Purification system

Abstract

To suppress discharge of ammonia.SOLUTION: A purification system S includes: an exhaust pipeline 12 through which exhaust gas of an engine 11 passes; a first purification device 21 that is provided in the exhaust pipeline 12 and collects particulate matters included in exhaust gas; a second purification device 22 that is provided between the first purification device 21 and the engine 11 in the exhaust pipeline 12 and eliminates nitrogen oxide included in exhaust gas; and a recirculation pipeline 31 that recirculates exhaust gas from a portion between the first purification device 21 and the second purification device 22 in the exhaust pipeline 12 to the upstream side of the second purification device 22 in the exhaust pipeline 12.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a purification system for purifying exhaust gas from an engine. [Background technology]

[0002] There are known purification devices that purify engine exhaust gas. Patent Document 1 discloses a purification device that has a selective catalytic reduction (SCR) catalyst that injects and supplies urea, which is a precursor of ammonia, into an exhaust passage, and uses the ammonia generated by hydrolysis of the urea as a reducing agent to selectively reduce nitrogen oxides (NOx). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2008-286096 A Summary of the Invention [Problem to be solved by the invention]

[0004] Incidentally, ammonia is adsorbed to the catalyst of the purifier and is not usually discharged from the purifier. However, when the temperature of the exhaust gas rises, ammonia may be desorbed from the catalyst and discharged from the purifier into the atmosphere. Since ammonia is an environmental pollutant, a technology to suppress ammonia emission into the atmosphere has been desired.

[0005] The present invention has been made in consideration of these points, and has an object to suppress the emission of ammonia. [Means for solving the problem]

[0006] One aspect of the present invention provides a purification system having an exhaust pipe through which engine exhaust passes, a first purification device provided in the exhaust pipe for collecting particulate matter contained in the exhaust, a second purification device provided in the exhaust pipe between the first purification device and the engine for purifying nitrogen oxides contained in the exhaust, and a return pipe for returning the exhaust from between the first purification device and the second purification device in the exhaust pipe to a portion of the exhaust pipe upstream of the second purification device.

[0007] The purification system includes a third purification device provided downstream of the first purification device in the exhaust pipe and purifying nitrogen oxides contained in the exhaust, and a valve control unit that controls a control valve that switches whether or not the exhaust that has passed through the second purification device is returned to the exhaust pipe upstream of the second purification device through the return pipe, and the valve control unit may control the control valve so as to return the exhaust that has passed through the second purification device upstream of the second purification device when a catalyst of the third purification device is in an inactive state.

[0008] The purification system may include a switching valve that switches between supplying exhaust gas recirculated from downstream of the second purification device and exhaust gas discharged from the engine to the second purification device, or supplying exhaust gas discharged from the engine to the recirculation pipe without supplying it to the second purification device, and the valve control unit may control the switching valve so that the exhaust gas discharged from the engine is supplied to the recirculation pipe without being supplied to the second purification device when a catalyst of the third purification device is in an active state.

[0009] The valve control unit may control the control valve and the switching valve to recirculate a portion of the exhaust gas that has passed through the second purification device upstream of the second purification device when a catalyst of the third purification device is in an inactive state and a risk that ammonia stored in the second purification device will be discharged from the second purification device is equal to or greater than a predetermined value.

[0010] The valve control unit may, when a catalyst of the third purification device is in an inactive state and the risk is less than the predetermined value, control the switching valve to supply exhaust gas discharged from the engine to the second purification device, and control the control valve to supply exhaust gas that has passed through the second purification device to the first purification device without supplying it to the return pipe.

[0011] The purification system may include a pump provided in the return pipe and operable when the catalyst of the third purification device is in an inactive state to return a portion of the exhaust gas that has passed through the second purification device to an area upstream of the second purification device in the exhaust pipe.

[0012] The purification system may include a bypass pipe provided so that exhaust gas flowing through the return pipe bypasses the pump, and a bypass valve that switches whether or not exhaust gas discharged from the engine and before reaching the second purification device is supplied downstream of the second purification device through the bypass pipe, and when a catalyst of the third purification device is in an active state, the valve control unit may control the bypass valve so that exhaust gas discharged from the engine and before reaching the second purification device passes through the bypass pipe, which bypasses the pump, and is supplied downstream of the second purification device.

[0013] The pump may be a rotary displacement pump capable of changing the direction in which the exhaust gas is discharged, and may recirculate a portion of the exhaust gas that has passed through the second purification device when the catalyst of the third purification device is in an inactive state, and may supply exhaust gas discharged from the engine downstream of the second purification device when the catalyst of the third purification device is in an active state.

[0014] The purification system may include a third purification device provided in the exhaust pipe downstream of the first purification device for purifying nitrogen oxides contained in the exhaust gas, a pump provided in the return pipe for returning a portion of the exhaust gas that has passed through the second purification device to the exhaust pipe upstream of the second purification device, a bypass pipe provided in the exhaust pipe so that exhaust gas before reaching the second purification device bypasses the second purification device, and a control valve for switching whether to supply exhaust gas discharged from the engine to the bypass pipe.

[0015] The purification system may include a valve control unit that controls the adjustment valve and the control valve so that exhaust gas is recirculated in the exhaust pipe from downstream of the second purification device to upstream of the second purification device when the catalyst of the third purification device is in a pre-inactive state, and controls the adjustment valve and the control valve so that exhaust gas discharged from the engine flows through the bypass pipe, bypassing the second purification device, and is supplied to the first purification device when the catalyst of the third purification device is in an active state. Effect of the Invention

[0016] The present invention has the effect of suppressing the emission of ammonia. [Brief description of the drawings]

[0017] [Figure 1] FIG. 2 is a diagram for explaining the configuration of a purification system. [Diagram 2] FIG. 13 is a diagram for explaining a slip risk. [Diagram 3] FIG. 4 is a diagram for explaining the flow of exhaust gas when exhaust gas is recirculated. [Figure 4] FIG. 2 is a diagram for explaining the flow of exhaust gas when the exhaust gas is passed through the exhaust gas passage; [Diagram 5] 10A and 10B are diagrams for explaining the flow of exhaust gas when the exhaust gas is bypassed. [Figure 6] 5 is a flowchart illustrating an example of a process for controlling an exhaust flow. [Figure 7]FIG. 11 is a diagram for explaining a purification system according to a second embodiment. [Figure 8] FIG. 11 is a diagram for explaining a purification system according to a third embodiment. [Figure 9] FIG. 13 is a diagram for explaining a purification system according to a fourth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] [Configuration of Purification System S] 1 is a diagram for explaining the configuration of a purification system S. The purification system S has an engine 11, an exhaust pipe 12, a first purification device 21, a second purification device 22, a return pipe 31, a control valve 41, and a switching valve 42.

[0019] The engine 11 is an internal combustion engine that generates power by burning and expanding a mixture of fuel and intake air (air). The engine 11 is, for example, a diesel engine installed in an automobile or a ship.

[0020] The exhaust pipe 12 is an exhaust pipe through which exhaust gas discharged from the engine 11 passes. The exhaust pipe 12 discharges the exhaust gas discharged from the engine 11 to the outside.

[0021] The exhaust pipe 12 is provided with a second purification device 22, a first purification device 21, and a third purification device 23 in that order from the engine 11. In other words, the second purification device 22 is provided in the exhaust pipe 12 between the first purification device 21 and the engine 11. The third purification device 23 is provided in the exhaust pipe 12 downstream of the first purification device 21.

[0022] The first purification device 21 collects particulate matter contained in the exhaust gas. The first purification device 21 has a filter (diesel particulate filter) that collects particulate matter. The first purification device 21 is also called a diesel particulate diffuser (DPD).

[0023] The first injector 221 is provided between the engine 11 and the second purification device 22 in the exhaust pipe 12. Specifically, the first injector 221 is provided immediately before the second purification device 22 in the exhaust pipe 12. The first injector 221 injects urea water, thereby supplying the urea water to the second purification device 22 in the exhaust pipe 12. Specifically, the first injector 221 injects the urea water immediately before the second purification device 22 in the exhaust pipe 12.

[0024] The second purification device 22 has a catalyst that purifies nitrogen oxides contained in the exhaust gas. The second catalyst 222 of the second purification device 22 is a selective catalytic reduction (SCR) catalyst that reacts nitrogen oxides contained in the exhaust gas with ammonia. The second catalyst 222 stores ammonia generated by hydrolysis of the urea water injected from the first injection part 221 by the heat of the exhaust gas by adsorbing the ammonia onto the surface of the catalyst. When the second catalyst 222 is at or above its activation temperature, it becomes active and purifies the stored ammonia by reacting it with the nitrogen oxides, and when the second catalyst 222 is below its activation temperature, it becomes inactive and cannot react the nitrogen oxides with ammonia.

[0025] The second injector 231 is provided in the exhaust pipe 12 between the first purification device 21 and the third purification device 23. The second injector 231 injects urea water in the exhaust pipe 12 between the first purification device 21 and the third purification device 23.

[0026] The third purification device 23 has a selective catalytic reduction (SCR) that purifies nitrogen oxides contained in the exhaust gas, similar to the second purification device 22. The third catalyst 232 of the third purification device 23 is in an active state when the temperature is equal to or higher than the activation temperature, and is in an inactive state when the temperature is lower than the activation temperature, similar to the second catalyst 222. The third purification device 23 notifies the valve control device 7 of state information indicating whether the catalyst is in an active state or an inactive state. For example, the third purification device 23 has a temperature sensor that detects the temperature of the third catalyst 232, and notifies the valve control device 7 of state information indicating the active state when the temperature of the third catalyst 232 is equal to or higher than the activation temperature, and of state information indicating the inactive state when the temperature of the catalyst is lower than the activation temperature. The activation temperature is a temperature at which ammonia and nitrogen oxides react with the catalyst, and is, for example, 200 degrees Celsius, but is not limited thereto. The third purification device 23 may notify the valve control device 7 of the temperature of the catalyst as state information.

[0027] As described above, the third purification device 23 of the purification system S of the first embodiment is provided downstream of the first purification device 21. The temperature of the exhaust gas reaching the third purification device 23 drops due to heat exchange between the first purification device 21 and the exhaust gas when the exhaust gas passes through the first purification device 21. As a result, the exhaust gas with a lowered temperature reaches the third purification device 23, so the temperature of the third catalyst 232 is unlikely to rise. For example, when the engine 11 is started after being stopped for a long period of time (hereinafter referred to as "cold start"), the temperature of the exhaust gas reaching the third purification device 23 does not increase unless the temperature of the first purification device 21 is raised, so the temperature of the third catalyst 232 does not rise. Therefore, the third catalyst 232 does not become active, and the third purification device 23 cannot purify the nitrogen oxides in the exhaust gas.

[0028] In order to purify nitrogen oxides even during a cold start of the engine 11, the purification system S of the first embodiment has a second purification device 22 between the engine 11 and the first purification device 21. The second purification device 22 is provided at a position closer to the engine 11 than the first purification device 21, so that the exhaust gas discharged from the engine 11 passes through the second purification device 22 before passing through other devices. Therefore, since the exhaust gas reaches the second purification device 22 without being cooled, the second catalyst 222 increases in temperature and becomes active in a shorter time than the third catalyst 232 becomes active. This enables the purification system S to purify nitrogen oxides even during a cold start.

[0029] Incidentally, the ammonia adsorbed and stored in the second catalyst 222 is more likely to be desorbed as the temperature of the second catalyst 222 increases. In other words, the maximum amount of ammonia that the second catalyst 222 can store decreases as the temperature of the catalyst increases. Therefore, when the amount of ammonia adsorbed to the catalyst becomes greater than the maximum amount of ammonia that has decreased as the temperature of the second catalyst 222 increases, the amount of ammonia that exceeds the maximum amount of ammonia is desorbed and discharged from the second purification device 22. Therefore, the purification system S has a return pipe 31 that returns the exhaust gas that has passed through the second purification device 22 to the upstream of the second purification device 22 so that the ammonia discharged from the second purification device 22 is not discharged to the outside from the exhaust pipe 12.

[0030] The return pipe 31 returns the exhaust gas that has passed through the second purification device 22 upstream of the second purification device 22. Specifically, the return pipe 31 returns the exhaust gas that has passed through the second purification device 22 from between the first purification device 21 and the second purification device 22 in the exhaust pipe 12 to between the engine 11 and the second purification device 22 in the exhaust pipe 12. The return pipe 31 not only returns the exhaust gas that has passed through the second purification device 22 upstream of the second purification device 22, but also allows the exhaust gas discharged from the engine 11 to bypass the second purification device 22. Specifically, the return pipe 31 allows the exhaust gas before reaching the second purification device 22 to bypass the second purification device 22, and supplies the exhaust gas downstream of the second purification device 22 in the exhaust pipe 12.

[0031] The control valve 41 is provided at a branch point 411 where the return pipe 31 branches off from the exhaust pipe 12. The control valve 41 switches whether or not to return the exhaust gas that has passed through the second purification device 22. Specifically, the control valve 41 switches whether to return a portion of the exhaust gas that has passed through the second purification device 22 to an area upstream of the second purification device 22 in the exhaust pipe 12 via the return pipe 31, or to supply all of the exhaust gas that has passed through the second purification device 22 to the first purification device 21.

[0032] The switching valve 42 is provided at a connection point 421 where the control valve 41 is connected to the exhaust pipe 12. The switching valve 42 switches the supply destination of the exhaust gas discharged from the engine 11. Specifically, the switching valve 42 switches whether the exhaust gas discharged from the engine 11 is supplied to the second purification device 22, or whether the exhaust gas discharged from the engine 11 is supplied to the return pipe 31 without being supplied to the second purification device 22. More specifically, the switching valve 42 switches whether the exhaust gas discharged from the engine 11 and the exhaust gas recirculated from downstream of the second purification device 22 are supplied to the second purification device 22, or whether the exhaust gas discharged from the engine 11 is supplied to the return pipe 31.

[0033] The valve control device 7 of the purification system S controls the control valve 41 and the switching valve 42 to supply a part of the exhaust gas that has passed through the second purification device 22 to the return pipe 31, and supplies the exhaust gas that has passed through the return pipe 31 and been returned to the second purification device 22. In this manner, even if exhaust gas containing ammonia is discharged from the second purification device 22, the valve control device 7 can supply the exhaust gas containing ammonia to the second purification device 22 again. The ammonia supplied to the second purification device 22 reacts with the nitrogen oxides contained in the exhaust gas passing through the second purification device 22 by the catalyst and is consumed. As a result, the valve control device 7 can suppress the ammonia discharged from the second purification device 22 from being discharged to the outside through the exhaust pipe 12. The configuration of the valve control device 7 will be described below.

[0034] The valve control device 7 has a storage unit 71 and a control unit 72. The storage unit 71 is a storage medium including a ROM (Read Only Memory), a RAM (Random Access Memory), a hard disk, etc. The storage unit 71 stores a program executed by the control unit 72.

[0035] The control unit 72 is a computation resource including a processor such as a CPU (Central Processing Unit). The control unit 72 executes a program stored in the storage unit 71 to realize functions as an acquisition unit 721 and a valve control unit 722.

[0036] The acquisition unit 721 acquires the activity state of the third catalyst 232. For example, the acquisition unit 721 acquires state information indicating whether the third catalyst 232 is in an active state or an inactive state from the third purification device 23. The acquisition unit 721 may also acquire state information indicating the temperature of the third catalyst 232, and acquire the active state when the catalyst temperature indicated by the state information is equal to or higher than the activation temperature, and acquire the inactive state when the catalyst temperature is lower than the activation temperature.

[0037] The acquisition unit 721 acquires the slippage risk of ammonia. The slippage risk is a risk that ammonia stored in the second purification device 22 will be desorbed from the second catalyst 222 and discharged from the second purification device 22. For example, the acquisition unit 721 acquires the slippage risk based on the maximum storage amount of ammonia that the second catalyst 222 can store, which is determined by the exhaust temperature, and the estimated storage amount of ammonia stored in the second catalyst 222. The slippage risk will be described below.

[0038] FIG. 2 is a diagram for explaining the slip risk. The horizontal axis of FIG. 2 indicates the exhaust temperature, and the vertical axis indicates the estimated storage amount. A straight line A sloping downward to the right indicates the relationship between the exhaust temperature and the maximum storage amount. In this embodiment, for ease of explanation, the relationship between the exhaust temperature and the maximum storage amount is represented by a straight line A, but the relationship between the exhaust temperature and the maximum storage amount is not limited to a straight line. As shown in FIG. 2, the maximum storage amount decreases as the exhaust temperature increases. More specifically, when the exhaust temperature increases and the temperature of the second catalyst 222 increases, ammonia becomes more likely to be desorbed from the catalyst, and the maximum storage amount of ammonia that can be stored in the catalyst decreases.

[0039] The maximum storage amount CM is the maximum amount of ammonia that can be stored in the second catalyst 222 when the exhaust temperature is a predetermined value B. The estimated storage amount CS is the estimated amount of ammonia currently stored in the second catalyst 222. When the difference D between the maximum storage amount CM and the estimated storage amount CS is large, even if the exhaust temperature rises and the maximum storage amount CM decreases, the estimated storage amount CS is unlikely to exceed the maximum storage amount CM, and the slippage risk is small. On the other hand, when the difference D between the maximum storage amount CM and the estimated storage amount CS is small, when the exhaust temperature rises and the maximum storage amount CM decreases, there is a risk that the estimated storage amount CS will exceed the maximum storage amount CM, and the slippage risk is large.

[0040] As described above, the slip risk decreases as the difference D between the maximum storage amount CM and the estimated storage amount CS increases. Therefore, the acquisition unit 721 acquires the reciprocal of the difference D between the maximum storage amount CM corresponding to the exhaust temperature B and the estimated storage amount CS as the slip risk. The maximum storage amount CM corresponding to the exhaust temperature B is stored in the memory unit 71, and the acquisition unit 721 acquires the maximum storage amount CM corresponding to the exhaust temperature B by referring to the memory unit 71. The estimated storage amount CS is, for example, the difference between the amount of ammonia supplied to the second catalyst 222 and the amount of ammonia consumed by the second catalyst 222.

[0041] The valve control unit 722 controls the control valve 41 and the switching valve 42 to switch whether or not to recirculate the exhaust gas. For example, the valve control unit 722 switches whether or not to recirculate the exhaust gas that has passed through the second purification device 22 depending on whether or not the third catalyst 232 is in an active state. Specifically, when the third catalyst 232 is in an inactive state, the valve control unit 722 controls the control valve 41 and the switching valve 42 to recirculate the exhaust gas that has passed through the second purification device 22 upstream of the second purification device 22. More specifically, when the third catalyst 232 is in an inactive state, the valve control unit 722 controls the control valve 41 to recirculate a part of the exhaust gas that has passed through the second purification device 22 upstream of the second purification device 22, and controls the switching valve 42 to supply the exhaust gas discharged from the engine 11 and the exhaust gas that has passed through the recirculation pipe 31 and has been recirculated to the second purification device 22.

[0042] The valve control unit 722 controls the control valve 41 and the switching valve 42 in accordance with changes in the exhaust pressure. Specifically, the valve control unit 722 controls the control valve 41 and the switching valve 42 so as to supply the recirculation gas RG in the recirculation pipe 31 to the exhaust pipe 12 in accordance with the timing when the pressure of the recirculation gas RG in the recirculation pipe 31 becomes higher than the pressure of the exhaust EG flowing through the exhaust pipe 12, and to supply the exhaust EG in the exhaust pipe 12 to the recirculation pipe 31 when the pressure of the recirculation gas RG in the recirculation pipe 31 becomes lower than the pressure of the exhaust EG flowing through the exhaust pipe 12. In this manner, the valve control unit 722 can recirculate the exhaust gas that has passed through the second purification device 22 upstream of the second purification device 22.

[0043] FIG. 3 is a diagram for explaining the flow of exhaust gas when the exhaust gas is recirculated. In FIG. 3, the control valve 41 and the switching valve 42 are omitted so that the flow of exhaust gas can be easily understood. The exhaust gas EG discharged from the engine 11 passes through a connection point 421 and is supplied to the second purification device 22. A part of the exhaust gas that has passed through the second purification device 22 passes through a branch point 411 and is supplied to the first purification device 21. The rest of the exhaust gas that has passed through the second purification device 22 is supplied to the recirculation pipe 31. The exhaust gas supplied to the recirculation pipe 31 becomes recirculation gas RG, and is supplied to the second purification device 22 via the exhaust pipe 12 after passing through the recirculation pipe 31.

[0044] In this manner, even if ammonia is discharged from the second purification device 22, the valve control unit 722 can supply the exhaust gas that has passed through the second purification device 22 back to the second purification device 22. The ammonia that has been supplied again to the second purification device 22 reacts with the nitrogen oxides contained in the exhaust gas passing through the second purification device 22 in the catalyst and is consumed. As a result, the valve control unit 722 can prevent the ammonia discharged from the second purification device 22 from being discharged to the outside through the exhaust pipe 12.

[0045] Incidentally, even if the third catalyst 232 is in an inactive state, if ammonia is not discharged from the second purification device 22, there is no need to recirculate the exhaust gas that has passed through the second purification device 22. Therefore, the valve control unit 722 recirculates the exhaust gas when the slip risk is high, and passes the exhaust gas through the second purification device 22 when the slip risk is low. Specifically, the valve control unit 722 recirculates a part of the exhaust gas when the slip risk is equal to or greater than a predetermined value, and supplies the entire exhaust gas to the first purification device 21 without recirculation when the slip risk is less than the predetermined value. Specifically, the valve control unit 722 controls the switching valve 42 so that the exhaust gas discharged from the engine 11 is supplied to the second purification device 22, and controls the control valve 41 so that the entire exhaust gas that has passed through the second purification device 22 is supplied to the first purification device 21.

[0046] FIG. 4 is a diagram for explaining the flow of exhaust gas when the exhaust gas is passed through. In FIG. 4, the control valve 41 and the switching valve 42 are omitted so that the flow of exhaust gas can be easily understood. The exhaust gas EG discharged from the engine 11 passes through the connection point 421 and is supplied to the second purification device 22. Then, the exhaust gas that has passed through the second purification device 22 is not supplied to the recirculation pipe 31, but passes through the branch point 411 and is supplied to the first purification device 21. In this way, when the slip risk is small, the valve control unit 722 does not recirculate the exhaust gas that has passed through the second purification device 22, so that the load of the process of controlling the control valve 41 and the switching valve 42 can be reduced.

[0047] However, when the temperature of the second catalyst 222 becomes high, not only the stored ammonia is desorbed, but also the ammonia may be oxidized to generate nitrogen oxides. For example, when the third catalyst 232 is in an active state after a sufficient amount of time has passed since the start of the engine 11, if the exhaust gas discharged from the engine 11 is supplied to the second purification device 22, the temperature of the second catalyst 222 may become too high. Therefore, when the third catalyst 232 is in an active state, the valve control unit 722 bypasses the second purification device 22 without recirculating the exhaust gas that has passed through the second purification device 22. Specifically, when the third purification device 23 is in an active state, the valve control unit 722 controls the switching valve 42 so that the exhaust gas discharged from the engine 11 is supplied to the recirculation pipe 31 without being supplied to the second purification device 22, and controls the control valve 41 so that the exhaust gas that has passed through the recirculation pipe 31 is supplied to the first purification device 21.

[0048] Fig. 5 is a diagram for explaining the flow of exhaust gas when the exhaust gas is bypassed. In Fig. 5, the control valve 41 and the switching valve 42 are omitted so that the flow of exhaust gas can be easily seen. All of the exhaust gas EG discharged from the engine 11 is supplied to the return pipe 31 from the connection point 421. Then, the exhaust gas EG that has passed through the return pipe 31 is supplied to the first purification device 21 from the branch point 411.

[0049] In this way, when the third catalyst 232 is in an active state, the valve control section 722 causes the exhaust gas discharged from the engine 11 to bypass the second purification device 22. In this way, when there is a risk that the temperature of the second catalyst 222 will become too high, the valve control section 722 can cause the exhaust gas to bypass the second purification device 22, thereby making it possible to suppress the oxidation of ammonia in the second purification device 22 to produce nitrogen oxides.

[0050] [Process to control exhaust flow] 6 is a flowchart showing an example of a process for controlling the flow of exhaust gas. The process for controlling the flow of exhaust gas is executed at a predetermined interval while the engine 11 is running. The predetermined interval is, for example, 100 milliseconds, but is not limited to this.

[0051] The acquisition unit 721 acquires the activity state of the third catalyst 232 (step S1). For example, the acquisition unit 721 acquires, from the third purification device 23, state information indicating whether the third catalyst 232 is in an active state or an inactive state.

[0052] The valve control unit 722 determines whether or not the state of the third catalyst 232 is an inactive state (step S2). Specifically, the valve control unit 722 determines whether the state of the third catalyst 232 indicated in the acquired state information is an inactive state.

[0053] When the third catalyst 232 is in an inactive state (Yes in step S2), the acquisition unit 721 acquires the slip risk (step S3). Specifically, first, the acquisition unit 721 acquires the exhaust temperature B of the exhaust gas discharged from the engine 11 and the estimated storage amount CS of ammonia stored in the second purification device 22. Next, the acquisition unit 721 acquires the maximum storage amount CM corresponding to the acquired exhaust temperature B by referring to the memory unit 71. Then, the acquisition unit 721 acquires the reciprocal of the difference D obtained by subtracting the estimated storage amount CS from the maximum storage amount CM as the slip risk.

[0054] The valve control unit 722 determines whether the slip risk is equal to or greater than a predetermined value (step S4). If the slip risk is equal to or greater than the predetermined value (Yes in step S4), the valve control unit 722 recirculates the exhaust gas that has passed through the second purification device 22 (step S5). Specifically, the valve control unit 722 controls the control valve 41 to recirculate a portion of the exhaust gas that has passed through the second purification device 22 upstream of the second purification device 22, and controls the switching valve 42 to supply the exhaust gas discharged from the engine 11 and the recirculated gas RG that has passed through the recirculation pipe 31 to the second purification device 22 (see FIG. 3).

[0055] If the slip risk is less than the predetermined value (No in step S4), the valve control unit 722 passes the exhaust gas discharged from the engine 11 (step S6). Specifically, the valve control unit 722 controls the switching valve 42 so that the exhaust gas discharged from the engine 11 is supplied to the second purification device 22, and controls the control valve 41 so that the entire amount of the exhaust gas that has passed through the second purification device 22 is supplied to the first purification device 21 (see FIG. 4).

[0056] When the third catalyst 232 is in an active state (No in step S2), the valve control unit 722 diverts the exhaust gas (step S7). Specifically, the valve control unit 722 controls the switching valve 42 so as to supply the exhaust gas discharged from the engine 11 to the return pipe 31 instead of to the second purification device 22, and controls the control valve 41 so that the exhaust gas that has passed through the return pipe 31 is supplied to the first purification device 21 (see FIG. 5).

[0057] [Effects of the Purification System S of the First Embodiment] As described above, the purification system S of the first embodiment has a first purification device 21 that is provided in the exhaust pipe 12 through which exhaust from the engine 11 passes and collects particulate matter contained in the exhaust, a second purification device 22 that is provided in the exhaust pipe 12 between the first purification device 21 and the engine 11 and that purifies nitrogen oxides contained in the exhaust, and a return pipe 31 that returns exhaust gas from between the first purification device 21 and the second purification device 22 in the exhaust pipe 12 to the upstream of the second purification device 22 in the exhaust pipe 12.

[0058] With the above configuration, the purification system S of the first embodiment is able to supply the exhaust gas that has passed through the second purification device 22 back to the second purification device 22, even if ammonia is discharged from the second purification device 22. The ammonia supplied to the second purification device 22 then reacts with the nitrogen oxides contained in the exhaust gas passing through the second purification device 22 in the catalyst and is consumed. As a result, the purification system S of the first embodiment can prevent the ammonia discharged from the second purification device 22 from being discharged to the outside through the exhaust pipe 12.

[0059] [Purification system S according to the second embodiment] The purification system S of the first embodiment recirculates the exhaust gas that has passed through the second purification device 22 upstream of the second purification device 22 by utilizing the change in exhaust gas pressure. However, when utilizing the change in exhaust gas pressure, the timing for recirculating the exhaust gas is limited, so that the exhaust gas cannot be recirculated at any timing. In this case, there is a risk that the ammonia discharged from the second purification device 22 will be discharged from the exhaust pipe 12.

[0060] Therefore, the purification system S according to the second embodiment has a pump for recirculating the exhaust gas. Fig. 7 is a diagram for explaining the purification system S according to the second embodiment. The purification system S according to the second embodiment has a pump 51 and a bypass pipeline 32. Below, the purification system S according to the second embodiment will be described in terms of differences from the purification system S according to the first embodiment, and the same points as the purification system S according to the first embodiment will be omitted.

[0061] The pump 51 is provided in the return pipe 31. The pump 51 is, for example, a turbo pump that sends out exhaust gas by centrifugal force caused by the rotation of a turbine, but is not limited to this and any known pump can be used. In addition, the pump 51 is not limited to a pump, and a compressor, a blower, a fan, or an air blower can be used as long as it is capable of sending out exhaust gas.

[0062] The pump 51 operates when the third purification device 23 is in an inactive state, and recirculates a portion of the exhaust gas that has passed through the second purification device 22 upstream of the second purification device 22 in the exhaust pipe 12. Specifically, the pump 51 recirculates the exhaust gas that has passed through the second purification device 22 to the exhaust pipe 12 at a pressure higher than the exhaust gas discharged from the engine 11. This makes it possible for the purification system S according to the second embodiment to recirculate the exhaust gas that has passed through the second purification device 22 to the exhaust pipe 12 at any timing. As a result, the purification system S according to the second embodiment can suppress the ammonia discharged from the second purification device 22 from being discharged from the exhaust pipe 12.

[0063] Incidentally, the pump 51 returns a portion of the exhaust gas that has passed through the second purification device 22 to the exhaust pipe 12 upstream of the second purification device 22, but is unable to send the exhaust gas before it reaches the second purification device 22 downstream of the second purification device 22. Therefore, even if the exhaust gas is made to bypass the second purification device 22 through the return pipe 31, the pump 51 becomes a resistance and the exhaust gas cannot bypass the pump 51. Therefore, the purification system S according to the second embodiment has a bypass pipe 32 that causes the exhaust gas to bypass the pump 51.

[0064] The bypass pipe 32 is a pipe that allows the exhaust gas before reaching the second purification device 22 to bypass the pump 51. The bypass pipe 32 is provided in the return pipe 31 so that the exhaust gas flowing through the return pipe 31 bypasses the pump 51. Specifically, the bypass pipe 32 connects the pump 51 and the branch point 411 from between the connection point 421 and the pump 51. The bypass pipe 32 allows the exhaust gas discharged from the engine 11 and before reaching the second purification device 22 to pass through, thereby bypassing the pump 51 and supplying the exhaust gas that has passed through the engine 11 downstream of the second purification device 22 without reaching the pump 51.

[0065] The first bypass valve 43 is provided at a branching point where the bypass pipe 32 branches off from the return pipe 31. The first bypass valve 43 switches the supply destination of the exhaust gas. The first bypass valve 43 switches whether or not to supply the exhaust gas discharged from the engine 11 and before it reaches the second purification device 22 to a location downstream of the second purification device 22 through the bypass pipe 32. Specifically, the first bypass valve 43 switches whether to supply the exhaust gas discharged from the engine 11 and flowing through the return pipe 31 to the bypass pipe 32, or to supply the exhaust gas sent out from the pump 51 to the exhaust pipe 12.

[0066] The second bypass valve 44 is provided at a connection point where the bypass pipe 32 is connected to the return pipe 31. The first bypass valve 43 switches the supply destination of the exhaust gas. Specifically, the second bypass valve 44 switches between supplying the exhaust gas that has passed through the bypass pipe 32 to the exhaust pipe 12 and supplying the exhaust gas that has been returned from the exhaust pipe 12 to the pump 51.

[0067] The valve control unit 722 controls the first bypass valve 43 and the second bypass valve 44 to switch between recirculating the exhaust gas that has passed through the second purification device 22 and bypassing the second purification device 22. For example, when the third catalyst 232 is in an active state, the valve control unit 722 controls the first bypass valve 43 and the second bypass valve 44 so that the exhaust gas discharged from the engine 11 and before reaching the second purification device 22 bypasses the second purification device 22. Specifically, the valve control unit 722 controls the first bypass valve 43 and the second bypass valve 44 so that the exhaust gas bypasses the pump 51 and is supplied downstream of the second purification device 22. More specifically, the valve control unit 722 controls the switching valve 42 so that the exhaust is supplied to the return pipeline 31, controls the first bypass valve 43 so that the exhaust flowing through the return pipeline 31 is supplied to the bypass pipeline 32, and controls the second bypass valve 44 so that the exhaust that has passed through the bypass pipeline 32 is directed to the exhaust pipeline 12 rather than to the pump 51.

[0068] Thereby, when there is a risk that the temperature of the second catalyst 222 will become too high, the valve control section 722 causes the exhaust gas to bypass the second purification device 22. As a result, the valve control section 722 can suppress the oxidation of ammonia in the second purification device 22 to produce nitrogen oxides.

[0069] [Purification system S according to the third embodiment] The purification system S according to the second embodiment has a turbo pump 51 whose exhaust gas sending direction is fixed. Therefore, the purification system S according to the second embodiment needs a bypass pipe 32 for causing the exhaust gas to bypass the pump 51 when the third catalyst 232 is in an active state, which makes the configuration of the purification system S according to the second embodiment complicated. Therefore, the purification system S according to the third embodiment has a rotary displacement pump instead of the turbo pump 51.

[0070] 8 is a diagram for explaining the configuration of a purification system S according to the third embodiment. The purification system S according to the third embodiment has a pump 52 instead of the pump 51 of the purification system S according to the second embodiment. Moreover, the purification system S according to the third embodiment does not have the bypass pipeline 32 of the purification system S according to the second embodiment.

[0071] The pump 52 is a pump capable of changing the direction in which the exhaust gas is discharged. The pump 52 is a rotary volumetric pump, but is not limited to this as long as the pump is capable of changing the direction in which the exhaust gas is discharged. When the third catalyst 232 is in an inactive state, the pump 52 discharges the exhaust gas in a direction in which a part of the exhaust gas that has passed through the second purification device 22 is recirculated. When the third catalyst 232 is in an active state, the pump 52 discharges the exhaust gas in a direction in which the exhaust gas discharged from the engine 11 and before reaching the second purification device 22 is supplied downstream of the second purification device 22.

[0072] The valve control unit 722 controls the pump 52 to reverse the direction in which the exhaust gas is discharged. Specifically, when the third catalyst 232 is in an inactive state, the valve control unit 722 operates the pump 52 so as to discharge the exhaust gas in a direction in which the exhaust gas that has passed through the second purification device 22 flows back upstream of the second purification device 22. Also, when the third catalyst 232 is in an active state, the valve control unit 722 operates the pump 52 so as to discharge the exhaust gas in a direction in which the exhaust gas discharged from the engine 11 and before reaching the second purification device 22 is supplied downstream of the second purification device 22. In this way, the purification system S according to the third embodiment has a simpler configuration than the purification system S according to the second embodiment by using the rotary displacement pump 52.

[0073] [Purification system S according to the fourth embodiment] The purification system S according to the second embodiment has the bypass pipe 32 in the return pipe 31 to bypass the pump 51. However, the pipe for bypassing the pump 51 is not limited to the return pipe 31, and may be provided in the exhaust pipe 12. The purification system S according to the fourth embodiment has a bypass pipe 33 instead of the bypass pipe 32 of the purification system S according to the second embodiment. The bypass pipe 33 is a pipe for causing the exhaust gas to bypass the second purification device 22. The bypass pipe 33 is provided in the exhaust pipe 12 so that the exhaust gas before reaching the second purification device 22 bypasses the second purification device 22. Specifically, the bypass pipe 33 branches off from a connection point 421 and is connected to a branch point 411.

[0074] The first four-way valve 45 is a four-way valve for switching the supply destination of the exhaust gas. The first four-way valve 45 switches between supplying the exhaust gas discharged from the engine 11 and the recirculated exhaust gas to the second purification device 22, and supplying the exhaust gas discharged from the engine 11 to the bypass pipe 33. The second four-way valve 46 is a four-way valve for switching the supply destination of the exhaust gas. The second four-way valve 46 switches between supplying all of the exhaust gas that has passed through the second purification device 22 to the first purification device 21, supplying a portion of the exhaust gas that has passed through the second purification device 22 to the recirculation pipe 31 and supplying the remainder to the first purification device 21, and supplying the exhaust gas that has passed through the bypass pipe 33 to the first purification device 21.

[0075] The valve control unit 722 controls the first four-way valve 45 and the second four-way valve 46 according to the activation state of the third catalyst 232 and the slippage risk of the second purification device 22 to control the supply destination of the exhaust gas. Specifically, when the third catalyst 232 is in an activation state, the valve control unit 722 controls the first four-way valve 45 and the second four-way valve 46 so that the exhaust gas flows through the bypass pipe 33. When the third catalyst 232 is in an inactive state and the slippage risk of the second purification device 22 is equal to or greater than a predetermined value, the valve control unit 722 controls the first four-way valve 45 and the second four-way valve 46 so that a part of the exhaust gas that has passed through the second purification device 22 is recirculated. When the third catalyst 232 is in an inactive state and the slippage risk of the second purification device 22 is less than a predetermined value, the valve control unit 722 controls the first four-way valve 45 and the second four-way valve 46 so that the exhaust gas does not pass through the second purification device 22 and is recirculated.

[0076] As described above, the purification system S according to the fourth embodiment can switch the supply destination of the exhaust gas simply by controlling the first four-way valve 45 and the second four-way valve 46. Therefore, the purification system S according to the fourth embodiment has a simpler process for controlling the valves than the purification system S according to the second embodiment.

[0077] Although the present invention has been described above using the embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments, and various modifications and changes are possible within the scope of the gist of the present invention. For example, all or part of the device can be configured by distributing or integrating functionally or physically in any unit. In addition, new embodiments resulting from any combination of multiple embodiments are also included in the embodiments of the present invention. The effect of the new embodiment resulting from the combination combines the effect of the original embodiment. [Explanation of symbols]

[0078] S Purification System 11 Engine 12 Exhaust pipe 21 First Purification Device 22 Second Purification Device 23 Third Purification Device 221 1st injection part 231 2nd injection part 31 Reflux line 32 Detour pipeline 33 Bypass Pipe 41 Control valve 42 Switching valve 43 First bypass valve 44 Second bypass valve 45 First four-way valve 46 Second four-way valve 411 Junction 421 Connection Point 51 Pump 52 Pump 7 Valve control device 71 Storage section 72 Control section 721 Acquisition Department 722 Valve control section

Claims

1. An exhaust pipe through which the engine exhaust passes; a first purification device provided in the exhaust pipe and configured to collect particulate matter contained in the exhaust gas; a second purification device that is provided in the exhaust pipe between the first purification device and the engine and that purifies nitrogen oxides contained in the exhaust gas; a return pipe that returns the exhaust gas from between the first purification device and the second purification device in the exhaust pipe to an area upstream of the second purification device in the exhaust pipe; A purification system having:

2. a third purification device provided downstream of the first purification device in the exhaust pipe and configured to purify nitrogen oxides contained in the exhaust gas; a valve control unit that controls a control valve that switches whether or not the exhaust gas that has passed through the second purification device is recirculated through the recirculation pipe to an area upstream of the second purification device in the exhaust pipe; and having The valve control unit controls the control valve to recirculate the exhaust gas that has passed through the second purification device upstream of the second purification device when a catalyst of the third purification device is in an inactive state. The purification system of claim 1 .

3. a switching valve that switches between supplying the exhaust gas recirculated from downstream of the second purification device and the exhaust gas discharged from the engine to the second purification device, and supplying the exhaust gas discharged from the engine to the recirculation pipe without supplying it to the second purification device, the valve control unit controls the switching valve so as to supply the exhaust gas discharged from the engine to the return pipe line without supplying it to the second purification device when the catalyst of the third purification device is in an active state. The purification system of claim 2.

4. The valve control unit controls the control valve and the switching valve so as to recirculate a portion of the exhaust gas that has passed through the second purification device upstream of the second purification device when a catalyst of the third purification device is in an inactive state and a risk that the ammonia stored in the second purification device will be discharged from the second purification device is equal to or greater than a predetermined value. The purification system of claim 3.

5. When a catalyst of the third purification device is in an inactive state and the risk is less than the predetermined value, the valve control unit controls the switching valve to supply the exhaust gas discharged from the engine to the second purification device, and controls the control valve to supply the exhaust gas that has passed through the second purification device to the first purification device without supplying it to the return pipe. The purification system of claim 4.

6. a pump provided in the return pipe, which operates when a catalyst of the third purification device is in an inactive state, and returns a portion of the exhaust gas that has passed through the second purification device to an area upstream of the second purification device in the exhaust pipe; The purification system according to any one of claims 3 to 5.

7. a bypass pipe provided so that exhaust gas flowing through the return pipe bypasses the pump; a bypass valve that switches whether or not exhaust gas discharged from the engine and before reaching the second purification device is supplied to the downstream of the second purification device through the bypass pipe, The valve control unit controls the bypass valve so that, when a catalyst of the third purification device is in an active state, exhaust gas discharged from the engine and before reaching the second purification device passes through the bypass pipe that bypasses the pump and is supplied to a downstream of the second purification device. The purification system of claim 6.

8. The pump comprises: a rotary displacement pump capable of changing the direction in which the exhaust gas is discharged, recirculating a portion of the exhaust gas that has passed through the second purification device when the catalyst of the third purification device is in an inactive state; supplying exhaust gas discharged from the engine downstream of the second purification device when the catalyst of the third purification device is in an active state; The purification system of claim 6.

9. a third purification device provided downstream of the first purification device in the exhaust pipe and configured to purify nitrogen oxides contained in the exhaust gas; a pump provided in the return pipe for returning a portion of the exhaust gas that has passed through the second purification device to an area upstream of the second purification device in the exhaust pipe; a bypass pipe provided in the exhaust pipe so that exhaust gas before reaching the second purification device bypasses the second purification device; and a control valve for switching whether or not exhaust gas discharged from the engine is supplied to the bypass pipe. The purification system of claim 1 .

10. a valve control unit that controls the adjustment valve and the control valve so that exhaust gas is recirculated in the exhaust pipe from downstream of the second purification device to upstream of the second purification device when the catalyst of the third purification device is in a pre-inactive state, and controls the adjustment valve and the control valve so that exhaust gas discharged from the engine flows through the bypass pipe, bypassing the second purification device, and is supplied to the first purification device when the catalyst of the third purification device is in an active state. The purification system of claim 9.

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