Patent Publication Number: US-8122983-B2

Title: Power output apparatus, hybrid vehicle with the same, and method for controlling power output apparatus

Description:
This is a 371 national phase application of PCT/JP2007/070044 filed 15 Oct. 2007, claiming priority to Japanese Patent Application No. JP 2006-334526 filed 12 Dec. 2006, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a power output apparatus for outputting power to a drive shaft, a hybrid vehicle with the same and method for controlling power output apparatus. 
     BACKGROUND ART 
     Conventionally, as such a power output apparatus, there has been known a power output apparatus including an internal combustion engine, two motors, a so-called ravigneaux planetary gear mechanism, a parallel shaft-type transmission capable of selectively coupling two output elements of the planetary gear mechanism to an output shaft (for example, see Patent Document 1). This power output apparatus is adapted for a front-wheel-drive vehicle, and this power output apparatus is configured such that the internal combustion engine is arranged transversely, and the rotating shafts of the internal combustion engine and the planetary gear mechanism, the two motors and the parallel shaft-type transmission extend in parallel to each other. In addition, conventionally, there has been known a power output apparatus including a planetary gear device having an input element connected to an internal combustion engine and two output elements; and a parallel shaft-type transmission having a countershaft connected to a corresponding output element of the planetary gear mechanism (for example, see Patent Document 2). According to this power output apparatus, each of the two output elements of the planetary gear device is fixed to an inner periphery of a corresponding rotor in an electric drive section.
     Patent Document 1: Japanese Patent Laid-Open No. 2005-155891   Patent Document 2: Japanese Patent Laid-Open No. 2003-106389   

     DISCLOSURE OF THE INVENTION 
     The power output apparatus disclosed in each of the above Patent Documents is capable of stopping the internal combustion engine as well as transmitting power output from any one of the motors to the output shaft with change in speed thereof by the transmission. However, the above individual Patent Documents do not disclose in detail how to perform such a motor drive for outputting power only by the motor. 
     In view of this, an object of the present invention is to start the internal combustion engine while properly outputting demanded power to the drive shaft when the power output from the motor is transmitted to the drive shaft with change in speed thereof by the speed change-transmission assembly. In addition, an object of the present invention is to provide a power output apparatus, a hybrid vehicle with the same, and a method for controlling the power output apparatus capable of efficiently transmitting demanded power to the drive shaft such that the speed change-transmission assembly changes the speed of the power output from the motor in a plurality of operation modes. 
     In order to achieve at least part of the above objects, the power output apparatus and the hybrid vehicle in accordance with the present invention adopt the following measures. 
     The present invention is directed to a power output apparatus for outputting power to a drive shaft. The power output apparatus includes: an internal combustion engine; a first motor capable of inputting and outputting power; a second motor capable of inputting and outputting power; an accumulator capable of supplying and receiving power to and from each of the first and second motors; a power distribution and integration mechanism having a first element connected to a rotating shaft of the first motor, a second element connected to a rotating shaft of the second motor, and a third element connected to an engine shaft of the internal combustion engine, the power distribution and integration mechanism being configured so that these three elements can be differentially rotated with respect to each other; a connection/disconnection device capable of performing a drive source element connection and a release of the drive source element connection which is one of a connection between the first motor and the first element, a connection between the second motor and the second element, and a connection between the internal combustion engine and the third element; a speed change-transmission assembly capable of selectively coupling one of or both of the rotating shaft of the first motor and the rotating shaft of the second motor to the drive shaft, the speed change-transmission assembly being capable of transmitting power from the first motor and power from the second motor to the drive shaft at predetermined respective speed ratios; a power demand setting device for setting power demand which is power required for the drive shaft; and a control device for controlling the first motor, the second motor, and the connection/disconnection device so that power based on the set power demand is output to the drive shaft, the control device, when an engine start condition for starting the internal combustion engine is satisfied in a state in which the drive source element connection is released, the internal combustion engine is stopped, only one of the first and second motors is coupled to the drive shaft by the speed change-transmission assembly, and the one of the first and second motors outputs power, controlling the first motor, the second motor, and the connection/disconnection device with a rotation speed adjusting process of adjusting a rotation speed of the other of the first and second motors so as to enable the drive source element connection, the drive source element connection, and an engine start process of cranking the internal combustion engine by the first or second motor. 
     Like this power output apparatus, in a state in which the drive source element connection is released, when the internal combustion engine is stopped, and only one of the first and second motors is coupled to the drive shaft by the speed change-transmission assembly, as well as one of the first and second motors is caused to output power, while the power based on the set power demand is being output to the drive shaft, the drive source element connection can be executed by executing the above rotation speed adjustment process; and when the drive source element connection is executed, the internal combustion engine can be started by causing one of the first and second motors to crank the internal combustion engine while the power based on the set power demand is being output to the drive shaft. As a result, according to this power output apparatus, in a state in which the drive source element connection is released, the internal combustion engine is stopped, and the power output from one of the first and second motors is changed in speed by the speed change-transmission assembly and is transmitted to the drive shaft. At this time, if the engine start condition is established, the internal combustion engine can be started while demanded power is more properly output to the drive shaft. 
     Alternatively, the rotation speed adjustment process may be a process of matching a rotation speed of the other of the first and second motors which is released from a coupling to the drive shaft, with a rotation speed of the first or second element at the drive source element connection based on a rotation speed of the one of the first and second motors which is coupled to the drive shaft. 
     Further, the speed change-transmission assembly may be a parallel shaft-type transmission which includes a first transmission mechanism having at least one parallel shaft-type gear train capable of coupling one of the first and second elements of the power distribution and integration mechanism to the drive shaft and a second transmission mechanism having at least one parallel shaft-type gear train capable of coupling the rotating shaft of the first motor or the second motor which corresponds to the other of the first and second elements to the drive shaft. According to such a speed change-transmission assembly which is a parallel shaft-type transmission, one of or both of the rotating shaft of the first motor and the rotating shaft of the second motor can be selectively coupled to the drive shaft. 
     Alternatively, the speed change-transmission assembly may be a planetary gear transmission which includes: a first transmission planetary gear mechanism having an input element connected to one of the first and second elements of the power distribution and integration mechanism, an output element connected to the drive shaft, and a fixable element, the first transmission planetary gear mechanism being configured so that these three elements can be differentially rotated with respect to each other; a first fixing mechanism capable of non-rotatably fixing the fixable element of the first transmission planetary gear mechanism; a second transmission planetary gear mechanism having an input element connected to the rotating shaft of the first or second motor which corresponds to the other of the first and second elements, and an output element connected to the drive shaft, and a fixable element, the second transmission planetary gear mechanism being configured so that these three elements can be differentially rotated with respect to each other; and a second fixing mechanism capable of non-rotatably fixing the fixable element of the second transmission planetary gear mechanism. According to such a speed change-transmission assembly which is a planetary gear transmission, if one of the first and second fixing mechanisms is placed in a fixed state, one of the rotating shaft of the first motor and the rotating shaft of the second motor can be coupled to the drive shaft. Alternatively, if both of the first fixing mechanism and second fixing mechanism are placed in a fixed state respectively, both of the rotating shaft of the first motor and the rotating shaft of the second motor can be coupled to the drive shaft. 
     In this case, the speed change-transmission assembly may further include a transmission connection/disconnection mechanism capable of performing a connection and a release of the connection between the output element of one of the first transmission planetary gear mechanism and the second transmission planetary gear mechanism and the fixable element. According to such a speed change-transmission assembly, an output element of the first or second transmission planetary gear mechanism which corresponds to the transmission connection/disconnection mechanism is connected to the fixable element by the transmission connection/disconnection mechanism, as well as a fixable element of the second or first transmission planetary gear mechanism which does not correspond to the transmission connection/disconnection mechanism is non-rotatably fixed. By doing so, both of the rotating shaft of the first motor and the rotating shaft of the second motor can be coupled to the drive shaft. In addition, according to this speed change-transmission assembly, an output element of the first or second transmission planetary gear mechanism which corresponds to the transmission connection/disconnection mechanism is connected to a fixable element, as well as a fixable element of the second or first transmission planetary gear mechanism which does not correspond to the transmission connection/disconnection mechanism is non-rotatably fixed. In this state, if the fixable element of the second or first transmission planetary gear mechanism is made to be rotatable, the individual elements of the first or second transmission planetary gear mechanism corresponding thereto by the transmission connection/disconnection mechanism are substantially locked to rotate integrally. Therefore, the power from one of the rotating shaft of the first motor and the rotating shaft of the second motor can be directly transmitted to the drive shaft. 
     The present invention is directed to a hybrid vehicle including drive wheels driven by power from a drive shaft. The hybrid vehicle includes: an internal combustion engine; a first motor capable of inputting and outputting power; a second motor capable of inputting and outputting power; an accumulator capable of supplying and receiving power to and from each of the first and second motors; a power distribution and integration mechanism having a first element connected to a rotating shaft of the first motor, a second element connected to a rotating shaft of the second motor, and a third element connected to an engine shaft of the internal combustion engine, the power distribution and integration mechanism being configured so that these three elements can be differentially rotated with respect to each other; a connection/disconnection device capable of performing a drive source element connection and a release of the drive source element connection which is one of a connection between the first motor and the first element, a connection between the second motor and the second element, and a connection between the internal combustion engine and the third element; a speed change-transmission assembly capable of selectively coupling one of or both of the rotating shaft of the first motor and the rotating shaft of the second motor to the drive shaft, the speed change-transmission assembly being capable of transmitting power from the first motor and power from the second motor to the drive shaft at predetermined respective speed ratios; a power demand setting device for setting power demand which is power required for the drive shaft; and a control device for controlling the first motor, the second motor, and the connection/disconnection device, so that power based on the set power demand is output to the drive shaft, the control device, when an engine start condition for starting the internal combustion engine is satisfied in a state in which the drive source element connection is released, the internal combustion engine is stopped, only one of the first and second motors is coupled to the drive shaft by the speed change-transmission assembly, and the one of the first and second motors outputs power, controlling the first motor, the second motor, and the connection/disconnection device with a rotation speed adjusting process of adjusting a rotation speed of the other of the first and second motors so as to enable the drive source element connection, the drive source element connection, and an engine start process of cranking the internal combustion engine by the first or second motor. 
     According to this hybrid vehicle, in a state in which the drive source element connection is released, when the internal combustion engine is stopped, and only one of the first and second motors is coupled to the drive shaft by the speed change-transmission assembly, as well as one of the first and second motors is caused to output power, if an engine start condition is established, the internal combustion engine can be started while demanded power is more properly output to the drive shaft. Therefore, this hybrid vehicle can well improve fuel consumption and drive performance by appropriately switching a running state in which power from one of the first and second motors is transmitted to the drive shaft and a running state in which power is output to the drive shaft with an operation of the internal combustion engine. 
     The present invention is directed to another power output apparatus for outputting power to a drive shaft. The power output apparatus includes: an internal combustion engine; a first motor capable of inputting and outputting power; a second motor capable of inputting and outputting power; an accumulator capable of supplying and receiving power to and from each of the first and second motors; a power distribution and integration mechanism having a first element connected to a rotating shaft of the first motor, a second element connected to a rotating shaft of the second motor, and a third element connected to an engine shaft of the internal combustion engine, the power distribution and integration mechanism being configured so that these three elements can be differentially rotated with respect to each other; a connection/disconnection device disposed between the first motor and the first element or between the second motor and the second element, the connection/disconnection device being capable of performing a connection and a release of the connection between the rotating shaft of the first or second motor and the first or second element; and a speed change-transmission assembly capable of selectively coupling the rotating shaft of the first or second motor corresponding to the connection/disconnection device and the first or second element not corresponding to the connection/disconnection device to the drive shaft, the speed change-transmission assembly being capable of transmitting power from a rotating shaft of the first or second motor and power from the first or second element to the drive shaft at predetermined respective speed ratios. 
     According to this power output apparatus, when the connection between a rotating shaft of the first or second motor and the first or second element is released by the connection/disconnection device, the connection between the first motor and the second motor through the power distribution and integration mechanism is released. Therefore, in a state in which the connection by the connection/disconnection device is released and the internal combustion engine is stopped, the speed change-transmission assembly is used to couple, to the drive shaft, one of the rotating shaft of the first or second motor corresponding to the connection/disconnection device and the first or second element not corresponding to the connection/disconnection device, as well as the power from one of the first and second motors is transmitted to the drive shaft. By doing so, the internal combustion engine and the other one of the first and second motors can be prevented from corotating, while the power from one of the first and second motors can be changed in speed by the speed change-transmission assembly, and demanded power can be efficiently transmitted to the drive shaft. Alternatively, in a state in which the connection by the connection/disconnection device is released and the internal combustion engine is stopped, the speed change-transmission assembly is used to couple, to the drive shaft, both of the rotating shaft of the first or second motor corresponding to the connection/disconnection device and the first or second element not corresponding to the connection/disconnection device. By doing so, the internal combustion engine can be prevented from corotating, and the power output from at least one of the first and the second motors can be transmitted to the drive shaft at a predetermined fixed speed ratio. Therefore, larger power can be output to the drive shaft in comparison with the case in which only one of the first and the second motors is caused to output power. Further, according to this power output apparatus, the connection by the connection/disconnection device can be maintained, while the power from one of the first and the second motors can be transmitted to the drive shaft by causing the speed change-transmission assembly to change the speed thereof. Thereby, according to this power output apparatus, in a plurality of drive modes, the power output from at least one of the first and the second motors can be changed in speed by the speed change-transmission assembly, and the demanded power can be efficiently transmitted to the drive shaft. 
     The present invention is directed to a method of controlling a power output apparatus including a drive shaft; an internal combustion engine; a first motor and a second motor capable of inputting and outputting power respectively; an accumulator capable of supplying and receiving power to and from each of the first and second motors; a power distribution and integration mechanism having a first element connected to a rotating shaft of the first motor, a second element connected to a rotating shaft of the second motor, and a third element connected to an engine shaft of the internal combustion engine and configured so that these three elements can be differentially rotated with respect to each other; a connection/disconnection device capable of performing a drive source element connection and a release of the drive source element connection which is one of a connection between the first motor and the first element, a connection between the second motor and the second element, and a connection between the internal combustion engine and the third element; a speed change-transmission assembly capable of selectively coupling one of or both of the rotating shaft of the first motor and the rotating shaft of the second motor to the drive shaft, the speed change-transmission assembly being capable of transmitting power from the first motor and power from the second motor to the drive shaft at predetermined respective speed ratios. The method of controlling the power output apparatus includes: (a) adjusting, when an engine start condition for starting the internal combustion engine is satisfied in a state in which the drive source element connection is released, the internal combustion engine is stopped, only one of the first and second motors is coupled to the drive shaft by the speed change-transmission assembly, and the one of the first and second motors outputs power, a rotation speed of the other of the first and second motors so as to enable the drive source element connection; (b) performing the drive source element connection; and (c) starting the internal combustion engine with cranking by the first or second motor. 
     Like this method, in a state in which the drive source element connection is released, the internal combustion engine is stopped, and only one of the first and second motors is coupled to the drive shaft by the speed change-transmission assembly, as well as one of the first and second motors is caused to output power. At this time, the drive source element connection can be executed by executing the above rotation speed adjustment process. When the drive source element connection is executed, the internal combustion engine can be started by causing one of the first and second motors to crank the internal combustion engine. 
     Alternatively, in the method of controlling the power output apparatus according to the present invention, torque commands to the first and second motors may be set so that power based on a power demand required for the drive shaft is output during the execution of steps (a) to (c). 
     Further, the step (a) may match a rotation speed of the other of the first and second motors which is released from a coupling to the drive shaft, with a rotation speed of the first or second element at the drive source element connection based on a rotation speed of one of the first and second motors which is coupled to the drive shaft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic configuration view of a hybrid vehicle  20  in accordance with an embodiment of the present invention; 
         FIG. 2  is an explanatory drawing illustrating a relationship of a rotation speed and a torque of major elements of a power distribution and integration mechanism  40  and a transmission  60  when the speed ratio of the transmission  60  is changed according to the speed change of the vehicle when the hybrid vehicle  20  of the present embodiment runs with an operation of an engine  22 ; 
         FIG. 3  is an explanatory drawing similar to  FIG. 2 ; 
         FIG. 4  is an explanatory drawing similar to  FIG. 2 ; 
         FIG. 5  is an explanatory drawing similar to  FIG. 2 ; 
         FIG. 6  is an explanatory drawing similar to  FIG. 2 ; 
         FIG. 7  is an explanatory drawing similar to  FIG. 2 ; 
         FIG. 8  is an explanatory drawing similar to  FIG. 2 ; 
         FIG. 9  is an explanatory drawing showing an example of an alignment chart representing a relationship of a rotation speed and a torque between an individual element of the power distribution and integration mechanism  40  and an individual element of a reduction gear mechanism  50  when a motor MG 1  functions as a generator and a motor MG 2  functions as a motor; 
         FIG. 10  is an explanatory drawing showing an example of an alignment chart representing a relationship of a rotation speed and a torque between an individual element of the power distribution and integration mechanism  40  and an individual element of the reduction gear mechanism  50  when the motor MG 2  functions as a generator and the motor MG 1  functions as a motor; 
         FIG. 11  is an explanatory drawing for explaining a motor drive mode in the hybrid vehicle  20  of the present embodiment; 
         FIG. 12  is a flowchart showing an example of a drive control routine for single motor drive executed by a hybrid ECU  70  when a clutch release single motor drive mode is selected for the hybrid vehicle  20  of the present embodiment; 
         FIG. 13  is a flowchart showing an example of a motor synchronization control routine executed by a hybrid ECU  70  of the present embodiment; 
         FIG. 14  is an explanatory drawing for explaining an operation while a motor synchronization control routine is being executed; 
         FIG. 15  is a flowchart showing an example of an engine start time drive control routine executed by a hybrid ECU  70  of the present embodiment; 
         FIG. 16  is a schematic configuration view showing another transmission  100  which can be applied to the hybrid vehicle  20  of the present embodiment; and 
         FIG. 17  is a schematic configuration view of a hybrid vehicle  20 A which is a variation of the present embodiment. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, the best mode for carrying out the invention will be described with reference to embodiments. 
       FIG. 1  is a schematic configuration view of a hybrid vehicle  20  in accordance with a present embodiment of the present invention. The hybrid vehicle  20  shown in the same figure is configured as a rear-wheel-drive vehicle, and includes an engine  22  arranged in a vehicle front portion; a power distribution and integration mechanism (differential rotation mechanism)  40  connected to a crankshaft  26  which is an output shaft of the engine  22 ; a generatable motor MG 1  connected to the power distribution and integration mechanism  40 ; a generatable motor MG 2  arranged coaxially with the motor MG 1  and connected to the power distribution and integration mechanism  40  through a reduction gear mechanism  50 ; a transmission  60  capable of transmitting power from the power distribution and integration mechanism  40  to a drive shaft  67  with a change in speed ratio; and a hybrid electronic control unit (hereinafter referred to as “hybrid ECU”)  70  for controlling the entire hybrid vehicle  20  and the like. 
     The engine  22  is an internal combustion engine which outputs power by receiving a supply of a hydrocarbonaceous fuel such as gasoline and a diesel oil, and receives control of a fuel injection amount, an ignition timing, an intake air amount, and the like from an engine electronic control unit (hereinafter referred to as “engine ECU”)  24 . The engine ECU  24  receives signals from various kinds of sensors which are provided with respect to the engine  22  and detect an operating state of the engine  22 . Moreover, the engine ECU  24  communicates with the hybrid ECU  70 , controls the operation of the engine  22  based on control signals from the hybrid ECU  70  and signals from the above sensors, and outputs data about the operating state of the engine  22  to the hybrid ECU  70  as needed. 
     Each of the motor MG 1  and the motor MG 2  is configured as a known synchronous generator/motor which can operate not only as a generator, but also as a motor; and supplies and receives electric power to and from a battery  35  which is a secondary battery through inverters  31  and  32 . Power lines  39  connecting the inverters  31  and  32  and the battery  35  are configured as a positive electrode bus line and a negative electrode bus line shared by the individual inverters  31  and  32 ; and are configured such that the power generated by one of the motors MG 1  and MG 2  can be consumed by the other motor. Therefore, the battery  35  is charged with electric power generated by one of the motors MG 1  and MG 2  and is discharged due to electric power shortage. If the electric power consumption and generation is balanced between the motors MG 1  and MG 2 , the battery  35  is assumed to be neither charged nor discharged. Both the motors MG 1  and MG 2  are drive-controlled by a motor electronic control unit (hereinafter referred to as “motor ECU”)  30 . The motor ECU  30  receives a signal necessary for drive-controlling the motors MG 1  and MG 2 , for example, a signal from rotational position detection sensors  33  and  34  for detecting a rotational position of a rotor of motors MG 1  and MG 2 ; and a phase current which is detected by a current sensor (not shown) and is applied to the motors MG 1  and MG 2 . The motor ECU  30  outputs a switching control signal to inverters  31  and  32  and the like. The motor ECU  30  executes a rotation speed calculation routine (not shown) based on a signal inputted from the rotational position detection sensors  33  and  34 , and calculates the rotation speeds Nm 1  and Nm 2  of rotors of the motors MG 1  and MG 2 . Moreover, the motor ECU  30  communicates with the hybrid ECU  70 , drive-controls the motors MG 1  and MG 2  based on control signals from the hybrid ECU  70 , and outputs data about the operating states of the motors MG 1  and MG 2  to the hybrid ECU  70  as needed. 
     The battery  35  is managed by a battery electronic control unit (hereinafter referred to as “battery ECU”)  36 . The battery ECU  36  receives a signal necessary for managing the battery  35 , for example, an inter-terminal voltage from a voltage sensor (not shown) provided between the terminals of the battery  35 ; a charge-discharge current from a current sensor (not shown) provided on the power line  39  connected to an output terminal of the battery  35 ; a battery temperature Tb from a temperature sensor  37  attached to the battery  35 , and the like. The battery ECU  36  outputs data about a state of the battery  35  to the hybrid ECU  70  and the engine ECU  24  through communication as needed. Further, the battery ECU  36  calculates a state of charge (SOC) based on an integrated value of charge and discharge currents detected by the current sensor in order to manage the battery  35 . 
     The power distribution and integration mechanism  40  is housed in a transmission case (not shown) together with the motors MG 1  and MG 2 , the reduction gear mechanism  50 , the transmission  60 , and arranged coaxially with a crankshaft  26  spaced at a predetermined distance from the engine  22 . The power distribution and integration mechanism  40  of the present embodiment is a double pinion planetary gear mechanism having a sun gear  41  which is an external gear; a ring gear  42  which is an internal gear arranged concentrically with the sun gear  41 ; a carrier  45  which rotatably and spinably holds at least one pair of two pinion gears  43  and  44  meshed with each other, one of which is meshed with the sun gear  41  and the other of which is meshed with the ring gear  42 ; and is configured such that the sun gear  41  (second element), the ring gear  42  (third element), and the carrier  45  (first element) can differentially rotate with each other. According to the present embodiment, the motor MG 1  (hollow rotor) serving as the second motor is connected to the sun gear  41  which is a second element of the power distribution and integration mechanism  40  through a hollow sun gear shaft  41   a  extending from the sun gear  41  to an opposite side (rear part of the vehicle) of the engine  22  and a hollow first motor shaft  46 . Moreover, the motor MG 2  (hollow rotor) serving as the first motor is connected to the carrier  45  which is a first element through the reduction gear mechanism  50  provided between the power distribution and integration mechanism  40  and the engine  22  and a hollow second motor shaft  55  extending toward the engine  22  from the reduction gear mechanism  50  (sun gear  51 ). Further, a crankshaft  26  of the engine  22  is connected to the ring gear  42  which is a third element through the ring gear shaft  42   a  and a damper  28  extending through the second motor shaft  55  and the motor MG 2 . 
     Moreover, as shown in  FIG. 1 , a clutch C 0  (connection/disconnection device) is provided between the sun gear shaft  41   a  and the first motor shaft  46  so as to perform connection (the drive source element connection) and disconnection therebetween. According to the present embodiment, the clutch C 0  is configured, for example, as a dog clutch which can mesh a dog fixed to a leading edge of the sun gear shaft  41   a  with a dog fixed to a leading edge of the first motor shaft  46  and can also release the mesh therebetween with less loss; and is driven by an electric, electromagnetic, or hydraulic actuator  88 . When the clutch C 0  releases the connection between the sun gear shaft  41   a  and the first motor shaft  46 , the connection between the motor MG 1  serving as the second motor and the sun gear  41  which is a second element of the power distribution and integration mechanism  40  is also released accordingly. In short, the functions of the power distribution and integration mechanism  40  can subsequently disconnect the engine  22  from the motors MG 1  and MG 2  and the transmission  60 . 
     Then, the first motor shaft  46  capable of being coupled to the sun gear  41  of the power distribution and integration mechanism  40  through the clutch C 0  further extends from the motor MG 1  to an opposite side (rear part of the vehicle) of the engine  22 , and is connected to the transmission  60 . Moreover, a carrier shaft (coupling shaft)  45   a  extends from the carrier  45  of the power distribution and integration mechanism  40  to an opposite side (rear part of the vehicle) of the engine  22  through the hollow sun gear shaft  41   a  and the first motor shaft  46 , and the carrier shaft  45   a  is also connected to the transmission  60 . Thereby, according to the present embodiment, the power distribution and integration mechanism  40  is provided between the motors MG 1  and MG 2  which are arranged coaxially with each other, and is arranged coaxially with both the motors MG 1  and MG 2 ; and the engine  22  is arranged in parallel to and coaxially with the motor MG 2  and faces the transmission  60  with the power distribution and integration mechanism  40  therebetween. That is, according to the present embodiment, the components of the power output apparatus such as the engine  22 , the motors MG 1  and MG 2 , the power distribution and integration mechanism  40 , and the transmission  60  are arranged starting with the front part of the vehicle, namely, the engine  22 , the motor MG 2 , (reduction gear mechanism  50 ), the power distribution and integration mechanism  40 , the motor MG 1 , and the transmission  60  in that order. This arrangement allows the power output apparatus to be compact in size, excellent in mountability, and preferable for the hybrid vehicle  20  which runs mainly by driving rear wheels. 
     Moreover, according to the present embodiment, as described above, the sun gear  41  which is a second element of the power distribution and integration mechanism  40  is connected to the transmission  60  through the sun gear shaft  41   a , and the clutch C 0 , and the first motor shaft  46 ; and the carrier  45  which is a first element of the power distribution and integration mechanism  40  is connected to the transmission  60  through the carrier shaft  45   a . Thereby, according to the hybrid vehicle  20 , one of the sun gear  41  and the carrier  45  of power distribution and integration mechanism  40  is set to a reaction element receiving a reaction of a torque outputted from the engine  22 , and the other is set to an output element; and thereby power can be outputted to the transmission  60 . If the sun gear  41  is set to the reaction element, the motor MG 1  functions as a generator. In this case, the power distribution and integration mechanism  40  receives power from the engine  22  through the ring gear  42  and distributes the power to the sun gear  41  side and the carrier  45  side according to the gear ratio; integrates the power from the engine  22  and power from the motor MG 2  functioning as a motor and outputs the integrated power to the carrier  45  side. If the carrier  45  is set to the reaction element, the motor MG 2  functions as a generator. In this case, the power distribution and integration mechanism  40  receives power from the engine  22  through the ring gear  42  and distributes the power to the sun gear  41  side and the carrier  45  side according to the gear ratio; integrates the power from the engine  22  and the power from the motor MG 1  functioning as a motor and outputs the integrated power to the sun gear  41  side. 
     The reduction gear mechanism  50  is a single pinion planetary gear mechanism having a sun gear  51  which is an external gear; a ring gear  52  which is an internal gear arranged concentrically with the sun gear  51 ; a plurality of pinion gears  53  which are meshed with both the sun gear  51  and the ring gear  52 ; and a carrier  54  which rotatably and spinably holds the plurality of pinion gears  53 . The sun gear  51  of the reduction gear mechanism  50  is connected to a rotor of the motor MG 2  through the above described second motor shaft  55 . Moreover, the ring gear  52  of the reduction gear mechanism  50  is fixed to the carrier  45  of the power distribution and integration mechanism  40 . Thereby, the reduction gear mechanism  50  is substantially integral with the power distribution and integration mechanism  40 . The carrier  54  of the reduction gear mechanism  50  is fixed with respect to the transmission case. Therefore, by the operation of the reduction gear mechanism  50 , the power from the motor MG 2  is reduced in speed and is inputted to the carrier  45  of the power distribution and integration mechanism  40 ; and at the same time, the power from the carrier  45  is increased in speed and is inputted to the motor MG 2 . It should be noted that as shown in the present embodiment, the power output apparatus can be more compact by placing the reduction gear mechanism  50  between the motor MG 2  and the power distribution and integration mechanism  40  so as to be formed integrally with the power distribution and integration mechanism  40 . 
     The transmission  60  is configured as an automatic parallel shaft-type transmission capable of setting the speed ratio at a plurality of stages, and includes: a first counter drive gear  61   a  and a first counter driven gear  61   b  constituting the first speed gear train; a second counter drive gear  62   a  and a second counter driven gear  62   b  constituting the second speed gear train; a third counter drive gear  63   a  and a third counter driven gear  63   b  constituting the third speed gear train; a fourth counter drive gear  64   a  and a fourth counter driven gear  64   b  constituting the fourth speed gear train; a countershaft  65  to which individual counter driven gears  61   b  to  64   b  and a gear  66   b  are fixed; clutches C 1  and C 2 ; a gear  66   a  attached to the drive shaft  67 ; further a reverse gear train (not shown) and the like (hereinafter, as needed, a “counter drive gear” and a “counter driven gear” are simply referred to as a “gear”). It should be noted that according to the transmission  60 , the first speed gear train has the largest speed ratio; and shifting from first to second to third and to fourth speed gear trains reduces the speed ratio accordingly. 
     As shown in  FIG. 1 , the first gear  61   a  of the first speed gear train is held rotatably and non-movably in the axial direction on the carrier shaft  45   a  extended from the carrier  45  which is the first element of the power distribution and integration mechanism  40  and is always meshed with the first gear  61   b  fixed to the countershaft  65 . Likewise, the third gear  63   a  of the third speed gear train is also held rotatably and non-movably in the axial direction on the carrier shaft  45   a , and is always meshed with the third gear  63   b  fixed to the countershaft  65 . According to the present embodiment, the clutch C 1  is provided on the carrier shaft  45   a  side (counter drive gear side) so as to selectively fix one of the first gear  61   a  (first speed gear train) and the third gear  63   a  (third speed gear train) to the carrier shaft  45   a  and to be able to rotatably release both the first gear  61   a  and the third gear  63   a  with respect to the carrier shaft  45   a . According to the present embodiment, the clutch C 1  is configured as, for example, a dog clutch which can mesh a dog which is held non-rotatably and movably in the axial direction on the carrier shaft  45   a  with one of the dog fixed to the first gear  61   a  and the dog fixed to the third gear  63   a  with less loss, and can release the mesh therebetween; and is driven by the above described actuator  88 . These gears  61   a  and  61   b  of the first speed gear train, the gears  63   a  and  63   b  of the third speed gear train, and the clutch C 1  constitute a first transmission mechanism of the transmission  60 . Moreover, the second gear  62   a  of the second speed gear train is held rotatably and non-movably in the axial direction on the first motor shaft  46  capable of being coupled with the sun gear  41  which is a second element of the power distribution and integration mechanism  40  through the clutch C 0 ; and is always meshed with the second gear  62   b  fixed to the countershaft  65 . Likewise, the fourth gear  64   a  of the fourth speed gear train is also held rotatably and non-movably in the axial direction on the first motor shaft  46 , and is always meshed with the fourth gear  64   b  fixed to the countershaft  65 . According to the present embodiment, the clutch C 2  is provided at the first motor shaft  46  side (counter drive gear side) so as to selectively fix one of the second gear  62   a  (second speed gear train) and the fourth gear  64   a  (fourth speed gear train) with respect to the first motor shaft  46  and to be able to rotatably release both the second gear  62   a  and the fourth gear  64   a  with respect to the first motor shaft  46 . According to the present embodiment, the clutch C 2  is also configured as, for example, a dog clutch which can mesh a dog which is held non-rotatably and movably in the axial direction on the first motor shaft  46  with one of the dog fixed to the second gear  62   a  and the dog fixed to the fourth gear  64   a  with less loss, and can release the mesh therebetween; and is driven by the above described actuator  88 . These gears  62   a  and  62   b  of the second speed gear train, the gears  64   a  and  64   b  of the fourth speed gear train, and the clutch C 2  constitute a second transmission mechanism of the transmission  60 . It should be noted that according to the present embodiment, the actuator  88  is illustrated as a single unit, but it is obvious that the clutches C 0 , C 1 , and C 2  may be driven individually. 
     Moreover, the power transmitted from the carrier shaft  45   a  or the first motor shaft  46  to the countershaft  65  is transmitted to the drive shaft  67  through the gears  66   a  and  66   b , and is finally output to the rear wheels  69   a  and  69   b  as the drive wheels through the differential gear  68 . It should be noted that, like the transmission  60  of the present embodiment, the clutches C 1  and C 2  are provided on the carrier shaft  45   a  and the first motor shaft  46  side, thereby enabling the reduction of the loss when the gears  61   a  to  64   a  are fixed to the carrier shaft  45   a  or the first motor shaft  46  by the clutches C 1  and C 2 . That is, depending on the gear ratio of an individual gear train, particularly regarding the second transmission mechanism including the fourth speed gear train having a small reduction gear ratio, the rotation speed of the gear  64   a  which is idle before being fixed to the first motor shaft  46  by the clutch C 2  is lower than the rotation speed of the corresponding gear  64   b  on the countershaft  65  side. Therefore, a dog of the gear  64   a  can be engaged with a dog of the first motor shaft  46  with less loss by at least installing the clutch C 2  on the first motor shaft  46  side. It should be noted that regarding the first transmission mechanism including the first speed gear train having a large reduction gear ratio, the clutch C 1  may be provided on the countershaft  65  side. 
     According to the transmission  60  configured as described above, when the clutch C 2  is released, as well as one of the first gear  61   a  (first speed gear train) and the third gear  63   a  (third speed gear train) is fixed to the carrier shaft  45   a  by the clutch C 1 , the power from the carrier shaft  45   a  can be transmitted to the drive shaft  67  through the first gear  61   a  (first speed gear train) or the third gear  63   a  (third speed gear train) and the countershaft  65 . Moreover, when the clutch C 0  is connected, the clutch C 1  is released, and the clutch C 2  is used to fix one of the second gear  62   a  (second speed gear train) and the fourth gear  64   a  (fourth speed gear train) to the first motor shaft  46 , the power from the first motor shaft  46  can be transmitted to the drive shaft  67  through the second gear  62   a  (second speed gear train) or the fourth gear  64   a  (fourth speed gear train) and the countershaft  65 . Hereinafter, as needed, a state of transmitting power using the first speed gear train is referred to as “first speed state (1st speed)”, a state of transmitting power using the second speed gear train is referred to as “second speed state (2nd speed)”, a state of transmitting power using the third speed gear train is referred to as “third speed state (3rd speed)” and a state of transmitting power using the fourth speed gear train is referred to as “fourth speed state (4th speed)”. Moreover, according to the transmission  60  of the present embodiment, the clutches C 1  and C 2  are provided on the carrier shaft  45   a  and the first motor shaft  46  side, thereby enabling the reduction of the loss when the gears  61   a  to  64   a  are fixed to the carrier shaft  45   a  or the first motor shaft  46  by the clutches C 1  and C 2 . That is, depending on the gear ratio of an individual gear train, particularly regarding the second transmission mechanism including the fourth speed gear train having a small reduction gear ratio, the rotation speed of the gear  64   a  which is idle before being fixed to the first motor shaft  46  by the clutch C 2  is lower than the rotation speed of the corresponding gear  64   b  on the countershaft  65  side. Therefore, a dog of the gear  64   a  can be engaged with a dog of the first motor shaft  46  with less loss by at least installing the clutch C 2  on the first motor shaft  46  side. It should be noted that regarding the first transmission mechanism including the first speed gear train having a large reduction gear ratio, the clutch C 1  may be provided on the countershaft  65  side. 
     The hybrid ECU  70  is configured as a microprocessor around a CPU  72 , and in addition to the CPU  72 , includes a ROM  74  for storing a processing program; a RAM  76  for temporarily storing data; an input/output port (not shown); and a communication port (not shown). The hybrid ECU  70  receives an ignition signal from an ignition switch (start switch)  80 ; a shift position SP from a shift position sensor  82  for detecting the shift position SP which is an operation position of a shift lever  81 ; an accelerator opening Acc from an accelerator pedal position sensor  84  for detecting the amount of depression of an accelerator pedal  83 ; a brake pedal position BP from a brake pedal position sensor  86  for detecting the amount of depression of a brake pedal  85 ; and a vehicle speed V from a vehicle speed sensor  87  through the input port. As described above, the hybrid ECU  70  is connected to the engine ECU  24 , the motor ECU  30 , and the battery ECU  36  through a communication port, and sends and receives various kinds of control signals and data to and from the engine ECU  24 , the motor ECU  30 , and the battery ECU  36 . Moreover, the hybrid ECU  70  also controls the actuator  88  which drives the clutch C 0 , and the clutches C 1  and C 2  of the transmission  60 . 
     Hereinafter, with reference to  FIGS. 2 to 8 , the outline of an operation of the hybrid vehicle  20  of the present embodiment will be described. When the above described hybrid vehicle  20  runs with an engagement of the clutch C 0  and an operation of the engine  22 , if the clutch C 2  is released and the first gear  61   a  (first speed gear train) is fixed to the carrier shaft  45   a  by the clutch C 1 , as shown in  FIG. 2 , the power from the carrier shaft  45   a  can be changed in speed (reduced) based on the speed ratio of the first speed gear train (first gears  61   a  and  61   b ) and can be output to the drive shaft  67 . Moreover, according to the change in vehicle speed V, as shown in  FIG. 3 , in the first speed state in which the first gear  61   a  (first speed gear train) is fixed to the carrier shaft  45   a  by the clutch C 1 , if the second gear  62   a  (second speed gear train) is fixed to the first motor shaft  46  by the clutch C 2  and each of the torque commands to the motors MG 1  and MG 2  is set to a value of 0, the power (torque) from the engine  22  can be transmitted mechanically (directly) to the drive shaft  67  without conversion to electrical energy at a fixed (constant) speed ratio (a value between the speed ratio of the first speed gear train and the speed ratio of the second speed gear train). Hereinafter, the state ( FIG. 3 ) in which the first speed gear train of the transmission  60  is used to couple the carrier  45  which is a first element of the power distribution and integration mechanism  40 , and the second speed gear train of the transmission  60  is used to couple the sun gear  41  which is a second element thereof to the drive shaft  67  respectively is referred to as “1st to 2nd simultaneous engagement state”. 
     Further, in the 1st to 2nd simultaneous engagement state shown in  FIG. 3 , when the clutch C 1  is released as shown by the two-dot chain line in  FIG. 4 , the clutch C 2  allows only the second gear  62   a  (second speed gear train) to be fixed to the first motor shaft  46  (sun gear  41 ) and thereby, the power from the first motor shaft  46  can be changed in speed based on the speed ratio of the second speed gear train (second gears  62   a  and  62   b ) and can be output to the drive shaft  67 . Moreover, according to the change in vehicle speed V, in the second speed state in which the clutch C 2  is used to fix the second gear  62   a  (second speed gear train) to the first motor shaft  46  as shown in  FIG. 5 , if the clutch C 1  is used to fix the third gear  63   a  (third speed gear train) to the carrier shaft  45   a  and set each of the torque commands to the motors MG 1  and MG 2  to a value of 0, the power (torque) from the engine  22  can be transmitted mechanically (directly) to the drive shaft  67  without conversion to electrical energy at a fixed (constant) speed ratio (a value between the speed ratio of the second speed gear train and the speed ratio of the third speed gear train) different from that in the 1st to 2nd simultaneous engagement state. Hereinafter, the state ( FIG. 5 ) in which the second speed gear train of the transmission  60  is used to couple the sun gear  41  which is a second element of the power distribution and integration mechanism  40 , and the third speed gear train of the transmission  60  is used to couple to carrier  45  which is a first element thereof to the drive shaft  67  is referred to as “2nd to 3rd simultaneous engagement state”. 
     Then, when the clutch C 2  is released in the 2nd to 3rd simultaneous engagement state shown in  FIG. 5 , as shown by the one-dot chain line in  FIG. 6 , the clutch C 1  allows only the third gear  63   a  (third speed gear train) to be fixed to the carrier shaft  45   a  (carrier  45 ). Thereby, the power from the carrier shaft  45   a  can be changed in speed based on the speed ratio of the third speed gear train (third gears  63   a  and  63   b ) and can be output to the drive shaft  67 . Further, according to the change in vehicle speed V as shown in  FIG. 7 , in the third speed state in which the clutch C 1  is used to fix the third gear  63   a  (third speed gear train) to the carrier shaft  45   a , when the clutch C 2  is used to fix the fourth gear  64   a  (fourth speed gear train) to the first motor shaft  46  and set the individual torque command to the motors MG 1  and MG 2  to a value of 0, the power (torque) from the engine  22  can be transmitted mechanically (directly) to the drive shaft  67  without conversion to electrical energy at a fixed (constant) speed ratio (a value between the speed ratio of the third speed gear train and the speed ratio of the fourth speed gear train) different from that in the 1st to 2nd simultaneous engagement state and the 2nd to 3rd simultaneous engagement state. Hereinafter, the state ( FIG. 7 ) in which the third speed gear train of the transmission  60  is used to fix the carrier  45  which is a first element of the power distribution and integration mechanism  40 , and the fourth speed gear train of the transmission  60  is used to fix the sun gear  41  which is a second element thereof to the drive shaft  67  respectively is referred to as “3rd to 4th simultaneous engagement state”. Afterward, in the 3rd to 4th simultaneous engagement state shown in  FIG. 7 , when the clutch C 1  is released as shown by the two-dot chain line in  FIG. 8 , the clutch C 2  allows only the fourth gear  64   a  (fourth speed gear train) to be fixed to the first motor shaft  46  (sun gear  41 ). Thereby, the power from the first motor shaft  46  can be changed in speed based on the speed ratio of the fourth speed gear train (fourth gears  64   a  and  64   b ) and can be output to the drive shaft  67 . Here, with reference to  FIGS. 2 to 8 , the S axis indicates a rotation speed (rotation speed Nm 1  of the motor MG 1 , namely, the first motor shaft  46 ) of the sun gear  41  of the power distribution and integration mechanism  40 ; the R axis indicates a rotation speed (rotation speed Ne of the engine  22 ) of the ring gear  42  of the power distribution and integration mechanism  40 ; and the C axis indicates a rotation speed (the carrier shaft  45   a  and the ring gear  52  of the reduction gear mechanism  50 ) of the carrier  45  of the power distribution and integration mechanism  40  respectively. Moreover, each of the  61   a  axis to  63   a  axis, the  65  axis, and the  67  axis indicates a rotation speed of the first gear  61   a  to the fourth gear  64   a  of the transmission  60 , the countershaft  65 , and the drive shaft  67  respectively. 
     While the hybrid vehicle  20  is running with an operation of the engine  22  as described above, when the transmission  60  is set to the first or third speed state, the carrier  45  of the power distribution and integration mechanism  40  becomes an output element, thereby allowing the motors MG 1  and MG 2  to be drive-controlled such that the motor MG 2  connected to the carrier  45  functions as a motor, and the motor MG 1  connected to the sun gear  41  which becomes a reaction element functions as a generator. Hereinafter, the mode in which the motor MG 1  functions as a generator and the motor MG 2  functions as a motor is referred to as a “first torque conversion mode”.  FIG. 9  shows an example of an alignment chart representing a relationship of a rotation speed and torque between an individual element of the power distribution and integration mechanism  40  and an individual element of the reduction gear mechanism  50  in the first torque conversion mode. Here, with reference to  FIG. 9 , the S axis, the R axis, and the C axis denote like elements shown in  FIGS. 2 to 8 ; the  54  axis denotes a rotation speed of the carrier  54  of the reduction gear mechanism  50 ; the  51  axis denotes a rotation speed (rotation speed Nm 2  of the motor MG 2 , namely, the second motor shaft  55 ) of the sun gear  51  of the reduction gear mechanism  50 ; ρ denotes a gear ratio (the number of teeth of the sun gear  41 /the number of teeth of the ring gear  42 ) of the power distribution and integration mechanism  40 ; and pr denotes a reduction gear ratio (the number of teeth of the sun gear  51 /the number of teeth of the ring gear  52 ) of the reduction gear mechanism  50  respectively. Moreover, in  FIG. 9 , a thick arrow indicates torque acting on an individual element. An upward arrow in the Figure indicates that the value of torque is positive; and a downward arrow in the Figure indicates that the value of torque is negative (same as in  FIGS. 2 to 8 ,  10  and  11 ). In the first torque conversion mode, the power distribution and integration mechanism  40  and the motors MG 1  and MG 2  perform torque conversion on power from the engine  22  and output the power to the carrier  45 ; and the ratio between the rotation speed Ne of the engine  22  and the rotation speed of the carrier  45  which is an output element can be changed steplessly and continuously by controlling the rotation speed of the motor MG 1 . While the hybrid vehicle  20  is running with an operation of the engine  22 , when the transmission  60  is set to the second or fourth speed state, the sun gear  41  of the power distribution and integration mechanism  40  becomes the output element, thereby allowing the motors MG 1  and MG 2  to be drive-controlled such that the motor MG 1  connected to the sun gear  41  functions as a motor, and the motor MG 2  connected to the carrier  45  which becomes a reaction element functions as a generator. Hereinafter, the mode in which the motor MG 2  functions as a generator and the motor MG 1  functions as a motor is referred to as a “second torque conversion model”.  FIG. 10  shows an example of an alignment chart representing a relationship of a rotation speed and torque between an individual element of the power distribution and integration mechanism  40  and an individual element of the reduction gear mechanism  50  in the second torque conversion mode. In the second torque conversion mode, the power distribution and integration mechanism  40  and the motors MG 1  and MG 2  perform torque conversion on power from the engine  22  and output the power to the sun gear  41 , and the ratio between the rotation speed Ne of the engine  22  and the rotation speed of the sun gear  41  which is an output element can be changed steplessly and continuously by controlling the rotation speed of the motor MG 2 . It should be noted that the reference characters in  FIG. 10  are the same as in  FIG. 9 . 
     As described above, according to the hybrid vehicle  20  of the present embodiment, with the change in speed ratio (speed state) of the transmission  60 , the first torque conversion mode and the second torque conversion mode can be alternately switched; and thus, particularly when the rotation speed Nm 2  or Nm 1  of the motor MG 2  or MG 1  which functions as a motor is increased, the rotation speed Nm 1  or Nm 2  of the motor MG 1  or MG 2  which functions as a generator can be prevented from having a negative value. Therefore, the hybrid vehicle  20  can prevent the occurrence of a power circulation in which as the rotation speed of the motor MG 1  becomes negative in the first torque conversion mode, the motor MG 2  uses part of the power output to the carrier shaft  45   a  to generate electric power and the motor MG 1  consumes the electric power generated by the motor MG 2  and outputs the power; and a power circulation in which as the rotation speed of the motor MG 2  becomes negative in the second torque conversion mode, the motor MG 1  uses part of the power output to the first motor shaft  46  to generate electric power and the motor MG 2  consumes the electric power generated by the motor MG 1  and outputs the power; and can improve power transmission efficiency in a wider driving area. Moreover, as such a power circulation is prevented, a maximum rotation speed of the motors MG 1  and MG 2  can also be suppressed, thereby allowing the motors MG 1  and MG 2  to be made compact. Further, the hybrid vehicle  20  can mechanically (directly) transmit power from the engine  22  to the drive shaft  67  at a speed ratio uniquely determined depending on the above described 1st to 2nd simultaneous engagement state, the 2nd to 3rd simultaneous engagement state, and the 3rd to 4th simultaneous engagement state, thereby increasing a chance of mechanically outputting the power from the engine  22  to the drive shaft  67  without conversion to electrical energy and further increasing power transmission efficiency in a wider driving area. In general, according to a power output apparatus using the engine, two motors, and the differential rotation mechanism such as a planetary gear mechanism, when the reduction gear ratio between the engine and the drive shaft is relatively large, more engine power is converted to electrical energy, and thus the power transmission efficiency is deteriorated, and the motors MG 1  and MG 2  tend to generate heat. Therefore, the above described simultaneous engagement mode is advantageous particularly when the reduction gear ratio between the engine  22  and the drive shaft is relatively large. Further, according to the hybrid vehicle  20  of the present embodiment, before the speed ratio of the transmission  60  is changed, the simultaneous engagement mode is performed between the first torque conversion mode and the second torque conversion mode. Therefore, a so-called torque loss does not occur at the time of change in speed ratio, and the change in speed ratio, namely, the switching between the first torque conversion mode and the second torque conversion mode can be performed very smoothly and without a shock. 
     Next, with reference to  FIG. 11  and the like, the outline of the motor drive mode will be described, in which, in an engine  22  stopped state, electric power from the battery  35  is used to cause the motor MG 1  and/or the motor MG 2  to output power, by which the hybrid vehicle  20  is driven. According to the hybrid vehicle  20  of the present embodiment, the motor drive mode is broadly classified into a clutch engagement single motor drive mode in which the clutch C 0  is connected and one of the motors MG 1  and MG 2  is caused to output power; a clutch release single motor drive mode in which the clutch C 0  is placed in a released state and one of the motors MG 1  and MG 2  is caused to output power; and a double motor drive mode in which the clutch C 0  is placed in a released state and power from both the motors MG 1  and MG 2  can be used. 
     When the clutch engagement single motor drive mode is performed, with the clutch C 0  being connected, the clutch C 2  of the transmission  60  is placed in a released state and the clutch C 1  is used to fix the first gear  61   a  of the first speed gear train or the third gear  63   a  of the third speed gear train to the carrier shaft  45   a  to cause only the motor MG 2  to output the power, or the clutch C 1  of the transmission  60  is placed in a released state and the clutch C 2  is used to fix the second gear  62   a  of the second speed gear train or the fourth gear  64   a  of the fourth speed gear train to the first motor shaft  46  to cause only the motor MG 1  to output the power. In such a clutch engagement single motor drive mode, the clutch C 0  allows the sun gear  41  of the power distribution and integration mechanism  40  and the first motor shaft  46  to be connected. Therefore, the motor MG 1  or MG 2  which does not output power is idle by being corotated by the motor MG 2  or MG 1  which is outputting power (see the broken line in  FIG. 11 ). Moreover, when the clutch release single motor drive mode is performed, the clutch C 0  is placed in a released state, the clutch C 2  of the transmission  60  is placed in a released state, and the clutch C 1  is used to fix the first gear  61   a  of the first speed gear train or the third gear  63   a  of the third speed gear train to the carrier shaft  45   a  to cause only the motor MG 2  to output power, or the clutch C 1  of the transmission  60  is placed in a released state, the clutch C 2  is used to fix the second gear  62   a  of the second speed gear train or the fourth gear  64   a  of the fourth speed gear train to the first motor shaft  46  to cause only the motor MG 1  to output power. In such a clutch release single motor drive mode, as shown by the one-dot chain line and the two-dot chain line in  FIG. 11 , the clutch C 0  is placed in a released state, and the connection between the sun gear  41  and the first motor shaft  46  is released. Therefore, the crankshaft  26  of the engine  22  which is stopped by a function of the power distribution and integration mechanism  40  is prevented from corotating. In addition, since the clutch C 2  or C 1  is placed in a released state, the motor MG 1  or MG 2  which is stopped can be prevented from corotating, thereby preventing power transmission efficiency from decreasing. When the double motor drive mode is performed, the clutch C 0  is placed in a released state, and the clutches C 1  and C 2  are used to set the transmission  60  to the above described the 1st to 2nd simultaneous engagement state, the 2nd to 3rd simultaneous engagement state, or the 3rd to 4th simultaneous engagement state, and then, at least one of the motors MG 1  and MG 2  is drive-controlled. This can prevent the engine  22  from corotating, can cause both the motors MG 1  and MG 2  to output power, and can transmit large power to the drive shaft  67  in the motor drive mode. Therefore, a so-called starting on a slope can be well performed and a good towing capability and the like at a motor drive can be well maintained. 
     Moreover, according to the hybrid vehicle  20  of the present embodiment, when the clutch release single motor drive mode is selected, the speed ratio (speed state) of the transmission  60  can be easily changed so as to efficiently transmit power to the drive shaft  67 . For example, in the clutch release single motor drive mode, the clutch C 1  of the transmission  60  is used to fix the first gear  61   a  of the first speed gear train or the third gear  63   a  of the third speed gear train to the carrier shaft  45   a  and causes only the motor MG 2  to output power. At this time, the rotation speed of the stopped motor MG 1  is synchronized with the rotation speed of the second gear  62   a  of the second speed gear train or the fourth gear  64   a  of the fourth speed gear train, and the clutch C 2  is used to fix the second gear  62   a  or the fourth gear  64   a  to the first motor shaft  46 . By doing so, the state can be changed to one of the above described 1st to 2nd simultaneous engagement state, the 2nd to 3rd simultaneous engagement state, and the 3rd to 4th simultaneous engagement state, namely, the double motor drive mode. Then, in this state, when the clutch C 1  of the transmission  60  is placed in a released state and only the motor MG 1  is caused to output power, the power output from the motor MG 1  can be transmitted to the drive shaft  67  through the second speed gear train or the fourth speed gear train of the transmission  60 . As a result, according to the hybrid vehicle  20  of the present embodiment, even in the motor drive mode, the transmission  60  can be used to change the rotation speed of the carrier shaft  45   a  and the first motor shaft  46  to increase torque. Therefore, the maximum torque required for the motors MG 1  and MG 2  can be decreased, thereby allowing the motors MG 1  and MG 2  to be made compact. Moreover, in such a motor drive mode, before the speed ratio of the transmission  60  is changed, the simultaneous engagement state of the transmission  60 , namely, the double motor drive mode is performed. Therefore, a so-called torque loss does not occur at the time of change in speed ratio, and thus, the speed ratio can be changed very smoothly and without a shock. It should be noted that when a required driving force is increased or the state of charge (SOC) of the battery  35  is decreased in these motor drive modes, one of the motors MG 1  and MG 2 , whichever does not output power according to the speed ratio of the transmission  60 , is used to perform cranking of the engine  22 , thereby starting the engine  22 . 
     Subsequently, with reference to  FIGS. 12 to 15 , a detailed description will be given to a procedure for controlling the hybrid vehicle  20  running in the clutch release single motor drive mode in which the clutch C 0  is placed in a released state so as to cause one of the motors MG 1  and MG 2  to output power.  FIG. 12  is a flowchart showing an example of a drive control routine for single motor drive executed by a hybrid ECU  70  when a clutch release single motor drive mode is selected. This routine is executed every predetermined time (e.g., every several msec). 
     When the drive control routine for single motor drive of  FIG. 12  starts, the CPU  72  of the hybrid ECU  70  executes an input process of data required for control such as an accelerator opening Acc from an accelerator pedal position sensor  84 ; a vehicle speed V from a vehicle speed sensor  87 ; rotation speeds Nm 1  and Nm 2  of the motors MG 1  and MG 2 ; a current speed ratio γ which is a current speed ratio of the transmission  60  and a target speed ratio γ*; a charge-discharge power demand pb*; and an input limit Win and an output limit Wout of the battery  35  (Step S 100 ). Here, the rotation speeds Nm 1  and Nm 2  of the motors MG 1  and MG 2  are assumed to be input from the motor ECU  40  through communication. In addition, the current speed ratio γ of the transmission  60  is a value stored in a predetermined area of the RAM  76  when the speed change of the transmission  60  is completed; the target speed ratio γ* is passed through a transmission process routine (not shown) and is set according to the vehicle speed V, the torque demand, and the like, and is stored in a predetermined area of the RAM  76 . Further, the charge/discharge power demand Pb* is assumed to be electric power which is input from the battery ECU  36  through communication, namely, the electric power which is set as electric power to be charged or discharged from the battery  35  by the battery ECU  36  based on the state of charge (SOC) of the battery  35  and the like. Moreover, the input limit Win as a charge allowable power which is electric power allowable to charge the battery  35  and the output limit Wout as a discharge allowable power which is electric power allowable to discharge the same are assumed to be input from the battery ECU  36  through communication, namely, the electric power which is set as the electric power based on the battery temperature Tb of the battery  35  detected by the temperature sensor  37  and the state of charge (SOC) of the battery  35 . It should be noted that the input limit Win and the output limit Wout of the battery  35  can be set by setting a basic value of the input limit Win and the output limit Wout based on the battery temperature Tb, setting an output limit correction coefficient and an input limit correction coefficient based on the state of charge (SOC) of the battery  35  and multiplying the set basic values of the input limit Win and the output limit Wout by the correction coefficients. 
     After the data input process in Step S 100 , the torque demand Tr* to be output to the drive shaft  67  is set based on the input accelerator opening Acc and the vehicle speed V, and the power demand P* required for the entire hybrid vehicle  20  is set (Step S 110 ). According to the present embodiment, a torque demand setting map (not shown) which preliminarily defines the relationship among the accelerator opening Acc, the vehicle speed V, and the torque demand Tr* is stored in ROM  74 . As the torque demand Tr*, a torque demand corresponding to a given accelerator opening Acc and a vehicle speed V is derived and set from the map. Moreover, according to the present embodiment, the power demand P* is calculated by multiplying the torque demand Tr* set in Step S 110  by the vehicle speed V indicating the rotation speed of the drive shaft  67  and by a conversion factor k; and adding the product to the charge/discharge power demand Pb* (assuming the charge demanding side as positive) plus a loss (Loss) Subsequently, for example, a determination is made as to whether the engine  22  remains stopped or not based on the power demand P* set in Step S 110 , the output limit Wout (or the state of charge (SOC)) input in Step S 100 , and the like (Step S 120 ). If a determination is made that the engine  22  remains stopped in Step S 120 , further a determination is made as to whether the current speed ratio γ of the transmission  60  matches the target speed ratio γ*, which are both input in Step S 100  (Step S 130 ). If the current speed ratio γ matches the target speed ratio γ*, further a determination is made as to which one of the first to fourth speed gear trains matches the current speed ratio γ (Step S 140 ). 
     If the current speed ratio γ corresponds to the first speed gear train or the third speed gear train, only the motor MG 2  should be caused to output power. First, the torque command Tm 1 * to the motor MG 1  is set to a value of 0 (Step S 150 ). Subsequently, each of the input limit Win and the output limit Wout of the battery  35  is divided by the rotation speed Nm 2  of the motor MG 2  input in Step S 100  to calculate the torque restrictions Tmin and Tmax as the upper and lower limits of torque allowed to be output from the motor MG 2  (Step S 160 ). Further, the torque demand Tr* set in Step S 110 , the current speed ratio γ, and the reduction gear ratio ρr of the reduction gear mechanism  50  are used to calculate the tentative motor torque Tm 2   tmp  as the torque to be output from the motor MG 2  based on the following expression (1) (Step S 170 ). The torque command Tm 2 * of the motor MG 2  is set as a value limiting the tentative motor torque Tm 2   tmp  by the torque restrictions Tmin and Tmax calculated in Step S 160  (Step S 180 ). Setting the torque command Tm 2 * of the motor MG 2  in this manner allows the torque to be output from the motor MG 2  to be set as the torque limited within the input limit Win and the output limit Wout of the battery  35 . After the torque commands Tm 1 * and Tm 2 * of the motors MG 1  and MG 2  are set, the set torque commands Tm 1 * and Tm 2 * are sent to the motor ECU  40  (Step S 190 ). Then, the processes following Step S 100  are executed again. The motor ECU  40  which received the torque commands Tm 1 * and Tm 2 * performs switching control on the switching elements of the inverters  31  and  32  so as to allow the motors MG 1  and MG 2  to be driven according to the torque commands Tm 1 * and Tm 2 *.
 
 Tm 2 tmp=−Tr*/γ·ρr   (1)
 
     On the contrary, if the current speed ratio γ corresponds to the second speed gear train or the fourth speed gear train, only the motor MG 1  is caused to output power. First, the torque command Tm 1 * to the motor MG 2  is set to a value of 0 (Step S 200 ) Next, each of the input limit Win and the output limit Wout of the battery  35  is divided by the rotation speed Nm 1  of the motor MG 1  input in Step S 100  to calculate the torque restrictions Tmin and Tmax as the upper and lower limits of torque allowed to be output from the motor MG 1  (Step S 210 ). Further, the torque demand Tr* set in Step S 110  and the current speed ratio γ are used to calculate the tentative motor torque Tm 1   tmp  as torque to be output from the motor MG 1  based on the following expression (2) (Step S 220 ). The torque command Tm 1 * of the motor MG 1  is set as a value limiting the tentative motor torque Tm 2   tmp  by the torque restrictions Tmin and Tmax calculated in Step S 210  (Step S 230 ). Setting the torque command Tm 1 * of the motor MG 1  in this manner allows the torque to be output from the motor MG 1  to be set as the torque limited within the input limit Win and the output limit Wout of the battery  35 . After the torque commands Tm 1 * and Tm 2 * of the motors MG 1  and MG 2  are set, the set torque commands Tm 1 * and Tm 2 * are sent to the motor ECU  40  (Step S 190 ). Then, the processes following Step S 100  are executed again.
 
 Tm 1 tmp=Tr*/γ   (2)
 
     On the contrary, if a determination is made in Step S 120  that the engine  22  should start, one of the motors MG 1  and MG 2  needs to be placed in a state capable of performing cranking of the engine  22  so as to be able to start the engine  22 . However, at this stage, the clutch C 0  is in a state of being released, and the motors MG 1  and MG 2  are substantially in a state of being disconnected from the engine  22 . Therefore, in order to start the engine  22 , a process of connecting the clutch C 0  needs to be executed before starting the engine  22 . For this reason, after the process in Step S 120 , a predetermined motor synchronization flag Fms is set to a value of 1 so as to be able to connect the clutch C 0  (Step S 240 ). Then, the present routine is terminated and the execution of a motor synchronization control routine described below is started. Moreover, if a determination is made in Step S 130  that the current speed ratio γ does not match the target speed ratio γ*, a change speed flag Fgc is set to a value of 1 (Step S 250 ) so as to instruct the execution of a process of changing the speed ratio (speed state) of the transmission  60  from current speed ratio γ to the target speed ratio γ*. Then, the present routine is terminated and a process of changing the speed ratio based on the aforementioned procedure is executed. It should be noted that the process of changing the speed ratio of the transmission  60  is not directly related to the present invention, and thus the detailed description is omitted here. 
       FIG. 13  is a flowchart showing an example of the motor synchronization control routine executed by the hybrid ECU  70 . When the motor synchronization flag Fms is set to a value of 1, this routine is executed every predetermined time (e.g., every several msec). When the motor synchronization control routine of  FIG. 13  starts, the CPU  72  of the hybrid ECU  70  executes a data input process required for control such as the accelerator opening Acc, the vehicle speed V, the rotation speeds Nm 1  and Nm 2  of the motors MG 1  and MG 2 , the current speed ratio γ of the transmission  60 , and the input limit Win and the output limit Wout of the battery  35  (Step S 300 ). Then, the torque demand Tr* to be output to the drive shaft  67  is set (Step S 310 ). Further, a determination is made as to which one of the first speed gear train to the fourth speed gear train corresponds to the current speed ratio γ (Step S 320 ). 
     If the current speed ratio γ corresponds to the first speed gear train or the third speed gear train, the carrier shaft  45   a  is coupled to the drive shaft  67  by the transmission  60 , and only the motor MG 2  outputs power. Since the clutch C 0  is released, the rotation speed Nm 1  (value 0) of the motor MG 1  (first motor shaft  46 ) is different from the rotation speed of the sun gear  41  of the power distribution and integration mechanism  40  when the clutch C 0  is engaged. For this reason, if the current speed ratio γ corresponds to the first speed gear train or the third speed gear train, in order to rotatably synchronize the motor MG 1 , namely, the first motor shaft  46  with the sun gear  41  of the power distribution and integration mechanism  40 , the rotation speed Nm 2  of the motor MG 2  input in Step S 300 , the gear ratio ρ of the power distribution and integration mechanism  40 , and the reduction gear ratio ρr of the reduction gear mechanism  50  are used to calculate the target rotation speed Nm 1 * of the motor MG 1  based on the following expression (3) (Step S 330 ). The expression (3) is an expression for matching the rotation speed Nm 1  of the motor MG 1  (second motor) which is released from the coupling to the drive shaft  67 , with the rotation speed of the sun gear (second element)  41  at the time of drive source element connection based on the rotation speed Nm 2  of the motor MG 2  (first motor) coupled to the drive shaft  67 , and can be easily derived from the alignment chart of  FIG. 9 . After the target rotation speed Nm 1 * of the motor MG 1  is obtained, a determination is made as to whether the absolute value of a deviation between the target rotation speed Nm 1 * and the rotation speed Nm 1  of the motor MG 1  input in Step S 300  is greater than a predetermined value a (Step S 340 ). If the absolute value is greater than the predetermined value α, the calculation of the following expression (4) is executed based on the target rotation speed Nm 1 * calculated in Step S 330  and the rotation speed Nm 1  of the motor MG 1  to set the torque command Tm 1 * of the motor MG 1  (Step S 350 ). It should be noted that the expression (4) is a relational expression in feedback control for rotating the motor MG 1  at the target rotation speed Nm 1 *; in the expression (4), “k 11 ” of the first term on the right-hand side is a gain of the proportional term; and “k 121 ” of the second term on the right-hand side is a gain of the integral term. Then, a deviation between the input limit Win and the output limit Wout of the battery  35  and the power consumption (generated electric power) of the motor MG 1  obtained as a product between the torque command Tm 1 * of the motor MG 1  set in S 350  and the current rotation speed Nm 1  of the motor MG 1  is divided by the rotation speed Nm 2  of the motor MG 2 . By doing so, calculation is made on torque restrictions Tmin and Tmax as the upper and lower limits of the torque allowed to be output from the motor MG 2  (Step S 360 ). Further, the tentative motor torque Tm 2   tmp  as the torque to be output from the motor MG 2  is calculated based on the above expression (1) (Step S 370 ). The torque command Tm 2 * of the motor MG 2  is set as a value limiting the tentative motor torque Tm 2   tmp  by the torque restrictions Tmin and Tmax calculated in Step S 360  (Step S 380 ). Setting the torque command Tm 2 * of the motor MG 2  in this manner allows the torque to be output from the motor MG 2  to be set as the torque limited within the input limit Win and the output limit Wout of the battery  35 . After the torque commands Tm 1 * and Tm 2 * of the motors MG 1  and MG 2  are set, the set torque commands Tm 1 * and Tm 2 * are sent to the motor ECU  40  (Step S 390 ). Then, the processes following Step S 300  are executed again.
 
 Nm 1 *=ρr·Nm 2/ρ·(1−ρ)  (3)
 
 Tm 1 *=k 11( Nm 1 *−Nm 1)+ k 12∫( Nm 1 *−Nm 1) dt   (4)
 
     On the contrary, if the current speed ratio γ corresponds to the second speed gear train or the fourth speed gear train, the first motor shaft  46  is coupled to the drive shaft  67  by the transmission  60 , and only the motor MG 1  outputs power. Since the clutch C 0  is released, the rotation speed (value 0) of the carrier shaft  45   a  is different from the rotation speed of the carrier  45  of the power distribution and integration mechanism  40  when the clutch C 0  is engaged. For this reason, if the current speed ratio γ corresponds to the second speed gear train or the fourth speed gear train, in order to synchronize (correspond to) the rotation speed Nm 2  of the motor MG 2  with the rotation speed of the carrier  45  which does not correspond to the clutch C 0  when the clutch C 0  is engaged, the rotation speed Nm 1  of the motor MG 1  input in Step S 300 , the gear ratio ρ of the power distribution and integration mechanism  40 , and the reduction gear ratio ρr of the reduction gear mechanism  50  are used to calculate the target rotation speed Nm 2 * of the motor MG 2  based on the following expression (5) (Step S 400 ). The expression (5) is an expression for matching the rotation speed Nm 2  of the motor MG 2  (first motor) which is released from the coupling to the drive shaft  67 , with the rotation speed of the carrier (first element)  45  at the time of drive source element connection based on the rotation speed Nm 1  of the motor MG 1  (second motor) coupled to the drive shaft  67 , and can be easily derived from the alignment chart of  FIG. 10 . After the target rotation speed Nm 2 * of the motor MG 2  is calculated in this manner, a determination is made as to whether the absolute value of a deviation between the target rotation speed Nm 2 * and the rotation speed Nm 2  of the motor MG 1  input in Step S 300  is greater than a predetermined value a (Step S 410 ). If the absolute value is greater than the predetermined value α, the calculation of the following expression (6) is executed based on the target rotation speed Nm 2 * calculated in Step S 400  and the rotation speed Nm 2  of the motor MG 2  to set the torque command Tm 2 * of the motor MG 2  (Step S 420 ). It should be noted that the expression (6) is a relational expression in feedback control for rotating the motor MG 2  at the target rotation speed Nm 2 *; in the expression (6), “k 21 ” of the first term on the right-hand side is a gain of the proportional term; and “k 22 ” of the second term on the right-hand side is a gain of the integral term. Then, a deviation between the input limit Win and the output limit Wout of the battery  35  and the power consumption (generated electric power) of the motor MG 2  obtained as a product between the torque command Tm 2 * of the motor MG 2  set in S 420  and the current rotation speed Nm 2  of the motor MG 2  is divided by the rotation speed Nm 1  of the motor MG 1 . By doing so, calculation is made on the torque restrictions Tmin and Tmax as the upper and lower limits of the torque allowed to be output from the motor MG 1  (Step S 430 ). Further, the tentative motor torque Tm 1   tmp  as the torque to be output from the motor MG 1  is calculated by the above expression (2) (Step S 440 ). The torque command Tm 1 * of the motor MG 1  is set as a value limiting the tentative motor torque Tm 2   tmp  by the torque restrictions Tmin and Tmax calculated in Step S 430  (Step S 450 ). After the torque commands Tm 1 * and Tm 2 * of the motors MG 1  and MG 2  are set in this manner, the set torque commands Tm 1 * and Tm 2 * are sent to the motor ECU  40  (Step S 390 ). Then, the processes following Step S 300  are executed again.
 
 Nm 2 *=Nm 1·ρ/(1−ρ)/ ρr   (5)
 
 Tm 2 *=k 21( Nm 2 *−Nm 2)+ k 22∫( Nm 2 *−Nm 2) dt   (6)
 
     As described above, the present routine is a process of matching the rotation speed Nm 1  or Nm 2  of the motor MG 1  or MG 2  which is released from the coupling to the drive shaft  67 , with the rotation speed of the sun gear  41  or the carrier  45  at the time of drive source element connection based on the rotation speed Nm 2  or Nm 1  of the motor MG 2  or MG 1  which is coupled to the drive shaft  67  (see  FIG. 14 ). When the above described processes are repeatedly executed, the rotation speed Nm 1  or Nm 2  of one of the motors MG 1  and MG 2  substantially matches the target rotation speed Nm 1 * or Nm 2 *, and thus a negative determination is made in Step S 340  or S 410 . At a stage in which a negative determination is made in Step S 340  or S 410 , the rotation speed Nm 1  of the motor MG 1  and the rotation speed Nm 2  of the motor MG 2  indicate a relationship when the clutch C 0  is engaged (see  FIG. 14 ). Therefore, the actuator  88  is drive-controlled so as to connect the sun gear  41  of the power distribution and integration mechanism  40  to the first motor shaft  46  by the clutch C 0 , and the motor synchronization flag Fms is set to a value of 0 (Step S 460 ). Then, in order to start the engine  22  by cranking thereof by one of the motors MG 1  and MG 2 , the engine start flag Fes is set to a value of 1 (Step S 470 ). Then, the present routine is terminated, and the execution of an engine start time drive control routine described later is started. 
       FIG. 15  is a flowchart showing an example of the engine start time drive control routine executed by the hybrid ECU  70 . When the engine start flag Fes is set to a value of 1, this routine is executed every predetermined time (e.g., every several msec). When the engine start time drive control routine of  FIG. 15  starts, the CPU  72  of the hybrid ECU  70  executes a data input process required for control such as the accelerator opening Acc, the vehicle speed V, the rotation speed Ne of the engine  22  (crankshaft  26 ), the rotation speeds Nm 1  and Nm 2  of the motors MG 1  and MG 2 , the current speed ratio γ of the transmission  60 , and the input limit Win and the output limit Wout of the battery  35  (Step S 500 ). Here, the rotation speed Ne of the engine  22  is assumed such that a value calculated based on a signal from a crank position sensor (not shown) attached to the crankshaft  26  is input from the engine ECU  24  through communication. After the data input process in Step S 500 , the torque demand Tr* is set based on the input accelerator opening Acc and the vehicle speed V (Step S 510 ). Further, a determination is made as to which one of the first speed gear train to the fourth speed gear train corresponds to the current speed ratio γ (Step S 520 ). 
     If the current speed ratio γ corresponds to the first speed gear train or the third speed gear train, which means the carrier shaft  45   a  is coupled to the drive shaft  67  by the transmission  60 , the motor MG 1  is used to crank the engine  22 . Therefore, a predetermined cranking torque setting map (not shown) is used to set the torque command Tm 1 * of the motor MG 1  as cranking torque for cranking the engine  22  according to the rotation speed Ne of the engine  22  input in Step S 500  and the elapsed time t since the present routine started (Step S 530 ). Moreover, if the current speed ratio γ corresponds to the second speed gear train or the fourth speed gear train, which means the first motor shaft  46  is coupled to the drive shaft  67  by the transmission  60 , the motor MG 2  is used to crank the engine  22 . Therefore, the above cranking torque setting map is used to set the torque command Tm 2 * of the motor MG 2  as cranking torque for cranking the engine  22  according to the rotation speed Ne of the engine  22  input in Step S 500  and the elapsed time t since the present routine started (Step S 570 ). The cranking torque setting map used in Steps S 530  and S 570  specifies the relationship among the torque commands Tm 1 * and Tm 2 * of the motors MG 1  and MG 2 , the rotation speed Ne of the engine  22 , and the elapsed time t since the start thereof when the engine  22  starts by cranking. According to the present embodiment, the cranking torque setting map is preliminarily created so as to set relatively large torque as the torque commands Tm 1 * and Tm 2 * using a rate process to rapidly increase the rotation speed Ne of the engine  22  immediately after the present routine started, as well as so as to set the torque capable of stably motoring the engine  22  at a predetermined rotation speed or more as the torque commands Tm 1 * and Tm 2 * after the rotation speed Ne of the engine  22  passed the resonance rotation speed region or the time required to pass the resonance rotation speed region elapsed, further so as to reduce the torque commands Tm 1 * and Tm 2 * up to a value of 0 using the rate process after the rotation speed Ne of the engine  22  reached the predetermined rotation speed. If such a cranking torque setting map is used, vibration which can occur when the engine  22  starts can be well suppressed. 
     After the torque command Tm 1 * of the motor MG 1  is set in Step S 530 , in the same manner as in Step S 360  of  FIG. 13 , the torque restrictions Tmax and Tmin are calculated as the upper and lower limits of torque allowed to be output from the motor MG 2  (Step S 540 ). Further, the torque demand Tr*, the torque command Tm 1 *, the gear ratio ρ of the power distribution and integration mechanism  40 , the reduction gear ratio ρr of the reduction gear mechanism  50  are used to calculate the tentative motor torque Tm 2   tmp  as the torque to be output from the motor MG 2  based on the expression (7) (Step S 550 ). It should be noted that the expression (7) can be easily derived from the alignment chart of  FIG. 9 . Then, the torque command Tm 2 * of the motor MG 2  is set by limiting the calculated tentative motor torque Tm 2   tmp  with the torque restrictions Tmax and Tmin calculated in Step S 540  (Step S 560 ). Moreover, after the torque command Tm 2 * of the motor MG 2  is set in Step S 570 , in the same manner as in Step S 430  of  FIG. 13 , the torque restrictions Tmax and Tmin are calculated as the upper and lower limits of the torque allowed to be output from the motor MG 1  (Step S 580 ). Further, the torque demand Tr*, the torque command Tm 2 *, the gear ratio ρ of the power distribution and integration mechanism  40 , and the reduction gear ratio ρr of the reduction gear mechanism  50  are used to calculate the tentative motor torque Tm 1   tmp  as the torque to be output from the motor MG 1  based on the expression (8) (Step S 590 ). It should be noted that the expression (8) can be easily derived from the alignment chart of  FIG. 10 . Then, the torque command Tm 1 * of the motor MG 1  is set by limiting the calculated tentative motor torque Tm 1   tmp  with the torque restrictions Tmax and Tmin calculated in Step S 580  (Step S 600 ).
 
 Tm 2 tmp=−ρr ·( Tr */γ+(1−ρ)/ρ· Tm 1)  (7)
 
 Tm 1 tmp=Tr */γ−ρ/(1−ρ)· Tm 2 */ρr   (8)
 
     After the torque commands Tm 1 * and Tm 2 * are set as described above, the set torque commands Tm 1 * and Tm 2 * are sent to the motor ECU  40  (Step S 610 ). Then, a determination is made as to whether the fuel injection start flag Ffire is set to a value of 0 or not (Step S 620 ). It should be noted that the flag Ffire is set to a value of 0 before the fuel injection control or the ignition control for the engine  22  starts; and the flag Ffire is set to a value of 1 when the fuel injection control or the ignition control starts. If the fuel injection start flag Ffire is set to a value of 0, further, a determination is made as to whether the rotation speed Ne of the engine  22  reaches the threshold Nfire (Step S 630 ). The threshold Nfire indicates a rotation speed of the engine  22  when the fuel injection control or the ignition control of the engine  22  starts, for example, a value of 1000 to 1200 rpm. If the rotation speed Ne of the engine  22  does not reach the threshold Nfire, the processes following the above Step S 500  are executed again. If the rotation speed Ne of the engine  22  reaches the threshold Nfire, the commands to start the fuel injection control and the ignition control are sent to the engine ECU  24 , and the fuel injection start flag Ffire is set a value of 1 (Step S 640 ). Then, a determination is made as to whether the engine  22  has reached its complete explosion (Step S 650 ). If the engine  22  has not reached its complete explosion, the processes following Step S 500  are executed again. It should be noted that if the fuel injection start flag Ffire is set to a value of 1 in Step S 640 , a determination is made in Step S 620  that the fuel injection start flag Ffire is set to a value of 1, the processes in Step S 630  and S 640  are skipped, and a determination is made as to whether the engine  22  has reached its complete explosion (Step S 650 ). If the engine  22  has reached its complete explosion, the fuel injection start flag Ffire and the engine start flag Fes are set to a value of 0 respectively (Step S 660 ), and then, the present routine is terminated. When the engine  22  is started in this manner, the hybrid ECU  70  starts executing a drive control routine for driving the hybrid vehicle  20  with an operation of the engine  22 . 
     As described above, according to the hybrid vehicle  20  of the present embodiment, if the drive source element connection, which is a connection between the sun gear  41  which is a second element of the power distribution and integration mechanism  40  and the first rotating shaft of the motor MG 1  as the second motor by the clutch C 0 , is released, the connection between the motor MG 1  and the motor MG 2  through the power distribution and integration mechanism  40  is released. Therefore, when the clutch release single motor drive mode is selected, the above drive source element connection is released, as well as in an engine  22  stopped state, one of the motor MG 1  corresponding to the clutch C 0 , namely, the first motor shaft  46  and the carrier  45  (carrier shaft  45   a ) which is a first element of the power distribution and integration mechanism  40  and which does not correspond to the clutch C 0  is coupled to the drive shaft  67  by the transmission  60 , and the power from one of the motors MG 1  and MG 2  is transmitted to the drive shaft  67 , the power based on the torque demand Tr* can be efficiently transmitted to the drive shaft  67  by causing the transmission  60  to change the speed of the power from one of the motors MG 1  and MG 2  while preventing the engine  22  and the other one of the motors MG 1  and MG 2  from corotating. Alternatively, when the double motor drive mode is selected, the drive source element connection by the clutch C 0  is released, as well as in an engine  22  stopped state, both the motor MG 1  corresponding to the clutch C 0 , namely, the first motor shaft  46  and the carrier  45  (carrier shaft  45   a ) which does not correspond to the clutch C 0  are coupled to the drive shaft  67  by the transmission  60 , the power output from at least one of the motors MG 1  and MG 2  can be transmitted to the drive shaft  67  at a predetermined fixed speed ratio while preventing the engine  22  from corotating. Therefore, larger power can be output to the drive shaft  67  in comparison with the mode in which only one of the motors MG 1  and MG 2  is caused to output power. Further, according to the hybrid vehicle  20 , if the clutch engagement single motor drive mode is selected, the power from one of the motors MG 1  and MG 2  can be transmitted to the drive shaft  67  by causing the transmission  60  to change the speed thereof with the above drive source element connection maintained. Thereby, according to the hybrid vehicle  20 , in a plurality of drive modes, the power based on the torque demand Tr* can be efficiently transmitted to the drive shaft  67  by causing the transmission  60  to change the speed of the power output from at least one of the motors MG 1  and MG 2 . It should be noted that in the clutch engagement single motor drive mode, when the power from one of the motors MG 1  and MG 2  is transmitted to the drive shaft  67  with the above drive source element connection maintained, the engine  22  is prevented from corotating; and in this state, if the engine  22  is cranked by the other one of the motors MG 1  and MG 2  which does not output power, the engine  22  can be quickly started. As described above, according to the hybrid vehicle  20 , in a plurality of drive modes, the power based on the torque demand Tr* can be efficiently transmitted to the drive shaft  67  by causing the transmission  60  to change the speed of the power output from at least one of the motors MG 1  and MG 2 , thereby well improving fuel consumption and drive performance. 
     Moreover, according to the hybrid vehicle  20  of the present embodiment, when the drive control routine for single motor drive of  FIG. 12  is executed, namely, when in a state in which the drive source element connection by the clutch C 0  is released, the engine  22  is stopped, and only one of the motors MG 1  and MG 2  is coupled to the drive shaft  67  by the transmission  60 , as well as one of the motors MG 1  and MG 2  is caused to output power, if a determination is made in Step S 120  of  FIG. 12  that an engine start condition is established, the motor synchronization control routine ( FIG. 13 ) is executed as a rotation speed adjustment process of adjusting the rotation speed Nm 1  or Nm 2  of the other one of the motors MG 1  and MG 2  which does not correspond to the current speed ratio γ and is not coupled to the drive shaft  67  so as to enable the drive source element connection while the power based on the torque demand Tr* can be output to the drive shaft  67 . Then, the actuator  88  is drive-controlled so as to connect the sun gear  41  of the power distribution and integration mechanism  40  to the first motor shaft  46  by the clutch C 0  (Step S 460  of  FIG. 13 ). Then, in a state of such a drive source element connection, when the engine start time drive control routine of  FIG. 15  is executed, the engine  22  can be started by cranking the engine  22  by one of the motors MG 1  and MG 2  while the power based on the torque demand Tr* can be output to the drive shaft  67 . As described above, while the hybrid vehicle  20  is running in the clutch release single motor drive mode, if the engine start condition is established, the drive source element connection can be executed by executing the motor synchronization control routine of  FIG. 13 . Then, if the drive source element connection is executed by the clutch C 0 , the engine  22  can be started while the power based on the torque demand Tr* is appropriately output to the drive shaft  67 . 
     Further, the transmission  60  of the present embodiment is a parallel shaft-type transmission which includes: a first transmission mechanism having the first speed gear train and the third speed gear train which are parallel shaft-type gear trains capable of coupling the carrier  45  which is a first element of the power distribution and integration mechanism  40  which does not correspond to the clutch C 0 , to the drive shaft  67 ; and a second transmission mechanism having the second speed gear train and the fourth speed gear train which are parallel shaft-type gear trains capable of coupling the first motor shaft  46  of the motor MG 1  corresponding to the clutch C 0 , to the drive shaft  67 . Therefore, such a transmission  60  can selectively couple, to the drive shaft  67 , one of or both of the first motor shaft  46  of the motor MG 1  corresponding to the clutch C 0  and the carrier  45  (carrier shaft  45   a ) not corresponding to the clutch C 0 . It should be noted that according to the hybrid vehicle  20  of the present embodiment, a planetary gear transmission may be adopted instead of the parallel shaft-type transmission  60 . 
       FIG. 16  is a schematic configuration view of a planetary gear-type transmission  100  applicable to the hybrid vehicle  20  of the present embodiment. The transmission  100  shown in the same Figure can also set the speed ratio (speed state) at a plurality of stages; and includes: a first transmission planetary gear mechanism  110  capable of coupling, to the drive shaft  67 , the carrier  45  (carrier shaft  45   a ) which is a first element of the power distribution and integration mechanism  40  and which does not correspond to the clutch C 0 ; a second transmission planetary gear mechanism  120  capable of coupling, to the drive shaft  67 , the first motor shaft  46  of the motor MG 1  which corresponds to the clutch C 0 ; a brake B 1  (first fixing mechanism) provided to the first transmission planetary gear mechanism  110 ; a brake B 2  (second fixing mechanism) provided to the second transmission planetary gear mechanism  120 ; a brake B 3  (third fixing mechanism); the clutch C 1  (transmission connection/disconnection mechanism), and the like. The first transmission planetary gear mechanism  110  and the brake B 1  constitute the first transmission mechanism of the transmission  100 ; and the second transmission planetary gear mechanism  120  and the brake B 2  constitute the second transmission mechanism of the transmission  100 . As shown in  FIG. 16 , the first transmission planetary gear mechanism  110  is a single pinion planetary gear mechanism which include: a sun gear  111  connected to the carrier shaft  45   a ; a ring gear  112  which is an internal gear arranged concentrically with this sun gear  111 ; a carrier  114  having a plurality of pinion gears  113  which are meshed with both the sun gear  111  and the ring gear  112 , as well as being connected to the drive shaft  67 ; and is configured such that the sun gear  111  (input element), the ring gear  112  (fixable element), and the carrier  114  (output element) can be differentially rotated with each other. In addition, the second transmission planetary gear mechanism  120  is a single pinion planetary gear mechanism which includes: a sun gear  121  connected to the first motor shaft  46 ; a ring gear  122  which is an internal gear arranged concentrically with this sun gear  121 ; and a carrier  114  common to the first transmission planetary gear mechanism  110  having a plurality of pinion gears  123  which are meshed with both the sun gear  121  and the ring gear  122 ; and is configured such that the sun gear  121  (input element), the ring gear  122  (fixable element), and the carrier  114  (output element) can be differentially rotated with each other. In the example of  FIG. 16 , the second transmission planetary gear mechanism  120  is arranged in parallel so as to be located coaxially and forward in the vehicle with respect to the first transmission planetary gear mechanism  110 , and the gear ratio ρ 2  (the number of teeth of the sun gear  121 /the number of teeth of the ring gear  122 ) of the second transmission planetary gear mechanism  120  is set so as to be a little larger than the gear ratio (the number of teeth of the sun gear  111 /the number of teeth of the ring gear  112 ) ρ 1  of the first transmission planetary gear mechanism  110 . 
     The brake B 1  can non-rotatably fix the ring gear  112  of the first transmission planetary gear mechanism  110  with respect to the transmission case and can release the ring gear  112  so as to be rotatable, and can be driven by the above described electric, electromagnetic, or hydraulic actuator  88 . In addition, the brake B 2  can non-rotatably fix the ring gear  122  of the second transmission planetary gear mechanism  120  with respect to the transmission case and can release the ring gear  122  so as to be rotatable; and can be driven by the actuator  88  in the same manner as for the brake B 1 . Further, the brake B 3  can non-rotatably fix the first motor shaft  46 , namely, the sun gear  41  which is a second element of the power distribution and integration mechanism  40  through the stator  130  fixed to the first motor shaft  46  with respect to the transmission case; can release the stator  130  to cause the first motor shaft  46  to be rotatable; and can be driven by the actuator  88  in the same manner as for the brakes B 1  and B 2 . In addition, the clutch C 1  can perform a connection and a release of the connection between the carrier  114  which is an output element of first transmission planetary gear mechanism  110  and the ring gear  112  which is a fixable element; and can be driven by the actuator  88  in the same manner as for the brakes B 1  to B 3 . The clutch C 1  can be configured as, for example, a dog clutch which can mesh a dog fixed to the carrier  114  with a dog fixed to the ring gear  112  with less loss and can release the mesh therebetween. Then, the power transmitted from the carrier  114  of the transmission  100  to the drive shaft  67  is finally output to the rear wheels  69   a  and  69   b  as the drive wheels through the differential gear  68 . The transmission  100  configured as described above can reduce the axial and radial sizes, for example, in comparison with the parallel shaft-type transmission. Moreover, the first transmission planetary gear mechanism  110  and the second transmission planetary gear mechanism  120  can be provided at a downstream side of and coaxially with the engine  22 , the motors MG 1  and MG 2 , and the power distribution and integration mechanism  40 . Therefore, the use of the transmission  100  can simplify the shaft bearing and can reduce the number of shaft bearings. 
     Moreover, the transmission  100  can set the speed ratio at a plurality of stages as described below. That is, the brake B 1  is used to non-rotatably fix the ring gear  112  of the first transmission planetary gear mechanism  110  with respect to the transmission case. By doing so, the power from the carrier shaft  45   a  which does not correspond to the clutch C 0  can be changed in speed at a speed ratio (ρ 1 /(1+ρ 1 )) based on the gear ratio ρ 1  of the first transmission planetary gear mechanism  110  and can be transmitted to the drive shaft  67  (hereinafter, this state is referred to as a “first speed state (1st speed)”). In addition, the brake B 2  is used to non-rotatably fix the ring gear  122  of the second transmission planetary gear mechanism  120  with respect to the transmission case. By doing so, the power from the first motor shaft  46  which corresponds to the clutch C 0  can be changed in speed at a speed ratio (ρ 2 /(1+ρ 2 )) based on the gear ratio ρ 2  of the second transmission planetary gear mechanism  120  and can be transmitted to the drive shaft  67  (hereinafter, this state is referred to as a “second speed state (2nd speed)”). Further, the clutch C 1  is used to connect the carrier  114  and the ring gear  112  of the first transmission planetary gear mechanism  110 . By doing so, the sun gear  111 , the ring gear  112 , and the carrier  114  constituting the first transmission planetary gear mechanism  110  are substantially locked to rotate integrally. Therefore, the power from the carrier shaft  45   a  which does not correspond to the clutch C 0  can be transmitted to the drive shaft  67  at a speed ratio of 1 (hereinafter, this state is referred to as a “third speed state (3rd speed)”). 
     In addition, according to the transmission  100 , the ring gear  112  is fixed by the brake B 1 , and the carrier  45  of the power distribution and integration mechanism  40  which does not correspond to the clutch C 0  is coupled to the drive shaft  67  by the first transmission planetary gear mechanism  110  (first transmission mechanism). In such a first speed state, when the ring gear  122  is fixed by the brake B 2  constituting the second transmission mechanism, the first motor shaft  46  which corresponds to the clutch C 0  is also coupled to the drive shaft  67  by the second transmission planetary gear mechanism  120  (second transmission mechanism). Therefore, the power from the engine  22  or the power from at least one of the motors MG 1  and MG 2  can be mechanically (directly) transmitted to the drive shaft  67  at a fixed speed ratio (this state is referred to as a “1st-2nd speed simultaneous engagement state”). Alternatively, the ring gear  122  of the second transmission planetary gear mechanism  120  which does not correspond to the clutch C 1  which is a transmission connection/disconnection mechanism is non-rotatably fixed. In such a second speed state, the clutch C 1  is used to connect between the carrier  114  which is an output element of the first transmission planetary gear mechanism  110  which corresponds to the clutch C 1  and the ring gear  112  which is a fixable element thereof. Even by doing so, both the first motor shaft  46  which corresponds to the clutch C 0  and the carrier  45  which does not correspond to the clutch C 0  can be coupled to the drive shaft  67 . Therefore, the power from the engine  22  or the power from at least one of the motors MG 1  and MG 2  can be mechanically (directly) transmitted to the drive shaft  67  at a fixed speed ratio different from the above 1st-2nd speed simultaneous engagement state (this state is referred to as a “2nd-3rd speed simultaneous engagement state”). Further, the clutch C 1  is used to connect the carrier  114  and the ring gear  112  of the first transmission planetary gear mechanism  110 . In such a third speed state, the brake B 3  is used to non-rotatably fix the first motor shaft  46 , namely, the sun gear  41  which is a second element of the power distribution and integration mechanism  40  to the transmission case through the stator  130  fixed to the first motor shaft  46 . By doing so, the power from the engine  22  and the motor MG 2  can be mechanically (directly) transmitted to the drive shaft  67  at a fixed speed ratio different from that of the above 1st-2nd speed simultaneous engagement state and the 2nd-3rd speed simultaneous engagement state (this state is referred to as a “3rd speed fixed state”). Alternatively, the ring gear  112  is fixed by the brake B 1  and the carrier  45  of the power distribution and integration mechanism  40  is coupled to the drive shaft  67  by the first transmission planetary gear mechanism  110 . In such a first speed state, the brake B 3  is used to non-rotatably fix the first motor shaft  46 , namely, the sun gear  41  which is a second element of the power distribution and integration mechanism  40  with respect to the transmission case through the stator  130  fixed to the first motor shaft  46 . By doing so, the power from the engine  22  and the motor MG 2  can be mechanically (directly) transmitted to the drive shaft  67  at a fixed speed ratio different from that of the above 1st-2nd speed simultaneous engagement state, the 2nd-3rd speed simultaneous engagement state, and the 3rd speed fixed state (this state is referred to as a “1st speed fixed state”). As described above, the planetary gear transmission  100  can also provide advantages similar to the parallel shaft-type transmission  60 . 
       FIG. 17  is a schematic configuration view of the hybrid vehicle  20 A of a variation of the present embodiment. While the above described hybrid vehicle  20  is configured as a rear-wheel-drive vehicle, the hybrid vehicle  20 A which is a variation thereof is configured as a front-wheel-drive vehicle. As shown in  FIG. 17 , the hybrid vehicle  20 A has a power distribution and integration mechanism  10  which is a single pinion planetary gear mechanism including the sun gear  11 ; the ring gear  12  arranged concentrically with the sun gear  11 ; and the carrier  14  having plurality of pinion gears  13  which are meshed with the sun gear  11  and the ring gear  12 . In this case, the engine  22  is arranged transversely, and the crankshaft  26  of the engine  22  is connected to the carrier  14  which is a third element of the power distribution and integration mechanism  10 . Moreover, the hollow ring gear shaft  12   a  is connected to the ring gear  12  which is a first element of the power distribution and integration mechanism  10 , and the motor MG 2  is connected to the ring gear shaft  12   a  through the reduction gear mechanism  50 A which is a parallel shaft-type gear train and the second motor shaft  55  which extends in parallel to the first motor shaft  46 . Moreover, the clutch C 1  can be used to selectively fix one of the first speed gear train (gear  61   a ) and the third speed gear train (gear  63   a ) which constitute the first transmission mechanism of the transmission  60  to the ring gear shaft  12   a . Further, the sun gear shaft  11   a  is connected to the sun gear  11  which is a second element of the power distribution and integration mechanism  10 , the sun gear shaft  11   a  is connected to the clutch C 0  through the hollow ring gear shaft  12   a , and can be connected to the first motor shaft  46 , namely, the motor MG 1  by the clutch C 0 . Then, the clutch C 2  can be used to selectively fix one of the second speed gear train (gear  62   a ) and the fourth speed gear train (gear  64   a ) constituting the second transmission mechanism of the transmission  60 . As described above, the hybrid vehicle in accordance with the present invention may be configured as a front-wheel-drive vehicle. 
     Hereinbefore, the embodiments of the present invention have been described with reference to drawings, but the present invention is not limited to the above embodiments. It will be apparent that various modifications can be made to the present invention without departing from the spirit and scope of the present invention. 
     That is, according to the above embodiments, the clutch C 0  is provided between the sun gear  41  which is a second element of the power distribution and integration mechanism  40  and the motor MG 1  as a second motor and is configured to perform a connection and a release of the connection therebetween, but the clutch C 0  may be provided between the carrier  45  which is a first element of the power distribution and integration mechanism  40  and the motor MG 2  as a first motor and may be configured to perform a connection and a release of the connection therebetween. Alternatively, the clutch C 0  may be provided between the ring gear  42  which is a third element of the power distribution and integration mechanism  40  and the crankshaft  26  of the engine  22  and may be configured to perform a connection and a release of the connection therebetween. Moreover, the power distribution and integration mechanism provided in the above hybrid vehicle  20  may be a planetary gear mechanism which includes a first sun gear and a second sun gear having a mutually different number of teeth; and a carrier having at least one stepped gear configured by coupling a first pinion gear meshed with the first sun gear and a second pinion gear meshed with the second sun gear. Further, the power distribution and integration mechanism provided in the hybrid vehicle  20  may be a single pinion planetary gear mechanism which includes a sun gear, a ring gear, and a carrier having at least one pinion gear meshed with both the sun gear and the ring gear. Further, the above hybrid vehicles  20  and  20 A may be configured as a rear-wheel-drive based or front-wheel-drive based four-wheel-drive vehicle. In addition, in the above embodiments, the power output apparatus has been described as being mounted on the hybrid vehicles  20  and  20 A, but the power output apparatus in accordance with the present invention may be mounted on a vehicle other than a car, and a mobile body such as vessel and aircraft, and may also be installed in fixed equipment such as construction equipment.