Method for operating a motor vehicle transmission

Abstract

The invention relates to a method for operating a motor vehicle transmission having a drive shaft, an output shaft, a first friction clutch, a second friction clutch, and a first sub-transmission and a second sub-transmission. To couple the drive shaft to the output shaft via the first sub-transmission in a first power flow path, the first friction clutch must be engaged. To couple the drive shaft to the output shaft via the second sub-transmission in a second power flow path that is at least partially parallel to the first power flow path, the second friction clutch must be engaged.As part of a braking function, the output shaft is braked by simultaneously coupling the input shaft to the output shaft, upon activation of the braking function, both via the first sub-transmission with a first gear ratio and via the second sub-transmission with a second gear ratio that differs from the first gear ratio. For this simultaneous coupling, both friction clutches are engaged simultaneously, and at least one of the friction clutches is engaged in a slipping mode during the braking function. When performing each slip mode, a load capacity of the at least one friction clutch is taken into account.

Description

[0001] The invention relates to a method for operating a motor vehicle transmission, in particular an agricultural machinery transmission, with a drive shaft which is provided for a drive-side coupling of the motor vehicle transmission to at least one drive engine of a motor vehicle, an output shaft which serves to establish a drive-side coupling of the motor vehicle transmission to drive wheels of the motor vehicle, a first friction clutch, a second friction clutch and a first partial transmission and a second partial transmission, wherein for a coupling of the drive shaft to the output shaft via the first partial transmission in a first power flow path, the first friction clutch is to be engaged, and wherein for a coupling of the drive shaft to the output shaft via the second partial transmission in a second power flow path which is at least partially parallel to the first power flow path, the second friction clutch is to be engaged.Furthermore, the invention relates to a control device, a computer program product and a data carrier.

[0002] Multi-speed transmissions are known in motor vehicle transmissions, each of which is composed of several sub-transmissions. In such a motor vehicle transmission, a respective power flow can be implemented between a respective input shaft and a respective output shaft via the sub-transmissions in power flow paths that are at least partially parallel to one another, with each power flow being guided via the respective sub-transmission usually via an associated friction clutch. This makes it possible to achieve powershift capability of the respective motor vehicle transmission in that, when the power flow is guided via one sub-transmission, a subsequent gear ratio is already engaged in the other sub-transmission, which is not currently in the power flow, and the gear change ultimately then has to be completed by simply switching between the friction clutches.When the two friction clutches are combined to form a double clutch, such a motor vehicle transmission is often referred to as a dual-clutch transmission.

[0003] Among other things, such motor vehicle transmissions are also used as a transmission group in group-designed agricultural machinery transmissions, whereby the motor vehicle transmission, which has multiple sub-transmissions, can then form a splitter or main group of the agricultural machinery transmission. This ensures that gear changes, which during continuous operation of the respective agricultural or municipal utility vehicle are usually only required in the splitter group and the main group, can be performed under load and thus with a high level of comfort.

[0004] EP 2 916 044 A1 discloses a transmission designed as a group, which is intended for use in an agricultural utility vehicle, such as a tractor. The transmission is composed of several transmission groups, one of which is implemented as a motor vehicle transmission designed in the manner of a dual-clutch transmission. This motor vehicle transmission has, in addition to a drive shaft, to which the motor vehicle transmission can be coupled on the drive side within a motor vehicle drive train of the commercial vehicle via the upstream, intermediate transmission group, and an output shaft, to which an output-side coupling of the motor vehicle transmission to drive wheels of the commercial vehicle can be established within the motor vehicle drive train, a first sub-transmission and a second sub-transmission.In each of the sub-transmissions, several different gear ratios can be engaged. When one of the gear ratios is engaged, the individual sub-transmission can couple the input shaft to the output shaft in a respective power flow path by engaging a corresponding friction clutch. Due to the power flow paths that can be realized via the sub-transmissions, powershift capability of the motor vehicle transmission can be achieved by switching between the friction clutches.

[0005] In the commercial vehicle sector, the use of continuous brakes to relieve the load on a service brake is well known. While retarders are often used as continuous brakes on trucks, continuous brakes on agricultural vehicles are often designed as engine brakes.

[0006] Based on the prior art described above, it is the object of the present invention to realize a braking function by means of a suitable operation of a motor vehicle transmission, whereby this should be designed in a simple and at the same time reliable manner.

[0007] This problem is solved from a process engineering perspective based on the preamble of claim 1 in conjunction with its characterizing features. The dependent claims that follow each of these claim further specify advantageous developments of the invention. A control device with which the aforementioned method can be carried out is further the subject of claims 11 and 12. Furthermore, claim 13 relates to a computer program product, and claim 14 relates to a data carrier.

[0008] According to the invention, within the scope of the method according to the invention, a motor vehicle transmission is operated which has a drive shaft which is provided for a drive-side coupling of the motor vehicle transmission to at least one drive engine of a motor vehicle, an output shaft which serves to establish an output-side coupling of the motor vehicle transmission to drive wheels of the motor vehicle, a first friction clutch, a second friction clutch as well as a first sub-transmission and a second sub-transmission. In order to couple the drive shaft to the output shaft via the first sub-transmission in a first power flow path, the first friction clutch must be engaged, whereas to couple the drive shaft to the output shaft via the second sub-transmission in a second power flow path which is at least partially parallel to the first power flow path.

[0009] The motor vehicle transmission operated within the scope of the method according to the invention thus comprises a drive shaft and an output shaft, of which the drive shaft, when the motor vehicle transmission is installed in a motor vehicle, serves as a drive-side connection of the motor vehicle transmission, and the output shaft serves as an output-side connection of the motor vehicle transmission. The motor vehicle transmission is designed to implement a power flow guide from at least one drive motor of the motor vehicle to drive wheels of the motor vehicle, so that the motor vehicle transmission is therefore a drive transmission. For this purpose, a coupling to at least one drive motor is established at the drive shaft in the installed state of the motor vehicle transmission and with power flow guide, while a connection to the drive wheels of the motor vehicle is established at the output shaft.

[0010] The motor vehicle transmission is preferably intended for use in an agricultural or municipal utility vehicle, so that the motor vehicle transmission is, in particular, an agricultural machinery transmission. Most preferably, the motor vehicle transmission is part of a group-type transmission, with the motor vehicle transmission being provided, in particular, as the main group of the group-type transmission. Thus, in the installed state of the motor vehicle transmission, a connection to a transmission group upstream on the input side can be established at the drive shaft and / or a connection to a transmission group downstream on the output side can be established at the output shaft.

[0011] Within the motor vehicle transmission, the input shaft can be coupled to the output shaft in a first power flow path via the first sub-transmission of the motor vehicle transmission, for which purpose the first friction clutch of the motor vehicle transmission is engaged. Alternatively, the input shaft can be coupled to the output shaft in a second power flow path that is at least partially parallel to the first power flow path by routing the power flow via the second sub-transmission of the motor vehicle transmission. For this purpose, the second friction clutch is engaged.When the coupling is established, a transmission ratio is realized between the input shaft and the output shaft by the respective sub-transmission, whereby different transmission ratios can preferably be switched within the respective sub-transmission, in particular by selectively actuating shifting elements of the respective sub-transmission. Furthermore, the two sub-transmissions preferably differ with regard to the transmission ratios that can be realized between the input shaft and the output shaft, so that by switching between the representable power flow paths, it is possible to switch between different transmission ratios as gears of the motor vehicle transmission.

[0012] These changes can be carried out by switching between the friction clutches under load due to the establishment of the respective coupling via the respective friction clutch.

[0013] The fact that the second power flow path is "at least partially" parallel to the first power flow path means, within the meaning of the invention, that the two power flow paths can also have overlapping sections. For example, if the power flow is routed via one sub-transmission, the power flow can be routed via a section of the other sub-transmission.

[0014] Within the respective sub-transmission, in particular, a plurality of gear ratios are provided, which are preferably each designed as spur gear ratios. It is also possible to provide gear ratios that can be used by both sub-transmissions for power flow control. In particular, a shifting element is assigned to each individual gear ratio, the actuation of which can integrate the respective associated gear ratio into the power flow. Furthermore, each sub-transmission preferably has an associated input shaft, which can then be coupled, in particular, to the output shaft via at least one of the gear ratios of the motor vehicle transmission by actuating the respective shifting element, and to the input shaft via the associated friction clutch.

[0015] The two friction clutches are each designed as clutches in which torque is transmitted by friction. In particular, the individual friction clutch is designed as a wet-running friction clutch. Specifically, the individual friction clutch can be a multi-disk clutch. The individual friction clutch is particularly preferably power-shiftable, i.e. it can establish the coupling of the input shaft with the output shaft when loads are applied to both clutch halves. To fully engage a transmission ratio between the input shaft and the output shaft, the respective friction clutch is fully engaged, with the torque being transmitted via the respective friction clutch initially slipping during engagement, i.e.if there are speed differences between the clutch halves of the friction clutch, takes place before the speeds between the clutch halves are equal and the clutch is fully closed.

[0016] The invention now comprises the technical teaching that, within the scope of a braking function, the output shaft is braked by coupling the input shaft to the output shaft, upon activation of the braking function, simultaneously via the first partial transmission with a first gear ratio and via the second partial transmission with a second gear ratio that differs from the first gear ratio. For this simultaneous coupling, both friction clutches are engaged simultaneously, with at least one of the friction clutches being engaged in a slipping mode during the braking function. When performing each slip mode, a load capacity of the at least one friction clutch is also taken into account.

[0017] In other words, when the motor vehicle transmission is in operation, a braking function can be carried out, during which the output shaft is braked. To carry out the braking function, both the first friction clutch and the second friction clutch are closed simultaneously, which results in the input shaft and output shaft being coupled simultaneously via both the first sub-transmission and the second sub-transmission. This results in a coupling with a first gear ratio via the first sub-transmission and a coupling with a different, second gear ratio via the second sub-transmission. However, at least one of the two friction clutches is only operated in slip mode and is therefore only slipping, i.e. not fully engaged.The slip operation of the at least one friction clutch is designed during the execution of the braking function taking into account a load capacity of the at least one friction clutch.

[0018] Such operation of the motor vehicle transmission has the advantage that the simultaneous engagement of the two friction clutches and the associated coupling of the input shaft to the output shaft with different gear ratios via the two power flow paths results in the braking of the output shaft and thus also of the drive wheels of the motor vehicle coupled to it. However, this does not result in the motor vehicle transmission locking, since at least one of the friction clutches is only engaged in a slipping manner in each slip mode. Thus, when engaged in each of its slip modes, the at least one friction clutch ensures a reduction of the respective speed differences, which results in the braking of the output shaft due to the complete engagement or the likewise slipping engagement of the other friction clutch due to the differing gear ratios of the two sub-transmissions.Since the design of the slipping operation of at least one friction clutch also takes into account the load capacity of this friction clutch, overloading of the slipping friction clutch and thus its damage can be reliably prevented. Overall, this allows a braking function similar to a retarder to be implemented via the vehicle transmission.

[0019] Essential to the invention is that, within the framework of the braking function, the input shaft is simultaneously coupled to the output shaft via the two sub-transmissions with differing gear ratios in order to decelerate the output shaft. However, to avoid locking the motor vehicle transmission and instead only decelerate the output shaft, at least one of the associated friction clutches must be engaged in a slipping manner during the coupling via the respective sub-transmission.

[0020] According to one embodiment of the invention, each slip operation of the at least one friction clutch is limited to a slip value range in which a friction power to be generated by the respective friction clutch during slip operation always lies within a fatigue strength range of the respective friction clutch. This reliably prevents the respective friction clutch from becoming overloaded during the braking function, since the respective friction clutch is always operated in a non-critical range during its slip operation. The fatigue strength range is defined in particular by a range in which the respective friction clutch can be operated for a long time in the respective slip value range, since sufficient cooling takes place here, in particular via lubricating oil.

[0021] According to an alternative embodiment of the invention, no limitation of the respective slip operation of the at least one friction clutch is carried out to a respective slip value range, wherein during operation of the at least one friction clutch in each slip operation a current load on this friction clutch is determined. Advantageously, due to the lack of limitation, a higher braking effect can also be achieved via the at least one friction clutch in slip. Preferably, each current load is determined based on a determination, preferably a calculation, of a respective current friction power of the individual friction clutch. The friction power can be used to reliably determine the load on the at least one friction clutch.

[0022] Alternatively or additionally, the braking function is terminated if the current load on the individual friction clutch exceeds a specified limit. This has the advantage that, by defining the limit accordingly, overloading of the respective friction clutch can be prevented by terminating the braking function before the overload occurs. If the current load is determined based on the current friction power, the specified limit is then a friction power limit value.

[0023] In a further embodiment of the invention, the respective slip mode of the at least one friction clutch is adjusted during the execution of the braking function depending on a braking torque to be applied to the output shaft. Within the scope of the invention, this adjustment can be implemented either in the form of a control or a regulation.

[0024] Preferably, for setting the respective slip mode of the at least one friction clutch, a respective actuating pressure is set as a function of the braking torque to be realized. This allows the respective slip of the respective friction clutch to be precisely set via a hydraulic or pneumatic actuating actuator of the respective friction clutch. Alternatively or additionally, the respective current gear ratio of the respective sub-transmission is taken into account when setting the respective slip mode of the at least one friction clutch.

[0025] According to one possible embodiment of the invention, both friction clutches are simultaneously operated in only one slip mode each during the braking function. This has the advantage that the load is distributed between both friction clutches. Particularly preferably, the slip modes of the friction clutches are configured such that the generation of a braking torque is distributed at least largely evenly between the two friction clutches, which accordingly also results in an even distribution of the load between both friction clutches.

[0026] In a further development of the invention, the drive shaft is decoupled on the drive side upon activation and upon execution of the braking function. This can prevent dragging of the at least one drive motor connected to the drive shaft.

[0027] The invention also relates to a control unit, which is in particular a transmission control unit of a motor vehicle transmission. This control unit is configured to initiate engagement of a first friction clutch in a motor vehicle transmission for coupling a drive shaft to an output shaft via a first partial transmission in a first power flow path. Furthermore, the control unit is designed to initiate engagement of the second friction clutch in the motor vehicle transmission for coupling the drive shaft to the output shaft via a second partial transmission in a second power flow path that is at least partially parallel to the first power flow path.The control unit is further configured to perform a braking function for braking the output shaft, the activation of which results in a simultaneous coupling of the input shaft to the output shaft both via the first partial transmission with a first gear ratio and via the second partial transmission with a second gear ratio that differs from the first gear ratio. In this case, the control unit is configured to cause both friction clutches to engage simultaneously to perform the braking function and, during the braking function, to engage at least one of the friction clutches in a slipping mode, each in a slipping mode. The control unit is configured to take into account a load capacity of the at least one friction clutch when performing each slipping mode.Furthermore, the control unit can then be configured to implement one or more of the aforementioned variants of a method according to the invention for controlling a control actuator.

[0028] The method according to the invention can also be embodied as a computer program product which, when running on a processor, for example a processor of an aforementioned control unit, instructs the processor via software to perform the associated method steps according to the invention. In this context, the subject matter of the invention also includes a computer-readable medium on which a computer program product described above is stored in a retrievable manner.

[0029] Advantageous embodiments of the invention, which are explained below, are illustrated in the drawings. It shows: Fig. 1 a schematic view of a motor vehicle drive train of an agricultural or municipal utility vehicle; Fig. 2 is a flowchart of a method for representing a braking function according to an embodiment of the invention; and Fig. 3 a flowchart of a method for representing a braking function according to one possible embodiment of the invention.

[0030] Out of Fig. 1 shows a schematic view of a motor vehicle drive train 1 of a motor vehicle in the form of an agricultural or municipal utility vehicle, which may in particular be a tractor.

[0031] Between a drive motor 2 in the form of an internal combustion engine and drive wheels 3, a group transmission 4 is provided, which, in addition to further transmission groups, also includes a powershift motor vehicle transmission 5. The motor vehicle transmission 5 is coupled to the upstream transmission group at an input shaft 6, while a coupling to the drive wheels 3 is established at an output shaft 7 of the motor vehicle transmission 5. In addition, the motor vehicle transmission 5 has a first sub-transmission 8 and a second sub-transmission 9, each of which is assigned an input shaft 10 and 11, respectively.

[0032] The input shaft 10 of the first sub-transmission 8 can, on the one hand, be coupled to the output shaft 7 via a spur gear stage 12 by actuating a shifting element 13, thereby establishing a first gear ratio of the first sub-transmission 8. On the other hand, a second gear ratio can be engaged in the first sub-transmission by coupling the input shaft 10 of the first sub-transmission via the spur gear stage 12 and a spur gear stage 14 to the input shaft 11 of the second sub-transmission 9 by actuating a shifting element 15, from which a further power flow is then transmitted to the output shaft 7 via a spur gear stage 16 by actuating a shifting element 17.

[0033] The switching of the second gear ratio of the first sub-transmission 8 simultaneously results in the representation of a first gear ratio of the second sub-transmission 9, in which the input shaft 11 of the second sub-transmission 9 is coupled to the output shaft 7 via the spur gear stage 16 upon actuation of the switching element 17. In addition, a coupling of the input shaft 11 of the second sub-transmission 9 with the output shaft 7 in a second gear ratio of the second sub-transmission 9 can be realized by simultaneously closing the switching elements 13 and 14 and thus coupling the input shaft 11 with the output shaft 7 via the spur gear stage 14. Due to the associated integration of the spur gear stage 12 into the power flow, this simultaneously results in the switching of the first gear ratio of the first sub-transmission 8.

[0034] In order to be able to couple the input shaft 6 to the output shaft 7 via the respective sub-transmission 8 or 9, the motor vehicle transmission 5 also features a first friction clutch 18 and a second friction clutch 19, each designed as a wet friction clutch. Engaging the first friction clutch 18 causes a coupling between the input shaft 10 of the first sub-transmission 8 and the input shaft 6 via a spur gear stage 20, whereby, with simultaneous switching of one of the gear ratios of the first sub-transmission 8, a power flow can be guided from the input shaft 6 to the output shaft 7 in a first power flow path.

[0035] If, however, the second friction clutch 19 is closed, this results in a coupling of the input shaft 6 with the input shaft 11 of the second partial transmission 9 via a spur gear stage 21. In this case, a power flow guidance from the input shaft 6 to the output shaft 7 can be carried out in a second power flow path that is at least partially parallel to the first power flow path, by simultaneously switching one of the transmission ratios of the second partial transmission 9, thereby representing a power flow guidance via the second partial transmission 9. The respective switching of the transmission ratios in the partial transmissions 8 and 9, the closing of the friction clutches 18 and 19 and also switching operations in the other transmission groups are carried out in the group transmission 4 via a control unit 22, which is in Fig. 1 is indicated.

[0036] Based on the motor vehicle transmission 5 described above, methods according to the invention will now be described, by means of which a braking function for braking the associated motor vehicle can be implemented. In principle, however, the methods according to the invention can also be implemented in a differently designed motor vehicle transmission comprising multiple sub-transmissions, in which gearshifts can be performed automatically or in an automated manner via a control unit.

[0037] Fig. Figure 2 shows a flowchart of a method according to a first embodiment of the invention for implementing the braking function. At the beginning of the method, a query is first made in a first step S1 as to whether a braking command is present, i.e., whether the respective motor vehicle needs to be braked at all. If the result in step S1 is negative, the system returns to step S1 and, preferably after a defined time window has elapsed, a new query is initiated in step S1.

[0038] If, however, a braking command is present, step S1 is followed by step S2, in which one of the possible gear ratios is simultaneously engaged in each of the two sub-transmissions 8 and 9 of the motor vehicle transmission 5. The engaged gear ratios differ from one another with regard to the ratio represented between the input shaft 6 and the output shaft 7. In step S2, the input shaft 6 is also decoupled from the upstream drive motor 2 by shifting the transmission group upstream of the motor vehicle transmission 5 into neutral.

[0039] Following step S2, the system proceeds to step S3, in which, in accordance with the braking specification, a braking torque T to be displayed on the output shaft 7 is determined. Brems is determined. From this braking torque T Brems In a step S4, actuation pressures p 1 and p 2of the two friction clutches 18 and 19 are determined, with which the two friction clutches 18 and 19 are to be controlled in order to carry out the simultaneous coupling of the output shaft 7 with the input shaft 6 via both partial transmissions 8 and 9 in the switched transmission ratios in slip modes of both friction clutches 18 and 19 and thereby to achieve the required braking torque T Brems on the output shaft 7. In this case, a slip value for each of the respective friction clutches 18 and 19 is determined, whereby the respective gear ratio of the respective sub-transmission 8 and 9 is taken into account when selecting the slip values, and an at least substantially equal distribution of the load is also carried out between the two friction clutches 18 and 19. Subsequently, in a step S5, the friction clutches 18 and 19 are actuated with the thus determined actuation pressures p 1 and p 2 controlled.

[0040] Following step S5, in step S6, current friction power P Reib1 and P Reib2 calculated, which are provided by the friction clutches 18 and 19 in the slip mode set in step S5. These calculated friction powers P Reib1 and P Reib2 of the friction clutches 18 and 19 are then subsequently in a step S7 with associated limits in the form of predetermined friction power limit values ​​P Reib, Grenz1 and P Reib, Grenz2 of the friction clutches 18 and 19. If both calculated friction powers P Reib1 and P Reib2 below the corresponding friction power limit values ​​P Reib, Grenz1 and P Reib, Grenz2 , the system proceeds to step S8 following step S7, while if one of the friction power limit values ​​P Reib, Grenz1 or P Reib, Grenz2 by the corresponding friction power P Reib1 or P Reib2to a step S9. In this step S9, the braking function is then terminated by fully opening both friction clutches 18 and 19 again.

[0041] However, in step S8, it is checked whether a braking request is still currently pending. If so, the system returns to step S3. If no braking request is pending, the system also proceeds to step S9.

[0042] Furthermore, Fig. 3 shows a flowchart of a method for implementing the braking function according to a second embodiment of the invention. This method largely corresponds to the preceding variant according to Fig. 2, with the difference that following step S3, the system proceeds to step S10. In this step S10, actuation pressures p 1* and p 2*of the two friction clutches 18 and 19 are determined, with which the two friction clutches 18 and 19 are to be controlled in order to achieve the required braking torque T Brems on the output shaft 7. A slip value and thus also the corresponding actuation pressure p 1* or p 2* of the respective friction clutch 18 or 19 is limited to a maximum value at which a friction power to be provided by the respective friction clutch 18 or 19 is in any case within a fatigue strength range of the respective friction clutch 18 or 19. Following this, the friction clutches 18 and 19 are then actuated in a step S11 with the thus selected actuating pressures p 1* and p 2* controlled.

[0043] Following step S11, a query is then made in step S12 as to whether a braking request is still currently present. If this is the case, the system returns to step S3. If the result in step S12 is negative, however, the system proceeds to step S3, in which the braking function is terminated by fully disengaging the two friction clutches 18 and 19.

[0044] By means of the operation of a motor vehicle transmission according to the invention, a braking function can be realized in a simple and at the same time reliable manner. Reference symbol 1 Motor vehicle powertrain 2 drive machine 3 drive wheels 4 group transmissions 5 Motor vehicle transmissions 6 Drive shaft 7 Output shaft 8 first partial transmission 9 second partial transmission 10 Input shaft 11 Input shaft 12 spur gear stage 13 Switching element 14 spur gear stage 15 Switching element 16 spur gear stage 17 Switching element 18 Friction clutch 19 Friction clutch 20 spur gear stage 21 Spur gear stage 22 Control unit T Brems Braking torque p 1 Actuation pressure p 2 Actuation pressure P Reib1 Friction power P Reib2 Friction power P Reib, Grenz1 Friction power limit P Reib, Grenz2 Friction power limit p 1* Actuation pressure p 2* Actuation pressure S1 to S13 single steps 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] EP 2 916 044 A1

[0004]

Claims

[1] Method for operating a motor vehicle transmission (5), in particular an agricultural machinery transmission, with a drive shaft (6) which is provided for a drive-side coupling of the motor vehicle transmission (5) to at least one drive engine (2) of a motor vehicle, an output shaft (7) which serves to establish an output-side coupling of the motor vehicle transmission (5) to drive wheels (3) of the motor vehicle, a first friction clutch (18), a second friction clutch (19) as well as a first partial transmission (8) and a second partial transmission (9), wherein for a coupling of the drive shaft (6) to the output shaft (7) via the first partial transmission (8) in a first power flow path, the first friction clutch (18) is to be closed, and wherein for a coupling of the drive shaft (6) to the output shaft (7) via the second partial transmission (9) in a power flow path which is at least partially parallel to the first power flow path,second force flow path, the second friction clutch (19) is to be closed, characterized by that, as part of a braking function, the output shaft (7) is braked by coupling the drive shaft (6), with the activation of the braking function, to the output shaft (7) simultaneously both via the first partial transmission (8) with a first transmission ratio and via the second partial transmission (9) with a second transmission ratio that differs from the first transmission ratio, wherein for this simultaneous coupling, both friction clutches (18, 19) are closed simultaneously and at least one of the friction clutches (18, 19) is closed in a slipping manner during the braking function, and wherein a load capacity of the at least one friction clutch (18, 19) is taken into account when carrying out each slipping operation. [2] Method according to claim 1, characterized bythat each slip operation of the at least one friction clutch (18, 19) is limited to a respective slip value range in which a friction power to be provided by the respective friction clutch (18, 19) in each slip operation lies in each case in a fatigue strength range of the respective friction clutch (18, 19). [3] Method according to claim 1, characterized by that no limitation of the respective slip operation of the at least one friction clutch (18, 19) is made to a respective slip value range, wherein during the operation of the at least one friction clutch (18, 19) in each respective slip operation a respective current load of this friction clutch (18, 19) is determined. [4] Method according to claim 3, characterized by that each current load is determined based on a determination, preferably a calculation, of a current friction power (P Reib1 , P Reib2 ) of the individual friction clutch (18, 19) is determined. [5] Method according to claim 3 or 4, characterized by that the braking function is terminated when a specified limit is exceeded by the current load on each friction clutch (18, 19). [6] Method according to one of the preceding claims, characterized by that the respective slip operation of the at least one friction clutch (18, 19) when carrying out the braking function is dependent on a braking torque (T Brems ) is set. [7] Method according to claim 6, characterized by that in the at least one friction clutch (18, 19) for setting the respective slip operation a respective actuating pressure (p1, p2; p 1* , p 2* ) depending on the braking torque to be represented (T Brems ) is set. [8] Method according to claim 6 or 7, characterized bythat when setting the respective slip operation of the at least one friction clutch (18, 19), the respective current transmission ratio of the respective partial transmission (8, 9) is taken into account. [9] Method according to one of the preceding claims, characterized by that in the course of carrying out the braking function both friction clutches (18, 19) are closed simultaneously in only one slip mode each. [10] Method according to claim 9, characterized by that the slip operations of the friction clutches (18, 19) are designed in such a way that a generation of a braking torque (T Brems ) is distributed at least largely evenly between the two friction clutches (18, 19). [11] Method according to one of the preceding claims, characterized by that the drive shaft (6) is decoupled on the drive side when activated and when the braking function is carried out. [12] Control unit (22), in particular a transmission control unit, which is designed to initiate the engagement of a first friction clutch (18) in a motor vehicle transmission (5) for coupling a drive shaft (6) to an output shaft (7) via a first partial transmission (8) in a first power flow path, wherein the control unit (22) is further designed to initiate the engagement of a second friction clutch (19) in the motor vehicle transmission (5) for coupling the drive shaft (6) to the output shaft (7) via a second partial transmission (9) in a second power flow path that is at least partially parallel to the first power flow path, wherein the control unit (22) is designed to carry out a braking function for braking the output shaft (7), the activation of which braking function results in a simultaneous coupling of the drive shaft (6) to the output shaft (7) both via the first partial transmission (8) with a first transmission ratio,as well as via the second partial transmission (9) with a second transmission ratio deviating from the first transmission ratio, wherein the control unit (22) is designed to cause both friction clutches (18, 19) to close simultaneously in order to carry out the braking function and, during the braking function, to close at least one of the friction clutches (18, 19) only in a slipping manner in a respective slip operation, and wherein the control unit (22) is designed to take into account a load capacity of the at least one friction clutch (18, 19) when carrying out each of the respective slip operations. [13] Control device (22) according to claim 12, which is further configured to carry out a method according to one or more of claims 2 to 11. [14] Computer program product for a control unit (22) according to claim 12 or 13, by means of which a method according to one or more of claims 1 to 11 can be carried out, wherein a routine for operating a motor vehicle transmission (5) is implemented by corresponding control commands stored in software. [15] A data carrier comprising a computer program product according to claim 14.

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