Abstract:
A motor vehicle power take off system for a motor vehicle engine includes a viscous coupling. The input rotor of the viscous coupling is provided with a variable geometry impeller section which allows direct control over the proportion of input torque on the input shaft transferred to the viscous fluid and thereby to an output shaft. Both passive and active control schemes are proposed.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The disclosure relates to viscous or shear fluid torque coupling devices and more particularly to a viscous coupling having a variably configurable input rotor allowing control over the transfer of power to an output rotor and to parasitic devices driven by the output rotor. 
         [0003]    2. Description of the Problem 
         [0004]    Fluid couplings using a viscous working fluid for transmitting torque from an input shaft to an output shaft are well known in the art. Such fluid couplings have typically included an output rotor and a cover which cooperate to define a fluid chamber, a valve plate dividing the fluid chamber into an operating chamber and a reservoir, and an input rotor disposed within the operating chamber and rotatable relative to an output rotor. The input and output rotors define a shear space such that rotation of the input rotor causes viscous fluid to circulate in the shear space and thereby exert a viscous drag on the output rotor, causing the output rotor to rotate. The valve plate defines a fill orifice, and a valving arrangement controls the flow of the working fluid from the reservoir chamber, through the fill orifice, into the operating chamber. When most of the viscous fluid is discharged from the operating chamber to the reservoir chamber the fluid coupling is considered to be “disengaged” and little or no power is transmitted through the device. When viscous fluid partially or fully fills the operating chamber, infiltrating the shear space, the coupling becomes partially or fully “engaged”. 
         [0005]    Conventional fluid couplings have exhibited relatively tight clearances between the outer periphery of the input member and the inner periphery of the output member, partly because the viscous fluid between these adjacent peripheries acts as a fluid bearing, and partly to maximize the available shear surface and the torque transmitting capacity. U.S. Pat. No. 4,132,299 taught such a fluid coupling. The &#39;299 patent is also an example of a form of valving to control the flow of fluid into the operating chamber to effect engagement or disengagement. 
         [0006]    Conventional fluid couplings have generally been of the type referred to as “full OD”, i.e., the outer surface of the input member and the inner surface of the output member are cylindrical and have a maximum diameter over the entire axial extent of the respective surfaces. A full OD input member provides maximum torque transmission when the fluid coupling is engaged. With the coupling disengaged, however, several problems arise in connection with the use of the full OD input member. One of these is the “cold-start” condition which arises after the coupling has been inoperative for a period of time and fluid has leaked from the reservoir into the operating chamber, causing the coupling to operate as though it were engaged when it is intended to be disengaged. Upon start-up of the coupling under this condition, it typically takes a full minute or more for enough of the fluid to be discharged from the operating chamber back into the reservoir chamber to reduce the speed of the output member to its normal, disengaged level. During this period of time, operation of the coupling may not be desired, e.g., the coupling is driving the radiator cooling fan of a vehicle engine and no cooling is required upon initial start-up of the vehicle engine. A relatively higher disengaged output speed (referred to as “idle speed”) results in a relatively higher horsepower consumption by the coupling and the associated cooling fan with no resultant benefit. 
         [0007]    As the interest in improved fuel efficiency in motor vehicles has grown, with the increasing cost of transportation fuel, and the need to comply with emission regimes has been compelled by mandatory government standards, interest has grown in limiting power drawn by parasitic devices which are conventionally driven off the engine or vehicle transmission. Such devices include not a only engine cooling fans, which have routinely been equipped with viscous couplings, but power steering pumps, alternators, compressors, power take-off equipment and even engine superchargers. The ability to disengage such devices when not needed can substantially reduce power demands, thus reducing fuel consumption and potentially easing meeting emission requirements. Also of interest would be controlling unintended engagement of the output rotor stemming from leakage of working fluid from its reservoir into the operating chamber and the ability to provide a rapid responding shear fluid coupling. 
       SUMMARY 
       [0008]    A motor vehicle power take off system for a motor vehicle engine includes a linkage from the engine for turning a rotatable input shaft. A viscous coupling including an operational chamber is connected to the rotatable input shaft. A rotatable output rotor and dependent output shaft are provided with the rotatable output rotor being located in the operational chamber. The operational chamber is substantially filled with a viscous working fluid for coupling the input rotor and the output rotor. A variable geometry impeller section is installed on the input rotor, the impeller section being configurable for varying the amount of input torque applied to the input rotor which is transferred to the working fluid. Both active and passive control schemes for the varying the geometry of the impeller section are provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a high level schematic of a motor vehicle power take off system. 
           [0010]      FIG. 2  is a side cross-sectional schematic of a viscous coupling used with the power take off system of  FIG. 1 . 
           [0011]      FIGS. 3A  and B are end sections of the viscous coupling of  FIG. 2 . 
           [0012]      FIG. 4  is a schematic of a representative embodiment of a motor vehicle power take off system. 
           [0013]      FIG. 5  is a schematic of a representative embodiment of a motor vehicle power take off system. 
           [0014]      FIG. 6  is a schematic of a representative embodiment of a motor vehicle power take off system. 
           [0015]      FIG. 7  is a schematic of a representative embodiment of a motor vehicle power take off system. 
           [0016]      FIG. 8  is a control schematic for still another embodiment of a motor vehicle power take off system. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Referring now to the figures and particularly to  FIGS. 1 ,  2  and  3 A-B, a viscous coupling  24  is described. Viscous coupling  24  provides for transmitting torque from (a vehicle) engine  10  to a parasitic device such as an engine coolant pump  26 . The amount of torque transmitted depends upon demand for the operation of the parasitic device, which here would be engine temperature as indicated by the temperature of coolant circulating through the coolant pump  26 . Control over torque transmission may be implemented either passively or actively. In the illustrated embodiment control is passive. Provision for a reservoir of working fluid is not required since the operating chamber may remained filled with torque transfer being varied by varying the geometry or alignment of impellers mounted to an input rotor in the viscous coupling. 
         [0018]    Engine  10  is equipped with a conventional power take-off mechanism (PTO)  12  which turns a shaft  14 . Shaft  14  is in turn connected into the viscous coupling  24  as the input shaft. Shaft  14  drives a cylindrical input rotor  28  of the viscous coupling  24 . Input rotor  28  carries a plurality of variable pitch/geometry fins  16 . Variable pitch fins  16  are mounted circumferentially about the input rotor  28 , oriented outwardly from the exterior surface of the rotor. When turned across the direction of rotation of the input rotor their rotation circulates the working fluid/oil within operating chamber  20 . Input rotor  28  is mounted within an output rotor  22  with the exterior surface of the input rotor opposite an interior surface of the output rotor. The variable pitch fins  16  extend from the surface of the input rotor  28  into near proximity with the interior surface of the output rotor  22  without physical contact between the fins and the interior surface of the output rotor. Both rotors are mounted for rotation. The fins  18  can be rotated from a substantially feathered or disengaged position ( FIG. 3B ) to an engaged position in which the fins are transverse to the tangent to the direction of rotation of the input rotor (as indicated by the arrows in  FIGS. 3A-B ) and working fluid is caused to circulate (shown in  FIG. 3A ) in operating chamber  20 . In the fins&#39;  16  feathered state the transmission of torque is minimized. As the fins  16  are progressively rotated across the direction of rotation of the input rotor  28  the transmission of torque increases. 
         [0019]    The viscous coupling  24  embodiment of  FIG. 1  does not include an active control system but relies instead on changes in shape of bi-metallic thermocouples  18  to reposition variable pitch fins  16 . The bi-metallic thermocouples  18  are mounted in communication with coolant circulating through a coolant pump  26  and extend from the pump into input rotor  28  of the viscous coupling  24 . By default, the fins  18 , when “disengaged”, maintain a minimum rotation of the output rotor  22  to assure that coolant circulates from the coolant pump  26  through the engine  10  and that thereby the temperature of the coolant accurately reflects engine temperature. This prevents local boiling and lockup of the coolant circulation system. It is easy to contemplate other passive control systems based, for example on temperature of the shear fluid. For example, a system is conceivable which would disengage the fins  18  with increasing temperature of the shear fluid to protect the operating characteristics of the fluid. Such a system could be used in combination with an active control system for a non-vital component to protect the coupling against damage. 
         [0020]    A generalized alternative embodiment is represented in  FIG. 4 , where engine  10  and PTO  12  are connected by a shaft  14  into a viscous coupling  124 . Viscous coupling  124  is connected by an output shaft  30  to a parasitic device  32 . A condition sensor may be associated with the coupling  124 , the parasitic device  32  or the output shaft  30 . Here for example an output shaft  30  with a sensor such as a strain gauge may be provided. A strain gauge could be applied to a shaft which was intended to be operated at constant output torque. Electrical power for the sensor and fin positioning could be provided by an electric generator built into one end of the input shaft  14  and a fixed point, such as the housing for the coupling  124 . A controller powered from the same power source would function to orient the fins  18  to maintain a fixed (or selected) strain on the output shaft. Such a system would represent a mild, but self contained, active control arrangement that would not involve other vehicle control systems. 
         [0021]    Fully active control arrangements typically vary with the application of the parasitic device to be powered. Referring to  FIG. 5 , provision may be made for active electronic control of the rotational positioning of fins  18  by use of a power take off controller  41 . An example of how such a controller  41  would operation would be its use to position a cam actuator located through input shaft  114  for engaging cam followers in the viscous coupling  224  to rotate the fins  18  for progressively engaging or disengaging the coupling. Of course a number of control arrangements may be provided and these are not limited to a cam actuator, cam follower system. Controller  41  would provide for moving the cam actuator in response to operating condition(s) relevant to the particular application.  FIG. 5  relates specifically to an arrangement for providing variable assist power steering. Variable boost is achieved by varying the pump speed of a power steering pump  226  driven by the output shaft  135  connected from the viscous coupling  224  to the power steering pump. Among possible variables that could be used for controlling boost are engine speed and vehicle speed. Typically less torque would be transferred through coupling  224  as vehicle speed increases and more torque would be transmitted as engine speed decreased, other variables being held equal. In essence the output torque required increases with decreasing vehicle speed, but the required proportion of available torque transmitted increases with decreasing engine speed. Given the transmission is changing gears, or even out of gear at times, these variables can be indicating opposite changes in fin  18  orientation and may even cancel. A transmission controller  37  coupled to a transmission could be used to generate a vehicle speed signal. An engine tachometer  39  connected to the engine  10  reports engine rotational speed. The PTO controller  41  develops a target output torque for the output shaft  135  at a given speed and varies transmitted torque based on changes in engine rotational speed. The fins are positioned to transfer the appropriate amount of torque to produce the target output shaft  135  speed for the power steering pump. Alternatively sets of variables are possible, for example vehicle speed and output shaft speed; power steering pump pressure and vehicle speed; or, vehicle speed and output shaft torque. 
         [0022]    Embodiments directed toward applications for the generation of vehicle electrical power may be considered desirable, and an alternating current system is illustrated in  FIG. 6 . In  FIG. 6  it is desirable to run an AC generator  626  at a constant speed notwithstanding changes in the load  627  supported by the generator. A tachometer  630  is illustrated connected to viscous coupling  224  which generates a rotational velocity signal for the output shaft  135 . In order to maintain a constant frequency AC output from AC generator  626  it is essential to maintain constant the rotation velocity of shaft  135 . Alternatively, if variable frequency alternating current were desired it would be a simple matter to vary the target speed of the shaft. For example, an alternating current electrical system capable of operating at 50, 60 or 400 Hertz may be desired. In any case the tachometer signal from tachometer  630  is provided as a feedback signal to controller  41 . Controller  41  may also receive engine operating variable signals from an engine controller  650  and may refer requests for increases in power from engine  10  to the EC  650 . For example, if coupling  224  is already set for maximum torque transfer and output frequency is falling the controller  41  can call on the engine controller  650  to increase engine output. 
         [0023]      FIG. 7  as illustrated describes an embodiment suited for vehicle operations such as maintaining air brake system pressure or vehicle electrical system battery charge. The specific illustration shows its use with an alternator  726 , a vehicle electrical system  728  and a battery voltage sensor  730 , however, an air pump/compressor could readily be substituted for alternator  726 , an air storage and brake system for electrical system  728  and a storage tank pressure gauge for voltage sensor  730 . It should be recognized that “battery voltage” is a proxy for the state of charge of a vehicle battery and may not reflect the actual state of charge of a battery, particularly a conventional lead-acid battery. Essentially the control arrangements are physically quite similar to the embodiment of  FIG. 5  except the source of the feedback control signal has changed. The use made of that variable also changes. Where battery voltage is the controlling variable it is expected that a minimum battery voltage must be maintained and that the system will run transferring maximum torque at low voltages to speed recharge of a system battery during periods when the battery can accept high input currents. Charging current is reduced to a trickle as the target voltage is approached. Keeping air pressure in a tank in a target range may make use of a different regimen, since there the air tank will not be damaged by high input rates, although there may be an interest in not overloading the engine. 
         [0024]    Referring to  FIG. 8 , a control schematic for an embodiment relating to supercharging of an engine  10  is illustrated. The arrangements are similar to those for the embodiment of  FIG. 7 , with the substitution of a supercharger  821  for alternator  726 . A boost pressure sensor  811  provides a boost pressure signal as a feedback signal to the PTO controller (PC)  41 . Providing boost to an engine  10  induction system presents different control issues than does maintaining air pressure for a pneumatic brake system. Superchargers, while providing quicker response times than turbochargers and providing boost at low engine RPMs when such boost is most needed, have suffered in comparison with turbochargers because they impose a parasitic loss on an engine while a turbocharger, in substantial part, operates on energy recaptured from a vehicle exhaust system. The parasitic loss represented by the supercharger has often been imposed whether or not boost was required. A system which allows control over the amount of torque transmitted from an engine drive system to a supercharger allows engine and transmission control (for automatic transmission equipped vehicles) to be integrated with control over the supercharger and for the supercharger to be disengaged when not needed. In the present embodiment coupling  224  can be disengaged at highway cruise to eliminate the parasitic losses imposed by supercharger  821 . At low engine RPMs and high throttle settings, full engagement of coupling  224  can be used to achieve the desired boost. “Desired boost” may be calculated by engine controller  650  based on engine and transmission operating variables, limited only by how much torque can be transmitted through coupling  224 . PC  41  than uses the pressure boost feedback signal from sensor  811  to control coupling  224  to achieve the target input.