Patent Publication Number: US-2015087457-A1

Title: Drive control device for hybrid vehicle

Description:
TECHNICAL FIELD 
     The present invention relates to a drive control device for a hybrid vehicle, and more particularly to an improvement of the drive control device for reducing generation of heat in a drive system to permit optimum running of the hybrid vehicle. 
     BACKGROUND ART 
     There is known a hybrid vehicle which has at least one electric motor in addition to an engine such as an internal combustion engine, which functions as a vehicle drive power source. Patent Document 1 discloses an example of such a hybrid vehicle, which is provided with an internal combustion engine, a first electric motor and a second electric motor. This hybrid vehicle is further provided with a brake which is configured to fix an output shaft of the above-described internal combustion engine to a stationary member, and an operating state of which is controlled according to a running condition of the hybrid vehicle, so as to improve energy efficiency of the hybrid vehicle and to permit the hybrid vehicle to run according to a requirement by an operator of the hybrid vehicle. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: JP-2008-265600 A1 
     SUMMARY OF THE INVENTION 
     Object Achieved by the Invention 
     According to the conventional arrangement of the hybrid vehicle described above, however, at least one of the first electric motor and the second electric motor suffers from generation of heat during running of the hybrid vehicle in a drive mode in which at least one of the first and second electric motors is used as the vehicle drive power source. Namely, it is required to reduce an output of the electric motor when the electric motor suffers from the heat generation, but the reduction of the output causes a drawback such as an adverse influence on the drivability of the hybrid vehicle. Thus, it has been difficult to achieve both the reduction of heat generation in the vehicle drive system and an improvement of the vehicle drivability. This problem was first discovered by the present inventors in the process of intensive studies in an attempt to improve the performance of the hybrid vehicle. 
     The present invention was made in view of the background art described above. It is therefore an object of the present invention to provide a drive control device for a hybrid vehicle, which permits reduction of heat generation in a drive system for assuring optimum running of the hybrid vehicle. 
     Means for Achieving the Object 
     The object indicated above is achieved according to a first aspect of the present invention, which provides a drive control device for a hybrid vehicle provided with: a first differential mechanism and a second differential mechanism which have four rotary elements as a whole; and an engine, a first electric motor, a second electric motor and an output rotary member which are respectively connected to said four rotary elements, and wherein one of the above-described four rotary elements is constituted by the rotary element of the above-described first differential mechanism and the rotary element of the above-described second differential mechanism which are selectively connected to each other through a clutch, and one of the rotary elements of the above-described first and second differential mechanisms which are selectively connected to each other through the above-described clutch is selectively fixed to a stationary member through a brake, the above-described hybrid vehicle being selectively placed in a plurality of drive modes according to respective combinations of engaged and released states of the above-described clutch and the above-described brake, the drive control device being characterized by switching the above-described drive mode on the basis of a temperature condition of a drive system of the hybrid vehicle. 
     Advantages of the Invention 
     According to the first aspect of the invention described above, the hybrid vehicle is provided with: the first differential mechanism and the second differential mechanism which have the four rotary elements as a whole; and the engine, the first electric motor, the second electric motor and the output rotary member which are respectively connected to said four rotary elements. One of the above-described four rotary elements is constituted by the rotary element of the above-described first differential mechanism and the rotary element of the above-described second differential mechanism which are selectively connected to each other through the clutch, and one of the rotary elements of the above-described first and second differential mechanisms which are selectively connected to each other through the clutch is selectively fixed to the stationary member through the brake. The hybrid vehicle is selectively placed in the plurality of drive modes according to respective combinations of the engaged and released states of the above-described clutch and the above-described brake. The drive control device is configured to switch the above-described drive mode on the basis of the temperature condition of its drive system. Accordingly, the hybrid vehicle can be suitably placed in the drive mode in which both of the above-described first and second electric motors are used as the vehicle drive power source, on the basis of the temperature condition of the drive system. Namely, the present invention provides the drive control device for the hybrid vehicle, which permits reduction of heat generation in the drive system, for assuring optimum running of the hybrid vehicle. 
     According to a second aspect of the invention, the drive control device according to the first aspect of the invention is configured to switch the drive mode to a drive mode in which the above-described clutch is placed in the engaged state, when a temperature of the drive system has become equal to or higher than a predetermined threshold value. According to this second aspect of the invention, it is possible to reduce the heat generation in the drive system, for assuring optimum running of the hybrid vehicle, by placing the drive system in the drive mode in which the required work is assigned to the above-described first and second electric motors, when the temperature of the drive system is comparatively high. 
     According to a third aspect of the invention, the drive control device according to the second aspect of the invention is configured to switch the drive mode from a drive mode in which the above-described engine is held at rest, and the above-described brake is placed in the engaged state while the above-described clutch is placed in the released state, to a drive mode in which the above-described brake and the above-described clutch are both placed in the engaged state, when a temperature of the above-described second electric motor has become equal to or higher than a predetermined threshold value. According to this third aspect of the invention, it is possible to reduce the heat generation in the second electric motor, for assuring optimum running of the hybrid vehicle, by placing the drive system in the drive mode in which the required work is assigned to the above-described first and second electric motors, when the temperature of the second electric motor is comparatively high. 
     According to a fourth aspect of the invention, the drive control device according to the second aspect of the invention is configured to switch the drive mode from a drive mode in which the above-described engine is operated, and the above-described brake is placed in the engaged state while the above-described clutch is placed in the released state, to a drive mode in which the above-described brake is placed in the released state while the above-described clutch is placed in the engaged state, when a temperature of the above-described first electric motor has become equal to or higher than a predetermined threshold value. According to this fourth aspect of the invention, it is possible to reduce the heat generation in the first electric motor, for assuring optimum running of the hybrid vehicle, by placing the drive system in the drive mode in which the required work is assigned to the above-described first and second electric motors, when the temperature of the first electric motor is comparatively high. 
     According to a fifth aspect of the invention, the drive control device according to any one of the first, second, third and fourth aspects of the invention is configured to control a ratio of amounts of work to be assigned to the above-described first electric motor and the above-described second electric motor in a drive mode in which the above-described clutch is placed in the engaged state, on the basis of a result of comparison of temperatures of the first and second electric motors with each other. According to this fifth aspect of the invention, it is possible to control the amounts of work to be assigned to the above-described first and second electric motors, while taking account of the heat generation by the first and second electric motors. 
     According to a sixth aspect of the invention, the drive control device according to the fifth aspect of the invention according to any one of the first, second, third and fourth aspects of the invention is configured to compare the temperatures of the above-described first and second electric motors, to reduce the ratio of the amount of work to be assigned to one of the first and second electric motors the temperature of which is higher to the amount of work to be assigned to the other of the first and second electric motors the temperature of which is lower, and to increase the ratio of the amount of work to be assigned to the other of the first and second electric motors to the amount of work to be assigned to the one. According to this sixth aspect of the invention, it is possible to control the amounts of work to be assigned to the above-described first and second electric motors, while taking account of the heat generation by the first and second electric motors. 
     According to a seventh aspect of the invention, the drive control device according to any one of the first, second, third and fourth aspects of the invention, or according to the fifth aspect of the invention according to any one of the first, second, third and fourth aspects of the invention, or according to the sixth aspect of the invention according to the fifth aspect of the invention according to any one of the first, second, third and fourth aspects of the invention is configured such that the above-described first differential mechanism is provided with a first rotary element connected to the above-described first electric motor, a second rotary element connected to the above-described engine, and a third rotary element connected to the above-described output rotary member, while the above-described second differential mechanism is provided with a first rotary element connected to the above-described second electric motor, a second rotary element, and a third rotary element, one of the second and third rotary elements being connected to the third rotary element of the above-described first differential mechanism, and wherein the above-described clutch is configured to selectively connect the second rotary element of the above-described first differential mechanism, and the other of the second and third rotary elements of the above-described second differential mechanism which is not connected to the third rotary element of the above-described first differential mechanism, to each other, while the above-described brake is configured to selectively fix the other of the second and third rotary elements of the above-described second differential mechanism which is not connected to the third rotary element of the above-described first differential mechanism, to the stationary member. According to this seventh aspect of the invention, it is possible to reduce the heat generation in the drive system, for assuring optimum running of the hybrid vehicle the drive system of which has a highly practical arrangement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view for explaining an arrangement of a hybrid vehicle drive system to which the present invention is suitably applicable; 
         FIG. 2  is a view for explaining major portions of a control system provided to control the drive system of  FIG. 1 ; 
         FIG. 3  is a table indicating combinations of operating states of a clutch and a brake, which correspond to respective five drive modes of the drive system of  FIG. 1 ; 
         FIG. 4  is a collinear chart having straight lines which permit indication thereon of relative rotating speeds of various rotary elements of the drive system of  FIG. 1 , the collinear chart corresponding to the modes 1 and 3 of  FIG. 3 ; 
         FIG. 5  is a collinear chart having straight lines which permit indication thereon of relative rotating speeds of various rotary elements of the drive system of  FIG. 1 , the collinear chart corresponding to the mode 2 of  FIG. 3 ; 
         FIG. 6  is a collinear chart having straight lines which permit indication thereon of relative rotating speeds of various rotary elements of the drive system of  FIG. 1 , the collinear chart corresponding to the mode 4 of  FIG. 3 ; 
         FIG. 7  is a collinear chart having straight lines which permit indication thereon of relative rotating speeds of various rotary elements of the drive system of  FIG. 1 , the collinear chart corresponding to the mode 5 of  FIG. 3 ; 
         FIG. 8  is a view for explaining transmission efficiency of the drive system of  FIG. 1 ; 
         FIG. 9  is a functional block diagram for explaining major control functions of an electronic control device provided for the drive system of  FIG. 1 ; 
         FIG. 10  is a flow chart for explaining a major portion of an example of a drive mode switching control implemented by the electronic control device provided for the drive system of  FIG. 1 ; 
         FIG. 11  is a flow chart for explaining a major portion of another example of the drive mode switching control implemented by the electronic control device provided for the drive system of  FIG. 1 ; 
         FIG. 12  is a flow chart for explaining a major portion of an example of a work assignment ratio control implemented by the electronic control device provided for the drive system of  FIG. 1 ; 
         FIG. 13  is a flow chart for explaining a major portion of another example of a work assignment ratio control implemented by the electronic control device provided for the drive system of  FIG. 1 ; 
         FIG. 14  is a schematic view for explaining an arrangement of a hybrid vehicle drive system according to another preferred embodiment of this invention; 
         FIG. 15  is a schematic view for explaining an arrangement of a hybrid vehicle drive system according to a further preferred embodiment of this invention; 
         FIG. 16  is a schematic view for explaining an arrangement of a hybrid vehicle drive system according to a still further preferred embodiment of this invention; 
         FIG. 17  is a schematic view for explaining an arrangement of a hybrid vehicle drive system according to a yet further preferred embodiment of this invention; 
         FIG. 18  is a schematic view for explaining an arrangement of a hybrid vehicle drive system according to still another preferred embodiment of this invention; 
         FIG. 19  is a schematic view for explaining an arrangement of a hybrid vehicle drive system according to yet another preferred embodiment of this invention; 
         FIG. 20  is a collinear chart for explaining an arrangement and an operation of a hybrid vehicle drive system according to another preferred embodiment of this invention; 
         FIG. 21  is a collinear chart for explaining an arrangement and an operation of a hybrid vehicle drive system according to a further preferred embodiment of this invention; and 
         FIG. 22  is a collinear chart for explaining an arrangement and an operation of a hybrid vehicle drive system according to a still further preferred embodiment of this invention. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     According to the present invention, the first and second differential mechanisms as a whole have four rotary elements while the above-described clutch is placed in the engaged state. In one preferred form of the present invention, the first and second differential mechanisms as a whole have four rotary elements while a plurality of clutches, each of which is provided between the rotary elements of the first and second differential mechanisms and which includes the above-described clutch, are placed in their engaged states. In other words, the present invention is suitably applicable to a drive control device for a hybrid vehicle which is provided with the first and second differential mechanisms represented as the four rotary elements indicated in a collinear chart, the engine, the first electric motor, the second electric motor and the output rotary member coupled to the respective four rotary elements, and wherein one of the four rotary elements is selectively connected through the above-described clutch to another of the rotary elements of the first differential mechanism and another of the rotary elements of the second differential mechanism, while the rotary element of the first or second differential mechanism to be selectively connected to the above-indicated one rotary element through the clutch is selectively fixed through the above-described brake to the stationary member. 
     In another preferred form of the present invention, the above-described clutch and brake are hydraulically operated coupling devices operating states (engaged and released states) of which are controlled according to a hydraulic pressure. While wet multiple-disc type frictional coupling devices are preferably used as the clutch and brake, meshing type coupling devices, namely, so-called dog clutches (claw clutches) may also be used. Alternatively, the clutch and brake may be electromagnetic clutches, magnetic powder clutches and any other clutches the operating states of which are controlled (which are engaged and released) according to electric commands. 
     The drive system to which the present invention is applicable is placed in a selected one of a plurality of drive modes, depending upon the operating states of the above-described clutch and brake. Preferably, EV drive modes in which at least one of the above-described first and second electric motors is used as a vehicle drive power source with the engine stopped include a mode 1 to be established in the engaged state of the brake and in the released state of the clutch, and a mode 2 to be established in the engaged states of both of the clutch and brake. Further, hybrid drive modes in which the above-described engine is operated while the above-described first and second electric motors are operated to generate a vehicle drive force and/or an electric energy as needed, include a mode 3 to be established in the engaged state of the brake and in the released state of the clutch, a mode 4 to be established in the released state of the brake and the engaged state of the clutch, and a mode 5 to be established in the released states of both of the brake and clutch. 
     In a further preferred form of the invention, the rotary elements of the above-described first differential mechanism, and the rotary elements of the above-described second differential mechanism are arranged as seen in the collinear charts, in the engaged state of the above-described clutch and in the released state of the above-described brake, in the order of the first rotary element of the first differential mechanism, the first rotary element of the second differential mechanism, the second rotary element of the first differential mechanism, the second rotary element of the second differential mechanism, the third rotary element of the first differential mechanism, and the third rotary element of the second differential mechanism, where the rotating speeds of the second rotary elements and the third rotary elements of the first and second differential mechanisms are indicated in mutually overlapping states in the collinear charts. 
     Referring to the drawings, preferred embodiments of the present invention will be described in detail. It is to be understood that the drawings referred to below do not necessarily accurately represent ratios of dimensions of various elements. 
     First Embodiment 
       FIG. 1  is the schematic view for explaining an arrangement of a hybrid vehicle drive system  10  (hereinafter referred to simply as a “drive system  10 ”) to which the present invention is suitably applicable. As shown in  FIG. 1 , the drive system  10  according to the present embodiment is of a transversely installed type suitably used for an FF (front-engine front-chive) type vehicle, and is provided with a main vehicle drive power source in the form of an engine  12 , a first electric motor MG1, a second electric motor MG2, a first differential mechanism in the form of a first planetary gear set  14 , and a second differential mechanism in the form of a second planetary gear set  16 , which are disposed on a common center axis CE. The drive system  10  is constructed substantially symmetrically with respect to the center axis CE. In  FIG. 1 , a lower half of the drive system  10  is not shown. This description applies to other embodiments which will be described. 
     The engine  12  is an internal combustion engine such as a gasoline engine, which is operable to generate a drive force by combustion of a fuel such as a gasoline injected into its cylinders. Each of the first electric motor MG1 and second electric motor MG2 is a so-called motor/generator having a function of a motor operable to generate a drive force, and a function of an electric generator operable to generate a reaction force, and is provided with a stator  18 ,  22  fixed to a stationary member in the form of a housing (casing)  26 , and a rotor  20 ,  24  disposed radially inwardly of the stator  18 ,  22 . 
     The first planetary gear set  14  is a single-pinion type planetary gear set which has a gear ratio ρ 1  and which is provided with rotary elements (elements) consisting of: a first rotary element in the form of a sun gear S1; a second rotary element in the form of a carrier C1 supporting a pinion gear P1 such that the pinion gear P1 is rotatable about its axis and the axis of the planetary gear set; and a third rotary element in the form of a ring gear R1 meshing with the sun gear S1 through the pinion gear P1. The second planetary gear set  16  is a single-pinion type planetary gear set which has a gear ratio ρ2 and which is provided with rotary elements (elements) consisting of: a first rotary element in the form of a sun gear S2; a second rotary element in the form of a carrier C2 supporting a pinion gear P2 such that the pinion gear P2 is rotatable about its axis and the axis of the planetary gear set; and a third rotary element in the form of a ring gear R2 meshing with the sun gear S2 through the pinion gear P2. 
     The sun gear S1 of the first planetary gear set  14  is connected to the rotor  20  of the first electric motor MG1. The carrier C1 of the first planetary gear set  14  is connected to an input shaft  28  which is rotated integrally with a crankshaft of the engine  12 . This input shaft  28  is rotated about the center axis CE. In the following description, the direction of extension of this center axis CE will be referred to as an “axial direction”, unless otherwise specified. The ring gear R1 of the first planetary gear set  14  is connected to an output rotary member in the form of an output gear  30 , and to the ring gear R2 of the second planetary gear set  16 . The sun gear S2 of the second planetary gear set  16  is connected to the rotor  24  of the second electric motor MG2. 
     The drive force received by the output gear  30  is transmitted to a pair of left and right drive wheels (not shown) through a differential gear device not shown and axles not shown. On the other hand, a torque received by the drive wheels from a roadway surface on which the vehicle is running is transmitted (input) to the output gear  30  through the differential gear device and axles, and to the drive system  10 . A mechanical oil pump  32 , which is a vane pump, for instance, is connected to one of opposite end portions of the input shaft  28 , which one end portion is remote from the engine  12 . The oil pump  32  is operated by the engine  12 , to generate a hydraulic pressure to be applied to a hydraulic control unit  60 , etc. which will be described. An electrically operated oil pump which is operated with an electric energy may be provided in addition to the oil pump  32 . 
     Between the carrier C1 of the first planetary gear set  14  and the carrier C2 of the second planetary gear set  16 , there is disposed a clutch CL which is configured to selectively couple these carriers C1 and C2 to each other (to selectively connect the carriers C1 and C2 to each other or disconnect the carriers C1 and C2 from each other). Between the carrier C2 of the second planetary gear set  16  and the stationary member in the form of the housing  26 , there is disposed a brake BK which is configured to selectively couple (fix) the carrier C2 to the housing  26 . Each of these clutch CL and brake BK is a hydraulically operated coupling device the operating state of which is controlled (which is engaged and released) according to the hydraulic pressure applied thereto from the hydraulic control unit  60 . While wet multiple-disc type frictional coupling devices are preferably used as the clutch CL and brake BK, meshing type coupling devices, namely, so-called dog clutches (claw clutches) may also be used. Alternatively, the clutch CL and brake BK may be electromagnetic clutches, magnetic powder clutches and any other clutches the operating states of which are controlled (which are engaged and released) according to electric commands generated from an electronic control device  40 . 
     As shown in  FIG. 1 , the drive system  10  is configured such that the first planetary gear set  14  and second planetary gear set  16  are disposed coaxially with the input shaft  28  (disposed on the center axis CE), and opposed to each other in the axial direction of the center axis CE. Namely, the first planetary gear set  14  is disposed on one side of the second planetary gear set  16  on a side of the engine  12 , in the axial direction of the center axis CE. The first electric motor MG1 is disposed on one side of the first planetary gear set  14  on the side of the engine  12 , in the axial direction of the center axis CE. The second electric motor MG1 is disposed on one side of the second planetary gear set  16  which is remote from the engine  12 , in the axial direction of the center axis CE. Namely, the first electric motor MG1 and second electric motor MG2 are opposed to each other in the axial direction of the center axis CE, such that the first planetary gear set  14  and second planetary gear set  16  are interposed between the first electric motor MG1 and second electric motor MG2. That is, the drive system  10  is configured such that the first electric motor MG1, first planetary gear set  14 , clutch CL, second planetary gear set  16 , brake BK and second electric motor MG2 are disposed coaxially with each other, in the order of description from the side of the engine  12 , in the axial direction of the center axis CE. 
       FIG. 2  is the view for explaining major portions of a control system provided to control the drive system  10 . The electronic control device  40  shown in  FIG. 2  is a so-called microcomputer which incorporates a CPU, a ROM, a RAM and an input-output interface and which is operable to perform signal processing operations according to programs stored in the ROM while utilizing a temporary data storage function of the RAM, to implement various drive controls of the drive system  10 , such as a drive control of the engine  12  and hybrid drive controls of the first electric motor MG1 and second electric motor MG2. In the present embodiment, the electronic control device  40  corresponds to a drive control device for a hybrid vehicle having the drive system  10 . The electronic control device  40  may be constituted by mutually independent control units as needed for respective controls such as an output control of the engine  12  and drive controls of the first electric motor MG1 and second electric motor MG2. 
     As indicated in  FIG. 2 , the electronic control device  40  is configured to receive various signals from sensors and switches provided in the drive system  10 . Namely, the electronic control device  40  receives: an output signal of an accelerator pedal operation amount sensor  42  indicative of an operation amount or angle A CC  of an accelerator pedal (not shown), which corresponds to a vehicle output required by a vehicle operator; an output signal of an engine speed sensor  44  indicative of an engine speed NE, that is, an operating speed of the engine  12 ; an output signal of an MG1 speed sensor  46  indicative of an operating speed N MG1  of the first electric motor MG1; an output signal of an MG2 speed sensor  48  indicative of an operating speed N MG2  of the second electric motor MG2; an output signal of an output speed sensor  50  indicative of a rotating speed N OUT  of the output gear  30 , which corresponds to a running speed V of the vehicle; an output signal of wheel speed sensors  52  indicative of rotating speeds N W  of wheels in the drive system  10 ; an output signal of a battery SOC sensor  54  indicative of a stored electric energy amount (state of charge) SOC of a battery not shown; an output signal of an ATF temperature sensor  62  indicative of a temperature Th ATF  of a working fluid in the drive system  10 ; an output signal of an MG1 temperature sensor  64  indicative of a temperature Th MG1  of the first electric motor MG1; and an output signal of an MG2 temperature sensor  66  indicative of a temperature Th MG2  of the second electric motor MG2. 
     The electronic control device  40  is also configured to generate various control commands to be applied to various portions of the drive system  10 . Namely, the electronic control device  40  applies to an engine control device  56  for controlling an output of the engine  12 , following engine output control commands for controlling the output of the engine  12 , which commands include: a fuel injection amount control signal to control an amount of injection of a fuel by a fuel injecting device into an intake pipe; an ignition control signal to control a timing of ignition of the engine  12  by an igniting device; and an electronic throttle valve drive control signal to control a throttle actuator for controlling an opening angle θ TH  of an electronic throttle valve. Further, the electronic control device  40  applies command signals to an inverter  58 , for controlling operations of the first electric motor MG1 and second electric motor MG2, so that the first and second electric motors MG1 and MG2 are operated with electric energies supplied thereto from a battery through the inverter  58  according to the command signals to control outputs (output torques) of the electric motors MG1 and MG2. Electric energies generated by the first and second electric motors MG1 and MG2 are supplied to and stored in the battery through the inverter  58 . Further, the electronic control device  40  applies command signals for controlling the operating states of the clutch CL and brake BK, to linear solenoid valves and other electromagnetic control valves provided in the hydraulic control unit  60 , so that hydraulic pressures generated by those electromagnetic control valves are controlled to control the operating states of the clutch CL and brake BK. 
     An operating state of the drive system  10  is controlled through the first electric motor MG1 and second electric motor MG2, such that the drive system  10  functions as an electrically controlled differential portion whose difference of input and output speeds is controllable. For example, an electric energy generated by the first electric motor MG1 is supplied to the battery or the second electric motor MG2 through the inverter  58 . Namely, a major portion of the drive force of the engine  12  is mechanically transmitted to the output gear  30 , while the remaining portion of the drive force is consumed by the first electric motor MG1 operating as the electric generator, and converted into the electric energy, which is supplied to the second electric motor MG2 through the inverter  58 , so that the second electric motor MG2 is operated to generate a drive force to be transmitted to the output gear  30 . Components associated with the generation of the electric energy and the consumption of the generated electric energy by the second electric motor MG2 constitute an electric path through which a portion of the drive force of the engine  12  is converted into an electric energy which is converted into a mechanical energy. 
     In the hybrid vehicle provided with the drive system  10  constructed as described above, one of a plurality of drive modes is selectively established according to the operating states of the engine  12 , first electric motor MG1 and second electric motor MG2, and the operating states of the clutch CL and brake BK.  FIG. 3  is the table indicating combinations of the operating states of the clutch CL and brake BK, which correspond to the respective five drive modes of the drive system  10 . In this table, “o” marks represent an engaged state while blanks represent a released state. The drive modes EV-1 and EV-2 indicated in  FIG. 3  are EV drive modes in which the engine  12  is held at rest while at least one of the first electric motor MG1 and second electric motor MG2 is used as a vehicle drive power source. The drive modes HV-1, HV-2 and HV-3 are hybrid drive modes (HV modes) in which the engine  12  is operated as the vehicle drive power source while the first electric motor MG1 and second electric motor MG2 are operated as needed to generate a vehicle drive force and/or an electric energy. In these hybrid drive modes, at least one of the first electric motor MG1 and second electric motor MG2 is operated to generate a reaction force or placed in a non-load free state. 
     As is apparent from  FIG. 3 , the EV drive modes of the drive system  10  in which the engine  12  is held at rest while at least one of the first electric motor MG1 and second electric motor MG2 is used as the vehicle drive power source consist of: a mode 1 (drive mode 1) in the form of the drive mode EV-1 which is established in the engaged state of the brake BK and in the released state of the clutch CL; and a mode 2 (drive mode 2) in the form of the drive mode EV-2 which is established in the engaged states of both of the brake BK and clutch CL. The hybrid drive modes in which the engine  12  is operated as the vehicle drive power source while the first electric motor MG1 and second electric motor MG2 are operated as needed to generate a vehicle drive force and/or an electric energy, consist of: a mode 3 (drive mode 3) in the form of the drive mode HV-1 which is established in the engaged state of the brake BK and in the released state of the clutch CL; a mode 4 (drive mode 4) in the form of the drive mode HV-2 which is established in the released state of the brake BK and in the engaged state of the clutch CL; and a mode 5 (drive mode 5) in the form of the drive mode HV-3 which is established in the released states of both of the brake BK and clutch CL. 
       FIGS. 4-7  are the collinear charts having straight lines which permit indication thereon of relative rotating speeds of the various rotary elements of the drive system  10  (first planetary gear set  14  and second planetary gear set  16 ), which rotary elements are connected to each other in different manners corresponding to respective combinations of the operating states of the clutch CL and brake BK. These collinear charts are defined in a two-dimensional coordinate system having a horizontal axis along which relative gear ratios ρ of the first and second planetary gear sets  14  and  16  are taken, and a vertical axis along which the relative rotating speeds are taken. The collinear charts indicate the relative rotating speeds when the output gear  30  is rotated in the positive direction to drive the hybrid vehicle in the forward direction. A horizontal line X1 represents the rotating speed of zero, while vertical lines Y1 through Y4 arranged in the order of description in the rightward direction represent the respective relative rotating speeds of the sun gear S1, sun gear S2, carrier C1 and ring gear R1. Namely, a solid line Y1 represents the relative rotating speed of the sun gear S1 of the first planetary gear set  14  (operating speed of the first electric motor MG1), a broken line Y2 represents the relative rotating speed of the sun gear S2 of the second planetary gear set  16  (operating speed of the second electric motor MG2), a solid line Y3 represents the relative rotating speed of the carrier C1 of the first planetary gear set  14  (operating speed of the engine  12 ), a broken line Y3′ represents the relative rotating speed of the carrier C2 of the second planetary gear set  16 , a solid line Y4 represents the relative rotating speed of the ring gear R1 of the first planetary gear set  14  (rotating speed of the output gear  30 ), and a broken line Y4′ represents the relative rotating speed of the ring gear R2 of the second planetary gear set  16 . In  FIGS. 4-7 , the vertical lines Y3 and Y3′ are superimposed on each other, while the vertical lines Y4 and Y4′ are superimposed on each other. Since the ring gears R1 and R2 are fixed to each other, the relative rotating speeds of the ring gears R1 and R2 represented by the vertical lines Y4 and Y4′ are equal to each other. 
     In  FIGS. 4-7 , a solid line L1 represents the relative rotating speeds of the three rotary elements of the first planetary gear set  14 , while a broken line L2 represents the relative rotating speeds of the three rotary elements of the second planetary gear set  16 . Distances between the vertical lines Y1-Y4 (Y2-Y4′) are determined by the gear ratios ρ1 and ρ2 of the first and second planetary gear sets  14  and  16 . Described more specifically, regarding the vertical lines Y1, Y3 and Y4 corresponding to the respective three rotary elements in the form of the sun gear S1, carrier C1 and ring gear R1 of the first planetary gear set  14 , a distance between the vertical lines Y1 and Y3 corresponds to “1”, while a distance between the vertical lines Y3 and Y4 corresponds to the gear ratio “ρ1”. Regarding the vertical lines Y2, Y3′ and Y4′ corresponding to the respective three rotary elements in the form of the sun gear S2, carrier C2 and ring gear R2 of the second planetary gear set  16 , a distance between the vertical lines Y2 and Y3′ corresponds to “1”, while a distance between the vertical lines Y3′ and Y4′ corresponds to the gear ratio “ρ2”. In the drive system  10 , the gear ratio ρ2 of the second planetary gear set  16  is higher than the gear ratio ρ1 of the first planetary gear set  14  (ρ2&gt;ρ1). The drive modes of the drive system  10  will be described by reference to  FIGS. 4-7 . 
     The drive mode EV-1 indicated in  FIG. 3  corresponds to the mode 1 (drive mode 1) of the drive system  10 , which is preferably the EV drive mode in which the engine  12  is held at rest while the second electric motor MG2 is used as the vehicle drive power source.  FIG. 4  is the collinear chart corresponding to the mode 1. Described by reference to this collinear chart, the carrier C1 of the first planetary gear set  14  and the carrier C2 of the second planetary gear set  16  are rotatable relative to each other in the released state of the clutch CL. In the engaged state of the brake BK, the carrier C2 of the second planetary gear set  16  is coupled (fixed) to the stationary member in the form of the housing  26 , so that the rotating speed of the carrier C2 is held zero. In this mode 1, the rotating direction of the sun gear S2 and the rotating direction of the ring gear R2 in the second planetary gear set  16  are opposite to each other, so that when the second electric motor MG2 is operated to generate a negative torque (acting in the negative direction), the ring gear R2, that is, the output gear  30  is rotated in the positive direction by the generated negative torque. Namely, the hybrid vehicle provided with the drive system  10  is driven in the forward direction when the negative torque is generated by the second electric motor MG2. In this case, the first electric motor MG1 is preferably held in a free state. In this mode 1, the carriers C1 and C2 are permitted to be rotated relative to each other, so that the hybrid vehicle can be driven in the EV drive mode similar to an EV drive mode which is established in a vehicle provided with a so-called “THS” (Toyota Hybrid System) and in which the carrier C2 is fixed to the stationary member. 
     The drive mode EV-2 indicated in  FIG. 3  corresponds to the mode 2 (drive mode 2) of the drive system  10 , which is preferably the EV drive mode in which the engine  12  is held at rest while at least one of the first electric motor MG1 and second electric motor MG2 is used as the vehicle drive power source.  FIG. 5  is the collinear chart corresponding to the mode 2. Described by reference to this collinear chart, the carrier C1 of the first planetary gear set  14  and the carrier C2 of the second planetary gear set  16  are not rotatable relative to each other in the engaged state of the clutch CL. Further, in the engaged state of the brake BK, the carrier C2 of the second planetary gear set  16  and the carrier C1 of the first planetary gear set  14  which is connected to the carrier C2 are coupled (fixed) to the stationary member in the form of the housing  26 , so that the rotating speeds of the carriers C1 and C2 are held zero. In this mode 2, the rotating direction of the sun gear S1 and the rotating direction of the ring gear R1 in the first planetary gear set  14  are opposite to each other, and the rotating direction of the sun gear S2 and the rotating direction of the ring gear R2 in the second planetary gear set  16  are opposite to each other, so that when the first electric motor MG1 and/or second electric motor MG2 is/are operated to generate a negative torque (acting in the negative direction), the ring gears R1 and R2 are rotated, that is, the output gear  30  is rotated in the positive direction by the generated negative torque. Namely, the hybrid vehicle provided with the drive system  10  is driven in the forward direction when the negative torque is generated by at least one of the first electric motor MG1 and second electric motor MG2. 
     In the mode 2, at least one of the first electric motor MG1 and second electric motor MG2 may be operated as the electric generator. In this case, one or both of the first and second electric motors MG1 and MG2 may be operated to generate a vehicle drive force (torque), at an operating point assuring a relatively high degree of operating efficiency, and/or with a reduced degree of torque limitation due to heat generation. Further, at least one of the first and second electric motors MG1 and MG2 may be held in a free state, when the generation of an electric energy by a regenerative operation of the electric motors MG1 and MG2 is inhibited due to full charging of the battery. Namely, the mode 2 is an EV drive mode in which amounts of work to be assigned to the first and second electric motors MG1 and MG2 can be adjusted with respect to each other, and which may be established under various running conditions of the hybrid vehicle, or may be kept for a relatively long length of time. Accordingly, the mode 2 is advantageously provided on a hybrid vehicle such as a plug-in hybrid vehicle, which is frequently placed in an EV drive mode. 
     The drive mode HV-1 indicated in  FIG. 3  corresponds to the mode 3 (drive mode 3) of the drive system  10 , which is preferably the HV drive mode in which the engine  12  is used as the vehicle drive power source while the first electric motor MG1 and second electric motor MG2 are operated as needed to generate a vehicle drive force and/or an electric energy.  FIG. 4  is the collinear chart corresponding to the mode 3. Described by reference to this collinear chart, the carrier C1 of the first planetary gear set  14  and the carrier C2 of the second planetary gear set  16  are rotatable relative to each other, in the released state of the clutch CL. In the engaged state of the brake BK, the carrier C2 of the second planetary gear set  16  is coupled (fixed) to the stationary member in the form of the housing  26 , so that the rotating speed of the carrier C2 is held zero. In this mode 3, the engine  12  is operated to generate an output torque by which the output gear  30  is rotated. At this time, the first electric motor MG1 is operated to generate a reaction torque in the first planetary gear set  14 , so that the output of the engine  12  can be transmitted to the output gear  30 . In the second planetary gear set  16 , the rotating direction of the sun gear S2 and the rotating direction of the ring gear R2 are opposite to each other, in the engaged state of the brake BK, so that when the second electric motor MG2 is operated to generate a negative torque (acting in the negative direction), the ring gears R1 and R2 are rotated, that is, the output gear  30  is rotated in the positive direction by the generated negative torque. 
     The drive mode HV-2 indicated in  FIG. 3  corresponds to the mode 4 (drive mode 4) of the drive system  10 , which is preferably the HV drive mode in which the engine  12  is used as the vehicle drive power source while the first electric motor MG1 and second electric motor MG2 are operated as needed to generate a vehicle drive force and/or an electric energy.  FIG. 6  is the collinear chart corresponding to the mode 4. Described by reference to this collinear chart, the carrier C1 of the first planetary gear set  14  and the carrier C2 of the second planetary gear set  16  are not rotatable relative to each other, in the engaged state of the clutch CL, that is, the carriers C1 and C2 are integrally rotated as a single rotary element. The ring gears R1 and R2, which are fixed to each other, are integrally rotated as a single rotary element. Namely, in the mode 4 of the drive system  10 , the first planetary gear set  14  and second planetary gear set  16  function as a differential mechanism having a total of four rotary elements. That is, the drive mode 4 is a composite split mode in which the four rotary elements consisting of the sun gear S1 (connected to the first electric motor MG1), the sun gear S2 (connected to the second electric motor MG2), the rotary element constituted by the carriers C1 and C2 connected to each other (and to the engine  12 ), and the rotary element constituted by the ring gears R1 and R2 fixed to each other (and connected to the output gear  30 ) are connected to each other in the order of description in the rightward direction as seen in  FIG. 6 . 
     In the mode 4, the rotary elements of the first planetary gear set  14  and second planetary gear set  16  are preferably arranged as indicated in the collinear chart of  FIG. 6 , that is, in the order of the sun gear S1 represented by the vertical line Y1, the sun gear S2 represented by the vertical line Y2, the carriers C1 and C2 represented by the vertical line Y3 (Y3′), and the ring gears R1 and R2 represented by the vertical line Y4 (Y4′). The gear ratios ρ1 and ρ2 of the first and second planetary gear sets  14  and  16  are determined such that the vertical line Y1 corresponding to the sun gear S1 and the vertical line Y2 corresponding to the sun gear S2 are positioned as indicated in the collinear chart of  FIG. 6 , namely, such that the distance between the vertical lines Y1 and Y3 is longer than the distance between the vertical lines Y2 and Y3′. In other words, the distance between the vertical lines corresponding to the sun gear S1 and the carrier C1 and the distance between the vertical lines corresponding to the sun gear S2 and the carrier C2 correspond to “1”, while the distance between the vertical lines corresponding to the carrier C1 and the ring gear R1 and the distance between the vertical lines corresponding to the carrier C2 and the ring gear R2 correspond to the respective gear ratios ρ1 and ρ2. Accordingly, the drive system  10  is configured such that the gear ratio ρ2 of the second planetary gear set  16  is higher than the gear ratio ρ1 of the first planetary gear set  14 . 
     In the mode 4, the carrier C1 of the first planetary gear set  14  and the carrier C2 of the second planetary gear set  16  are connected to each other in the engaged state of the clutch CL, so that the carriers C1 and C2 are rotated integrally with each other. Accordingly, either one or both of the first electric motor MG1 and second electric motor MG2 can receive a reaction force corresponding to the output of the engine  12 . Namely, one or both of the first and second electric motors MG1 and MG2 can be operated to receive the reaction force during an operation of the engine  12 , in other words, the amounts of work to be assigned to the first and second electric motors MG1 and MG2 can be adjusted with respect to each other. That is, in the mode 4, each of the first and second electric motors MG1 and MG2 can be operated at an operating point assuring a relatively high degree of operating efficiency, and/or with a reduced degree of torque limitation due to heat generation. 
     For example, one of the first electric motor MG1 and second electric motor MG2 which is operable with a higher degree of operating efficiency is preferentially operated to generate a reaction force, so that the overall operating efficiency can be improved. When the hybrid vehicle is driven at a comparatively high running speed V and at a comparatively low engine speed NE, for instance, the operating speed N MG1  of the first electric motor MG1 may have a negative value, that is, the first electric motor MG1 may be operated in the negative direction. In the case where the first electric motor MG1 generates the reaction force acting on the engine  12 , the first electric motor MG1 is operated in the negative direction so as to generate a negative torque with consumption of an electric energy, giving rise to a risk of reduction of the operating efficiency. In this respect, it will be apparent from  FIG. 6  that in the drive system  10 , the operating speed of the second electric motor MG2 indicated on the vertical line Y2 is less likely to have a negative value than the operating speed of the above-indicated first electric motor MG1 indicated on the vertical line Y1, and the second electric motor MG2 may possibly be operated in the positive direction, during generation of the reaction force. Accordingly, it is possible to improve the operating efficiency to improve the fuel economy, by preferentially controlling the second electric motor MG2 so as to generate the reaction force, while the operating speed of the first electric motor MG1 has a negative value. Further, where there is a torque limitation of one of the first electric motor MG1 and second electric motor MG2 due to heat generation, it is possible to ensure the generation of the reaction force required for the engine  12 , by controlling the other electric motor so as to perform a regenerative operation or a vehicle driving operation, for providing an assisting vehicle driving force. 
       FIG. 8  is the view for explaining transmission efficiency of the drive system  10 , wherein a speed ratio is taken along the horizontal axis while theoretical transmission efficiency is taken along the vertical axis. The speed ratio indicated in  FIG. 8  is a ratio of the input side speed of the first and second planetary gear sets  14  and  16  to the output side speed, that is, the speed reduction ratio, which is for example, a ratio of the rotating speed of the input rotary member in the form of the carrier C1 to the rotating speed of the output gear  30  (ring gears R1 and R2). The speed ratio is taken along the horizontal axis in  FIG. 8  such that the left side as seen in the view of  FIG. 8  is a side of high gear positions having comparatively low speed ratio values while the right side is a side of low gear positions having comparatively high speed ratio values. Theoretical transmission efficiency indicated in  FIG. 8  is a theoretical value of the transmission efficiency of the drive system  10 , which has a maximum value of 1.0 when an entirety of the drive force is mechanically transmitted from the first and second planetary gear sets  14  and  16  to the output gear  30 , without transmission of an electric energy through the electric path. 
     In  FIG. 8 , a one-dot chain line represents the transmission efficiency of the drive system  10  placed in the mode 3 (HV-1), while a solid line represents the transmission efficiency in the mode 4 (HV-2). As indicated in  FIG. 8 , the transmission efficiency of the drive system  10  in the mode 3 (HV-1) has a maximum value at a speed ratio value γ1. At this speed ratio value γ1, the operating speed of the first electric motor MG1 (rotating speed of the sun gear S1) is zero, and an amount of an electric energy transmitted through the electric path is zero during generation of the reaction force, so that the drive force is only mechanically transmitted from the engine  12  and the second electric motor MG2 to the output gear  30 , at an operating point corresponding to the speed ratio value γ1. This operating point at which the transmission efficiency is maximum while the amount of the electric energy transmitted through the electric path is zero will be hereinafter referred to as a “mechanical point (mechanical transmission point)”. The speed ratio value γ1 is lower than “1”, that is, a speed ratio on an overdrive side, and will be hereinafter referred to as a “first mechanical transmission speed ratio value γ1”. As indicated in  FIG. 8 , the transmission efficiency in the mode 3 gradually decreases with an increase of the speed ratio from the first mechanical transmission speed ratio value γ1 toward the low-gear side, and abruptly decreases with a decrease of the speed ratio from the first mechanical transmission speed ratio value γ1 toward the high-gear side. 
     In the mode 4 (HV-2) of the drive system  10 , the gear ratios ρ1 and ρ2 of the first planetary gear set  14  and second planetary gear set  16  having the four rotary elements in the engaged state of the clutch CL are determined such that the operating speeds of the first electric motor MG1 and second electric motor MG2 are indicated at respective different positions along the horizontal axis of the collinear chart of  FIG. 6 , so that the transmission efficiency in the mode 4 has a maximum value at a mechanical point at a speed ratio value γ2, as well as at the speed ratio value γ1, as indicated in  FIG. 8 . Namely, in the mode 4, the rotating speed of the first electric motor MG1 is zero at the first mechanical transmission speed ratio value γ1 at which the amount of the electric energy transmitted through the electric path is zero during generation of the reaction force by the first electric motor MG1, while the rotating speed of the second electric motor MG2 is zero at the speed ratio value γ2 at which the amount of the electric energy transmitted through the electric path is zero during generation of the reaction force by the second electric motor MG2. The speed ratio value γ2 will be hereinafter referred to as a “second mechanical transmission speed ratio value γ2”. This second mechanical transmission speed ratio value γ2 is smaller than the first mechanical transmission speed ratio value γ1. In the mode 4, the drive system  10  has the mechanical point located on the high-gear side of the mechanical point in the mode 3. 
     As indicated in  FIG. 8 , the transmission efficiency in the mode 4 more abruptly decreases with an increase of the speed ratio on a low-gear side of the first mechanical transmission speed ratio value γ1, than the transmission efficiency in the mode 3. In a region of the speed ratio between the first mechanical transmission speed ratio value γ1 and second mechanical transmission speed ratio value γ2, the transmission efficiency in the mode 4 changes along a concave curve. In this region, the transmission efficiency in the mode 4 is almost equal to or higher than that in the mode 3. The transmission efficiency in the mode 4 decreases with a decrease of the speed ratio from the second mechanical transmission speed ratio value γ2 toward the high-gear side, but is higher than that in the mode 3. That is, the drive system placed in the mode 4 has not only the first mechanical transmission speed ratio value γ1, but also the second mechanical transmission speed ratio value γ2 on the high-gear side of the first mechanical transmission speed ratio value γ1, so that the transmission efficiency of the drive system can be improved in high-gear positions having comparatively low speed ratio values. Thus, a fuel economy during running of the vehicle at a relatively high speed is improved owing to an improvement of the transmission efficiency. 
     As described above referring to  FIG. 8 , the transmission efficiency of the drive system  10  during a hybrid running of the vehicle with an operation of the engine  12  used as the vehicle drive power source and operations of the first and second electric motors MG1 and MG2 as needed to generate a vehicle drive force and/or an electric energy can be improved by adequately switching the vehicle drive mode between the mode 3 (HV-1) and mode 4 (HV-2). For instance, the mode 3 is established in low-gear positions having speed ratio values lower than the first mechanical transmission speed ratio value γ1, while the mode 4 is established in high-gear positions having speed ratio values higher than the first mechanical transmission speed ratio value γ1, so that the transmission efficiency can be improved over a wide range of the speed ratio covering the low-gear region and the high-gear region. 
     The drive mode HV-3 indicated in  FIG. 3  corresponds to the mode 5 (drive mode 5) of the drive system  10 , which is preferably the hybrid drive mode in which the engine  12  is operated as the vehicle drive power source while the first electric motor MG1 is operated as needed to generate a vehicle drive force and/or an electric energy. In this mode 5, the engine  12  and first electric motor MG1 may be operated to generate a vehicle drive force, with the second electric motor MG2 being disconnected from a drive system.  FIG. 7  is the collinear chart corresponding to this mode 5. Described by reference to this collinear chart, the carrier C  1  of the first planetary gear set  14  and the carrier C2 of the second planetary gear set  16  are rotatable relative to each other in the released state of the clutch CL. In the released state of the brake BK, the carrier C2 of the second planetary gear set  16  is rotatable relative to the stationary member in the form of the housing  26 . In this arrangement, the second electric motor MG2 can be held at rest while it is disconnected from the drive system (power transmitting path). 
     In the mode 3 in which the brake BK is placed in the engaged state, the second electric motor MG2 is kept in an operated state together with a rotary motion of the output gear  30  (ring gear R2) during running of the vehicle. In this operating state, the operating speed of the second electric motor MG2 may reach an upper limit value (upper limit) during running of the vehicle at a comparatively high speed, or a rotary motion of the ring gear R2 at a high speed is transmitted to the sun gear S2. In this respect, it is not necessarily desirable to keep the second electric motor MG2 in the operated state during running of the vehicle at a comparatively high speed, from the standpoint of the operating efficiency. In the mode 5, on the other hand, the engine  12  and the first electric motor MG1 may be operated to generate the vehicle drive force during running of the vehicle at the comparatively high speed, while the second electric motor MG2 is disconnected from the drive system, so that it is possible to reduce a power loss due to dragging of the unnecessarily operated second electric motor MG2, and to eliminate a limitation of the highest vehicle running speed corresponding to the permissible highest operating speed (upper limit of the operating speed) of the second electric motor MG2. 
     It will be understood from the foregoing description, the drive system  10  is selectively placed in one of the three hybrid drive modes in which the engine  12  is operated as the vehicle drive power source, namely, in one of the drive mode HV-1 (mode 3), drive mode HV-2 (mode 4) and drive mode HV-3 (mode 5), which are selectively established by respective combinations of the engaged and released states of the clutch CL and brake BK. Accordingly, the transmission efficiency can be improved to improve the fuel economy of the vehicle, by selectively establishing one of the three hybrid drive modes according to the vehicle running speed and the speed ratio, in which the transmission efficiency is the highest. 
       FIG. 9  is the functional block diagram for explaining major control functions of the electronic control device  40 . A drive mode determining portion  70 , shown in  FIG. 9 , is configured to determine one of the drive modes of the drive system  10  to be established. The drive mode determining portion  70  is basically configured to select one of the modes 1-5 described above by reference to  FIG. 3 , according to a predetermined relationship and on the basis of the accelerator pedal operation amount A CC  detected by the accelerator pedal operation amount sensor  42 , the vehicle running speed V corresponding to the output speed N OUT  detected by the output speed sensor  50 , and the battery SOC detected by the battery SOC sensor  54 , for example. When the battery SOC detected by the battery SOC sensor  54  is smaller than a predetermined threshold value, for instance, the drive mode determining portion  70  selects one of the hybrid drive modes in the form of the modes 3-5 in which the engine  12  is operated as the vehicle drive power source. Upon starting of the hybrid vehicle, namely, upon a releasing action of a brake pedal (not shown) (from the operated position to the non-operated position) when the vehicle running speed V corresponding to the output speed N OUT  detected by the output speed sensor  50  is zero while the battery SOC detected by the battery SOC sensor  54  is not smaller than the predetermined threshold value, for instance, the drive mode determining portion  70  selects the EV drive mode in the form of the mode 1 in which the engine  12  is held at rest while the first electric motor MG1 is primarily used as the vehicle drive power source. The drive mode determining portion  70  determines one of the drive modes according to the specific running state of the hybrid vehicle provided with the drive system  10 , so as to improve the transmission efficiency and the fuel economy of the engine  12 . 
     A clutch engagement control portion  72  is configured to control the operating state of the clutch CL through the hydraulic control unit  60 . For instance, the clutch engagement control portion  72  controls an output hydraulic pressure of an electromagnetic control valve provided in the hydraulic control unit  60  to control the clutch CL, so as to place the clutch CL in an engaged state or a released state. A brake engagement control portion  74  is configured to control the operating state of the brake BK through the hydraulic control unit  60 . For instance, the brake engagement control portion  74  controls an output hydraulic pressure of an electromagnetic control valve provided in the hydraulic control unit  60  to control the brake BK, so as to place the brake BK in an engaged state or a released state. The clutch engagement control portion  72  and the brake engagement control portion  74  are basically configured to control the operating states of the clutch CL and the brake BK to establish the drive mode selected by the drive mode determining portion  70 . Namely, the clutch and brake engagement control portions  72  and  74  establish one of the combinations of the operating states of the clutch CL and the brake BK indicated in  FIG. 3 , which corresponds to one of the modes 1-5 to be established. 
     The drive mode determining portion  70  implements a drive mode switching control on the basis of a temperature condition of the drive system in the drive system  10 . Preferably, the drive mode determining portion  70  switches the drive mode to the mode in which the clutch CL is placed in the engaged state, that is, to the mode 2 (EV-2) or mode 4 (HV-2), when the temperature of the drive system in the drive system  10  has become equal to or higher than a predetermined threshold value (reference value). The “temperature of the drive system in the drive system  10 ” is preferably a temperature of a given portion of the drive system  10 , or the temperature of the working fluid, and most preferably, a temperature regarding the first electric motor MG1 and the second electric motor MG2. Namely, the temperature of the drive system in the drive system  10  is preferably the first electric motor temperature Th MG1  detected by the MG1 temperature sensor  64 , or the second electric motor temperature Th MG2  detected by the MG2 temperature sensor  66 , but may be the ATF temperature Th ATF  detected by the ATF temperature sensor  62 . The temperature of the drive system in the drive system  10  may be determined by a difference of the first electric motor temperature Th MG1  detected by the MG1 temperature sensor  64  or the second electric motor temperature Th MG2  detected by the MG2 temperature sensor  66 , with respect to respective predetermined upper limits. The temperature of the drive system in the drive system  10  may be determined by a rate of change of the first electric motor temperature Th MG1  detected by the MG1 temperature sensor  64  or the second electric motor temperature Th MG2  detected by the MG2 temperature sensor  66 , or by a cumulative amount of load acting on the first electric motor MG1 or the second electric motor MG2. Further, the temperature of the drive system in the drive system  10  may be determined by taking account of a temperature of a cooling water of the engine  12 , or the ambient temperature. 
     Preferably, the drive mode determining portion  70  switches the drive mode from the mode in which the engine  12  is held at rest and the brake BK is placed in the engaged state while the clutch CL is placed in the released state, that is, from the mode 1 (EV-1), the mode in which the brake BK and the clutch CL are both placed in the engaged state, that is, to the mode 2 (EV-2), when the temperature Th MG2  of the second electric motor MG2 is equal to or higher than a predetermined threshold value Th A . That is, the drive mode determining portion  70  determines that the mode 2 should be established if the temperature Th MG2  of the second electric motor MG2 is equal to or higher than the predetermined threshold value Th A , even when the mode 1 is established according to the vehicle running speed V and the accelerator pedal operation amount A CC . The above-indicated threshold value Th A  corresponds to the upper limit of the temperature of the second electric motor MG2, preferably, a temperature lower than the upper limit by a predetermined margin value. When the temperature Th MG2  of the second electric motor MG2 is equal to or higher than the threshold value Th A , the operation (output torque) of the second electric motor MG2 is limited by establishing the mode 2 to assign required work to the first electric motor MG1 and the second electric motor MG2, for preventing an excessively large amount of generation of heat by either of these first and second electric motors MG1 and MG2. 
     The drive mode determining portion  70  may switch the drive mode from the mode 1 (EV-1) to the mode in which the brake BK and the clutch CL are both placed in the engaged state, that is, to the mode 2 (EV-2), when the ATF temperature Th ATF  has become equal to or higher than the predetermined threshold value (which is not necessarily equal to the above-indicated threshold value Th A ). When the ATF temperature Th ATF  is comparatively high, the temperature of the second electric motor MG2 (first electric motor MG1) is considered to be accordingly high, so that the mode 2 is established to assign the required work to the first electric motor MG1 and the second electric motor MG2, for thereby preventing an excessively large amount of generation by either of the electric motors MG1 and MG2. 
     Preferably, the drive mode determining portion  70  switches the drive mode from the mode in which the engine  12  is operated and the brake BK is placed in the engaged state while the clutch CL is placed in the released state, that is, from the mode 3 (HV-1), to the mode in which the brake BK is placed in the released state while and the clutch CL is placed in the engaged state, that is, to the mode 4 (HV-2), when the temperature Th MG1  of the first electric motor MG1 is equal to or higher than a predetermined threshold value Th B . That is, the drive mode determining portion  70  determines that the mode 4 should be established if the temperature Th MG1  of the first electric motor MG1 is equal to or higher than the predetermined threshold value Th B  even when the mode 3 is established according to the vehicle running speed V and the accelerator pedal operation amount A CC . The above-indicated threshold value Th B  corresponds to the upper limit of the temperature of the first electric motor MG1, preferably, a temperature lower than the upper limit by a predetermined margin value. When the temperature Th MG1  of the first electric motor MG1 is equal to or higher than the threshold value Th B , the operation (output torque) of the first electric motor MG1 is limited by establishing the mode 4 to assign the required work to the first electric motor MG1 and the second electric motor MG2, for preventing an excessively large amount of generation of heat by either of these first and second electric motors MG1 and MG2. 
     The drive mode determining portion  70  may switch the drive mode from the mode 3 (HV-1) to the mode in which the brake BK and the clutch CL are both placed in the engaged state, that is, to the mode 4 (HV-2), when the ATF temperature Th ATF  has become equal to or higher than the predetermined threshold value (which is not necessarily equal to the above-indicated threshold value Th B ). When the ATF temperature Th ATF  is comparatively high, the temperature of the first electric motor MG1 (second electric motor MG2) is considered to be accordingly high, so that the mode 4 is established to assign the required work to the first electric motor MG1 and the second electric motor MG2, for thereby preventing an excessively large amount of generation by either of the electric motors MG1 and MG2. 
     A work assignment ratio control portion  76  is configured to control a ratio of the amounts of work to be assigned to the first electric motor MG1 and the second electric motor MG2, in the hybrid drive modes in which the first and second electric motors MG1 and MG2 are operated as the vehicle drive power source. Preferably, the work assignment ratio control portion  76  controls the ratio of the amounts of work to be assigned to the first and second electric motors MG1 and MG2, on the basis of a result of comparison of the temperature Th MG1  of the first electric motor MG1 and the temperature Th MG2  of the second electric motor MG2 with each other, when a drive mode in which the clutch CL is placed in an engaged state is established. For example, the work assignment ratio control portion  76  compares the temperature Th MG1  of the first electric motor MG1 and the temperature Th MG2  of the second electric motor MG2 with each other, and controls the ratio such that the amount of work to be assigned to one of the electric motors the temperature of which is higher is reduced while the amount of work to be assigned to the other electric motor is increased. That is, if the temperature Th MG1  of the first electric motor MG1 is higher than the temperature Th MG2  of the second electric motor MG2 (Th MG1 &gt;Th MG2 ), the work assignment ratio control portion  76  reduces the amount of work to be assigned to the first electric motor MG1 and increases the amount of work to be assigned to the second electric motor MG2. If the temperature Th MG2  of the second electric motor MG2 is higher than the temperature Th MG1  of the first electric motor MG1 (Th MG2 &gt;Th MG1 ), the work assignment ratio control portion  76  reduces the amount of work to be assigned to the second electric motor MG2 and increases the amount of work to be assigned to the first electric motor MG1. Preferably, the work assignment ratio control portion  76  controls the ratio of the amounts of work to be assigned to the first and second electric motors MG1 and MG2 such that a ratio of the temperature Th MG1  of the first electric motor MG1 to the temperature Th MG2  of the second electric motor MG2 (=temperature Th MG1  of the first electric motor MG1/temperature Th MG2  of the second electric motor MG2) is inversely proportional to the ratio of the amounts of work to be assigned to the first and second electric motors MG1 and MG2 (=amount of work to be assigned to the first electric motor MG1/amount of work to be assigned to the second electric motor MG2). 
       FIG. 10  is the flow chart for explaining a major portion of an example of a drive mode switching control implemented by the electronic control device  40 . The drive mode switching control is repeatedly implemented with a predetermined cycle time. 
     The drive mode switching control is initiated with step SA1 (“step” being hereinafter omitted), to determine whether the drive mode in which the engine  12  is held at rest and the brake BK is placed in the engaged state while the clutch CL is placed in the released state, that is, the mode 1 (EV-1) is presently established. If a negative determination is obtained in SA1, the present control routine is terminated. If an affirmative determination is obtained in SA1, the control flow goes to SA2 to determine whether the temperature Th MG2  of the second electric motor MG2 is equal to or higher than the predetermined threshold value Th A . If a negative determination is obtained in SA2, the present control routine is terminated. If an affirmative determination is obtained in SA2, the control flow goes to SA3 to switch the drive mode to the drive mode in which the engine  12  is held at rest while the brake BK and the clutch CL are both placed in the engaged state, that is, to the mode 2 (EV-2), so that the required work is assigned to the first electric motor MG1 and the second electric motor MG2. Then, the present control routine is terminated. 
       FIG. 11  is the flow chart for explaining a major portion of another example of the drive mode switching control implemented by the electronic control device  40 . This drive mode switching control is repeatedly implemented with a predetermined cycle time. 
     The drive mode switching control is initiated with step SB1 to determine whether the drive mode in which the engine  12  is operated and the brake BK is placed in the engaged state while the clutch CL is placed in the released state, that is, the mode 3 (HV-1) is presently established. If a negative determination is obtained in SB1, the present control routine is terminated. If an affirmative determination is obtained in SB1, the control flow goes to SB2 to determine whether the temperature Th MG1  of the first electric motor MG1 is equal to or higher than the predetermined threshold value Th B . If a negative determination is obtained in SB2, the present control routine is terminated. If an affirmative determination is obtained in SB2, the control flow goes to SB3 to switch the drive mode to the drive mode in which the engine  12  is operated and the brake BK is placed in the released state while the clutch CL is placed in the engaged state, that is, to the mode 4 (HV-2), so that the required work is assigned to the first electric motor MG1 and the second electric motor MG2. Then, the present control routine is terminated. 
       FIG. 12  is the flow chart for explaining a major portion of an example of a work assignment ratio control implemented by the electronic control device  40 . The work assignment ratio control is repeatedly implemented with a predetermined cycle time. 
     The work assignment ratio control is initiated with SC1, to determine whether the drive mode in which the engine  12  is held at rest while the brake BK and the clutch CL are both placed in the engaged state, that is, the mode 2 (EV-2) is presently established. If a negative determination is obtained in SC  1 , the present control routine is terminated. If an affirmative determination is obtained in SC  1 , the control flow goes to SC2 to determine whether the temperature Th MG2  of the second electric motor MG2 is higher than the temperature Th MG1  of the first electric motor MG1 (Th MG2 &gt;Th MG1 ). If an affirmative determination is obtained in SC2, the control flow goes to SC3 to reduce the amount of work to be assigned to the second electric motor MG2 and increase the amount of work to be assigned to the first electric motor MG1. Then, the present control routine is terminated. If a negative determination is obtained in SC2, for instance, if the temperature Th MG1  of the first electric motor MG1 is higher than the temperature Th MG2  of the second electric motor MG2 (Th MG1 &gt;Th MG2 ), the control flow goes to SC4 to reduce the amount of work to be assigned to the first electric motor MG1 and increase the amount of work to be assigned to the second electric motor MG2. Then, the present control routine is terminated. 
       FIG. 13  is the flow chart for explaining a major portion of another example of the work assignment ratio control implemented by the electronic control device  40 . This work assignment ratio control is repeatedly implemented with a predetermined cycle time. 
     The work assignment ratio control is initiated with SD1 to determine whether the drive mode in which the engine  12  is operated and the brake BK is placed in the released state while the clutch CL is placed in the engaged state, that is, the mode 4 (HV-2) is presently established. If a negative determination is obtained in SD1, the present control routine is terminated. If an affirmative determination is obtained in SD  1 , the control flow goes to SD2 to determine whether the temperature Th MG2  of the second electric motor MG2 is higher than the temperature Th MG1  of the first electric motor MG1 (Th MG2 &gt;Th MG1 ). If an affirmative determination is obtained in SD2, the control flow goes to SD3 to reduce the amount of work to be assigned to the second electric motor MG2 and increase the amount of work to be assigned to the first electric motor MG1. Then, the present control routine is terminated. If a negative determination is obtained in SD2, for instance, if the temperature Th MG1  of the first electric motor MG1 is higher than the temperature Th MG2  of the second electric motor MG2 (Th MG1 &gt;Th MG2 ), the control flow goes to SD4 to reduce the amount of work to be assigned to the first electric motor MG1 and increase the amount of work to be assigned to the second electric motor MG2. Then, the present control routine is terminated. 
     It will be understood from the foregoing description by reference to  FIGS. 10-13  that SA1, SA3, SB1, SB3, SC1 and SD1 correspond to the operation of the drive mode determining portion  70 , and SA3 and SB3 correspond to the operations of the clutch engagement control portion  72  and the brake engagement control portion  74 , while SC2-SC4 and SD2-SD4 correspond to the operation of the work assignment ratio control portion  76 . 
     Other preferred embodiments of the present invention will be described in detail by reference to the drawings. In the following description, the same reference signs will be used to identify the same elements in the different embodiments, which will not be described redundantly. 
     Second Embodiment 
       FIG. 14  is the schematic view for explaining an arrangement of a hybrid vehicle drive system  100  (hereinafter referred to simply as a “drive system  100 ”) according to another preferred embodiment of this invention. In this drive system  100  shown in  FIG. 14 , the second planetary gear set  16 , clutch CL and brake BK are disposed on one side of the first planetary gear set  14  remote from the engine  12 , such that the second electric motor MG2 is interposed between the first planetary gear set  14 , and the second planetary gear set  16 , clutch CL and brake BK, in the axial direction of the center axis CE. Preferably, the clutch CL and brake BK are disposed at substantially the same position in the axial direction of the center axis CE. That is, the drive system  100  is configured such that the first electric motor MG1, first planetary gear set  14 , second electric motor MG2, second planetary gear set  16 , clutch CL, and brake BK are disposed coaxially with each other, in the order of description from the side of the engine  12 , in the axial direction of the center axis CE. The hybrid vehicle drive control device according to the present invention is equally applicable to the present drive system  100  configured as described above. 
     Third Embodiment 
       FIG. 15  is a schematic view for explaining an arrangement of a hybrid vehicle drive system  110  (hereinafter referred to simply as a “drive system  110 ”) according to a further preferred embodiment of this invention. In this drive system  110  shown in  FIG. 15 , the first planetary gear set  14 , clutch CL, second planetary gear set  16  and brake BK which constitute a mechanical system are disposed on the side of the engine  12 , while the first electric motor MG1 and second electric motor MG2 which constitute an electric system are disposed on one side of the mechanical system remote from the engine  12 . That is, the drive system  110  is configured such that the first planetary gear set  14 , clutch CL, second planetary gear set  16 , brake BK, second electric motor MG2, and first electric motor MG1 are disposed coaxially with each other, in the order of description from the side of the engine  12 , in the axial direction of the center axis CE. The hybrid vehicle drive control device according to the present invention is equally applicable to the present drive system  110  configured as described above. 
     Fourth Embodiment 
       FIG. 16  is the schematic view for explaining an arrangement of a hybrid vehicle drive system  120  (hereinafter referred to simply as a “drive system  120 ”) according to a still further preferred embodiment of this invention. In this drive system  120  shown in  FIG. 16 , a one-way clutch OWC is disposed in parallel with the brake BK, between the carrier C2 of the second planetary gear set  16  and the stationary member in the form of the above-indicated housing  26 . The one-way clutch OWC permits a rotary motion of the carrier C2 in one of opposite directions relative to the housing  26 , and inhibits a rotary motion of the carrier C2 in the other direction. Preferably, this one-way clutch OWC permits the rotary motion of the carrier C2 in the positive or forward direction relative to the housing  26 , and inhibits the rotary motion of the carrier C2 in the negative or reverse direction. Namely, in a drive state where the carrier C2 is rotated in the negative direction, that is, where the second electric motor MG2 is operated to generate a negative torque, for example, the modes 1-3 can be established without the engaging action of the brake BK. The hybrid vehicle drive control device according to the present invention is equally applicable to the present drive system  120  configured as described above. 
     Fifth Embodiment 
       FIG. 17  is the schematic view for explaining an arrangement of a hybrid vehicle drive system  130  (hereinafter referred to simply as a “drive system  130 ”) according to a yet further preferred embodiment of this invention. This drive system  130  shown in  FIG. 17  is provided with a second differential mechanism in the form of a double-pinion type second planetary gear set  16 ′ disposed on the center axis CE, in place of the single-pinion type second planetary gear set  16 . This second planetary gear set  16 ′ is provided with rotary elements (elements) consisting of: a first rotary element in the form of a sun gear S2′; a second rotary element in the form of a carrier C2′ supporting a plurality of pinion gears P2′ meshing with each other such that each pinion gear P2′ is rotatable about its axis and the axis of the planetary gear set; and a third rotary element in the form of a ring gear R2′ meshing with the sun gear S2′ through the pinion gears P2′. 
     The ring gear R1 of the first planetary gear set  14  is connected to the output rotary member in the form of the output gear  30 , and to the carrier C2′ of the second planetary gear set  16 ′. The sun gear S2′ of the second planetary gear set  16 ′ is connected to the rotor  24  of the second electric motor MG2. Between the carrier C1 of the first planetary gear set  14  and the ring gear R2′ of the second planetary gear set  16 ′, there is disposed the clutch CL which is configured to selectively couple these carrier C1 and ring gear R2′ to each other (to selectively connect the carrier C1 and ring gear R2′ to each other or disconnect the carrier C1 and ring gear R2′ from each other). Between the ring gear R2′ of the second planetary gear set  16 ′ and the stationary member in the form of the housing  26 , there is disposed the brake BK which is configured to selectively couple (fix) the ring gear R2′ to the housing  26 . 
     As shown in  FIG. 17 , the drive system  130  is configured such that the first planetary gear set  14  and second planetary gear set  16 ′ are disposed coaxially with the input shaft  28 , and opposed to each other in the axial direction of the center axis CE. Namely, the first planetary gear set  14  is disposed on one side of the second planetary gear set  16 ′ on the side of the engine  12 , in the axial direction of the center axis CE. The first electric motor MG1 is disposed on one side of the first planetary gear set  14  on the side of the engine  12 , in the axial direction of the center axis CE. The second electric motor MG2 is disposed on one side of the second planetary gear set  16 ′ which is remote from the engine  12 , in the axial direction of the center axis CE. Namely, the first electric motor MG1 and second electric motor MG2 are opposed to each other in the axial direction of the center axis CE, such that the first planetary gear set  14  and second planetary gear set  16 ′ are interposed between the first electric motor MG1 and second electric motor MG2. That is, the drive system  130  is configured such that the first electric motor MG1, first planetary gear set  14 , clutch CL, second planetary gear set  16 ′, second electric motor MG2, and brake BK are disposed coaxially with each other, in the order of description from the side of the engine  12 , in the axial direction of the center axis CE. The hybrid vehicle drive control device according to the present invention is equally applicable to the present drive system  130  configured as described above. 
     Sixth Embodiment 
       FIG. 18  is the schematic view for explaining an arrangement of a hybrid vehicle drive system  140  (hereinafter referred to simply as a “drive system  140 ”) according to still another preferred embodiment of this invention. In this drive system  140  shown in  FIG. 18 , the second planetary gear set  16 ′, clutch CL and brake BK are disposed on one side of the first planetary gear set  14  remote from the engine  12 , such that the second electric motor MG2 is interposed between the first planetary gear set  14 , and the second planetary gear set  16 ′, clutch CL and brake BK, in the axial direction of the center axis CE. Preferably, the clutch CL and brake BK are disposed at substantially the same position in the axial direction of the center axis CE. That is, the drive system  140  is configured such that the first electric motor MG1, first planetary gear set  14 , second electric motor MG2, second planetary gear set  16 ′, clutch CL, and brake BK are disposed coaxially with each other, in the order of description from the side of the engine  12 , in the axial direction of the center axis CE. The hybrid vehicle drive control device according to the present invention is equally applicable to the present drive system  140  configured as described above. 
     Seventh Embodiment 
       FIG. 19  is the schematic view for explaining an arrangement of a hybrid vehicle drive system  150  (hereinafter referred to simply as a “drive system  150 ”) according to yet another preferred embodiment of this invention. In this drive system  150  shown in  FIG. 19 , the first electric motor MG1 and second electric motor MG2 which constitute an electric system are disposed on the side of the engine  12  with regard to the direction of the center axis CE, while the second planetary gear set  16 ′, first planetary gear set  14 , clutch CL, and brake BK which constitute a mechanical system are disposed on one side of the electric system remote from the engine  12 . Preferably, the clutch CL and the brake BK are positioned at substantially the same position in the direction of the center axis CE. That is, the drive system  150  is configured such that the first electric motor MG1, second electric motor MG2, second planetary gear set  16 ′, first planetary gear set  14 , clutch CL, and brake BK are disposed coaxially with each other, in the order of description from the side of the engine  12 , in the axial direction of the center axis CE. The hybrid vehicle drive control device according to the present invention is equally applicable to the present drive system  150  configured as described above. 
     Eighth Embodiment 
       FIGS. 20-22  are the collinear charts for explaining arrangements and operations of respective hybrid vehicle drive systems  160 ,  170  and  180  according to other preferred embodiments of this invention in place of the drive system  10 . In  FIGS. 20-22 , the relative rotating speeds of the sun gear S1, carrier C1 and ring gear R1 of the first planetary gear set  14  are represented by the solid line L1, while the relative rotating speeds of the sun gear S2, carrier C2 and ring gear R2 of the second planetary gear set  16  are represented by the broken line L2, as in  FIGS. 4-7 . In the drive system  160  for the hybrid vehicle shown in  FIG. 20 , the sun gear S1, carrier C1 and ring gear R1 of the first planetary gear set  14  are respectively connected to the first electric motor MG1, engine  12  and second electric motor MG2, while the sun gear S2, carrier C2 and ring gear R2 of the second planetary gear set  16  are respectively connected to the second electric motor MG2 and output gear  30 , and to the housing  26  through the brake BK. The sun gear S1 and the ring gear R2 are selectively connected to each other through the clutch CL. The ring gear R1 and the sun gear S2 are connected to each other. In the drive system  170  for the hybrid vehicle shown in  FIG. 21 , the sun gear S1, carrier C1 and ring gear R1 of the first planetary gear set  14  are respectively connected to the first electric motor MG1, output gear  30  and engine  12 , while the sun gear S2, carrier C2 and ring gear R2 of the second planetary gear set  16  are respectively connected to the second electric motor MG2 and output gear  30 , and to the housing  26  through the brake BK. The sun gear S1 and the ring gear R2 are selectively connected to each other through the clutch CL. The clutches C1 and C2 are connected to each other. In the drive system  180  for the hybrid vehicle shown in  FIG. 22 , the sun gear S1, carrier C1 and ring gear R1 of the first planetary gear set  14  are respectively connected to the first electric motor MG1, output gear  30  and engine  12 , while the sun gear S2, carrier C2 and ring gear R2 of the second planetary gear set  16  are respectively connected to the second electric motor MG2, to the housing  26  through the brake BK, and to the output gear  30 . The ring gear R1 and the carrier C2 are selectively connected to each other through the clutch CL. The carrier C1 and ring gear R2 are connected to each other. 
     The drive systems for the hybrid vehicle shown in  FIGS. 20-22  are identical with each other in that each of these drive systems for the hybrid vehicle is provided with the first differential mechanism in the form of the first planetary gear set  14  and the second differential mechanism in the form of the second planetary gear set  16 ,  16 ′, which have four rotary elements (whose relative rotating speeds are represented) in the collinear chart, and is further provided with the first electric motor MG1, second electric motor MG2, engine  12  and output rotary member (output gear  30 ) which are connected to the respective four rotary elements, and wherein one of the four rotary elements is constituted by the rotary element of the first planetary gear set  14  and the rotary element of the second planetary gear set  16 ,  16 ′ which are selectively connected to each other through the clutch CL, and the rotary element of the second planetary gear set  16 ,  16 ′ selectively connected to the rotary element of the first planetary gear set  14  through the clutch CL is selectively fixed to the housing  26  as the stationary member through the brake BK, as in the drive system for the hybrid vehicle shown in  FIGS. 4-7 . Namely, the hybrid vehicle drive control device of the present invention described above by reference to  FIG. 9  and the other figures is suitably applicable to the drive systems shown in  FIGS. 20-22 . 
     As described above, the illustrated embodiments are configured such that the hybrid vehicle is provided with: the first differential mechanism in the form of the first planetary gear set  14  and the second differential mechanism in the form of the second planetary gear set  16 ,  16 ′, which have the four rotary elements as a whole when the clutch CL is in an engaged state (and thus the first planetary gear set  14  and the second planetary gear set  16 ,  16 ′ are described as the four rotary elements in the collinear charts such as  FIGS. 4 to 7  and the like); and the engine  12 , the first electric motor MG1, the second electric motor MG2 and the output rotary member in the form of the output gear  30  which are respectively connected to the four rotary elements. One of the four rotary elements is constituted by the rotary element of the above-described first differential mechanism and the rotary element of the above-described second differential mechanism which are selectively connected to each other through the clutch CL, and one of the rotary elements of the first and second differential mechanisms which are selectively connected to each other through the clutch CL is selectively fixed to the stationary member in the form of the housing  26  through the brake BK. The hybrid vehicle is selectively placed in the plurality of drive modes according to the respective combinations of the engaged and released states of the clutch CL and brake BK. The drive control device is configured to switch the drive mode on the basis of the temperature condition of its drive system. Accordingly, the hybrid vehicle can be suitably placed in the drive mode in which both of the first and second electric motors MG1 and MG2 are used as the vehicle drive power source, on the basis of the temperature condition of the drive system. Namely, the illustrated embodiments provide the electronic control device  40  as the drive control device for the hybrid vehicle, which permits reduction of heat generation in the drive system, for assuring optimum running of the hybrid vehicle. 
     The drive mode is switched to the drive mode in which the clutch CL is placed in the engaged state, when the temperature of the drive system has become equal to or higher than the predetermined threshold value. Accordingly, it is possible to reduce the heat generation in the drive system, for assuring optimum running of the hybrid vehicle, by placing the drive system in the drive mode in which the required work is assigned to the above-described first and second electric motors MG1 and MG2, when the temperature of the drive system is comparatively high. 
     The drive mode is switched from the mode 1 (EV-1) that is a drive mode in which the engine  12  is held at rest, and the brake BK is placed in the engaged state while the clutch CL is placed in the released state, to the mode 2 (EV-2) that is a drive mode in which the brake BK and the clutch CL are both placed in the engaged state, when the temperature Th MG2  of the second electric motor MG2 has become equal to or higher than the predetermined threshold value Th A . Accordingly, it is possible to reduce the heat generation in the second electric motor MG2, for assuring optimum running of the hybrid vehicle, by placing the drive system in the drive mode in which the required work is assigned to the above-described first and second electric motors MG1 and MG2, when the temperature of the second electric motor MG2 is comparatively high. 
     The drive mode is switched from the mode 3 (HV-1) that is a drive mode in which the engine  12  is operated, and the brake BK is placed in the engaged state while the clutch CL is placed in the released state, to the mode 4 (HV-2) that is a drive mode in which the brake BK is placed in the released state while the clutch CL is placed in the engaged state, when the temperature Th MG1  of the first electric motor MG1 has become equal to or higher than the predetermined threshold value Th B . Accordingly, it is possible to reduce the heat generation in the first electric motor MG1, for assuring optimum running of the hybrid vehicle, by placing the drive system in the drive mode in which the required work is assigned to the above-described first and second electric motors MG1 and MG2, when the temperature of the first electric motor MG1 is comparatively high. 
     The ratio of the amounts of work to be assigned to the first electric motor MG1 and the second electric motor MG2 in the drive mode in which the clutch CL is placed in the engaged state is controlled on the basis of the result of comparison of the temperatures Th MG1  and Th MG2  of the first and second electric motors MG1 and MG2 with each other. Accordingly, it is possible to the control the amounts of work to be assigned to the first and second electric motors MG1 and MG2, while taking account of the heat generation by the first and second electric motors MG1 and MG2. 
     The temperatures Th MG1  and Th MG2  of the first and second electric motors MG1 and MG2 are compared with each other, and the amount of work to be assigned to one of the first and second electric motors MG1 and MG2 the temperature of which is higher is reduced, while the amount of work to be assigned to the other of the first and second electric motors MG1 and MG2 is increased. Accordingly, it is possible to control the amounts of work to be assigned to the first and second electric motors MG1 and MG2, while taking account of the heat generation by the first and second electric motors MG1 and MG2. 
     The first planetary gear set  14  is provided with a first rotary element in the form of the sun gear S1 connected to the first electric motor MG1, a second rotary element in the form of the carrier C1 connected to the engine  12 , and a third rotary element in the form of the ring gear R1 connected to the output gear  30 , while the second planetary gear set  16  ( 16 ′) is provided with a first rotary element in the form of the sun gear S2 (S2′) connected to the second electric motor MG2, a second rotary element in the form of the carrier C2 (C2′), and a third rotary element in the form of the ring gear R2 (R2′), one of the carrier C2 (C2′) and the ring gear R2 (R2′) being connected to the ring gear R1 of the first planetary gear set  14 . The clutch CL is configured to selectively connect the carrier C1 of the first planetary gear set  14  and the other of the carrier C2 (C2′) and the ring gear R2 (R2′) which is not connected to the ring gear R1, to each other, while the brake BK is configured to selectively fix the other of the carrier C2 (C2′) and the ring gear R2 (R2′) which is not connected to the ring gear R1, to a stationary member in the form of the housing  26 . Accordingly, it is possible to reduce the heat generation in the drive system, for assuring optimum running of the hybrid vehicle the drive system  10  of which has a highly practical arrangement. 
     While the preferred embodiments of this invention have been described by reference to the drawings, it is to be understood that the invention is not limited to the details of the illustrated embodiments, but may be embodied with various changes which may occur without departing from the spirit of the invention. 
     NOMENCLATURE OF REFERENCE SIGNS 
     
         
           10 ,  100 ,  110 ,  120 ,  130 ,  140 ,  150 ,  160 ,  170 ,  180 : Hybrid vehicle drive system 
           12 : Engine 
           14 : First planetary gear set (First differential mechanism) 
           16 ,  16 ′: Second planetary gear set (Second differential mechanism) 
           18 ,  22 : Stator 
           20 ,  24 : Rotor 
           26 : Housing (Stationary member) 
           28 : Input shaft 
           30 : Output gear (Output rotary member) 
           32 : Oil pump 
           40 : Electronic control device (Drive control device) 
           42 : Accelerator pedal operation amount sensor 
           44 : Engine speed sensor 
           46 : MG1 speed sensor 
           48 : MG2 speed sensor 
           50 : Output speed sensor 
           52 : Wheel speed sensors 
           54 : Battery SOC sensor 
           56 : Engine control device 
           58 : Inverter 
           60 : Hydraulic control unit 
           62 : ATF temperature sensor 
           64 : MG1 temperature sensor 
           66 : MG2 temperature sensor 
           70 : Drive mode determining portion 
           72 : Clutch engagement control portion 
           74 : Brake engagement control portion 
           76 : Work assignment ratio control portion 
         BK: Brake 
         CL: Clutch 
         C1, C2, C2′: Carrier (Second rotary element) 
         MG1: First electric motor 
         MG2: Second electric motor 
         OWC: One-way clutch 
         P1, P2, P2′: Pinion gear 
         R1, R2, R2′: Ring gear (Third rotary element) 
         S1, S2, S2′: Sun gear (First rotary element)