Abstract:
A compact fluid accumulator both stores a relatively large amount of fluid and provides good fluid pressure stability. The accumulator includes a piston slidably disposed in a cylinder having a fluid inlet/outlet at one end which communicates with a first chamber and one face of the piston. Engaging the opposite face of the piston, and disposed in a second chamber, is a compression spring. The second chamber is filled with a gas which is at atmospheric pressure when the accumulator is relaxed. When pressurized hydraulic fluid fills the first chamber, the piston moves against the pressure of the spring and gas in the second chamber. The present invention thus provides an accumulator having the relatively small size of a gas filled accumulator without the leakage problem of a super-atmospheric gas charge—the extra force being provided by the compression spring.

Description:
FIELD 
       [0001]    The present disclosure relates to accumulators for hydraulic fluid systems and more particularly to combination spring biased and gas filled accumulators for hydraulic fluid systems. 
       BACKGROUND 
       [0002]    The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art. 
         [0003]    Accumulators which are essentially pressurized fluid storage devices are common components of hydraulic fluid systems. They serve two related functions in such systems. On one hand, when a supply pump is operating, they function as a reservoir or storage site for excess pumped fluid resulting simply from pumped fluid volume exceeding system fluid consumption. On the other hand, when a supply pump is not operating or system fluid consumption exceeds pumped volume, the accumulator supplies pressurized fluid until the pump re-starts, supplies pressurized fluid while the pump restarts or until pump output exceeds fluid consumption. Thus, accumulators maintain and create both desired fluid pressure and flow in a hydraulic fluid system, improve the match between the instantaneous volume of fluid supplied by the pump and the instantaneous volume of fluid consumed by the system and thereby improve system operation. 
         [0004]    Accumulators are a common component of many automatic transmission configurations in which selective flows of hydraulic fluid are utilized to manipulate spool valves and operate actuators, clutches and brakes to sequentially engage desired speed or gear ratios. The majority of automatic transmission accumulators take two forms: a super-atmospheric gas charged accumulator and a spring biased accumulator. In the first design, one face of a free piston in a cylinder is acted upon by the hydraulic fluid and the adjacent region defines a fluid storage volume; the opposite face of the piston and adjacent volume is charged with, for example, super-atmospheric pressurized nitrogen. The compressed (and compressible) gas provides a fluid spring against which the hydraulic fluid acts. The spring biased accumulator replaces the gas with a mechanical compression spring which biases the piston and maintains the pressure of the hydraulic fluid. 
         [0005]    Notwithstanding their popularity, these devices each have shortcomings. For example, given the operating pressures of automatic transmissions, the most practical size gas filled accumulator will, as noted above, include a gas charged to a pressure above atmospheric pressure. Over the life of the accumulator, this pressurized gas will slowly leak out, rendering the accumulator without optimal functionality. This slow change will slowly but inexorably affect the operation of the transmission where there may not be enough fluid storage volume for operations such as re-engaging the clutches for engine start—stop vehicle launches. The alternative to a super-atmospheric pressure charged accumulator is an atmospheric pressure charged accumulator but this choice results in a much larger accumulator which is especially undesirable given the current trend toward highly efficient packaging. A spring accumulator is also generally larger than a gas filled accumulator and thus suffers from the same packaging related problems. Though size may appear to be a minor issue, it is a major issue and has major consequences in automotive component design. Thus, there is a need for an efficiently packaged accumulator for use in hydraulic systems such as those in automatic transmissions. 
       SUMMARY 
       [0006]    The present invention provides a compact fluid accumulator which both stores a relatively large amount of fluid and provides good fluid pressure stability. The accumulator includes a piston slidably disposed in a cylindrical housing having a fluid inlet/outlet at one end which communicates with a first chamber and one face of the piston. Engaging the opposite face of the piston, and disposed in a second chamber, is a compression spring. The second chamber is filled with a gas which is at atmospheric pressure when the accumulator is relaxed. When pressurized hydraulic fluid begins to fill the first chamber, the piston moves against the pressure of the spring and gas in the second chamber. The accumulator of the present invention is especially suited for engine start—stop applications. 
         [0007]    The present invention thus provides an accumulator having the small size of a gas filled accumulator without the leakage problem of a super-atmospheric gas charged chamber—the extra force being provided by the compression spring. 
         [0008]    It is thus an object of the present invention to provide an accumulator for a hydraulic fluid system. 
         [0009]    It is a further object of the present invention to provide an accumulator for a hydraulic fluid system of an automatic transmission. 
         [0010]    It is a still further object of the present invention to provide an accumulator for a hydraulic control system of an automatic transmission. 
         [0011]    It is a further object of the present invention to provide an accumulator having a piston disposed in a cylindrical housing. 
         [0012]    It is a further object of the present invention to provide an accumulator having a cylindrical housing with an inlet/outlet at one end. 
         [0013]    It is a further object of the present invention to provide an accumulator having a piston biased by both a compression spring and gas disposed in a cylindrical housing. 
         [0014]    It is a further object of the present invention to provide an accumulator having a piston biased by both a compression spring and gas disposed in a cylindrical housing and adapted to engine start—stop applications. 
         [0015]    It is a further object of the present invention to provide a compact accumulator having a piston biased by both a compression spring and gas. 
         [0016]    Further objects, advantages and areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0017]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0018]      FIG. 1A  is a diagrammatic view of a hydraulic fluid supply system incorporating a fluid accumulator according to the present invention; 
           [0019]      FIG. 1B  is a fragmentary, diagrammatic view of a portion of a hydraulic fluid supply system incorporating a fluid accumulator according to the present invention which is especially suited to engine start—stop applications; 
           [0020]      FIG. 2A  is an enlarged, side view of a fluid accumulator according to the present invention in an unfilled state; 
           [0021]      FIG. 2B  is an enlarged, side view of a fluid accumulator according to the present invention in a filled state; and 
           [0022]      FIG. 3  is a multiple plot graph presenting the performance of a prior art gas filled accumulator at three different temperatures and the performance of an accumulator according to the present invention at the same three temperatures. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
         [0024]    With reference to  FIG. 1A , a typical and representative hydraulic fluid supply system is illustrated and generally designated by the reference number  10 . The hydraulic fluid supply system  10  may find application in, for example, vehicular automatic transmissions and numerous other devices having hydraulic control and hydraulic operating systems. The hydraulic fluid supply system  10  typically includes a sump  12  which is disposed at the lowest region of a housing (not illustrated) or other device or fluid containment component. Typically, a filter  14  is disposed in the sump  12  which filters the hydraulic fluid passing from the sump  12  to a suction line  16  to remove foreign particulate matter. The suction line  16  is in fluid communication with a suction or inlet port  18  of a hydraulic pump  20 . Typically the pump  20  will be a positive displacement pump such as a vane pump (illustrated), a gear pump or a gerotor pump. The pump  20  also includes a pressure or outlet port  22  which communicates with a first pressure line  24 . If desired, the supply system  10  may include a blow-off or pressure relief valve  28 . The pressure relief valve  28  is pre-set at a pressure limit and when that pressure limit is exceeded in the pressure line  24 , the pressure relief valve  28  opens, reducing the pressure in the first pressure line  24 , and, typically, returning hydraulic fluid to the sump  12 . 
         [0025]    The first pressure line  24  is also in fluid communication with a filtration assembly  30 . The filtration assembly  30  includes a second particulate filter  32 , typically having finer filtration media and pores than the sump filter  14 . Also contained in the filtration assembly  30  and in fluid parallel with the second filter  32  is a flow bypass valve  34 . The flow bypass valve  34  is pre-set at a pressure differential and when this pressure differential is exceeded, due to flow restriction or plugging of the second filter  32 , the flow bypass valve  34  opens to allow hydraulic fluid to flow around the second filter  32 , thereby avoiding starving the supplied hydraulic system of hydraulic fluid. 
         [0026]    A second pressure line  36  communicates with the outlet of the filtration assembly  30  and an inlet of a one-way or ball check valve  38 . The ball check valve  38  allows hydraulic fluid flow from the filtration assembly  30  into the rest of the hydraulic system but prevents reverse flow from the system back into the filtration assembly  30  and other upstream components. 
         [0027]    The outlet of the ball check valve  38  communicates with a fluid accumulator  40  and a main fluid supply line  42  which may optionally include a fluid pressure sensor or similar transducer  44  which provides a signal indicative of the pressure of the hydraulic fluid in the fluid supply line  42  to associated control equipment (not illustrated). 
         [0028]    Referring now to  FIG. 1B , a portion of a hydraulic fluid supply system  10 ′ incorporating a fluid accumulator  40  according to the present invention, which is specific to engine start—stop applications, is illustrated. The components illustrated in  FIG. 1B  are associated with and in communication with the main fluid supply line  42  and reside generally on the right side of  FIG. 1A . In communication with the main fluid supply line  42  is a flow restricting orifice  46  which, in turn, communicates with another one-way or ball check valve  48 A which is configured to permit fluid flow toward a hydraulic line  49  and the accumulator  40  but prevent reverse flow. The accumulator  40  in this application is the same as the accumulator  40  in  FIG. 1A , includes a piston  58  and a spring  70  and is further described below. Also in fluid communication with the accumulator  40  and the hydraulic line  49  is a solenoid valve  50 . The solenoid valve may be electrically energized to open and provide fluid communication and flow therethrough out of the accumulator  40  to an additional one-way or ball check valve  48 B. The additional check valve  48 B is configured to permit fluid flow toward the main supply line  42  but prevent reverse flow. 
         [0029]    Referring now to  FIGS. 1A ,  1 B,  2 A and  2 B, the fluid accumulator  40  includes a generally cylindrical housing  52  having an inlet/outlet port  54  which communicates with the fluid supply line  42  in  FIG. 1A  and the hydraulic line  49  in  FIG. 1B . The housing  52  defines a cylinder  56  having the piston  58  which divides the cylinder  56  into a first, fluid chamber  62  and a second, gas chamber  64  on the opposite side or face of the piston  58 . The piston  58  defines a circumferential channel or groove  66  which receives an O-ring seal  68  which provides a fluid tight seal between the piston  58 , the wall of the cylinder  56  and between the chambers  62  and  64 . An additional groove  66  and O-ring seal  68  may be utilized in the piston  58 , as well as other seal types, if desired. Additional glide rings may be incorporated if deemed necessary. Typical oil storage volumes of the accumulator  40  in automatic transmission hydraulic systems will be less than about 0.3 liters. 
         [0030]    Disposed within the second, gas chamber  64  is the compression spring  70 . The compression spring  70  may take many forms and have a spring constant (rate) that varies significantly depending upon the particular application and system pressure. In applications such as vehicular automatic transmissions, spring constants (rates) in the range of about 20 newtons/meter to about 28 newtons/meter have been found suitable and a nominal value of 24 newtons/meter has been found preferable. Additionally, the compression spring  70  is preloaded for automatic transmission service to between about 600 and 650 newtons and a nominal value of 622 newtons has been found preferable. Other spring rates and preloads of the compression spring  70  are within the purview of the present invention and can vary significantly from the values recited above based upon the application, system operating pressure and other design criteria. Preferably, as well, the compression spring  70  is a coil spring, as illustrated, although helical (spiral) springs or stacked spring washers or Belleville springs, for example, and other spring configurations may be utilized. 
         [0031]    In  FIG. 2A , the accumulator  40  is presented in a relaxed state with essentially no hydraulic fluid in the first, fluid chamber  62 . In this case, the second, gas chamber  64  is filled with a gas, essentially at atmospheric pressure having a volume V 1 . Depending upon design and application parameters, there may or may not be a preload on the compression spring  70 . In  FIG. 2B , the accumulator  40  is presented with a full fluid charge and the piston  58  has translated a full stroke to its travel limit. Now the first, fluid chamber  62  is at its maximum volume and is fully filled with pressurized hydraulic fluid. The second, gas chamber  64  is at its minimum volume V 2 , determined by the stack of the compression spring  70 . The general equation for the instantaneous pressure of the accumulator  40  which essentially represents a force balance on the piston  58  is 
         [0000]    
       
         
           
             
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         [0000]    where P 1  is the initial pressure in the second, gas chamber  64 , P 2  is the final pressure of the hydraulic fluid in the first, fluid chamber  62 , V 1  is the initial volume of the second, gas chamber  64 , V 2  is the final volume of the second, gas chamber  64 , K is the spring constant of the compression spring  70 , D is the displacement of the piston  58  from its relaxed position illustrated in  FIG. 2A  and its energized position illustrated in  FIG. 2B , B is the preload of the compression spring  70  and R is the radius of the piston  58 . 
         [0032]    Referring now to  FIG. 3 , a multiple plot graph illustrates the performance of a prior art gas charged accumulator and an accumulator  40  according to the present invention having both a compression spring and gas charge. Both accumulators define the same interior volume. The graph plots hydraulic fluid volume along the horizontal (X) axis and hydraulic fluid pressure along the vertical (Y) axis. The three lower plots  80 A,  80 B and  80 C present data from a prior art accumulator having only a gas charge at 20° C., 80° C. and 120° C., respectively. The three upper plots  82 A,  82 B and  82 C present data from a combination spring and gas filled accumulator  40  according to the present invention also at 20° C., 80° C. and 120° C., respectively. Note that a gas charged accumulator has a usable volume of only 126 cc whereas the accumulator  40  according to the present invention has a usable volume of 199 cc. As a general observation, the accumulator  40  according to the present invention, with the same stored hydraulic fluid volume, operates and provides a higher pressure at essentially all operating conditions when compared to the accumulator having only a gas charge. 
         [0033]    The foregoing description of the invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be within the scope of the invention and the following claims. Such variations are not to be regarded as a departure from the spirit and scope of the invention.