WO2015011540A2

WO2015011540A2 - Power transmission system of vehicle and method of controlling the same

Info

Publication number: WO2015011540A2
Application number: PCT/IB2014/001338
Authority: WO
Inventors: Yasuhiro Hiasa; Kazuyuki Shiiba; Atsushi Tabata; Tooru Matsubara; Tatsuya Imamura; Takeshi Kitahata; Kenta Kumazaki; Munehiro KATSUMATA
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2013-07-26
Filing date: 2014-07-17
Publication date: 2015-01-29
Also published as: JP2015024764A; WO2015011540A3

Abstract

A power transmission system of a vehicle includes an engine, a first rotating machine, a second electric machine, a differential unit, an engaging device disposed between the engine and the differential unit, and a controller. The differential unit has an input rotational element connected to the engaging device, a rotational element connected to the first rotating machine, and an output rotational element connected to the second rotating machine and drive wheels. When required driving force is equal to or larger than a first predetermined value when the vehicle is started, the controller is configured to control a slip amount of the engaging device and increase torque of the engine.

Description

POWER TRANSMISSION SYSTEM OF VEHICLE AND METHOD OF

CONTROLLING THE SAME

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to a power transmission system of a vehicle, and a method of controlling the power transmission system.

2. Description of Related Art

[0002] Power transmission systems of hybrid vehicles are generally known. For example, a power transmission system of a hybrid vehicle as disclosed in Japanese Patent Application Publication No. 2008-120234 (JP 2008-120234 A) includes a first rotating machine that has a function of generating electric power and is driven by power generated by an internal combustion engine, and a second rotating machine that operates with electric power supplied from the first rotating machine to apply power to an output member, and the first rotating machine and the second rotating machine are disposed on the same axis. In the power transmission system disclosed in the above-identified publication, a power split device for distributing power generated by the internal combustion engine to the first rotating machine and the output member is disposed between the first rotating machine and the second rotating machine.

SUMMARY OF THE INVENTION

[0003] It has not been sufficiently studied to curb or prevent overload of the rotating machine included in the power transmission system. For example, when large driving force is required to be generated, it is desired to produce the required driving force while curbing overload of the rotating machine.

[0004] Also, in hybrid vehicles, the amount of electric power generated may be restricted. When the amount of electric power generated is restricted, it is desirable to produce required driving force while curbing overload of the rotating machine.

[0005] The invention provides a power transmission system of a vehicle that can produce required driving force while curbing overload of a rotating machine, and a method of controlling the power transmission system.

[0006] A power'transmission system of a vehicle according to a first aspect of the invention includes an engine, a first rotating machine, a second rotating machine, a differential unit, an engaging device disposed between the engine and the differential unit, and a controller. The differential unit includes an input rotational element connected to the engaging device, a rotational element connected to the first rotating machine, and an output rotational element connected to the second rotating machine and drive wheels. The controller is configured to control a slip amount of the engaging device, and increase torque of the engine, when required driving force is equal to or larger than a first predetermined value when the vehicle is started.

[0007] In the power transmission system according to the above aspect of the invention, the controller may be configured to increase the slip amount of the engaging device, when the required driving force is equal to or larger than a second predetermined value that is larger than the first predetermined value.

[0008] The power transmission system according to the above aspect of the invention may further include a power storage device that supplies and receives electric power to and from the first rotating machine, and the controller may be configured to change the slip amount according to a maximum value of electric power that is allowed to be received by the power storage device.

[0009] In the power transmission system as described above, the controller may be configured to determine the slip amount so that the slip amount increases as the maximum value of the allowed electric power is smaller.

[0010] In the power transmission system according to the above aspect of the invention, the controller may control the rotational speed of the first rotating machine when the controller controls the slip amount, so that the rotational speed becomes lower than that in the case where the engaging device is fully engaged. [0011] In the power transmission system as described above, the controller may be configured to control the rotational speed of the first rotating machine so that the slip amount of the engaging device becomes equal to or smaller than an allowable maximum slip amount.

[0012] In the power transmission system according to the above aspect of the invention, the controller may be configured to reduce the required driving force, instead of controlling the slip amount when the engaging device is not allowed to slip.

[0013] According to a second aspect of the invention, a method of controlling a power transmission system of a vehicle is provided. The power transmission system includes an engine, a first rotating machine, a second rotating machine, a differential unit including an input rotational element, a rotational element connected to the first rotating machine, and an output rotational element connected to the second rotating machine and drive wheels, an engaging device that is disposed between the engine and the differential unit and connected to the input rotational element, and a controller. The method includes controlling, by the controller, a slip amount of the engaging device, and increasing torque of the engine, when required driving force is equal to or larger than a first predetermined value when the vehicle is started.

[0014] According to the above aspects of the invention, torque transmitted from the engine is increased, so as to produce required driving force while curbing overload of the rotating machine.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the , accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a skeleton diagram of a vehicle according to a first embodiment of the invention;

FIG. 2 is a view illustrating the relationships of inputs and outputs of the vehicle according to the first embodiment of the invention; FIG. 3 is a view showing an operation engagement table of the vehicle according to the first embodiment;

FIG 4 is a collinear diagram concerning a single-motor EV mode;

FIG. 5 is a collinear diagram concerning a both-motor EV mode;

FIG. 6 is a collinear diagram concerning a HV LOW mode;

FIG. 7 is a collinear diagram concerning a HV HIGH mode;

FIG. 8 is a view showing a map used for mode selection in the first embodiment;

FIG. 9 is a monographic chart useful for explaining slip control of the first embodiment;

FIG. 10 is a flowchart concerning control of the first embodiment;

FIG. 11 is a view useful for explaining change of an engine operating point in the first embodiment;

FIG. 12A through FIG. 12J are time charts related to control of the first embodiment; and

FIG. 13 is a skeleton diagram of a vehicle according to a second embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0016] In the following, a power transmission system according to one embodiment of the invention will be described in detail with reference to the drawings. It is to be understood that this invention should not be limited to this embodiment. Also, constituent elements of the invention include those of the following embodiment, those that can be easily conceived by a person skilled in the art, and those that are substantially identical with the elements of the embodiment.

[0017] As shown in FIG. 1, a vehicle 100 according to this embodiment is a hybrid vehicle (HV) having an engine 1, a first rotating electric machine MGl, and a second rotating electric machine MG2, as power sources. The vehicle 100 may be a plug-in hybrid vehicle (PHV) that can be charged with electric power supplied from an external power supply. As shown in FIG. 1 and FIG. 2, the vehicle 100 includes the engine 1, battery 3, first planetary gear unit 10, second planetary gear unit 20, first rotating machine MG1, second rotating machine MG2, clutch CL1, brake BK1, HV_ECU 50, MG_ECU 60, and an engine ECU 70.

[0018] A power transmission system 1-1 according to this embodiment includes the engine 1, battery 3, first rotating machine MG1, second rotating machine MG2, second planetary gear unit 20, clutch CL1, HV_ECU 50, MG_ECU 60, and the engine ECU 70. The power transmission system 1-1 can be used in an FF (front-engine, front-drive) vehicle, or an RR (rear-engine, rear-drive) vehicle, or the like. For example, the power transmission system 1-1 is installed on the vehicle 100 such that the axial direction of the system 1-1 corresponds to the vehicle width direction.

[0019] In the power transmission system 1-1 according to this embodiment, the second planetary gear unit 20 is one example of differential unit. Also, the clutch CL1 is one example of engaging device disposed between the engine 1 and the second planetary gear unit 20 as the differential unit. Also, in this embodiment, the HV_ECU 50, MG_ECU 60, and the engine ECU 70 constitute one example of controller.

[0020] The engine 1 as one example of engine converts combustion energy of fuel into rotary motion of an output shaft thereof. The output shaft of the engine 1 is connected to an input shaft 2. The input shaft 2 is an input shaft of a power transmission mechanism. The power transmission mechanism includes the first rotating machine MG1 , second rotating machine MG2, clutch CL1, brake BK1, differential device 30, and so forth. The input shaft 2 is disposed coaxially with the output shaft of the engine 1, on an extended line of the output shaft. The input shaft 2 is connected to a first carrier 14 of the first planetary gear unit 10.

[0021] The first planetary gear unit 10 of this embodiment is able to change the speed of rotation of the engine 1 , and deliver the rotation to the second planetary gear unit 20. In this embodiment, the first planetary gear unit 10, clutch CLJ, and brake BK1 constitute a speed changing unit. The first planetary gear unit 10 is disposed between the engine 1 and the second planetary gear unit 20. The first planetary gear unit 10 is of a single pinion type, and has a first sun gear 11, a first pinion gear 12, a first ring gear 13, and the first carrier 14.

[0022] The first ring gear 13, which is on the same axis as the first sun gear 11, is disposed radially outwardly of the first sun gear 11. The first pinion gear 12 is disposed between the first sun gear 11 and the first ring gear 13, and meshes with the first sun gear 11 and the first ring gear 13, respectively. The first pinion gear 12 is rotatably supported by the first carrier 14. The first carrier 14 is coupled to the input shaft 2, and rotates as a unit with the input shaft 2. Accordingly, the first pinion gear 12 can rotate about the center axis of the input shaft 2, along with the input shaft 2, and can also rotate about the center axis of the first pinion gear 12 while being supported by the first carrier 14.

[0023] The clutch CLl is a clutch device that can couple the first sun gear 11 with the first carrier 14. The clutch CLl of this embodiment is a friction engagement type clutch. For example, the clutch CL is hydraulically controlled to be engaged or released. The clutch CLl, when it is in a fully engaged state, couples the first sun gear 11 with the first carrier 14, so that the first sun gear 11 and the first carrier 14 can rotate as a unit. The clutch CLl in the fully engaged state restricts differential operation of the first planetary gear unit 10. On the other hand, the clutch CLl, when it is in a released state, disconnects the first sun gear 11 and the first carrier 14 from each other, so as to allow the first sun gear 11 and the first carrier 14 to rotate relative to each other. Namely, the clutch CLl in the released state allows differential operation of the first planetary gear unit 10. The clutch CLl can be controlled to a half-engaged state. The clutch CLl in the half-engaged state allows differential operation of the first planetary gear unit 10.

[0024] The clutch CLl is an engaging device disposed between the engine 1 and the second planetary gear unit 20, and connects and disconnects the engine 1 and the second planetary gear unit 20 to and from each other. When the clutch CL is in the engaged state, a power transmission path between the engine 1 and the second planetary gear unit 20 is connected, so that power is transmitted between the engine 1 and the second planetary gear unit 20. On the other hand, when the clutch CL is in the released state, the power transmission path between the engine 1 and the second planetary gear unit 20 is cut off or disconnected, so that power is inhibited from being transmitted between the engine 1 and the second planetary gear unit 20.

[0025] The brake BK1 is a brake device that can restrict rotation of the first sun gear 11. The brake BK1 has an engaging element connected to the first sun gear 11, and an engaging element connected to the vehicle body side, e.g., a case of the power transmission mechanism. While the brake BK1 may be a friction engagement type clutch device like the clutch CLl, a clutch device, such as a meshing clutch, may be used as the brake BK1. For example, the brake BK1 is hydraulically controlled to be engaged or released. The brake BK1, when it is in a fully engaged state, can couple the first sun gear 11 with the vehicle body side, so as to restrict rotation of the first sun gear 11. On the other hand, the brake BK1, when it is in a released state, disconnects the first sun gear 11 from the vehicle body side, so as to allow rotation of the first sun gear 11. The brake BK1 can be controlled to a half-engaged state. The brake BK1 in the half-engaged state allows rotation of the first sun gear 11.

[0026] The second planetary gear unit 20 of this embodiment connects the first planetary gear unit 10 with drive wheels 32. The second planetary gear unit 20 is of a single pinion type, and has a second sun gear 21 , a second pinion gear 22, a second ring gear 23, and a second carrier 24. The second planetary gear unit 20 is disposed coaxially with the first planetary gear unit 10, and is opposed to the engine 1 with the first planetary gear unit 10 interposed therebetween.

[0027] The second ring gear 23, which is on the same axis as the second sun gear 21, is disposed radially outwardly of the second sun gear 21. The second pinion gear 22 is disposed between the second sun gear 21 and the second ring gear 23, and meshes with the second sun gear 21 and the second ring gear 23, respectively. The second pinion gear 22 is rotatably supported by the second carrier 24. The second carrier 24 is connected to the first ring gear 13, and rotates as a unit with the first ring gear 13. The second pinion gear 22 can rotate about the center axis of the input shaft 2, along with the second carrier 24, and can also rotate about the center axis of the second pinion gear 22 while being supported by the second carrier 24. The first ring gear 13 is an output element of the first planetary gear unit 10, and can deliver rotation received from the engine 1, to the second carrier 24. The second carrier 24 is an input rotational element connected to the clutch CL1 via the first planetary gear unit 10.

[0028] A rotary shaft 33 of the first rotating machine MG1 is connected to the second sun gear 21. The rotary shaft 33 of the first rotating machine MG1 is disposed coaxially with the input shaft 2, and rotates as a unit with the second sun gear 21. The second sun gear 21 is a rotational element connected to the first rotating machine MG1. A counter drive gear 25 is connected to the second ring gear 23. The counter drive gear 25 is- an output gear that rotates as a unit with the second ring gear 23. The second ring gear 23 is an output rotational element connected to the second rotating machine MG2 and the drive wheels 32. The second ring gear 23 can deliver rotation received from the first rotating machine MG1 or the first planetary gear unit 10, to the drive wheels 32.

[0029] The counter drive gear 25 meshes with a counter driven gear 26. The counter driven gear 26 is connected to a drive pinion gear 28 via a counter shaft 27. The counter driven gear 26 and the drive pinion gear 28 rotate as a unit. Also, a reduction gear 35 meshes with the counter driven gear 26. The reduction gear 35 is connected to a rotary shaft 34 of the second rotating machine MG2. Namely, rotation of the second rotating machine MG2 is transmitted to the counter driven gear 26 via the reduction gear 35. The reduction gear 35, which has a smaller diameter than the counter driven gear 26, reduces the speed of rotation of the second rotating machine MG2, and transmits the resulting rotation to the counter driven gear 26.

[0030] The drive pinion gear 28 meshes with a differential ring gear 29 of the differential device 30. The differential device 30 is connected to the drive wheels 32 via right and left drive axles 31. The second ring gear 23 is connected to the drive wheels 32, via the counter drive gear 25, counter driven gear 26, drive pinion gear 28, differential device 30, and the drive axles 31. Also, the second rotating machine MG2 is connected to the power transmission path between the second ring gear 23 and the drive wheels 32, and is able to transmit power to the second ring gear 23 and the drive wheels 32, respectively.

[0031] Each of the first rotating machine MG1 and the second rotating machine MG2 functions as a motor (electric motor) and also function as a generator. The first rotating machine MGl and the second rotating machine MG2 are connected to the battery 3 via respective inverters. The battery 3 is a power storage device capable of charging and discharging, and supplies and receives electric power to and from the first rotating machine MGl and the second rotating machine MG2. The first rotating machine MGl and the second rotating machine MG2 can convert electric power supplied from the battery 3 into mechanical power, and generate the mechanical power, and can also convert mechanical power into electric power when driven, by power received. The electric power generated by the rotating machines MGl, MG2 can be stored in the battery 3. As the first rotating machine MGl and second rotating machine MG2, three-phase AC synchronous motor- generators may be used, for example.

[0032] In the vehicle 100 of this embodiment, the brake BK1, clutch CL1 , first planetary gear unit 10, counter drive gear 25, second planetary gear unit 20, and the first rotating machine MGl are arranged in this order as viewed from one side close to the engine 1, on the same axis as the engine 1. Also, the power transmission system 1-1 of this embodiment is of a plural-axis type in which the input shaft 2 and the rotary shaft 34 of the second rotating machine MG2 are located on different axes.

[0033] As shown in FIG. 2, the vehicle 100 has the HV_ECU 50, MG_ECU 60, and the engine ECU 70. Each of the ECUs 50, 60, 70 is an electronic control unit having a computer. The HV ECU 50 has a function of performing integrated control on the vehicle 100 as a whole. The MG_ECU 60 and the engine ECU 70 are electrically connected to the HV_ECU 50.

[0034] The MG_ECU 60 can control the first rotating machine MGl and the second rotating machine MG2. For example, the MG_ECU 60 can adjust a value of current supplied to the first rotating machine MGl, so as to control output torque of the first rotating machine MGl, and can adjust a value of current supplied to the second rotating machine MG2, so as to control output torque of the second rotating machine MG2.

[0035] The engine ECU 70 can control the engine 1. For example, the engine ECU 70 can control the opening of an electronic throttle valve of the engine 1 , output an ignition signal so as to perform ignition control of the engine 1, and perform fuel injection control, etc. on the engine 1. The engine ECU 70 can control the output torque of the engine 1, through the opening control of the electronic throttle valve, fuel injection control, ignition control, and so forth.

[0036] To the HV ECU 50 are connected a vehicle speed sensor, accelerator pedal position sensor, MGl speed sensor, MG2 speed sensor, output shaft speed sensor, battery sensor, and so forth. From these sensors, the HV ECU 50 can obtain the vehicle speed, accelerator pedal angle, the rotational speed of the first rotating machine MGl, the rotational speed of the second rotating machine MG2, the rotational speed of the output shaft of the power transmission mechanism, the SGC of the battery, and so forth.

[0037] The HV_ECU 50 can calculate required driving force, required power, required torque, and the like, which are required to be generated by the vehicle 100, based on the obtained information. The HV_ECU 50 determines the output torque of the first rotating machine MGl (which will also be denoted as "MGl torque"), the output toque of the second rotating machine MG2 (which will also be denoted as "MG2 torque"), and the output torque of the engine 1 (which will also be denoted as "engine torque"), based on the calculated required values. The HV ECU 50 outputs a command value of the MGl torque and a command value of the MG2 torque to the MG_ECU 60. Also, the HV_ECU 50 outputs a command value of the engine torque to the engine ECU 70.

[0038] The HV_ECU 50 controls the clutch CL 1 and the brake BK1 , respectively, based on a running mode, etc. which will be described later. The HV_ECU 50 outputs a command value (PbCLl) of a hydraulic pressure supplied to the clutch CL1, and a command value (PbBKl) of a hydraulic pressure supplied to the brake BK1. A hydraulic control device (not shown) controls the hydraulic pressures supplied to the clutch CL1 and the brake BK1 in accordance with the respective command values PbCLl, PbBKl.

[0039] The vehicle 100 is able to run in a running mode selected from hybrid (HV) running and EV running. The HV running is a running mode in which the vehicle 100 runs using the engine 1 as a power source. In the HV running, the second rotating machine MG2 may be further used as a power source, in addition to the engine 1. [0040] The EV running is a running mode in which the vehicle 100 runs using at least one of the first rotating machine MG1 and the second rotating machine MG2 as a power source(s). In the EV running, the vehicle 100 is able to run with the engine 1 stopped. The power transmission system 1-1 according to this embodiment has two EV running modes, i.e., a single-motor EV mode (single drive EV mode) in which the vehicle 100 runs using the second rotating machine MG2 as a single power source, and a both-motor EV mode (both drive EV mode) in which the vehicle 100 runs using the first rotating machine MG1 and the second rotating machine MG2 as power sources.

[0041] In the operation, engagement table of FIG. 3, circles in some columns of the clutch CLl and some columns of the brake BKl indicate engagement, and blanks indicate release. Also, triangles indicate engagement of one of the clutch CLl and the brake BKl and release of the other. The single-motor EV mode is established by releasing both the clutch CLl and the brake BKl . In each of collinear diagrams shown in FIG. 4 and other figures, symbols SI, CI, Rl represent the first sun gear 11, the first carrier 14, and the first ring gear 13, respectively, and symbols S2, C2, R2 represent the second sun gear 21 , the second carrier 24, and the second ring gear 23.

[0042] In the single-motor EV mode shown in FIG. 4, the clutch CLl and the brake BKl are released. With the brake BKl thus released, the first sun gear 11 is allowed to rotate. With the clutch CLl thus released, the first planetary gear unit 10 is able to perform differential operation. To run the vehicle 100 forward, the HV_ECU 50 causes the second rotating machine MG2 to produce positive torque, via the MG_ECU 60, so as to generate forward driving force. The second ring gear 23 rotates in the positive direction in accordance with rotation of the drive wheels 32. . The positive direction mentioned herein is the direction of rotation of the second ring gear 23 when the vehicle 100 runs forward. The HV ECU 50 causes the first rotating machine MG1 to operate as a generator, so as to reduce a drag loss. More specifically, the HV_ECU 50 causes slight torque to be applied to the first rotating machine MG1 so that the first rotating machine MG1 generates electric power, and the rotational speed of the first rotating machine MG1 becomes substantially equal to 0. As a result, the drag loss of the first rotating machine MG1 can be reduced. If it is possible to keep the MG1 speed substantially equal to 0 by utilizing cogging torque even where the MG1 torque is substantially equal to 0, the MG1 torque may not be applied to the first rotating machine MG1. Alternatively, the MG1 speed may be made substantially equal to 0 by locking the d-axis of the first rotating machine MG1.

[0043] The first ring gear 13 is forced to rotate in the positive direction along with the second carrier 24. Since the first planetary gear unit 10 is in a neutral state where the clutch CLl and the brake BKl are released, the engine 1 is not forced to rotate along with the first ring gear 13, and rotation of the first carrier 14 is stopped. Thus, a large amount of regeneration can be taken. The first sun gear 11 rotates idle in the negative direction.

[0044] During running in the single-motor EV mode, the state of charge of the battery 3 may become full (100%), and the battery 3 may not be able to take regeneration energy any more. In this case, engine brake may be used at the same time. By engaging the clutch CLl or the brake BKl, it is possible to connect the engine 1 with the drive wheels 21, and apply engine brake to the drive wheels 32. If the clutch CLl or brake BKl is engaged in the single-motor EV mode, as indicated by the triangles in FIG. 3, the engine 1 is forced to rotate, and the engine speed is raised by the first rotating machine MG1 so that the vehicle is brought into an engine brake condition.

[0045] In the both-motor EV mode as shown in FIG. 5, the HV_ECU 50 engages the clutch CLl and the brake BKl. With the clutch CLl thus engaged, differential operation of the first planetary gear unit 10 is restricted. With the brake BKl thus engaged, rotation of the first sun gear 11 is restricted. Accordingly, rotation of all rotational elements of the first planetary gear unit 10 is stopped. Since the rotation of the first ring gear 13 as the output element is restricted, the second carrier 24 connected to the first ring gear 13 is locked, i.e., its rotational speed becomes equal to 0. ?

[0046] The HV_ECU 50 causes the first rotating machine MG1 and the second rotating machine MG2 to produce torque for driving the vehicle. The second carrier 24, which is inhibited from rotating, takes reaction force against the torque of the first rotating machine MG1, so that the torque of the first rotating, machine MG1 can be delivered from the second ring gear 23. The first rotating machine MGl produces negative torque and rotates in the negative direction when the vehicle runs forward, so that positive torque is delivered from the second ring gear 23. When the vehicle runs backward, on the other hand, the first rotating machine MGl produces positive torque and rotates in the positive direction, so that negative torque can be delivered from the second ring gear 23.

[0047] In the HV running, the second planetary gear unit 20 as the differential unit is basically placed in a differentially operating condition, and the first planetary gear unit 10 as the speed changing unit is switched between LOW and HIGH. FIG. 6 is a collinear diagram concerning the HV running mode in the LOW state (which will also be called "HV LOW mode"), and FIG. 7 is a collinear diagram concerning the HV running mode in the HIGH state (which will also be referred to as "HV HIGH mode").

[0048] In the HV LOW mode, the HV_ECU 50 engages the clutch CL1, and releases the brake BK1. With the clutch CL1 thus engaged, differential operation of the first planetary gear unit 10 is restricted, and the rotational elements 11, 13, 14 rotate as a unit. Accordingly, the speed of rotation of the engine 1 is not increased nor reduced, and the rotation of the engine 1 is transmitted at the same speed from the first ring gear 13 to the second carrier 24.

[0049] In the HV HIGH mode, on the other hand, the HV_ECU 50 releases the clutch CL1, and engages the brake BK1. With the brake BK1 thus engaged, rotation of the first sun gear 11 is restricted. As a result, the first planetary gear unit 10 is brought into an overdrive (OD) state in which the rotation of the engine 1 received by the first carrier 14 is delivered from the first ring gear 13 such that the engine speed is increased. Thus, the first planetary gear unit 10 is able to deliver rotation of the engine 1 while increasing the speed of rotation of the engine 1. The speed ratio of the first planetary gear unit 10 in the overdrive (OD) state may be set to, for example, 0.7.

[0050] As described above, a switching device consisting of the clutch CL1 and the brake BK1 switches the first planetary gear unit 10 between a condition where its differential operation is restricted, and a condition where its differential operation is allowed, so that the speed of rotation between the input and output elements of the first planetary gear unit 10 is changed. The power transmission system 1-1 can be switched between the HV HIGH mode and the HV LOW mode, using the speed changing unit including the first planetary gear unit 10, so as to improve the transmission efficiency of the vehicle 100. Also, the second planetary gear unit 20 as the differential unit is connected in series with the downstream or output side of the speed changing unit. Since the first planetary gear unit 10 is operable in the overdrive state, the first rotating machine MG1 need not provide significantly increased torque.

[0051], For example, the HV_ECU 50 selects the HV HIGH mode when the vehicle runs at high speeds, and selects the HV LOW mode when the vehicle runs at middle to low speeds. In the map as shown in FIG. 8, the horizontal axis indicates the vehicle speed, and the vertical axis indicates the required driving force. As shown in FIG. 8, a low-load region in which the vehicle speed is low and the required driving force is small is a motor running region. In the motor running region, the EV running is selected. In the motor running region, the single-motor EV mode is selected when the vehicle runs at a low load, and the both-drive EV mode is selected when the vehicle runs at a high load.

[0052] In an engine running region, the vehicle speed and the load are higher than those of the motor running region. The engine running region is further divided into a direct-coupling (LOW) region and an OD (HIGH) region. The direct-coupling region is an engine running region in which the HV LOW mode is selected. The OD region is an engine running region in which the HV HIGH mode is selected. The OD region is a high-vehicle-speed region, and the direct-coupling region is a middle- to low-vehicle-speed region. The direct-coupling region is set on the higher load side of the OD region. By placing the speed changing unit in the overdrive state when the vehicle speed is high and the load is low, the fuel efficiency can be improved.

[0053] In this embodiment, the rotation of the engine 1 is delivered such that the speed of rotation is changed through switching between the HV HIGH mode and the HV LOW mode, so as to provide two mechanical points, and thus improve the fuel efficiency. The mechanical point is a highly efficient operating point at which the entire power received by the planetary gear units 10, 20 is mechanically transmitted to the counter drive gear 25 without passing through any electric path.

[0054] In the power transmission system 1-1 according to this embodiment, the first planetary gear unit 10 can increase the speed of rotation of the engine 1, and delivers the rotation from the first ring gear 13. Accordingly, in addition to a mechanical point in the case where the first planetary gear unit 10 is not provided and the engine 1 is directly connected to the second carrier 24, the power transmission system 1-1 has another mechanical point on the high gear side. Namely, the power transmission system 1-1 has two mechanical points on the high gear side. Thus, the power transmission system 1-1 can realize a hybrid system with which the fuel efficiency can be improved due to improvement in the transmission efficiency during high-speed running.

[0055] Also, the power transmission system 1-1 can restrict rotation of the input element of the second planetary gear unit 20, by engaging the clutch CL1 and the brake B 1, so as to enable the vehicle to run in the both-motor EV mode. Therefore, there is no need to separately provide a clutch, or the like, for establishing the both-motor EV mode, and the arrangement of the system is simplified. With the above layout of this embodiment, the speed reduction ratio of the second rotating machine MG2 can be set to a large value. Also, a compact arrangement can be achieved by the FF or RR layout.

[0056] When the vehicle runs backward while the engine is running, the first rotating machine MG1 operates as a generator to generate electric power, and the second rotating machine MG2 operating as a motor rotates in the negative direction to produce negative torque, so as to run the vehicle backward. When the battery 3 is in a sufficiently charged state, the second rotating machine MG2 may rotate alone in the reverse direction in the single-motor EV mode, to run the vehicle with the motor. It is also possible to run the vehicle backward in the both-motor EV mode by fixing the second carrier 24.

[0057] When switching between the HV HIGH mode and the HV LOW mode, the HV_ECU 50 can perform coordinated shift control for shifting the first planetary gear unit 10 and the second planetary gear unit 20 at the same time. In the coordinated shift control, the HV_ECU 50 increases the speed ratio of one of the first planetary gear unit 10 and the second planetary gear unit 20, and reduces the speed ratio of the other. [0058] When switching from the HV HIGH mode to the HV low mode, the HV_ECU 50 changes the speed ratio of the second planetary gear unit 20 to the higher gear in synchronization with switching of the modes. This makes it possible to curb or reduce discontinuous change of the speed ratio in the whole system from the engine 1 to the drive wheels 32 of the vehicle 100, and reduce the degree of change of the speed ratio. By curbing change of the speed ratio in the system from the engine 1 to the drive wheels 32, it is possible to reduce the amount of adjustment of the engine speed that would be required by change of the speed, or make the adjustment of the engine speed unnecessary. The HV_ECU 50 changes the speed ratios of the first planetary gear unit 10 and the second planetary gear unit 20 in coordination with each other, so as to continuously change the speed ratio of the vehicle 100 as a whole to the lower gear.

[0059] When switching from the HV LOW mode to the HV HIGH mode, the HV ECU 50 changes the speed ratio of the second planetary gear unit 20 to the lower gear in synchronization with switching of the modes. This makes it possible to curb or reduce discontinuous change of the speed ratio in the vehicle 100 as a whole, and reduce the degree of change of the speed ratio. For example, the HV_ECU 50 changes the speed ratios of the first planetary gear unit 10 and the second planetary gear unit 20 in coordination with each other, so as to continuously change the speed ratio of the vehicle 100 as a whole to the higher gear.

[0060] The speed ratio of the second planetary gear unit 20 is adjusted by controlling the rotational speed of the first rotating machine MG1, for example. For example, the HV_ECU 50 controls the first rotating machine MG1 so as to continuously or steplessly change the speed ratio between the input shaft 2 and the counter drive gear 25. As a result, the whole system including the planetary gear units 10, 20, first rotating machine MG1, clutch CL1 and the brake BK1, namely, a speed changing device including the differential unit and the speed changing unit, operates as an electric continuously variable transmission. Since the speed ratio of the speed changing device including the differential unit and the speed changing unit is variable in a wide range (the range of the speed ratio is larger than that of the differential unit alone), the speed ratio from the differential unit to the drive wheels 32 can be set to a relatively large value. Also, power circulation during high-speed running of the vehicle in the HV running mode is reduced.

[0061] When the engine 1 starts from the single-motor EV mode, the HV ECU 50 engages the clutch CL1 or the brake BKl, and increases the engine speed for ignition. If the clutch CL1 or the brake BKl is engaged, torque is transmitted from the first ring gear 13 to the first carrier 14, so that positive torque is applied to the engine 1. With the positive torque thus applied, the engine 1 starts rotating, and the engine speed rises. If the engine speed becomes equal to or higher than a predetermined ignition speed, the HV ECU 50 causes ignition in the engine 1 so as to complete startup of the engine 1.

[0062] In hybrid vehicles, it is generally desired to curb overload of a rotating machine. When rotation of the rotating machine is stopped, or the rotating machine rotates at a low speed, phase change is less likely to take place, and electric current may keep flowing into one phase. At this time, electric current may concentrate in motor winding (or coil) or IGBT devices of an inverter, which may result in an increase of a thermal load. Such a condition where electric current keeps flowing into one phase is called "single-phase lock". If the first rotating machine MGl or the second rotating machine MG2 is kept in a condition of low-speed rotation and high load, overload may occur to the rotating machine due to the single-phase lock.

[0063] In the power transmission system 1-1 of this embodiment, the second rotating machine MG2 rotates in association with rotation of the drive wheels 32. The second rotating machine MG2 is connected to a power transmission path closer to the drive wheels 32 than the second planetary gear unit 22 as the differential unit. Therefore, unlike the first rotating machine MGl connected to the drive wheels 32 via the second planetary gear unit 30, the second rotating machine MG2 cannot change the rotational speed as desired relative to the vehicle speed. Since rotation of the drive wheels 32 is stopped when the vehicle 100 is stopped, rotation of the second rotating machine MG2 is also stopped. Also, the rotational speed of the second rotating machine MG2 during running of the vehicle is uniquely determined, and cannot be changed as desired.

[0064] Accordingly, if large driving force is required to be generated when the vehicle 100 is started, for example, overload is likely to occur due to the single-phase lock. For example, when the vehicle is supposed to get over a step when starting, or the driver tries to start the vehicle against a steep slope, the rotational speed of the second rotating machine MG may not increase, and single-phase lock may takes place, which may result in overload due to flow of large current into the second rotating machine MG2. In this specification, starting or startup means starting of the vehicle 100, and indicates a running condition where the rotational speed of the second rotating machine MG2 (which will be called "MG2 road") is equal to 0, or a running condition where the MG2 speed is low. The above-mentioned starting or startup may also indicate a condition where the MG2 speed is within a rotational speed range equal to or lower than a detection limit of the MG speed sensor. Also, the starting or startup may indicate a condition where the vehicle speed may be within a vehicle speed range equal to or lower than a detection limit of the vehicle sensor.

[0065] When the required driving force is equal to or larger than a first predetermined value Fl at startup, the power transmission system 1-1 according to this embodiment controls a slip amount of the clutch CL1 (the power transmission system 1-1 according to this embodiment performs slip control to cause the clutch CL1 to slip), and increases engine torque. With the engine torque thus increased, torque transmitted from the engine 1 to the drive wheels 32 via the first planetary gear unit 10 and the second planetary gear unit 20 increases. As a result, torque required to be produced by the second rotating machine MG2 is less likely to be large or prevented from being large. Accordingly, the power transmission system 1-1 according to this embodiment is able to produce required driving force while curbing overload of the second rotating machine MG2. Although a loss of torque may appear due to slipping of the clutch CL1, the torque transmitted from the engine 1 to the drive wheels 32 increases by an amount large than the loss. Accordingly, it is possible to increase the output torque, without increasing torque (current) of the second rotating machine MG2.

[0066] When the required driving force is equal to or larger than the first predetermined value Fl at startup, the power transmission system 1-1 performs slip control to cause the clutch CL1 to slip, as will be explained with reference to FIG. 9. In FIG. 9, solid lines indicate a collinear diagram prior to start of slip control of the clutch CL1, and broken lines indicate a collinear diagram at the time when slip control of the clutch CL1 is executed. In the following description, the slip control of the clutch CL1 executed when the required driving force is equal to or larger than the first predetermined value Fl at startup will be simply referred to as "slip control". Before the slip control is executed, the clutch CL1 is in a fully engaged state, and the rotational speed of the first sun gear 11 (SI axis) coincides with the rotational speed of the first carrier 14 (CI axis).

[0067] If the slip control is executed, and the clutch CL1 slips, differential rotation of the first planetary gear unit 10 is allowed. Accordingly, as indicated by the broken line in FIG. 9, the rotational speeds of the first sun gear 11 and the first ring gear 13 can be changed to be different from the rotational speed of the first carrier 14. The power transmission system 1-1, when it executes the slip control, reduces the MG1 speed to be lower than that in the case where the clutch CL1 is fully engaged, namely, reduces the absolute value of the MG1 speed. Thus, the amount of electric power generated by the first rotating machine MG1 can be reduced. Accordingly, the power transmission system 1-1 of this embodiment is able to produce the required driving force while curbing overload of the second rotating machine MG2, and reducing the amount of electric power generated.

[0068] Referring FIG. 10 through FIG. 12, the control of the first embodiment will be described. The engine speed is indicated in the time chart of FIG. 12 A, the engine torque is indicated in the time chart of FIG. 12B, the MG1 torque is indicated in the time chart of FIG. 12C, the MG1 speed is indicated in the time chart of FIG. 12D, the MG2 torque is indicated in the time chart of FIG. 12E, the amount of charge of the battery 3 is indicated in the time chart of FIG. 12F, the hydraulic pressure applied to the clutch CL1 is indicated in the time chart of FIG. 12G, the output torque is indicated in the time chart of FIG. 12H, the accelerator pedal angle is indicated in the time chart of FIG 121, and the vehicle speed is indicated in the time chart of FIG. 12 J. In FIG. 12A through FIG. 12 J, the operation performed when the accelerator pedal is depressed by an increased degree at time tl, from a condition where the accelerator pedal is depressed and the vehicle 100 is stopped, as in the case where the vehicle gets over a step, for example, is illustrated. A solid line of each value in FIG. 12A through FIG. 12J indicates changes in the value in the case where slip control is performed, and a broken line indicates changes in the case where slip control is not performed.

[0069] A control routine_^ illustrated in the flowchart of FIG. 10 is repeatedly executed at given intervals, for example. Here, the case where the control routine illustrated in FIG. 10 is carried out in the HV LOW mode will be described. Initially, in step S10, the HV ECU 50 determines whether the vehicle speed is 0, and the required driving force is large. The HV ECU 50 makes an affirmative decision (YES) in step S10 if the required driving force is equal to or larger than the first predetermined value Fl when the vehicle is started. The HV ECU 50 determines that the vehicle is started when the vehicle speed currently detected is equal to 0 [km/h]. In this embodiment, it is determined whether the required driving force is large, based on the accelerator pedal angle.

[0070] More specifically, when the accelerator pedal angle is equal to or larger than a predetermined angle Θ 1 , it is determined that the required driving force is equal to or larger than the first predetermined value Fl. The predetermined angle Θ1 and the first predetermined value Fl are determined based on the single-phase lock region of the second rotating machine MG2, for example. The single-phase lock region is a region determined with respect to an operating point of the second rotating machine MG2 (which will be called "MG2 operating point") as a combination of the value of current flowing through the second rotating machine MG2 and the MG2 speed. The single-phase lock region is a region of operating points in which the MG2 speed is low, and the value of current flowing through the second rotating machine MG2 is large.

[0071] The HV_ECU 50 determines a target value of the engine torque and a target value of the MG2 torque, so that the engine torque and the MG2 torque cooperate to provide output torque according to the'required driving force, in the HV LOW mode. The amount of torque to be produced by the engine 1 and the amount of torque to be produced by the second rotating machine MG2 are calculated based on a calculation formula or map stored in advance, for example.

[0072] According to the calculation formula or map, the torque to be produced by the second rotating machine MG2 may increase as the required driving force increases, and the value of current flowing through the second rotating machine MG2 may increase. When the required driving force is large, overload is not likely to arise if the vehicle speed increases and the MG2 speed increases; however, the second rotating. machine MG2 may be overloaded if it keeps producing torque while the vehicle speed is not increased, such as when the vehicle gets over a step on the road.

[0073] The HVJECU 50 of this embodiment determines that the required driving force is large, when the MG2 operating point corresponding to the required driving force lies within the single-phase lock region, or it is an operating point located just ahead of (or in the vicinity) of the single-phase lock region and on the low load side. The first predetermined value Fl as a threshold value based on which it is determined whether the required driving force is large is determined in advance, based on the result of experiments, or the like. The predetermined angle Θ1 is an accelerator pedal angle corresponding to the first predetermined value Fl . If the accelerator pedal angle is equal to or larger than the predetermined angle Θ1, it is determined that the required driving force is large. In FIG. 121, the accelerator pedal angle becomes equal to or larger than the predetermined angle Θ1 at time t2, and it is thus determined that the required driving force is large. If it is determined in step S 10 that the vehicle is started, and the required driving force is equal to or larger than the first predetermined value Fl (YES in step S10), the control proceeds to step S20. If not (NO in step S 10), this cycle of the control routine ends.

[0074] In step S20, the HV_ECU 50 determines whether the oil temperature Tw is higher than a predetermined temperature Twl . The oil temperature Tw is the temperature of hydraulic oil supplied to the clutch CL1. If the oil temperature Tw is equal to or lower than the predetermined temperature Twl, the HV ECU 50 does not perform slip control of the clutch CL1. If it is determined in step S20 that the oil temperature Tw is higher than the predetermined temperature Twl (YES in step S20), the control proceeds to step S50. If not (NO in step S20), the control proceeds to step S30.

[0075] In step S30, the HV_ECU 50 causes the clutch CL1 to be fully engaged. The HV_ECU 50 restricts or inhibits differential operation of the first planetary gear unit 10 without slipping the clutch CL1. After executing step S30, the control proceeds to step S40.

[0076] In step S40, the HV_ECU 50 cuts down the driving force. If a negative decision (NO) is made in step S20, and the slip control of the clutch CL1 is not allowed, the HV_ECU 50 reduces the actual driving force relative to the required driving force, instead of controlling the slip amount, so as to cut down the driving force. As shown in FIG. 12B and FIG. 121, the HV_ECU 50 increases engine torque to the extent possible, after the accelerator pedal angle exceeds the predetermined angle Θ1. The HV_ECU 50 increases output torque, by increasing engine torque according to increase of the accelerator pedal angle, from time t2 to time t3. The MG torque, which is already right ahead of the single-phase lock region, is not increased after time tl .

[0077] As the engine torque increases, the engine speed increases, and the MGl speed also increases with the engine speed. Also, as the engine torque increases, the magnitude of the MGl torque as reaction force torque increases. As a result, the amount of electric power generated by the first rotating machine MGl increases, and the amount of charge of the battery 3 (electric power with which the battery 3 is charged) increases from time tl to time t3. At time t3, the amount of charge reaches its allowable upper limit (the maximum input power Win which will be described later), and the MGl torque and the MGl speed cannot be further changed. Thus* the HV ECU 50 stops increasing the engine torque after time t3. In other words, the HV_ECU 50 executes control for cutting down the driving force from time t3, and the actual output torque indicated by a broken t line is kept smaller than the required output torque (solid line) that varies with the accelerator pedal angle. Once step S40 is executed, this cycle of the control routine ends.

[0078] In step S50, the HV_ECU 50 executes slip control of the clutch CL1. The HV_ECU 50 reduces the command value of the hydraulic pressure supplied to the clutch CL1, and brings the clutch CL1 that has been fully engaged, into a half-engaged state, so that the clutch CL1 slips. The HV_ECU 50 performs feedback control on the hydraulic pressure, so that the slip amount (rotational speed difference) of the clutch CL1 becomes equal to a target slip amount. In FIG. 12G, the hydraulic pressure supplied to the clutch CL1 is reduced from time t3, at which slip control is started. After executing step S50, the control proceeds to step S60.

[0079] In step S60, the HV_ECU 50 changes the engine operating point. For example, the HV_ECU 50 changes the engine operating point, as will be explained with reference to FIG. 11. In FIG. 11, the horizontal axis indicates the engine speed, and the vertical axis indicates the engine torque. The engine operating point indicates a combination of the engine speed and the engine torque. Curve LI represents a collection of predetermined operating points, for example, a collection of operating points at which the engine 1 can be operated with high fuel efficiency. In this embodiment, curve LI connects operating points at which the engine 1 can be operated with the minimum fuel consumption, with respect to each engine speed. The HV_ECU 50 changes the engine operating point from an operating point XI before start of the slip control, to an operating point X2 after start of the slip control. With the operating point thus changed, the engine torque increases from a value denoted by Tel, to a value denoted by Te2, as shown in FIG. 12B, and the engine speed increases from a value denoted by Nel, to a value denoted by Ne2, as shown in FIG. 12 A. In FIG. 12, the engine operating point is changed from time t3 to time t4. After executing step S60, the control proceeds to step S70.

[0080] In step S70, the HV_ECU 50 changes the MG1 operating point. The HV_ECU 50 instructs the MG_ECU 60 to reduce the MG1 speed to be lower than the rotational speed in the case where the clutch CL1 is fully engaged. Referring to FIG. 9, the engine speed increases from the rotational speed Nel before start of slip control, to the rotational speed Ne2 after start of slip control, in step S60. If the clutch CL is kept fully engaged, the rotational speed of the first ring gear 13 (Rl axis) and the rotational speed of the second carrier 24 (C2 axis) become equal to the engine speed (Ne2). In this case, the MG1 speed increases from the rotational speed Ngl before start of slip control, to the rotational speed denoted by Ng2. [0081] If the MGl speed increases, the amount of electric power generated by the first rotating machine MGl increases. On the other hand, electric power consumed by the second rotating machine MG2 is limited, so that overload of the second rotating machine MG2 is curbed. Therefore, when slip control is not executed, the amount of charge of the battery 3 (i.e., the amount of electric power with which the battery 3 is charged) is likely to increase. In this connection, the electric power allowed to be received by the battery 3 has the maximum value (which may be called "maximum input power Win"). The maximum input power Win varies according to the temperature, etc. of the battery 3.

[0082] The HV ECU 50 restricts the amount of electric power generated by the first rotating machine MGl, so that an excess of the electric power generated by the first rotating machine MGl, which cannot be consumed or used up, does not exceed the maximum input power Win. As a result, if slip control is not executed, the MGl torque is restricted, and reaction torque becomes insufficient; as a result, sufficient engine torque may not be transmitted to the drive wheels 32. In FIG. 12F, the electric power with which the battery 3 is charged reaches the maximum input power Win at time t3. In the case where slip control is not executed (as indicated by each broken line), the MGl speed and the engine speed cannot be increased, and the magnitude of the MGl torque cannot be increased, after time t3, as already described above.

[0083] On the other hand, the power transmission system 1-1 of this embodiment performs slip control, so as to reduce the amount of electric power generated by the first rotating machine MGl . By executing the slip control, it is possible to achieve the required driving force, and keep the electric power supplied to the battery 3 equal to or smaller than the maximum input power Win. The HV ECU 50 determines the target values of engine toque and engine speed during execution of the slip control, according to the required driving force, as explained above with reference to FIG. 11. More specifically, the HV_ECU 50 initially determines the target value of the MG2 torque according to the required driving force, so that the MG2 torque falls within a range in which the second rotating machine MG2 can be prevented from being overloaded. The HV ECU 50 then determines the target value of the engine torque so as to cause the engine 1 to produce torque that makes up for a shortage caused by the MG2 torque, relative to the required driving force. The HV_ECU 50 sets the target value of engine speed, to the rotational speed at which the engine 1 can be operated with the optimum fuel efficiency, to provide the target engine torque.

[0084] Also, the HVJECU 50 determines the target value of the MGl torque, so that the target engine torque can be delivered from the second carrier 24 to the second ring gear 23. Also, the HV_ECU 50 determines the target value of the MGl speed, so that the electric power stored in the battery 3 becomes equal to or smaller than the maximum input power Win. The MGl speed thus determined is, for example, the rotational speed denoted by Ng3 in FIG. 9. The MGl speed Ng3 is lower than the rotational speed Ng2 in the case where the clutch CLl is fully engaged, and the absolute value of the MGl speed Ng3 is smaller than that of the speed Ng2. The MGl speed Ng3 may be lower than the rotational speed Ngl of the first rotating machine MGl before the slip control is started, for example. The slip amount Nsl of the clutch CLl is determined by the determined target value Ne2 of the engine speed and the determined target value Ng3 of the MGl speed. The slip amount Nsl of the clutch CLl is a difference between the rotational speed of the first sun gear 11 (SI axis) and the rotational speed of the first carrier 14 (CI axis).

[0085] The HV_ECU 50 adjusts the hydraulic pressure supplied to the clutch CL 1 , the electric power generated by the first rotating machine MGl, etc., so that the engine speed becomes equal to the target speed Ne2, and the MGl speed becomes equal to the target speed Ng3. In FIG. 12C and FIG. 12D, the MGl operating point is changed from time t3 to time t4. With the MGl operating point thus changed, the electric power supplied to the battery 3 is kept at a value that does not exceed the maximum input power Win; for example, it is kept at a constant value equal to or smaller than the maximum input power Win. After step S70 is executed, this cycle of the control routine ends.

[0086] As described above, in this embodiment, the target values of engine torque and engine speed are determined, so as to achieve the required driving force. Accordingly, the HV_ECU 50 changes the slip amount Nsl of the clutch CLl according to the magnitude of the required driving force. The HV_ECU 50 increases the engine torque as the required driving force increases. When the engine torque is increased, the engine speed is often increased so as to achieve high fuel efficiency. When the required driving force is large, the HV_ECU 50 may make the slip amount Nsl of the clutch CL1 larger than that in the case where the required driving force is small. When the required driving force changes during slip control, the HV_ECU 50 may change the slip amount Nsl of the clutch CL1, according to changes in the value of the required driving force.

[0087] When the required driving force is equal to or larger than a second predetermined value F 11, the HV_ECU 50 may increase the slip amount Nsl of the engaging device. The second determined value Fl l is preferably a value of driving force larger than the first predetermined value F 1.

[0088] The HV_ECU 50 of this embodiment changes the MG1 speed according to the maximum input power Win. Namely, the HV_ECU 50 changes the slip amount Nsl of the clutch CL1 according to the maximum input power Win. It is deemed desirable to reduce the MG1 speed as the maximum input power Win decreases. When the maximum input power Win is small, the HV_ECU 50 may make the slip amount Nsl of the clutch CL1 larger than that in the case where the maximum input power Win is large. When the maximum input power Win changes during execution of slip control, the HV ECU 50 may change the slip amount Nsl of the clutch CL1 , according to changes in the value of the maximum input power Win.

[0089] When the required driving force is equal to or larger than the first predetermined value Fl, the HV ECU 50 of this embodiment uses engine torque to produce driving force of an amount by which the required driving force exceeds the first predetermined value Fl . After the required driving force becomes equal to or larger than the first predetermined value Fl , increase of the MG2 torque is restricted or inhibited, for example, the MG2 torque is kept constant. Thus, the second rotating machine MG2 is prevented from being overloaded.

[0090] The HV_ECU 50 of this embodiment causes the clutch CL1 to slip under slip control. The HV ECU 50 may perform slip control on the brake BK1 when the brake BK1 is engaged at startup.

[0091] In the first embodiment as described above, the rotational speed of the first rotating machine MGl may be reduced according to the amount by which the clutch CLl is able to slip. For the clutch CLl, the desirable upper limit of the slip mount (which will be called "maximum slip amount") is determined, from the viewpoint of assurance of the durability, for example. The maximum slip amount is an allowable maximum slip amount. During execution of slip control, the HV_ECU 50 adjusts the rotational speed of the first rotating machine MGl so as to make the slip amount Nsl of the clutch CLl equal to or smaller than the maximum slip amount.

[0092] The HV_ECU 50 determines the maximum slip amount based on the amount of heat absorbed by the clutch CLl, for example. The heat absorption amount of the clutch CLl may be calculated according to the following equation (1), for example. Heat Absorption Amount = Torque Capacity Tel x Slip Amount Nsl x Time Tsl - Cooling Amount (1)

In this equation, the torque capacity Tel is the torque capacity of the clutch CLl, time Tsl is a length of time for which the clutch CLl is caused to slip with the torque capacity Tel, and the cooling amount is the amount of heat which hydraulic oil takes from the clutch CLl during slip control. The cooling amount depends on the oil temperature of the hydraulic oil.

[0093] As the heat absorption amount of the clutch CLl increases, the maximum slip amount is reduced. The maximum slip amount is determined in advance so that the heat absorption amount of the clutch CLl does not exceed the maximum heat absorption amount. When the HV_ECU 50 determines that slip control is to be executed, it calculates the amount of heat absorbed by the clutch CLl up to this point in time, and determines the maximum slip amount, based on a map, or the like, stored in advance. The HV_ECU 50 performs an upper-limit guarding operation on the slip amount Nsl, so that, in slip control, the slip amount Nsl of the clutch CLl does not exceed the maximum slip amount. For example, when the target value of the slip amount Nsl determined based on the maximum input power Win exceeds the maximum slip amount, the HV_ECU 50 corrects the target value of the MGl speed. The HV_ECU 50 corrects the target value of the MGl speed, so that the slip amount Nsl becomes equal to or smaller than the maximum slip amount. This makes it possible to perform slip control while assuring the durability of the clutch CL1.

[0094] The maximum slip amount may be determined based on the target torque capacity Tel of the clutch CL1. For example, the HV ECU 50 may determine the maximum slip amount, referring to a pre-stored map indicating the correspondence relationship between the torque capacity Tel and the maximum slip amount. In this case, if the HV_ECU 50 determines the target torque capacity Tel of the clutch CL1 so that the target engine torque can be transmitted to the drive wheels 32, the HV_ECU 50 determines the maximum slip amount from the torque capacity Tel thus determined, referring to the map. In this manner, the HV ECU 50 can easily determine the maximum slip amount. During execution of slip control, the HVJECU 50 may change the maximum slip amount according to change of the target torque capacity Tel .

[0095] Also, the maximum slip amount may be determined based on the oil temperature. For example, the HV ECU 50 may determine the maximum slip amount, referring to a pre-stored map indicating the correspondence relationship between the oil temperature of the hydraulic oil and the maximum slip amount. In this case, when the HV_ECU 50 performs slip control, it determines the maximum slip amount with reference to the map, based on the oil temperature. During execution of the slip control, the HV ECU 50 may monitor the oil temperature, and change the maximum slip amount.

[0096] FIG. 13 is a skeleton diagram of a vehicle according to a second embodiment of the invention. A power transmission system 2-1 according to the second embodiment has an input clutch CO, in place of the speed changing unit of the first embodiment. The input clutch CO is a friction engagement type clutch, and is engaged or released according to the hydraulic pressure supplied thereto, for example. A rotary shaft la of the engine 1 is connected to the input shaft 2 via the input clutch CO. The input shaft 2 is connected to the second carrier 24 of the second planetary gear unit 20.

[0097] The input clutch CO of this embodiment functions as an engaging device disposed between the engine 1 and the second planetary gear unit 20 as the differential unit. The second carrier 24 is an input rotational element connected to the input clutch CO. Like the power transmission system 1-1 of the above-described first embodiment, the power transmission system 2-1 includes the HV_ECU 50, MG_ECU 60, and the engine ECU 70 (see FIG. 2). The HV_ECU 50 outputs a command value of hydraulic pressure supplied to the input clutch CO.

[0098] The input clutch CO connects and disconnects the second planetary gear unit 20 to and from the engine 1. The HV_ECU 50 releases the input clutch CO in the EV running mode. On the other hand, the HV_ECU 50 engages the input clutch CO in the HV running mode. If the required driving force is equal to or larger than a first predetermined value F2 when the vehicle is started, the power transmission system 2-1 performs slip control to cause the input clutch CO to slip, and increases torque of the engine 1. The first predetermined value F2 of this embodiment may be the same value as the first predetermined value Fl of the first embodiment.

[0099] With the torque of the engine 1 thus increased, engine torque transmitted to the drive wheels 32 via the input clutch CO and the second planetary gear unit 20 can be increased. Accordingly, the power transmission system 2-1 can deliver the required driving force while preventing the second rotating machine MG2 from being overloaded. Also, the power transmission system 2-1 performs slip control on the input clutch CO, so as to deliver the required driving force while preventing the second rotating machine MG2 from being overloaded, and suppressing or reducing the amount of electric power generated by the first rotating machine MG1.

[0100] The power transmission system 2-1 can determine the target value of the engine torque, the target value of the engine speed, the target value of the MG1 torque, the target value of the MG1 speed, etc., in substantially the same manners as the power transmission system 1-1 of the above-described first embodiment. The power transmission system 2-1 determines the slip amount of the input clutch CO, based on these target values. Also, the power transmission system 2-1 controls the hydraulic pressure supplied to the input clutch CO, the engine 1, the first rotating machine MG1, etc., so as to achieve these target values.

[0101] An oil pump 4 is connected to an end portion of the input shaft 2 opposite to the engine 1. The oil pump 4 is a mechanical pump that is driven by rotation of the input shaft 2 to discharge hydraulic oil. The oil pump 4 may be driven irrespective of whether the vehicle is running or stopped. For example, in the HV running mode, the oil pump 4 is driven by rotation of the engine 1 during running. While the vehicle is stopped, the input clutch CO may be released, and the oil pump 4. may be driven using the MG1 torque. In the EV running mode, the oil pump 4 can be driven using the MG1 torque, irrespective of whether the vehicle is. stopped or running.

[0102] In the second embodiment, the slip amount of the input clutch CO may be restricted according to the maximum slip amount, as in the modified example of the first embodiment.

[0103] In the first embodiment and second embodiment as described above, the power storage device is not limited to the battery 3. For example, the power storage device may be a capacitor, or the like. Also, the engine is not limited to the engine 1. The engine may be a motor, or any other type of engine. Also, the differentia unit is not limited to the illustrated single-pinion-type second planetary gear unit 20. For example, the differential unit may be a double-pihion-type planetary gear unit, or any other type of differential mechanism. The clutch CL1 or the input clutch CO may be engaged or released under force other than the hydraulic pressure.

[0104] In the first embodiment and second embodiment as described above, the slip control is performed on the assumption that the engine 1 is in operation. Instead, the engine 1 that has been stopped may be started before start of the slip control. For example, the HV_ECU 50 may start the engine 1 when the required driving force becomes equal to or larger than a threshold value F3 that is smaller than the first predetermined value Fl, F2, and executes the slip control when the required driving force then becomes equal to or larger than the first predetermined value Fl, F2, so as to increase the torque of the engine 1.

[0105] In the first embodiment and second embodiment as described above, it is determined whether the required driving force is large, based on the accelerator pedal angle. Instead, the required driving force may be calculated, and the calculated required driving force may be compared with the first predetermined value Fl , F2. The HV ECU 50 calculates the required driving force, based on the accelerator pedal angle and the vehicle speed, for example. The HV ECU 50 determines that the required driving force is large, when the calculated required driving force is equal to or larger than the first predetermined value F1, F2.

[0106] Through each of the above-described embodiments and modified examples, the following control method is presented: in a method of controlling an engine and an automatic transmission comprising an electric differential unit, the electric differential unit has a first rotating machine and a second rotating machine, an engaging device is provided between the engine and an input part of the electric differential unit, and a controller performs slip control to cause the engaging device to slip control when rotation of the second rotating machine is stopped, and required driving force is equal to or larger than a first predetermined value.

[0107] The contents disclosed in the respective embodiments and modified examples as described above may be combined as appropriate and implemented.

Claims

CLAIMS:

1. A power transmission system of a vehicle, comprising:

an engine;

a first rotating machine;

a second rotating machine;

a differential unit including an input rotational element, a rotational element connected to the first rotating machine, and an output rotational element connected to the second rotating machine and drive wheels;

an engaging device disposed between the engine and the differential unit, the engaging device being connected to the input rotational element; and

a controller configured to control a slip amount of the engaging device, and increase torque of the engine, when required driving force is equal to or larger than a first predetermined value when the vehicle is started.

2. The power transmission system according to claim 1, wherein

the controller is configured to increase the slip amount of the engaging device, when the required driving force is equal to or larger than a second predetermined value that is larger than the first predetermined value.

3. The power transmission system according to claim 1 or 2, further comprising: a power storage device that supplies and receives electric power to and from the first rotating machine, wherein

the controller is configured to change the slip amount according to a maximum value of electric power that is allowed to be received by the power storage device.

4. The power transmission system according to claim 3, wherein

the controller is configured to determine the slip amount so that the slip amount increases as the maximum value of the allowed electric power is smaller.

5. The power transmission system according to any one of claims 1 to 4, wherein the controller is configured to control a rotational speed of the first rotating machine when the controller controls the slip amount, so that the rotational speed becomes lower than that when the engaging device is fully engaged.

6. The power transmission system according to claim 5, wherein

the controller is configured to control the rotational speed of the first rotating machine so that the slip amount of the engaging device becomes equal to or smaller than an allowable maximum slip amount.

7. The power transmission system according to any one of claims 1 to 6, wherein the controller is configured to reduce a driving force, instead of controlling the slip amount, when the engaging device is not allowed to slip.

8. A method of controlling a power transmission system of a vehicle including an engine, a first rotating machine, a second rotating machine, a differential unit including an input rotational element, a rotational element connected to the first rotating machine, and an output rotational element connected to the second rotating machine and drive wheels, an engaging device that is disposed between the engine and the differential unit and connected to the input rotational element, and a controller, the method comprising:

controlling, by the controller, a slip amount of the engaging device, and increasing torque of the engine, when required driving force is equal to or larger than a first predetermined value when the vehicle is started.