Exhaust gas recirculation system

Abstract

To enable the discharge of condensate generated in the low-pressure EGR passage into the exhaust passage while suppressing a decrease in the effectiveness of reducing NOx contained in the exhaust. [Solution] The exhaust gas recirculation system 100 comprises a high-pressure EGR device 40, a low-pressure EGR device 50, and a control device 60. The high-pressure EGR device 40 has a high-pressure EGR passage 41 connected to the portion of the intake passage 13 downstream of the compressor 32. The low-pressure EGR device 50 has a low-pressure EGR passage 51 connected to the portion of the intake passage 13 upstream of the compressor 32. The control device 60 identifies the operating range of the internal combustion engine 10 and calculates the retention amount, which is an estimated value of the amount of condensate retained in the low-pressure EGR passage 51. If the retention amount is greater than a determination value corresponding to the current operating range, the control device 60 operates the low-pressure EGR device 50 to reduce the return flow rate of the low-pressure EGR gas.

Description

Technical Field

[0001] The present invention relates to an exhaust gas recirculation device provided in an internal combustion engine.

Background Art

[0002] The exhaust gas recirculation device disclosed in Patent Document 1 includes a high-pressure EGR device, a low-pressure EGR device, and a control device. The high-pressure EGR device includes a high-pressure EGR passage that refluxes the exhaust flowing through the exhaust passage of the internal combustion engine as high-pressure EGR gas to the intake passage of the internal combustion engine, and an adjustment valve that adjusts the reflux amount of the high-pressure EGR gas. The high-pressure EGR passage is connected to a portion of the intake passage downstream of the compressor of the supercharger.

[0003] The low-pressure EGR device includes a low-pressure EGR passage that refluxes the exhaust flowing through the exhaust passage as low-pressure EGR gas to the intake passage, a cooling device that cools the low-pressure EGR gas flowing through the low-pressure EGR passage, and an adjustment valve that adjusts the reflux amount of the low-pressure EGR gas. The low-pressure EGR passage is connected to a portion of the intake passage upstream of the compressor of the supercharger.

[0004] In the low-pressure EGR passage, condensed water is generated when the cooling device cools the low-pressure EGR gas. When the flow rate of the low-pressure EGR gas in the low-pressure EGR passage is high, the condensed water flows together with the low-pressure EGR gas and is discharged to the intake passage. On the other hand, when the flow rate of the low-pressure EGR gas in the low-pressure EGR passage is low or the flow of the low-pressure EGR gas in the low-pressure EGR passage stops, the condensed water flows toward the first end of the low-pressure EGR passage due to its own weight. Then, the condensed water is discharged to the exhaust passage.

[0005] If the estimated amount of condensed water accumulated in the low-pressure EGR passage exceeds a standard value, the control device activates the adjustment valves for the high-pressure EGR system and the low-pressure EGR passage so that the proportion of low-pressure EGR gas in the total amount of EGR gas recirculated to the intake passage decreases. In this way, the control device can reduce the amount of condensed water remaining in the low-pressure EGR passage by discharging the condensed water from the low-pressure EGR passage to the exhaust passage. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2012-163061 [Overview of the project] [Problems that the invention aims to solve]

[0007] Depending on the operating conditions of the internal combustion engine, recirculating low-pressure EGR gas into the intake passage can reduce NOx emissions more effectively than recirculating high-pressure EGR gas. However, in the exhaust gas recirculation system described above, even under operating conditions where recirculating low-pressure EGR gas into the intake passage enhances NOx reduction, if the amount of condensate exceeds a standard value, the recirculation rate of low-pressure EGR gas into the intake passage is reduced. Therefore, there is room for improvement in exhaust gas recirculation systems in terms of suppressing the reduction in NOx emissions. [Means for solving the problem]

[0008] The exhaust gas recirculation device for solving the above problems is a device provided in an internal combustion engine comprising a cylinder, an intake passage through which air introduced into the cylinder flows, an exhaust passage through which exhaust gas discharged from the cylinder flows, and a supercharger for supercharging the air flowing through the intake passage. The supercharger has a turbine installed in the exhaust passage and a compressor installed in the intake passage and operating in synchronization with the turbine. The exhaust gas recirculation device comprises a high-pressure EGR device that recirculates the exhaust gas flowing through the exhaust passage to the intake passage as high-pressure EGR gas, a low-pressure EGR device that recirculates the exhaust gas flowing through the exhaust passage to the intake passage as low-pressure EGR gas, and a control device that controls the high-pressure EGR device and the low-pressure EGR device to adjust the recirculation rate of the high-pressure EGR gas and the recirculation rate of the low-pressure EGR gas. The high-pressure EGR device has a high-pressure EGR passage whose first end is connected to a portion of the exhaust passage upstream of the turbine and whose second end is connected to a portion of the intake passage downstream of the compressor. The low-pressure EGR device has a low-pressure EGR passage whose first end is connected to a portion of the exhaust passage downstream of the turbine and whose second end is connected to a portion of the intake passage upstream of the compressor, and a cooling device for cooling the low-pressure EGR gas flowing through the low-pressure EGR passage. The control device performs the following actions: to identify the operating range of the internal combustion engine based on the output torque and engine rotational speed of the internal combustion engine; to calculate the retention amount, which is an estimated value of the amount of condensed water retained in the low-pressure EGR passage; and to operate the low-pressure EGR device to reduce the return flow rate of the low-pressure EGR gas when the retention amount is greater than a determination value corresponding to the operating range. [Effects of the Invention]

[0009] The above exhaust gas recirculation system has the effect of discharging condensate generated in the low-pressure EGR passage into the exhaust passage while suppressing a decrease in the NOx reduction effect contained in the exhaust gas. [Brief explanation of the drawing]

[0010] [Figure 1]Figure 1 is a schematic diagram showing an internal combustion engine equipped with one embodiment of an exhaust gas recirculation device. [Figure 2] Figure 2 shows the operating range of the internal combustion engine shown in Figure 1. [Figure 3] Figure 3 shows the relationship between the multiple operating regions and the judgment values ​​in Figure 2. [Figure 4] Figure 4 is a flowchart showing a series of processes performed by the control device included in the exhaust gas recirculation system shown in Figure 1. [Figure 5] Figure 5 is a block diagram showing the condensate calculation process. [Modes for carrying out the invention]

[0011] One embodiment of the exhaust gas recirculation device will be described with reference to Figures 1 to 5. <Configuration of an internal combustion engine> Figure 1 shows an internal combustion engine 10 equipped with an exhaust gas recirculation device 100. The internal combustion engine 10 is mounted in a vehicle. The internal combustion engine 10 comprises a plurality of cylinders 11, a crankshaft 12, an intake passage 13, a plurality of fuel injectors 14, and an exhaust passage 16.

[0012] The intake passage 13 is a passage through which air flows to be introduced into multiple cylinders 11. An intercooler 21 and a throttle valve 22 are installed in the intake passage 13. The intercooler 21 cools the air flowing through the intake passage 13. The throttle valve 22 is located downstream of the intercooler 21 in the intake passage 13. By adjusting the opening of the throttle valve 22, the amount of air introduced into the multiple cylinders 11 changes.

[0013] The fuel injector 14 injects fuel into the corresponding cylinder 11. A mixture of air and fuel is burned in the cylinders 11. This causes the crankshaft 12 to rotate. The combustion of the mixture also generates exhaust gas in the cylinders 11. The exhaust gas discharged from the cylinders 11 then flows through the exhaust passage 16.

[0014] The internal combustion engine 10 is equipped with a supercharger 30. The supercharger 30 has a turbine 31 installed in the exhaust passage 16 and a compressor 32 installed in the intake passage 13. The turbine 31 is operated by the flow rate of the exhaust gases flowing through the exhaust passage 16. The compressor 32 is located in the intake passage 13 upstream of the intercooler 21. The compressor 32 operates in sync with the turbine 31, thereby supercharging the air flowing through the intake passage 13.

[0015] <Configuration of exhaust gas recirculation system> The exhaust gas recirculation system 100 includes a high-pressure EGR device 40, a low-pressure EGR device 50, and a control device 60.

[0016] The high-pressure EGR device 40 recirculates the exhaust gas flowing through the exhaust passage 16 as high-pressure EGR gas to the portion of the intake passage 13 downstream of the compressor 32. The high-pressure EGR device 40 includes a high-pressure EGR passage 41 connecting the exhaust passage 16 and the intake passage 13, and a high-pressure EGR valve 42, which is a valve installed in the high-pressure EGR device 40. The first end of the high-pressure EGR passage 41 is connected to the portion of the exhaust passage 16 upstream of the turbine 31. The second end of the high-pressure EGR passage 41 is connected to the portion of the intake passage 13 downstream of the compressor 32. For example, the second end of the high-pressure EGR passage 41 is connected to the portion of the intake passage 13 between the compressor 32 and the intercooler 21.

[0017] The high-pressure EGR valve 42 is an electronically controlled valve. The opening degree of the high-pressure EGR valve 42 can be controlled by the control device 60, which will be described later. The higher the opening degree of the high-pressure EGR valve 42, the greater the flow rate of high-pressure EGR gas returned to the intake passage 13 by the high-pressure EGR device 40. On the other hand, when the high-pressure EGR valve 42 is closed, the return of high-pressure EGR gas to the intake passage 13 by the high-pressure EGR device 40 is stopped.

[0018] The low-pressure EGR device 50 recirculates the exhaust flowing through the exhaust passage 16 as low-pressure EGR gas to a portion upstream of the compressor 32 in the intake passage 13. The low-pressure EGR device 50 includes a low-pressure EGR passage 51 connecting the exhaust passage 16 and the intake passage 13, a cooling device 52 and a low-pressure EGR valve 53 installed in the low-pressure EGR passage 51. The first end of the low-pressure EGR passage 51 is connected to a portion downstream of the turbine 31 in the exhaust passage 16. The second end of the low-pressure EGR passage 51 is connected to a portion upstream of the compressor 32 in the intake passage 13.

[0019] In a state where the internal combustion engine 10 is mounted on a vehicle, the first end of the low-pressure EGR passage 51 is positioned below the second end of the low-pressure EGR passage 51. For example, the low-pressure EGR passage 51 is configured to be positioned upward as it approaches the intake passage 13 in a state where the internal combustion engine 10 is mounted on a vehicle.

[0020] The cooling device 52 cools the low-pressure EGR gas flowing through the low-pressure EGR passage 51. The low-pressure EGR valve 53 is disposed in a portion of the low-pressure EGR passage 51 between the connection point with the intake passage 13 and the cooling device 52. The low-pressure EGR valve 53 is an electronically controlled valve. The opening degree of the low-pressure EGR valve 53 can be controlled by a control device 60 described later. The higher the opening degree of the low-pressure EGR valve 53, the greater the recirculation amount of the low-pressure EGR gas to the intake passage 13 by the low-pressure EGR device 50. On the other hand, when the low-pressure EGR valve 53 is closed, the recirculation of the low-pressure EGR gas to the intake passage 13 by the low-pressure EGR device 50 is stopped.

[0021] The low-pressure EGR gas in the low-pressure EGR passage 51 is cooled by the cooling device 52. As a result, condensation occurs in the low-pressure EGR passage 51 due to the decrease in the temperature of the low-pressure EGR gas. When the flow velocity of the low-pressure EGR gas flowing through the low-pressure EGR passage 51 toward the intake passage 13 is relatively high, the condensation is discharged into the intake passage 13 along with the low-pressure EGR gas. On the other hand, when the flow velocity of the low-pressure EGR gas flowing through the low-pressure EGR passage 51 toward the intake passage 13 is relatively low, or when the flow of low-pressure EGR gas is stopped, the condensation flows through the low-pressure EGR passage 51 toward the exhaust passage 16 due to its own gravity. The condensation is then discharged into the exhaust passage 16.

[0022] The control device 60 controls the high-pressure EGR device 40 and the low-pressure EGR device 50 to adjust the return flow rate of high-pressure EGR gas and low-pressure EGR gas. Detection signals are input to the control device 60 from multiple sensors.

[0023] The multiple sensors include a crank angle sensor 71, an airflow meter 72, a water temperature sensor 73, an intake air temperature sensor 74, and an exhaust temperature sensor 75. The crank angle sensor 71 detects the rotation angle of the crankshaft 12. The rotational speed of the crankshaft 12 based on the detection signal from the crank angle sensor 71 is described as "engine rotational speed NE". The airflow meter 72 detects the airflow rate in the intake passage 13. The airflow rate based on the detection signal from the airflow meter 72 is described as "intake air volume GA". The water temperature sensor 73 detects the temperature of the coolant flowing through the cooling system 52. The temperature of the coolant based on the detection signal from the water temperature sensor 73 is described as "coolant temperature". The intake air temperature sensor 74 detects the temperature of the air introduced into the multiple cylinders 11 from the intake passage 13. The temperature of the air based on the detection signal from the intake air temperature sensor 74 is described as "intake air temperature". The exhaust temperature sensor 75 detects the temperature of the exhaust gas flowing downstream of the turbine 31 in the exhaust passage 16. The temperature of the exhaust gas based on the detection signal from the exhaust temperature sensor 75 is described as "exhaust temperature".

[0024] An example of a control device 60 is an electronic control device. In this case, the control device 60 has a CPU 61, a first memory 62, and a second memory 63. The first memory 62 stores a control program executed by the CPU 61. The second memory 63 stores the calculation results of the CPU 61. By the CPU 61 executing the control program in the first memory 62, the control device 60 can adjust the opening degree of the high-pressure EGR valve 42 and the opening degree of the low-pressure EGR valve 53.

[0025] <Operating range of internal combustion engines> Referring to Figure 2, the relationship between the operating range of the internal combustion engine 10 and the distribution ratio of high-pressure EGR gas and low-pressure EGR gas will be explained. The operating range of the internal combustion engine 10 is determined by the engine torque TQ, which is the output torque of the internal combustion engine 10, and the engine rotational speed NE.

[0026] The operating range of the internal combustion engine 10 can be divided into a first operating range DA1, a second operating range DA2, and a fifth operating range DA5. The second operating range DA2 can be divided into a third operating range DA3 and a fourth operating range DA4.

[0027] The first operating region DA1 is an operating region with high torque and low rotation speed. In the first operating region DA1, it is permitted to recirculate low-pressure EGR gas into the intake passage 13, while it is prohibited to recirculate high-pressure EGR gas into the intake passage 13.

[0028] The fifth operating region, DA5, is a high-torque, high-speed operating region. In the fifth operating region, DA5, it is permitted to recirculate high-pressure EGR gas into the intake passage 13, while it is prohibited to recirculate low-pressure EGR gas into the intake passage 13.

[0029] In the second operating region DA2, both high-pressure and low-pressure EGR gases are permitted to be recirculated into the intake passage 13. The fourth operating region DA4 is an operating region with low torque and low rotation speed. In the fourth operating region DA4, the temperature of the high-pressure EGR gas is not relatively high, and the amount of NOx emitted from multiple cylinders 11 is not very large. Therefore, in the fourth operating region DA4, the difference between the NOx emission reduction effect by recirculating high-pressure EGR gas and the NOx emission reduction effect by recirculating low-pressure EGR gas is relatively small. On the other hand, in the third operating region DA3, the temperature of the high-pressure EGR gas is relatively high compared to the fourth operating region DA4, so the NOx emission reduction effect by recirculating low-pressure EGR gas is high. In other words, the fourth operating region DA4 is an operating region in which the effect of improving exhaust characteristics by recirculating low-pressure EGR gas into the intake passage 13 is lower compared to the third operating region DA3.

[0030] <Control of high-voltage and low-voltage EGR devices> The control device 60 calculates the retention amount Qwc, which is an estimated value of the amount of condensate remaining in the low-pressure EGR passage 51. If the retention amount Qwc is greater than a judgment value, the control device 60 operates the low-pressure EGR device 50 to reduce the return flow rate of the low-pressure EGR gas. For example, the control device 60 controls the low-pressure EGR device 50 and the high-pressure EGR device 40 so that the proportion of low-pressure EGR gas in the return flow rate of EGR gas to the intake passage 13 is reduced. The return flow rate of EGR gas to the intake passage 13 is the sum of the return flow rate of high-pressure EGR gas and the return flow rate of low-pressure EGR gas.

[0031] At this time, the control device 60 compares a determination value corresponding to the operating region at that time with the stagnant amount Qwc. For example, if the operating region is the fourth operating region DA4, the control device 60 operates the low-pressure EGR device 50 to reduce the return flow rate of low-pressure EGR gas if the stagnant amount Qwc is greater than the determination value QwcThA corresponding to the fourth operating region DA4. For example, if the operating region is the third operating region DA3, the control device 60 operates the low-pressure EGR device 50 to reduce the return flow rate of low-pressure EGR gas if the stagnant amount Qwc is greater than the determination value QwcThB corresponding to the third operating region DA3. For example, if the operating region is the first operating region DA1, the control device 60 operates the low-pressure EGR device 50 to reduce the return flow rate of low-pressure EGR gas if the stagnant amount Qwc is greater than the determination value QwcThC corresponding to the first operating region DA1.

[0032] As shown in Figure 3, all of the multiple judgment values ​​QwcThA, QwcThB, and QwcThC are smaller than the retention limit value QwcLm. The retention limit value QwcLm is the upper limit of condensate that can be retained in the low-pressure EGR passage 51.

[0033] As described above, the fourth operating region DA4 is an operating region in which the effect of reducing NOx emissions by recirculating low-pressure EGR gas is relatively low. The third operating region DA3 is an operating region in which the effect of reducing NOx emissions by recirculating low-pressure EGR gas is higher compared to the fourth operating region DA4. The first operating region DA1 is an operating region in which the effect of reducing NOx emissions by recirculating low-pressure EGR gas is higher compared to the third operating region DA3 and the fourth operating region DA4.

[0034] Therefore, among the three judgment values ​​QwcThA, QwcThB, and QwcThC, judgment value QwcThA is the smallest, judgment value QwcThB is the second smallest, and judgment value QwcThC is the largest.

[0035] Referring to Figure 4, a series of processes performed by the control device 60 when operating the high-pressure EGR device 40 and the low-pressure EGR device 50 will be explained. The control device 60 is repeatedly executed at predetermined control cycles while the internal combustion engine 10 is in operation.

[0036] In step S11, the control device 60 identifies an operating region from among multiple operating regions DA1, DA3, DA4, and DA5 that corresponds to the current engine torque TQ and engine rotational speed NE of the internal combustion engine 10. In the following step S13, the control device 60 determines whether the operating region identified in step S11 is the fifth operating region DA5. If the current operating region is the fifth operating region DA5 (S13: YES), the control device 60 proceeds to step S15. On the other hand, if the current operating region is not the fifth operating region DA5 (S13: NO), the control device 60 proceeds to step S21.

[0037] In step S15, the control device 60 operates the low-pressure EGR device 50 so that the recirculation of low-pressure EGR gas is stopped. For example, the control device 60 closes the low-pressure EGR valve 53. After that, the control device 60 terminates the series of processes.

[0038] In step S21, the control device 60 calculates the retention amount Qwc, which is an estimated value of the amount of condensate remaining in the low-pressure EGR passage 51. The calculation process for the retention amount Qwc will be described later with reference to Figure 5. Once the control device 60 has calculated the retention amount Qwc, it proceeds to step S23.

[0039] In step S23, the control device 60 determines whether the stagnant amount Qwc calculated in step S21 is greater than the determination value QwcThA corresponding to the fourth operating area DA4. If the stagnant amount Qwc is greater than the determination value QwcThA (S23: YES), the control device 60 proceeds to step S25. On the other hand, if the stagnant amount Qwc is less than or equal to the determination value QwcThA (S23: NO), the control device 60 terminates the series of processes.

[0040] In step S25, the control device 60 determines whether the current operating area identified in step S11 is the fourth operating area DA4. If the current operating area is the fourth operating area DA4 (S25: YES), the control device 60 proceeds to step S27. On the other hand, if the current operating area is not the fourth operating area DA4 (S25: NO), the control device 60 proceeds to step S31.

[0041] In step S27, the control device 60 operates the low-pressure EGR device 50 and the high-pressure EGR device 40 so that the return flow rate of low-pressure EGR gas decreases and the return flow rate of high-pressure EGR gas increases. Preferably, the control device 60 stops the return of low-pressure EGR gas and increases the return flow rate of high-pressure EGR gas. At this time, the control device 60 closes the low-pressure EGR valve 53 and increases the opening of the high-pressure EGR valve 42. Then, the control device 60 terminates the series of processes.

[0042] In step S31, the control device 60 determines whether the amount of accumulated material Qwc calculated in step S21 is greater than the determination value QwcThB corresponding to the third operating area DA3. If the amount of accumulated material Qwc is greater than the determination value QwcThB (S31: YES), the control device 60 proceeds to step S33. On the other hand, if the amount of accumulated material Qwc is less than or equal to the determination value QwcThB (S31: NO), the control device 60 terminates the series of processes.

[0043] In step S33, the control device 60 determines whether the current operating area identified in step S11 is the third operating area DA3. If the current operating area is the third operating area DA3 (S33: YES), the control device 60 proceeds to step S35. On the other hand, if the current operating area is not the third operating area DA3 (S33: NO), the control device 60 proceeds to step S41.

[0044] In step S35, the control device 60 operates the low-pressure EGR device 50 and the high-pressure EGR device 40 so that the return flow rate of low-pressure EGR gas decreases and the return flow rate of high-pressure EGR gas increases. Preferably, the control device 60 stops the return of low-pressure EGR gas and increases the return flow rate of high-pressure EGR gas. At this time, the control device 60 closes the low-pressure EGR valve 53 and increases the opening of the high-pressure EGR valve 42. Then, the control device 60 terminates the series of processes.

[0045] In step S41, the control device 60 determines whether the amount of accumulated material Qwc calculated in step S21 is greater than the determination value QwcThC corresponding to the first operating area DA1. If the amount of accumulated material Qwc is greater than the determination value QwcThC (S41: YES), the control device 60 proceeds to step S43. On the other hand, if the amount of accumulated material Qwc is less than or equal to the determination value QwcThC (S41: NO), the control device 60 terminates the series of processes.

[0046] In step S43, the control device 60 determines whether the current operating area identified in step S11 is the first operating area DA1. If the current operating area is the first operating area DA1 (S43: YES), the control device 60 proceeds to step S45. On the other hand, if the current operating area is not the first operating area DA1 (S43: NO), the control device 60 terminates the series of processes.

[0047] In step S45, the control device 60 operates the low-pressure EGR device 50 to reduce the return flow rate of the low-pressure EGR gas. Preferably, the control device 60 stops the return flow of the low-pressure EGR gas. At this time, the control device 60 closes the low-pressure EGR valve 53. Then, the control device 60 terminates the series of processes.

[0048] <Calculation process for the amount of water remaining (Qwc)> Referring to Figure 5, the calculation process for the retention amount Qwc in step S21 described above will be explained. The control device 60 repeatedly executes the calculation process at predetermined calculation cycles.

[0049] The calculation process includes the generation amount calculation process M10, the emission amount calculation process M20, the difference calculation process M30, and the integration process M40. The generation amount calculation process M10 is a process that calculates the generation amount Qwg, which is an estimate of the amount of condensate produced in the low-pressure EGR passage 51 within a unit time. The unit time is, for example, equal to the length of the time interval between the execution of the calculation process for the retention amount Qwc.

[0050] In the production amount calculation process M10, the control device 60 calculates the production amount Qwg based on the ambient temperature, intake air volume, fuel injection volume, and moisture content in the air. For example, the control device 60 calculates the amount of moisture contained in the exhaust flowing through the exhaust passage 16 based on the ambient temperature, intake air volume, fuel injection volume, and moisture content in the air. Subsequently, the control device 60 calculates the production amount Qwg as the difference between the moisture content calculated as described above and the amount of moisture in the exhaust with a relative humidity of 100% after it has been cooled in the low-pressure EGR passage 51.

[0051] The emission calculation process M20 is a process that calculates the emission amount Qwd, which is an estimate of the amount of condensate discharged from the low-pressure EGR passage 51 within a unit time. The emission calculation process M20 includes the first emission calculation process M21, the second emission calculation process M22, and the summation process M23.

[0052] The first emission calculation process M21 is a process that calculates the first emission amount Qwd1, which is an estimated value of the amount of condensed water discharged from the low-pressure EGR passage 51 to the intake passage 13 within a unit time. In the first emission calculation process M21, the control device 60 calculates the first emission amount Qwd1 based on the flow rate of the low-pressure EGR gas. In this process, the control device 60 calculates the first emission amount Qwd1 such that the value increases as the flow rate of the low-pressure EGR gas increases. The control device 60 can also calculate the flow rate of the low-pressure EGR gas based on the amount of exhaust gas flowing through the exhaust passage 16 and the opening degree of the low-pressure EGR valve 53.

[0053] The second emission calculation process M22 is a process that calculates the second emission amount Qwd2, which is an estimated value of the amount of condensed water discharged from the low-pressure EGR passage 51 to the exhaust passage 16 within a unit time. In the second emission calculation process M22, the control device 60 calculates the second emission amount Qwd2 based on the stop time TM, which is the duration of the low-pressure EGR valve 53 being closed.

[0054] Here, there is a correlation between the amount of condensed water discharged from the low-pressure EGR passage 51 to the exhaust passage 16 and the stopping time TM. Figure 5 shows a map MP illustrating this correlation. The control device 60 uses this map MP to derive a value based on the stop time TM as the second emission Qwd2. This allows the control device 60 to calculate the second emission Qwd2 such that the value increases as the stop time TM increases.

[0055] In the aggregation process M23, the control device 60 calculates the sum of the first emission Qwd1 and the second emission Qwd2 as the emission Qwd. In the difference calculation process M30, the control device 60 calculates the difference ΔQ between the generated amount Qwg and the emitted amount Qwd by subtracting the emitted amount Qwd from the generated amount Qwg. If the generated amount Qwg is greater than the emitted amount Qwd, the difference ΔQ will be a positive value. On the other hand, if the generated amount Qwg is less than the emitted amount Qwd, the difference ΔQ will be a negative value.

[0056] In the integration process M40, the control device 60 calculates the stagnant amount Qwc by integrating the difference ΔQ. However, if the integrated value of the difference ΔQ is negative, the control device 60 calculates 0 (zero) as the stagnant amount Qwc.

[0057] <Operation and Effects of This Embodiment> (1) The NOx reduction effect of recirculating low-pressure EGR gas into the intake passage 13 differs depending on the operating range of the internal combustion engine 10. Therefore, in operating ranges where the NOx reduction effect of recirculating low-pressure EGR gas into the intake passage 13 is relatively low, even if the process of reducing the recirculation rate of low-pressure EGR gas is performed at a stage where the amount of condensate accumulating in the low-pressure EGR passage 51 is relatively small, the NOx reduction effect does not decrease significantly. On the other hand, in operating ranges where the NOx reduction effect of recirculating low-pressure EGR gas into the intake passage 13 is relatively high, if the process of reducing the recirculation rate of low-pressure EGR gas is performed at a stage where the amount of condensate accumulating in the low-pressure EGR passage 51 is relatively small, the NOx reduction effect will decrease.

[0058] Therefore, the control device 60 identifies the operating range of the internal combustion engine 10 based on the engine torque TQ and the engine rotational speed NE. The control device 60 calculates the retention amount Qwc, which is an estimated value of the amount of condensed water remaining in the low-pressure EGR passage 51. Then, if the retention amount Qwc is greater than a determination value corresponding to the identified operating range, the control device 60 operates the low-pressure EGR device 50 to reduce the return flow rate of the low-pressure EGR gas.

[0059] The timing for initiating the treatment to reduce the return flow rate of low-pressure EGR gas is determined by comparing a judgment value corresponding to the current operating range with the retention amount Qwc. This allows the exhaust gas recirculation device 100 to optimize the timing for initiating the treatment to reduce the return flow rate of low-pressure EGR gas, compared to the case where the judgment value is fixed regardless of the operating range. Therefore, the exhaust gas recirculation device 100 can discharge the condensate generated in the low-pressure EGR passage 51 into the exhaust passage 16 while suppressing a decrease in the NOx reduction effect.

[0060] (2) The first operating region DA1 is an operating region in which the recirculation of high-pressure EGR gas into the intake passage 13 is prohibited. On the other hand, the second operating region DA2 is an operating region in which the recirculation of both high-pressure EGR gas and low-pressure EGR gas into the intake passage 13 is permitted.

[0061] Therefore, in the exhaust gas recirculation device 100, the determination values ​​QwcThA and QwcThB corresponding to the second operating region DA2 are smaller than the determination value QwcThC corresponding to the first operating region DA1. Therefore, when the current operating range is the first operating range DA1, it becomes more difficult to perform the process of reducing the return flow rate of low-pressure EGR gas at a stage where the accumulated amount Qwc is small, compared to when the current operating range is the second operating range DA2. As a result, the exhaust gas recirculation device 100 can suppress a decrease in the NOx reduction effect.

[0062] On the other hand, when the current operating range is the second operating range DA2, the process of reducing the return flow rate of low-pressure EGR gas is performed while the accumulated amount Qwc is relatively small, compared to when the current operating range is the first operating range DA1. As a result, the exhaust gas recirculation device 100 can reduce the amount of condensed water accumulated in the low-pressure EGR passage 51.

[0063] (3) The second operating region DA2 can be divided into the third operating region DA3 and the fourth operating region DA4. The fourth operating region DA4 is an operating region in which the NOx reduction effect by recirculating low-pressure EGR gas into the intake passage 13 is lower compared to the third operating region DA3.

[0064] Therefore, in the exhaust gas recirculation device 100, the determination value QwcThA corresponding to the fourth operating region DA4 is smaller than the determination value QwcThB corresponding to the third operating region DA3. As a result, when the current operating region is the fourth operating region DA4, the process of reducing the return flow rate of low-pressure EGR gas is performed with a relatively small amount of accumulated Qwc compared to when the current operating region is the third operating region DA3. Consequently, the exhaust gas recirculation device 100 can discharge the condensate generated in the low-pressure EGR passage 51 into the exhaust passage 16 while suppressing a decrease in the NOx reduction effect.

[0065] (4) The control device 60 calculates the generation amount Qwg and the discharge amount Qwd at predetermined control cycles. The control device 60 then calculates the retained amount Qwc by integrating the difference between the generation amount Qwg and the discharge amount Qwd. At this time, the control device 60 calculates the discharge amount Qwd as the sum of the first discharge amount Qwd1, which is the discharge of condensate from the low-pressure EGR passage 51 to the intake passage 13, and the second discharge amount Qwd2, which is the discharge of condensate from the low-pressure EGR passage 51 to the exhaust passage 16. The exhaust gas recirculation device 100 can improve the accuracy of calculating the retained amount Qwc by considering the second discharge amount Qwd2 in addition to the first discharge amount Qwd1 when calculating the discharge amount Qwd.

[0066] <Example of changes> The above embodiment can be implemented with the following modifications. The above embodiment and the following modifications can be combined with each other to the extent that they do not contradict each other technically.

[0067] The control device 60 does not need to calculate the second emission Qwd2 when calculating the emission Qwd. For example, the control device 60 may calculate the emission Qwd by increasing the first emission Qwd1 according to the execution time of the process of reducing the opening of the low-pressure EGR gas. For example, it is preferable that the control device 60 calculates the emission Qwd by correcting the first emission Qwd1 such that the amount of increase correction increases as the execution time increases.

[0068] The determination value QwcThA corresponding to the fourth operating region DA4 may be equal to the determination value QwcThB corresponding to the third operating region DA3. In steps S27 and S35 shown in Figure 4, the control device 60 does not need to close the low-pressure EGR valve 53 if it can reduce the return flow rate of low-pressure EGR gas to the intake passage 13.

[0069] The control device 60 is not limited to one that includes a CPU and ROM and executes software processing. In other words, the control device 60 may have any of the following configurations: (a), (b), and (c).

[0070] (a) The control device 60 comprises one or more processors that perform various processes according to a computer program. The processors include a CPU and memory such as RAM and ROM. The memory stores program code or instructions configured to cause the CPU to perform processes. The memory, i.e., computer-readable media, includes any available media that can be accessed by a general-purpose or dedicated computer.

[0071] (b) The control device 60 includes one or more dedicated hardware circuits that perform various processes. Examples of dedicated hardware circuits include application-specific integrated circuits, i.e., ASICs or FPGAs. ASIC is an abbreviation for "Application Specific Integrated Circuit," and FPGA is an abbreviation for "Field Programmable Gate Array."

[0072] (c) The control device 60 comprises one or more processors that execute a portion of the various processes according to a computer program, and one or more dedicated hardware circuits that execute the remaining processes of the various processes. [Explanation of symbols]

[0073] 10...Internal combustion engine, 11...Cylinder, 13...Intake passage, 16...Exhaust passage, 30...Supercharger, 31...Turbine, 32...Compressor, 40...High-pressure EGR device, 41...High-pressure EGR passage, 50...Low-pressure EGR device, 51...Low-pressure EGR passage, 52...Cooling device, 60...Control device, 100...Exhaust gas recirculation device.

Claims

1. An exhaust gas recirculation device provided in an internal combustion engine comprising a cylinder, an intake passage through which air introduced into the cylinder flows, an exhaust passage through which exhaust gas discharged from the cylinder flows, and a supercharger for supercharging the air flowing through the intake passage, The supercharger comprises a turbine installed in the exhaust passage and a compressor installed in the intake passage and operating in sync with the turbine. The exhaust gas recirculation device is, A high-pressure EGR device that recirculates the exhaust gas flowing through the exhaust passage as high-pressure EGR gas into the intake passage, A low-pressure EGR device that recirculates the exhaust gas flowing through the exhaust passage as low-pressure EGR gas into the intake passage, The system includes a control device for controlling the high-pressure EGR device and the low-pressure EGR device in order to adjust the return flow rate of the high-pressure EGR gas and the return flow rate of the low-pressure EGR gas, The high-pressure EGR device has a high-pressure EGR passage, the first end of which is connected to the portion of the exhaust passage upstream of the turbine, and the second end of which is connected to the portion of the intake passage downstream of the compressor. The low-pressure EGR device is A low-pressure EGR passage, the first end of which is connected to the portion of the exhaust passage downstream of the turbine, and the second end of which is connected to the portion of the intake passage upstream of the compressor, The system includes a cooling device for cooling the low-pressure EGR gas flowing through the low-pressure EGR passage, The control device is Based on the output torque and engine rotational speed of the internal combustion engine, the operating range of the internal combustion engine is determined. The accumulation amount is calculated as an estimated value of the amount of condensed water remaining in the low-pressure EGR passage. If the amount of stagnant gas is greater than the determination value corresponding to the operating range, the low-pressure EGR device is operated to reduce the return flow rate of the low-pressure EGR gas. Exhaust gas recirculation device.

2. The operating region includes a first operating region in which the recirculation of the high-pressure EGR gas into the intake passage is prohibited, and a second operating region in which both the high-pressure EGR gas and the low-pressure EGR gas are permitted to be recirculated into the intake passage. The determination value corresponding to the second operating region is smaller than the determination value corresponding to the first operating region. The exhaust gas recirculation device according to claim 1.

3. The second operating region includes a third operating region and a fourth operating region in which, compared to the third operating region, the effect of improving exhaust characteristics by recirculating the low-pressure EGR gas into the intake passage is lower. The determination value corresponding to the fourth operating region is smaller than the determination value corresponding to the third operating region. The exhaust gas recirculation device according to claim 2.

4. The control device is The amount of condensed water generated in the low-pressure EGR passage and the amount of condensed water discharged from the low-pressure EGR passage are estimated at predetermined control cycles. The amount of material left is derived by accumulating the difference between the amount of material generated and the amount of material discharged for each control cycle. The control device calculates the discharge amount as the sum of the discharge amount of condensed water from the low-pressure EGR passage to the exhaust passage and the discharge amount of condensed water from the low-pressure EGR passage to the intake passage. Exhaust gas recirculation device according to any one of claims 1 to 3.

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