WO2007132241A1

WO2007132241A1 - Kinetic energy storage device

Info

Publication number: WO2007132241A1
Application number: PCT/GB2007/001800
Authority: WO
Inventors: Christopher William Henderson Ellis
Original assignee: Hykinesys Inc
Priority date: 2006-05-16
Filing date: 2007-05-16
Publication date: 2007-11-22
Also published as: GB0609646D0

Abstract

According to the present invention there is provided a kinetic energy storage device (10) for a road vehicle, the device (10) comprising a casing (12) having an interior space defining a rotor chamber (24), a substantially hollow rotor (14) having a length greater than its diameter provided within the rotor chamber (24) and having opposed shafts (30) mounted for rotation about opposed rotor bearings (15) of the casing (12), one of said shafts (30) extending through an aperture (94) of the casing (12), wherein the rotor (14) comprises opposing end caps (44) connected by tubular assembly, said tubular assembly comprising an inner tubular member (46) and an outer cover (48), said inner tubular member (46) having a larger mass than the outer cover (48).

Description

Kinetic Energy Storage Device

The present invention relates to a kinetic energy storage device, and particularly, though not exclusively, to a kinetic energy storage device for a road vehicle.

The engines in most motor cars and other road vehicles spend only a small fraction of their operating time running at more than 60% of peak power, typically for only a few seconds. If an alternative means could be found to provide brief surges of high power, the engines in most applications could be made smaller and therefore more economical at all speeds. Engine designs could also be optimised to deliver the average level of power required in each application as economically as practicable and with lower emission levels than engines that need to provide power flexibly across the full speed range. This is one of the principles behind the concept of hybrid vehicles.

A car travelling along a level road at a steady 30 mph might consume fuel at only 60 mpg. However, its fuel consumption will be much higher in stop/start city traffic. The most significant reason is the energy wasted by continually braking and accelerating, with the vehicle's expensively acquired kinetic energy being wasted as heat rejected by the brakes. The vehicle's kinetic energy is built up again during acceleration by burning more fuel. Another reason is the perceived need to keep the engine running while the vehicle is stationary or travelling slowly. If an energy store could capture braking energy and then allow it to be re-used to restart and accelerate the vehicle, a major reduction in fuel consumption would result, and the engine could be switched off much of the time in heavy city traffic.

As a vehicle accelerates from rest, energy from the fuel tank, whether stored in the form of, for example, petrol, diesel, natural gas or hydrogen, is transformed by the engine or fuel cell into the increasing kinetic energy of the vehicle itself, a function of the mass of the vehicle and its contents times the square of the velocity of the vehicle. However, with a conventional engine, the majority of fuel is merely transformed into heat, which is then dissipated into the atmosphere via the exhaust pipe, radiator, etc. The rest of the energy is used to run ancillaries and to overcome aerodynamic and rolling resistance losses. Ideally, once fuel energy has been transformed into kinetic energy via the vehicle transmission, any kinetic energy that needs to be temporarily shed should be saved in kinetic form because this avoids the inevitable losses which occur if energy is transformed from one form to another.

An alternative approach to the provision of a temporary energy store is to use batteries. However, in braking and then accelerating, four energy transformations must take place, first from the kinetic energy of the vehicle to electrical energy in the generator, then from electrical energy to chemical energy in the battery. The third transformation is back from chemical energy into electrical, and the fourth is from electrical to kinetic energy in the motor. As a consequence, few current hybrid vehicles are capable of achieving more than 35% efficiency in a full regenerative braking cycle.

To illustrate how this unsatisfactory level of efficiency can come about, assume that the kinetic-to-electric and electric-to-kinetic transformations take place at 80% efficiency and the battery energy transformations are 75% efficient. On these figures, only 60% of the vehicle's available kinetic energy will make it into the battery, and only 36% will make it back into the vehicle. A similar efficiency problem also occurs when the engine is used to re-charge the battery, for example when the engine restarts after a period of engine-off running. By contrast, the use of a kinetic energy storage device requires only energy transfers, which typically have efficiencies in the high nineties. It is reasonable therefore to expect that such a device should be capable of a total efficiency in excess of 80% when fully developed, with first generation devices achieving at least 60%.

Kinetic energy storage devices have been successfully exploited in spacecraft, as a superior alternative to batteries, with an electric motor/generator directly connected to the axle shaft of the flywheel. The vacuum of space, low or zero gravity and the absence of external kinetic shocks combine to make this a relatively benign environment for flywheels. In unmanned applications, the maximum potential specific energy can be used without the need for a heavy containment vessel, because unprotected catastrophic rotor failure can be an acceptable risk. There have been a number of unsuccessful attempts to exploit aerospace-derived kinetic energy storage systems in ground vehicles. Among the problems have been developing rotor bearings capable of coping with the full range of road shocks, and the requirement to provide safe containment of highly stressed rotors, particularly in a severe accident.

The principal objective of the present innovation is a high-speed kinetic energy storage device which can achieve a satisfactory level of specific energy while the probability of catastrophic failure even at maximum rotational speed remains low enough that massive containment is not required. The challenge is to design a high-speed rotor which requires only basic containment, in the form of a rotor casing essentially there to keep the outside world, including the atmosphere, away from the surface of the rotor, and to provide location of the rotor bearings and hence the rotor.

According to a first aspect of the present invention there is provided a kinetic energy storage device comprising a casing having an interior space defining a rotor chamber, a substantially hollow rotor having a length greater than its diameter provided within the rotor chamber and having opposed shafts mounted for rotation about opposed rotor bearings of the casing, one of said shafts extending through an aperture of the casing, wherein the rotor comprises opposing end caps connected by tubular assembly, said tubular assembly comprising an inner tubular member and an outer cover, said inner tubular member having a larger mass than the outer cover.

The inner tubular member is comprised of a material or materials which are relatively high in density and strong, for example an isotropic material such as stainless steel. The outer cover is comprised of a material or materials which are of a lower density and very strong in at least one direction to provide a high hoop strength. The outer cover may comprise an anisotropic material such as, for example, carbon reinforced plastic. The outer cover provides the majority of the circumferential hoop strength for the rotor, while the inner tubular member provides the majority of the mass and general structural strength of the rotor. The rotor is constructed with the principal of self-containment in mind, with the outer cover acting to contain the inner tubular member in the event of catastrophic failure of the device. The rotor has a length that is greater than its diameter. The rotor may have a length that is from to two to five times greater than its diameter. Preferably, the rotor has a length that is from two and a half to three and a half times greater than its diameter. More preferably, the rotor has a length that is around three times its diameter.

The outer cover preferably fully envelopes the inner tubular member and is in intimate contact with the inner tubular member. The inner tubular member preferably has a substantially uniform inner diameter over its entire length. The inner tubular member has a substantially uniform wall thickness along the majority of its length, and a reduced wall thickness at each end thereof. In such an embodiment, the outer diameter of the tubular member reduces at each end of the tubular member. The outer cover has a substantially uniform wall thickness over its entire length. The outer cover has a substantially uniform diameter along the majority of its length and a reduced diameter at each end thereof. The outer cover has a substantially uniform diameter for the portion of its length which overlaps the inner tubular member.

Each end cap of the rotor comprises a substantially circular base from the centre of which projects a stub shaft. On the opposite side of the base to the stub shaft there is provided a circumferential seat configured to receive the inner tubular member. The seat preferably includes an abutment surface against which an end of the tubular member rests, in use. The abutment surface may be defined by an annular step provided around the exterior of the base. In a preferred embodiment, the height of the step is substantially equal to the wall thickness of the inner tubular member at each end thereof. A recess may be provided in said opposing side of the base to the stub shaft such that at least a part of the circumferential seat of the base may be defined by a skirt of the base.

Each end cap preferably also includes an inclined outer shoulder which, in use, receives the reduced diameter portion of the outer cover provided at each end thereof.

Each end cap is preferably provided with one of an annular recess or an annular lip which, in use is received with a clearance fit within a complementarily shaped annular lip or recess of the casing. These features of the end cap and casing define a plain bearing which may be used in the event that one of both of the rotor bearings fails. The casing defines a substantially hermetically sealed rotor chamber which can be evacuated such that, in use, the rotor rotates in a vacuum. In order to maintain the desired vacuum pressure within the chamber, the casing aperture through which the rotor shaft projects is provided with a vacuum seal. The casing is preferably connected to a means to evacuate the rotor chamber such as, for example, a vacuum pump. Accordingly, the casing may be provided with a through aperture in communication with the rotor chamber which is connectable to a vacuum pump. In a preferred embodiment the casing is further provided with an externally operable valve which may be used to equalise the pressure within the chamber with that of the atmosphere surrounding the casing. The valve may be integrated into the casing. Alternatively, the casing may be provided with a through aperture in communication with the rotor chamber which is connectable via an appropriate conduit to a valve.

In a preferred embodiment, the casing is defined by opposed bulk heads which are retained in association with one another by a tubular member extending therebetween.

The device may comprise a pair of contra rotatable rotors aligned upon mutually parallel axes which are geared together for rotation at equal speeds. The rotors may be provided in a common casing. Alternatively, the rotors may be provided in separate casings. In such an embodiment, the casing may comprise opposed bulkheads which are retained in association with one another by a pair of tubular members. In such an embodiment, the rotors may be geared together about a common input/output shaft of the device.

According to a second aspect of the present invention there is provided a substantially hollow rotor for a kinetic energy storage device, the rotor comprising opposed end caps connected by tubular assembly, said tubular assembly comprising an inner tubular member and an outer cover, said inner tubular member having a larger mass than the outer cover.

The inner tubular member is comprised of a material or materials which are relatively high in density and strong, for example an isotropic material such as stainless steel. The outer cover is comprised of a material or materials which are of a lower density and very strong in at least one direction to provide a high hoop strength. The outer cover may comprise an anisotropic material such as, for example, carbon reinforced plastic. The outer cover provides the majority of the circumferential hoop strength for the rotor, while the inner tubular member provides the majority of the mass and general structural strength of the rotor. The rotor has a length that is greater than its diameter. The rotor may have a length that is from to two to five times greater than its diameter. Preferably, the rotor has a length that is from two and a half to three and a half times greater than its diameter. More preferably, the rotor has a length that is around three times its diameter.

Each end cap is preferably provided with one of an annular recess or an annular lip which, in use is received with a clearance fit within a complementarily shaped annular lip or recess of the casing. These features of the end cap and casing define a plain bearing which may be used in the event that one of both of the rotor bearings fails.

Further features of the invention will appear from the following description of embodiments of the invention and will now be more particularly described by way of example with reference to the accompanying drawings, in which: -

Figure 1 shows, in perspective, the kinetic energy storage device with part of the casing cut away to show one of the two rotors; Figure 2 is a perspective view of one of the rotors with its associated rolling element bearings and vacuum seals;

Figure 3 is a perspective view of one of the rotors with the outer rim removed;

Figure 4 is a perspective view of a long-section through one end of one of the rotors; Figure 5 shows a long-section through one end of one of the rotors assembled in its casing; and

Figure 6 is a perspective view of an alternative way of mounting two similar rotors in an all-wheel-drive vehicle.

Referring firstly to figure 1 there is shown a kinetic energy storage device suitable for a road vehicle generally designated 10. The device 10 includes a casing 12 which is shown to be partially cutaway. Within the casing 12 there are provided a pair of rotors 14, one of which is shown. Each rotor 14 is mounted on bearings 15 for rotation within the casing 12. In one embodiment of the present invention the rotors may each have a diameter of approximately 200 mm and a length of 600mm, and be rotatable at speeds of between 5,000 to 20,000 rpm. The construction of the rotors 14 will described in greater detail below.

The casing 12 comprises front and rear bulkheads 16,18 between which are retained a pair of tubular members 20,22. The bulkheads 16,18 may be manufactured from metal such as, for example, stainless steel or aluminium alloy, or a composite material such as a fibre reinforced composite. Similarly, the tubular members 20,22 may be manufactured from metal such as, for example, stainless steel or aluminium alloy, or a composite material such as a fibre reinforced composite. The terms front and rear are construed with reference to the orientation of the device shown in figure 1 and are not intended to be limiting upon the orientation of the device when installed, for example, in a road vehicle. The bulkheads 16,18 define with each tubular member 20,22 a rotor chamber 24. In the embodiment shown the casing 12 is configured so as to define two separate rotor chambers 24. In an alternative embodiment the casing 12 may be provided with a single tubular wall member which defines a common rotor chamber 24.

The front bulkhead 16 of the casing 12 is provided with a common input/output shaft 26 having spur gear 28. The rotor 14 which is shown is provided with an input/output shaft 30 which extends through the front bulkhead 16 along an axis substantially parallel to that of the common input/output shaft 26. The rotor input/output shaft 30 is provided with a spur gear 32 which connects to the common shaft spur gear 28 via an idler gear 34 mounted to the front bulkhead 16. The hidden rotor 14 is also provided with an input/output shaft 36 which extends through the front bulkhead 16 along an axis substantially parallel to that of the common input/output shaft 26. This shaft 36 is provided with a spur gear 38 which meshes directly with the common shaft spur gear 28. It will thus be appreciated that the rotors 14 are geared for rotation in opposite directions. The rotor shaft spur gears 32,38 and the bulkhead idler gear 34 are sized such that the rotors 14 contra-rotate at the same speed. The gears 32,34,38 are contained in a gear housing portion 17 of the front bulkhead An input torque may be applied to the common input/output shaft 26 in order to drive the rotors 14, and that the rotors themselves may in turn supply an output torque to the common shaft 26. The common input/output shaft may be connected either directly or indirectly to one or more electric motor/generators to add or remove energy from the rotors 14. Alternatively, the shaft 26 may carry energy to and from an external mechanical or electro-mechanical transmission. In yet a further embodiment, an electric motor/generator many be integrated with the rotors 14 within the casing 12, with energy being converted by electrical means from within or into the casing 12.

The rotors 14 are mounted in a pair, with their principal axes parallel to each other. Both rotors 14 are of identical design and moment of inertia and can be made to revolve at the same speed but in opposite directions. It will thus be appreciated that the gyroscopic moments that are generated in the rotors 14 from the yawing and pitching of the host vehicle when it is underway will be equal and opposite. These moments place large loads on the rotor bearings 15 when rates of change of yaw and pitch are high, but they are balanced within the casing 12 being conveyed through the bulkheads 16,18 from one set of bearings to the other, effectively conveying no gyroscopic effects to the rest of the vehicle.

In use, the rotor chambers 24 are evacuated so that the rotors 14 are surrounded by a near vacuum to minimise windage losses. In order to maintain the vacuum within the rotor chambers 24 the casing 12 is connected to a vacuum pump 40. The casing 12 is further provided with an externally operable valve 42 which may be utilised to permit air to enter the chambers 24 and thus equalise the pressure within the chamber with the external pressure surrounding the casing 12.

The configuration of a rotor 14 will now be described with reference to figures 2 to 5. The rotor 14 comprises a pair of opposed end caps 44, of which one is visible in figures 2,3 and 5, which are connected to one another by an inner tubular member 46 and an outer cover 48. The end cap 44 includes a substantially circular base 50 having an inner side 52 and outer side 54. The outer side 54 faces away from the opposing end cap 44 of the rotor 14 and the inner side 52 faces towards the opposing end cap 44 of the rotor 14. The input/output shaft 30 described above extends from the outer side 54 of the base 50. Surrounding the shaft 30 is an annular groove 56 which is surmounted by a lip 58. The annular groove 56 includes a substantially flat base 57 and an inclined radially outer wall 59. Radially outwardly of the lip 58 there is provided an inclined annular shoulder 66. An annular undercut 68 is provided between the lip 58 and the radially inner edge 70 of the shoulder 66. The radially outer edge 72 of the shoulder 66 terminates at an annular step 74. The inner side 52 of the base 50 is provided with a recess 76 having a flat bottom 78 and an inclined peripheral wall 80. The bottom and wall 78,80 define a skirt 82 of the end cap 44. The end cap 44 is manufactured from a strong, isotropic, dense material such as, for example, austenitic stainless steel or titanium.

In the embodiment shown the shaft 30 is formed integrally with the end cap 44. In an alternative embodiment, the shaft 30 may be formed separate from the end cap 44 and affixed thereto by appropriate means. The shaft 30 is configured so as to be able to receive and support a bearing 15, a vacuum seal 84 and a spur gear 32. An annular step 86 is provided at the base of the shaft 30 ensure that the bearing 15 and seal 84 maintain the correct longitudinal alignment on the shaft 30.

The end cap 44 which is not shown has a similar configuration to that described above. The shaft of the end cap 44 may be shorter such that it does not project through the rear bulkhead 18 and hence does not carry a spur gear. In an alternative embodiment, the end caps 44 may mirror each other with a stub shaft 30 extending through both the front and rear bulkheads 16,18.

The inner tubular member 46 is fitted to the end cap 44 such that the ends 88 of the member 46 abuts the annular steps 74 of the end caps 44. The inner tubular member 46 may be retained in association with the end cap 44 by virtue of there being an interference fit between the two. Alternatively, other forms of mechanical connection which prevent relative movement of the inner tubular member 46 to the end cap 44 may be employed such as, for example, welding. The tubular member 46 has a substantially uniform wall thickness over the majority of its length, wherein the wall thickness is greater that the height of the annular step 74. The outer wall 90 of the tubular member 46 is tapered at each end 88 such that the wall thickness is reduced to that of the height of the step 74. The rotor 14 may be provided with a single inner tubular member 46 of a desired wall thickness as shown in the accompanying figures. Alternatively, the rotor 14 may be provided with a plurality of concentric inner tubular members 46 which cumulatively provide a desired wall thickness, hi such an embodiment, the concentric members 46 may be of the same material or differing materials such as, for example, stainless steel or titanium.

The outer cover 48 of the rotor 14 is comprised of a strong anisotropic material which is of a lower density than the inner tubular member 46. The outer cover 48 may, for example, comprise circumferentially wound carbon reinforced plastic (CRP). Such a material provides an outer cover 48 having a high hoop strength. The outer cover is relatively thin and may typically have a wall thickness of less than 15mm. As can readily be seen from figure 5, the outer cover 48 fully envelopes the inner tubular member 46 and extends over the annular step 74 and shoulder 66 to the annular undercut 68 provided between the lip 58 and the radially inner edge 70 of the shoulder 66. The diameter of the outer cover 48 thus reduces in the direction of end cap shafts 30 from a longitudinal position on the inner side of annular step 74 to the annular undercut 68. The inner surface of the outer cover 48 is in contact with the outer surface of the inner tubular member 46 and the outer cover 48 may be considered to be an interference fit with the inner tubular member 46. The outer cover 48 may comprise a single layer of CRP. In an alternative embodiment the outer cover 48 may comprise a plurality of concentric layers of anisotropic material which cumulatively provide a desired wall thickness. In such an embodiment the layers may all be of the same material, such as CRP, or be of differing materials, such as layers of differing composite materials.

The rotors 14 have a length which is greater that than their diameter. The ratio of diameter to length may range from 1:2 to 1:5. Typically, the ratio is in the region of 1:3. The above described configurations of the end caps 44, inner tubular member 46 and outer cover 48 combine to provide a rotor 14 which is hollow and has an internal cavity 60. The cavity 60 may be sealed, in which case the cavity 60 is evacuated during production of the rotor 14. Evacuation is necessary so as to reduce internal windage losses in use. Alternatively, the rotor 14 may be provided with one or more apertures which allow fluid communication between the cavity 60 and the rotor chamber 24. In this embodiment it will be understood that the cavity 60 is able to be evacuated along with the rotor chamber 24 by an external vacuum pump 40.

As can be seen from figure 5, the interior of the front bulkhead 16 is provided with an annular ridge 92 which surrounds an aperture 94 of the bulkhead 16 through which the shaft 30 of the end cap 44 projects, in use. The ridge 92 mirrors the shape of the annular groove 56 of the end cap 44 having a radially outer wall 96 which is inclined, and a substantially flat crown 98. As can be seen from figure 5, the ridge 92 is received in the end cap groove 56 when the rotor 14 is fitted to the bulk head 16 and is spaced from the groove 56 by the bearing 15. The ridge 92 and groove 56 are shaped internally to form a plain bearing in he event that the bearing 15 fails. Should the bearing 15 fail, the close tolerances and conforming shapes of the end cap ridge 92 and the bulkhead groove 56 co-operate as a one- time plain bearing to ensure the outer cover 48 of the rotor 14 does not make contact with the tubular member 20 or the bulkheads 16. The objective of the plain bearing is to avoid a rapid transfer of momentum from the failed rotor 14 to the rest of the vehicle to which the device 10 is fitted. However, it is also desirable to remove energy from the rotor 14 quickly. To help this happen in a smooth fashion, the rotor end cap 44 and the bulkhead 16 are shaped to build up heat rapidly in the area of the vacuum seal 84 in the event of bearing failure. The heat build up will cause the vacuum seal 84 to fail automatically, allowing air at atmospheric pressure to enter the rotor chamber 24 surrounding the rotor 14. If the rotor 14 is still rotating at high-speed, the effect of windage, i.e. resistance against the air admitted to the rotor chamber 24, will gradually bring its speed down to a safer level. The plain bearing arrangement is replicated between the rear end cap 44 and bulkhead 18.

It will be noted that the lip 58 of the end cap 44 is the most radially displaced part of the isotropic rotor core which is not sheathed by the outer cover 48. This is a consequence of the way the ends of the outer cover 48 reduce in radius towards the ends of the rotor 14. This tapering ensures that the stresses in the exposed lip 58 at high speed are much lower than in the inner tubular member 46 because the radius of the lip 58 is significantly less than the radius of the inner tubular member. No other part of the rotor 14 not contained by the outer cover 48 is subjected to higher stresses due to high-speed rotation.

As the outer cover 48 of the rotor 14 is relatively thin and of much lower density than the inner tubular member 46, the majority of the energy stored by the rotor 14 is stored in the heavier inner tubular member 46. Consequently, the principal function of the outer cover 48 is the containment of the much more highly stressed inner tubular member 46. While a flywheel having a diameter of 200mm and a length of 600mm, and made entirely of CRP might have a burst speed of 70,000 rpm and an operational maximum of 50,000 rpm, the rotors 14 of the present invention will typically never exceed 20,000 rpm, except in testing during manufacture. As the stresses in a simple tubular member rise in proportion to the square of the rotational speed, it will be appreciated that the outer cover 48 could, in principle, be tested on its own at 50,000 rpm, which might stress it at more than six times the level it would be under if spun on its own at 20,000 rpm. However, when combined with the inner tubular member 46 and spun up to peak operating speed, the rotor stresses become more complex because the inner tubular member 46 will stretch in the radial direction and lean into the outer cover 48, which reduces the stresses on the inner tubular member 46 and increases them on the outer cover 48. The outer cover is thus required to provide considerable support to the inner tubular member 46. Achieving an optimal balance between the stress conditions in the inner tubular member 46 and the outer cover 48 depends on the precise characteristics of the materials of both the member 46 and cover 48, and also the cross-sectional dimensions of the rotor 14 . Once the materials have been chosen and the overall diameter of the rotor 14 has been set, the thicknesses of the tubular member 46 and cover 48 will determine the balance of stresses at a particular speed, and the available energy. In a preferred implementation, one objective is for the peak circumferential stresses in the outer cover 48 of the fully assembled rotor 14 to not exceed 30% of maximum, and the corresponding stresses in the stainless steel inner tubular member 46 to not exceed 70% of maximum during normal operation.

Before initial rotational testing during manufacture, the inner tubular member 46 is integrated with the end caps 44 and bearings 15, as shown in Figure 3. The inner tubular member 46 is designed to run 'naked' (i.e. without the outer cover 48) during initial rotational testing, which will include repeated runs at up to 80% of 'naked' burst speed. A variety of methods for detecting flaws remain practical because most of the rotor core is still accessible at this stage, and a selection of these checks can be carried out to determine the integrity of the rotor core after these runs but before its outer cover 48 is fitted. It will be understood that various grades of stainless steel have differing fatigue limits, unlike many other metals and alloys. This is one major reason for choosing stainless steel for the end caps 44 and inner tubular member 46 comprising the core of the rotor 14. If the core is able to survive for 100,000 cycles of running up to high speed and down again and subsequent cycles are no more stressful, the rotor 14 may last indefinitely. Another important characteristic is the high ductility of austenitic stainless steel, which essentially means it will stretch rather than snap under high stress. This should cause the rotor 14 to go out of balance well before any form of serious failure occurs. Relatively inexpensive sensors (not shown) will pick up any out-of-balance vibrations and cause the device 10 to shut down.

The tubular member 20 surrounding the rotor 14 fulfils several functions. Firstly, it isolates the rotor 14 from external influences, including the ambient atmosphere surrounding the casing 12. Second, it acts to prevent the bulkheads 16,18 from moving away from each other. Given the inherent resistance Of the rotors 14 to catastrophic failure by the provision of the outer cover 48, it will be appreciated the most significant risk associated with the device 10 is the escape of a spinning rotor 14 from the casing 12. As a general statement, as long as the rotors 14 are kept together with the bulkheads 16,18, the external risk the device 10 poses is small, arguably less than a conventional engine after an accident. Consequently, as keeping the bulkheads 16,18 from separating is paramount, it is the tensile strength of the tubular member 20 which is critical. To this end, it can prove advantageous to construct the tubular member from both isotropic and anisotropic materials. However, in contrast to the hoop-wound anisotropic layer of the outer cover 48, the anisotropic layer of the tubular member 20 will have its maximum tensile strength along the line of the common principal axis of the tubular member 20 and the rotor 14.

In the event of a severe side impact, the tubular member 20 is configured so as to yield relatively easily at the area of impact but within a few millimetres of deformation will come in contact with the outer cover 42. As the rotor 14 is far stronger than the casing 12, even allowing for its maximum internal stresses at speed, the rotor 14 will act as a massive beam supported by the bulkheads 16,18 at each end. If the impact is very severe, the most likely outcome is that the complete device 10 will be torn from its mountings in the vehicle. As long as the casing 12 has not been torn open, the kinetic energy storage device 10 sitting by the side of the road will be less of a threat than a very hot engine. The tubular member 20 is likely to be intact after most impacts because of support from the rotor 14, and it may well have sprung partially back to its original form. The device 10 itself will typically be going through a process of automatic controlled energy dissipation. This will include opening the rotor chambers 24 to the air. The external manual valve 42 is provided to allow emergency services to reassure themselves that energy is being or has been drained from the device 10. This should make the device more friendly and acceptable in an emergency than a battery or a large capacitor.

Figure 6 illustrates an alternative layout for a kinetic energy storage device, generally designated 100 which is particularly effective in a four-wheel-drive vehicle, as shown here. In this layout the rotors 14 (one of which is shown) are arranged still with their principal axes parallel but with the rotors 14 one behind the other rather than side-by-side. However, the principal axes of the rotors 14 are offset one from the other to allow a matched pair of meshed spur gears each attached to one of the rotors 14 to ensure that the rotors 14 always rotate in opposite directions at identical speeds. The synchronising pair of spur gears are encased in the transfer case 102, which also serves to connect the front and rear rotor casings 20 (one of which is shown) rigidly together, supporting cancellation of gyroscopic moments. In this layout, a planetary gear reduction gearbox 104 and a continuously variable transmission 106 are fitted between each rotor 14 and the nearest transmission axle 108,110. This provides a particularly effective form of four-wheel-drive, with no need for a centre differential and the ability to power each axle 108,110 completely independently.

Claims

1. A kinetic energy storage device for a road vehicle, the device comprising a casing having an interior space defining a rotor chamber, a substantially hollow rotor having a length greater than its diameter provided within the rotor chamber and having opposed shafts mounted for rotation about opposed rotor bearings of the casing, one of said shafts extending through an aperture of the casing, wherein the rotor comprises opposing end caps connected by tubular assembly, said tubular assembly comprising an inner tubular member and an outer cover, said inner tubular member having a larger mass than the outer cover.

2. A kinetic energy storage device as claimed in claim 1 wherein the inner tubular member is comprised of an isotropic material or materials which are relatively high in density and strong, and the outer cover is comprised of an anisotropic material or materials which are of a lower density and very strong in at least one direction to provide a high hoop strength.

3. A kinetic energy storage device as claimed in claim 1 or claim 2 wherein the rotor has a length that is around three times its diameter.

4. A kinetic energy storage device as claimed in any preceding claim wherein the outer cover preferably fully envelopes the inner tubular member and is in intimate contact with the inner tubular member.

5. A kinetic energy storage device as claimed in any preceding claim wherein each end cap of the rotor comprises a substantially circular base from the centre of which projects a stub shaft, wherein on the opposing side to the base to the stub shaft there is provided an outer circumferential seat to which the inner tubular member is mounted.

6. A kinetic energy storage device as claimed in any preceding claim wherein each end cap includes an inclined outer shoulder to which the outer cover is mounted

7. A kinetic energy storage device as claimed in any preceding claim wherein one of the end caps is provided with one of an annular recess or an annular lip which is received with a clearance fit within a complementarily shaped annular lip or recess of the casing.

8. A kinetic energy storage device as claimed in any preceding claim wherein the casing is provided with a through aperture in communication with the rotor chamber which is connectable to a vacuum pump.

9. A kinetic energy storage device as claimed in any preceding claim wherein the device is further provided with an externally operable valve which may be used to equalise the pressure within the chamber with that of the atmosphere surrounding the casing.

10. A kinetic energy storage device as claimed in any preceding claim wherein the casing is defined by opposed bulk heads which are retained in association with one another by a tubular member extending therebetween.

11. A kinetic energy storage device as claimed in any preceding claim wherein the device comprises a pair of contra rotatable rotors aligned upon mutually parallel axes which are geared together for rotation at equal speeds.

12. A substantially hollow rotor for a road vehicle kinetic energy storage device, the rotor comprising opposed end caps connected by tubular assembly, said tubular assembly comprising an inner tubular member and an outer cover, said inner tubular member having a larger mass than the outer cover.

13. A rotor as claimed in claim 12 wherein the inner tubular member is comprised of an isotropic material or materials which are relatively high in density and strong, and the outer cover is comprised of an anisotropic material or materials which are of a lower density and very strong in at least one direction to provide a high hoop strength.

14. A rotor as claimed in claim 12 or claim 13 wherein the rotor has a length that is around three times its diameter.

15. A rotor as claimed in any of claims 12 to 14 wherein the outer cover fully envelopes the inner tubular member and is in intimate contact with the inner tubular member.

16. A rotor as claimed in any of claims 12 to 15 wherein the inner tubular member has a substantially uniform inner diameter over its entire length.

17. A rotor as claimed in claim 16 wherein the inner tubular member has a substantially uniform wall thickness along the majority of its length, and a reduced wall thickness at each end thereof.

18. A rotor as claimed in claim 17 wherein the outer diameter of the tubular member reduces at each end of the tubular member.

19. A rotor as claimed in any of claims 12 to 18 wherein the outer cover has a substantially uniform wall thickness over its entire length.

20. A rotor as claimed in claim 19 wherein the outer cover has a substantially uniform diameter along the majority of its length and a reduced diameter at each end thereof.

21. A rotor as claimed in claim 20 wherein the outer cover has a substantially uniform diameter for the portion of its length which overlaps the inner tubular member.

22. A rotor as claimed in any of claims 12 to 21, wherein each end cap of the rotor comprises a substantially circular base from the centre of which projects a stub shaft.

23. A rotor as claimed in claim 22 wherein the opposite side to the base to the stub shaft is provided with a circumferential seat to which the inner tubular member is mounted.

24. A rotor as claimed in claim 23 wherein the circumferential seat includes an abutment surface against which an end of the tubular member rests.

25. A rotor as claimed in claim 24 wherein the abutment surface is defined by an annular step provided around the exterior of the base.

26. A rotor as claimed in any of claims 12 to 25 wherein each end cap includes an inclined outer shoulder against which an end of the outer cover rests.

27. A rotor as claimed in any of claims 12 to 26 wherein each end cap is provided with one of an annular recess or an annular lip which, in use is received with a clearance fit within a complementarily shaped annular lip or recess of the casing.