JP4561236B2 - Internal combustion engine supercharging system - Google Patents

Description

  The present invention relates to a supercharging system for an internal combustion engine in which a plurality of turbochargers are combined.

As a supercharging system for an internal combustion engine, a low-pressure turbocharger and a high-pressure turbocharger are used, and the high-pressure turbocharger turbine is located upstream of the low-pressure turbocharger in the exhaust passage, and the high-pressure turbocharger. There is known a two-stage supercharging system in which a compressor of a supercharger is disposed downstream of an intake passage from that of a low-pressure turbocharger. As an EGR device of an internal combustion engine provided with this type of supercharging system, an EGR device that takes out EGR gas from an exhaust passage between turbines and introduces EGR gas upstream of a compressor of a low-pressure turbocharger (Patent Document 1). Reference is also made to an EGR device (see Patent Document 2) that takes out EGR gas from the upstream of the turbine of the high-pressure turbocharger and introduces EGR gas downstream of the compressor of the high-pressure turbocharger. In a two-stage supercharging system, a configuration has also been proposed in which EGR gas is taken out from an exhaust passage between turbines and introduced into an intake passage between compressors (see Patent Document 3). However, in Patent Document 3, the capacity of the subsequent turbocharger is shown to be larger than that of the preceding turbocharger.
Utility Model Registration No. 2522482 Japanese Patent No. 2759375 JP 2000-220480 A

  When EGR gas is introduced upstream of the preceding (low pressure stage) compressor (Patent Document 1), the volume of the flow path until the EGR gas reaches the combustion chamber increases, and the response delay of EGR is large. When the EGR gas is taken out from the upstream of the turbine of the high-pressure turbocharger (Patent Document 2), the exhaust energy led to the turbine of the high-pressure turbocharger is reduced by the removal of the EGR gas, and the supercharging effect is impaired. . In the configuration of Patent Document 3, since the capacity of the turbocharger at the rear stage is larger than that of the turbocharger at the front stage, the EGR channel volume is increased, and there is a concern regarding the responsiveness of EGR.

  An object of the present invention is to provide a supercharging system for an internal combustion engine that is excellent in EGR response and can execute EGR without impairing the supercharging effect.

The present invention relates to a low-pressure turbocharger, a turbine disposed upstream of an exhaust passage from a turbine of the low-pressure turbocharger, and a compressor disposed downstream of an intake passage from a compressor of the low-pressure turbocharger. A high-pressure turbocharger having a smaller capacity than the low-pressure turbocharger, and an EGR take-out position set between the turbine of the high-pressure turbocharger and the turbine of the low-pressure turbocharger An EGR passage that connects an EGR introduction position set between the compressor of the low-pressure turbocharger and the compressor of the high-pressure turbocharger, and an EGR valve that adjusts an EGR amount that passes through the EGR passage An EGR valve control means for controlling the EGR valve so as to obtain an EGR amount corresponding to the operating state of the internal combustion engine; and an EGR valve downstream of the EGR extraction position in the exhaust passage. Variable throttle means capable of adjusting the opening, throttle control means for controlling the opening of the variable throttle means, and EGR amount detection means for detecting the EGR amount introduced into the intake passage, The EGR valve control means feedback-controls the EGR valve so that a deviation of the detected value with respect to the target EGR amount is reduced by using a detected value of the EGR amount by the EGR amount detecting means as a feedback amount, and the throttle control means Determines whether or not the feedback control of the EGR valve exceeds a predetermined limit, and when it exceeds the limit, changes the opening of the variable throttle means in a direction in which the deviation decreases, As the supercharger, a variable nozzle turbocharger capable of changing the flow path cross-sectional area of the inlet portion of the turbine is provided, so that the low-pressure turbocharger and the variable throttle means The throttle control means acquires a change range of the nozzle opening of the low-pressure turbocharger based on a map prepared in advance as a thing to be executed when the limit is exceeded, and the acquired The open degree of the nozzle opening of the low-pressure turbocharger is controlled according to the change width (Claim 1).

According to the supercharging system of the present invention, the EGR introduction position, which is the connection position of the EGR passage to the intake passage, is set between the compressors of the low pressure turbocharger and the high pressure turbocharger. Compared with the case where the EGR passage is connected upstream of the compressor of the supercharger, the EGR passage volume is reduced. Therefore, the response of EGR is improved and the control accuracy of EGR is improved. In addition, since the capacity of the compressor of the high-pressure turbocharger is set smaller than the capacity of the compressor of the low-pressure turbocharger, the high-pressure turbocharger is set to have a larger capacity than the low-pressure turbocharger. In comparison, the EGR channel volume is small and the EGR response is excellent. Since the EGR take-out position, which is the connection position of the EGR passage to the exhaust passage, is set between the turbines of the low-pressure turbocharger and the high-pressure turbocharger, the EGR take-out position is set to the turbine of the high-pressure turbocharger. Compared with the case where it is set upstream, a large exhaust energy can be given to the turbine of the high-pressure turbocharger. Thereby, EGR can be executed without impairing the supercharging effect. In addition, while controlling the EGR amount with high accuracy by the feedback control of the EGR valve, when the control reaches the limit, the variable throttle means is operated to change the EGR amount, thereby bringing the feedback control of the EGR valve within the limit. Can be returned. Furthermore, since the control accuracy of EGR by the variable nozzle turbocharger is inferior to the control accuracy of the EGR amount by the EGR valve, the control of the EGR amount by adjusting the nozzle opening is set as an open loop control and the target EGR amount of EGR amount By controlling the EGR valve so as to eliminate the deviation of the EGR amount while suppressing the deviation amount with respect to the amount within a predetermined control range, the variable nozzle as the EGR adjustment means and the EGR valve can be used properly. The EGR amount can be controlled efficiently and with high accuracy to the target EGR amount.

  As described above, according to the present invention, the EGR passage volume is made smaller compared with the case where the EGR passage is connected upstream of the compressor of the low-pressure turbocharger to improve the EGR response. , EGR control accuracy can be improved. In addition, since the capacity of the compressor of the high-pressure turbocharger is set smaller than the capacity of the compressor of the low-pressure turbocharger, the high-pressure turbocharger is set to have a larger capacity than the low-pressure turbocharger. In comparison, the EGR channel volume is reduced, and the EGR response is excellent. Compared to the case where the EGR take-out position is set upstream of the turbine of the high-pressure turbocharger, a larger exhaust energy is given to the turbine of the high-pressure turbocharger, thereby performing EGR without impairing the supercharging effect. it can.

[First embodiment]
FIG. 1 shows an embodiment of a supercharging system according to the present invention. This supercharging system is applied to a diesel engine as an internal combustion engine (hereinafter sometimes referred to as an engine). The engine 1 includes an intake passage 2 and an exhaust passage 3, and a low-pressure turbocharger 4 and a high-pressure turbocharger 5 are provided between the passages 2 and 3. These turbochargers 4 and 5 collect exhaust energy by turbines 4b and 5b arranged in the exhaust passage 3, and drive the compressors 4a and 5a arranged in the intake passage 2 by the energy to take in intake air. It is a known device for compressing. The turbine 5 b of the high-pressure turbocharger 5 is arranged upstream of the exhaust passage 3 than the turbine 4 b of the low-pressure turbocharger 4, and the compressor 5 a is downstream of the intake passage 2 than the compressor 4 a of the low-pressure turbocharger 4. Placed in. Accordingly, the intake air compressed by the low-pressure turbocharger 4 is further compressed by the high-pressure turbocharger 5 and sent into a cylinder (not shown) of the engine 1. The capacity of the high-pressure turbocharger 5 is smaller than the capacity of the low-pressure turbocharger 4. That is, the capacities of the compressor 5a and the turbine 5b of the high pressure turbocharger 5 are smaller than the capacities of the compressor 4a and the turbine 4b of the low pressure turbocharger 4, respectively.

  An air cleaner 6 for filtering the intake air and an air flow meter 7 for outputting a signal corresponding to the intake air amount are provided upstream of the compressor 4a. A throttle valve 8 for changing the intake air amount is provided between the compressors 4a and 5a. On the other hand, between the downstream of the compressor 5a of the high-pressure turbocharger 5 and the intake manifold 9, the intake passage 2 is branched into a main passage 2a and an intake bypass passage 2b. An intercooler 10 for cooling the intake air compressed by the turbochargers 4 and 5 is provided in the main passage 2a. As a flow path selection means for selecting whether the intake air is guided to the intercooler 10 or the intake bypass passage 2b, a first switching valve 11 is provided in the vicinity of the junction of the intake bypass passage 2b with the main passage 2a, and the intercooler A second switching valve 12 is provided in the main passage 2a upstream of 10 and downstream of the branch portion of the intake bypass passage 2b. It is desirable to arrange the first switching valve 11 as close to the junction as possible, and it is desirable to arrange the second switching valve 12 as close to the branch as possible.

  When the flow path lengths from the branching portion to the merge portion are compared between the main passage 2a and the intake bypass passage 2b, the flow passage length of the intake bypass passage 2b is omitted from the internal passage of the intercooler 10. Thus, it is sufficiently shorter than the flow path length of the main passage 2a. The intake bypass passage 2b is used only during execution of EGR, and when the intake air amount is large as in sudden acceleration, EGR is prohibited and the intake bypass passage 2b is closed. Therefore, the cross-sectional area of the intake bypass passage 2b may be set according to the maximum intake air amount during execution of EGR, and may be smaller than the cross-sectional area of the main passage 2a. As a result, the flow volume of the intake bypass passage 2b is sufficiently smaller than that of the main passage 2a (from the branch portion to the merge portion).

  On the other hand, in the exhaust passage 3, a main passage 3 a that guides exhaust from the exhaust manifold 13 to the turbine 5 b of the high-pressure turbocharger 5, and the exhaust passage 13 bypasses the turbine 5 b and joins the main passage 3 a downstream of the turbine 5 b. An exhaust bypass passage 3b is provided. The exhaust bypass passage 3b is provided with an exhaust bypass valve 14 for adjusting the flow rate of the exhaust gas passing through the exhaust bypass passage 3b. The exhaust bypass valve 14 can be adjusted between a minimum opening and a maximum opening. When the exhaust bypass valve 14 is set to the minimum opening, the exhaust bypass passage 3b is closed and the entire amount of exhaust gas in the exhaust manifold 13 is guided to the main passage 3a.

  Between the turbines 4b and 5b in the exhaust passage 3, an exhaust purification device 15 for purifying the exhaust is provided. The exhaust purification device 15 is disposed downstream of the joining position of the main passage 3a and the exhaust bypass passage 3b. Therefore, the exhaust gas that has passed through either the turbine 5 b or the exhaust bypass passage 3 b of the high-pressure turbocharger 5 passes through the exhaust gas purification device 15. As the exhaust purification device 15, a particulate filter that captures particulate matter in the exhaust and a catalyst having oxidation ability can be used alone or in combination. The catalyst having oxidation ability may be an oxidation catalyst or a NOx storage reduction catalyst. The exhaust gas purification device 15 may be configured by supporting a catalytic material having oxidizing ability on these particulate filters.

  The exhaust passage 3 and the intake passage 2 are connected by an EGR passage 16. The EGR passage 16 includes an EGR extraction position set downstream of the exhaust purification device 15 in the exhaust passage 3 and upstream of the turbine 4b of the turbocharger 4, and downstream of the throttle valve 8 in the intake passage 2 and high-pressure turbocharger. The EGR introduction position set upstream of the compressor 5a of the feeder 5 is provided. The EGR passage 16 is provided with a water-cooled EGR cooler 17 that cools EGR gas and an EGR valve 18 that adjusts the flow rate of gas passing through the EGR passage 16. As the cooling water of the EGR cooler 17, the cooling water of the engine 1 is guided. The EGR valve 18 can be adjusted between a minimum opening and a maximum opening. When the EGR valve 18 is set to the minimum opening, the EGR passage 16 is closed and the introduction of EGR gas into the intake passage 2 is prevented.

  In the supercharging system described above, the operations of the first switching valve 11, the second switching valve 12, the exhaust bypass valve 14, and the EGR valve 18 are controlled by an engine control unit (hereinafter referred to as ECU) 20. The ECU 20 is provided as a computer unit that controls fuel injection and the like for the engine 1, and each valve 11, 12, 14 is executed by executing a specific program prepared for controlling the supercharging system of the present invention. And 18 functions as a means for controlling the operation. In addition to the air flow meter 7, output signals from various sensors such as the intake pressure sensor 21 are input to the ECU 20 in order to determine the operating state of the engine 1, but details thereof are omitted.

  FIG. 2 shows an EGR valve control routine that the ECU 20 repeatedly executes at a constant cycle in order to control the EGR valve 18. By executing this EGR control routine, the ECU 20 functions as an EGR valve control means. In the EGR valve control routine of FIG. 2, the ECU 20 first identifies the operating state of the engine 1 with reference to various sensor signals in step S1. In subsequent step S2, the ECU 20 determines whether or not an EGR execution condition is satisfied. This determination may be performed by a method similar to the known EGR valve control. For example, when the engine 1 is required to output at the time of starting or accelerating, the establishment of the EGR execution condition is denied. In an engine that can be operated in a premixed compression ignition mode that requires a large amount of EGR gas, the EGR execution condition is satisfied when the premixed compression ignition mode is selected.

  If the establishment of the EGR execution condition is denied, the ECU 20 resets the EGR execution flag to 0 in step S3, and then closes the EGR passage 16 by closing the EGR valve 18 in step S4. On the other hand, if the EGR execution condition is satisfied, the ECU 20 proceeds to step S5, sets 1 to the EGR execution flag, and acquires the target EGR amount corresponding to the operating state in the subsequent step S6. The target EGR amount can be specified by, for example, storing a map in which the operating state and the target EGR amount are associated with each other in a ROM provided in the ECU 20 in advance, and using the map. In subsequent step S7, the ECU 20 detects the EGR amount (EGR gas amount). The EGR amount can be obtained by various means. As an example, the intake air amount is calculated from the intake air amount specified based on the intake pressure acquired by the intake pressure sensor 21 and the intake air temperature detected by an unillustrated intake air temperature sensor, and the air flow meter 7 detects from that value. By subtracting the intake air amount, the amount of EGR gas flowing into the intake passage 2 via the EGR passage 16 can be obtained. By performing the process of step S7, the ECU 20 functions as an EGR amount detection means in cooperation with the air flow meter 7 and the intake pressure sensor 21.

  After detecting the EGR amount, the ECU 20 proceeds to step S8 and specifies the deviation of the detected value of the EGR amount from the target EGR amount. In subsequent step S9, the ECU 20 sets the opening degree of the EGR valve 18 based on the deviation of the EGR amount. In this case, when the detected value of the EGR amount is insufficient with respect to the target EGR amount, the opening degree of the EGR valve 18 is changed to the open side, and when the detected value of the EGR amount is excessive with respect to the target EGR amount. The opening degree of the EGR valve 18 is changed to the closed side. The opening change width of the EGR valve 18 may be a constant width in one routine, or the opening change width may be set larger as the deviation is larger.

  After operating the EGR valve 18 in step S4 or step S9, the ECU 20 ends the current EGR valve control routine. By repeatedly executing the above control, the opening degree of the EGR valve 18 is feedback-controlled so that the detected value of the EGR amount is a feedback amount and the deviation of the detected value from the target EGR amount is reduced. The EGR amount here is the amount of EGR gas introduced from the EGR passage 16 into the intake passage 2 per predetermined time, and is a physical quantity to be grasped as a flow rate.

  In parallel with the above-described EGR valve control routine, the ECU 20 repeatedly executes the intake bypass control routine of FIG. By executing this intake bypass control routine, the ECU 20 functions as intake bypass control means. In the intake bypass control routine of FIG. 3, the ECU 20 first determines whether or not 1 is set in the EGR execution flag in step S11. If 1 is not set, the ECU 20 proceeds to step S12, closes the first switching valve 11, and opens the second switching valve 12 in the subsequent step S13. Therefore, when the EGR valve 18 is closed, that is, when EGR is prohibited, the intake bypass passage 2 b is closed, and the intake air passing through the intake passage 2 is guided to the intake manifold 9 via the intercooler 10. On the other hand, when the EGR execution flag is set to 1, the ECU 20 proceeds to step S14 to open the first switching valve 11, and then closes the second switching valve 12 in step S15. As a result, during execution of EGR, intake air containing EGR gas bypasses the intercooler 10 and is guided from the intake bypass passage 2b to the intake manifold 9.

  The ECU 20 repeatedly executes the exhaust bypass valve control routine of FIG. 4 at a predetermined cycle in parallel with the routines of FIGS. 2 and 3 described above. In the exhaust bypass valve control routine of FIG. 4, the ECU 20 first acquires the exhaust gas flow rate (exhaust flow rate) in step S21. The exhaust flow rate can be obtained from, for example, the intake air amount specified based on the rotational speed of the engine 1 and the output of the intake pressure sensor 21. In subsequent step S22, the ECU 20 acquires the opening degree of the exhaust bypass valve 14 corresponding to the exhaust flow rate from, for example, a map stored in the ROM. For example, when exhaust gas having a flow rate exceeding the limit amount of exhaust gas that can be received by the turbine 5b of the high-pressure turbocharger 5 is discharged to the exhaust manifold, the exhaust gas exceeding the limit amount is introduced into the exhaust bypass passage 3b. The opening degree of the exhaust bypass valve 14 is determined so as to be adjusted. In the following step S23, the ECU 20 sets the opening degree of the exhaust bypass valve 14 to the opening degree acquired in step S22, and thereafter ends the exhaust bypass valve control routine.

  According to the above supercharging system, the connection position (EGR introduction position) of the EGR passage 16 with respect to the intake passage 2 is set between the compressors 4 a and 5 a of the low pressure turbocharger 4 and the high pressure turbocharger 5. Therefore, the EGR channel volume is smaller than when the EGR passage 16 is connected upstream of the compressor 4a of the low-pressure turbocharger 4. Therefore, the response of EGR is improved and the control accuracy of EGR is improved. Moreover, since the capacity of the compressor 5a of the high-pressure turbocharger 5 is set smaller than the capacity of the compressor 4a of the low-pressure turbocharger 4, the high-pressure turbocharger 5 is larger than the low-pressure turbocharger 4. Compared to the conventional example set to capacity, the EGR channel volume is small and the EGR response is excellent. Since the connection position (EGR extraction position) of the EGR passage 16 to the exhaust passage 3 is set between the turbines 4b and 5b of the low-pressure turbocharger 4 and the high-pressure turbocharger 5, the EGR extraction position is set to the turbine. Compared with the case where it is set upstream of 5b, it is possible to give a larger exhaust energy to the turbine 5b of the high-pressure turbocharger 5, thereby performing EGR without impairing the supercharging performance.

  Further, according to the supercharging system, the first switching valve 11 is opened while the EGR valve 18 is open, the second switching valve 12 is closed, and all intake air including EGR gas bypasses the intercooler 10. Then, the air is guided from the intake bypass passage 2b to the intake manifold 9. As described above, since the flow volume of the intake bypass passage 2b is sufficiently smaller than that of the main passage 2a, the responsiveness of EGR is further improved. The contamination of the intercooler 10 due to EGR gas and the deterioration of the cooling efficiency of the intercooler 10 due to the contamination can be prevented. Furthermore, since the second switching valve 12 is disposed upstream of the intercooler 10, there is no possibility that intake air containing EGR gas will stay in the intercooler 10. Therefore, when the EGR execution condition is no longer satisfied due to sudden acceleration or the like, air that does not contain EGR gas is rapidly sent to the engine 1 by opening the second switching valve 12 and closing the first switching valve 11. Can do. For this reason, it is possible to suppress the occurrence of smoke due to the intake air containing EGR gas being introduced into the engine 1. Since the first switching valve 11 is provided at the end of the intake bypass passage 2b, that is, in the vicinity of the junction with the main passage 2a, the EGR from the intake bypass passage 2b to the intake manifold 9 by closing the first switching valve 11 The introduction of gas can be stopped immediately. This can also suppress the occurrence of smoke.

  On the other hand, regarding the exhaust passage 3, when exhaust gas having a flow rate exceeding the capacity of the turbine 5 b of the high-pressure turbocharger 5 is discharged to the exhaust manifold 13, if the exhaust bypass passage 3 b does not exist, the turbine 5 b The amount of exhaust gas guided to the connection position of the EGR passage 16 is limited to a limit amount that can pass through the turbine 5b. On the other hand, in the above supercharging system, by opening the exhaust bypass valve 14 provided in the exhaust bypass passage 3b, exhaust gas having a flow rate exceeding the limit amount of the turbine 5b is guided to the connection position of the EGR passage 16. be able to. For this reason, a large amount of EGR gas can be introduced into the intake passage 2. In particular, in the case of the engine 1 that can be operated in the premixed compression ignition mode, the maximum amount of EGR gas that can be introduced into the intake passage 2 is increased, thereby suppressing premature ignition due to lack of EGR gas and premixed compression. The operating range of the engine 1 suitable for the ignition mode can be expanded to the high load side.

  Further, by flowing exhaust gas having a flow rate exceeding the limit amount of the turbine 5b to the exhaust bypass passage 3b, an increase in the pressure (exhaust back pressure) of the exhaust manifold 13 is suppressed, thereby preventing deterioration of the intake charging efficiency of the engine 1. Thus, the output of the engine 1 can be reliably increased. Further, by opening the exhaust bypass valve 14 and guiding a part of the exhaust gas pressure to the EGR extraction position, the pressure at the EGR extraction position of the EGR passage 16 is increased, and thereby the EGR passage 16 is connected to the exhaust passage 3. The pressure at the connection position (EGR extraction position) can be further increased with respect to the pressure at the connection position (EGR introduction position) of the EGR passage 16 with the intake passage 2 to increase the efficiency of EGR. Further, since the exhaust gas that has passed through either the turbine 5b or the exhaust bypass passage 3b of the high-pressure turbocharger 5 passes through the exhaust purification device 15 and is guided to the EGR extraction position, the particulate matter introduced into the EGR passage 16 As a result, the contamination of various devices through which EGR gas passes, such as the EGR passage 16, the EGR cooler 17, the EGR valve 18, and the turbine 5b of the high-pressure turbocharger 5, can be suppressed.

  In the above embodiment, when the target EGR amount cannot be achieved by the opening degree control of the EGR valve 18, the opening amount of the throttle valve 8 is changed to the closed side and the fresh air amount (air cleaner) guided to the EGR introduction position of the EGR passage 16 is reached. 6), the target EGR amount may be ensured by increasing the EGR amount. However, in the present invention, the throttle valve 8 is not necessarily provided, and the intake air amount may be adjusted by other means.

[Second form]
FIG. 5 shows a supercharging system according to the second embodiment of the present invention. In the present embodiment, the low-pressure turbocharger 4 is a so-called variable nozzle turbocharger that can change the flow path cross-sectional area of the inlet portion of the turbine 4b. The function is different from the embodiment of FIG. The cooling water of the EGR cooler 17 is guided to an EGR radiator 31 provided separately from an engine cooling radiator (not shown). The cooling system connecting the EGR radiator 31 and the EGR cooler 17 is separated from the engine cooling system including the engine cooling radiator. That is, the cooling water circulation paths of both cooling systems are separated without crossing each other. A water pump 32 for circulating cooling water is connected to the flow path (which may be either the forward path or the return path) connecting the EGR cooler 17 and the radiator 31. The rest of the configuration of the supercharging system is the same as that of FIG. Therefore, in FIG. 5, the same reference numerals are given to the portions common to the above-described embodiment of FIG.

  The ECU 20 controls the nozzle opening degree of the low-pressure turbocharger 4 so that a supercharging effect according to the operating state of the engine 1 is obtained. Control of the nozzle opening with respect to the target supercharging pressure may be performed in the same procedure as a known supercharging system using a variable nozzle turbocharger, and details thereof are omitted here. The ECU 20 repeatedly executes the routines shown in FIGS. 2 to 4 at a predetermined cycle, and in parallel with these controls, repeatedly executes the nozzle opening correction control routine shown in FIG. 6 at a predetermined cycle. By doing so, it functions as the aperture control means of the present invention. This nozzle opening correction control routine is executed to supplement the control of the EGR amount by the EGR valve 18. In first step S31, the ECU 20 determines whether or not the deviation amount of the opening degree of the EGR valve 18 from the reference opening degree has reached a limit. For example, when the engine 1 is started, the opening degree of the EGR valve 18 is initialized to the reference opening degree, and every time step S9 in FIG. 2 is executed, the deviation amount from the reference opening degree is changed. It can be identified by calculation. The limit of the deviation amount may be appropriately determined from the reference opening degree in the opening direction and the closing direction as a range in which the EGR amount can be controlled only by the opening degree control of the EGR valve 18.

  If it is determined in step S31 that the deviation amount is within the limit, the ECU 20 ends the current control routine. On the other hand, when the deviation amount exceeds the limit, the ECU 20 proceeds to step S32, and obtains the correction amount of the nozzle opening according to the deviation amount from the reference opening from the map recorded in advance in the ROM. This map describes the correlation between the deviation amount of the opening of the EGR valve 18 and the correction amount of the nozzle opening, and can be created by a bench fit test or simulation. The relationship between the deviation amount of the opening degree of the EGR valve 18 and the correction amount of the nozzle opening degree is as follows.

  When the opening degree of the EGR valve 18 is deviated in the direction to open the EGR valve 18 with respect to the reference opening degree, it is necessary to increase the EGR amount beyond the control limit of the EGR amount by the EGR valve 18. The correction amount of the nozzle opening is given in the direction in which the nozzle of the low-pressure turbocharger 4 is closed. When the nozzle opening degree of the low-pressure turbocharger 4 is changed to the closed side, the pressure at the EGR take-out position upstream of the low-pressure turbocharger 4 increases, and even if the opening degree of the EGR valve 18 remains constant, more EGR gas is introduced into the intake passage 2 via the EGR passage 16. On the other hand, when the opening degree of the EGR valve 18 is deviated in the direction to close the EGR valve 18 with respect to the reference opening degree, it is necessary to reduce the EGR amount beyond the control limit of the EGR amount by the EGR valve 18. Therefore, the correction amount of the nozzle opening is given in the direction in which the nozzle of the low-pressure turbocharger 4 is opened. When the nozzle opening degree of the low-pressure turbocharger 4 is changed to the open side, the pressure at the EGR take-out position upstream of the low-pressure turbocharger 4 is reduced, and the EGR passage 16 is allowed to pass even if the opening degree of the EGR valve 18 remains constant. Thus, the amount of EGR gas introduced into the intake passage 2 decreases.

  After acquiring the correction amount of the nozzle opening in step S32, the ECU 20 proceeds to step S33, and corrects the nozzle opening of the low-pressure turbocharger 4 according to the acquired correction amount. Thereafter, the ECU 20 ends the current routine. The correction of the nozzle opening performed here is open loop control that drives the actuator of the nozzle by the amount corresponding to the correction amount specified in the map.

  FIG. 7 shows an example of a correspondence relationship between the EGR valve opening and the nozzle opening (VN opening) when the above-described nozzle opening correction control is executed. In this example, as a result of the feedback control shown in FIG. 2, the opening degree of the EGR valve 18 gradually changes to the open side with respect to the reference opening degree. When the EGR valve opening exceeds the upper limit (open side limit), the nozzle opening of the low-pressure turbocharger 4 is changed to the closed side by a predetermined correction amount by the routine of FIG. As a result, the amount of EGR gas guided to the intake passage 2 via the EGR passage 16 increases, so the opening degree of the EGR valve 18 is controlled in the decreasing direction by the feedback control of FIG. 2 and is adjusted again near the reference opening degree. It becomes like this.

  Thus, according to the present embodiment, the EGR valve 18 can be controlled when the EGR amount cannot be controlled to the target EGR amount as long as the opening degree of the EGR valve 18 is feedback controlled within a predetermined range (between the upper limit and the lower limit in FIG. 7). The feedback control is regarded as exceeding a predetermined limit, the nozzle opening of the low-pressure turbocharger 4 is corrected, and the opening of the EGR valve 18 is returned to a predetermined range with respect to the reference opening. Since the control accuracy of the EGR amount by the control of the nozzle opening of the turbine 4b is inferior to the control accuracy of the EGR amount by the feedback control of the EGR valve 18, the EGR valve 18 is executed by executing the control as in this embodiment. The EGR amount can be controlled with high accuracy toward the target EGR amount while avoiding the occurrence of an uncontrollable state due to the deviation of the opening from the reference opening.

  Further, in the present embodiment, the cooling efficiency of the EGR cooler 17 is compared with the case where the cooling water of the engine 1 is used by releasing the heat of the cooling water of the EGR cooler 17 from the radiator 31 provided as a dedicated product for the EGR cooler 17. Can be significantly increased. As a result, the temperature of the EGR gas introduced from the EGR passage 16 into the intake passage 2 can be sufficiently lowered, the thermal load on the compressor 5a of the high-pressure turbocharger 5 can be reduced, and the reliability thereof can be improved. Further, when the exhaust purification device 15 has an oxidizing ability, the temperature of the EGR gas rises due to the oxidation heat of HC and CO in the exhaust. However, if the EGR gas is sufficiently cooled using the radiator 31, the oxidation is performed. Heat corresponding to the temperature rise due to heat is taken away from the EGR gas, and the temperature of the EGR gas introduced into the intake passage 2 can be sufficiently lowered.

  The present invention is not limited to the form described above, and may be implemented in various forms. The arrangement and passage configuration of various devices such as the intake passage 2 and the exhaust passage 3 shown in FIGS. 1 and 5 are merely examples of the present invention, and various modifications can be made. For example, the intake bypass passage 2b, the first switching valve 11 and the second switching valve 12 provided therewith, and the exhaust bypass passage 3b and the exhaust bypass valve 14 are not necessarily provided in the present invention. In the simplest form of the present invention, the EGR extraction position of the EGR passage 16 is set between the turbines 4b and 5b, the EGR introduction position is set between the compressors 4a and 5a, and the capacity of the high-pressure turbocharger 5 is What is necessary is just to set smaller than the capacity | capacitance of the low pressure turbocharger 4. FIG. When the intake bypass passage 2b is provided, as long as the volume of the flow path passing through the intake bypass passage 2b is kept smaller than the volume of the flow path passing through the main passage 2a, intake cooling such as an EGR cooler is provided in the intake bypass path 2b. Means may be provided. The flow path selection means of the intake passage 2 is not limited to the combination of the first switching valve 11 and the second switching valve 12, and various valve means may be used.

  As shown in FIG. 8, an intercooler 33 may be further provided in the intake passage 2 downstream of the compressor 4 a of the low-pressure turbocharger 4 and upstream of the connection position with the EGR passage 16. Thereby, the intake air compressed by the low-pressure turbocharger 4 can be cooled to increase the supercharging efficiency. If the intercooler 33 is disposed upstream of the connection position of the EGR passage 16, the flow volume of the EGR gas is kept small without being affected by the intercooler 33, and the responsiveness of EGR is not impaired at all. When the throttle valve 8 is provided in the intake passage 2, the intercooler 33 can be disposed upstream of the throttle valve 8.

  In the second embodiment, the nozzle of the low-pressure turbocharger 4 is used as the variable throttle means provided downstream from the EGR take-out position of the exhaust passage 3, but EGR is performed using various variable throttle means such as an exhaust throttle valve. The feedback control of the valve 18 may be supplemented. The control of the exhaust bypass valve 14 is not necessarily performed based on the exhaust flow rate. The opening degree of the exhaust bypass valve 14 may be appropriately adjusted according to the operating state of the engine 1 and the required output. Depending on the operating state of the engine 1, the exhaust bypass valve 14 may be switched between two positions, a fully closed state and a fully open state.

The figure which shows the structure of the supercharging system which concerns on the 1st form of this invention. The flowchart which shows the EGR valve control routine which ECU performs in order to control an EGR valve. The flowchart which shows the intake bypass control routine which ECU for opening and closing an intake bypass passage performs. 6 is a flowchart showing an exhaust bypass valve control routine executed by the ECU to adjust the amount of exhaust gas passing through the exhaust bypass passage. The figure which shows the structure of the supercharging system which concerns on the 2nd form of this invention. 6 is a flowchart showing a nozzle opening correction control routine executed by the ECU of FIG. 5 to control the opening of the variable nozzle of the low-pressure turbocharger. The figure which shows an example of the relationship between the control state of the opening degree of an EGR valve, and the variable nozzle opening degree controlled by the routine of FIG. The figure which shows the structure of the supercharging system which concerns on the further form of this invention.

Explanation of symbols

1 engine (internal combustion engine)
2 Intake passage 2a Main passage 2b Intake bypass passage 3 Exhaust passage 3a Main passage 3b Exhaust bypass passage 4 Low pressure turbocharger 4a Compressor 4b Turbine 5 High pressure turbocharger 5a Compressor 5b Turbine 8 Throttle valve 10 Intercooler 11 First switching Valve (channel selection means)
12 Second switching valve (channel selection means)
DESCRIPTION OF SYMBOLS 14 Exhaust bypass valve 15 Exhaust gas purification device 16 EGR passage 17 EGR cooler 18 EGR valve 31 EGR radiator 32 Water pump 33 Intercooler

Claims (1)

A low-pressure turbocharger,
The low-pressure turbocharger has a turbine disposed upstream of the exhaust passage from the turbine of the low-pressure turbocharger and a compressor disposed downstream of the intake passage from the compressor of the low-pressure turbocharger. A high-pressure turbocharger with a smaller capacity than
Between the EGR take-out position set between the turbine of the high-pressure turbocharger and the turbine of the low-pressure turbocharger, and between the compressor of the low-pressure turbocharger and the compressor of the high-pressure turbocharger An EGR passage connecting the set EGR introduction position ;
An EGR valve that adjusts the amount of EGR passing through the EGR passage;
EGR valve control means for controlling the EGR valve so as to obtain an EGR amount corresponding to the operating state of the internal combustion engine;
Variable throttle means provided downstream of the EGR extraction position of the exhaust passage and capable of adjusting the opening;
Throttle control means for controlling the opening of the variable throttle means;
EGR amount detection means for detecting the amount of EGR introduced into the intake passage,
The EGR valve control means feedback-controls the EGR valve so that a deviation of the detected value with respect to the target EGR amount is reduced by using the detected value of the EGR amount by the EGR amount detecting means as a feedback amount,
The throttle control means determines whether or not feedback control of the EGR valve exceeds a predetermined limit, and when the limit is exceeded, the opening degree of the variable throttle means is changed in a direction in which the deviation decreases. Let
As the low-pressure turbocharger, by providing a variable nozzle turbocharger capable of changing the flow path cross-sectional area of the inlet portion of the turbine, the low-pressure turbocharger functions as the variable throttle means,
The throttle control means acquires a change width of the nozzle opening of the low-pressure turbocharger based on a map prepared in advance as that to be executed when the limit is exceeded, and according to the acquired change width The deviation of the EGR amount is reduced by open-loop control of the nozzle opening of the low-pressure turbocharger.
Supercharging system that is characterized in that.

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