Abstract:
A hydraulic control system provides lubrication fluid to a gearbox via two flow paths in parallel. One of the flow paths includes a passive thermal valve in series with a heat exchanger. When the fluid temperature is elevated, the thermal valve opens such that fluid flows through the heat exchanger for cooling. The thermal valve restricts flow to the heat exchanger when transmission fluid is in a normal operating temperature range, reducing flow demands. When the flow demands are reduced, a variable displacement pump requires less torque improving fuel economy. The thermal valve may also open when the fluid is below the normal operating temperature such that the heat exchanger heats the fluid. The parallel path may include a regulator valve. When the pressure of fluid in the lubrication circuit is elevated, the regulator valve reduces flow through the parallel path and may also divert flow to a sump.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to the field of automotive transmission hydraulic control systems. More particularly, the disclosure pertains to a hydraulic control system designed to passively regulate the temperature of the hydraulic fluid. 
       BACKGROUND 
       [0002]      FIG. 1  illustrates a vehicle powertrain. Heavy lines indicate mechanical power flow whereas thin lines indicate flow of transmission fluid. Dotted lines represent control signals. 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 . Regulator valve  20  diverts some portion of the flow back to the sump in order to controls the pressure in line pressure circuit  22 . Regulator valve  22  responds to a control signal from electronic controller  23  indicating the desired line pressure value. A component that acts in response to a control signal is called an actively controlled component. In response to commands from controller  23 , valve body  24  routes the pressurized fluid to the torque converter circuit  26  and the appropriate clutch circuits  28  to establish the desired gear ratio in gearbox  14 . Fluid exiting the torque converter goes into bypass valve  28 . When the fluid temperature is below a threshold, bypass valve  28  routes the fluid directly to lubrication circuit  30 . When the temperature of the fluid is above the threshold, bypass valve  28  routes the fluid through heat exchanger  32  before routing it to lubrication circuit  30 . Components which do not require any control signals, such as bypass valve  28 , are called passively controlled components. Fluid in the lubrication circuit provides lubrication to gearbox  14  and absorbs heat. The fluid then returns to sump  18 . 
         [0003]    When fluid is routed through first through one circuit and then through the other circuit, the two circuits are said to be in series. The flow rate through the first circuit is equal to the flow rate through the second circuit and the sum of the pressure drops across the circuits is equal to the pressure drop across the combination. When fluid is routed through only one of the circuits, on the other hand, the circuits are said to be in parallel. The pressure drop across the first circuit is equal to the pressure drop across the parallel circuit and the flow rate of the combined circuit is equal to the sum of the flow rates through each of the parallel circuits. 
         [0004]    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 selectively routing the lubrication fluid through heat exchanger  32 . When the fluid temperature is high, lubrication fluid is routed through heat exchanger  32  such that heat is dissipated either directly to the air or to an intermediate fluid such as engine coolant. When the fluid temperature is low, on the other hand, bypass valve  28  routes the fluid directly to gearbox  14  bypassing the heat exchanger and thus permitting the fluid to warm up. Note that, although regulator valve  20 , valve body  24 , and bypass valve  28  are illustrated in  FIG. 1  as distinct components, some embodiments may integrate regulator valve  20  and bypass valve  28  into the valve body. 
         [0005]    Most transmissions use positive displacement pumps. The volume of oil that is pressurized by the pump per unit time is dependent on the pump displacement and the engine speed. The torque required to drive the pump is dependent on the displacement and the pressure to which the fluid is pressurized. The power loss of the pump is proportional to the torque and the speed. Some transmissions utilize fixed displacement pumps. If the pump pressurizes more fluid than required at a particular time, the excess volume in discharged by regulator valve  20  with no reduction in either pump speed or pump torque. To reduce pump power loss and improve fuel economy, some transmissions utilize a variable displacement pump. The pump displacement is adjusted as fluid flow requirements change. When less fluid is required, the reduced pump displacement results in lower pump torque and reduced pump power loss. 
       SUMMARY OF THE DISCLOSURE 
       [0006]    A hydraulic control system includes a heat exchanger flow path and a parallel flow path from a torque converter to a lubrication circuit. The heat exchanger flow path includes a heat exchanger and a passive valve arranged in series. The passive valve reduces the flow rate when fluid temperature is less than a first threshold. The passive valve may also increase the flow rate when fluid temperature is below a lower threshold. The control system may also include a flow path in parallel with the valve and in series with the heat exchanger. The pressurized fluid may be provided by a variable displacement pump. A regulator valve in the parallel flow path may reduce the flow when lubrication circuit pressure exceeds a pressure threshold. The regulator valve may also exhaust fluid from the lubrication circuit to relieve pressure. 
         [0007]    In another embodiment, a hydraulic control system includes a heat exchanger in series with a passive valve. The valve responds to changes in fluid temperature by permitting full flow when the temperature is below a lower threshold and permitting full flow when temperature is above an upper threshold while blocking flow when temperature is between the two thresholds. A parallel flow path may provide flow at all temperatures. A regulator valve in the parallel flow path may reduce the flow when lubrication circuit pressure exceeds a pressure threshold. The regulator valve may also exhaust fluid from the lubrication circuit to relieve pressure. 
         [0008]    A valve suitable for use in a hydraulic control system includes a valve bore, a moveable spool, and a wax cartridge. The wax cartridge causes the spool to move within the bore in response to fluid temperature. The valve bore defines three ports. An annulus in the spool permits flow between the first and second ports in a first position, permits flow between the second and third ports in a second position, and blocks flow from the second port in a third position between the first and second positions. The valve bore may define a fourth port to provide thermal communication between a fluid and the wax cartridge. The first and third ports may be directly connected to a common hydraulic circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic representation of transmission hydraulic network. 
           [0010]      FIG. 2  is a schematic representation of a first transmission hydraulic network according to the present invention. 
           [0011]      FIG. 3  is a cross sectional view of a thermal valve when fluid is colder than a normal operating temperature range. 
           [0012]      FIG. 4  is a cross sectional view of the thermal valve of  FIG. 3  when fluid is within the normal operating temperature range. 
           [0013]      FIG. 5  is a cross sectional view of the thermal valve of  FIG. 3  when fluid is warmer than the normal operating temperature range. 
           [0014]      FIG. 6  is a schematic representation of a second transmission hydraulic network according to the present invention. 
           [0015]      FIG. 7  is a cross sectional view of a lubrication regulator valve when lubrication pressure is within a normal range. 
           [0016]      FIG. 8  is a cross sectional view of the lubrication regulator valve of  FIG. 7  when lubrication pressure is above the normal range. 
           [0017]      FIG. 9  is a cross sectional view of the lubrication regulator valve of  FIG. 7  when lubrication pressure is farther above the normal range. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    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. 
         [0019]    A portion of a transmission hydraulic control system is illustrated schematically in  FIG. 2 . Heavy lines indicate mechanical power flow whereas thin lines indicate flow of transmission fluid. Dotted lines represent control signals. Engine  10  drives torque converter  12  which, in turn, drives gearbox  14 . The gear ratio of gearbox  14  is selected by providing pressurized fluid to hydraulically actuated clutches. Variable displacement pump  16 , driven mechanically by engine  10 , draws fluid from sump  18  and delivers it to the valve body in line pressure circuit  22 . In response to a signal from controller  23 , regulator  34  adjusts the displacement of pump  16  in order control the pressure in line pressure circuit  22 . The control signals from line pressure circuit  22  to regulator  34  and from regulator  34  to pump  16  may take the form of a pressure in a hydraulic connection with negligible flow rate. Valve body  24  routes the pressurized fluid to the torque converter circuit and the appropriate clutch circuits  28  to establish the desired gear ratio in gearbox  14 . Fluid exiting the torque converter is split into two parallel circuits on its way to lubrication circuit  30 . Fluid in the lubrication circuit provides lubrication to gearbox  14  and absorbs heat before returning to sump  18 . 
         [0020]    A portion of the fluid exiting the torque converter flows to the lubrication circuit through orifice  36 . The remainder of the flow is routed through heat exchanger  32 . When the fluid temperature is in the normal operating range, fluid in the heat exchanger circuit flows through orifice  38  into heat exchanger  32  at a relatively low flow rate. When the fluid temperature is higher than a threshold, thermal valve  40  permits a considerably higher flow rate through heat exchanger  32 . Since pump  16  is a variable displacement pump, the reduced flow rate through the heat exchanger when the temperature is in the normal operating range permits pump  16  to operate with reduced displacement and therefore reduced power consumption. 
         [0021]    Some embodiments may be configured to provide accelerated transmission fluid warm-up. In these embodiments, heat exchanger  32  provides heat transfer between transmission fluid and engine coolant. Furthermore, thermal valve  40  is configured to provide an increased flow rate when the transmission fluid is below the normal operating temperature range. Since the engine coolant tends to heat up sooner during a drive cycle, during the early stages of a drive cycle, heat flows from the engine coolant to the transmission fluid. As the transmission fluid warms up toward normal operating temperatures, its viscosity decreases and transmission parasitic losses decrease. Since the transmission spends less time subject to the increased parasitic loss associated with cold transmission fluid, fuel consumption for the drive cycle improves. 
         [0022]      FIG. 3  shows a cross section of thermal valve  40 . Valve  40  is axisymetric with respect to centerline  50 . Spool  52  slides axially within valve bore  54 . Plug  56  is stationary with respect to valve bore  54 . Spool  52  and plug  56  define a chamber which contains a wax cartridge  58 . The line pressure circuit flows past the valve such that the temperature of the wax approximates the temperature of the fluid in the line pressure circuit. As the wax heat up, it expands pushing spool  52  to the left. As the wax cools, spool  52  moves to the right. The valve bore includes several lands  60 ,  62 , and  64  which define three ports  66 ,  68 , and  70 . Middle port  68  is connected to the torque converter outlet circuit. Left and right ports  66  and  70  are connected to the heat exchanger inlet circuit. Spool  52  includes an annulus  72  that, in certain spool positions, allows fluid to flow from port  68  to either port  66  or port  70 .  FIG. 3  shows the spool in the position corresponding to fluid below the normal operating temperature. In this position, fluid flows freely from the converter outlet through port  68  to port  70  to the heat exchanger. This provides a high flow rate when the fluid is cold such that the transmission fluid is heated by the engine coolant for faster transmission warm-up. In embodiments that do not implement this warm-up feature, port  70  would not be present. 
         [0023]      FIG. 4  shows the thermal valve in the position corresponding to normal operating temperature. Expansion of the wax has pushed the spool to the left relative to the position in  FIG. 3 . In this position, flow between port  68  and ports  66  and  70  is blocked off. The only flow to the heat exchanger is the minimal level permitted by orifice  38 . Since the flow rate of fluid is reduced, the displacement of pump  16  may be reduced, thus reducing pump torque and improving fuel economy. 
         [0024]      FIG. 5  shows the thermal valve in the position corresponding to temperatures above normal operating temperature. Expansion of the wax has pushed the spool further to the left relative to the positions in  FIGS. 3 and 4 . In this position, fluid flows freely from port  68  to port  66 . This provides a high flow rate when the fluid is warm such that heat is transferred from the transmission fluid to engine coolant and eventually to ambient air. 
         [0025]    The system is even more efficient in the normal operating temperature range when lube regulator valve  80  is included as shown in  FIG. 6 . Regulator valve  80  responds to changes in the pressure in circuit  30 . When the pressure in circuit  30  is less than a first threshold, regulator valve  80  permits fluid to flow freely from the torque converter outlet to circuit  30 . When the pressure in circuit  30  exceeds the first threshold, regulator valve begins to restrict the flow, completely blocking the flow when the pressure reaches a second threshold. In response, regulator  34  can reduce the displacement of pump  16 , reducing the torque required to drive the pump. If the pressure continues to rise above a third threshold, regulator valve  80  diverts fluid from circuit  30  to sump  18 . In addition to reducing pump flow requirements, regulator valve  80  ensures that the pressure in circuit  30  will not increase excessively. Excessive pressure in circuit  30  may result in excessive lubrication flow which may cause excessive drag. In addition to supplying lubrication flow, circuit  30  may supply fluid to balance chambers of transmission clutches. Excessive pressure in a balance chamber may cause a clutch to partially disengage. When a clutch piston is stroked, fluid is pushed out of the balance chamber which can cause a pressure increase in circuit  30  unless a relief path is available. 
         [0026]      FIG. 7  shows a cross section of lube regulator valve  80 . Valve  80  is axisymetric with respect to centerline  90 . Spool  92  slides axially within valve bore  94 . The valve bore includes several lands  96 ,  98 , and  100  which define four ports  102 ,  104 ,  106 , and  108 . The lube pressure circuit  30  is connected to ports  102  and  106 . The position of spool  92  is determined by the pressure of chamber  110  which is connected to lube circuit  30 . When the pressure in chamber  110  is low, spring  112  forces spool  92  to the left against a stop. In this position, fluid from the torque converter outlet flows freely to the lube circuit through annulus  114 . If pressure in lube circuit  30  increases, spool  92  will move to the right as shown in  FIG. 8 , blocking the flow of fluid to lube circuit  30 . If pressure in lube circuit  30  increases even further, spool  92  will move further to the right to the position shown in  FIG. 9 . In this position, fluid from lube circuit  30  flows to the sump through annulus  116  to relieve the pressure. 
         [0027]    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.