Abstract:
A lightweight, low permeation, piston-in-sleeve high pressure accumulator is provided. The accumulator includes a cylindrical composite pressure vessel with two integral rounded ends. A piston slidably disposed in a thin nonpermeable internal sleeve in the accumulator separates two chambers, one adapted for containing a working fluid and the other adapted for containing gas under pressure. Working fluid is provided in a volume between the nonpermeable internal sleeve and the composite pressure vessel wall. Further means 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. A method for pre-charging the device is also presented.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority from U.S. Provisional Application No. 60/551,271, of the same name, filed on Mar. 8, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention 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. This invention further relates to the potential use of such accumulators in conjunction with fuel efficient hydraulic hybrid motor vehicles.  
         [0004]     2. Description of the Related Art  
         [0005]     1. Hydraulic Hybrid Vehicles  
         [0006]     Hybrid powertrains are an increasingly popular approach to improving the fuel utilization of motor vehicles. “Hybrid” refers to the combination of one or more conventional internal combustion engines with a secondary power system. The secondary power system 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 frees the internal combustion engine to operate more efficiently, while the secondary power system acts in concert with it to make sure that enough power is available to meet road load demands and any excess power is stored for later use.  
         [0007]     Several forms of secondary power systems are known in the art. For example, various electrical systems, such as batteries and electric motor/generators, are becoming popular in commercial applications. Recently, however, hydraulic power systems have been demonstrated to have comparable or even better efficiency, while offering potential advantages in power density, cost, and service life. 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 that energy may be stored by compressing a gas. An accumulator&#39;s pressure vessel contains a captive charge of gas, typically nitrogen, which becomes compressed as a hydraulic pump pumps liquid into the vessel. The liquid thereby becomes pressurized and when released may be used to drive a hydraulic motor. A hydraulic accumulator thus utilizes two distinct working media, one a compressible gas and the other a relatively incompressible liquid. Throughout this document, the term “gas” shall refer to the gaseous medium and the term “fluid” shall refer to the liquid working medium, as is customary in the art.  
         [0008]     U.S. Pat. No. 5,495,912 (“Hybrid Powertrain Vehicle”) and U.S. Pat. No. 6,719,080 (“Hydraulic Hybrid Vehicle”) provide additional context on the role of hydraulic accumulators in hydraulic hybrid powertrains, as well as detailed descriptions on the preferred series and parallel hydraulic hybrid motor vehicle configurations for use with the accumulator of the present invention, and are therefore incorporated herein by reference in their entirety.  
         [0009]     2. High Pressure Accumulator Design Considerations—Permeation  
         [0010]     Care must be taken in the design of a high pressure accumulator to minimize the extent to which the gaseous medium mixes with and dissolves within the hydraulic fluid. The dissolution of gas molecules in hydraulic fluid may cause several significant problems, particularly in a hydraulic hybrid vehicle application. For example, when highly pressurized fluid is discharged from the accumulator to drive a hydraulic motor, the fluid pressure will drop rapidly and dramatically (e.g., from 5000 psi to 100 psi in less than one second) as the fluid flows through the motor. Such a pressure drop causes any dissolved gas present in the fluid to immediately come out of solution and form small bubbles or gas pockets. As the depressurized fluid is discharged from the motor, the gas travels with it, generally into a low pressure accumulator (or reservoir) where the fluid is stored until needed again. A significant quantity of gas may thus over time become trapped in the low pressure accumulator. Although it is possible to develop means to vent the gas after it collects at the fluid surface in the low pressure accumulator, the loss of this gas would nevertheless also represent a gradual depletion of the gas pre-charge of the high pressure accumulator and thus would lead to the need for occasional gas recharging (which would not be a desirable result for consumers, particularly for use in a motor vehicle application). In addition, such gas in the low pressure accumulator may also become entrained as bubbles or gas pockets in fluid that is later pumped back out, causing the pump to experience potentially damaging effects such as cavitation or torque fluctuations. Further, the accumulating volume of gas reduces the effective fluid capacity of the low pressure accumulator.  
         [0011]     Systems that operate at very high pressures are particularly vulnerable to all of these difficulties because high pressures encourage greater dissolution of gas in the fluid, and the greater degree of pressure drop increases the rate at which gas comes out of solution.  
         [0012]     Prior art high pressure accumulator designs therefore go to great lengths to minimize gas dissolution and ensure physical separation of the charge gas from the fluid. Several separation means are known, including the use of an elastic bladder or diaphragm, a flexible bellows, or a piston in a cylinder. However, thus far prior art separation means do not minimize gas dissolution to the degree that would be preferred in a commercially acceptable hydraulic hybrid vehicle. Such an application calls for a high pressure system that is preferably “closed,” that is, capable of operating indefinitely without frequent adjustment of gas or fluid levels in the accumulator.  
         [0013]     3. High Pressure Accumulator Designs in the Present State of the Art  
         [0014]     In the present state of the art, most commonly available accumulators employ an elastic bladder. Although the pressure in the hydraulic fluid on one side of the bladder is generally the same as the pressure of the compressed gas on the other side of the bladder, molecules of gas tend to permeate through the bladder and dissolve in the fluid, seeking an equilibrium concentration. Some elastic bladder materials have properties that minimize permeation, but due to the molecular nature of elastomers, permeation cannot be eliminated completely. In addition, permeation resistant, flexible coatings such as poly-vinyl alcohol can be, used on the gas side of the bladder, but even with such coatings the permeation level is still unacceptable at high pressures.  
         [0015]     Some success has been achieved. by replacing the elastic bladder with a flexible metallic or metallic-coated bellows structure. For example, the inventions disclosed in U.S. Pat. No. 5,771,936 granted to Sasaki et al. (1998) and U.S. Pat. No. 6,478,051 to Drumm et al. (2002) depict such a bellows. However, a principal shortcoming of this approach lies in the potential for the bellows to experience stresses and longitudinal disorientation that would soon lead to failure under a severe duty cycle, such as would be present in an automotive power system application.  
         [0016]     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 increases 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). Furthermore, the need to assemble the cylinder with a piston inside traditionally requires that the cylinder have at least one removable end cap for use in assembly and repair, rather than the integral rounded ends that are more structurally desirable in efficiently meeting pressure containment demands. Composite pressure vessels are not constructed effectively with removable end caps.  
         [0017]     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.  
         [0018]     4. Prior Art Regarding Piston-in-Sleeve Accumulator Designs  
         [0019]     One piston accumulator concept, considered, for example, for military aircraft applications, 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. 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 (aka “interstitial”) volume between the sleeve and vessel wall which may be filled with the charge gas to provide a safety factor against explosion in the event of puncture of the pressure vessel (e.g., by a bullet in military combat). U.S. Pat. No. 2,417,873 (Huber 1947), U.S. Pat. No. 2,703,108 (McQuistion 1955), U.S. Pat. Re24,223 (Ford 1956), and later U.S. Pat. No. 4,714,094 (Tovagliaro 1987) each teach the use of a piston and sleeve assembly on a high pressure accumulator. Such designs comprise a generally thickwalled 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.  
         [0020]     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.  
         [0021]     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 if a bullet or shrapnel were to enter the fluid side during military combat. With the charge gas residing in the interstitial space at the cylindrical periphery of the vessel, the displacement caused by the bullet would be largely absorbed by compression of the gas in this space. 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.  
         [0022]     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 (Tovagliaro 1987) 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.  
         [0023]     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.  
         [0024]     5. Disadvantages of the Prior Art  
         [0025]     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 lightweight, low cost, durable under stresses, and does not have permeation difficulties at high fluid pressures. 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  
       [0026]     The object of the present invention is to provide a high pressure piston and sleeve accumulator design that is lightweight, adaptable to service in mobile vehicles, and can be mass produced at low cost, while maintaining or exceeding the superior low gas permeability properties of state of the art prior art piston accumulators.  
         [0027]     The present invention utilizes a piston and sleeve design, but through various means enables use of a thoroughly lightweight and easy to manufacture composite pressure vessel therewith, thereby providing a lightweight low permeation 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 placing working fluid instead of charge gas between the sleeve and vessel wall, assembling and using the piston-in-sleeve device with domed vessel ends and without the aid of removable end caps, using fatigue-resistant plastic internal liner material, and/or providing means to withstand harmful effects of radial flexing of the composite vessel wall at high pressures. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]      FIG. 1  shows a sectional view of a preferred embodiment of the invention.  
         [0029]      FIG. 2  depicts an alternate embodiment of the invention, in which the piston has an oil- or grease-filled double seal that is pressure balanced by means of a pressure balancing channel filled with grease.  
         [0030]      FIG. 3  depicts another embodiment of the invention, in which the double seal is pressure balanced by a channel that contains an inline free piston instead of grease.  
         [0031]      FIG. 4  shows an alternative piston embodiment for the present invention, with both bearings on the same (oil) side of the piston seal.  
         [0032]      FIG. 5  presents an alternative embodiment for the piston sleeve of the present invention, utilizing S-shaped transitions through areas of stress on the piston sleeve. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     With reference to  FIG. 1 , depicting a preferred embodiment of the invention, a lightweight composite cylindrical outer pressure vessel  10  with rounded ends is presented. Suitable materials for vessel  10  comprise carbon fiber wrap, E-glass, or one of many other strong and lightweight materials, such as may be found for high pressure bladder accumulators of the prior art. Suitable materials for vessel  10  may include materials that are gas-permeable at high pressures (e.g., 5000 psi). Vessel  10  is preferably lined with a thin liner  12  made of fatigue-resistant plastic, but may also be made of HDPE or other suitable material, as will also be understood in the art. Metal end bosses  12   a  and  12   b  reside at the ends of the vessel  10  to provide access to the interior of the vessel and are preferably embedded within liner  12 , if liner  12  is provided.  
         [0034]     Non-permeable sleeve unit  13  resides within vessel  10  (and liner  12  if provided), and is thin relative to the wall of pressure vessel  10 . Sleeve unit  13  is preferably welded to metal end boss  12   a  by means of a weld joint such as that depicted in the position of weld  15 , or at a similar location such as at other points on the interior of metal end boss  12   a . Other joining means (for example, a threaded connection with an appropriate sealing means) may alternatively be employed.  
         [0035]     Charge gas port  23  communicates with inner working medium chamber  24 . Hydraulic fluid port  26  communicates with outer working medium chamber  25 , which includes interstitial volume  16  between sleeve  13  and liner  12  (or if no liner, between sleeve  13  and cylinder wall  10 ). Shutoff valve  27  resides in port  26  and acts to close port  26  as the fluid volume approaches zero. Piston  14  is slidably contained within sleeve  13 . The inner working medium chamber  24  formed by piston  14  and sleeve  13  is filled with charge gas at a pressure typical of the art. Chamber  24  may also contain foam to avoid heat increase in chamber  24  as the charge gas is compressed, as will be understood in the art. The addition of foam in chamber  24  may also be utilized to provide structural support for sleeve  13  if desired. Outer chamber  25  is filled with-hydraulic fluid.  
         [0036]     As is known in the art, as hydraulic fluid enters and exits via port  26 , piston  14  will move longitudinally within sleeve  13  in reaction to forces resulting from the balancing of pressure between the gas in chamber  24  and the fluid in chamber  25 . Charge gas is prevented from contacting the fluid by means of piston seal  19 . Slider bearings  31  and  32  preferably encircle piston  14  and act to facilitate its longitudinal movement within sleeve  13 .  
         [0037]     Flexing of the composite pressure vessel causes stress on the area of the sleeve near its attachment to the pressure vessel. Using gas within the closed chamber  24  within the sleeve  13 , instead of fluid as in prior art piston/sleeve accumulators, reduces this stress because of the lighter weight of gas than fluid. Sleeve  13  of the present invention additionally preferably contains a finite element analysis (FEA) engineered fatigue resistant, flexing end-dome section between the cylindrical portion of sleeve  13  and its attachment to metal end boss  12   a , to assure fatigue resistance associated with sleeve radial movement in response to vibrations (e.g., in vehicle travel). The preferred example of such an end-dome uses classic “S shape” transitions  35  through areas of high stress for the sleeve, as shown in  FIG. 5  and as will be known and understood in the art.  
         [0038]     A preferred method to prepare the accumulator for operation begins by introducing fluid working medium into chamber  25  through fluid port  26  so as to cause interstitial space  16  and chamber  25  (which may be larger or smaller than depicted depending on the position of piston  14 ) to fill entirely with fluid to the exclusion of any residual gases that may be present from manufacturing and assembly. A charge gas such as nitrogen is then introduced through gas charge port  23  at a designated pre-charge pressure, perhaps for example 1000 psi. The pressure of the initial gas charge will cause piston  14  to move longitudinally toward the opposite end of the vessel, expelling fluid from chamber  25  as the piston sweeps through it. Valve bumper  29  will eventually exert pressure on shutoff valve stem  27  causing fluid port  26  to close and fluid to cease exiting. Fluid will continue to be present in interstitial space  16  and represents a volume of non-working fluid that will always be present in this space. To retain the charge gas, charge port  23  is sealed by conventional gas valve means as is known in the art. In this manner the accumulator is brought to its proper pre-charge pressure. To store energy in the accumulator, fluid is pumped into chamber  25  through valve port  26  by a hydraulic pump/motor or other means as is known in the art. Also as known in the art, this causes charge gas in chamber  24  to become compressed as fluid causes piston  14  to move into it.  
         [0039]     As pressure inside the accumulator increases to very high levels in operation, perhaps 5000 psi, 7000 psi, or more, the pressure vessel  10  will exhibit a natural tendency to expand slightly in diameter, particularly near its center. This causes the interstitial volume  16  between pressure vessel/liner wall  12  and sleeve  13  to increase. In response, fluid will flow into interstitial space  16  causing pressure to balance across sleeve  13 . Because pressure is balanced among interstitial space  16 , gas chamber  24 , and fluid chamber  25 , little if any force is exerted on sleeve  13 . Also, because sleeve  13  is not radially connected to pressure vessel wall  10  or liner  12 , and the fluid in the interstitial space is relatively incompressible (as opposed to a compressible gas), the vessel&#39;s radial expansion does not exert any distortive force on sleeve  13  or result in any change of position of sleeve  13 . In this way sleeve  13  maintains a good seal with piston  14  despite the vessel&#39;s moderate radial expansion under pressure.  
         [0040]     Utilizing oil instead of gas in the interstitial space  16  outside of the sleeve  13  in the present invention avoids gas loss problems that would otherwise occur from an attempted use of composite pressure vessel  10  in a high pressure accumulator of this type, as the gas is thus contained solely within  24  surrounded by non-permeable sleeve  13  and piston  14 . Fluid (unlike gas) will not permeate from interstitial space  16  through composite pressure vessel  10  at high pressures, thus also eliminating the necessity for a non-gas-permeable metallic barrier between space  16  and composite vessel  10 . Maintaining gas on the inside of sleeve  13  away from vessel wall  10  also reduces heat transfer from the compressed gas to outside of the vessel, therefore providing some reduction in overall energy loss for the system.  
         [0041]     As an additional embodiment, supposing that sleeve  13  is made of a deformable material such as (for example) a semi-rigid plastic, piston  14  may take on the additional role of shaping the cross sectional area of sleeve  13  so as to cause it to conform to the shape of piston  14 , thus ensuring a good seal regardless of any local deformations in the sleeve at points not in contact with the piston.  
         [0042]      FIG. 2  depicts an alternative embodiment of the piston used with the invention, employing a dual seal design for the piston. Piston  14  is encircled by seals  19  and  20 , which are separated by an oil- or grease-filled space  21  that also encircles the piston. The dual seals provide additional anti-permeation sealing above that of the single seal of the previous embodiment. Because the oil in oil-filled space  21  must be pressure balanced with the other working media in order not to exert pressure on the sealing interface, pressure balancing passage  22  is provided to allow pressure communication between space  21  and fluid chamber  25 . A non-flowing but pressure transmitting viscous medium such as (for example) a highly viscous grease is interposed in passage  22  in order that space  21  and chamber  25  do not physically communicate, but only reach a pressure balance via the interposed column of viscous medium.  
         [0043]      FIG. 3  depicts yet another embodiment that employs a similar pressure balancing dual seal. In this embodiment, pressure balancing piston  30  is interposed in passage  22  instead of the viscous medium means employed in the previous embodiment, and similarly acts to prevent physical contact between media in space  21  and chamber  25  while providing for pressure balancing between the two.  
         [0044]      FIG. 4  depicts a third embodiment of the piston for use with the present invention. In this embodiment, bearings  31  and  32  are both located on the same side of the piston seal  19 , preferably toward the fluid (oil) chamber&#39;s side of piston seal  19 . By placing both bearings on the oil side of piston seal  19 , both bearings will stay fully lubricated by the fluid (oil) from chamber  25  in operation, and therefore may reduce wear and debris formation that can otherwise result over time with use of a dry bearing.  
         [0045]     The arrangements for the present invention set forth above provide several advantages over the prior art as discussed above, and as additionally explained below.  
         [0046]     Because relatively incompressible fluid rather than gas now resides in the interstitial space  16  between the sleeve  13  and the vessel wall  10  in the present invention, the forces exerted on the sleeve  13  as fluid flows around it in this restricted space will be smaller and more uniform. In the prior art, due to the compressibility of gas relative to liquid, having gas reside on the outside of the sleeve increased the forces likely to act on the piston and sleeve assembly as gas flows through the interstitial space around the sleeve in response to rapid charging and discharging of the device, again leading to the need to make the sleeve thicker and more rigid than would otherwise be required for piston sealing only. This is avoided in the present invention: a smaller and more uniform volumetric flow is generated in such conditions. This reduces the required rigidity of the sleeve, allowing consideration of previously unsuitable materials for the sleeve, such as moldable polymers that are lower in cost, easier to manufacture, or provide superior sealing properties over the metal sleeves existing in the prior art.  
         [0047]     The reduced stress acting on the sleeve  13  also alleviates the problem of retaining and centering the sleeve  13  within the vessel  10 . Previous designs rigidly attach the fixed inner end of the cylindrical sleeve to the inner surface of one end of the vessel, thereby forming a cantilever-style connection with the end surface. This was done to provide sufficient retention and centering for the sleeve, and also causes the surface of the inner end of the vessel to serve as one wall of the chamber that will contain working medium within the sleeve. Because the present invention reduces stress on the sleeve, such a rigid connection is no longer necessary. Instead the sleeve  13  may be formed integrally with its own chamber end dome section and also integrally with the gas port  23  that supplies the chamber  24 . This enables the sleeve  13  to be fixed within the vessel by inserting the sleeve unit within the socket for gas port  23 , and reduces manufacturing effort and cost. Because the integral sleeve unit  13  does not need to be physically joined with an inner end surface of the vessel, it also helps enable use of vessels with a thoroughly composite construction. The effect of unit vibration on the fixed end of the sleeve  13  may also be reduced by allowing the thinner necked end portion of the sleeve unit to flex somewhat, thereby accommodating these forces without causing fatigue to a rigid joint.  
         [0048]     In the present invention, the aforementioned reduced stresses on the sleeve also obviate the need to involve the pressure vessel walls  10  in the function of centering or reinforcing the sleeve  13 . The use of composite materials for the pressure vessel construction now becomes more practical, because effective sleeve centering and retention is no longer dependent on resisting pressure-related changes in the vessel diameter.  
         [0049]     These developments lead to additional advantages in design and fabrication. For example, the inner sleeve  13  may be very thin, because it need not support any substantial differential pressure, and thus may be inexpensively manufactured. The sleeve  13  may also be constructed of deformable material that is locally retained in a circular shape for sealing by a circular piston  14  as it moves longitudinally within the sleeve.  
         [0050]     Additionally, the inner sleeve  13  may be fit relatively tightly into the outer pressure vessel  10  to maximize the potential storage space for the compressed gas and working volume for the accumulator size. Use of a thin sleeve fitting tightly within the pressure vessel (i.e., with minimum slip clearance between the sleeve and the pressure vessel&#39;s inside wall) at atmospheric assembly pressure is not done in the prior art. This is because gas is located on the outside of the sleeve in prior art, and a rapid discharge of oil from the interior of the sleeve would rapidly drop the accumulator pressure, and as the compressed gas in the annulus between the sleeve and pressure vessel wall dropped in pressure and expanded (i.e., flowed from the annulus), a significant pressure drop would develop from the sealed end of the annulus to the open end causing pressure loading on the sleeve. A larger separation with spacers is instead used in prior art, which allows the expanding gas to flow with minimum restriction. Moving the oil to the outside of the sleeve  13  as in the present invention avoids the need for spacing and spacers since oil is essentially incompressible and little flow from the annulus would occur, thus enabling a tight clearance and greater utilization of the inside volume of the pressure vessel.  
         [0051]     As can be seen from the foregoing, various advantages may result from the present invention. For example, the invention is adaptable to service in mobile vehicles, is lightweight, and can be mass produced at low cost, while maintaining the low gas permeability properties of prior art piston accumulators. While particularly useful for high pressure accumulators for the reasons as discussed above, it will also be understood that the device of the present invention may be used for other purposes as well, including, for example, as a lower pressure accumulator for a wide variety of applications.  
         [0052]     From the foregoing it will also be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.