Abstract:
An accumulator includes a cylinder for containing a working fluid and a first volume of pressurized gas, the gas and fluid being separated by a displaceable piston and a first seal contacting the piston and the cylinder, a reservoir carried on the piston for containing a second volume of pressurized gas, and a device that permits gas flow from the second volume into the first volume.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims priority to and the benefit of U.S. provisional application No. 61/812,778, filed Apr. 17, 2013, the full disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF INVENTION 
       [0002]    This invention relates generally to an accumulator that holds a pressurized hydraulic working fluid and automatically replenishes a volume of gas that keeps the accumulator charged. 
         [0003]    Fuel economy of a vehicle can be increased by a stop-start system that automatically stops an internal combustion engine when the vehicle is stopped and restarts the engine when an operator indicates intent to accelerate the vehicle. A vehicle equipped with an automatic transmission and the stop-start system requires that automatic transmission fluid be maintained pressurized while the engine is stopped. An engine driven hydraulic pump that normally pressurizes the automatic transmission fluid while the engine is running is incapable of maintaining the fluid pressurized while the engine is stopped. An accumulator containing pressurized transmission fluid is continually connected to the hydraulic system of the transmission so that hydraulic system pressure is maintained until the engine restarts. The accumulator uses a gas pre-charge to maintain the pressure of the transmission fluid. 
         [0004]    However, slow leakage through and around a piston seal in the accumulator depletes the gas pre-charge over the life of a gas-charged accumulator. This changes the amount of working fluid that the accumulator holds and eventually lowers the peak working pressure of the accumulator when the piston reaches a stroke limit. 
       SUMMARY OF INVENTION 
       [0005]    An accumulator includes a cylinder containing a working fluid and a first volume of pressurized gas, the gas and fluid being separated by a displaceable piston and a first seal sealing between the piston and the cylinder, a reservoir carried on the piston containing a second volume of pressurized gas, and a device that permits gas flow from the second volume into the first volume. 
         [0006]    The accumulator provides a solution to permeation and sliding seal gas loss by automatically replenishing pressure and a volume of gas that keeps the accumulator charged. 
         [0007]    The accumulator contains the high pressure replenishment reservoir within its piston, making manufacturing and assembly easier. 
         [0008]    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. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a schematic cross section at a diametric plane of a hydraulic fluid accumulator according to the prior art. 
           [0010]      FIG. 2  is a schematic cross section at a diametric plane of a hydraulic fluid accumulator containing a permeable plug. 
           [0011]      FIG. 3  is a schematic cross section at a diametric plane of a hydraulic fluid accumulator having a position sensing valve. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Referring now to  FIG. 1 , there is illustrated a prior art accumulator  10  including a cylinder  12 ; a piston  14 , displaceable along an axis  15  of the cylinder  12 ; an O-ring seal  16  located between a radial outer surface of the piston  14  and an inner surface of the cylinder  12 ; a working fluid  18  located below the piston  14 , which may be an automatic transmission fluid; and a gas pressure chamber  20  in the cylinder  12 , above the piston  14 , containing a pressurized pre-charge gas and sealed by a plug  22  in a passage  24 . As a level of the working fluid  18  in the cylinder  12  changes, the piston  14  strokes along a length  26 . Slow leakage of the pre-charge gas through and around the seal  16  (flow illustrated by arrow  28 ) depletes the pressurized pre-charge gas over a service life of the accumulator  10 . 
         [0013]      FIG. 2  illustrates an accumulator  130  including a cylinder  112 ; a piston  114 , displaceable along an axis  115  of the cylinder  112 ; an O-ring seal  116  located between a radial outer surface of the piston  114  and an inner surface of the cylinder  112 ; a working fluid  118  located below the piston  114 , which may be an automatic transmission fluid; and a gas pressure chamber  120  in the cylinder  112 , above the piston  114 , containing a pressurized pre-charge gas and sealed by a plug  122  in a passage  124 . As a level of the working fluid  118  in the cylinder  112  changes, the piston  114  strokes along a length  126 . 
         [0014]    The accumulator  130  includes a high pressure reservoir  132 , fitted in the piston  114 , containing a re-charge gas under pressure greater than the pre-charge gas in the chamber  120 . The pre-charge and re-charge gases may both be the same gas. For example, the pre-charge and recharge gases may both be nitrogen. Alternatively, different gases may be used for the pre-charge and re-charge gases. A lower end of reservoir  132  is closed by a plug  134  in a passage through the piston  114 . The reservoir  132  fluidly communicates with the chamber  120  through a permeable seal  136  extending through an upper cell  137 . The permeable seal  136  is configured to allow the re-charge gas in the reservoir  132  to flow into the chamber  120  (flow illustrated by arrow  138 ) slightly more slowly than the pre-charge gas leaks from the chamber  120 , between a seal  116  and inner surface of a cylinder  112 , into a working fluid  118  (flow illustrated by arrow  128 ). The permeable seal  136  may be fabricated from the same material as the seal  116 . For example, the permeable seal  136  may be fabricated from an elastomeric material. 
         [0015]    In this way, the high pressure reservoir  132  contained in the piston  114  replenishes the pre-charge gas in the chamber  120  with the re-charge gas through the seal  136 . The cross sectional area of the seal  136  may be scaled with the pressure differential between the chamber  120  and reservoir  132  such that the re-charge gas from the reservoir  132  enters the chamber  120  slightly more slowly than the pre-charge gas from the chamber  120  leaks into the fluid  118 . 
         [0016]      FIG. 3  illustrates an accumulator  140 . Because the accumulator  140  is similar to the accumulator  130  of  FIG. 2 , like reference numerals designate corresponding elements in the drawings. 
         [0017]    The accumulator  140  of  FIG. 3  includes a reservoir  132 , fitted in a piston  114 , containing a re-charge gas under pressure greater than a pre-charge gas in a chamber  120 . A lower end of reservoir  132  is closed by a plug  134  in a passage through the piston. The reservoir  132  fluidly communicates with the chamber  120  through a valve  142  whose operating state varies between open and closed depending on a stroke position of the piston  114 . 
         [0018]    When the stroke position of the piston  114  is low, i.e., the piston  114  is located at or near a bottom  152  of a cylinder  112 , the valve  142  is closed due to a magnitude of upward force on the valve  142 , produced by pressure of the re-charge gas in the reservoir  132 , exceeding a magnitude of downward force on the valve  142 , produced by pressure of the pre-charge gas in the chamber  120 . 
         [0019]    The valve  142  opens when the stroke position of the piston  114  is large, i.e., the piston  114  moves upward towards a top  154  of the cylinder  112  due to loss of pre-charge gas pressure in the chamber  120  and pressure of a working fluid  118 . When the piston  114  strokes near the top  154 , the valve  142  is opened by the top  154  displacing the valve  142  towards the reservoir  132 . When the valve  142  is open, high pressure re-charge gas in the reservoir  132  replenishes the pre-charge gas in the chamber  120  by flowing through the valve  142 . The valve  142  may include a spring to ensure that the valve  142  reseats as increasing pressure in the chamber  120  forces the piston  114  downward towards the bottom  152 . 
         [0020]    A relief valve  144 , which may be a one-way ball valve, opens when pressure of the working fluid  118  exceeds a reference pressure of the valve  144  as determined by force of a compression spring  146  acting on a ball  148 . The relief valve  144  responds to pressure of the working fluid  118  to seat or unseat the ball  148  on an opening at an end of a passage  150 , thereby closing or opening, respectively, the valve  144 . 
         [0021]    While certain embodiments of the present 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 as defined by the following claims.