Patent Publication Number: US-10315507-B2

Title: Hybrid vehicle

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a hybrid vehicle and, more particularly, to a hybrid vehicle including first and second rotary electric machines and a transmission unit. 
     2. Description of Related Art 
     There is known a hybrid vehicle including not only an engine, two rotary electric machines and a power split mechanism but also a transmission mechanism between the engine and the power split mechanism. 
     A hybrid vehicle described in International Application Publication No. 2013/114594 employs a series-parallel hybrid system. In the vehicle having a series-parallel hybrid system, the power of an engine is transmitted to a first rotary electric machine (first motor generator) and is used to generate electric power, while part of the power of the engine is also transmitted to drive wheels via a power split mechanism. 
     The vehicle described in International Application Publication No. 2013/114594 is allowed to travel by setting both a first rotary electric machine and a second rotary electric machine in a motoring state while stopping an engine. In this case, the torque of each of the two motor generators is used as torque for rotating drive wheels. However, with the configuration of the vehicle described in the International Application Publication No. 2013/114594, such a driving method is allowed only when the rotation speed of the engine is zero. When the engine is rotated, the vehicle is operated in the above-described series-parallel hybrid mode, so the first rotary electric machine is used to generate electric power. Because the first rotary electric machine is in a regeneration state, torque during motoring of the first rotary electric machine cannot be directly used to rotate the drive wheels. 
     Even when the rotation speed of the engine is not zero, but when the torque of the first rotary electric machine is allowed to be directly used to rotate the drive wheels, it is desirable because it is possible to increase driving torque of the vehicle. 
     SUMMARY OF THE INVENTION 
     The invention provides a hybrid vehicle that has an increased number of opportunities that the torque of each of two motor generators is used to drive a wheel. 
     An aspect of the invention provides a hybrid vehicle. The hybrid vehicle includes an internal combustion engine, a first rotary electric machine, a second rotary electric machine, a power transmission unit, a differential unit, a clutch and a controller. 
     The second rotary electric machine is configured to output power to a drive wheel. The power transmission unit includes an input element and an output element. The input element is configured to receive power from the internal combustion engine. The output element is configured to output power input to the input element. The power transmission unit is configured to switch between a non-neutral state where power is transmitted between the input element and the output element and a neutral state where power is not transmitted between the input element and the output element. 
     The differential unit includes a first rotating element, a second rotating element and a third rotating element. The first rotating element is connected to the first rotary electric machine. The second rotating element is connected to the second rotary electric machine and the drive wheel. The third rotating element is connected to the output element of the power transmission unit. The differential unit is configured such that, when rotation speeds of any two of the first rotating element, the second rotating element and the third rotating element are determined, a rotation speed of the remaining one of the first rotating element, the second rotating element and the third rotating element is determined. 
     The clutch is configured to switch between an engaged state where power is transmitted from the internal combustion engine to the first rotary electric machine and a released state where transmission of power from the internal combustion engine to the first rotary electric machine is interrupted. Power from the internal combustion engine is transmitted to the first rotary electric machine though at least one of a first path or a second path. The first path is a path through which power is transmitted from the internal combustion engine to the first rotary electric machine via the power transmission unit and the differential unit, and the second path is a path through which power is transmitted from the internal combustion engine to the first rotary electric machine via a path different from the first path. The clutch is provided in the second path. 
     The controller is configured to (i) control the internal combustion engine, the first rotary electric machine, the power transmission unit and the clutch, and (ii) set the power transmission unit to the non-neutral state, set the clutch to the engaged state, and then cause the vehicle to travel by using driving force from the first rotary electric machine and driving force from the second rotary electric machine. 
     With the above-described hybrid vehicle, the vehicle is configured as described above, and the power transmission unit, the clutch, the first rotary electric machine and the second rotary electric machine are controlled as described above. Thus, the vehicle is allowed to be propelled by operating both the first rotary electric machine and the second rotary electric machine to carry out motoring even in a state where the rotation speed of the internal combustion engine is not zero. Therefore, it is possible to increase the opportunity that the torque of the two rotary electric machines is used as driving torque of the vehicle, so the flexibility of control over the vehicle increases in the case where large driving force is required during traveling. 
     In the hybrid vehicle, the controller may be configured to switch a drive mode of the vehicle between a first mode and a second mode in response to a vehicle speed. The first mode is a drive mode in which a rotation speed of the internal combustion engine is fixed to zero, the clutch is set to the released state and then the vehicle is caused to travel by using driving force from the first rotary electric machine and driving force from the second rotary electric machine. The second mode is a drive mode in which the power transmission unit is set to the non-neutral state, the clutch is set to the engaged state and then the vehicle is caused to travel by using driving force from the first rotary electric machine and driving force from the second rotary electric machine. 
     With the above-described hybrid vehicle, because the second mode is provided as the drive mode, even when the rotation speed of the engine is not zero like a transition from a state where the engine is operated to an EV mode, the vehicle is allowed to travel with large driving force using driving force from the first rotary electric machine and driving force from the second rotary electric machine. 
     In the hybrid vehicle, the controller may be configured to (i) when the vehicle speed is lower than a determination threshold, set the drive mode to the first mode, and (ii) when the vehicle speed is higher than the determination threshold, set the drive mode to the second mode. 
     With the above-described hybrid vehicle, when the drive mode is selected as described above, even when the vehicle speed increases and the vehicle is not allowed to travel in first mode because of the limitation of the rotation speed of the first rotary electric machine, the vehicle is allowed to travel with large driving force using driving force from the first rotary electric machine and driving force from the second rotary electric machine when the second mode is used. 
     In the hybrid vehicle, the controller may be configured to (i) in third mode that is the drive mode of the vehicle, set the power transmission unit to the non-neutral state, set the clutch to the released state, and then cause the first rotary electric machine to generate electric power in a state where the internal combustion engine is operated, and cause the second rotary electric machine to generate driving force for propelling the vehicle, and (ii) when the drive mode is changed from the third mode to the first mode, change the drive mode via the second mode. 
     With the above-described hybrid vehicle, when the drive mode is changed from the third mode to the first mode, it is possible not to cause a driver to experience a feeling of output torque loss by changing the drive mode via the second mode. 
     In the hybrid vehicle, the controller may be configured to, when fuel is not supplied to the internal combustion engine in the case where the vehicle is caused to travel in second mode, change open or close timing of at least one of an intake valve or exhaust valve such that resistance is reduced during rotation of the internal combustion engine. 
     When the vehicle is caused to travel in second mode and fuel is not supplied to the internal combustion engine, the internal combustion engine is forcibly rotated by the first rotary electric machine and the second rotary electric machine. In this case, an energy loss is smaller when the rotation resistance of the internal combustion engine is small. In order to reduce the rotation resistance of the internal combustion engine, it is desirable that the compressibility and expansion coefficient of air in a cylinder be small. Therefore, with the above-described hybrid vehicle, the controller reduces the rotation resistance of the internal combustion engine by changing the open/close timing of the intake valve or exhaust valve, thus reducing an energy loss. 
     In the hybrid vehicle, the controller may be configured to set the power transmission unit to the non-neutral state, set the clutch to the engaged state, and then cause the vehicle to travel by using driving force from the internal combustion engine in addition to driving force from the first rotary electric machine and driving force from the second rotary electric machine. 
     With the above-described hybrid vehicle, through the above-described control, it is possible to further increase the maximum driving force of the vehicle as compared to the EV mode in which the internal combustion engine is stopped and the first rotary electric machine and the second rotary electric machine are operated to carry out motoring. 
     In the hybrid vehicle, the controller may be configured to, in fourth mode as the drive mode of the vehicle, set the power transmission unit to the non-neutral state, set the clutch to the engaged state, and then cause the vehicle to travel by using driving force from the internal combustion engine in a state where the first rotary electric machine and the second rotary electric machine are not caused to generate torque. 
     With the above-described hybrid vehicle, through the above-described control, in an operating range in which the internal combustion engine is efficiently operable, the power of the internal combustion engine is allowed to be directly transmitted to the drive wheel without being converted to electric power, so it is possible to improve fuel economy. 
     In the hybrid vehicle, the controller may be configured to, in fifth mode as the drive mode of the vehicle, set the power transmission unit to the neutral state, set the clutch to the engaged state, and then cause the first rotary electric machine to generate electric power by using power of the internal combustion engine, and cause the second rotary electric machine to generate driving force for propelling the vehicle. 
     With the above-described hybrid vehicle, in the above-described series HV mode, a shock at a startup of the internal combustion engine is interrupted by the power transmission unit in the neutral state, and is not transmitted to the drive wheel. Thus, it is possible to reduce a shock at a startup of the internal combustion engine, which is experienced by a user. 
     In the hybrid vehicle, the power transmission unit may be configured to be able to change the ratio of a rotation speed of the input element to a rotation speed of the output element. 
     With the above-described hybrid vehicle, it is possible to increase the state of the vehicle where it is possible to set the EV mode in which large driving force is generated by operating both the two rotary electric machines to carry out motoring. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a view that shows the overall configuration of a hybrid vehicle including a drive system according to an embodiment of the invention; 
         FIG. 2  is a block diagram that schematically shows power transmission paths of components of the vehicle in  FIG. 1 ; 
         FIG. 3  is a block diagram that shows the configuration of a controller for the vehicle in  FIG. 1 ; 
         FIG. 4  is a view that schematically shows the configuration of a hydraulic circuit mounted on the hybrid vehicle shown in  FIG. 1 ; 
         FIG. 5  is a chart that shows each drive mode in the hybrid vehicle and controlled statuses of clutches and brake of a transmission unit in each drive mode; 
         FIG. 6  is a nomograph for illustrating an operation of a one-motor EV mode (E 1  line in  FIG. 5 ) in the hybrid vehicle; 
         FIG. 7  is a nomograph for illustrating an operation of a two-motor EV mode (E 3  line in  FIG. 5 ) in the hybrid vehicle; 
         FIG. 8  is a nomograph for illustrating an operation of a (series-parallel) HV mode (H 1 , H 2  lines in  FIG. 5 ) in the hybrid vehicle; 
         FIG. 9  is a nomograph for illustrating an operation of a (series) HV mode (H 4  line in  FIG. 5 ) in the hybrid vehicle; 
         FIG. 10  is a nomograph for illustrating an operation of a two-motor EV mode (E 4 , E 5  lines in  FIG. 5 ) in the hybrid vehicle; 
         FIG. 11  is a nomograph for illustrating an operation of a (parallel) HV mode (H 7 , H 9  lines in  FIG. 5 ) in the hybrid vehicle; 
         FIG. 12  is a nomograph for illustrating an operation of an engine drive mode (Z 1  line in  FIG. 5 ) in the hybrid vehicle; 
         FIG. 13  is a nomograph for illustrating an operation of an engine drive mode (Z 2  line in  FIG. 5 ) in the hybrid vehicle; 
         FIG. 14  is a graph that shows the relationship between a vehicle speed and a maximum driving force in each drive mode in the hybrid vehicle; 
         FIG. 15  is a flowchart for illustrating control over the clutches and the brake in two-motor EV mode in the hybrid vehicle; 
         FIG. 16  is a view that shows an example of a map for determining the drive mode in the hybrid vehicle; and 
         FIG. 17  is an operation waveform chart that shows an example of a change from the (series-parallel) HV mode to the two-motor EV mode in the hybrid vehicle. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the invention will be described with reference to the accompanying drawings. Like reference numerals denote the same or corresponding portions in the following embodiment, and the description thereof will not be repeated. 
       FIG. 1  is a view that shows the overall configuration of a hybrid vehicle including a drive system according to the embodiment of the invention. 
     As shown in  FIG. 1 , the hybrid vehicle  1  (hereinafter, also referred to as vehicle  1 ) includes an engine  10 , the drive system  2 , drive wheels  90  and a controller  100 . The drive system  2  includes a first motor generator (hereinafter, referred to as first MG)  20  that is a first rotary electric machine, a second motor generator (hereinafter, referred to as second MG)  30  that is a second rotary electric machine, a transmission unit  40 , a differential unit  50 , a clutch CS, an input shaft  21 , a counter shaft  70  that is an output shaft of the drive system  2 , a differential gear set  80  and a hydraulic circuit  500 . 
     The hybrid vehicle  1  is a front-engine front-drive (FF) hybrid vehicle that travels by using the power of at least any one of the engine  10 , the first MG  20  or the second MG  30 . The hybrid vehicle  1  may be a plug-in hybrid vehicle of which an in-vehicle battery (not shown) is rechargeable from an external power supply. 
     The engine  10  is, for example, an internal combustion engine, such as a gasoline engine and a diesel engine. Each of the first MG  20  and the second MG  30  is, for example, a permanent magnet synchronous motor that includes a rotor in which permanent magnets are embedded. The drive system  2  is a double-axis drive system in which the first MG  20  is provided along a first axis  12  coaxial with the crankshaft of the engine  10  and the second MG  30  is provided along a second axis  14  different from the first axis  12 . The first axis  12  and the second axis  14  are parallel to each other. 
     The transmission unit  40 , the differential unit  50  and the clutch CS are further provided along the first axis  12 . The transmission unit  40 , the differential unit  50 , the first MG  20  and the clutch CS are arranged from the side close to the engine  10  in the stated order. 
     The first MG  20  is provided so as to be able to receive power from the engine  10 . More specifically, the input shaft  21  of the drive system  2  is connected to the crankshaft of the engine  10 . The input shaft  21  extends along the first axis  12  in a direction away from the engine  10 . The input shaft  21  is connected to the clutch CS at its distal end extending from the engine  10 . A rotary shaft  22  of the first MG  20  extends in a cylindrical shape along the first axis  12 . The input shaft  21  passes through the inside of the rotary shaft  22  at a portion before the input shaft  21  is connected to the clutch CS. The input shaft  21  is connected to the rotary shaft  22  of the first MG  20  via the clutch CS. 
     The clutch CS is provided in a power transmission path from the engine  10  to the first MG  20 . The clutch CS is a hydraulic friction engagement element that is able to couple the input shaft  21  to the rotary shaft  22  of the first MG  20 . When the clutch CS is placed in an engaged state; the input shaft  21  and the rotary shaft  22  are coupled to each other, and transmission of power from the engine  10  to the first MG  20  is allowed. When the clutch CS is placed in a released state, coupling of the input shaft  21  to the rotary shaft  22  is released, and transmission of power from the engine  10  to the first MG  20  via the clutch CS is interrupted. 
     The transmission unit  40  shifts power from the engine  10  and then outputs the power to the differential unit  50 . The transmission unit  40  includes a single-pinion-type planetary gear mechanism, a clutch C 1  and a brake B 1 . The single-pinion-type planetary gear mechanism includes a sun gear S 1 , pinions P 1 , a ring gear R 1  and a carrier CA 1 . 
     The sun gear S 1  is provided such that the rotation center of the sun gear S 1  coincides with the first axis  12 . The ring gear R 1  is provided coaxially with the sun gear S 1  on the radially outer side of the sun gear S 1 . The pinions P 1  are arranged between the sun gear S 1  and the ring gear R 1 , and are in mesh with the sun gear S 1  and the ring gear R 1 . The pinions P 1  are rotatably supported by the carrier CA 1 . The carrier CA 1  is connected to the input shaft  21 , and rotates integrally with the input shaft  21 . Each of the pinions P 1  is provided so as to be revolvable about the first axis  12  and rotatable around the central axis of the pinion P 1 . 
     As shown in  FIG. 6  to  FIG. 13  (described later), the rotation speed of the sun gear S 1 , the rotation speed of the carrier CA 1  (that is, the rotation speed of the engine  10 ) and the rotation speed of the ring gear R 1  are in the relationship represented by points that are connected by a straight line in each of the nomographs (that is, the relationship that, when any two rotation speeds are determined, the remaining one rotation speed is also determined). 
     In the present embodiment, the carrier CA 1  is provided as an input element to which power is input from the engine  10 , and the ring gear R 1  is provided as an output element that outputs the power input to the carrier CA 1 . By the use of the planetary gear mechanism including the sun gear S 1 , the pinions P 1 , the ring gear R 1  and the carrier CA 1 , power input to the carrier CA 1  is shifted and output from the ring gear R 1 . 
     The clutch C 1  is a hydraulic friction engagement element that is able to couple the sun gear S 1  to the carrier CA 1 . When the clutch C 1  is placed in an engaged state, the sun gear S 1  and the carrier CA 1  are coupled to each other, and rotate integrally with each other. When the clutch C 1  is placed in a released state, integral rotation of the sun gear S 1  and the carrier CA 1  is cancelled. 
     The brake B 1  is a hydraulic friction engagement element that is able to restrict (lock) the rotation of the sun gear S 1 . When the brake B 1  is placed in an engaged state, the sun gear S 1  is fixed to the case body of the drive system, and the rotation of the sun gear S 1  is restricted. When the brake B 1  is placed in a released state (disengaged state), the sun gear S 1  is separated from the case body of the drive system, and the rotation of the sun gear S 1  is allowed. 
     A separating wall W 1  is provided between the planetary gear mechanism and the brake B 1 . It is preferable to provide the separating wall W 1  at this position, because it is possible to provide an oil passage in the separating wall W 1  in order to supply activating oil of the brake B 1  and the clutch C 1 . Furthermore, a hole which is provided in the separating wall W 1  can be small by providing the separating wall W 1 , the brake B 1 , the clutch C 1  and the engine  10  in this described order and by making the carrier CA 1  serve as an inner rotating element and the sun gear S 1  serve as an outer rotating element. 
     A speed ratio (the ratio of the rotation speed of the carrier CA 1  that is the input element to the rotation speed of the ring gear R 1  that is the output element, specifically, Rotation Speed of Carrier CA 1 /Rotation Speed of Ring Gear R 1 ) of the transmission unit  40  is changed in response to a combination of the engaged/released states of the clutch C 1  and brake B 1 . When the clutch C 1  is engaged and the brake B 1  is released, a low gear position Lo in which the speed ratio is 1.0 (directly coupled state) is established. When the clutch C 1  is released and the brake B 1  is engaged, a high gear position Hi in which the speed ratio is smaller than 1.0 (for example, 0.7, and a so-called over-drive state) is established. When the clutch C 1  is engaged and the brake B 1  is engaged, the rotation of the sun gear S 1  and the rotation of the carrier CA 1  are restricted, so the rotation of the ring gear R 1  is also restricted. 
     The transmission unit  40  is configured to be able to switch between a non-neutral state and a neutral state. In the non-neutral state, power is transmitted. In the neutral state, power is not transmitted. In the present embodiment, the above-described directly coupled state and over-drive state correspond to the non-neutral state. On the other hand, when both the clutch C 1  and the brake B 1  are released, the carrier CA 1  is allowed to coast about the first axis  12 . Thus, the neutral state in which power transmitted from the engine  10  to the carrier CA 1  is not transmitted from the carrier CA 1  to the ring gear R 1  is obtained. 
     The differential unit  50  includes a single-pinion-type planetary gear mechanism and a counter drive gear  51 . The single-pinion-type planetary gear mechanism includes a sun gear S 2 , pinions P 2 , a ring gear R 2  and a carrier CA 2 . 
     The sun gear S 2  is provided such that the rotation center of the sun gear S 2  coincides with the first axis  12 . The ring gear R 2  is provided coaxially with the sun gear S 2  on the radially outer side of the sun gear S 2 . The pinions P 2  are arranged between the sun gear S 2  and the ring gear R 2 , and are in mesh with the sun gear S 2  and the ring gear R 2 . The pinions P 2  are rotatably supported by the carrier CA 2 . The carrier CA 2  is connected to the ring gear R 1  of the transmission unit  40 , and rotates integrally with the ring gear R 1 . Each of the pinions P 2  is provided so as to be revolvable about the first axis  12  and rotatable around the central axis of the pinion P 2 . 
     The rotary shaft  22  of the first MG  20  is connected to the sun gear S 2 . The rotary shaft  22  of the first MG  20  rotates integrally with the sun gear S 2 . The counter drive gear  51  is connected to the ring gear R 2 . The counter drive gear  51  is an output gear of the differential unit  50 . The output gear rotates integrally with the ring gear R 2 . 
     As shown in  FIG. 6  to  FIG. 13  (described later), the rotation speed of the sun gear S 2  (that is, the rotation speed of the first MG  20 ), the rotation speed of the carrier CA 2  and the rotation speed of the ring gear R 2  are in the relationship represented by points that are connected by a straight line in each of the nomographs (that is, the relationship that, when any two rotation speeds are determined, the remaining one rotation speed is also determined). Therefore, when the rotation speed of the carrier CA 2  is a predetermined value, it is possible to steplessly change the rotation speed of the ring gear R 2  by adjusting the rotation speed of the first MG  20 . 
     The counter shaft  70  extends parallel to the first axis  12  and the second axis  14 . The counter shaft  70  is arranged parallel to the rotary shaft  22  of the first MG  20  and a rotary shaft  31  of the second MG  30 . A driven gear  71  and a drive gear  72  are provided on the counter shaft  70 . The driven gear  71  is in mesh with the counter drive gear  51  of the differential unit  50 . That is, the power of the engine  10  and the power of the first MG  20  are transmitted to the counter shaft  70  via the counter drive gear  51  of the differential unit  50 . 
     The transmission unit  40  and the differential unit  50  are connected in series with each other in a power transmission path from the engine  10  to the counter shaft  70 . Therefore, power from the engine  10  is shifted in the transmission unit  40  and the differential unit  50  and then transmitted to the counter shaft  70 . 
     The driven gear  71  is in mesh with a reduction gear  32  connected to the rotary shaft  31  of the second MG  30 . That is, the power of the second MG  30  is transmitted to the counter shaft  70  via the reduction gear  32 . 
     The drive gear  72  is in mesh with a differential ring gear  81  of the differential gear set  80 . The differential gear set  80  is connected to the right and left drive wheels  90  via corresponding right and left drive shafts  82 . That is, the rotation of the counter shaft  70  is transmitted to the right and left drive shafts  82  via the differential gear set  80 . 
     With the above-described configuration in which the clutch CS is provided, the hybrid vehicle  1  is allowed to operate in a mode in which a series-parallel system is used (hereinafter, referred to as series-parallel mode) and is also allowed to operate in a mode in which a series system is used (hereinafter, referred to as series mode). In terms of this point, how power is transmitted from the engine in each mode will be described with reference to the schematic view shown in  FIG. 2 . 
       FIG. 2  is a block diagram that schematically shows power transmission paths of components of the vehicle in  FIG. 1 . As shown in  FIG. 2 , the hybrid vehicle  1  includes the engine  10 , the first MG  20 , the second MG  30 , the transmission unit  40 , the differential unit  50 , a battery  60  and the clutch CS. 
     The second MG  30  is provided so as to be able to output power to the drive wheels  90 . The transmission unit  40  includes the input element and the output element. The power of the engine  10  is input to the input element. The output element outputs the power input to the input element. The transmission unit  40  is configured to be able to switch between the non-neutral state and the neutral state. In the non-neutral state, power is transmitted between the input element and the output element. In the neutral state, power is not transmitted between the input element and the output element. 
     The battery  60  supplies electric power to the first MG  20  or the second MG  30  during motoring of a corresponding one of the first MG  20  and the second MG  30 , and stores electric power generated by the first MG  20  or the second MG  30  during regeneration of a corresponding one of the first MG  20  and the second MG  30 . 
     The differential unit  50  includes a first rotating element, a second rotating element and a third rotating element. The first rotating element is connected to the first MG  20 . The second rotating element is connected to the second MG  30  and the drive wheels  90 . The third rotating element is connected to the output element of the transmission unit  40 . The differential unit  50  is configured as in the case of, for example, the planetary gear mechanism, or the like, such that, when the rotation speeds of any two of the first to third rotating elements are determined, the rotation speed of the remaining one of the first to third rotating elements is determined. 
     The hybrid vehicle  1  is configured to be able to transmit power from the engine  10  to the first MG  20  with the use of at least any one of two paths K 1 , K 2  through which power is transmitted. The path K 1  is a path through which power is transmitted from the engine  10  to the first MG  20  via the transmission unit  40  and the differential unit  50 . The path K 2  is different from the path K 1 , and is a path through which power is transmitted from the engine  10  to the first MG  20 . The clutch CS is provided in the path K 2 , and is able to switch between the engaged state and the released state. In the engaged state, power is transmitted from the engine  10  to the first MG  20 . In the released state, transmission of power from the engine  10  to the first MG  20  is interrupted. 
     In HV mode in which the engine is operated, any one of the clutch C 1  and the brake B 1  is placed in the engaged state, and the other one of the clutch C 1  and the brake B 1  is placed in the released state. Thus, when the transmission unit  40  is controlled to the non-neutral state, power is transmitted from the engine  10  to the first MG  20  through the path K 1 . At this time, when the clutch CS is placed in the released state to interrupt the path K 2  at the same time, the vehicle is operable in series-parallel mode. 
     On the other hand, in HV mode in which the engine is operated, when power is transmitted through the path K 2  by directly coupling the engine  10  to the first MG  20  with the clutch CS and the path K 1  is interrupted by controlling the transmission unit  40  such that the transmission unit  40  is placed in the neutral state by placing both the clutch C 1  and the brake B 1  in the released state, the vehicle is operable in series mode. At this time, in the differential unit  50 , the rotating element connected to the transmission unit  40  is freely rotatable, so the other two rotating elements do not influence each other and are rotatable. Therefore, it is possible to independently perform the operation of generating electric power by rotating the first MG  20  with the use of the rotation of the engine  10  and the operation of rotating the drive wheels by driving the second MG  30  with the use of generated electric power or electric power charged in the battery  60 . 
     When the engine  10  and the first MG  20  are directly coupled to each other by the clutch CS and the transmission unit  40  is controlled to the non-neutral state in a state where the engine  10  is operated, the rotation of the engine  10  is transmitted to the drive wheels at a fixed gear ratio. At this time, power is not transmitted via a route, such as the paths K 1 , K 2 , but power is transmitted from the engine  10  to the drive wheels  90  via the differential unit  50 . 
     The transmission unit  40  does not always need to be able to change the speed ratio. As long as it is possible to interrupt transmission of power between the engine  10  and the differential unit  50  in the path K 1 , a mere clutch is applicable. 
       FIG. 3  is a block diagram that shows the configuration of the controller  100  of the vehicle shown in  FIG. 1 . As shown in  FIG. 3 , the controller  100  includes an HV ECU  150 , an MG ECU  160  and an engine ECU  170 . Each of the HV ECU  150 , the MG ECU  160  and the engine ECU  170  is an electronic control unit including a computer. The number of ECUs is not limited to three. An integrated single ECU may be provided as a whole, or two or four or more of split ECUs may be provided. 
     The MG ECU  160  controls the first MG  20  and the second MG  30 . The MG ECU  160 , for example, controls the output torque of the first MG  20  by adjusting the value of current that is supplied to the first MG  20 , and controls the output torque of the second MG  30  by adjusting the value of current that is supplied to the second MG  30 . 
     The engine ECU  170  controls the engine  10 . The engine ECU  170 , for example, controls the opening degree of an electronic throttle valve of the engine  10 , controls ignition of the engine by outputting an ignition signal, or controls injection of fuel to the engine  10 . The engine ECU  170  controls the output torque of the engine  10  through opening degree control over the electronic throttle valve, injection control, ignition control, and the like. 
     The HV ECU  150  comprehensively controls the entire vehicle. A vehicle speed sensor, an accelerator operation amount sensor, an MG 1  rotation speed sensor, an MG 2  rotation speed sensor, an output shaft rotation speed sensor, a battery sensor, and the like, are connected to the HV ECU  150 . With these sensors, the HV ECU  150  acquires a vehicle speed, an accelerator operation amount, the rotation speed of the first MG  20 , the rotation speed of the second MG  30 , the rotation speed of the output shaft of a power transmission system, a battery state SOC, and the like. 
     The HV ECU  150  calculates a required driving force, a required power, a required torque, and the like, for the vehicle on the basis of acquired information. The HV ECU  150  determines the output torque of the first MG  20  (hereinafter, also referred to as MG 1  torque), the output torque of the second MG  30  (hereinafter, also referred to as MG 2  torque) and the output torque of the engine  10  (hereinafter, also referred to as engine torque) on the basis of the calculated required values. The HV ECU  150  outputs a command value of the MG 1  torque and a command value of the MG 2  torque to the MG ECU  160 . The HV ECU  150  outputs a command value of the engine torque to the engine ECU  170 . 
     The HV ECU  150  controls the clutches C 1 , CS and the brake B 1  on the basis of the drive mode (described later), and the like. The HV ECU  150  outputs, to the hydraulic circuit  500  shown in  FIG. 1 , a command value (PbC 1 ) of hydraulic pressure that is supplied to the clutch C 1 , a command value (PbCS) of hydraulic pressure that is supplied to the clutch CS and a command value (PbB 1 ) of hydraulic pressure that is supplied to the brake B 1 . The HV ECU  150  outputs a control signal NM and a control signal S/C to the hydraulic circuit  500  shown in  FIG. 1 . 
     The hydraulic circuit  500  shown in  FIG. 1  controls hydraulic pressures that are respectively supplied to the clutch C 1  and the brake B 1  in response to the command values PbC 1 , PbB 1 , controls an electric oil pump in response to the control signal NM, and controls whether to allow or prohibit simultaneous engagement of the clutch C 1 , the brake B 1  and the clutch CS in response to the control signal S/C. 
     Next, the configuration of the hydraulic circuit will be described.  FIG. 4  is a view that schematically shows the configuration of the hydraulic circuit  500  mounted on the hybrid vehicle  1 . The hydraulic circuit  500  includes a mechanical oil pump (hereinafter, also referred to as MOP)  501 , the electric oil pump (hereinafter, also referred to as EOP)  502 , pressure regulating valves  510 ,  520 , linear solenoid valves SL 1 , SL 2 , SL 3 , simultaneous supply prevention valves  530 ,  540 ,  550 , an electromagnetic change-over valve  560 , a check valve  570 , and oil passages LM, LE, L 1 , L 2 , L 3 , L 4 . 
     The MOP  501  is driven by the rotation of the carrier CA 2  of the differential unit  50  to generate hydraulic pressure. Therefore, when the carrier CA 2  is rotated by, for example, driving the engine  10 , the MOP  501  is also driven; whereas, when the carrier CA 2  is stopped, the MOP  501  is also stopped. The MOP  501  outputs generated hydraulic pressure to the oil passage LM. 
     The hydraulic pressure in the oil passage LM is regulated (reduced) to a predetermined pressure by the pressure regulating valve  510 . Hereinafter, the hydraulic pressure in the oil passage LM, regulated by the pressure regulating valve  510 , is also referred to as line pressure PL. The line pressure PL is supplied to each of the linear solenoid valves SL 1 , SL 2 , SL 3 . 
     The linear solenoid valve SL 1  generates hydraulic pressure for engaging the clutch C 1  (hereinafter, referred to as C 1  pressure) by regulating the line pressure PL in response to the hydraulic pressure command value PbC 1  from the controller  100 . The C 1  pressure is supplied to the clutch C 1  via the oil passage L 1 . 
     The linear solenoid valve SL 2  generates hydraulic pressure for engaging the brake B 1  (hereinafter, referred to as B 1  pressure) by regulating the line pressure PL in response to the hydraulic pressure command value PbB 1  from the controller  100 . The B 1  pressure is supplied to the brake B 1  via the oil passage L 2 . 
     The linear solenoid valve SL 3  generates hydraulic pressure for engaging the clutch CS (hereinafter, referred to as CS pressure) by regulating the line pressure PL in response to the hydraulic pressure command value PbCS from the controller  100 . The CS pressure is supplied to the clutch CS via the oil passage L 3 . 
     The simultaneous supply prevention valve  530  is provided in the oil passage L 1 , and is configured to prevent the clutch C 1  and at least one of the brake B 1  or the clutch CS from being simultaneously engaged. Specifically, the oil passages L 2 , L 3  are connected to the simultaneous supply prevention valve  530 . The simultaneous supply prevention valve  530  operates by using the B 1  pressure and the CS pressure through the oil passages L 2 , L 3  as signal pressures. 
     When both signal pressures that are the B 1  pressure and the CS pressure are not input to the simultaneous supply prevention valve  530  (that is, when both the brake B 1  and the clutch CS are released), the simultaneous supply prevention valve  530  is in a normal state in which the C 1  pressure is supplied to the clutch C 1 .  FIG. 4  illustrates the case where the simultaneous supply prevention valve  530  is in the normal state. 
     On the other hand, when at least one of the signal pressures that are the B 1  pressure and the CS pressure is input to the simultaneous supply prevention valve  530  (that is, when at least one of the brake B 1  or the clutch CS is engaged), even when the clutch C 1  is engaged, the simultaneous supply prevention valve  530  switches into a drain state in which supply of the C 1  pressure to the clutch C 1  is cut off and the hydraulic pressure in the clutch C 1  is released to the outside. Thus, the clutch C 1  is released, so the clutch C 1  and at least one of the brake B 1  or the clutch CS are prevented from being simultaneously engaged. 
     Similarly, the simultaneous supply prevention valve  540  operates in response to the C 1  pressure and the CS pressure as signal pressures to prevent the brake B 1  and at least one of the clutch C 1  or the clutch CS from being simultaneously engaged. Specifically, when both the signal pressures that are the C 1  pressure and the CS pressure are not input to the simultaneous supply prevention valve  540 , the simultaneous supply prevention valve  540  is in a normal state in which the B 1  pressure is supplied to the brake B 1 . On the other hand, when at least one of the signal pressures that are the C 1  pressure and the CS pressure is input to the simultaneous supply prevention valve  540 , the simultaneous supply prevention valve  540  switches into a drain state in which supply of the B 1  pressure to the brake B 1  is cut off and the hydraulic pressure in the brake B 1  is released to the outside.  FIG. 4  illustrates the case where the C 1  pressure is input to the simultaneous supply prevention valve  540  as the signal pressure and the simultaneous supply prevention valve  540  is in the drain state. 
     Similarly, the simultaneous supply prevention valve  550  operates by using the C 1  pressure and the B 1  pressure as signal pressures to prevent the clutch CS and at least one of the clutch C 1  or the brake B 1  from being simultaneously engaged. Specifically, when both the signal pressures that are the C 1  pressure and the B 1  pressure are not input to the simultaneous supply prevention valve  550 , the simultaneous supply prevention valve  550  is in a normal state in which the CS pressure is supplied to the clutch CS. On the other hand, when at least one of the signal pressures that are the C 1  pressure and the B 1  pressure is input to the simultaneous supply prevention valve  550 , the simultaneous supply prevention valve  550  switches into a drain state in which supply of the CS pressure to the clutch CS is cut off and the hydraulic pressure in the clutch CS is released to the outside.  FIG. 4  illustrates the case where the C 1  pressure is input to the simultaneous supply prevention valve  550  and the simultaneous supply prevention valve  550  is in the drain state. 
     The EOP  502  is driven by a motor (hereinafter, also referred to as internal motor)  502 A provided inside to generate hydraulic pressure. The internal motor  502 A is controlled by the control signal NM from the controller  100 . Therefore, the EOP  502  is operable irrespective of whether the carrier CA 2  is rotating. The EOP  502  outputs generated hydraulic pressure to the oil passage LE. 
     The hydraulic pressure in the oil passage LE is regulated (reduced) to a predetermined pressure by the pressure regulating valve  520 . The oil passage LE is connected to the oil passage LM via the check valve  570 . When the hydraulic pressure in the oil passage LE is higher by a predetermined pressure or more than the hydraulic pressure in the oil passage LM, the check valve  570  opens, and the hydraulic pressure in the oil passage LE is supplied to the oil passage LM via the check valve  570 . Thus, during a stop of the MOP  501  as well, it is possible to supply hydraulic pressure to the oil passage LM by driving the EOP  502 . 
     The electromagnetic change-over valve  560  is switched to any one of an on state and an off state in response to the control signal S/C from the controller  100 . In the on state, the electromagnetic change-over valve  560  communicates the oil passage LE with the oil passage L 4 . In the off state, the electromagnetic change-over valve  560  interrupts the oil passage LE from the oil passage L 4 , and releases the hydraulic pressure in the oil passage L 4  to the outside.  FIG. 4  illustrates the case where the electromagnetic change-over valve  560  is in the off state. 
     The oil passage L 4  is connected to the simultaneous supply prevention valves  530 ,  540 . When the electromagnetic change-over valve  560  is in the on state, the hydraulic pressure in the oil passage LE is input to the simultaneous supply prevention valves  530 ,  540  via the oil passage L 4  as a signal pressure. When the signal pressure from the oil passage L 4  is input to the simultaneous supply prevention valve  530 , the simultaneous supply prevention valve  530  is forcibly fixed to the normal state irrespective of whether the signal pressure (B 1  pressure) is input from the oil passage L 2 . Similarly, when the signal pressure is input from the oil passage L 4  to the simultaneous supply prevention valve  540 , the simultaneous supply prevention valve  540  is forcibly fixed to the normal state irrespective of whether the signal pressure (C 1  pressure) is input from the oil passage L 1 . Therefore, by driving the EOP  502  and switching the electromagnetic change-over valve  560  to the on state, the simultaneous supply prevention valves  530 ,  540  are simultaneously fixed to the normal state. Thus, the clutch C 1  and the brake B 1  are allowed to be simultaneously engaged, and two-motor mode (described later) is enabled. 
     Hereinafter, the details of control modes of the hybrid vehicle  1  will be described with reference to an operation engagement chart and the nomographs. 
       FIG. 5  is a chart that shows each drive mode and controlled statuses of the clutch C 1  and brake B 1  of the transmission unit  40  in each drive mode. 
     The controller  100  causes the hybrid vehicle  1  to travel in motor drive mode (hereinafter, referred to as EV mode), hybrid mode (hereinafter, referred to as HV mode) or engine drive mode. The EV mode is a control mode in which the engine  10  is stopped and the hybrid vehicle  1  is caused to travel by using the power of at least one of the first MG  20  or the second MG  30 . The HV mode is a control mode in which the hybrid vehicle  1  is caused to travel by using the power of the engine  10  and the power of the second MG  30 . The engine drive mode is a control mode in which the first MG  20  and the second MG  30  are not used and the vehicle is caused to travel by using the driving force of the engine  10 . Each of the EV mode, the HV mode and the engine drive mode is further divided into some control modes. 
     In  FIG. 5 , C 1 , B 1 , CS, MG 1  and MG 2  respectively denote the clutch C 1 , the brake B 1 , the clutch CS, the first MG  20  and the second MG  30 . The circle mark (◯) in each of the C 1 , B 1 , CS columns indicates the engaged state, the cross mark (×) indicates the released state, and the triangle mark (Δ) indicates that any one of the clutch C 1  and the brake B 1  is engaged during engine brake. The sign G in each of the MG 1  column and the MG 2  column indicates that the MG 1  or the MG 2  is mainly operated as a generator. The sign M in each of the MG 1  column and the MG 2  column indicates that the MG 1  or the MG 2  is mainly operated as a motor. 
     In EV mode, the controller  100  selectively switches between one-motor mode and two-motor mode in response to a user&#39;s required torque, and the like. In one-motor mode, the hybrid vehicle  1  is caused to travel by using the power of the second MG  30  alone. In two-motor mode, the hybrid vehicle  1  is caused to travel by using the power of both the first MG  20  and the second MG  30 . 
     When the load of the drive system  2  is low, the one-motor mode is used. When the load of the drive system  2  becomes high, the drive mode is changed to the two-motor mode. 
     As shown in E 1  line of  FIG. 5 , when the hybrid vehicle  1  is driven (moved forward or reversed) in one-motor EV mode, the controller  100  places the transmission unit  40  in the neutral state (state in which no power is transmitted) by releasing the clutch C 1  and releasing the brake B 1 . At this time, the controller  100  causes the first MG  20  to mainly operate as fixing means for fixing the rotation speed of the sun gear S 2  to zero and causes the second MG  30  to mainly operate as a motor (see  FIG. 6  (described later)). In order to cause the first MG  20  to operate as the fixing means, the current of the first MG  20  may be controlled by feeding back the rotation speed of the first MG  20  such that the rotation speed becomes zero. When the rotation speed of the first MG  20  is kept zero even when torque is zero, cogging torque may be utilized without adding current. When the transmission unit  40  is placed in the neutral state, the engine  10  is not co-rotated during regenerative braking, so a loss is smaller by that amount, and it is possible to recover large regenerated electric power. 
     As shown in the E 2  line in  FIG. 5 , when the hybrid vehicle  1  is braked in one-motor EV mode and engine brake is required, the controller  100  engages any one of the clutch C 1  and the brake B 1 . For example, when braking force is insufficient with only regenerative brake, engine brake is used together with regenerative brake. For example, when the SOC of the battery  60  is close to a full charge state, regenerated electric power cannot be charged, so it is conceivable to establish an engine brake state. 
     By engaging any one of the clutch C 1  and the brake B 1 , a so-called engine brake state is established. In the engine brake state, the rotation of the drive wheels  90  is transmitted to the engine  10 , and the engine  10  is rotated. At this time, the controller  100  causes the first MG  20  to mainly operate as a motor, and causes the second MG  30  to mainly operate as a generator. 
     On the other hand, as shown in the E 3  line in  FIG. 5 , when the hybrid vehicle  1  is driven (moved forward or reversed) in two-motor EV mode, the controller  100  restricts (locks) the rotation of the ring gear R 1  of the transmission unit  40  by engaging the clutch C 1  and engaging the brake B 1 . Thus, the rotation of the carrier CA 2  of the differential unit  50  coupled to the ring gear R 1  of the transmission unit  40  is also restricted (locked), so the carrier CA 2  of the differential unit  50  is kept in a stopped state (Engine Rotation Speed Ne=0). The controller  100  causes the first MG  20  and the second MG  30  to mainly operate as motors (see  FIG. 7  (described later)). 
     In EV mode (one-motor mode or two-motor mode), the engine  10  is stopped, so the MOP  501  is also stopped. Therefore, in EV mode, the clutch C 1  or the brake B 1  is engaged by using hydraulic pressure that is generated by the EOP  502 . 
     E 4  and E 5  lines in EV mode will be described. These modes as well as E 3  line are two-motor modes, and differ from E 3  line in that these modes are operable even when the engine rotation speed Ne is not zero (Ne free in  FIG. 5 ). The details of these modes will be described later with reference to the nomograph of  FIG. 10 . 
     The HV mode may be divided into three modes, that is, a series-parallel mode, a series mode and a parallel mode. In series-parallel mode or series mode, the controller  100  causes the first MG  20  to operate as a generator, and causes the second MG  30  to operate as a motor. In parallel mode, the controller  100  causes only the second MG  30  to operate as a motor (one-motor mode) or causes both the first MG  20  and the second MG  30  to operate as motors (two-motor mode). 
     In HV mode, the controller  100  sets the control mode to any one of the series-parallel mode, the series mode and the parallel mode. 
     In series-parallel mode, part of the power of the engine  10  is used in order to drive the drive wheels  90 , and the remaining part of the power of the engine  10  is used as power for generating electric power in the first MG  20 . The second MG  30  drives the drive wheels  90  by using electric power generated by the first MG  20 . In series-parallel mode, the controller  100  changes the speed ratio of the transmission unit  40  in response to the vehicle speed. 
     When the hybrid vehicle  1  is caused to move forward in an intermediate or low speed range, the controller  100  establishes the low gear position Lo (see the continuous line in  FIG. 8  (described later)) by engaging the clutch C 1  and releasing the brake B 1  as shown in the H 2  line in  FIG. 5 . On the other hand, when the hybrid vehicle  1  is caused to move forward in a high speed range, the controller  100  establishes the high gear position Hi (see the dashed line in  FIG. 8  (described later)) by releasing the clutch C 1  and engaging the brake B 1  as shown in the H 1  line in  FIG. 5 . Either when the high gear position is established or when the low gear position is established, the transmission unit  40  and the differential unit  50  operate as a continuously variable transmission as a whole. 
     When the hybrid vehicle  1  is reversed, the controller  100  engages the clutch C 1  and releases the brake B 1  as shown in the H 3  line in  FIG. 5 . When there is an allowance in the SOC of the battery, the controller  100  rotates the second MG  30  alone in the reverse direction; whereas, when there is no allowance in the SOC of the battery, the controller  100  generates electric power with the use of the first MG  20  by operating the engine  10  and rotates the second MG  30  in the reverse direction. 
     In series mode, the entire power of the engine  10  is used as power for generating electric power with the use of the first MG  20 . The second MG  30  drives the drive wheels  90  by using electric power generated by the first MG  20 . In series mode, when the hybrid vehicle  1  is moved forward or when the hybrid vehicle  1  is reversed, the controller  100  releases both the clutch C 1  and the brake B 1  and engages the clutch CS (see  FIG. 9  (described later)) as shown in the H 4  line and the H 5  line in  FIG. 5 . 
     In HV mode, the engine  10  is operating, so the MOP  501  is also operating. Therefore, in HV mode, the clutch C 1 , the clutch CS or the brake B 1  is engaged mainly by using hydraulic pressure generated by the MOP  501 . 
     The controlled statuses in parallel HV mode are shown in H 6  to H 9  lines. These are also the HV mode; however, the first MG  20  does not operate as a generator. The two-motor (parallel) HV mode significantly differs from the series-parallel mode or the series mode in that the first MG  20  operates to carry out motoring as a motor and outputs torque for rotating the drive wheels. In parallel mode, any one of the clutch C 1  and the brake B 1  is engaged, the other one of the clutch C 1  and the brake B 1  is released, and the clutch CS is engaged. The details of these modes will be described later with reference to the nomograph of  FIG. 13 . 
     The vehicle  1  is able to travel in engine drive mode in which the vehicle  1  travels without using the first MG  20  or the second MG  30 . When the traveling state of the vehicle coincides with a rotation speed and a torque where the efficiency of the engine is high, the efficiency is high when the power of the engine is directly used to rotate the drive wheels without using the power of the engine for power generation, or the like. The controlled statuses in engine drive mode are shown in Z 1  and Z 2  lines in  FIG. 5 . In engine drive mode, as well as the parallel HV mode, any one of the clutch C 1  and the brake B 1  is engaged, the other one of the clutch C 1  and the brake B 1  is released, and the clutch CS is engaged. The details of these modes will be described later with reference to the nomographs of  FIG. 12  and  FIG. 13 . 
     Hereinafter, the statuses of the rotating elements in typical modes among the operation modes shown in  FIG. 5  will be described with reference to the nomographs. 
       FIG. 6  is a nomograph for illustrating the operation of the one-motor EV mode (E 1  line in  FIG. 5 ).  FIG. 7  is a nomograph for illustrating the operation of the two-motor EV mode (E 3  line in  FIG. 5 ).  FIG. 8  is a nomograph for illustrating the operation of the series-parallel HV mode (H 1 , H 2  lines in  FIG. 5 ).  FIG. 9  is a nomograph for illustrating the operation of the series HV mode (H 4  line in  FIG. 5 ). 
     In  FIG. 6  to  FIG. 9 , S 1 , CA 1  and R 1  respectively denote the sun gear S 1 , the carrier CA 1  and the ring gear R 1  of the transmission unit  40 , S 2 , CA 2  and R 2  respectively denote the sun gear S 2 , the carrier CA 2  and the ring gear R 2  of the differential unit  50 . 
     The controlled statuses in one-motor EV mode (E 1  line in  FIG. 5 ) will be described with reference to  FIG. 6 . In one-motor EV mode, the controller  100  releases the clutch C 1 , the brake B 1  and the clutch CS of the transmission unit  40 , stops the engine  10 , and causes the second MG  30  to mainly operate as a motor. Therefore, in one-motor EV mode, the hybrid vehicle  1  travels by using the torque of the second MG  30  (hereinafter, referred to as MG 2  torque Tm 2 ). 
     At this time, the controller  100  executes feedback control over the torque of the first MG  20  (hereinafter, referred to as MG 1  torque Tm 1 ) such that the rotation speed of the sun gear S 2  becomes zero. Therefore, the sun gear S 2  does not rotate. However, because the clutch C 1  and brake B 1  of the transmission unit  40  are released, the rotation of the carrier CA 2  of the differential unit  50  is not restricted. Therefore, the ring gear R 2  and carrier CA 2  of the differential unit  50  and the ring gear R 1  of the transmission unit  40  are rotated (coasted) interlocking with the rotation of the second MG  30  in the same direction as the second MG  30 . 
     On the other hand, the carrier CA 1  of the transmission unit  40  is kept in a stopped state because the engine  10  is stopped. The sun gear S 1  of the transmission unit  40  is rotated (coasted) interlocking with the rotation of the ring gear R 1  in a direction opposite to the rotation direction of the ring gear R 1 . 
     In order to decelerate the vehicle in one-motor EV mode, it is allowed to activate engine brake in addition to regenerative brake using the second MG  30 . In this case (E 2  line in  FIG. 5 ); by engaging any one of the clutch C 1  and the brake B 1 , the engine  10  is also rotated at the time when the carrier CA 2  is driven from the drive wheels  90  side, so engine brake is activated. 
     Next, the controlled status in two-motor EV mode (E 3  line in  FIG. 5 ) will be described with reference to  FIG. 7 . In two-motor EV mode, the controller  100  engages the clutch C 1  and the brake B 1 , releases the clutch CS, and stops the engine  10 . Therefore, the rotation of each of the sun gear S 1 , carrier CA 1  and ring gear R 1  of the transmission unit  40  is restricted such that the rotation speed becomes zero. 
     Because the rotation of the ring gear R 1  of the transmission unit  40  is restricted, the rotation of the carrier CA 2  of the differential unit  50  is also restricted (locked). In this state, the controller  100  causes the first MG  20  and the second MG  30  to mainly operate as motors. Specifically, the second MG  30  is rotated in the positive direction by setting the MG 2  torque Tm 2  to a positive torque, and the first MG  20  is rotated in the negative direction by setting the MG 1  torque Tm 1  to a negative torque. 
     When the rotation of the carrier CA 2  is restricted by engaging the clutch C 1 , the MG 1  torque Tm 1  is transmitted to the ring gear R 2  by using the carrier CA 2  as a supporting point. The MG 1  torque Tm 1  (hereinafter, referred to as MG 1  transmission torque Tm 1   c ) that is transmitted to the ring gear R 2  acts in the positive direction, and is transmitted to the counter shaft  70 . Therefore, in two-motor EV mode, the hybrid vehicle  1  travels by using the MG 1  transmission torque Tm 1   c  and the MG 2  torque Tm 2 . The controller  100  adjusts the distribution ratio between the MG 1  torque Tm 1  and the MG 2  torque Tm 2  such that the sum of the MG 1  transmission torque Tm 1   c  and the MG 2  torque Tm 2  meets the user&#39;s required torque. 
     The controlled state in series-parallel HV mode (H 1  to H 3  lines in  FIG. 5 ) will be described with reference to  FIG. 8 .  FIG. 8  illustrates the case where the vehicle is traveling forward in the low gear position Lo (see H 2  line in  FIG. 5 , and the continuous common line shown in the nomograph of S 1 , CA 1  and R 1  in  FIG. 8 ) and the case where the vehicle is traveling forward in the high gear position Hi (see H 1  line in  FIG. 5 , and the dashed common line shown in the nomograph of S 1 , CA 1  and R 1  in  FIG. 8 ). For the sake of convenience of description, it is assumed that the rotation speed of the ring gear R 1  is the same either when the vehicle is traveling forward in the low gear position Lo or when the vehicle is traveling forward in the high gear position Hi. 
     When the low gear position Lo is established in series-parallel HV mode, the controller  100  engages the clutch C 1 , and releases the brake B 1  and the clutch CS. Therefore, the rotating elements (the sun gear S 1 , the carrier CA 1  and the ring gear R 1 ) rotate integrally with one another. Thus, the ring gear R 1  of the transmission unit  40  also rotates at the same rotation speed as the carrier CA 1 , and the rotation of the engine  10  is transmitted from the ring gear R 1  to the carrier CA 2  of the differential unit  50  at the same rotation speed. That is, the torque of the engine  10  (hereinafter, referred to as engine torque Te) input to the carrier CA 1  of the transmission unit  40  is transmitted from the ring gear R 1  of the transmission unit  40  to the carrier CA 2  of the differential unit  50 . When the low gear position Lo is established, the torque that is transmitted from the ring gear R 1  (hereinafter, referred to as transmission unit output torque Tr 1 ) is equal to the engine torque Te (Te=Tr 1 ). 
     The rotation of the engine  10 , transmitted to the carrier CA 2  of the differential unit  50 , is steplessly shifted by the use of the rotation speed of the sun gear S 2  (the rotation speed of the first MG  20 ), and is transmitted to the ring gear R 2  of the differential unit  50 . At this time, the controller  100  basically causes the first MG  20  to operate as a generator to apply the MG 1  torque Tm 1  in the negative direction. Thus, the MG 1  torque Tm 1  serves as reaction force for transmitting the engine torque Te, input to the carrier CA 2 , to the ring gear R 2 . 
     The engine torque Te transmitted to the ring gear R 2  (hereinafter, referred to as engine transmission torque Tec) is transmitted from the counter drive gear  51  to the counter shaft  70 , and acts as driving force of the hybrid vehicle  1 . 
     In series-parallel HV mode, the controller  100  causes the second MG  30  to mainly operate as a motor. The MG 2  torque Tm 2  is transmitted from the reduction gear  32  to the counter shaft  70 , and acts as driving force of the hybrid vehicle  1 . That is, in series-parallel HV mode, the hybrid vehicle  1  travels by using the engine transmission torque Tec and the MG 2  torque Tm 2 . 
     On the other hand, when the high gear position Hi is established in series-parallel HV mode, the controller  100  engages the brake B 1 , and releases the clutch C 1  and the clutch CS. Because the brake B 1  is engaged, the rotation of the sun gear S 1  is restricted. Thus, the rotation of the engine  10 , input to the carrier CA 1  of the transmission unit  40 , is increased in speed, and is transmitted from the ring gear R 1  of the transmission unit  40  to the carrier CA 2  of the differential unit  50 . Therefore, when the high gear position Hi is established, the transmission unit output torque Tr 1  is smaller than the engine torque Te (Te&gt;Tr 1 ). 
     The controlled status in series HV mode (H 4  line in  FIG. 5 ) will be described with reference to  FIG. 9 . In series HV mode, the controller  100  releases the clutch C 1  and the brake B 1 , and engages the clutch CS. Therefore, when the clutch CS is engaged, the sun gear S 2  of the differential unit  50  rotates at the same rotation speed as the carrier CA 1  of the transmission unit  40 , and the rotation of the engine  10  is transmitted from the clutch CS to the first MG  20  at the same rotation speed. Thus, electric power is allowed to be generated with the use of the first MG  20  by using the engine  10  as a power source. 
     On the other hand, because both the clutch C 1  and the brake B 1  are released, the rotation of each of the sun gear S 1  and ring gear R 1  of the transmission unit  40  and the rotation of the carrier CA 2  of the differential unit  50  are not restricted. That is, because the transmission unit  40  is in the neutral state and the rotation of the carrier CA 2  of the differential unit  50  is not restricted, the power of the first MG  20  and the power of the engine  10  are not transmitted to the counter shaft  70 . Therefore, the MG 2  torque Tm 2  of the second MG  30  is transmitted to the counter shaft  70 . Accordingly, in series HV mode, while electric power is generated with the use of the first MG  20  by using the engine  10  as a power source, the hybrid vehicle  1  travels by using the MG 2  torque Tm 2  generated by the use of part or all of the generated electric power. 
     Because the series mode is allowed to be achieved, it is possible to select the operating point of the engine without concern for occurrence of tooth contact noise of the gear mechanism due to engine torque fluctuations, to which attention needs to be paid in series-parallel mode, when the vehicle travels at a low vehicle speed. Thus, a vehicle state that enables both quietness of the vehicle and improvement in fuel consumption increases. 
     In series HV mode, the controller  100  sets the transmission unit  40  to the neutral state and sets the clutch CS to the engaged state, and then causes the first MG  20  to generate electric power by the use of the power of the engine  10 , and causes the second MG  30  to generate driving force for propelling the vehicle. In the above-described series HV mode, a shock at a startup of the engine  10  is interrupted by the transmission unit  40  in the neutral state, and is not transmitted to the drive wheels  90 . Thus, it is possible to reduce a shock at a startup of the engine  10 , which is experienced by a user. 
       FIG. 10  is a nomograph for illustrating the operation of the two-motor EV mode (E 4 , E 5  lines in  FIG. 5 ). The controlled statuses in two-motor EV mode will be described with reference to  FIG. 10 .  FIG. 10  illustrates the case where the vehicle is traveling forward in the low gear position Lo (see the continuous common lines) and the case where the vehicle is traveling in the high gear position Hi (see the dashed common lines). For the sake of convenience of description, it is assumed that the rotation speed of the ring gear R 1  is the same either when the vehicle is traveling forward in the low gear position Lo or when the vehicle is traveling forward in the high gear position Hi. 
     When the low gear position Lo is established in two-motor EV mode (E 5  line in  FIG. 5 ), the controller  100  engages the clutch C 1  and the clutch CS and releases the brake B 1 . Therefore, the rotating elements (the sun gear S 1 , the carrier CA 1  and the ring gear R 1 ) of the transmission unit  40  rotate integrally with one another. When the clutch CS is engaged, the carrier CA 1  of the transmission unit  40  and the sun gear S 2  of the differential unit  50  rotate integrally with each other. Thus, all the rotating elements of the transmission unit  40  and differential unit  50  rotate integrally at the same rotation speed. Therefore, when the MG 1  torque Tm 1  is generated in the positive rotation direction by the first MG  20  together with the second MG  30 , it is possible to cause the hybrid vehicle  1  to travel by using both the motors. Because the engine  10  is not autonomously driven in EV mode, the engine  10  is in a driven state where the engine  10  is driven by the torque of both the first MG  20  and the second MG  30 . Therefore, it is desirable that the open/close timing of each valve be operated such that resistance during rotation of the engine reduces. 
     The MG 1  transmission torque Tm 1   c  transmitted to the ring gear R 2  is transmitted from the counter drive gear  51  to the counter shaft  70 , and acts as the driving force of the hybrid vehicle  1 . At the same time, the MG 2  torque Tm 2  is transmitted from the reduction gear  32  to the counter shaft  70 , and acts as the driving force of the hybrid vehicle  1 . That is, when the low gear position Lo is established in two-motor EV mode, the hybrid vehicle  1  travels by using the MG 2  torque Tm 2  and the MG 1  torque Tm 1  transmitted to the ring gear R 2 . 
     On the other hand, when the high gear position Hi is established in two-motor EV mode (E 4  line in  FIG. 5 ), the controller  100  engages the brake B 1  and the clutch CS and releases the clutch C 1 . Because the brake B 1  is engaged, the rotation of the sun gear S 1  is restricted. 
     Because the clutch CS is engaged, the carrier CA 1  of the transmission unit  40  and the sun gear S 2  of the differential unit  50  rotate integrally with each other. Therefore, the rotation speed of the sun gear S 2  is equal to the rotation speed of the engine  10 . 
       FIG. 11  is a nomograph for illustrating the operation of the parallel HV mode (H 7 , H 9  lines in  FIG. 5 ). The controlled statuses in two-motor parallel stepped HV mode will be described with reference to  FIG. 11 .  FIG. 11  illustrates the case where the vehicle is traveling forward in the low gear position Lo (see the continuous common lines) and the case where the vehicle is traveling in the high gear position Hi (see the dashed common lines). 
     As is apparent from the comparison between  FIG. 10  and  FIG. 11 , in two-motor parallel stepped HV mode, the engine  10  is autonomously driven, so the engine torque Te is applied to the carrier CA 1  shown in  FIG. 11 . Therefore, the engine torque Te is also added to the ring gear R 2 . The remaining points of the nomograph shown in  FIG. 11  are the same as those of  FIG. 10 , so the description will not be repeated. 
     In two-motor parallel stepped HV mode, the engine torque Te, the MG 1  torque Tm 1  and the MG 2  torque Tm 2  all are allowed to be used for the forward rotation torque of the drive wheels, so it is particularly effective when a large torque is required of the drive wheels. 
     The controlled statuses in one-motor parallel: stepped HV mode (H 6 , H 8  lines in  FIG. 5 ) correspond to the case where Tm 1 =0 in  FIG. 11 . In parallel stepped HV mode, the vehicle is allowed to travel (engine drive mode) by setting Tm 1 =0 and Tm 2 =0 and using only engine torque. 
       FIG. 12  is a nomograph for illustrating the operation of the engine drive mode (Z 1  in  FIG. 5 ).  FIG. 13  is a nomograph for illustrating the operation of the engine drive mode (Z 2  in  FIG. 5 ). The nomograph of  FIG. 12  corresponds to a nomograph at the time when Tm 1 =0 and Tm 2 =0 in the nomograph indicated by the continuous lines in  FIG. 11 . The nomograph of  FIG. 13  corresponds to a nomograph at the time when Tm 1 =0 and Tm 2 =0 in the nomograph indicated by the dashed lines in  FIG. 11 . 
     As shown in  FIG. 12  and  FIG. 13 , the hybrid vehicle  1  further has the engine drive mode (Z 1 , Z 2  lines in  FIG. 5 ). In engine drive mode (Z 1 , Z 2  lines in  FIG. 5 ), the controller  100  sets the transmission unit  40  to the non-neutral state and sets the clutch CS to the engaged state, and then causes the vehicle to travel by using the engine  10  in a state where torque is not generated by the first MG  20  or the second MG  30 . 
     In this way, by setting the transmission unit  40  to a high-gear fixed position or a low-gear fixed position and engaging the clutch CS, it is possible to directly transmit the torque of the engine  10  to the drive shafts. Under the condition that the energy efficiency of the engine  10  is high, fuel economy is high when the engine drive mode is used. 
     With the above-described control, in a state where the engine  10  is efficiently operable, the power of the engine  10  is allowed to be directly transmitted to the drive wheels  90  without being converted to electric power, so it is possible to improve fuel economy. 
     Next, a difference in driving force among the drive modes will be described. As described above, the hybrid vehicle  1  according to the present embodiment is able to travel in many drive modes, such as the one-motor EV mode; the two-motor EV mode, the two-motor HV mode and the engine drive mode. For this reason, it is required to study which drive mode is used in what situation. 
       FIG. 14  is a graph that shows the relationship between a vehicle speed and a maximum driving force in each drive mode. In  FIG. 14 , the line L 1  indicates the maximum driving force in engine drive mode, the line L 2  indicates the maximum driving force in one-motor EV mode, the line L 3  indicates the maximum driving force in two-motor EV mode, and the line L 4  indicates the maximum driving force in two-motor parallel HV mode. 
     The line L 1  indicates the driving force at the time when the engine is set to a maximum power in the case where the vehicle travels in engine drive mode while the engine is directly coupled to the output shaft (the gear position is Lo) as shown in  FIG. 12 . The line L 2  indicates the driving force generated by the torque of only the second MG  30  as shown in  FIG. 6 . 
     The line L 3  indicates the driving force generated by the torque of both the first MG  20  and the second MG  30  as shown in  FIG. 7 . However, when the vehicle speed exceeds V 1 , the maximum driving force reduces at a stroke. This is because, when the vehicle speed becomes V 1 , the rotation speed of the first MG  20  decreases in the negative direction in  FIG. 7  and then reaches a limit value and, as a result, the operating state is changed. Specifically, when the vehicle speed is higher than V 1 , the clutch CS is engaged, and any one of the clutch C 1  and the brake B 1  is engaged and the other one of the clutch C 1  and the brake B 1  is released as shown in  FIG. 10 . Thus, the state of the first MG  20  is changed such that the first MG  20  generates positive torque, and the rotation speed is lower than that in the state of  FIG. 7 . In this state, because the torque of the first MG  20  is not increased by the planetary gear mechanism and there is a loss for idling of the engine  10 , the driving force remarkably reduces in a stepwise manner at the vehicle speed V 1 . 
     When a driving force larger than that in two-motor mode indicated by the line L 3  is required, the torque of the engine  10  is used in addition to the torque of both the first MG  20  and the second MG  30  as indicated by the line L 4 . 
     In this case, the controller  100  sets the transmission unit  40  to the non-neutral state and sets the clutch CS to the engaged state, and then causes the vehicle to travel by using driving force from the engine  10  in addition to driving force from the first MG  20  and driving force from the second MG  30  (H 7 , H 9  lines in  FIG. 5 ). 
     With the above-described control, it is possible to further increase the maximum driving force of the vehicle (the line L 4  in  FIG. 14 ) as compared to the EV mode (the line L 3  in  FIG. 14 ) in which the engine  10  is stopped and the first MG  20  and the second MG  30  are operated to carry out motoring. 
     When the gear position is switched at the vehicle speed V 1 , the Lo gear indicated by the continuous line in  FIG. 11  is used when the vehicle speed is lower than V 1 , and the Hi gear indicated by the dashed line in  FIG. 11  is used when the vehicle speed is higher than V 1 , with the result that the driving force reduces in a stepwise manner. 
     Next, control over the clutches and the brake in the case where the drive mode is changed to the two-motor EV mode will be described. In the above-description, the controlled statuses of the clutches C 1 , CS and brake B 1  in each drive mode are mainly described. Hereinafter, control at the time of switching the drive mode in the case where the drive mode is changed will be described. 
       FIG. 15  is a flowchart for illustrating control over the clutches and the brake in two-motor EV mode, which is executed by the controller  100 . As shown in  FIG. 15 , when the process of this flowchart is started, it is initially determined in step S 10  whether the drive mode is switched to the two-motor mode. 
     For example, determination as to switching of the drive mode is carried out on the basis of a map in which ranges are determined on the basis of a vehicle speed and a vehicle load;  FIG. 16  is a view that shows an example of such a map for determining the drive mode. As shown in  FIG. 16 , in positive and negative low load ranges, the one-motor EV mode is used. Basically, it is not necessary to assume a startup of the engine  10 , a relatively wide range in which reaction force compensation torque resulting from a startup of the engine  10  is not required may be allocated to the one-motor EV mode. 
     In a high load range, torque is insufficient in one-motor mode, so the two-motor mode is selected. That is, in a range in which the vehicle speed is lower than a predetermined value and the load is small, the one-motor EV mode is selected; whereas, when the load is larger than a predetermined value, the two-motor EV mode is selected. 
     When the vehicle speed exceeds the predetermined value V 1  in two-motor mode, because the rotation speed of each of the first MG  20  and the pinion gears has an upper limit, the state of the vehicle changes from the two-motor mode ( FIG. 7 ) in which the engine rotation speed Ne is zero to the two-motor mode ( FIG. 10 ) in which the engine rotation speed Ne is not zero. 
     When the vehicle speed exceeds V 2 , because the energy efficiency at the time when the vehicle travels by using the electric power of the battery tends to deteriorate, any one of the series-parallel HV mode (Lo), the series-parallel HV mode (Hi) and the series HV mode is selected. 
     When the drive mode is not switched to the two-motor mode in step S 10 , the process proceeds to step S 60 , and the control is returned to the main routine. On the other hand, when the drive mode is switched to the two-motor mode in step S 10 , the process proceeds to step S 20 . 
     In step S 20 , it is determined whether the drive mode is changed from the series-parallel mode to the two-motor EV mode. For example, when the state is changed from the one-motor EV mode as shown in  FIG. 6  to the two-motor EV mode, because the engine rotation speed is zero, it is relatively easy to directly change into the state shown in  FIG. 7 . However, in series-parallel mode as shown in  FIG. 8 , the engine rotation speed is not zero. Therefore, in order to change from the state shown in  FIG. 8  to the state shown in  FIG. 7 , it is required to decrease the rotation speed of the engine, coasting by inertia force, to zero. Therefore, when the drive mode before a change is the series-parallel mode in step S 20  (YES in S 20 ), the process proceeds to step S 50 , the clutch CS is engaged, the state shown in  FIG. 10  is once set, and the drive mode is placed in two-motor EV Mode in which the engine rotation speed Ne is not zero. 
     Even when the drive mode before a change is not the series-parallel mode in step S 20  (YES in S 20 ), but when it is required in step S 30  to set the engine rotation speed Ne to a predetermined rotation speed or higher, the process proceeds to step S 50  because of a similar reason. In step S 50 , the clutch CS is engaged, the state shown in  FIG. 10  is once set, and the drive mode is placed in two-motor EV mode in which the engine rotation speed Ne is not zero. For example, when it is required to drive the MOP  501  for lubrication or when it is required to avoid a rotation speed range in which the vibration of the vehicle increases because of resonance, it is determined that it is required to set the engine rotation speed Ne to the predetermined rotation speed or higher. 
     When it is not required to increase the engine rotation speed Ne in step S 30  (NO in S 30 ), the process proceeds to step S 40 . In step S 40 , the drive mode is switched to the two-motor EV Mode by engaging the clutch C 1  and the brake B 1 . 
     When the states of the clutches C 1 , CS and brake B 1  are determined in step S 40  or step S 50 , the process proceeds to step S 60 , and the control is returned to the main routine. 
     As described above, as shown in  FIG. 14  to  FIG. 16 , the controller  100  sets the drive mode to a first mode (E 3  line in  FIG. 5 : two-motor EV mode (Ne=0)) when the vehicle speed is lower than the determination threshold V 1 , and sets the drive mode to a second mode (E 5  line in  FIG. 5 : two-motor EV mode (Ne free)) when the vehicle speed is higher than the determination threshold. 
     When the drive mode is selected as described above, even when the vehicle speed increases and the vehicle is not allowed to travel in first mode because of the limitation of the rotation speed of the first MG  20 , the vehicle is allowed to travel with large driving force by using the first MG  20  and the second MG  30  at the same time when the second mode is used. 
     Preferably, the hybrid vehicle  1  further has a third mode (H 7 , H 9  lines in  FIG. 5 : two-motor parallel HV mode) as the drive mode. In third mode, the controller  100  sets the transmission unit  40  to the non-neutral state and sets the clutch CS to the released state, and then causes the first MG  20  to generate electric power in a state where the engine  10  is operated, and causes the second MG  30  to generate driving force for propelling the vehicle. When the controller  100  changes the drive mode from the third mode to the first mode, the controller  100  changes the drive mode via the second mode. 
     In this way, when the drive mode is changed from the third mode to the first mode, it is possible not to cause a driver to experience a feeling of output torque loss by changing the drive mode via the second mode. 
     Subsequently, an example at the time of a change of the drive mode will be described with reference to the operation waveform chart.  FIG. 17  is an operation waveform chart that shows an example of a change from the series-parallel HV mode to the two-motor EV mode. 
     As shown in  FIG. 17 , in the initial state at time t 0 , the hybrid vehicle is traveling in series-parallel HV mode. At this time, the clutch C 1  is controlled to the engaged state, the brake B 1  is controlled to the released state, and the transmission unit  40  establishes the Lo gear position. The clutch CS is controlled to the released state. 
     From time t 0  to time t 1 , the second MG  30  is outputting a positive torque at a positive rotation speed, and is being operated to carry out motoring. The first MG  20  is outputting a negative torque at a negative rotation speed, and is generating electric power through regenerative operation. The engine  10  is operating at a positive torque and a positive rotation speed. 
     At time t 1 , in response to the fact that the vehicle speed becomes lower than the threshold V 2 , it is determined to change the drive mode to the two-motor mode. At this time, the rotation speed of the first MG  20  is lower than the rotation speed of the engine  10  as shown by the nomograph indicated by the continuous lines in  FIG. 8 . When engagement of the clutch CS is started in a state where there is a difference in rotation speed, a shock at the time of engagement is large, so the process of synchronizing the rotation speed of the engine  10  with the rotation speed of the first MG  20  is executed from time t 1  to time t 2 . 
     At time t 2 , when the rotation speed of the engine  10  and the rotation speed of the first MG  20  are substantially equal to each other, the CS pressure begins to increase from zero. From time t 2  to time t 3 , the CS pressure increases, the engine torque reduces, and the torque (MG 1  torque) of the first MG  20  changes from a negative value to a positive value. 
     At time t 3 , the clutch CS completes engagement, and the rotation speeds of the engine  10 , first MG  20  and second MG  30  are equal to one another (the state indicated by the continuous lines in  FIG. 10 ). After time t 3 , the vehicle travels in two-motor mode. At this time, the engine  10  is being rotated by the first MG  20  and the second MG  30 , and negative torque is indicated as rotation resistance. After time t 3 , when fuel is not supplied to the engine  10  in the case where the vehicle is caused to travel in second mode (E 5  line in  FIG. 5 : two-motor EV mode (Ne free)), the controller  100  changes the open/close timing of an intake valve or exhaust valve such that resistance during rotation of the engine  10  is reduced. 
     When the vehicle is caused to travel in second mode and fuel is not supplied to the engine  10 , the engine  10  is forcibly rotated by the first MG  20  and the second MG  30 . In this case, an energy loss is smaller when the rotation resistance of the engine  10  is small. In order to reduce the rotation resistance of the engine  10 , it is desirable that the compressibility and expansion coefficient of air in a cylinder be small. Therefore, the controller  100  reduces the rotation resistance of the engine  10  by changing the open/close timing of the intake valve or exhaust valve, thus reducing an energy loss. 
     Lastly, the hybrid vehicle  1  according to the present embodiment is summarized with reference to  FIG. 1 , and the like, again. As shown in  FIG. 1 , the hybrid vehicle  1  includes the engine  10 , the first MG  20 , the second MG  30 , the transmission unit  40 , the differential unit  50 , the clutch CS and the controller  100 . The controller  100  controls the engine  10 , the first MG  20 , the transmission unit  40  and the clutch CS. The controller  100  sets the transmission unit  40  to the non-neutral state and sets the clutch CS to the engaged state, and then causes the vehicle to travel by using driving force from the first MG  20  and driving force from the second MG  30  at the same time ( FIG. 10 ). 
     By providing such a drive mode, the vehicle is allowed to be propelled by operating both the first MG  20  and the second MG  30  to carry out motoring even in a state where the rotation speed of the engine  10  is not zero (indicated as Ne free in E 4 , E 5  lines in  FIG. 5 ). Therefore, it is possible to increase the opportunity that the two rotary electric machines are allowed to be used, so the flexibility of control over the vehicle increases in the case where large driving force is required in EV mode. 
     Preferably, the controller  100  switches the drive mode of the vehicle between the first mode (E 3  line in  FIG. 5 ) and the second mode (E 4 , E 5  lines in  FIG. 5 ) in response to the vehicle speed. The first mode is a drive mode in which the rotation speed of the engine  10  is fixed to zero and the clutch CS is set to the released state, and then the vehicle is caused to travel by using driving force from the first MG  20  and driving force from the second MG  30  at the same time. The second mode is a drive mode in which the transmission unit  40  is set to the non-neutral state and the clutch CS is set to the engaged state, and then the vehicle is caused to travel by using driving force from the first MG  20  and driving force from the second MG  30  at the same time. 
     Because the second mode is provided as the drive mode as described above, even when the rotation speed of the engine is not zero like a change from a state where the engine is being operated to the EV mode, the vehicle is able to travel with large driving force using the first MG  20  and the second MG  30  at the same time. 
     The embodiment described above is illustrative and not restrictive in all respects. The scope of the invention is defined by the appended claims rather than the above description. The scope of the invention is intended to encompass all modifications within the scope of the appended claims and equivalents thereof.