Patent Publication Number: US-2015087458-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 
     SUMMARY OF THE INVENTION 
     Object Achieved by the Invention 
     However, the conventional arrangement of the hybrid vehicle described above has a risk of generation of noises and vibrations during an operation of the engine, due to coincidence of an engine revolution 0.5-order component (a component of pulsation generated at a time interval equal to a half of a period of revolution of the engine), with a resonance frequency of a power transmitting system by a variation of combustion among cylinders of the engine. 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 generation of noises and vibrations. 
     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 switching an operating state of the above-described clutch when the engine is operated in a loaded condition while a torque of the above-described second electric motor falls within a predetermined narrow range including zero. 
     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 switch the operating state of the above-described clutch when the engine is operated in the loaded condition while the torque of the above-described second electric motor falls within the predetermined narrow range including zero. According to this first aspect of the invention, an inertia balance of a power transmitting system is changed to change a resonance point of the power transmitting system when the torque of the second electric motor is close to zero and the power transmitting system is likely to generate a resonance, so that generation of the resonance in the power transmitting system can be effectively reduced. Namely, the present invention provides a drive control device for a hybrid vehicle, which permits reduction of generation of vibrations in a power transmitting system 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 operating state of the above-described clutch when the above-described engine is operated in the loaded condition while the torque of the above-described second electric motor falls within the predetermined narrow range including zero, and when generation of a resonance has been detected or forecasted. According to this second aspect of the invention, the inertia balance of the power transmitting system is changed to change the resonance point of the power transmitting system when the torque of the second electric motor is close to zero and generation of the resonance in the power transmitting system is detected or forecasted, so that generation of the resonance in the power transmitting system can be effectively reduced. 
     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 first differential mechanism is provided with a first rotary element connected to the above-described first electric motor, and 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 third aspect of the invention, it is possible to reduce generation of the vibrations in the power transmitting system of 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 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 view schematically illustrating different resonance frequency values of a power transmitting system in the drive system of  FIG. 1 , which correspond to the respective different operating states of the clutch; 
         FIG. 11  is a view for explaining different characteristics (resonance frequency characteristics) of the power transmitting system in the drive system of  FIG. 1 , which correspond to the respective different operating states of the clutch; 
         FIG. 12  is a view schematically illustrating different resonance frequency values of the power transmitting system in the drive system of  FIG. 1 , which correspond to respective different combinations of the operating states of the clutch and brake; 
         FIG. 13  is a view for explaining different characteristics (resonance frequency characteristics) of the power transmitting system in the drive system of  FIG. 1 , which correspond to the respective different combinations of the operating states of the clutch and brake; 
         FIG. 14  is a view illustrating regions of an operating point of an engine in which noises are generated due to resonance when the clutch is placed in a released state; 
         FIG. 15  is a view illustrating regions of the operating point of the engine in which the noises are generated due to resonance when the clutch is placed in an engaged state; 
         FIG. 16  is a flow chart for explaining a major portion of a resonance point change control implemented by the electronic control device provided for the drive system of  FIG. 1 ; 
         FIG. 17  is a flow chart for explaining a major portion of another resonance point change control implemented by the electronic control device provided for the drive system of  FIG. 1 ; 
         FIG. 18  is a schematic view for explaining an arrangement of a hybrid vehicle drive system according to another preferred embodiment of this invention; 
         FIG. 19  is a schematic view for explaining an arrangement of a hybrid vehicle drive system according to a further preferred embodiment of this invention; 
         FIG. 20  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. 21  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. 22  is a schematic view for explaining an arrangement of a hybrid vehicle drive system according to still another preferred embodiment of this invention; 
         FIG. 23  is a schematic view for explaining an arrangement of a hybrid vehicle drive system according to yet another preferred embodiment of this invention; 
         FIG. 24  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. 25  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. 26  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, and the engine, the first electric motor, the second electric motor and the output rotary member which are connected 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 while the engine is held at rest 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. 
     In a further preferred form of the invention, the operating state of the clutch is switched to the engaged state when the engine is operated in a loaded condition while the torque of the above-described second electric motor falls within the predetermined narrow range including zero. Namely, the clutch is placed in the engaged state even when the presently selected drive mode is a drive mode established by releasing the clutch. More preferably, the clutch is placed in the engaged state when the above-described engine is operated in a loaded condition while the torque of the second electric motor falls within the predetermined narrow range including zero, and when generation of a resonance is detected or forecasted. 
     In a further preferred form of the invention, it is determined that generation of a resonance in the power transmitting system is detected or forecasted when a temperature of the power transmitting system is equal to or lower than a predetermined threshold value. Preferably, it is determined that generation of a resonance in the power transmitting system is detected or forecasted when an EGR device is operated to return a portion of an exhaust gas of the engine into an intake gas. Preferably, it is determined that generation of a resonance in the power transmitting system is detected or forecasted when the engine is operated to warm up a catalytic converter. 
     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 MG 1 , a second electric motor MG 2 , 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 MG 1  and second electric motor MG 2  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 S 1 ; a second rotary element in the form of a carrier C 1  supporting a pinion gear P 1  such that the pinion gear P 1  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 R 1  meshing with the sun gear S 1  through the pinion gear P 1 . 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 S 2 ; a second rotary element in the form of a carrier C 2  supporting a pinion gear P 2  such that the pinion gear P 2  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 R 2  meshing with the sun gear S 2  through the pinion gear P 2 . 
     The sun gear S 1  of the first planetary gear set  14  is connected to the rotor  20  of the first electric motor MG 1 . The carrier C 1  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 R 1  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 R 2  of the second planetary gear set  16 . The sun gear S 2  of the second planetary gear set  16  is connected to the rotor  24  of the second electric motor MG 2 . 
     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 C 1  of the first planetary gear set  14  and the carrier C 2  of the second planetary gear set  16 , there is disposed a clutch CL which is configured to selectively couple these carriers C 1  and C 2  to each other (to selectively connect the carriers C 1  and C 2  to each other or disconnect the carriers C 1  and C 2  from each other). Between the carrier C 2  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 C 2  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 MG 1  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 MG 1  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 MG 1  and second electric motor MG 2  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 MG 1  and second electric motor MG 2 . That is, the drive system  10  is configured such that the first electric motor MG 1 , first planetary gear set  14 , clutch CL, second planetary gear set  16 , brake BK and second electric motor MG 2  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 MG 1  and second electric motor MG 2 . 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 MG 1  and second electric motor MG 2 . 
     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 MG 1  speed sensor  46  indicative of an operating speed N MG1  of the first electric motor MG 1 ; an output signal of an MG 2  speed sensor  48  indicative of an operating speed N MG2  of the second electric motor MG 2 ; 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 an oil temperature sensor  52  indicative of a temperature T OIL  of a working fluid to be supplied to various parts of the drive system  10 ; and an output signal of a shift position sensor  54  indicative of a presently selected one of shift positions P S  of a manually operated shifting device not shown. 
     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; an electronic throttle valve drive control signal to control a throttle actuator for controlling an opening angle θ TH  of an electronic throttle valve; and an EGR valve drive signal to control an angle of opening (opening and closing actions) of an EGR valve  34 . The EGR valve  34  is provided to control an amount of recirculation of an exhaust gas of the engine  12  into an intake pipe to implement an EGR operation (Exhaust-Gas Recirculation) for returning a portion of the exhaust gas into an intake gas. Further, the electronic control device  40  applies command signals to an inverter  58 , for controlling operations of the first electric motor MG 1  and second electric motor MG 2 , so that the first and second electric motors MG 1  and MG 2  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 MG 1  and MG 2 . Electric energies generated by the first and second electric motors MG 1  and MG 2  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 MG 1  and second electric motor MG 2 , 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 MG 1  is supplied to the battery or the second electric motor MG 2  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 MG 1  operating as the electric generator, and converted into the electric energy, which is supplied to the second electric motor MG 2  through the inverter  58 , so that the second electric motor MG 2  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 MG 2  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 MG 1  and second electric motor MG 2 , 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 MG 1  and second electric motor MG 2  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 MG 1  and second electric motor MG 2  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 MG 1  and second electric motor MG 2  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 MG 1  and second electric motor MG 2  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 MG 1  and second electric motor MG 2  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 X 1  represents the rotating speed of zero, while vertical lines Y 1  through Y 4  arranged in the order of description in the rightward direction represent the respective relative rotating speeds of the sun gear S 1 , sun gear S 2 , carrier C 1  and ring gear R 1 . Namely, a solid line Y 1  represents the relative rotating speed of the sun gear S 1  of the first planetary gear set  14  (operating speed of the first electric motor MG 1 ), a broken line Y 2  represents the relative rotating speed of the sun gear S 2  of the second planetary gear set  16  (operating speed of the second electric motor MG 2 ), a solid line Y 3  represents the relative rotating speed of the carrier C 1  of the first planetary gear set  14  (operating speed of the engine  12 ), a broken line Y 3 ′ represents the relative rotating speed of the carrier C 2  of the second planetary gear set  16 , a solid line Y 4  represents the relative rotating speed of the ring gear R 1  of the first planetary gear set  14  (rotating speed of the output gear  30 ), and a broken line Y 4 ′ represents the relative rotating speed of the ring gear R 2  of the second planetary gear set  16 . In  FIGS. 4-7 , the vertical lines Y 3  and Y 3 ′ are superimposed on each other, while the vertical lines Y 4  and Y 4 ′ are superimposed on each other. Since the ring gears R 1  and R 2  are fixed to each other, the relative rotating speeds of the ring gears R 1  and R 2  represented by the vertical lines Y 4  and Y 4 ′ are equal to each other. 
     In  FIGS. 4-7 , a solid line L 1  represents the relative rotating speeds of the three rotary elements of the first planetary gear set  14 , while a broken line L 2  represents the relative rotating speeds of the three rotary elements of the second planetary gear set  16 . Distances between the vertical lines Y 1 -Y 4  (Y 2 -Y 4 ′) 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 Y 1 , Y 3  and Y 4  corresponding to the respective three rotary elements in the form of the sun gear S 1 , carrier C 1  and ring gear R 1  of the first planetary gear set  14 , a distance between the vertical lines Y 1  and Y 3  corresponds to “1”, while a distance between the vertical lines Y 3  and Y 4  corresponds to the gear ratio “ρ1”. Regarding the vertical lines Y 2 , Y 3 ′ and Y 4 ′ corresponding to the respective three rotary elements in the form of the sun gear S 2 , carrier C 2  and ring gear R 2  of the second planetary gear set  16 , a distance between the vertical lines Y 2  and Y 3 ′ corresponds to “1”, while a distance between the vertical lines Y 3 ′ and Y 4 ′ 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 MG 2  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 C 1  of the first planetary gear set  14  and the carrier C 2  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 C 2  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 C 2  is held zero. In this mode 1, the rotating direction of the sun gear S 2  and the rotating direction of the ring gear R 2  in the second planetary gear set  16  are opposite to each other, so that when the second electric motor MG 2  is operated to generate a negative torque (acting in the negative direction), the ring gear R 2 , 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 MG 2 . In this case, the first electric motor MG 1  is preferably held in a free state. In this mode 1, the carriers C 1  and C 2  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 C 2  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 MG 1  and second electric motor MG 2  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 C 1  of the first planetary gear set  14  and the carrier C 2  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 C 2  of the second planetary gear set  16  and the carrier C 1  of the first planetary gear set  14  which is connected to the carrier C 2  are coupled (fixed) to the stationary member in the form of the housing  26 , so that the rotating speeds of the carriers C 1  and C 2  are held zero. In this mode 2, the rotating direction of the sun gear S 1  and the rotating direction of the ring gear R 1  in the first planetary gear set  14  are opposite to each other, and the rotating direction of the sun gear S 2  and the rotating direction of the ring gear R 2  in the second planetary gear set  16  are opposite to each other, so that when the first electric motor MG 1  and/or second electric motor MG 2  is/are operated to generate a negative torque (acting in the negative direction), the ring gears R 1  and R 2  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 MG 1  and second electric motor MG 2 . 
     In the mode 2, at least one of the first electric motor MG 1  and second electric motor MG 2  may be operated as the electric generator. In this case, one or both of the first and second electric motors MG 1  and MG 2  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 MG 1  and MG 2  may be held in a free state, when the generation of an electric energy by a regenerative operation of the electric motors MG 1  and MG 2  is inhibited due to full charging of the battery. Namely, the mode 2 is an EV drive mode 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 MG 1  and second electric motor MG 2  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 C 1  of the first planetary gear set  14  and the carrier C 2  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 C 2  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 C 2  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 MG 1  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 S 2  and the rotating direction of the ring gear R 2  are opposite to each other, in the engaged state of the brake BK, so that when the second electric motor MG 2  is operated to generate a negative torque (acting in the negative direction), the ring gears R 1  and R 2  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 MG 1  and second electric motor MG 2  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 C 1  of the first planetary gear set  14  and the carrier C 2  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 C 1  and C 2  are integrally rotated as a single rotary element. The ring gears R 1  and R 2 , 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 S 1  (connected to the first electric motor MG 1 ), the sun gear S 2  (connected to the second electric motor MG 2 ), the rotary element constituted by the carriers C 1  and C 2  connected to each other (and to the engine  12 ), and the rotary element constituted by the ring gears R 1  and R 2  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 S 1  represented by the vertical line Y 1 , the sun gear S 2  represented by the vertical line Y 2 , the carriers C 1  and C 2  represented by the vertical line Y 3  (Y 3 ′), and the ring gears R 1  and R 2  represented by the vertical line Y 4  (Y 4 ′). The gear ratios ρ1 and ρ2 of the first and second planetary gear sets  14  and  16  are determined such that the vertical line Y 1  corresponding to the sun gear S 1  and the vertical line Y 2  corresponding to the sun gear S 2  are positioned as indicated in the collinear chart of  FIG. 6 , namely, such that the distance between the vertical lines Y 1  and Y 3  is longer than the distance between the vertical lines Y 2  and Y 3 ′. In other words, the distance between the vertical lines corresponding to the sun gear S 1  and the carrier C 1  and the distance between the vertical lines corresponding to the sun gear S 2  and the carrier C 2  correspond to “1”, while the distance between the vertical lines corresponding to the carrier C 1  and the ring gear R 1  and the distance between the vertical lines corresponding to the carrier C 2  and the ring gear R 2  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 C 1  of the first planetary gear set  14  and the carrier C 2  of the second planetary gear set  16  are connected to each other in the engaged state of the clutch CL, so that the carriers C 1  and C 2  are rotated integrally with each other. Accordingly, either one or both of the first electric motor MG 1  and second electric motor MG 2  can receive a reaction force corresponding to the output of the engine  12 . Namely, one or both of the first and second electric motors MG 1  and MG 2  can be operated to receive the reaction force during an operation of the engine  12 , and each of the first and second electric motors MG 1  and MG 2  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 MG 1  and second electric motor MG 2  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 N E , for instance, the operating speed N MG1  of the first electric motor MG 1  may have a negative value, that is, the first electric motor MG 1  may be operated in the negative direction. In the case where the first electric motor MG 1  generates the reaction force acting on the engine  12 , the first electric motor MG 1  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 MG 2  indicated on the vertical line Y 2  is less likely to have a negative value than the operating speed of the above-indicated first electric motor MG 1  indicated on the vertical line Y 1 , and the second electric motor MG 2  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 MG 2  so as to generate the reaction force, while the operating speed of the first electric motor MG 1  has a negative value. Further, where there is a torque limitation of one of the first electric motor MG 1  and second electric motor MG 2  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 C 1  to the rotating speed of the output gear  30  (ring gears R 1  and R 2 ). 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 MG 1  (rotating speed of the sun gear S 1 ) 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 MG 2  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 MG 1  and second electric motor MG 2  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 MG 1  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 MG 1 , while the rotating speed of the second electric motor MG 2  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 MG 2 . 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 MG 1  and MG 2  to generate a vehicle drive force and/or an electric energy as needed 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 regions having speed ratio values lower than the first mechanical transmission speed ratio value γ1, while the mode 4 is established in high-gear regions 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 MG 1  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 MG 1  may be operated to generate a vehicle drive force, with the second electric motor MG 2  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 C 2  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 C 2  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 MG 2  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 MG 2  is kept in an operated state together with a rotary motion of the output gear  30  (ring gear R 2 ) during running of the vehicle. In this operating state, the operating speed of the second electric motor MG 2  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 R 2  at a high speed is transmitted to the sun gear S 2 . In this respect, it is not necessarily desirable to keep the second electric motor MG 2  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 MG 1  may be operated to generate the vehicle drive force during running of the vehicle at the comparatively high speed, while the second electric motor MG 2  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 MG 2 , 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 MG 2 . 
     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 . An engine loaded-operation determining portion  70  shown in  FIG. 9  is configured to determine whether the engine  12  is operated in a loaded condition. Described more specifically, the engine loaded-operation determining portion  70  determines whether the engine  12  is operated so as to generate an engine torque T E  equal to or larger than a predetermined value. The engine loaded-operation determining portion  70  obtains a negative determination if the engine  12  is placed in an idling state. Preferably, this determination is made on the basis of engine drive control commands applied from the electronic control device  40  to the engine control device  56 . Alternatively, the determination may be made on the basis of the engine speed N E  detected by the engine speed sensor  44 , the accelerator pedal operation amount A CC  detected by the accelerator pedal operation amount sensor  42 , an intake air quantity Q A  of the engine  12  detected by an intake air quantity sensor not shown, etc. For example, the engine loaded-operation determining portion  70  calculates (estimates) the torque T E  of the engine  12  on the basis of the intake air quantity Q A  and a predetermined relationship, and determines that the engine  12  is operated in the loaded condition, if the calculated torque T E  is equal to or larger than a predetermined threshold value. 
     An MG 2  torque determining portion  72  is configured to determine whether the torque of the second electric motor MG 2  falls within a predetermined narrow range including zero. Preferably, the MG 2  torque determining portion  72  makes this determination on the basis of a second electric motor operation control command applied from the electronic control device  40  to the inverter  58 . For example, the predetermined narrow range is a range between zero and a predetermined value T id  which is a torque value of the second electric motor MG 2  when the hybrid vehicle provided with the drive system  10  is in a coasting run while the accelerator pedal operation amount A CC  detected by the accelerator pedal operation amount sensor  42  is zero (while the accelerator pedal is placed in the non-operated position). Preferably, the MG 2  torque determining portion  72  determines whether an absolute value of the torque of the second electric motor MG 2  falls within the predetermined narrow range. The MG 2  torque determining portion  72  may determine whether the torque of the second electric motor MG 2  is considered to be close to zero or substantially zero. 
     A resonance determining portion  74  is configured to determine whether or not a power transmitting system of the hybrid vehicle provided with the drive system  10  has a resonance. Namely, the resonance determining portion  74  detects or forecasts generation of a resonance in the power transmitting system. In other words, the resonance determining portion  74  determines whether a pulsation of a given frequency that causes generation of a resonance in the power transmitting system of the drive system  10  is likely to be generated. The “power transmitting system” means a system so-called “a drive line” for power transmission from the vehicle drive power source to the drive wheels. In the hybrid vehicle provided with the drive system  10 , the power transmitting system is a power transmission system which is provided in a power transmitting path from the vehicle drive power source in the form of the engine  12 , first electric motor MG 1  and second electric motor MG 2  to the drive wheels in the form of tires  68  (shown in  FIG. 12 ), and which includes the first planetary gear set  14 , second planetary gear set  16 , input shaft  28  and output gear  30 , and a damper  62 , a drive shaft  64 , the tires  66 , and a body  68  (which are shown in  FIGS. 10 and 12 ). 
     Preferably, as shown in  FIG. 9 , the resonance determining portion  74  includes a pulsation input determining portion  76 , a low temperature determining portion  78 , an EGR operation determining portion  78  and a catalyst warm-up determining portion  82 , the determining portions are configured to determine whether the power transmitting system has a resonance. The pulsation input determining portion  76  is configured to determine whether the power transmitting system has a resonance, on the basis of the vehicle running speed V and the operating speed N E  of the engine  12 , and according to a predetermined relationship. For instance, the pulsation input determining portion  76  calculates a frequency of a pulsation (of an input torque) received from the roadway surface on which the vehicle is running (from the drive wheels), on the basis of the vehicle running speed V corresponding to the output speed N OUT  detected by the output speed sensor  50  and the engine speed N E  detected by the engine speed sensor  44 , and determines that the pulsation received by the power transmitting system has been detected or forecasted, if the calculated frequency of the pulsation is substantially coincident with a resonance frequency of the power transmitting system, that is, falls within a predetermined range (band) of frequency a center point of which is equal to the resonance frequency. Alternatively, the pulsation input determining portion  76  calculates a frequency of a pulsation input as a result of a rotary motion of the engine  12 , on the basis of the engine speed N E  detected by the engine speed sensor  44 , and determines that the pulsation received by the power transmitting system has been detected or forecasted, if the calculated frequency of the pulsation is substantially coincident with the resonance frequency of the power transmitting system, that is, falls within the predetermined range (band) of frequency the center point of which is equal to the resonance frequency. The resonance frequency of the power transmitting system is determined by inertial values of various portions of the drive system  10 , and by the operating states of the clutch CL and brake BK, as described below. That is, the resonance frequency values of the drive system  10  which correspond to the different combinations of the operating states of the clutch CL and brake BK are obtained by experimentation and stored in a memory. The pulsation input determining portion  76  determines whether or not the frequency of the pulsation received from the roadway surface, which is calculated on the basis of the vehicle running speed V and the engine speed N E , or the frequency of the pulsation input as a result of the rotary motion of the engine  12  is substantially coincident with the resonance frequency value of the drive system  10  corresponding to the present combination of the operating states of the clutch CL and brake BK. If an affirmative determination is obtained, the pulsation input determining portion  76  determines that the input of the pulsation into the power transmitting system has been detected or forecasted. 
     The low temperature determining portion  78  is configured to determine whether the power transmitting system has a resonance, depending upon whether a temperature of the power transmitting system is equal to or lower than a predetermined threshold value. For instance, the low temperature determining portion  78  determines that the power transmitting system in the hybrid vehicle has a resonance, if the oil temperature T OIL  detected by the oil temperature sensor  52  is equal to or lower than a predetermined threshold value T bo  (e.g., about −20° C.). In other words, the low temperature determining portion  78  determines that there is a high degree of probability that the power transmitting system generates vibrations, if the oil temperature T OIL  representing the temperature of the power transmitting system is equal to or lower than the predetermined threshold value T bo . Although the temperature of the power transmitting system corresponds to the oil temperature T OIL  of the working fluid supplied to the various parts of the drive system  10 , the oil temperature T OIL  may be replaced by a cooling water temperature of the engine  12 , a temperature of the battery connected to the first and second electric motors MG 1  and MG 2 , or an average value of the above-indicated temperature of the working fluid, engine cooling water temperature and battery temperature. 
     The EGR operation determining portion  80  is configured to determine whether the power transmitting system has a resonance, depending upon whether an EGR device is operated to return a portion of the exhaust gas of the engine  12  into the intake gas. For example, the EGR operation determining portion  80  determines whether the EGR valve  34  is placed in an open state (in which the exhaust gas is returned into the intake pipe), on the basis of the engine drive control commands applied from the electronic control device  40  to the engine control device  56 . If this determination is affirmative, that is, if the EGR valve  34  is placed in the open state, the EGR operation determining portion  80  determines that the power transmitting system of the hybrid vehicle has a resonance. In other words, the EGR operation determining portion  80  determines that there is a high degree of probability that the power transmitting system generates vibrations, if the EGR valve  34  is placed in the open state. 
     The catalyst warm-up determining portion  82  is configured to determine whether the power transmitting system has a resonance, depending upon whether the engine  12  is operated to warm up a catalytic converter. For instance, the catalyst warm-up determining portion  82  determines whether the engine  12  is operated to warm up the catalytic converter, depending on the basis of the engine drive control commands applied from the electronic control device  40  to the engine control device  56 . If this determination is affirmative, that is, if the engine  12  is operated to warm up the catalytic converter, the catalyst warm-up determining portion  82  determines that the power transmitting system in the hybrid vehicle has a resonance. In other words, the catalyst warm-up determining portion  82  determines that there is a high degree of probability that the power transmitting system generates vibrations, if the engine  12  is operated to warm up the catalytic converter. 
     In the drive system  10  according to the present embodiment wherein the internal combustion engine in the form of the engine  12  is provided as a vehicle drive power source, a vibration damping torsional damper is provided between the engine  12  and transaxles. The power transmitting system (drive line) including the torsional damper has a specific resonance frequency determined by its specific constructional arrangement. In the prior art, there is a risk of instability of combustion of the engine  12  and likeliness of occurrence of a variation of combustion among the cylinders of the engine  12 , while the temperature of the power transmitting system is comparatively low, while the EGR device is operated, or while the catalytic converter is warmed up. In such condition as described above in which the variation of combustion among the cylinders of the engine  12  is likely to occur, there is a risk of generation of noises and vibrations as a result of amplification of a revolution 0.5-order component of the engine  12 , namely, a component of pulsation generated at a time interval equal to a half of the period of the engine revolution, which amplification takes place due to coincidence of the revolution 0.5-order component with the resonance frequency of the power transmitting system including a damper main in the form of the torsional damper, within an ordinary operation band of frequency of the engine  12  (e.g., a band of about 1000-2000 [rpm]). 
       FIG. 10  is the view schematically illustrating different resonance frequency values of the power transmitting system in the above-descried drive system  10 , which correspond to the respective different operating states of the clutch CL.  FIG. 11  is the view for explaining different characteristics of the power transmitting system (resonance frequency characteristics) of the power transmitting system corresponding to the respective different operating states of the clutch CL. In  FIG. 11 , a solid line represents the characteristic in the released state of the clutch CL, while a broken line represents the characteristic in the engaged state of the clutch CL. In the drive system  10 , its resonance point (resonance frequency) changes depending upon whether the clutch CL is placed in the engaged state or the released state, while the brake BK is placed in the released state. Namely, the second electric motor MG 2  is not connected to the power transmitting system between the engine  12  and the first electric motor MG 1 , in the released state of the clutch CL, as indicated in an upper part of  FIG. 10 . When the clutch CL is switched from the released state to the engaged state, the second electric motor MG 2  is connected to the power transmitting system between the engine  12  and the first electric motor MG 1 , as indicated in a lower part of  FIG. 10 . Accordingly, the components such as the rotor  24  of the second electric motor MG 2  is added to the power transmitting system, so that the resonance point of the power transmitting system is changed as a result of a change of characteristic relating to the inertia (inertia balance), as indicated in  FIG. 11 . In particular, a resonance point relating to an arrangement around the damper  62  (damper main) disposed between the engine  12  and the first electric motor MG 1  is changed as a result of switching of the operating state of the clutch CL, as indicated in  FIG. 10 . 
       FIG. 12  is the view schematically illustrating different resonance frequency values of the power transmitting system in the drive system  10 , which correspond to the respective different operating states of at least one of the clutch CL and brake BK.  FIG. 13  is the view for explaining different characteristics of the power transmitting system (resonance frequency characteristics) of the power transmitting system which correspond to respective different combinations of the operating states of the clutch CL and brake BK. In  FIG. 13 , a solid line represents the characteristic in the released state of the clutch CL and in the engaged state of the brake BK, while a broken line represents the characteristic in the engaged state of the clutch CL and in the released state of the brake BK. In particular,  FIGS. 12 and 13  represent a characteristic of the damper main when the torque of the second electric motor MG 2  is close to zero (substantially zero). As indicated in  FIGS. 12 and 13 , the resonance point (resonance frequency) in the drive system  10  changes as a result of switching of the operating state of the brake BK, in addition to or in place of switching of the operating state of the clutch CL. Namely, the second electric motor MG 2  is not connected to the power transmitting system between the engine  12  and the first electric motor MG 1 , in the released state of the clutch CL and in the engaged state of the brake BK, that is, when the mode 3 (HV-1) indicated in  FIG. 3  is established, as indicated in an upper part of  FIG. 12 . In the engaged state of the clutch CL and in the released state of the brake BK, that is, when the mode 4 (HV-2) indicated in  FIG. 3  is established, on the other hand, the second electric motor MG 2  is connected to the power transmitting system between the engine  12  and the first electric motor MG 1 , as indicated in a lower part of  FIG. 12 . Namely, the second electric motor MG 2  is connected to an input-side power transmitting system. Accordingly, the resonance point of the power transmitting system is changed as a result of a change of the characteristic relating to the inertia (inertia balance), as indicated in  FIG. 13 . 
       FIGS. 14 and 15  are the views illustrating regions of an operating point of the engine  12  in which noises are generated due to resonance.  FIG. 14  illustrates the regions when the clutch CL is placed in the released state, while  FIG. 15  illustrates the regions when the clutch CL is placed in the engaged state. In  FIGS. 14 and 15 , a dotted area represents an impermissible region of generation of noises due to the engine explosion 1-order component (pulsation generated at the time interval equal to the period of the engine explosion), while a hatched area represents an impermissible region of generation of noises due to the engine revolution 0.5-order component (pulsation generated at the time interval equal to the half of the period of the engine revolution). In  FIGS. 14 and 15 , a broken line represents the resonance frequency (resonance point) of the damper main, while a solid line represents a highest fuel economy line of the engine  12 . This highest fuel economy line is a curve connecting highest fuel economy points on an iso-fuel-economy curve, which are moved through a highest fuel economy area with a rise of the engine speed N E  and which are obtained preliminarily by experimentation. The highest fuel economy line may also be considered as a succession of highest fuel economy points of the engine  12  predetermined by experimentation so as to provide a good compromise between drivability and fuel economy of the hybrid vehicle. 
     It will be understood from  FIGS. 14 and 15  that the hatched impermissible region of generation of the noises due to the engine revolution 0.5-order component, which noises are included in the noises generated due to pulsation of the rotary motion of the engine  12 , is moved when the operating state of the clutch CL is switched or changed. Namely, the impermissible region of generation of the noises due to the engine revolution 0.5-order component, which region is represented by the hatched area in  FIG. 14 , is located on the side of a comparatively high operating speed of the engine (on a comparatively high engine speed side), so that an area of overlapping of this impermissible region with respect to the impermissible region of generation of the noises due to the engine explosion 1-order component, which region is represented by the dotted area, is comparatively narrow, whereby a permissible region of generation of the noises due to pulsation of the rotary motion of the engine  12  is comparatively narrow. In this respect, it is noted that according to characteristics of the drive line in the drive system  10 , the resonance frequency (resonance point) of the damper main is lowered as a result of addition of an inertia of the second electric motor MG 2  to the power transmitting system between the engine  12  and the first electric motor MG 1 . Accordingly, by switching the clutch CL to the engaged state, the hatched impermissible region of generation of the noises due to the engine revolution 0.5-order component is moved toward the side of a comparatively low operating speed of the engine (on a comparatively low engine speed side), with respect to the region indicated in  FIG. 14 , as indicated in  FIG. 15 . Accordingly, the area of overlapping of the hatched impermissible region with respect to the dotted impermissible region of generation of the noises due to the engine explosion 1-order component is broadened, whereby the permissible region of generation of the noises due to pulsation of the rotary motion of the engine  12  is broadened. That is, the operating point of the engine  12  can be located in a better region for improving the fuel economy. 
     Depending upon a design of the drive system  10 , on the other hand, the impermissible region of generation of the noises due to the engine revolution 0.5-order component when the clutch CL is placed in the engaged state may be located on the side of an excessively low operating speed of the engine (on a comparatively low engine speed side), so that the area of overlapping of this impermissible region with respect to the impermissible region of generation of the noises due to the engine explosion 1-order component is narrow, whereby the permissible region of generation of the noises due to pulsation of the rotary motion of the engine  12  is narrow, contrary to the example of  FIGS. 14 and 15 . In this case, the clutch CL is brought into the released state, so that the impermissible region of generation of the noises due to the engine revolution 0.5-order component is moved toward the side of the comparatively high operating speed of the engine (on the comparatively high engine speed side). Accordingly, the area of overlapping of this impermissible region with respect to the impermissible region of generation of the noises due to the engine explosion 1-order component is broadened, so that the permissible range of generation of the noises due to pulsation of the rotary motion of the engine  12  is broadened. That is, the operating point of the engine  12  can be located in a better region for improving the fuel economy. 
     In view of the characteristics of the drive system  10  described, above, a resonance point change control portion  84  shown in  FIG. 9  is configured to change the operating state of the clutch CL, when both of the engine loaded-operation determining portion  70  and the MG 2  torque determining portion  72  make affirmative determinations, that is, when the engine  12  is operated in a loaded condition while the torque of the second electric motor MG 2  falls within the predetermined narrow range including zero. Preferably, the resonance point change control portion  84  switches the clutch CL to the engaged state when the engine  12  is operated in a loaded condition while the torque of the second electric motor MG 2  falls within the predetermined narrow range including zero. As described above by reference to  FIGS. 12-15 , the resonance frequency (resonance point) of the damper main in the power transmitting system is changed by switching the operating state of the clutch CL in the drive system  10 . Accordingly, and described more specifically, the resonance point change control portion  84  implements a control for changing the resonance point in the power transmitting system, by switching the operating state of the clutch CL through the hydraulic control unit  60 . For example, the resonance point change control portion  84  implements the control for switching the operating state of the clutch CL to the engaged state, even where the clutch CL should be placed in the released state to establish the mode 1 (HV-1), when the engine  12  is operated in a loaded condition while the torque of the second electric motor MG 2  falls within the predetermined narrow range including zero. 
     When each of the engine loaded-operation determining portion  70 , the MG 2  torque determining portion  72  and the resonance determining portion  74  makes an affirmative determination, that is, when the generation of a resonance has been detected or forecasted while the engine  12  is operated in a loaded condition and while the torque of the second electric motor MG 2  falls within the predetermined narrow range including zero, the resonance point change control portion  84  preferably implements the control for switching the operating state of the clutch CL. When both of the engine loaded-operation determining portion  70  and the MG 2  torque determining portion  72  make affirmative determinations, and when at least one of the pulsation input determining portion  76 , the low temperature determining portion  78 , the EGR operation determining portion  80  and the catalyst warm-up determining portion  82  makes an affirmative determination, the resonance point change control portion  84  preferably implements the control for switching the operating state of the clutch CL. Preferably, the resonance point change control portion  84  implements the control for switching the operating state of the clutch CL to the engaged state when the engine  12  is operated in a loaded condition while the torque of the second electric motor MG 2  falls within the predetermined narrow range including zero, and while the generation of a resonance has been detected or forecasted. 
     The resonance point change control portion  84  is preferably configured to selectively implement the above-described controls depending upon results of the determinations by the engine loaded-operation determining portion  70 , the MG 2  torque determining portion  72  and the resonance determining portion  74 , when the drive system  10  is placed in a drive position “D”, namely, when the selected shift position detected by the shift position sensor  54  is a forward drive position. The drive system  10  has a risk of generation of noises due to the revolution 0.5-order component of the engine  12  in addition to noises (rattling noises) due to the explosion 1-order component of the engine  12  when the torque of the second electric motor MG 2  is close to zero, that is, falls within the predetermined narrow range including zero, while the engine  12  is operated in a loaded condition and while the EGR device is operated. Where the frequency of this engine revolution 0.5-order component is coincident with the resonance point of the damper main in the power transmitting system, in particular, the drive system  10  has a drawback of deterioration of the fuel economy since it is not possible to avoid the generation of the former noises unless a threshold line (corresponding to a one-dot chain line in  FIG. 14 , for example) for avoiding the generation of these noises is located on a higher-speed smaller-torque side of a threshold line (corresponding to a two-dot chain line in  FIG. 14 , for example) for avoiding the generation of the explosion 1-order component. The present embodiment is configured to change the characteristic of the drive line from the relationship illustrated in  FIG. 14  to the relationship illustrated in  FIG. 15 , by switching the operating state of the clutch CL when the torque of the second electric motor MG 2  falls within the predetermined narrow range including zero, while the engine  12  is operated in a loaded condition. Accordingly, it is possible to narrow the impermissible range of generation of noises and vibrations so that the operating point of the engine  12  can be located in a better region for improving the fuel economy, than in the prior art. 
     The resonance point change control portion  84  is preferably configured to change the resonance point of the power transmitting system of the drive system  10 , depending upon results of the determinations by the engine loaded-operation determining portion  70  and the resonance determining portion  74 , when the drive system  10  is placed in a parking position “P”, namely, when the selected shift position detected by the shift position sensor  54  is a the parking position. While the hybrid vehicle provided with the drive system  10  is stationary (parked) in the parking position “P”, the engine revolution 0.5-order component is likely to be generated during an operation of the engine  12  when the engine is operated in a loaded condition during a cold state or for warming up the catalyst converter. In this state, the operating state of the clutch CL is switched, preferably, to the engaged state, to establish the drive line characteristic in which the resonance point is comparatively different from the pulsation frequency of the engine  12 , so that the generation of noises and vibrations can be effectively reduced. 
       FIG. 16  is the flow chart for explaining a major portion of a resonance point change control implemented by the electronic control device  40 . The resonance point change control is repeatedly implemented with a predetermined cycle time. 
     The resonance point change control is initiated with step S 1  (“step” being hereinafter omitted), to determine whether the engine  12  is operated in a loaded condition. If a negative determination is obtained in S 1 , the present control routine is terminated. If an affirmative determination is obtained in S 1 , on the other hand, the control flow goes to S 2  to determine whether the torque of the second electric motor MG 2  is close to zero, that is, falls within the predetermined narrow range including zero. If a negative determination is obtained in S 2 , the present control routine is terminated. If an affirmative determination is obtained in S 2 , the control flow goes to S 3  to determine whether the frequency of pulsation of the engine  12  is likely to generate a resonance in the power transmitting system, due to an operation of the engine  12  in a loaded condition in a cold state or with an operation of the EGR device, for instance. If a negative determination is obtained in S 3 , the present routine is terminated. If an affirmative determination is obtained in S 3 , the control flow goes to S 4  to switch the operating state of the clutch CL, and preferably, after the clutch CL is placed in the engaged state, the present routine is terminated. It will be understood that S 1  corresponds to the operation of the engine loaded-operation determining portion  70  while S 2  corresponds to the operation of the MG 2  torque determining portion  72 , and that S 3  corresponds to the operation of the resonance determining portion  74  while S 4  corresponds to the operation of the resonance point change control portion  84 . 
       FIG. 17  is the flow chart for explaining a major portion of another example of the resonance point change control implemented by the electronic control device  40 . This resonance point change control is repeatedly implemented with a predetermined cycle time. In  FIG. 17 , the same step numbers as used in  FIG. 16  are used to identify the same steps, which will not be described redundantly. If the affirmative determination is obtained in S 1  in the control of  FIG. 17 , that is, if it is determined that the engine  12  is operated in a loaded condition, the control flow goes to S 3 . 
     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. 18  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. 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 MG 2  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 MG 1 , first planetary gear set  14 , second electric motor MG 2 , 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. 19  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. 19 , 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  in the axial direction of the center axis CE, while the first electric motor MG 1  and second electric motor MG 2  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 MG 2 , and first electric motor MG 1  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. 20  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. 20 , a one-way clutch OWC is disposed in parallel with the brake BK, between the carrier C 2  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 C 2  in one of opposite directions relative to the housing  26 , and inhibits a rotary motion of the carrier C 2  in the other direction. Preferably, this one-way clutch OWC permits the rotary motion of the carrier C 2  in the positive or forward direction relative to the housing  26 , and inhibits the rotary motion of the carrier C 2  in the negative or reverse direction. Namely, in a drive state where the carrier C 2  is rotated in the negative direction, that is, where the second electric motor MG 2  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. 21  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. 21  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 S 2 ′; a second rotary element in the form of a carrier C 2 ′ supporting a plurality of pinion gears P 2 ′ meshing with each other such that each pinion gear P 2 ′ 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 R 2 ′ meshing with the sun gear S 2 ′ through the pinion gears P 2 ′. 
     The ring gear R 1  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 C 2 ′ of the second planetary gear set  16 ′. The sun gear S 2 ′ of the second planetary gear set  16 ′ is connected to the rotor  24  of the second electric motor MG 2 . Between the carrier C 1  of the first planetary gear set  14  and the ring gear R 2 ′ of the second planetary gear set  16 ′, there is disposed the clutch CL which is configured to selectively couple these carrier C 1  and ring gear R 2 ′ to each other (to selectively connect the carrier C 1  and ring gear R 2 ′ to each other or disconnect the carrier C 1  and ring gear R 2 ′ from each other). Between the ring gear R 2 ′ 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 R 2 ′ to the housing  26 . 
     As shown in  FIG. 21 , 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 MG 1  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 MG 2  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 MG 1  and second electric motor MG 2  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 MG 1  and second electric motor MG 2 . That is, the drive system  130  is configured such that the first electric motor MG 1 , first planetary gear set  14 , clutch CL, second planetary gear set  16 ′, second electric motor MG 2 , 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. 22  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. 22 , 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 MG 2  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 MG 1 , first planetary gear set  14 , second electric motor MG 2 , 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. 23  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. 23 , the first electric motor MG 1  and second electric motor MG 2  which constitute an electric system are disposed on the side of the engine  12  in the axial 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 MG 1 , second electric motor MG 2 , 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. 24-26  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. 24-26 , the relative rotating speeds of the sun gear S 1 , carrier C 1  and ring gear R 1  of the first planetary gear set  14  are represented by the solid line L 1 , while the relative rotating speeds of the sun gear S 2 , carrier C 2  and ring gear R 2  of the second planetary gear set  16  are represented by the broken line L 2 , as in  FIGS. 4-7 . In the drive system  160  for the hybrid vehicle shown in  FIG. 24 , the sun gear S 1 , carrier C 1  and ring gear R 1  of the first planetary gear set  14  are respectively connected to the first electric motor MG 1 , engine  12  and second electric motor MG 2 , while the sun gear S 2 , carrier C 2  and ring gear R 2  of the second planetary gear set  16  are respectively connected to the second electric motor MG 2  and output gear  30 , and to the housing  26  through the brake BK. The sun gear S 1  and the ring gear R 2  are selectively connected to each other through the clutch CL. The ring gear R 1  and the sun gear S 2  are connected to each other. In the drive system  170  for the hybrid vehicle shown in  FIG. 25 , the sun gear S 1 , carrier C 1  and ring gear R 1  of the first planetary gear set  14  are respectively connected to the first electric motor MG 1 , output gear  30  and engine  12 , while the sun gear S 2 , carrier C 2  and ring gear R 2  of the second planetary gear set  16  are respectively connected to the second electric motor MG 2  and output gear  30 , and to the housing  26  through the brake BK. The sun gear S 1  and the ring gear R 2  are selectively connected to each other through the clutch CL. The clutches C 1  and C 2  are connected to each other. In the drive system  180  for the hybrid vehicle shown in  FIG. 26 , the sun gear S 1 , carrier C 1  and ring gear R 1  of the first planetary gear set  14  are respectively connected to the first electric motor MG 1 , output gear  30  and engine  12 , while the sun gear S 2 , carrier C 2  and ring gear R 2  of the second planetary gear set  16  are respectively connected to the second electric motor MG 2 , to the housing  26  through the brake BK, and to the output gear  30 . The ring gear R 1  and the carrier C 2  are selectively connected to each other through the clutch CL. The carrier C 1  and ring gear R 2  are connected to each other. 
     The drive systems for the hybrid vehicle shown in  FIGS. 24-26  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 MG 1 , second electric motor MG 2 , 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 stationary member in the form of the housing  26  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. 24-26 . 
     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 MG 1 , the second electric motor MG 2  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 switch the operating state of the clutch CL when the engine  12  is operated in the loaded condition while the torque of the second electric motor MG 2  falls within the predetermined narrow range including zero. Accordingly, when the torque of the second electric motor is close to zero and the resonance in the power transmitting system is likely to be generated, an inertia balance of the power transmitting system is changed to change the resonance point of the power transmitting system, so that generation of a resonance in the power transmitting system can be effectively reduced. Namely, the illustrated embodiments provide a drive control device in the form of the electronic control device  40  for a hybrid vehicle, which permits reduction of generation of vibrations in the power transmitting system of the hybrid vehicle. 
     The illustrated embodiments are further configured to switch the operating state of the clutch CL when the engine  12  is operated in the loaded condition while the torque of the second electric motor MG 2  falls within the predetermined narrow range including zero, and when generation of a resonance has been detected or forecasted. Accordingly, the inertia balance of the power transmitting system is changed to change the resonance point of the power transmitting system when the torque of the second electric motor MG 2  is close to zero and generation of the resonance in the power transmitting system is likely to be detected or forecasted, so that generation of the resonance in the power transmitting system can be effectively reduced. 
     The first planetary gear set  14  is provided with a first rotary element in the form of the sun gear S 1  connected to the first electric motor MG 1 , a second rotary element in the form of the carrier C 1  connected to the engine  12 , and a third rotary element in the form of the ring gear R 1  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 S 2  (S 2 ′) connected to the second electric motor MG 2 , a second rotary element in the form of the carrier C 2  (C 2 ′), and a third rotary element in the form of the ring gear R 2  (R 2 ′), one of the carrier C 2  (C 2 ′) and the ring gear R 2  (R 2 ′) being connected to the ring gear R 1  of the first planetary gear set  14 . The clutch CL is configured to selectively connect the carrier C 1  of the first planetary gear set  14  and the other of the carrier C 2  (C 2 ′) and the ring gear R 2  (R 2 ′) which is not connected to the ring gear R 1 , to each other, while the brake BK is configured to selectively fix the other of the carrier C 2  (C 2 ′) and the ring gear R 2  (R 2 ′) which is not connected to the ring gear R 1 , to a stationary member in the form of the housing  26 . Accordingly, it is possible to reduce the generation of vibrations of the power transmitting system of the hybrid vehicle drive system  10  having 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  34 : EGR valve 
           40 : Electronic control device (Drive control device) 
           42 : Accelerator pedal operation amount sensor  44 : Engine speed sensor 
           46 : MG 1  speed sensor  48 : MG 2  speed sensor  50 : Output speed sensor 
           52 : Oil temperature sensor  54 : Shift position sensor 
           56 : Engine control device  58 : Inverter  60 : Hydraulic control unit 
           62 : Damper  64 : Drive shaft  66 : Tires  68 : Body 
           70 : Engine loaded-operation determining portion 
           72 : MG 2  torque determining portion  74 : Resonance determining portion 
           76 : Pulsation input determining portion 
           78 : Low temperature determining portion 
           80 : EGR operation determining portion 
           82 : Catalyst warm-up determining portion 
           84 : Resonance point change control portion 
         BK: Brake CL: Clutch C 1 , C 2 , C 2 ′: Carrier (Second rotary element) 
         MG 1 : First electric motor MG 2 : Second electric motor 
         OWC: One-way clutch P 1 , P 2 , P 2 ′: Pinion gear 
         R 1 , R 2 , R 2 ′: Ring gear (Third rotary element) 
         S 1 , S 2 , S 2 ′: Sun gear (First rotary element)