Abstract:
A liquid cooled viscous fan clutch transfers torque from a driving plate driven by an engine crankshaft to a driven plate connected to a cooling fan. Heat generated in a working fluid is transferred to a second fluid, such as engine coolant or transmission fluid, flowing through a cooling jacket. The cooling jacket is located in a stationary housing which may be fixed to the engine. The working fluid circulates through a working zone, a passageway in the stationary housing, and a passageway in the driving plate. Rotation of the driving plate provides the motive force to circulate the fluid, independent of the rotation speed of the driven plate. A controllable valve may be closed to block circulation, trapping working fluid in a reservoir, to disengage the clutch.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to the field of automotive fan clutches. More particularly, the disclosure pertains to a viscous fan clutch having a stationary housing and liquid cooling of the working fluid. 
       BACKGROUND 
       [0002]    Many automotive engines are cooled with liquid coolant. The coolant absorbs heat while circulating within the engine and then transfers that heat to ambient air while circulating through a radiator. During operation in the most demanding operating conditions, an engine driven fan may be used to increase the flow of ambient air through the radiator. In less demanding conditions, it is desirable not to operate the fan to reduce the load on the engine. To achieve this intermittent fan operation, the engine crankshaft may drive the fan via either an actively controlled or thermostatically controlled fan clutch. 
         [0003]    A fan clutch is illustrated in  FIG. 1 . Input shaft  10  is driven by the engine crankshaft either directly or via some power transfer mechanism such as an accessory drive belt. Output shaft  12  drives the fan. Input plate  14  is fixed to input shaft  10  while output plate  16  is fixed to output shaft  12  via a clutch cover  18 . Ribs on input plate  14  are interspersed with ribs on output plate  16  such that the ribs are close to one another but do not touch. To engage the clutch, a working fluid is released from reservoir  20 . As the fluid flows through the narrow gap between the ribs, viscous shear in the fluid exerts torque on the input plate and output plate. This narrow gap is called the working zone. The magnitude of the torque depends upon the relative speed between the plates and on the quantity of fluid in the working zone. When the fluid reaches the perimeter of the working zone, it is moving circumferentially. Some of the fluid enters return channel  22  in the clutch cover. If the output shaft is moving slower than the input shaft, then the fluid slows as it enters the return channel, causing an increase in pressure. When the speed difference between the input and output plates is sufficient, the increased pressure forces the fluid through return channel  22 , against centrifugal force, back to reservoir  20 . Thus, in the engaged state, fluid circulates continuously from the reservoir, through the working zone, through the return channel, and back to the reservoir. The output shaft speed stabilizes at a speed less than the input shaft speed. 
         [0004]    To disengage the clutch, valve  24  is moved into a position in which it blocks the flow of fluid out of the reservoir  20 . Once the fluid that was in the working zone exits the working zone, all torque transfer stops. Once the torque capacity is reduced, drag causes the fan to slow down. As the fan slows down, all of the fluid is returned to reservoir  20  through return channel  22 . The position of valve  24  may be controlled via an actuator  26 . For example, actuator  26  may be a stationary electro-magnetic actuator that pulls valve  24  into the engaged position shown in  FIG. 1  by exerting a magnetic force. A return spring  28  pushes the valve into the disengaged position when the magnetic force is removed. 
       SUMMARY OF THE DISCLOSURE 
       [0005]    A powertrain includes an engine, a cooling fan selectively driven by the engine via a viscous fan clutch, and a transmission. A working fluid circulates within the viscous clutch through a working zone between a drive plate and a driven plate. The drive plate and driven plate are supported in a stationary housing fixed to the engine. Liquid coolant, such as transmission fluid or engine coolant, may be routed through a coolant jacket in the clutch to remove heat generated by viscous shear and prevent the clutch from overheating. A valve may selectively block circulation of the working fluid to disengage the clutch. 
         [0006]    A viscous fan clutch includes a drive plate fixed to an input shaft, a driven plate fixed to an output shaft, and a stationary housing. A plurality of cylindrical ridges on the drive plate are interspersed with a plurality of cylindrical ridges on the driven plate to define a working zone. Viscous shear in a working fluid flowing through the working zone transfers torque from the drive plate to the driven plate. A passageway in the drive plate connects an inlet port to an outlet port radially inside the working zone. A second passageway in the stationary housing connects an inlet port located radially outside the working zone to an outlet port adjacent to the inlet port of the first passageway. Rotation of the drive plate propels the working fluid through the first passageway, working zone, and then through the second passageway. The stationary housing may further define a coolant jacket proximate to the second passageway to provide heat transfer from the working fluid to a coolant flowing through the coolant jacket. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a cross sectional diagram of a prior art viscous fan clutch. 
           [0008]      FIG. 2  is a schematic diagram of a vehicle powertrain including a fan clutch cooled with transmission fluid. 
           [0009]      FIG. 3  is a cross sectional diagram of a first embodiment of a liquid cooled fan clutch suitable for use in the powertrain of  FIG. 2 . 
           [0010]      FIG. 4  is a cross sectional diagram of a second embodiment of a liquid cooled fan clutch suitable for use in the powertrain of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
         [0012]      FIG. 2  schematically illustrates a vehicle powertrain. The flow of mechanical power is illustrated by solid lines. Dashed lines indicate the flow of engine coolant while dotted lines indicate flow of transmission fluid. Engine  40  generates power to turn a crankshaft by burning fuel. A transmission  42  conditions the mechanical power by adjusting the speed and torque based on current vehicle needs. At low speed, transmission  42  reduces the speed and multiplies the torque to improve performance. At higher speed, transmission  42  increases the speed such that the engine can run at an efficient crankshaft speed. Differential  44  divides the power between left and right drive wheels  46  and  48  while permitting slight speed differences as the vehicle turns. 
         [0013]    Heat is removed from the engine by circulating engine coolant through the engine block and through radiator  54 . A thermostatic valve shuts off circulation through the radiator whenever the engine coolant is below a desired operating temperature. The engine coolant may also circulate through a heat exchanger called a heater core when cabin heat is requested. To control the temperature of the transmission fluid, the transmission fluid may be circulated through radiator  54  (although separated from engine coolant) or may be circulated through a liquid to liquid heat exchanger to transfer heat to engine coolant. Both the engine and the transmission operate less efficiently when the temperature is below the normal operating temperature, so warming up quickly to the normal operating temperature is desirable. During heavy load operating conditions, such as towing a trailer up an incline, the natural flow of ambient air through radiator  54  may be insufficient to control the temperature of the engine coolant. In these conditions, clutch  56  may engaged to drive fan  58  to increase the flow rate of ambient air through radiator  54 . When clutch  56  is fully or partially engaged, some of the engine power is diverted to the fan as opposed to propelling the vehicle, reducing vehicle performance. Therefore, it is desirable to engage clutch  56  only when necessary and only to the degree necessary. 
         [0014]    When a viscous fan clutch is transferring torque, heat is generated in the working fluid. The rate of heat generation is proportional to the torque and also proportional to the speed difference between the input shaft and the output shaft. In the prior art fan clutch of  FIG. 1 , the only significant mechanism for dissipating this heat is via convection to ambient air through either output plate  16  or cover  18 . Even when these parts are designed with fins to facilitate convection, the heat dissipation capability is limited. Consequently, the clutch must be carefully controlled to avoid operation with combinations of speeds and torque capacity that would generate excessive heat in the working fluid. When the engine speed is high, the clutch must either be disengaged to reduce the torque or engaged sufficiently to reduce the speed difference. Operation at intermediate torque capacities with substantial slip must be avoided. This limits the control system&#39;s ability to set the fan speed to the optimum level to provide adequate engine cooling with minimum parasitic loss. In the prior art clutch of  FIG. 1 , the motive force circulating the working fluid is based on a speed difference between the driving plate and the driven plate. Therefore, some degree of slip is required in order to maintain working fluid circulation though the working zone to maintain torque capacity. 
         [0015]    Fan clutch  56  of  FIG. 2  provides an additional heat dissipation mechanism by routing transmission fluid from the transmission to the clutch via circuit  60  and then back to the transmission via circuit  62 .  FIGS. 3 and 4  show alternative fan clutches configured to transfer heat from the working fluid to the transmission fluid. Each of these embodiments include a stationary clutch housing through which both the working fluid and the transmission fluid flow, providing an opportunity for heat transfer. In alternative embodiments, engine coolant may be routed through clutch  56  as opposed to transmission fluid. 
         [0016]    A viscous fan clutch  56  with a stationary housing  70  and liquid cooling is illustrated in  FIG. 3 . Input shaft  10  and output shaft  12  are both supported for rotation by bearings. Input shaft  10  is driven by the engine crankshaft either directly or via some power transfer mechanism such as an accessory drive belt. Output shaft  12  drives the fan. Input plate  14  is fixed to input shaft  10  while output plate  16  is fixed to output shaft  12 . Ribs on input plate  14  are interspersed with ribs on output plate  16  such that the ribs are close to one another but do not touch. To engage the clutch, a working fluid is released from reservoir  20 . As the fluid flows between through the narrow gap between the ribs, viscous shear in the fluid exerts torque on the input plate and output plate. The magnitude of the torque depends upon the relative speed between the plates and on the quantity of fluid in the working zone. When the fluid reaches the perimeter of the working zone, the input and output plates propel the fluid circumferentially around the interior of housing  70 . The fluid enters return channel  72  near the top of the housing  70 . Gravity causes the fluid to travel through return channel  72  back towards the rotational axis. Momentum of the fluid imparted by the input plate also propels the fluid through return channel  72 . This mechanism of circulating the working fluid does not rely on slip between the input shaft and output shaft. Since the housing  70  is not rotating, it is not necessary to overcome centrifugal forces to return the working fluid to reservoir  20 . From return channel  72 , the fluid flows through a gap between housing  70  and input shaft  10  into a channel in input shaft  10  which returns the fluid to reservoir  20 . Seals define the gap between housing  70  and input shaft  10  and direct the flow into the input shaft channel. Thus, in the engaged state, fluid circulates continuously from the reservoir, through the working zone, through the return channel, and back to the reservoir. As the output shaft speed approaches the input shaft speed, the torque capacity decreases due to reduced shear rate in the working zone. However, the quantity of working fluid in the working zone does not decrease. The output shaft speed stabilizes at a speed slightly less than the input shaft speed. 
         [0017]    A coolant jacket  74  is formed into housing  70 . Transmission fluid is routed from circuit  60  through coolant jacket  74  and then back to the circuit  62 . Alternatively, engine coolant may be circulated through the coolant jacket. Return channel  70  is routed through coolant jacket  74  to provide opportunity for efficient heat transfer. Although only a single, straight path is shown, return channel  70  may divide into multiple paths which may take a circuitous route through the coolant jacket to maximize the surface area available for heat transfer. 
         [0018]    To disengage the clutch, valve  24  is moved into a position in which it blocks the flow of fluid out of the reservoir  20 . Once the fluid that was in the working zone exits the working zone, all torque transfer stops. The position of valve  24  may be controlled by a stationary electro-magnetic actuator in stationary housing  70  that pulls valve  24  into the engaged position by exerting a magnetic force. A return spring  28  pushes the valve into the disengaged position shown in  FIG. 3  when the magnetic force is removed. 
         [0019]    A second viscous fan clutch  56  with liquid cooling is shown in  FIG. 4 . In this embodiment, reservoir  20  is in the stationary housing  70  as opposed to the rotating input shaft  10 . Since valve  24  is not in a rotating component, the actuator can be simplified. When the valve is open, gravity causes the working fluid to flow into feed channel  76  in input shaft  10  to engage the clutch. Scavenging of the fluid back to reservoir  20  and transfer of heat to cooling jacket  74  is accomplished as in the embodiment of  FIG. 3 . 
         [0020]    While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and can be desirable for particular applications.