Patent Publication Number: US-9889733-B2

Title: Power transmission system and vehicle comprising the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and benefits of Chinese Patent Application Nos. 201510024314.X and 201520033349.5, both filed with the State Intellectual Property Office of P. R. China on Jan. 16, 2015. The entire contents of the above-identified applications are incorporated herein by reference. 
     FIELD 
     Embodiments of the present disclosure relate to vehicles, and more particularly to a power transmission system for a vehicle, and a vehicle including the power transmission system. 
     BACKGROUND 
     To reduce energy consumption, the development and utilization of energy-efficient vehicles have become a trend. As an energy-efficient vehicle, a hybrid vehicle is driven by at least one of an internal combustion engine and a motor and has various operation modes, and consequently may operate with improved transmission efficiency and fuel efficiency. 
     However, in the related art, the power transmission system in the hybrid vehicle is generally complex in structure, provides fewer transmission modes, and is low in transmission efficiency. Besides, for most hybrid vehicles, the charging process is always carried out during the running of the vehicle. Therefore, a conventional hybrid vehicle has relatively fewer charging modes and charging passage, and lower charging efficiency. 
     SUMMARY 
     Embodiments of the present disclosure seek to solve at least one of the problems existing in the prior art to at least some extent. 
     Embodiments of a broad aspect of the present disclosure provide a power transmission system for a vehicle. The power transmission system for a vehicle according to embodiments of the present disclosure includes: an engine; a plurality of input shafts, at least one of the input shafts being configured to selectively engage with the engine, each of the input shafts being provided with a shift driving gear thereon; a plurality of output shafts, each of the output shafts being provided with a shift driven gear configured to mesh with a corresponding shift driving gear; a motor power shaft configured to rotate together with one of the output shafts; and a first motor generator configured to rotate together with the motor power shaft, when the motor power shaft is rotated together with one of the output shafts, the first motor generator is configured to generate electric power utilizing at least parts of power generated by the engine while a vehicle in a running state or a parking state. 
     Embodiments of another broad aspect of the present disclosure provide a vehicle. The vehicle, according to embodiments of the present disclosure, includes the above-identified power transmission system for a vehicle. 
     With the power transmission system and the vehicle according to embodiments of the present disclosure, the transmission modes are increased, and various conditions, such as charging the vehicle while parking or charging the vehicle while driving, may be accomplished. 
     Additional aspects and advantages of embodiments of the present disclosure will be given in part in the following descriptions, become apparent in part from the following descriptions, or be learned from the practice of the embodiments of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects and advantages of embodiments of the present disclosure will become apparent and more readily appreciated from the following descriptions made with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic view of an exemplary power transmission system according to an embodiment of the present disclosure; 
         FIG. 2  is a schematic view of an exemplary power transmission system according to an embodiment of the present disclosure; 
         FIG. 3  is a schematic view of an exemplary power transmission system according to an embodiment of the present disclosure; 
         FIG. 4  is a schematic view of an exemplary power transmission system according to an embodiment of the present disclosure; 
         FIG. 5  is a schematic view of an exemplary power transmission system according to an embodiment of the present disclosure; and 
         FIG. 6  is a schematic view of an exemplary power transmission unit of a power transmission system according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will be made in detail to embodiments of the present disclosure. The embodiments described herein with reference to drawings are explanatory, illustrative, and used to generally understand the present disclosure. The embodiments shall not be construed to limit the present disclosure. The same or similar elements and the elements having same or similar functions are denoted by like reference numerals throughout the descriptions. 
     In the specification, it should be understood that the terms such as “central”, “longitudinal”, “lateral”, “width”, “thickness”, “above”, “below”, “front”, “rear”, “right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, “inner”, “outer”, “clockwise”, “counter-clockwise” should be construed to refer to the orientation as then described or as shown in the drawings. These terms are merely for convenience and concision of description and do not alone indicate or imply that the device or element referred to must have a particular orientation. Thus, it cannot be understood to limit the present disclosure. 
     In addition, terms such as “first” and “second” are used herein for purposes of description and are not intended to indicate or imply relative importance or significance or impliedly indicate quantity of the technical feature referred to. Thus, the feature defined with “first” and “second” may comprise one or more of these features. In the description of the present disclosure, “a plurality of” means two or more than two of these features, unless specified otherwise. 
     In the present invention, unless specified or limited otherwise, the terms “mounted,” “connected,” “coupled,” “fixed” and the like are used broadly, and may be, for example, fixed connections, detachable connections, or integral connections; may also be mechanical or electrical connections; may also be direct connections or indirect connections via intervening structures; may also be inner communications of two elements, which can be understood by those skilled in the art according to specific situations. 
     In the present invention, unless specified or limited otherwise, a structure in which a first feature is “on” or “below” a second feature may include an embodiment in which the first feature is in direct contact with the second feature, and may also include an embodiment in which the first feature and the second feature are not in direct contact with each other, but are contacted via an additional feature formed therebetween. Furthermore, a first feature “on,” “above,” or “on top of” a second feature may include an embodiment in which the first feature is right or obliquely “on,” “above,” or “on top of” the second feature, or just means that the first feature is at a height higher than that of the second feature; while a first feature “below,” “under,” or “on bottom of” a second feature may include an embodiment in which the first feature is right or obliquely “below,” “under,” or “on bottom of” the second feature, or just means that the first feature is at a height lower than that of the second feature. 
     A power transmission system according to embodiments of the present disclosure may be described below with reference to  FIGS. 1-5 . The power transmission system according to embodiments of the present disclosure may be used in vehicles such as hybrid vehicles as a power system, which may provide sufficient power and electric power for driving the vehicle. 
     In some embodiments, a power transmission system  100  may generally include a power unit and a transmission unit. The power unit may be an engine  4 , a motor generator, and so on. In some embodiments, the transmission unit  101  as shown in  FIG. 6  may transmit power output from the power unit, thus driving or charging the vehicle. 
     In some embodiments, as shown in  FIGS. 1-5 , the power transmission system  100  may include, but is not limited to, an engine  4 , a first motor generator  51  and a transmission unit  101 . 
     In some embodiments as shown in, for example,  FIG. 1 , the transmission  101  unit includes a plurality of input shafts (e.g., a first input shaft  11 , a second input shaft  12 ), a plurality of output shafts (e.g., a first output shaft  21 , a second output shaft  22 ), a motor power shaft  3 , a plurality of gears provided on related shafts (such as the input shaft, the output shaft, and the motor power shaft), and a gear shift member such as a synchronizer. 
     In some embodiments, the engine  4  is configured to selectively engage with at least one of the input shafts, when the engine  4  performs power transmission with the input shaft(s). For example, when the engine  4  is transmitting power to the input shaft, the engine  4  may selectively engage with one of the input shafts to transmit power. In some embodiments, the engine  4  may be selectively engage with two or more of the input shafts simultaneously to transmit power. 
     In some embodiments, as shown in  FIGS. 1-5 , the plurality of input shafts include a first input shaft  11  and a second input shaft  12 . The engine  4  may selectively engage with one of the first and second input shafts  11 ,  12  to transmit power. In some embodiments, the engine  4  may engage with the first and second input shafts  11 ,  12  simultaneously to transmit power. It should be noted that the engine  4  may be disengaged from the first and second input shafts  11 ,  12  simultaneously. 
     It is known to a person skilled in the art that the engagement between the engine  4  and the input shaft(s) is related to specific conditions of the power transmission system  100 . The engagement between the engine  4  and the input shaft(s) will be described below in detail with reference to detailed embodiments. 
     In some embodiments, the power transmission between the input shaft(s) and the output shaft(s) is achieved by shaft gear pairs. For example, each of the input shafts has a shaft driving gear provided thereon, each of the output shafts has a shaft driven gear provided thereon, so that a plurality of gear pairs with different velocity ratios are formed by meshes of corresponding shaft driving gears and shaft driven gears. 
     In some embodiments, the transmission unit may be a six-speed transmission, i.e., the transmission unit may include a first-gear gear pair, a second-gear gear pair, a third-gear gear pair, a fourth-gear gear pair, a fifth-gear gear pair and a sixth-gear gear pair. There are no particular limits in the present disclosure, a person skilled in the art may increase or reduce the number of gear pairs accordingly based on transmission requirements, and the transmission unit may not be limited to the six-speed transmission as disclosed in the present embodiment. 
     In some embodiments, as shown in, for example,  FIGS. 1-6 , the motor power shaft  3  is configured to rotate together with one of the output shafts, such as the second output shaft  22 . In some embodiments, when power (such as power transmitted to an output shaft from the engine  4 ) needs to be transmitted to the motor power shaft  3 , the motor power shaft  3  can rotate together with the output shaft while receiving the power. In some embodiments, when power (such as power transmitted to the motor power shaft  3  from a first motor generator  51 ) needs to be transmitted to an output shaft, this output shaft can rotate together with the motor power shaft  3  while receiving power. 
     In some specification of the present disclosure, the expression “rotate together with” means that related components (such as two components) may rotate together. In an embodiment where one component rotates together with the other one component, when the one component rotates, the other one component rotates also. 
     In some embodiments, where a gear rotates together with a shaft, when the gear rotates, the relative rotates together; alternatively, when the shaft rotates, the relative gear rotates also. 
     In some embodiments, where one shaft rotates together with the other shaft, when one shaft rotates, the other shaft rotates also. 
     In some embodiments, where one gear rotates together with the other one gear, when the one gear rotates, the other one gear rotates also. 
     In the following description, the expression “rotate together with” may be understood as described above, unless specified or limited otherwise. 
     In some embodiments, the first motor generator  51  may be configured to rotate together with the motor power shaft  3 . For example, when functioning as a motor, the first motor generator  51  outputs the power to the motor power shaft  3 . In some embodiments, when functioning as a generator, power from the motor power shaft  3  may be transmitted to the first motor generator  51 , thereby driving the first motor generator  51  to generate electric power. 
     In the specification of the present disclosure, a motor generator (such as the first motor generator  51 ) may be understood as an apparatus which can function as a motor and a generator, unless specified or limited otherwise. 
     In some embodiments, the motor power shaft  3  may rotate together with one of the output shafts, such as the second output shaft  22 . In some embodiments, when the motor power shaft  3  is rotating together with the one of the output shafts, the first motor generator  51  may use at least a part of power output by the engine  4  so as to generate electric power when the vehicle is parking or running. 
     In some embodiments, when the vehicle is in a running state and the motor power shaft  3  is rotating together with one of the output shafts, a part of power output by the engine  4  may be transmitted to the first motor generator  51  via the motor power shaft  3  such that the first motor generator  51  is driven to generate electric power, thus accomplishing a condition of charging the vehicle battery while driving the vehicle. In some embodiments, when the vehicle is in a parking state (e.g., the vehicle stops running but the engine is still working) and the motor power shaft  3  is rotating together with one of the input shafts, a part of power output by the engine  4  may be transmitted to the first motor generator  51  via the motor power shaft  3  such that the first motor generator  51  is driven to generate electric power, thus accomplishing a condition of charging the vehicle while parking (such as charging the vehicle while the vehicle is not running). 
     In some embodiments, the motor power shaft  3  may be a motor shaft of the first motor generator  51 . In some embodiments, the motor power shaft  3  may be a shaft different from the motor shaft of the first motor generator  51 . 
     With the power transmission system  100  according to embodiments of the present disclosure, the number of charging modes of the vehicle can be increased. For example, the charging of a vehicle battery can take place either when the vehicle is running or when the vehicle is parked. Therefore, different charging modes can be provided, and charging efficiency can be improved. 
     The detailed configuration of the transmission unit  101  may be described in detail below with reference to detailed embodiments as shown in  FIGS. 1-6 . 
     In some embodiments, as shown in  FIGS. 1-6 , the output unit may rotate with one of the output shafts, such as the second output shaft  22 , at a different speed. In other words, the output unit  221  and the corresponding output shaft may rotate at different speeds independently. 
     In some embodiments, the output unit  221  may selectively engage one of the output shafts and rotate together with the output shaft. In other words, the output unit  221  may engage one of the output shafts and rotate together with the output shaft thereof. In some embodiments, the output unit  221  and one of the output shafts may rotate at different speeds. 
     In some embodiments, as shown in  FIGS. 1-6 , the output unit  221  may fit over one of the output shafts without particular limits in the present disclosure. In some embodiments, as shown in  FIGS. 1-5 , the output unit  221  may fit over the second output shaft  22 . In other words, the output unit  221  and the second output shaft  22  may rotate at different speeds. 
     In some embodiments, as mentioned above. Corresponding output unit synchronizers  221   c  may configure to synchronizing the output unit  221  with one of the output shafts. 
     In some embodiments, the output unit synchronizer  221   c  may dispose on one of the output shafts and engage the output unit  221 . In other words, as shown in  FIG. 1 , when the output unit synchronizer  221   c  is in a disengaged state, the output unit  221  and the second output shaft  22  may rotate at different speeds. When the output unit synchronizer  221   c  is in an engaged state, the output unit  221  may rotate together with the second output shaft  22 . 
     In some embodiments, the output unit  221  may be an output idler gear, and the output idler gear  221  may fit over one of the output shafts. The output idler gear  221  may mesh with the shift driven gear  74  of a main reducer. In the present embodiments, the output unit synchronizer  221   c  may be the output idler gear  221   c , and the output idler gear  221   c  may configure to synchronize the output idler gear  221  with one of the output shafts, such as the second output shaft  22 . 
     It should be noted that the output idler gear  221  as the output unit  221  and the output idler gear  221   c  as the output unit synchronizer  221   c  are being applied in specific cases may be schematic examples provided for better understanding the present disclosure, which may not be construed a limitation. 
     In some embodiments, the fixed output gear  211  may configure to fix on the other output shafts. In the present embodiments, the output shafts include a first output shaft  21  and a second output shaft  22 . The output unit  221  may fit over the second output shaft  22 , and the fixed output gear  211  may be fixed on the first output shaft  21 , which may not be construed as a limitation. 
     The motor power shaft  3  may rotate together with one of the output shafts according to embodiments of the present disclosure may be described below with reference to  FIGS. 1-6 . 
     In some embodiments of the present disclosure, the motor power shaft  3  may rotate together with one of the output shafts via a gear pair. The gear mechanism has simple structure and is convenient for using in power transmission. In addition, with the gear mechanism, a required transmission ration may be obtained and the power transmission may be reliable. The gear pair may include two meshed gears, a generator gear  73  and a motor power shaft gear  31 . 
     In some embodiments, the generator gear  73  may be fixed on one of the output shafts. In other words, the generator gear  73  is fixed on an output shaft. The output shaft and the output unit  221  may rotate at different speeds or rotate together with each other. In some embodiments, the generator gear  73  may be fixed on the second output shaft  22  without particular limits in the present disclosure. The motor power shaft gear  31  may be disposed on the motor power shaft  3 , and the motor power shaft gear  31  may configure to mesh with the generator gear  73 . In other words, power may transmit from the motor power shaft gear  31  to the generator gear  73 . 
     A reverse unit of the power transmission system  100  according to embodiments of the present disclosure may be described below in detail. 
     In some embodiments, the reverse unit includes a reverse output gear  72  and a reverse idler gear. The reverse output gear  72  may configure to rotate together or disengage from one of the shift driving gears, such as a shift driving gear  2   a . In some embodiments, the reverse output gear  72  may rotate together with the shift driving gear, the power generated by the engine  4  and/or the power generated by the first motor generator  51  may transmit to the reverse output gear  72 . In some embodiments, the reverse output gear  72  may disengage from the shift driving gear, and power may not transmit to the reverse output gear  72 . 
     In some embodiments, the reverse output gear  72  may selectively rotate together with the shift driving gear via reverse idler gears, such as a first reverse idler gear  711 , a second reverse idler gear  712  and a third reverse idler gear  713 . 
     In the present embodiments, the reverse idler gear may configure to rotate together with one of the shift driven gears and the reverse output gear may selectively rotate together with the reverse idler gear. In other words, in some embodiments, the reverse output gear  72  may rotate together with the reverse idler gear, the power generated by the engine  4  and/or the power generated by the first motor generator  51  may transmit to the reverse output gear  72 . In some embodiments, the reverse output gear  72  may disengage from the reverse idler gear, and power may not transmit to the reverse output gear  72 . 
     In some embodiments, the reverse output gear  72  may synchronize with the reverse idler gear via the reverse synchronizer  72   c . In the embodiments of the present disclosure, the reverse output gear  72  may configure to rotate together with the reverse idler gear via a synchronization of the reverse synchronizer  72 . In some embodiments, the reverse output gear  72  and the reverse idler gear may rotate at different speeds when the reverse synchronizer  72   c  is in a disengage state. 
     In some embodiments, the reverse synchronizer  72   c  and the output unit synchronizer  221   c  may share a shift fork mechanism. The reverse synchronizer  72   c  may synchronize the reverse output gear  72  with the reverse idler gear. At the same time, the output unit synchronizer  221   c  is in a disengaged state. The output unit synchronizer  221   c  may synchronize the output unit  21  with one of the output shafts. At the same time, the reverse synchronizer  72   c  is in a disengaged state. In some embodiments, as shown in  FIG. 1 , the engaging sleeve of the reverse synchronizer  72   c  may move to right to engage the third reverse idler gear  713 , and the output unit synchronizer  221   c  is in a disengaged state. In some embodiments, the engaging sleeve of the output unit synchronizer  221   c  may move to the left to engage the output unit  221 , and the reverse synchronizer  72   c  is in a disengaged state. 
     Therefore, both of the synchronization of the reverse synchronizer  72  and the output unit synchronizer  221   c  can be controlled by one shift fork mechanism. The number of the shift fork mechanisms can be saved, and the power transmission system  100  can have a more compact structure, a smaller axial and diametric size, and thus more convenient to arrange on vehicles. 
     In some embodiments, as shown in  FIGS. 1-5 , the reverse idler gear may include the first reverse idler gear  711 , the second reverse idler gear  712  and the third reverse idler gear  713 . Specially, the first reverse idler gear  711  may configure to mesh with one of the shift driving gears, such as the second-gear shift driving gear  2   a . The first reverse idler gear  711  may rotate together with the second reverse idler gear  712  in the same direction and the same velocity. The second reverse idler gear  712  may rotate together with the third reverse idle gear  713 , and the reverse synchronizer  72   c  may configure to selectively synchronize the reverse output gear  72  with the third reverse idler gear  713 . 
     In some embodiments, the reverse output gear  72  and the third reverse idler gear  713  may be arranged coaxially. The reverse synchronizer  72   c  may be disposed on the reverse output gear  72  to engage with the third reverse idler gear  713  or the reverse synchronizer  72   c  may be disposed on the third reverse idler gear  713  to engage with the reverse output gear  72 . In some embodiments of the present disclosure, as shown in  FIGS. 1-5 , both of the reverse output gear  72  and the third reverse idler gear  713  are fitted over the motor power shaft  3 , such that the reverse shaft can be saved and the transmission unit  101  can have a more compact structure. In some embodiments, the reverse synchronizer  72   c  may be disposed on the reverse output gear  72  to engage the third reverse idler gear  713 , which may not be construed as a limitation. 
     In some embodiments, as shown in  FIGS. 1-6 , the first reverse idler gear  711  and the second reverse idler gear  712  may form an integrated structure so as to be a joint gear structure, such that the axial size of the first reverse idler gear  711  and the second idler gear  712  may be reduced and arranged on vehicles more conveniently. 
     The input shaft(s), the output shaft(s), the shift driving gears and the shift driven gears of the power transmission system  100  will be described below with reference to embodiments shown in  FIGS. 1-6 . 
     In some embodiments, as shown in  FIGS. 1-5 , two input shafts are provided. In the present embodiment, the plurality of input shafts includes a first input shaft  11  and a second input shaft  12 . The second input shaft  12  may be hollow and the first input shaft  11  may be solid. One part of the first input shaft  11  may be inserted within the second input shaft  12 , and the other part of the first input shaft  11  may extend out of the second input shaft  12  along an axial direction of the second input shaft  12 . The first input shaft  11  and the second input shaft  12  may be arranged coaxially. 
     In some embodiments, two output shafts are provided. In the present embodiment, the plurality of output shafts may include a first output shaft  21  and a second output shaft  22 . The first output shaft  21  and the second output shaft  22  may be arranged coaxially with the input shafts (such as the first input shaft  11  and the second input shaft  12 ). Both the first output shaft  21  and the second output shaft  22  may be solid. 
     In some embodiments, the power transmission system  100  according to embodiments of the present disclosure may have six gear transmission types. Specifically, odd number-gear shift driving gears may be arranged on the first input shaft  11 , while even number-gear shift driving gear may be arranged on the second input shaft  12 . The first input shaft  11  may transmit power from gear pairs of odd-numbered gear, and the second input shaft  12  may transmit power from gear pairs of even-numbered gear. 
     In some embodiments, as shown in  FIGS. 1-5 , a first-gear shift driving gear  1   a , a third-gear shift driving gear  3   a  and a fifth-gear shift driving gear  5   a  may be arranged on the first input shaft  11 , and a second-gear shift driving gear  2   a  and a fourth-sixth-gear shift driving gear  46   a  may be arranged on the second input shaft  12 . Each of the first-gear to fourth-sixth-gear shift driving gears  1   a ,  2   a ,  3   a ,  46   a , and  5   a  may rotate together with a corresponding input shaft. 
     In some embodiments, a first-gear shift driven gear  1   b , a second-gear shift driven gear  2   b , a third-gear shift driven gear  3   b  and a fourth-gear shift driven gear  4   b  may be disposed on the first output shaft  21 , and a fifth-gear shift driven gear  5   b  and a sixth-gear shift driven  6   b  may be disposed on the second output shaft  22 . Each of the shift driven gears  1   b ,  2   b ,  3   b ,  4   b ,  5   b  and  6   b  may be fitted over a corresponding output shaft. Each of the shift driven gears and the corresponding output shafts thereof may rotate at different speeds. 
     In some embodiments, the first-gear shift driving gear  1   a  may mesh with the first-gear shift driven gear  1   b  to form one gear pair, the second-gear shift driving gear  2   a  may mesh with the second-gear shift driven gear  2   b  to form one gear pair, the third-gear shift driving gear  3   a  may mesh with the second-gear shift driven gear  3   b  to form one gear pair, the fourth-sixth-gear shift driving gear  46   a  may mesh with the fourth-gear shift driven gear  4   b  to form one gear pair, the fifth-gear shift driving gear  5   a  may mesh with the fifth-gear shift driven gear  5   b  to form one gear pair, and the fourth-and-sixth-gear shift driving gear  46   a  may mesh with the fifth-gear shift driven gear  6   b  to form one gear pair and six pairs of gear pairs can be formed. 
     In the present embodiment, the fourth-gear gear pair and the sixth-gear gear pair share the fourth-sixth shift driving gear  46   a , so that the number of shift driving gears can be reduced to make the power transmission system  100  have a more compact structure. 
     As the shift driven gear is fitted over the corresponding output shaft, a synchronizer is provided to synchronize the shift driven gear and the corresponding output shaft, thus achieving the object of power transmission. 
     In some embodiments, as shown in  FIGS. 1-5 , the power transmission system  100  includes a first-third gear synchronizer  13   c , a second-fourth gear synchronizer  24   c , and a fifth-sixth gear synchronizer  56   c.    
     In some embodiments, as shown in  FIG. 1 , the first-third gear synchronizer  13   c  is disposed on the first output shaft  21  and between the first-gear shift driven gear  1   b  and the third-gear shift driven gear  3   b . The first-third gear synchronizer  13   c  may engage the first output shaft  21  with the first-gear shift driven gear  1   b  or the third-gear shift driven gear  3   b , such that the shift driven gear may rotate together with the corresponding output shaft, e.g., the first-gear shift driven gear  1   b  and may rotate together with the first output shaft  21 , and the third-gear shift driven gear  3   b  and may rotate together with the first output shaft  21 . 
     In some embodiments, as shown in  FIG. 1 , the first-third gear synchronizer  13   c  includes an engaging sleeve. In some embodiments, the engaging sleeve of the first-third gear synchronizer  13   c  may move to the left so as to engage the third-gear shift driven gear  3   b  with the first output shaft  21 , such that the third-gear shift driven gear  3   b  may rotate together with the first output shaft  21 . In some embodiments, the engaging sleeve of the first-third gear synchronizer  13   c  may move to the right so as to engage first-gear shift driven gear  1   b  with the first output shaft  21 , such that the first-gear shift driven gear  1   b  may rotate together with the first output shaft  21 . 
     In some embodiments, as shown in  FIG. 1 , the second-fourth gear synchronizer  24   c  is disposed on the first output shaft  21  and between the second-gear shift driven gear  2   b  and the fourth-gear shift driven gear  4   b . The second-fourth gear synchronizer  24   c  may engage the second-gear shift driven gear  2   b  with the first output shaft  21  or engage the fourth-gear shift driven gear  4   b  with the first output shaft  21 , such that the shift driven gear may rotate together with the corresponding output shaft, e.g. the second-gear shift driven gear  2   b  may rotate together with the first output shaft  21 , and the fourth-gear shift driven gear  4   b  may rotate together with the first output shaft  21 . 
     In some embodiments, as shown in  FIG. 1 , the second-fourth gear synchronizer  24   c  includes an engaging sleeve. In some embodiments, the engaging sleeve of the second-fourth gear synchronizer  24   c  may move to the left so as to engage the second-gear shift driven gear  2   b  with the first output shaft  21 , such that the second-gear shift driven gear  2   b  may rotate together with the first output shaft  21 . In some embodiments, the engaging sleeve of the second-fourth gear synchronizer  24   c  may move to the right so as to engage fourth-gear shift driven gear  4   b  with the first output shaft  21 , such that the fourth-gear shift driven gear  4   b  may rotate together with the first output shaft  21 . 
     In some embodiments, as shown in  FIG. 1 , the fifth-sixth gear synchronizer  56   c  is disposed on the second output shaft  22  and between the fifth-gear shift driven gear  5   b  and the sixth-gear shift driven gear  6   b . The fifth-sixth gear synchronizer  56   c  may engage the fifth-gear shift driven gear  5   b  with the second output shaft  22  or engage the sixth-gear shift driven gear  6   b  with the second output shaft  22 . The fifth-sixth gear synchronizer  56   c  includes an engaging sleeve. In some embodiments, the engaging sleeve of the fifth-sixth gear synchronizer  56   c  may move to the left so as to engage the sixth-gear shift driven gear  6   b  with the second output shaft  22 , such that the sixth-gear shift driven gear  6   b  may rotate together with the second output shaft  22 . In some embodiments, the engaging sleeve of the fifth-sixth gear synchronizer  56   c  may move to the right so as to engage fifth-gear shift driven gear  5   b  with the second output shaft  22 , such that the fifth-gear shift driven gear  5   b  may rotate together with the second output shaft  22 . 
     In some embodiments of the present disclosure, the engine  4  may transmit power to, or disengage from, the first input shaft  11  and the second input shaft  12  via a dual clutch  2   d.    
     In some embodiments of the present disclosure, as shown in  FIGS. 1-5 , the dual clutch  2   d  includes an input terminal  23   d , a first output terminal  21   d  and a second output terminal  22   d . The engine  4  is connected with the input terminal  23   d  of the dual clutch  2   d . In some embodiments, the engine  4  is connected with the input terminal  23   d  by at least one selected from a group consisting of a flywheel, a damper, a torsional disk, etc. 
     In some embodiments, the first output terminal  21   d  is connected with the first input shaft  11 , such that the first output terminal  21   d  may rotate together with the first input shaft  11 . In some embodiments, the second output terminal  22   d  is connected with the second input shaft  12 , such that the second output terminal  22   d  may rotate together with the second input shaft  12 . 
     In some embodiments, the input terminal  23   d  may include a shell of the dual clutch  2   d , and each of the first output terminal  21   d  and the second output terminal  22   d  may include one driven disk of the dual clutch  2   d . In some embodiments, the shell is disengaged from the driven disk, i.e., the input terminal  23   d  is disengaged from the first output terminal  21   d  and is disengaged from the second output terminal  22   d . When the shell is to be engaged with one driven disk, the shell can be controlled to engage with a corresponding driven disk, thus the shell and this driven disk may rotate together. In the present embodiment, the input terminal  23   d  may engage with one of the first output terminal  21   d  and the second output terminal  22   d  to transmit power from the input terminal  23   d  to one of the first output terminal  21   d  and the second output terminal  22   d , to output the transmitted power. 
     In some embodiments, the shell may be engaged with two driven disks simultaneously. In the present embodiment, the input terminal  23   d  is engaged with both the first output terminal  21   d  and the second output terminal  22   d , and thereby power from the input terminal  23   d  may be transmitted to the first output terminal  21   d  and the second output terminal  22   d  so as to be output. 
     A person with ordinary skill in the art will appreciate that the engaging state of the dual clutch  2   d  may be controlled according to practical condition, and that the engaging state may also be adjusted accordingly based on a current transmission mode. In some embodiments, the input terminal  23   d  may disengage from the two output terminals including, for example, the first output terminal  21   d  and the second output terminal  22   d . In some embodiments, the input terminal  23   d  may engage with at least one of the two output terminals including, for example, the first output terminal  21   d  and the second output terminal  22   d.    
     In some embodiments, the power transmission system  100  further includes three power output shafts, i.e., a first output shaft  21 , a second output shaft  22 , and a motor power shaft  3 . These power output shafts, a differential  75 , and relationships therebetween may be described below in detail with reference to  FIGS. 1-5 . 
     In some embodiments, the differential  75  may be disposed between a pair of front wheels  76  of the vehicle. In some embodiments, the differential  75  may be disposed between a pair of rear wheels  77  of the vehicle. The differential  75  may drive the wheels to the left or to the right when the vehicle is turning or running on a rough road, such that the wheels may roll with different angular speeds, and therefore driving wheels at both sides of the vehicle may perform only rolling on the ground. In some embodiments, a shift driven gear  74  of a main reducer may be disposed on the differential  75 , for example, the shift driven gear  74  may be disposed on a shell of the differential  75 . In some embodiments, the shift driven gear  74  may be a bevel gear, which may not be construed a limitation. 
     In some embodiments, as mentioned above, the fixed output gear  211  and the output unit  221 , i.e. the output idler gear  221 , may output the power transmitted to the output shafts, such that both of the fixed output gear  211  and the output unit may mesh with the shift driven gear of a main reducer. 
     In some embodiments, as the output reverse output gear  72  may output the reverse power, the reverse output gear  72  may mesh with the shift driven gear  74 . 
     The power transmission system  100  according to embodiments of the present disclosure may be used in various different conditions, such as a parking-charging condition (for example, charging the vehicle while the vehicle is parking), a running-charging condition (for example, charging the vehicle while the vehicle is running and both clutch parts of dual clutch  2   d  are engaged), and the reverse mode. 
     In the parking-charging condition, the engine  4  is configured to generate power and output the power to the first motor generator  51  via the generator gear  73  and the motor power shaft gear  31 , thereby driving the first motor generator  51  to generate electric power. 
     In some embodiments, as shown in  FIGS. 1-5 , in the parking-charging state, the engine  4  generates power and transmits the power to the second output shaft  22  via the first input shaft  11 , one component of the fifth-gear gear pair and the second input shaft  12  therebetween and the six-gear gear pair sequentially. The first motor generator  51  may be driven to generate electric power as a generator by the power generated by the engine  4  transmitted by the generator gear  73 , the motor power shaft gear  31  and the motor power shaft  3  sequentially. 
     Therefore, charging the vehicle when the vehicle is parking may be achieved, and the number of charging modes is increased. In the parking-charging mode, the vehicle is not running, all power from the engine  4  may be used to charge the vehicle, thus providing a fast charging performance and enhancing the charging efficiency. 
     In the running-charging condition, the input terminal  23   d  is engaged with the first output terminal  21   d  and engaged with the second output terminal  22   d  simultaneously, a part of power generated by the engine  4  may be output to one of the output shafts to drive the wheels of the vehicle, and the other part of power may be transmitted to the first motor generator  51  via the generator gear  73  and the motor power shaft gear  31 , thus driving the first motor generator  51  to generate electric power. 
     In the running-charging condition, as shown in  FIGS. 1-5 , a part of power generated by the engine  4  may be transmitted to the first motor generator  51  via the first input shaft  11 , the fifth-gear gear pair, the generator gear  73  and the motor power shaft gear  31  sequentially, thus driving the first motor generator  51  to generate electric power. The other part of the power generated by the engine  4  may be output via the second input shaft  12  and one component of the second-gear gear pair and the fourth-gear gear pair therebetween. 
     In the running-charging condition, as shown in  FIG. 1 , a part of power generated by the engine  4  may be transmitted to the first motor generator  51  via the second input shaft  12 , the sixth-gear gear pair, the generator gear  73  and the motor power shaft gear  31  sequentially, thus driving the first motor generator  51  to generate electric power. The other part of the power generated by the engine  4  may be output via the first input shaft  11  and one component of the first-gear gear pair and the third-gear gear pair therebetween. 
     It is known to those skilled in the art that a conventional dual clutch generally has two gear parts, and only one gear part is used when the dual clutch is working. In the power transmission system  100  according to embodiments of the present disclosure, however, two gears parts of the dual clutch  2   d  may be both engaged (for example, the input terminal  23   d  is engaged with the first output terminal  21   d  and engaged with the second output terminal  22   d  simultaneously) when the dual clutch  2   d  is working. In the present embodiment, a part of power from the engine  4  may be output to wheels of the vehicle via one output shaft to drive the vehicle to run, and the other part of power from the engine  4  may be transmitted to the first motor generator  51  to drive the first motor generator  51  to generate electric power. In this way, transmission modes of the vehicle are increased, and charging the vehicle while the vehicle is running may be achieved. 
     In the power transmission system  100  according to embodiments of the present disclosure, a mechanical reverse mode, an electric reverse mode and a hybrid (both mechanic and electric) reverse mode may be achieved. 
     In the mechanical reverse mode, the reverse of the vehicle is accomplished with power from the engine  4 . Specifically, the engine  4  generates power and transmits the power to the reverse idler gear, and then transmits to the reverse output gear  72  via synchronization of reverse synchronizer  72   c  (synchronizing the reverse idler gear). 
     In the mechanical reverse mode, as shown in  FIG. 1 , power generated by the engine  4  may transmit to the third reverse idler gear  713  via the second input shaft  12 , the first reverse idler gear  711  and the second reverse idler gear  712 . The engaging sleeve of the reverse synchronizer  72   c  may move to the right to engage with the third reverse idler gear  713 , thus transmitting the power generated by the engine  4  to the reverse output gear  72  via the reverse idler gear. 
     In the mechanical reverse mode, as shown in  FIG. 1 , the reverse synchronizer  72   c  may engage with the third reverse idler gear  713 . 
     In the electric reverse mode, the reverse of the vehicle can be enabled with power from the first motor generator  51 . Specifically, the first motor generator  51  may generate power and transmit the power to an output shaft via the generator gear  73 , so as to be output. The generator gear  73  may dispose on the output shaft. In some embodiments, as shown in  FIGS. 1-5 , the output unit synchronizer  221   c  may engage with the output unit  221 . Power generated by the first motor generator  51  may transmit to the output unit  221  via the motor power shaft gear  31 , the generator gear  73  and the second output shaft  22 , so as to be output. Only the output unit synchronizer  221   c  is in an engaged state in this transmission passage. 
     In some embodiments, the first motor generator  51  may generate power and transmit the power to the reverse output gear  72  via the reverse idler gear and a synchronization of the reverse synchronizer  72   c . In some embodiments, as shown in  FIGS. 1-5 , the output unit synchronizer  221   c  may disengage from the output unit  221 , and the fifth-sixth gear synchronizer  56   c  may engage with the sixth-gear shift driven gear  6   b , at the same time, the reverse synchronizer  72   c  may engage with the third reverse idler gear  713 , thus transmitting power generated by the first motor generator  51  to reverse output gear  72  via the motor power shaft gear  31 , the generator gear  73 , the sixth-gear gear pair, the second input shaft  12  and the reverse idler gear, so as to be output. The reverse synchronizer  72   c  is in an engaged state and the fifth-sixth gear synchronizer  56   c  may engage with the sixth-gear shift driven gear  6   b  in this transmission passage. 
     In the hybrid reverse mode, the reverse of the vehicle may be achieved with the engine  4  and the first motor generator  51 . The hybrid reverse mode may be a combination of the above mechanical reverse mode and the electric reverse mode. 
     In the hybrid reverse mode, the engine  4  may generate first power and transmit the first power to the reverse idler gear, and then the first power may be transmitted to the reverse output gear  72  via a synchronization of the reverse synchronizer  72   c  (synchronizing the reverse idler gear), so as to be output. 
     In addition, the first motor generator  51  may generate second power and transmit the second power to the reverse idler gear via the generator gear  73 , and then the second power may be transmitted to reverse output gear  72  via a synchronization of the reverse synchronizer  72   c . The reverse synchronizer  72   c  is in an engaged state and the fifth-sixth gear synchronizer  56   c  may engage with the sixth-gear shift driven gear  6   b  in this transmission passage. 
     In some embodiments as shown in, for example,  FIG. 1 , when the power transmission system  100  in the hybrid reverse mode, combines the above mechanical reverse mode and the electric reverse mode. The engine  4  may transmit the first power to the second input shaft  12  as the above mechanical reverse mode described. The first motor generator  51  may transmit the second power to the second output shaft  12  as the above electric reverse mode described. The first power and the second power may be coupled together before being output to the wheels. In some embodiments, the first power and the second power may be coupled at the second input shaft  12 , and the coupled power may be transmitted to the wheels via the reverse idler gear and the reverse output gear  72  so as to reverse the vehicle. 
     In the hybrid reverse mode, the first motor generator  51  may adjust the speed, such that the second input shaft  12  may synchronously receive the first power from the engine  4  and the second power from the first motor generator  51 , to provide a smooth and harmonious power transmission. 
     As described, with the power transmission system  100  according to embodiments of the present disclosure, three reverse modes including the mechanical reverse mode, the electric reverse mode and the hybrid reverse mode may be achieved, thus increasing the reverse modes and facilitating a user to shift within the three reverse modes according to a practical condition, and therefore different driving requirements may be satisfied. 
     When the vehicle has sufficient electric power, the electric reverse mode may be used. In the electric reverse mode, harmful exhaust gases can be minimized, and the energy consumption can be reduced. It is known to those skilled in the art that an unskilled driver will take longer time and more maneuvers to park the vehicle at a predetermined position. Considering that the engine  4  may generate more harmful gases during a low-speed reverse process and that the engine  4  has relatively higher fuel consumption, because the engine is at an uneconomical rotating speed during the reverse process, the electric reverse mode of the present disclosure is highly effective in reducing fuel consumption during such a low-speed reverse process. In addition, with the generator being used as a power source, harmful exhaust gases can be minimized, and the energy consumption in a low-speed reverse process can also be decreased. Therefore, the fuel economy of the engine  4  may be enhanced. 
     When the vehicle has insufficient or relatively less electric power, the mechanical reverse mode may be used. In a case that the vehicle needs to be reversed quickly or that the vehicle needs to be reversed with a larger power, the hybrid reverse mode may be used, thus enhancing the power of the vehicle and providing better driving experience to the user. 
     It should be noted that the above three reverse modes being applied in specific cases may be schematic examples provided for better understanding the present disclosure, which may not be construed that the described reverse mode should be applied when the vehicle is in the corresponding case. It is well known to those skilled in the art that, in a specific condition, a corresponding reverse mode may be selected according to specific requirements and a practical condition. 
     With the power transmission system  100  according to embodiments of the present disclosure, a number of the reverse modes of the vehicle are increased, which provide a driver more options to reverse the vehicle. In this way, the driver may be provided with more driving fun and reverse of the vehicle in different road conditions may be satisfied. 
     In some embodiments, the power transmission system  100  further includes a second motor generator  52 . With the second motor generator  52 , the power of the power transmission system  100  may be improved, and more transmission modes can be provided. 
     In some embodiments, the second motor generator  52  may perform power transmission with the shift driven gear  74  of the main reducer. For example, a gear may be disposed on a motor shaft of the second motor generator  52 , and the gear is configured to directly mesh with the shift driven gear  74  so as to perform power transmission. In some embodiments, the second motor generator  52  is configured to connect with the first input shaft  11  or the first output shaft  21 . In some embodiments, the second motor generator  52  may be integral with the differential  75 . In some embodiments, the engine  4  and the first motor generator  51  are configured to drive front wheels of the vehicle, and the second motor generator  52  may be a wheel-side motor and configured to drive rear wheels. In some embodiments, the second motor generator  52  may drive the pair of rear wheels via a reducing mechanism. In some embodiments, two second motor generators  52  are provided, and each second motor generator  52  is configured to drive one rear wheel via a reducing mechanism. 
     In some embodiments, as shown in  FIGS. 2-5 , the power transmission system  100  may include an electric differential lock unit. The electric differential lock unit may lock a pair of driving wheels when the vehicle is skidding, thus enhancing the antiskid performance and the pass performance of the vehicle. 
     In some embodiments, as shown in  FIGS. 2-5 , the electric differential lock unit may include a third motor generator  201 , a fourth motor generator  301  and an antiskid synchronizer  503 . The engine  4  and/or the first motor generator  51  is configured to drive a first pair of wheels  76 , the third motor generator  201  and the fourth motor generator  301  are configured to driven a second pair of wheels  77 , the first pair of wheels  76  are one pair of the pair of front wheels and the pair of the rear wheels, and the second pair of wheels  77  are the other one pair of the pair of front wheels and the pair of the rear wheels. In some embodiments, as shown in  FIGS. 2-5 , the engine and the first motor generator  51  may drive the pair of front wheels, and the third motor generator  201  and the fourth motor generator  301  may drive the pair of rear wheels. 
     In some embodiments, as shown in Figs. 2 - 5 , the third motor generator  201  is configured to rotate together with one of the second pair of wheels  77 . In the present embodiment, the third motor generator  201  may output power to this one wheel so as to drive this one wheel to rotate. In some embodiments, power from this one wheel may be transmitted to the third motor generator  201 , thus driving the third motor generator  201  to generate electric power. 
     In some embodiments, the fourth motor generator  301  is configured to rotate together with the other one of the second pair of wheels  77 . In the present embodiment, the fourth motor generator  301  may output power to the other one wheel so as to drive the other wheel to rotate. In some embodiments, power from the other wheel may be transmitted to the fourth motor generator  301 , thus driving the fourth motor generator  301  to generate electric power. In some embodiments, as shown in  FIG. 2-5 , the third motor generator  201  is configured to rotate together with a left rear wheel of the vehicle, and the fourth motor generator  301  is configured to rotate together with a right rear wheel of the vehicle. This embodiment is provided as an example, and the present disclosure should not be construed to be limited by this embodiment. 
     In some embodiments, the antiskid synchronizer  503  is configured to selectively synchronize the second pair of wheels  77 , such that the second pair of wheels  77  may rotate together. In the present embodiment, the antiskid synchronizer  503  may synchronize the second pair of wheels  77 , i.e., the antiskid synchronizer  503  is in an engaged state, such that the second pair of wheels  77  may form a fixed engagement. In this way, the second pair of wheels  77  may rotate together, without rotating at different rotating speeds. 
     In some embodiments, when the antiskid synchronizer  503  is in a disengaged state, and the third motor generator  201  and the fourth motor generator  301  may drive corresponding wheels respectively, such that the corresponding wheels may rotate at different rotating speeds, thus the object that different wheels rotates at different speeds may be achieved. In some embodiments, when the antiskid synchronizer  503  is in a disengaged state, the third motor generator  201  and the fourth motor generator  301  may drive the second pair of wheels  77  to rotate at a same rotating speed. 
     With the power transmission system  100  according to embodiment of the present disclosure, the third motor generator  201  and the fourth motor generator  301  are provided and configured to drive the second pair of wheels  77  respectively, and therefore the second pair of wheels  77  rotating at different rotating speeds may be achieved. When one of the second pair of wheels  77  is skidding, the antiskid synchronizer  503  may synchronize the second pair of wheels  77  such that the second pair of wheels  77  rotate together. In this way, powers output by two motors (for example, the third motor generator  201  and the fourth motor generator  301 ) or one motor (for example, the third motor generator  201  or the fourth motor generator  301 ) may be coupled to drive the second pair of wheels  77  together, thus enhancing the antiskid capability and passing performance of the vehicle. 
     The power transmission system  100 , according to embodiments of the present disclosure, includes the antiskid synchronizer  503 , and therefore a mechanical self-locking differential mechanism commonly used in an axle (such as a rear axle) a conventional power transmission system may be avoided. In addition to the functions of the antiskid synchronizer  503  itself, the function of a mechanical self-locking differential mechanism is performed by the antiskid synchronizer  503 , and therefore the power transmission system  100  according to embodiments of the present disclosure may have a more compact structure and relatively lower cost. 
     The third motor generator  201 , the fourth motor generator  301 , and transmission method thereof will be described below in detail with references to  FIGS. 2-5 . 
     In some embodiments, as shown in  FIGS. 2-4 , the third motor generator  201  may perform power transmission with the corresponding wheel via a gear mechanism. In some embodiments, the fourth motor generator  301  may perform power transmission with the corresponding wheel via a gear mechanism. 
     The gear mechanism has a simple structure and is convenient for use in power transmission. In addition, with the gear mechanism, a required transmission ratio may be obtained and the power transmission may be reliable. In some embodiments, the third motor generator  201  and the fourth motor generator  301  may perform power transmission with corresponding wheel(s) via a same gear mechanism. In the present embodiment, the gear mechanism is common, and the power transmission system  100  may be highly symmetric, thus avoiding the center of gravity moving to one side. With one common gear mechanism, the center of gravity may be located right in the middle or substantially the middle of the two wheels, and both the stability and reliability of the power transmission system  100  may be improved. 
     In some embodiments, as shown in  FIGS. 3-5 , the gear mechanism between the third motor generator  201  and the corresponding wheel may include a first gear  401 , a second gear  402 , a third gear  403 , and a fourth gear  404 . 
     In some embodiments, the first gear  401  may be disposed on the first output shaft  202  corresponding to the third motor generator  201 , and the first gear  401  is configured to rotate together with the first output shaft  202 . In some embodiments, the first output shaft  202  may output power generated by the third motor generator  201 . In some embodiments, the first output shaft  202  may transmit power generated by the corresponding wheel to the third motor generator  201 . In some embodiments, the first output shaft  202  and the third motor generator  201  may share a same motor shaft. In some embodiments, the motor shaft of the first output shaft  202  and the motor shaft of the third motor generator  201  may be two individual parts different from each other. In the present embodiment, the motor shaft of the first output shaft  202  and the motor shaft of the third motor generator  201  may be connected to each other. 
     In some embodiments, a first drive shaft  204  is connected with a wheel corresponding to the third motor generator  201 , and the second gear  402  is disposed on the first drive shaft  204  and configured to rotate together with the first drive shaft  204 . The third gear  403  and the first gear  401  are configured to mesh with each other, and the fourth gear  404  and the second gear  402  are configured to mesh with each other. The third gear  403  and the fourth gear  404  are coaxially arranged and may rotate together. 
     In some embodiments, as shown in  FIGS. 2-4 , the gear mechanism between the fourth motor generator  301  and the corresponding wheel may include a fifth gear  405 , a sixth gear  406 , a seventh gear  407 , and an eighth gear  408 . The fifth gear  405  may be disposed on the second output shaft  302  corresponding to the fourth motor generator  301 , and the fifth gear  405  is configured to rotate together with the second output shaft  302 . In some embodiments, the second output shaft  302  may output power generated by the fourth motor generator  301 . In some embodiments, the second output shaft  302  may transmit power generated by the corresponding wheel to the fourth motor generator  301 . In some embodiments, the second output shaft  302  and the fourth motor generator  301  may share one motor shaft. In some embodiments, the motor shaft of the second output shaft  302  and the motor shaft of the fourth motor generator  301  may be two individual parts different from each other. In the present embodiment, the motor shaft of the second output shaft  302  and the motor shaft of the fourth motor generator  301  may be connected to each other. 
     In some embodiments, a second drive shaft  304  is connected with a wheel corresponding to the fourth motor generator  301 , and the sixth gear  406  is disposed on the second drive shaft  304  and configured to rotate together with the second drive shaft  304 . The seventh gear  407  and the fifth gear  405  are configured to mesh with each other, and the eighth gear  408  and the sixth gear  406  are configured to mesh with each other. The seventh gear  407  and the eighth gear  408  are coaxially arranged and may rotate together. 
     In some embodiments, the first gear  401  and the fifth gear  405  may have a same structure, such as having the same size and a same teeth number. In some embodiments, the second gear  402  and the sixth gear  406  may have a same structure, such as having the same size and the same teeth number. In some embodiments, the third gear  403  and the seventh gear  407  may have a same structure, such as having the same size and the same teeth number. In some embodiments, the fourth gear  404  and the eighth gear  408  may have a same structure, such as having the same size and the same teeth number. Therefore, versatility of the gear mechanism may be improved. 
     In some embodiments, the third gear  403  and the fourth gear  404  may be fixed on the first gear shaft  501 , and the seventh gear  407  and the eighth gear  408  may be fixed on the second gear shaft  502 . In some embodiments, the third gear  403  and the fourth gear  404  may form a substantial ladder shape or a joint gear structure. In some embodiments, the seventh gear  407  and the eighth gear  408  may form a substantial ladder shape or a joint gear structure. 
     In some embodiments, as shown in  FIG. 2 , the antiskid synchronizer  503  may be disposed on the first drive shaft  204  and configured to selectively engage with the sixth gear  406 . In some embodiments, a gear ring may be provided on a side of the sixth gear  406  facing the antiskid synchronizer  503 , and the antiskid synchronizer  503  may include an engaging sleeve to adapt to the gear ring. With the engagement of the antiskid synchronizer  503 , the second pair of wheels  77  may rotate together. 
     In some embodiments, as shown in  FIG. 3 , the antiskid synchronizer  503  may be disposed on the first output shaft  202  and configured to selectively engage with the fifth gear  405 . In some embodiments, a gear ring may be provided on a side of the fifth gear  405  facing the antiskid synchronizer  503 , and the antiskid synchronizer  503  may include an engaging sleeve to adapt to the gear ring. With the engagement of the antiskid synchronizer  503 , the second pair of wheels  77  may rotate together. 
     In some embodiments, as shown in  FIG. 4 , the antiskid synchronizer  503  may be disposed on the first gear shaft  501  and configured to selectively engage with the seventh gear  407 . In some embodiments, a gear ring may be provided on a side of the seventh gear  407  facing the antiskid synchronizer  503 , and the antiskid synchronizer  503  may include an engaging sleeve to adapt to the gear ring. With the engagement of the antiskid synchronizer  503 , the second pair of wheels  77  may rotate together. 
     In some embodiments, as shown in  FIG. 5 , the third motor generator  201  may be connected coaxially with a corresponding wheel, and the fourth motor generator  301  may be connected coaxially with a corresponding wheel. In some embodiments, both the third motor generator  201  and the fourth motor generator  301  may be wheel-side motors, thus shortening the transmission passage, reducing the power transmission loss and enhancing the transmission efficiency. 
     In some embodiments, as shown in  FIGS. 5 , the antiskid synchronizer  503  may be disposed on the first output shaft  202  corresponding to the third motor generator  201 , and configured to selectively engage with the second output shaft  302  corresponding to the fourth motor generator  301 . With the engagement of the antiskid synchronizer  503 , the second pair of wheels  77  may rotate together. 
     The power transmission system  100  and the condition the power transmission system  100  may be used will be described below with reference to  FIGS. 1-5 . 
     Embodiment 1 
     As shown in  FIG. 1 , the engine  4  is connected with the input terminal  23   d  of the dual clutch  2   d , the first output terminal  21   d  of the dual clutch  2   d  is connected with the first input shaft  11 , and the second output terminal  22   d  of the dual clutch  2   d  is connected with the second input shaft  12 . The input terminal  23   d  may be disengaged from both the first output terminal  21   d  and the second output terminal  22   d , or the input terminal  23   d  may be engaged with one of the first output terminal  21   d  and the second output terminal  22   d , or the input terminal  23   d  may be engaged with both the first output terminal  21   d  and the second output terminal  22   d.    
     The second input shaft  12  may be a hollow shaft, and the first input shaft  11  may be a solid shaft. The second input shaft  12  is coaxially fitted over the first input shaft  11 , and a part of the first input shaft  11  extends outside of the second input shaft  12  along an axial direction of the second input shaft  12 . 
     The first-gear shift driving gear  1   a , the third-gear shift driving gear  3   a  and the fifth-gear shift driving gear  5   a  are disposed on the first input shaft  11  and configured to rotate together with the first input shaft  11 . The first-gear shift driving gear  1   a  is positioned in the right of the fifth-gear shift driving gear  5   a , and the third-gear shift driving gear  3   a  is positioned in the left of the fifth-gear shift driving gear  5   a.    
     The second-gear shift driving gear  2   a  and the fourth-sixth-gear shift driving gear  46   a  are disposed on the second input shaft  12  and configured to rotate together with the second input shaft  12 . 
     The first output shaft  21  is arranged parallel to the two input shafts, i.e., the first and second input shafts  11 ,  12 . The first-gear shift driven gear  1   b , the second-gear shift driven gear  2   b , the third-gear shift driven gear  3   b  and the fourth-gear shift driven gear  4   b  are fitted over the first output shaft  21 . The first-gear shift driven gear  1   b  is configured to mesh directly with the first-gear shift driving gear  1   a , the second-gear shift driving gear  2   a  is configured to mesh directly with the second-gear shift driven gear  2   b , the third-gear shift driving gear  3   a  is configured to mesh directly with the third-gear shift driven gear  3   b , and the fourth-sixth-gear shift driving gear  46   a  is configured to mesh directly with the fourth-gear shift driven gear  4   b.    
     The first-third gear synchronizer  13   c , the second-fourth gear synchronizer  24   c  are disposed on the first output shaft  21 , and the first-third gear synchronizer  13   c  is positioned between the first-gear shift driven gear  1   b  and the third-gear shift driven gear  3   b  and configured to selectively synchronize the first output shaft  21  with the first-gear shift driven gear  1   b  or the third-gear shift driven gear  3   b . The second-fourth gear synchronizer  24   c  is positioned between the second-gear shift driven gear  2   b  and the fourth-gear shift driven gear  4   b  and configured to selectively synchronize the first output shaft  21  with the second-gear shift driven gear  2   b  or the fourth-gear shift driven gear  4   b.    
     The second output shaft  22  is arranged parallel to the two input shafts, i.e., the first and second input shafts  11 ,  12 . The fifth-gear shift driven gear  5   b  and the sixth-gear  6   b  are fitted over the second output shaft  22 . The fifth-gear shift driven gear  5   b  may mesh with the fifth-gear shift driving gear  5   a  directly. The sixth-gear shift driven gear  6   b  may mesh with the fourth-sixth-gear shift driving gear  46   a  directly. The fifth-sixth gear synchronizer  56   c  is disposed on the second output shaft  22  and is configured to synchronize the second output gear with the fifth-gear shift driven gear  5   b  or the sixth-gear shift driven gear  6   b.    
     The fixed output gear  211  is fixed on the first output shaft  21  and configured to mesh with the shift driven gear  74 . The output unit  221 , i.e., output idler gear  221 , is fixed on the second output shaft  22  and configured to mesh with the shift driven gear  74 . 
     The output unit synchronizer  221   c , i.e., the output idler gear synchronizer  221   c , is positioned to the right of the output idler gear  221  and may engage with the output idler gear and with the second output shaft  22 . The generator gear  73  is fixed on the second output shaft  22 . 
     The first reverse idler gear  711  and the second reverse idler gear  712  are both fitted over the second output gear  22  to form a duplex gear. The first reverse idler gear  71  may mesh with the second-gear shift driving gear  2   a.    
     The motor power shaft  3  is disposed coaxially with the two input shafts such as the first and second input shafts  11 ,  12  and the two output shafts such as the first and second output shafts  21 ,  22 . The reverse output gear  72  and the third reverse idler gear  713  are fitted over the motor power shaft  3 . The first motor gear  31  is fixed on the motor power shaft  3  and may mesh with the generator gear  73 . The reverse synchronizer  72   c  is disposed on the reverse output gear  72  and may engage with the third reverse idler gear  713 . The third idler gear  713  may mesh with the second reverse idler gear  712 . The first motor generator  51  and the motor power shaft  3  are coaxially connected. 
     A condition in which the power transmission system  100  according to embodiments of the present disclosure may be used will be discussed below in detail with reference to  FIG. 1 . 
     Parking-Charging Condition 
     In the parking-charging condition, the engine  4  can drive the first motor generator  51  via two different transmission passages. 
     Transmission Passage  1   
     The fifth-sixth gear synchronizer  56   c  may engage with the fifth-gear shift driven gear  5   b . Power generated by the engine  4  may transmit to the first motor generator  51  via the first input shaft  11 , the fifth-gear gear pair, the second output shaft  22 , the generator gear  73  and the motor power shaft gear  31 , thus driving the first motor generator  51  to generate electric power. 
     Transmission Passage  2   
     The fifth-sixth gear synchronizer  56   c  may engage with the sixth-gear shift driven gear  6   b . Power generated by the engine  4  may transmit to the first motor generator  51  via the second input shaft  12 , the sixth-gear gear pair, the second output shaft  22 , the generator gear  73  and the motor power shaft gear  31 , thus driving the first motor generator  51  to generate electric power. 
     In the parking-charging condition, charging the vehicle with a fixed velocity ratio may be achieved, and the power transmission efficiency may be increased. Those with ordinary skill in the art will appreciate that the velocity ratio relates to parameters such as the rotating speed of the engine  4  in the parking state, the type of the first motor generator  51 , and maximum rotating speed acceptable by the peripheral parts such as bearings, and so on. In the present disclosure, the velocity ratio may be designed according to the above parameters and the power transmission ratio may be flexibly designed, thus making maximum use of the power from the engine  4  and achieving the object of fast charging. In the parking-charging condition, power from the engine  4  may be transmitted via a transmission passage consisting of the first input shaft  11 , the fifth-gear gear pair and the generator gear  73  or a transmission passage consisting of the second output shaft  22 , the sixth-gear gear pair and the generator gear  73 , and therefore the object of charging with an optimal fixed velocity ratio may be achieved, and both the charging efficiency and the fuel economy of the engine are improved. 
     Pure Electric Condition 
     First Electric Condition 
     The output unit synchronizer  221   c  engages the output unit  221 , and power generated by the first motor generator  51  is transmitted to the output unit  221  via the motor power shaft gear  31  and the generator gear  73 . This transmission passage has less transmission components and higher efficacy. 
     Second Electric Condition 
     Power generated by the first motor generator  51  is transmitted to the second-gear gear pair or the fourth-gear gear pair via the generator gear  73 , the sixth-gear gear pair and the second input shaft  12 . 
     Third Electric Condition 
     Power generated by the first motor generator  51  is transmitted to the first-gear gear pair or the third-gear gear pair via the generator gear  73 , the fifth-gear gear pair and the first input shaft  11 . 
     In the electric condition such as the first electric condition or the second electric condition, power from the first motor generator  51  may be transmitted to wheels of the vehicle via three power transmission passages having different velocity ratios, thus driving the vehicle to run. In cases when the first motor generator  51  is used to start, to accelerate, to climb or to run, different velocity ratios may be selected accordingly to ensure that the first motor generator  51  has the highest operation efficiency. 
     First First-Gear Hybrid Condition 
     The output unit synchronizer  221   c  engages the output unit  221 , and power generated by the first motor generator  51  is transmitted to the output unit  221  via the motor power shaft gear  31  and the generator gear  73 . This transmission passage has less transmission components and higher efficacy. 
     The first power generated by the engine  4  can be transmitted to any of the first-gear to fourth-gear gear pairs. The first power and second power are coupled at the driven gear  74 , and then output together to the wheels of the vehicle. 
     The first power generated by the engine  4  can be transmitted to either the fifth-gear or the sixth-gear gear pair. The first power and the second power are coupled at the second output shaft  22  and then output together to the wheels of the vehicle. 
     In the hybrid reverse mode, the first motor generator  51  may adjust the speed, such that the shift driven gear  74  or the second output shaft  22  may synchronously receive the first power from the engine  4  and the second power from the first motor generator  51 , to provide a smooth and harmonious power transmission. 
     First Second-Gear Hybrid Condition 
     In the first second-gear hybrid condition, the output unit synchronizer  221   c  is in a disengaged state. The power generated by the first motor generator  51  may transmit to the second input shaft  12  via the generator gear  73  and the sixth-gear gear pair. The power generated by the engine  4  may transmit to the second input shaft  12 . The first power and the second power generated by the first motor generator  51  are coupled at the second input shaft  12 , and then transmitted to either the second-gear gear pair or the fourth-gear gear pair, so as to be output. In some embodiments, the power generated by the engine  4  may transmit to either the first-gear gear pair or the third-gear gear pair via the first input shaft  11 . Two powers are coupled at the first output shaft  21  and then output together. 
     In the hybrid reverse mode, the first motor generator  51  may adjust the speed, such that the second input shaft  12  or the first output shaft  21  may synchronously receive the first power from the engine  4  and the second power from the first motor generator  51 , to provide a smooth and harmonious power transmission. 
     First Third-Gear Hybrid Condition 
     In the first third-gear hybrid condition, the output unit synchronizer  221   c  is in a disengaged state. The power generated by the first motor generator  51  may transmit to the first input shaft  11  via the generator gear  73  and the fifth-gear gear pair. The power generated by the engine  4  may transmit to the first input shaft  11 . The first power and the second power generated by the first motor generator  51  are coupled at the first input shaft  11 , and then transmitted to either the first-gear gear pair or the third-gear gear pair. In some embodiments, the power generated by the engine  4  may transmit to either the second-gear gear pair or the fourth-gear gear pair via the first output shaft  21 . Two powers are coupled at the first output shaft  21  and then output together. 
     In the hybrid reverse mode, the first motor generator  51  may adjust the speed, such that the first input shaft  11  or the first output shaft  21  may synchronously receive the first power from the engine  4  and the second power from the first motor generator  51 , to provide a smooth and harmonious power transmission. 
     In the present disclosure, a person skilled in the art may flexibly select any of the above hybrid conditions and power transmission passages thereof according to practical requirements. With these hybrid conditions, more driving fun may be provided to the users. In addition, the vehicle may be used in different road conditions, thus enhancing both the power and the fuel economy of the vehicle. 
     First First-Gear Driving-Charging Condition 
     In the first first-gear driving-charging condition, the power generated by the engine  4  can be transmitted to any of the first-gear to fourth-gear gear pairs. The output unit synchronizer  221   c  may engage with the output unit  221 . Power generated by the corresponding wheel via the output unit  221  and the second output shat  22  may configure the generator gear  73  and the motor power shaft gear  31  rotate together with the motor power shaft  3 . In some embodiments, the first motor generator  51  may drive to generate electric power by the power generated by corresponding wheel. 
     In some embodiments, the first power generated by the engine  4  can be transmitted to either the fifth-gear or the six-gear gear pair. At the same time, the output unit synchronizer  221   c  may engage with the output unit  221 . The first power generated by the engine  4  may transmit to the second output shaft  22  via the fifth-gear gear pair or the sixth-gear gear pair. One part of the power may transmit to the output unit  221  to drive the wheels of the vehicle. The other part of the power may transmit to the first motor generator  51  via the generator gear  73 , the motor power shaft gear  31  and the motor power shaft  3 , thus driving the first motor generator  51  to generate electric power. 
     First Second-Gear Driving-Charging Condition 
     In the first-gear driving-charging condition, one of the two gear parts of the dual clutch  2   d  is engaged when performing power transmission, for example, the input terminal  23   d  is engaged with the first output terminal  21   d  or engaged with the second output terminal  22   d . In the third first-gear driving-charging condition, the input terminal  23   d  is engaged with both the first output terminal  21   d  and the second output terminal  22   d , thus achieving a new driving-charging condition. 
     Condition 1 
     In the first second-gear driving—charging condition, the fifth-sixth gear synchronizer  56   c  may engage with the fifth-gear shift driven gear  5   b . The output unit synchronizer  221   c  is in a disengaged state. A part of the power generated by the engine  4  may transmit to the first motor generator  51  via the first input shaft  11 , the fifth-gear gear pair, the second output shaft  22 , the generator gear  73  and the motor power shaft gear  31 , thus driving the first motor generator  51  to generate electric power. The other part of the power generated by the engine  4  may transmit to the first output shaft  21  to drive the wheels of the vehicle via the second input shaft  12 , the second-gear gear pair or the fourth-gear gear pair. 
     Condition 2 
     In the first second-gear driving—charging condition, the fifth-sixth gear synchronizer  56   c  may engage with the sixth-gear shift driven gear  6   b . The output unit synchronizer  221   c  is in a disengaged state. A part of the power generated by the engine  4  may transmit to the first motor generator  51  via the second input shaft  12 , the sixth-gear gear pair, the second output shaft  22 , the generator gear  73  and the motor power shaft gear  31 , thus driving the first motor generator  51  to generate electric power. The other part of the power generated by the engine  4  may transmit to the first output shaft  21  to drive the wheels of the vehicle via the first input shaft  11 , the second-gear gear pair or the fourth-gear gear pair. 
     In the present disclosure, a person skilled in the art may flexibly select any of the above hybrid conditions and power transmission passages thereof according to practical requirements. With these hybrid conditions, more driving fun may be provided to the users. In addition, the vehicle may be used in different road conditions, thus enhancing both the power and the fuel economy of the vehicle. 
     In the driving-charging conditions, a part of power from the engine  4  may be transmitted via a passage consisting of the first input shaft  11 , the fifth-gear gear pair, and the generator gear  73 , or a passage consisting of the second input shaft  12 , the sixth-gear gear pair and the generator gear  73 , and therefore the object of charging with an optimal fixed velocity ratio may be achieved, and both the charging efficiency and the fuel economy of the engine  4  are improved. 
     Mechanical Reverse Condition 
     In the mechanical reverse condition, the reverse synchronizer  72   c  may engage with the third reverse idler gear  713 , such that the power generated by the engine  4  may transmit to the reverse output gear  72  via the second input shaft  12 , the second-gear shift driving gear  2   a , the first reverse idler gear  711 , the second reverse idler gear  712  and the third reverse idler gear  713 . 
     Electric Reverse Condition 
     In the electric reverse mode, the output unit synchronizer  221   c  may engage with the output unit  221 , and the power generated by the first motor generator  51  may transmit to the output unit via the motor power shaft gear  31 , the generator gear  73  and the second output shaft  22 . 
     In the electric reverse mode, the output unit synchronizer  221   c  is in a disengaged state, and the fifth-sixth gear synchronizer  56   c  may engage with the sixth-gear shift driven gear  6   b . At the same time, the reverse synchronizer  72   c  may engage with the third reverse idler gear  713 , such that the power generated by the first motor generator  51  may transmit to the reverse output gear  72  via the generator gear  73 , the sixth-gear gear pair, the second input shaft  12  and the reverse idler gear. 
     Hybrid (Electric-Mechanic) Reverse Condition 
     In the hybrid reverse mode, the reverse synchronizer  72   c  is in an engaged state and the fifth-sixth gear synchronizer  56   c  may engage with the sixth-gear shift driven gear  6   b . The power generated by the engine  4  may transmit to the second input shaft  12 , and the power generated by the first motor generator  51  may transmit to the second input shaft  12  via the generator gear  73  and the sixth-gear gear pair. The first power and the second power are coupled at the second input shaft  12 , and then output together via the reverse idler gear. In the hybrid reverse mode, the first motor generator  51  may adjust the speed, such that the shift driven gear  74  may synchronously receive the first power from the engine  4  and the second power from the first motor generator  51 , to provide a smooth and harmonious power transmission. 
     In the parking-charging condition and the running-charging condition, the power generated by the engine  4  may transmit to the first motor generator  51  via the generator gear  73  and the motor power shaft gear  31 . The first motor generator  51  may always rotate along the original rotating direction (the predetermined rotating direction such as the clockwise direction). When the first generator is regarded as the power producer, such as the pure electric conditions and the hybrid conditions, the first motor generator  51  may always rotate along the original rotating direction (the predetermined rotating direction such as the clockwise direction). In the reverse conditions, when the power generated by the first motor generator  51  may output via a transmission passage consisting of the generator gear  73 , the reverse idler gear and the reverse output gear  72 , the first motor generator  51  may always rotate along the original rotating direction (the predetermined rotating direction such as the clockwise direction). 
     With the power transmission system  100  according to embodiments of the present disclosure, the first motor generator  51  may rotate along the predetermined rotating direction in all the above-mentioned conditions. In other words, the first motor generator  51  may always rotate along the predetermined rotating direction when functioning as a motor or as a generator. Even during the power transmission system  100  switching from one condition to the reverse condition, the rotating direction of the first motor generator  51  needs not to be changed. Therefore, the first motor generator  51  may always rotate along the predetermined rotating direction in all related conditions, such that problems of shock and interruption due to direction change of the motor may be avoided, and the life of the power transmission system  100  may be prolonged. 
     Embodiments 2-5 
     As shown in  FIGS. 2-5 , the power transmission system  100  in the present embodiment is substantially the same as that in Embodiment 1, with the following exceptions that a rear-wheel driving mechanism, a third motor generator  201 , a fourth motor generator  301  and an antiskid synchronizer  503  are added respectively. 
     Embodiment 6 
     As shown in  FIG. 6 , the power transmission system  100  in the present embodiment is substantially the same as that in Embodiment 1, with the following exceptions that the engine  4 , the dual clutch  2   d , the first motor generator  51  and the differential may be avoided. 
     Embodiments of the present disclosure further provide a vehicle including the above-identified power transmission system  100 . Other configuration such as the driving system, the turning system and the braking system may be well known to those skilled in the art, thus details thereof are omitted herein. 
     Reference throughout this specification to “an embodiment,” “some embodiments,” “one embodiment”, “another example,” “an example,” “a specific example,” or “some examples,” means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present disclosure. Thus, the appearances of the phrases such as “in some embodiments,” “in one embodiment”, “in an embodiment”, “in another example,” “in an example,” “in a specific example,” or “in some examples,” in various places throughout this specification are not necessarily referring to the same embodiment or example of the present disclosure. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or examples. 
     Although explanatory embodiments have been shown and described, it would be appreciated by those skilled in the art that the above embodiments cannot be construed to limit the present disclosure, and changes, alternatives, and modifications can be made in the embodiments without departing from spirit, principles and scope of the present disclosure.