Patent Publication Number: US-8991433-B2

Title: Energy storage system including an expandable accumulator and reservoir assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to co-pending U.S. Provisional Patent Application No. 61/369,214 filed on Jul. 30, 2010, and co-pending U.S. Provisional Patent Application No. 61/248,573 filed on Oct. 5, 2009, the entire contents of both of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to hybrid drive systems for vehicles, and more particularly to hybrid hydraulic drive systems for vehicles. 
     BACKGROUND OF THE INVENTION 
     A typical vehicle hybrid hydraulic drive system uses a reversible pump/motor to absorb power from and add power to or assist a conventional vehicle drive system. The system absorbs power by pumping hydraulic fluid from a low pressure reservoir into a hydraulic energy storage system. This hydraulic energy storage system typically includes one or more nitrogen-charged hydraulic accumulators. Hybrid hydraulic drive systems typically add power to conventional vehicle drive systems by utilizing the hydraulic energy stored in the hydraulic accumulators to drive the reversible pump/motor as a motor. 
     SUMMARY OF THE INVENTION 
     The present invention provides, in one aspect, an expandable accumulator and reservoir assembly including a reservoir defining an interior chamber containing working fluid therein, and an expandable accumulator at least partially positioned in the reservoir and at least partially immersed in the working fluid contained within the interior chamber. The accumulator is configured to exchange working fluid with the reservoir. 
     The present invention provides, in another aspect, an energy storage system including a reservoir defining an interior chamber containing working fluid therein, a reversible pump/motor in fluid communication with the reservoir, and an expandable accumulator at least partially positioned in the reservoir and at least partially immersed in the working fluid contained within the interior chamber. The accumulator contains working fluid, and is in selective fluid communication with the reversible pump/motor to deliver pressurized working fluid to the reversible pump/motor when operating as a motor, and to receive pressurized working fluid discharged by the reversible pump/motor when operating as a pump. 
     The present invention provides, in yet another aspect, a method of operating an energy storage system. The method includes providing a reservoir defining an interior chamber containing working fluid therein, positioning an expandable accumulator at least partially within the interior chamber, immersing the expandable accumulator at least partially into the working fluid contained within the interior chamber, returning working fluid to the reservoir with a reversible pump/motor when operating as a motor, and drawing working fluid from the reservoir when the reversible pump/motor is operating as a pump. 
     The present invention provides, in another aspect, an expandable accumulator including a body having an inner layer defining an interior space and an outer layer at least partially surrounding the inner layer. The accumulator also includes an inlet/outlet port in fluid communication with the interior space. The inner layer includes a higher fracture strain than the outer layer. 
     The present invention provides, in yet another aspect, an expandable accumulator and reservoir assembly including a reservoir defining an interior chamber containing working fluid therein and an expandable accumulator. The expandable accumulator includes an inner layer and an outer layer at least partially surrounding the inner layer. The inner layer includes a higher fracture strain than the outer layer. The accumulator is at least partially positioned in the reservoir and at least partially immersed in the working fluid contained within the interior chamber. The accumulator is configured to exchange working fluid with the reservoir. 
     The present invention provides, in another aspect, an expandable accumulator and reservoir assembly including a reservoir defining a central axis and an interior chamber containing working fluid therein, and an expandable accumulator coaxial with the central axis, at least partially positioned in the reservoir, and at least partially immersed in the working fluid contained within the interior chamber. The accumulator is configured to exchange working fluid with the reservoir. The assembly also includes a support coaxial with the reservoir and extending for at least the length of the accumulator. The support is engageable with an outer periphery of the accumulator to limit expansion of the accumulator upon receipt of pressurized working fluid from the reservoir. 
     The present invention provides, in yet another aspect, an expandable accumulator and reservoir assembly including a reservoir defining an interior chamber containing working fluid therein and a single expandable accumulator at least partially positioned in the reservoir and at least partially immersed in the working fluid contained within the interior chamber. The accumulator is configured to exchange working fluid with the reservoir. The reservoir includes an internal volume, and the accumulator occupies between about 40% and about 70% of the internal volume of the reservoir depending upon the amount of working fluid in the accumulator. 
     Other features and aspects of the invention will become apparent by consideration of the following detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a first construction of an energy storage system of the present invention, illustrating a reservoir and an expandable accumulator positioned within the reservoir. 
         FIG. 2  is a schematic of the energy storage system of  FIG. 1 , illustrating the accumulator in an expanded configuration in response to receiving pressurized working fluid from the reversible pump/motor when operating as a pump. 
         FIG. 3  is a schematic of a second construction of an energy storage system of the present invention, illustrating a reservoir and multiple accumulators positioned within the reservoir. 
         FIG. 4  is a cross-sectional view of a multi-layer bladder which can be used in the expandable accumulator of  FIGS. 1-3 . 
         FIG. 5  is a cross-sectional view of a multi-layer tube or bladder which can be used in the expandable accumulator of  FIGS. 1-3 . 
         FIG. 6  is a cross-sectional view of a tube or bladder, which can be used in the expandable accumulator of  FIGS. 1-3 , having a non-circular inner surface. 
         FIG. 7  is a perspective view of a reservoir and an expandable accumulator assembly 
         FIG. 8  is an exploded perspective view of the assembly of  FIG. 7 , illustrating several constructions of the expandable accumulator. 
         FIG. 9  is a cross-sectional view of the assembly of  FIG. 7  along line  9 - 9 , illustrating the accumulator in an unexpanded state. 
         FIG. 10  is a cross-sectional view of the assembly of  FIG. 9 , illustrating the accumulator in a partially expanded state. 
         FIG. 11  is a cross-sectional view of the assembly of  FIG. 9 , illustrating the accumulator in a fully expanded state. 
         FIG. 12  is a cross-sectional view of the assembly of  FIG. 7  with the accumulator configured as a multi-layer bladder, illustrating the bladder in an unexpanded state. 
         FIG. 13  is a cross-sectional view of the assembly of  FIG. 12 , illustrating the bladder in a partially expanded state. 
         FIG. 14  is a cross-sectional view of the assembly of  FIG. 12 , illustrating the bladder in a fully expanded state. 
     
    
    
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an energy storage system  10  for a hybrid vehicle. However, the system  10  may be utilized in other applications (e.g., a mobile or industrial hydraulic application, etc.). Specifically, the system  10  is configured as a parallel hydraulic regenerative drive system  10  including an accumulator and reservoir assembly  14  and a reversible pump/motor  18  operably coupled to the assembly  14 . Alternatively, the system  10  may be configured as a series hydraulic regenerative drive system, in which the pump/motor  18  is directly coupled to a wheel or drive axle of a vehicle. As a further alternative, the system  10  may include more than one pump/motor  18 . 
     The assembly  14  includes a reservoir  22  and an accumulator  26  in selective fluid communication with the reservoir  22  via the pump/motor  18 . The reversible pump/motor  18  is configured as a variable displacement, axial-piston, swashplate-design pump/motor  18 , such as a Bosch Rexroth Model No. A4VSO variable displacement, axial piston reversible pump/motor  18 . Alternatively, the reversible pump/motor  18  may be configured having a constant displacement rather than a variable displacement. The reversible pump/motor  18  is drivably coupled to a rotating shaft  30  (e.g., an output shaft of an engine, an accessory drive system of the engine, a drive shaft between a transmission and an axle assembly, a wheel or drive axle, etc.). As is described in more detail below, the pump/motor  18  transfers power to the rotating shaft  30  when operating as a motor, and the pump/motor  18  is driven by the rotating shaft  30  when operating as a pump. 
     With continued reference to  FIG. 1 , the reservoir  22  contains working fluid (e.g., hydraulic fluid) and is in fluid communication with the reversible pump/motor  18  by a fluid passageway  34 . A heat exchanger and/or a working fluid filter (not shown) may be situated in the fluid passageway  34  to facilitate cooling and filtering of the working fluid. The reversible pump/motor  18  is in fluid communication with the reservoir  22  to draw low-pressure working fluid (in the direction of arrow A in  FIG. 2 ) from the reservoir  22  via the fluid passageway  34  when operating as a pump. The reversible pump/motor  18  is also in fluid communication with the reservoir  22  to return low-pressure working fluid (in the direction of arrow B in  FIG. 1 ) to the reservoir  22  via the fluid passageway  34  when operating as a motor. 
     The reversible pump/motor  18  is in fluid communication with the accumulator  26  via a fluid passageway  42  to deliver pressurized working fluid (in the direction of arrow A in  FIG. 2 ) to the accumulator  26  when operating as a pump. The reversible pump/motor  18  is also in fluid communication with the accumulator  26  via the fluid passageway  42  to receive pressurized working fluid (in the direction of arrow B in  FIG. 1 ) from the accumulator  26  when operating as a motor. An isolation valve  46  is situated in the fluid passageway  42  and blocks the flow of working fluid through the passageway  42  when in a closed configuration, and permits the flow of working fluid through the passageway  42  when in an open configuration. 
     With continued reference to  FIG. 1 , the reservoir  22  defines an interior chamber  50  in which the working fluid is contained. In the illustrated construction of the energy storage system  10 , the accumulator  26  is positioned within the reservoir  22  and is at least partially immersed in the working fluid contained within the interior chamber  50 . Alternatively, the accumulator  26  may only be at least partially positioned within the reservoir  22 , such that less of the accumulator  26  is immersed in the working fluid compared to the position of the accumulator  26  in  FIG. 1 . Also, in the illustrated construction of the energy storage system  10 , the accumulator  26  includes a flange  54  to facilitate mounting the accumulator  26  to the reservoir  22 . Any of a number of different structural elements (e.g., fasteners, etc.), processes (e.g., welding, adhering, etc.), or a combination of structural elements and processes may be employed to secure the flange  54 , and therefore the accumulator  26 , to the reservoir  22 . 
     With continued reference to  FIG. 1 , the reservoir  22  includes a single, low-pressure inlet/outlet port  58  in fluid communication with the fluid passageway  34  through which working fluid passes to enter or exit the reservoir  22 . Likewise, the accumulator  26  includes a single, high-pressure inlet/outlet port  62  in fluid communication with the fluid passageway  42  through which working fluid passes to enter or exit the accumulator  26 . Alternatively, the reservoir  22  may include more than one low-pressure inlet/outlet port  58 . In such a configuration of the reservoir, the plurality of low-pressure inlet/outlet ports  58  may be paired with respective fluid passageways  34 . 
     In the illustrated construction of the system  10 , the reservoir  22  is substantially air-tight (i.e., “closed”) and is capable of maintaining air within the reservoir  22  at atmospheric pressure (e.g., 0 psi gauge) or at a pressure higher than atmospheric pressure. Alternatively, the reservoir  22  may be open to the atmosphere and include a breather to permit an exchange of air with the atmosphere. The interior chamber  50  of the reservoir  22  includes an air space  66  surrounding the accumulator  26 , above the working fluid. As previously mentioned, the air space  66  may include air at atmospheric pressure or at a pressure higher than atmospheric pressure. Pressurization of the reservoir  22  (i.e., providing air in the air space  66  at a pressure higher than atmospheric pressure) substantially ensures that the pressure of the working fluid at the inlet of the pump/motor  18  (and the inlet/outlet port  58  of the reservoir  22 ) is maintained at a level sufficient to substantially prevent cavitation of the pump/motor  18  when operating as a pump. 
     In the illustrated construction of the system  10 , the reservoir  22  is schematically illustrated as having a generally cylindrical shape. However, the reservoir  22  may be configured having any of a number of different shapes to conform with the structure of a hybrid vehicle within which the reservoir  22  is located. In addition, the reservoir  22  may be made from any of the number of different materials (e.g., metals, plastics, composite materials, etc.). Also, in the illustrated construction of the system  10 , the reservoir  22  is schematically illustrated in a vertical orientation. However, the reservoir  22  may be positioned in any of a number of different orientations in the hybrid vehicle incorporating the system  10 . For example, the reservoir  22  may be oriented upright (i.e., vertical) in the vehicle, laid flat (i.e., horizontal), or positioned at an incline at any angle between a horizontal orientation of the reservoir  22  and a vertical orientation of the reservoir  22 . 
     With continued reference to  FIG. 1 , the accumulator  26  is configured as an expandable accumulator  26 , in which the internal volume or space of the accumulator  26  is variable depending upon the amount of working fluid contained within the accumulator  26 . In the illustrated construction of the system  10 , the accumulator  26  includes an expandable tube  70  having opposed ends  74 ,  78  and an interior space  82  between the ends  74 ,  78 . The inlet/outlet port  62  is positioned in the top end  74  (as viewed in  FIG. 1 ) of the tube  70 , and a clamp  86  couples the inlet/outlet port  62  to the tube  70 . The clamp  86  also functions as a seal to substantially prevent leakage of working fluid between the top end  74  and the inlet/outlet port  62 . One or more seals (e.g., O-rings, gaskets, etc.) may also be utilized to seal the clamp  86  to the inlet/outlet port  62 , and the clamp  86  to the top end  74  of the tube  70 . Another clamp  90  is coupled to the bottom end  78  (as viewed in  FIG. 1 ) of the tube  70  to close the bottom end  78  of the tube  70  and prevent the exchange of working fluid between the accumulator  26  and the reservoir  22  via the bottom end  78 . One or more seals (e.g., O-rings, gaskets, etc.) may be utilized to seal the clamp  90  to the bottom end  78  of the tube  70 . Alternatively, a bladder  118  having only a single open end (i.e., the end adjacent the inlet/outlet port  62 ) may be used with the accumulator  26  in place of the tube  70  ( FIG. 4 ). 
     With reference to  FIG. 1 , the accumulator  26  may include a de-aerating valve  94  coupled to the clamp  90  and in fluid communication with the interior space  82  of the tube  70 . Such a de-aerating valve  94  (e.g., a spring-biased ball valve) assumes an open configuration when the accumulator  26  is not pressurized to permit the escape of entrained air from the accumulator  26  to the reservoir  22 , where the entrained air is allowed to rise through the working fluid to the air space  66 . The de-aerating valve  94  then assumes a closed configuration when the accumulator  26  is pressurized to prevent the pressurized working fluid in the accumulator  26  from leaking into the reservoir  22 . 
     With continued reference to  FIG. 1 , the accumulator  26  includes a plurality of supports  98  that are engageable with the outer periphery of the tube  70  to limit the extent to which the tube  70  may expand when pressurized working fluid is transferred from the reservoir  22  to the accumulator  26 . Although discrete supports  98  “smooth formers” are shown with the illustrated accumulator  26 , a single cage may alternatively be positioned around the outer periphery of the tube  70  and spaced from the outer periphery of the tube  70  by a particular distance corresponding with the desired extent to which the tube  70  may expand. Such a cage may also be shaped to define and limit the expanded shape of the accumulator  26  (e.g., to the expanded shape of the accumulator  26  shown in  FIG. 2 ). 
     The expandable tube  70  or bladder is made from an elastomeric material (e.g., polyurethane, natural rubber, polyisoprene, fluoropolymer elastomers, nitriles, etc.) to facilitate deformation of the tube  70  in response to pressurized working fluid being pumped into the accumulator  26  when the reversible pump/motor  18  is operating as a pump. Specifically, as shown in  FIG. 2 , a radial dimension D corresponding with the outer diameter of a middle portion of the tube  70  varies in response to pressurized working fluid filling and exiting the accumulator  26 . However, the outer diameter of the tube  70  adjacent each of the ends  74 ,  78  is maintained substantially constant by the respective clamps  86 ,  90 . The accumulator  26  is operable to exert a compressive force on the working fluid in the tube  70  as the radial dimension D increases from a value corresponding with the unstretched or undeformed tube  70  (see  FIG. 1 ). In other words, the pressurized working fluid entering the accumulator  26  performs work on the tube  70  to stretch or expand the tube  70  to the shape shown in  FIG. 2 . This energy is stored in the tube  70  at a molecular level, and is proportional to the amount of strain experienced by the tube  70 . 
     Applicants have discovered through testing that when the interior of a homogeneous tube  70  (i.e., a tube  70  having only a single layer, without reinforcing fibers) is pressurized, most of the strain energy stored in the tube  70  is concentrated near the inner surface of the tube  70 . Applicants have also discovered that the concentration of strain energy stored in the tube  70  decreases with an increasing radial position along the thickness of the tube  70 . In other words, the material proximate the outer surface of the tube  70  contributes less to the storage of strain energy than the material proximate the inner surface of the tube  70 . To increase the uniformity of distribution of strain energy along the thickness of the tube  70 , a multi-layer construction may be used in which an innermost layer of the tube includes a higher fracture strain (i.e., the strain at which fracture occurs during a tensile test) than an outermost layer, and in which the outermost layer includes a higher stiffness than the innermost layer. Because such a multi-layer tube can more efficiently store strain energy along its thickness, the maximum internal pressure that the tube is capable of handling would also be increased compared to the single-layer tube  70 . 
     As shown in  FIG. 4 , the bladder  118  includes an inner layer  122  defining an interior space  126  in which working fluid is contained, and an outer layer  130  surrounding the inner layer  122 . It should also be understood that the same configuration could be implemented as a tube having opposed open ends. The outer layer  130  is in contact with the working fluid in the reservoir  22  when the bladder  118  is used with the accumulator, and the accumulator  26  is immersed in the working fluid. The inner layer  122  includes a higher fracture strain than the outer layer  130 , and the outer layer  130  includes a higher stiffness (i.e., modulus of elasticity) than the inner layer  122 . In a construction of the bladder  118  in which at least 200 kJ of strain energy may be stored at an internal pressure between about 3,000 psi and about 6,000 psi, the fracture strain of the inner layer  122  may be between about 30% and about 70% greater than the fracture strain of the outer layer  130 . Likewise, under the same conditions, the stiffness of the outer layer  130  may be between about 30% and about 70% greater than the stiffness of the inner layer  122 . 
     In addition to providing the performance characteristics discussed above, the materials comprising the inner and outer layers  122 ,  130  of the bladder  118  may be selected such that each of the layers  122 ,  130  may be resistant to the working fluid such that deterioration of either of the layers  122 ,  130  after prolonged contact with the working fluid is substantially inhibited. For example, the inner and outer layers  122 ,  130  of the bladder  118  may be made from an elastomer including a nitrile butadiene rubber (NBR), a fluoropolymer elastomer (e.g., VITON), a polyurethane polymer, an elastic hydrocarbon polymer (e.g., natural rubber), and so forth. Each of the inner and outer layers  122 ,  130  may be made from different grades of material within the same material family. Alternatively, the inner and outer layers  122 ,  130  may be made from materials having distinctly different chemistry. 
     With continued reference to  FIG. 4 , the inner and outer layers  122 ,  130  of the bladder  118  may be separately formed and assembled such that the inner surface of the outer layer  130  conforms to the outer surface of the inner layer  122 . The outer layer  130  may or may not be bonded to the inner layer  122  (e.g., using adhesives, etc.). Alternatively, the inner and outer layers  122 ,  130  of the bladder  118  may be co-molded such that subsequent assembly of the layers  122 ,  130  is not required. For example, concentric inner and outer layers of a multi-layer tube (not shown) may be co-extruded layer by layer. 
     With reference to  FIG. 5 , another multi-layer construction of a tube or bladder  134  is shown that may be used in the accumulator  26  of  FIGS. 1-3 . The tube or bladder  134  includes four layers—an inner layer  138 , an outer layer  142 , and two interior layers  146 ,  150 . Like the bladder  118  of  FIG. 4 , the inner layer  138  includes a higher fracture strain than the outer layer  142 , and the outer layer  142  includes a higher stiffness than the inner layer  138 . In some constructions of the tube or bladder  134 , the fracture strain of the layers  138 ,  146 ,  150 ,  142  may progressively decrease from the inner layer  138  to the outer layer  142 . For example, the fracture strain of the layers  138 ,  146 ,  150 ,  142  may progressively decrease in accordance with a linear or nonlinear (e.g., a second order, third order, etc.) relationship. Likewise, the stiffness of the layers  138 ,  146 ,  150 ,  142  may progressively increase from the inner layer  138  to the outer layer  142  in accordance with a linear or nonlinear (e.g., a second order, third order, etc.) relationship. 
     The layers  138 ,  146 ,  150 ,  142  may be made from the same materials discussed above with respect to the bladder  118  of  FIG. 4 . However, only the inner and outer layers  138 ,  142  of the tube or bladder  134  need to be made from a material that is resistant to the working fluid because the interior layers  146 ,  150  are not in contact with the working fluid when the accumulator  26  is immersed in the working fluid. As such, the interior layers  146 ,  150  may be made from a material that possesses desirable strain energy properties, yet lacks resistivity to the working fluid. In one construction of the tube or bladder  134 , the thicknesses of the layers  138 ,  142  may be relatively small compared to the thicknesses of the interior layers  146 ,  150 , such that the interior layers  146 ,  150  are primarily used for energy storage, while the inner and outer layers  138 ,  142  are primarily used as barriers to shield the interior layers  146 ,  150  from the working fluid. In such a construction, the layers  138 ,  142  may contribute a very small or negligible amount to the overall energy storage capability of the tube or bladder  134 , such that the fracture strain or stiffness values of the layers  138 ,  142  need not be chosen in relation to those values of the interior layers  146 ,  150 . In other words, the “inner” interior layer  146  may include a higher fracture strain than the “outer” interior layer  150 , however, the inner layer  138  need not have a higher fracture strain than the interior layer  146 . 
     The individual layers  138 ,  146 ,  150 ,  142  may be separately formed and assembled such that the mating surfaces of the layers  138 ,  146 ,  150 ,  142  conform to each other. The layers  138 ,  146 ,  150 ,  142  may or may not be bonded together. Alternatively, the layers  138 ,  146 ,  150 ,  142  may be co-molded such that subsequent assembly of the layers  138 ,  146 ,  150 ,  142  is not required. For example, when configured as a tube  134 , the layers  138 ,  146 ,  150 ,  142  may be co-extruded layer by layer. 
     With reference to  FIG. 6 , another construction of a tube or bladder  154  is shown having a single layer with an inner surface  158  defining a non-circular cross-sectional shape. Particularly, the inner surface  158  of the tube or bladder  154  includes alternating peaks  162  and valleys  166  spanning the length of the tube or bladder  154  (i.e., into the page of  FIG. 6 ). Such a configuration of the tube or bladder  154  would also increase the uniformity of distribution of strain energy along the thickness of the tube or bladder  154 . 
     In operation, when the system  10  recovers kinetic energy from the rotating shaft  30 , the pump/motor  18  operates as a pump to draw working fluid from the reservoir  22  (via the inlet/outlet port  58 ) in the direction of arrow A (see  FIG. 2 ), pressurize the working fluid, and pump the pressurized working fluid into the interior space  82  of the tube  70  through the open isolation valve  46  and the inlet/outlet port  62 . The accumulator  26  expands or stretches in response to the pressurized working fluid entering the tube  70 . The expansion of the accumulator  26  occurs progressively along the length of the accumulator  26  as working fluid is pumped into the accumulator  26  (see, for example, the expansion of the accumulators  26   a ,  26   b  in  FIGS. 9-11  and  12 - 13 ) at a substantially constant pressure. 
     As working fluid exits the reservoir  22 , the volume of the air space  66  above the working fluid is substantially unchanged because the working fluid is merely transferred from outside the tube  70  (as shown in  FIG. 1 ) to inside the tube  70  (as shown in  FIG. 2 ). In other words, the combination of the accumulator  26  and the reservoir  22  substantially mimics a control volume, in which the volume of working fluid exiting the reservoir  22  is substantially equal to the volume of working fluid entering the accumulator  26 . Likewise, the volume of working fluid exiting the accumulator  26  is substantially equal to the volume of working fluid returning to the reservoir  22 . 
     Consequently, the total volume of working fluid maintained within the accumulator  26  and the reservoir  22  at any given time during operation of the system  10  is substantially constant. In addition, because the volume of the air space  66  is maintained substantially constant during operation of the system  10 , working fluid may be drawn from the reservoir  22  and returned to the reservoir  22  without an exchange of gas or air with the atmosphere (i.e., drawing replacement air from the atmosphere or venting air to the atmosphere). After the kinetic energy of the rotating shaft  30  is recovered, the isolation valve  46  is actuated to a closed configuration, and the tube  70  exerts a compressive force on the working fluid to maintain the working fluid at a high pressure within the accumulator  26 . 
     When the hybrid vehicle requires propulsion assistance, the isolation valve  46  is actuated to an open configuration to permit the flow of pressurized working fluid in the direction of arrow B (see  FIG. 1 ) from the accumulator  26 . As mentioned above, the energy used for propulsion assistance is stored in the tube  70  at a molecular level, and is proportional to the amount of strain experienced by the tube  70 . High-pressure working fluid flows from the accumulator  26 , through the fluid passageway  42 , and into the pump/motor  18  to operate the pump/motor  18  as a motor to drive the shaft  30 . The pump/motor  18  then returns the low-pressure working fluid to the reservoir  22  via the fluid passageway  34  and the inlet/outlet port  58 . As working fluid is returned to the reservoir  22 , the volume of the air space  66  above the working fluid is substantially unchanged because the working fluid is merely transferred from inside the tube  70  (as shown in  FIG. 2 ) to outside the tube  70  (as shown in  FIG. 1 ). As previously mentioned, the combination of the accumulator  26  and the reservoir  22  substantially mimics a control volume, in which the total volume of working fluid maintained within the accumulator  26  and the reservoir  22  at any given time during operation of the system  10  is substantially constant. 
     With reference to  FIG. 3 , a second construction of an energy storage system  110  is shown including an assembly  114  having dual accumulators  26  positioned in the reservoir  22  to enhance the energy storage capacity of the system  110 . Like components are labeled with like reference numerals, and will not be described again in detail. 
       FIGS. 7 and 8  illustrate an accumulator and reservoir assembly  14   a  that may be used in the system  10  of  FIGS. 1 and 2 . Like components are labeled with like reference numerals with the letter “a.” In the illustrated construction of the reservoir  22   a , the flange  54   a  is fastened (i.e., using bolts  168 ) to a corresponding flange  170  on the reservoir  22   a  to seal the interior chamber  50   a  ( FIG. 8 ). A gasket  174  is positioned between the flange  54   a  and the reservoir  22   a  to facilitate sealing the flange  54   a  to the reservoir  22   a . Alternatively, any of a number of different seals (e.g., O-rings, etc.) may be positioned between the flange  54   a  and the reservoir  22   a  to facilitate sealing. Alternatively, any of a number of different fasteners or quick-release arrangements may be utilized to secure the flange  54   a  to the reservoir  22   a.    
     With reference to  FIG. 9 , the expandable accumulator  26   a  is configured as a single-layer bladder  178  having an open end  182  in fluid communication with the high-pressure inlet/outlet port  62   a , and a closed end  186 . Alternatively, the accumulator  26   a  may be configured as a multi-layer bladder  190 , a single-layer tube  194 , or a multi-layer tube  198  having material properties as discussed above ( FIG. 8 ). With reference to  FIG. 9 , the assembly  14   a  also includes a support or a cage  202  coaxial with a central axis  206  ( FIG. 8 ) of the reservoir  22   a  and the inlet/outlet port  62   a . In the illustrated construction of the assembly  14   a , the cage  202  is configured as a cylindrical, rigid tube extending the length of the bladder  178 . The flange  54   a  is fastened (i.e., using bolts  168 ) to a corresponding flange  210  on the cage ( FIG. 8 ) to maintain the cage  202  coaxial with the reservoir  22   a . The clamp  86   a  is also fastened (i.e., using bolts) to the flange  54   a  to maintain the accumulator  26   a  coaxial with the reservoir  22   a  and the cage  202 . In the illustrated construction of the assembly  14   a  as shown in  FIG. 9 , the clamp  86   a  is configured as a ring configured to secure an end or lip portion  214  of the accumulator  26   a  between the clamp  86   a  and the flange  54   a . Alternatively, the clamp  86   a  may be configured in any of a number of different ways to secure the accumulator  26   a  to the flange  54   a , and therefore to the reservoir  22   a.    
     As discussed above, the cage  202  is spaced from the outer periphery of the bladder  178  by a particular distance corresponding with the desired extent to which the bladder  178  may expand. The end of the cage  202  proximate the low-pressure inlet/outlet port  58   a  is also spaced from the end of the reservoir  22   a  a sufficient distance to permit free-flow of working fluid between locations in the interior chamber  50   a  inside the cage  202  and outside the cage  202 . With reference to  FIGS. 7-9 , the reservoir  22   a  includes a fill port  218  in fluid communication with the interior chamber  50   a  to permit the reservoir  22   a  to be refilled with working fluid when necessary. Although not shown, a cap may be secured to the fill port  218  to seal the reservoir  22   a.    
     With reference to  FIG. 9 , the bladder  178  includes a variable internal volume  222  which increases as working fluid is received within the bladder  178  at a relatively constant pressure. As discussed above, Applicants have discovered through testing that most of the strain energy stored in the bladder  178  is concentrated near the inner surface of the bladder  178 . In other words, the material proximate the inner surface of the bladder  178  is compressed in a radially outward direction as pressurized working fluid is received in the bladder  178  (see  FIGS. 10 and 11 ), effectively causing the internal volume  222  of the bladder  178  to progressively increase along the length of the bladder  178 . In some constructions of the bladder  178 , the variable internal volume  222  is configured to be increased up to about 13 times an initial internal volume corresponding with an unexpanded state of the bladder  178  ( FIG. 9 ). As a result, up to about 75% of the working fluid in the reservoir  22   a  can be exchanged with the bladder  178  as the bladder  178  is expanded from its unexpanded state ( FIG. 9 ) to its fully expanded state ( FIG. 11 ). In the illustrated construction of the assembly  14   a , the reservoir  22   a  is configured to contain 30 liters of working fluid, while the bladder  178  is configured to contain at least 22 liters of the working fluid when it is fully expanded as shown in  FIG. 11 . Alternatively, the reservoir  22   a  may be sized appropriately to contain more or less working fluid. 
     With reference to  FIGS. 9 and 11 , the bladder  178  may occupy between about 40% and about 70% of the internal volume (which is defined by the interior chamber  50   a ) of the reservoir  22   a  depending upon the amount of working fluid in the bladder  178 . For example, as shown in  FIG. 9 , the bladder  178  occupies about 40% of the internal volume of the reservoir  22   a  when in its unexpanded state. However, when the bladder  178  is filled with working fluid as shown in  FIG. 11 , the bladder  178  occupies about 70% of the internal volume of the reservoir  22   a . When operating at a system pressure of about 3,000 psi, the bladder  178  is configured to store at least about 150,000 ft-lbs of energy when completely filled with working fluid as shown in  FIG. 11 , which is sufficient to provide propulsion assistance to a two-ton vehicle (e.g., a car or pickup truck). When operating at a system pressure of about 6,000 psi, the bladder  178  is configured to store at least about 750,000 ft-lbs of energy when completely filled with working fluid as shown in  FIG. 11 , which is sufficient to provide propulsion assistance to a ten-ton vehicle (e.g., a single axle delivery truck). 
     In one construction, the assembly  14   a  occupies only about 3.6 cubic feet of space. Such a relatively small package is possible as a result of positioning the bladder  178  within the reservoir  22   a , and by permitting the bladder  178  to occupy up to about 70% of the internal volume of the reservoir  22   a  when the bladder  178  is fully charged with pressurized working fluid. With the available energy storage capabilities of the assembly  14   a  when operating between system pressures of 2,000 psi and 6,000 psi, the energy density (i.e., the stored energy divided by the occupied space of the storage device) of the assembly  14   a  may range between about 41,500 ft-lbs/cubic foot and about 208,500 ft-lbs/cubic foot. In comparison, the energy density of a conventional hybrid hydraulic system including a gas-charged accumulator and a separate low-pressure reservoir is about one-third to about one-fifth the energy density of the assembly  14   a . Because the energy density of the assembly  14   a  is much higher than that of a conventional hybrid hydraulic system including a gas-charged accumulator and a separate low-pressure reservoir, the assembly  14   a  may be packaged much more efficiently within a vehicle or other machinery with which the assembly  14   a  is used. 
       FIGS. 12-14  illustrate another construction of an accumulator and reservoir assembly  14   b  which may be used in the system  10  of  FIGS. 1 and 2 . Like components are labeled with like reference numerals with the letter “b.” The assembly  14   b  is identical to the assembly  14   a  of  FIGS. 7-11 , however, a multi-layer bladder  190 , such as the bladder  118  shown in  FIG. 4  and described above, replaces the single-layer bladder  178 . The bladder  190  includes an inner layer  226  and an outer layer  230 , and may be manufactured in a similar manner as described above with respect to the bladder  118 . Alternatively, the bladder  190  may be configured having more than two layers, such as the tube or bladder  134  shown in  FIG. 5 . 
     In one construction of the multi-layer bladder  190  which Applicants have tested, the inner layer  226  includes an inner diameter D 1  of about 2.25 inches and an outer diameter D 2  of about 10.25 inches, and the outer layer  230  includes an inner diameter D 3  of about 10.25 inches and an outer diameter D 4  of about 13.25 inches. Therefore, the wall thickness T 1  of the inner layer  226  is about 4 inches, while the wall thickness T 2  of the outer layer  230  is about 1.5 inches. The values of these dimensions D 1 -D 4 , T 1 , T 2  correspond with the unexpanded state of the bladder  190 , as shown in  FIG. 12 . After filling the bladder  190  with working fluid at a pressure of about 5,000 psi, Applicants measured an increase in each of the dimensions D 1 -D 4 , and a decrease in each of the thicknesses T 1 , T 2 . Particularly, Applicants measured a decrease in the thickness T 1  of about 47%, and a decrease in the thickness T 2  of about 21%. Considering the total reduction of thickness associated with the dimensions T 1 , T 2 , up to about 85% of the total amount of reduced thickness occurs in the inner layer  226 . Consequently, only about 15% of the total amount of reduced thickness occurs in the outer layer  230 . Therefore, the particular materials, or grades of the same material, from which the inner and outer layers  226 ,  230  are made may be chosen to increase the uniformity of distribution of strain energy along the thickness of the bladder  190 , thereby leading to increased performance and more predictable operation of the assembly  14   b.    
     Operation of either of the assemblies  14   a ,  14   b  is substantially similar to the operation of the assembly  14  as described above. 
     Various features of the invention are set forth in the following claims.