Patent Publication Number: US-2015088350-A1

Title: Drive control device for hybrid vehicle

Description:
TECHNICAL FIELD 
     The present invention relates to an improvement of a drive control device for a 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 
     Patent Document 2: JP-4038183 B2 
     SUMMARY OF THE INVENTION 
     Object Achieved by the Invention 
     According to the conventional arrangement of the hybrid vehicle described above, however, the engine cannot generate a vehicle drive force in the event of a failure of the first electric motor. In this event, therefore, the hybrid vehicle is run in an EV drive mode as a substitutive drive mode, with the second electric motor being primarily operated as a vehicle drive power source. But, the running of the hybrid vehicle in the substitutive drive mode cannot be continued after an entire amount of an electric energy stored in a vehicle driving battery is consumed, so that a maximum distance of running of the hybrid vehicle in the substitutive drive mode is undesirably limited. 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 the hybrid vehicle to be run in an adequate substitutive drive mode in the event of a failure of an electric motor. 
     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 the above-described 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 drive control device being characterized by implementing, in the event of a failure of the above-described first electric motor, an substitutive drive control wherein the above-described engine is operated to generate a vehicle drive force while the above-described second electric motor receives a reaction force of the vehicle drive force. 
     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 the 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 drive control device is configured to implement, in the event of a failure of the above-described first electric motor, the substitutive drive control wherein the above-described engine is operated to generate the vehicle drive force while the above-described second electric motor receives the reaction force of the vehicle drive force. Accordingly, the hybrid vehicle can be run in the substitutive drive mode, with a torque transmitted directly from the above-described engine, to an extent of the reaction force received by the above-described second electric motor. Namely, the present invention provides a drive control device for a hybrid vehicle, which permits the hybrid vehicle to be run in an adequate substitutive drive mode in the event of a failure of an electric motor. 
     According to a second aspect of the invention, the drive control device according to the first aspect of the invention is configured such that the above-described clutch is placed in an engaged state while the above-described brake is placed in a released state, in the above-described substitutive drive control. According to this second aspect of the invention, the above-described second electric motor can receive the reaction force of the vehicle drive force generated by the above-described engine, in a highly practical manner, in the event of the failure of the above-described first electric motor. 
     According to a third aspect of the invention, the drive control device according to the first or second aspect of the invention is configured such that the above-described brake is placed in a slipping state when an excessive speed rise of at least one of the above-described engine and the above-described first electric motor is estimated. According to this third aspect of the invention, it is possible to effectively prevent the excessive speed rise immediately after a moment of determination of the failure of the above-described first electric motor. 
     According to a fourth aspect of the invention, the drive control device according to the first or second aspect of the invention, or according to the third aspect of the invention according to the first or second aspect of the invention is configured such that the above-described brake is placed in a slipping state when it is estimated that an input upper limit of a vehicle driving battery is smaller than a predetermined threshold value. According to this fourth aspect of the invention, it is possible to effectively prevent an excessive amount of input to the above-described vehicle driving battery immediately after a moment of determination of the failure of the above-described first electric motor. 
     According to a fifth aspect of the invention, the drive control device according to the first or second aspect of the invention, or according to the third aspect of the invention according to the first or second aspect of the invention, or according to the fourth aspect of the invention according to the first or second aspect of the invention, or according to the fourth aspect of the invention according to the third aspect of the invention according to the first or second aspect of the invention is configured such that a torque of the above-described second electric motor is controlled so as to prevent an excessive speed rise of at least one of the above-described engine and the above-described first electric motor, when it is estimated that an input upper limit of a vehicle driving battery is equal to or larger than a predetermined threshold value, while the above-indicated excessive speed rise is estimated. According to this fifth aspect of the invention, it is possible to effectively prevent the excessive speed rise immediately after a moment of determination of the failure of the above-described first electric motor. 
     According to a sixth aspect of the invention, the drive control device according to the first or second aspect of the invention, or according to the third aspect of the invention according to the first or second aspect of the invention, or according to the fourth aspect of the invention according to the first or second aspect of the invention, or according to the fourth aspect of the invention according to the third aspect of the invention according to the first or second aspect of the invention, or according to the fifth aspect of the invention according to the first or second aspect of the invention, or according to the fifth aspect of the invention according to the third aspect of the invention according to the first or second aspect of the invention, or according to the fifth aspect of the invention according to the fourth aspect of the invention according to the first or second aspect of the invention, or according to the fifth aspect of the invention according to the fourth aspect of the invention according to the third aspect of the invention according to the first or second aspect 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 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 sixth aspect of the invention, the hybrid vehicle can be run in an adequate substitutive drive mode in the event of a failure of an electric motor in the drive system of the hybrid vehicle, 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 functional block diagram for explaining major control functions of an electronic control device of  FIG. 2 ; 
         FIG. 9  is a collinear chart for explaining a substitutive drive control implemented by the electronic control device of  FIG. 2  in the event of a failure of a first electric motor; 
         FIG. 10  is a collinear chart for explaining an excessive speed rise preventive control in the substitutive drive control implemented by the electronic control device of  FIG. 2  in the event of the failure of the first electric motor; 
         FIG. 11  is a collinear chart for explaining an excessive speed rise preventive control in the substitutive drive control implemented by the electronic control device of  FIG. 2  in the event of the failure of the first electric motor; 
         FIG. 12  is a flow chart for explaining a major portion of one example of the substitutive drive control implemented by the electronic control device of  FIG. 2 ; 
         FIG. 13  is a schematic view for explaining an arrangement of a hybrid vehicle drive system according to another preferred embodiment of this invention; 
         FIG. 14  is a schematic view for explaining an arrangement of a hybrid vehicle drive system according to a further preferred embodiment of this invention; 
         FIG. 15  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. 16  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. 17  is a schematic view for explaining an arrangement of a hybrid vehicle drive system according to still another preferred embodiment of this invention; 
         FIG. 18  is a schematic view for explaining an arrangement of a hybrid vehicle drive system according to yet another preferred embodiment of this invention; 
         FIG. 19  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; 
         FIG. 20  is a collinear chart for explaining an arrangement and an operation of a hybrid vehicle drive system according to a yet further preferred embodiment of this invention; and 
         FIG. 21  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-drive) 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 ρ1 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 N E , 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 ; and an output signal of a battery SOC sensor  54  indicative of a stored electric energy amount (state of charge) SOC of a battery  55 . 
     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 the battery  55  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  55  through the inverter  58 . That is, the battery  55  corresponds to a vehicle driving battery in the drive system  10 . 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  55 . 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 , and 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 and second electric motor MG1 and MG2 which is operable with a high degree of operating efficiency is preferentially operated to generate a reaction force, so that the overall operating efficiency can be improved. 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. 
     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 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 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. 8  is the functional block diagram for explaining major control functions of the electronic control device  40 . An electric motor operation control portion  70 , shown in  FIG. 8 , is configured to control the operations of the first and second electric motors MG1 and MG2 through the inverter  58 . Described more specifically, the electric motor operation control portion  70  controls the amounts of electric energy to be supplied from the battery  55  to the first and second electric motors MG1 and MG2 through the inverter  58 , so that each of the first and second electric motors MG1 and MG2 provides a required output, that is, a target torque (target electric motor output). When the first or second electric motor MG1, MG2 is operated as an electric generator, the electric motor operation control portion  70  implements a control for storing an electric energy generated by the first or second electric motor MG1, MG2, in the battery  55  through the inverter  58 . 
     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 according to the running state of the hybrid vehicle as described above. 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. 
     An MG1 failure determining portion  76  is configured to determine a failure of the first electric motor MG1. Namely, the MG1 failure determining portion  76  determines whether the first electric motor MG1 has an operational defect preventing its normal operation. For example, the MG1 failure determining portion  76  makes the determination as to whether the first electric motor MG1 has an operational defect, on the basis of a difference of the first electric motor speed N MG1  detected by the MG1 speed sensor  46 , with respect to a value of a command generated from the electric motor operation control portion  70  to specify an output of the first electric motor MG1. Described more specifically, the MG1 failure determining portion  76  determines a failure of the first electric motor MG1, if the difference of the first electric motor speed N MG1  detected by the MG1 speed sensor  46  with respect to the value of the command generated from the electric motor operation control portion  70  to specify the output of the first electric motor MG1 has become equal to or larger than a predetermined value. Preferably, the MG1 failure determining portion  76  determines an abnormality of the first planetary gear set  14 . The MG1 failure determining portion  76  is preferably configured to determine whether the first planetary gear set  14  has an operational defect that prevents normal transmission of an output of the engine  12  to the output gear  30 . For example, the MG1 failure determining portion  76  makes the determination as to whether the first planetary gear set  14  has an operational defect, on the basis of the engine speed N E  detected by the engine speed sensor  44 , or the output speed N OUT  detected by the output speed sensor  50 , as well as the gear ratio ρ1 of the first planetary gear set  14 . 
     An excessive speed estimating portion  78  is configured to estimate excessively high values of the operating speed N E  of the engine  12 , the operating speed N MG1  of the first electric motor MG1, and a rotating speed N P1  of the pinion gear P1 of the first planetary gear set  14 . For instance, the excessive speed estimating portion  78  determines whether each of the speed values N E , N MG1  and N P1  (hereinafter referred to simply as speed values “N”, unless otherwise specified) is higher than a predetermined threshold value N lim , or has a risk of rise to a value higher than the predetermined threshold value N lim . This determination is made on the basis of the speed values N and change rates dN E /dt, dN MG1 /dt and dN P1 /dt (hereinafter referred to simply as change rates “dN/dt”, unless otherwise specified). Preferably, the excessive speed estimating portion  78  calculates an estimated upper limit value N max  (N, dN/dt) to which each speed value N can be temporarily raised due to a failure of the first electric motor MG1 or the like. This calculation is made on the basis of the speed value N and the change rate dN/dt. The excessive speed estimating portion  78  determines whether the calculated estimated upper limit value N max  is higher than the predetermined threshold value N lim . In other words, the excessive speed estimating portion  78  determines whether the speed value N falls within a predetermined excessive speed range, or has a risk of rise to within the predetermined excessive speed range. Preferably, this determination is made immediately after a moment of the determination by the MG1 failure determining portion  76  that the first electric motor MG1 or the first planetary gear set  14  has a failure. Preferably, the operating speed N E  of the engine  12  and the operating speed N MG1  of the first electric motor MG1, which are used for the above-indicated determination, are the values respectively detected by the engine speed sensor  44  and the MG1 speed sensor  46 . Preferably, the rotating speed N P1  of the pinion gear P1 is calculated on the basis of the engine speed N E  detected by the engine speed sensor  44 , the first electric motor speed N MG1  detected by the MG1 speed sensor  46  and the output speed N OUT  detected by the output speed sensor  50 , as well as the gear ratio ρ1 of the first planetary gear set  14 . 
     A battery charge determining portion  80  is configured to determine whether the amount of electric energy stored in the battery  55  is within a predetermined range. Preferably, the battery charge determining portion  80  determines whether the battery-stored electric energy amount SOC detected by the battery SOC sensor  54  is equal to or larger than a predetermined threshold value SOC lim . For example, this threshold value SOC lim  corresponds to a charging limit value of the battery  55 . In other words, the battery charge determining portion  80  determines whether a permissible amount of electric energy input to the battery  55 , that is, an input limit value W in  of electric power that is permitted to be input to the battery  55  is smaller than a predetermined threshold value (whether the permissible amount of electric energy input to the battery  55  is comparatively small). 
     A substitutive drive control portion  82  is configured to implement a substitutive drive control in the event of a failure of the first electric motor MG1. Described more specifically, where the MG1 failure determining portion  76  determines a failure of the first electric motor MG1 or the first planetary gear set  14 , the substitutive drive control portion  82  implements a substitutive drive control wherein the engine  12  is operated to generate a vehicle drive force while the second electric motor MG2 receives a reaction force with respect to the vehicle drive force (generates a reaction torque). Preferably, the substitutive drive control portion  82  implements the substitutive drive control by commanding the clutch engagement control portion  72  to engage the clutch CL, commanding the brake engagement control portion  74  to release the brake BK, commanding the engine control device  56  to operate the engine  12  for generating a vehicle drive force, and commanding the electric motor operation control portion  70  to control the second electric motor MG2 so as to receive the reaction force with respect to the vehicle drive force. The substitutive drive control implemented by the substitutive drive control portion  82  will be described in detail. 
       FIG. 9  is the collinear chart for explaining the substitutive drive control implemented in the event of a failure of the first electric motor MG1. In the event of a failure (an operational defect) of the first electric motor MG1, this first electric motor MG1 cannot generate a torque (reaction torque), as indicated in  FIG. 9 , so that the first electric motor MG1 cannot receive a reaction force of a vehicle drive force generated by the engine  12 . The present embodiment is configured such that in the event of a failure of the first electric motor MG1, the clutch CL is brought into the engaged state, to permit transmission of a drive force from the engine  12  to the carrier C2 of the second planetary gear set  16 , so that the second electric motor MG2 receives the reaction force with respect to the vehicle drive force generated by the engine  12 , whereby the vehicle drive force generated by the engine  12  can be transmitted to the output gear  30 . Namely, the hybrid vehicle can be run in the substitutive drive mode, with the torque transmitted directly from the engine. According to the drive system  10 , the EV drive mode in which the second electric motor MG2 is primarily operated to generate the vehicle drive force, in the released state of the clutch CL and in the engaged state of the brake BK, can be used as the substitutive drive mode in the event of a failure of the first electric motor MG1. That is, the vehicle drive mode is switched to the EV drive mode to continue the substitutive drive mode, where the battery  55  has a risk of excessive charging, so that the maximum distance of running of the hybrid vehicle in the substitutive drive mode is not limited by the battery-stored electric energy amount. 
     In the event of estimation of an excessively high operating speed of at least one of the engine  12  and the first electric motor MG1, the substitutive drive control portion  82  preferably controls an amount of engagement (torque capacity) of the brake BK to prevent the excessive rise. Where the excessive speed estimating portion  78  estimates an excessively high value of at least one of the operating speed N E  of the engine  12 , the operating speed N MG1  of the first electric motor MG1 and the rotating speed N P1  of the pinion gear P1 of the first planetary gear set  14 , the substitutive drive control portion  82  commands the brake engagement control portion  74  to place the brake BK in a slipping state (partially engaged state), for preventing the excessively high value of the speed value N. In this control of the slipping state, the amount of engagement of the brake BK is controlled on the basis of the speed value N and the change rate dN/dt, and according to a predetermined relationship, so as to prevent the excessively high value of the speed value N. Namely, the torque capacity (engaging force) of the brake BK that can prevent the excessively high value of the speed value N is calculated, and the brake engagement control portion  74  controls the amount of engagement of the brake BK so as to establish the calculated torque capacity. Alternatively, the control of the slipping state of the brake BK may be implemented such that the torque capacity of the brake BK is controlled to a predetermined value (constant value) irrespective of the speed value N and the change rate dN/dt. This predetermined value of the torque capacity of the brake BK is preferably obtained by experimentation such that the speed value N will not have an excessively high value at the predetermined torque capacity value of the brake BK. 
     The substitutive drive control portion  82  is preferably configured to place the brake BK in a slipping state, where the permissible amount of electric energy input to the battery  55  is estimated to be smaller than the predetermined threshold value. When the battery charge determining portion  80  determines that the battery-stored electric energy amount SOC detected by the battery SOC sensor  54  is equal to or larger than the predetermined threshold value SOC lim , that is, the input limit value W in  of electric power that is permitted to be input to the battery  55  is smaller than a predetermined threshold value P mode , for example, the substitutive drive control portion  82  commands the brake engagement control portion  74  to place the brake BK in a slipping state (partially engaged state) in order to prevent the excessive speed rise. In this control of the slipping state of the brake BK, the amount of engagement of the brake BK is controlled on the basis of the input limit value W in , and according to a predetermined relationship, so as to prevent the amount of generation of an electric energy by the second electric motor MG2 from exceeding the input limit value W in . Namely, the torque capacity (engaging force) of the brake BK that can prevent the amount of generation of the electric energy by the second electric motor MG2 from exceeding the input limit value W in  is calculated, and the brake engagement control portion  74  controls the amount of engagement of the brake BK so as to establish the calculated torque capacity. Alternatively, the control of the slipping state of the brake BK may be implemented such that the torque capacity of the brake BK is controlled to a predetermined value (constant value) irrespective of the input limit value W in . This predetermined value of the torque capacity of the brake BK is preferably obtained by experimentation such that the battery  55  will not be excessively charged at the predetermined torque capacity value of the brake BK. 
       FIGS. 10 and 11  are the collinear charts for explaining a control to prevent an excessively high rotating speed of each rotary element or an excessive amount of charging of the battery, in the substitutive drive control implemented in the event of the failure of the first electric motor MG1. If the first electric motor MG1 which has received a reaction force (at least a part of the reaction force) with respect to a vehicle drive force generated by the engine  12  has a failure (an operational defect), for example, the first electric motor MG1 no longer receives the reaction force, so that a balance between a torque received by the output gear  30  based on a roadway surface resistance vehicle inertia and a sum of a torque of the engine  12  and a torque of the second electric motor MG2 is lost, giving excessive speed rise to a racing of the engine  12 , and an excessive rise of the operating speed of the first or second electric motor MG1, MG2, which may result in excessive charging of the battery  55 . In the event of a failure of the first electric motor MG1 to generate a reaction force, a loss of the reaction force of the first electric motor MG1 can be compensated for by an increase of the reaction force of the second electric motor MG2, in the engaged state of the clutch CL, as indicated in  FIG. 10 . Namely, it is possible to prevent an excessively high rotating speed of each rotary element, by controlling the output torque of the second electric motor MG2. Where the output torque of the second electric motor MG2 is limited due to overheating of the second electric motor MG2, for instance, it is difficult to control the output torque of the second electric motor MG2. In addition, the battery  55  may be excessively charged, where the input limit value W in  of the battery  55  is comparatively small. Where the first electric motor MG1 fails to generate a reaction force, the loss of the reaction force of the first electric motor MG1 can be compensated for by placing the brake BK in the slipping state to increase its torque capacity, as indicated in  FIG. 11 . This control makes it possible to effectively prevent an excessively high value of the operating speed N E  of the engine  12 , the operating speed N MG1  of the first electric motor MG1 and the rotating speed N P1  of the pinion gear P1 of the first planetary gear set  14 , without suffering from another problem of excessive charging of the battery  55 , and to effectively prevent the excessive charging of the battery  55 . 
       FIG. 12  is the flow chart for explaining a major portion of an example of the substitutive drive control implemented by the electronic control device  40 . The substitutive drive control is repeatedly implemented with a predetermined cycle time. 
     The substitutive drive control is initiated with step S1 (“step” being hereinafter omitted), to determine whether the first electric motor MG1 is normally operable, namely, whether the first electric motor MG1 is free from a failure. If a negative determination is obtained in S1, that is, if it is determined that the first electric motor MG1 has a failure, the control flow goes to S7. If an affirmative determination is obtained in S1, that is, if it is determined that the first electric motor MG1 is free from a failure, the control flow goes to S2 to determine whether the first differential mechanism in the form of the first planetary gear set  14  is normally operable, namely, whether the first planetary gear set  14  is free from a failure. If a negative determination is obtained in S2, that is, if it is determined that the first planetary gear set  14  has a failure, the control flow goes to S7. If an affirmative determination is obtained in S2, that is, if it is determined that the first planetary gear set  14  is free from a failure, the control flow goes to S3. 
     S3 is implemented to obtain, as a running condition of the hybrid vehicle, the accelerator pedal operation amount A CC  detected by the accelerator pedal operation amount sensor  42 , the engine speed N E  detected by the engine speed sensor  44 , the output speed N OUT  detected by the output speed sensor  50 , the battery-stored electric energy amount SOC detected by the battery SOC sensor  54 , etc. The control flow then goes to S4 to select a vehicle drive mode (a normal drive mode other than the substitutive drive mode). For instance, one of the modes 1-5 indicated in  FIG. 3  which permits the required vehicle drive force to be obtained with a highest degree of fuel economy is selected depending upon the running condition of the hybrid vehicle obtained in S3. Then, the control flow goes to S5 to control the operating states of the clutch CL and the brake BK, according to the drive mode selected in S4. The control flow then goes to S6 to implement a normal drive control (non-substitutive drive control), which is not the substitutive drive control. Then, the present routine is terminated. 
     S7 is implemented to obtain or calculate the operating speed N E  of the engine  12 , the operating speed N MG1  of the first electric motor MG1 and the rotating speed N P1  of the pinion gear P1 of the first planetary gear set  14 , and the change rates dN E /dt, dN MG1 /dt and dN P1 /dt of the speeds N E , N MG1  and N P1 . The control flow then goes to S8 to calculate estimated upper limits N max  (N, dN/dt), namely, the upper limits of the speed values N (to which the speed values N may be temporarily raised due to a failure of the first electric motor MG1, for example) based on the speed value N and the change rate dN/dt which are obtained in S7. Then, the control flow goes to S9 to determine whether there is a possibility of an excessive rise of any one of the speed values N, that is, whether any one of the estimated upper limits N max  calculated in S8 is higher than the predetermined threshold value N lim . If a negative determination is obtained in S9, the control flow goes to S10. If an affirmative determination is obtained in S9, the control flow goes to S14. 
     S10 is implemented to obtain, as the running condition of the hybrid vehicle, the accelerator pedal operation amount A CC  detected by the accelerator pedal operation amount sensor  42 , the engine speed N E  detected by the engine speed sensor  44 , the output speed N OUT  detected by the output speed sensor  50 , the battery-stored electric energy amount SOC detected by the battery SOC sensor  54 , etc. Then, the control flow goes to S11 to select the substitutive drive mode, that is, the drive mode to be established in the event of a failure of the first electric motor MG1 or the first planetary gear set  14 . The control flow then goes to S12 to control the operating states of the clutch CL and the brake BK, according to the substitutive drive mode selected in S11. Preferably, the clutch CL is placed in the engaged state while the brake BK is placed in the released state. Then, the control flow goes to S13 to implement the substitutive drive control wherein the engine  12  is operated to generate a vehicle drive force while the second electric motor MG2 is controlled to receive a reaction force with respect to the vehicle drive force. Then, the present routine is terminated. 
     S14 is implemented to determine whether the estimated upper limit N max  calculated in S8 is lower than a predetermined threshold value N mode  (N mode &gt;N lim ). If an affirmative determination is obtained in S14, the control flow goes to S15. If a negative determination is obtained in S14, the control flow goes to S18 to implement an excessive speed rise preventive control 2 wherein the brake BK is placed in a slipping state to prevent an excessive rise of the speed value N. For instance, the torque capacity (engaging force) of the brake BK that can prevent excessive rise of the speed value N is calculated on the basis of the estimated upper limit N max  calculated in S8, and according to a predetermined relationship, and the slipping state of the brake BK is controlled so as to establish the calculated torque capacity. Then, the control flow goes back to S7. S15 is implemented to estimate the input limit value W in  of the battery  55  on the basis of the battery-stored electric energy amount SOC detected by the battery SOC sensor  54 . The control flow then goes to S16 to determine whether the input limit value W in  of the battery  55  calculated in S15 is larger than the predetermined threshold value P mode . If a negative determination is obtained in S16, the control flow goes to S18. If an affirmative determination is obtained in S16, the control flow goes to S17 to implement an excessive speed rise preventive control 1 wherein the torque (reaction torque) of the second electric motor MG2 is controlled to prevent the excessive rise of the speed value N. Then, the control flow goes back to S7. 
     It will be understood from the foregoing description by reference to  FIG. 12  that S6, S13 and S17 correspond to the operation of the electric motor operation control portion  70 , and S5 and S12 correspond to the operation of the clutch engagement control portion  72 , while S5, S12 and S18 correspond to the operation of the brake engagement control portion  74 , and that S1 and S2 correspond to the operation of the MG failure determining portion  76 , while S7-S9 correspond to the operation of the excessive speed estimating portion  78 , and that S14 corresponds to the operation of the battery charge determining portion  80 , while S1-S18 correspond to the operation of the substitutive drive control portion  82 . 
     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 
       FIGS. 13-18  are the schematic views for explaining arrangements of respective hybrid vehicle drive systems  100 ,  110 ,  120 ,  130 ,  140  and  150  according to other preferred modes of this invention. The hybrid vehicle drive control device of the present invention is also applicable to drive systems such as the drive system  100  shown in  FIG. 13  and the drive system  110  shown in  FIG. 14 , which have respective different arrangements of the first electric motor MG1, first planetary gear set  14 , second electric motor MG2, second planetary gear set  16 , clutch CL and brake BK in the direction of the center axis CE. The present hybrid vehicle driving control device is also applicable to drive systems such as the drive system  120  shown in  FIG. 15 , which have a one-way clutch OWC disposed between the carrier C2 of the second planetary gear set  16  and the stationary member in the form of the housing  26 , in parallel with the brake BK, such that the one-way clutch OWC permits a rotary motion of the carrier C2 relative to the housing  26  in one of opposite directions and inhibits a rotary motion of the carrier C2 in the other direction. The present hybrid vehicle drive control device is further applicable to drive systems such as the drive system  130  shown in  FIG. 16 , the drive system  140  shown in  FIG. 17  and the drive system  150  shown in  FIG. 18 , which are provided with a second differential mechanism in the form of a second planetary gear set  16 ′ of a double-pinion type, in place of the second planetary gear set  16  of a single-pinion type. 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′. 
     Third Embodiment 
       FIGS. 19-21  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. 19-21 , 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. 19 , 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. 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, 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. 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, 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. 19-21  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. 8  and the other figures is suitably applicable to the drive systems shown in  FIGS. 19-21 . 
     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 placed in the engaged state (and thus the first planetary gear set  14  and the second planetary gear set  16 ,  16 ′ are represented as the four rotary elements in the collinear charts such as  FIGS. 4-7 ); 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 drive control device is configured to implement, in the event of a failure of the first electric motor MG1, the substitutive drive control wherein the engine  12  is operated to generate a vehicle drive force while the second electric motor MG2 receives a reaction force of the vehicle drive force. Accordingly, the hybrid vehicle can be run in the substitutive drive mode, with a torque transmitted directly from the engine  12 , to an extent of the reaction force received by the second electric motor MG2. Namely, the illustrated embodiments provide a drive control device in the form of the electronic control device  40  for a hybrid vehicle, which permits the hybrid vehicle to be run in an adequate substitutive drive mode in the event of a failure of an electric motor. 
     The drive control device is configured such that the clutch CL is placed in the engaged state while the brake BK is placed in the released state, in the substitutive drive control. Accordingly, the second electric motor MG2 can receive the reaction force of the vehicle drive force generated by the engine  12 , in a highly practical manner, in the event of the failure of the first electric motor MG1. 
     The drive control device is configured such that the brake BK is placed in the slipping state when an excessive speed rise of at least one of the engine  12  and the first electric motor MG1 is estimated. Accordingly, it is possible to effectively prevent the excessive speed rise immediately after the moment of the determination of the failure of the first electric motor MG1. 
     The drive control device is configured such that the brake BK is placed in the slipping state when it is estimated that the input upper limit W in  of the battery  55 , that is a vehicle driving battery, is smaller than the predetermined threshold value P mode . Accordingly, it is possible to effectively prevent an excessive amount of input to the battery  55  immediately after the moment of the determination of the failure of the first electric motor MG1. 
     The drive control device is configured such that the torque of the second electric motor MG2 is controlled so as to prevent an excessive speed rise of at least one of the engine  12  and the first electric motor MG1, when it is estimated that the input upper limit W in  of the battery  55  is equal to or larger than the predetermined threshold value P mode , while the above-indicated excessive speed rise is estimated. Accordingly, it is possible to effectively prevent the excessive speed rise immediately after the moment of the determination of the failure of the above-described first electric motor MG1. 
     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, the hybrid vehicle the drive system  10  of which has a highly practically arrangement can be run in an adequate substitutive drive mode in the event of a failure of an electric motor. 
     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 55: Battery (Vehicle 
               
               
                 driving battery) 
               
               
                 56: Engine control device 58: Inverter 60: Hydraulic control unit 
               
               
                 70: Electric motor operation control portion 
               
               
                 72: Clutch engagement control portion 
               
               
                 74: Brake engagement control portion 
               
               
                 76: MG1 failure determining portion 
               
               
                 78: Excessive speed estimating portion 
               
               
                 80: Battery charge determining portion 
               
               
                 82: Substitutive drive 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)