BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a drive train of a solely electrically drivable motor vehicle, having an axle which has a differential, and two electric machines, wherein the axle is drivable by means of the electric machines via at least one gearing. 
         [0003]    2. Description of the Related Art 
         [0004]    Such a drive train, which is used for an electrically drivable earth-moving vehicle or for an agricultural vehicle with four-wheel drive, is known from DE 600 13 340 T2. Said drive train has two electric machines which are arranged above the rear axle of the motor vehicle in the direction of travel and interact with a spur gearing which is arranged in front of the rear axle. The gearing is connected via one shaft or two shafts to the differentials which are assigned to the two axles, therefore to the rear axle and to the front axle of the motor vehicle. 
         [0005]    It is the object of the present invention to provide a drive train in a motor vehicle to be operated solely electrically, which drive train makes it possible to drive in different driving situations with particularly good efficiency. 
       SUMMARY OF THE INVENTION 
       [0006]    The drive train of the solely electrically drivable motor vehicle therefore has two electric machines. One of the two electric machines, referred to below as the first electric machine, interacts with a first gearing, and the other of the two electric machines, referred to below as the second electric machine, interacts with a second gearing. The first gearing is connectable via a first switchable clutch to an input gear of the differential, for driving two axle sections of the axle, which axle sections are connected to different outputs of the differential. The first gearing is connectable via a second clutch to a first axle section of the axle when the first clutch is open, and the second gearing is connectable via a third clutch to a second axle section of the axle when the first clutch is open. 
         [0007]    This configuration of the drive train with the two electric machines, the two gearings assigned to the latter and the three clutches makes it possible either to drive the two axle sections of the axle by driving only the first electric machine via the first gearing assigned thereto and the differential assigned to said gearing, or else not to introduce the driving force into the differential and, instead, to drive each axle section directly by means of the electric machine assigned thereto. This independent drive of the respective axle section of the axle permits an individual wheel drive of the motor vehicle wheel assigned to the respective axle section and of the running wheel of the motor vehicle. As a result, a torque vectoring of the axle sections of the axle or of the wheels of the axle is possible. Said torque vectoring does not cause any loss due to a braking engagement on the axle section of the one or other wheel. 
         [0008]    In the driving mode, in which force is transmitted via the differential, only the one electric machine—the first electric machine—is operated, when the first clutch is closed, while the two other clutches—the second and third clutches—are open. By contrast, in individual wheel drive, the first clutch is open and the second and third clutches are closed. 
         [0009]    The driving of the motor vehicle only by the one electric machine—the first electric machine—is advantageous if driving situations depending on low energy consumption are to prevail. In particular whenever critical driving situations in terms of driving dynamics are to prevail, the switch is made to the individual wheel drive. Said critical driving situations in terms of driving dynamics are, in particular, those which are critical under safety aspects and a stabilizing intervention on the vehicle is required. 
         [0010]    The two electric machines and the three clutches preferably have control means, by means of which, when the first electric machine is in operation, the first clutch is closed and the second and third clutches are open, or, when the electric machines are in operation, the first clutch is open and the second and third clutches are closed. 
         [0011]    The gearings are preferably designed as spur gearings. They can be accommodated in the relatively small construction space. In particular, the gearings have identical transmission ratios. 
         [0012]    The electric machines are arranged in particular transversely with respect to the direction of travel of the motor vehicle. 
         [0013]    The drive train is preferably used in a motor vehicle which is in the form of a passenger vehicle. Said passenger vehicle is in particular a sports car. Said motor vehicle, in particular the passenger vehicle or the sports car, is preferably in the form of a rear drive. The two electric machines are therefore arranged in the rear region of the motor vehicle or of the drive train. It is considered to be particularly advantageous if the two electric machines are arranged behind the rear axle. 
         [0014]    In principle, however, the motor vehicle can be in the form of a front drive. 
         [0015]    The wheels assigned to the drive train are suspended in particular individually via propeller shafts. The drive train therefore does not have a rigid axle. 
         [0016]    Further features of the invention emerge from the dependent claims, the attached drawing and the description of the preferred exemplary embodiment, which is reproduced in the drawing, without being limited thereto. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0017]      FIG. 1  shows a schematic diagram of a preferred embodiment of the drive train according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    The exemplary embodiment according to  FIG. 1  illustrates a drive train for a solely electrically drivable motor vehicle, which is in particular a passenger vehicle, specifically a sports car. The rear axle of the drive train assigned to the motor vehicle and, furthermore, a non-driven front axle of the motor vehicle is shown. 
         [0019]    The drive train  1  with individual wheel suspension has the first, rear axle  2 . With respect to the forward direction of travel  3  of the motor vehicle, referred to below as direction of travel, the rear axle  2  has a left axle section  4  and a right axle section  5 . The left and the right wheel of the rear axle  2  are denoted by the reference number  6 , and bearings for the axle sections  4  and  5  of the rear axle  2  are denoted by the reference number  7 . The axle sections  4  and  5  of the rear axle  2  have propeller shafts. 
         [0020]    The motor vehicle furthermore has a second, front axle  8  which is not driven. This axle  8  also has individual wheel suspension. The front axle  8  has a left axle section  9  and a right axle section  10 . The left and right wheel of the front axle  8  are denoted by the reference number  11 , and the bearings for the axle sections  9  and  10  of the front axle  8  are denoted by the reference number  12 . The axle sections  9  and  10  of the front axle  8  likewise have propeller shafts. 
         [0021]    The rear axle  2  is drivable by means of two electric machines  13  and  14 . In this case, the electric machine  13  interacts with a gearing  15  and the electric machine  14  interacts with a gearing  16 . The gearings  15  and  16  are arranged substantially behind the rear axle  2 , and the two electric machines  13  and  14  are each arranged transversely with respect to the direction of travel  3 . The axis of rotation of the respective electric machine  13  or  14 , illustrated by the driven shaft  17  thereof, is therefore arranged in the direction of travel  3 . 
         [0022]    The stator of the respective electric machine  13  or  14  is denoted by the reference number  18 , and the rotor of the respective electric machine  13  or  14 , to which rotor the driven shaft  17  is connected, is denoted by the reference number  19 . The respective driven shaft  17  is mounted in bearings  20 . 
         [0023]    The driven shafts  17  of the two electric machines  13 ,  14  are arranged on the same geometrical axis, and therefore the axis of rotation of the stators  18  of the electric machines  13  and  14  corresponds to said geometrical axis. 
         [0024]    Apart from the minor difference still to be described below, the two gearings  15 ,  16  are arranged mirror-symmetrically with respect to the longitudinal axis of the vehicle and are of identical design. They therefore have identical transmission ratios. The gearings are designed as spur gearings. 
         [0025]    The respective gearing  15  or  16  has a pinion  21  which is connected to the driven shaft  17  for rotation therewith and is designed as a spur gear. Said pinion  21  meshes with a spur gear  22  of the respective gearing  15  or  16 , which spur gear is connected to a shaft  23  for rotation therewith. Said shaft is mounted on the end sides in bearings  24 . A pinion  25  is arranged next to the spur gear  22  and is connected to the shaft  23  for rotation therewith. The pinion  25  of the gearing  16  meshes with a spur gear  26  which is mounted in a freely rotatable manner in the right axle section  5  of the axle  2 . The spur gear  26  is connected to a clutch part  27  for rotation therewith, which clutch part can be brought into an operative position with a clutch part  28  which is connected to the right axle section  5  for rotation therewith. 
         [0026]    A spur gear  29  which is modified in relation to the spur gear  26  interacts with the pinion  25  of the other gearing  15 . In the same manner as the spur gear  26 , said spur gear  29  is mounted in a freely rotatable manner in the axle section of the axle  2 , in the present case the left axle section  4 , and is connected to a clutch part  30 , which is designed in a manner corresponding to the clutch part  27 , for rotation therewith. Said clutch part  30  can be brought into an operative position with a clutch part  31  which is connected to the left axle section  4  for rotation therewith and in this respect corresponds to the clutch part  28  with regard to construction and arrangement. 
         [0027]    The spur gear  29  which is assigned to the left axle section  4  basically differs from the spur gear  26  which is assigned to the right axle section  5  in that the spur gear  29  non-rotatably receives a clutch part  32 . The latter can be brought into operative connection with a clutch part  33 , which is switchable and with the aid of which a non-rotatable connection can be produced between the spur gear  29  and an input gear  34  of a differential  35 . The two axle sections  4  and  5  are connected to two outputs of the differential  35  for rotation therewith. 
         [0028]    Accordingly, the clutch parts  32  and  33  form a first switchable clutch  36 , the clutch parts  30  and  31  form a second switchable clutch  37 , and the clutch parts  27  and  28  form a third switchable clutch  38 . In a first driving situation, in which it suffices to drive the vehicle by means of one of the two electric machines, specifically the electric machine  13 , and in which an energy-saving driving manner occurs, the first clutch  36  is closed and the second and third clutches  37 ,  38  are open. The electric machine  14  is switched off. Accordingly, only the electric machine  13  transmits the torque via the gearing  15 , and accordingly, the spur gear  29  and the clutch  36  to the differential  35  and from there via the two axle sections  4  and  5  to the wheels  6 . 
         [0029]    If, by contrast, a critical situation in terms of driving dynamics prevails, the wheels  6  of the rear axle  2  can be driven individually. The first clutch  36  is open and the second and third clutches  37 ,  38  are closed. In this case, force is not transmitted via the differential  35  to the axle sections  4  and  5  of the rear axle  2 , but rather the transmission takes place in a first torque train from the electric machine  13  via the gearing  15  assigned thereto to the clutch  37  and from there to the left axle section  4  with the wheel  6  assigned thereto. The electric machine  14  is connected by a second torque path via the gearing  16  to the closed clutch  38 , and therefore the right axle section  5  and the wheel  6  assigned thereto are driven via said clutch. Different torques of the electric machines  13 ,  14  make is possible for different torques to be introduced into the two axle sections  4  and  5 , thus enabling torque vectoring of the rear axle  2  by means of individual wheel drive. 
         [0030]    The two electric machines  13  and  14  and the three clutches  36 ,  37 ,  38  have control means, by means of which, when the first electric machine  13  is in operation, the first clutch  36  is closed and the second and third clutches  37 ,  38  are open, or, when the two electric machines  13 ,  14  are in operation, the first clutch  36  is open and the second and third clutches  37 ,  38  are closed.
A drive train ( 1) of a solely electrically driven motor has an axle ( 2) with first and second axle sections ( 4, 5) connected to different outputs of a differential ( 35). The drive train also has a first and second electric motors ( 13, 14) that co-operate with first and second transmissions ( 15) respectively to drive the axle ( 2). A first clutch can connect the first transmission ( 15) to an entry wheel ( 34) of the differential ( 35) for driving the two axle sections ( 4, 5) of the axle ( 2). A second clutch ( 37) can connect the first transmission ( 15) to the first axle section ( 4) while the first clutch ( 36) is open and a third clutch ( 38) can connect the second transmission ( 16) to a second axle section ( 5) while the first clutch ( 36) is open.