Abstract:
A clutch unit ( 47 ) comprises a wet friction clutch for controllable transmission of a torque from an input element ( 41 ) to an output element ( 45 ), housing that contains the friction clutch and oil for cooling the friction clutch, and an actuator ( 51 ) for actuating the friction clutch. The actuator is attached to the housing in a thermally conductive way and has a temperature sensor ( 108 ) for sensing a temperature of the actuator. In order to computationally determine the oil temperature (T Öl ) in the clutch unit ( 47 ), a thermal input power to the clutch unit is determined as a function of at least a speed of the input element and/or of the output element. The difference between the thermal input power and the thermal output power is determined, and the oil temperature is determined as a function of the difference that was determined.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/EP2009/003729, filed May 26, 2009. This application claims the benefit and priority of German Application No. 10 2008 026 553.5, filed Jun. 3, 2008. The entire disclosures of each of the above applications are incorporated herein by reference. 
     FIELD 
     The present disclosure concerns a method for computational determination of the oil temperature in a clutch unit for a drive train of a motor vehicle, wherein the clutch unit contains, at a minimum, a wet friction clutch for controllable transmission of a torque from an input element to an output element of the clutch unit, a housing that contains the friction clutch and oil for cooling the friction clutch, and an actuator for actuating the friction clutch that is attached to the housing in a thermally conductive way and has a temperature sensor for sensing a temperature of the actuator. The present disclosure also concerns a torque transmission arrangement that has an input element, an output element, a control unit, and a clutch unit of the aforementioned type. 
     BACKGROUND 
     This section provides background information related to the present disclosure which is not necessarily prior art. 
     A clutch unit may be used, for example, in a transfer case of a motor vehicle with four-wheel drive for controllable transmission of a drive torque to a primary axle and/or a secondary axle of the motor vehicle. In the case of a so-called “torque on demand” transfer case, the wheels of the primary axle are continuously driven while a portion of the drive torque can be selectively transmitted to the wheels of the secondary axle by means of the clutch unit. The transfer case can also be designed as a controllable center differential in which the clutch unit is associated with a differential lock in order to adjust the distribution of the drive torque in the longitudinal direction of the vehicle. A clutch unit can also be used in a torque transmission arrangement, which, in a motor vehicle with a continuously driven front axle, permits the transmission of part of the torque to the rear axle, wherein the unit is located on the front axle differential or the rear axle differential, for example. Such different applications and arrangements are known from U.S. Pat. No. 7,111,716 B2, for example. 
     A clutch unit can also act in the transverse direction of the motor vehicle, for example for a differential lock of an axle differential or in a torque superposition arrangement of an axle differential (known as “torque vectoring”). In all of the aforementioned cases, the clutch unit can frictionally connect a rotating input element (e.g., input shaft) and a rotating output element (e.g., output shaft), particularly in order to transmit a drive torque. As an alternative thereto, the clutch unit can be configured as a brake with a stationary input element or a stationary output element, particularly in order to transmit a braking torque. 
     In the aforementioned applications of the clutch unit, the clutch unit is located after the main transmission of the drive train (e.g., after the manual or automatic transmission or CVT transmission) with respect to the direction of power flow. Normally, the clutch torque—which is to say the torque transmitted by the friction clutch—is variably adjusted as a function of the relevant driving situation. Thus, a change in the torque to be transmitted by the clutch unit takes place in accordance with the requirements of vehicle dynamics, which may depend on such factors as the driving situation or environmental influences (e.g., smooth road surface with slip of the drive wheels occurring). This requires not only controlled engagement of the friction clutch, but frequently also requires a relatively long period of operation with precisely adjusted clutch torque, for which reason the friction clutch usually is designed as a wet plate clutch in the aforementioned applications. Typically, the friction clutch is integrated into a housing, which contains oil for cooling and lubricating the frictional components. For example, an oil sump is provided at the bottom of the housing, whence an oil pump continuously pumps oil during the operation of the clutch and drips it on the friction surfaces. The oil returns to the oil sump from the friction surfaces. 
     The clutch unit further includes an actuator for actuating the friction clutch. The actuator often has an electric motor, and is attached to the housing of the clutch unit in a thermally conductive way in order to use the housing as a heat sink for the actuator&#39;s waste heat. Under certain operating conditions, overheating of the actuator can occur. Consequently, the actuator is typically equipped with a temperature sensor that continuously senses the temperature of the actuator. In this way, clutch operation can be discontinued in the event of impending overheating of the actuator. If the actuator has an electric motor, the temperature sensor can, for example, be attached to the housing of the electric motor or within the same. 
     A clutch unit of the aforementioned type and a method for calibrating such a clutch unit are known from WO 2003/025422 A1 (corresponding to U.S. Pat. No. 7,032,733 B2), the content of which is expressly incorporated in the disclosure content of the present application. As is described in greater detail in WO 2003/025422 A1, the setting of a specific desired clutch torque does not necessarily require the provision of direct torque control (with the measured actual clutch torque as the control variable). Instead, the control of the friction clutch can take place by indirect means through controlling the position of the actuator based on an appropriate calibration of the clutch unit. Thus, to set the desired torque to be transmitted, the angle of rotation of the electric motor, for example, or another position variable of the actuator, is employed as a control variable and is set to a value that corresponds to the desired clutch torque. To this end, a clutch torque/actuator position dependence is empirically determined, which is stored as a characteristic curve, for example in the form of a table (lookup table, LUT) or a function (which is to say an algorithm). Using this dependence, the applicable target value of the relevant position variable of the actuator (e.g., angle of rotation) is determined for a specific torque requirement and is regulated. 
     For a variety of control tasks relating to operation of the clutch unit, it is necessary to determine the current temperature of the oil located in the clutch housing. To this end, a suitable temperature sensor could be provided in the oil sump, for example. However, an arrangement of this nature is associated with increased effort and additional costs. 
     SUMMARY 
     This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
     It is an object of the present disclosure to provide a computational determination of the oil temperature in a clutch unit that can be implemented simply, economically, and reliably. 
     This object is attained by a method for computationally determining the oil temperature in a clutch unit with the following steps: determining a thermal input power to the clutch unit as a function of at least a speed of the input element and/or of the output element of the clutch unit; determining a thermal output power of the clutch unit as a function of at least the actuator temperature; determining a difference between the thermal input power and the thermal output power; and determining the oil temperature as a function of the difference that was determined. 
     In the calculation of the oil temperature, therefore, the heat input brought into the clutch unit and the heat output taken out of the clutch unit are taken into account and placed into relation to one another in order to determine a corresponding change in the oil temperature in the clutch unit, and thus to determine a current value of the oil temperature. In order to determine the thermal input power to the clutch unit, at least the speed of the input element of the clutch unit or the speed of the output element of the clutch unit—or a difference between these two speeds—is taken into account. These speeds are customarily available in any case, for example because of the wheel speed sensors of the motor vehicle that are usually present. In order to determine the thermal output power from the clutch unit, at least the temperature of the actuator of the clutch unit is taken into account, something which is normally measured in any case as well, as was explained above, and thus is available without additional effort. By offsetting the thermal input power with the thermal output power, it is possible to estimate whether the oil temperature within the clutch unit has increased or decreased. To this end, the oil temperature is ultimately equated with a function of the difference determined between the thermal input power and the thermal output power of the clutch unit. This computational determination of the oil temperature can be achieved in an especially simple and economical manner, since additional sensors are not strictly necessary. 
     Within the scope of the present disclosure, it has been recognized in particular that, because of the thermally conductive connection between the actuator and the clutch housing, the temperature of the actuator, which is measured in any case for monitoring purposes, permits certain inferences to be drawn about the surrounding temperature and thus can be used as a substitute quantity for that temperature. It has additionally been recognized that the thermal output power of the clutch unit can consequently be estimated on the basis of the actuator temperature using a heat flow model. By placing this thermal output power in relation to the thermal input power to the clutch unit, it is possible to determine the oil temperature by purely computational means. As a result, an additional temperature sensor in the oil sump can be avoided. 
     The steps of the method to be carried out need not necessarily be performed in the specified order. It is also not strictly necessary that they be performed one after the other chronologically, i.e., at least some of them may also be performed simultaneously with one another. 
     In a wet friction clutch, the aforementioned dependence between actuator position and transmitted torque is dependent on the consistency of the oil, in particular its viscosity. Since the viscosity of the oil is temperature-dependent, undesirable deviations between the required torque (target value) and the actually transmitted torque (actual value) can occur during clutch operation. 
     Consequently, it is an additional object of the present disclosure to reduce such deviations between target value and actual value of the clutch torque. This is achieved in a method for controlling a clutch unit by the means that the oil temperature in the clutch unit is computationally determined using the above-described method, and the clutch unit is controlled as a function of the oil temperature thus determined. In particular, the above-described characteristic curve of the friction clutch, which describes the dependence between the clutch torque and an actuator control variable, can be adapted as a function of the current oil temperature that has been determined. Temperature-induced deviations of the clutch characteristic from the behavior described by the characteristic curve can be compensated for in this way, thus increasing the accuracy of adjustment of the clutch unit. The actuator control variable can be, for instance, an actuator position (angular position, in particular) or a hydraulic pressure. 
     Preferably, in order to computationally determine the oil temperature, a time integral of the thermal input power during operation of the clutch unit and/or a time integral of the thermal output power during operation of the clutch unit and/or a time integral of the difference between the thermal input power and the thermal output power during operation of the clutch unit is calculated. Such calculation of an integral makes it possible to ascertain a heat quantity based on the applicable thermal power in order to determine the oil temperature therefrom. 
     An especially simple and precise determination of the oil temperature results when the product of the oil temperature (to be determined) and a thermal capacity of the clutch unit is set equal to the difference determined between the thermal input power and the thermal output power (or the difference between the heat input quantity and the heat output quantity). This corresponds to the thermodynamically supported assumption that the heat introduced into the clutch unit and not carried off to the outside leads to a corresponding increase in the oil temperature. The thermal capacity of the clutch unit to be used here can be empirically determined for the applicable type of clutch unit, wherein the thermal capacity can in turn depend on additional parameters. 
     According to one embodiment, the determination of the thermal input power to the clutch unit takes place as a function of a speed difference between the input element and the output element. By this means, a speed difference between, e.g., the inner plates and the outer plates of the friction clutch can be taken into account in order to determine the frictional heat generated in the clutch. 
     Preferably, the determination of the thermal input power also takes place as a function of a clutch torque. This clutch torque is, for example, a torque requirement (target value) or the calculated or measured torque actually transmitted (actual value). In particular, it is possible to find the product of the clutch torque, the speed of the input element or output element, and a constant that depends on an efficiency of a transmission component and/or an oil pump of the clutch unit. In place of the speed of the input element or output element, a speed difference between the input element and output element can also enter into the product. By this means, it is possible to determine a power dissipation of the clutch unit, which stands in relation to the thermal input power. 
     For an especially accurate determination of the thermal output power of the clutch unit, it can be assumed that this quantity itself depends in turn on the oil temperature (to be determined). In this case, the above-described equating of the oil temperature to a function of the difference determined between thermal input power and thermal output power results in a differential equation. As an alternative hereto, the simplifying assumption can be made for determining the thermal output power that the output power depends on the last calculated oil temperature. 
     Preferably, the determination of the thermal output power of the clutch also takes place as a function of a coefficient of thermal conductivity of the clutch unit. This takes into account the thermal conductivity properties resulting from the particular features of the housing material and the design features. The thermal conductivity coefficient can also be empirically determined for the specific type of clutch unit. 
     Preferably, the thermal conductivity coefficient is chosen as a function of the speed of the input element and/or output element. It has been demonstrated that the accuracy of the determination of the clutch unit&#39;s thermal output power can be improved still further by this means. The associated relationship between speed and thermal conductivity coefficient can be empirically determined and stored in the form of a lookup table, for example. 
     An initialization of the calculation process can be accomplished by the means that the oil temperature at a startup of the motor vehicle is set equal to an initial value that depends on the current actuator temperature. Hence, use is made of the recognition that the actuator temperature and oil temperature in a vehicle that is stopped gradually approach one another, since both components use the clutch housing as a heat sink. The initial value of the oil temperature may additionally depend on a value of the oil temperature that was last determined during a preceding operation of the motor vehicle. Furthermore, the initial value can additionally be determined as a function of a duration of a preceding stoppage of the motor vehicle. In this way, it is possible to take into account the fact that the clutch represents a heat reservoir for the oil, which is to say the oil cools more slowly in general than the actuator, for example. 
     The duration of the preceding stoppage of the motor vehicle is preferably determined as a function of the difference between the current actuator temperature and the last actuator temperature detected during a preceding operation of the motor vehicle. This duration thus determined, which is also called the inactive time, can also be used for other control tasks. Thus, a dedicated timer is not strictly necessary in order to determine the duration of the preceding inactive time of the motor vehicle. Alternatively, however, the inactive time can also be detected separately, for example by means of a timer. 
     According to a preferred embodiment, the actuator has an electric motor, with which the temperature sensor is associated. Electric motors must be protected in a specific manner from overheating, which is why the temperature sensor here serves to output an alarm signal when a threshold temperature that is considered hazardous is exceeded. 
     As already mentioned above, the method for computational determination of the oil temperature can be used to good advantage for controlling the clutch unit. Preferably, the characteristic curve of the friction clutch that describes the dependence of the clutch torque on the actuator control variable is adapted as a function of the oil temperature that has been determined. By adapting the characteristic curve, temperature-dependent influencing factors in the wet friction clutch can be taken into account in a simple way. It is useful if the characteristic curve is adapted by modifying a slope and/or an offset. In particular, the adaptation of the characteristic curve can be performed on a periodic or continuous basis during operation of the clutch. 
     The present disclosure also relates to a torque transmission arrangement that has an input element, an output element, a clutch unit, and a control unit, wherein the clutch unit has, at a minimum, a wet friction clutch for controllable transmission of a torque from the input element to the output element, a housing that contains the friction clutch and oil for cooling the friction clutch, and an actuator for actuating the friction clutch that is attached to the housing in a thermally conductive way and has a temperature sensor for sensing a temperature of the actuator, wherein the control unit is designed to determine a thermal input power to the clutch unit as a function of at least a speed of the input element and/or of the output element of the clutch unit, to determine a thermal output power of the clutch unit as a function of at least the actuator temperature, to determine a difference between the thermal input power and the thermal output power, and to determine an oil temperature as a function of the difference that was determined. 
     The clutch unit or torque transmission arrangement can be used in different configurations in order to transmit a torque along a drive train of a motor vehicle, as was explained at the outset. The features of the present disclosure are explained below with reference to the drawings, in connection with a “torque on demand” transfer case solely by way of example. 
     Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
         FIG. 1  shows a schematic view of a drive train of a motor vehicle; 
         FIG. 2  shows a schematic view of a transfer case; 
         FIG. 3  shows a cross-sectional view of the transfer case from  FIG. 2 ; 
         FIG. 4  shows a schematic view of a clutch actuator; and 
         FIG. 5  shows a flow diagram of a method according to the present disclosure for computationally determining the oil temperature in a clutch unit. 
     
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     Example embodiments will now be described more fully with reference to the accompanying drawings. 
       FIG. 1  schematically shows a drive train of a motor vehicle with selectable four-wheel drive. The drive torque produced by an internal combustion engine  11  is delivered to a transfer case  15  through a main transmission  13  (manual transmission or automatic transmission). A first output of the transfer case  15  is coupled to a rear axle differential  19  through a drive shaft  17 . In this way, the wheels  21  of the rear axle  23  are driven continuously. The rear axle  23  thus constitutes the primary axle of the motor vehicle. A second output of the transfer case  15  is coupled to a front axle differential  27  through a drive shaft  25 . In this way, a portion of the drive torque of the internal combustion engine  11  can be selectively transmitted to the wheels  29  of the front axle  31 . The front axle  31  thus constitutes the secondary axle of the motor vehicle. 
     Also shown in  FIG. 1  is a vehicle dynamics controller  33 . This is connected to wheel speed sensors  35 ,  37 , which are associated with the wheels  21  of the rear axle  23  and the wheels  29  of the front axle  31 . The vehicle dynamics controller  33  is also connected to additional sensors  39 , for example a yaw-rate sensor. As a function of the signals from the sensors  35 ,  37 ,  39 , the vehicle dynamics controller  33  generates a control signal, which is delivered to a control unit (not shown in  FIG. 1 ) of the transfer case  15 , in order to set a specific distribution of the drive torque between the two axles  23 ,  31  of the vehicle by this means. The aforementioned control signal is, in particular, a target value of a clutch torque, which is to say a torque requirement for a clutch unit of the transfer case  15 . 
       FIG. 2  shows a schematic cross-sectional view of the transfer case  15  from  FIG. 1 . The transfer case  15  has an input shaft  41 , a first output shaft  43 , and a second output shaft  45 . The first output shaft  43  is coaxial to the input shaft  41  and is designed to be rotationally fixed therewith, preferably as a single piece. The second output shaft  45  is parallel to and offset from the input shaft  41 . 
     The transfer case  15  has a clutch unit  47  with a friction clutch  49  and an actuator  51 . The friction clutch  49  has a clutch basket  53  that is attached in a rotationally fixed manner to the input shaft  41  and the first output shaft  43  and that carries multiple clutch plates. The friction clutch  49  also has a rotatably supported clutch hub  55 , which likewise carries multiple clutch plates that engage in an alternating arrangement with the plates of the clutch basket  53 . The clutch hub  55  is connected in a rotationally fixed manner to an input gear  57  of a chain drive  59 . An output gear  61  of the chain drive  59  is connected in a rotationally fixed manner to the second output shaft  45 . A gear drive, for example with an intermediate gear between the aforementioned gears  57 ,  61 , may be provided in place of the chain drive  59 . 
     By actuating the actuator  51  in the engagement direction of the friction clutch  49 , an increasing fraction of the drive torque introduced into the transfer case  15  through the input shaft  41  can be transmitted to the second output shaft  45 . 
       FIG. 3  shows details of the transfer case  15  from  FIG. 2  in a cross-sectional view. In particular, it is evident that the actuator  51  has a bearing ring  63  and an adjusting ring  65 , which are rotatably supported with respect to the axis of rotation A of the input shaft  41  and the first output shaft  43 . The bearing ring  63  is axially supported on the input gear  57  by means of a thrust bearing. In contrast, the adjusting ring  65  is supported in an axially displaceable manner. The bearing ring  63  and adjusting ring  65  each have multiple ball races  67  and  69  on their sides facing one another. These ball races extend in the circumferential direction with respect to the axis A and are inclined in a ramp-like manner in the circumferential direction with respect to a plane perpendicular to the axis A, which is to say that the ball races  67 ,  69  have a depth that varies in the circumferential direction. In each case, a ball race  67  of the bearing ring  63  and a ball race  69  of the adjusting ring  65  are located opposite one another and enclose an associated ball  71 . By rotating the bearing ring  63  and the adjusting ring  65  relative to one another, an axial displacement of the adjusting ring  65  can thus be accomplished, wherein the adjusting ring  65  works together with a pressure ring  73  of the friction clutch  49  through a thrust bearing. The pressure ring  73  is preloaded in the disengagement direction of the friction clutch  49  by means of a spring washer arrangement  75 . 
     An actuating lever  77  or  79  is integrally formed on the bearing ring  63  and the adjusting ring  65 , respectively. A roller  81  or  83  is rotatably supported at the free end of each relevant lever  77 ,  79 . By means of the rollers  81 ,  83 , the actuating levers  77 ,  79  work together with the two end faces  85 ,  87  of a disk cam  89 , which is rotatable relative to an axis C. The end faces  85 ,  87  are inclined in the circumferential direction relative to a plane perpendicular to the axis C, i.e., the disk cam  89  is wedge-shaped in cross-section. By rotating the disk cam  89 , the actuating levers  77 ,  79  can thus be moved in a scissoring manner in order to rotate the bearing ring  63  and the adjusting ring  65  relative to one another. The disk cam  89  has an integrally formed splined projection  91 . By means of the projection, the disk cam  89  can be connected to an electric motor and associated reduction gear (not shown in  FIG. 3 ) in a manner that is effective for driving. 
     In this way, by appropriate control of the aforementioned electric motor the disk cam  89  can be driven into a rotary motion so as to thereby pivot the actuating levers  77 ,  79  relative to one another. The rotation of the bearing ring  63  and the adjusting ring  65  relative to one another that is produced thereby causes an axial motion of the adjusting ring  65 . The pressure ring  73  thus causes an engagement of the friction clutch  49 , or—assisted by the spring washer arrangement  75 —a disengagement of the friction clutch  49 . 
       FIG. 4  shows the actuator  51  from  FIG. 2  and  FIG. 3  in a schematic view. The actuator  51  has a controllable electric motor  93  with an armature shaft  95 , a reduction gearbox  97  with a worm  99  and worm wheel  101 , and a deflection device  103 . By means of the deflection device  103 , a rotational motion of an output shaft  105  of the reduction gearbox  97  is converted into a translational, i.e., straight-line, motion of the pressure ring  73  ( FIG. 3 ). The deflection device  103  comprises the disk cam  89  as well as the bearing ring  63  and the adjusting ring  65  with the actuating levers  77 ,  79  and the balls  71  as shown in  FIG. 3 . A sensor  107 , which is designed as an incremental encoder for example, is located on the armature shaft  95  of the electric motor  93 . Alternatively, as shown in  FIG. 4 , the sensor  107  may also be located on the output shaft  105  as a sensor  107 ′. In addition, a temperature sensor  108  that outputs a temperature signal T is attached to the electric motor  93 . 
     The sensor  107  produces a signal that corresponds to an actuator position value. In the exemplary embodiment shown, this is the actual angular position value α′ of the armature shaft  95 . This signal α′ is delivered to a control unit  109  of the transfer case  15 . The control unit  109  also receives a torque requirement M, which is to say a target value of the clutch torque, from the vehicle dynamics controller  33  of the motor vehicle ( FIG. 1 ). From a clutch torque/angular position characteristic curve  111 , which is stored in a nonvolatile memory  113  of the control unit  109 , the control unit  109  determines a target angular position value α on the basis of the torque requirement M. As a function of the difference between the target angular position value α and the actual angular position value α′, the control unit  109  generates a control signal for the electric motor  93  in order to adjust the friction clutch  49  ( FIGS. 2 and 3 ) accordingly. The control unit  109  thus acts as a position controller. 
     The way the oil temperature in the clutch unit  47  can be ascertained and taken into account by means of the control unit  109  shown in  FIG. 4  will now be explained with reference to  FIG. 5 . 
     A step S 1  consists of waiting until the motor vehicle is started up. As soon as the vehicle has been started (the control unit  109  from  FIG. 4  receives the “ignition on” signal), in a step S 2  the last oil temperature T Öl ′ that was determined and the last actuator temperature T Akt ′ that was sensed are retrieved from a memory associated with the control unit  109 . T Öl ′ and T Akt ′ can be loaded with suitable initial values by the factory in order to ensure that the method can be carried out the very first time the vehicle is started up. Then, in a step S 3 , the current temperature T Akt  of the actuator  51  is sensed by the temperature sensor  108 . An initialization of the oil temperature T Öl  on the basis of T Akt , T Akt ′, and T Öl ′ takes place in a step S 4 . In addition, the inactive time of the motor vehicle can be taken into account for the initialization. After the initialization, the current actuator temperature T Akt  is measured again in a step S 5 . 
     In a step S 6 , the thermal input power W in  to the clutch unit  47  is determined. In the embodiment described here, the power dissipation of the chain drive  59  (or of a corresponding associated gear drive) associated with the clutch unit  47  and the power dissipation of an oil pump (not shown) associated with the clutch unit  47  are taken into account for the thermal input power W in . Alternatively or in addition, a power dissipation of the clutch plates can be taken into account. The aforementioned power dissipation of the chain drive  59  is calculated on the basis of a product of the required clutch torque M ( FIG. 4 ), the speed of the second output shaft  45 , and an empirically determined constant that is associated with the efficiency of the chain drive  59 . The speed of the second output shaft  45  can be determined in a simple manner from the signals of the wheel speed sensors  37  of the front wheels  29  ( FIG. 1 ), which usually are available in any case through the data bus of the vehicle. The power dissipation of the oil pump is determined as a function of the speed of the input shaft  41  or the first output shaft  43 , wherein this speed is multiplied by, e.g., a constant that again is empirically determined. The speed of the input shaft  41  or first output shaft  43  can be determined in a simple manner from the signals of the wheel speed sensors  35  of the rear wheels  21 . 
     In a step S 7 , the thermal output power W aus  of W f the clutch unit  47  is determined as a function of the current actuator temperature T Akt . In a preferred method, the thermal output power W aus  is approximated in that the difference between the oil temperature T Öl  to be determined in the current calculation cycle and the current actuator temperature T Akt  is multiplied by a coefficient of thermal conductivity. Again, an empirically determined constant can be used for this thermal conductivity coefficient. However, it has been determined that the thermal output power W aus  can be approximated with an especially high accuracy if the thermal conductivity coefficient of the clutch unit depends on the speed of the input shaft  41  or the second output shaft  45 . This dependence is taken into account in the described method in that the thermal conductivity coefficient is retrieved, as a function of the speed that has been determined, from a lookup table stored in a memory associated with the control unit  109 . Intermediate values can be found by interpolation if necessary. 
     In a step S 8 , the difference is obtained between the thermal input power W in  that has been determined and the thermal output power W aus  that has been determined, and the oil temperature T Öl  to be determined is set equal to a function of this difference. In particular, a time integral of the difference is calculated, and is set equal to a product of the oil temperature T Öl  to be determined and a thermal capacity of the clutch unit  47 . The thermal capacity, in turn, can be inserted as an empirically determined constant. The time integral is calculated beginning with the startup of the vehicle, with the values that were determined in the initialization step S 4  being used as initial values. Insofar as it is assumed that the thermal output power W aus  itself depends—as explained above—on the oil temperature T Öl  to be determined, setting the oil temperature T Öl  equal to a function of the difference between the thermal input power W in  and the thermal output power W aus  ultimately yields a differential equation. The oil temperature T Öl  can be determined from this equation, for example analytically, iteratively, or using a lookup table. When determining the thermal output power W aus , a value of the oil temperature T Öl  that was determined in a preceding computational step can be used as an alternative to taking into consideration the oil temperature T Öl  that is to be determined. This simplifies the determination of the current oil temperature T Öl . 
     The integral calculation need not take place using the difference, but instead it is possible to separately integrate the thermal input power W in  and the thermal output power W aus . 
     A step S 9  checks whether the motor vehicle has been switched off. If the control unit  109  receives a corresponding signal (“ignition off”), the initialization values T Öl ′ and T Akt ′ are overwritten with the current values T Öl  and T Akt  and a return to step S 1  takes place. If no “ignition off” signal is received in step S 9 , a return to step S 5  takes place and the determination of the oil temperature T Öl  continues according to the steps S 5  through S 8 . 
     The oil temperature T Öl  that is determined can be used, in particular, to adapt the clutch characteristic curve  111  ( FIG. 4 ), for example by correcting the slope and/or the offset. In this way, it is possible, for example, to take into account the fact that the viscosity of the lubricating oil decreases with increasing operating temperature of the clutch unit  47 , thus changing the clutch characteristics. As a result of the compensation of the temperature influences, the accuracy of the clutch torque control can be increased. The oil temperature T Öl  that has been determined can also be used for additional control tasks as part of vehicle operation, however. To this end, it can be output to a CAN bus, for example, in order to thus be available to other control units. 
     While the present disclosure finds especially advantageous application in a transfer case with electromechanical actuation of the friction clutch, the present disclosure should not be limited to the above-described exemplary embodiment. Other arrangements in the drive train of a motor vehicle are also possible, as explained at the outset. Furthermore, the actuator  51  can be designed in a different manner than that described above in conjunction with the figures. For example, a different type of reduction gearbox  97  or a different type of deflection device  103  may be provided. In place of the electromechanical actuation of the friction clutch  49  shown, an electromagnetic, hydraulic, or electrohydraulic actuation may also be provided, for instance. 
     The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure. 
     Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail. 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     When an element or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments. 
     Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.