Abstract:
A lightweight, optimally efficient, easily serviced, piston-in-sleeve high pressure accumulator is provided. The accumulator includes one or more cylindrical composite pressure vessel separate end cap manifolds. A piston slidably disposed in a thin impermeable internal sleeve in the accumulator separates two chambers, one adapted for containing a working fluid and the other adapted for containing gas under pressure. Gas is provided in a volume between the impermeable internal sleeve and the composite pressure vessel wall. Additional gas is optionally provided in gas cylinders. Further components are provided for withstanding harmful effects of radial flexing of the composite vessel wall under high pressures, and from stresses present in use in mobile applications such as with a hydraulic power system for a hydraulic hybrid motor vehicle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/943,930, filed on Jun. 14, 2007. The entire disclosure of the above application is hereby incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present disclosure relates generally to accumulators for high pressure applications, and more particularly to high pressure accumulators of the piston-in-sleeve (or “piston and sleeve”) type. The disclosure further relates to the potential use of such accumulators in conjunction with fuel efficient hydraulic hybrid motor vehicles. 
   BACKGROUND OF THE INVENTION 
   1. Hydraulic Hybrid Vehicles 
   Hybrid powertrains are an increasingly popular approach to improving the fuel utilization of motor vehicles. The term “hybrid” refers to the combination of a conventional internal combustion engine with an energy storage system, which typically serves the functions of receiving and storing excess energy produced by the engine and energy recovered from braking events, and redelivering this energy to supplement the engine when necessary. This decouples the production and consumption of power, thereby allowing the internal combustion engine to operate more efficiently, while making sure that enough power is available to meet load demands. 
   Several forms of energy storage are known in the art, with electrical storage using batteries being the best known. Recently, hydraulic hybrids have been demonstrated to offer better efficiency, greater power density, lower cost, and longer service life than electric hybrids. A hydraulic power system takes the form of one or more hydraulic accumulators for energy storage and one or more hydraulic pumps, motors, or pump/motors for power transmission. Hydraulic accumulators operate on the principle of storing energy by compressing a gas. An accumulator&#39;s pressure vessel contains a charge of gas, typically nitrogen, which becomes compressed as the hydraulic pump pumps liquid into the vessel. The liquid thereby becomes pressurized and may be used to drive the hydraulic motor when released. A hydraulic accumulator thus utilizes two distinct working media; one a compressible gas and the other a relatively incompressible liquid. More generally, an accumulator utilizes two distinct working media, at least one of which is a gas and the other a gas or a fluid. Throughout this disclosure, the term “gas” shall refer to the gaseous medium and the term “fluid” shall refer to the gaseous or the liquid working medium, as is customary in the art. 
   2. High Pressure Accumulator Designs in the Present State of the Art 
   In the present state of the art, there are three basic configurations for hydraulic accumulators: spring type, bladder type and piston type. 
   Spring type is typically limited to accumulators with small fluid volumes due to the size, cost, mass, and spring rates of the springs. 
   Bladder accumulators typically suffer from high gas permeation rates and poor reliability. Some success has been achieved by replacing the elastic bladder with a flexible metallic or metallic-coated bellows structure, for example, as disclosed in U.S. Pat. No. 5,771,936 to Sasaki et al. and U.S. Pat. No. 6,478,051 to Drumm et al. However, a principal shortcoming of this approach lies in the potential for the bellows to experience stresses and longitudinal disorientation that may rapidly lead to failure under a severe duty cycle, such as would be present in an automotive power system application. 
   Of the three basic configurations in the present state of the art, the piston type is the least costly design that can store desirable volumes of fluid. In addition, properly designed piston accumulators are physically robust, efficient, and reliable. 
   Standard piston accumulators are also well represented in the art. In a standard piston accumulator, the hydraulic fluid is separated from the compressed gas by means of a piston which seals against the inner walls of a cylindrical pressure vessel and is free to move longitudinally as fluid enters and leaves and the gas compresses and expands. Because the piston does not need to be flexible, it may be made of a gas impermeable material such as steel. However, the interface between the piston and the inner wall of the cylinder must be controlled tightly to ensure a good seal, and the degree of dimensional tolerance necessary to ensure a good seal may increase the cost of manufacturing. It also requires that the pressure vessel be extremely rigid and resistant to expansion near its center when pressurized, which would otherwise defeat the seal by widening the distance between the piston and cylinder wall. This has eliminated the consideration of composite materials for high pressure piston accumulator vessels, as composite materials tend to expand significantly under pressure (e.g., about 1/10 of an inch diametrically for a 12 inch diameter vessel at 5,000 psi pressure). 
   As a result of the foregoing, standard piston accumulator vessels tend to be made of thick, high strength steel and are very heavy. Standard piston accumulators have a much higher weight to energy storage ratio than either steel or composite bladder accumulators, which makes them undesirable for mobile vehicular applications (as such increased weight would, for example, reduce fuel economy for the vehicle). More specifically, piston accumulators for the same capacity (i.e., size) and pressure rating are many times heavier (e.g., by up to 10 times) than an accumulator with a lightweight composite pressure vessel design, as would be preferred in such applications where accumulator weight is an issue. Therefore, despite their potentially superior gas impermeability, piston accumulators are largely impractical for vehicular applications. 
   3. Prior Art Regarding Piston-in-Sleeve Accumulator Designs 
   One piston accumulator concept utilizes a piston and sleeve assembly, in which the piston resides within and seals against a cylindrical sleeve that is separate from the inner wall of the pressure vessel. As defined herein, the term “sleeve” includes a hollow member substantially incapable of withstanding stresses that would be applied thereto were a full pressure differential of the accumulator to be applied across the hollow member. This piston-in-sleeve approach provides at least two benefits over the prior art for high pressure accumulators, namely (i) separating the pressure containment function of the vessel wall from its piston sealing function, allowing an effective seal to be pursued with a sleeve independently of issues relating to pressure vessel construction, and (ii) providing an intervening or “interstitial” volume between the sleeve and vessel wall which may be filled with the charge gas to allow tailoring of the ratio of gas to fluid to optimize performance and which also allows shaping” the pressure profile of the discharged fluid. In each of U.S. Pat. No. 2,417,873 to Huber, U.S. Pat. No. 2,703,108 to McQuistion, U.S. Pat. No. RE24,223 to Ford, and later U.S. Pat. No. 4,714,094 to Tovagliaro, the use of a piston and sleeve assembly on a high pressure accumulator is taught. Such designs comprise a generally thick-walled strong cylindrical pressure vessel constructed of a steel alloy, and a metal sleeve which is thin relative to the vessel walls. The sleeve is permanently attached to the inner surface of one end of the pressure vessel near its circumference, creating (with the piston) a closed or “inside” chamber for the working fluid. The other end of the sleeve extends toward the other end of the vessel and is generally left open to create an “outside” chamber that consists of the open volume of the sleeve, the remaining volume of the pressure vessel, and the intervening/interstitial space between the outer wall of the sleeve and the inner wall of the pressure vessel, each filled with the gaseous medium of the accumulator. 
   In operation of these prior art piston-in-sleeve accumulator designs, the sleeve must be tightly retained and centered within the vessel to prevent radial movement, for example, due to vibrations in use with mobile (e.g. aircraft) applications. Sleeve movement would fatigue the rigid fixed end of the sleeve possibly leading to leakage due to cracking, distortion, or wear of the sealing gasket if one is present. This requires the sleeve to either be stiffened by connecting it at points to the vessel wall, or requires the sleeve to be thicker than the minimum that would be necessary to withstand the small pressure differentials normally encountered in charging and discharging. Further, the outer walls of the vessel must be thicker than would be necessary for pressure containment alone because the walls must be prevented from expanding and thus loosening the sleeve or distorting it from the true circular form necessary for piston sealing. 
   Prior art piston-in-sleeve designs also uniformly contain the fluid within the closed (inside) chamber, with the charge gas residing on the other side of the piston and in the interstitial space between the sleeve and vessel wall. This fluid-inside, gas-outside arrangement is used in the prior art for at least two reasons. First, as mentioned above, the prior inventors sought a resistance to structural splitting. Second, this arrangement is naturally preferable because it maximizes the fluid capacity and hence energy capacity of the device. That is, the working medium that resides inside the sleeve may be discharged completely, while some portion of the medium outside the sleeve will always remain trapped in the interstitial space; because working capacity is determined by how much fluid may be discharged, it is a natural choice to have the fluid reside on the inside of the sleeve and gas on the outside. 
   Like standard piston accumulators discussed above, these prior art piston-in-sleeve accumulators are unacceptably heavy for a hydraulic hybrid motor vehicle application or other application where accumulator weight is a significant issue. Notably, U.S. Pat. No. 4,714,094 to Tovagliaro attempts to reduce the weight of such piston-in-sleeve accumulators through the use of lightweight composite materials in place of steel for the pressure containment function in the vessel wall. However, the Tovagliaro device still requires an internal metallic core to the vessel wall (in addition to the composite envelope, likely at least in part to resist permeation of the gas under pressure out through the composite vessel wall) and a thickened metal area at one (flat) end of the accumulator (to enable providing a removable end cap and to tightly retain and center the sleeve, as discussed above). As such, the Tovagliaro device would still remain undesirably heavy for a hydraulic hybrid motor vehicle application, and would also entail significantly greater manufacturing cost than desired (e.g., because of complexity of the design and entailing vessel construction with both a composite envelope and metallic core and end). In addition, the internal metallic core (or liner) used in conjunction with composite materials would be unacceptable for use in hydraulic hybrid vehicles. The intense duty cycle experienced by the accumulator (i.e., the extremely large number of charge-discharge cycles, in some cases exceeding one million cycles) and the significant radial expansion of composite materials (about 1/10 of one inch diametrically for a 12 inch diameter vessel at 5,000 psi pressure) together would result in expected fatigue failure of the metal core or liner. 
   In addition, the flat end construction (on at least one end) of prior art piston accumulators also adds significantly to the complexity, weight and cost of the accumulator. 
   The most recent attempt to resolve these short-comings is disclosed in U.S. Pat. No. 7,108,016 to Moskalik. The Moskalik device employs a thermoplastic sleeve, with a carbon-fiber wound pressure vessel shell. The Moskalik device also places the fluid outside the core, thus the fluid fills the interstitial spaces. There are several significant short-comings with this design. First, the physical size of the accumulator is larger than necessary to enclose the same volume of useful working fluid as the fluid in the interstitial spaces cannot be used. Second, as will be described below, optimal accumulator design requires that the gas volume be greater than the fluid volume. Third, the design cannot be serviced—any failure of any component requires that the entire cylinder be discarded. Fourth, the thickness of the pressure vessel wrapping is thicker than needed because the wrapping must counter both axial and tangential loads. Fifth, the design does not provide the means to protect the integrity of the sleeve should the fluid pressure exceed that of the gas pressure. 
   4. Disadvantages of the Prior Art 
   In summary, as has been explained above, despite the many years of development for accumulator designs, the prior art has thus far failed to provide a high pressure accumulator design that is modular, lightweight, low cost, durable under stresses, easy to assemble and maintain, all while providing optimal performance. Prior art bladder accumulators have unacceptable permeation. Prior art metal bellows accumulators are not sufficiently durable under stresses. Prior art piston accumulators of all types are unacceptably heavy and costly. As a result, the prior art has failed to provide a high pressure accumulator that is satisfactory for hydraulic hybrid motor vehicle applications, as is desired for the present invention. 
   SUMMARY OF THE INVENTION 
   In concordance with the instant disclosure, a high pressure piston and sleeve accumulator design that is optimally efficient, extremely light weight, easily serviceable, adaptable to service in mobile vehicles, and can be mass produced at low cost, is surprisingly discovered. 
   The present invention utilizes a piston and sleeve design, but through various means enables use of a extremely lightweight, easy to assemble, and easy to manufacture composite pressure vessel therewith, thereby providing an optimized high pressure accumulator and satisfying a long felt need for such an accumulator in the art. As will be described in greater detail hereafter, applicant meets these needs through one or more of various modifications from the piston accumulator prior art, including using a modular design that comprises accumulator cylinders and auxiliary gas cylinders, providing manifolds with tension members as removable end caps, including a specially configured valve to prevent possible damage to the sleeve, and facilitating assembly and repair by using non-permanent seals. 
   In one embodiment, an accumulator assembly includes at least one accumulator cylinder. The accumulator cylinder has a cylindrical, gas-impermeable shell, and a cylindrical, gas-impermeable sleeve disposed within and substantially concentric with the shell. An interstitial space is formed between the sleeve and the shell. A piston is slidably disposed within the sleeve. The piston separates an interior of the sleeve into a first chamber configured to contain a compressed gas, and a second chamber configured to contain a pressurized fluid. A pair of axial closures is disposed at opposing ends of the accumulator cylinder. The axial closures may include a gas manifold and a fluid manifold. At least one tension member is further disposed between the axial closures and coupled thereto for holding the accumulator assembly together. 
   In another embodiment, an accumulator system includes the accumulator assembly and at least one auxiliary gas cylinder in communication therewith. 
   In a further embodiment, a pressure relief valve for the accumulator system includes a hollow cylinder body having a fluid valve port formed at a first end of the cylinder body, a gas valve port formed at a second end of the cylinder body, and a drain relief port formed through a wall of the cylinder body. A valve piston is slidably disposed within the cylinder body. A biasing means is disposed in the cylinder body between the piston and the second end. 

   
     DRAWINGS 
     The above, as well as other advantages of the present disclosure, will become readily apparent to those skilled in the art from the following detailed description, particularly when considered in the light of the drawings described herein. 
       FIG. 1  shows a schematic view for of an accumulator system according to the present disclosure; 
       FIG. 2  shows an isometric view of one embodiment of an accumulator assembly for use in the accumulator system depicted in  FIG. 1 ; 
       FIG. 3  shows a cross sectional view of the accumulator assembly depicted in  FIG. 2 , taken along section line  3 - 3 ; 
       FIG. 4A  shows a cross sectional view of a differential pressure relief valve for use with the accumulator system of the present disclosure; 
       FIG. 4B  shows a cross sectional view of the differential pressure relief valve depicted in  FIG. 4A , the valve shown with a piston blocking a relief port; and 
       FIG. 4C  shows a cross sectional view of the differential pressure relief valve depicted in  FIGS. 4A and 4B , the valve shown with the relief port opened. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should also be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. In respect of the methods disclosed, the order of the steps presented is exemplary in nature, and thus, is not necessary or critical. 
   An overall schematic representation of an accumulator system  2  is shown in  FIG. 1 . The accumulator system  2  is comprised of at least one accumulator cylinder  4 , whose fluid-side connections are joined to a common fluid network  6 ; zero or more auxiliary gas cylinders  8 , whose gas-side connections share a common gas network  10  with the accumulator cylinders  4 , and a gas-mediated, differential pressure relief valve  12 . The accumulator system  2  is coupled to the primary hydraulic circuitry by connector  14 , and the relief valve  12  has a connection to a reservoir tank  16 . 
   As shown in  FIG. 2 , a plurality of the accumulator cylinders  4  may be bound together as an accumulator assembly  200  for use in the accumulator system  2 . A skilled artisan should appreciate that any number of the accumulator cylinders  4  may be disposed in the accumulator assembly  200 . In a particular embodiment shown, the accumulator assembly  200  includes four of the accumulator cylinders  4 . The accumulator cylinders  4  are disposed between a pair of axial closures  202 ,  204 , for example, a fluid manifold  202  and a gas manifold  204 . It should be appreciated that the fluid manifold  202  and the gas manifold  204  may be positioned at either the same or opposing ends of the accumulator assembly  200 , as desired. 
   The axial closures  202 ,  204  are coupled together with at least one tension member  206  to thereby hold together the accumulator assembly  200 . The tension member  206  can be a substantially rigid member such as a steel bolt, for example, or a pliable member such as a carbon fiber cable, for example. Other materials suitable for holding together the accumulator assembly  200  may be selected, as desired. Methods of fastening the tension member  206  include threaded fasteners, wedges embedded within the tension member  206 , loops of a single tension member  206  material passed around stays, or ‘weaving’ the tension member  206 . Other suitable methods of fastening the tension member  206  may be selected, as desired. It should be appreciated that the tension member  206  facilitates a minimization of the amount of material required to form the accumulator cylinder  4 , since the tension member  206  is configured to bear the axial stresses of the accumulator cylinder  4 . 
   Referring now to  FIG. 3 , the accumulator assembly  200  according to the present disclosure includes a plurality of the accumulator cylinders  4 . Each of the accumulator cylinders  4  has an outer, substantially gas-impermeable shell  300 . The shell  300  may be formed from any suitable material, such as at least one of a metal, a polymer, and a composite material, as desired. The shell may be formed from a material that is optimized for strength in relation to directional stresses, such as one of an axial stress and a hoop stress, for example. The shell  300  may have an over-wrap  302 , as desired. The over-wrap  302  is typically formed of a strong lightweight material, such as carbon fiber, E-glass, or other suitable material as known in the art. The material of the over-wrap  302  may be wrapped to maximize an angle between the over-wrap  302  and an axial axis of the accumulator cylinder  4 . A first metal boss  304  resides at the gas-side of the accumulator cylinder  4  and a second metal boss  306  resides at the fluid-side of the accumulator cylinder  4 . The shell  300  is affixed to the first metal boss  304  and the second metal boss  306  using any substantially gas-impermeable means known in the art, such as welding, adhesive, sealant, and the like. 
   An inner sleeve  308  is disposed between the first metal boss  304  and the second metal boss  306 . The inner sleeve  308  is divided into two chambers; a gas-side  310  first chamber and a fluid-side  312  second chamber. The gas-side  310  first chamber is configured to contain a gas, such as a nitrogen, helium, or other suitable gas as known in the art. In particular examples, the gas-side  310  first chamber additionally contains a foam. The fluid-side  312  second chamber is configured to contain a fluid, such as a hydrocarbon oil or other suitable fluid or gas as known in the art. 
   The inner sleeve  308  may be readily removable and replaceable. In other embodiments, the inner sleeve  308  may be serviceable. For example, the inner sleeve  308  may be selectively held in place by the first metal boss  304  and the second metal boss  306 . Thus, it should be appreciated that a damaged accumulator assembly  200 , wherein the damage is to the inner sleeve  308 , may be inexpensively and easily repaired with the accumulator assembly  200  of the present disclosure. 
   The inner sleeve  308  may be constructed from a light-weight, gas-impermeable material. In one embodiment, the cylindrical impermeable sleeve  308  is made of a thin, non-metallic material, such as a composite material, for example. In another embodiment, the cylindrical impermeable sleeve is formed from a sheet metal, such as steel, for example. Other suitable gas-impermeable materials may be selected as desired. 
   It should be further understood that having the first metal boss  304  and the second metal boss  306  disposed at opposite ends of the sleeve  308  may militate against numerous potential sleeve  308  failure concerns. Since the sleeve  308  is generally not cantilevered, the sleeve  308  is unlikely to deflect due to motion of the underlying accumulator cylinder  4 . Accordingly, fatigue failure issues are obviated with the accumulator assembly  200  of the present disclosure. 
   An interstitial space  314  is formed between the shell  300  and the inner sleeve  308 . For example, a specified clearance may be provided between the cylindrical impermeable inner sleeve  308  and the shell  300 . The size of the inner sleeve  308  and the shell  300  may be chosen to contain a desired quantity of gas within the interstitial space  314 . On the gas-side  310  of the accumulator cylinder  4 , an opening  316  is provided in the first metal boss  304 . The opening  316  allows passage of a gas from the gas-side  310  of the inner sleeve  308  to the interstitial space  314 . 
   The gas-side  310  first chamber and the fluid-side  312  second chamber of the inner sleeve  308  are divided by a slidable, gas-impermeable piston  318 . Sealing between the gas-side  310  first chamber and the fluid-side  312  second chamber is accomplished with a seal  320 . The seal  320  may comprise two O-rings, for example. The seal  320  comprising two O-rings may be separated by a backup ring, or other methods typical of the art. The piston  318  may include at an annular ring  322 , which is preferably an O-ring, and an alignment bearing  324 . 
   To ensure that piston  318  maintains good sealing in sleeve  6  during the useful lifetime of the accumulator assembly  200 , the seals  320 ,  324  should be as widely spaced as possible. However, in so doing, the pressure on the outside of the sleeve  308  can cause micro-deformations of said sleeve  6 , causing the sleeve  308  to rub on the piston  318 . To militate against the rubbing of the sleeve  308  and the piston  318 , the annular ring  322  is produced on the piston  318  and a hole  326  is produced in the piston  318  to allow the gas-side  310  first chamber to communicate with the annular ring  322 . The hole  326  provides a mechanically stable piston  318  that does not produce undue stresses on the sleeve  308 , thereby allowing for a lighter sleeve  308 . The communication of the first chamber with the annular ring  322  may minimize the sleeve  308  and piston  318  contact area. To effectively seal the piston  318 , two of the seal  320  are generally placed on the outside of the piston  318  adjacent the fluid-side  312  face of the piston  318 . To ensure that the piston  318  moves without cocking or jamming, an optional third seal may be placed adjacent the gas-side  310  face of the piston  318  distal the seal  320 . 
   The axial closures  202 ,  204  seal the ends of the accumulator assembly  200 . The axial closures  202 ,  204  may have at least one planar surface. In particular embodiments, the axial closures  202 ,  204  are fluid and gas manifolds  202 ,  204  that seal at least one of the accumulator cylinders  4  disposed in the accumulator assembly  200 . The gas manifold  204  may further have a gas port  328  formed therein that connects the gas-side  310  of each the accumulator cylinders  4  disposed in the accumulator assembly  200  and all auxiliary gas cylinders  8 , if present in the accumulator system  2 . The fluid manifold  204  may further have a fluid port  330  that connects the fluid-side  312  of each of the accumulator cylinders  4  in the accumulator assembly  200 . 
   It should be appreciated that when used, the auxiliary gas cylinders  8  may have substantially the same structure as the accumulator cylinder  4 , less the sleeve  308  and the piston  318 , for example. A skilled artisan should further appreciate that other auxiliary gas cylinder  8  designs may be employed, as desired. 
   Assembly and maintenance of the accumulator system  2  and the accumulator assembly  200  according to the present invention is greatly facilitated by the design of the accumulator assembly  200 . Assembly is achieved by the steps of placing the piston  318  into sleeve  308 , inserting metal boss  304  and metal boss  306  into the wrapped shell  300 , inserting the sleeve  308  into the shell  300 , and placing the at least one accumulator cylinder  4  and/or more auxiliary air cylinders  8  between the axial closures  202 ,  204 . The at least one tension member  206  is then added to the accumulator assembly  200  and tightened, as desired. Should the performance of the accumulator system  2  indicate a wearing or possible failure of an internal part, the assembly procedure is simply reversed to open the accumulator assembly  200 . 
   Since the material and thickness of sleeve  308  are not intended to carry load, as the pressure on the gas-side  310  first chamber and fluid-side  312  second chamber are nominally equal, a mechanism is provided to deal with the two instances in which this assumption can fail. These cases are an anomalous high fluid pressure or a loss of gas pressure. In both cases, the piston  318  would move to the left, as shown in  FIG. 3 , until such time as the piston  318  bottoms out on the gas manifold  204 . Once this occurs, any further fluid pressure buildup or gas pressure reduction would cause the sleeve  308  to undesirably bear load. The differential pressure relief valve  12  can mitigate the undesirable bearing of load by the sleeve  308 . 
   An illustrative embodiment of the gas-mediated, differential pressure relief valve  12  of the present disclosure is depicted in  FIGS. 4A ,  4 B, and  4 C. The differential pressure relief valve  12  includes a valve piston  400  and a cylinder body  402 . The piston  400  is slidably disposed and confined within the cylinder body  402 . Like the accumulator cylinder  4 , the differential pressure relief valve  12  has a valve gas-side  404  and a valve fluid-side  406 . The sides  404 ,  406  are separated by a seal  408 . The seal  408  is particularly embodied as a pair of O-rings with a spacer, for example. 
   The differential pressure relief valve  12  may be connected to, and in communication with, the fluid-manifold  202  of the accumulator assembly  200  by a valve fluid port  410 , for example. The valve fluid port  410  may be formed in the cylinder body  402  at the valve fluid-side  406  thereof, for example. The differential pressure relief valve  12  may be connected to, and in communication with, the gas manifold  204  by a valve gas port  412 . The valve gas port  412  may be formed in the cylinder body  402  at the valve gas-side  404  thereof, for example. The differential pressure relief valve  12  may be connected, and in communication with, the reservoir  16  by a drain relief port  414 . 
   A differential pressure bias may be provided by a biasing means  416  disposed between the piston  400  and an end of the cylinder body  402  adjacent the valve gas port  412 , for example. The biasing means  416  may be a spring, for example. One of ordinary skill in the art should understand that other suitable biasing means  416  for providing the difference pressure bias may be employed, as desired. 
   In a further embodiment, the differential pressure relief valve  12  may include at least one sensor  418 . The sensor  418  may be disposed within the piston  400 , on the piston  400 , or adjacent the piston  400 , for example. As a further nonlimiting example, the sensor  418  may be configured to monitor a position of the piston  400  within the cylinder body  402 . The sensor  418  may include one of an electrical switch, a hydraulic switch, and a pneumatic switch. It should be appreciated that if the sensor  418  is activated, it may be desirable that the sensor  418  inform an operator that the accumulator system  2  may need to be served before losing operational performance of the accumulator system  2 . 
   The causing of the sleeve  308  to undesirably bear load is militated against by use of the gas-mediated, differential pressure relief valve  12 , depicted in  FIGS. 4A to 4C . Under normal operations, the valve piston  400 , with substantially equal area on both faces thereof, for example, would be displaced all the way to the left of the cylinder body  402 , thereby blocking the drain relief port  414 . The valve piston  400  is held in this position by a combination of the gas pressure, which is supplied to the cylinder body  400  by the valve gas port  412  and by the biasing means  416 . The biasing means  416  is sized such, and has a sufficient spring constant, that when the difference between gas and fluid pressure, which enters the cylinder body  402  by the fluid port  410 , exceeds a predetermined value, the drain relief port  414  is opened. Excess fluid, such as oil, is thereby allowed to return to the reservoir tank  16 . 
   The performance of the accumulator system  2  can also be assessed using a thermodynamic model. By modeling the accumulator system  2 , such as the one depicted in  FIG. 1 , energy stored can be calculated as a function of the gas-side  310  volume, the fluid-side  312  volume, and a minimum operating pressure for the accumulator system  2 . Further manipulation of the model may yield useful results, such as one manipulation that yields maximum energy storage as a function of the minimum pressure, maximum pressure and fluid volume of the accumulator system  2 . The model may be useful for the design engineer whom, guided by common practice and application needs, typically has a priori knowledge of the minimum pressure, maximum pressure, and fluid volume values. A final set of manipulation may further result in identification of a desired ratio of minimum and maximum pressure that yields optimal performance of the accumulator system  2 . When the desired ratio is substituted into the model, it is found that to optimize stored energy, which is the goal of the design engineer of an hydraulic hybrid vehicle, the gas-side volume should be greater than the fluid-side volume. 
   There are at least two ways to achieve an optimal design for the accumulator system  2 . A first method requires increasing a diameter of the shell  300  with respect to the sleeve  308 , thereby creating a larger volume within the interstitial space  314 . It should be appreciated that the approach of the first method causes the accumulator cylinder  4  to become larger and requires a thicker over-wrap  4  of the shell  300 . A second method employs both the interstitial space  314  and at least one auxiliary cylinder  8 . The approach of the second method provides greater design flexibility as the ratio of gas-side  310  to fluid-side  312  can be more easily varied. In addition, since the gas manifold  204  can be comprised of multiple connected sections, packaging the accumulator system  2  into a vehicle, for example, is facilitated with the approach of the second method. 
   While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes may be made without departing from the scope of the disclosure, which is further described in the following appended claims.