Recirculation system

Abstract

To cool exhaust air, and to suppress the accumulation of soot in a cooler.SOLUTION: A recirculation system S has: an intake air conduit 12 which supplies intake air to an engine 11; an exhaust air conduit 14 in which the exhaust air of the engine 11 flows; a first cooler 13 which is provided at the intake air conduit 12, and cools the intake air; a second cooler 19 which cools the exhaust air of the engine 11; and a recirculation conduit 15 which returns the exhaust air cooled by the second cooler 19 to the intake air conduit 12. The recirculation conduit 15 has: a first conduit 16 which returns the exhaust air cooled by the second cooler 19 to an upstream side of the first cooler 13 in the intake air conduit 12; a second conduit 17 which returns the exhaust air cooled by the second cooler 19 to a downstream side of the first cooler 13 in the intake air conduit 12; and a first control valve 211 and a second control valve 212 which can switch a return of the exhaust air cooled by the second cooler 19 to the intake air conduit through either of the first conduit 16 or the second conduit 17.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a recirculation system for introducing exhaust gas into a combustion chamber of an engine. [Background technology]

[0002] There is a known technology for reducing nitrogen oxides in exhaust gas by introducing the exhaust gas discharged from the engine into the engine. Patent Document 1 discloses a technology for supplying exhaust gas that has passed through a cooler (EGR cooler) that cools exhaust gas downstream of a cooler (intercooler) that cools intake air. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2009-270476 A Summary of the Invention [Problem to be solved by the invention]

[0004] When exhaust gas is supplied downstream of a cooler that cools intake air, the exhaust gas may not be cooled sufficiently if its temperature is high. However, if exhaust gas is supplied upstream of the cooler to further cool the exhaust gas, soot in the exhaust gas accumulates in the cooler, reducing the cooling performance of the cooler.

[0005] The present invention has been made in consideration of these points, and has an object to cool exhaust gas and suppress the accumulation of soot in a cooler. [Means for solving the problem]

[0006] In one aspect of the present invention, there is provided a recirculation system having an intake pipe that supplies intake air to an engine, an exhaust pipe through which exhaust gas from the engine flows, a first cooler provided in the intake pipe that cools the intake air, a second cooler that cools the exhaust gas from the engine, and a recirculation pipe that returns the exhaust gas cooled by the second cooler to the intake pipe, wherein the recirculation pipe has a first pipe that returns the exhaust gas cooled by the second cooler to the intake pipe upstream of the first cooler, a second pipe that returns the exhaust gas cooled by the second cooler to the intake pipe downstream of the first cooler, and one or more control valves that can switch whether the exhaust gas cooled by the second cooler is returned to the intake pipe through the first pipe or the second pipe.

[0007] The second cooler may be provided between a first branch point where the recirculation line branches off from the exhaust line and a second branch point in the recirculation line where the first line and the second line branch off.

[0008] The recirculation system may include a control unit that controls the control valve to recirculate the exhaust gas upstream of the first cooler in the intake pipe through the first pipe when an excess air ratio, which indicates the degree of air surplus in a mixture of intake air and fuel supplied to a combustion chamber of the engine, is equal to or greater than a predetermined value, and controls the control valve to recirculate the exhaust gas downstream of the first cooler in the intake pipe through the second pipe when the excess air ratio is less than the predetermined value.

[0009] The recirculation system may include a valve control unit that controls the control valve to recirculate the exhaust gas through the second pipe to the intake pipe when the engine speed is below a low speed threshold.

[0010] The recirculation system may include a control unit that controls the control valve to recirculate the exhaust gas through the second pipe to the intake pipe when the engine speed is equal to or greater than a high-speed threshold and the engine load is equal to or greater than a load threshold.

[0011] The control unit may control the control valve to recirculate the exhaust gas to the intake pipe through the first pipe when the rotation speed is less than the high rotation threshold or when the load is less than the load threshold.

[0012] The recirculation system may include a control unit that controls the control valve to recirculate the exhaust gas to the intake pipe through the first pipe when the engine speed is greater than or equal to a low speed threshold and less than a high speed threshold.

[0013] The recirculation system may include a turbine provided in the exhaust pipe line that rotates as the exhaust passes through, and a compressor provided in the intake pipe line upstream of the first cooler that supercharges the intake air by rotating the turbine, wherein the first pipe line recirculates the exhaust gas from upstream of the turbine in the exhaust pipe line to between the first cooler and the compressor in the intake pipe line, and the second pipe line recirculates the exhaust gas from upstream of the turbine in the exhaust pipe line to between the first cooler and the engine in the intake pipe line. Effect of the Invention

[0014] According to the present invention, it is possible to cool exhaust gas and suppress the accumulation of soot on the cooler. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a diagram for explaining a configuration of a recirculation system according to a first embodiment. [Diagram 2] 10 is a flowchart showing an example of a process for determining a reflux destination in the first embodiment. [Diagram 3] FIG. 11 is a diagram for explaining the configuration of a recirculation system according to a second embodiment. [Figure 4] 13 is a flowchart showing an example of a process for determining a reflux destination in the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] [Configuration of the recirculation system S according to the first embodiment] 1 is a diagram for explaining the configuration of a recirculation system S of the first embodiment. The recirculation system S of the first embodiment is a system that recirculates exhaust gas discharged from an engine 11 to an intake pipe 12. The recirculation system S has the engine 11, the intake pipe 12, a first cooler 13, an exhaust pipe 14, a recirculation pipe 15, a first pipe 16, a second pipe 17, a second cooler 19, a first control valve 211, and a second control valve 212.

[0017] 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.

[0018] The intake pipe 12 is a pipe that supplies intake air to the engine 11. A first cooler 13 is provided in the intake pipe 12. The first cooler 13 cools the intake air supplied to the engine 11. The first cooler 13 is a heat exchanger that cools the intake air by exchanging heat between the intake air and the cooling water of the engine 11 or outside air.

[0019] The intake pipe 12 is provided with a sensor 5. The sensor 5 is, for example, an intake flow rate sensor that detects the flow rate of intake air passing through the intake pipe 12. The sensor 5 may also be a pressure sensor that detects the pressure of the intake air, or an oxygen concentration sensor that detects the concentration of oxygen contained in the intake air. Note that it is sufficient that at least one of the above three sensors is provided in the intake pipe 12, but two or more types of sensors out of the three sensors may also be provided.

[0020] The exhaust pipe 14 is a pipe that discharges exhaust gas from the engine 11 to the outside. A turbine 41 is provided in the exhaust pipe 14. The turbine 41 rotates when the exhaust gas passes through it. The compressor 42 is provided upstream of the first cooler 13 in the intake pipe 12. The compressor 42 is connected to the turbine 41. The compressor 42 rotates in conjunction with the rotation of the turbine 41 to supercharge the intake air.

[0021] The recirculation pipe 15 branches off from the exhaust pipe 14 at a first branch point 181 upstream of the turbine 41, and is a pipe that recirculates exhaust from the engine 11 to the intake pipe 12. The recirculation pipe 15 has a pipe 182 that branches off from the first branch point 181 of the exhaust pipe 14, and a first pipe 16 and a second pipe 17 that branch off from the pipe 182 at a second branch point 183.

[0022] A second cooler 19 is provided upstream of the second branch point 183 in the pipe 182. The second cooler 19 cools the exhaust gas recirculated to the intake pipe 12. The second cooler 19 is a heat exchanger that cools the recirculated exhaust gas by exchanging heat between the cooling water of the engine 11 or outside air and the exhaust gas recirculated to the intake pipe 12.

[0023] The first pipe 16 recirculates the exhaust gas upstream of the first cooler 13 in the intake pipe 12. Specifically, the first pipe 16 recirculates the exhaust gas that has passed through a second cooler 19 provided in a pipe 182 that branches off at a first branch point 181 upstream of the turbine 41 in the exhaust pipe 14, to between the first cooler 13 and the compressor 42 in the intake pipe 12.

[0024] A first check valve 31 is provided in the first pipe 16. The first check valve 31 is provided in the first pipe 16 between the second cooler 19 and the intake pipe 12. The first check valve 31 allows exhaust air flowing from the first pipe 16 toward the intake pipe 12 to pass therethrough and blocks intake air flowing from the intake pipe 12 toward the first pipe 16.

[0025] The second pipe 17 recirculates the exhaust gas downstream of the first cooler 13 in the intake pipe 12. Specifically, the second pipe 17 recirculates the exhaust gas that has passed through a second cooler 19 provided in a pipe 182 that branches off at a first branch point 181 upstream of the turbine 41 in the exhaust pipe 14, to between the first cooler 13 and the engine 11 in the intake pipe 12.

[0026] A second check valve 32 is provided in the second pipe 17. The second check valve 32 is provided in the second pipe 17 between the second cooler 19 and the intake pipe 12. The second check valve 32 allows exhaust air flowing from the second pipe 17 to the intake pipe 12 to pass, and blocks intake air flowing from the intake pipe 12 to the second pipe 17.

[0027] The recirculation pipe 15 is provided with one or more control valves capable of switching whether the exhaust gas that has passed through the second cooler 19 is recirculated to the intake pipe 12 through the first pipe 16 or the second pipe 17. For example, the recirculation pipe 15 is provided with a first control valve 211 and a second control valve 212. The first control valve 211 switches whether the exhaust gas is recirculated to the intake pipe 12 upstream of the first cooler 13. The second control valve 212 switches whether the exhaust gas is recirculated to the intake pipe 12 downstream of the first cooler 13. Specifically, the first control valve 211 is fully opened and the second control valve 212 is fully closed, whereby the exhaust gas is recirculated to the intake pipe 12 upstream of the first cooler 13. The first control valve 211 is fully closed and the second control valve 212 is fully opened, whereby the exhaust gas is recirculated to the intake pipe 12 downstream of the first cooler 13.

[0028] The first control valve 211 and the second control valve 212 can adjust the amount of exhaust gas passing through them. Specifically, the first control valve 211 and the second control valve 212 can adjust the amount of exhaust gas recirculated to the intake pipe 12 by adjusting the opening degree of the valve.

[0029] The valve control device 6 has a storage unit 61 and a control unit 62. The storage unit 61 is a storage medium including a ROM (Read Only Memory), a RAM (Random Access Memory), a hard disk, etc. The storage unit 61 stores a program executed by the control unit 62.

[0030] The control unit 62 is a computation resource including a processor such as a CPU (Central Processing Unit). The control unit 62 executes a program stored in the storage unit 61 to realize functions as an acquisition unit 621 and a valve control unit 622.

[0031] The acquisition unit 621 acquires an excess air ratio indicating the degree of excess air in the mixture of intake air and fuel supplied to the combustion chamber of the engine 11. For example, the acquisition unit 621 acquires an excess air ratio based on the flow rate of intake air detected by an intake flow rate sensor and the amount of fuel injected into the combustion chamber of the engine 11. The acquisition unit 621 may also acquire an excess air ratio based on the concentration of oxygen or the pressure of intake air detected by the sensor 5 and the amount of fuel injected into the combustion chamber of the engine 11. The excess air ratio is the ratio of the amount of air actually supplied to the amount of air required when the oxygen and fuel in the mixture react with each other without excess or deficiency. The amount of soot discharged from the engine 11 is determined by the excess air ratio, and increases as the excess air ratio decreases and the amount of air supplied to the engine 11 decreases.

[0032] The valve control unit 622 controls the opening and closing of the first control valve 211 and the second control valve 212 to switch whether the exhaust gas that has passed through the second cooler 19 is to be recirculated to the intake pipe 12 through the first pipe 16 or the second pipe 17. For example, when the excess air ratio is equal to or greater than a predetermined value, the valve control unit 622 opens the first control valve 211 so as to recirculate the exhaust gas through the first pipe 16, and fully closes the second control valve 212. The predetermined value is, for example, a value for determining whether the amount of soot contained in the exhaust gas is an amount that does not cause a problem in the first cooler 13. The predetermined value is determined based on the specifications of the first cooler 13, experiments, and the like. In this way, when the amount of soot is small, the valve control unit 622 recirculates the exhaust gas upstream of the first cooler 13 in the intake pipe 12, and therefore the exhaust gas can be cooled more than when the exhaust gas is recirculated downstream of the first cooler 13.

[0033] When the excess air ratio is less than a predetermined value, the valve control unit 622 fully closes the first control valve 211 and opens the second control valve 212 so as to recirculate the exhaust gas through the second pipe 17. In this manner, the valve control unit 622 recirculates the exhaust gas containing soot downstream of the first cooler 13 in the intake pipe 12 when soot is generated relatively much, and therefore it is possible to suppress the accumulation of soot in the first cooler 13. When soot accumulates in the first cooler 13, the heat exchange between the first cooler 13 and the intake air is hindered and the cooling efficiency is reduced, but as described above, the valve control unit 622 can suppress the accumulation of soot in the first cooler 13, and therefore it is possible to suppress the deterioration of the cooling performance of the first cooler 13.

[0034] [Process for determining return destination in the first embodiment] 2 is a flowchart showing an example of a process for determining a reflux destination in the first embodiment. The process for determining the reflux destination in the first embodiment is executed at a predetermined interval while the engine 11 is running. The predetermined interval is, for example, one second, but is not limited to this.

[0035] The acquisition unit 621 acquires an excess air ratio (step S11). For example, the acquisition unit 621 acquires an excess air ratio based on the pressure of the intake air and the amount of fuel injected into the combustion chamber of the engine 11.

[0036] The valve control unit 622 determines whether the acquired excess air ratio is less than a predetermined value (step S12). When the excess air ratio is less than the predetermined value (Yes in step S12), the valve control unit 622 closes the first control valve 211 and opens the second control valve 212 (step S13). Specifically, the valve control unit 622 fully closes the first control valve 211 and opens the second control valve 212. As a result, when the amount of discharged soot increases, the valve control unit 622 recirculates the exhaust gas downstream of the first cooler 13 in the intake pipe 12, thereby suppressing the accumulation of soot in the first cooler 13.

[0037] When the excess air ratio is equal to or greater than a predetermined value (No in step S12), the valve control unit 622 closes the second control valve 212 and opens the first control valve 211 (step S14). Specifically, the valve control unit 622 fully closes the second control valve 212 and opens the first control valve 211. As a result, when the amount of soot discharged is small, the valve control unit 622 recirculates the exhaust gas upstream of the first cooler 13 in the intake pipe 12, so that the exhaust gas can be cooled more effectively than when the exhaust gas is recirculated downstream of the first cooler 13.

[0038] [Effects of the recirculation system S according to the first embodiment] As described above, the recirculation system S of the first embodiment has the first pipe 16 for recirculating the exhaust gas of the engine 11 upstream of the first cooler 13 that is provided in the intake pipe 12 for supplying intake air to the engine 11 and cools the intake air, and the second pipe 17 for recirculating the exhaust gas downstream of the first cooler 13 in the intake pipe 12. The recirculation system S of the first embodiment also has the second cooler 19 that is provided between the engine 11 and the second branch point 183 where the first pipe 16 and the second pipe 17 branch off, and cools the exhaust gas recirculated to the intake pipe 12, and the first control valve 211 and the second control valve 212 that can switch whether the exhaust gas that has passed through the second cooler 19 is recirculated to the intake pipe 12 through the first pipe 16 or the second pipe 17.

[0039] The recirculation system S of the first embodiment can switch between recirculating the exhaust gas upstream of the first cooler 13 in the intake pipe 12 and recirculating the exhaust gas downstream of the first cooler 13 by controlling the opening and closing of the first control valve 211 and the second control valve 212. As a result, when the amount of soot contained in the exhaust gas is large, the recirculation system S of the first embodiment can suppress the accumulation of soot in the first cooler 13 by recirculating the exhaust gas downstream of the first cooler 13 in the intake pipe 12. Furthermore, when the amount of soot contained in the exhaust gas is small, the recirculation system S of the first embodiment can further cool the exhaust gas by recirculating the exhaust gas upstream of the first cooler 13 in the intake pipe 12.

[0040] [Second embodiment of recirculation system S] 3 is a diagram for explaining the configuration of the recirculation system S of the second embodiment. Hereinafter, the recirculation system S of the second embodiment will be described with respect to the differences from the recirculation system S of the first embodiment, and the similarities will be omitted. In the recirculation system S of the second embodiment, a three-way valve 213 is provided at the location where the second branch point 183 of the first embodiment was provided, instead of the first control valve 211 and the second control valve 212 of the recirculation system S of the first embodiment. The three-way valve 213 is a three-way valve that can switch whether the exhaust gas that has passed through the second cooler 19 is returned to the intake pipe 12 through either the first pipe 16 or the second pipe 17.

[0041] The recirculation system S according to the second embodiment has a flow rate control valve for adjusting the flow rate of the recirculated exhaust gas. The flow rate control valve is provided, for example, in the recirculation pipe 15. As a specific example, the flow rate control valve is provided between the first branch point 181 and the second cooler 19 in the recirculation pipe 15, or between the second cooler 19 and the three-way valve 213. In addition, not only one flow rate control valve but also a plurality of flow rate control valves may be provided. In this case, one flow rate control valve is provided in each of the first pipe 16 and the second pipe 17. The recirculation system S according to the second embodiment can adjust the amount of recirculated exhaust gas by adjusting the opening degree of the flow rate control valve.

[0042] The acquisition unit 621 of the second embodiment acquires, instead of the excess air ratio, the rotation speed and the load of the engine 11. The acquisition unit 621 acquires the rotation speed of the engine 11, for example, from a sensor that detects the rotation speed of the output shaft of the engine 11. The acquisition unit 621 also acquires, as the load, the torque detected by a torque sensor that acquires the load applied to the output shaft of the engine 11.

[0043] The valve control section 622 of the second embodiment switches whether the air is to be recirculated to the intake pipe 12 through the first pipe 16 or the second pipe 17 based on at least one of the rotation speed and the load of the engine 11, instead of the excess air ratio.

[0044] For example, when the rotation speed of the engine 11 is less than the low rotation threshold for determining whether the rotation speed is low, the amount and pressure of the exhaust gas discharged are reduced, and the rotation speed of the turbine 41 is reduced. When the rotation speed of the turbine 41 is reduced, the rotation speed of the compressor 42 is reduced, and the compression ratio of the intake air is reduced. In this case, the amount of intake air supplied to the engine 11 is reduced, and the excess air ratio is reduced, and the amount of soot in the exhaust gas is increased. Therefore, when the acquired rotation speed is less than the low rotation threshold, the valve control unit 622 controls the three-way valve 213 to recirculate the exhaust gas to the intake pipe 12 through the second pipe 17. As a result, when the compression ratio of the intake air is reduced, the excess air ratio is reduced and the amount of soot is increased, the valve control unit 622 recirculates the exhaust gas to the intake pipe 12 through the second pipe 17, and therefore, it is possible to suppress the deposition of soot in the first cooler 13.

[0045] When the engine 11 is at a medium speed, more air is supplied than when the engine is at a low speed, so that less soot is generated than when the engine is at a low speed. Therefore, when the engine speed is equal to or higher than the low speed threshold and lower than the high speed threshold, the valve control unit 622 controls the three-way valve 213 to recirculate the exhaust gas to the intake pipe 12 through the first pipe 16. As a result, when soot generation is suppressed, the recirculation system S can further cool the exhaust gas by recirculating the exhaust gas upstream of the first cooler 13 in the intake pipe 12.

[0046] Incidentally, the amount of soot in the exhaust also increases when the rotation speed and load of the engine 11 are high. Specifically, even if the compressor 42 supercharges the intake air, the amount of intake air supplied to the engine 11 is insufficient, causing a decrease in the excess air ratio, and thus increasing the amount of soot.

[0047] Therefore, when the rotation speed and load of the engine 11 are high, the valve control unit 622 recirculates the exhaust gas to the intake pipe 12 through the second pipe 17. Specifically, when the rotation speed is equal to or higher than a high rotation threshold for determining whether or not the rotation speed is high, and the load is equal to or higher than a load threshold for determining whether or not the load is high, the valve control unit 622 controls the three-way valve 213 to recirculate the exhaust gas to the intake pipe 12 through the second pipe 17. As a result, even when the amount of soot increases due to a shortage of the amount of intake air supplied to the engine 11, the valve control unit 622 recirculates the exhaust gas to the intake pipe 12 through the second pipe 17, so that the accumulation of soot in the first cooler 13 can be suppressed.

[0048] As described above, when the rotation speed and load of the engine 11 are high, the amount of intake air supplied to the engine 11 is insufficient, and the amount of soot generated increases. On the other hand, when the load of the engine 11 is low, the amount of intake air required for combustion can be supplied to the engine 11, and soot generation is suppressed.

[0049] Therefore, when the load of the engine 11 is less than the load threshold, the valve control unit 622 recirculates the exhaust gas to the intake pipe 12 through the first pipe 16. Specifically, when the load is less than the load threshold even if the rotation speed is equal to or greater than the high rotation threshold, the valve control unit 622 controls the three-way valve 213 to recirculate the exhaust gas upstream of the first cooler 13 in the intake pipe 12 through the first pipe 16. In this way, the valve control unit 622 can further cool the exhaust gas when the load of the engine 11 is low and soot generation is suppressed.

[0050] [Process for determining return destination in the second embodiment] 4 is a flowchart showing an example of a process for determining a reflux destination in the second embodiment. The process for determining a reflux destination in the second embodiment is executed at a predetermined interval while the engine 11 is running. The predetermined interval is, for example, one second, but is not limited to this.

[0051] The acquisition unit 621 acquires the rotation speed of the engine 11 (step S21). For example, the acquisition unit 621 acquires the rotation speed of the engine 11 from a sensor that detects the rotation speed of the output shaft of the engine 11.

[0052] The valve control unit 622 determines whether the rotation speed of the engine 11 acquired by the acquisition unit 621 is equal to or higher than the low rotation threshold (step S22). If the rotation speed is less than the low rotation threshold (No in step S22), the valve control unit 622 recirculates the exhaust gas through the second pipe 17 (step S26). Specifically, the valve control unit 622 controls the three-way valve 213 to recirculate the exhaust gas through the second pipe 17 to the intake pipe 12 downstream of the first cooler 13.

[0053] If the rotation speed of the engine 11 is equal to or higher than the low rotation threshold (Yes in step S22), the valve control unit 622 determines whether or not the rotation speed of the engine 11 is equal to or higher than the high rotation threshold (step S23). If the valve control unit 622 determines that the rotation speed is equal to or higher than the high rotation threshold (Yes in step S23), the acquisition unit 621 acquires the load of the engine 11 (step S24). For example, the acquisition unit 621 acquires the torque detected by a torque sensor that acquires the load on the output shaft of the engine 11 as the load.

[0054] The valve control unit 622 determines whether the load acquired by the acquisition unit 621 is equal to or greater than a load threshold (step S25). If the load is equal to or greater than the threshold (Yes in step S25), that is, if the engine 11 is rotating at high speed and under high load, the valve control unit 622 recirculates the exhaust gas through the second pipe 17 (step S26).

[0055] When the rotation speed is equal to or greater than the low rotation threshold and less than the high rotation threshold (No in step S23), or when the rotation speed is equal to or greater than the high rotation threshold and the load is less than the load threshold (No in step S25), the valve control unit 622 recirculates the exhaust gas through the first pipe 16 (step S27). Specifically, the valve control unit 622 controls the three-way valve 213 to recirculate the exhaust gas through the first pipe 16 to the intake pipe 12 upstream of the first cooler 13.

[0056] [Effects of the recirculation system S of the second embodiment] As described above, the recirculation system S of the second embodiment has a three-way valve 213 that can switch whether the air is returned to the intake pipe 12 through the first pipe 16 or the second pipe 17, instead of the first control valve 211 and the second control valve 212 of the recirculation system S of the first embodiment.

[0057] The recirculation system S of the second embodiment can switch between recirculating the exhaust gas upstream of the first cooler 13 in the intake pipe 12 and recirculating the exhaust gas downstream of the first cooler 13 by controlling the three-way valve 213. As a result, when the amount of soot contained in the exhaust gas is large, the recirculation system S of the second embodiment can suppress the accumulation of soot in the first cooler 13 by recirculating the exhaust gas downstream of the first cooler 13 in the intake pipe 12. Furthermore, when the amount of soot contained in the exhaust gas is small, the recirculation system S of the second embodiment can further cool the exhaust gas by recirculating the exhaust gas upstream of the first cooler 13 in the intake pipe 12.

[0058] (Modification) The recirculation system S may combine the configuration of the first embodiment with the configuration of the second embodiment. For example, the recirculation system S may have a three-way valve 213, and may control the three-way valve 213 to recirculate the exhaust gas to the intake pipe 12 through the second pipe 17 when the excess air ratio is less than a predetermined value, and may control the three-way valve 213 to recirculate the exhaust gas to the intake pipe 12 upstream of the first cooler 13 through the first pipe 16 when the excess air ratio is equal to or greater than the predetermined value. The recirculation system S may also control the opening and closing of the first control valve 211 and the second control valve 212 based on at least one of the rotation speed and the load of the engine 11, to switch whether the exhaust gas that has passed through the second cooler 19 is to be recirculated to the intake pipe 12 through the first pipe 16 or the second pipe 17.

[0059] 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]

[0060] S Recirculation System 11 Engine 12 Intake pipe 13 1st cooler 14 Exhaust pipe 15 Recirculation line 16 1st pipeline 17 2nd pipeline 181 First Junction 182 Pipeline 183 Second Junction 19 Second cooler 31 First check valve 32 Second check valve 41 Turbine 42 Compressor 5 Sensors 6 Valve control device 61 Storage section 62 Control section 211 First control valve 212 Second control valve 213 Three-way valve 621 Acquisition Department 622 Valve control section

Claims

1. an intake pipe for supplying intake air to the engine; an exhaust pipe through which exhaust from the engine flows; a first cooler provided in the intake pipe for cooling the intake air; A second cooler for cooling exhaust gas from the engine; a recirculation pipe that recirculates the exhaust gas cooled by the second cooler to the intake pipe; having The recirculation line includes: a first pipe that recirculates the exhaust gas cooled by the second cooler to an upstream of the first cooler in the intake pipe; a second pipe that recirculates the exhaust gas cooled by the second cooler to a downstream of the first cooler in the intake pipe; one or more control valves capable of switching whether the exhaust gas cooled by the second cooler is returned to the intake pipe through the first pipe or the second pipe; A recirculation system having

2. The second cooler is provided between a first branch point where the recirculation pipe branches off from the exhaust pipe and a second branch point where the first pipe and the second pipe branch off in the recirculation pipe.

2. The recirculation system of claim 1.

3. a control unit that controls the control valve to recirculate the exhaust gas through the first pipe to the upstream of the first cooler in the intake pipe when an excess air ratio indicating an excess of air in a mixture of intake air and fuel supplied to a combustion chamber of the engine is equal to or greater than a predetermined value, and controls the control valve to recirculate the exhaust gas through the second pipe to the downstream of the first cooler in the intake pipe when the excess air ratio is less than the predetermined value.

2. The recirculation system of claim 1.

4. and a valve control unit that controls the control valve to recirculate the exhaust gas to the intake pipe through the second pipe when the engine speed is less than a low speed threshold.

2. The recirculation system of claim 1.

5. and a control unit that controls the control valve to recirculate the exhaust gas to the intake pipe through the second pipe when the engine speed is equal to or higher than a high rotation threshold and the engine load is equal to or higher than a load threshold.

2. The recirculation system of claim 1.

6. The control unit controls the control valve to recirculate the exhaust gas to the intake pipe through the first pipe when the rotation speed is less than the high rotation threshold or when the load is less than the load threshold.

6. The recirculation system of claim 5.

7. and a control unit that controls the control valve to recirculate the exhaust gas to the intake pipe through the first pipe when the engine speed is equal to or higher than a low speed threshold and lower than a high speed threshold.

2. The recirculation system of claim 1.

8. a turbine provided in the exhaust pipe and rotated by the passage of the exhaust gas; a compressor provided in the intake pipe upstream of the first cooler and configured to supercharge the intake air by rotation of the turbine; the first pipe recirculates the exhaust gas from an upstream of the turbine in the exhaust pipe to a portion between the first cooler and the compressor in the intake pipe, The second pipe recirculates the exhaust gas from an upstream of the turbine in the exhaust pipe to a position between the first cooler and the engine in the intake pipe. A recirculation system according to any one of claims 1 to 7.

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