GB2559203A

GB2559203A - Kinetic energy recovery system for a vehicle

Info

Publication number: GB2559203A
Application number: GB1701609.8A
Authority: GB
Inventors: hughes Steve; Jonathan Deakin Andrew
Original assignee: Flybrid Automotive Ltd
Current assignee: Flybrid Automotive Ltd
Priority date: 2017-01-31
Filing date: 2017-01-31
Publication date: 2018-08-01
Also published as: GB201701609D0

Abstract

A kinetic energy recovery system (KERS) 11 for a vehicle is provided. The KERS 11 is arranged to be connected between an energy storage flywheel and an output couplable to a powertrain of the vehicle. The KERS 11 further includes an energy transfer transmission including a power-split arrangement comprising a hydrostatic continuously variable transmission (CVT) 6. The energy transfer transmission may also include an epicyclic gearset 5 and is provided between the input and the output of the KERS. The power-split arrangement between the CVT 6 and the epicyclic gearset 5 may comprise an input coupled shunt (Figure 1) or an output coupled shunt (Figure 2).

Description

(54) Title of the Invention: Kinetic energy recovery system for a vehicle Abstract Title: A kinetic energy recovery system for a vehicle (57) A kinetic energy recovery system (KERS) 11 for a vehicle is provided. The KERS 11 is arranged to be connected between an energy storage flywheel and an output couplable to a powertrain of the vehicle. The KERS 11 further includes an energy transfer transmission including a power-split arrangement comprising a hydrostatic continuously variable transmission (CVT) 6. The energy transfer transmission may also include an epicyclic gearset 5 and is provided between the input and the output of the KERS. The power-split arrangement between the CVT 6 and the epicyclic gearset 5 may comprise an input coupled shunt (Figure 1) or an output coupled shunt (Figure 2).

7b

Figure 1

1/3

10b

Output

7b

Figure 2

2/3

Figure 3

Figure 4

3/3

Figure 5

100%

KERS efficiency

Efficiency/Power Output Percentage %

Percentage of KERS output power transferred by the CVT

0%

KERS speed ratio (% of maximum)

0% 100% (Vehicle stationary)

Figure 6

Kinetic Energy Recovery System for a Vehicle

The present invention relates to a kinetic energy recovery system (KERS) and in particular, to a KERS in which energy is stored as rotational kinetic energy in a flywheel. The invention has particular, but not exclusive, application to a hybrid powertrain for a vehicle. The abbreviation “KERS” will be used to refer to a kinetic energy recovery system.

It is known to attach a KERS to a part of vehicle drivetrain such as the engine, the main transmission output or the rear axle itself. In these two latter two arrangements, an input to the KERS turns at a speed that is directly related to the vehicle road speed.

The KERS comprises typically a flywheel system and a transmission for transmitting power to and from a power sink/source, this sink/source typically being the wheels of the vehicle (although the sink/source may equally be the prime mover such as an electric motor or internal combustion engine). The significance of the sink/source is that power flow transmitted to or from the flywheel may be in either direction. The transmission is required to accommodate a continuously varying speed ratio between the flywheel and the power source/sink as the speed of at least the flywheel and potentially both the flywheel and the power source/sink varies continuously as power is exchanged between the two. Typically, power flow to the flywheel causes the flywheel to accelerate thus increasing its speed whilst (in this case) the sink/source acts as a source and is decelerated. Thus, power flow to the flywheel causes the source/sink to decelerate; examples of this event include vehicle regenerative braking. Conversely, power flow from the flywheel causes the flywheel to decelerate thus decreasing its speed whilst, in this case, the sink/source acts as a sink and is accelerated; accordingly, power flow from the flywheel causes the source/sink to accelerate; examples of this event include vehicle acceleration using flywheel energy to do so. The case whereby the power source/sink is a vehicle will be used in the following description.

There are two principal elements of a KERS: the energy storage device, and the energy transfer transmission. The energy storage device principally considered in this invention is a rotary mechanical storage device, or a flywheel. Typically this will be a high speed flywheel which is adapted to run up to a maximum speed of 20,000 rpm or more. A high speed flywheel enables high levels of energy to be stored in the flywheel. The energy transmission has requirements that it should have a continuously variable ratio, and preferably also has a high efficiency and a suitable ratio spread. Challenges can exist in achieving acceptable transmission characteristics in a low cost, reliable, compact, and proven solution. In vehicle applications in particular, achieving a solution which offers packaging flexibility can be a great advantage when the device is to be fitted to a range of different vehicle types and sizes. The importance, and challenges associated with, each of these aspects will now be explained more fully.

High efficiency is desirable since the energy which can be transferred to the flywheel, and subsequently returned to the vehicle, is related to the square of the One-way’ energy transmission efficiency. In the case where a mechanical continuously variable transmission (CVT) is used (and note that CVTs are traditionally less efficient than fixed ratio power transfer devices such as gear sets) the one-way efficiency may be 80-90%, depending on the gear stages and any CVT actuation power required; in the case where 84% is assumed, then 0.84x0.84 = 70% - thus 70% of the vehicle kinetic energy may be returned by the KERS to the vehicle if the KERS is available to accommodate the vehicle speed fully during an deceleration and subsequent acceleration event. If the oneway transmission efficiency were increased to 92% then 85% of the energy may be returned, representing a 20% increase (85/70) in returned energy. Furthermore, a mechanical CVT will typically have a finite range of ratios that do not include a ‘zero’ ratio (that is, a stationary vehicle cannot be accommodated with a fully engaged KERS transmission whilst the flywheel is rotating), so that as the vehicle comes to rest, the CVT must disengage from the vehicle driveline. The actual energy which may be returned would therefore be reduced below the previously stated 70%. A CVT which can be engaged across the entire applicable speed range of the vehicle, and which has good efficiency, will be advantageous for improving energy recovery characteristics of vehicles and other machinery.

As the bulk of transmissions in production are fixed ratio transmissions, CVTs may be perceived by some vehicle suppliers as a higher technical risk option and may therefore prefer to use more conventional transmissions in the KERS, if possible. The ability to provide a solution which provides packaging flexibility using standard or widely used subsystems can be advantageous in reducing development cost and time, and in mitigating perceived technical risk.

An aim of this invention is to improve the efficiency and ratio spread of an energy transfer transmission in a KERS for a vehicle or a machine. A further aim of this invention is to provide a KERS which is able to recover increased energy in a vehicle or machine. A further aim of this invention is to provide a flexible, low technical risk, low cost and compact KERS for use in a vehicle or a machine.

Therefore according to the present invention there is provided a Kinetic Energy Recovery System (KERS) for a vehicle, the KERS comprising: an energy storage flywheel; an input couplable, in use, to the flywheel; an output couplable to a powertrain of the vehicle; and an energy transfer transmission including a power-split arrangement comprising a hydrostatic continuously variable transmission (CVT) wherein the energy transfer transmission is provided between the input and the output.

The power-split arrangement is so termed because it includes two power transmission paths. This preferably includes a variable ratio path (i.e. a power transmission path whose ratio is continuously variable) and a fixed ratio path, the latter typically being of higher efficiency than the former. The power paths are preferably arranged to be in parallel with one another. This layout is termed a ‘shunt’. In one form of shunt, the speeds from the variable and fixed ratio paths are summed at one end (i.e. at an input end or an output end) in the differential speed device, and the torques are summed at the other end (i.e. the other of the input and output end) at a node termed the ‘coupling point’, or ‘torque summing junction’. Since only a portion of the total power passes through the less efficient variable ratio path in a power-split shunt, the overall efficiency of the device is increased compared with an arrangement which includes only a variable ratio path. This may be illustrated from the simplified expression which adds estimated losses of each path together to form a combined efficiency of the KERS transmission:

KERS transmission efficiency = Pcvt x (1 - E_Cvt) + (1 - Pcvt) x (1 - E_Frp)

Pcvt = Proportion of total power passing through CVT

Ecvt = Efficiency of CVT

Efrp = Efficiency of fixed ratio path

If 25% of the total power passes through the CVT at a particular operating point, and the fixed ratio path has an efficiency of 96% and the CVT path has an efficiency of 84%„ then the KERS efficiency may be estimated to be:

KERS transmission efficiency ~ 1 - { [0.25 x (1 - 0.84)] + [(1 - 0.25) x (1- 0.96)]} = 93%

This is significantly higher than the 84% of the CVT path alone. The above estimate excludes efficiency effects from other drive ratios such as the final drive, but is sufficient for illustrative purposes.

The power-split arrangement may comprise a differential or speed-mixing device with at least three inputs. The differential or speed-mixing arrangement preferably has exactly three inputs. Typically, the differential or speed-mixing arrangement comprises an epicyclic gearset. The three inputs in this gearset may be a sun, carrier and an annulus, but other forms of epicyclic such as a bevel gear type (as is found in final drive transmissions), or an idler epicyclic (in which pairs of planets in mutual engagement with one another, and mounted within the carrier, mesh with the sun and annulus) are also known. The epicyclic may suitably comprise a simple sun, carrier and annulus, or may alternatively comprise two sun gears and a carrier comprising bevel gears; in another example the epicyclic comprises a sun gear, an idler planet carrier arrangement and annulus.

The arrangement may comprise a shunt, and this may be an input or an output coupled shunt. In an input coupled arrangement, the torque summing junction is at the input side (in this description, the flywheel side) of the transmission. In such an arrangement, the output from the flywheel is divided into two separate paths, optionally using a splitter gear or other arrangement, as required. In such an arrangement, one path may be operatively coupled via one or more fixed ratios to the input of the hydrostatic CVT whilst the other path can be operatively coupled via one or more fixed ratios to a first input of the differential device such as the epicyclic unit. The output side of the CVT can be operatively coupled to a second input of the differential device or epicyclic, whilst the third element of the differential device or epicyclic may then be operatively coupled to the output of the KERS, which is operatively coupled to (for example) the wheels of the vehicle, optionally via a transmission, prop-shaft or final drive of the vehicle.

Preferably the KERS transmission comprises an output coupled shunt, where the torque summing junction is on the KERS output side (in this example, on the vehicle wheel side). In such an arrangement, the flywheel is operatively coupled to a first element of the differential device or epicyclic, a second input of the differential device or epicyclic is coupled to the input of the CVT, and the third element of the differential device or epicyclic is coupled to the CVT output and the KERS output at the torque summing junction.

The hydrostatic CVT may comprise two pumping units (these may each be configured as either a hydraulic pump and/or a motor), and may have a ratio that is varied by adjusting the fluid displacement of one or both units. Such adjustment may be made by adjusting a swash plate angle in an axial piston pump, or it may be made by adjusting the angle of a bent-axis piston pump arrangement, as required. Relative displacement settings of the pumping elements can be used to set the speed ratio (and torque ratio) of the hydrostatic CVT. One or both of the pumping units may be able to set to a substantially zero displacement.

Returning to the output coupled shunt arrangement, if the displacement of the pumping unit on the CVT input side is set substantially to zero then the KERS may be engaged (i.e. the flywheel is connected via an engaged KERS to the wheels) even when the vehicle is stationary. The hydrostatic CVT ratio is effectively locked so that its output which is connected to the KERS output, has a zero speed. Thus torque can be applied by the KERS to the vehicle wheels even when the vehicle is near a stationary condition, thus maximising energy transfer possibilities. Further, there is no need to include a disconnect, clutch or launch device between the KERS and the wheels of the vehicle, thus saving on weight, cost and complexity. It should be noted that in order to achieve this ‘engaged KERS neutral’ function with the input coupled arrangement, the epicyclic unit would need to mix non-zero speeds from the flywheel and from the output of the hydrostatic CVT in order to provide a zero output speed condition on its third element, thus providing the zero speed condition at the vehicle wheels. When torque is applied to the wheels with such an input-coupled arrangement, power recirculation within the KERS transmission can result in an undesirably high level of power loss. The output coupled shunt is therefore preferable as it is offers greater efficiency under the launch condition as well as being able to accommodate frequent vehicle stops and launches. This is important in urban cycle vehicles such as city cars, refuse trucks, delivery trucks, and especially in buses but also in other applications such as off-highway vehicles such as loaders and other plant and machinery. As the ratio shifts away from the stationary vehicle (‘KERS engaged neutral’) condition, all power passes through the CVT, which as mentioned previously may be relatively inefficient; however, this power level is relatively low since vehicle speed is close to zero, and drag losses will be low because there is no fluid circulating within the hydrostatic CVT. This output coupled power-split shunt is a preferable arrangement to the recirculating power condition of the input coupled shunt, where much energy would be lost when the vehicle is stationary, such as a bus waiting in a depot or at a bus stop.

A further advantage of this output coupled arrangement can be realised when the pumping element on the output side (in this example, the wheel side) of the hydrostatic CVT is able to adopt a substantially zero displacement condition such that the CVT input (and also the second input of the differential device or epicyclic) is constrained to have a substantially zero speed. In this condition, the KERS transmission may be at an overdrive ratio (that is, high vehicle speed relative to flywheel speed) but none of the total power passing through the transmission passes through the CVT. Thus, cruising at higher vehicle speeds is very efficient, and the high powers associated with energy recovery from high vehicle speeds can be recovered very efficiently as all power passes through the mechanical drive path of the KERS transmission.

The hydrostatic CVT may therefore be capable of achieving a condition at which its speed ratio, defined as a CVT output speed divided by a CVT input speed, is substantially zero.

Alternatively or additionally, the hydrostatic CVT may be capable of achieving a condition at which the reciprocal of its speed ratio, defined as a CVT input speed divided by a CVT output speed, is substantially zero. The hydrostatic CVT preferably comprises two pumping elements, each comprising a respective variable fluid displacement mechanism. Such mechanism is adapted to vary the fluid displaced by the pumping element per revolution of the pump’s driveshaft.

Preferably both the hydrostatic CVT pumping elements can achieve a fluid displacement setting which is substantially zero. In the case of both the input or output coupled KERS, a zero vehicle speed condition (zero KERS ratio) may be provided as described earlier.

In such cases, a clutch or mechanical disconnect between the KERS transmission torque summing junction and the driven wheels of the vehicle may be eliminated; the KERS output may thus, in use, be permanently coupled to the vehicle road wheels.

The present invention further provides a vehicle comprising a KERS such as that described above wherein the KERS comprises an input which is operatively coupled to an energy storage flywheel and an output which is operatively coupled to the drivetrain of the vehicle. The KERS output may be coupled between the output of a vehicle main drive transmission and the vehicle wheels. The KERS output may be operatively coupled to a vehicle prop-shaft or vehicle main drive transmission, or it may be operatively coupled to a vehicle final drive arrangement. The KERS output may alternatively be operatively coupled to a power take-off of the vehicle, which can be advantageous in reducing installation or integration costs. Said power take-off may be a power-take-off of the vehicle main drive transmission.

As the drivetrain to which the KERS is connected may be stationary in some situations, in the case of both the input or output coupled KERS transmission arrangements, a zero vehicle speed condition (zero KERS ratio or ‘engaged KERS neutral’) may be provided, as described earlier; thus a clutch or mechanical disconnect between the KERS and the driven wheels may be eliminated. The KERS output may thus, in use, be permanently coupled to the road wheels.

One or more clutches or mechanical disconnects, such as a friction clutch or a dog clutch, may optionally be provided to alleviate driveline drag losses arising from the pumping elements, to help in improving the efficiency of the system. If only one clutch or disconnect is used, this is optionally placed between the pumping elements and the road wheels, between the pumping elements and the epicyclic or between the epicyclic and the road wheels. If two clutches or disconnects are provided, then these may be placed in various combinations, preferably to eliminate the pumping element drag referred to both the flywheel and the vehicle wheels. For example, one may be placed between the pumping elements and the road wheels and the other between the pumping elements and the epicyclic.

The invention may be implemented in a variety of vehicles or machines. It is particularly advantageous for applications where the vehicle or machine is often changing speed or making frequent stops in use, allowing the KERS to recover energy that would otherwise be lost into the brakes or elsewhere. The vehicle or machine may be an urban passenger vehicle, such a bus, a coach, or a city car. Alternatively, the vehicle may be a construction, industrial or agricultural vehicle. The system may be used on wheeled or tracked vehicles and on vehicles for either on or off-highway use. The vehicle or machine may be adapted and/or arranged for repeated cyclic working operations. Also, the vehicle or machine may be a loading, or a material shifting vehicle or machine, such as an excavator, shovel, fork-lift or wheeled loader.

The present invention will now be described in more detail with reference to the attached drawings in which:

Figure 1 shows an input coupled shunted KERS transmission;

Figure 2 shows an output coupled shunted KERS transmission;

Figure 3 shows a KERS mounted on a main drive vehicle transmission output;

Figure 4 shows a KERS mounted on a vehicle final drive arrangement of a vehicle;

Figure 5 shows an alternative embodiment of a KERS mounted on a vehicle final drive arrangement of a vehicle; and

Figure 6 shows the efficiency and CVT power factor characteristics of a KERS transmission for an output coupled shunt transmission.

Figure 1 shows an example of an input coupled shunted KERS transmission 11 according to the invention. The transmission comprises an input 8 coupled to an energy storage flywheel (not shown), and output 9 (coupled to vehicle road wheels), a hydrostatic CVT 6 comprising two pumping elements 7 which are hydraulically coupled as is known, preferably each with a variable fluid displacement mechanism such as a variable swash plate or bent axis pump arrangement. The input 8 is also coupled (optionally via one or both ratios R1 10a and R2 10b) to a first connection 1 of the epicyclic gearset 5. One pumping element 7a drive shaft is coupled to the input side of the shunt transmission 11 whilst the drive shaft of the other pumping element 7b is coupled to a second input 2 of the epicyclic 5. The epicyclic third element 3 is coupled to the KERS output 9 which is coupled to the powertrain of a machine, or a vehicle, such as a transmission output, final drive or power take-off of a vehicle or main transmission. Other ratios, e.g. R4 10d, R5 10e may suitably be included. Suitable selection of the ratio values R1, R2, R3, R4 and R5 allows the provision of the required KERS ratio range to suit the breadth of flywheel vehicle speeds in any particular application.

Figure 2 shows a preferred power-split output coupled shunt KERS transmission comprising an input 8 coupled to an energy storage flywheel (not shown), and output 9 (coupled to vehicle road wheels), a hydrostatic CVT 6 comprising two pumping elements 7 which are hydraulically coupled, each preferably with a variable fluid displacement mechanism such as a variable swash plate or bent axis mechanism. The input 8 is coupled (optionally via a ratio R1 10a) to a first connection 1 of the epicyclic gearset 5. One pumping element 7a drive shaft is coupled to a second input 2 of the epicyclic 5 (optionally via a ratio R3 10c). The epicyclic third element 3 is coupled optionally via ratios R2 10b and/or R5 10e to the KERS output 9 which is coupled to the powertrain of the vehicle, such as a transmission output, final drive or power take-off of the vehicle or main transmission. The CVT output is coupled, optionally via ratio R4 10d and/or R510 e, to the KERS output 9. Other ratios 10a-e may suitably be included in order to create the required breadth of KERS ratios to suit the required flywheel and vehicle speed range in any particular application.

In the above embodiments, epicyclic connection 1 is a sun gear, epicyclic connection 2 is an annulus gear and epicyclic connection 3 is a planet carrier. The epicyclic is a simple epicyclic comprising a sun gear, planet carrier with a single ring of planet gears, and an annulus (ring) gear. However, a great many variations of epicyclic are possible, as will be known to the skilled person and the invention is not limited to this specific type of epicyclic configuration.

Figure 3 shows an installation of a KERS in the powertrain of a vehicle such as a bus. In this example the engine 12, main drive transmission 13, prop-shaft 15, final drive 16 and wheels 18, are all connected in series (as is known). The engine 12 drives the wheels 18 through the transmission 13 and final drive 16. All elements may be located as required in the vehicle, but in the case of a bus the engine 12, transmission 13, prop-shaft 15 and KERS 11 may be located in the rear of the vehicle behind the rear wheels 18. A transfer box 14 transfers drive from the transmission 13 output to the input of the KERS 11. In this example, the KERS 11 is located to the side of the transmission 13.

Figure 4 shows a similar vehicle powertrain as Figure 3, except that the transfer box 14 transfers drive from the input of the final drive 16 to the input of the KERS 11 via an additional KERS driveshaft 19. In this example the KERS 11 is again located to the side of the transmission 13.

Figure 5 shows a similar vehicle powertrain as Figures 3 and 4, except that the transfer box (not labelled) transfers drive from the input of the final drive 16 to the input of the KERS 11 via an angled driveshaft (not labelled). The KERS 11 is again located to the side of the transmission 13.

Figure 6 shows a graph of the characteristics of the output coupled power-split KERS with KERS speed ratio (shown on the horizontal axis of the graph). Percentage efficiency is shown by the dashed line, while the percentage of power passing through the CVT as a proportion of the KERS total transferred power (the corresponding proportion metric is termed the ‘power factor’) is shown by the dash-dot line. The power factor is shown to be 100%, that is, all of the KERS power transferred is passing through the hydrostatic CVT, when the flywheel is rotating but the vehicle is stationary. This is shown at the left-most end of the graph, and describes the condition where the flywheel is launching the vehicle from rest and passing previously captured vehicle kinetic energy back to the vehicle wheels.

The efficiency at this condition will be dominated by the hydrostatic efficiency which may be between 60 and 90%. However, it should be noted that although this is a little lower than the efficiency of power-split systems or indeed other types of CVT, the amount of power transferred is limited because the vehicle speed is very low or in fact zero. Importantly, power is not recirculated within the KERS (as it would be in the input coupled arrangement of Figure 1), a situation which would create higher losses than a simple power-split with all of the KERS power passing through the CVT. At this ‘zero vehicle speed’ condition the pumping element of the CVT that is connected to the epicyclic second connection 2 has a fluid displacement setting which is substantially zero whereas the pumping element connected to the KERS output has a non-zero fluid displacement, and its displacement may be at a maximum setting. As the displacement of the pumping element 7a of the CVT that is connected to the epicyclic second connection 2 is increased above zero, the output of the KERS, and therefore the vehicle wheels, will begin to rotate. (Note that this is for illustrative purposes - the wheels may also be made to rotate by receiving torque from another source such as the main drive transmission; however, the KERS must still accommodate the change in vehicle speed by shifting its ratio). Power starts to flow through the mechanical path as well as the CVT, thus the efficiency of the KERS starts to increase, as is shown by moving from left to right in Figure 6.

As the displacement of the said pumping element 7a is increased further, the vehicle speed continues to increase and/or the flywheel speed continues to decrease as energy is drawn from the flywheel to accelerate the vehicle, until the displacement of this pumping element 7a reaches a maximum setting. At this point, the KERS ratio may be increased further by decreasing the displacement setting of the pumping element 7b whose driveshaft is connected to the KERS output 9. Efficiency continues to increase with increasing KERS ratio (and therefore vehicle speed) until the displacement of the KERS output-side pumping element 7b reaches zero. At this point, no fluid is transferred through the hydrostatic fluid circuit with the result that the pumping element 7a connected to the second input 2 of the epicyclic 5 is now substantially locked. In this condition, all of the power passing through the KERS transmission passes through the mechanical elements of the ratios 10a, 10b, 10e and epicyclic 5 rather than the CVT 6, thus KERS efficiency is at a maximum. This is shown in the right-most region of the graph of Figure 6. In this way, the vehicle speed range may be accommodated by the KERS without use of a clutching or vehicle launch system. Efficiency of the KERS increases as vehicle speed, and hence kinetic energy of the vehicle, increases. Recirculating power may also be avoided at all forward speeds of the vehicle, which enables low power consumption by the KERS at all KERS ratios.

However, it should be noted that in another embodiment of the output coupled KERS transmission, the KERS ratio range can be extended to an even higher range by continuing to sweep the displacement varying mechanism of the pumping element whose shaft is operatively coupled to the KERS output such that the driveshaft of the other pumping element (which is connected to the epicyclic second element) may start to rotate in the opposite direction. This will cause power to flow in the opposite sense (i.e. from output to input) within the CVT. Such power reversal will lead to some recirculating power within the KERS shunt, and thus may slightly increase power losses at higher KERS transmission ratios. However, this may be acceptable as there may be a net benefit in fuel saving, overall efficiency or KERS function by extending the KERS overdrive ratio in this way.

Therefore, in such embodiments the use of a hydrostatic CVT enables the zero vehicle speed condition, and the 100% mechanical power path at full, or at least a high, KERS ratio, due to the ability for this type of CVT to achieve a zero and infinite speed ratio. Although hydrostatic CVTs are not known for having the highest of efficiencies compared to other CVTs, they are perceived as being well developed and robust and are widely used in main drives and industrial applications. Use of the power-split architectures described in this invention enable the use of such hydrostatic CVTs, reducing their losses and increasing their efficiency so that they become an attractive option for use in KERS applications.

Claims

1. A Kinetic Energy Recovery System (KERS) for a vehicle, the KERS comprising: an input couplable, in use, to an energy storage flywheel;

an output couplable to a powertrain of the vehicle; and an energy transfer transmission including a power-split arrangement comprising a hydrostatic continuously variable transmission (CVT) wherein the energy transfer transmission is provided between the input and the output.

2. A Kinetic Energy Recovery System according to claim 1 wherein the power-split arrangement comprises a differential or speed-mixing device with at least three inputs.

3. A Kinetic Energy Recovery System according to claim 2 wherein the differential or speed-mixing arrangement has exactly three inputs.

4. A Kinetic Energy Recovery System according to claim 2 or 3 wherein the differential or speed-mixing arrangement comprises an epicyclic gearset.

5. A Kinetic Energy Recovery System according to claim 4, wherein the epicyclic gearset has a sun gear, an annulus gear and a planet carrier gear and wherein the sun gear is connected to the KERS input, the planet carrier gear is connected to the KERS output and the annulus gear is connected to one side of the hydrostatic CVT.

6. A Kinetic Energy Recovery System according to claim 5 wherein the planet carrier gear is connected to the other side of the hydrostatic CVT.

7. A Kinetic Energy Recovery System according to claim 5 wherein the sun gear is connected to the other side of the hydrostatic CVT.

8. A Kinetic Energy Recovery System according to any one of claims 1 to 6 in which the power-split arrangement comprises an output coupled shunt.

9. A Kinetic Energy Recovery System according to any preceding claim wherein the hydrostatic CVT is configurable to a condition at which its speed ratio, defined as a CVT output speed divided by a CVT input speed, is substantially zero.

10. A Kinetic Energy Recovery System according to any preceding claim wherein the hydrostatic CVT is configurable to a condition at which the reciprocal of its speed ratio, defined as a CVT input speed divided by a CVT output speed, is substantially zero.

11. A Kinetic Energy Recovery System according to any preceding claim wherein the hydrostatic CVT comprises two pumping elements, each with an associated fluid displacement, wherein at least one of said pumping elements comprises a mechanism for varying said fluid displacement.

12. A Kinetic Energy Recovery System according to claim 11 wherein the fluid displacement of said at least one variable displacement pumping element can be varied to be substantially zero.

13. A Kinetic Energy Recovery System according to claim 11 or claim 12 wherein each of said pumping elements comprises a respective mechanism for varying the respective fluid displacement.

14. A Kinetic Energy Recovery System according to claim 13 wherein both of said pumping elements can achieve a fluid displacement which is substantially zero.

15. A Kinetic Energy Recovery System according to any preceding claim further comprising a flywheel, wherein said KERS input is coupled to the flywheel.

16. A vehicle comprising a KERS according to any one of claims 1 to 14, a flywheel and a powertrain, wherein said KERS input is coupled to the flywheel and said KERS output is operatively coupled to the powertrain.

17. A vehicle according to claim 16 wherein the KERS output is operatively coupled between the output of a vehicle main drive transmission and the vehicle wheels or tracks.

18. A vehicle according to claim 16 wherein the KERS output is operatively coupled to a vehicle final drive arrangement.

19. A vehicle according to claim 16 or claim 17 wherein the KERS output is 5 operatively coupled to a vehicle prop-shaft or a vehicle main drive transmission.

20. A vehicle according to any one of claims 16 to 19 wherein the KERS output is operatively coupled to a power take-off of the vehicle.

10

21. A vehicle according to claim 20 wherein said power take-off is a power-take-off of a vehicle main drive transmission.

22. A vehicle according to any one of claims 16 to 21 wherein the KERS output is permanently engaged with the vehicle driven wheels or tracks.

23. A vehicle according to any of claims 16 to 22 wherein the KERS output, in use, is permanently engaged with the vehicle driven wheels or tracks.

Intellectual

Property

Office

Application No: GB1701609.8 Examiner: Mr Kevin Hewitt