Abstract:
In a hydraulic system for an automatic transmission, a method for automatically draining fluid from an oil cooler and related hydraulic lines when power is off to prevent fluid leakage, escape and outflow when they are disconnected from the system. A control system includes a source of relatively high pressure when power is on, an oil cooler, fluid circuit lines for supplying transmission oil to the cooler, a source of low pressure for containing fluid, such as an oil sump, and a valve hydraulically connected to the circuit, high pressure source, and low pressure source, the valve having a first state at which a hydraulic connection through the valve between the circuit and the source of low pressure is closed when power is on, and a second state at which a hydraulic connection through the valve between the circuit and the low pressure source is open when power is off.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to hydraulic fluid systems for a motor vehicle automatic power transmission. More particularly, it pertains to draining oil automatically from an oil cooler and fluid supply lines to a transmission sump. 
   Generally, automatic transmission fluid for operating and lubricating an automatic transmission is contained in an oil reservoir such as oil pan and the like located at the underside of the automatic transmission case. The automatic transmission fluid contained in the oil pan is inducted by an oil pump and is supplied to a torque converter, miscellaneous lubrication circuits, and a hydraulic control system, which produces various magnitudes of pressure and provides circuit paths between the pressure sources and the appropriate components that employ the pressure to perform their functions. 
   For example, the various speed ratios produced by the transmission result by selectively engaging and disengaging various friction elements, hydraulically actuated clutches and brakes. The applied and released condition of the friction elements operate to interconnect and disconnect elements of the planetary gearsets in order to produce multiple forward drive gear ratios and reverse drive. The friction elements are applied and released in response to the pressurized and vented state of a hydraulic servo through which the friction elements are actuated. 
   The magnitude of torque transmitted by the various friction elements in the several gear ratios is reflected in the magnitude of pressure applied to each friction element. When the magnitude of transmitted torque is high, the magnitude of actuating pressure is high. Generally, during operation in the lowest forward drive gears and reverse gear, the transmitted torque magnitude is high. A control system for an automatic transmission produces line pressure up to about 300 psi. The lubrication circuit is continually supplied with fluid during normal operation, and typically fluid is present in the cooler and its supply lines even after the pump is stopped by turning off the vehicle&#39;s engine. 
   Under operating conditions when the automatic transmission fluid is at its normal, elevated temperature, fluid from the control system directed to the lubrication circuits passes first through an oil cooler, a heat exchanger usually incorporated in the radiator, where an exchange of heat from the transmission fluid to ambient air or other fluid occurs. The lubrication circuits are supplied from the cooler outlet. Fluid used to lubricate various friction surfaces throughout the transmission returns by gravity to the reservoir, from which it is inducted at the pump inlet. 
   The cooler is connected to the transmission control system by hydraulic lines, which extend from the transmission case through hydraulic fittings, which connect the lines to the cooler. The cooler is located in the engine compartment in the vicinity of the air inlet shroud, cooling fan, and radiator. The cooler fittings are located above the fittings that connect the lines to the transmission case. The transmission oil pan or reservoir are located at an elevation that is lower than that of the cooler. 
   Because the cooler lines remain full when the engine is not running, whenever the transmission is disconnected from the cooler supply lines, or the lines are disconnected at the fittings from the cooler, transmission fluid contained in the cooler and lines can pour out into the service area or onto equipment in the engine compartment. To prevent this oil spillage when servicing the transmission, either a catch basin is used to hold the fluid in the lines and cooler when opening the hydraulic fittings at the cooler lines, or the fittings are immediately plugged after disconnecting the lines from the cooler. 
   There is a need, therefore, for a reliable, low cost technique to prevent spillage and outflow of transmission fluid in this way while servicing the transmission, radiator, or cooler. 
   SUMMARY OF THE INVENTION 
   A hydraulic system according to this invention includes an anti-drainback valve, which not only prevents drainback of both the apply and release converter circuits, but also permits the “case out” cooler line to drain into the transmission when the engine is not running. Because of the higher location of the cooler hydraulic fittings relative to that of the transmission case hydraulic fittings and reservoir, gravity is employed to move the fluid out of the cooler line into the fluid reservoir. 
   An anti-drain back valve according to this invention eliminates transmission fluid leakage and escape from an oil cooler and related supply lines when they are disconnected from the transmission. The valve also maintains the torque converter filled with fluid after power is turned off so that there is no delay in providing the torque converter function after starting the engine. Otherwise, time would be required to refill the torque converter with fluid after the engine is restarted and before the torque converter is able to assist in vehicle launch by amplifying the torque produced by the engine. 
   In a hydraulic system for an automatic transmission, a method according to this invention permits unobstructed fluid flow to the cooler when power is on, but it eliminates fluid leakage from the oil cooler and related hydraulic lines when they are mechanically disconnected from the system when power is off. Loss of fluid by spilling and leaking is avoided automatically by draining fluid from the cooler and its supply lines when power is off. To accomplishing this result, a control system includes a source of relatively high pressure when power is on, an oil cooler, fluid circuit lines for supplying transmission oil to the cooler, a fluid reservoir for containing fluid at relatively low pressure, and a valve hydraulically connected to the circuit, high pressure source, and reservoir. The valve has a first state at which a hydraulic connection through the valve between the circuit and the source of low pressure is closed when power and the high pressure source are on, and a second state at which a hydraulic connection through the valve between the circuit and the low pressure reservoir is open when power is off. 
   Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side elevation view of a automatic transmission, oil cooler, and hydraulic lines connecting the cooler and transmission; 
       FIG. 2  is a side elevation view, partially in cross section, showing an automatic transmission to which this invention can be applied; 
       FIG. 3  is a schematic diagram of a hydraulic system showing certain valves in a power-on position; and 
       FIG. 4  is a schematic diagram of the hydraulic system of  FIG. 3  showing certain valves in a power-off position. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring first to  FIG. 1 , an automatic transmission  10  includes a case  12 , a transmission fluid filler tube  14 , fluid outlet  16 , fluid inlet  18 , fluid reservoir or oil pan  20  located below the case, a torque converter  22 , gearing  24  and an output shaft  26 . The fluid outlet and inlets  16 ,  18  are used to circulate fluid under pressure from within the transmission to an oil cooler  28 , which extracts heat from the transmission fluid circulating in the cooler. Heat from the transmission fluid can be exchanged in the cooler by convection to air passing at high speed between fins radiating from the lines that carry the fluid through the cooler and by conduction to surrounding fluid. This heat exchange occurs in a section of a radiator having an inlet at fitting  30  and an outlet at fitting  32 . The cooler inlet  30  is directly connected to fluid outlet  16  through a suitable tubular hydraulic line  34 . Similarly, the cooler outlet  32  is connected to fluid inlet  18  through a tubular hydraulic line  36 . 
   The torque converter  22  includes a bladed impeller wheel  38  driven by an engine (not shown). A bladed turbine  40 , arranged in toroidal fluid flow relationship with respect to the impeller  38 , is driven hydrokinetically by the impeller and is driveably connected to the gearing  24 . A bladed stator wheel  42 , mounted on an overrunning brake  43 , makes it possible for hydrokinetic torque multiplication to occur in the converter  22 . 
   A lockup or bypass clutch  44 , which has a spring damper  46 , establishes a direct mechanical connection between impeller  38  and turbine  40  when clutch  44  is engaged, thereby bypassing the hydrokinetic drive connection between the impeller and turbine that is present when bypass clutch  44  is disengaged. The torque converter is supplied with fluid at converter apply pressure by a hydraulic control system located in the transmission. Transmission fluid fills the toroidal cavity and causes clutch  44  to frictionally engage. However, clutch  44  is disengaged when fluid at converter release pressure is supplied to the space  48  between the friction surfaces of the clutch by hydraulic control system. Converter release pressure in space  48  forces the friction surface of clutch  44  apart disengaging the clutch. 
   The engine drives a positive displacement, duocentric pump  50 . A control body  52  and pump body, containing various hydraulic control valves and fluid passages, surrounds the pump  50 . The pump  50  includes an internal rotor gear  54  supported rotatably, the rotor having nine exterior teeth. An external stator gear  56  having ten internal teeth or lobes meshes with the internal rotor and is fixed to the pump cover. The impeller  38  and internal pump rotor  54  rotate at the speed of the engine shaft. Spaces between the meshing teeth of the internal rotor  54  and pump stator  56  are pumping chambers in which fluid travels about the axis of the pump from the inlet of the pump to the outlet. Fluid in those spaces is compressed as the volume of the spaces decreases from the inlet to the outlet due to rotation of the rotor within the stator. Pump  50  is supplied with fluid from an oil sump or reservoir  20 ,  58  through a suction filter  60 , and with fluid contained in a passage  62  leading to the pump inlet from a main regulator valve  64 . 
   The magnitude of line pressure is controlled at regulator valve  64 . Regulated line pressure in passage  66  is connected through port  68  with the chamber  74  of an anti-drain back valve  70 , which includes a spool  72  formed with lands  78 ,  80 . A compression spring  76  forces spool  72  downward within the chamber against the effect of a pressure force produced by line pressure on the lower surface of land  78 . A converter release port  82  and a converter apply port  84  communicate with chamber  74 . Converter apply port  84  is connected through passage  86 , valve  64 , passage  88 , and converter clutch control valve  90  to a source of converter apply pressure  92 . Similarly, converter release port  82  is connected sequentially through passage  92 , converter pressure limit valve  94 , passage  96 , valve  90 , and passage  98  to a source of converter release pressure  100 . 
   Cooler bypass valve  102  includes a spool  104  that is movable within a chamber  106  due to the force of a compression spring  108 , which biases the spool upward against the stem  110  of a thermostat  111 , which senses and responds to the temperature of the transmission fluid. When the temperature of the hydraulic fluid is elevated to its normal temperature range, stem  110  is extended to the position shown in  FIG. 3 , and land  112  closes or blocks a connection through valve  102  between an inlet port  116  and outlet port  114 . Outlet port  114  is connected through passage  118  to lubrication fluid connections  120  and  138 . 
   Cooler bypass valve  102  need not contain a thermostat. Instead, any valve that changes its state as the temperature of hydraulic fluid in the system changes can be substituted for a cooler bypass valve having a thermostat. The change of state that occurs is such that a hydraulic passage through the valve closes when fluid temperature is equal to or greater than a predetermined temperature, and the hydraulic passage through the valve opens when fluid temperature is less than the predetermined temperature. For example, when the fluid temperature is equal to or less than 158° F., the stem  110  of valve  102  is fully retracted and the valve is in the position of  FIG. 4 , where a connection between ports  114  and  116  through valve  102  is fully open. When the fluid temperature increases to 185° F., the stem  110  of valve  102  extends to the position of  FIG. 3 , where a connection between ports  114  and  116  through valve  102  is fully closed. When the fluid temperature is between 158° F. and 185° F., valve  102  partially closes the connection between ports  114  and  116  through valve  102 . A normal operating temperature range for automatic transmission fluid is 180° F.-200° F. 
   When the temperature of the transmission fluid is relatively low, i.e., below its normal operating range, stem  110  retracts into the thermostat  111 , to the position shown in  FIG. 4 , and spool  104  moves upward in the valve chamber due to the force of spring  108 , permitting land  112  to open a connection between inlet port  116  and outlet port  114 . Passages  122  and  124  connect inlet port  116  to a case outlet port  126 . 
   Supply line  34  connects port  126  to the oil cooler  28  and to a pressure-side filter  132 , arranged in parallel with cooler  28 . Fluid passing through the cooler  28  and filter  132  enters a rear lube circuit  134 , a center lube circuit  136 , and a front lube circuit  138 . The lubrication circuits, which are located within transmission case  12 , supply lubricant to friction surfaces on various shafts, bearings and journal surfaces of the transmission. Lubrication fluid returns by gravity to the oil pan or reservoir  20 ,  58  after exiting the lubrication circuits. 
   In operation, the spool of valve  70  is in the position shown in  FIG. 3  when power is on, i.e., when the engine is running and regulated line pressure is supplied to the hydraulic system. The spools of valves  90 ,  94  can be in various positions controlled by external commands when the engine is running. The thermostat  111  moves spool  104  to the position shown in  FIG. 3  when the oil temperature is greater than a thermostat switch temperature. When the engine is turned off, the spools of those valves  70 ,  90 , and  94  move to the bottom of the corresponding valve chamber, to the positions shown in FIG.  4 . Spool  104  returns to the position shown in  FIG. 4  as the thermostat and fluid cool. 
   When the engine is running and the transmission oil temperature is relatively high, lubrication fluid is supplied to the cooler  28  and filter  132  from the converter apply source  92 . The path between the converter apply source  92  to the cooler  28  and filter  132  includes sequentially valve  152 , passages  154 ,  88 , valve  64 , passage  86  valve  70 , passages  140 ,  124 , case out port  16 ,  126 , and fitting  30 . Lube fluid leaving the cooler and filter is delivered to the lube circuits  134 ,  136 ,  138 . The connection to front lube circuit  138  is made through lube port  120  and passage  118 . A connection between ports  114  and  116  through the cooler bypass valve  120  is closed by land  112 . 
   When the engine is running and the transmission oil temperature is relatively low, lubrication fluid is supplied to the cooler  28  and filter  132  from the converter apply source  92  through the path described above. A lubrication fluid path, parallel to the path that supplies cooler  28  and filter  132 , is opened between ports  114  and  116  through the cooler bypass valve  102 . The flow rate of lubrication fluid through the cooler and filter is low due to the high viscosity of the transmission oil at low temperature. Lubrication fluid exiting case out port  142  of the anti-drain back valve  70  enters cooler bypass valve  102  through passages  124 ,  122  and port  116 , exits valve  102  through port  114 , and flows to the lubrication circuits  134 ,  136 ,  138 . 
   When power is off, i.e. when the engine and pump  50  are stopped, spool  72  of the converter anti-drain back valve  70  moves downward within its chamber due to the force of spring  76 . This movement causes land  80  to open a connection between oil cooler  28  through line  34 , passages  124 ,  140 , and port  142  to the fluid reservoir  20 ,  58 . Exhaust port  144  connects port  142  through the valve chamber  74  to the fluid reservoir  20 ,  58 . In this way, lubrication fluid contained in the cooler supply line  34 , as well as any fluid contained in the cooler  28  at an elevation above the cooler fittings  30 ,  32  returns through valve  70  to the oil reservoir  20 ,  58  because the cooler fittings and the line  34  are located at a higher elevation than that of the oil pan and fluid reservoir  20 ,  58 . 
   When power is off, the spool  152  of converter clutch control valve  90  moves downward within its chamber to the position of  FIG. 4 , thereby opening a connection between the converter release source  100  and port  82  of the anti-drain back valve  70 . The path from converter release source  100  to port  82  includes passage  98 , valve  90 , passage  96 , port  154 , valve  94 , and passage  92 . However, port  82  of the anti-drain back valve  70  is blocked by land  78 , preventing flow from the torque converter release circuit when the engine is off. 
   Similarly, when power is off, valve  90  opens a connection between the converter apply pressure source  92  and port  84  of the anti-drain-back valve  70 . This connection is made through valve  90 , passages  154 ,  88 , valve  64 , and passage  86 . However, when power is off, lands  80  and  78  close port  84  from communication with other portions of the system, thereby preventing flow from the torque converter apply circuit when the engine is off. In this way, the torque converter is maintained full of hydraulic fluid in the power-off condition. This feature avoids any delay in producing torque multiplication by torque converter upon starting the engine and launching the vehicle from a stopped position. 
   In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.