Patent Publication Number: US-2023150357-A1

Title: Vehicle system with multiple electric drive axles

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of U.S. Non-Provisional Patent Application No. 16/795,263, entitled “VEHICLE SYSTEM WITH MULTIPLE ELECTRIC DRIVE AXLES”, and filed on Feb. 19, 2020. The entire contents of the above-listed application are hereby incorporated by reference for all purposes. 
    
    
     FIELD 
     The present description relates generally to electric drive axles in vehicles, and more particularly to a vehicle system including multiple electric drive axles. 
     BACKGROUND 
     Electrified axles have been incorporated into electric as well as hybrid vehicles to generate and/or augment vehicle propulsion. Drive axles have been provided in both the front and rear of the vehicles. For example, in certain vehicle designs, one drive axle receives rotational energy from an engine and another drive axle receives rotational energy from an electric motor. Other axle systems have utilized electric motors coupled to each drive wheel to provide four wheel drive operation. 
     However, the inventors have recognized that using multiple electrified axles (e.g., a front and rear electric drive axle) in a vehicle may present challenges with regard to manufacturing costs, vehicle handling, axle structural integrity, as well as drive axle control. For example, providing a drivetrain with a separate electric motor for each drive wheel may significantly increase the drivetrain&#39;s manufacturing costs. The inventors have also recognized that cost reductions may be achieved by using similar components in multiple vehicle drive axles. However, vehicle packaging may, in certain cases, impede an identical electrified axle housing from being used in both the front and rear of the vehicle. Furthermore, packaging constraints may lead to the re-orientation of the front and rear drive axles, in some instances, presenting motor control issues with regard to coordinated control of the different motors in the drivetrain. 
     SUMMARY 
     To overcome at least some of the aforementioned drawbacks, a vehicle system is provided. In one example, the vehicle system includes a first electric drive axle assembly with a first gear train having a first planetary gear set axially offset from a first electric motor-generator, where the first planetary gear set is rotationally coupled to a first differential. Further, the vehicle system has a second axle assembly with a second gear train having a second planetary gear set axially offset from a second electric motor-generator, where the second planetary gear set is rotationally coupled to a second differential. Arranging the gears in this manner allows the first and second electric drive axle assemblies to achieve a space efficient arrangement. Consequently, the axles may be less susceptible to damage from road debris, obstacles, etc. 
     In another example, the first gear train may include multiple selectable gear sets rotationally coupled to the first planetary gear set and the second gear train includes multiple selectable gear sets rotationally coupled to the second planetary gear set. At least a portion of the gears in the selectable gears sets in the first gear train may have a substantially equivalent size in relation to corresponding gears in the second gear train. Using similar gear sizes in each of the drive axles allows the manufacturing cost of the drive axle system to be reduced. 
     In yet another example, the first and second drive axle assemblies may be oriented such that the first and second planetary gear sets are positioned inboard or outboard from the first and second electric motor-generators, correspondingly. In this example, the electric motor-generators may be controlled to rotate in opposite directions during forward or reverse drive. In this way, front-rear motor control is coordinated to implement forward or reverse vehicle drive modes. 
     It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic representation of a vehicle including a vehicle system. 
         FIG.  2    shows a perspective view of an example of a vehicle system including multiple electric drive axle assemblies. 
         FIG.  3    shows a top-down view of a first arrangement of the front and rear electric drive axle assemblies. 
         FIG.  4    shows a top-down view of a second arrangement of the front and rear electric drive axle assemblies. 
         FIG.  5    shows a top-down view of a third arrangement of the front and rear electric drive axle assemblies. 
         FIG.  6    shows a top-down view of a fourth arrangement of the front and rear electric drive axle assemblies. 
         FIG.  7    shows a top-down view of a fifth arrangement of the front and rear electric drive axle assemblies. 
         FIG.  8    shows a top-down view of a sixth arrangement of the front and rear electric drive axle assemblies. 
         FIG.  9    shows an example of a routine for operating a vehicle equipped with electric drive axle assemblies which may be any of the arrangements of  FIGS.  3 - 8   . 
     
    
    
     DETAILED DESCRIPTION 
     A vehicle system with two electric drive axles is described herein. Different features of the vehicle system allow the system to achieve a compact design which may be less expensive to manufacture than previous electrified axles. To elaborate, in one example, the orientation of a front electric drive axle relative to a rear electric drive axle may be selected to allow substantially equivalent gears in the drive axles to be used while also meeting packaging constraints in the vehicle. For instance, planetary gear sets in each of the axles may be positioned inboard or outboard in relation to the electric motor-generators. In one example, the front axle housing may mirror the rear axle housing but the direction of motor rotation in the front and rear electric motors may be equivalent during forward or reverse drive. In this way, the electric motors can run in the same direction so the drive and coast flanks of the gears can be improved for forward drive, regeneration, and reverse modes. However, in such an example, in production, the front and rear axle constituents may need separate part numbers and/or identifying markers. In another example, the front axle housing and the rear axle housing may have a similar geometric profile but the electric motors in each axle may be positioned inboard or outboard with regard to a central section of the vehicle. In such an example, the direction of motor rotation in the front and rear electric motors may be opposite one another, during forward or reverse drive. In this way, motor control may be coordinated to enable forward and reverse drive operation and allow the housing of each axle to have an equivalent geometry to reduce system manufacturing costs. 
       FIG.  1    schematically illustrates a vehicle with a vehicle system designed with multiple gear ratios. An example of the vehicle system, including a front axle assembly and a rear axle assembly, is shown in  FIG.  2    from a perspective view. Various orientations of the front and rear axle assemblies are illustrated in  FIGS.  3 - 8   . An example of a routine for operating the electric drive axle assemblies which may be implemented in the vehicle is depicted in  FIG.  9   . 
       FIG.  1    shows a schematic depiction of a vehicle  100  having a vehicle system  102  with an electric drive axle assembly  103  including a gear train  104  and an electric motor-generator  106 . The stick diagram of  FIG.  1    provides a high-level topology of the vehicle, gear train, and corresponding components. However, it will be understood that the vehicle, gear train, and corresponding components have greater structural complexity than is captured in  FIG.  1   . The structural details of various facets of the vehicle system  102  are illustrated, by way of example, in greater detail herein with regard to  FIGS.  2 - 8   . 
     The electric motor-generator  106  is electrically coupled to an energy storage device  108  (e.g., battery, capacitor, and the like). Arrows  109  signify the energy transfer between the electric motor-generator  106  and the energy storage device  108  that may occur during different modes of system operation. The electric motor-generator  106  may include conventional components for generating rotational output (e.g., forward and reverse drive rotational output) and/or electrical energy for recharging the energy storage device  108  such as a rotor electromagnetically interacting with a stator, to provide the aforementioned energy transfer functionality. The electric motor-generator  106  is shown including a rotor shaft  180  with a first bearing  181  and a second bearing  182  coupled thereto. The first bearing  181  may be a fixed bearing and the second bearing  182  may be a floating bearing. Although the second bearing  182  is shown positioned within the motor-generator, it will be understood that in some embodiments, bearing  182  may be coupled to the input shaft to facilitate rotation thereof. Other bearing arrangements with regard to the motor-generator have been contemplated such as arrangements with alternate quantities and/or types of bearings. 
     The vehicle may take a variety of forms in different embodiments. For example, the vehicle  100  may be hybrid vehicle where both the electric motor-generator  106  and an internal combustion engine (not shown) are utilized for motive power generation. For instance, in one use-case hybrid vehicle configuration, the internal combustion engine may assist in recharging the energy storage device  108 , during certain conditions. In another use-case hybrid vehicle configuration, the internal combustion engine may be configured to provide rotational energy to a differential  110  or other suitable locations in the gear train  104 . Further, in other examples, the vehicle may be a battery electric vehicle (BEV) where the internal combustion engine is omitted. 
     The rotor shaft  180  of the electric motor-generator  106  is coupled to an input shaft  112 . For instance, the rotor shaft  180  may be transition fit, slip fit, mechanically attached, in splined engagement, combinations thereof, etc., with an end of the input shaft  112 . A first gear  114  is positioned or formed on the input shaft  112 . A bearing  183  is shown coupled to the input shaft  112 . The bearing  183  may be a fixed bearing, in one example. However, in other examples, the bearing  183  may be another suitable type of bearing or in some cases may be omitted from the system. 
     A second gear  116  is rotationally coupled to the first gear  114  and resides on an intermediate shaft  118 . As described herein, rotational coupling between gears or other components may include an interface between the gears where teeth of the gears mesh to facilitate rotational energy transfer therebetween. As such, rotational coupling of the components allows rotational energy transfer to be transferred between the corresponding components. Conversely, rotational decoupling may include a state between two components when rotational energy is substantially inhibited from being transferred between the components. 
     A third gear  120  and a fourth gear  122  are additionally included on the intermediate shaft  118 , although other gearing arrangements have been envisioned. Bearings  184  (e.g., tapered roller bearings) are coupled to either axial end of the intermediate shaft  118  to support the shaft and facilitate rotation thereof. The tapered roller bearings may decrease the axle package width when compared to other types of bearing such as ball bearings. However, other suitable intermediate shaft bearing types and/or arrangements have been envisioned. The bearing arrangement on the intermediate shaft as well as the other bearing arrangements described herein may be selected based on expected shaft loading (e.g., radial and thrust loading), gear size, shaft size, etc. 
     Continuing with the gear train description, the fourth gear  122  is rotationally coupled to a fifth gear  124  and the third gear  120  is rotationally coupled to a sixth gear  126 . The first gear  114 , the second gear  116 , the third gear  120 , the fourth gear  122 , the fifth gear  124 , and the sixth gear  126  are included in a gear assembly  130 , in the illustrated embodiment. However, the gear assembly may include an alternate number of gears and/or have a different layout, in other embodiments. The number of gears in the assembly and the assembly layout may be selected based on end-use design goals related to desired gear range and packaging, for instance. 
     The first gear  114 , the second gear  116 , the fourth gear  122 , and the fifth gear  124 , may be included in a first gear set  127 . Additionally, the first gear  114 , the second gear  116 , third gear  120 , and the sixth gear  126 , may be included in a second gear set  129 . The first gear set  127  may have a higher gear ratio than the second gear set  129 , in one example. However, other gear arrangements in the different gear sets may be used, in other examples. Clutch assemblies in the system  102  allow the first gear set  127  or the second gear set  129  to be placed in an operational state. To elaborate, the clutch assemblies allow the gear ratio delivered to drive wheels  128  on driving surfaces  133 , by way of the gear assembly  130 , a planetary gear assembly  138 , and the differential  110 , to be adjusted. For instance, the clutch assemblies may be operated to engage the first gear set  127 , during certain conditions (e.g., towing, lower speed vehicle operation, etc.), and engage the second gear set  129 , during other conditions (e.g., higher speed vehicle operation). As such, the system may transition between the different gear sets based on vehicle operating conditions, driver input, etc. In this way, the gear train has distinct selectable gear ratios, allowing the gear train to be adapted for different driving conditions, as desired. It will be appreciated that the gear ratio adjustability may also be utilized to increase electric motor efficiency, in some cases. 
     The system  102  may specifically include a first clutch assembly  132  and a second clutch assembly  134 . The first clutch assembly  132  is configured to rotationally couple and decouple the fifth gear  124  from an output shaft  136 . Likewise, the second clutch assembly  134  functions to rotationally couple and decouple the sixth gear  126  from the output shaft  136 . The first clutch assembly  132  may include a one-way clutch  185  (e.g., sprag clutch) and a locking clutch  186  working in conjunction to accomplish the coupling/decoupling functionality, in a compact arrangement. However, other clutch designs have been contemplated, such as a friction clutch (e.g., wet friction clutch), a hydraulic clutch, an electromagnetic clutch, and the like. 
     The one-way clutch  185  may be designed to transfer rotational energy from the fifth gear  124  to the output shaft  136  when the fifth gear is rotating in the forward drive direction and the rotational speed exceeds that speed of the output shaft. Conversely, when the fifth gear is rotating in the reverse drive direction of the output shaft speed is greater than the fifth gear speed, the one-way clutch freewheels, allowing the fifth gear and the output shaft to be rotationally decoupled. Furthermore, the locking clutch  186  may be configured to rotationally couple and decouple the fifth gear from the output shaft. For instance, the locking clutch may be activated during a reverse drive mode or during a regeneration mode. 
     The second clutch assembly  134  may be a wet friction clutch providing smooth engagement/disengagement between the sixth gear  126  and the output shaft  136 , in one embodiment. However, in other examples, the second clutch assembly  134  may include additional or alternate types of suitable clutches (e.g., hydraulic, electromagnetic, etc.). 
     The output shaft  136  is rotationally coupled to the planetary gear assembly  138 , in the illustrated embodiment. The planetary gear assembly  138  may include an annulus  187  also referred to as a ring gear, a carrier  188  with planet gears  189  mounted thereon, and a sun gear  190  providing a space efficient design capable of providing a relatively high gear ratio in comparison to non-planetary arrangements. However, non-planetary gear layouts may be used in the system, in certain embodiments, when for example, space efficient packaging is less favored. In the illustrated embodiment, the sun gear  190  is rotationally coupled to the output shaft  136  and the carrier  188  is rotationally coupled to the differential  110  (e.g., a differential case). However, in alternate examples, different gears in the planetary assembly may be rotationally coupled to the output shaft and the differential. Further, in one example, the components of the planetary gear assembly  138  may be non-adjustable with regard to the components that are held stationary and allowed to rotate. Thus, in one-use case example, the annulus  187  may be held substantially stationary and the carrier  188 , planet gears  189 , and the sun gear  190  and the gears stationary/rotational state may remain unchanged during gear train operation. In the illustrated embodiment, the annulus  187  is fixedly coupled to the motor-generator housing, to increase system space efficiency. However, the annulus may be fixedly coupled to other vehicle structures, in other instances. By using a non-adjustable planetary assembly, gear train operation may be simplified when compared to planetary arrangements with gears having rotational state adjustability. However, adjustable planetary arrangements may be used in the system, in other embodiments. 
     Various bearings may be coupled to the output shaft  136  and the planetary gear assembly  138  to enable rotation of components coupled to the shaft and assembly and in some cases support the components with regard to radial and/or thrust loads. A bearing  191  (e.g., needle roller bearing) is shown coupled to the output shaft  136  and the second clutch assembly  134 . Additionally, a bearing  192  (e.g., tapered roller bearing) is shown coupled to the second clutch assembly  134 . A bearing  193  (e.g., floating bearing) is also shown coupled to the second clutch assembly  134  and the output shaft  136 . A bearing  194  (e.g., thrust bearing) may also be positioned axially between and coupled to the sixth gear  126  and the first clutch assembly  132 . A bearing  196  (e.g., fixed bearing) may also be coupled to the one-way clutch  185 . Additionally, a bearing  197  (e.g., ball bearing) is shown coupled to the planetary gear assembly  138  and a bearing  198  (e.g., ball bearing) is shown coupled to the differential case  142 . However, other suitable bearing arrangements have been contemplated, such as arrangements where the quantity and/or configurations of the bearings are varied. 
     Additionally,  FIG.  1    depicts the planetary gear assembly  138  directly rotationally coupled to the differential  110 . Directly coupling the planetary gear assembly to the differential increases system compactness and simplifies system architecture. In other examples, however, intermediate gearing may be provided between the planetary gear assembly and the differential. In turn, the differential  110  is designed to rotationally drive an axle  140  coupled to the drive wheels  128 . The axle  140  is shown including a first shaft section  141  and a second shaft section  143  coupled to different drive wheels  128 . Furthermore, the axle  140  is shown arranged within (e.g., co-axial with) the output shaft  136  which allows more space efficient design to be achieved. However, offset axle-output shaft arrangements may be used, in other examples. 
     Further in one example, the axle  140  may be a beam axle. A beam axle, also referred to in the art as a solid axle or rigid axle, may be an axle with mechanical components structurally supporting one another and extending between drive wheels coupled to the axle. Thus, wheels coupled to the axle may move in unison when articulating, during, for example, vehicle travel on uneven road surfaces. For instance, the beam axle may be a structurally continuous axle spanning the drive wheels on a lateral axis, in one embodiment. In another embodiment, the beam axle may include co-axial shafts receiving rotational input from different gears in the differential and structurally supported by the differential. 
     The differential  110  may include a case  142  housing gearing such as pinion gears, side gears, etc., to achieve the aforementioned energy transfer functionality. To elaborate, the differential  110  may be an electronic locking differential, in one example. In another example, the differential  110  may be an electronic limited slip differential or a torque vectoring dual clutch. In yet other examples, an open differential may be used. Referring to the locking differential example, when unlocked, the locking differential may allow the two drive wheels to spin at different speeds and conversely, when locked, the locking differential may force the drive wheels to rotate at the same speed. In this way, the gear train configuration can be adapted to increase traction, under certain driving conditions. In the case of the limited slip differential, the differential allows the deviation of the speed between shafts  144  coupled to the drive wheels  128  to be constrained. Consequently, traction under certain road conditions (e.g., low traction conditions such as icy conditions, wet conditions, muddy conditions, etc.) may be increased due to the wheel speed deviation constraint. Additionally, in the torque vectoring dual clutch example, the differential may allow for torque delivered to the drive wheels to be independently and more granularly adjusted to again increase traction during certain driving conditions. The torque vectoring dual clutch may therefore provide greater wheel speed/torque control but may, in some cases, be more complex than the locking or limited slip differentials. 
     The vehicle  100  may also include a control system  150  with a controller  152 . In some examples, as described herein with reference to  FIGS.  3 - 9   , the vehicle system  102  may include more than one electric drive axle assembly. As such, the electric drive axle assembly  103  including the gear train  104 , electric motor-generator  106 , etc., may be a rear electric drive axle assembly, in one use-case. In such a use-case, a similar electric drive axle assembly may be provided at the front axle which includes a similar gear train, electric motor-generator, etc. Thus, in some embodiments, the front and rear electric drive axle assemblies may each include gear trains with similar gear ratios and clutches enabling gear ratio selection functionality. Both the first and second electric drive axle assemblies may be controlled by the control system  150  and controller  152 . In addition, in some examples, the motor-generator of each drive axle may be coupled to the energy storage device  108 . In other examples, each motor-generator may be coupled to an individual energy storage device. 
     The controller  152  includes a processor  154  and memory  156 . The memory  156  may hold instructions stored therein that when executed by the processor cause the controller  152  to perform the various methods, control techniques, etc., described herein. The processor  154  may include a microprocessor unit and/or other types of circuits. The memory  156  may include known data storage mediums such as random access memory, read only memory, keep alive memory, combinations thereof, etc. Furthermore, it will also be understood that the memory  156  may include non-transitory memory. 
     The controller  152  may receive various signals from sensors  158  coupled to various locations in the vehicle  100  and the vehicle system  102 . The sensors may include a motor-generator speed sensor  160 , an energy storage device temperature sensor  162 , an energy storage device state of charge sensor  164 , wheel speed sensors  166 , clutch position sensors  168 , etc. The controller  152  may also send control signals to various actuators  170  coupled at different locations in the vehicle  100  and the vehicle system  102 . For instance, the controller  152  may send signals to the electric motor-generator  106  and the energy storage device  108  to adjust the rotational speed and/or direction (e.g., forward drive rotational direction and reverse drive rotational direction) of the motor-generator. The controller  152  may also send signals to the first clutch assembly  132  and the second clutch assembly  134  to adjust the operational gear ratio in the gear train  104 . For instance, the first clutch assembly  132  may be disengaged and the second clutch assembly  134  may be engaged to place the second gear set  129  in an operational state (transferring rotational energy between the electric motor-generator  106  and the output shaft  136 ). The other controllable components in the vehicle and gear system may function in a similar manner with regard to command signals and actuator adjustment. For instance, the differential  110  may receive command signals from the controller  152 . 
     The vehicle  100  may also include an input device  172  (e.g., a gear selector such as a gear stick, gear lever, etc., console instrument panel, touch interface, touch panel, keyboard, combinations thereof, etc.) The input device  172 , responsive to driver input, may generate a mode request indicating a desired operating mode for the gear train. For instance, in a use-case example, the driver may shift a gear selector into a gear mode (e.g., first gear mode or second gear mode) to generate a gear set modal transition request at the controller. In response, the controller commands gear train components (e.g., the first clutch assembly  132  and the second clutch assembly  134 ) to initiate a transition into a first gear mode, where the first gear set  127  is operational, from a second gear mode, where the second gear set  129  is operational, or vice versa. The controller  152  may also be configured to transition the vehicle system  102  into a regenerative mode. In the regenerative mode energy is extracted from the gear train using the electric motor-generator  106  and transferred to the energy storage device  108 . For instance, the electric motor-generator  106  may be placed in a generator mode configured to convert at least a portion of the rotational energy transferred from the drive wheels to the generator by way of the gear train into electrical energy. It will be appreciated that the other electric drive axle assembly included in the vehicle may be operated in similar modalities, in some embodiments. 
       FIG.  2    shows a vehicle system  200  of a vehicle  210 . It will be appreciated that the vehicle system  200 , shown in  FIG.  2   , serves as an example of the vehicle system  102  shown in  FIG.  1   . As such, at least a portion of the functional and structural features of the vehicle system  102  shown in  FIG.  1    may be embodied in the vehicle system  200  shown in  FIG.  2    or vice versa, in certain embodiments. 
     The vehicle system  200  includes a first electric drive axle assembly  202  and a second electric drive axle assembly  204 . A set of reference axes  201  are provided for reference in  FIGS.  2 - 8   , indicating a y-axis, an x-axis, and a z-axis. The z-axis may be a longitudinal axis, the x-axis may be a lateral axis, and/or the y-axis may be a vertical axis, in one example. However, the axes may have other orientations, in other examples. 
     As one example, the first electric drive axle assembly  202  may be a front electric drive axle of the vehicle  210  and the second electric drive axle assembly  204  may be a rear electric drive axle of the vehicle  210 . In one example, the front axle may be a beam axle coupled to front drive wheels  203  of the vehicle  210  and the rear axle may also be a beam axle coupled to rear drive wheels  205  of the vehicle  210 . Therefore, in such an example, the front electric drive axle assembly  202  may be arranged proximate to a front end  206  of the vehicle  210  and the rear electric drive axle assembly  204  may be arranged proximate to a rear end  208  of the vehicle  210 . 
     Components of the front electric drive axle assembly  202  will now be described. It will be appreciated that, although positioned in a different orientation, the rear electric drive axle assembly  204  may be similarly configured to the front electric drive axle assembly  202 , with regard to internal componentry such as gearing, clutches, electric motor generators, etc.) and as such, description of the components of the front electric drive axle assembly  202  may be applicable to components of the rear electric drive axle assembly  204 . Details of the relative orientations of the front electric drive axle assembly  202  and the rear electric drive axle assembly  204  are provided further below with reference to  FIGS.  3 - 8   . 
     The front electric drive axle assembly  202  includes a first electric motor-generator with a rotor shaft  213  connected to a first input shaft  214 . The first electric motor-generator is omitted in  FIG.  2    for but depicted in  FIGS.  3 - 8   . It will be appreciated that other suitable electrical interfaces other than the electrical interface shown in  FIGS.  3 - 8    may be used, in other examples. The first motor-generator may be coupled to a first gear train  212  which may include the first input shaft  214 , a first intermediate shaft  216 , and a first output shaft, obscured from view in  FIG.  2   . 
     The first input shaft  214  receives rotational input (forward or reverse drive rotation) from the rotor shaft  213  of the first electric motor-generator, while the system is operating in forward and reverse drive modes. The first input shaft  214  is rotationally coupled to the first intermediate shaft  216  and the first output shaft by way of the first gear train  212 . Rotational axes  222 ,  224 , and  226  of the first input shaft  214 , the first intermediate shaft  216 , and the first output shaft, respectively, are provided for reference in  FIG.  2   .  FIG.  2    additionally shows a first planetary gear assembly  228  rotationally coupled to a first differential  230  in the first gear train  212 . It will be appreciated that placing the first planetary gear assembly  228  next to the first differential  224  allows less torque to be carried through the first gear train  212 , enabling the drive train to have fewer and/or smaller components, if wanted. 
     The first planetary gear assembly  228  can achieve a targeted gear ratio (e.g., a relatively high gear ratio, such as a ratio greater than 20:1, in one use-case) in a compact arrangement relative to non-planetary gear arrangements. Thus, the planetary gear assembly can achieve a desired gear ratio with less components (e.g., gears and shafts) than non-planetary gear assemblies, if desired. Furthermore, in embodiments where the planetary gear assembly exhibits a relatively high torque output, the planetary assembly can attain a more compact packaging due to the load sharing between the planet gears, if desired. 
     The first gear train  212  includes a gear assembly  289  that may include six gears (a first gear  290 , a second gear  291 , a third gear (obscured from view), a fourth gear  292 , a fifth gear  293 , and a sixth gear  294 ), where the second, third, and fourth gears are coupled to the first intermediate shaft  216 , as described above with regard to  FIG.  1   . The fifth and sixth gears are coupled to the first output shaft. It will be understood, that during different modes of system operation different sets of gears may be operational. The first, second, fourth, and fifth gears may be included in a first gear set and the first gear, second, third, and sixth gears may be included in a second gear set, as described above for  FIG.  1   . A park gear may also be included in the first gear train  212 , in some examples. However, the gear sets may include different gear combinations, in other examples. 
     It will also be understood that the first and the second gear sets have different gear ratios. In this way, the gear train may include multiple gear ratios to increase gear train adaptability. Additionally, the gear sets may share a few common gears (i.e., the first and second gears in the illustrated embodiment). Fixing the first ratio (i.e., the first and second gears) in the gear train can allow the accuracy of the gears to be increased, if wanted, thereby reducing noise, vibration, and harshness (NVH) in the system. However, embodiments where the gear sets do not include overlapping gears have been envisioned. Clutch assemblies, which may be similar to the clutch assemblies  132  and  134  shown in  FIG.  1   , are included in the first gear train  212  to enable the first gear set and the second gear set to be coupled/decoupled to/from the first output shaft. In this way, the different gear sets may be operationally selected to, for example, more aptly suite the driving environment and/or increase electric motor efficiency. Thus, the first and second gear sets may be conceptually included in a selectable gear assembly. 
     The first planetary gear assembly  228  is rotationally coupled to the first output shaft and the first differential  230  in the first gear train  212  is rotationally coupled to the first planetary gear assembly  228 . However, other gear layouts may be used in other examples, such as non-planetary gear assemblies, gear trains with gears positioned between the planetary assembly and the differential, etc. The first planetary gear assembly  228  allows a desired gear ratio to be realized in a compact arrangement. For instance, the first planetary gear assembly  228  may achieve a relatively high gear ratio and space efficiency, if desired. However, non-planetary gear arrangements may be used, in other examples. 
     Furthermore, the first planetary gear assembly  228  and the first differential  230  are shown positioned on a lateral side relative to the first motor-generator (e.g., where the first motor-generator is coupled to the first input shaft  214  and centered about the rotational axis  222  of the first input shaft  214 ). In other words, the first planetary gear assembly  228  and the first differential  230  are positioned on lateral side of the first motor-generator along the z-axis. More specifically, the first planetary gear assembly  228  and the first differential  230  are axially offset from the rotor shaft  313  of the first motor-generator. Therefore, the rotational axis  222  of the rotor shaft  213  is not co-axial with the rotational axis  226  of the output shaft, the first planetary gear assembly  228 , and the first differential  230 . It will be appreciated that the first planetary gear assembly  228  may be axially offset from the first motor-generator due to the planetary gear assembly&#39;s ability to be integrated into the gear train without a mating gear parallel thereto, if wanted. In this way, the planetary gear assembly may be placed in a space which has remained unused in certain electrified gearboxes. Thus, positioning the planetary gear assembly on the side of the motor allows the compactness of the axle system to be increased. As a result, the packaging constraints arising during axle installation in the vehicle may pose less of an issue. However, in other examples, the first planetary gear assembly  228  may be positioned in other suitable locations. 
     As described above, the vehicle system  200  includes two electric drive axle assemblies. In the illustrated embodiment, each electric drive axle assembly has substantially similar gear components (e.g., gear shafts, gears, and/or clutches). To elaborate, the gears in the front electric drive axle assembly  202  and the second electric drive axle assembly  204  may include individual gears in their respective gear trains that are substantially equivalent in size and/or profile. For instance, equivalent gears may have a similar inner diameter, outer diameter, width, and/or tooth pattern. Thus, the gears may be jointly manufactured to allow for reductions in vehicle manufacturing costs. However, in other examples, only a portion of the gears and/or other components in each drive axle may have a substantially equivalent size and/or profile. 
     Therefore, as shown in  FIG.  2   , the rear electric drive axle assembly  204  also includes a second gear train  250 , a second input shaft  252 , a second intermediate shaft  254  and a second output shaft  256 . A second motor-generator with a rotor shaft  257  may be connected to the second input shaft  252 . The second motor-generator may be axially (e.g., along the x-axis) offset from a second planetary gear assembly  258  and a second differential  260  of the second gear train  250 . Furthermore, the gears in the second gear train  250  are shown including six gears (a first gear  295 , a second gear  296 , a third gear  297 , a fourth gear (obscured from view), a fifth gear  298 , and a sixth gear  299 ). However, as mentioned above, the gear trains may have an alternate number of gears, in other embodiments. 
     Components of the rear electric drive axle assembly  204  may be oriented in a similar, or equivalent, arrangement relative to one another as the front electric drive axle assembly  202 . However, relative to the front end  206  and rear end  208  of the vehicle  210 , the rear electric drive axle assembly  204  and the front electric drive axle assembly  202  may each be positioned such that their respective motor generator rotor shafts ( 213  and  257 ) are positioned outboard with regard to a central vehicle region  259 . Thus, in such an example, the orientations of the rear electric drive axle assembly  204  and the front electric drive axle assembly  202 , shown in  FIG.  2   , are rotated 180° about the y-axis with regard to one another. Although, the vehicle  210  is shown in an arrangement where the rotor shafts are positioned in an outboard location, it will be appreciated that the front and rear electric drive axle assemblies  202  and  204  may be positioned in different orientations, in other embodiments.  FIGS.  3 - 8    depict different possible orientations of front and rear drive axles in the vehicle system  200  with the drive axle&#39;s componentry enclosed in outer housings. It will also be understood, that the vehicle system  200  and the other vehicle system arrangements described herein may be controlled by a suitable controller, such as the controller  152 , shown in  FIG.  1   . 
       FIG.  3    shows a first vehicle system arrangement  300 . As shown in  FIG.  3   , the first vehicle system arrangement  300  includes a front electric drive axle assembly  302 , providing torque to front drive wheels  303  of the vehicle  210 , and a rear electric drive axle assembly  304 , providing torque to rear drive wheels  305 . As previously discussed, each electric drive axle assembly may include axles  390 ,  392  with axle shafts coupled to a differential and the drive wheels. 
     In the illustrated embodiment, the rear electric drive axle assembly  304  is rotated 180° about the y-axis relative to the front electric drive axle assembly  302  so that a first motor-generator  306  of the front electric drive axle assembly  302  and a second motor-generator  308  of the rear electric drive axle assembly  304  are positioned in an outboard configuration. In the outboard configuration of the first vehicle system arrangement  300 , each of the first motor-generator  306  and the second motor-generator  308  are spaced away from a central region  310  of the vehicle  210 . The central region  310 , may be longitudinally bounded, in one example, by front and rear axles  390  and  392  extending between the front and rear drive wheels  303 ,  305 , respectively. 
     The front electric drive axle assembly  302  is depicted with a first outer housing  312  and the rear electric drive axle assembly  304  is depicted with a second outer housing  314 . The first and second outer housings  312 ,  314  may enclose the first and second motor-generators  306 ,  308  as well as a first gear train (e.g., the first gear train  212 , shown in  FIG.  2   ) and a second gear train (e.g., the second gear train  250 , shown in  FIG.  2   ), respectively. The first and second outer housings  312 ,  314  may be similarly configured and the following description of the outer housing is applicable to both. 
     Each of the first and second outer housings  312 ,  314  may include a first section  350 , a second section  352 , a third section  354 , and a fourth section  356 . It will be appreciated that each of the front and rear drive axle assemblies shown in  FIGS.  4 - 8    include an outer housing having similar sections to the first and outer housing  312 ,  314 , described for  FIG.  3    and will not be re-introduced for brevity. However, the relative positions and orientations of the housing sections may vary between the different vehicle system arrangements, shown in  FIGS.  4 - 8   . Further, in some embodiments, the axle housings may have a mirrored profile from front to rear, as discussed in greater detail herein with regard to the vehicle system arrangements shown in  FIGS.  5 - 6   . 
     The sections of the outer housings may be configured to be removably coupled via fasteners, interference fitting, clamps, etc., to allow access to inner components of the vehicle system  200 . To elaborate, the first section  350  may be coupled to the second section  352  at an attachment interface  380  and the third section  354  may be coupled to the first section  350  at an attachment interface  382 . The first section  350  may at least partially enclose portions of the motor-generator, the differential (e.g., the first differential  230 , shown in  FIG.  2   ), and the gear train (e.g., the first gear train  212 , shown in  FIG.  2   ) of the axle assembly. The second section  352  may at least partially surround a portion of the gear train and, when coupled to the first section  350 . Additionally, the third section  354  may at least partially enclose and provide access to the planetary gear assembly (e.g., the first planetary gear assembly  228 , shown in  FIG.  2   ) and the differential (e.g., the first differential  230 , shown in  FIG.  2   ). 
     The first section  350  of the outer housing may include a first shaft opening  358 . It will be understood that a first section  394  of the rear axle  392  coupled to one of the rear drive wheels  305  (or one of the front drive wheels  303 ) of the vehicle  210  may extend through the first shaft opening  358 . The second section  352  of the outer housing may include a second shaft opening  360 . Again, a second section  396  of the rear axle  392 , coupled to one of the rear drive wheels  305  (or one of the front drive wheels  303 ), may extend through the second shaft opening  360 . Furthermore, it will be appreciated that the shaft openings in the axle housings may be arranged co-axial with the output shafts (e.g., output shaft  136 , shown in  FIG.  1   ) in the gear trains. 
     When assembled (e.g., all sections of the outer housing are coupled to one another) the outer housing provides an outer shell to the axle assembly and shields inner components of the axle assembly from contact with flying debris during vehicle navigation. 
     In the first vehicle system arrangement  300 , the front and rear axle assemblies  302 ,  304  are in the outboard configuration, as described above. In this arrangement, the first shaft opening  358  of the first outer housing  312  is proximate to a first vehicle frame section  370  of the vehicle  210  (e.g., on the left side of  FIG.  3   ), and the second shaft opening  360  of the first outer housing  312  is proximate to a second vehicle frame section  372  of the vehicle  210  (e.g., on the right side of  FIG.  3   ). Thus, the first and second housings may be interchangeable and manufactured based on a single prototype, thereby decreasing manufacturing costs, if desired. 
     Each of the first and second motor-generators  306 ,  308  also includes an electrical interface  307  protruding upwards from the motor-generators, along the y-axis. The electrical interface  307  allows electrical cables to be connected to the motor-generators. Although the gear trains in the front and rear electric drive axle assemblies  302 ,  304  may be similarly configured (e.g., components of each gear train are similarly aligned and positioned relative to one another) a drive direction of the gear trains may be different. For example, as shown in  FIG.  2   , a forward direction of the vehicle  210  is indicated by arrow  262 . Both the front drive wheels  203  and the rear drive wheels  205  rotate in a first direction, as indicated by arrows  264 , to enable forward navigation of the vehicle  210 . To facilitate the rotation of the front drive wheels  203  in the first direction for forward motion of the vehicle, the first input shaft  214  may be driven by the first motor-generator to rotate in a clockwise (cw) direction, as indicated by arrow  266 , when viewing the front electric drive axle assembly  202  along arrow  207 . The first intermediate shaft  216  may be driven by the first input shaft  214  to rotate in a counterclockwise (ccw) direction, as indicated by arrow  268 . Rotation of the first intermediate shaft  216  in the ccw direction may, in turn, drive rotation of the first output shaft  218  in the cw direction, as indicated by arrow  270 . As a result of the rotation of the first output shaft  218 , the drive wheel  203  propel the vehicle  210  in the forward direction, indicated via arrow  262 . 
     As shown in  FIG.  2   , at the rear axle of the vehicle  210 , in a forward drive mode, components of the rear electric drive axle assembly  204  may be configured to rotate in opposite directions in relation to components of the front electric drive axle assembly  202  due to the rotated configuration of the front and rear electric drive axles. For example, when viewing the rear electric drive axle assembly  204  along arrow  209 , the second input shaft  252  may be driven by the second motor-generator to rotate in the ccw direction, as indicated by arrow  270 , the second intermediate shaft  254  may rotate in the cw direction, as indicated by arrow  272 , and the second output shaft  256  may rotate in the ccw direction, as indicated by arrow  274 . As a result of the rotation of the second output shaft  256 , the drive wheel  205  propel the vehicle  210  in the forward direction, indicated via arrow  262 . 
     As such, the first vehicle system arrangement  300 , as shown in  FIG.  3   , the first motor-generator  306 , and the second motor-generator  308  may generate opposite rotational outputs to achieve both forward and reverse motion of the vehicle  210 . As such, a controller (e.g., the controller  152  of  FIG.  1   ) may be configured with instructions specific to the arrangement of the vehicle system  200  to accommodate the relative orientations of the front and rear electric drive axle assemblies. Thus, when the vehicle system  200  is configured in the first vehicle system arrangement  300 , the controller of the vehicle  210  may be adapted with executable instructions to operate the first motor-generator and the second motor-generator in opposite rotational directions to achieve forward and reverse vehicle drive modes. It will also be understood, that during regeneration modes in both the front and rear electric motor-generators the rotational input received by each motor-generator may be in an opposite rotational direction. 
     In the outboard orientation of the front and rear electric drive axle assemblies (e.g., where the first and second motor-generators are rotated away from the central region  310  of the vehicle  210 ) the arrangement of the motor-generators may allow vehicle components (e.g., battery banks, frame sections, etc.) to be efficiently located in the central region  310  with regard to space. Thus, by positioning the motor-generators away from the central region, motor-generator packaging may not interfere with the centrally located vehicle components. Furthermore, positioning the electrical interfaces  307  on upper sides of the motor-generators decreases their exposure to road debris, stationary objects, etc., decreasing the chance of damage to the electrical interfaces during vehicle operation. 
     In other instances, when space is available in the central region  310  of the vehicle  210 , the motor-generators may be rotated inwards in an inboard configuration. For example, as shown in  FIG.  4   , a front electric drive axle assembly  402  and a rear electric drive axle assembly  404  may oriented such that they are rotated 180° about the y-axis in relation to one another in a second vehicle system arrangement  400 . In the second vehicle system arrangement  400 , a first motor-generator  406  of the front electric drive axle assembly  402  and a second motor-generator  408  of the rear electric drive axle assembly  404  are positioned in an inboard configuration where the first and second motor-generators  406 ,  408  are in the central vehicle region  310 . As previously discussed, the central vehicle region  310  may be longitudinally bounded via the front and rear axles extending between the drive wheels  303  and the drive wheel  305 , respectively. 
     In the second vehicle system arrangement  400 , the first shaft opening  358  of a first outer housing  450  of the front electric drive axle assembly  402  may be proximate to the second vehicle frame section  372  and the second shaft opening  360  of the first outer housing  450  may be proximate to the first vehicle frame section  370 . The first shaft opening  358  of a second outer housing  452  of the rear electric drive axle assembly  404  may be proximate to the first vehicle frame section  370  and the second shaft opening  360  of the second outer housing  452  may be proximate to the second vehicle frame section  372 . Thus, the outer housings in the second vehicle system arrangement  400  and the first vehicle system arrangement  300  of  FIG.  3    are oriented opposite one another. 
     In the second vehicle system arrangement  400 , by positioning the first and second motor-generators in an inboard region (e.g., the central region  310 ) a likelihood of contact between the motor-generators and flying debris may be decreased. Therefore, the motor-generators may be more shielded in the inboard configuration of  FIG.  4    than the outboard configuration of  FIG.  3   . 
     Similar to the first vehicle system arrangement  300  of  FIG.  3   , the front electric drive axle assembly  402  may be equivalently configured to the rear electric drive axle assembly  404  (e.g., positioning of a gear train, planetary gear assembly, differential, and/or the geometry of an outer housing of the axle assemblies may be the same). To elaborate, the gear train of each electric drive axle assembly may each have a plurality of selectable gear sets which may be substantially equivalent to one another, in one embodiment. In this way, manufacturing costs of the vehicle system can be reduced, if desired. Electrical interfaces  403  of each of the front and rear electric drive axle assemblies  402 ,  404  may extend upwards, along the y-axis, from each of the axle assemblies. However, the first motor-generator  406  may be configured to rotate in opposite rotational directions than the second motor-generator  408  to achieve forward and reverse motion of the vehicle  210 , as described above. 
     For example, when viewing the front electric drive axle assembly along arrow  412 , the first motor-generator  406  may spin in the ccw direction to achieve forward motion of the vehicle  210 , as indicated by arrow  414 . Viewing the second electric drive axle assembly  404  along arrow  416 , an equivalent perspective relative to a geometry of the axle assemblies to arrow  412 , the second motor-generator  408  may spin in the cw direction to similarly achieve forward motion of the vehicle  210 . Conversely, when reverse motion (e.g., vehicle motion is opposite of the direction indicated by arrow  414 ) of the vehicle  210  is desired, the first motor-generator  406  may spin in the cw direction and the second motor-generator  408  may spin in the ccw direction. 
     By implementing the first electric drive axle assembly and the second electric drive axle assembly of both the first and second arrangements  300 ,  400  of  FIGS.  3  and  4    with substantially equivalent configurations, a manufacturing of the axle assemblies may be simplified. A single electric drive axle assembly unit may, in some instances, be fabricated and used for both the front and rear electric drive axle assemblies therefore precluding separate identification and packaging of front versus rear axle assemblies. 
     In other examples, the vehicle system may have distinct front and rear electric drive axle assembly housings. As shown in  FIG.  5    in a third vehicle system arrangement  500 , a front electric drive axle assembly  502  and a rear electric drive axle assembly  504  may not be mirrored. For example, the front electric drive axle assembly  502  and the rear electric drive axle assembly  504  may be mirror images across a mirror plane  506 , coplanar with the y-x plane. The layout of inner components, such as multiple selectable gear sets in each electric drive axle assembly, planetary gear sets, and differentials, of the axle assemblies may also be mirrored. As such, the internal components may have a similar form and size but a mirror arrangement. Thus, in such an example, at least a portion of the inner components of the drive axle assemblies may be substantially equivalent. Put another way, the internal gear train parts (e.g., gears, shafts, etc.) may be fabricated with manufacturing equivalency where the inner components may be constructed using an analogous manufacturing process to produce components having substantially similar sizes, profiles, and material constructions. It will be appreciated, however, that relatively small dimensional variances and other minor inconsistencies resulting from the type of manufacturing process may be present in components having a manufacturing equivalency. In this way, the manufacturing cost of the vehicle system can be reduced. 
     A first outer housing  508  of the front electric drive axle assembly  502  may be manufactured (e.g., cast, machined, etc.) as a mirror image of a second outer housing  510  of the rear electric drive axle assembly  504 . Thus, the first and second outer housings  508 ,  510  may have different geometric profiles. For example, as shown in  FIG.  5   , the first shaft opening  358  of the first outer housing  508  may be proximate to the second vehicle frame section  372  and the second shaft opening  360  of the first outer housing  508  may be proximate to the first vehicle frame section  370 . The first shaft opening  358  of the second outer housing  510  is also proximate to the second vehicle frame section  372  and the second shaft opening  360  of the second outer housing  510  is also proximate to the first vehicle frame section  370 . However, a first motor-generator  512  of the front electric drive axle assembly  502  is positioned on an opposite side of the first and second shaft openings  358 ,  360  than a second motor-generator  514  of the rear electric drive axle assembly  504 . 
     The inner components may be arranged within the first housing  508  in an opposite orientation from inner components enclosed in the second housing  510 . As such, the first motor-generator  512  of the front electric drive axle assembly  502  may be configured to spin in the same direction as a second motor-generator  514  of the rear electric drive axle assembly  504  to achieve forward and reverse propulsion of the vehicle  210 . 
     For example, when viewing the front and rear electric drive axle assemblies  502 ,  504  from an equivalent perspective relative to the geometries of the electric drive axle assemblies along arrow  516 , the first motor-generator  512  may rotate in the ccw direction to rotate the front drive wheels  303  of the vehicle  210  also in the ccw direction to enable forward travel of the vehicle  210 , as indicated by arrow  414 . The second motor-generator  514  may also spin in the ccw direction to rotate the rear drive wheels  305  of the vehicle  210  in the ccw direction. Executable instructions implemented at the controller of the vehicle  210  may be simplified relative to the arrangements of  FIGS.  3  and  4    to operate the first and second motor-generators  512 ,  514  in the same rotational direction for forward and reverse motion of the vehicle  210 . 
     In addition, the mirrored configuration of the vehicle system  200  may allow drive and coast flanks of the gear trains of both the front and rear electric drive axle assemblies  502 ,  504  to be improved (e.g., optimized) for drive, regeneration, and reverse modes of vehicle operation. As mentioned above, when the front-rear electric drive axles are mirror the motor-generator may be rotated in a similar direction in forward and reverse drive modes. However, with regard to manufacturing, separate parts numbers and/or identifying markers may be needed when the mirrored configuration is implemented. Thus, gears, shafts, etc., of front and rear axles of the vehicle  210  may be manufactured with separate part numbers and identifying markers, in some cases. 
     In the third vehicle system arrangement  500 , the front and rear electric drive axle assemblies  502 ,  504  are positioned in the outboard configuration where the first and second motor-generators  512 ,  514  are rotated away from the central region  310  of the vehicle  210 . Electrical interfaces  503  of both the electric drive axle assemblies may extend upwards, along the y-axis, from the electric drive axle assemblies. The third vehicle system arrangement  500  may be implemented when operation of the motor-generators in the same rotational direction is desired while the central region  310  of the vehicle  210  is occupied by other vehicle components. In instances where space in the central region  310  is available, the vehicle system  200  may be arranged in the inboard configuration while maintaining the mirrored orientation of the electric drive axle assemblies, as shown in  FIG.  6   . 
       FIG.  6    shows a fourth vehicle system arrangement  600 . Similar to the third arrangement of  FIG.  5   , a front electric drive axle assembly  602  and a rear electric drive axle assembly  604  are mirrored. Electrical interfaces  603  of both axle assemblies extend upwards, along the y-axis. The fourth vehicle system arrangement  600  includes orienting the front and rear axle assemblies  602 ,  604  in the inboard configuration where the axle assemblies are rotated so that a first motor-generator  606  of the front electric drive axle assembly  602  and a second motor-generator  608  of the rear electric drive axle assembly  604  are situated in the central region  310  of the vehicle. In this way, the first and second motor-generators  606 ,  608  may be spun in the same rotational direction to compel forward or reverse drive of the vehicle  210 , as well as activation of a regeneration mode, while maintaining the motor-generators in a position which reduces interactions between road debris and the motor-generators during vehicle motion. 
     In an alternate configuration of the vehicle system  200 , the electric drive axle assemblies may have a same orientation in the vehicle. For example,  FIG.  7    shows a fifth vehicle system arrangement  700 , including a front electric drive axle assembly  702  and a rear electric drive axle assembly  704 . The axle assemblies are oriented so that electrical interfaces  703  extend upwards, along the y-axis from the axle assemblies. The front and rear axle assemblies  702 ,  704  have substantially equivalent geometries, with components of each electric drive axle assembly configured similarly. In one example, the front and rear electric drive axle assemblies  702 ,  704  may be interchangeable without affecting operation of the axle assemblies. 
     The substantially equivalent geometries of the axle assemblies enables a first motor-generator  706  of the front electric drive axle assembly  702  and a second motor-generator  708  of the rear electric drive axle assembly  704  to be spun in a same rotational direction to achieve forward motion, as indicated by arrow  414 , or reverse motion of the vehicle  210 . A single electric drive axle assembly unit may be manufactured and applied to both the front axle and rear axle of the vehicle  210 , thereby providing a simplified manufacturing process to implement the fifth vehicle system arrangement  700  of  FIG.  7    (as well as a sixth vehicle system arrangement  800  shown in  FIG.  8   ) in the vehicle  210  compared to the mirrored configurations of  FIGS.  5  and  6   . Furthermore, operation of the electric drive axle assemblies in the same rotational direction allows control of the axle assemblies to be streamlined compared to the rotated configurations of  FIGS.  3  and  4   . 
     The front and rear electric drive axle assemblies  702 ,  704  are oriented in  FIG.  7    so that the first motor-generator  706  is behind, relative to the forward direction of motion indicated by arrow  710 , an output shaft in a first gear train  712  of the front electric drive axle assembly  702  and the second motor-generator  708  is behind an output shaft in a second gear train  714  of the rear electric drive axle assembly  704 . As described above, the output shafts of the gear trains are arranged co-axial to the shaft openings  358  and the shaft openings  360 . The first motor-generator  706  is positioned in the central region  310  of the vehicle  210  and the second motor-generator  708  is positioned proximate to a rear end of the vehicle. The fifth vehicle system arrangement  700  may be used in a vehicle when space is available in the central region  310  behind the front axle  390  but not in front of the rear axle  392 . 
     Alternatively, the axle assemblies may be rotated 180° about the y-axis to be oriented opposite from the fifth vehicle system arrangement  700  while maintaining the substantially equivalent and interchangeable geometries of the axle assemblies. For example, as shown in  FIG.  8   , a sixth vehicle system arrangement  800  includes positioning a front electric drive axle assembly  802  and a rear electric drive axle assembly  804  in a same orientation with electrical interfaces  803  of the axle assemblies extending upwards along the y-axis. As in the fifth vehicle system arrangement  700  of  FIG.  7   , the front and rear axle assemblies  802 ,  804  in the sixth vehicle system arrangement  800  may be interchangeable. However, in the sixth vehicle system arrangement  800  of  FIG.  8   , the axle assemblies may be manufactured and assembled so that a first motor-generator  806  is in front of an output shaft in a first gear train  808  of the front electric drive axle assembly  802  relative to the forward direction of motion of the vehicle  210 , as indicated by arrow  414 . Similarly, a second motor-generator  810  is positioned in front of an output shaft of a second gear train  812  of the rear electric drive axle assembly  804 . 
     The second motor-generator  810  may be situated in the central region  310  of the sixth vehicle system arrangement  800  and the first motor-generator  806  may be positioned proximate to a front end of the sixth vehicle system arrangement. Thus, the sixth vehicle system arrangement  800  may be deployed when space is available in front of the rear axle  392  in the central region  310  but not behind the front axle  390 . 
     In this way, an arrangement of the electric drive axle system may be selected based on a desired level of complexity of operation, manufacturing efficiency, packaging efficiency, and accommodation of other vehicle components according to available packaging space. In each of the vehicle system arrangements illustrated in  FIGS.  3 - 8   , the electric drive axle assemblies may be configured to maintain electrical interfaces oriented upwards to maintain electrical cables away from the ground and maintain a compact, space efficient footprint. The gear trains of each of the front and rear drive axle assemblies may have manufacturing equivalency with substantially equivalent gear ratios in multiple selectable gear sets of each gear train. 
     An example of a routine  900  for operating a vehicle equipped with any of the examples of a vehicle system, as shown in  FIGS.  2 - 8   , is depicted in  FIG.  9   . The vehicle system may include a first electric drive axle assembly, rotationally coupled to a front axle of the vehicle and a second electric drive axle assembly, rotationally coupled to a rear axle of the vehicle. The front axle is attached to front drive wheels of the vehicle and the rear axle is attached to rear drive wheels of the vehicle. Each of the first and second axle assemblies includes an electric motor-generator, a gear train, a planetary gear set and a differential rotationally coupled to the planetary gear set. The axle assemblies may be coupled to the vehicle axles so that output shafts of the gear trains are co-axial with the vehicle axles and the planetary gear sets and differentials are axially offset from the motor-generators. 
     Additionally, in some embodiments, the gear trains of the first and second axle assemblies may have substantially equivalent selectable gear ratios. The planetary gear sets of each of the gear trains may be both positioned longitudinally inboard from the motor-generators, longitudinally outboard from the motor-generators, or one planetary gear set may be longitudinally inboard from the corresponding motor-generator and the other planetary gear set may be longitudinally outboard from the corresponding motor-generator. Each of the first and axle assemblies may have outer housing which may have similar geometric profiles or different geometric profiles. However, the axle assemblies may be oriented so that electrical interfaces of each assembly extends upwards from the axle assemblies. 
     Routine  900  may be implemented when a change in vehicle operation mode occurs, such as when forward or reverse motion of the vehicle is requested from a stationary mode. Routine  900  may also be executed during vehicle navigation (e.g., the vehicle is already in motion, and a change in speed is requested). 
     At  902 , the routine includes adjusting operation of the motor-generators of the first and second electric drive axle assemblies. As an example, the vehicle may be stationary and the motor-generators may be inactive. Upon receiving a request for vehicle motion at a vehicle controller (e.g., as indicated by an accelerator pedal tip-in for example) the controller may command activation of the motor-generators to drive rotation of the front and rear drive wheels. Alternatively the vehicle may already be in motion and an increase or decrease in vehicle speed may be indicated by a change in position of the accelerator pedal. A power of the motor-generators may be adjusted accordingly. 
     Rotational energy is transferred from the motor-generators to the front and rear drive wheels of the vehicle at  904 . The energy may be transferred via rotation of rotor shafts of the motor-generators. As previously discussed, when the electric drive axle have inboard or outboard motor arrangements and their gear packaging is mirrored front-rear, the electric motor-generators in each axle may be rotated in a similar direction in both forward and reverse drive modes. However, when the electric drive axle have inboard or outboard motor arrangement and their gear packaging is symmetric front-rear, the electric motor-generators in each axle may be rotated in opposite directions in both forward and reverse drive modes. 
     At  906 , the routine includes selecting gears according to vehicle operating conditions such as vehicle speed and electric motor efficiency. Clutch assemblies may be operated to engage and/or disengage components of the gear trains to adjust a gear ratio delivered to the front and rear drive wheels. The routine then returns to the start. 
     The technical effect of providing a vehicle system with multiple electric drive axles with planetary gear sets and selectable gears it to provide compact axle arrangements with increase gear range adaptability in the vehicle. Consequently, the vehicle&#39;s gearing may be selected to more aptly suite the vehicle&#39;s driving environment. 
       FIGS.  2 - 8    are shown approximately to scale. However, other relative dimensions may be used, in other embodiments. 
       FIGS.  2 - 8    show example configurations with relative positioning of the various components. If shown directly contacting each other, or directly coupled, then such elements may be referred to as directly contacting or directly coupled, respectively, at least in one example. Similarly, elements shown contiguous or adjacent to one another may be contiguous or adjacent to each other, respectively, at least in one example. As an example, components laying in face-sharing contact with each other may be referred to as in face-sharing contact. As another example, elements positioned apart from each other with only a space there-between and no other components may be referred to as such, in at least one example. As yet another example, elements shown above/below one another, at opposite sides to one another, or to the left/right of one another may be referred to as such, relative to one another. Further, as shown in the figures, a topmost element or point of element may be referred to as a “top” of the component and a bottommost element or point of the element may be referred to as a “bottom” of the component, in at least one example. As used herein, top/bottom, upper/lower, above/below, may be relative to a vertical axis of the figures and used to describe positioning of elements of the figures relative to one another. As such, elements shown above other elements are positioned vertically above the other elements, in one example. As yet another example, shapes of the elements depicted within the figures may be referred to as having those shapes (e.g., such as being circular, straight, planar, curved, rounded, chamfered, angled, or the like). Further, elements shown intersecting one another may be referred to as intersecting elements or intersecting one another, in at least one example. Further still, an element shown within another element or shown outside of another element may be referred as such, in one example. 
     In a first embodiment, a system includes a first electric drive axle assembly with a first gear train having a first planetary gear set axially offset from a first electric motor-generator, wherein the first planetary gear set is rotationally coupled to a first differential, and a second electric drive axle assembly with a second gear train having a second planetary gear set axially offset from a second electric motor-generator, wherein the second planetary gear set is rotationally coupled to a second differential. In a first example of the system, the first gear train includes multiple selectable gear sets rotationally coupled to the first planetary gear set and the second gear train includes multiple selectable gear sets rotationally coupled to the second planetary gear set. A second example of the system optionally includes the first example, and further includes, wherein gear ratios of the multiple selectable gear sets in the first gear train are substantially equivalent to gear ratios of the multiple selectable gear sets in the second gear train. A third example of the system optionally includes one or more of the first and second examples, and further includes, wherein the first electric motor-generator includes a first housing and the second electric motor-generator includes a second housing having a different geometric profile than the first housing. A fourth example of the system optionally includes one or more of the first through third examples, and further includes, wherein one or more gears in the first gear train have a manufacturing equivalency to one or more gears in the second gear train. A fifth example of the system optionally includes one or more of the first through fourth examples, and further includes a first beam axle rotationally coupled to the first differential and a second beam axle rotationally coupled to the second differential. A sixth example of the system optionally includes one or more of the first through fifth examples, and further includes, wherein the first beam axle is arranged co-axial with a first output shaft in the first gear train and where the second beam axle is arranged co-axial with a second output shaft in the second gear train. A seventh example of the system optionally includes one or more of the first through sixth examples, and further includes, wherein a first electrical interface of the first electric motor-generator and a second electrical interface of the second electric motor-generator each extend upward from a housing of the corresponding electric motor-generator. An eighth example of the system optionally includes one or more of the first through seventh examples, and further includes, wherein the first planetary gear set is positioned longitudinally inboard from the first electric motor and where the second planetary gear set is positioned longitudinally inboard from the second electric motor and wherein housings of the first and electric drive axle assemblies have a mirrored geometric profile. A ninth example of the system optionally includes one or more of the first through eighth examples and further includes a controller including instructions stored in memory that when executed, during a forward or reverse drive mode, cause the controller to rotate the first electric motor-generator and the second electric motor-generator in opposite directions. 
     In another embodiment, a method includes transferring rotational energy from a first electric motor-generator through a first gear train to a first set of drive wheels, and transferring rotational energy from a second electric motor-generator through a second gear train to a second set of drive wheels, wherein the first gear train includes a first planetary gear set axially offset from the first electric motor-generator, wherein the first planetary gear set is rotationally coupled to a first differential, wherein the second gear train includes a second planetary gear set axially offset from the second electric motor-generator, and wherein the second planetary gear set is rotationally coupled to a second differential. In a first example of the method, transferring rotational energy from the first electric motor-generator through the first gear train to the first set of drive wheels includes rotating a first rotor shaft of the first electric motor-generator in a first rotational direction, and transferring rotational energy from the second electric motor-generator through the second gear train to the second set of drive wheels includes rotating a second rotor shaft of the second electric motor-generator in a second rotational direction opposite the first rotational direction. A second example of the method optionally includes the first example, and further includes, wherein each of the first and second planetary gear sets is positioned longitudinally inboard or outboard from their respective electric motor-generators. A third example of the method optionally includes one or more of the first and second examples, and further includes, wherein the steps of transferring rotational energy from the first electric motor-generator and the second motor-generator are implemented during a forward drive mode or a reverse drive mode. A fourth example of the method optionally includes one or more of the first through third examples, and further includes transitioning between selectable gear sets in the first gear train, and transitioning between selectable gear sets in the second gear train, wherein gear ratios of the selectable gear sets in the first gear train are substantially equivalent to gear ratios of the selectable gear sets in the second gear train. 
     In yet another embodiment, a system includes a first electric drive axle assembly with a first gear train having a plurality of selectable gear sets rotationally coupled to a first planetary gear set, wherein the first planetary gear set is axially offset from a first electric motor-generator and wherein the first planetary gear set is directly rotationally coupled to a first differential, and a second electric drive axle assembly with a second gear train having a plurality of selectable gear sets rotationally coupled to a second planetary gear set, wherein the second planetary gear set is axially offset from a second electric motor-generator and wherein the second planetary gear set is directly rotationally coupled to a second differential, wherein gear ratios of the plurality of selectable gear sets in the first gear train are substantially equivalent to gear ratios of the plurality of selectable gear sets in the second gear train. In a first example of the system, a first beam axle rotationally coupled to the first differential and a second beam axle rotationally coupled to the second differential and where the first beam axle is arranged co-axial to a first output shaft in the first gear train and where the second beam axle is arranged co-axial to a second output shaft in the second gear train. A second example of the system optionally includes the first example, and further includes, wherein the first electric motor-generator includes a first housing and the second electric motor-generator includes a second housing has a mirrored geometric profile in comparison to the first housing. A third example of the system optionally includes one or more of the first and second examples, and further includes, wherein each of the first and second planetary gear sets is positioned longitudinally inboard or outboard from their respective electric motor-generators. A fourth example of the system optionally includes one or more of the first through third examples, and further includes a controller including instructions stored in memory that when executed, during a forward or reverse drive mode, cause the controller to rotate the first electric motor-generator and the second electric motor-generator in opposite directions. 
     The following claims particularly point out certain combinations and sub-combinations regarded as novel and non-obvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and sub-combinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.