Abstract:
A propulsion device for a vehicle includes a differential gear, which is driven by the electric machine via a planetary gear and operatively connected to an axle on the vehicle, with the planetary gear including at least three rotatable gear elements. A switching device is switchable by an actuator between a first switch position, in which one gear element is secured against a rotation about the axis of rotation, and a second switch position, in which the one gear element is connected by the switching device to another gear element in a torsion-proof manner. A parking brake including at least one parking brake element is movable by the actuator between a park position, in which the vehicle is prevented from rolling away, and at least one release position.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of German Patent Application, Serial No. Serial No. 102015217521.9, filed Sep. 14, 2015, pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is incorporated herein by reference in its entirety as if fully set forth herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a propulsion device for a vehicle, especially an electric or hybrid vehicle. 
         [0003]    The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention. 
         [0004]    Vehicles that are embodied as electric or hybrid vehicles are known in the art. Such a vehicle includes at least one electric machine to propel the vehicle. In other words, the electric machine is embodied for driving the vehicle, so that the electric machine is also called a traction machine. 
         [0005]    In order to drive the vehicle, the electric machine is supplied with electric current. For this purpose, the vehicle has at least one electric energy store, especially in the form of a battery, where electric current or electric energy can be stored in the electric energy store. In this case, the electric machine can be supplied with the electric current stored in the electric energy store, in order to drive the vehicle electrically, especially purely electrically. Purely electric drive is to be understood as the vehicle moving purely under electric power, such that the vehicle is driven exclusively with the aid of electric energy in the absence of a combustion engine. 
         [0006]    Usually the vehicle also includes a gear unit that is driven by the electric machine, so that the vehicle can be driven via the gear unit by the electric machine. In this case, the vehicle has ground contact elements, especially in the form of wheels. While on the move, the vehicle rolls on a road via the ground contact elements. To drive the vehicle, the wheels are driven by the electric machine via the gear unit. 
         [0007]    Such a vehicle, which includes an electric machine for driving the vehicle and a battery for supplying the electric machine with electric current, is also called a battery-electric vehicle (BEV). With battery-electric vehicles, the performance, and thus the size, meaning the outer dimensions, of the electric machine, as well as the gear ratio of the gear unit is usually determined by requirements for initial acceleration. The vehicle can be accelerated by virtue of the electric machine via the gear unit, especially as part of a process of the vehicle starting from rest. Usually the possible top speed of the vehicle, as well as the capability to accelerate at high speeds suffer as a rule from being restricted to a single-gear gear unit, which is to be understood as a gear unit with precisely one gear. 
         [0008]    It would be desirable and advantageous to provide an improved propulsion device for a vehicle to obviate prior art shortcomings and to realize high initial acceleration and high top speed in a space-saving and cost-effective way. 
       SUMMARY OF THE INVENTION 
       [0009]    According to one aspect of the present invention, a propulsion device includes an electric machine, a differential gear driven by the electric machine and operatively connected to an axle on the vehicle, with ground contact elements of the axle being driven by the electric machine via the differential gear, a planetary gear via which the differential gear is driven by the electric machine, the planetary gear comprising at least three gear elements that are rotatable about an axis of rotation, with one of the at least three gear elements being configured as a planet carrier, and at least one planet gearwheel which is different from the at least three gear elements, the planet gearwheel being rotatably supported on one of the at least three gear elements and in engagement with the other ones of the at least three gear elements via respective sets of teeth, a first one of the at least three gear elements being driven by the electric machine, and a second one of the at least three gear elements being driven by the electric machine via the first of the at least three gear elements and coupled to the differential gear, a switching device including at least one switching element and switchable between a first switch position, in which a third one of the at least three gear elements is secured against a rotation about the axis of rotation, and at least one second switch position in which the third one of the at least three gear elements is connected by the switching device to the first one of the at least three gear elements in a torsion-proof manner, an actuator configured to move the at least one switching element of the switching device between the first and second switch positions, and a parking brake including at least one parking brake element, the parking brake being movable by the actuator between a park position, in which the vehicle is prevented from rolling away, and at least one release position. 
         [0010]    In accordance with the present invention, the propulsion device for a vehicle, especially an electric or hybrid vehicle, includes a differential gear, which is driven by an electric machine and connected to an axle on the vehicle. Ground contact elements of the axle are driven by the electric machine. The ground contact elements may comprise wheels, such that the vehicle, embodied for example as an automobile, rolls when traveling on a road. The vehicle is driven by the electric machine via the wheels. 
         [0011]    The differential gear is also referred to as a differential transmission or differential, and allows the wheels to rotate at different speeds, when the vehicle is negotiating a curve, so that for example, the wheel on the outside of the curve can rotate faster than the wheel on the inside of the curve. 
         [0012]    The planetary gear of the propulsion device has three gear elements that are rotated about an axis of rotation. One is embodied as the planet carrier of the planetary gear. The planetary gear further includes at least one planet gearwheel differing from the gear elements, which is supported rotationally on a one gear element, i.e. on the planet carrier, and is in engagement with the other gear elements via respective sets of teeth. 
         [0013]    In this case, a first of the gear elements is driven by the electric machine. Furthermore, a second of the gear elements can be driven via the first gear element by the electric machine and is coupled to the differential gear, such that the differential gear is driven by the electric machine via the second gear element and the first gear element. Overall, the vehicle can thus be driven by the electric machine via the differential gear, the second gear element and the first gear element. 
         [0014]    The switching device of the propulsion device is switched or adjusted between at least two switch positions. In a first switch position of the switching device, the third gear element is secured from rotating about the axis of rotation via the switching device. For example, the propulsion device, especially the planetary gear has a housing. The gear elements are rotated about the axis of rotation relative to the housing. In the first switch position of the switching device, the third gear element is fixed via the switching device on the housing, so that the third gear element cannot rotate about the axis of rotation relative to the housing. 
         [0015]    In the second switch position of the switching device, the third gear element is connected in a torsion-proof manner to the first gear element via the switching device, so that when the first gear element is driven by the electric machine. The first gear element and the third gear element orbit together as a block, and in doing so, rotate about the axis of rotation relative to the housing. 
         [0016]    The second gear element is thus a take-off of the planetary gear and the first gear element is a drive of the planetary gear. The planetary gear is driven via the drive from the electric machine and provides torques via the take-off, which are transmitted to the differential gear, so that the differential gear can be driven via the take-off, and thus by the torques provided via the take-off. 
         [0017]    The fact that the switching device is provided, and can be switched between the switch positions, means that the planetary gear is switchable, so that both an especially high initial acceleration as well as an especially high top speed of the vehicle can be realized by means of the propulsion device. The initial acceleration is to be understood as an acceleration able to be brought about by means of the propulsion device, with which the vehicle can be accelerated from rest as part of a starting process. The top speed is the maximum realizable traveling speed of the vehicle. The high initial acceleration and the high top speed can be realized by means of the propulsion device in an especially space-saving and cost-effective way, since as a result of the fact that the planetary gear is switchable, so that for example at least two different gear ratios are able to be set, the external dimensions of the propulsion device and in particular of the electric machine can be kept especially small. 
         [0018]    Advantageously, the number of parts and thus the weight, the installation space required and the costs of the propulsion device can be kept low, since only the switching device is provided and required in order to switch the planetary gear, and thus to obtain a high initial acceleration and a high top speed. 
         [0019]    The electric machine of the propulsion device can be embodied as a traction machine for driving the vehicle. The electric machine can be operated, for example, in a motor mode, and thus as an electric motor. The vehicle can be driven via the planetary gear and the differential gear by the electric machine in the motor mode. It is further conceivable for the electric machine to be able to be operated in a generator mode and thus as a generator. In this generator mode the electric machine is driven for example via the differential gear and the planetary gear by the moving vehicle and thus by means of kinetic energy of the vehicle, wherein, in the generator mode, at least a part of the kinetic energy of the vehicle is converted into electric energy by means of the electric machine. Through this the vehicle is slowed down or braked for example. The electric machine provides the electric energy for example, so that at least one electric load can be supplied with the electric energy. As an alternative or in addition it is conceivable to store the electric energy in an electric storage device, in particular in a battery. In this case there can be provision for the propulsion device to include at least one such storage device. 
         [0020]    According to another advantageous feature of the present invention, an actuator is provided, by means of which at least one switching element of the switching device is able to be moved between the switch positions. In the first switch position, the third gear element is secured by the switching element against a rotation about the axis of rotation, wherein the third gear element in the second switch position is connected to the first gear element in a torsion-proof manner. By means of the actuator automatic or automated or partly-automatic or partly-automated switching of the switching device, in particular of the switching element, can be realized, so that for example an especially advantageous transition from the first switch position into the second switch position and vice versa can be realized. In particular it is possible, after the startup process, to move the switching element by means of the actuator from the first switch position into the second switch position or vice versa, in order to realize an especially advantageous operation of the propulsion device and thus of the vehicle as a whole thereby. 
         [0021]    According to another advantageous feature of the present invention, a parking brake with at least one parking brake element is provided, which is moved between a park position to prevent the vehicle rolling away and at least one released position. The propulsion device, in this case, includes at least one take-off shaft. The vehicle is driven by the electric machine. This take-off shaft, for example, involves one of the gear elements, but also a different shaft of the propulsion device from the gear elements. The take-off shaft is rotated about a second axis of rotation, in particular, relative to the housing. The second axis of rotation can be spaced away from the aforementioned first axis of rotation of the gear elements, or can coincide with the first axis of rotation. In the park position, the parking brake element acts to make a form fit together with the take-off shaft, such that the take-off shaft is secured via the brake element located in the park position from a rotation about the second axis of rotation. This advantageously enables the vehicle, when it is parked on an incline for example, not to roll away as a result of gravity, since the take-off shaft, and thus the ground contact elements, which are driven via the take-off shaft, cannot rotate. 
         [0022]    In the release position, however, the parking brake element releases the take-off shaft, and thus the ground contact elements (wheels) of the vehicle, so that the take-off shaft and the ground contact elements can rotate. As a result, the vehicle is driven by the electric machine. 
         [0023]    In order to keep the installation space required for the propulsion device small, there is provision for the parking brake element to be moved via the actuator. This means that the actuator is embodied to move the parking brake element from the park position into the release position and/or from the release position into the park position. This enables a separate actuator for actuating or moving the parking brake element. Consequently, the number of parts, the weight, the costs and the installation space required for the propulsion device can be kept low. In other words, there is provision for the parking brake element and the switching element to be adjusted or moved via the same actuator. 
         [0024]    According to another advantageous feature of the present invention, the first one of the at least three gear elements can be configured as a hollow gearwheel, the second one of the at least three gear elements can be configured as a planet carrier, and the third one of the at least three gear elements can be configured as a sun gearwheel of the planetary gear. Thus the planet carrier represents the take-off of the planetary gear, wherein the hollow gearwheel represents the drive of the planetary gear. The sun gearwheel, as required, can be fixed to the housing by means of the switching device or can be secured against a rotation about the axis of rotation but also connected in a torsion-proof manner to the hollow gearwheel, so that both an especially high initial acceleration and also an especially high top speed are able to be realized in a cost-effective and space-saving manner. 
         [0025]    According to another advantageous feature of the present invention, the actuator can be an electromechanical actuator, a hydraulic actuator, an electro-hydraulic actuator, or an electromagnetic actuator. As a result costs, weight and installation space required for the propulsion device can be kept low. This also enables short switching times of the switching device, in particular of the switching element, to be obtained. 
         [0026]    According to another advantageous feature of the present invention, the planetary gear can have a stationary gear ratio ranging from 1.5 to 4. In other words the mathematical amount of the stationary gear ratio of the planetary gear lies in a range from 1.5 to 4, wherein the stationary gear ratio is usually designated i 0 . For example the stationary gear ratio or its value lies in a range of −4 to −1.5. 
         [0027]    According to another advantageous feature of the present invention, the planetary gear in the second switch position can have a gear ratio which is smaller than a gear ratio in the first switch position. A rotational speed, at which, the first gear element rotates about the axis of rotation, when the first gear element is driven by the electric machine, is also referred to as the rotational drive speed, since the first gear element forms the drive of the planetary gear. Since the second gear element forms the take-off of the planetary gear, a rotational speed, at which, the second gear element rotates about the axis of rotation, when the second gear element is driven via the first gear element by the electric machine, is referred as the rotational take-off speed. 
         [0028]    Since the planetary gear in the second switch position advantageously has now a smaller gear ratio than in the first switch position, the rotational take-off speed is smaller in the first switch position than in the second switch position when the rotational drive speed remains the same, so that the first switch position is embodied for example as a slow drive stage and the second switch position as a fast drive stage. Thus, by means of the first switch position an especially high initial acceleration can be realized, wherein the provision of the second switch position enables an especially high top speed of the vehicle can be realized. 
         [0029]    According to another advantageous feature of the present invention, the planetary gear can have a gear step of at least 1.3, when the switching device switches from the first switch position to the second switch position. The gear step is to be understood in particular as the quotient of the gear ratio of the planetary gear in the first switch position and the gear ratio of the planetary gear in the second switch position. In the first switch position, the vehicle can thus be moved or driven in a so-called low speed range, wherein, in the second switch position, the vehicle can thus be moved or driven in a so-called high speed range, wherein the high speed range is greater than the low speed range. Thus, provision is made for the gear step to amount to at least 1.3 during switching from the low to the high speed range. 
         [0030]    According to another advantageous feature of the present invention, the gear step can range from 1.3 to 1.6. As a result, a high initial acceleration and a high top speed are realized. This is especially advantageous for an electric vehicle, which can be propelled purely electrically, i.e. not by a combustion motor. With a hybrid vehicle the gear step can be greater than 1.6, wherein a hybrid vehicle differs from an electric vehicle in that the hybrid vehicle, by contrast with the hybrid vehicle, has an internal combustion engine for propelling the hybrid vehicle. 
         [0031]    According to another advantageous feature of the present invention, at least one rotational speed sensor can be provided to detect a rotational speed of one of the at least three gear elements, in particular, of the second one of the gear elements. By detection of the rotational speed it is possible for example to move or to switch the switching element via the actuator depending on the rotational speed detected by means of the rotational speed sensor, so that especially advantageous switching processes of the propulsion device can be realized. 
         [0032]    According to another advantageous feature of the present invention, the third one of the at least three gear elements can be secured via the switching device in the first switch position by a form fit against a rotation about the axis of rotation and can be connected in the second switch position by a form fit to the first one of the at least three gear elements in a torsion-proof manner. This enables an especially high efficiency of the propulsion device to be realized. 
         [0033]    According to another advantageous feature of the present invention, the third one of the at least three gear elements can be secured via the switching device in the first switch position by a friction fit against a rotation about the axis of rotation and connected in the second switch position by a friction fit to the first one of the at least three gear elements in a torsion-proof manner This enables especially convenient switching processes to be realized. 
         [0034]    According to another aspect of the present invention, a vehicle, in particular, an electric or hybrid vehicle, includes at least one electric machine embodied as a traction machine for driving the vehicle, and a propulsion device, as set forth above. Advantages and embodiments of the inventive propulsion device are to be seen as advantages and embodiments of the inventive vehicle, and vice versa. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0035]    Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
           [0036]      FIG. 1  is a schematic illustration of a first embodiment of a propulsion device according to the present invention; 
           [0037]      FIG. 2  is a schematic illustration of a section of a second embodiment of a propulsion device according to the present invention; 
           [0038]      FIG. 3  is a schematic illustration of a section of a third embodiment of a propulsion device according to the present invention; 
           [0039]      FIG. 4  is a schematic illustration of a section of a fourth embodiment of a propulsion device according to the present invention; 
           [0040]      FIG. 5  is a schematic illustration of a section of a fifth embodiment of a propulsion device according to the present invention; and 
           [0041]      FIG. 6  is a schematic illustration of a section of a sixth embodiment of a propulsion device according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0042]    Throughout all the figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments may be illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. 
         [0043]    Turning now to the drawing, and in particular to  FIG. 1 , there is shown a schematic illustration of a first embodiment of a propulsion device according to the present invention, generally designated by reference numeral  10 , for use in a vehicle, in particular, an electric vehicle. The propulsion device  10  includes an electric machine  12  shown in  FIG. 1  schematically, wherein in particular the arrangement of the electric machine  12  is by way of example in  FIG. 1 . The electric machine  12  includes a housing  14 , in which a stator  16  and a rotor  18  of the electric machine  12  are at least partly accommodated. The stator  16  is fixed to the housing  14 , wherein the rotor  18  is able to be rotated about a first axis of rotation relative to the housing  14  and relative to the stator  16 . For example the rotor  18  is able to be driven by the stator  16 , wherein the rotor  18  is connected to a shaft  20  in a torsion-proof manner. Thus the shaft  20  is able to be rotated about the said first axis of rotation relative to the housing  14 , wherein the electric machine  12  can provide torques via the rotor  18  and the shaft  20  to drive the vehicle. 
         [0044]    The electric machine  12  is embodied as a traction machine, by means of which the vehicle is able to be driven. To this end the electric machine  12  is able to be operated in a motor mode and thus as a motor or electric motor. In the motor mode the electric machine  12  is supplied with electric energy or electric current respectively, through which the electric machine  12  provides torques to drive the vehicle via the shaft  20 . 
         [0045]    In order to supply the electric machine  12  with electric current, the propulsion device  10  for example includes at least one electric energy store not shown in  FIG. 1 , which is embodied as a battery for example. The electric machine  12  is connected via power electronics to the battery for example, so that the electric machine  12  can be supplied with electric current from the battery via the power electronics. Electric energy or electric current can namely be stored by means of the battery, wherein the electric current stored in the battery can be fed via the power electronics to the electric machine  12 . 
         [0046]    It is further conceivable for the electric machine  12  to be able to be operated in a generator mode. In the generator mode the electric machine  12  functions as a generator and is driven by the moving vehicle and thus by means of kinetic energy of the vehicle. By means of the electric machine  12  at least a part of the kinetic energy of the vehicle is converted in generator mode into electric energy or electric current, wherein this electric current is provided by the electric machine  12 . The electric current provided by the electric machine  12  in the generator mode can be fed into the battery for example and stored there and/or fed to at least one electric load, which can be operated by means of the electric energy. 
         [0047]    The propulsion device  10  further includes a differential gear  22 , which is assigned to an axle of the vehicle labeled overall by the number  24 . The axle  24  is for example a rear axle or a front axle and has ground contact elements in the form of wheels  26 . While driving along a road the vehicle rolls on the road via the wheels  26  rotating about an axis of rotation. The wheels  26 —as is explained in greater detail below—are able to be driven via the differential gear  22  by the electric machine  12  in its motor mode. 
         [0048]    The differential gear  22  is also simply referred to as a differential and, when the vehicle is negotiating a curve for example, allows the wheels  26  to rotate at different speeds, so that for example the wheel on the outside of the curve can rotate faster than the wheel on the inside of the curve. This enables disproportionate stresses in the propulsion device  10  or in a drive train of the vehicle to be avoided. 
         [0049]    The differential gear  22  includes a cage  28 , on which a bolt element  30  is held. Differential gearwheels  32  of the differential gear  22  are supported rotationally on the bolt element  30 , wherein the differential gearwheels  32  are embodied as toothed gearwheels and here as bevel gearwheels. The differential gear  22  also includes toothed gearwheels in the form of drive gearwheels  34 , which are embodied here as bevel gearwheels. The drive gearwheels  34  engage with the differential gearwheels  32  and are connected in a torsion-proof manner to shafts  36 . The shafts  36  are embodied for example as articulated shafts and are coupled to the wheels  26 , so that the wheels  26  are able to be driven via the shafts  36  by the electric machine  12 . 
         [0050]    If the cage  28  is driven by means of the electric machine  12  and during this process is rotated about a second axis of rotation, the differential gearwheels  32  are driven via the bolt element  30  and are rotated during the process about the second axis of rotation, so that once again the drive gearwheels  34  and via these the shafts  36  and thus the wheels  26  are driven about the second axis of rotation. It can be seen from  FIG. 1  here that the first axis of rotation, about which the rotor  18  and the shaft  20  are able to be rotated, is at a distance from the second axis of rotation and runs in parallel to the second axis of rotation. 
         [0051]    The propulsion device  10  also includes a planetary gear labeled overall by the number  38 , which is embodied as a simple planetary gear and includes a first gear element in the form of a hollow gearwheel  40 , a second gear element in the form of a planet carrier  42  and a third gear element in the form of a sun gearwheel  44 . The planetary gear  38  further includes planet gearwheels  46  different from the gear elements (hollow gearwheel  40 , planet carrier  42  and sun gearwheel  44 ), which are each supported rotatably on the planet carrier  42 . The planet carrier  42  is also referred to as a web and is coupled here to the differential gear  22 , so that the differential gear  22  is able to be driven via the planetary gear  38 , in particular the planet carrier  42 , by the electric machine  12  in its motor mode. To this end the planet carrier  42  is connected to the cage  28  in a torsion-proof manner for example. In particular the planet carrier  42  can be embodied in one piece with the cage  28 . 
         [0052]    The hollow gearwheel  40  has a first set of teeth in the form of inner teeth, wherein the sun gearwheel  44  has a second set of teeth in the form of outer teeth. Furthermore the respective planet gearwheel  46  has a third set of teeth in the form of outer teeth, so that the gear elements are embodied as toothed gearwheels. The planetary gear  38  is thus embodied as a toothed gearwheel gear, wherein the planet gearwheels  46  engage via the respective sets of teeth with the sun gearwheel  44  and the hollow gearwheel  40 . In other words the planet gearwheels  46  mesh both with the hollow gearwheel  40  and also with the sun gearwheel  44 . 
         [0053]    It can be seen in this case from  FIG. 1  that the hollow gearwheel  40  (first gear element) is able to be driven by the electric machine  12 , so that the planet carrier  42  is able to be driven by the hollow gearwheel  40  and via said gearwheel by the electric machine  12 . This means that the differential gear  22 , in particular the cage  28 , is able to be driven via the planet carrier  42  and the hollow gearwheel  40  by the electric machine  12 . To this end the hollow gearwheel  40  is connected in a torsion-proof manner to the toothed gearwheel  48 , wherein the toothed gearwheel  48  is embodied for example as a cylindrical gear or as a ring gear. For example the hollow gearwheel  40  is embodied in one piece with the toothed gearwheel  48 . In addition a toothed gearwheel  50  is connected to the shaft  20  in a torsion-proof manner, wherein the toothed gearwheel  50  is embodied as a cylindrical gear for example. The toothed gearwheel  50  is also referred to as the pinion or drive pinion and is able to be driven via the shaft  20  by the rotor  18  or by the electric machine  12 . 
         [0054]    The toothed gearwheel  50  is in engagement with the toothed gearwheel  48  via the respective set of teeth, so that the toothed gearwheel  48  and via said gearwheel the hollow gearwheel  40  are able to be driven via the toothed gearwheel  50  and the shaft  20  by the electric machine  12 . The hollow gearwheel  40  thus represents a drive or a drive element of the planetary gear  38 , since the torques provided by the electric machine  12  in its motor mode for driving the vehicle via the toothed gearwheels  48  and  50  and thus via the hollow gearwheel  40  are introduced into the planetary gear  38 . The web (planet carrier  42 ) represents a take-off or a take-off element of the planetary gear  38 , since the planetary gear  38  provides the torques for driving the vehicle via the web and introduces them into the differential gear  22 . In other words the torques for driving the vehicle via the web are derived from the planetary gear  38  and transmitted to the differential gear  22 , in particular the cage  28 . 
         [0055]    The propulsion device  10  further includes a switching device  52  with a first switching element  54  and a second switching element  56 . The second switching element  56  is connected to the sun gearwheel  44  in a torsion-proof manner. To this end a shaft  58  is provided, to which both the second switching element  56  and also the sun gearwheel  44  are connected in a torsion-proof manner. For example the sun gearwheel  44  is embodied in one piece with the shaft  58 . Thus the second switching element  56  is connected via the shaft  58  in a torsion-proof manner to the sun gearwheel  44 . 
         [0056]    The first switching element  54  and thus the switching device  52  overall are able to be switched between a first switch position S 1  and a second switch position S 2 . To this end the first switching element  54  is able to be moved relative to the second switching element  56  between the switch positions S 1  and S 2 , wherein the first switching element  54  is able to be moved in an axial direction of the sun gearwheel  44  between the switch positions S 1  and S 2  and thus translationally. 
         [0057]    The propulsion device  10  includes a housing  60  especially shown schematically in  FIG. 1 , in which the switching device  52  and/or the planetary gear  38  and/or the differential gear  22  are each at least partly accommodated. In this case the gear elements (hollow gearwheel  40 , planet carrier  42  and sun gearwheel  44 ) are able to be rotated relative to the housing  60  about the said second axis of rotation, about which the cage  28  and the shafts  36  are also able to be rotated. 
         [0058]    In the first switch position S 1  the sun gearwheel  44  is fixed by means of the first switching element  54  on the housing  60 , so that the sun gearwheel  44  is secured by means of the switching device  52  against a rotation about the second axis of rotation. In the first switch position S 1  the sun gearwheel  44  is supported via the shaft  58 , the second switching element  56  and the first switching element  54  on housing  60 , so that the sun gearwheel  44  cannot rotate about the second axis of rotation. A switching element  62  fixed to the housing  60  is provided for this purpose for example, with which the first switching element  54  interacts in the first switch position S 1 . As a result of this interaction the sun gearwheel  44  is fixed to the housing  60  and cannot rotate about the second axis of rotation relative to housing  60 . 
         [0059]    In the second switch position the sun gearwheel  44  is connected via the shaft  58 , the second switching element  56  and the first switching element  54  in a torsion-proof manner to the hollow gearwheel  40 , so that the hollow gearwheel  40  and the sun gearwheel  44 —when the hollow gearwheel  40  is driven via the toothed gearwheels  48  and  50  by the electric machine  12 —orbit as a block and thus rotate together about the second axis of rotation relative to the housing  60 . 
         [0060]    For example a fourth switching element  63  is connected in a torsion-proof manner to the hollow gearwheel  40 , wherein the first switching element  54  in the second switch position S 2  interacts with the fourth switching element  63 , so that through this the sun gearwheel  44  is connected in a torsion-proof manner via the shaft  58 , the second switching element  56 , the first switching element  54  and the fourth switching element  63  to the hollow gearwheel  40 . 
         [0061]    Overall it can be seen that the sun gearwheel  44 , in the first switch position S 1 , is coupled to the housing  60  and in the second switch position S 2  to the hollow gearwheel  40  in a torsion-proof manner. It is further conceivable that the first switching element  54  is able to be moved into neutral position, in which the sun gearwheel  44  is decoupled both from the housing  60  and also from the hollow gearwheel  40 . 
         [0062]    In the first switch position S 1  the planetary gear  38  has a first gear ratio i 1 , which essentially amounts to at least 1.5. In the second switch position S 2  the planetary gear  38  advantageously has a second gear ratio i 2 , which essentially at least amounts to 1. Thus the first switch position S 1  is a slow gear or a starting ratio, in which an especially high initial acceleration can be realized. This enables the vehicle to be accelerated especially strongly by means of the electric machine  12 . The second switch position S 2  is a fast gear, by means of which an especially high top speed of the vehicle can be realized by means of the electric machine  12 . 
         [0063]    The propulsion device  10  advantageously includes an actuator  64  especially shown schematically in  FIG. 1  and coupled to the switching device  52 , in particular to the first switching element  54 , in a way not shown in any greater detail, by means of which the switching element  54  is able to be switched or moved. The actuator  64  is embodied for example as an electromechanical actuator or hydraulic actuator, in particular an electrohydraulic actuator, or electromagnetic actuator, so that the first switching element  54  can be switched by means of the actuator  64  automatically or in an automated manner or semi-automatically or in a semi-automated manner. 
         [0064]    As an especially simple solution the switching device  52  operates purely by making a form fit while interrupting the flow of power during the changing of switching stages S 1  and S 2 , wherein this change is also called a gear change. Thus, provision is advantageously made for the first switching element  54  to interact in the first switch position S 1  by a form fit with the third switching element  62  and in the second switch position S 2  to interact by a form fit with the fourth switching element  63 , so that a form-fit coupling of the sun gearwheels  44  with the housing  60  or the hollow gearwheel  40  respectively is provided. To this end the switching device  52  is embodied as a claw switch for example, so that the first switching element  54  is embodied as the switching claw. The switching claw in each case has teeth for example, wherein die switching elements  62  and  63  are embodied as respective sets of teeth. Through this the teeth act in the respective switch positions S 1  and S 2  in a form fit with one another. 
         [0065]    In a comparatively more complex version the switch positions S 1  and S 2 , also referred to as gears, can be changed without interrupting the tractive power. This means for example that the first switching element  54  in the first switch position S 1  interacts by a friction fit with the third switching element  62  and in the second switch position S 2  by a friction fit with the fourth switching element  63 , so that then the sun gearwheel  44  is coupled in each case by a friction fit with the housing  60  or with the hollow gearwheel  40  respectively. 
         [0066]    The design of the actuator  64  is oriented for example to the respective design of other actuators used in the propulsion device  10 , so that these actuators use the same operating principle. The switching element  54  is a separation element, which is used for coupling and decoupling or separating the sun gearwheel  44 . For this separating element for example an axial form fit in particular in the form of a claw coupling similar to a synchronizing unit, or a friction fit, in particular with flat or cone-shaped friction surfaces, is conceivable. 
         [0067]    Through the first switch position S 1  a low speed range is able to be realized, in which the vehicle is moved or driven respectively, i.e. can be driven by the electric machine  12 . Through the second switching stage S 2  for example a so-called high speed range is able to be realized, in which the vehicle can be driven, wherein the high speed range is higher than the low speed range. Advantageously, provision is also made for the planetary gear  38  to have a stationary gear ratio i 0 , which lies for example in a range from −4 inclusive to −1.5 inclusive. Through the integration of the planetary gear  38  an additional gear ratio is realized for the low speed range, for example an effective gear ratio of i i =1−i 0   −1 , with retention of the direction of rotation. Advantageously, provision is made for a gear step when switching from a low speed range to a high speed range, wherein this gear step lies in a range from 1.3 inclusive to 1.6 inclusive, which has been shown to be advantageous for an electric vehicle, in particular a Battery-Electric Vehicle (BEV). For a Hybrid Vehicle (HEV) a greater gear step can be advantageous, wherein the limits of the concept are able to be expanded by using a kinematic equivalent planet set. This means that  FIG. 1  shows a first form of embodiment of the propulsion device  10 , wherein  FIGS. 2 to 7  illustrate further possible forms of embodiment of the propulsion device  10 . 
         [0068]    In the low speed range, i.e. in the first switch position S 1 , the sun gearwheel  44  is firmly held via the switching device  52 , in particular in a form fit, wherein the drive takes place via the hollow gearwheel  40  with a ratio retaining the direction of rotation into the slow range. In the high speed range, i.e. in the second switch position S 2 , the sun gearwheel  44  is connected with the aid of the switching device  52 , in particular in a form fit, to the hollow gearwheel  40 , so that the planetary gear  38  or the planet set then orbits as a block. 
         [0069]    Thus overall a two-gear stage is realized. For an especially simple way of realizing the two-gear stage the integration of a rotational speed sensor, in particular on housing  60 , is advantageous, in order for example either for the simple variant with tractive power interruption, to synchronize by rapid electric regulation or in order with the more complex variant capable of load switching, to enable the slip behavior of the friction-fit power-guiding components to be better regulated. 
         [0070]    It is further advantageous that both the variant with and also the variant without tractive power interruption are able to be designed with just one active element as actuator  64 . It is further conceivable to dispose the differential gear  22  and the switching device  52  differently along the axis  24  in the vehicle, which in particular involves the axial location of the differential ring gearwheel, i.e. the toothed gearwheel  48 . 
         [0071]    It might also be additionally possible to integrate a parking brake with at least one parking brake element into the propulsion device  10  and to actuate the parking brake element by means of the same actuator  64  as the first switching element  54 , i.e. to move it. Then a primary actuator of the parking brake can be dispensed with, wherein for reasons of functional safety only a significantly more simple secondary actuator is still provided for the parking brake element. 
         [0072]    Also conceivable are other arrangements of the switching device  52 , in particular as a coaxial construction element directly on the electric machine  12  or as a parallel arrangement based on cylindrical gears. By means of the propulsion device  10  shown in  FIG. 1  the installation space requirement can be kept especially low however. The advantage of the propulsion device  10  in accordance with  FIG. 1  is that the two-gear stage as a constructional unit with the differential gear  22  provides the opportunity for compressing functions, since the differential gear  22  and the switching stage or switching device  52  respectively can form one unit. In addition it is possible in an especially simple manner to actuate the parking brake element by means of the actuator  64 , by means of which the first switching element  54  is also actuated. 
         [0073]      FIG. 2  shows a schematic illustration of a second embodiment of a propulsion device  10  according to the present invention. Parts corresponding with those in  FIG. 1  are denoted by identical reference numerals. The description below will center on the differences between the embodiments. In this embodiment, provision is made for planetary gear  38  of the planetary gear  38 , which has a stationary gear ratio i 0  in an advantageous range of i 0 =−0.54 . . . −53, with 
         [0000]    
       
         
           
             
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         [0000]    wherein z A  designates the number of teeth of the hollow gearwheel  40 , and z B  designates the number of teeth of the hollow gearwheel  40 . In addition, the respective planet gearwheel  46  is embodied as a double planet gearwheel, so that the planet gearwheel  46  has two planet gearwheel elements  66  and  68 , which are connected to one another in a torsion-proof manner. In this case z P1  designates the number of teeth of the planet gearwheel element  66  and z P2  number of teeth of the planet gearwheel element  68 . The planet gearwheel element  66  is in engagement with the hollow gearwheel  40  and the planet gearwheel element  68  is in engagement with the hollow gearwheel  40 . 
         [0074]      FIG. 3  shows a schematic illustration of a third embodiment of a propulsion device  10  according to the present invention. In this embodiment, the stationary gear ratio i 0  ranges according to i 0 =−1.2 . . . −11, with 
         [0000]    
       
         
           
             
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         [0075]      FIG. 4  shows a schematic illustration of a fourth embodiment of a propulsion device  10  according to the present invention. In this embodiment, a second hollow gearwheel  41  is provided as the third gear element instead of a sun gearwheel in addition to hollow gearwheel  40 . The stationary gear ratio i 0  ranges according to i 0 =1 . . . 2.7, with 
         [0000]    
       
         
           
             
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         [0000]    wherein z B  designates the number of teeth of the hollow gearwheel  40  and z A  the number of teeth of the hollow gearwheel  41 . 
         [0076]      FIG. 5  shows a schematic illustration of a firth embodiment of a propulsion device  10  according to the present invention. In this embodiment, a further planet gearwheel  70  is provided in addition to planet gearwheel  46 , wherein the planet gearwheels  46  and  70  are not connected to one another in a torsion-proof manner, but engage with one another via their respective teeth, so that the planet gearwheels  46  and  70  mesh with one another and can be rotatable relative to one another. In this case the planet gearwheel  46  is in engagement with the sun gearwheel  44  as well as being in engagement with the planet gearwheel  70 , which is in engagement with the planet gearwheel  46  and in engagement with the hollow gearwheel  40 . The stationary gear ratio i 0  ranges according to i 0 =1.2 . . . −17.6, with 
         [0000]    
       
         
           
             
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         [0077]      FIG. 6  shows a schematic illustration of a sixth embodiment of a propulsion device  10  according to the present invention. In this embodiment, both the third gear element and also the first gear element are embodied as sun gearwheels  44  and  45 , wherein the planet gearwheel  46  is embodied as a double planet gearwheel. In this case, the planet gearwheel elements  66  and  68  are connected to one another in a torsion-proof manner, wherein the planet gearwheel element  66  is in engagement with the sun gearwheel  44  and the planet gearwheel element  68  is in engagement with the sun gearwheel  45 . The stationary gear ratio i 0  ranges according to i 0 =1.2 . . . 41, with 
         [0000]    
       
         
           
             
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         [0000]    wherein z B  designates the number of teeth of the sun gearwheel  45  and z A  the number of teeth of the sun gearwheel  44 . 
         [0078]    While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit and scope of the present invention. The embodiments were chosen and described in order to explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. 
         [0079]    What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein: