WO2007116220A1

WO2007116220A1 - Transmission systems

Info

Publication number: WO2007116220A1
Application number: PCT/GB2007/001329
Authority: WO
Inventors: Luke Barker; John Samuel Russell; John George Martin
Original assignee: Integral Powertrain Ltd
Priority date: 2006-04-08
Filing date: 2007-04-10
Publication date: 2007-10-18

Abstract

An apparatus for transmitting power from an engine to an electrical machine, the apparatus comprising a first shaft (10), a second shaft (1) and a multi-speed transmission device, the first and second shafts being coupled to the transmission device, the transmission device being operable to provide a first ratio of relative rotational speed between the first shaft and the second shaft when the first shaft is driven by the second shaft and a second ratio of relative rotational speed between the first shaft and the second shaft when the second shaft is driven by the first shaft, the first shaft comprising coupling means for rigidly coupling it to an engine crankshaft (10), wherein the transmission device is operable to decouple the second shaft from the first shaft, thereby to allow independent rotation thereof.

Description

TRANSMISSION SYSTEMS

The present invention relates generally to transmission systems, for example transmission systems for use in automotive applications. More specifically, although not exclusively, the present invention relates to transmission systems comprising gearing mechanisms for connecting, for example, the rotor of an electrical machine to an engine.

Internal combustion engines are often fitted with an electrical machine for generation of electrical energy (typically an alternator). This is usually driven using a belt or gear drive system by the crankshaft either directly or using an intermediate shaft (for example camshaft, power take off or balance shaft). A coupling mechanism, such as a fluid coupling, may also be provided at some stage between the crankshaft and the electrical machine.

It is also known for the electrical machine to be driven by the main gearbox of a vehicle, which is itself driven by the engine crankshaft.

In some cases the electrical machine is configured to allow it to start the engine. In this case it may be referred to as an integrated starter generator (ISG) or stop-start generator (SSG). Where connected to the engine by a belt drive the device may be referred to as a belt driven starter generator (BSG).

The electrical machine may also be configured so that it can operate as an extra source of motive power, using stored electrical energy to enhance engine output. This may be referred to as a motoring or electric assist mode. Starting mode typically occurs at crankshaft speeds of up to 15% of maximum. For electrical machines with a power rating substantially lower than the peak power rating of the engine to which it is attached, motoring mode is also typically of most benefit at crankshaft speeds below 25% to 40% of maximum. The electrical machine must however typically act as a generator throughout the entire speed range.

Where the electrical machine is to be used as a starter and / or a motor, the size and cost of the machine and its associated control and electrical storage systems are strongly related to its maximum torque output. This in turn is a consequence of the maximum torque required to be applied to the engine and the ratio of the speed of the electrical machine to the engine.

In some cases the electrical machine may be configured to operate as a regenerative braking system, converting kinetic or rotational energy into electrical energy. In this case the torque requirement is typically significantly greater than when the machine is operating conventionally as a generator. According to typical duty cycles for certain applications it is common that most of the benefit of regenerative braking can be gained in the lowest 30% of the engine speed range.

Hence, it is desirable to provide a transmission system configured such that the ratio of the speed of the electrical machine to the engine is substantially higher during starting and motoring modes and/or during regenerative braking than when the electrical machine is acting as a generator. This allows a smaller and lower cost electrical machine to meet the requirements of a given application.

US2153856 discloses a transmission device which includes an epicyclic type gear set consisting of two sun gears and one series of planet pinions associated with each sun gear. Each planet pinion of one series is fixed relative to one planet pinion of the other series, both series being rotatably secured to a pulley. A first of the sun gears is fixed to a crankshaft. A second sun gear is movable between a first position in which it is fixed relative to a fixed casing and a second position in which it is fixed to the other sun gear. When the second sun gear is in the first position and the pulley drives the planet pinions, the first sun gear is driven at a lower angular velocity to that of the pulley. When the engine begins to operate under its own power, the first sun gear becomes the driving element. The teeth are shaped such that, when the first sun gear is the driving element, the second sun gear is moved from the first position to the second position, thus causing the pulley to be driven at the same angular velocity as the crank shaft. The teeth are arranged to urge the second sun gear from one position to the other in response to a change in the direction of torque transmission.

Electrical machines often have significant rotational inertia and, under some circumstances such as during gear changes, engine speed may reduce rapidly and the torque required to decelerate the electrical machine in-line with the engine may cause high loads in the drive system. In the case of a belt drive system this may result in undesirable slippage or noise.

In order to protect against such occurrences, known devices are sometimes configured to de-couple the electrical machine using a one-way clutch when certain conditions are met. This arrangement is sometimes referred to as a de-coupler.

Such a simple de-coupler cannot be fitted to an electrical machine configured to operate as a starter or in motoring mode as it is not possible to transmit torque back to the engine through such a de-coupler. EP0591617 discloses a transmission for mounting to an ancillary device such as an alternator. The transmission includes an epicyclic type gear set consisting of two sun gears and a series of planet pinions associated with each sun gear. Each planet pinion of one series is fixed to one planet pinion of the other series and rotatably secured to a carrier. The transmission includes a first shaft for connection with the rotor of the ancillary device and a pulley for connection with the engine via a belt drive. One of the sun gears is fixed to the first shaft and the other sun gear is fixed to a second shaft. The transmission includes a clutch, which is operable to couple the first shaft directly to the second shaft. The transmission also includes a brake which is operable to prevent rotation of the carrier, thereby directing the torque through the gear set which results in the first shaft having a higher rotational speed than the second shaft.

Providing a direct connection between the first and second shaft is advantageous as it reduces the amount of play or slack in the assembly.

In certain applications with ISG systems, when the vehicle comes to rest and certain conditions are met, the engine is stopped instead of idling. In these circumstances it may be beneficial for the air-conditioning pump or other ancillary device to be powered by the ISG unit. This configuration is not possible in the case of the aforementioned arrangements.

Accordingly, it would be advantageous if a transmission system included a means for decoupling the electric machine from the engine in such a situation. According to a first aspect of the invention, there is provided an apparatus for transmitting power from an engine to an electrical machine, the apparatus comprising a first shaft, a second shaft and a multi-speed transmission device, the first and second shafts being coupled to the transmission device, the transmission device being operable to provide a first ratio of relative rotational speed between the first shaft and the second shaft when the first shaft is driven by the second shaft and a second ratio of relative rotational speed between the first shaft and the second shaft when the second shaft is driven by the first shaft, the first shaft comprising coupling means for rigidly coupling it to an engine crankshaft, wherein the transmission device is operable to decouple the second shaft from the first shaft, thereby to allow independent rotation thereof.

The power impulses which result from the nature of a typical reciprocating engine result in torsional vibration of the crankshaft. The power stroke of each piston causes a surge of force to be transmitted to the crankshaft through the piston rod, which results in an elastic torsional deformation of the crankshaft. At the end of the power stroke, the crankshaft returns to its original configuration and the power stroke of a further piston repeats this cycle. This oscillating deformation may be amplified by torsional resonance, which is dependent on the crankshaft design.

According to a second aspect of the invention, there is provided an apparatus for transmitting power from an engine to an electrical machine, the apparatus comprising a first shaft, a second shaft, a multi-speed transmission device and damper means, the first and second shafts being rigidly coupled to the transmission device, the transmission device being operable to provide a first ratio of relative rotational speed between the first shaft and the second shaft when the first shaft is driven by the second shaft and a second ratio of relative rotational speed between the first shaft and the second shaft when the second shaft is driven by the first shaft, the first shaft comprising coupling means for rigidly coupling it to an engine crankshaft, wherein the damper means is secured to the first shaft and arranged to dampen, in use, torsional vibration of the first shaft.

The rigid coupling between the shafts and the transmission device provides an arrangement with minimal play or slack. This feature in combination with the provision of securing a damper means to the first shaft ensures that damping of the torsional vibration of the crankshaft is achieved without additional slack that may be associated with incorporating the damper within the coupling.

The transmission device may further comprise a first clutch means arranged to releasably connect the first shaft to the second shaft. The first clutch means may be arranged to provide a direct connection between the first shaft and the second shaft. The first clutch means comprises a one-way clutch or a two-way clutch.

According to a third aspect of the invention, there is provided an apparatus for transmitting power from an engine to an electrical machine, the apparatus comprising a first shaft, a second shaft and a multi-speed transmission device, the first and second shafts being coupled to the transmission device, the transmission device comprising first clutch means and second clutch means, the first clutch means being operable to directly connect the first shaft to the second shaft when the second shaft is driven by the first shaft, the second clutch means being operable to provide a first ratio of relative rotational speed between the first shaft and the second shaft when the first shaft is driven by the second shaft, wherein each of the first clutch means and the second clutch means comprises a one-way clutch. The apparatus may further comprise damper means, wherein the damper means is secured to the first shaft and arranged to dampen, in use, torsional vibration of the first shaft.

The damper means may comprise a mounting member and a substantially disc shaped member. The damper means may further comprise a resilient member between the mounting member and the substantially disc shaped member.

The transmission device preferably comprises an epicyclic gear set, for example comprising a first sun gear, wherein the first sun gear is preferably rigidly connected to the first shaft, for example bolted thereto or integrally formed therewith. The transmission device may also comprise one or more first planet gears rotatably secured to a planet carrier and arranged to mesh with the first sun gear.

In one embodiment, the transmission device comprises one or more second planet gears rigidly connected to a respective one or more first planet gears and arranged to mesh with a second sun gear. The second sun gear is preferably rigidly connected to the second shaft. The transmission device may also comprise a second clutch means, for example a one-way or a two-way clutch, arranged to releasably connect the planet carrier relative to a fixed casing.

In an alternative embodiment, the planet carrier is rigidly connected to the second shaft. The transmission device may comprise a ring gear arranged to mesh with the one or more planet gears. The transmission device may further comprise a second clutch means, for example a one-way or a two-way clutch, arranged to releasably connect the ring gear relative to a fixed casing. The second shaft preferably comprises a pulley for engaging a belt coupled to an electric machine. The pulley may comprise a depression, wherein the damper means may be at least partially located within the depression.

A further aspect of the invention provides a transmission device including a positive type epicyclic gearset comprising a first sun gear which is in engagement with a plurality of first planet wheels, and a second sun wheel which is in engagement with a plurality of second planet wheels and is connected to a second sun shaft, each first planet wheel being connected to a respective second planet wheel, each connected pair of first and second planet wheels being carried by a respective common planet shaft, the planet shafts being connected to a common carrier. The device is also connected to at least two clutches. One clutch is provided between the first sun gear and the crankshaft. Another clutch is provided between the planet carrier and the engine casing. The pulley is connected to the electrical machine either directly or through a fixed ratio transmission such as a belt drive. The second sun gear is connected directly or via a second sun shaft to the crankshaft.

When the clutch between the planet carrier and the engine casing is open and the clutch between the pulley and the crankshaft is closed, this gives a ratio of 1 :1 between the second sun gear and the pulley.

When the clutch between the pulley and the crankshaft is open and the clutch between the planet carrier and the engine casing is closed, this gives a higher ratio typically between 1.5:1 and 4:1 between the crankshaft and the pulley. Conventionally, the clutch between the pulley and the crankshaft and the clutch between the planet carrier and the engine casing would both require actuation mechanisms and control adding cost and complexity.

In operation the rotational direction of the engine or transmission is generally always in the same sense and hence the rotational direction of the pulley is always in the same sense (i.e. always clockwise or always anti-clockwise).

This gives the opportunity to configure one or both of the clutches to be of the self- actuating one-way type, that is to say of a type that can only transmit torque in one direction. There are various types of self-actuating one-way clutches including sprag, ratchet or roller and ramp.

Many internal combustion engines require a torsional damper to be rigidly connected to the crankshaft. The transmission device preferably allows for such a connection.

With many modern engine applications it is important to achieve a low overall length. It is therefore preferable to minimize the transmission and crankshaft damper package length.

In one embodiment a torsional damper is located within the pulley, thereby minimising the impact on overall package size.

In a preferred embodiment the clutch between the pulley and the crankshaft is of the one-way type and the clutch between the planet carrier and the engine casing is of the two-way type. In this embodiment a single actuation mechanism will be required for the two-way clutch. This embodiment will also perform the function of a de- coupler when the electrical machine is generating. Such an embodiment is able to utilise the high ratio for starting and motoring, and also provide a de-coupling capability, thus enhancing regenerative braking capacity and allowing the reduction of belt loads during rapid decelerations.

In a further preferred embodiment the clutch between the pulley and the crankshaft is of the one-way type and the clutch between the planet carrier and the engine casing is also of the one-way type. In this embodiment no external actuation is required and the cost and complexity of the device are minimised. This configuration can also perform the function of a de-coupler when the electrical machine is generating but cannot normally be used to enhance regenerative braking capacity or operate ancillary devices when the engine is not operating.

According to a further aspect of the invention, the device includes a fixed casing, a power transfer shaft, a 3 branch epicyclic gearset with a ring gear, sun, sun shaft and a plurality of planet wheels, which are carried by a common carrier, and at least two clutches. One clutch is provided between the power transfer shaft and the sun shaft.

Another clutch is provided between the annulus and the casing. The power transfer shaft is connected to the engine crankshaft either directly or through a fixed ratio transmission such as a belt drive. The power transfer shaft is also connected to the planet carrier. The sun gear is connected directly or through a fixed ratio transmission such as a belt drive to the rotor of the electrical machine.

When the electrical machine is operating under steady conditions as a generator, the clutch between the annulus and the casing is open and the clutch between the power transfer shaft and the sun shaft is closed. This gives a ratio of 1:1 between the sun shaft and the power transfer shaft. When the electrical machine is operating in starting or motoring mode, the clutch between the power transfer shaft and the sun shaft is open and the clutch between the annulus and the casing is closed. This gives a ratio typically between 3:1 and 6:1 between the sun shaft and the power transfer shaft.

However, this ratio may be varied depending on the requirements of the application in question.

Conventionally the clutch between the power transfer shaft and the sun shaft and the clutch between the annulus and the casing would both require actuation mechanisms and control adding cost and complexity.

In operation the rotational direction of the engine is generally always in the same sense and hence the rotational direction of the power transfer shaft is always in the same sense (i.e. always clockwise or always anti-clockwise).

This gives the opportunity to configure one or both of the clutches to be of the self- actuating one-way type, that is to say of a type that can only transmit torque in one direction. There are various types of self-actuating one-way clutches including spragg, ratchet or roller and ramp.

In a preferred embodiment the clutch between the power transfer shaft and the sun shaft is of the one-way type and the clutch between the annulus and the casing is of the two-way type. In this embodiment a single actuation mechanism will be required for the two-way clutch. This embodiment will also perform the function of a decoupler when the electrical machine is generating. Such an embodiment can also be employed to enhance regenerative braking capacity by increasing the ratio of the speed of the electrical machine to that of the engine.

In another embodiment the clutch between the annulus and the casing is of the one- way type and the clutch between the power transfer shaft and the sun shaft is of the two-way type. In this embodiment a single actuation mechanism will be required. This embodiment does not perform the function of a de-coupler and cannot normally be used to enhance regenerative braking capacity.

In a further preferred embodiment the clutch between the power transfer shaft and the sun shaft is of the one-way type and the clutch between the annulus and the casing is also of the one-way type. In this embodiment no external actuation is required and the cost and complexity of the mechanism are minimised. This configuration will also perform the function of a de-coupler when the electrical machine is generating but cannot be used to enhance regenerative braking capacity.

In all of the above embodiments the 3 branch epicyclic gearset with annulus, sun, sun shaft and plurality of planet wheels can be substituted by a different positive type 3 branch epicyclic gearset comprising a first sun gear which is in engagement with a plurality of first planet wheels, and a second sun wheel which is in engagement with a plurality of second planet wheels and is connected to a second sun shaft, each first planet wheel being connected to a respective second planet wheel, each connected pair of first and second planet wheels being carried by a respective common planet shaft, the planet shafts being connected to a common carrier. In all the above embodiments this can be effected by substituting the planet carrier, first sun, second sun and second sun shaft of the positive type for the annulus, planet carrier, sun and sun shaft respectively.

In the case where the positive type epicyclic gearset is employed the choice of ratio under motoring and starting can be much wider and range of ratios between 1 and 9 for example would be practical. This configuration also has the advantage of requiring only simple gears with external teeth.

It is also noted that the mechanism could be applied to enhance the devices refered to in "Automotive Air Blowers" patent application GB 0304047.4

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a partial section view from the side of a first embodiment of the invention;

Figure 2 is a simplified schematic view of the arrangement of Figure 1 ; Figure 3 is a partial section view from the side of a second embodiment of the invention; and

Figure 4 is a simplified schematic view of a third embodiment of the invention.

The embodiment shown in Figures 1 and 2 includes a drive pulley 1 , an epicyclic gear set which includes a first sun gear 2, three first planet gears 3, three second planet gears 4, a second sun gear 5, three planet shafts 6, a planet carrier 7, a first clutch 8, a second clutch 9 and a vibration damper 11. Figure 1 also shows the engine crankshaft 10 and engine casing 12. The drive pulley 1 is rigidly connected to the first sun gear 2, for example using bolts. Each first planet gear 3 is connected by one of the planet shafts 6 to one of the second planet gears 4. The planet shafts 6 are rotatably mounted to the planet carrier 7. The first planet gears 3 are in engagement with the first sun gear 2 and the second planet gears 4 are in engagement with the second sun gear 5.

The second sun gear 5 is rigidly connected to the crankshaft 10. The drive pulley 1 is connected through a belt (not shown) to the rotor of an electrical machine and an air-conditioning compressor (not shown).

The first clutch 8 is of the one-way type and releasably connects the pulley 1 directly to the crankshaft 10. The second clutch 9 is of the two-way type and releasably connects the planet carrier 7 to the engine casing 12.

The first clutch 8 acts on the crankshaft 10 via the torsional-vibration damper 11 , which is mounted rigidly to the crankshaft 10, thereby rotating therewith. The torsional vibration damper 11 includes an external ring member 13, a flexible ring member 14 and a central mounting portion 15. The external ring member 13 and the central mounting portion 15 are both made from steel, while the flexible ring member 14 is made from a material having elastic properties, such as rubber. The function of the damper 11 will be understood by those skilled in the art.

In operation, when the electrical machine is operating as a generator at high engine speed, the second clutch 9 is disengaged automatically or by an actuator (not shown) and the first clutch 8 is automatically engaged. This produces a ratio of 1 :1 between the crankshaft 10 and the pulley 1. During conditions where the engine slows down whilst the second clutch 9 is disengaged, the first clutch 8 will automatically disengage reducing the torque transmitted through the belt drive. This may occur, for example, during gear changes.

When the electrical machine is operating as a motor or as a generator with high power demand at relatively low engine speed, such as during regenerative braking, the second clutch 9 may be engaged, thus connecting the planet carrier 7 to the engine casing 12. It will be appreciated that the first clutch 8 disengages automatically. This produces a predefined ratio between the crankshaft 10 and the pulley 1.

If it is desired to rotate the air conditioning compressor whilst the engine is stationary, this can be achieved by providing power to the electrical machine and opening the second clutch 9.

The embodiment shown in Figure 3 includes a first clutch 101, a drive pulley 102, a second clutch 103, three planet gears 104, a sun gear 105, a power transmission shaft 106, a ring gear 109, a casing 110 and a sun shaft 111.

The drive pulley 102 is connected to the power transmission shaft 106. The power transmission shaft incorporates a planet carrier on which planet gears 104 are rotatably mounted. The planet gears 104 engage the ring gear 109 and the sun gear 105. The first clutch 101 is of the one-way sprag type and provided between the power transfer shaft 106 and the sun shaft 111 via the drive pulley 102. The second clutch 103 is of the one-way sprag type and is provided between the rign gear 109 and the casing 110.

The sun gear 105 is directly connected to the sun shaft 111. The sun shaft 111 is directly connected to the rotor of an electrical machine (not shown). The drive pulley 102 is connected by a belt (not shown) to the crankshaft of an engine.

In operation, when the electrical machine is operating as a generator, the second sprag clutch 103 is disengaged and the first sprag clutch 101 is engaged. This produces a ratio of 1:1 between the sun shaft 111 and the power transfer shaft 106.

When the electrical machine is operating as a motor the second sprag clutch 103 is engaged connecting the annulus 109 to the casing 110. The first sprag clutch 101 is disengaged. This produces a ratio of 4:1 between the sun shaft 111 and the power transfer shaft 106. During conditions where the engine slows down very rapidly both the first sprag clutch 101 and the second sprag clutch 103 will automatically disengage reducing the torque transmitted through the belt drive.

Figure 4 shows another embodiment employing a positive type epicyclic gearbox. In this embodiment pulley 202 is connected by a belt to the crankshaft of an engine. The drive pulley 202 is also connected to the power transmission shaft 206 which is connected to the first sun 205 of a positive type epicyclic gearset comprising first planets 204 in engagement with the first sun connected by common shafts to second planets 208 in engagement with second sun 212 and mounted on a common carrier 213. The second sun 212 is connected to second sun shaft 211 which is directly connected to the rotor of an electrical machine. A first clutch 203 of the two-way type is provided between the planet carrier 213 and the casing 210. A second clutch of the one-way type 201 is provided between the power transfer shaft 206 and the second sun shaft 211.

In operation, when the electrical machine is operating as a generator at high engine speed, the first clutch 203 is disengaged by an actuator (not shown) and the second clutch 201 is automatically engaged. This produces a ratio of 1 :1 between the second sun shaft 211 and the power transfer shaft 206. When the electrical machine is operating as a motor or as a generator with high power demand at relatively low engine speed (such as during regenerative braking), the first clutch 203 is engaged connecting the planet carrier 213 to the casing 210. The second clutch 201 is automatically disengaged. This produces a ratio of 4:1 between the sun shaft 211 and the power transfer shaft 206. During conditions where the engine slows down whilst the first clutch is disengaged, the second clutch 201 will automatically disengage reducing the torque transmitted through the belt drive.

The term two-way clutch refers to any suitable clutch means which is able to transfer torque in both directions and is separately actuated.

It will be understood by those skilled in the art that several variations are envisaged without departing from the scope of the invention.

Claims

1. An apparatus for transmitting power from an engine to an electrical machine, the apparatus comprising a first shaft, a second shaft and a multi-speed transmission device, the first and second shafts being coupled to the transmission device, the transmission device being operable to provide a first ratio of relative rotational speed between the first shaft and the second shaft when the first shaft is driven by the second shaft and a second ratio of relative rotational speed between the first shaft and the second shaft when the second shaft is driven by the first shaft, the first shaft comprising coupling means for rigidly coupling it to an engine crankshaft, wherein the transmission device is operable to decouple the second shaft from the first shaft, thereby to allow independent rotation thereof.

2. An apparatus according to Claim 1 , further comprising damper means, wherein the damper means is secured to the first shaft and arranged to dampen, in use, torsional vibration of the first shaft.

3. An apparatus for transmitting power from an engine to an electrical machine, the apparatus comprising a first shaft, a second shaft, a multi-speed transmission device and damper means, the first and second shafts being rigidly coupled to the transmission device, the transmission device being operable to provide a first ratio of relative rotational speed between the first shaft and the second shaft when the first shaft is driven by the second shaft and a second ratio of relative rotational speed between the first shaft and the second shaft when the second shaft is driven by the first shaft, the first shaft comprising coupling means for rigidly coupling it to an engine crankshaft, wherein the damper means is secured to the first shaft and arranged to dampen, in use, torsional vibration of the first shaft.

4. An apparatus as claimed in any preceding Claim, wherein the transmission device further comprises a first clutch means arranged to releasably connect the first shaft to the second shaft.

5. An apparatus as claimed in Claim 4, wherein the first clutch means is arranged to provide a direct connection between the first shaft and the second shaft.

6. An apparatus as claimed in Claim 4 or Claim 5, wherein the first clutch means comprises a one-way clutch.

7. An apparatus as claimed in Claim 4 or Claim 5, wherein the first clutch means comprises a two-way clutch.

8. An apparatus as claimed in any preceding Claim, wherein the transmission device further comprises a second clutch means.

9. An apparatus as claimed in Claim 8, wherein the second clutch means comprises a one-way clutch.

10. An apparatus as claimed in Claim 8, wherein the second clutch means comprises a two-way clutch.

11. An apparatus for transmitting power from an engine to an electrical machine, the apparatus comprising a first shaft, a second shaft and a multi-speed transmission device, the first and second shafts being coupled to the transmission device, the transmission device comprising first clutch means and second clutch means, the first clutch means being operable to directly connect the first shaft to the second shaft when the second shaft is driven by the first shaft, the second clutch means being operable to provide a first ratio of relative rotational speed between the first shaft and the second shaft when the first shaft is driven by the second shaft, wherein each of the first clutch means and the second clutch means comprises a one-way clutch.

12. An apparatus according to Claim 11 , further comprising damper means, wherein the damper means is secured to the first shaft and arranged to dampen, in use, torsional vibration of the first shaft.

13. An apparatus as claimed in any one of Claims 2, 3 or 12, wherein the damper means comprises a mounting member and a substantially disc shaped member.

14. An apparatus as claimed in Claim 13, wherein the damper means further comprises a resilient member between the mounting member and the substantially disc shaped member.

15. An apparatus as claimed in any preceding Claim, wherein the transmission device comprises an epicyclic gear set.

16. An apparatus as claimed in any preceding Claim, wherein the transmission device comprises a first sun gear, the first sun gear being rigidly connected to the first shaft.

17. An apparatus as claimed in Claim 16, wherein the transmission device comprises one or more first planet gears rotatably secured to a planet carrier and arranged to mesh with the first sun gear.

18. An apparatus as claimed in Claim 17, wherein the transmission device comprises one or more second planet gears rigidly connected to a respective one or more first planet gears and arranged to mesh with a second sun gear.

19. An apparatus as claimed in Claim 18, wherein the second sun gear is rigidly connected to the second shaft.

20. An apparatus as claimed in any one of Claims 17 to 19, wherein the second clutch means is arranged to releasably connect the planet carrier relative to a fixed casing.

21. An apparatus as claimed in Claim 17, wherein the planet carrier is rigidly connected to the second shaft.

22. An apparatus as claimed in Claim 21 , wherein the transmission device comprises a ring gear arranged to mesh with the one or more planet gears.

23. An apparatus as claimed in Claim 22, wherein the transmission device further comprises a second clutch means arranged to releasably connect the ring gear relative to a fixed casing.

24. An apparatus as claimed in Claim 23, wherein the second clutch means comprises a one-way clutch.

25. An apparatus as claimed in Claim 24, wherein the second clutch means comprises a two-way clutch.

26. An apparatus as claimed in any preceding Claim, wherein the second shaft comprises a pulley for engaging a belt coupled to an electric machine.

27. An apparatus as claimed in Claim 26, wherein the pulley comprises a depression, the damper means being at least partially located within the depression.