Method and control unit for operating a motor vehicle having an electric all-wheel drive

Abstract

The invention relates to a control unit for a motor vehicle, which has a first electric drive machine, which is designed to apply a first axle torque to a first axle of the motor vehicle via a first axle transmission, and which has a second electric drive machine, which is designed to apply a second axle torque to a second axle of the motor vehicle via a second axle transmission. The control unit is designed to determine that the motor vehicle is being operated in a manoeuvring operating range. The control unit is also designed, while the motor vehicle is being operated in the manoeuvring operating range, to cause the first drive machine to always set only a first axle torque in a first direction; in such a way that the second drive machine always only sets a second axle torque in a second direction which is opposite to the first direction; and in that predominantly an axle torque is set by only one of the two drive machines.

Description

[0001] Method and control unit for operating a motor vehicle with an electric all-wheel drive

[0002] The invention relates to a method and a corresponding control unit designed to enable particularly efficient and comfortable operation of a motor vehicle with electric all-wheel drive during a maneuver.

[0003] A (motor) vehicle with an electric all-wheel drive system can have a first electric drive motor for driving a first axle (e.g., the rear axle) and a second electric drive motor for driving a second axle (e.g., the front axle). The drive motors are typically each coupled to the respective axle via an axle drive. The axle drive often has backlash, which can lead to noises (known as load change clacking) when load changes occur. This noise can be unpleasant for the driver, especially at relatively low speeds (e.g., when maneuvering the vehicle).

[0004] This document addresses the technical problem of efficiently increasing the comfort of a vehicle with an electric all-wheel drive during maneuvering of the vehicle. This problem is solved by each of the independent claims. Advantageous embodiments are described, among other things, in the dependent claims. It is pointed out that additional features of a patent claim dependent on an independent patent claim, without the features of the independent patent claim or only in combination with a subset of the features of the independent patent claim, can form a separate invention independent of the combination of all features of the independent patent claim, which can be made the subject of an independent claim, a divisional application, or a subsequent application.This applies equally to technical teachings described in the description, which may constitute an invention independent of the features of the independent patent claims.

[0005] According to one aspect, a control unit for a motor vehicle is described, wherein the motor vehicle has a first electric drive machine which is configured to provide a first axle torque to a first axle (e.g., to the rear axle) of the motor vehicle via a first axle transmission, and which has a second electric drive machine which is configured to provide a second axle torque to a second axle (e.g., to the front axle) of the motor vehicle via a second axle transmission. In an alternative example, the first axle is the front axle and the second axle is the rear axle of the motor vehicle. All-wheel drive and / or 4WD operation of the motor vehicle can be provided via the two drive machines. It should be noted that the first drive machine can have a dedicated (partial) drive machine for each of the two wheels of the first axle. In a corresponding manner, if necessary,the second drive motor for each of the two wheels of the second axle has a dedicated (partial) drive motor. The control unit is configured to determine that the motor vehicle is being operated in a shunting operating range. The control unit can be configured to determine, based on the current driving speed of the motor vehicle and / or on the (current) requirement for positive or negative acceleration of the motor vehicle, whether the motor vehicle is being operated in the shunting operating range or not. The shunting operating range can, for example, exist (in particular, only exist) if,

[0006] • the travel speed is equal to or less than a maximum permissible speed (e.g. a basic creep speed), e.g. 2 m / s; and / or

[0007] • the requirement for positive or negative acceleration is equal to or less than a maximum permissible acceleration (e.g. 0.1g).

[0008] The control unit can in particular be configured to detect that

[0009] • a driving mode is activated in which the vehicle automatically travels at a crawling speed (especially at the base crawling speed) on a level road surface even when the vehicle's accelerator pedal is not depressed. This can be the case, for example, if driving mode D is engaged.

[0010] • the accelerator pedal of the motor vehicle is not pressed.

[0011] It can thus be detected that the vehicle is crawling (without pressing the accelerator pedal). In such a driving mode, the change in driving speed can be achieved by pressing the brake pedal. In particular, the driving speed can be reduced compared to the base crawling speed by pressing the brake pedal.

[0012] In response to the detection (i.e., in particular, if it is recognized that both of the above-mentioned conditions are met), it can be determined that the motor vehicle is operating in the maneuvering operating range. Operation in the maneuvering operating range can thus occur when the motor vehicle is crawling and when (simultaneously) the accelerator pedal is not being actuated. As explained further below, such (crawling) operation of the motor vehicle can be effected in a particularly efficient and convenient manner using the first and second electric drive motors.

[0013] The control unit is further configured to effect a specific use of the first and second electric drive motors while the motor vehicle is operated in the shunting operating range, in particular during the total period in which the motor vehicle is operated in the shunting operating range.

[0014] The specific use of the first and second prime movers includes the first prime mover always providing only a first axle torque in a first direction (where the axle torque provided by the first prime mover is referred to as the "first" axle torque). That is, the first prime mover is restricted to providing axle torques in the first direction. Conversely, the first prime mover does not provide axle torques in the opposite second direction. The first direction may correspond to the forward direction, and the second direction may correspond to the reverse direction. Alternatively, the first direction may correspond to the reverse direction, and the second direction may correspond to the forward direction.Because the first drive motor only provides axle torques in the first direction, a flank change of the first axle drive can be avoided in an efficient and reliable manner (solely due to inertia and / or friction).

[0015] The specific use of the first and second drive motors further includes the second drive motor always providing only a second axle torque in the second direction (where the axle torque provided by the second drive motor is referred to as the "second" axle torque). This means that the second drive motor is restricted to providing axle torques in the second direction. On the other hand, no axle torques are provided by the second drive motor in the opposite first direction. By only providing axle torques in the second direction by the second drive motor, a flank change of the second axle drive (solely due to inertia and / or friction) can be efficiently and reliably avoided.

[0016] The specific use of the first and second drive motors further includes the fact that an axle torque is provided predominantly and / or predominantly by only one of the two drive motors. Thus, a predominantly and / or predominantly mutually exclusive operation of the first and second drive motors can be effected. In particular, it can be effected that an axle torque is provided in a mutually exclusive manner by only one of the two drive motors for more than 50%, in particular for more than 80%, of the total period in which the motor vehicle is operated in the shunting operating area.

[0017] The control unit can be set up to cause, while the motor vehicle is operated in the shunting operating area, predominantly and / or mostly and / or for more than 50% of the total period, that

[0018] • the first axle torque is zero if the second axle torque is greater than zero; and / or

[0019] • the second axle torque is zero if the first axle torque is greater than zero.

[0020] Alternatively or additionally, the control unit can be set up to cause, while the motor vehicle is operated in the shunting operating area, predominantly and / or mostly and / or for more than 50% of the total period, that • the first drive motor is operated in passive overrun mode (without current supply) when a second axle torque is actively provided by the second drive motor; and / or

[0021] • the second drive motor is operated in passive overrun mode (without power supply) when a first axle torque is actively provided by the first drive motor.

[0022] The mutually exclusive operation of the first and second drive units enables particularly energy-efficient operation in shunting operations. Bracing of the two drive units is not necessary, as flank changes of the axle drives are avoided solely by passive overrun operation and the inertia and / or friction of the respective drive train (with drive unit and axle drive).

[0023] The control unit can be configured to ensure that, while the motor vehicle is operated in the shunting operating area (i.e. within the scope of the specific use), apart from one or more transition phases between the exclusive operation of the first drive motor or the second drive motor (and possibly apart from a detected special situation), an axle torque is always only provided by one of the two drive motors. In other words, the simultaneous operation of both drive motors can be limited to the individual transition phases between the exclusive operating phases of the individual drive motors (and possibly to a detected special situation). The one or more transition phases can each have a total duration of 5 seconds or less, in particular of 1 second or less.

[0024] Furthermore, the one or more transition phases can be designed in such a way that in the one or more transition phases • the axle torque provided by one of the two drive machines is increased in amount starting from zero (continuously, along a ramp); and

[0025] • the axle torque provided by the other drive machine is reduced to zero starting from an initial value (continuously, along a ramp).

[0026] In the individual transition phases, a (ramp-like) reduction in the axle torque of one of the two drive units and a (ramp-like) increase in the axle torque of the other drive unit can be achieved (if necessary exclusively). This allows for particularly efficient and comfortable operation in the shunting area.

[0027] The control unit can be configured to cause the first drive motor to set a base axle torque when the brake pedal of the motor vehicle is not depressed while the motor vehicle is operating in the maneuvering mode, in order to effect a base creep speed of the motor vehicle (where the base creep speed may be effected when the motor vehicle is traveling on a level roadway). The first drive motor can thus be used to simulate creeping, which is known from vehicles powered by an internal combustion engine at idle.

[0028] The control unit can also be configured to determine, depending on the position of the brake pedal effected by the driver of the motor vehicle

[0029] • to cause the second drive motor to provide a second axle torque in order to reduce the driving speed of the motor vehicle compared to the basic crawling speed; and / or

[0030] • to cause the first drive motor to produce a first axle torque in order to increase the driving speed of the motor vehicle to the base creep speed. This makes it possible for the driver of the motor vehicle to change the creep speed of the motor vehicle simply by pressing the brake pedal. In this case, the second drive motor is used (exclusively) to reduce the creep speed (apart from one or more transition phases, and possibly apart from a detected special situation). Furthermore, the first drive motor is used (exclusively) to increase the creep speed (apart from one or more transition phases, and possibly apart from a detected special situation). This makes it possible to achieve particularly efficient and comfortable creep operation of the motor vehicle.

[0031] The control unit can be configured (particularly while the motor vehicle is being operated in the shunting operation area) to detect that a special situation exists in which a flank change could occur in the first axle drive or in the second axle drive. In particular, a special situation can be detected in which there is an increased risk of such a flank change. A flank change can be brought about if the force acting on the respective axle drive in the direction of the flank change is greater than the (inertial and / or frictional) force maintaining the current flank position. A force acting in the direction of the flank change can be caused, for example, by driving resistance (e.g. headwind) of the motor vehicle.

[0032] A special situation may arise if

[0033] • neither of the two drive machines provides an axle torque that is greater than zero; and

[0034] • the motor vehicle is nevertheless accelerated or decelerated (this can be detected on the basis of the sensor data from an acceleration sensor of the motor vehicle). The control unit can further be configured, in response to the detection, to bring about one or more measures aimed at preventing or reversing the edge change. As a measure, for example, the first drive motor and the second drive motor can be tensioned (temporarily, e.g. limited to the duration of the special situation). The first drive motor can then be caused to set a first axle torque (in the first direction) with a first value. Furthermore, the second drive motor can be caused to set a second axle torque (in the second direction) with a second value. The first and second values ​​can be the same. In this way, an edge change can be reliably avoided or reversed.Furthermore, the comfort of operating the vehicle in shunting operations can be further increased (without significantly impairing efficiency).

[0035] According to a further aspect, a (road) motor vehicle (in particular a passenger car or a truck or a bus or a motorcycle) is described which comprises the control unit described in this document.

[0036] According to one aspect, a method for operating a motor vehicle is described, which has a first electric drive motor configured to provide a first axle torque to a first axle (e.g., to the rear axle) of the motor vehicle via a first axle transmission, and which has a second electric drive motor configured to provide a second axle torque to a second axle (e.g., to the front axle) of the motor vehicle via a second axle transmission. The method comprises determining that the motor vehicle is being operated in a maneuvering operating range (at a relatively low driving speed, in particular at a crawling speed, and / or with a relatively low acceleration or deceleration requirement).The method further comprises causing, while the motor vehicle is operated in the shunting operating range, that the first drive machine always only provides a first axle torque in a first direction (and not in the second direction), that the second drive machine always only provides a second axle torque in a second direction which is opposite to the first direction (and not in the first direction), and that predominantly (in particular for more than 50% of the total time period of operation in the shunting operating range) only one of the two drive machines provides an axle torque.

[0037] It should be noted that the aspects described in connection with the control unit, in particular the claims described in connection with the control unit, are also to be applied to the method as corresponding method features.

[0038] According to another aspect, a software (SW) program is described. The SW program can be configured to be executed on a processor (e.g., on a vehicle control unit) and thereby to carry out the method described in this document.

[0039] According to a further aspect, a storage medium is described. The storage medium can comprise a software program configured to be executed on a processor and thereby to carry out the method described in this document.

[0040] It should be noted that the methods, devices, and systems described in this document can be used both alone and in combination with other methods, devices, and systems described in this document. Furthermore, any aspects of the methods, devices, and systems described in this document can be combined in a variety of ways. In particular, the features of the claims can be combined in a variety of ways. Furthermore, features listed in parentheses are to be understood as optional features.

[0041] The invention will be described in more detail below using exemplary embodiments.

[0042] Figure 1 schematically shows the flank clearances on both axles of a motor vehicle; Figure 2a shows the possible axle torque components during operation within the shunting operating range;

[0043] Figure 2b shows exemplary speed and acceleration limits for defining the shunting operating area;

[0044] Figure 2c shows an example of the driving speed during a shunting manoeuvre; and

[0045] Figure 3 is a flowchart of an exemplary method for operating a motor vehicle.

[0046] As initially stated, this document deals with efficiently increasing the comfort of a motor vehicle with electric all-wheel drive. Fig. 1 schematically shows the flank play 102, 107 on the two axles VA and HA of a motor vehicle 1. The vehicle 1 comprises a drive arrangement 10 with a first electric drive motor 4, which is designed to drive a first axle (HA) via a first axle transmission (GHA). Furthermore, the drive arrangement 10 comprises a second electric drive motor 2, which is designed to drive a second axle (VA) via a second axle transmission (GVA). In the example shown in Fig. 1, the first axle is the rear axle (HA) and the second axle is the front axle (VA). A first axle torque (MHA) can be set (i.e., caused) on the first axle (HA) by the first drive motor 4.A second axle torque (MVA) can be provided (i.e., caused) on the second axle (VA) by the second drive machine 2. The vehicle 1 comprises a control unit S that is designed to control the first and second drive machines. The axle transmissions each have thrust flanks 105, 106 that abut one another when a thrust torque is provided by the respective axle transmission (to cause a negative acceleration a). Furthermore, the axle transmissions each have traction flanks 104, 107 that abut one another when a traction torque is provided by the respective axle transmission (to cause a positive acceleration a). A thrust torque is typically an axle torque with a negative sign or in a negative (backward) direction, and a traction torque is typically an axle torque with a positive sign or in a positive (forward) direction.

[0047] To avoid noise from the axle gears during load changes, in particular to avoid the load change clatter, the control unit S can cause

[0048] • the first drive machine 4 always only produces a first axle torque MHA in a first direction or with a first sign; and

[0049] • the second drive machine 2 always only produces a second axle torque MVA in a second direction (opposite to the first direction) or with a second sign (opposite to the second sign).

[0050] In the example shown in Fig. 1, the first direction is the positive (forward) direction, and the second direction is the negative (backward) direction. However, the opposite assignment can also be chosen.

[0051] In the example shown, it is thus possible to ensure that the traction flanks 104 of the first axle drive (GHA) are permanently in contact with each other, and that the thrust flanks 105 of the second axle drive (GVA) are permanently in contact. Thus, flank changes during load changes and the resulting noise can be avoided.

[0052] Furthermore, it is possible to ensure that only one of the two drive motors 2, 4 actively provides an axle torque at any one time. The other drive motor 4, 2 is dragged along, which (due to inertia and friction) automatically results in the flank position of the dragged axle drive being maintained. This allows for particularly efficient operation of the drive arrangement 10 without the need for active tensioning of the two drive motors 2, 4.

[0053] Fig. 2a illustrates the possible total axle torques that can be provided by the drive assembly 10 with the above-mentioned operating strategy. If the first axle torque MHA>0, then the second axle torque MVA=0. On the other hand, if the second axle torque MVA<0, then the first axle torque MHA=0. In the example shown in Fig. 2a,

[0054] • the first direction is the forward direction and the first sign is the positive sign; and

[0055] • the second direction is the reverse direction and the second sign is the negative sign.

[0056] As already explained, the assignment can be exactly the opposite in an alternative example. Thus, as an operating strategy, it can be ensured that (whenever) one of the axle torques is not equal to zero, the other axle torque is equal to zero.

[0057] The above-mentioned operating strategy for the drive arrangement 10 is preferably limited to a specific, predefined maneuvering operating range RB, as shown by way of example in Fig. 2b. Fig. 2b shows exemplary limits for the application of the described operating strategy. In the diagram, the speed change a is plotted against the speed v. The inner, dashed ellipsoid represents the maneuvering operating range RB of the motor vehicle 1. Outside the maneuvering operating range RB, both drive motors 2, 4 can be used to actively set axle torques, so that 4WD functionality is available here. Within the maneuvering operating range RB, the request for a speed change a for an acceleration a>0 and / or a deceleration a<0 can be effected (possibly solely) by means of the position of the brake pedal (while the accelerator pedal is not being operated).Depending on the brake pedal position, the drive motors 2 and 4 are then controlled by the control unit S according to the described operating strategy.

[0058] The control unit S can be configured to cause one of the two drive motors 2, 4 to generate a base axle torque while the vehicle 1 is operating in the shunting operating range RB. For example, the first drive motor 4 can be caused to generate the base axle torque when the vehicle 1 is to travel in the first (e.g., forward) direction (e.g., when the forward gear of the vehicle 1 is engaged). On the other hand, the second drive motor 2 can be caused to generate the base axle torque when the vehicle 1 is to travel in the second (e.g., reverse) direction (e.g., when the reverse gear of the vehicle 1 is engaged).

[0059] The base axle torque can be used to cause vehicle 1 (on a level road surface without an incline) to travel at a specific crawling speed (e.g., approximately 2 m / s). The base axle torque can also be applied when the accelerator pedal of vehicle 1 is not depressed. The base axle torque can be used to simulate the typical idling behavior of a vehicle 1 with a combustion engine.

[0060] This makes it possible to provide a driving mode that mimics the driving behavior of a combustion engine (e.g., driving mode D), in which, without pedal operation, the vehicle 1 crawls, as with a torque converter transmission, and in which, by operating the brake pedal, a reduction in driving speed and, if necessary, a stop to a standstill can be achieved. In a vehicle with a combustion engine, the reduction in driving speed is typically achieved using the vehicle's friction brake. When using an electric all-wheel drive, the electric drive motors 2, 4 can be used to reduce the driving speed starting from the base crawling speed when the brake pedal is operated.

[0061] Fig. 2c shows an example of operation of vehicle 1 in the shunting operating range. The first drive motor 4 provides the (positive) base axle torque (for forward travel), which results in vehicle 1 traveling at the base crawling speed (in the section between times "a" and "b"). No axle torque is provided by the second drive motor 2. At time "b", the driver actuates the brake pedal to reduce the driving speed v of vehicle 1 to a reduced crawling speed. This is achieved by the second drive motor 2 providing a (negative) axle torque and by the first drive motor 4 providing no axle torque.

[0062] The reduced creep speed corresponding to the position of the brake pedal is reached at time “c”, which means that the second drive motor 2 no longer provides any axle torque, and the first drive motor 4 provides a (positive) reduced axle torque. At time “d”, the driver reduces the actuation angle of the brake pedal in order to increase the creep speed back towards the base creep speed. This results in the axle torque provided by the first drive motor 4 being increased, while the second drive motor 2 continues to provide no axle torque. From time “f ' the actuation angle of the brake pedal is increased again in order to decelerate the vehicle 1 to a standstill. As a result, the second drive motor 2 provides a (negative) axle torque, while the first drive motor 4 no longer provides any axle torque.As soon as standstill is reached (at time “g”), it can be ensured that no axle torque is provided by either of the two drive machines 2, 4.

[0063] As can be seen from Fig. 2c, the alternating, mutually exclusive operation of the two electric drive motors 2, 4 can thus result in a particularly comfortable and efficient shunting operation of the vehicle 1.

[0064] It should be noted that during the individual transitions between the first and second drive motors 2, 4, a second axle torque can be provided by the second drive motor 2 and a first axle torque can be provided by the first drive motor 4 simultaneously in a time-limited transition phase, with one of the two axle torques being increased starting from zero and the other axle torque being reduced to zero. The two axle torques can thus exhibit temporal gradients with opposite signs in the individual transition phases. This makes it possible to achieve a smooth transition between the first and second drive motors 2, 4, which can potentially further increase driving comfort.

[0065] In the shunting operating range RB (e.g. in the low-speed range and / or when crawling), the front and rear axles can thus be operated with different torque signs without the respective torque sign being changed (during the entire duration of operation in the shunting operating range RB). For forward travel, the (first) drive motor on the rear axle can be used for acceleration, and the (second) drive motor on the front axle can be used for deceleration. In this case, there is no active tensioning of the electric drive motors against each other. The mass moment of inertia and / or the friction in the respective drive motor ensure that the correct gear flank is permanently applied and that both accelerating and decelerating forces can be applied without going through the reversing play.

[0066] To enter and / or leave the shunting operating area RB from or into a driving state in which the drive motors have identical torque signs, a change of the applied flank on (exactly) one of the two drive motors can be actively effected.

[0067] The control unit S can be configured to detect a driving condition (i.e., a special situation) in which there is an increased risk of the gear flanks changing their contact side (e.g., during travel with deceleration caused solely by driving resistance, and in which a zero torque is set by both drive motors). One or more measures can then be initiated (such as actively bracing both electric drive motors) to prevent a flank change or to restore the correct contact side of the gear flanks.

[0068] Fig. 3 shows a flowchart of a (possibly computer-implemented) method 300 for operating a motor vehicle 1, which has a first electric drive machine 4, which is configured to provide a first axle torque MHA on a first axle HA of the motor vehicle 1 via a first axle transmission GHA, and which has a second electric drive machine 2, which is configured to provide a second axle torque MVA on a second axle VA of the motor vehicle 1 via a second axle transmission GVA. The motor vehicle 1 preferably has no further drive machine for driving the motor vehicle 1 apart from the first and second drive machines 2, 4. Furthermore, the motor vehicle 1 preferably has only the first and second axles. The method 300 can be carried out by a control unit S of the motor vehicle 1.

[0069] The method 300 includes determining 301 that the motor vehicle 1 is operated in a shunting operating area RB. The shunting operating area RB may exist if

[0070] • the driving speed v of the motor vehicle 1 is equal to or less than a maximum permissible (basic) creep speed, e.g. 2 m / s; and / or

[0071] • the acceleration requirement a or the acceleration a of the motor vehicle 1 is equal to or less than a maximum permissible acceleration (e.g. 0.1g, where g is the normal acceleration due to gravity).

[0072] A further condition for the existence of operation in the shunting operating area RB may be that the accelerator pedal of motor vehicle 1 is not actuated.

[0073] The method 300 further comprises causing 302, while the motor vehicle 1 is operated in the shunting operating area RB (ie during the entire period in which the motor vehicle 1 is operated in the shunting operating area RB), that

[0074] • the first drive machine 4 always only provides a first axle torque MHA in the first direction (and thus never an axle torque in the opposite second direction);

[0075] • the second drive motor 2 always only provides a second axle torque MVA in the second direction, which is opposite to the first direction (and thus never an axle torque in the first direction). Furthermore, it can be ensured that, while the motor vehicle 1 is operated in the shunting operating range RB (i.e., during the entire period in which the motor vehicle 1 is operated in the shunting operating range RB), an axle torque is predominantly (in particular for more than 50% of the entire period) provided by only one of the two drive motors 2, 4.

[0076] By such mutually exclusive operation of the two drive motors 2, 4, load change noises of the motor vehicle 1 during shunting operation can be avoided in a particularly (energy) efficient manner.

[0077] The present invention is not limited to the embodiments shown. In particular, it should be noted that the description and figures are intended only to illustrate the principle of the proposed methods, devices, and systems by way of example.

Claims

Claims 1) Control unit (S) for a motor vehicle (1), which has a first electric drive machine (4) which is designed to provide a first axle torque (MHA) on a first axle (HA) of the motor vehicle (1) via a first axle transmission (GHA), and which has a second electric drive machine (2) which is designed to provide a second axle torque (MVA) on a second axle (VA) of the motor vehicle (1) via a second axle transmission (GVA); wherein the control unit (S) is designed - to determine that the motor vehicle (1) is operated in a shunting area (RB); and - while the motor vehicle (1) is operated in the shunting area (RB), to ensure that - the first drive machine (4) always only provides a first axle torque (MHA) in a first direction; - the second drive machine (2) always provides only a second axle torque (MVA) in a second direction which is opposite to the first direction; and - an axle torque is predominantly provided by only one of the two drive machines (2, 4). 2) Control unit (S) according to claim 1, wherein the control unit (S) is designed, while the motor vehicle (1) is operated in the shunting operating area (RB), to predominantly cause - the first axle torque (MHA) is zero if the second axle torque (MVA) is greater than zero; and - the second axle torque (MVA) is zero if the first axle torque (MHA) is greater than zero. 3) Control unit (S) according to one of the preceding claims, wherein the control unit (S) is designed, while the motor vehicle (1) is operated in the shunting operating area (RB), to predominantly cause - the first drive unit (4) is operated in passive overrun mode when a second axle torque (MVA) is actively provided by the second drive unit (2); and - the second drive machine (2) is operated in passive overrun mode when a first axle torque (MHA) is actively provided by the first drive machine (4). 4) Control unit (S) according to one of the preceding claims, wherein the control unit (S) is configured to ensure that, while the motor vehicle (1) is operated in the shunting operating range (RB), an axle torque is always provided only by one of the two drive machines (2, 4), apart from one or more transition phases between exclusive operation of the first drive machine (4) and exclusive operation of the second drive machine (2). 5) Control unit (S) according to claim 4, wherein in the one or more transition phases - the axle torque provided by one of the two drive motors (2, 4) is increased in amount starting from zero; and - the axle torque provided by the other drive machine (2, 4) is reduced to zero starting from an initial value. 6) Control unit (S) according to one of claims 4 to 5, wherein the one or more transition phases each have a total duration of 5 seconds or less, in particular of 1 second or less. 7) Control unit (S) according to one of the preceding claims, wherein the control unit (S) is configured to ensure that, while the motor vehicle (1) is operated in the shunting operating range (RB), an axle torque is provided in a mutually exclusive manner by only one of the two drive machines (2, 4) for more than 50%, in particular more than 80%, of the period in which the motor vehicle (1) is operated in the shunting operating range. 8) Control unit (S) according to one of the preceding claims, wherein the control unit (S) is configured to determine, on the basis of a current driving speed of the motor vehicle (1) and on the basis of a request for a positive or negative acceleration of the motor vehicle (1), whether the motor vehicle (1) is operated in the shunting operating range (RB) or not. 9) Control unit (S) according to one of the preceding claims, wherein the control unit (S) is arranged - to detect that - a driving mode is activated in which the motor vehicle (1) automatically drives at a crawling speed on a level road surface even if the accelerator pedal of the motor vehicle (1) is not actuated; and - the accelerator pedal of the motor vehicle (1) is not actuated; and - to determine in response to the detection that the motor vehicle (1) is operated in the shunting operating area (RB). 10) Control unit (S) according to one of the preceding claims, wherein the control unit (S) is set up while the motor vehicle (1) is operated in the shunting operating area (RB), - to cause the first drive machine (4) to set a base axle torque when the brake pedal of the motor vehicle (1) is not actuated to effect a basic creep speed of the motor vehicle (1); and - depending on the position of the brake pedal effected by the driver of the motor vehicle (1) - to cause the second drive motor (2) to provide a second axle torque in order to reduce the driving speed (1) of the motor vehicle (1) compared to the basic crawling speed; and - to cause the first drive motor (4) to produce a first axle torque in order to increase the driving speed (1) of the motor vehicle (1) towards the basic crawling speed. 11) Control unit (S) according to one of the preceding claims, wherein the control unit (S) is set up while the motor vehicle (1) is operated in the shunting operating area (RB), - to detect that a special situation exists in which a flank change could occur in the first axle drive (GHA) or in the second axle drive (GVA); and - in response to the detection, to take one or more measures aimed at preventing or reversing the flank change. 12) Method (300) for operating a motor vehicle (1) having a first electric drive machine (4) configured to provide a first axle torque (MHA) on a first axle (HA) of the motor vehicle (1) via a first axle transmission (GHA), and having a second electric drive machine (2) configured to provide a second axle torque (MVA) on a second axle (VA) of the motor vehicle (1) via a second axle transmission (GVA); wherein the method (300) comprises - determining (301) that the motor vehicle (1) is operated in a shunting operating area (RB); and - causing (302), while the motor vehicle (1) is operated in the shunting operating area (RB), that - the first drive machine (4) always only has a first Axle torque (MHA) in a first direction; - the second drive machine (2) always only provides a second axle torque (MVA) in a second direction which is opposite to the first direction; and - an axle torque is predominantly provided by only one of the two drive machines (2, 4).

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