AU2019426487A1

AU2019426487A1 - Supercharging system

Info

Publication number: AU2019426487A1
Application number: AU2019426487A
Authority: AU
Inventors: Tomohito OZEKI
Original assignee: Toyota Industries Corp
Current assignee: Toyota Industries Corp
Priority date: 2019-01-31
Filing date: 2019-10-25
Publication date: 2021-08-12
Anticipated expiration: 2039-10-25
Also published as: JP2020122467A; JP7056596B2; AU2019426487B2; WO2020158074A1

Abstract

This supercharging system comprises: primary and secondary superchargers (30, 40) including turbines (32, 42) driven by exhaust discharged form an engine, variable nozzle mechanisms (35, 45) that via the degrees of opening thereof adjust the flow rate of exhaust flowing into the turbines (32, 42), and compressors (31, 41) that supercharge air taken into the engine (1); a control device (200) that switches between a single supercharging mode in which air supercharged in the primary supercharger (30) is supplied and a twin supercharging mode in which air supercharged in the primary and secondary superchargers (30, 40) is supplied; and a memory that stores data indicating a map indicating the characteristics of the compressors (31, 41). The map includes a first axis for the amount of air taken in and a second axis for a pressure ratio of discharge pressure to intake pressure. When the control device (200) as switched to twin supercharging mode, the control devices switches to single supercharging mode when an operating point identified in the map by the pressure ratio and the amount of air taken in falls below a switch line on the map (S115, 117).

Description

DESCRIPTION SUPERCHARGING SYSTEM TECHNICAL FIELD

[0001] This disclosure relates to a supercharging system, particularly a supercharging system having a plurality of superchargers connected in parallel to each other.

BACKGROUNDART

[0002] There has been conventionally known a supercharging system having two superchargers connected in parallel to each other (e.g. see Patent Document 1). A supercharging system of the Patent Document 1 has a single supercharging mode in which one supercharger pumps intake air into an engine and a twin supercharging mode in which two superchargers pump intake air into the engine. The supercharging system switches the two modes in accordance with an engine load.

[0003] The single supercharging mode in which only a primary supercharger is driven is set at a low speed area. This is to improve a responsiveness of boost pressure with one supercharger in a speed area in which exhaust energy is small. The twin supercharging mode in which the primary supercharger and a secondary supercharger are driven is set at a high speed area. This is to realize a high boost pressure with two superchargers in a speed area in which the exhaust energy is large.

Citation List Patent Document

[0004] Patent Document 1: Japanese Patent Application Publication No. 2009

SUMMARY OF INVENTION

Technical Problem

[0005] Patent Document 1 discloses a technique having a feature regarding the switching from the single supercharging mode to the twin supercharging mode. However, Patent Document 1 does not disclose a technique having a feature regarding the switching from the twin supercharging mode to the single supercharging mode. Thus, the technique having the feature regarding the switching from the twin supercharging mode to the single supercharging mode has not been conventionally considered so much. In a generally used technique, the mode is switched from the twin supercharging mode to the single supercharging mode in accordance with an engine speed and an engine load (fuel injection amount).

[0006] FIG. 14 is an explanatory view of the conventional switching from the twin supercharging mode to the single supercharging mode. FIG. 14 at (A) illustrates a variation in an engine speed. FIG. 14 at (B) illustrates a variation in a fuel injection amount. FIG. 14 at (C) illustrates a variation in boost pressure. At a time T1, the engine speed is gradually reduced by turning off the accelerator, or the like, and the fuel injection amount is cut off at once. Then, target boost pressure determined by the engine speed and the fuel injection amount also decreases at once. However, a decrease of actual boost pressure is delayed relative to that of the target boost pressure, and the actual boost pressure gradually decreases, thereby causing a gap between the target boost pressure, which is determined by the engine speed and the fuel injection amount, and the actual boost pressure. This means that the switching from the twin supercharging mode to the single supercharging mode in accordance with the engine speed and the fuel injection amount does not appropriately work under a transitional state in which the engine speed and the fuel injection amount continue to change.

[0007] This disclosure has been made to solve the problem described above, and is directed to providing a supercharging system that appropriately switches the mode from the twin supercharging mode to the single supercharging mode even under a transitional condition.

Solution to Problem

[0008] A supercharging system according to the present disclosure includes: a first supercharger including a first turbine that is driven by exhaust gas discharged from an engine, a first variable nozzle mechanism that is configured to adjust a flow rate of the exhaust gas flowing into the first turbine by an opening degree, and a first compressor that is configured to pump intake air into the engine; a second supercharger including a second turbine that is driven by the exhaust gas discharged from the engine, and a second variable nozzle mechanism that is configured to adjust a flow rate of the exhaust gas flowing into the second turbine by an opening degree, wherein the second supercharger is configured to pump the intake air into the engine; a control device that is configured to switch a supercharging mode from a single supercharging mode to a twin supercharging mode or from a twin supercharging mode to a single supercharging mode, wherein in the single supercharging mode, the air pumped by the first supercharger is fed to the engine, and in the twin supercharging mode, the air pumped by the first supercharger and the air pumped by the second supercharger are fed to the engine; and a storage unit that stores data showing a compressor map that illustrates a characteristic of the first compressor. The compressor map includes a first axis that shows an air amount drawn to the first compressor of the first supercharger and a second axis that shows a pressure ratio of discharge pressure to intake pressure in the first compressor.

[0009] The control device switches the supercharging mode from the twin supercharging mode to the single supercharging mode, when an operating point that is specified by the pressure ratio and the air amount on the compressor map stored in the storage unit falls below a specified line on the compressor map during operation in the twin supercharging mode.

[0010] Preferably, the specified line is a border between an area in which the pressure ratio is more quickly increased in the twin supercharging mode than in the single supercharging mode and an area in which the pressure ratio is more quickly increased in the single supercharging mode than in the twin supercharging mode, when fuel injection is restarted in the engine. Preferably, the specified line is a constant rotational speed line of the first supercharger during a steady traveling.

[0011] Preferably, when the operation point is not below the specified line in the twin supercharging mode, the control device calculates, from quantities of state of an intake passage and an exhaust passage, target opening degrees of the first variable nozzle mechanism and the second variable nozzle mechanism that the first supercharger and the second supercharger require to obtain target boost pressures, and the control device switches the supercharging mode to the single supercharging mode when the calculated opening degrees are out of a controllable range.

Advantageous Effects of Invention

[0012] According to the disclosure, the supercharging system is provided that appropriately switches the supercharging mode from the twin supercharging mode to the single supercharging system even under a transitional condition.

BRIEF DESCRIPTION OF DRAWINGS

[0013] FIG. 1 is a view illustrating an example of a schematic configuration of an engine according to a present embodiment.

FIG. 2 is an explanatory view illustrating an operation of a supercharging system in a single supercharging mode.

FIG. 3 is an explanatory view illustrating an operation of the supercharging system in a run-up mode.

FIG. 4 is an explanatory view illustrating an operation of the supercharging system in a twin supercharging mode.

FIG. 5 is flowchart illustrating an example of a switching procedure to the single supercharging mode according to the present embodiment.

FIG. 6 is a first explanatory view of physical equations of a supercharger.

FIG. 7 is a second explanatory view of physical equations of the supercharger.

FIG. 8 is an explanatory view of an example of a variation of a target VN opening degree calculated by the physical equations.

FIG. 9 is a view illustrating an example of a compressor operation map that illustrates a relationship between an intake air amount of a compressor of a primary supercharger and a pressure ratio between upstream pressure and downstream pressure of the compressor.

FIG. 10 is an explanatory view illustrating a variation of boost pressure by reacceleration in the cases where an operating point on the compressor operation map reaches and does not reach a switching line L1.

FIG. 11 is an explanatory view illustrating, on the compressor operation map, different operating points in accordance with the accelerator opening degrees in a steady operation.

FIG. 12 is an explanatory view illustrating different boost pressures by reacceleration after deceleration in each of the accelerator opening degrees.

FIG. 13 is an explanatory view illustrating a calculation of a target VN opening degree according to a modified embodiment.

FIG. 14 is an explanatory view of the conventional switching from a twin supercharging mode to a single supercharging mode.

DESCRIPTION OF EMBODIMENTS

[0014] The following will describe an embodiment of the present disclosure with reference to the accompany drawings. In the following description, the same components are denoted by the same reference numerals. The same components also have the same names and functions. Accordingly, a detailed description of the same components is not reiterated.

[0015] [Configuration of a supercharging system] FIG. 1 is a view illustrating an example of a schematic configuration of an engine 1 according to the present embodiment. Referring to FIG. 1, the engine 1 is mounted on a vehicle as a driving source for travelling, for example. In one example, the engine 1 will be described as a diesel engine in the present embodiment. The engine 1 may be a gasoline engine.

[0016] The engine 1 includes a cylinder bank 10A, a cylinder bank 10B, an air cleaner 20, an intercooler 25, an intake manifold 28A, an intake manifold 28B, a primary supercharger 30, a secondary supercharger 40, an exhaust manifold 50A, an exhaust manifold 50B (hereinafter, also referred to as "ex-mani"), an exhaust gas treatment device 81, and a control device 200.

[0017] The cylinder bank 10A has a plurality of cylinders 12A. Thecylinderbank

10B has a plurality of cylinders 12B. A piston (not illustrated) is accommodated in each of the cylinders 12A, 12B, and a top portion of the piston and an inner wall of the cylinder cooperate to form a combustion chamber (a space in which fuel is combusted). The piston slides in each of the cylinders 12A, 12B to change volume of the combustion chamber. Each of the cylinders 12A, 12B has an injector (not illustrated). The injector is configured to inject fuel into each of the cylinders 12A, 12B at a timing during an operation of the engine 1. The timing and a fuel injection amount are set by the control device 200. It is noted that the fuel injection amount that is injected by the injector and the timing are set by the control device 200 in accordance with, for example, an engine speed, an intake air amount Ga, a stepping amount of an accelerator pedal, a vehicle speed, or the like.

[0018] The piston of each of the cylinders 12A, 12B is coupled toa common crank shaft (not illustrated) through a connecting rod. The fuel is combusted in each of the cylinders 12A, 12B in a specified order to cause the piston to slide in each of the cylinders 12A, 12B in an up-to-down direction. The up-to-down motion of the piston is converted into a rotational motion of the crank shaft through the connecting rod.

[0019] The primary supercharger 30 (a first supercharger) is a turbocharger that includes a compressor 31 (a first compressor) and a turbine 32 (a first turbine). The compressor 31 of the primary supercharger 30 is disposed in an intake passage (that is, a passage from the air cleaner 20 to the intake manifolds 28A, 28B) of the engine 1. The turbine 32 of the primary supercharger 30 is disposed in an exhaust passage (that is, a passage from the exhaust manifolds 50A, 50B to the exhaust gas treatment device 81) of the engine 1.

[0020] A compressor wheel 33 is accommodated in the compressor 31 and rotatable therein. The turbine 32 has a turbine wheel 34 and a variable nozzle mechanism 35 therein. The turbine wheel 34 is accommodated in the turbine 32 and rotatable therein. The compressor wheel 33 and the turbine wheel 34 are coupled to each other by a rotary shaft 36, and integrally rotate. The compressor wheel 33 is driven to rotate by energy of exhaust gas (exhaust energy) that is fed into the turbine wheel 34.

[0021] The variable nozzle mechanism 35 (a first variable nozzle mechanism) changes a flow rate of the exhaust gas that operates the turbine 32. The variable nozzle mechanism 35 includes a plurality of nozzle vanes (not illustrated) and a driving device (not illustrated). The nozzle vanes are disposed on an outer circumferential side of the turbine wheel 34 and introduce the exhaust gas fed from an exhaust inlet to the turbine wheel 34. The driving device is configured to rotate the plurality of nozzle vanes to change a gap between adjacent two of the nozzle vanes (hereinafter, the gap is referred to as a VN opening degree). In one example, the variable nozzle mechanism 35 rotates the nozzle vanes by the driving device in response to a control signal VN1 from the control device 200 to change the VN opening degree.

[0022] The secondary supercharger 40 (a second supercharger) is a turbocharger that includes a compressor 41 and a turbine 42 (a second turbine). In this embodiment, the secondary supercharger 40 has the same configuration and size as those of the primary supercharger 30. The compressor 41 of the secondary supercharger 40 is arranged in parallel to the compressor 31 in the intake passage of the engine 1, and configured to pump intake air into the engine 1. The turbine 42 of the secondary supercharger 40 is arranged in parallel to the turbine 32 in the exhaust passage of the engine 1.

[0023] A compressor wheel 43 is accommodated in the compressor 41 and rotatable therein. The turbine 42 has a turbine wheel 44 and a variable nozzle mechanism 45 (a second variable nozzle mechanism) therein. Theturbinewheel 44 is accommodated in the turbine 42 and rotatable therein. The compressor wheel 43 and the turbine wheel 44 are coupled to each other by a rotary shaft 46, and integrally rotate. The compressor wheel 43 is driven to rotate by energy of exhaust gas that is fed into the turbine wheel 44.

[0024] Since the variable nozzle mechanism 45 has the same configuration as that of the variable nozzle mechanism 35, the detailed configuration of the variable nozzle mechanism 45 is not reiterated. In one example, the variable nozzle mechanism 45 rotates nozzle vanes by a driving device in response to a control signal VN2 from the control device 200 to change a VN opening degree.

[0025] The air cleaner 20 is configured to remove foreign substances from air drawn from an air inlet (not illustrated). The air cleaner 20 is connected to one end of an intake pipe 23. The other end of the intake pipe 23 branches into two that are each connected to one end of an intake pipe 21 and one end of an intake pipe 22.

[0026] The other end of the intake pipe 21 is connected to an air inlet of the compressor 31 of the primary supercharger 30. An air outlet of the compressor 31 of the primary supercharger 30 is connected to one end of an intake pipe 37. The other end of the intake pipe 37 is connected to the intercooler 25. The compressor 31 pumps air drawn through the intake pipe 21 by the rotation of the compressor wheel 33 to feed the air to the intake pipe 37.

[0027] The other end of the intake pipe 22 is connected to an air inlet of the compressor 41 of the secondary supercharger 40. An air outlet of the compressor 41 of the secondary supercharger 40 is connected to one end of an intake pipe 47. The other end of the intake pipe 47 is connected to a connection C3 in a middle of the intake pipe 37. The compressor 41 pumps air drawn through the intake pipe 22 by the rotation of the compressor wheel 43 to feed the air to the intake pipe 47.

[0028] A first control valve 62 is disposed in a middle of the intake pipe 47. The first control valve 62 is, for example, a VSV (negative pressure switching valve) that is normally closed and controlled to be ON (open)/OFF (closed) in response to a control signal CV1 from the control device 200.

[0029] In addition, a connection C4 that is located on an upstream side of the first control valve 62 (near the compressor 41) in the intake pipe 47 is connected to one end of a recirculation pipe 48. The other end of the recirculation pipe 48 is connected to the intake pipe 21. The recirculation pipe 48 is a passage through which at least a part of air flowing through the intake pipe 47 flows to an upstream side of the compressor 31 of the primary supercharger 30. The air flowing to the intake pipe 21 through the recirculation pipe 48 to recirculate is fed to the compressor 31.

[0030] A second control valve 64 is disposed in a middle of the recirculation pipe 48. The second control valve 64 is, for example, an electromagnetic valve (solenoid valve) that is normally closed and controlled to be ON (open)/OFF (closed) in response to a control signal CV2 from the control device 200.

[0031] The air pumped by the compressor 31 and the air pumped by the compressor 41 and passing through the first control valve 62 are fed into the connectionC3. These streams of the air join together at the connection C3, and flow into the intercooler 25.

[0032] The intercooler 25 is configured to cool the air flowing thereto. The intercooler 25 is, for example, an air-cooled type heat exchanger or a liquid-cooled type heat exchanger. An air outlet of the intercooler 25 is connected to one end of an intake pipe 27A and one end of an intake pipe 27B through a diesel throttle 68. The diesel throttle 68 has a configuration in which an opening degree of the diesel throttle 68 is adjustable by an electric actuator, and is configured to adjust a flow rate of the intake air in response to a control signal from the control device 200. The other end of the intake pipe 27A is connected to the intake manifold 28A. The other end of the intake pipe 27B is connected to the intake manifold 28B.

[0033] The intake manifolds 28A, 28B are respectively coupled to intake ports

(not illustrated) of the cylinders 12A, 12B in the cylinder banks 10A, 1OB. On the other hand, the exhaust manifolds 50A, 50B are respectively coupled to exhaust ports (not illustrated) of the cylinders 12A, 12B in the cylinder banks 10A, 1OB.

[0034] The exhaust gas (combusted gas) flows out of the cylinders from the combustion chambers of the cylinders 12A, 12B through the exhaust ports, and then, is discharged out of the engine 1 through the exhaust passage of the engine 1. The exhaust passage described above includes the exhaust manifolds 50A, 50B, exhaust pipes 51A, 51B, a connection C1, exhaust pipes 52A, 52B, 53A, 53B, and a connection C2. One end of the exhaust pipe 51A is connected to the exhaust manifold 50A. One end of the exhaust pipe 51B is connected to the exhaust manifold 50B. The other end of the exhaust pipe 51A and the other end of the exhaust pipe 51B join together at the connection C1 to one. The joined exhaust pipe branches into two that are each connected to one end of the exhaust pipe 52A and one end of the exhaust pipe 52B.

[0035] The other end of the exhaust pipe 52A is connected to the exhaust inlet of the turbine 32. An exhaust outlet of the turbine 32 is connected to one end of the exhaust pipe 53A. The other end of the exhaust pipe 52B is connected to an exhaust inlet of the turbine 42. An exhaust outlet of the turbine 42 is connected to one end of the exhaust pipe 53B.

[0036] A third control valve 66 is disposed in a middle of the exhaust pipe 52B. The third control valve 66 is, for example, a VSV (negative pressure switching valve) that is normally open and controlled to be ON (open)/OFF (closed) in response to a control signal CV3 from the control device 200.

[0037] The other end of the exhaust pipe 53A and the other end of the exhaust pipe 53B join together at the connection C2 to one that is connected to the exhaust gas treatment device 81. The exhaust gas treatment device 81 is formed from an SCR catalyst, an oxidation catalyst, a PM removing filter, or the like, and purifies the exhaust gas flowing from the exhaust pipe 53A and the exhaust pipe 53B.

[0038] The operation of the engine 1 is controlled by the control device 200. The control device 200 includes a CPU (Central Processing Unit) that performs a variety of processes, a memory (storage unit) including a ROM (Read Only Memory) that stores programs and data and a RAM (Random Access Memory) that stores results of processes by the CPU, and the like, and input/output ports through which information is transmitted/received to/from the outside (None of the CPU, the memory, and the input/output ports is illustrated.). The input ports are connected to a variety of sensors (for example, an air flow meter 102, a first pressure sensor 106, and a second pressure sensor 108). The output ports are connected to some pieces of apparatus to be controlled (for example, a plurality of injectors, the variable nozzle mechanisms 35, 45, the first control valve 62, the second control valve 64, and the third control valve 66).

[0039] The control device 200 is configured to control the various pieces of apparatus in accordance with signals from each of sensors, apparatus, and a map and program that are stored in the memory so as to make the engine 1 in a desired driving state. It is noted that a variety of controls may be performed by not only processing by software but also processing by dedicated hardware (electronic circuit). The control device 200 has therein a timer circuit (not illustrated) for measuring time intervals.

[0040] The air flow meter 102 detects the intake air amount Ga. The air flow meter 102 sends a signal indicating the detected intake air amount Ga to the control device 200.

[0041] An engine rotational number sensor detects a number of rotations of the engine. The engine rotational number sensor sends a signal indicating the detected number of rotations of the engine to the control device 200.

[0042] The first pressure sensor 106 detects pressure Pp (hereinafter, referred to as first boost pressure) at the connection C3 in the intake pipe 37. The first pressure sensor 106 sends a signal indicating the detected first boost pressure Pp to the control device 200.

[0043] The second pressure sensor 108 detects pressure (hereinafter, referred to as second boost pressure Ps) at the connection C4 in the intake pipe 47. The second pressure sensor 108 sends a signal indicating the second boost pressure Ps to the control device 200.

[0044] In this embodiment, the primary supercharger 30, the secondary supercharger 40, and the control device 200 cooperate to form "a supercharging system".

[0045] The control device 200 is configured to control the switching from a single supercharging mode to a twin supercharging mode, or vice versa by controlling the first control valve 62, the second control valve 64, and the third control valve 66. In the single supercharging mode, only the primary supercharger 30 (primary turbo) supercharges the engine. In the second supercharging mode, both of the primary supercharger 30 (primary turbo) and the secondary supercharger 40 (secondary turbo) supercharge the engine. In addition, when the control device 200 switches the mode from the single supercharging mode to the twin supercharging mode, the control device 200 executes driving in a run-up mode that is switched from the single supercharging mode, and then switches the mode to the twin supercharging mode. In the run-up mode, boost pressure is increased to a constant pressure or more by the secondary supercharger 40.

[0046] The following will describe an operation of the supercharging system in each of the single supercharging mode, the run-up mode, and the twin supercharging mode with reference to FIGS. 2, 3, and 4.

[0047] <Single supercharging mode> The control device 200 operates the supercharging system in the single supercharging mode when a specified execution condition is satisfied. The specified execution condition includes a condition that a driving state of the engine 1 in accordance with the number of rotations of the engine 1 and the intake air amount Ga is a driving state with a low load. The control device 200 makes all of the first control valve 62, the second control valve 64, and the third control valve 66 closed (in an off-state) when a supercharging mode is the single supercharging mode.

[0048] FIG. 2 is an explanatory view illustrating an operation of the supercharging system in the single supercharging mode. As illustrated by arrows in FIG. 2, the exhaust gas flowing through the exhaust manifolds 50A, 50B flows into the turbine 32 of the primary supercharger 30 through the exhaust pipe 52A, and then, flows into the exhaust gas treatment device 81 through the exhaust pipe 53A. The exhaust gas fed into the turbine 32 rotates the turbine wheel 34, so that the compressor wheel 33 rotates with the rotation of the turbine wheel 34.

[0049] The air drawn from the air cleaner 20 flows into the compressor 31 through the intake pipe 23 and the intake pipe 21. The intake air discharged from the compressor 31 flows into the intercooler 25 through the intake pipe 37. The intake air flowing into the intercooler 25 branches into two streams that each flow into the intake manifold 28A through the intake pipe 27A and the intake manifold 28B through the intake pipe 27B.

[0050] <Run-up mode> The control device 200 determines that the switching from the single supercharging mode to the twin supercharging mode is required, for example, when the supercharging mode is the single supercharging mode, and the number of rotations of the primary supercharger 30 exceeds a threshold value.

[0051] When the switching from the single supercharging mode to the twin supercharging mode is required, the control device 200 executes the run-up mode before the control device 200 switches the supercharging mode to the twin supercharging mode. That is, the control device 200 makes both of the second control valve 64 and the third control valve 66 open (in an on-state), while the control device 200 makes the first control valve 62 closed (in the off-state).

[0052] FIG. 3 is an explanatory view illustrating an operation of the supercharging system in the run-up mode. As illustrated by arrows in FIG. 3, the exhaust gas flowing through the exhaust manifolds 50A, 50B join together at the connection C1. Then, the exhaust gas branches into two streams that each flow into the turbine 32 of the primary supercharger 30 through the exhaust pipe 52A and the turbine 42 of the secondary supercharger 40 through the exhaust pipe 52B. The two streams finally flow into the exhaust gas treatment device 81 through the respective exhaust pipes 53A, 53B.

[0053] The exhaust gas fed into the turbine 32 rotates the turbine wheel 34, so that the compressor wheel 33 rotates with the rotation of the turbine wheel 34. The exhaust gas fed into the turbine 42 rotates the turbine wheel 44, so that the compressor wheel 43 rotates with the rotation of the turbine wheel 44.

[0054] The air drawn from the air cleaner 20 flows through the intake pipe 23, and branches into two streams that each flow into the compressors 31 through the intake pipe 21 and the compressor 41 through the intake pipe 22. The intake air discharged from the compressor 31 flows into the intercooler 25 through the intake pipe 37. The intake air discharged from the compressor 41 flows to the recirculation pipe 48 from the intake pipe 47 through the connection C4. Then, the intake air flows into the compressor 31 from the recirculation pipe 48 through the intake pipe 21.

[0055] The intake air flowing into the intercooler 25 branches into two streams that each flow into the intake manifold 28A through the intake pipe 27A and the intake manifold 28B through the intake pipe 27B. In the run-up mode, while the primary supercharger 30 pumps the intake air flowing to the intercooler 25 into the engine 1, the number of rotations of the secondary supercharger 40 is increased. Asthe number of rotations of the secondary supercharger 40 is increased, pressure of the intake air discharged from the compressor 41 of the secondary supercharger 40 is increased.

[0056] <Twin supercharging mode> The control device 200 operates the supercharging system in the twin supercharging mode at a timing when supercharging performance of the secondary supercharger 40 in the run-up mode becomes high enough. When the supercharging mode is the twin supercharging mode, the control device 200 makes the first control valve 62 open (in the on-state), makes the second control valve 64 closed (in the off-state), and makes the third control valve 66 open (in the on-state).

[0057] FIG. 4 is an explanatory view illustrating an operation of the supercharging system in the twin supercharging mode. Whereas in the run-up mode, the intake air discharged from the compressor 41 of the secondary supercharger 40 flows into the intake pipe 21 from a middle of the intake pipe 47 through the recirculation pipe 48, in the twin supercharging mode, the intake air discharged from the compressor 41 of the secondary supercharger 40 flows into the intercooler 25 from the intake pipe 47 through the intake pipe 37, as illustrated by arrows in FIG. 4.

[0058] It is noted that the flow of the exhaust gas and the intake air excluding the flow described above is the same as the flow of the exhaust gas and the intake air in the run-up mode. Thus, a detailed description of the flow of the exhaust gas and the intake air excluding the flows described above is not reiterated.

[0059] [Conventional problem] In a generally used technique, a supercharging mode is switched from a twin supercharging mode to a single supercharging mode in accordance with an engine speed and an engine load (fuel injection amount). As illustrated in FIG. 14 described above, the switching from the twin supercharging mode to the single supercharging mode in accordance with the engine speed and the fuel injection amount does not appropriately work in a transitional state in which the engine speed and the fuel injection amount continue to change.

[0060] [Control in the present embodiment] Thus, in the present embodiment, when the supercharging mode is the twin supercharging mode, the control device 200 calculates, from quantities of state of the intake passage and the exhaust passage, target VN opening degrees (target opening degrees) of the variable nozzle mechanisms 35, 45 that the primary supercharger 30 and the secondary supercharger 40 require to obtain target boost pressures. The control device 200 switches the supercharging mode to the single supercharging mode when the calculated target VN opening degrees are out of a controllable range. With this control, it is possible that the supercharging mode is appropriately switched from the twin supercharging mode to the single supercharging mode even in the transitional state.

[0061] In the present embodiment, the memory of the control device 200 stores data showing a compressor map that illustrates a characteristic of the first compressor. The compressor map includes a first axis that shows an air amount drawn to the first compressor of the first supercharger and a second axis that shows a pressure ratio of discharge pressure to intake pressure in the first compressor. An operating point is specified by the pressure ratio and the air amount on the compressor map stored in the memory. When the supercharging mode is the twin supercharging mode and the operating point falls below a specified line on the compressor map, the control device 200 switches the supercharging mode to the single supercharging mode. With this control, it is possible to appropriately switch the supercharging mode from the twin supercharging mode to the single supercharging mode even in the transitional state.

[0062] Specifically, the following procedure is executed so as to execute the control described above. FIG. 5 is flowchart illustrating an example of a switching procedure to the single supercharging mode according the present embodiment. Referring to FIG. 5, the control device 200 determines whether a supercharging mode frag has a value indicating the twin supercharging mode (at Step S111). The supercharging mode frag indicates the supercharging mode which is currently controlled, and has any one of values indicating the single supercharging mode, the twin supercharging mode, and the run-up mode.

[0063] In response to determining that the supercharging mode frag does not indicate the twin supercharging mode (NO at Step 111), the control device 200 returns a processing to be executed to a procedure of a calling source of this procedure.

[0064] In response to determining that the supercharging mode frag indicates the twin supercharging mode (YES at Step S111), the control device 200 calculates the target VN opening degrees from quantities of state of the respective units by using physical equations of a supercharger (Step S112). As the quantities of state of the intake passage and the exhaust passage used for calculating the target VN opening degrees, the quantity of state of the intake passage before branching toward the primary supercharger 30 and the secondary supercharger 40 and the quantity of state of the exhaust passage after the downstream passages of the primary supercharger 30 and the secondary supercharger 40 join together are respectively used.

[0065] FIG. 6 is a first explanatory view of the physical equations of a supercharger. Referring to FIG. 6, in the physical equations of the supercharger, target compressor downstream pressure P3 is firstly calculated from target diesel throttle upstream pressure and intercooler pressure loss. The target diesel throttle upstream pressure is target pressure between the diesel throttle 68 and the intercooler 25. The intercooler pressure loss is pressure loss caused by the intercooler 25. The target compressor downstream pressure P3 is calculated by adding the intercooler pressure loss to the target diesel throttle upstream pressure (hereinafter, called "target D-throttle upstream pressure").

[0066] Next, target compressor work is calculated from the target compressor downstream pressure P3, the intake air amount Ga, an intake air temperature Tha, and compressor upstream pressure P2 by using Equation 1. A symbol Cpa represents specific heat at a constant temperature (0.24), and a symbol k represents a ratio of specific heat of air (1.4). The intake air amount Ga is determined in response to a detection signal from the air flow meter 102.

[0067] [Equation 1] k-1

TARGET COMPRESSOR WORK = Cpa x Ga x Tha x 1 W• .-3 ( 1

)

[0068] Next, target turbine work is calculated from the target compressor work and turbo total efficiency qtot by using Equation 2. The turbo total efficiency qtot is calculated from a quality of state by using a generally known equation by the control device 200.

[0069] [Equation 2]

TARGET COMPRESSOR WORK TURBO TOTAL EFFICIENCY 1tt =TARGET TURBINE WORK(2)

[0070] Next, target ex-mani pressure is calculated from the target turbine work, an ex-mani gas temperature T4, turbo downstream pressure P6, and a turbine passed gas amount Ga+Gf by using Equation 3. A symbol Cpg represents specific heat at constant pressure (0.26), and a symbol K represents a ratio of specific heat of exhaust gas (1.33). A mass flow rate Gf of injected fuel is calculated from a fuel injection amount that is calculated for fuel injection by the control device 200.

[0071] [Equation 3]

TARGET EX-MANI PRESSURE P4 = P6 (3) TARGET TURBINE WORK 1~Cpg x (G;a+ Gf) x T41

[0072] FIG. 7 is a second explanatory view of physical equations of a supercharger. Referring to FIG. 7, protection by restriction is applied to target ex mani pressure P4. When the target ex-mani pressure P4 is higher than a limited ex-mani pressure, this protection by restriction sets the target ex-mani pressure P4 to the limited ex-mani pressure. The limited ex-mani pressure is predetermined as a value with which fuel does not flow across an oil seal of a valve stem of an exhaust valve and the exhaust valve does not open.

[0073] A target effective opening area pA is calculated from the target ex-mani pressure P4, the ex-mani gas temperature T4, the turbo downstream pressure P6, and the turbine-passed gas amount Ga+Gf by using a nozzle equation of Equation 4. A symbol A represents an actual opening area; a symbol pA represents a target effective opening area; a symbol R represents a gas constant (287); and symbols a, b represent constant values predetermined for P6 and P4, respectively.

[0074] [Equation 4]

(Ga + Gf) - 24 P6 -a+ P4b

[0075] Next, the target VN opening degrees are calculated from the calculated target effective opening area pA by using an opening degree characteristic map that illustrates a relationship between a VN opening degree and an effective opening area.

[0076] Turning to FIG. 5, the control device 200 determines whether the calculated target VN opening degrees are out of a controllable range (Step S113). A VN opening degree is not controllable to be completely open (100%) or closed (0%) because of a structure of the nozzle vanes. Thus, the controllable range of the VN opening degree is, for example, a range from 10% to 96%. In response to determining that the calculated target VN opening degrees are out of the controllable range (YES at Step S113), the control device 200 determines whether the target VN opening degrees keep out of the controllable range for a predetermined time (Step S114).

[0077] In response to determining that it keeps for the predetermined time (YES at Step S114), the control device 200 substitutes a value indicating the single supercharging mode for the present value of the supercharging mode frag (Step S117), and returns the processing to be executed to a procedure of a calling source of the procedure described above. Thus, the supercharging mode is switched to the single supercharging mode.

[0078] FIG. 8 is an explanatory view of an example of a variation of the target VN opening degree calculated by the physical equations. Referring to FIG. 8, as illustrated in FIG. 8 at (B), when a fuel injection amount is drastically decreased by turning off the accelerator, or the like, the engine speed of the engine 1 is gradually decreased, as illustrated in FIG. 8 at (A). Thus, the target D-throttle upstream pressure that is determined by the engine speed of the engine 1 and the fuel injection amount is also drastically decreased, so that the target boost pressure (= the target compressor downstream pressure P3) is drastically decreased as well. With these decreases, the target VN opening degree for determination calculated by the physical equations described above is also decreased. However, when the target boost pressure exceeds the actual boost pressure at a time T1, the target VN opening degree for the determination starts to increase. Then, the target VN opening degree keeps exceeding a specified value of an upper limit on a closed side of the nozzle vanes in the controllable range for a predetermined time, and the supercharging mode is switched from the twin supercharging mode to the single supercharging mode at a time T2.

[0079] Turning to FIG. 5, in response to determining that the calculated VN opening degrees are not out of the controllable range (NO at Step S113), or the calculated target VN opening degrees do not keep out of the controllable range for the predetermined time (NO at Step S114), the control device 200 determines whether the intake air amount Ga of the primary supercharger 30 is not more than a threshold value calculated from a pressure ratio of the compressor downstream pressure P3 to the compressor upstream pressure P2 in the primary supercharger 30 (Step S115).

[0080] FIG. 9 is a view illustrating an example of a compressor operation map (compressor map) that illustrates a relationship between an intake air amount of the compressor 31 of the primary supercharger 30 and a pressure ratio of upstream pressure and downstream pressure of the compressor 31. Referring to FIG. 9, the compressor operation map uses the intake air amount and the pressure ratio of the compressor 31 of the primary supercharger 30 as parameters, and illustrates an operation area of the compressor 31. A horizontal axis and a vertical axis of the compressor operation map represent the intake air amount and the pressure ratio of the compressor 31, respectively. It is noted that the intake air amount of the compressor 31 is estimated from the intake air amount detected by the air flow meter 102, for example.

[0081] "A surge line" (long-dashed double-short dashed line) represents a border adjacent to a surge area in which surging easily occurs in the primary supercharger 30. "Constant rotational speed lines" (thin solid lines) mean a group of lines for different rotational speeds of the compressor 31. Each of the lines is drawn by connecting operation points at the constant rotation speed of the compressor 31. The rotational speed of the compressor 31 has a larger value, as the intake air amount of the compressor 31 is larger or the pressure ratio of the compressor 31 is larger. The rotational speed of the compressor 31 has a larger value, as the engine speed is larger in the single supercharging mode.

[0082] "A switching line L2" (bold solid line) is a line on which the supercharging mode is switched from the single supercharging mode to the twin supercharging mode. During acceleration, an operation point of the compressor 31 moves along an operation line illustrated in FIG. 9. When the operation point of the compressor 31 reaches the switching line L2, the supercharging mode is switched to the run-up mode that is a preparation phase for the switching from the single supercharging mode to the twin supercharging mode, and then, switched to the twin supercharging mode.

[0083] "A switching line L" (bold solid line) is aline on which the supercharging mode is switched from the twin supercharging mode to the single supercharging mode. The switching line L1 corresponds to a specified line in the present invention. During deceleration, the operation point of the compressor 31 moves along the operation line illustrated in FIG. 9. When the operation point of the compressor 31 reaches the switching line L1, the supercharging mode is switched from the twin supercharging mode to the single supercharging mode.

[0084] Turning to FIG. 5, in response to determining that the intake air amount Ga of the primary supercharger 30 is not more than the threshold value calculated from the pressure ratio of the primary supercharger 30 when the operation point reaches the switching line L1 (YES at Step S115), the control device 200 substitutes the value indicating the single supercharging mode for the present value of the supercharging mode frag (Step S117), and returns the processing to be executed to a procedure of a calling source of the procedure described above. Thus, the supercharging mode is switched to the single supercharging mode.

[0085] On the other hand, in response to determining that the intake air amount Ga of the primary supercharger 30 is more than the threshold value calculated from the pressure ratio of the primary supercharger 30 when the operation point does not reach the switching line L1 (NO at Step S115), the control device 200 keeps the supercharging mode frag as the value indicating the twin supercharging mode (Step S116), and returns the processing to be executed to a procedure of a calling source of the procedure described above.

[0086] FIG. 10 is an explanatory view illustrating a variation of boost pressure by reacceleration in the cases where the operating point of the compressor operation map reaches and does not reach the switching line L1. Referring to FIG. 10, as illustrated in FIG. 10 at (A), when an accelerator opening degree is decreased from 100 % to 20%, target boost pressure drastically decreases, as illustrated in FIG. 10 at (B). However, as illustrated in FIG. 10 at (B), actual boost pressure gradually decreases.

[0087] As the conventional technique, when the switching from the twin supercharging mode to the single supercharging mode is determined in accordance with the engine speed of the engine 1 and the fuel injection amount, the supercharging mode is switched to the single supercharging mode at a time T1 when the target boost pressure is drastically decreased. When the vehicle immediately reaccelerates after the switching of the supercharging mode, the boost pressure has a poor responsiveness because of a gap between the target boost pressure and the actual boost pressure.

[0088] On the other hand, in the present embodiment, when the operating point of the compressor operation map falls below the switching line Li (at a time T3 in FIG. 10), the supercharging mode is switched from the twin supercharging mode to the single supercharging mode. As illustrated by a bold dashed line of (I) in FIG. 10, when the vehicle reaccelerates with the accelerator opening degree set to 100% at a time T2 just before a time T3, the time T2 is a time before the operation point reaches the switching line L1, so that the boost pressure has a great responsiveness even in the continuous twin supercharging mode.

[0089] In addition, as illustrated by a bold solid line of (II) in FIG. 10, when the vehicle reaccelerates with the accelerator opening degree set to 100% at a time T4 after the supercharging mode is switched to the single supercharging mode at the time T3, the time T4 is a time after the operation point has been below the switching line L1, so that the boost pressure has a great responsiveness, because reacceleration starts in the single supercharging mode. Then, when the rotational speed of the compressor 31 is increased in some degree in the single supercharging mode at a time T5 to increase the intake air amount, the supercharging mode is switched to the run-up mode for the switching to the twin supercharging mode, so that the boost pressure is increased.

[0090] FIG. 11 is an explanatory view illustrating, on the compressor operation map, different operating points in accordance with accelerator opening degrees in a steady operation. Referring to FIG. 11, the accelerator opening degrees are illustrated by Al% to A5% that have a relationship of Al > A2 > A3 > A4> A5. On the compressor operation map, as the accelerator opening degree becomes larger in the steady operation, the operation point moves toward an upper-right area in which the intake air amount and the pressure ratio P3/P2 are large. The operation point of the accelerator opening degree A3% is on the switching line L1. The operation point of the accelerator opening degree A4% falls below the switching line L1.

[0091] FIG. 12 is an explanatory view illustrating different boost pressures by reacceleration after deceleration in each of the accelerator opening degrees. Referring to FIG. 12, as illustrated in FIG. 12 at (A), when the vehicle reaccelerates with the accelerator opening degree A4% at a time T1 in the continuous twin supercharging mode, the compressor downstream pressure P3 is increased, as illustrated by a bold solid line. When the vehicle reaccelerates with the accelerator opening degree A4% at the time T1 after the supercharging mode is switched to the single supercharging mode, the compressor downstream pressure P3 is increased, as illustrated by the bold dashed line. As illustrated in FIG. 12 at (A), when the vehicle reaccelerates with the accelerator opening degree A4%, the responsiveness of the boost pressure after the supercharging mode is switched to the single supercharging mode is superior to that in the continuous twin supercharging mode.

[0092] As illustrated in FIG. 12 at (B), when the vehicle reaccelerates with the accelerator opening degree A3% at the time T1 in the continuous twin supercharging mode, the compressor downstream pressure P3 is increased, as illustrated by a bold solid line. When the vehicle reaccelerates with the accelerator opening degree A3% at the time T1 after the supercharging mode is switched into the single supercharging mode, the compressor downstream pressure P3 is increased, as illustrated by a bold dashed line. As illustrated in FIG. 12 at (B), when the vehicle reaccelerates with the accelerator opening degree A3%, the responsiveness of the boost pressure after the supercharging mode is switched to the single supercharging mode is substantially the same as that in the continuous twin supercharging mode.

[0093] As illustrated in FIG. 12 at (A) and (B), in either accelerator opening degree of A3% and A4%, when the vehicle reaccelerates in the single supercharging mode, the compressor downstream pressure P3 is increased in some degree, and then, the second control valve 64 is switched to be in the open state so that the supercharging mode is switched to the run-up mode. Then, the first control valve 62 is switched to be in the open state, so that the supercharging mode is switched to the twin supercharging mode.

[0094] From a result of FIG. 12, when the operation point on the compressor operation map falls below the constant rotational speed line that passes through the operation point of the accelerator opening degree A3% in the steady state, the responsiveness of the boost pressure in the single supercharging mode is superior to that in the twin supercharging mode. Thus, the constant rotational speed line is defined as the switching line L1.

[0095] [Modified embodiments] (1) In the embodiment described above, as described by FIGS. 6 and 7, the target VN opening degrees are calculated by using the compressor downstream pressure P3 calculated from the D-throttle upstream pressure. FIG. 13 is an explanatory view illustrating a calculation of a target VN opening degree in a modified embodiment. Referring to FIG. 13, in this case, as illustrated in FIG. 13 at (A), when an accelerator opening degree is increased, actual boost pressure gradually increases, while target boost pressure drastically increases, as illustrated in FIG. 12 at (B). Thus, as illustrated in FIG. 13 at (C), when the target VN opening degree is calculated by the physical equations described by FIGS. 6 and 7, the calculated target VN opening degree is drastically close to a value on a closed side of the nozzle vanes. As a result, the target VN opening degree in Step S113 of FIG. 5 is unstably determined.

[0096] To deal with the situation described above, as illustrated in FIG. 13 at (D), the target VN opening degree may be calculated from target boost pressure to which a smoothing process is performed. This process makes the target VN opening degree stably determined.

[0097] (2) In the embodiment described above, the primary supercharger 30 and the secondary supercharger 40 are disposed in the intake passage of the engine 1. In one example, in addition to the primary supercharger 30 and the secondary supercharger 40, an intake throttle valve and an EGR (Exhaust Gas Recirculation) gas inlet of an exhaust gas recirculation device may be disposed in the intake passage of the engine 1.

[0098] (3) In the embodiment described above, a V6 engine is described as an example of the engine 1. In one example, the engine 1 may be an engine that has the other cylinder layouts (e.g. inline-type engine or flat-type engine).

[0099] (4) In the embodiment described above, the engine 1 includes two superchargers as the supercharging system. The engine 1 may include three superchargers or more.

[0100] (5) The embodiment described above maybe thought as a disclosure of a supercharging system formed of the primary supercharger 30, the secondary supercharger 40, and the control device 200, a disclosure of an internal combustion engine such as the engine 1, a disclosure of a control device such as the ECU 100 of the internal combustion engine such as the engine 1, a disclosure of a control method by the control device, or a disclosure of an internal combustion engine system including the internal combustion engine and the control device.

[0101] [Effect] (1-1) As illustrated in FIGS. 1 to 4, the supercharging system includes the primary supercharger 30, the secondary supercharger 40, and the control device 200. The primary supercharger 30 includes the turbine 32 that is driven by exhaust gas discharged from the engine 1, the variable nozzle mechanism 35 that is configured to adjust a flow rate of the exhaust gas flowing into the turbine 32 by an opening degree, and the compressor 31 that is configured to pump intake air intotheengine1. The secondary supercharger 40 includes the turbine 42 that is driven by the exhaust gas discharged from the engine 1, and the variable nozzle mechanism 45 that is configured to adjust a flow rate of the exhaust gas flowing into the turbine 42 by an opening degree. The secondary supercharger 40 is configured to pump the intake air into the engine 1. The control device 200 is configured to switch the supercharging mode from the single supercharging mode to the twin supercharging mode or from the twin supercharging mode to the single supercharging mode. In the single supercharging mode, the air pumped by the primary supercharger 30 is fed to the engine 1. In the twin supercharging mode, the air pumped by the primary supercharger 30 and the air pumped by the secondary supercharger 40 are fed to the engine 1.

[0102] As illustrated by Step S112, Step S113, and Step S117 in FIG. 5, and in FIG. 6 and FIG. 7, when the operation point is not below the switching line Li in the twin supercharging mode, the control device 200 calculates, from quantities of state of the intake passage and the exhaust passage, target VN opening degrees of the variable nozzle mechanisms 35, 45 that the primary supercharger 30 and the secondary supercharger 40 require to obtain target boost pressures. The control device 200 switches the supercharging mode to the single supercharging mode when the calculated target VN opening degrees are out of the controllable range.

[0103] With this control, it is possible to appropriately switch the supercharging mode from the twin supercharging mode to the single supercharging mode even in the transitional state. In the conventional control by an engine speed and an injection amount, a correction map is used to correct a change in an environment. However, in this disclosure, the target VN opening degrees that are used for determination are calculated by the physical equations, so that a correction to the change in the environment is not required. In addition, time and an effort for creating the correction map are reduced.

[0104] (1-2)As illustrated in FIGS. 6 and 7, the control device 200 uses equations of a simulation based on physical laws of the intake passage and the exhaust passage of the engine 1 as an equation for calculating the target VN opening degree. The target VN opening degree for the determination is appropriately calculated by these equation.

[0105] (1-3) As the quantities of state of the intake passage and the exhaust passage when the target VN opening degrees are calculated, the control device 200 uses the quantity of state of the intake passage before branching toward the primary supercharger 30 and the secondary supercharger 40 and the quantity of state of the exhaust passage after passages from the primary supercharger 30 and the secondary supercharger 40 join together, respectively. The target VN opening degrees for the determination are appropriately calculated by these quantities of state.

[0106] (1-4) As illustrated in FIGS. 1 to 4, the primary supercharger 30 and the secondary supercharger 40 are the same type supercharger. The control device 200 regards the primary supercharger 30 and the secondary supercharger 40 as one supercharger, and calculates a target VN opening degree. The control device 200 performs determination by using the calculated target VN opening degree as the target VN opening degrees of the variable nozzle mechanisms 35,

45. This simplifies a calculation of the target VN opening degree and a control of the supercharging mode.

[0107] (1-5) As illustrated by Step S114 in FIG. 5, when the calculated target VN opening degrees keep out of the controllable range for the predetermined time, the control device 200 switches the supercharging mode to the single supercharging mode. This prevents hunting of the switching of the supercharging mode.

[0108] (2-1) As illustrated in FIGS. 1 to 4, the supercharging system includes the primary supercharger 30, the secondary supercharger 40, the control device 200, and the memory. The primary supercharger 30 includes the turbine 32 that is driven by exhaust gas discharged from the engine 1, the variable nozzle mechanism 35 that is configured to adjust a flow rate of the exhaust gas flowing into the turbine 32 by an opening degree, and the compressor 31 that is configured to pump intake air into the engine 1. The secondary supercharger 40 includes the turbine 42 that is driven by the exhaust gas discharged from the engine 1, and the variable nozzle mechanism 45 that is configured to adjust a flow rate of the exhaust gas flowing into the turbine 42 by an opening degree. The secondary supercharger 40 is configured to pump the intake air into the engine 1. The control device 200 is configured to switch the supercharging mode from the single supercharging mode to the twin supercharging mode or from the twin supercharging mode to the single supercharging mode. In the single supercharging mode, the air pumped by the primary supercharger 30 is fed to the engine 1. In the twin supercharging mode, the air pumped by the primary supercharger 30 and the air pumped by the secondary supercharger 40 are fed to the engine 1. The memory stores the data showing the compressor operation map that illustrates the characteristic of the compressor 31. As illustrated in FIG. 9, the compressor operation map includes the first axis that shows an air amount drawn to the compressor 31 of the primary supercharger 30 and the second axis that shows a pressure ratio P3/P2 of discharge pressure to intake pressure in the compressor 31.

[0109] As illustrated by Step S115 and Step S117 in FIG. 5, when the operating point falls below the switching line Li on the compressor operation map in the twin supercharging mode, the control device 200 switches the supercharging mode to the single supercharging mode. The operating point is specified by the pressure ratio P3/P2 and the air amount on the compressor operation map stored in the memory.

[0110] With this control, it is possible to appropriately switch the supercharging mode from the twin supercharging mode to the single supercharging mode even in the transitional state. In the conventional control by an engine speed and an injection amount, a correction map is used to correct a change in an environment. However, in this disclosure, the determination is performed by the compressor operation map, so that a correction to the change in the environment is not required. In addition, time and an effort for creating the correction map are reduced.

[0111] (2-2) As illustrated in FIGS. 11 and 12, the switching line Li is a border between an area in which the pressure ratio is more quickly increased in the twin supercharging mode than in the single supercharging mode and an area in which the pressure ratio is more quickly increased in the single supercharging mode than in the twin supercharging mode, when fuel injection is restarted in the engine. Thus, the switching line Li is appropriately set.

[0112] (2-3) As illustrated in FIG. 9, the switching line Li is a constant rotational speed line of the primary supercharger 30 during a steady traveling.

[0113] The presently disclosed embodiments may be combined as appropriate. The presently disclosed embodiments are to be considered in all respects as illustrative and not restrictive. The scope of the present disclosure is indicated by the appended claims rather than the description of the embodiments described above. All modifications that come within the meaning and scope equivalent to the claims are intended to be embraced therein.

Reference Signs List

[0114] 1 engine 10A, 1OB cylinder bank 12A, 12B cylinder 20 air cleaner 21, 22, 23, 27A, 27B, 37, 47 intake pipe 25 intercooler 28A, 28B exhaust manifold 30 primary supercharger 31, 41 compressor 32, 42 turbine 33, 43 compressor wheel 34, 44 turbine wheel 35, 45 variable nozzle mechanism 36, 46 rotary shaft 40 secondary supercharger 48 recirculation pipe 50A, 50B exhaust manifold 51A, 51B, 52A, 52B, 53A, 53B exhaust pipe 62 first control valve 64 second control valve 66 third control valve 68 diesel throttle 81 exhaust treatment device 102 air flow meter 106 first pressure sensor 108 second pressure sensor 200 control device

Claims

1. A supercharging system comprising: a first supercharger including a first turbine that is driven by exhaust gas discharged from an engine, a first variable nozzle mechanism that is configured to adjust a flow rate of the exhaust gas flowing into the first turbine by an opening degree, and a first compressor that is configured to pump intake air into the engine; a second supercharger including a second turbine that is driven by the exhaust gas discharged from the engine, and a second variable nozzle mechanism that is configured to adjust a flow rate of the exhaust gas flowing into the second turbine by an opening degree, the second supercharger being configured to pump the intake air into the engine; a control device that is configured to switch a supercharging mode from a single supercharging mode to a twin supercharging mode or from a twin supercharging mode to a single supercharging mode, wherein in the single supercharging mode, the air pumped by the first supercharger is fed to the engine, and in the twin supercharging mode, the air pumped by the first supercharger and the air pumped by the second supercharger are fed to the engine; and a storage unit that stores data showing a compressor map that illustrates a characteristic of the first compressor, wherein the compressor map includes a first axis that shows an air amount drawn to the first compressor of the first supercharger and a second axis that shows a pressure ratio of discharge pressure to intake pressure in the first compressor, the control device switches the supercharging mode from the twin supercharging mode to the single supercharging mode when an operating point that is specified by the pressure ratio and the air amount on the compressor map stored in the storage unit falls below a specified line on the compressor map during operation in the twin supercharging mode.

2. The supercharging system according to claim 1, wherein the specified line is a border between an area in which the pressure ratio is more quickly increased in the twin supercharging mode than in the single supercharging mode and an area in which the pressure ratio is more quickly increased in the single supercharging mode than in the twin supercharging mode, when fuel injection is restarted in the engine.

3. The supercharging system according to claim 1, wherein the specified line is a constant rotational speed line of the first supercharger during a steady traveling.

4. The supercharging system according to claim 1, wherein when the operation point is not below the specified line in the twin supercharging mode, the control device calculates, from quantities of state of an intake passage and an exhaust passage, target opening degrees of the first variable nozzle mechanism and the second variable nozzle mechanism that the first supercharger and the second supercharger require to obtain target boost pressures, and the control device switches the supercharging mode to the single supercharging mode when the calculated target opening degrees are out of a controllable range.