Abstract:
The present invention provides an accessory drive system for a hybrid vehicle. The accessory drive system includes a planetary gear set having a first, second, and third planetary member. A torque transfer device operatively connects an engine with the first planetary member. A motor/generator is operatively connected to the second planetary member, and a plurality of accessories are operatively connected to the third planetary member. A one-way clutch is preferably operatively connected to either the first planetary member or the torque transfer device such that the accessories can be effectively driven by the motor/generator when the engine is off. Engine output is transferable through the planetary gear set to drive the accessories, and the speed at which the accessories are driven is selectable by controlling the speed of the motor/generator.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a vehicle transmission having a latched-pump applied clutch circuit with an accumulator for providing a backside boost pressure. 
       BACKGROUND OF THE INVENTION 
       [0002]    In a vehicle having an automatic transmission, a clutch assembly smoothly engages a rotating engine crankshaft with a stationary driveshaft for transmission of power to the drive wheels, and also disengages the respective shafts to interrupt power transfer therebetween to permit, for instance, smooth shifting between various gears of a planetary gear set. Clutch assemblies or clutches are torque-transmitting devices typically having a series of friction elements, i.e. a clutch pack, located within a clutch housing and actuated by a clutch piston, the piston being powered or energized by a supply of hydraulic fluid. The hydraulic fluid supply is typically pressurized by a controllable pump. When hydraulic clutch pressure is reduced, the clutch is released or disengaged, and likewise, when clutch pressure is increased, the clutch is actuated or engaged. The hydraulic system may be further controllable to actuate other transmission components, such as a specific clutch or a series of clutches within an automatic transmission. 
         [0003]    One such hydraulically-actuated clutch is a latched-pump applied clutch (LPAC). In an automatic transmission containing an LPAC, a controllable pump pressure engages the LPAC while a latching valve closed to thereby trap and substantially seal a supply of pressurized hydraulic fluid within the LPAC circuit. Once adequate clutch-apply pressure has been trapped or sealed within the LPAC circuit, controllable pressure in the main pump circuit may then be reduced as required in order to minimize spin losses elsewhere in the transmission without thereby diminishing available clutch-apply pressure within the LPAC circuit. 
         [0004]    A leak or series of leaks within a sealed LPAC circuit may cause a decrease or reduction in available clutch-apply pressure, the effect varying with the severity and/or number of leaks in the circuit. Leakage or bypass might occur at various points within the circuit, including around piston seals, valve body gaskets, various component connections, within latching valves, or through the inherent porosity of cast transmission components. Such leaks, particularly in a relatively stiff of low-compliance system, can deplete available clutch-apply pressure within the LPAC circuit. When applied to a rotating-type clutch in particular, that is a stationary piston is used to clamp or apply a rotating clutch pack, a plurality of thrust bearings are commonly used to allow relative motion without an undue increase in drag. Under these circumstances, however, thrust bearing spin loss may increase along with the increase in clutch-apply pressure. 
       SUMMARY OF THE INVENTION 
       [0005]    Accordingly, an LPAC circuit is provided for use in a vehicle transmission, the LPAC having a clutch with a clutch-apply pressure, at least one valve in fluid communication with a controllable pump, and an accumulator in fluid communication with the clutch and valve for adding compliance or resiliency to the LPAC circuit. The accumulator provides a controllable backside boost pressure to boost or increase the clutch-apply pressure to quickly adjust to transient pressure spikes during periods requiring increased torque capacity. 
         [0006]    In one aspect of the invention, the LPAC circuit has a shift-type valve in fluid communication with the controllable pump and the accumulator for admitting a backside boost pressure to the accumulator. 
         [0007]    In another aspect of the invention, the LPAC circuit has a no-leak latch-type valve in fluid communication with the controllable pump and the clutch piston for admitting a clutch-apply pressure to the LPAC circuit. 
         [0008]    In another aspect of the invention, a controllable electro-mechanical device is operatively connected to the accumulator to independently apply a direct force or boost pressure to actuate a clutch-apply piston disposed within an exhaustible accumulator. 
         [0009]    The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1A  is a schematic illustration of a hydraulic circuit according to the invention; 
           [0011]      FIG. 1B  is a table describing the operation of the hydraulic circuit of  FIG. 1A ; 
           [0012]      FIG. 2  is a schematic illustration of a hydraulic circuit according to another embodiment of the invention; 
           [0013]      FIG. 3  is a graph showing three discrete latched-pressure levels (TPLs) for a LPAC over a representative duration; and 
           [0014]      FIG. 4  is a graph showing an LPAC system having a discrete latched-pressure level and a backside accumulator pressure according to the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    Referring to the drawings wherein like reference numbers correspond to like or similar components throughout the several figures, there is shown in  FIG. 1A  a main hydraulic pump circuit  15  in fluid communication with a latched-pump applied clutch (LPAC) circuit  10 . Main pump circuit  15  includes a controllable hydraulic pump  28  operatively connected to a main sump  32 . Pump  28  is preferably a positive displacement pump, but alternatively may be a fixed displacement pump, variable displacement pump, or other pump suitable for use within an automatic transmission. Main sump  32  contains hydraulic fluid  48 , represented in  FIG. 1A  by dotted lines within sum  32 . Fluid  48  is drawn out of sump  32  through a first fluid passage  36 , then pressurized by the pump  28  to a controlled pressure, represented by arrow  29 , for fluid communication with a second fluid passage  37 . Second passage  37  is in fluid communication with a third and fourth fluid passage  38 ,  39  of LPAC circuit  10 , and optionally feeds additional clutch circuits or other components. 
         [0016]    The fluid passage  38  is in fluid communication with a first valve  18 , preferably a solenoid actuated no-leak latch-type valve operable to admit a fluid  48  having a controlled fluid pressure (arrow  29 ) from main pump circuit  15  to LPAC circuit  10 , then latch or close to thereby capture and seal off the line pressure in the form of pressurized fluid  48  within circuit  10  as required, where the pressure of fluid  48  is then useable to affect clutch apply pressure of the fluid  48 , represented by arrow  23 . Additionally, fourth fluid passage  39  is in fluid communication with a second valve  22 , preferably a solenoid-actuated shift-type valve operable to toggle or shift between two primary states (open/closed), and further configured to selectively exhaust or discharge fluid  48  back to sump  32 . Additionally, a fifth fluid passage  24  is in fluid communication with the second valve  22  and with an accumulator  20 , the accumulator  20  being disposed between passage  24  and a sixth fluid passage  16 . 
         [0017]    Accumulator  20  is a pressure storage supply or reservoir for holding fluid  48  under pressure, and is preferably a spring-loaded design in which an accumulator spring  13  is energized by the pressure of fluid  48  to exert a compressive force or backside boost pressure, represented by arrow  21 , on an accumulator piston  17  disposed within the volume of the accumulator  20 . Accumulator  20  is in fluid communication with a sixth fluid passage  16 , the accumulator  20  being further operable to build or accumulate a commandable standby or backside boost pressure (arrow  21 ) of fluid  48  within the volume of the accumulator  20  in order to rapidly supplement or boost the clutch-apply pressure (arrow  23 ) when so required, such as during leak-induced pressure loss within LPAC circuit  10 . Finally, sixth fluid passage  16  is in fluid communication with a clutch cylinder  30  having an internally-disposed clutch piston  14 , wherein the clutch piston  14  is operable to engage a rotating hydraulic clutch  12  when actuated. 
         [0018]    One factor in designing an LPAC circuit as described hereinabove and by  FIG. 1A  is the trade-off between clutch-apply circuit response time and spin losses through, for example, the thrust bearings  50 ,  51  disposed on either side of clutch  12  within the LPAC circuit  10 . For example, if a clutch-apply pressure (arrow  23 ) is set at the maximum design capacity of the clutch  12 , relatively high spin-losses may result, particularly through the thrust bearings  50 ,  51  of the LPAC circuit  10 . Likewise, if the clutch-apply pressure (arrow  23 ) is set at a more ideal point short of maximum clutch design capacity, spin losses through the thrust bearings  50 ,  51  may be thereby minimized, however a transient increase or spike in transmission torque may lead to an increased likelihood of clutch slippage. Therefore, to balance spin loss versus response time during transient periods requiring increased torque capacity, an LPAC preferably will have a plurality of discrete clutch torque capacity levels as depicted in  FIG. 3 . 
         [0019]    Turning to  FIG. 1B , which shows the various operating conditions available with the embodiment of  FIG. 1A  in which a 2-state shift-type solenoid valve is used as valve  22 , condition  1  shows a clutch  12  in the process of engaging, i.e. clutch engagement has been initiated but has not yet been fully completed, valve  18  opens and valve  22  closes. In this initial state, clutch-apply pressure (arrow  23 ) of fluid  48  within LPAC circuit  10  builds or increases with the controlled pressure (arrow  29 ) of main pump circuit  15 . Upon entering condition  2 , a clutch  12  is fully engaged. Valve  18  is closed to latch or seal clutch-apply pressure (arrow  23 ) of fluid  48  within clutch-apply cylinder  30 . Valve  22  can then be opened, thus allowing accumulator  20  to act as a compliance device. Accumulator  20  also is operatively connected to controlled pressure (arrow  29 ) of fluid  48  and thus capable of providing a readily available backside boost pressure (arrow  21 ) of fluid  48  for modulating the torque-capacity of clutch  12 , without opening valve  18 , particularly in situations requiring rapid and fluctuating increases or spikes in transient torque capacity, as depicted in the graph of  FIG. 4 . 
         [0020]    By using accumulator  20  as herein described, compliance or resiliency is added to LPAC circuit  10  so that, for example, a small leak or leaks within the circuit will have less of a negative impact on available clutch torque capacity or clutch-apply pressure (arrow  23 ) of fluid  48 , thereby helping to preserve LPAC response time and the resulting vehicle drive performance. Clutch piston  14  is preferably a non-rotating piston employing a double-sided fluid seal to minimize the potential for fluid leaks or bypass within clutch cylinder  30 . Backside boost pressure (arrow  21 ) of fluid  48  will be commandable when clutch-apply pressure (arrow  23 ) of fluid  48  is insufficient to properly engage clutch  12  during transient periods requiring increased torque capacity, e.g. a step-in throttle condition as shown in  FIG. 4 . 
         [0021]    As rotating clutch  12  begins disengaging but is not yet fully disengaged or released, LPAC circuit  10  enters condition  3  in which both of valves  18 ,  22  are open to controlled pressure (arrow  29 ) and backside boost pressure (arrow  21 ) of fluid  48  within accumulator  20  is vented through the valve  22  to sump  32 . Clutch  12  thereby reaches the level of controlled pressure (arrow  29 ) of fluid  48  until fully disengaged, or condition  4  of  FIG. 1B . Upon entering the fully disengaged condition, valves  18 ,  22  are both closed, clutch-apply pressure (arrow  23 ) of fluid  48  is minimal, and backside pressure (arrow  21 ) of the fluid  48  in accumulator  2  is fully exhausted. The engagement/disengagement cycle may then repeat as herein described. 
         [0022]    In a second embodiment shown in  FIG. 2 , in which a main hydraulic pump circuit  115  is in fluid communication with a latched-pump applied clutch (LPAC) circuit  110 , an electro-mechanical device  40  energizes or pressurizes a hydraulic accumulator  120  within the LPAC circuit  110  by directly applying force to an accumulator piston  117  disposed within the volume of accumulator  120 . A controllable pump  128  is in fluid communication via a first passage  136  with a main sump  132  containing hydraulic fluid  148 . Pump  128  pressurizes fluid  148  to a controlled pressure, represented by arrow  129 , and transmits the fluid  148  through a second fluid pressure  137 , through a third fluid passage  138 , and to a valve  118 , preferably a no-leak latch-type valve operable to admit a controlled pressure (arrow  129 ) from main pump circuit  115  to LPAC circuit  110 . Valve  188  then latches or closes to thereby capture and seal off controlled pressure within circuit  110  as required. The sealed off controlled pressure is then useable as a clutch-apply pressure, represented by arrow  123 . Fluid passage  137  may optionally feed additional clutch circuits or other components elsewhere in the transmission as shown. 
         [0023]    Valve  118  is in fluid communication with a clutch cylinder  130  through a fourth fluid passage  116 , cylinder  130  having a clutch piston  114  disposed therewithin. Piston  114  is operable to engage or actuate a rotating clutch  112  disposed between thrust bearings  150 ,  151 , as shown in  FIG. 2 . An accumulator  120  is in fluid communication with a fourth fluid passage  116 , the accumulator  120  further having an exhaust port  44  configure for continuous venting or exhausting of fluid  148  (and any entrained air) to main sump  132 . Accumulator  120  is operatively connected to an electro-mechanical device  40 , preferably a motorized ball-screw, for direct application of backside for or pressure, represented by arrow  121 , of fluid  148  to accumulator piston  117  when clutch-apply pressure (arrow  123 ) of fluid  148  is insufficient to actuate or engage clutch  112 . In this manner, boost pressure (arrow  121 ) may increase or boost clutch-apply pressure (arrow  123 ) when needed. 
         [0024]    While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.