Abstract:
A hybrid powered vehicle is capable of running forward and backward under various conditions (e.g., by a motor) and enables the regeneration of energy during the reduction of speed without increasing the full length, weight, cost and the like of a transmission. The hybrid powered vehicle has a planetary gear unit, which comprises the following three elements: a ring gear, a sun gear, and a carrier pivotally supporting a planetary pinion arranged between the carrier and the sun gear. The planetary gear unit is disposed between an engine and an electric motor. The hybrid powered vehicle also has a forward/backward rotation switching mechanism, which comprises a brake for braking a first element that is one of the above-mentioned three elements and a clutch for connecting and disconnecting two elements among the above-mentioned three elements. An output shaft of the engine connects to one of the two elements other than the first element, and an output shaft of the electric motor and an input shaft of the transmission connect to the other.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a hybrid powered vehicle that has an engine and an electric motor. 
     2. Description of Related Art 
     A hybrid powered vehicle is known which has both an engine and an electric motor and is selectively driven by the engine and/or the electric motor. Hybrid powered vehicles are roughly classified into series hybrid powered vehicles and parallel hybrid powered vehicles. The series hybrid powered vehicle uses an output of the engine for power generation only, and is driven exclusively by an output of the electric motor. On the other hand, the parallel hybrid powered vehicle can be driven by the output of the engine and/or the electric motor depending on the driving conditions. Thus, the parallel hybrid powered vehicle requires a drive system that is capable of transmitting the output of the engine and the electric-motor to an axle. 
     FIG. 6 schematically shows the structure of the drive system in the conventional parallel hybrid powered vehicle. As shown in FIG. 6, an electric motor  72  is arranged between an engine  70  and a transmission  80 , and a rotor  73  of the electric motor  72  is axially and fixedly integrated with an output shaft  71  of the engine  70 . Reference numeral  74  denotes a stator, which is fixed to a transmission case  81 . Accordingly, the output of the engine and the electric motor is transmitted from the output shaft  71  of the engine  70  to an input shaft  82 . Then, the output of the engine and the electric motor goes through a forward/backward rotation switching mechanism  83 , and is converted by a continuously variable transmission mechanism  84 . The output is finally transmitted to an axle. 
     In the above-mentioned conventional hybrid powered vehicle, the engine  70  and the electric motor  72  always rotate as one, and it is therefore impossible to use only the electric motor  72  as the drive source for the vehicle with the engine  70  being stopped. It is also impossible to regenerate energy solely by the electric motor  72  without running an engine brake during speed reduction. 
     To address this problem, it is necessary to provide a clutch mechanism between the engine  70  and the motor  72  so as to stop the transmission of the output between the engine  70  and the input shaft  82 . This increases the full length, weight, cost and the like of the transmission  80 . 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a hybrid powered vehicle that is capable of running forward and backward under various conditions (i.e., the vehicle is driven by either one or both of an electric motor and an engine) and enables the regeneration of energy during speed reduction without increasing the full length, weight, cost and the like of a transmission. 
     The above object may be accomplished according to a principle of the present invention, which provides a hybrid powered vehicle, which has an engine and a transmission, the hybrid powered vehicle comprising: a forward/backward rotation switching mechanism arranged between the engine and the electric motor; wherein the forward/backward rotation switching mechanism comprises: a planetary gear unit composed of the following three elements: a ring gear, a sun gear and a carrier pivotally supporting a planetary pinion arranged between the ring gear and the sun gear; and a brake for braking a first element that is one of the three elements; connecting means for connecting and disconnecting two elements among the three elements; and wherein an output shaft of the engine is connected to a second element that is one of two elements except the first element, and an output shaft of the electric motor and an input shaft of the transmission are connected to a third element that is the other element of the two elements except the first element. 
     Therefore, disengaging the brake and the clutch enables the vehicle to be driven solely by the electric motor and makes possible the regeneration of energy during speed reduction. The vehicle is driven forward by disengaging the brake, engaging the clutch and rotating the electric motor and the engine forward. The vehicle is driven backward by engaging the brake, disengaging the clutch, rotating the electric motor backward and rotating the engine forward. 
     The planetary gear unit is preferably a double pinion gear, which substantially equalizes a reduction gear ratio during the forward driving and the backward driving. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The nature of this invention, as well as other objects and advantages thereof, will be explained in the following with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures and wherein: 
     FIG. 1 is a view schematically showing the structure of a hybrid powered vehicle according to the first embodiment of the present invention; 
     FIG. 2 is a table showing a relationship between the vehicle driving directions and the rotating directions of an engine and an electric motor and the ON/OFF state of a clutch and a brake in each driving mode of a hybrid powered vehicle according to the first embodiment of the present invention; 
     FIG. 3 is a view schematically showing a driving force transmission mechanism in a hybrid powered vehicle according to the second embodiment of the present invention; 
     FIG. 4 is a view schematically showing a driving force transmission mechanism in a hybrid powered vehicle according to the third embodiment of the present invention; 
     FIG. 5 is a view schematically showing a driving force transmission mechanism in a hybrid powered vehicle according to the fourth embodiment of the present invention; and 
     FIG. 6 is a view schematically showing a driving force transmission mechanism in a conventional hybrid powered vehicle. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention will be described in further detail by way of example with reference to the accompanying drawings. 
     FIGS. 1 and 2 are directed to a hybrid powered vehicle according to a first embodiment of the invention, wherein FIG. 1 outlines the structure of the hybrid powered vehicle with rotary shafts  5 ,  24 ,  25 ,  27 ,  33 ,  34  arranged in parallel. 
     As shown in FIG. 1, the hybrid powered vehicle has an engine  1 , an electric motor  2  and a transmission  3 . A transmission case  9  for the transmission  3  is fixedly integrated with the engine  1 . The electric motor  2  is coaxial with an output shaft  5  of the engine  1 . The output shaft  5  goes through an output shaft  6  of the electric motor  2 , which is coaxially integrated with a rotor  7 . A stator  8  of the electric motor  2  is fixed to the transmission case  9 . 
     A forward/backward rotation switching mechanism  4  is arranged between the engine  1  and the electric motor  2  and a continuously variable transmission mechanism  20  of the transmission  3 . The rotations inputted from the engine  1  and the electric motor  2  are inputted to the continuously variable transmission mechanism  20  through the forward/backward rotation switching mechanism  4 . 
     A planetary gear unit is used as the forward/backward rotation switching mechanism  4 , and the output shaft  5  of the engine  1  connects to a sun gear  10  (the second element) of the planetary gear unit. A carrier (the third element) for holding a pinion gear (a planetary pinion)  11  connects to the output shaft  6 , which is coaxially fixed to the rotor  7  of the electric motor  2 . Accordingly, the rotation of the engine  1  is inputted from the sun gear  10 , and the rotation of the electric motor  2  is inputted from the carrier  12 . The pinion gear  11  is a double pinion gear composed of an inner pinion (an inner gear)  11   a  and an outer pinion (an outer gear)  11   b,  which are engaged with one another. The inner gear  11   a  is engaged with the sun gear  10 , and the outer gear  11   b  is engaged with a ring gear (the first element)  13 . 
     The rotation of the forward/backward rotation switching mechanism  4  is outputted to the continuously variable transmission mechanism  20  through the carrier  12 . More specifically, an input shaft  24  coaxially integrated with a primary pulley  21  of the continuously variable transmission mechanism  20  connects to the continuously variable transmission mechanism  20  of the carrier  12 . The rotation of the engine  1  and the electric motor  2  is inputted to the sun gear  10  and the carrier  12 , respectively, and is then outputted from the carrier  12  to the input shaft  24 . 
     The ring gear  13  has a brake  14 , which is fixed to the transmission case  9 . Therefore, engaging the brake  14  restricts the rotation of the ring gear  13 . If the brake  14  is disengaged, the ring gear  13  becomes capable of rotating with the rotation and revolution of the pinion gear  11 . 
     A clutch  15  is arranged inside the input shaft  24  of the continuously variable transmission mechanism  20 . The clutch  15  connects and disconnects a shaft  16 , which extends from the sun gear  10  to the continuously variable transmission mechanism  20 , and the input shaft  24 . The engagement of the clutch  15  causes the sun gear  10  integrated with the shaft  16  and the carrier  12  integrated with the input shaft  24  to rotate as one. 
     As stated above, the rotation inputted from the engine  1  and the electric motor  2  is outputted from the forward/backward rotation switching mechanism  4  to the continuously variable transmission mechanism  20 . The continuously variable transmission mechanism  20  is composed of the primary pulley  21 , a secondary pulley  22  and a belt  23 . The rotation inputted from the forward/backward rotation switching mechanism  4  is inputted from the primary pulley  21  coaxially integrated with the input shaft  24  to the secondary pulley  22  through the belt  23 . 
     The primary pulley  21  is composed of two sheaves  21   a,    21   b,  and the secondary pulley  22  is composed of two sheaves  22   a,    22   b.  The sheaves  21   a,    22   a  are fixed in the axial direction, and the sheaves  21   b,    22   b  are movable in the axial direction by means of a hydraulic actuator (not illustrated). The movable sheaves  21   b,    22   b  of the primary pulley  21  and the secondary pulley  22  are synchronously movable in the axial direction. To reduce the driving speed, the movable sheaves  21   b,    22   b  are driven in such a manner as to narrow a groove of the secondary pulley  22  and widen a groove of the primary pulley  21 . To increase the driving speed, the width of the groove of the primary pulley  21  is narrowed, and the groove of the secondary pulley  22  is widened. 
     The rotation transmitted by the continuously variable transmission mechanism  20  is outputted from a second shaft  25  coaxially integrated with the secondary pulley  22 , and is inputted from a drive gear  26  coaxially integrated with the second shaft  25  to a driven gear  28  pivotally supported by a third shaft  27 . The third shaft  27  has a starting clutch  29  for connecting and disconnecting the third shaft  27  and the driven gear  28 . The engagement of the starting clutch  29  integrates the driven gear  28  and the third shaft  27 , and the rotation inputted in the driven gear  28  is transmitted to the third shaft  27  and is inputted from a differential output gear  30  axially integrated with the third shaft  27  to a ring gear  32  of a front differential gear  31 . The front differential gear  31  distributes the output to the right and left axle shafts  33 ,  34 . 
     According to the first embodiment, the hybrid powered vehicle that is constructed in the above-mentioned manner is able to run forward and backward in each driving mode by regulating the rotating directions of the engine  1  and the electric motor  2  and engaging and disengaging the clutch  15  as shown in FIG.  2 . 
     The engagement of the clutch  15  integrates the sun gear  10  and the carrier  12 , which are restricted by one another. This restricts the revolution of the pinion gear  11  supported by the carrier  12  around the sun gear  12  and the rotation of the inner gear  11   a  engaged with the sun gear  10 . Additionally, the rotation of the outer gear  11   b  engaged with the inner gear  11   a  is restricted, and the relative rotation of the ring gear  13  engaged with the outer gear  11   b  with respect to the pinion gear  11  is restricted. In short, the sun gear  10 , the carrier  12  and the ring gear  13  are integrated. 
     Accordingly, disengaging the brake  14  to permit the rotation of the ring gear  13  causes the sun gear  10 , the carrier  12  and the ring gear  13  to rotate as one. 
     If the engine  1  and the electric motor  2  are rotated forward, a resultant force from the output of the engine  1  and the output of the electric motor  2  is inputted to the input shaft  24 , and the engine  1  and the electric motor  2  are used as the drive source for the vehicle. It is also possible to rotate only the engine  1  forward with the electric motor  2  being idled so that the engine  1  can be used as the drive source for the vehicle. In this case, the output of the engine I rotates the rotor  7  of the electric motor  2 , and this enables the use of the electric motor  2  for power generation. 
     On the other hand, the disengagement of the clutch  15  cancels the integration of the sun gear  10  and the carrier  12 , and consequently, the sun gear  10  and the carrier  12  become capable of rotating relatively to one another. Therefore, the pinion gear  11  supported by the carrier  12  becomes capable of revolving around the sun gear  10 . In the following description, the revolutions of the inner gear  11   a,  the outer gear  11   b,  the carrier  12  and the ring gear  13  are seen from the sun gear  10 . 
     The inner gear  11   a  a engaged with the sun gear  10  also rotates in association with the revolution. The inner gear  11   a  rotates in the same direction as the revolving direction seen from the sun gear  10 . In association with the rotation of the inner gear  11   a,  the outer gear  11   b  rotates in an opposite direction to the rotating direction of the inner gear  11   a.  The rotation of the outer gear  11   b  causes the ring gear  13  engaged with the outer gear  11   b  to rotate with respect to the pinion gear  11 , in other words, with respect to the carrier  12  in the same direction as the rotating direction of the outer gear  11   b.  More specifically, the ring gear  13  rotates at a lower speed than the carrier  12  in a direction in that the pinion gear  11  (the carrier  12 ) revolves with respect to the sun gear  10 . Seen from the sun gear  13 , the pinion gear  11  revolves around the sun gear  10  in the opposite direction to the rotating direction of the sun gear  10 . 
     Thus, if the brake  14  is engaged to restrict the rotation of the ring gear  13  with respect to the transmission case  9 , the forward rotation of the sun gear  10  results in the backward rotation of the carrier  12  and the backward rotation of the sun gear  10  results in the forward rotation of the carrier  12 . In fact, however, the sun gear  10  is rotated forward and the carrier  12  is rotated backward because the engine  1  connected to the sun gear  10  is capable of rotating only forward. 
     The backward rotation is inputted to the continuously variable transmission mechanism  20  by rotating the engine  1  forward and rotating the electric motor  2  backward. Consequently, the vehicle is driven backward by the engine  1  and the electric motor  2 . It is also possible to rotate only the engine  1  forward with the electric motor  2  being idled, and in this case, the vehicle is driven backward by the engine  1 . The output of the engine  1  rotates the rotor  7  of the electric motor  2 , and this enables the electric motor  2  to function as a power generator. 
     As stated above, the forward driving and the backward driving can be switched by engaging and disengaging the clutch  15  and the brake  14 . The engine  1  must be rotating whether the vehicle is running forward or backward. More specifically, idling the engine  1  requires an engine brake to operate, and driving the vehicle solely by the electric motor  2  increases a load and deteriorates an energy regeneration efficiency during the speed reduction. 
     In order to drive the vehicle solely by the electric motor  2  and improve the energy regeneration efficiency during the speed reduction, the clutch  15  is disengaged first. This cancels the integration of the sun gear  10  and the carrier  12 , so that they can become capable of rotating relatively to one another. If the rotation of the ring gear  13  is restricted, however, the carrier  12  cannot rotate freely since the rotation of the carrier  12  follows that of the gear  10 . To address this problem, the brake  14  and the clutch  15  are disengaged so as to permit the free rotation of the ring gear  13 , and this enables the sun gear  10  and the carrier  12  to rotate independently of one another. 
     Consequently, the carrier  12  can rotate freely even when the engine  1  and the sun gear  10  are stopped. The forward rotation of the electric motor  2  causes the carrier  12  to rotate forward, and the forward rotation is inputted to the continuously variable transmission mechanism  20  to thereby drive the vehicle forward. The backward rotation of the electric motor  2  causes the carrier  12  to rotate backward, and this drives the vehicle backward. If the electric motor  2  is stopped to function as the power generator, the driving speed is reduced solely by a power generation resistance of the electric motor  2  whether the engine  1  is operating or stopped. Therefore, the energy can be regenerated efficiently. 
     Thus, the hybrid powered vehicle of the present invention has advantages as described below. The transmission of the rotation from the engine  1  to the transmission  3  is stopped by disengaging the clutch  15  and the brake  14  in the forward/backward rotation switching mechanism  4 . Thus, the vehicle can be driven solely by the electric motor  2 , and the energy can be regenerated during the speed reduction. Moreover, the vehicle can be driven solely by the engine  1  or by both the engine  1  and the electric motor  2 . The energy efficiency can be improved by selecting a driving mode according to the driving conditions. 
     Since the vehicle can be driven by either the engine  1  or the electric motor  2 , the vehicle is able to run continuously even if the engine  1  or the electric motor  2  experiences a problem or if the amount of a battery runs low. 
     The hybrid powered vehicle of the present invention uses the double pinion type planetary gear unit as the forward/backward rotation switching mechanism  4 , and this substantially equalizes a reduction gear ratio during the forward driving and the backward driving. This eliminates the necessity of adjusting the reduction gear ratio of the continuously variable transmission mechanism  20 . 
     Moreover, the hybrid powered vehicle of the present invention is able to start the engine  1  without using a starter when the vehicle is driven by the electric motor  2  with the engine  1  being stopped. More specifically, when the vehicle is driven forward by the forward rotation of the electric motor  2 , the clutch  15  is engaged to input the rotation of the electric motor  2  to the engine  1  to thereby start the engine  1 . When the vehicle is driven backward by the backward rotation of the electric motor  2 , the brake  14  is engaged to convert the backward rotation of the motor  2  into the forward rotation, which is inputted to the engine  1  to start the engine  1 . 
     The structure of the forward/backward rotation switching mechanism  4  in FIG. 4 is only an example among many. For example, the forward/backward rotation switching mechanism  4  may also be constructed in the second to fourth embodiments as described below. 
     A description will now be given of a forward/backward rotation switching mechanism in the hybrid powered vehicle according to a second embodiment of the present invention. As shown in FIG. 3, the second embodiment is different from the first embodiment only in the position of the clutch. More specifically, a clutch  56  is provided inside the output shaft  6  of the electric motor  2  in such a manner as to connect and disconnect the output shaft  6  of the electric motor  2  and the output shaft  5  of the engine  1 . The engagement of the clutch  56  causes a sun gear  51  integrated with the output shaft  5  and a carrier  53  integrated with the output shaft  6  to be restricted by one another to rotate as one. In this embodiment, a pinion gear  52  is a double pinion gear composed of an inner gear  52   a  and an outer gear  52   b  as is the case with the first embodiment. 
     Accordingly, engaging the clutch  56  and disengaging the brake  55  cause the sun gear  51  and the carrier  53  to be restricted by one another to rotate as one. The rotation of the pinion gear  52  is also restricted, and this restricts the rotation of the ring gear  54 . Thus, the sun gear  51 , the carrier  53  and the ring gear  54  rotate as one. 
     If the engine  1  and the electric motor  2  are rotated forward, the vehicle is driven by both the engine  1  and the electric motor  2 . If the engine  1  is rotated forward with the electric motor  2  being idled, the vehicle is driven solely by the engine  1 . 
     If the sun gear  51  is rotated forward by disengaging the clutch  56  and engaging the brake  55 , the carrier  53  rotates backward. If the engine  1  is rotated forward and the electric motor  2  is rotated backward, the vehicle is driven by the engine  1  and the electric motor  2 . If the engine I is rotated forward with the electric motor  2  being idled, the vehicle is driven solely by the engine  1 . 
     If the engine  1  and the sun gear  51  are stopped by disengaging the clutch  56  and disengaging the brake  55 , the carrier  53  becomes capable of rotating freely. If the electric motor  2  is rotated forward, the carrier  53  also rotates forward to enable the vehicle to be driven solely by the electric motor  2 . If the electric motor  2  is rotated backward, the carrier  53  also rotates backward to enable the backward driving of the vehicle. 
     A description will now be given of a forward/backward rotation switching mechanism in the hybrid powered vehicle according to a third embodiment of the present invention. As shown in FIG. 4, the third embodiment is different from the first embodiment only in the position of the clutch. A clutch  46  is provided outside the input shaft  24  of the continuously variable transmission mechanism  20  in such a manner as to connect and disconnect a ring gear  44  and the input shaft  24 . The engagement of the clutch  46  causes a ring gear (the first element)  44  and a carrier (the third element)  43  integrated with the input shaft  24  to be restricted by one another to rotate as one. In this embodiment, a pinion gear  42  is a double pinion gear composed of an inner gear  42   a  and an outer gear  42   b  as is the case with the first embodiment. 
     Accordingly, the engagement of the clutch  46  causes the ring gear  44  and the carrier  43  to be restricted by one another to rotate as one. This restricts the rotation of the pinion gear  42 , which is supported by the carrier  43 , with respect to the ring gear  44 , and also restricts the rotation of the outer gear  42   b  engaged with the ring gear  44 . Moreover, the rotation of the inner gear  42   a  is restricted since it is engaged with the outer gear  42   b,  and the rotation of the sun gear  42  is restricted since it is engaged with the inner gear  42   a.  Thus, the sun gear  41 , the carrier  43  and the ring gear  43  are integrated. 
     The sun gear  41 , the carrier  43  and the ring gear  44  are rotated as one by disengaging the brake  45  to permit the rotation of the ring gear  44 . Thus, the vehicle can be driven by both the engine  1  and the electric motor  2  by rotating the engine  1  and the electric motor  2  forward. The vehicle can be driven solely by the engine  1  by rotating the engine  1  with the electric motor  2  being idled. 
     The disengagement of the clutch  46  cancels the integration of the ring gear  44  and the carrier  43  to thereby make the carrier  43  rotatable with respect to the ring gear  44 . Since the sun gear  41  connects to the ring gear  44  through the double pinion type pinion gear  42 , the sun gear  41  rotates at a lower speed than the carrier  43  in the same direction as the rotating direction of the carrier  43  in association with the rotation of the carrier  43  with respect to the ring gear  44 . 
     Thus, the carrier  43  is rotated backward by engaging the brake  45  and restricting the rotation of the ring gear  44  with respect to the transmission case  9 . Therefore, if the engine  1  is rotated forward and the electric motor  2  is rotated backward, the vehicle is driven backward by both the engine I and the electric motor  2 . If the engine  1  is rotated forward with the electric motor  2  being idled, the vehicle is driven backward solely by the engine  1 . 
     If the clutch  46  is disengaged to cancel the integration of the ring gear  44  and the carrier  43  to enable their relative rotation and the brake  45  is disengaged to permit the free rotation of the ring gear  44 , the carrier  43  becomes capable of rotating freely while the engine  1  and the sun gear  41  are stopped. Therefore, if the electric motor  2  is rotated forward, the carrier  43  rotates forward to drive the vehicle forward, and if the electric motor  2  is rotated backward, the carrier  43  rotates backward to drive the vehicle backward. 
     A description will now be given of a forward/backward rotation switching mechanism in the hybrid powered vehicle according to a fourth embodiment of the present invention. As shown in FIG. 5, the fourth embodiment is different from the third embodiment only in the position of the clutch. In the fourth embodiment, a clutch  66  is provided outside the output shaft  6  of the electric motor  2  in such a manner as to connect and disconnect the output shaft  6  of the electric motor  2  and a ring gear  64 . The engagement of the clutch  66  causes the ring gear  64  and a carrier  63  integrated with the ring gear  64  to be restricted by one another to rotate as one. 
     Accordingly, engaging the clutch  66  and disengaging a brake  65  causes the ring  64  and the carrier  63  to be restricted by one another to rotate as one. The rotation of a pinion gear  62 , which includes inner gear  62   a  and outer gear  62   b,  is also restricted, and this restricts the free rotation of the sun gear  61 . Thus, the sun gear  61 , the carrier  63  and the ring gear  64  rotate as one. 
     Therefore, if the engine  1  and the electric motor  2  are rotated forward, the vehicle is driven by the engine  1  and the electric motor  2 . If the engine  1  is rotated forward with the electric motor  2  being idled, the vehicle is driven solely by the engine  1 . 
     If the sun gear  61  is rotated forward by disengaging the clutch  66  and engaging the brake  65 , the carrier  63  rotates backward. In this case, if the engine  1  is rotated forward and the electric motor  2  is rotated backward, the vehicle is driven backward by the engine  1  and the electric motor  2 . If the engine  1  is rotated forward with the electric motor  2  being idled, the vehicle is driven solely by the engine  1 . 
     The carrier  63  can rotate freely even if the engine  1  and the sun gear  61  are stopped by disengaging the clutch  66  and the brake  65 . If the electric motor  2  is rotated forward, the carrier  63  rotates forward to drive the vehicle forward solely by the electric motor  2 , and if the electric motor  2  is rotated backward, the carrier  63  rotates backward to drive the vehicle backward. 
     As is the case with the first embodiment, the hybrid powered vehicle according to the second to fourth embodiments can be driven solely by the electric motor  2  and can regenerate the energy during the speed reduction. It is also possible to select whether to drive the vehicle solely by the engine  1  or both the engine  1  and the electric motor  2 . The energy efficiency can be improved by selecting the driving mode according to the driving conditions. The second to fourth embodiments have the same advantages as the first embodiment. 
     It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the invention is to cover all modifications within the spirit and scope of the invention. 
     In the above-described embodiments, the second element is the sun gear, the third element is the carrier, and the first element is the ring gear. The clutch connects and disconnects the sun gear (the second element) and the carrier (the third element), or the carrier (the third element) and the ring gear (the first element). If, however, the motor  2  is arranged at the continuously variable  20 , not at the forward/backward rotation switching mechanism, the clutch may be provided in such a manner as to connect and disconnect the sun gear (the second element) and the ring gear (the first element). 
     The ring gear, the sun gear and the carrier should not necessarily be the first element, the second element and the third element, respectively. Of course, it is possible to adopt alternate combinations on condition that the output shaft of the engine connects to one element (the second element) among the ring gear, the sun gear and the carrier; the output shaft of the electric motor and the input shaft of the transmission connect to one element (the third element) among the three elements; and one element (the first element) among the three elements is braked by the brake. 
     In the above-described embodiments, the transmission  3  is provided with the continuously variable transmission mechanism  20 , but the present invention should not be restricted to this. The transmission  3  may also be provided with a normal gear transmission mechanism. 
     As set forth hereinabove, the transmission of the rotation from the engine to the transmission can be stopped by disengaging the clutch and the brake of the forward/backward rotation switching mechanism. Therefore, the vehicle can be driven solely by the electric motor, and the energy can be regenerated during the speed reduction. It is also possible to select whether to drive the vehicle by solely the engine or by both the engine and the electric motor. The energy efficiency can be improved by selecting the driving mode according to the driving conditions. 
     Moreover, the vehicle can be driven forward and backward by either the engine or the electric motor. Thus, the vehicle can continue to run in an emergency, e.g., if the electric motor goes wrong and if the amount of the battery runs short. 
     It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the invention is to cover all modifications, alternate constructions and equivalents falling within the spirit and scope of the invention as expressed in the appended claims.