Abstract:
A flywheel assembly for storing energy and rotatable in response to the rotation of a shaft includes a hub having a transition portion having a constant stress intermediate region operatively coupled to the shaft and an outer connecting portion forming a flexible cylinder, wherein the shaft and the flexible cylinder are substantially coaxial, and an outer cylinder wherein a majority of the mass of the flywheel assembly is concentrated. The flexible cylinder includes connecting pads disposed at opposing edges of the outer side of the flexible cylinder thereby permitting connection to the outer cylinder. The outer cylinder increases radially responsive to a corresponding increase in rotational speed of the flywheel assembly, while the diameter of the transition portion of the hub follows the radial increase of the outer cylinder.

Description:
This is a Continuation of Ser. No. 08/637,649 (PCT/US94/11809), now U.S. Pat. No. 5,767,595, which was filed on Apr. 30, 1996, which, in turn, is a combined Continuation of Ser. No. 08/148,361, now U.S. Pat. No. 5,559,381, which was filed on Nov. 8, 1993, and entitled “FLYWHEEL SUPPORT SYSTEM FOR MOBILE ENERGY STORAGE,” Ser. No. 08/242,647, now U.S. Pat. No. 5,628,232, which was filed on May 13, 1994, and entitled “FLYWHEEL ROTOR WITH CONICAL HUB AND METHODS OF MANUFACTURE THEREFOR,” which is a Continuation-in-Part of application Ser. No. 08/181,038 now U.S. Pat. No. 5,566,588, filed Jan. 14, 1994, also entitled “FLYWHEEL ROTOR WITH CONICAL HUB AND METHODS OF MANUFACTURE THEREFOR,” and Ser. No. 08/199,897, which was filed on Feb. 22, 1994, and entitled “FLYWHEEL ENERGY STORAGE SYSTEM WITH INTEGRAL MOLECULAR PUMP.” 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a flywheel energy storage device. More specifically, the present invention is related to a flywheel-motor-generator combination providing surge power, dynamic braking, and energy storage for a hybrid electric motor vehicle. The present invention is particularly advantageous when adapted for use in a hybrid electric motor vehicle. 
     One aspect of the present invention relates to the maintenance of a vacuum within the space occupied by a high speed flywheel rotor. More specifically, the use of a molecular pump incorporated into the flywheel assembly of a flywheel energy storage system to pump gases from a rotor environment into a separate chamber is disclosed. The separate chamber advantageously can contain molecular sieves for adsorbing gas molecules given off by the rotor. 
     BACKGROUND OF THE INVENTION 
     The manufacture of electric vehicles powered by chemical batteries is being encouraged by air quality control agencies in an effort to reduce the air pollution created by the internal combustion engines in current use. Even though the electric power utilities which supply the energy used to charge the batteries are themselves polluters, the net result is favorable with respect to air quality. However, the relatively poor characteristics of chemical batteries, in terms of weight, cycle life, and cost make it difficult for them to compete in the marketplace with internal-combustion engines as the power system of choice. 
     A hybrid electric power train, consisting of a turbogenerator which generates the average power consumed by the vehicle, a flywheel surge power generator, an electric traction motor, and an electronic power control system can achieve the low pollution levels needed for good air quality, but with performance characteristics which exceed those of the internal combustion engine. Even though the turbine burns hydrocarbon fuels, its use of a catalytic combustor results in less air pollution than that created by the utilities which provide the electricity needed to charge the chemical batteries in vehicles so powered. The separation of the power sources into elements separately optimized to supply the average and the peak power, respectively, coupled with the ability to use dynamic braking, causes the efficiency over most driving schedules to be enhanced and, thus, less fuel is consumed. 
     A description of a turbogenerator suitable for use in a hybrid electric vehicle is given in a paper by Robin Mackay for the SAE International Congress and Exposition, March, 1994, entitled “Development of a 24-kW Gas Turbine Generator Set for Hybrid Vehicles,” which paper is incorporated herein by reference for all purposes. Many different types of electric motors have been used for traction of electrically propelled vehicles for over a century. The present disclosure relates to the design of the flywheel energy storage system. The electric power control system, the fourth major element of the electric power train, is described in a U.S. Pat. No. 5,568,023, which is entitled “ELECTRIC POWER TRAIN CONTROL” and which is incorporated herein for all purposes. 
     Modem high strength-to-weight ratio fibers make it possible to construct high energy density flywheels, which, when combined with a high power motor-generators, are an attractive alternative to electrochemical batteries for use as energy buffers in hybrid electric vehicles. A properly designed flywheel system would provide higher energy density, higher power density, higher efficiency, and longer life than a conventional electrochemical battery. 
     The vehicle environment, however, presents special challenges to successful implementation of a flywheel to motor vehicle applications. Among these challenges are the need to deal with the gyroscopic torques resulting from the vehicle&#39;s angular motions and the need to accommodate the translational accelerations of the vehicle. Several safety issues resulting from the high energy and momentum stored in the flywheel also need to be taken into account, as does the difficulty of cooling the motor-generator operating in a vacuum chamber. In addition, energy conservation considerations and user convenience dictate the requirement that the flywheel storage system possess a slow self-discharge rate. 
     Flywheel energy storage systems have been proposed for many years; many of the storage systems have even been proposed for use in motor vehicles. U.S. Pat. No. 3,741,034, for example, discloses a flywheel contained in an evacuated sphere which is surrounded by a liquid and having various safety features. However, the &#39;034 patent does not address waste heat production and the requirement for cooling the motor-generator. In addition, the &#39;034 patent does not address itself to the dynamics of the driving environment, or the minimization of the power drain when parked. U.S. Pat. Nos. 4,266,442, 4,285,251 and 4,860,611, on the other hand, disclose different ways of constructing high speed rotors. However, the above referenced patents do not recognize, let alone describe, design features needed for compatibility with the environment of a motor vehicle. 
     Moreover, in order to accommodate a rim speed of about about 1000 meters per second, a housing containing the flywheel should be maintained at a very low pressure, e.g., a pressure below 0.01 Pascal, to limit windage losses. While this pressure can be readily achieved before sealing the housing, the fiber composite materials used in the construction of high energy density flywheels have a residual gas evolution rate which make it difficult to achieve this desired degree of pressure, i.e., near vacuum conditions, in a sealed container. Thus, continuous pumping of the evolving gases from the container is often needed. Most often, an external pump is employed to maintain the desired pressure. 
     U.S. Pat. Nos. 4,023,920, 4,732,529 and 4,826,393 describe various implementations of molecular pumps, which are a class of high vacuum pump wherein the dimensions of the critical elements are comparable to the mean free path of the gas molecules at the pressure of interest. Two types are generally known, a turbo-molecular pump, which is similar in construction to an axial flow compressor in a gas turbine employing interleaved rotor and stator blades, and a molecular drag pump, which uses helical grooves cut in the stator, which, in turn, is disposed in close proximity to a high speed rotor so as to direct gas flow through the pump. It will be appreciated that hybrid molecular pumps, which pumps contains separate sections of each of these types or molecular pumps, are also known. More specifically, U.S. Pat. No. 4,023,920 discloses a turbo-molecular pump using magnetic bearings to support the pump rotor at high rotational speeds. U.S. Pat. Nos. 4,732,529 and 4,826,393 disclose hybrid molecular pumps in which a turbo-molecular section is used on the high vacuum input side and a spiral groove drag pump is used on the discharge side. 
     All of these pumps are designed as self-contained systems, each with its own shaft, bearing system and power source, i.e., motor. While this solution is satisfactory for stationary systems, it is more difficult to apply in mobile applications because the space and weight for its implementation is not readily available. 
     As discussed above, flywheel systems currently being designed for mobile energy storage are generally intended to replace batteries in electrically powered vehicles. In such applications, multiple units are needed to store the required energy, so that each motor-generator need supply only a small portion of the vehicle&#39;s power. In systems where all of the surge power must be supplied by a single flywheel, the relatively large size of the single motor-generator makes it difficult to provide the needed energy density without reducing safety factors, e.g., for radial stresses, to unacceptable low levels or raising manufacturing costs to exorbitantly high levels. 
     The above-mentioned U.S. Pat. No. 3,741,034 discloses rotor designs using high strength-to-weight ratio filament wound composites in relatively thin concentric cylinders, which cylinders are separated by radial springs. While this arrangement limits the radial stresses to tolerable values, it is expensive to manufacture. U.S. Patent No. 3,859,868 discloses techniques for varying the elasticity-density ratio of the rotor elements to minimize radial stresses. On the other hand, U.S. Pat. Nos. 4,341,001 and 4,821,599 describe the use of curved metallic hubs to connect the energy storage elements to the axle. Additionally, U.S. Pat. No. 5,124,605 discloses a flywheel system employing counter-rotating flywheels, each of which includes a hub, a rim and a plurality of tubular assemblies disposed parallel to the hub axis for connecting the hub to the rim while allowing for differential radial expansion between the hub and the rim. 
     None of the latter references deal with the integration of a large, high power motor-generator into the flywheel energy storage system currently being designed for vehicles. 
     The present invention was, thus, motivated by a desire to provide an improved flywheel-motor-generator energy storage system suitable for moving vehicles. More specifically, the present invention was motivated by a desire to correct the perceived weaknesses and identified problems associated with conventional flywheel energy storage systems. 
     SUMMARY OF THE INVENTION 
     The principal purpose of the present invention is to provide a flywheel energy storage system that is optimized for the motor vehicle environment. According to one aspect of the invention, the flywheel energy storage system provides substantial surge power needed to accommodate transient load requirements associated with the automobile. 
     An object to the present invention is to provide isolation for the flywheel from the vehicle&#39;s angular motions. 
     Another object of the present invention is to provide support for the rotor during omni-directional accelerations, while maintaining small radial gaps between the spinning and stationary elements. 
     Yet another object of the present invention is to provide an efficient and compact cooling system for a high-power motor-generator. 
     Another object of the present invention is to provide protection for the vehicle in which it is contained from accidental release of stored energy and angular momentum. 
     Still another object of the present invention is to provide an energy storage device having a slow self-discharge rate. 
     A further object of the present invention is to provide a system located within a sealed chamber for maintaining pressure below a predetermined threshold. 
     Another object of the present invention is to provide a pressure regulating system for a flywheel energy storage system disposed within a sealed housing wherein a shaft of the flywheel drives a pump for moving gas molecules from a first chamber to a second chamber within the housing. 
     Yet another object of the present invention is to provide a pressure regulating system for a flywheel energy storage system disposed within a sealed housing wherein bearings supporting a shaft of a flywheel supports rotating elements of a pump moving gas molecules from a first chamber to a second chamber within the housing. 
     Still another object of the present invention is to provide a pressure regulating system for a flywheel energy storage system disposed within a sealed housing wherein a pump for moving gas molecules from a first chamber to a second chamber within the housing is provided at a low incremental cost. 
     An additional object of the present invention is to provide a pressure regulating system for a flywheel energy storage system disposed within a sealed housing wherein the pressure is maintained by adsorbing gas molecules moving from a first chamber to a second chamber within the housing on a molecular sieve. 
     Still another object of the present invention is to provide a high energy density rotor. 
     Another object according the present invention is to provide a high energy density rotor which includes ample space within its volume for a large, relatively high power motor-generator. 
     Still another object according the present invention is to provide a high energy density rotor which can be easily manufactured. 
     Yet another object according the present invention is to provide a high energy density rotor which can be manufactured at a reasonable cost. 
     These and other objects, features and advantages of the present invention are accomplished by a flywheel energy storage system including a fiber composite energy-storing rotor, a high-powered, liquid-cooled motor-generator supported by ball bearings in an evacuated sphere, which sphere floats in a liquid contained in an outer spherical housing. The energy storage system includes a flywheel-motor-generator assembly having a low center of mass location with respect to the evacuated sphere so as to provide a vertical orientation of the flywheel-motor-generator along a rotor axis. 
     These and other objects, features and advantages according to the present invention are provided by an integral flywheel energy storage system combining a molecular pump into a flywheel energy storage system for vacuum control purposes. The integral flywheel energy storage system includes a sealed housing, a baffle including an orifice dividing the housing into a low pressure first chamber and a relatively high pressure second chamber, a shaft suspended between first bearings located in the first chamber and second bearing in the second chamber, the shaft being disposed within the orifice, a flywheel disposed within the first chamber spinning at high speed, and a molecular pump operatively connected for driving by the shaft for pumping gas molecules from the first chamber to the second chamber. It will be appreciated that other bearing arrangements for operatively supporting the shaft can be used without departing from the spirit and scope of the present invention. 
     According to one aspect of the invention, the molecular pump is designed into the flywheel assembly so as to permit the high speed motor, shaft, and bearing needed by the molecular pump to be supplied by components already present in the energy storage system. Preferably, the molecular pump transfers the gases evolving from the flywheel rotor and its environs into a separate chamber within the housing of the energy storage system, i.e., contained within the overall vacuum housing. This chamber advantageously may contain so-called molecular sieve materials designed to adsorb the most prevalent of the gases given off by the flywheel rotor. It will be appreciated that other getter materials may also be used throughout the vacuum housing to adsorb trace elements not adsorbed by the molecular sieves. 
     These and other objects, features and advantages according to the present invention are provided by a molecular pump disposed with a sealed housing of a flywheel energy storage system, wherein the shaft supporting the flywheel powers the molecular pump to maintain gas pressure in the vicinity of the flywheel rotor at or below a predetermined pressure producing negligible drag on the spinning flywheel. It will be appreciated that the molecular pump transfers gas molecules generated by the flywheel rotor material to a receiving chamber which advantageously contains so-called molecular sieves, which adsorb these gas molecules, thereby maintaining the pressure of the receiving chamber at a predetermined second pressure. 
     These and other objects, features and advantages according to the present invention are provided by a rotor including a generally cylindrical outer portion for storing most of the energy, and a hub portion attaching the outer portion to the shaft. In an exemplary case, the hub portion includes an engineered metallic disc member which can be attached to the outer cylindrical portion via an inner cylindrical member having a relatively short axial extent. 
     According to another aspect of the invention, the arrangement of rotor components provides the desired geometric properties in a readily manufacturable configuration. 
     These and other features and advantages of the present invention will become more apparent from the following detailed description, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments are described with reference to the drawings, in which like elements are denoted by like numbers, and in which: 
     FIG. 1 is a cutaway sketch of a hybrid electric vehicle showing respective elements of its power train; 
     FIG. 2 is a high-level block diagram illustrating the power control system of the vehicle shown in FIG. 1; 
     FIG. 3 is an illustration showing the general arrangement of a flywheel assembly according to the present invention; 
     FIG. 4 is a cross-sectional view taken perpendicular to the axis of the flywheel illustrated in FIG. 3, FIG. 4B is a sectional view of the disc member, which is included in FIG. 4A, is useful in understanding the construction and operation of the disc member, while FIG. 4C illustrates radial stress and FIG. 4D illustrates tangential stress in the disc member profiled in FIG. 4B; 
     FIG. 5 is a detailed illustration of the upper bearing assembly and its lubrication system of the flywheel illustrated in FIG. 3; 
     FIG. 6 is a detailed illustration which is useful in understanding the construction and operation of lower bearing system and the associated lubrication system for the flywheel illustrated in FIG. 3; 
     FIG. 7 illustrates the molecular drag pump used to maintain adequate vacuum in the chamber containing the flywheel rotor for the flywheel illustrated in FIG. 3; 
     FIG. 8 is a detailed illustration of an exemplary mechanical gimbal supporting the flywheel assembly shown in FIG. 3; 
     FIG. 9 is an exemplary illustration showing an external protective barrier and the external radiator; and 
     FIG.  10 A and FIG. 10B are illustrations which are useful in explaining the construction and operation of a squeeze film damper employed by the flywheel shown in FIG. 3 in the bearing of FIG.  6 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows the power train elements of a hybrid electric vehicle using a flywheel  1  as an energy buffer. In this configuration, the flywheel  1  provides surge power for accelerating the vehicle and for hill climbing, complementing the relatively low, steady power provided by a fuel-burning power source  3 , e.g., a turbogenerator set. The flywheel  1  is also used to absorb energy by storing it during dynamic braking and downhill driving. An electric motor  4  converts the electric power from either the flywheel  1  or power source  3  to mechanical motive power. Preferably, all of these elements are regulated by the electronic controller  2 . 
     FIG. 2 is high level a block diagram of a power control system showing how the electronic controller  2  regulates the vehicle&#39;s power flow in response to the driver&#39;s inputs, which inputs are supplied by the accelerator pedal  5  and the brake pedal  6 . Controller  2  channels power to the drive motor  4  from the turbogenerator  3  during cruise conditions and augments this power with power from flywheel  1  for accelerating or hill climbing. Controller  2  advantageously charges the flywheel  1  with power from the drive motor  4  which is acting as a generator during braking or downhill driving. Preferably, controller  2  maintains the speed of flywheel  1  within a predetermined range by charging it from power source  3  to avoid its lower limit or giving flywheel  1  a higher share of the driving load to thus avoid the flywheel&#39;s  1  upper limit. Controller  2  also channels power from the flywheel  1  to the power source  3  for starting. In FIG. 2, power leads are designated by solid lines and signal leads are designated by dashed lines. 
     FIG. 3 is a cross-sectional view of the entire flywheel assembly showing the general arrangement of its parts. An outer housing  8  surrounds the assembly and provides mechanical and electrical connections to the vehicle. The space between housing  8  and a vacuum housing  10  is filled with a liquid  9  in which the vacuum housing  10  floats. It will be noted that bearings  14  and  15  are part of the mechanical gimbal system  80 , which advantageously is provided between housings  8  and  10 . The gimbal system  80  is discussed in greater detail below while referring to FIG.  8 . 
     The rotating assembly  100  includes a metal shaft  18  and is supported by an upper bearing assembly  12  and a lower bearing assembly  16 . A squeeze film damper  145  operates in conjunction with the lower bearing assembly  16 . The rotating assembly  100  is powered by a motor-generator  17  including rotor  21   a  and a stator  21   b.    
     The stator  21   b  is in good thermal contact with the re-entrant portion  25  of the vacuum housing, i.e., a metal cylinder  20  perforated with axial holes  20   a,  which provide passageways for flow of the liquid  9 . Advantageously, alternate holes  20   a  can be used for upward and downward flow. All holes  20   a  are connected together in the top section of cylinder  25  but are separated at the bottom into respective inlet and outlet manifolds  25   a,    25   b.  Flow separator  10   a,  which advantageously has a small clearance with respect to outer housing  8 , causes the liquid which is pumped by an external pump  54  through an external radiator  55  to first flow bidirectionally past the stator  21   a,  removing its heat, and then through the annular space between the outer housing  8  and the vacuum housing  10 . It will be appreciated from FIG. 9 that radiator  55  can be a heat exchanger cooled by a dedicated fan  56 . It will also be appreciated from FIG. 3 that flow separator is positioned so as to permit fluid flow through member  25  at all but the severest angles of vehicle operation. Since periods during which the vehicle negotiates large angles are expected to be extremely short, minimal flow interruptions will not produce unacceptable temperature increases in motor-generator  17 . 
     Preferably, the relatively cool liquid  9  pumped from the radiator  55  enters the flywheel  1  via the inlet port  36  and exits the flywheel via outlet port  37  to return to the radiator  55  via pump  54 . 
     The fiber composite cylinder  11  of assembly  100  is connected to the shaft  18  by means of a metallic hub  22  and an optional axially short fiber composite cylinder  24 . Preferably, the metallic hub  22  is formed of aluminum, and, most preferably, the hub  22  is formed of titanium. It should be mentioned that any metal, metallic composite or compound having a substantially similar, i.e., similarly high, ultimate strength to modulus of elasticity ratio, can be used. The assembly  100  stores energy in the form of rotational kinetic energy, most of it in cylinder  11 . A toroidal magnet  23  advantageously can be provided to produce a lifting force equal to the weight of the rotating assembly  100 . 
     A molecular drag pump  26  pumps residual gases evolving from material in the low pressure compartment  28  into compartment  27 , which contains molecular sieves  27   a  to adsorb these gases. These compartments are separated by a metal disc  29 . 
     FIG. 4A is a sectional view taken perpendicular to the axis of rotation of the flywheel  1  shown in FIG. 3, showing a titanium hub  22  used to connect the shaft  18  to the cylinder  11  through the optional intermediate cylinder  24 . The hub  22 , which is shown in the cross-section in FIG. 4B, has an axial thickness which decreases with increasing radius in its main portion  22   a.  It will be noted that the main portion  22   a  accounts for the majority of the hub  22 . This shape advantageously provides a nearly constant stress at each point along the radius. It will be appreciated that this constant stress profile permits maximal radial growth in this respective portion of hub  22 . It will also be appreciated that the hub  22  can be either a single piece having various regions or a single piece fabricated from several discrete components. For example, the hub  22  advantageously can be assembled from a main portion piece and a cylindrical portion piece. Moreover, the main portion piece can itself be fabricated from several smaller piece to minimize wastage during the fabrication of the hub. 
     At an outermost portion  22   b  of the radius, the axial thickness increases abruptly to thereby form a radially thin outer cylindrical section  22   c.  It should be noted that this cylindrical section  22   c  includes terminating pads  22   d  and  22   e,  which advantageously can be bonded to the intermediate composite cylinder  24  shown in FIGS. 3,  4 A and  4 B. It will also be noted the cylindrical portion  22   c  flexes in response to variations in applied centrifugal force. It will be understood that the combination of the stretch of the main portion  22   a  with the flexibility of the cylindrical portion  22   c  permits pads  22   d,    22   e  to follow the radial growth of the cylinder  24  without overstressing any point of the hub  22 . 
     Preferably, rotating assembly  100 , which in an exemplary case is 12 inches in diameter, stores approximately 2 kilowatt-hours, i.e., 7,200,000 joules, of energy at a maximum rotational speed of about 6500 radians per second. It will be appreciated that this corresponds to a surface speed of about 1000 meters per second. It will be noted that this high speed dictates that the rotating assembly be enclosed in an evacuated container. Moreover, the high centrifugal accelerations require that the rotating assembly  100  be constructed primarily of high strength fiber composites, e.g., a filament wound in the circumferential direction. 
     Preferably, rotating assembly  100 , which is shown in detail in FIG. 3, includes two major elements, an outer, primarily cylindrical portion  11 , which in an exemplary case can be up to 12 inches long, and the metallic hub  22 . The optional inner composite cylinder  24  connects hub  22  with outer composite cylinder  11 ; alternatively, the hub  22  advantageously can be directly connected to the outer composite cylinder  11 . The outer composite cylinder  11 , which is shown in FIG. 3, consists of two regions, an outermost region  11   a,  which preferably is a filament wound composite using the highest strength graphite fiber available to sustain the centrifugal acceleration of one million G&#39;s, and an innermost region  11   b,  which is a filament wound fiber composite, whose combination of density and modulus of elasticity create a moderate compressive load on the outermost member  11   a.  This advantageously minimizes the radial tension in the outermost member  11   a.  The radial and tangential stresses achieved with this material are shown in FIGS. 4C and 4D, respectively, as discussed in greater detail below. 
     The highest strength graphite fiber, which is used in fabrication of outermost region  11   a , advantageously has a minimum tensile strength of about 924,000 lb/in 2  (924 kpsi) while the wound fiber used in the fabrication of composite cylinder  24  has a tensile strength of about 450 kpsi. The optional cylinder  24  advantageously can be manufactured using a material sold under the brand name “Spectra.” It should be noted that the moderate strength graphite fiber used in innermost cylinder region  11   b  has a minimum tensile strength of about 714 kpsi. High strength aluminum with a minimum tensile strength of about 75 kpsi advantageously can be used in the construction of hub  22 , as discussed in greater detail above. 
     The rotating assembly  100  advantageously can be fabricated as two separate pieces, the hub  22  and outer cylindrical portion including both optional cylinder  24  and cylinder  11 . These two pieces advantageously are then mated with an interference fit. It will be appreciated that the interference fit results in compression of the terminating pads  22   d,    22   e  in the direction of shaft  18 . 
     The fiber properties in cylinders  24  and  11  important for this application are tensile strength and modulus of elasticity. The radial stress in these cylinders, which extend from the inner radius of cylinder  24  of 3.7 inches to the outer radius of cylinder  11  of 6 inches, is shown in FIG. 4C to be less than 4000 pounds per square inch at the highest rotational speed, well within the capability of the epoxy matrix material. The matrix material alone bears this stress, since the fiber, being circumferentially wound, makes no contribution to the radial strength. The gradation of the modulus of elasticity of the fibers from 24 million psi in optional cylinder  24  to 33 million psi for the inner portion of cylinder  11   b  to 43 million psi for the outer portion of cylinder  11   a  accounts for the shape of the radial stress curve and its desirably low maximum value. 
     The hoop stresses in the cylinders are shown in FIG.  4 D. They are seen to be a maximum of 100,000 psi in optional cylinder  24  and 200,000 psi in cylinder  11 . These stresses are borne by the fibers, and are well below the ultimate capabilities of the materials employed. The fiber used in optional cylinder  24  has an ultimate tensile strength of 435,000 psi, which is reduced by the fill factor of two thirds in the composite to 290,000 psi. The fiber in the inner portion of cylinder  11  has a reduced ultimate strength of 476,000 psi, and the fiber in the outer portion has a reduced ultimate strength of 616,000 psi. The factor of three in strength indicated allows for both degradation due to fatigue and a substantial margin of safety. 
     The cylinder  11  advantageously can be assembled onto optional cylinder  24  with an interference fit, as is the cylinder  11  onto the hub  22 . This causes the hub to be in compression when the rotor is at rest, which reduces its radial growth and tension when the rotor is spinning. This technique allows the metal hub to match the radial growth of the composite cylinders without being overstressed. 
     FIG. 5 gives details of the upper bearing assembly  12 . Preferably, an angular contact bearing  30 , using ceramic balls  30   a  to provide long bearing life, supports the spinning shaft  18  disposed in vacuum housing  10 . Bearing  12  advantageously can be lubricated by means of a circulating oil system in which oil pumping action is provided by a combination of centrifugal and gravitational forces. When oil in a spinning reservoir  36 , whose free surface forms a vertical cylinder when the shaft  18  is spinning, exceeds its desired level, a scoop  32  connected to a stationary shaft  37  scoops the excess oil into stationary reservoir  39 . Preferably, the oil then flows by gravity from reservoir  39  to central chamber  40 . The oil thus collected is discharged to spinning chamber  35 . Advantageously, the flow rate is regulated by the oil flow metering plug  34  through which the oil passes between central chamber  40  and spinning chamber  35 . Centrifugal force in spinning chamber  35  throws the thus-introduced oil radially outward. This advantageously permits the flow of oil to pass through oil flow holes  33  so as to enter the bearing  30 . The centrifugal force in the rotating portions of the bearing  30  slings oil into the spinning reservoir  36 , thus permitting the cycle to begin anew. 
     It will be appreciated that the small gap  31  between the stationary and rotating conical surfaces of bearing  12  shown in FIG. 5 acts as an effective seal or trap which prevents oil droplets from escaping from the vicinity of bearing  12  into flywheel chamber  27 . Any oil droplets which might enter gap  31  advantageously can be accelerated outwardly by the spinning wall of conical member  41  and, thus, caused to reenter the spinning reservoir  36 . 
     It should be noted that before shaft  18  begins to rotate, the oil resides in spinning chamber  35 . Once shaft rotation begins, the above-described oil circulation cycle begins. 
     FIG. 6 is an illustration which finds use in explaining the operation of the lower bearing assembly  16 . Preferably, bearing  140  is of the angular contact type which advantageously uses ceramic balls  140   a  to accommodate long life, just as in the upper bearing  12 . Bearing  140  can be lubricated by a circulating oil system. 
     Preferably, the circulating oil system  130  includes a rotating disc  141  which slings lubricating oil from the rotating part of bearing  140  outward into a reservoir  142 . It should be noted that the oil level in reservoir  142  is indicated by the dashed line. Lubricating oil flows through hole  143  into a squeeze film damper  145 , whose narrow annulus formed by concentric metal cylinders  145   a,    145   b  contains a radial spring  145   c  as well as lubricating oil. Details of the squeeze film damper  145  are shown in FIG. 10, wherein FIG. 10A is an axial view of a small arc of squeeze film damper  145  illustrating the annular space between concentric cylinders  145   a  and  145   b  occupied by radial spring  145   c.    
     Preferably, radial spring  145   c  is a chemically etched part whose etch pattern is as illustrated in FIG.  10 B. It will be appreciated that when the radial spring  145   a  is wrapped around cylinder  145   a,  the half rectangles of the pattern will stick out substantially, forming hundreds of elementary springs whose ends contact the inner surface of cylinder  145   b.  The space between the cylinders  145   a,    145   b  not occupied by the radial spring  145   c  is filled with lubricating oil. Advantageously, the spring  145   c  gives a restoring force to counteract the radial displacement of the outer cylinder  145   a,  which is connected to the vacuum sphere  10 , with respect to the inner cylinder  145   b,  which is rotably coupled to the spinning shaft  18  via bearing  140 . 
     The presence of viscous oil in this annulus produces a radial force proportional to the rate of this displacement. The squeeze film damper  145  acts as a means for limiting the amplitude of vibrations at shaft critical frequencies caused by residual unbalance of the rotating assembly  100 . 
     Referring to FIG. 6, lubricating oil enters reservoir  144  through hole  149  at the bottom of squeeze film damper  145 . It should be noted that the oil level in reservoir  144  is indicated by the dashed line. Lubricating oil enters the vertical hole  146  in spinning cone  150  and flows out through radial holes  147  to thereby impinge on the rotating part of bearing  140 , and thereby begin its circulatory cycle anew. 
     Advantageously, a double Belleville washer  148  can be used to preload both bearing  12  and bearing  16 . It will be noted washer  148  produces an axial force on the curved races of bearings  12 ,  16 , which advantageously squeezes the balls in each respective bearing radially. The stress thus produced creates the desired area of contact between the balls and the associated races, which, in turn, produces the desired radial stiffness of the bearing assembly. It will be appreciated that since most of the service life of the bearings is spent with the preload as the only load, the preload is kept as small as consistent with the radial stiffness requirement, thus maximizing bearing life. 
     FIG. 7 shows the construction of the molecular drag pump  26  which advantageously maintains the pressure in vacuum housing  10  at a predetermined pressure. It will be noted that gases slowly evolve from the flywheel materials. Preferably, molecular drag pump  26  pumps the offending gas molecules from the chamber  28  in which the shaft  18  spins into chamber  27 , which contains molecular sieves  27   a.  It will further be noted that molecular sieves  27   a  preferentially adsorb the pumped gas molecules. This pumping action advantageously maintains the gas pressure in chamber  28  low enough to achieve low aerodynamic drag and, thus, minimize heat generation due to the spinning fiber composite cylinder  11  of assembly  100 , whose surface speed can easily exceed 1000 meters per second. Drag pump  26  consists of a spiral groove on the inside of the stationary cylinder  38  in close proximity to the spinning shaft  18 . Since the bearing assemblies  12 ,  16  and motor  17  used for powering drag pump  26  are those required for the flywheel  1 , the additional cost of adding this important function is negligible. 
     More specifically, a separate gas storage chamber  27 , located proximate to one of the bearings  12 ,  16  is formed by a baffle plate  29 . It will be appreciated from FIG. 7 that baffle plate  29  includes an orifice  29   a  for positioning of the shaft  18 . Preferably, the bearing  12  is disposed within molecular pump  26 , which advantageously may be a molecular drag pump  26 . Preferably, gas storage chamber  27  contains so-called molecular sieves  27   a,  which will be discussed in greater detail below. 
     The purpose of the present invention is to maintain a high vacuum in the space in which the flywheel rotor spins so that a negligible drag on the flywheel rotating assembly  100  will be produced. It will be appreciated that at a preferred rim speeds of about 1000 meters per second, the pressure in housing  10  should be less than to 0.01 Pascal. It will also be noted that the fiber composite materials used in the construction of high energy density flywheels, i.e., flywheel assembly  100 , have a propensity for residual gas evolution at a rate which make it difficult to achieve this desired degree of vacuum in a sealed container. Therefore, continuous pumping of the evolved gases from the container in conventional systems is often performed using an external pump. 
     In contrast to these conventional systems, a molecular pump, which is designed into the flywheel  1 , and which employs the high speed motor, shaft, and bearing system already present in the flywheel energy storage system, transfers the gases evolving from the flywheel assembly  100  and its environs into a separate chamber  27 , which chamber is fully contained within the overall vacuum housing  10 . Advantageously, chamber  27  contains molecular sieves  27   a  designed to adsorb the most prevalent of the gases generated by, e.g., cylinder  11 . Preferably, getters are disposed throughout the vacuum housing  10  to adsorb trace quantities of gases which are not readily adsorbed by molecular sieves  27   a.    
     The flywheel assembly  100 , in an exemplary case, is 12 inches in diameter and has a maximum rotational speed of 6500 radians per second. This rotational speed corresponds to a surface speed of 1000 meters per second, which high speed requires that the surrounding gas pressure be maintained at a pressure less than 0.01 Pascal in order to permit a sufficiently long self discharge time. 
     It will be appreciated that even though the flywheel assembly  100  will be exposed to a high temperature bakeout while vacuum housing  10  is being evacuated prior to being sealed, the high mass of the volatile materials of the composites, particularly the epoxy, employed in the construction of flywheel assembly  100  can be expected to produce a residual gas evolution rate which could exceed the allowable pressure for the vacuum housing  10  in a relatively short time. The molecular drag pump  26  advantageously can be used to pump these gases into gas storage chamber  27  where the gases can be adsorbed by the molecular sieves  27   a.  It will be appreciated that the pressure in housing  10  can, thus, be maintained in the vicinity of the flywheel cylinder  11 , even though the pressure in the storage chamber  27  may rise as high as one Pascal. 
     It will also be appreciated that, e.g., molecular drag pump  26  would be too expensive an item to be used for maintaining the pressure of housing  10  below its maximum allowable pressure if molecular drag pump  26  were to be provided as a self-contained item, principally because of the cost of the high speed bearings and motor required by stand alone molecular pumps of any configuration. By integrating molecular drag pump  26  into the design of flywheel assembly  100 , the shaft, bearings, and motor of the flywheel assembly  100  advantageously can be used by molecular drag pump  100 . It will be noted that the incremental cost of incorporating the molecular pump into the flywheel energy storage system is very low. 
     Molecular sieves are adsorbents whose pores are tailored in size to the dimensions of the molecules to be adsorbed. They are available under the trade name MOLSIV from the Union Carbide Corporation. Their ability to adsorb is strongly influenced by pressure, e.g., the adsorption ability is low at the pressure normally applied to flywheel assembly  100 . It should also be noted that at the normal operating pressure of gas storage chamber  27 , i.e., a pressure P 2  which is approximately one thousand times higher than a pressure P 1  felt throughout housing  10 , the molecular sieves  27   a  are capable of adsorbing all of the gases evolved from flywheel assembly  100 . In other words, at the upstream pressure P 1  of the molecular drag pump  26 , the adsorption rate of the target gas molecules produced by the flywheel assembly  100  is low. The adsorption rate increases as the pressure P 2  in chamber  27  is increased. Preferably, molecular sieve material is selected so that a minimum adsorption rate, e.g., the minimum adsorption rate necessary to match the gas molecule evolution rate of flywheel assembly  100 , is achieved at a pressure lower than the shut off head of the molecular drag pump  26 . 
     Preferably, a helical groove  26   a  cut into the stator of drag pump  26  provides the flow path for the evolved gases from the high vacuum chamber, at pressure P 1 , e.g., 0.01 Pascal, to the chamber  27  containing the molecular sieves  27   a  in which the pressure P 2  may be as high as 10.0 Pascal. 
     It will be appreciated that an alternate embodiment of the present invention wherein a turbo-molecular pump  26 ′ is substituted for molecular drag pump  26 . The pump  26 ′ consists of a multiplicity of turbine blades connected to the shaft  18  of the pump  26 ′, interleaved with stator blades supported by plate  29 . It will be appreciated that pump  26 ′ serves the same function as pump  26  in pumping gases evolving from the flywheel rotor  100  into gas storage chamber  27  containing the molecular sieves  27   a.  Turbo-molecular pump  26 ′ may be used advantageously with some flywheel configurations in which more space is available along the shaft than in the configuration shown in FIG.  3 . 
     FIG. 8 illustrates the mechanical gimbal assembly  80 , consisting of a steel band  50  in the annular space between the outer housing  8  and vacuum housing  10 . Band  50  is attached to the vacuum housing  10  by means of journal bearings  14  and  15 , which are diametrically opposed to one another. A second set of journal bearings,  51  (shown) and  52  (not shown) also diametrically opposed to one another and are rotated by 90° (rotational degrees) from the first set of journal bearings  14 ,  15  connected to the band  50  on the outer surface of vacuum housing  10 . This arrangement isolates the vacuum housing  10  which contains the flywheel assembly  100  from pitch and roll angular motions of the vehicle. The motor-generator torques are reacted by the gimbal  80 , which also transmits the residual acceleration loads which result from the small departure from neutral buoyancy of the vacuum sphere in the flotation liquid  9 . The journal bearing shafts are sized to shear under the high torque overloads which would occur in the event of a flywheel failure corresponding to bearing seizure. This is a safety feature to prevent the flywheel from jerking the vehicle. 
     In addition to these functions, the gimbal assembly also provides mechanical support for the power leads which must be routed from the outer housing into the vacuum housing to connect to the motor-generator. 
     The operation of the flywheel-motor-generator assembly will now be described in detail. 
     An object of the support system is to permit the flywheel  1  to safely perform its function as an energy buffer during all driving conditions, while consuming negligible power when the vehicle is parked, even on a steep hill. Since the surface speed of the rotor  100  may exceed 1000 meters per second at peak charge, the assembly  100  must be maintained in a vacuum. The small, oil lubricated ceramic ball bearings  30 ,  140  can provide the desired service life provided the mechanical loads are kept as low as possible. The overall design of this flywheel system is aimed at minimizing these loads. 
     It will be appreciated that placing the vacuum housing  10  in a gimbal system  80  makes the flywheel  1  nearly impervious to vehicle rotations. If the flywheel  1  were not gimbaled, a vehicle rotation would cause a gyroscopic torque of magnitude (HdP/dt), where H is the angular momentum of the flywheel  1  and dP/dt is the pitch or roll angular velocity of the vehicle. The reaction at each bearing of the unit depicted in FIG. 3, which preferably is capable of storing 2 KWH of energy at full charge, would be 6000 newtons per radian per second of vehicle pitch or roll. It will be appreciated that this represents a load that would shorten the life of the bearings on all but the smoothest of roads. The use of the gimbal system  80  described above reduces the moments exerted on the bearings  30 ,  140  to those produced by hydrodynamic forces on the vacuum housing  10  and the spring forces produced by the power leads. Because the liquid  9  provides nearly neutral buoyancy to the inner housing, the mechanical gimbal need not support the bulk of the acceleration loads, i.e., these loads mainly are borne by liquid  9 . The mechanical gimbal need only react to the spin-up and spin-down torques developed by the motor-generator  17 , which are 12.5 newton-meters when the flywheel  1  is delivering or accepting 80 kilowatts of power at its quiescent operating speed of 6400 radians per second. Thus, gimbal  80  preferably can have a small enough drag area to make the hydrodynamic torques it develops during vehicle pitching and rolling negligibly small. 
     During steady driving the orientation of the rotor axis is vertical, a consequence of the center of mass of the vacuum housing  10  and its contents being below the center of buoyancy, which arrangement advantageously produces a righting moment on vacuum housing  10 . In this orientation, the weight of the assembly  100  is borne by the toroidal magnet  23  and the forces on the bearings are those produced by the preload spring  148 . This advantageously can be made as small as the radial stiffness requirement permits. 
     When the vehicle is accelerating or braking, the spin axis is no longer vertical, aligning itself, after a transient, to the equivalent gravitational field which is the vector sum of the earth&#39;s gravitational acceleration and the vehicle&#39;s acceleration. Thus, the bearing load during steady accelerations is primarily axial. During transients, which cause a damped precessional motion of the axis, the bearings react to the small torques associated with this motion by exerting radial forces. 
     When the vehicle is parked, even on a hill, the spin axis is very close to vertical, just as in steady driving. The spring forces exerted by the power leads routed along the gimbal system  80  produce a torque tending to align the axis perpendicular to the hill, but these forces advantageously are small enough to keep the resulting offset from vertical negligibly small. With a vertical orientation of the rotor axis when the vehicle is stationary, the rotor weight is exactly offset by the magnet  23 , thus minimizing the load on the bearings  12 ,  16 , thereby maximizing bearing life. 
     Another object of the present invention is to provide adequate cooling of the motor-generator  17  under all driving conditions, the most demanding of which is a repetitive stop and go driving schedule. During this cyclic use, the motor-generator  17  is alternately delivering power as a generator when accelerating the vehicle or accepting power as a motor during dynamic braking. Even though it is advantageously very efficient in both operating modes, the high powers involved, e.g., many tens of kilowatts, create iron and copper losses which would lead to destructive temperatures in the motor-generator  17  if cooling were not provided. 
     Advantageously, one preferred embodiment according to the present invention provides effective cooling of the motor-generator stator  21   a  by circulating flotation liquid  9  through axial holes  20   a  in the metal cylinder  25 , as previously described. Since the bearings  12 ,  16  provide very little thermal conduction from the rotating shaft  18 , the rotor  21   b  of the motor-generator is cooled primarily by radiation. The shaft temperature needed for this thermal radiation can be maintained within acceptable limits by using a motor-generator design which minimizes rotor losses, such as a synchronous reluctance machine. The relatively cool spherical boundary, i.e., the vacuum housing  10 , into which the rotating assembly  100  radiates helps keep the rotor temperature within acceptable limits. 
     Another object of the present invention is to protect the vehicle and its passengers from (a) an accidental sudden release of the stored energy or (b) transfer of angular momentum, events which could be caused either by vehicle collision or by mechanical failure of the flywheel  1 . Although the energy of a full charge is only equivalent to that resulting from the burning of six ounces of gasoline, its potentially dangerous form of release, i.e., sudden release, must be considered. Preferably, four barriers are provided between the rotating assembly  100  and the outside: the vacuum housing  10 , the liquid  9 , the outer enclosure  8 , and an outer wrapping of fiber composite material  52  which surrounds and supports the housing  8  using foam pads  53  in the intervening space. See FIG.  9 . 
     The heat released by a full charge will produce an increase in the temperature in the fluid of approximately a few hundred degrees, causing no significant hazard. The sudden transfer of the rotor&#39;s angular momentum to the vehicle could jerk the vehicle dangerously, if such were permitted to happen. This is precluded in the preferred embodiment of the present invention by allowing the vacuum housing  10  to spin down gradually in the liquid  9  when pins in the mechanical gimbal shear in the event of bearing seizure or of rotor disintegration. This detail is shown in FIG.  8 . 
     The foregoing description of a preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and many modifications and variations are possible in light of the above teaching. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to electric vehicles, thereby enabling others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular vehicle use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.