WO2014076216A1

WO2014076216A1 - Flywheel

Info

Publication number: WO2014076216A1
Application number: PCT/EP2013/073892
Authority: WO
Inventors: John Fuller
Original assignee: Torotrak Development Ltd
Current assignee: Torotrak Development Ltd
Priority date: 2012-11-14
Filing date: 2013-11-14
Publication date: 2014-05-22
Anticipated expiration: 2015-05-14
Also published as: GB201220542D0

Abstract

A flywheel assembly having a flywheel mounted on a rotatable shaft, a housing for the flywheel capable of evacuation, a chamber for a lubricating fluid through which the shaft passes into the housing and the housing and chamber are separated by a seal such that the lubricating fluid is capable of acting on the seal to form a hermetic seal between the chamber and the housing and further comprising evacuation means to effect evacuation of the housing during rotation of the shaft. Upon rotation of the shaft, air is forced to the innermost radius of the seal around the shaft and may then bleed through the seal into the housing which has been evacuated thereby expelling air from the cavity. The evacuation means maintains the vacuum in the housing and ensures that air passes from the cavity, through the evacuated housing and out of the system.

Description

FLYWHEEL

This invention relates to a flywheel, to a kinetic energy recovery system comprising a high speed flywheel and a variable-ratio drive and to a drive train including such a system. More particularly, the invention relates to a high speed flywheel that provides for kinetic energy recovery and delivery during operation of a motor vehicle, for example a road-going motor vehicle that is powered by a combustion engine. It has particular, but not exclusive, application to a transmission system for a passenger carrying vehicle. In operation of conventional vehicles powered by combustion engines, kinetic energy of the vehicle in motion is lost by being converted to heat in the braking systems of the vehicle when the vehicle is decelerating or having its speed checked during a descent. If, instead, the kinetic energy can be recovered and stored, it can be used later to accelerate or drive the vehicle, thereby reducing the amount of fuel that is consumed by the engine.

Kinetic energy recovery is commonplace in electrically-driven vehicles, where its implementation is comparatively straightforward. However, in a vehicle driven by a combustion engine, means separate from the combustion engine must be provided to convert the kinetic energy of the vehicle into some form in which it can be stored and subsequently recovered.

One approach for implementing kinetic energy recovery in a vehicle powered by a combustion engine is to transfer kinetic energy between the vehicle itself and a flywheel that is carried within the vehicle. Through careful engineering to reduce losses, a flywheel can act as an effective store of energy. However, connecting the flywheel to the drive train of a vehicle presents considerable technical challenges. In particular, when the brakes of a vehicle are applied to slow it, energy is transferred to the flywheel thereby increasing its speed. When the vehicle is subsequently accelerated, the flywheel slows. That is, the angular speed of the flywheel changes in the opposite sense to the change in road speed of the vehicle. Therefore, in addition to the flywheel, a kinetic energy recovery system typically incorporates a variable-ratio drive and one or more clutches, and a complex control system. There is a desire to avoid complexity and to keep costs as low as possible to enable mass market use of kinetic energy recovery systems, particularly in motor cars and commercial vehicles, for example buses. Flywheels typically comprise a body mounted on a shaft for rotation at high speed. And typically are used to aid acceleration or deceleration of a vehicle and to store energy for other purposes for example for conversion into electrical energy. Typically a balance of mass, inertia and rotational speed of the flywheel must be achieved for efficient usage.

The flywheel may be driven using direct mechanical means for example a shaft. Flywheels rotate at a very high rotational speed in order to maximise the amount of energy which may be stored. High speed flywheels are typically mounted in an evacuated chamber in a housing to reduce energy losses for example due to drag and friction with surrounding air. It is known to evacuate the chamber during build and initial commissioning to avoid complexity of manufacture and in operating and running the flywheel.

However, in such systems, the direct drive needs to access the evacuated chamber whilst avoiding significant air ingress in order to maintain the vacuum. It is accordingly necessary to provide a seal between the housing and the shaft in order to allow a vacuum to be maintained within the housing.

GB-A-2448930, the disclosure of which is incorporated herein by reference, discloses a seal for a high speed flywheel mounted on a shaft, the seal comprising a housing and a cavity within the housing, a lip seal being provided either side of the cavity. The lip seals contact and encircle the shaft. The cavity has a volume which can be filled with fluid and includes means for inserting fluid into the cavity, and means for allowing the expulsion of air from the cavity during insertion of the fluid into the cavity, whereby the fluid can form a hermetical seal against the shaft. This document discloses the use of the seal with a flywheel While the lubricating fluid in this system is inserted into the cavity, air is expelled from the cavity via a bleed nipple. The fluid used should not vaporise at the pressure of the evacuated core of the flywheel. This mechanical arrangement, and associated method of preparing the hardware, adds mechanical complexity.

We have now devised a fly wheel assembly having a housing in which the flywheel is located and which is capable of being evacuated while the flywheel and its shaft are spinning and a fluid supply to a cavity for lubricating the shaft which avoids the complexity of known systems.

The invention provides in a first aspect a flywheel assembly, preferably a high speed flywheel assembly comprising a flywheel mounted on a rotatable shaft, a housing for the flywheel capable of evacuation, a chamber defining a cavity adapted to receive a lubricating fluid and through which chamber the shaft passes into the housing wherein the chamber and housing are separated by a seal such that the lubricating fluid is capable of acting on the seal to form a hermetic seal between the chamber and the housing and further comprising evacuation means to effect evacuation of the housing during rotation of the shaft.

Suitably, the flywheel assembly is configured to rotate at a rate of at least 5000 preferably at least 10000 rpm, for example at least 20000rpm.

During assembly of the flywheel assembly, the cavity may be charged with lubricating fluid and air within the cavity may be expelled during initial set-up or commissioning of the flywheel. The arrangement of the invention is such that upon rotation of the shaft, air is forced to the innermost radius of the seal around the shaft by virtue of its low density relative to the lubricating fluid and may then bleed through the seal into the housing which has been evacuated thereby expelling air from the cavity in the chamber without the need for separate constructional features in the chamber to allow expulsion of air directly from the chamber. The evacuation means maintains the vacuum in the housing and ensures that air passes from the cavity, through the evacuated housing and out of the system.

The invention further provides a flywheel assembly comprising a flywheel mounted on a rotatable shaft in a housing, an adjacent lubricant chamber through which the shaft passes into the housing, the housing and chamber being separated by a seal through which air may pass upon rotation of the shaft and further comprising evacuation means to effect evacuation of the housing during rotation of the shaft and a lubricant inlet to allow charging of lubricant into the chamber.

In referring to the flywheel being mounted on the shaft, this term is intended to encompass the case where the flywheel is formed integrally with the shaft. The flywheel and shaft may be constructed of known materials. The shaft does not require a particular grade of stainless steel however a high strength material may be used, thereby allowing the diameter of the shaft to be reduced so as to reduce drag and heat generation which improves the efficiency of the flywheel, and decreases the potential for overheating of the seal. The seal suitably encircles the shaft to form the hermetic seal between the chamber and the housing. Suitably, a seal, preferably a lip-seal is provided where the shaft passes into the chamber from externally of the device. The seal may be smooth or textured to better allow air to seep past the seal upon rotation of the shaft but without material quantities of the lubricant seeping through into the housing.

The seal suitably comprises a lip seal. In a preferred embodiment, the cavity is closed by a pair of lip seals around the shaft, one seal of the pair positioned at either end of the cavity with the lubricating fluid being inserted into the cavity between the lip-seals whereby hermetic sealing of the cavity relative to the housing is achieved.

The seal is suitably made of a polymer or a polymer blend for example comprising PTFE. Preferably the seal includes an insert provided with a bore, wherein the size of the bore determines the volume of the annular cavity, and accordingly the amount of lubricant which must to be inserted to fill the cavity. Suitably, the bore is circular, and may be formed so as to be eccentric with the outer diameter of the insert, and eccentric with the shaft on which the flywheel is supported to provide for improved fluid flow on operation of the flywheel and improving thermal heat transfer between the fluid and the housing, and accordingly reducing the potential for overheating of the seal or other components.

The chamber suitably comprises a lubricant inlet port. Other than comprising means for enabling the shaft to be located through the chamber and the lubricant inlet port, the chamber is preferably free of any other inlets or outlet. In particular, the chamber preferably does not comprise additional means for allowing expulsion of the air from the cavity.

The evacuation means suitably comprises a vacuum pump or system in fluid communication with the housing which is capable of evacuating the housing whilst the shaft is rotating. Any lubricant that passes through the seal into the housing is suitably removed from the housing, for example using the evacuation means. In a further embodiment, any lubricant which has passed into the housing may be removed using a scavenge device for example a pump, within the evacuation means or separately of the evacuation means. Lubricant is accordingly supplied to the cavity and upon rotation of the shaft, some lubricant may leave the cavity along the shaft into the housing Suitably, insofar as this occurs, the lubricant is recovered from the housing and may be recycled to the inlet port. The flywheel drive mechanism may be lubricated and the lubricant may act both as a lubricant and also to effect sealing between the cavity and the housing. The lubricant supply may comprise an accumulator. The invention preferably comprises a renewable or continuous supply of lubricant in order to maintain appropriate levels of lubricant in the event of egress or entrainment of lubricant from the chamber. Advantageously, the chamber cavity and adjacent components are continuously cooled by circulating lubricant. The risk of overheating may be reduced, therefore enabling the operation of flywheels at higher speeds and thereby providing greater energy recovery and storage capacity. The lubricant may be fed back to the lubricant inlet as desired.

The lubricating fluid may be inserted into the cavity in situ, i.e. once the flywheel has been mounted onto the shaft, via a fill nipple. Whilst the cavity is being filled with fluid, air can be expelled from the cavity by rotating the shaft whereby air passes through the seal into the housing and from there is evacuated.

Suitably, the chamber is provided with a means of varying the volume of the cavity such that excessive pressure-build up may be avoided or reduced to ensure satisfactory operation of the seal between the cavity and the housing.

The housing for the flywheel is suitably mounted on the shaft via a bearing arrangement comprising at least one bearing. Suitably, the bearing arrangement may be offset to one or other side of the flywheel, for example a cantilevered flywheel arrangement.. The bearing arrangement may be located within the evacuated chamber containing the flywheel or, preferably, outside the chamber. The bearing arrangement may be located about the shaft inboard of the seal, that is between the seal and the flywheel or, preferably outboard of the seal. A cantilevered design with single bearing arrangement advantageously requires only a single seal.

The flywheel may comprise two or more bearing arrangements, one either side of the flywheel, for example where the shaft extends beyond the housing at both ends. A seal arrangement and bearing arrangement will typically be required at each point at which the shaft passes from the inside to the outside of the housing. Suitably, a first bearing arrangement comprises a pair of angular contact bearings on the shaft on one side of the flywheel, and a second bearing arrangement on the shaft on the other side of the flywheel, wherein the second bearing arrangement is arranged so as to allow axial displacement of the bearing with respect to the shaft. Such movement also compensates for different coefficients of thermal expansion of the flywheel shaft and vacuum chamber, and different working and non-working temperatures. Relative axial movement of the bearing arrangement also allows for deflection of the vacuum chamber due to a lower internal chamber pressure than the external environment pressure.

Examples of preferred bearing arrangements are shown in GB-A-2463282, the disclosure of which is incorporated herein by reference. The lubricant in the cavity may also be employed to lubricate the at least one bearing. As the assembly comprises a lubricant inlet but not a lubricant outlet, the transport of lubricant may not be sufficient to provide adequate cooling of the shaft and/or at least one bearing. Relatively small quantities of lubricant may pass through the seal along the shaft to the evacuated chamber and provide some cooling effect but there may be a requirement to provide further means to cool the shaft.

In a further embodiment, the shaft is cooled by a coolant, for example a second lubricant supply. As desired, the second lubricant supply may be employed to lubricate the at least one bearing. The second lubricant supply may comprise the same lubricant as supplied to the cavity and, desirably is supplied to the shaft and/or at least one bearing and to the cavity from a common source.

In a preferred embodiment, the flywheel assembly comprises a coolant supply mechanism for providing coolant to the shaft and/or the bearing where present. The coolant supply mechanism suitably comprises a line for supplying coolant to the at least one bearing and the shaft to reduce the risk of overheating.

In a further aspect, the invention provides a kinetic energy recovery system comprising a flywheel, preferably a high-speed flywheel, according to the invention and a variable-ratio drive operably linked to the flywheel to transfer power to or from the flywheel.

The variable ratio drive suitably comprises a variator. A variator is a transmission component that interconnects two rotatable elements whereby, when rotating, the two elements have rotational speeds related to one another by a ratio (referred to as the "variator ratio") that can vary between a minimum variator ratio and a maximum variator ratio in a substantially stepless manner. The rotatable elements are suitably mounted on co-axial shafts and the shaft of one element has the flywheel mounted on it or is operably connected, directly or indirectly to the shaft upon which the flywheel is mounted. The variable drive may comprise gearing, for example epicyclic gearing to extend the operating range of the variable ratio drive. In a preferred embodiment, the variable-ratio drive is a continuously-variable-ratio variator or an infinitely-variable-ratio drive variator and preferably a full-toroidal variator. The variator may be a ratio-controlled device but is preferably torque-controlled.

Suitably, the kinetic energy recovery system of this invention may be incorporated into existing vehicle models without a requirement to re-engineer major components of the vehicle, to allow these systems to be fitted during production alongside conventional vehicles or retrofitted into existing vehicles. Desirably, no major component of the drive train of an existing vehicle should require modification to install the kinetic energy recovery system.

In a further aspect, the invention provides a drive train for a vehicle comprising:

a transmission system that includes a vehicle transmission and a final drive unit; and

a kinetic energy recovery system according to the invention

in which:

the kinetic energy recovery system is operatively coupled to the transmission system between the vehicle transmission and the final drive unit.

Suitably, the kinetic energy recovery system is operatively coupled to a single point of the transmission system.

The drive train may include a propshaft that is connected to transmit drive between the kinetic energy recovery system and the final drive unit. Preferably, the flywheel and the variable-ratio drive are disposed to the same lateral side of a drive axis that extends through the vehicle transmission towards the final drive.

In a preferred embodiment, one of the variable ratio drive and the flywheel is located longitudinally forwardly of the point at which the kinetic energy recovery system is connected to the transmission system and the other of the variable-ratio drive and the energy storing means is located rearwardly of the said point. The invention is illustrated in the accompanying figures in which:

Figure 1 shows a schematic diagram of a flywheel assembly according to the invention;

Figures 2 and 3 are schematic diagrams of a drive train for a passenger carrying vehicle that incorporates a drive system embodying the invention;

Figure 4 is a representation of a kinetic energy recovery system being part of the embodiment of Figures 2 and 3;

Figure 5 is a representation of an alternative kinetic energy recovery system being part of the embodiment of Figures 2 and 3;

Figure 6 is a schematic diagram of a drive train for a passenger carrying vehicle that incorporates a drive system embodying the invention wherein the variable-ratio drive and energy storage means are located laterally of the drive axis,

Figure 1 shows a flywheel assembly according to the invention having a chamber 4 defining a cavity 1 for receiving lubricating fluid 2 from lubricant inlet 3, a housing 5 for the flywheel 6, a rotatable shaft 7 passing through the chamber 4 into the housing 5 upon which the flywheel 6 is mounted. The shaft 7 may also in another embodiment pass through the housing 5 in which case a further sealing assembly comprising a chamber with a lubricant inlet and a seal, preferably substantially identical to that shown in Figure 1 may be employed. The assembly has a seal 8 between the chamber 4 and a further seal 9 between the cavity 1 and atmospheric pressure outside the chamber 4. The evacuatable housing 5 has means to evacuate the housing 10, such as a vacuum pump or management system. The lubricating fluid 2 is capable of acting on the seal 8 to form a hermetic seal between the cavity 1 in chamber 4 and the evacuated volume in housing 5.

The assembly may include a bearing within the housing 5 or outside of the housing 5 and the sealing assembly.

Upon commissioning the flywheel, the shaft 7 is rotated and lubricant 2 supplied through lubricant inlet 3. The cavity 1 is charged to the desired level with the lubricant 2. There is no exit point from cavity 1 through which the lubricant 2 is intended to pass. As lubricant is charged to cavity 2, the air in the cavity has no exit point other than via seal 8 into the evacuated chamber in housing 5. This mechanism allows air to be expelled from cavity 1 without the need for any exit port in the cavity 1 and thereby reduces complexity and costs of manufacture. A small quantity of lubricant however may also escape through seal 8 into the evacuated chamber by virtue of rotation of the shaft 7. Any such lubricant is then suitably removed from the housing 5 by the vacuum management system 10 or separately employing a scavenging device (not shown). With reference first to Figure 2, an embodiment of the invention is constituted within the drive system of a single-deck passenger carrying vehicle. The drive system is represented in Figure 2 with the rear of the vehicle towards the top of the drawing, and a front-to-rear axis of the vehicle extending vertically with respect to the drawing. The drive train includes an internal combustion engine 10 mounted longitudinally at the rear of the vehicle. An output shaft extends in a forward direction to connect through a coupling such as a torque converter or a clutch to a multi-speed vehicle transmission 12. Output from the vehicle transmission 12 is taken to a transfer box 14 through an intermediate shaft 34 (shown in Figures 4 and 5 only). From the transfer box 14, drive passes through a propshaft 16, to a final drive unit 18, which is contained within a live axle 20. Within the axle 20, drive is taken from the final drive unit through half shafts to the driven road wheels 22. The engine 10, vehicle transmission 12, transfer box 14, and an end of the propshaft 16 closest to the transfer box have a drive axis that is approximately parallel to the front-to-rear axis of the vehicle.

With the exception of the transfer box 14, the components described above are those that make up a conventional drive train of a public carrying vehicle, in particular, a single- deck passenger bus. Each component may take a variety of forms. The engine 10 will typically be a diesel engine, but might, by way of example only, be fuelled by petrol or gas. The vehicle transmission 12 will typically be an epicyclic automatic vehicle transmission (in which case the coupling will typically be a torque convertor), but could also, by way of example only, be a manual, semi-automatic, manually-controlled automatic, automatically-controlled manual, or continuously or infinitely variable ratio transmission, each of which will be paired with an appropriate clutch. An independent suspension arrangement might replace the live axle.

The drive train includes a kinetic energy recovery system, of which the transfer box 14 is a component. The kinetic energy recovery system further includes a flywheel assembly 30 and continuously-variable-ratio drive 32. In this embodiment, the variable-ratio drive includes a full-toroidal variator, but other embodiments may use one of the many other types of continuously-variable-ratio drive that will be familiar to those skilled in this technical field. The flywheel assembly 30 and the variable-ratio drive 32 may be self- contained units, each being connected to the transfer box 14.

In relation to the vehicle as a whole, the flywheel assembly 30 extends towards the front from the transfer box 14 and the variable-ratio drive 32 extends towards the rear from the transfer box 14, and both of the flywheel assembly 30 and the variable-ratio drive 32 are laterally to one side (in this case, the right side) of the drive axis. The particular disposition of these components may be rearranged to avoid conflict with existing components of the vehicle. For example, the flywheel assembly 30 might extend towards the rear from the transfer box 14 and the variable-ratio drive 32 extends towards the front from the transfer box 14. Likewise, the variable-ratio drive 32 and the flywheel assembly 30 may be disposed laterally to opposite sides of the drive axis.

There are various limitations imposed upon the configuration of a drive train for a passenger carrying vehicle. In particular, the rear overhang of the vehicle (the distance between the rear axle line and the rear of the vehicle) cannot exceed a maximum that is set by statutory regulation. This imposes a maximum length on the drive train along the front-to-rear axis of the vehicle. The propshaft 16 must be capable of articulation to accommodate relative movement between the axle 20 (which moves in relation to the vehicle as a whole to provide suspension travel) and the vehicle transmission (which is fixed to a frame of the vehicle). This imposes a minimum length on the propshaft 16. Together, these limitations impose a requirement, or, at least, a strong preference, that the thickness of the transfer box (that is, the length that it occupies along the front-to-rear axis of the vehicle) is kept to a minimum. The arrangement of this embodiment provides this, because drive from the vehicle transmission 12, through the transfer box 14, to the propshaft 16 is not carried across the front-to-rear axis of the vehicle.

The internal arrangement of the kinetic energy recovery system will now be described briefly with respect to Figures 4 and 5.

The intermediate shaft 34 is connected to a first clutch 40 within the transfer box 14. A first output of the first clutch 40 is connected to the propshaft 16. A second output of the first clutch 40 is connected to a first spur gear 42 that is coaxial with the intermediate shaft 34, and positioned axially between the vehicle transmission 12 and the first clutch 40. The first spur gear 42 is in mesh with a second spur gear 44, to which it transmits drive transversely to the drive axis, the second spur gear 44 being connected to an input of the variable-ratio drive 32. An output of the variable-ratio drive 32 is connected through a third spur gear 46 to an input of a step-up gearset 48 within the transfer box 14 that has an axis that is parallel with the intermediate shaft 34. The step-up gearset 48 is arranged to cause the flywheel to rotate at a speed that is greater than the output of the third spur gear 46. In this embodiment, the step-up gearset 48 is an epicyclic gearset, which is advantageous for its compact size. An output of the step-up gearset 48 is connected through a second clutch 50 to the flywheel 30.

Operation of the kinetic energy recovery system is in accordance with existing flywheel- based kinetic energy recovery systems, so will not be described in detail. The first clutch 40 is used to optionally connect the propshaft 16 direct to the vehicle transmission 12, to drive the vehicle in a conventional manner using the combustion engine 10, it can connect the propshaft 16 (and therefore, the final drive 18) through the variable-ratio drive 32 and the step-up gearset 48 to the flywheel 30 to allow kinetic energy to be transferred between the flywheel 30 and the vehicle. The second clutch 50 can be disengaged to allow the flywheel 30 to spin freely to act as an energy store with a minimum of energy loss.

As has been previously mentioned, the range of rotational speed at the output of the vehicle transmission 12 is greater than at the input. The transfer box 14 shown in Figure 4 is intended to address this. The first clutch 14 can connect the intermediate shaft 34 to the output 16, to the first spur gear 42 or to a fourth spur gear 54. The fourth spur gear 54 is in mesh with a fifth spur gear 56, which is fixed for rotation with the second spur gear 44. Provided that the first and fourth spur gears 42; 54 are not the same size, connection of the intermediate shaft 34 to one or other of the first or fourth spur gear 42; 54 results in a different ratio between the intermediate shaft 34 and the input to the variable-ratio drive 32. Selection of the respective sizes of the first and second spur gears 42; 44 and the fourth and fifth spur gears 54; 56 allows the range in the speed of the input to the variable-ratio drive 32 to be kept within an operational range over a greater range of input speeds as compared with the embodiment of Figure 4.

As implemented in a vehicle, it is common for the engine 10 in a drive train such as is described to be mounted on longitudinal elements of the frame of the vehicle. In practice, engine mounts will typically be bolted through mounting holes in the longitudinal elements. To install a drive train embodying the invention in such a vehicle, it will often be possible to remove the engine mounts from the longitudinal elements, form additional mounting holes in the longitudinal elements to the rear of the original mounting holes by a distance equal to the thickness of the transfer box 14, and re-install the engine and its mounts using the new mounting holes. This provides the additional space required to accommodate the transfer box 14 between the engine 10 and the propshaft 16. While vehicles may have alternative arrangements for mounting their engine, essentially the same procedure can be used to re-position the engine in order to install a kinetic energy recovery system of the type described.

New vehicles can be produced with a frame that includes two sets of mounting holes or other mounting apparatus, whereby an engine can be installed in one of two positions dependent upon whether or not the vehicle will be provided with a kinetic energy recovery system.

Figure 6 shows a drive train according to the invention comprising the engine 401 , the vehicle transmission 403, the propshaft 405 which is operatively coupled to the final drive unit (not shown). The kinetic energy recovery system comprises the energy storage means 407, the variable ratio drive 409 and these are operatively coupled to the transmission system at the output of the vehicle transmission 403 or to the propshaft 405 via the transfer box 41 1. The variable-ratio drive 409 is located rearwardly of the line A- A at which the kinetic energy recovery system is operatively coupled to the transmission system and the energy storage means 407 is located forwardly of the line A-A. The variable-ratio drive 407 and energy storage means 409 are operatively linked by layshaft 413. The disc of the variable ratio drive which is located most rearwardly provides the take-off drive for the layshaft.

Claims

1 . A flywheel assembly comprising a flywheel mounted on a rotatable shaft, a housing for the flywheel capable of evacuation, a chamber defining a cavity adapted to receive a lubricating fluid and through which chamber the shaft passes into the housing wherein the chamber and housing are separated by a seal such that the lubricating fluid is capable of acting on the seal to form a hermetic seal between the chamber and the housing and further comprising evacuation means to effect evacuation of the housing during rotation of the shaft.

A flywheel assembly comprising a flywheel mounted on a rotatable shaft in a housing, an adjacent lubricant chamber through which the shaft passes into the housing, the housing and chamber being separated by a seal through which air may pass upon rotation of the shaft and further comprising evacuation means to effect evacuation of the housing during rotation of the shaft.

A flywheel assembly according to claim 1 or claim 2 which further comprises a lubricant inlet to allow charging of lubricant into the chamber.

A flywheel assembly according to any one of claims 1 to 3 wherein the seal is textured thereby to allow air from the cavity to seep past the seal upon rotation of the shaft and insertion of lubricant into the cavity but to avoid material quantities of the lubricant seeping through the seal into the housing.

A flywheel assembly according to any one of claims 1 to 4 wherein the shaft for the flywheel is mounted on the housing via a bearing arrangement comprising at least one bearing.

A flywheel assembly according to claim 5 wherein the bearing is located on the opposite side of the seal to the flywheel.

A flywheel assembly according to any one of claims 1 to 6 wherein the shaft for the flywheel is mounted on the housing via a bearing arrangement comprising at least one bearing and the assembly further comprises a coolant supply mechanism for providing coolant to the shaft and/or the bearing .

A kinetic energy recovery system comprising a flywheel as defined in any one of claims 1 to 7 and a variable-ratio drive operably linked to the flywheel to transfer power to or from the flywheel.

9. A kinetic energy recovery system according to claim 8 in which the variable-ratio drive is a continuously-variable-ratio or infinitely-variable-ratio drive.

10. A kinetic energy recovery system according to claim 9 wherein the variator is torque- controlled.

1 1 . A kinetic energy recovery system according to claim 9 or 10 in which the variable- ratio drive includes a full-toroidal variator.

12. A drive train for a vehicle comprising

a transmission system that includes a vehicle transmission and a final drive unit; and a kinetic energy recovery system as defined in any one of claims 8 to 1 1

in which the kinetic energy recovery system is operatively coupled to the transmission system between the vehicle transmission and the final drive unit.

13. A drive train for a vehicle according to claim 12 wherein the kinetic energy recovery system is operatively coupled to a single point of the transmission system.

14. A drive train for a vehicle according to claim 12 or claim 13 that further includes a propshaft that is connected to transmit drive between the kinetic energy recovery system and the final drive unit.

15. A drive train for a vehicle according to any one of claims 12 to 14 in which the flywheel and the variable-ratio drive are disposed to the same lateral side of a drive axis that extends through the vehicle transmission towards the final drive.

16. A drive train for a vehicle according to claim 15 in which one of the variable ratio drive and the flywheel is located longitudinally forwardly of the point at which the kinetic energy recovery system is connected to the transmission system and the other of the variable-ratio drive and the energy storing means is located rearwardly of the said point.

17. A motor vehicle that comprises a drive train according to any one of claims 12 to 16.

18. A vehicle according to claim 17 that is a passenger carrying vehicle.