyst is provided and detects an air-fuel ratio of the exhaust gas; The control unit is When a second excess air ratio, which is an air excess ratio calculated using a second detected air-fuel ratio, which is an air-fuel ratio detected by the second air-fuel ratio sensor, indicates that the second excess air ratio is leaner than a lean-side set excess air ratio previously set as a reference for the lean-side excess air ratio, the target excess air ratio is corrected to a rich excess air ratio; An exhaust gas purification device that corrects the target excess air ratio to a lean excess air ratio when the second excess air ratio indicates that the second excess air ratio is richer than a rich-side set excess air ratio that is preset as a standard for the rich-side excess air ratio. [Application Example 7] 1. An engine system comprising: An exhaust gas purification device according to any one of Application Examples 1 to 6, a reformer that reforms ammonia into hydrogen; The internal combustion engine; an ammonia supply unit that supplies ammonia to be used for combustion in the internal combustion engine; The internal combustion engine of the engine system burns hydrogen when burning ammonia. [Explanation of symbols]

[0059] 1,1a…Engine system 10, 10a...Exhaust gas purification device 11. Tank 11J…First injector 13...Reformer 13J…Second injector 15…Intake pipe 17…Intake valve 20…Internal combustion engine 21…Combustion chamber 23…Spark plug 25…Piston 35…Exhaust pipe 37…Exhaust valve 40, 40a…Exhaust gas purification device 41…First catalyst 42…Second catalyst 43…Third catalyst 45...Control section S1…First air-fuel ratio sensor S2: Second air-fuel ratio sensor

Claims

1. An exhaust gas purification device that purifies exhaust gas discharged from an internal combustion engine that obtains driving force using at least one of ammonia and hydrogen as fuel, a first catalyst which is an oxidation catalyst provided in an exhaust pipe through which the exhaust gas flows; a second catalyst which is a three-way catalyst provided in the exhaust pipe downstream of a position where the first catalyst is provided; a first air-fuel ratio sensor that is provided in the exhaust pipe between a position where the first catalyst is provided and a position where the second catalyst is provided, and that detects an air-fuel ratio of the exhaust gas; a control unit that controls the amount of fuel supplied to the internal combustion engine in accordance with a first air excess ratio, which is an air excess ratio calculated using a first detected air-fuel ratio, which is an air-fuel ratio detected by the first air-fuel ratio sensor, so that the first air excess ratio becomes a target air excess ratio.

2. The exhaust gas purification device according to claim 1, An exhaust gas purification device, wherein the first catalyst does not have an ability to decompose ammonia.

3. The exhaust gas purification device according to claim 1, The exhaust gas purification device, wherein the first catalyst does not have an oxygen storage capacity.

4. The exhaust gas purification device according to claim 1, The second catalyst has an oxygen storage capacity.

5. The exhaust gas purification device according to claim 1, further comprising: a third catalyst which is a selective reduction catalyst provided downstream of a position where the second catalyst is provided in the exhaust pipe, The third catalyst has an ability to adsorb ammonia and an ability to selectively reduce nitrogen oxides.

6. The exhaust gas purification device according to claim 1, further comprising: a second air-fuel ratio sensor that is provided in the exhaust pipe downstream of a position where the second catalyst is provided and detects an air-fuel ratio of the exhaust gas; The control unit is When a second excess air ratio, which is an air excess ratio calculated using a second detected air-fuel ratio, which is an air-fuel ratio detected by the second air-fuel ratio sensor, indicates that the second excess air ratio is leaner than a lean-side set excess air ratio previously set as a reference for the lean-side excess air ratio, the target excess air ratio is corrected to a rich excess air ratio; When the second excess air ratio indicates that the second excess air ratio is richer than a preset rich side set excess air ratio that is set as a standard for the rich side excess air ratio, the target excess air ratio is corrected to a lean side excess air ratio.

7. 1. An engine system comprising: An exhaust gas purification device according to any one of claims 1 to 6; a reformer that reforms ammonia into hydrogen; The internal combustion engine; an ammonia supply unit that supplies ammonia to be used for combustion in the internal combustion engine; The internal combustion engine of the engine system burns hydrogen when burning ammonia.

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