Abstract:
A fan apparatus is disclosed which includes a fan coupled to a drive mechanism via a viscous clutch. The viscous clutch includes a drive chamber containing a hydraulic fluid with an amount of hydraulic fluid in the drive chamber being related to a torque transmitted from said drive mechanism to the fan; a valve coupled to the drive chamber controlling flow of hydraulic fluid into the drive chamber; a bimetallic strip coupled to the valve; and a heating element coupled to the viscous clutch close to the bimetallic strip. The bimetallic strip is located in a position open to ambient air.

Description:
FIELD OF THE INVENTION 
   The present invention pertains to a viscous clutch disposed between an internal combustion engine and a cooling fan, and more particularly, to a method and apparatus to control the actuation of the viscous clutch. 
   BACKGROUND OF THE INVENTION 
   It is well known in the art to use a fan to provide cooling to an internal combustion engine. The fan is typically driven by a belt connected to a pulley on the engine. It is also known to employ a viscous clutch between the fan&#39;s pulley, which receives drive power from the engine, and output of the clutch coupled to the fan. 
   A typical system is shown in  FIG. 1 . An engine  10  is disposed in a vehicle  12 . On the front of engine is a pulley (not visible in  FIG. 1 ) which drives a second pulley  14 . Pulley  14  is coupled to a shaft  16 . Shaft  16  is coupled to a viscous clutch  18 . Fan  20  is affixed to viscous clutch  18 . 
   A front view of engine  10  is shown in  FIG. 2  in which pulley  24 , which is affixed to the crankshaft of the engine, is coupled to pulley  14  via belt  26 . Pulley  16  is coupled to viscous clutch  14 , which is in turn coupled to fan  20 . 
   A detail of the viscous clutch  18  is shown in  FIG. 3 . Input shaft  16  to clutch  18  is coupled to plate  30 . Output shaft  22  is coupled to plate  32 . When the space between plates  30  and  32  is filled with a liquid, e.g., hydraulic fluid, shaft  22  is caused to rotate at a speed somewhat less than the rotational speed of shaft  16 , such speed of shaft  22  depending on speed of shaft  16 , viscosity of the fluid and other parameters. When the space contains little or no liquid, shaft  22  rotates at a much slower speed than shaft  16 . Viscous clutch  18  contains a storage reservoir  28  containing a hydraulic fluid. The space in between plates  30  and  32  is called a drive chamber. There is a valve (not shown) between storage reservoir  28  and the drive chamber. In one example, the valve is an orifice that can be occluded or not by a plate. In a second example, the valve is an orifice that can be occluded by a pin. When the valve is open, hydraulic fluid from the storage reservoir is forced through the valve orifice into the drive chamber, thereby filling the space between plates  30  and  32 . Typically, plates  30  and  32  have annular ridges on their surfaces to inhibit the hydraulic fluid movement toward the outside edges and to increase surface area for more torque capacity due to the shear force of the hydraulic fluid. Hydraulic fluid returns to storage chamber  28  through a passageway, not shown. When the valve is open, hydraulic fluid is continually flowing through the circuit described: storage chamber  28 , through the valve to the drive chamber (between plates  30  and  32 ), and back to storage chamber  28  through the passageway. 
   The opening and closing of the valve is controlled by a bimetallic strip coupled to the valve. One example is a bimetallic coil made with metals of dissimilar expansion coefficients. When the temperature of the coil changes, the coil either coils further or becomes uncoiled, depending on whether the metal with the higher expansion coefficient is on the outside or inside surface of the bimetallic coil. When the bimetallic coil is connected to a small plate that can occlude the valve&#39;s orifice, then, the amount of the valve orifice occlusion is a function of how much the bimetallic coil is coiled up, and hence a function of the temperature. In another example, the bimetallic strip is a strip which changes from flat to cupped when it is heated. When the strip becomes heated, it can depress a pin in the valve to cause the valve to open. The bimetallic strip/valve system can be designed to an on-off or fully variable device. 
   In the prior art, the bimetallic strip is exposed to under-hood air. As the under-hood temperature changes, the bimetallic strip changes shape and changes the position of the valve coupled to the drive chamber and, hence, the amount of fluid in the drive chamber. 
   In U.S. Pat. No. 4,351,426, a heater is disposed next to the bimetallic strip so that by controlling the current or voltage applied to the heater, the state of the bimetallic strip, and hence the valve opening position, can be controlled. In &#39;426, the bimetallic strip and heating element are enclosed at the back of the viscous clutch. The inventor of the present invention has recognized a disadvantage in such a configuration. 
   SUMMARY OF THE INVENTION 
   The inventor of the present invention has recognized a new arrangement for controlling flow into the drive chamber of viscous clutch. 
   To overcome deficiencies in prior art approaches, a fan apparatus including a fan coupled to a drive mechanism via a viscous clutch is disclosed. The viscous clutch further includes a drive chamber containing a hydraulic fluid. The amount of hydraulic fluid in the drive chamber is related to torque transmitted from the drive mechanism to the fan. The clutch further includes a valve coupled to the drive chamber controlling flow of hydraulic fluid into the drive chamber and a bimetallic strip coupled to the valve. The bimetallic strip moves the valve to a more open position when the temperature of the bimetallic strip increases, thereby increasing the amount of hydraulic fluid supplied to the drive chamber. The clutch also includes a heating element coupled to the viscous clutch in close proximity to, or attached to, the bimetallic strip. Further, the bimetallic strip is located in a position open to ambient air. The drive mechanism is an internal combustion engine and the ambient air is the air flowing in and around the engine, in the under-hood engine compartment. 
   The heating element is electronically coupled to an electronic control unit and is supplied current based on sensors coupled to the electronic control unit providing such information as coolant temperature, oil temperature, engine speed, vehicle speed, and engine torque. The current is supplied via a slip ring or an inductive coupling. The electronic control unit is capable of detecting an open circuit to the heating element. If such a condition is determined a code in memory is set. 
   The clutch further includes an input shaft coupled to the drive mechanism via a pulley and an output shaft coupled to the fan. The input shaft is coupled to a first plate and the output shaft is coupled to a second plate. The volume in between the plates is the drive chamber. The amount of torque transmitted from the first plate to the second plate depends on the viscosity and amount of fluid contained between the plates. 
   In one embodiment, the bimetallic element is a flat strip. Alternatively, the bimetallic element is a coil. The bimetallic element changes shape as a function of temperature change. The bimetallic element is coupled to the valve and arranged so that the valve is moved toward a more open position when temperature of the bimetallic element rises. 
   An advantage of the present invention is that the bimetallic strip is in communication with both the heating element and the under-hood air meaning that control of the bimetallic strip is modified or controlled by both. If a break in the circuit to the heating element occurs, the bimetallic strip is still actuated when the under-hood temperature rises above a predetermined temperature, i.e., the set point of the bimetallic strip, thereby avoiding engine overheating. In other words, the heating element is not the solely controlling the viscous fan clutch—the under-hood air temperature also controls. 
   Yet another advantage of the present invention is that engine parameters, such as coolant temperature, oil temperature, engine speed, or any other parameters measured or inferred in the engine control unit can be used to control the current supply to the heater strip, thereby causing control of the viscous clutch&#39;s valve to be based on more information than simply under-hood temperature. In this way, control of the fan is tailored to be more accurate to meet the desired cooling. 
   Another advantage provided by the present invention is that the fuel economy of the vehicle is improved by providing the appropriate cooling level to the engine. When the engine is overcooled, the engine oil is brought to a cooler temperature than optimal and the oil is more viscous. When the oil is more viscous than necessary, the shear force in creating relative movement between engine parts, e.g., pistons reciprocating in engine cylinders, is higher than need be thereby consuming extra fuel. The present invention alleviates overcooling. In addition, by allowing overall lower fan speeds, the fan consumes less engine power, thereby decreasing fuel consumption. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages described herein will be more fully understood by reading an example of an embodiment in which the invention is used to advantage, referred to herein as the Detailed Description, with reference to the drawings wherein: 
       FIG. 1  is a schematic of an automotive vehicle with an internal combustion engine and a fan coupled to the engine via a viscous clutch; 
       FIG. 2  is a schematic of a front view of an internal combustion engine of an internal combustion engine showing pulleys, a belt and a fan; 
       FIG. 3  is a schematic of a viscous clutch according to an aspect of the present invention; and 
       FIG. 4  is a cross-section of the inductive coupling. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An internal combustion engine  10  is shown disposed in an automotive vehicle  12  is shown in  FIG. 1 . At the front of the engine are two pulleys  14  and  24 . Pulley  14  is coupled to a shaft  16 , the input shaft to viscous clutch  18 . Viscous clutch  18  has an output shaft  22  coupled to fan  20  which forces air to flow across engine  10  to provide cooling. Engine  10  has one or more sensors  6 ,  8  connected to it for measuring such quantities as: engine coolant temperature, engine oil temperature, engine speed, barometric pressure, ambient temperature, EGR flow rate, as examples. The output of the engine sensors is communicated to engine control unit  60 . 
   Referring to  FIG. 2 , a front view of the engine is shown in which the relationship between pulleys  14  and  24  is more clearly shown. Pulley  24  is coupled to the crankshaft of engine  10 . Typically, pulley  14  is coupled to a water pump. Alternatively, pulley  14  is coupled to engine camshafts or other engine component. Pulley  14  is belt  26  driven by pulley  24 . Alternatively, element  26  is a chain. In  FIG. 2 , pulley  24  is shown with a larger diameter than pulley  14 . Alternatively, pulley  24  has a smaller diameter than pulley  14 . Also shown in  FIG. 2  is fan  20 . 
   Shown in  FIG. 3  is a detail of the viscous clutch  18  according to the present invention. Input shaft  16  of clutch  18  is coupled to plate  30 ; output shaft  22  is coupled to plate  32 . The amount of torque transmitted between input shaft  16  and output shaft  22  is the amount of fluid that is contained between plates  30  and  32 . If the volume between the plates is nearly full of fluid, shaft  22  rotates at nearly the same speed as shaft  16 , unless shaft  16  is rotating at very high speed. When there is little fluid in the volume between plates  30  and  32 , shaft  22  is nearly stationary, being hardly affected by the speed of shaft  16 . In the present invention, a heating element  38  is placed in the vicinity of the bimetallic strip  36 . When additional cooling is desired, current is caused to flow to heating element  38 , which causes bimetallic strip  36  to open a valve  35  thereby allowing more fluid to flow in between plates  30  and  32 , which in turn causes the fan to rotate at a higher speed. The current is provided through slip rings  34 , rotating electrical connecting device and wires  40  which connect to heating element  38 . Engine control unit  60  is connected directly to the slip rings  34  to provide a controlled current supply to the heating element  38 . Alternatively, engine control unit  60  supplies a signal to an intermediary device to control the current supplied to the heating element  38 . 
   In an alternative embodiment, the current flow to the heating element  38  is supplied inductively, as shown in  FIG. 4 . Coil  44  is the stationary coil mounted to a nonrotating element  46  or mounting surface. Element  46  is a water pump housing, a surface of the engine, or other nonrotating part. In  FIG. 4 , rotating coil  42  is mounted to pulley  14 . Alternatively, coil  42  is mounted to any element rotating at the same speed as and concentrically with shaft  16 , which is the input shaft to viscous clutch  18 . By causing current to flow through stationary coil  44  an alternating current is induced in rotating coil  42 . Coil  42  is electrically connected to heating element  38  along shaft  16 , (electrical connection not shown). Electronic control unit  60  controls the current in stationary coil  44 , thereby adjusting current induced in rotating coil  12 , thereby controlling the amount of heating provided by heating element  38 . 
   In the prior art, the bimetallic strip temperature is affected by the air flowing in and around it and the temperature of the material in which it is in contact. This approximates engine temperature poorly, as it is an inference based on underhood temperature, which is influenced by the air flowing through the radiator and/or condenser into the engine compartment, surface temperature of underhood components, etc. If, for example, temperature A is the temperature at which the bimetallic strip opens the valve  35  to cause the fan to increase engine cooling in the average situation, to protect for a worst case scenario, that temperature of opening the valve  35  is set to a temperature less than temperature A. This results in running the fan more often than desired, thus providing more cooling than necessary, in most situations simply to protect against the unusual situation. 
   In the &#39;426 prior art, the bimetallic strip is provided with a heating element connected to a microprocessor. The microprocessor sends a control signal to the heating element, based on input from the engine coolant temperature sensor. The control signal is a binary on/off signal to the heating element, which modifies the shape of an actuating disk into either a concave or convex position. However, this configuration allows no fail safe for a break in the electrical connection. If the electrical connection is severed, the actuating disk cannot be modified to provide increased fan speed, thus the engine is susceptible to an overheat condition. It also does not allow for modulation of the opening in the orifice for the hydraulic fluid path from the storage chamber to the drive chamber, thus degrading its ability to modulate fan speed, and thus airflow, for increased control of cooling performance and fuel economy. 
   In the present invention the bimetallic strip is placed in the presence of under-hood air flow so that if the connection to the heating element becomes an open circuit or other anomalous condition occurs, the bimetallic strip continues to function based on the under-hood air flow providing satisfactory, albeit less than optimized, control of the cooling provided by the fan. Further, when the heating element is operating properly, the control is based on at least one of: engine coolant temperature, engine oil temperature, under-hood temperature, and engine operating condition in addition to the influence of under-hood air temperature flowing by the bimetallic strip. 
   While several modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize alternative designs and embodiments for practicing the invention. The above-described embodiments are intended to be illustrative of the invention, which may be modified within the scope of the following claims.