Abstract:
A hydraulic system for supplying fluid to a torque converter of an automatic transmission for a vehicle driven by a power source includes a torque converter including an impeller, a turbine, and an impeller clutch for alternately engaging and disengaging a drive connection between the impeller and the power source, a hydraulic control system producing line pressure and converter charge pressure that communicates with the impeller clutch, a discharge line communicating with the impeller clutch and through which hydraulic fluid discharges from the torque converter at a discharge pressure similar to the first pressure, thereby minimizing a pressure differential across the impeller clutch tending to disengage the impeller clutch, and an oil cooler to which fluid is supplied from at least one of the source of line pressure and the converter discharge line and from which fluid returns to the control system.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a torque converter for an automatic transmission, and, in particular, to a hydraulic system that actuates an impeller clutch of the torque converter and provides a continuous supply of hydraulic lubricant to transmission components. 
         [0003]    2. Description of the Prior Art 
         [0004]    A torque converter is a modified form of a hydrodynamic fluid coupling, and like a fluid coupling, is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. A torque converter is able to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear. 
         [0005]    In a torque converter there are at least three rotating elements: an impeller, which is mechanically driven by the prime mover; a turbine, which drives the load; and a stator, which is interposed between the impeller and turbine so that it can alter oil flow returning from the turbine to the impeller to multiply torque. The stator is mounted on an overrunning clutch, which prevents the stator from counter-rotating the prime mover but allows for forward rotation. The torque converter is encased in a housing, which contains with automatic transmission fluid (ATF), sometimes referred to as “oil,” “lube” or “lubricant.” 
         [0006]    Hydrodynamic parasitic losses within the torque converter reduce efficiency and generate waste heat. In modern automotive applications, this problem is commonly avoided by use of a bypass clutch (also called lock-up clutch), which physically links the impeller and turbine, effectively changing the converter into a purely mechanical coupling. The result is no slippage, and therefore virtually no power loss and improved fuel economy. 
         [0007]    Torque converter clutch designs include two basic types, a closed piston design and an open piston design. A closed piston design requires a dedicated hydraulic circuit into the torque converter, which communicates only with the apply side of the clutch piston. When pressure is high, the clutch applies. When pressure is low, the clutch releases. A more uncommon form is to have this circuit on the release side where high pressure releases the clutch and low pressure applies the clutch. 
         [0008]    An open piston design involves flowing ATF through the torque converter and across the piston, flowing from the apply side to the release side. The piston is applied by the pressure difference between the apply and release sides. This pressure differential can be controlled by either controlling apply and release pressure directly or by controlling flow rate with a designed pressure drop restriction across the piston. Normally, this same flow of ATF is used to cool the torque converter, so a relatively high flow rate is required in this hydraulic circuit. A barrier to achieving the intended flow rate is the limitation on converter charge pressure to prevent converter ballooning (axial distortion of the torque converter). This commonly results in a high gain clutch design where small pressure drop changes across the piston result in large changes in apply force, which makes clutch controllability a challenge. 
         [0009]    Most torque converters only have one converter clutch, the bypass clutch which alternately connects and releases a drive connection between the impeller and turbine. A torque converter can also provide an impeller clutch to connect and release a drive connection between the impeller and a power source, such as an engine, electric motor, starter/generator or hydraulic motor. The intent of the impeller clutch is to reduce load on the power source during idle, which reduces fuel consumption. This functionality is commonly referred to as idle-disconnect or neutral idle. 
         [0010]    When two clutches are present within a torque converter, usually one piston is an open piston design while the other is a closed piston design. Having two closed piston designs within a torque converter is not practical because this requires four hydraulic circuits to communicate with the torque converter—one for each clutch, and two more to flow across the converter for cooling. Having two as open piston clutches presents a complicated design problem for controlling the apply and release of the two clutches independently. This leads to the more practical approach of using a closed piston design for the bypass clutch and an open piston design for the impeller clutch. 
         [0011]    In an open piston design, the impeller clutch is actuated by a pressure differential between a converter charge circuit and a converter discharge circuit. A relatively high flow rate is required to cool the converter when the impeller clutch is engaged. Low flow restrictions across the closed clutch to reduce the pressure drop and a high gain clutch to maintain capacity would be required to avoid an excessive charge pressure. To disengage the impeller clutch, the pressure drop must be reduced lower yet. Because there is no direct control over flow restrictions across the clutch, pressure drop can only be reduced by reducing flow rate through the converter. During vehicle launch, the ramp rate of pressure drop across the converter clutch can be varied to achieve a variable “k factor” across the clutch for better launch feel. 
         [0012]    When the torque converter is multiplying torque, power loss occurs which significantly increases the temperature of ATF in the torque converter and must be cooled before returning to the transmission. Cooler return oil is usually routed into the transmission lubrication circuit to cool internal clutches, gear sets and bearings. The lubrication circuit is also used to fill or charge balance dams, which are intended to keep disengaged clutch pistons from drifting on when internal rotational speeds increase. 
         [0013]    The converter clutch control and hydraulic layout described above reduces flow to the downstream lube circuit when in idle-disconnect mode. When in idle-disconnect mode, the balance dams will drain down and result in an error state during the upcoming drive-away unless a minimum lubrication circuit flow rate is maintained, which cannot easily be met and still have the low pressure drop needed to disengage the impeller clutch. This error state could cause unintended clutch application including an early gear shift, an unintended gear state, or a tie up in the gearbox. 
         [0014]    There is a need in the industry to control an impeller clutch in a torque converter during idle-disconnect mode without introducing risk to the transmission lubrication system. 
       SUMMARY OF THE INVENTION 
       [0015]    A hydraulic system for supplying fluid to a torque converter of an automatic transmission for a vehicle driven by a power source includes a torque converter including an impeller, a turbine, and an impeller clutch for alternately engaging and disengaging a drive connection between the impeller and the power source, a hydraulic control system producing line pressure and converter charge pressure that communicates with the impeller clutch, a discharge line communicating with the impeller clutch and through which hydraulic fluid discharges from the torque converter at a discharge pressure greater than the first pressure, thereby producing a pressure differential across the impeller clutch tending to disengage the impeller clutch, and an oil cooler to which fluid is supplied from at least one of the source of line pressure and the converter discharge line and from which fluid returns to the control system. 
         [0016]    The system supplies fluid to the impeller clutch in a torque converter and provides ample flow of lube to the transmission in all operating conditions including neutral-idle. 
         [0017]    The system also improves fuel economy in a vehicle equipped with an automatic transmission having a torque converter impeller clutch. 
         [0018]    The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0020]      FIG. 1  is a cross section through a torque converter having a bypass clutch and impeller clutch; and 
           [0021]      FIG. 2  is schematic diagram of a hydraulic system through which a torque converter having an impeller clutch is supplied with ATF while the impeller clutch is engaged; 
           [0022]      FIG. 3  is schematic diagram of a hydraulic system through which a torque converter having an impeller clutch is supplied with ATF while the impeller clutch is disengaged; 
           [0023]      FIG. 4  is schematic diagram of an alternate embodiment of a hydraulic system for supplying ATF to the torque converter while the impeller clutch is disengaged; and 
           [0024]      FIG. 5  is schematic diagram of another embodiment of a hydraulic system for supplying ATF to the torque converter while the impeller clutch is disengaged. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0025]    Referring now to the drawings, there is illustrated in  FIG. 1  a torque converter  10 , which is arranged about a central axis  12  and includes an impeller  14 , turbine  16 , and stator  18 . The impeller, stator and turbine define a toroidal fluid flow circuit, whereby the impeller is hydrokinetically connected to the turbine. 
         [0026]    The stator  18  is secured to, and supported for rotation on a stationary stator sleeve shaft  20 . An overrunning brake  22  anchors the stator to shaft  20  to prevent rotation of the stator in a direction opposite the direction of rotation of the impeller, although free-wheeling motion in the direction of rotation of the impeller is permitted. The turbine  16  is secured to a rotating transmission input shaft  24 , which transmits torque to the transmission gear box (not shown). A torque converter housing  26 , surrounding the turbine, impeller and stator, is driveably connected to the crankshaft of an internal combustion engine (not shown) or another power source, such as an electric motor. 
         [0027]    Located within a torque converter housing  26  is an impeller clutch  28  for alternately opening and closing a drive connection between the impeller  14  and engine. Impeller clutch  28  includes a disc  30 , supported for rotation on a turbine hub  32  by a bearing  34 ; a ring  36  secured to a shroud  38 , which is attached to the periphery of each blade of the impeller  14 ; and friction plates  40 , located between ring  36  and disc  30 . A ring  42 , secured to disc  30 , is connected also to a torsion damper  44 , which resiliently connects the engine shaft  45  through the cover  26  to disc  30 . The engine shaft  45  is secured to cover  26 . 
         [0028]    Also located within the torque converter housing  26  is a lockup clutch  46  for alternately driveably connecting and releasing the turbine  16  and engine through cover  26 . Clutch  46  includes a first set of friction discs  48 , splined at their outer circumference to a surface of ring  42 , and a second set of friction discs  50 , each interleaved between consecutive first discs and secured to the turbine  16 . Lockup clutch  46  is actuated by a piston  52 , which is supported on turbine hub  32  and disc  30  allowing axial movement along axis  12  and will transfer torque to turbine hub  32  through a spline  56 . A disc  54 , secured by a spline  56  to turbine hub  32 , is separated from piston  52  by a volume  58 , which, when pressurized, moves piston  52  rightward forcing discs  50 ,  52  into mutual frictional contact and engaging clutch  46 . When lockup clutch  46  is engaged, the engine shaft  45  and turbine  16  are mechanically interconnected and driveably connected to the transmission input shaft  24 . When lockup clutch  46  is disengaged, the turbine  16  and engine shaft  45  are mechanically disconnected, and the turbine may be hydrokinetically driven by the impeller  14 , provided impeller clutch  28  is fully engaged or slipping. 
         [0029]    ATF that causes lockup clutch  46  alternately to engage or apply and to disengage or release is supplied from a converter apply pressure circuit of the hydraulic system, whose magnitude is varied and regulated by the hydraulic control and actuation system of the transmission. Converter apply pressure C APY  is supplied from the converter apply pressure circuit of the hydraulic system to volume  58  through an fluid passage  60 , passage  62  formed in input shaft  24 , passage  64 , and passage  66  formed in turbine hub  32 . 
         [0030]    A converter charge pressure hydraulic circuit of the hydraulic system includes passage  68 , which communicates through radial fluid passage  70  to the toroidal volume of the torque converter  10 . Converter charge pressure C CL  supplied from the converter charge pressure circuit of the hydraulic system fills the torque converter  10  and develops a pressure force against the surface of impeller clutch disc  30  that is directed axially into impeller clutch  28  and ring  36 . 
         [0031]    A converter discharge hydraulic circuit of the hydraulic system includes passage  72  and communicates with passages  74 ,  75  and  76 . Converter discharge pressure C OUT  is controlled by the converter discharge pressure circuit of the hydraulic system fills a volume  78  between impeller shroud  38  and cover  26  and develops a pressure force against the left surface of disc  30  that opposes the force created by converter charge pressure. The engaged, disengaged and slipping state of impeller clutch  28  is determined by the magnitude of the pressure differential across disc  30 , i.e., (ΔC APY  C OUT ). 
         [0032]    When the engine is idling and the transmission is in neutral gear, the pressure differential across disc  30 , i.e. the difference between charge pressure and discharge pressure, must be low. When this differential pressure is low, impeller clutch  28  opens, thereby decoupling impeller  14  from the engine shaft  45  during the engine idle condition. Decoupling the impeller reduces load on the engine caused by the torque converter and reduces fuel consumption in drive, reverse and neutral operation. 
         [0033]      FIG. 2  illustrates the ATF flow paths through a hydraulic system  90  when the impeller clutch  28  is engaged. The hydraulic system  90 , which supplies ATF to the torque converter  10 , includes a hydraulic control system  92 , which provides a source of torque converter charge pressure and line pressure, includes an oil cooler  94 ; a lube circuit  98 , which supplies ATO to clutches, shafts, bearings, and gears and balance dams in the transmission; an oil sump or reservoir  100 ; an oil filter  102 ; and an oil pump assembly  104 , whose output provides to the hydraulic system  90  a source of line pressure  106 , which is regulated by the hydraulic control  92 . 
         [0034]    Converter charge pressure CCL is carried in line  110  to converter charge passage  68 . Flow at torque converter discharge pressure COUT is carried from converter passage  72  in discharge passage  112 , which exits the transmission case  124 , flows through oil cooler  94  located outside the case, reenters case  124 , and supplies lubricant at low temperature to lube circuit  98 . ATF exiting lube circuit  98  enters the oil sump  100 , from which it enters the oil filter  102 . The inlet  126  of oil pump  104  is supplied from filter  102 , and the pump outlet  128  supplies oil at line pressure to the hydraulic control  92 . 
         [0035]      FIG. 3  illustrates a first embodiment in which the hydraulic system  90  causes the impeller clutch  28  to disengage. Converter charge pressure CCL is carried in line  110  to converter passage  68 . Flow at torque converter discharge pressure COUT is carried from converter passage  72  in discharge passage  112  and through an orifice  132 , which is sized to control flow rate to levels required by the cooling and lubrication circuits. The control system hydraulically connects converter charge pressure to converter discharge pressure  96  to reduce the pressure differential across the impeller clutch  28  sufficient to disengage the impeller clutch  28 . Torque converter discharge exits transmission case  124 , flows through oil cooler  94  located outside the case, reenters case  124 , and supplies lubricant at low temperature to lube circuit  98 . ATF exiting lube circuit  98  enters the oil sump  100 , from which it enters the oil filter  102 . The inlet  126  of oil pump  104  is supplied from filter  102 , and the pump outlet  128  supplies oil at line pressure to the hydraulic control  92 . 
         [0036]      FIG. 4  illustrates a second embodiment in which the hydraulic system  90  causes the impeller clutch  28  to disengage. Converter charge pressure CCL is carried in line  110  to converter passage  68 . Flow at torque converter discharge pressure COUT is carried from converter passage  72  in discharge passage  112  and through an orifice  130 , which is sized to reduce the flow rate sufficiently to reduce the pressure differential across the impeller clutch  28  sufficient to disengage the impeller clutch. Torque converter discharge flows directly to the oil sump  100 , without exiting the case  124  or entering the oil cooler. Flow at line pressure regulated by hydraulic control system  92  is carried in line  136  through orifice  134 , transmission case  124  and oil cooler  94  located outside the case, reenters case  124  and supplies lubricant at low temperature to lube circuit  98 . ATF exiting lube circuit  98  enters the oil sump  100 , from which it enters the oil filter  102 . The inlet  126  of oil pump  104  is supplied from filter  102 , and the pump outlet  128  supplies oil at line pressure to the hydraulic control system  92 . 
         [0037]      FIG. 5  illustrates a third embodiment in which the hydraulic system  90  causes the impeller clutch  28  to disengage the impeller clutch  28 . Converter charge pressure CCL is carried in line  110  to converter passage  68 . Flow at torque converter discharge pressure COUT is carried from converter passage  72  in discharge passage  112 , where it is deadheaded at  138 , permitting no flow to exit converter discharge passage  72  and producing a pressure in discharge passage  72  that is nearly the same as the pressure in converter charge pressure in line  110  and passage  68 . Flow at line pressure regulated by hydraulic control system  92  is carried in line  136  through orifice  134 , transmission case  124  and oil cooler  94  located outside the case, reenters case  124  and supplies lubricant at low temperature to lube circuit  98 . ATF exiting lube circuit  98  enters the oil sump  100 , from which it enters the oil filter  102 . The inlet  126  of oil pump  104  is supplied from filter  102 , and the pump outlet  128  supplies oil at line pressure to the hydraulic control system  92 . 
         [0038]    In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.