CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of PCT/DE2011/001519 filed Jul. 27, 2011, which in turn claims the priority of DE 10 2010 035 123.7 filed Aug. 23, 2010 and DE 10 2010 047 524.6 filed Oct. 5, 2010, the priority of these applications is hereby claimed and these applications are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a friction clutch and especially a clutch bell housing air cooler, 
     Friction clutches in motor vehicles as a rule have a clutch bell housing. With current clutch temperature models, the bell air temperature in the clutch bell housing might not be calculated with sufficient precision online—i.e., in the vehicle—in a transmission with a dry friction clutch and active cooling. Thus it makes sense to apply a temperature sensor and thereupon to optimize the clutch temperature model. 
     The clutch temperature model delivers a signal which is used for a protection strategy. The clutch protection strategy protects the clutch from excess heating and thus from effects that appear at higher temperatures, such as fading, thermal shock, etc. Through introduced friction loss, the clutch disks and also the clutch plates and other components of the clutch such as diaphragm springs, reset systems, bearings, etc, become warm. These heated parts become cool, among other locations, also in the bell air—i.e., the air in the clutch bell housing. The be air in turn has a heat transmission path in the direction of the engine, engine compartment and transmission, as is shown in  FIG. 1 . 
     The be air temperature can currently be determined in various ways:
         1.) A bell air temperature sensor within the dutch bell housing, but without active cooling. This is disclosed by DE 41 00 091 A1 for example.   2.) A temperature sensor on the outer wall outside the clutch bell housing with active ventilation. This is disclosed in DE 10 2009 044 385 A1, for example.   3.) A calculated bell air temperature in dependence on friction loss and on heat-transmission behavior of the vehicle, engine, transmission and the clutch itself in a system without active cooling   The known prior art permits no flow simulation online in the vehicle, since currently too high a computational capacity is required. The drawbacks of the individual solutions are the following:   1.) Without active cooling, clutch applicability, and thus vehicle availability, is thermally limited, or the clutch will be thermally overloaded, leading to a shortening of service life.   2.) With a temperature sensor on the outer wall of the bell housing, outside the bell, the value of the bell air temperature can be assessed only with insufficient precision, since the temperature determined on the outer wall of the bell is dependent on the capacity of the bell casting, the heat flux within the engine compartment and also the bell air. This results in the clutch model being imprecise, which leads to a limitation in mobility and availability of the vehicle, or causes an overheating of, or damage to, the clutch.   3.) Calculation of the bell air temperature with active cooling involves increased computational complexity, which requires increased computing performance. By increasing the complexity, rather many input signals are required, which impairs the precision of the model, which leads to a limitation in mobility and availability of the vehicle or can cause an overheating of, or damage to, the clutch.       

     SUMMARY OF THE INVENTION 
     The object that is the basis for the present invention is to determine the dutch temperature while using active bell air cooling with increased precision by means of a clutch temperature model. 
     According to the invention a friction clutch is proposed in a motor vehicle with a clutch bell housing as well as at least one air inlet duct situated on the clutch bell housing and at least one air outlet duct situated on the clutch bell housing and at least one temperature sensor, wherein the temperature sensor in the clutch bell housing is situated at an angular range of 90 degrees to 360 degrees, especially of 180 degrees to 360 degrees, along the circumference of the clutch bell housing proceeding from the air inlet duct in the flow direction of the air fed through the air inlet duct, or wherein the temperature sensor is situated in the air outlet duct. In advantageous fashion, the temperature in the clutch bell housing can thus be determined. 
     In one preferred embodiment, the inlet duct and/or the outlet duct are/is situated essentially tangentially in the circumferential direction to the clutch bell housing, or placed at a preset angle to the tangent line of the clutch bell housing. The preset angle is selected to be small for reasons of optimized aerodynamics, for example between 0 and 45 degrees, and in an especially preferred embodiment between 0 and 25 degrees. 
     In a further preferred embodiment, the ends of the inlet duct and/or the outlet duct extend into the clutch bell housing. 
     In a further preferred embodiment, the ends of the ducts are curved and/or the ends of the ducts have deflection vanes such as, guide vanes in the area of the ends, so that the ends are aerodynamically optimized. 
     In a further preferred embodiment, the ends terminate flush with the clutch bell housing. 
     In a further preferred embodiment, provision is made that the temperature sensor situated within the clutch bell housing is placed at a distance from the clutch bell housing wall by a spacing device and/or thermal insulation, such as a ceramic insulator, so that the temperature sensor measures the temperature of the bell air. 
     In a further preferred embodiment, provision is made that the air fed through the air inlet duct is cooled as compared with the recirculating air of the vehicle, which occurs by means of connection to an air conditioner and/or to the vehicle cooling system and/or to an engine cooling system and/or to a low-temperature circuit. 
     In a further preferred embodiment, provision is made that the air fed through the air inlet duct is heated as compared with the recirculating air of the vehicle, which occurs by means of connection to the vehicle heating system. 
     According to the invention, a method is also proposed for determining a dutch temperature of a friction clutch in a motor vehicle, wherein the friction clutch is designed according to the designs implemented above. The method according to the invention makes provision that, by means of a clutch temperature model which takes the values of the at least one temperature sensor, it determines the clutch temperature and by means of the determined clutch temperature controls the active cooling device. 
     In a preferred embodiment of the method, provision is made that if the sensor fails, either the method is continued by an additional functionally capable temperature sensor, or the active cooling device is run continuously, wherein in this case the clutch temperature is determined using a clutch temperature model which takes no active cooling into account. 
     In a further preferred embodiment of the method, provision is made that the active cooling device is controlled by means of a transmission control device or by means of an engine control device. 
     The friction clutch according to the invention, as well as the method according to the invention, has the following advantages:
     a) avoiding excessively high computing capacity, or computing capacity which is not available   b) avoiding limited vehicle availability due to a clutch protection strategy that engages too strongly   c) Preventing damage to the clutch because the model is insufficiently precise   

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the Drawing 
         FIG. 1  is a schematic diagram of a prior art clutch bell housing showing heat transmission paths; 
         FIG. 2  is a schematic diagram of the friction clutch according to the invention; and 
         FIG. 3  is a schematic block diagram of a control for the friction clutch of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For determination of the clutch temperature when using an active bell housing cooling device  100 , a temperature sensor  120  is provided within the bell housing air volume  150 . By reference number  120 , both the temperature sensor and its position are designated. In  FIG. 2 , several positions are indicated as examples. Especially appropriate sensor positions  120  can be determined using an investigation of the flow behavior within the clutch or the clutch bell housing. As viewed from the inlet duct  130 , advantageous sensor positions  120  are the angular range between 90 degrees  265  and 360 degrees  250 , especially between 180 degrees  260  and 360 degrees  250 , in the flow direction  270  of the air fed through the inlet duct  130  or a sensor  120  in the outlet duct  140 . Thus an advantageous measurement position would, generally formulated, be the bell inner air  150 , not yet with admixture of the fresh air  160 , i.e. not in the direct flow of the admixed outer air  160 . Therefore the range of angles from 180 degrees to 360 degrees has an advantage relative to the range of angles from 90 degrees to 360 degrees in that the fresh air  160  has already been more thoroughly mixed with the inner bell air  150 . Also, the inlet duct  130  and outlet duct  140  are aerodynamically optimized. As depicted in  FIG. 2 , this, for example, occurs through ends of the ducts  300  that extend radially into the clutch or the clutch bell housing; through a curvature of the ends  300  extending into the clutch; through deflection vanes such as guide vanes in the area of the ends of the ducts  300 , or in the clutch; or a combination of these possibilities. Also it is advantageous for the ends  300  and the clutch bell housing to terminate in flush fashion. The inlet duct  130  and/or the outlet duct  140 —especially in the area where they empty into the dutch bell housing—are situated essentially tangentially in the circumferential direction to the dutch be housing or at a preset angle to the tangent line of the clutch bell housing  210 . To optimize the aerodynamics, the preset angle is chosen to be small, for example between 0 and 25 degrees. 
     In the event the sensor fails, there are in principle two possibilities for solution. One is that a second sensor can be provided as a check or as a replacement, and the method runs aided by the second sensor. Another is that if a sensor fails, provision can be made to run the fan of the active cooling device  100  continuously, but with the conventional clutch temperature model—thus assuming that no active cooling is present—to determine the temperature. 
     The active cooling device  100 —which is designed in a preferred embodiment as a fan in FIG.  2 —can be controlled by a control device  350  (see  FIG. 3 ), such as the transmission control device or the engine control device. The advantage in actuating by means of the transmission control device is that the active cooling device  100  can react directly to the determined signals of the clutch temperatures. 
     The device can undergo active cooling  100  in various ways for use with a bell air temperature sensor  120  and optimized temperature model, regardless of whether the auxiliary cooling is generated by a fan, via compressed air through the inlet duct  130  or via suction—for example, through reduced pressure in the outlet duct  140 . A possibility also exists to cool the air flow  160 —for example by means of connection to the air conditioner  310 , or integrating it into the cooling systems of the vehicle  320  or engine  330 , for example a low-temperature circuit. Likewise, a combination of the above-named options is provided. Also a possibility exists to heat the air flow  160 , for example by connection to a vehicle heating system  340 . 
     Characteristic of this design are meaningful measurement location  120  for the bell air temperature sensor  120  and adaptation of the clutch temperature model. The temperature sensor  120  should not determine the temperature directly on the transmission housing wall or the bell housing wall, but rather measure the pure bell air temperature, which requires a certain distance to or insulation from the bell housing cast material—for example a ceramic insulator between the bell housing cast material and the sensor  120 . The sensor cable can be run through an existing actuator duct—such as with a lever actuator—or via another sensor feed. 
     An actively cooled dutch with an air temperature sensor within the dutch bell housing results in improved cooling behavior of the clutch and in a precise clutch temperature model. A clutch system without a bell air temperature sensor and active cooling can lead to impaired vehicle availability or to thermal overloading of the clutch, since there would be very many parameters to be identified, and thus the complexity could no longer be handled. Determining the flow behavior and the value of the bell air temperature determined thereon on-line would require excessively high computing capacity. The measurement position of the bell air temperature sensor must be determined after an assessment of the aerodynamic circumstances in the clutch bell housing. One possible measurement location which is recommended for this is within the expelled heated air; here the aerodynamic behavior should not result in excessively large changes (such as dead-flow zones) whether the active cooling is switched on or turned off. A possibility exists to actuate the active cooling device either via the engine control device or the transmission control device. There are various options on how to proceed if the sensor fails: conventional calculation without active cooling, wherein the active cooling is in operation, or a second bell air temperature sensor. The sensor should not measure the cast-structure temperature directly, since this would result in inaccuracy of the determined cast-structure temperature. The cable of the sensor can be run via the clutch actuator. 
     LIST OF REFERENCE SYMBOLS 
     
         
         
           
               100  active cooling device 
               120  advantageous positions for a temperature sensor 
               130  inlet duct 
               140  outlet duct 
               150  clutch bell housing air 
               160  cool air or fresh air 
               170  warm air 
               200  turning direction of clutch 
               210  clutch bell housing 
               250  angle of 0 degrees or 360 degrees 
               260  angle of 180 degrees 
               265  angle of 90 degrees 
               270  flow direction of the air fed through the inlet duct 
               300  ends of the ducts
The invention relates to a friction clutch in a motor vehicle, comprising a clutch bell housing ( 210), at least one air inlet duct ( 130) arranged on the clutch bell housing for cooling the clutch bell housing, at least one air outlet duct ( 140) arranged on the clutch bell housing, and at least one temperature sensor ( 120). The invention further relates to a method for determining a clutch temperature of a friction clutch.