Abstract:
A thermostatic bypass valve functions to regulate fluid temperature and also to act as a pressure relief valve using a single valve bore. The poppet valve includes a cylinder with a chamber that is thermally immersed in a source passageway such that the valve state is determined by the temperature of the fluid flowing through the source passageway as opposed to the fluid flowing through a return passageway. When the fluid in the source passageway is hot, a poppet is forced against the return passageway side of a valve seat. The poppet may either be rigidly attached to the cylinder or may slide with respect to the cylinder and be forced against the valve seat by a spring. A piston may either be rigidly attached to the housing or may be forced toward the valve seat by a spring.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to the field of valves. More particularly, the disclosure pertains to a transmission cooler bypass valve. 
       BACKGROUND 
       [0002]      FIG. 1  illustrates a vehicle powertrain. Heavy lines indicate mechanical power flow whereas thin lines indicate flow of transmission fluid. Engine  10  drives torque converter  12  which, in turn, drives gearbox  14 . Gearbox  14  may adjust the speed and torque before transmitting the mechanical power to an output shaft. The gear ratio of gearbox  14  is selected by providing pressurized fluid to hydraulically actuated clutches. Pump  16 , driven mechanically by engine  10 , draws fluid from sump  18 . Valve body  20  routes the pressurized fluid to the torque converter and to the clutches within gearbox  14  that establish the desired gear ratio. The fluid also provides lubrication to gearbox  14  and absorbs heat. The fluid then returns to sump  18 . 
         [0003]    The transmission operates most efficiently when the fluid is at an optimal temperature. When the fluid is too cold, its viscosity is higher increasing parasitic drag. If the fluid gets too hot, the viscosity is too low resulting in increased leakage around the pump and elsewhere. This increased leakage reduces the pressure available from pump  16  reducing the torque capacity of the clutches within gearbox  14 . If the fluid temperature remains high for a sufficient period of time, the friction characteristics of the clutches change and shift quality degrades. The temperature of the fluid is controlled by routing the lubrication fluid through cooler  22  and bypass valve  24 . The cooler is a heat exchanger with a fluid loop designed to facilitate heat transfer either directly to ambient air, or to an intermediate medium such as liquid coolant. When the fluid temperature is high, lubrication fluid is routed through cooler  22  before entering the gearbox  14 . When the fluid temperature is low, on the other hand, bypass valve  24  routes the fluid directly to gearbox  14  bypassing the cooler and thus permitting the fluid to warm up quicker. Note that, although valve body  20  and bypass valve  24  are illustrated in  FIG. 1  as distinct components, some embodiments may integrate bypass valve  24  into the valve body. 
         [0004]    Some pressure drop is normal as the fluid flows through cooler  22 . However, in some conditions, the resistance may be excessive resulting in an unacceptable pressure drop. This can occur, for example, when the fluid in the cooler fluid loop is very cold and therefore has very high viscosity. The fluid within the cooler can be cold even when the fluid circulating within the transmission has warmed up because the bypass valve has been segregating the fluid. In some cases, the resistance is high enough to completely block the flow through the lubrication and cooling circuit of the transmission, risking damage to components of the gearbox. 
       SUMMARY OF THE DISCLOSURE 
       [0005]    A transmission system includes a heat exchanger with an inlet and an outlet. A housing defines a source passageway between a source of pressurized fluid and the inlet, a return passageway between the outlet and a transmission lubrication circuit, and a bypass passageway between the source passageway and the return passageway. A poppet valve within the bypass passageway performs several functions. First, the poppet valve permits flow through the bypass passageway when a temperature in the source passageway is less than a predefined temperature. Second, the poppet valve permits flow through the bypass passageway when a pressure difference between the source passageway and the return passageway exceeds a predefined value. Finally, the poppet valve blocks flow through the bypass passageway in other conditions. The poppet valve includes a cylinder defining a chamber containing a phase change material such that the chamber is thermally immersed in the source passageway. The cylinder slides within the bypass passageway towards the source passageway in response to an increase in a volume of the phase change material which may be a wax. A return spring forces the cylinder away from the source passageway in response to a decrease in the volume of the phase change material. In some embodiments, the poppet valve may include a poppet rigidly attached to the cylinder. A pressure relief valve may force a piston towards the source passageway. In other embodiments, the piston may be fixed with respect to the housing. The poppet valve may include a poppet that slides with respect to the cylinder and a pressure relief spring that pushes the poppet toward the valve seat. 
         [0006]    A thermostatic bypass valve includes a housing defining a bypass passageway between a source passageway and a return passageway. The bypass passageway includes a valve seat. A piston and a cylinder define a chamber containing a phase change material which may be wax. The chamber is thermally immersed in the source passageway. A spring forces a poppet against the valve seat, on the return passageway side, when the phase change material is in a liquid state. In some embodiments, the poppet may be rigidly fixed to the cylinder. In other embodiments, the poppet may slide with respect to the cylinder and a pressure relief spring may force the poppet toward the valve seat. In some embodiments, the piston may be rigidly fixed to the housing. In other embodiments, a pressure relief spring may force the piston towards the valve seat. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic representation of a vehicle powertrain. 
           [0008]      FIG. 2  is a diagram showing a first embodiment of a thermostatic bypass valve when the fluid is cold. 
           [0009]      FIG. 3  is a diagram showing a first embodiment of a thermostatic bypass valve when the fluid is hot. 
           [0010]      FIG. 4  is a diagram showing a first embodiment of a thermostatic bypass valve when the cooler fluid loop is blocked. 
           [0011]      FIG. 5  is a diagram showing a second embodiment of a thermostatic bypass valve when the fluid is cold. 
           [0012]      FIG. 6  is a diagram showing a second embodiment of a thermostatic bypass valve when the fluid is hot. 
           [0013]      FIG. 7  is a diagram showing a second embodiment of a thermostatic bypass valve when the cooler fluid loop is blocked. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    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. 
         [0015]      FIGS. 2-4  illustrates a first bypass valve designed to regulate fluid temperature and also to route fluid directly to the gearbox when the pressure difference between the inlet and the outlet is excessive.  FIG. 2  shows bypass valve  24  when the fluid is cold. The bypass valve includes a housing  30  with three passageways. Source passageway  32  conducts pressurized fluid between inlet port  34  from the valve body to a first cooler port  36  connected and one end of the cooler fluid loop. Similarly, return passageway  38  conducts cooled fluid between a second cooler port  40  connected to the opposite end of the cooler fluid loop to outlet port  42  connected to the gearbox lubrication circuit. Bypass passageway  44  conducts fluid from source passageway  32  to return passageway  38 , bypassing the cooler fluid loop. Bypass passageway  44  includes valve seat  46  between the source passageway and the return passageway. When poppet  48  is displaced away from valve seat  46 , as shown in  FIG. 2 , fluid can flow through the bypass passageway between valve seat  46  and poppet  48 . Poppet  48  is rigidly attached to cylinder  50  which is supported to move within bypass passageway  44 . Cylinder  50  also includes a face  51  that blocks return passageway  38  when Cylinder  50  is in the position illustrated in  FIG. 2 . Alternatively, the flow could be blocked by other means such as a second poppet and a second valve seat within the return passageway. 
         [0016]    Piston  52  slides within cylinder  50 . Wax is contained within chamber  54  that is formed by cylinder  50  and piston  52 . Cylinder  50  is designed such that chamber  54  is thermally immersed in the source passageway. In other words, based on the location of the chamber and the thermal conductivity of the materials, the temperature of the wax is determined primarily by the temperature of fluid within the source passageway. The temperature of fluid in the return passageway has much less, if any, impact on the temperature of the wax. The wax is formulated to have a melting point that corresponds to the desired operating temperature of the transmission fluid. When the wax is solid, its volume is relatively low, permitting piston  52  to slide deep into cylinder  50 . Piston  52  is restrained by lip  58  in housing  30 . Return spring  56 , acting in compression, ensures that piston  50  slides as deeply into cylinder  50  as permitted by the volume of wax. Consequently, poppet  48  is held away from valve seat  46 , permitting fluid to flow through the bypass passageway. 
         [0017]      FIG. 3  shows bypass valve  24  when the fluid is hot. Because the wax in chamber  54  is thermally immersed in source passageway  32 , the wax melts. Wax increases substantially in volume when it melts. Therefore, the volume of chamber  54  is relatively high forcing cylinder  50  to slide relative to piston  52 , overcoming return spring  56 . Piston  52  is restrained from moving by pressure relief spring  60 , which acts in compression. Cylinder  50  moves toward source passageway  32  forcing poppet  48  into contact with valve seat  46 . When poppet  48  is forced into contact with valve seat  46 , flow through bypass passageway  44  is prevented, forcing the fluid to flow through the cooler fluid loop. The cooled fluid returns to the transmission through the return passageway  38  which is no longer blocked by face  51 . Because wax chamber  54  is thermally immersed in the source passageway, bypass valve  24  will remain in this condition as long as the temperature of the fluid in the source passageway remains above the melting point of the wax. If the wax chamber were located in the return passageway, then the cooled fluid would tend to return the valve to the bypass condition. 
         [0018]      FIG. 4  shows bypass valve  24  when the fluid is hot and the cooler fluid loop is blocked. Although pressure relief spring  60  continues to push poppet  48  toward valve seat  46 , this force is overcome by a pressure difference between source passageway  32  and return passageway  38 . The pressure difference displaces poppet  48  from valve seat  46  permitting fluid to flow through the bypass passageway between valve seat  46  and poppet  48 . The pressure difference is determined by the force generated by pressure relief spring  60  and the area of poppet  48 . These parameters are selected such that this feature is only activated when the resistance through the cooler fluid loop is excessive. If the blockage of the cooler fluid loop is only a partial blockage, then a fraction of the fluid will continue to flow through the cooler. If the partial blockage is caused by low temperatures, this flow of hot fluid will relieve the blockage. In the meantime, the transmission is provided with adequate lubrication flow at acceptable pressure. 
         [0019]      FIGS. 5-7  illustrates a second bypass valve designed to regulate fluid temperature and also to route fluid directly to the gearbox when the pressure difference between the inlet and the outlet is excessive.  FIG. 5  shows bypass valve  24  when the fluid is cold. Source passageway  32 , return passageway  38 , and bypass passageway  44  are similar to the corresponding passageways in the first bypass valve. Piston  62  is rigidly attached to housing  30 . Cylinder  64  slides with respect to piston  62 . Wax in chamber  64  is thermally immersed in source passageway  32 . When the wax melts, it pushes cylinder  64  toward the source passageway. When the wax solidifies, return spring  68  pushes cylinder  64  towards return passageway  38 . Poppet  72  slides with respect to cylinder  62 . When the wax is in a liquid state, pressure relief spring  74  pushes poppet  72  into valve seat  46 . However, the free length of spring  74  is selected such that it does not force poppet  72  against valve seat  46  when the wax is in a solid state. Alternatively, a feature on the cylinder may restrict spring  74  from extending to its free length such that poppet  72  is not forced against valve seat  46  when the wax is solid. 
         [0020]      FIG. 5  shows the bypass valve when the fluid in the source passageway is cold and the wax is solid. Spring  68  pushes cylinder  64  away from the source passageway such that face  70  blocks return passageway  70 . Fluid in the source passageway easily pushes poppet  72  away from valve seat  46  permitting can flow through the bypass passageway between valve seat  46  and poppet  72 . 
         [0021]      FIG. 6  shows bypass valve  24  when the fluid is hot. Because the wax in chamber  54  is thermally immersed in source passageway  32 , the wax melts forcing cylinder  64  to move against spring  68 . Spring  74  forces poppet  72  into contact with valve seat  46 . When poppet  48  is forced into contact with valve seat  46 , flow through bypass passageway  44  is prevented, forcing the fluid to flow through the cooler fluid loop. Because wax chamber  54  is thermally immersed in the source passageway, bypass valve  24  will remain in this condition as long as the temperature of the fluid in the source passageway remains above the melting point of the wax. 
         [0022]      FIG. 7  shows bypass valve  24  when the fluid is hot and the cooler fluid loop is blocked. Although pressure relief spring  74  continues to push poppet  72  toward valve seat  46 , this force is overcome by a pressure difference between source passageway  32  and return passageway  38 . The pressure difference displaces poppet  72  from valve seat  46  permitting fluid to flow through the bypass passageway. Therefore, despite the blockage, the transmission is provided with adequate lubrication flow at acceptable pressure. 
         [0023]    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. These attributes can include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. 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.