Method for controlling a motor vehicle, control unit for a motor vehicle and motor vehicle with such a control unit

Abstract

The invention relates to a method and a control unit (2) for controlling a motor vehicle (1), the service brake system (12) of which has a first wheel brake unit (13) arranged on a drivable primary axle (A1) of the motor vehicle (1), a second wheel brake unit (14) arranged on a secondary axle (A2) of the motor vehicle (1), and a traction unit (9). In the method, a standby state is established in which the first wheel brake unit (13) is set to a wheel-release position, in which rotation of wheels (5, 6) mounted on the primary axle (A1) is released, and the second wheel brake unit (14) is set to a wheel-blocking position, in which wheels (7, 8) mounted on the secondary axle (A2) are blocked against rotation.Then, in the standby state, a first drive torque is applied to the wheels (5, 6) of the primary axle (A1) by means of the traction unit (9), which is dimensioned such that the motor vehicle (1) does not move against the wheels (7, 8) of the secondary axle (A2) blocked by the second wheel brake unit (14). Furthermore, a motor vehicle (1) equipped with the control unit (2) is proposed.

Description

[0001] The present invention relates to a method for controlling a motor vehicle, more specifically for performing a drive-off start. Furthermore, the invention relates to a control unit for a motor vehicle that is configured to carry out such a method or to control a motor vehicle according to such a method. Furthermore, the invention relates to a motor vehicle equipped with such a control unit.

[0002] Nowadays, motor vehicles are often equipped with a start-start system (also known as a launch control system). This provides the driver of the motor vehicle with an assistance system that supports particularly fast starting, i.e., accelerating from a standstill, particularly for sporting use of the motor vehicle and / or for demonstration purposes, in each case on routes closed off from public traffic. Conventional launch control systems apply a starting drive torque to one or more of the motor vehicle's drive axles via a motor vehicle's drive train, while the motor vehicle is held stationary by a service brake system. The starting drive torque is specifically dimensioned to achieve the fastest possible start or move off as soon as the service brake allows the motor vehicle's wheels to rotate.In other words, conventional start-up systems pre-tension the drive train against the service brake system.

[0003] After the service brake is released, wheel slip, which is necessary for optimal acceleration, must first build up. Since the coefficient of friction between the wheels and the road surface is unknown, transient processes occur that are not optimal. For example, too little torque may be delivered to a wheel, even though more torque could have been transmitted to that wheel. If more torque is then delivered to that wheel, the wheel may spin on the road, requiring the torque to be removed. Furthermore, translational vibration of the wheel may occur.

[0004] In order to create a particularly advantageous coefficient of friction between a wheel of a motor vehicle and the road surface during a start-off event, US 2020 / 0122534 A1 proposes performing a controlled burnout of the wheels of a drive axle of the motor vehicle. In a line-lock mode, other wheels mounted on a non-driveable axle of the motor vehicle are braked by a braking system of the motor vehicle, even though a brake pedal has already been released by a user. In this way, the wheels mounted on the drive axle are heated. Multi-axle drive systems are also known that can be switched between front-wheel drive and all-wheel drive depending on the current driving condition. US 4,784,236 A discloses such a system in which the all-wheel drive is deactivated by releasing a brake band, thus giving the vehicle the operating characteristics of a pure front-wheel drive vehicle.

[0005] The object of the invention is to enable a particularly efficient starting of a motor vehicle.

[0006] This problem is solved by the subject matter of the independent claims. Further possible embodiments of the invention are disclosed in the subclaims, the description, and the figures. Features, advantages, and possible embodiments presented in the description for one of the subject matter of the independent claims are to be regarded, across categories and embodiments, at least analogously as features, advantages, and possible embodiments of the respective subject matter of the other independent claims, as well as any possible combination of the subject matter of the independent claims, optionally in conjunction with one or more of the subclaims.

[0007] According to the invention, a method for controlling a motor vehicle is proposed. The motor vehicle has a primary axle and a secondary axle, wherein the primary axle forms a drive axle of the motor vehicle. The secondary axle can be designed as a driveless trailing axle. Alternatively, the secondary axle can optionally be designed as another drive axle, for example, as an auxiliary drive axle or as part of a multi-axle drive of the motor vehicle. A wheel (rim-tire combination) of the motor vehicle is mounted on each of the lateral wheel hubs of the axles, via which the motor vehicle is positioned on a surface.

[0008] Furthermore, the motor vehicle has a traction unit, which, for example, has an internal combustion engine as the traction motor. Alternatively or additionally, the traction unit has an electric motor as the traction motor. Two or more electric motors can be provided, for example, one electric motor per axle or one electric motor per wheel (as a traction motor close to the wheel, as a traction motor integrated into the wheel hub, etc.).

[0009] In addition, the motor vehicle has a service braking system comprising a first wheel braking unit arranged on the primary axle and a second wheel braking unit arranged on the secondary axle. Each wheel braking unit has, in particular, a wheel brake for each wheel mounted on the corresponding axle. If the motor vehicle is equipped with a parking braking system, the parking braking system is not part of the service braking system. The service braking system is designed to enable braking of the wheels on an axle-specific or wheel-specific basis. The service braking system is, in particular, designed as an electromechanical braking system or has such an electromechanical braking system, since in this case, undesirable haptic feedback on the brake pedal and acoustic feedback are avoided in the event of a braking operation not initiated by the driver.Nevertheless, it is conceivable that the service brake system is or has a pneumatic and / or hydraulic brake system.

[0010] The motor vehicle, i.e., at least its traction unit and service brake system, are controllable to execute the method, for example, by means of a control unit. According to the invention, such a control unit is proposed, which is configured to execute the method. This means that the control unit is configured to control at least the service brake system and the traction unit according to the method.

[0011] The method according to the invention for controlling the motor vehicle comprises the steps described below for starting the motor vehicle. If this has not already happened, the motor vehicle is stopped, i.e. braked to a standstill. A standby state is then established in which the first wheel brake unit is set in a wheel release position in which rotation of the wheels mounted on the primary axle (hereinafter referred to as the primary wheels) is permitted. In addition, in the standby state, the second wheel brake unit is set in a wheel blocking position in which the wheels mounted on the secondary axle (hereinafter referred to as the secondary wheels) are blocked from rotating. For example, the first wheel brake unit or the first wheel brakes are completely released so that the primary wheels are released to roll, wherein the second wheel brake unit orthe maximum braking force is generated by means of the second wheel brakes, whereby the secondary wheels are braked to the maximum, i.e. they are locked against rolling. In this respect, the service braking system is designed in such a way that axle-individual or wheel-individual braking of the wheels is possible. The wheel brake units can be adjusted simultaneously or sequentially. Starting from a fully braked state of the motor vehicle, only the first wheel brake unit needs to be released in order to establish the standby state. Analogously, from an unbraked state of the motor vehicle, only the second wheel brake unit needs to be closed in order to establish the standby state. If the motor vehicle is in a partially braked state in which only a portion of a maximum braking force is applied to the respective associated wheels by means of the first and second wheel brake units, both wheel brake units must be adjusted as described above.The establishment of the ready state can be initiated, for example, by a user or driver input, such as by activating a launch control function of the vehicle. Furthermore, the establishment of the ready state can be initiated automatically, for example, after each stop of the vehicle and / or before each start of the vehicle.

[0012] In the standby state, i.e. while the primary wheels are released and the secondary wheels are blocked, a first drive torque is then applied to the wheels of the primary axle by means of the traction unit, which is dimensioned such that the motor vehicle does not move against the wheels of the secondary axle blocked by the second wheel brake unit. The first drive torque is in particular greater than zero and drives the motor vehicle in the forward direction of travel against the blocked secondary wheels. It is conceivable that for reversing, the first drive torque drives the motor vehicle against the forward direction of travel; optionally, the assignment of the primary and secondary axle functions to a rear or front axle of the motor vehicle is reversed for this purpose.In any case, the first drive torque is delivered to the primary wheels at a maximum value sufficient to prevent the locked secondary wheels from slipping on the ground. This preloads a drive train of the vehicle between the wheels of the primary axle, i.e., the primary wheels, including wheel suspension and any gear ratios arranged between the traction unit and the respective primary wheel, and the traction unit. The vehicle is held stationary by the second wheel brake unit. The primary wheels are rotated by the first drive torque, only until the idle rotation distances or tolerances required to preload the drive train are overcome.If the first wheel brake unit is released in this preconditioning state, in which the first wheel brake unit is released, the second wheel brake unit is opened, and the first drive torque is applied to the primary wheels, the vehicle accelerates at an increasing speed using the first drive torque. The preload or preconditioning state of the drivetrain significantly reduces vibrations when starting off.

[0013] The start can be a sporty start, in which the motor vehicle is accelerated from a standstill to the highest possible speed as quickly as possible. However, the method can also be used in less extreme situations, for example when starting off normally on public roads, for example after stopping at a traffic light or the like. Furthermore, the method can be used to start off a vehicle combination, whereby the primary axle or the first wheel brake unit is installed on board the towing vehicle, and the secondary axle is installed on board the trailer vehicle coupled to the towing vehicle. The vehicles in the vehicle combination are coupled together by means of a coupling device (trailer coupling). If no countermeasures are taken, unwanted impacts or jolts will occur when the vehicle combination starts off.Vibrations that occur when a coupling element on the towing vehicle and a trailer vehicle come into contact with each other during start-up. This method allows the coupling device to be pre-tensioned before the actual start-up, thus preventing unwanted shocks or vibrations during start-up.

[0014] According to a possible further development, while the first drive torque is applied to the primary wheels, the first wheel brake unit is adjusted to its wheel-locking position so that the primary wheels are blocked against further rotation. Then, by means of the traction unit, a second drive torque is applied to the primary wheels. This second drive torque is greater than the first drive torque and is dimensioned such that the motor vehicle does not move against the wheels of both axles that are blocked by the wheel brake units. In this way, after the wheel brake units are released, a greater drive torque can be efficiently used to move off and accelerate the motor vehicle, namely the second drive torque. However, when the motor vehicle is stationary, the second drive torque is initially not converted into propulsion because both wheel brake units are blocking the wheels.This is because the first drive torque acting between the primary wheels and the traction unit is frozen between the primary wheels and the respective wheel brake when the first wheel brake unit is applied. However, between the wheel brakes and the traction unit, the remaining drivetrain is preloaded with the second drive torque when stationary, so that the second drive torque is available to accelerate the vehicle immediately after the wheel brake units are released. This enables a particularly sporty start.

[0015] In a further possible embodiment, it is provided that before the first drive torque is applied to the wheels of the primary axle - for example, before or after the standby state is established - a friction coefficient is detected that exists between one of the wheels (i.e., the corresponding contact patch) and the surface on which the motor vehicle is positioned via its wheels. Based on the determined friction coefficient, the first or second drive torque, or both the first and second drive torques, are set. The friction coefficient determined in this embodiment has, in particular, a static friction component and a sliding friction component. Based on the determined friction coefficient, the corresponding drive torque can be dosed particularly precisely for even more efficient starting or moving off.The risk of the vehicle's driven wheels receiving too much drive torque during acceleration, taking the coefficient of friction into account, and thus causing the wheels to spin on the road, is avoided. Furthermore, it can be prevented that the driven wheels receive too little drive torque, even though, given the coefficient of friction, they could transmit more drive torque without spinning.

[0016] A possible further development provides for the coefficient of friction to be determined by detecting a limiting drive torque at which the wheel for which the coefficient of friction is being determined begins to spin on the ground. The associated wheel brake unit is or will be adjusted to its wheel release position to determine the coefficient of friction. In the wheel release position, the wheels mounted on the corresponding axle are - as already explained above - both released to rotate or roll. To prevent the motor vehicle from moving in an undesired manner when the coefficient of friction is being determined, the wheel brake unit assigned to the axle for whose wheels the coefficient of friction is not currently being determined is or will be adjusted to its wheel blocking position when the coefficient of friction is being determined. It can be seen that the standby state can be used, for example, to determine the coefficient of friction of the primary wheels.To determine the coefficient of friction between the secondary wheels, the wheel brake units are adjusted as described above. If the wheels of the corresponding axle (i.e., the two primary wheels or the two secondary wheels) are supplied with drive torque by a common traction motor via a transverse or axle differential, a lower coefficient of friction between the two wheels and the ground is determined and used to adjust the first and / or second drive torque. However, if the wheels can be driven individually, for example, via individual wheel traction motors or due to the lockability of the axle differential, wheel-specific friction values ​​can be recorded and used to adjust the first and / or second drive torque.Alternatively, the associated wheel brake unit is adjusted to a wheel-part release position to determine the coefficient of friction. In this position, only one of the wheels mounted on the corresponding axle, namely the one for which the coefficient of friction is being determined, is released to rotate, while the other wheel on the same axle is blocked from rotating. This allows the coefficient of friction to be determined particularly efficiently for each individual wheel. Optionally, the wheel brake unit assigned to the axle for whose wheels the coefficient of friction is not currently being determined is or will be adjusted to its wheel-lock position when the coefficient of friction is being determined. This reliably prevents unwanted movement of the vehicle while the coefficient of friction is being determined.

[0017] According to a further possible embodiment, the corresponding friction coefficient is determined sequentially for two or more of the wheels. For example, the corresponding friction coefficients for the wheels of the motor vehicle can be recorded sequentially in any order. It is equally conceivable that the corresponding friction coefficient is determined simultaneously for two of the wheels of the motor vehicle, for example first for the primary wheels and then for the secondary wheels, or vice versa. Furthermore, the friction coefficient can be determined individually for each of the wheels of the motor vehicle at the same time, in particular crosswise one after the other, i.e. simultaneously for one of the primary wheels and for the secondary wheel on the other side of the vehicle and optionally then for the corresponding other wheels. Simultaneous determination of the friction coefficients for each axle is also conceivable.In conjunction with determining the wheel-specific friction coefficient, it is then possible to provide the wheels with wheel-specific first and / or wheel-specific second drive torques, which are determined / calculated based on the wheel-specific friction coefficients. This reliably prevents the vehicle from skidding, at least at the beginning of acceleration, especially during a sporty start.

[0018] According to another possible embodiment, a rear axle of the motor vehicle acts as the primary axle. This is particularly advantageous for motor vehicles with pure rear-axle drive and for motor vehicles with rear-biased multi-axle drive, since more torque can be transferred to the ground via the wheels mounted on the rear axle than via the wheels mounted on a front axle of the motor vehicle. If, however, the multi-axle drive is front-biased, it is also advantageous to use the rear axle as the primary axle, although in this case, bracing of the drive train is only possible between the traction unit and the wheel brakes acting on the front wheels, but not all the way to the wheel hub.Otherwise, the standby state cannot be achieved with wheel brakes mounted on the front axle (here, the primary axle) (here, the first wheel brake unit) – the desired tension throughout the entire drivetrain would be lost due to the front wheels rolling. In vehicles with front-wheel drive only, the front axle acts as the primary axle.

[0019] With a view to particularly efficient, in particular fast, sporty acceleration, a further possible embodiment provides that a maximum torque that can be reliably provided by the traction unit is used to provide the first drive torque or the second drive torque. For this purpose, the internal combustion engine of the traction unit is operated / fired in such a way that its maximum torque is delivered by the internal combustion engine. It is further advantageous in the case in which the traction unit has an electric traction machine that a permanently excited synchronous motor is used for this purpose, which can provide its maximum torque to accelerate its rotor from a standstill. Therefore, it is provided here that such a permanently excited synchronous motor is used as the electric traction machine acting on the primary wheels.For example, an asynchronous motor can be used on the secondary axle, given that asynchronous motors cannot generate their full torque when stationary. By preloading the drive train, the primary axle does not move significantly when the wheel brake units are released. However, the secondary axle is preloaded in exactly the opposite direction and therefore moves significantly more when the wheel brake units are released.

[0020] Further features of the invention can be derived from the following description of the figures and from the drawings. The features and combinations of features mentioned above in the description, as well as the features and combinations of features shown below in the description of the figures and / or in the figures alone, can be used not only in the respective combinations specified, but also in other combinations or on their own, without departing from the scope of the invention.

[0021] The drawing shows Fig. 1 a motor vehicle in schematic representation and in Fig. 2 a flowchart to illustrate a method for controlling the motor vehicle.

[0022] In the following, a method for controlling a motor vehicle 1, more specifically for performing a drive-off start, as well as a control unit 2 and the motor vehicle 1 having the control unit 2 are explained in a joint description. Identical or functionally equivalent elements are provided with the same reference numerals in the figures. Fig. 1 shows a coordinate system in which a longitudinal direction x and a forward direction of the motor vehicle 1 coincide.

[0023] In this example, motor vehicle 1 is designed as a multi-axle drive vehicle, which means that both a rear axle 3, which functions here as a primary axle A1, and a front axle 4, which functions here as a secondary axle A2, are drive axles of motor vehicle 1. Two wheels are mounted on each of the axles 3 and 4: a left primary wheel 5 and a right primary wheel 6 on the rear or primary axle 3, A1, and a left secondary wheel 7 and a right secondary wheel 8 on the front or secondary axle 4, A2. Motor vehicle 1 is positioned on a surface or roadway via wheels 5-8. Furthermore, the motor vehicle has a traction unit 9, which in this example has an electric motor as the traction machine 10.In addition to or as an alternative to the traction machine 10, two or more further electrical machines 11 can be provided, for example one electrical machine per axle 3, 4 or one electrical machine per wheel 5-8, which in . Fig. 1 are shown in dashed lines. Furthermore, the motor vehicle 1 has a service brake system 12 comprising an electromechanical braking system, which has a first wheel brake unit 13 arranged on the primary axle A1 and a second wheel brake unit 14 arranged on the secondary axle A2. The respective wheel brake unit 13, 14 has, in particular, a wheel brake 15, 16, 17, 18 for each wheel 5, 6, 7, 8 mounted on the corresponding axle A1, A2. The service brake system 12 is designed in this case such that axle-individual or wheel-individual braking of the wheels 5-8 is possible.

[0024] The traction unit 9 and the service brake system 12 of the motor vehicle 1 are controllable to execute the method for controlling the starting motion. In the present case, the motor vehicle 1 has the control unit 2, which is configured to control the service brake system 12 and the traction unit 9 according to the method. For this purpose, the control unit 2 and the traction unit 9, as well as the control unit 2 and the service brake system 12, in particular the wheel brakes 15-18, are or can be coupled to one another for control signal transmission.

[0025] In the method, starting from a standstill of the motor vehicle 1 (driving speed is zero), a ready state for the start-up is established, which is Fig.2 is designated with step S1. If the first wheel brake unit 13 is arranged entirely or partially in a wheel-locking position in which the primary wheels 5, 6 are blocked against rotation by means of the first wheel brake unit 13 or by means of the wheel brakes 15, 16, the establishment of the standby state or step S1 comprises a step S2. In step S2, the first wheel brake unit 13 or the wheel brakes 15, 16 are placed in a wheel-release position in which rotation of the primary wheels 5, 6 is permitted. If the second wheel brake unit 14 is arranged entirely or partially in a wheel-release position in which rotation of the secondary wheels 7, 8 is permitted by means of the second wheel brake unit 14 or by means of the wheel brakes 17, 18, the establishment of the standby state or step S1 comprises a step S3. In step S3, the second wheel brake unit 14 orThe wheel brakes 15, 16 are placed in a wheel-locking position, in which rotation of the secondary wheels 7, 8 is locked or blocked. The standby state is established in this case when the first wheel brakes 15, 16 are completely released, so that the primary wheels 5, 6 are released to roll, and the maximum braking force is generated by the second wheel brakes 17, 18, whereby the secondary wheels 7, 8 are braked to the maximum, i.e., are locked against rolling.

[0026] In the standby state, i.e. while the primary wheels 5, 6 are released and the secondary wheels 7, 8 are blocked, in a step S4 a first drive torque is applied to the primary wheels 5, 6 by means of the traction unit 9, which is dimensioned such that the motor vehicle 1 does not move against the secondary wheels 7, 8 blocked by the second wheel brake unit 14. The first drive torque is greater than zero and drives the motor vehicle 1 in the forward direction x against the blocked secondary wheels 7, 8. The first drive torque is delivered to the primary wheels 5, 6 with a maximum value such that the contact surface of the blocked secondary wheels 7, 8 just does not slide off the ground.This preloads a drive train of the motor vehicle 1 between the primary wheels 5, 6 and the traction unit 9, including the wheel suspensions and any gear ratios arranged between the traction unit 9 and the respective primary wheel 5, 6. Meanwhile, the motor vehicle 1 is held stationary by the second wheel brake unit 14. The primary wheels 5, 6 are rotated by the first drive torque only until the idle rotation distances or tolerances in the drive train required to preload the drive train are overcome.

[0027] In the present case, after step S4, a further step S5 is executed, in which—while the first drive torque is applied to the primary wheels 5, 6—the first wheel brake unit 13 is adjusted to its wheel-locking position, so that the primary wheels 5, 6 are locked against further rotation or rolling. Then, in a step S6 of the method, a second drive torque is applied to the primary wheels 5, 6 by means of the traction unit 9. This second drive torque is greater than the first drive torque and is dimensioned such that the motor vehicle 1 does not start moving against the wheels 5-8 locked by the wheel brake units 13, 14.

[0028] If, following step S6 for executing the start, the wheel brake units 13, 14 are released, the motor vehicle 1 is accelerated with increasing speed using the second drive torque already present or provided. Due to the preload of the drive train, vibrations during start-up, which is carried out using the method, are significantly reduced compared to conventional start-up or start-assist systems. For a particularly sporty start, the example here provides for a maximum torque that can be reliably provided by the traction unit to be used to provide the second drive torque. In order to achieve this from a standstill of the motor vehicle 1, the traction machine 10 and / or the traction machine(s) 11 are / are designed as permanent-magnet synchronous machines.

[0029] According to the present example, it is also provided - see step S7 - that before the first drive torque is applied to the primary wheels 5, 6, for example before or after the standby state is established, a friction coefficient prevailing between one of the wheels 5-8 and the ground is detected. Based on the determined friction coefficient, the first and second drive torques are set or metered to the corresponding wheel 5-8 in this example. The friction coefficient is determined according to the example described here by detecting a limit drive torque at which the wheel 5-8, for which the friction coefficient is being determined, begins to spin on the ground. In this case, the associated one of the wheel brake units 13, 14 is or will be adjusted to its wheel release position to determine the friction coefficient.Alternatively, the associated one of the wheel brake units 13, 14 is adjusted to a wheel-part release position for determining the coefficient of friction, in which position the wheel 5-8 for which the coefficient of friction is being determined is released to rotate, whereas the other wheel 5-8 of the same axle A1, A2 is blocked from rotating. In the present case, it is provided that at least the wheel brake unit 13, 14 assigned to the axle A1, A2 for whose wheels 5-8 the coefficient of friction is not currently being determined is / remains or will be adjusted to its wheel-locking position when the coefficient of friction is being determined. It is further provided here that the respective associated coefficient of friction is determined successively for two or more of the wheels 5-8, thereby making it possible to provide the wheels 5-8 with wheel-specific first and / or wheel-specific second drive torques, which are determined / calculated based on the wheel-specific determined coefficients of friction.

[0030] The method for controlling motor vehicle 1, the control unit 2, and the motor vehicle 1 itself demonstrate solutions for enabling particularly efficient starting of motor vehicles. The basic idea here is to preload the drive train before a start, in particular before a start using launch control, which results in less jerk in the drive train and a faster buildup or increase in the drive torque. For a particularly efficient start of motor vehicle 1, it has also proven advantageous if the friction coefficient is determined on at least one of the wheels 5-8 and used to provide the corresponding drive torque. This allows the maximum drive torque to be better regulated. List of reference symbols 1 motor vehicle 2 control unit 3 rear axle 4 front axle 5 left primary gear 6 right primary gear 7 left secondary wheel 8 right secondary wheel 9 Traction unit 10 traction machine 11 additional traction machines 12 Service brake system 13 first wheel brake unit 14 second wheel brake unit 15 Wheel brake 16 Wheel brake 17 Wheel brake 18 Wheel brake A1 Primary Axis A2 secondary axis S1-S7 process step QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 2020 / 0122534 A1

[0004] US 4 784 236 A

[0004]

Claims

[1] Method for controlling a motor vehicle (1), the service brake system (12) of which comprises a first wheel brake unit (13) arranged on a drivable primary axle (A1) of the motor vehicle (1) and a second wheel brake unit (14) arranged on a secondary axle (A2) of the motor vehicle (1), as well as a traction unit (9), wherein the method for starting the motor vehicle (1) comprises the following steps: - Establish a standby state in which - the first wheel brake unit (13) is set in a wheel release position in which rotation of wheels (5, 6) mounted on the primary axle (A1) is released, - the second wheel brake unit (14) is set in a wheel blocking position in which wheels (7, 8) mounted on the secondary axle (A2) are blocked against rotation, - in the standby state: applying a first drive torque to the wheels (5, 6) of the primary axle (A1) by means of the traction unit (9), which is dimensioned such that the motor vehicle (1) does not start against the wheels (7, 8) of the secondary axle (A2) blocked by means of the second wheel brake unit (14). [2] Method according to claim 1, characterized by the steps: - Adjusting the first wheel brake unit (13) into a wheel locking position while the first drive torque is applied to the wheels (5, 6) of the primary axle (A1), - applying a second drive torque to the wheels (5, 6) of the primary axle (A1) by means of the traction unit (9), which second drive torque is greater than the first drive torque and is dimensioned such that the motor vehicle (1) does not start moving against the wheels (5, 6, 7, 8) blocked by the wheel brake units (13, 14). [3] Method according to claim 1 or 2, characterized bythat before applying the first drive torque to the wheels (5, 6) of the primary axle (A1), the following steps are carried out: - determining a coefficient of friction between one of the wheels (5, 6, 7, 8) and a surface on which the motor vehicle (1) is placed via its wheels (5, 6, 7, 8), - based on the determined friction coefficient: setting the first drive torque and / or the second drive torque. [4] Method according to claim 3, characterized by that the coefficient of friction is determined by detecting a limiting drive torque at which the wheel (5, 6, 7, 8) for which the coefficient of friction is being determined begins to spin on the ground, whereby - the associated wheel brake unit (13, 14) is or will be adjusted to its wheel release position, wherein the wheel brake unit (13, 14) assigned to the axle (A1, A2) for whose wheels (5, 6, 7, 8) the coefficient of friction is not currently being determined is or will be adjusted to its wheel blocking position when the coefficient of friction is being determined, or - the associated wheel brake unit (13, 14) is adjusted to a wheel part release position in which the wheel (5, 6, 7, 8) for which the coefficient of friction is determined is released for rotation and the other wheel (5, 6, 7, 8) of the same axle is blocked against rotation. [5] Method according to one of the preceding claims, characterized by that the coefficient of friction is determined successively for two or more of the wheels (5, 6, 7, 8). [6] Method according to one of the preceding claims, characterized by , that - a rear axle (3) of the motor vehicle (1) acts as the primary axle (A1) or - a front axle (4) of the motor vehicle (1) acts as the primary axle (A1). [7] Method according to one of the preceding claims, characterized by that a maximum torque that can be reliably provided by means of the traction unit (9) is used to provide the first drive torque or the second drive torque. [8] Control unit (2) for a motor vehicle (1), which is configured to control a service brake system (12) and a traction unit (9) of the motor vehicle (1) according to the method according to one of the preceding claims. [9] Motor vehicle (1) with the control unit (2) designed according to claim 8.

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