Patent Publication Number: US-2021162862-A1

Title: A powertrain system for driving at least one wheel of a vehicle

Description:
TECHNICAL FIELD 
     The invention relates to a powertrain system for driving at least one wheel of a vehicle, and to a powertrain module comprising such a powertrain system. The invention also relates to a driven wheel system for a vehicle comprising at least one such powertrain system, and to a vehicle comprising at least one such driven wheel system. The invention further relates to a method for charging a battery set of a vehicle, with such a powertrain module. 
     BACKGROUND 
     Transport industry is moving to electric-powered vehicles to deal with emission regulation requirements which are becoming more and more demanding. Electric vehicles require significant space for batteries, which means that other vehicle components need to be fairly compact. Indeed, the transverse width of a vehicle, in particular a truck, is limited to a maximum value given by regulatory requirements. This applies in particular to the powertrain system, especially on a driven wheel system which includes a device providing a differential effect, in order to allow the outer drive wheel to rotate faster than the inner drive wheel during a turn. 
     Such a space constraint is even more significant in some vehicle configurations: 
     for vehicles having an independent wheel suspension configuration, as they require an increased length in the transverse direction of the vehicle, to get enough space for the drive shafts to work with acceptable angles, specifically if the vehicle is designed without wheel reduction; 
     for vehicles not comprising a mechanical differential, but implementing a torque vectoring solution. This creates packaging issues since the left and right wheels need to be independently driven, duplicating the transmission from the motor to the wheel(s). 
     Another issue in electric drivelines is the significant diversity in terms of powers, uses and applications. Therefore, powertrain systems are required to fit various needs, without impairing compactness nor efficiency. 
     SUMMARY 
     An object of the invention is to provide an improved powertrain system, which can solve at least one of the above-mentioned problems of the prior art. 
     In particular, an object of the invention is to provide a powertrain system which is more compact in the vehicle transverse direction, while preferably offering advantageous functionalities. 
     To that end, and according to a first aspect, the invention concerns a powertrain system for driving at least one wheel of a vehicle, comprising: 
     a motor having an output shaft; 
     a transmission system between the motor and a drive shaft connected to said wheel, the transmission system comprising two epicyclic gear trains each including the following components: a ring, a sun, a planet carrier and planet gears, wherein:
         a first epicyclic gear train has a first axis, a component forming a first input component connected to the motor output shaft, and a component forming a first output component;   a second epicyclic gear train has a second axis parallel to the first axis, a component forming a second input component meshing with the first output component, and a component forming a second output component configured to be connected to the drive shaft.       

     It is specified that “motor” means “electric motor”. 
     The powertrain system thus includes two epicyclic gear trains which are structurally arranged in parallel and functionally arranged in series. Such a powertrain system is highly advantageous, as it provides a high gear ratio as well as a significant compactness in the transverse direction of the vehicle, therefore leaving space for other components, especially batteries. 
     Another advantage of the invention is that, because of the compactness of the powertrain system, it can be implemented on a vehicle having an independent wheel suspension arrangement, with dual mounted tires and without wheel reduction. This is all the more significant as independent wheel suspension is a key solution to develop an optimized electrified driveline. The powertrain system of the invention can also be advantageous when implementing torque vectoring solutions, as it frees up some space to receive the necessary components of such solutions. 
     According to an implementation, the ring, an intermediate part slidably mounted on the outer part of the ring, or the sun of one of the epicyclic gear trains is movable, preferably parallel to the first and second axes, between at least: 
     a first position, in which the motor is able to transmit torque to the drive shaft according to a first gear ratio; 
     and a second position, in which the motor is able to transmit torque to the drive shaft according to a second gear ratio. 
     Such a gear ratio change makes it possible to better fit the various customer needs with one and the same powertrain system, and can further improve efficiency. 
     More specifically, a slow gear ratio is needed to ensure startability performances (i.e. torque demand at start). Having a fixed gear ratio would mean that in cruising conditions, the motor runs at high speed/low torque conditions, which are poor efficiency conditions for an electric motor. By providing at least a second gear ratio, the invention ensures improved efficiency in many conditions, including starting and cruising. Owing to the use of the epicyclic gear trains, providing a second gear ratio—which involves a corresponding gear changing device—does not significantly impair the compactness of the powertrain system. 
     It may be envisaged that the ring, an intermediate part slidably mounted on the outer part of the ring, or the sun of one of the epicyclic gear trains be movable, preferably parallel to the first and second axes, between: 
     at least one position in which the motor is able to transmit torque to the drive shaft; 
     and a neutral position in which no torque can be transmitted from the motor to the drive shaft. 
     In such a neutral position, the motor can transmit torque to one component of the second epicyclic gear train, but not to the drive shaft. This arrangement may be useful with a powertrain system including two motors, each motor having an output shaft connected to a dedicated epicyclic gear train, said two epicyclic gear trains each meshing with another common epicyclic gear train connected to the drive shaft. This has a specific interest during a battery charging phase, as will be explained later with more details. This arrangement may also be useful to tow a disabled vehicle, in particular a truck, without running the motors. 
     The piece that is moveable between at least a first position and a second position, and/or between at least one position and a neutral position can be the ring of the second epicyclic gear train, or an intermediate part slidably mounted on the outer part of said ring. Such an implementation is advantageous in terms of compactness and output rotation speed. It is furthermore required in case the powertrain system comprises two motors each provided with a dedicated epicyclic gear train and sharing the second epicyclic gear train. 
     As regards the intermediate part slidably mounted on the outer part of said ring, it can be provided with splines engaging the outer teeth of the ring, for allowing relative axial movement but preventing relative rotational movement. 
     In an embodiment, the ring of one of the epicyclic gear trains can form the first output component or respectively the second input component. Furthermore, said ring can have inner teeth for meshing with the planet gears of the same epicyclic gear train and outer teeth for meshing with the second input component or respectively the first output component. This makes it possible to have an even more compact powertrain system in the vehicle transverse direction. The inner teeth and outer teeth may preferably be located in one and the same plane orthogonal to the first and second axes. 
     The ring of the first epicyclic gear train can form the first output component. Furthermore, said ring can have inner teeth for meshing with the planet gears of the first epicyclic gear train and outer for meshing with the second input component. This implementation provides a particularly satisfactory gear ratio. 
     According to one embodiment, the first input component is the sun and the first output component is the ring. The planet carrier of the first epicyclic gear train may be fastened to the casing which houses the transmission system. Preferably, the first epicyclic gear train may be of “Type I”, i.e. with the sun as an inner component, the ring as an outer component, with the planet gears arranged in between. 
     According to one embodiment, the second input component is the sun and the second output component is the planet carrier, planet gears being rotationally mounted on the planet carrier and being arranged between the sun and the ring. Preferably, the second epicyclic gear train may be of “Type I”, i.e. with the sun as an inner component, the ring as an outer component, with the planet gears arranged in between. 
     In an embodiment, in the first epicyclic gear train, the sun can be configured as an inner component, the ring can be configured as an outer component, with the planet gears arranged in between and rotationally mounted on the planet carrier, the planet carrier being fastened to a casing of the motor (or fastened to a casing housing the powertrain system, for example made as a single piece with such a casing housing the powertrain system). Furthermore, said ring of the first epicyclic gear train can have inner teeth for meshing with the planet gears of the first epicyclic gear train and outer teeth for meshing with the sun of the second epicyclic gear train or a hub secured to said sun, the inner teeth and outer teeth preferably being located in one and the same plane orthogonal to the first and second axes. 
     With a specific configuration, in which the ring bears both inner and outer teeth located in a same plane, compactness is greatly increased. 
     According to a second aspect, the invention relates to a powertrain module comprising at least one powertrain system as previously described, and a casing which houses the at least one transmission system. 
     The ring of the second epicyclic gear train—or an intermediate part slidably mounted on the outer part of the ring—can be movable relative to the casing, parallel to the first and second axes, between: 
     the first position, in which said ring is rotationally fastened to the sun of the second epicyclic gear train or to a hub secured to said sun; 
     and the second position, in which said ring is rotationally fastened to the casing. 
     The second position results in a higher gear reduction ratio than the first position. For example, the ratio in the first position can be 1, while the ratio in the second position can be 3.4. 
     The ring of the second epicyclic gear train—or an intermediate part slidably mounted on the outer part of the ring—can be movable relative to the casing, parallel to the first and second axes, between: 
     at least one of: a first position, in which said ring is rotationally fastened to the sun of the second epicyclic gear train or to a hub secured to said sun; and a second position, in which said ring is rotationally fastened to the casing; 
     and a neutral position in which said ring is rotationally uncoupled from both the sun of the second epicyclic gear train and the casing. 
     The casing may comprise at least one receiving area for an additional epicyclic gear train configured to be connected to the output shaft of an additional motor, the additional epicyclic gear train having an axis parallel to the first and second axes and meshing with the second epicyclic gear train. 
     Owing to this arrangement, the invention provides a modular and/or scalable power train module which can include one, two or three motors depending on the needs in terms of power, or application. Preferably, the additional epicyclic gear train is identical to first epicyclic gear train. Besides, to improve spatial layout, the two motors—or more—may be mounted on the same side of the casing along the axes. 
     The powertrain module may thus further comprise at least one additional motor and at least one additional epicyclic gear train connected to or connectable to the additional motor, the additional epicyclic gear train being mounted or configured to be mounted in the or one of the receiving area(s) of the casing. With this arrangement, the invention can provide on the one hand a basic powertrain module with only one motor, and on the other hand an upgrade kit including an additional motor and its dedicated epicyclic gear train, for being connected to the basic powertrain module. 
     The plane including the first axis and the second axis and the plane including the second axis and the axis of the additional epicyclic gear train can form an angle comprised between 80° and 180°, preferably between 100° and 160°. In other words, the casing can substantially have the shape of a V, when seen along the direction of the axes. In case the angle is 180°, the above mentioned planes are coincident and thus form one and a single plane. 
     This makes it possible to mount on the vehicle chassis two modules each including two motors—or one motor and one motor receiving area—located at one end of the V-shaped casing, while arranging one motor or a first powertrain module between the two motors of the second powertrain module. This results in space gain and reduces the impact on ground clearance. 
     According to an embodiment, the powertrain module, preferably the casing, comprises connecting means for cooperating with the connecting means of an identical powertrain module arranged in a facing relationship with said powertrain module, for mechanically connecting said two powertrain modules, with the second axes substantially coincident, the casings being rotationally offset the one relative to the other. 
     Such an arrangement allows significantly increasing compactness, and further improves stress transmission between the powertrain modules, therefore improving the overall mechanical strength. 
     The connecting means can comprise a dog clutch connection. For example, indentations can be arranged in a circle around the second axis. 
     According to another embodiment, the casing is made as a single body including: 
     a left casing portion for a left powertrain system, comprising at least one area housing a first epicyclic gear train connected to a motor and an area housing a second epicyclic gear train of said left powertrain system, wherein the second output component of the second epicyclic gear train of said left powertrain system is configured to be connected to a left drive shaft; 
     a right casing portion for a right powertrain system, comprising at least one area housing a first epicyclic gear train connected to a motor and an area housing a second epicyclic gear train of said right powertrain system, wherein the second output component of the second epicyclic gear train of said right powertrain system is configured to be connected to a right drive shaft. 
     This arrangement is specifically used when one powertrain system is provided for each left or right wheel, to achieve the differential effect through torque vectoring technology. Each powertrain system is configured to vary the torque to the corresponding wheel, depending on the driving conditions (straight road or curve, ground surface slipping properties, etc.). 
     According to still another embodiment, the casing comprises connecting means for connection with a differential carrier housing receiving a differential configured to link the drive shaft of one wheel to the drive shaft of another wheel. 
     This arrangement is specifically used when only one powertrain system is provided for the left and right wheels of the driven wheel system, to drive both left and right wheels with the appropriate torque, through the differential. 
     According to a third aspect, the invention relates to a driven wheel system for a vehicle, comprising at least one left wheel and one right wheel, each wheel being connected to a drive shaft, the driven wheel system further comprising at least one powertrain system or at least one powertrain module as previously described, the second output component of the second epicyclic gear train being connected to at least one drive shaft. It has to be noted that the term “connected” does not necessarily means “directly connected”; an intermediate part can be provided to achieve this. 
     In a torque vectoring mode, one powertrain module is provided for each wheel. Each module may include one or two motors (or more), or one motor and a receiving area for an additional motor. The two modules may be arranged in a facing and interlinked relationship, with their casings mechanically connected to one another, as previously explained. 
     Alternatively, in case a mechanical differential is provided, the driven wheel system comprises a single power train module, the second output component of which is connected to a differential unit which appropriately drives each drive shaft and therefore each wheel. 
     According to an embodiment, commonly referred to as “rigid axle configuration”, the driven wheel system forms an axle and comprises: 
     a first axle housing secured to one side of the casing of one powertrain module, preferably opposite the motor, and receiving a first drive shaft connected between a first wheel of the driven wheel system and the powertrain system of said powertrain module; 
     a second axle housing receiving a second drive shaft, said second drive shaft being connected between a second wheel of the driven wheel system and:
         said powertrain system of said powertrain module, the second axle housing being secured to an opposite side of the same casing. This is typically if a differential is provided. Then, the second axle housing also receives the differential;   or the powertrain system of another powertrain module, the second axle housing being secured to the casing of said other powertrain module. This is typically when torque vectoring technology is used, and when two separate casings can be provided, each for one transmission system;   or another powertrain system of said powertrain module, wherein the casing of said powertrain module is made as a single body, the second axle housing being secured to an opposite side of said casing. This is typically when torque vectoring technology is used, and when there is provided only one casing, in a single piece.       

     It has to be noted that the term “secured” does not necessarily means “directly secured”; an intermediate part can be provided to achieve this. 
     According to another embodiment, commonly referred to as “independent wheel suspension”, the connection between one wheel and one powertrain module is made by means of a connection device including a driveshaft, at least one joint (such as a universal joint, an homocinetic joint, etc.), at least one lower arm articulated at both ends and preferably at least one upper arm articulated at both ends. Furthermore: 
     the driven wheel system may comprise a single powertrain module, a first wheel being connected to the powertrain module by means of a first connection device, the second wheel being connected to the powertrain module by means of a second connection device, and a differential unit being provided between the drive shaft of the first wheel and the drive shaft of the second wheel; 
     or the driven wheel system may comprises two powertrain modules, a first wheel being connected to a first powertrain module by means of a first connection device, and a second wheel being connected to a second powertrain module by means of a second connection device. This is typically when torque vectoring technology is used, and when two separate casings can be provided, each for one transmission system; 
     or the driven wheel system may comprise a single powertrain module comprising a casing which is made as a single body and which includes a first powertrain system and a second powertrain system, a first wheel being connected to the powertrain module by means of a first connection device, with the drive shaft of the first wheel connected to the first powertrain system, and a second wheel being connected to the powertrain module by means of a second connection device, with the drive shaft of the second wheel connected to the second powertrain system. This is typically when torque vectoring technology is used, and when there is provided only one casing, in a single piece. 
     According to a fourth aspect, the invention relates to a vehicle comprising at least one driven wheel system as previously described. 
     According to a fifth aspect, the invention relates to a method for charging a battery set of a vehicle with such a powertrain module, which comprises an additional motor and an additional epicyclic gear train connected to the output shaft of the additional motor, the additional epicyclic gear train being mounted in a receiving area of the casing, having an axis parallel to the first and second axes and meshing with the second epicyclic gear train. The method comprises: 
     connecting the motor to a power supply located outside the vehicle; 
     connecting the additional motor to the battery set; 
     bringing the ring or the sun of the second epicyclic gear train in the neutral position, as previously described, so that the motor is able to transmit torque to the additional motor through the epicyclic gear trains and no torque can be transmitted from the motor to the drive shaft. 
     Then, the motor acts as an electrical/mechanical converter and the additional motor acts as a mechanical/electrical converter, the mechanical energy being transmitted through the epicyclic gear trains. This makes it possible to charge the battery set. Furthermore, using the neutral position allows conducting the charging process without rotating the wheels. 
     Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples. 
       In the drawings: 
         FIG. 1  is a sectional view of a powertrain module according to an embodiment of the invention, comprising one motor and a transmission system including a first and a second epicyclic gear trains; 
         FIG. 2  is an exploded perspective view of the powertrain module of  FIG. 1 ; 
         FIGS. 3 a  and 3 b    are perspective views of a ring pertaining to the first epicyclic gear train; 
         FIG. 4  is a perspective partial view of the transmission system of  FIG. 1 ; 
         FIGS. 5   a,    5   b  and  5   c  are sectional views showing various positions of a ring of the second epicyclic gear train; 
         FIG. 6  is a perspective view of a powertrain module according to another embodiment of the invention, comprising an additional motor; 
         FIG. 7  is a sectional view of the powertrain module of  FIG. 6 ; 
         FIG. 8  is a partially exploded perspective view of the powertrain module of  FIG. 6 ; 
         FIG. 9  shows an implementation of the powertrain module of  FIG. 1  on a vehicle driven wheel system; 
         FIG. 10  shows two powertrain modules mechanically connected to one another; 
         FIG. 11  is a perspective view of an embodiment of a casing of a powertrain module; 
         FIG. 12  shows an implementation of two powertrain modules of  FIG. 10  on a vehicle driven wheel system; 
         FIG. 13  is a sectional view of the powertrain modules of  FIG. 12 ; 
         FIG. 14  shows another implementation of two powertrain modules of  FIG. 10  on a vehicle driven wheel system. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION 
       FIGS. 1 and 2  show a powertrain module  10  according to an embodiment of the invention. 
     The powertrain module  10  is designed to be implemented in a vehicle  1 . As shown in  FIGS. 9, 12 and 14 , the vehicle  1  comprises at least one driven wheel system  2  which has an axis A 2  and which comprises at least one left wheel  3 ,  3   a  and one right wheel  3 ,  3   b.  Each wheel  3  is connected to a drive shaft  4 ,  4   a,    4   b.  The driven wheel system  2  further comprises at least one powertrain module  10  arranged between the drive shafts  4 . It has to be noted that the drive shafts  4  are not illustrated in  FIG. 12 , and are not visible in  FIG. 14 . 
     The vehicle  1  may comprise a front axle connected to front wheels (not shown), and at least one driven rear wheel system  2 —for example a first driven rear wheel system and a second driven rear wheel system located rearwards from the first driven rear wheel system. Each rear wheel system  2  can comprise two wheels  3  on either side, thus forming a dual mounted tires arrangement. 
     The invention can be applied in heavy-duty vehicles, such as trucks, buses and construction equipment, as well as medium-duty vehicles. Although the following description is made with reference to a rear wheel system, it has to be noted that the invention can be used on another driven wheel system, for example on a driven front wheel system. 
     As shown in  FIGS. 12 and 14 , direction X is defined as the longitudinal direction of the vehicle  1  and direction Y is defined as the transverse direction of the vehicle  1 , i.e. the direction of axis A 2  of the driven wheel system  2 . Directions X and Y are substantially horizontal when the vehicle  1  is on a horizontal surface. Moreover, direction Z is defined as the vertical direction—when the vehicle  1  is on a horizontal surface. 
     The invention will be described when the vehicle  1  is on a horizontal surface. 
     The powertrain module  10  essentially comprises a casing  11  and a powertrain system  12 . The powertrain system  12  is configured to drive at least one wheel  3  and comprises: 
     a motor  5  having an output shaft  6 ; 
     a transmission system  13  between the motor  5  and the drive shaft  4  connected to said wheel  3 , the transmission system  13  being housed in the casing  11 . 
     As shown in  FIGS. 1 to 4 , the transmission system  13  comprises a first epicyclic gear train  100  having a first axis A 100 , and a second epicyclic gear train  200  having a second axis A 200  which is parallel to the first axis A 100 . 
     In the operating position, i.e. when the powertrain module  10  is mounted on the vehicle  1 , as shown in  FIGS. 9, 12 and 14 , the axes A 100  and A 200  are parallel to direction Y. 
     The first epicyclic gear train  100  comprises: 
     a sun  101  which is connected to the motor output shaft  6  and forms a first input component of the transmission system  13 . The sun  101  is arranged as an inner component of the first epicyclic gear train  100 , as better seen in  FIG. 4 ; 
     a ring  102  which is arranged as an outer component of the first epicyclic gear train  100 , as better seen in  FIG. 4 , and which forms a first output component of the transmission system  13 ; 
     a planet carrier  103  which is fixedly secured to the casing  11 , or could be made as a single piece with the casing  11 ; 
     planet gears  104  (for example four planet gears) arranged between the sun  101  and the ring  102 . The planet gears  104  are rotationally mounted on the planet carrier  103 . 
     The second epicyclic gear train  200  comprises: 
     a sun  201  which is fixedly secured to a hub  205 , said hub  205  meshing with the ring  102  of first epicyclic gear train  100 . The sun  201  is arranged as an inner component of the second epicyclic gear train  200  and forms a second input component of the transmission system  13 ; 
     a ring  202  which is arranged as an outer component of the second epicyclic gear train  200 ; 
     a planet carrier  203  which forms a second output component of the transmission system  13 ; 
     planet gears  204  (for example four planet gears) arranged between the sun  201  and the ring  202 . The planet gears  204  are rotationally mounted on the planet carrier  203 . 
     As better shown in  FIGS. 3 a    and  3 , the ring  102  of the first epicyclic gear train  100  can have outer teeth  105  for meshing with the hub  205  of the second epicyclic gear train  200 , i.e. for driving the sun  201  around the second axis A 200 . The ring  102  can further have inner teeth  106  for meshing with the planet gears  104  of the first epicyclic gear train  100 . 
     Such an arrangement is advantageous as it results in a layout which is even more compact in the transverse direction Y. This is even more significant if the ring  102  is configured so that the inner teeth  106  and outer teeth  105  are located in one and the same plane P orthogonal to the first and second axes A 100 , A 200 , as shown in  FIG. 1 . Another advantage of such a double-teeth configuration is that only one piece is necessary, which decreases the time needed for assembling the transmission system. However, this specific configuration of the ring  102  shall not be considered as limitative. 
     The transmission system  13  therefore makes it possible to transmit torque from the motor  5  to the wheels  3 , and to multiply said torque according to at least one gear ratio. 
     It may be desirable to improve efficiency to provide two different gear ratios. Indeed, slow gear ratio allows good slope startability, while fast gear ratio allows good efficiency in cruising conditions. 
     To that end, the ring  202  of the second epicyclic gear train  200  can be movable relative to the casing  11 , parallel to the first and second axes A 100 , A 200 , between at least: 
     a first position, in which said ring  202  is rotationally fastened to the sun  201  of the second epicyclic gear train  200  or to the hub  205 . In this first position, depicted in  FIG. 5 a   , the motor  5  is able to transmit torque to the drive shaft  4  according to a first gear ratio which here is a fast gear ratio; 
     and a second position, in which said ring  202  is rotationally fastened to the casing  11 . In this second position, depicted in  FIG. 5 b   , the motor  5  is able to transmit torque to the drive shaft  4  according to a second gear ratio which here is a slow gear ratio. 
     Therefore, the powertrain module  10  of the invention can adapt to the driving phase or condition. 
     In addition to the first and/or second position, the ring  202  of the second epicyclic gear train  200  can be placed in a neutral position, in which said ring  202  is rotationally uncoupled from both said sun  201  and the casing  11 . In this neutral position, illustrated in  FIG. 5 c   , no torque can be transmitted from the motor  5  to the drive shaft  4 . In other words, along the transverse direction Y, the ring  202  is located between the part of the sun  201  of the second epicyclic gear train  200  and the part of the casing  11  which are configured to engage with the ring  202 . 
     Alternatively, the ring  202  can remain at the same axial position, and there may be provided an intermediate part slidably mounted on the outer part of the ring  202 , which can be placed in the first position, second position or neutral position. 
     Reference is now made to  FIGS. 6 to 8 . 
     It may be desirable that the powertrain module  10  comprises or is designed to receive at least one additional motor  5 ′. Providing a powertrain module  10  which can include more than one motor makes it possible to better meet the performance demands, in terms of power, startability, gross combined weight rating (GCW), etc. The motors  5 ,  5 ′ may be identical. Preferably, the motors  5 ,  5 ′ are mounted on a same side of the casing  11 , along the transverse direction Y. 
     As shown on  FIG. 8 , the casing  11  may comprise at least one receiving area  14  for an additional epicyclic gear train  300  configured to be connected to the output shaft  6 ′ of an additional motor  5 ′. The additional epicyclic gear train  300 , when mounted on the receiving area  14  of the casing  11 , has an axis A 300  which is parallel to the first and second axes A 100 , A 200 , and meshes with the second epicyclic gear train  200 . 
     In practice, the additional epicyclic gear train  300  may be identical to the first epicyclic gear train  100 . Then, as shown in  FIGS. 7 and 8 , the additional epicyclic gear train  300  can comprise: 
     a sun  301  connected to the motor output shaft  6 ′; 
     a ring  302  which meshes with the hub  205  fixedly secured to the sun  201  of the second epicyclic gear train  200   
     a planet carrier  303  which is fixedly secured to the casing  11  or could be made as a single piece with the casing  11 ; 
     planet gears  304  (for example four planet gears) arranged between the sun  301  and the ring  302  and rotationally mounted on the planet carrier  303 . 
     In  FIG. 6 , the additional epicyclic gear train  300  (which is not visible) is mounted in the receiving area  14  of the casing  11  and meshes with the second epicyclic gear train  200 . It is further connected to the output shaft  6 ′ of the additional motor  5 ′. Thus, the powertrain module  10  comprises two motors. Powertrain modules with more than two motors, for example with three motors, can be envisaged. 
     According to a variant, as shown in  FIG. 8 , there may be provided: 
     on the one hand, a powertrain module  10  with a single motor  5 , but with a casing  11  comprising a receiving area  14  for an additional motor, if need be; 
     and, on the other hand, an additional motor  5 ′ and an additional epicyclic gear train  300  which is configured to be mounted in the receiving area  14  of the casing  11  and to be connected to the additional motor  5 ′. 
     As a result, it is possible to equip a vehicle  1  with a powertrain module  10  including a single motor  5  and to upgrade this powertrain module  10  by implementing an additional motor  5 ′ if needed. 
     When the powertrain module  10  comprises at least two motors  5 ,  5 ′, the invention provides a high advantage over the prior art when it comes to charging the vehicle batteries. Indeed, during a charging phase, galvanic isolation is required for safety reasons, which means that the power supply—typically provided by an electric network outside the vehicle  1 —has to be electrically isolated from the batteries of the vehicle  1 . 
     Conventional solutions, which use electronic devices, are not fully satisfactory as they have a significant cost. 
     With the method according to the invention, the battery set of the vehicle  1  can be charged as follows. One motor  5  of the powertrain module  10  is connected to a power supply located outside the vehicle  1 , while the other motor  5 ′ of the same powertrain module  10  is connected to the battery set. In other words, the motor  5  acts as a motor, and the other motor  5 ′ acts as an alternator. Because of the gap between the rotor and the stator of the motor, a galvanic isolation is obtained without electronic devices. 
     Moreover, the method of the invention advantageously comprises bringing the ring  202  of the second epicyclic gear train  200  in the neutral position. Thus, the motor  5  is able to transmit torque to the other motor  5 ′ through the epicyclic gear trains  100 ,  200 ,  300  and no torque can be transmitted from the motor  5  to the drive shaft  4 . In other words, the invention makes it possible to declutch the second epicyclic gear train  200  during the battery charging process, while keeping the vehicle  1  standstill, as the wheels  3  are not driven by the motors  5 ,  5 ′. 
     According to an embodiment, the invention can be implemented on a driven wheel system  2  including only one powertrain module  10  and a differential  20 . With such a configuration, the power from the motor  5 , or from the two motors  5 ,  5 ′, is transmitted through the transmission system  13  to the differential  20 , more specifically by the planet carrier  203  of the second epicyclic gear train  200  connected to a differential housing which houses the differential  20 . The differential housing drives the differential  20 , which then appropriately drives the driveshafts  4   a,    4   b,  and ultimately the wheels  3   a,    3   b.  The differential  20  therefore links the drive shaft  4  of one wheel  3  to the drive shaft  4  of the other wheel  3 . 
       FIG. 9  illustrates such an implementation of the invention, for a driven wheel system  2  which forms a rigid axle, bearing in mind that it could alternatively be a driven wheel system of having an independent wheel suspension configuration. It has to be noted that  FIG. 9  shows a powertrain module  10  comprising a single motor  5 ; however, the powertrain module  10  could comprise at least one additional motor  5 ′, or at least one or more receiving area(s)  14  for such an additional motor  5 ′ or additional motors  5 ′. 
     In the illustrated non-limiting embodiment, the differential  20  comprises differential side pinions  21 , for example four differential side pinions, which are fitted on a joint cross  22 , and two differential side gears  23 . Each differential side gear  23  meshes with at least one differential side pinion  21  and is fastened to a first end of one of the drive shafts  4 , the other end of each drive shaft being connected to a wheel  3 . The differential  20  may further comprise a blocking system  25  for blocking the differential operation. Besides, the differential  20  is preferably housed in a differential carrier housing  26 . 
     In this embodiment, the casing  11  preferably comprises connecting means  16  for connection with the differential carrier housing  26 . For example, the casing  11  and differential carrier housing  26  can be assembled by fasteners such as screws. 
     According to another embodiment, the invention can be used to implement a torque vectoring solution. Then, one powertrain module  10  is provided to drive each wheel  3  of the driven wheel system  2  (or each set of wheels of the driven wheel system  2 , on one given side of the vehicle  1 ). Thus, the differential effect, i.e. the fact that the outer drive wheel rotates faster than the inner drive wheel during a turn, is achieved by the fact that the wheels are driven independently by the dedicated powertrain module  10 . 
     In a torque vectoring solution, as two powertrain modules  10  are provided on the driven wheel system  2 , the space required is larger. Consequently, there is a need to make the set of two powertrain modules  10  as compact as possible. 
     According to an embodiment, illustrated in  FIG. 10 , the powertrain modules  10  are distinct but designed and assembled in a specific way. 
     More precisely, on the one hand, as shown In  FIG. 6 , the plane including the first axis A 100  and the second axis A 200  and the plane including the second axis A 200  and the axis A 300  of the additional epicyclic gear train  300  can form an angle α the tip of which is coincident with the second axis A 200 . The casing  11  can thus have the shape of a V, when seen along the transverse direction Y, i.e. the direction of the axes A 100 , A 200 . Angle α can be comprised between 80° and 180°, preferably between 100° and 160°. However, the value of α depends on various parameters such as the diameter of the motor(s)  5 ,  5 ′, the diameter of the epicyclic gear trains  100 ,  200 ,  300 . Said value is further determined taking into account the departure angle, the wheel diameter, the required ground clearance, etc. 
     This V-shaped configuration allows assembling two powertrain modules  10  as a particularly compact assembly. Indeed, as shown in  FIG. 10 , the powertrain modules  10  can be arranged in a facing relationship, with the second axes A 200  substantially coincident, the casing  11  of one powertrain module  10  being rotationally offset—around axis A 200 —relative to the casing  11  of the other powertrain module  10 . Then, one motor  5  of one powertrain module  10  is located between the two legs of the V-shaped casing  11  of the other powertrain module  10 . As a result, the two powertrain modules  10  occupy a limited space which is preferably included in the cylindrical transverse space defined by the wheels  3  of the driven wheel system  2 . There remains quite a lot of space available for other purposes. 
     Moreover, because of the assembly according to a facing relationship, the motors  5 ,  5 ′ of one powertrain module  10  extend from the corresponding casing  11  in one direction, and the motors  5 ,  5 ′ of the other powertrain module  10  extend from the corresponding casing  11  in the opposite direction, as can be seen in  FIG. 13 . Consequently, the assembly comprising the two powertrain modules  10  is included, along the transverse direction Y, between the casing  11  of one powertrain module  10  and the casing  11  of the other powertrain module  10 . This enhances the overall compactness in the transverse direction Y. 
     On the other hand, the powertrain module  10 —for example the casing  11 —may comprise connecting means  15  for cooperating with the connecting means  15  of an identical powertrain module  10 , for mechanically connecting said two powertrain modules  10 . Owing to this feature, the assembly comprising the two casings  11  forms a unit which is capable of handling gear reduction forces/torques. Furthermore, this provides a fairly compact design. The connecting means  15  of said two powertrain modules  10  may be configured to cooperate by engagement. 
     In the embodiment illustrated in the figures, the connecting means  15  comprise a dog clutch connection. However, other connecting means can be implemented, provided they ensure an appropriate mechanical connection, such as splines, teeth, etc. 
     According to another embodiment, illustrated in  FIG. 11 , the two powertrain modules  10  can share one and the same casing  11 , made as a single body. Such a one-piece casing  11  includes: 
     a left casing portion  11   a  for a left powertrain system  12 , comprising at least one area  17  housing a first epicyclic gear train  100  connected to a motor  5  and an area  18  housing a second epicyclic gear train  200  of said left powertrain system  12 ; 
     a right casing portion  11   b  for a right powertrain system  12 , comprising at least one area  17  housing a first epicyclic gear train  100  connected to a motor  5  and an area  18  housing a second epicyclic gear train  200  of said right powertrain system  12 . 
     The areas  18  housing the second epicyclic gear trains  200  of the left and right powertrain systems  12  can be arranged one as the extension of the other, for example as one and the same cylindrical portion having an axis coincident with the axis A 200  of the second epicyclic gear train  200 . 
     The powertrain module  10  of the invention can be implemented on a vehicle  1  having an independent wheel suspension configuration, as illustrated in  FIGS. 12 and 13 . 
     In an independent wheel suspension configuration, the connection between one wheel  3  and one powertrain module  10  is made by means of a connection device including a driveshaft  4 , at least one joint, at least one lower arm articulated at both ends and preferably at least one upper arm articulated at both ends (not shown in the figures). 
     With a torque vectoring solution, as illustrated in  FIGS. 12 and 13 , the driven wheel system  2  comprises two powertrain modules  10 , each of the left wheel(s)  3   a  and the right wheel(s)  3   b  being connected to a dedicated powertrain module  10  by means of a dedicated connection device, as described above. One drive shaft  4   a,    4   b  is rotated by the corresponding planet carrier  203 . The powertrain modules  10  can be assembled as shown on  FIG. 10 , or through a casing  11  made as a single body, as shown on  FIG. 11 . 
     According to an alternative not shown, the differential effect is achieved through a mechanical differential. Then, the driven wheel system  2  comprises a single powertrain module  10 . A first wheel  3 ,  3   a  is connected to the powertrain module  10  by means of a first connection device (as described above). The second wheel  3 ,  3   b  is connected to the powertrain module  10  by means of a second connection device. A differential unit links the drive shaft  4 ,  4   b  of the second wheel  3 ,  3   b  to the drive shaft  4 ,  4   a  of the first wheel  3 ,  3   a.  The differential housed in the differential unit is driven by the planet carrier  203  of the powertrain module  10  and then rotates the drive shafts  4   a,    4   b.    
     It has to be noted that, although the powertrain modules  10  in  FIGS. 12 and 13  include two motors  5 ,  5 ′, this should not be considered as limitative. 
     According to a variant, the powertrain module  10  of the invention can be implemented on a vehicle  1  having a rigid axle configuration, as illustrated in  FIG. 14 . The driven wheel system  2  then forms an axle and comprises: 
     a first axle housing  7   a  secured to one side of the casing  11  of one powertrain module  10 , preferably opposite the motor(s)  5 ,  5 ′, and receiving a first drive shaft  4   a  connected to a first wheel  3   a  of the driven wheel system  2 ; 
     a second axle housing  7   b  receiving a second drive shaft  4   b  connected to a second wheel  3   b  of the driven wheel system  2 . 
     With a torque vectoring solution, as illustrated in  FIG. 14 , the second axle housing  7   b  can be secured to a casing  11  housing the transmission system  13  of another powertrain system  10 . The powertrain modules  10  can be assembled as shown on  FIG. 10 , two separate casings  11  being provided, or through a casing  11  made as a single body, as shown on  FIG. 11 . 
     According to an alternative not shown, the differential effect is achieved through a mechanical differential. Then, the second axle housing  7   b  can be secured to an opposite side of the same casing  11 , the second axle housing  7   b  thus also receiving the differential. 
     It has to be noted that, although the powertrain modules  10  in  FIG. 14  include two motors  5 ,  5 ′, this should not be considered as limitative. 
     As can be seen on  FIGS. 12 to 14 , the powertrain modules  10  are substantially arranged in a virtual cylindrical transverse space defined by the wheels  3  of the driven wheel system  2 , which makes the arrangement fairly compact. 
     The invention therefore provides a compact powertrain system which however is modular and/or scalable, as it is based on a “basic” powertrain module: 
     which can be upgraded with at least one additional motor; 
     which can be assembled in different ways and/or quantities, to fit the customer needs. For example, the powertrain module can be duplicated to pass from a 4×2 to a 6×4 solution. 
     Having a unitary modular powertrain module which can be used on various uses/applications/ranges makes it possible: 
     to have a high volume and low cost parts, i.e. a cost efficient solution; 
     to manufacture and assemble the module at reasonable investment level; 
     to automatize the manufacturing and assembly; 
     to reduce the parts to manage and maintain 
     It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.