WO2020183118A1

WO2020183118A1 - Transmission system

Info

Publication number: WO2020183118A1
Application number: PCT/GB2019/052958
Authority: WO
Inventors: Neil Quinn
Original assignee: Zeroshift Transmissions Limited
Priority date: 2019-03-11
Filing date: 2019-10-16
Publication date: 2020-09-17
Also published as: GB2574291B; GB2574291A8; GB201903297D0; GB2574291A

Abstract

A transmission system, including: a first input shaft (1) that is arranged to receive drive directly from a drive source (80); a second input shaft (3) that is arranged to receive drive from the drive source (80) via a drive interruption means (86), such as a friction clutch device; a layshaft (5); a first gear element (23) rotatably mounted on the lay shaft (5); a second gear element (22) rotatably mounted on the layshaft (5); a first selector assembly (29) arranged to selectively lock the first and second gear elements (22,23) for rotation with the layshaft (5), the first selector assembly (29) comprising a single engagement ring (35) that includes a first side (35a) having a first set of engagement elements (28a), wherein each engagement element (28a) has a drive face (43a) arranged to drivingly engage in a first rotational direction a first set of drive formations (20a) associated with the first gear element (23) and a non-driving face (45a), such as a ramp, that is arranged to slip with respect to the first set of drive formations (20a) in a second rotational direction, thereby preventing driving engagement with the first set of drive formations (20a), and a second side (35b) having a second set of engagement elements (28b), wherein each engagement element (28b) has a drive face (43b) arranged to drivingly engage in the first rotational direction a second set of drive formations (20b) associated with the second gear element (22) and a non-driving face (45b), such as a ramp, that is arranged to slip with respect to the second set of drive formations (20b) in the second rotational direction, thereby preventing driving engagement of the second set of drive formations (20b). A transmission system according to claim 1, wherein the non-driving face (45a, 45b) for each engagement element (28a, 28b) extends along a curved path from a position adjacent its respective drive face (43a, 43b).

Description

Transmission system

The present invention relates to a transmission system, in particular to a transmission system that is configured for use in vehicles having a combustion engine and/or electric motor for providing drive to the wheels, and a selector assembly for use in the transmission system.

In conventional single clutch synchromesh transmission systems for vehicles it is necessary to disengage the transmission from the power source, such as an engine or motor, by operating the clutch before the current gear is deselected and the new gear is engaged. If the power is not disengaged when attempting to engage a new gear the synchromesh is unable to engage the new gear element or has to be forced into engagement with the risk of damaging the transmission and creating torque spikes in the transmission. This is because in most cases the speed of the engine is not matched to the speed of the new gear. For motor vehicles such as cars having conventional gearboxes and powered by an engine, the selection of a new gear ratio typically takes between 0.5 and 1 second to complete. So, for example, when a higher gear is selected the time delay allows the engine to reduce its speed [due to its own inertia] to more closely match the speed of the new gear before the clutch re-connects the engine and the transmission, thereby reducing the possibility of torque spikes occurring when the power is reapplied.

Dual Clutch Transmission (DCT) systems have tried to address this problem by using two clutches to handover the transfer of torque seamlessly when shifting between gears. However there are a number of drawbacks to DCTs, for example they require the use of two friction clutches which are heavy and expensive, they are complex to control and have parasitic losses and therefore are not very efficient.

Another type of seamless transmission system is referred to as an instantaneous type transmission system. This family of transmission systems includes at least one selector assembly that includes first and second sets of engagement elements that are arranged to selectively engage drive formations on the or each gear element associated with it. The first and second set sets of engagement elements are arranged such that a new gear can be selected while the current gear is still engaged and therefore the new gear can be selected under power for some shift types. A selector assembly of this type has four modes of operation with respect to the or each rotatably mounted gear element associated with it: fully engaged in both torque directions (fully in gear); disengaged in both torque directions (neutral); engaged in the forward torque direction while disengaged in the reverse torque direction; and disengaged in the forward torque direction while engaged in the reverse torque direction.

It is the last two modes that enable a discrete ratio gearbox to have the ability to shift up or down ratios instantly under load without torque interruption. Instantaneous transmissions are described in WO 2004/099654, WO 2005/005868, WO 2005/005869, WO 2005/024261 and WO 2005/026570, WO 2006/095140, WO 2006/123128, WO2006/123166, W02007/132209, W02008/062192, W02008/096140, W02008/145979, W02009/068853, WO2010/046654, WO2010/046655, WO2010/046652, and WO2012/164237 the contents of which are incorporated by reference.

It is desirable to provide torque support using a relatively simple transmission layout and relatively simple selector assemblies in order to reduce control complexity, where possible. The inventors have determined that a simplification can be made to the transmission system by requiring torque support during power on upshifts only. It with this background that the present invention seeks to provide an improved transmission system that can be used on a vehicle such as a motorcar or motorcycle, powered by a combustion engine and/or an electric motor, and that has a simplified control system.

Accordingly, the invention seeks to mitigate at least one of the aforementioned problems, or at least to provide an alternative transmission system to existing transmission systems.

According to one aspect there is provided a transmission system according to claim 1. The invention enables the transmission system to provide torque support during power on upshifts only. This is due to the arrangement of the drive faces and ramps on each side of the drive ring. In particular, because the drive faces on each side of the drive ring face in the same rotational direction. Likewise, the non-driving faces on each side of the drive ring face in the same rotational direction. The invention provides is a simplified transmission system compared with a traditional instantaneous transmission system since torque support is provided for only one shift type, power on upshifts. Furthermore a single shift drum can be used to control operation of the first selector assembly since movement of the drive ring is reversible. This enables a relatively compact transmission system to be provided.

According to another aspect there is provided a transmission system.

The transmission system can include a first input shaft that is arranged to receive drive directly from a drive source.

The transmission system can include a second input shaft that is arranged to receive drive from the drive source via a drive interruption means, such as a friction clutch device.

The transmission system can include a layshaft.

The transmission system can include a first gear element rotatably mounted on the layshaft.

The transmission system can include a second gear element rotatably mounted on the layshaft.

The transmission system can include a first selector assembly arranged to selectively lock the first and second gear elements for rotation with the layshaft, the first selector assembly comprising a single engagement ring.

The engagement ring can include a first side having a first set of engagement elements, wherein each engagement element has a drive face arranged to drivingly engage, in a first rotational direction, a first set of drive formations associated with the first gear element. Each engagement element can include a non-driving face, such as a ramp, that is arranged to slip with respect to the first set of drive formations in a second rotational direction, thereby preventing driving engagement with the first set of drive formations.

The engagement ring can include a second side having a second set of engagement elements, wherein each engagement element has a drive face arranged to drivingly engage in the first rotational direction a second set of drive formations associated with the second gear element. Each engagement element can include a non-driving face, such as a ramp, that is arranged to slip with respect to the second set of drive formations in the second rotational direction, thereby preventing driving engagement of the second set of drive formations.

Each non-driving face can extend along a respective curved path. Each non-driving face can follow a curved path around a part of its respective side of the engagement ring, and typically a peripheral part of its respective side of the engagement ring. For example, the curved path can start at a position adjacent its respective drive face.

Each non-driving face can extend along a respective arcuate path from a position adjacent its respective drive face. Each non-driving face can extend along its respective arcuate path towards an adjacent one of the engagement elements on a respective side of the engagement ring. Each non-driving face can extend circumferentially along its respective arcuate path. Each non-driving face can follow an arcuate path around a peripheral part of its respective side of the engagement ring. The long sloping and curved arrangement of the non-driving faces prevents driving engagement of the driving formations from occurring. It also reduces the magnitude of slipping noise generated be the interaction of the non-driving faces with the driving formations, and the frictional forces experienced.

Each non-driving face can extend circumferentially along its respective arcuate path from a position adjacent its respective drive face to a position adjacent the drive face of an adjacent one of the engagement elements on a respective side of the engagement ring. It is particularly helpful to minimise the magnitude of slipping noise generated and the frictional forces experienced by the interaction of the non-driving faces that extend circumferentially along arcuate paths and the driving formations. This is particularly useful where the relative speeds between gear elements are high, such as in electric vehicle transmissions.

The number of engagement elements on each side of the engagement ring can be less than or equal to four, less than or equal to three, or less than or equal to two. Low numbers of engagement elements are particularly suited to electric vehicle applications that have relatively few gear ratios as the speed differences between gear ratios are high. Having a small number of engagement elements increases the size of the "engagement windows" between engagement elements, which makes engagement easier, and reduces the frequency of slipping engagements between the engagement elements and the drive formations. This reduces the magnitude of the slipping noise generated. It also ensures that the non-drive faces can have a larger arcuate extent, which reduces the rate of separation of the between the engagement ring and the drive formations when slipping occurs, providing more gentle slipping.

In preferred embodiments, the arcuate extent of each non-driving face is in the range 90° to 360°. This is particularly helpful to minimise the magnitude of slipping noise generated and the frictional forces experienced by the interaction of the non-driving faces that extend circumferentially along arcuate paths and the driving formations. This is particularly useful where the relative speeds between gear elements are high, such as in electric vehicle transmissions. The measurement can be taken with reference to a central axis of the engagement ring. For embodiments having four engagement elements per side of the engagement ring the arcuate extent of each non-driving face is typically around 90°. For embodiments having three engagement elements per side of the engagement ring the arcuate extent of each non-driving face can be in the range 90° to 120°, is typically in the range 100° to 120°. For embodiments having two engagement elements per side of the engagement ring the arcuate extent of each non-driving face can be in the range 90° to 180°, and is typically in the range 150° to 180°. The number of engagement elements on the first side of the engagement ring can equal the number of engagement elements on the second side of the engagement ring.

In some embodiments only a single engagement element is provided on each side of the engagement ring. This maximises the size of the engagement window and provides the largest arcuate extent of each non-driving face. For embodiments having one engagement element per side of the engagement ring the arcuate extent of each non-driving face can be in the range 90° to 360°, can be in the range 180° to 360°, can be in the range 270° to 360°, and is typically in the range 300° to 360°.

Each non-driving face can be substantially planar along the arcuate path. This helps to provide a smooth slipping arrangement. Each drive face can be planar.

Each drive face can be arranged radially from the centre of the engagement ring. That is, the plane of each drive face is aligned with a respective radius of the engagement ring. This helps to ensure that drive faces face in a single rotational direction.

The engagement ring can include internal splines. The internal splines can be arranged to mate with external splines formed on the layshaft or with external splines formed on an intermediate component, such as a sleeve member, located between the engagement ring and the first input shaft. Preferably the transmission system includes a sleeve member including internal splines for a splined connection with the layshaft. The sleeve member can include external splines for a splined connection with the internal splines of the engagement ring. This provides a very robust connection between the engagement ring and the first input shaft. The splined arrangement enables the engagement ring to move axially along input shaft while rotationally locking the engagement ring for rotation with the first input shaft. This differs from the mounting arrangement of instantaneous type selector assemblies, which have a more complicated mounting arrangement since it can be necessary to accommodate movement of two engagement rings.

The transmission system can include an actuator mechanism for controlling axial movement of the engagement ring. The actuator mechanism can include a shift fork engaged with the engagement ring. The actuator mechanism can include a single shift drum for controlling movement of the shift fork and engagement ring. Only a single shift drum is required for the first selector assembly because movement of the engagement ring can be reversible. Thus the transmission system can provide torque support during at least some shift types, with a relatively simple transmission layout and a relatively simple actuator mechanism.

The actuator mechanism can include a spring cradle. This helps to mitigate against shocks, for example during a slipping condition, where some axial movement of the engagement ring may occur.

The transmission can consists of a two-speed, three-speed, four-speed, five-speed, six-speed, seven-speed or eight-speed transmission. The transmission can include any practicable number of gears.

The transmission can include a first sleeve rotatably mounted on the second input shaft. The transmission can include a second sleeve rotatably mounted on the second input shaft. The transmission can include a second selector assembly for selectively locking each of the first and second sleeves for rotation with the second input shaft.

Gear elements that are each part of an odd gear (e.g. 1^st gear, 3^rd gear, 5^th gear, 7^th gear) can be mounted on the first sleeve. Gear elements that are each part of an even gear (e.g. 2^nd gear, 4^th gear, 6^th gear, eighth) can be mounted on the second sleeve. The second selector assembly is able to switch between the odd and even sides be selectively engaging the first and second sleeves, and selectively locking the first and second sleeves for rotation with the second input shaft. The second selector assembly can be mounted on the second input shaft between the first and second sleeves.

The transmission system can include 1^st gear. 1^st gear can comprise a gear element mounted on the first sleeve and a gear element rotatably mounted on an output shaft. The gear elements are in meshing engagement.

The transmission can include 3^rd gear. 3^rd gear can comprise a gear element mounted on the first sleeve and a gear element rotatably mounted on the output shaft. The gear elements are in meshing engagement.

The transmission can include a third selector assembly. The third selector assembly can be arranged to selectively lock gear elements of 1^st and 3^rd gears, which are rotatably mounted on the output shaft, for rotation with the output shaft. The third selector assembly can be mounted on the output shaft between the gear elements of 1 ^st and 3^rd gears, which are rotatably mounted on the output shaft.

The transmission can include 2^nd gear. 2^nd gear can comprise a gear element mounted on the second sleeve and a gear element mounted on the output shaft. The gear elements are in meshing engagement. The gear element mounted on the output shaft can be one of: fixedly mounted to the output shaft, and arranged to rotate therewith; and rotatably mounted on the output shaft.

The transmission can include 4^th gear. 4^th gear can comprise a gear element mounted on the second sleeve and a gear element rotatably mounted on the output shaft. The gear elements are in meshing engagement.

The transmission can include a fourth selector assembly arranged to selectively lock gear elements of 2^nd and 4^th gears, which are rotatably mounted on the output shaft, for rotation with the output shaft. The fourth selector assembly can be mounted on the output shaft between the gear elements of 2^nd and 4^th gears, which are rotatably mounted on the output shaft.

At least one of the second selector assembly, third selector assembly and fourth selector assembly is a conventional selector assembly such as a synchromesh or dog clutch type assembly. Preferably each of the second selector assembly, third selector assembly and fourth selector assembly is a conventional selector assembly. Each gear element that is selectable by one of second, third and fourth selector assemblies includes appropriate drive formations and, where applicable, synchronising cones.

The transmission can be arranged to provide a torque path between 1^st gear and the first input shaft, and a torque path between 1^st gear and the second input shaft. The transmission can be arranged to provide a torque path between 2^nd gear and the first input shaft, and to provide a torque path between 2^nd gear and the second input shaft. The transmission can be arranged to provide a torque path between 3^rd gear and the first input shaft, and to provide a torque path between 3^rd gear and the second input shaft. The transmission can be arranged to provide a torque path between 4^th gear and the first input shaft, and to provide a torque path between 4^th gear and the second input shaft.

The transmission system can include a torque path between the layshaft and the second sleeve. The transmission can include a gear element mounted on the second sleeve which meshes with the second gear element. Torque can be transmitted between the layshaft, the second input shaft and the output shaft via a gear train connected to the second sleeve.

The transmission system can include a torque path between the layshaft and the first sleeve. The transmission can include a gear element mounted on the first sleeve which meshes with the first gear element. The gear element mounted on the first sleeve which meshes with the first gear element can be a gear element that is part of 1 ^st gear. With this arrangement, torque can be transferred between the layshaft, second input shaft and the output shaft.

The transmission can include a torque path between the first input shaft and the layshaft. The transmission can include a gear element mounted on the first input shaft and a gear element mounted on the layshaft. The gear elements are in meshing engagement. Preferably the gear element mounted on the first input shaft is fixed for rotation with the first input shaft. Preferably the gear element mounted on the layshaft is fixed for rotation with the layshaft.

One of the first and second input shafts can be tubular and can house at least part of the other of the first and second input shafts. The arrangement can be such that at least parts of the shafts are substantially co-axial. In preferred embodiments the first input shaft is tubular and houses at least part of the second input shaft.

The transmission can include a control system for controlling operation of the transmission system.

The transmission can be arranged such that when driving in 1^st gear, torque is routed from the second input shaft to the output shaft via the second selector assembly, the first sleeve, 1^st gear gear elements and the third selector assembly.

The transmission can be arranged such that when driving in 2^nd gear torque is routed from the second input shaft to the output shaft via the second selector assembly, the second sleeve, and 2^nd gear gear elements. Optionally, when driving in 2^nd gear, torque can be routed from the second input shaft to the output shaft, in addition to the aforementioned route, via the fourth selector assembly. The transmission can be arranged such that when driving in 3^rd gear torque is routed from the second input shaft to the output shaft via the second selector assembly, the first sleeve, 3^rd gear gear elements and the third selector assembly.

The transmission can be arranged to supply torque continually to the output shaft during power on upshifts. For example, gearshifts from 1^st gear to 2^nd gear, 2^nd gear to 3^rd gear, 3^rd gear to 4^th gear, 4^th gear to 5^th gear, 5^th gear to 6^th gear, 6^th gear to 7^th gear, 7^th gear to eighth gear, and so on.

The transmission can be arranged to, during a power on upshift, switch torque flow from a first torque path that includes the second input shaft, the second selector assembly, and one of the first and second sleeves; to a second torque path that includes the first input shaft, the layshaft and first selector assembly; to a third torque path that includes the second input shaft the second selector assembly, and the other of the first and second sleeves.

The transmission can be arranged to operate at least one of the first selector assembly, the second selector assembly and a slippable drive, such as a friction clutch device, to switch between the first, second and third torque paths.

According to another aspect there is provided a drive train including a transmission system according to any configuration described herein. The drive train can include at least one of: a drive source and a slippable drive such as friction clutch device. The friction clutch device can be a wet friction clutch or a dry friction clutch. According to another aspect there is provided vehicle including a transmission system or a drive train according to any configuration described herein.

According to another aspect there is provided a transmission system, including: a first shaft; a first gear element rotatably mounted on the first shaft; a second gear element rotatably mounted on the first shaft; and a first selector assembly arranged to selectively lock the first and second gear elements for rotation with the first shaft. The first selector assembly can comprise a single engagement ring. The engagement ring can include a first side having a first set of engagement elements. Each engagement element has a drive face arranged to drivingly engage, in a first rotational direction, a first set of drive formations associated with the first gear element, and a non-driving face that is arranged to slip with respect to the first set of drive formations in a second rotational direction, thereby preventing driving engagement with the first set of drive formations. The engagement ring can include a second side having a second set of engagement elements, wherein each engagement element has a drive face arranged to drivingly engage, in the first rotational direction, a second set of drive formations associated with the second gear element, and a non-driving face that is arranged to slip with respect to the second set of drive formations in the second rotational direction, thereby preventing driving engagement of the second set of drive formations.

The first shaft can be a layshaft.

The transmission system can arranged according to any configuration described herein.

The invention has many advantages, which include:

• The transmission system is significantly less complex and easier to control than a DCT type transmission system, and is less complex and easier to control than an instantaneous type transmission system.

• The capacity of the synchronisers is small because the layout of the transmission means that the size of the inertia that has to be synchronised is relatively small. In DCTs the capacity of the synchronisers is significantly higher.

• Only one type of synchroniser is required, which reduces manufacturing costs. For DCTs it is often required to have many different types of synchronisers in different parts of the transmission.

• The transmission layout and shift strategies employed enables torque support to be provide during at least some gearshifts.

• The transmission layout is very compact. It is also versatile in the sense that it can be used front wheel drive vehicles, rear wheel drive vehicles and all wheel drive vehicles.

• The layout uses many regular transmission components so existing production lines can easily be adapted to manufacture this transmission layout.

• The layout and shift strategies enables the torque in the transmission to be controlled during a gearshift in a manner that eliminates engagement torque spikes so it is not necessary for any of the gears to include dampers.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which like references indicate equivalent features, wherein: Figure 1 a is a schematic of a three-speed transmission system, suitable for use in a vehicle, such as a motorcar, in accordance with a first embodiment to the invention;

Figure 1 b is a schematic of a drive train including the three-speed transmission system of Figure 1 a and a control system for the drive train;

Figure 2 is an isometric view of a gear selector ring and an actuator mechanism used in the three-speed transmission system of Figure 1a;

Figures 3a and 3b provide 4 views of the gear selector ring of Figure 2;

Figures 4 to 4g show the steps and torque pathways for a power on upshift in the three- speed transmission system of Figure 1a from 1^st gear to 2^nd gear;

Figures 5a to 5g show the steps and torque pathways for a power on upshift in the three-speed transmission system of Figure 1 a from 2^nd gear to 3^rd gear; and

Figure 6 is a schematic of a four-speed transmission system, suitable for use in a vehicle, such as a motorcar, in accordance with a second embodiment to the invention.

Figure 1 b shows schematically a drive system including a drive train having a drive source 80, a slippable friction clutch 86, and a transmission 88. The drive source 80 is typically one that is suitable for use in a vehicle for example an electric motor (electric vehicle), petrol engine, diesel engine, or a hybrid arrangement. The output of the drive source 80 is largely determined by the driver loading a throttle input device 81, typically a throttle pedal, which is connected to the drive source via a throttle interface 83 and a drive source control unit 82. The drive source control unit 82, such as an engine control unit or a motor control unit, is arranged to monitor and adjust the output of the drive source 80 in accordance with instructions received from the user and/or a transmission control unit 90. The drive source control unit 82 may be a throttle potentiometer type system or alternatively an electronic control system, which is sometimes called a "drive by wire" system.

The drive source control unit 82 communicates with the transmission control unit 90 via a Controller Area Network (CAN) bus.

The layout of the transmission 88 is shown schematically in Figure 1 a. The transmission 88 includes a first input shaft 1, a second input shaft 3, a layshaft 5 (sometimes referred to as a counter shaft) an output shaft 9, a first selector assembly 29, a second selector assembly 31, and a third selector assembly 33.

The layout depicted in Figure 1 is a three-speed transmission 88, which is suited for use with a vehicle, such as a motorcar or motorcycle. The transmission 88 includes 1^st, 2^nd and 3^rd gears. 1^st gear includes a gear element 15 rotatably mounted on the second input shaft 3, and a gear element 17 rotatably mounted on the output shaft 9, which meshes with the gear element 15. 2^nd gear includes a gear element 19 rotatably mounted on the second input shaft 3, and a gear element 21 mounted on the output shaft 9 and fixed for rotation therewith, which meshes with the gear element 19. 3^rd gear includes a gear element 16 rotatably mounted on the second input shaft 3 and a gear element 18 rotatably mounted on the output shaft 9, which meshes with the gear element 16. 1^st and 3^rd gears are mounted onto an "odd gears" side of the transmission and 2^nd gear is mounted onto an "even gears" side of the transmission 88.

The output shaft 9 is connected to a differential which transmits drive to vehicle wheels via an axles. The first input shaft 1 is directly connected to the drive source 80. By directly connected it is meant there is an uninterruptible drive from the drive source 80 to the transmission 88. In practicable terms, torque from the drive source does not pass through the friction plates of the clutch 86, it may however pass through other intermediate components that provide a non- slippable drive between the drive source and the transmission, such as gears, shafts, etc.

The second input shaft 3 is connected to the drive source 80 via an interruptible torque pathway. Typically the second input shaft 3 is connected to the drive source 80 via a slippable drive device such as the friction clutch 86, which may be a dry friction clutch or a wet friction clutch. Drive from the drive source 80 to the transmission 88 is interruptible by the friction clutch 86.

Preferably the first input shaft 1 is tubular and houses at least part of the second input shaft 3, the arrangement being such that the first and second input shafts 1,3 are substantially co-axial. This is a very compact arrangement.

A gear element 23 is rotatably mounted on the layshaft 5 and meshes with the gear element 15. A gear element 25 is fixed for rotation with the layshaft 5 and meshes with a gear element

20 which is fixed for rotation with the first input shaft 1. A gear element 22 is rotatably mounted on the layshaft 5 and meshes with a gear element 27, which is rotatably mounted on the second input shaft 3.

The gear elements 15 and 16 are fixedly mounted onto a first sleeve 24 (the odd side sleeve). The first sleeve 24 is rotatably mounted on the second input shaft 3 by bearings. Accordingly the gear elements 15 and 16 rotate with the first sleeve 24. The gear elements 19 and 27 are fixedly mounted onto a second sleeve 26 (the even side sleeve). The second sleeve 26 is rotatably mounted on the second input shaft 3 by bearings. Accordingly the gear elements 19 and 27 rotate with the second sleeve 26. Preferably the gear ratio of the gear element 15 to the gear element 23 is the same as the gear ratio of the gear element 27 to the gear element 22. The first selector assembly 29 is mounted on the layshaft between gear elements 22 and 23. The first selector assembly 29 is arranged to selectively lock each of the adjacent gear elements 22 and 23 for rotation with layshaft 5. The first selector assembly 29 is a novel selector assembly and is described further below. The first selector assembly 29 is arranged to provide torque support during power on upshifts. The first selector assembly 29 can be moved to a neutral position.

The second selector assembly 31 is mounted on the second input shaft 3 between the first and second sleeves 24,26. The second selector assembly 31 is arranged to selectively lock the first sleeve 24, and hence gear elements 15 and 16, for rotation with the second input shaft 3, and to selectively lock the second sleeve 26, and hence gear elements 19 and 27, for rotation with the second input shaft 3. The second selector assembly 31 is arranged to select between the odd (1^st and 3^rd gears) and even (2^nd gear) sides of the transmission by selectively locking either one of the first and second sleeves 24,26 for rotation with the second input shaft 3.

The second selector assembly 31 is typically a conventional selector assembly, for example can be a conventional synchromesh type selector assembly. The first and second sleeves 24,26 include appropriate drive formations 22 and synchronising cones 24 to compliment those of the second selector assembly 31. Because of the transmission layout, the synchronising cones can be small. Also, only one type of synchronising cone is required. Although the second selector assembly 31 is preferably of the synchromesh type, other types of selector assembly can be used, such as a dog selector assembly.

A third selector assembly 33 is located on output shaft 9 between gear elements 17 and 18. The third selector assembly 33 is arranged to selectively lock the gear elements 17 and 18 for rotation with the output shaft. The third selector assembly 33 is arranged to select between 1 ^st and 3^rd gears by selectively locking either the gear element 17 or 18 for rotation with the output shaft 9. Typically, the third selector assembly 33 is used to pre-select the next gear to be selected prior to operating the second selector assembly 31 to make the shift. The third selector assembly 33 is typically a conventional selector assembly, for example can be a conventional synchromesh type selector assembly. The gear elements 17,18 include appropriate drive formations 22 and synchronising cones 24 to compliment those of the third selector assembly 33. Because of the transmission layout, the synchronising cones can be small. Also, only one type of synchronising cone is required. Although the third selector assembly 33 is preferably of the synchromesh type, other types of selector assembly can be used, such as a dog selector assembly.

The first, second and third selector assemblies 29,31 ,33 are arranged to create torque pathways within the transmission system according to the selections made by each of the assemblies 29,31,33. This enables the transmission 88 to select between 1^st, 2^nd and third gears. The first, second and third selector assemblies 29,31,33 are arranged to selectively provide a torque path between 1^st gear and the first input shaft 1 , and to selectively provide a torque path way between 1 ^st gear and the second input shaft 3. The first, second and third selector assemblies 29,31,33 are arranged to selectively provide a torque path between 2^nd gear and the first input shaft 1, and to selectively provide a torque path way between 2^nd gear and the second input shaft 3. The first, second and third selector assemblies 29,31 ,33 are arranged to selectively provide a torque path between 3^rd gear and the first input shaft 1, and to selectively provide a torque path way between 3^rd gear and the second input shaft 3.

The first selector assembly 29 is arranged to engage a first set of drive formations 20a that are arranged to drive the gear element 22 ("the second gear element"). The first selector assembly 29 is arranged to engage a second set of drive formations 20b that are arranged to drive the gear element 23 ("the first gear element"). The drive formations 20a, 20b comprise first and second sets of dogs. The dogs 20a, 20b in each set typically comprise three dogs evenly circumferentially distributed about the gear face/shaft, i.e. the angles subtended between the centres of a pair of dogs is approximately 120 degrees. Three dogs are used because the arrangement provides relatively large engagement windows, that is the spaces between the dogs, to receive engagement elements from the first selector assembly 29. Also, three dogs provide inherent self-centring and even load distribution. Large engagement windows provide greater opportunities for the first selector assembly 29 to fully engage the gear elements 22 and 23 before transmitting drive thereto. It will of course be appreciated that a different number of dogs 20a, 20b can be used.

The first selector assembly 29 is mounted on the layshaft 5 in between the first and second gear elements 23,22 and hence the first set of dogs 20a and second the second set of dogs 20b.

The first selector assembly 29 consists of a single gear selector ring 35 (see Figure 3a and 3b) and an actuator mechanism 38 (see Figure 2), which is arranged to move the gear selector ring 35. The gear selector ring 35 is mounted on the first input shaft 1 and is moveable along the axis of the first input shaft 1 by the actuator mechanism 38. The gear selector ring 35 has a main body 35d, a first side 35a that faces towards gear element 22 and a second side 35b that faces towards gear element 23. The gear selector ring 35 includes a first set of engagement elements 28a on the first side 35a. The gear selector ring 35 includes a second set of engagement elements 28b on the second side 35b. The first set of engagement elements 28a are evenly circumferentially distributed about the first side 35a of the gear selector ring. The second set of engagement elements 28b are evenly circumferentially distributed about the second side 35b of the gear selector ring.

In Figure 3a, it can be seen that three engagement elements 28a, 28b are provided on each side 35a, 35b of the engagement ring. Typically, each side 35a, 35b of the engagement ring includes one to four engagement elements 28a, 28b. Typically, the engagement elements 28a, 28b are uniformly distributed about a circumferentially peripheral portion of each side 35a, 35b of the engagement ring. Having a low number of engagement elements 28a, 28b on each side 35a, 35b of the engagement ring reduces the frequency with which an engagement elements 28a, 28b engages dogs 20a, 20b in a slipping condition and provides for larger engagement windows. Typically the number of dogs 20a, 20b matches the number of engagement members 28a, 28b formed on a respective side or an engagement ring 35.

Each engagement element 28a, 28b includes a drive face 43a, 43b and a non-driving face, preferably in the form of a ramp 45a, 45b. The drive faces 43a are arranged to drivingly engage the dogs 20a, and the drive faces 43b are arranged to drivingly engage the dogs 20b. The drive faces 43a, 43b are planar and are preferably arranged perpendicularly to the respective first and second sides of the gear selector ring 35. Typically the drive faces 43a, 43b are radially arranged. That is, each face 43a, 43b is aligned with a respective radius of the engagement ring 35. In some embodiments the drive faces may be angled to complement dogs 20a, 20b having angled sides. This can help to reduce wear of the engagement elements 28a, 28b when they rotate into engagement, there is face- to-face contact to reduce wear.

The purpose of the non-driving ramps 45a, 45b is to prevent locking engagement between the drive formations 20a, 20b and the gear selector ring 35 in one rotational direction, that is, in a rotational direction opposite to the rotational direction in which the drive faces 43a, 43b are oriented. Thus the ramps 45a, 45b are non-driving faces. Each ramp 45a, 45b slopes away from its respective drive face 43a, 43b in the direction opposite to the rotational direction in which the drive faces 43a, 43b are oriented. Each ramp 45a, 45b slopes gradually towards the base of an adjacent drive face 43a, 43b. The angle of slope is substantially constant along each ramp 45a, 45b. The upper surface of each ramp 45a, 45b is substantially planar. Each ramp 45a, 45b follows an arcuate path around a part of the circumference of it respective side 35a, 35b of the gear selector ring 35. The arcuate extent AE of the path is typically in the range 90° to 360° degrees. The arcuate extent AE of the path may be determined by the number of engagement elements 28a, 28b on each side 35a, 35b of the engagement ring. For example, for arrangements having four engagement elements 28a, 28b per side 35a, 35b the arcuate extent AE of each non driving face is typically in the range 70° to 90°. For embodiments having three engagement elements 28a, 28b per side 35a, 35b the arcuate extent AE of each non-driving face is typically in the range 100° to 120°. For embodiments having two engagement elements 28a, 28b per side 35a, 35b the arcuate extent AE of each non-driving face is typically in the range 150° to 180°. For embodiments having one engagement element 28a, 28b per side 35a, 35b the arcuate extent AE of each non-driving face is typically in the range 300° to 360°.

The first set of engagement elements 28a is arranged on the first side 35a of the gear selector ring such that the drive faces 43a of the first set of engagement elements 28a all face in a first rotational direction. The first set of engagement elements 28a are arranged on the first side 35a of the engagement ring such that the ramps 45a of the first set of engagement elements 28a all slope to a lower height in a second rotational direction, which is opposite to the first rotational direction. The ramps 45a slope from a respective drive face 43a towards an adjacent drive face 43a. In use, when there is relative rotational movement between the first set of engagement elements 28a and the first set of dogs 20a that causes the ramps 45a to move in the second rotational direction in a manner wherein the drive faces 43a do not drivingly engage the first set of dogs 20a, the first set of dogs 20a slide across the ramps 45a. The sloped surfaces of the ramps 45a cause the gear selector ring 35 to move axially along the input shaft 1 by a small amount away from the gear 19. As the dogs 20a slide pass the drive faces 43a, the ring 35 moves axially along the input shaft by a small amount towards gear 19. Thus the ramps 45a provide a ratcheting effect, and the first selector assembly 29 is arranged to slip relative to its associated gear element 22 under certain operational conditions.

The second set of engagement elements 28b is arranged on the second side 35b of the gear selector ring such that the drive faces 43b of the second set of engagement elements 28b all face in the first rotational direction. The second set of engagement elements 28b are arranged on the second side 35b of the gear selector ring such that the ramps 45a of the second set of engagement elements 28b all slope to a lower height in the second rotational direction, which is opposite to the first rotational direction, from a respective drive face 43b. The ramps 45b slope towards an adjacent drive face 43b. In use, when there is relative rotational movement between the second set of engagement elements 28b and the second set of dogs 20b that causes the ramps 45b to move in the second rotational direction, so the drive faces 43b do not drivingly engage the second set of dogs 20b of the gear element 23, the dogs 20b slide across the ramps 45b. The sloped surfaces of the ramps 45b cause the gear selector ring 35 to move axially along the input shaft 1 by a small amount away from the layshaft 5. As the dogs 20b slide pass the drive faces 43b, the ring 35 moves axially along the input shaft by a small amount towards gear 19. Thus the ramps 45b provide a ratcheting effect, and the first selector assembly 29 is arranged to slip relative to its associated gear element 23 under certain operational conditions. Thus the first and second sets of engagement elements 28a, 28b have a similar configuration. The first set of engagement elements 28a is arranged to engage the first set of dogs 20a during acceleration (forward torque direction) and the second set of engagement elements 28b is arranged to engage the second set of dogs 20b during acceleration (forward torque direction).

Preferably the drive faces 43a on the first side 35a of the engagement ring are rotationally aligned with the drive faces 43b on the second side 35b of the engagement ring (see Figure 3a).

The gear selector ring 35 includes internal splines 36. A sleeve member 200 is provided that includes internal splines 201 for a splined connection with the layshaft 5, and external splines 203 for a splined connection with the internal splines 36 of the engagement ring 35 (see Figure 3b). This provides a very robust connection between the engagement ring 35 and the first input shaft 1. The splined arrangement 36,201,203 enables the engagement ring 35 to move axially along first layshaft 5 while rotationally locking the engagement ring 35 for rotation with the layshaft 5 according to operation of the actuator mechanism 38.

The first selector assembly 29 has the following modes of operation:

1. Locking the second gear element 22 for rotation with the layshaft 5 in the first rotational direction, but not locking the second gear element 22 for rotation with the layshaft 5 in the second rotational direction (the first gear element 23 is not engaged);

2. Neutral; and

3. Locking the first gear element 23 for rotation with the layshaft 5 in the first rotational direction, but not locking the first gear element 23 for rotation with the layshaft 5 in the second direction (the second gear element 22 is not engaged).

It will be apparent from the above description that first selector assembly 29 cannot lock either of the gear elements 22 and 23 for rotation with the layshaft 5 in both of the first and second rotational directions, nor can it lock the either of the gear elements 22 and 23 for rotation with the layshaft 5 in the second direction at all. Furthermore, the first selector assembly 29 is only cable of selecting one of the first and second gear elements 23,22 at a time. Thus the first selector assembly 29 has a different structure and operating modes from known instantaneous selector assemblies. The new arrangement is a simplified selector assembly.

The actuator mechanism 38 is arranged to control the movement of the gear selector ring 35. The actuator mechanism 38 includes a shift fork 48, a shift cradle 100 and a shift drum 102. Optionally the actuator mechanism 38 may include resilient means, such as helical springs 104. The springs 104 are arranged to bias the shift fork 48, and hence the gear selector ring 35, into a neutral position. The springs 104 are located in the shift cradle 100. The gear selector ring 35 has a groove 35c formed in its outer curved surface that extends circumferentially around the curved surface. The shift fork 48 mates with the groove 35c and drives the gear selector ring 35 axially according to the rotational orientation of the shift drum 102. The shift drum 102 controls operation of the shift fork 48. The shift drum 102 has a relatively simple construction because movement of the ring 35 is reversible. An advantage of the current arrangement is that only a single shift drum 102 is required to obtain all of the necessary movement of the selector ring 35 when providing torque support during gearshifts.

Movement of the gear selector ring 35 is controlled by the transmission control unit 90 for automatic and semi-automatic versions of the transmission 88, however with this layout it is possible to have an entirely manual transmission, in which case the transmission control unit is not strictly required.

The transmission control unit 90 is an electronic logic control system driven by software that is arranged to control operation of the actuator mechanism 38 and hence the gear selector ring 35.

For fully automatic transmissions 88 gear selections are made by the transmission control unit 90 when the drive source control unit 82 detects predetermined operational conditions, for example when the drive source 80 reaches a particular speed in a particular gear. For semi automatic transmissions 88 gear selections are made by the user of the drive system by initiating the gear selection input device 94, for example a gear lever (manual) or switches located adjacent the steering wheel (semi-automatic). The transmission 88 can be arranged such that it is possible to select between the automatic and manual modes.

The transmission 88 is arranged to provide torque support to the output shaft 9 during power on upshifts only. During operation of the transmission 88, the first and second input shafts 1,3 can be considered sources of torque from which torque can be provided selectively to the 1^st, 2^nd and 3^rd gears. The transmission 88 is arranged to change torque path during a gearshift, for power on upshifts, from a first torque path that includes one of the first and second input shafts 1,3, to a second torque path that includes the other of the first and second input shafts 1,3, to a third torque path that includes the one of the first and second input shafts 1,3. For example, in preferred arrangements when driving normally in 1^st gear, torque is provided to 1^st gear along a first torque path that includes the second input shaft 3. During a power on upshift, the torque path is changed temporarily from the first torque path to a second torque path that includes the first input shaft 1. This is achieved by operating at least one of the first selector assembly 29, the second selector assembly 31 and the clutch 86, and typically at least the first selector assembly 29 and the clutch 86. This provides torque support to the output shaft 9 during the power on up shift. Subsequently the torque path is changed again to a third torque path for the driving in 2^nd gear. The third torque path includes the second input shaft 3. This provides torque support to the output shaft 9 during a power on up shift from 1^st gear to 2^nd gear. When driving normally in 2^nd gear, torque is provided to 2^nd gear along a first torque path that includes the second input shaft 3. For a power on up shift, third gear is pre-selected by the third selector assembly 33. The torque path is changed temporarily from the first torque path to a second torque path that includes the first input shaft 1. This is achieved by operating at least one of the first selector assembly 29, second selector assembly 31 and the clutch 86, and typically at least the first selector assembly 29 and the clutch 86. This provides torque support to the output shaft 9 during a power on up shift from 2^nd gear to 3^rd gear. Subsequently the torque path is changed again to a third torque path for the driving in 3^rd gear. The third torque path includes the second input shaft 3. This switching between input shafts 1,3 (torque sources) from the second input shaft 3 to the first input shaft 1 and then back to the second input shaft 3, will be explained further below. Operation of the transmission will now be described with reference to Figures 4 to 5g. In Figures 4 to 5g the stages marked "a" to "g" on the graphs correspond with respective Figures a to g showing the positions of the first selector assembly 29 with respect to first and second gear elements 23,22.

Figures 4 to 4g show a power on upshift strategy from 1^st gear to 2^nd gear.

Figure 4a shows the initial condition of the transmission when the power on upshift is requested, that is, driving in 1^st gear. The clutch 86 is closed, the second selector assembly 31 is in engagement with the first sleeve 24 (and hence gear element 15), the gear selector ring 35 of the first selector assembly 29 is in a neutral position. The torque path through the transmission 88 is second input shaft 3, second selector assembly 31, first sleeve 24, gear element 15, gear element 17, the third selector assembly 33 and the output shaft 9. The arrows A and B in Figure 4a show the relative speeds of the gear elements 22 and 23. The triple headed arrow B indicates that the gear element 23 is rotating faster than the gear element 22. In response to a request for a power on upshift, the first selector mechanism 29 moves the gear selector ring 35 into engagement with gear element 23, thereby locking the gear element 23 for rotation with the layshaft 5 in the acceleration direction, and the clutch 86 is opened (Figure 4b). The second selector assembly 31 does not move at this stage. The effect of opening the clutch 86 on relative torques and speeds of the drive source 80, clutch 86, 1^st gear, 2^nd gear and output are shown in the graph. The torque path through the transmission 88 changes to the first input shaft 1, gear element 20, gear element 25, the layshaft 5, the first selector assembly 29, gear elements 23,15,17, the third selector mechanism 33 and the output shaft 9. The arrow C indicates the direction of torque. The next step is for the clutch 86 speed to be synchronised with the speed of 2^nd gear (gear element 19) (Figure 4c). This is achieved by moving the second selector assembly 31 out of engagement with the first sleeve 24 and into engagement with the second sleeve 26 and hence the gear element 19. The clutch 86 torque capacity is then gradually increased to substantially match the clutch torque to the drive source torque (Figure 4d). The torque path through transmission 88 is then changed to the second input shaft 3, the second selector assembly 31 , second sleeve 26, gear elements 19 and 21 and the output shaft 9. The speed of the drive source 80 is then synchronised to the speed of the 2^nd gear (gear element 19) and the gear selector ring 35 is moved out of engagement with the gear element 23 to a neutral position (Figure 4e). The clutch 86 is then closed (Figure 4f) and the shift is completed. Thus torque is supplied to the output shaft 9 continually throughout the shift. This helps to improve the performance of the vehicle.

Figures 4g to 5g show a power on up shift strategy from 2^nd gear to 3^rd gear.

Figure 4g shows the third selector assembly 33 pre-selecting gear element 18, thereby locking gear element 18 for rotation with the output shaft 9.

Figure 5a shows the torque path when driving in 2^nd gear. The clutch 86 is closed, the second selector assembly 31 is in engagement with second sleeve 26 (and hence gear element 19), the first selector assembly 29 is in a neutral position. Torque passes from the drive source 80, through the clutch 86, the second input shaft 3, the second selector assembly 31 and gear elements 19 and 21 (2^nd gear) to the output shaft 9. The second selector assembly 31 engages the second sleeve 26 thereby locking gear element 19 for rotation with the second input shaft 3. The first selector assembly 29 is in neutral. The arrows A and B in Figure 5a show the relative speeds of the gear elements 22 and 23. The double-headed arrow A indicates that the gear element 22 is rotating faster than the gear element 23. When a request is made for a power on up shift, the first selector assembly 29 engages gear element 22 with the engagement ring 35, thereby locking gear element 22 for rotation with the layshaft 5 in the acceleration direction, and the clutch 86 is opened. (Figure 5b). The effect of opening the clutch 86 on relative torques and speeds of the drive source 80, clutch 86, 2^nd gear, 3^rd gear and output are shown in the graph. The torque path through the transmission 88 changes to the first input shaft 1, gear element 20, gear element 25, the layshaft 5, the first selector assembly 29, gear elements 22,27, the second sleeve 26, gear elements 19 and 21 and the output shaft 9. The arrow C indicates the direction of torque. The speed of the clutch 86 is then synchronised to the speed of 3rd gear (and hence gear element 16) and the second selector assembly 33 is actuated to engage the first sleeve 24 (Figure 5c). The clutch 86 torque capacity is then increased until the clutch 86 matches the drive source 80 torque (Figure 5d). As this happens, the output torque begins to fall and the torque generated by 3^rd gear increases. The torque path through the transmission 88 changes to second input shaft 3, the second selector assembly 33, the first sleeve 24, gear elements 16 and 18, the third selector assembly 33 and the output shaft 9. The drive source speed is then synchronised to 3^rd gear (and hence gear element 16) and the first selector assembly 29 is operated to disengage the gear element 22 and move to a neutral position (Figure 5e). The clutch 86 torque capacity is then quickly increased until the clutch 86 is closed. (Figure 5f). Shift completed and driving takes place in 3^rd gear (Figure 5g). Thus torque is supplied to the output shaft 9 continually throughout the power on up shift. This helps to improve the performance of the vehicle.

A second embodiment is shown in Figure 6. The second embodiment is similar to the first embodiment except that it comprises a four-speed transmission. The transmission includes a 4^th gear comprising a gear element 128 mounted on the second sleeve 26 and a gear element 130 rotatably mounted on the output shaft 9. The gear element 128 meshes with the gear element 130. The transmission further includes a fourth selector assembly 134 mounted on the output shaft 9 between the gear elements 21 and 130. The fourth selector assembly 134 has a similar preselect function with respect to the even gears as the third selector assembly 33 has with respect to the odd gears. The fourth sector assembly 134 can be similarly arranged to the third selector assembly 33. Also each gear element 21 ,130 that is mounted on the output shaft 9 that is part of an even gear is rotatably mounted on the output shaft 9 in a similar fashion to the gear elements 17 and 18. The fourth selector assembly 134 is arranged to selectively lock the gear elements 21,130 for rotation with the output shaft.

It will be appreciated that the inventive transmission layout is easily scalable to include additional gears, for example the transmission can be a five-speed transmission or a six-speed transmission.

For a five-speed transmission, 5^th gear can be added on the odd side by including an additional gear element mounted on the first sleeve 24 and a meshing additional gear element rotatably mounted on the output shaft 9. An additional selector assembly can be provided for selectively locking the additional gear for rotation with the output shaft 9. For a six-speed transmission, in addition to 5^th gear mentioned above, 6^th gear can be added on the even side by including an additional gear element mounted on the second sleeve 26 and a meshing additional gear element rotatably mounted on the output shaft 9. An additional selector assembly can be provided for selectively locking the additional gear for rotation with the output shaft 9.

It will be appreciated that the above embodiments are examples only of the invention and that modifications can be made to the above-embodiments that fall within the scope of the claims, for example, additional gears can be added to the transmission. The transmission can be arranged as a seven speed transmission by adding an additional gear train (7^th gear) on the odd side of the second embodiment, or arranged as an eight speed transmission by, in addition to 7^th gear, adding an additional gear train on the even side of the second embodiment (8^th gear). Any practicable number of gears can be included, and where required, additional selector assemblies on the output shaft.

The first set of engagement elements on the first side of the engagement ring can be rotationally offset from the second set of engagement elements on the second side of the engagement ring.

Claims

1. A transmission system, including: a first input shaft that is arranged to receive drive directly from a drive source; a second input shaft that is arranged to receive drive from the drive source via a drive interruption means; a layshaft; a first gear element rotatably mounted on the lay shaft; a second gear element rotatably mounted on the layshaft; a first selector assembly arranged to selectively lock the first and second gear elements for rotation with the layshaft, the first selector assembly comprising a single engagement ring that includes a first side having a first set of engagement elements, wherein each engagement element has a drive face arranged to drivingly engage in a first rotational direction a first set of drive formations associated with the first gear element and a non-driving face that is arranged to slip with respect to the first set of drive formations in a second rotational direction, thereby preventing driving engagement with the first set of drive formations, and a second side having a second set of engagement elements, wherein each engagement element has a drive face arranged to drivingly engage in the first rotational direction a second set of drive formations associated with the second gear element and a non-driving face that is arranged to slip with respect to the second set of drive formations in the second rotational direction, thereby preventing driving engagement of the second set of drive formations.

2. A transmission system according to claim 1, wherein the non-driving face for each engagement element extends along a curved path from a position adjacent its respective drive face.

3. A transmission system according to claim 1 or 2, wherein the arcuate extent of each non- driving face is in the range 90° to 360°.

4. A transmission system according to claim 2 or 3, wherein the non-driving face extends circumferentially along an arcuate path from a position adjacent its respective drive face.

5. A transmission system according to claim 4, wherein the non-driving face extends circumferentially along the arcuate path from a position adjacent its respective drive face to a position adjacent the drive face of an adjacent one of the engagement elements on a respective side of the engagement ring.

6. A transmission system according to any one of the preceding claims, wherein the number of engagement elements on each side of the engagement ring is less than or equal to four, less than or equal to three, or less than or equal to two.

7. A transmission system according to claim 6, wherein only a single engagement element is provided on each side of the engagement ring.

8. A transmission system according to any one of the preceding claims, wherein each non driving face on the first side of the engagement ring slopes away from its respective drive face in the second rotational direction and each non-driving face on the second side of the engagement ring slopes away from its respective drive face in the first rotational direction.

9. A transmission system according to claim 8 when dependent on claim 4, wherein the angle of slope of each non-driving face along its respective circumferentially extending arcuate path is constant.

10. A transmission system according to any one of the preceding claims, wherein each non driving face is substantially planar along its arcuate path.

1 1. A transmission system according to any one of the preceding claims, wherein each drive face is planar.

12. A transmission system according to any one of the preceding claims, wherein each drive face is arranged radially from the centre of the engagement ring.

13. A transmission system according to any one of the preceding claims, wherein the engagement ring includes internal splines arranged to engage external splines formed on the first input shaft or with an intermediate component in the form of a sleeve member located between the engagement ring and the first input shaft, said sleeve member including internal splines for a splined connection with the first input shaft and external splines for a splined connection with the internal splines of the engagement ring.

14. A transmission system according to any one of the preceding claims, including an actuator mechanism for controlling axial movement of the engagement ring, wherein the actuator mechanism includes a shift fork engaged with the engagement ring and a single shift drum for controlling movement of the shift fork and engagement ring.

15. A transmission system according to claim 14, wherein the actuator mechanism includes a spring cradle.

16. A transmission system according to any one of the preceding claims, consisting of one of: a two-speed, three-speed, four-speed, five-speed, six-speed, seven-speed and eight-speed transmission.

17. A transmission system according to any one of the preceding claims, including a first sleeve rotatably mounted on the second input shaft, a second sleeve rotatably mounted on the second input shaft and a second selector assembly for selectively locking each of the first and second sleeves for rotation with the second input shaft.

18. A transmission system according to claim 17, wherein gear elements that are part of odd gears are mounted on the first sleeve and gear elements that are part of even gears are mounted on the second sleeve.

19. A transmission system according to claim 17 or 18, including 1^st gear comprising a gear element mounted on the first sleeve and a gear element rotatably mounted on an output shaft, wherein the gear elements are in meshing engagement.

20. A transmission system according to claim 19, including 3^rd gear comprising a gear element mounted on the first sleeve and a gear element rotatably mounted on an output shaft, wherein the gear elements are in meshing engagement.

21. A transmission system according to claim 20, including a third selector assembly arranged to selectively lock gear elements of 1^st and 3^rd gears, which are rotatably mounted on the output shaft, for rotation with the output shaft.

22. A transmission system according to claim 21, wherein the third selector assembly is located between the gear elements of 1^st and 3^rd gears, which are rotatably mounted on the output shaft.

23. A transmission system according to any one of claims 19 to 22, including 2^nd gear comprising a gear element mounted on the second sleeve and a gear element mounted on the output shaft, wherein the gear elements are in meshing engagement.

24. A transmission system according to claim 23, including 4^th gear comprising a gear element mounted on the second sleeve and a gear element rotatably mounted on the output shaft, wherein the gear elements are in meshing engagement.

25. A transmission system according to claim 24, including a fourth selector assembly arranged to selectively lock gear elements of 2^nd and 4^th gears, which are rotatably mounted on the output shaft, for rotation with the output shaft.

26. A transmission system according to claim 25, wherein the fourth selector assembly is located between the gear elements of 2^nd and 4^th gears, which are rotatably mounted on the output shaft.

27. A transmission system according to any one of claims 17 to 26, wherein at least one of the second selector assembly, third selector assembly and fourth selector assembly is a synchromesh selector assembly or dog clutch selector assembly.

28. A transmission system according to any one of claims 17 to 27, including a gear element mounted on the second sleeve which meshes with the second gear element.

29. A transmission system according to any one of the preceding claims, including a torque path between the layshaft and the first sleeve.

30. A transmission system according to claim 29 when dependent on claim 17, wherein the first gear element is in meshing engagement with a gear element mounted on the first sleeve.

31. A transmission system according to any one of the preceding claims, including a torque path between the layshaft and the second sleeve.

32. A transmission system according to claim 31 when dependent on claim 17, wherein the second gear element is in meshing engagement with a gear element mounted on the second sleeve.

33. A transmission system according to any one of the preceding claims, including a torque path between the first input shaft and the layshaft.

34. A transmission system according to claim 33, including a gear element mounted on the first input shaft and a gear element mounted on the layshaft, wherein the gear elements are in meshing engagement.

35. A transmission system according to any one of the preceding claims, arranged such that, in use, when driving in 1^st gear, torque is routed from the second input shaft to the output shaft via the second selector assembly, the first sleeve, 1^st gear gear elements and the third selector assembly.

36. A transmission system according to any one of the preceding claims, arranged such that, in use, when driving in 2^nd gear, torque is routed from the second input shaft to the output shaft via the second selector assembly, the second sleeve, and 2^nd gear gear elements.

37. A transmission system according to any one of the preceding claims, arranged such that, in use, when driving in 3^rd gear, torque is routed from the second input shaft to the output shaft via the second selector assembly, the first sleeve, 3^rd gear gear elements and the third selector assembly.

38. A transmission system according to any one of the preceding claims, arranged to supply torque continually to the output shaft during a power on upshifts.

39. A transmission system according to any one of the preceding claims, arranged during a power on upshift, to switch torque flow from a first torque path that includes the second input shaft, the second selector assembly, and one of the first and second sleeves , to a second torque path that includes the first input shaft, the layshaft 5 and first selector assembly, to a third torque path that includes the second input shaft the second selector assembly, and the other of the first and second sleeves.

40. A transmission system according to claim 39, including operating at least one of the first selector assembly, the second selector assembly and a slippable clutch device to switch between the first, second and third torque paths.

41. A transmission system according to any one of the preceding claims, wherein one of the first and second input shafts is tubular and houses at least part of the other of the first and second input shafts, wherein at least part of the shafts are substantially co-axial. The first input shaft is tubular and houses part of the second input shaft.

42. A transmission system according to any one of the preceding including a control system for controlling operation of the transmission system.

43. A transmission system according to any one of the preceding claims, wherein the drive interruption means comprises a friction clutch.

44. A transmission system according to any one of the preceding claims, wherein each non-drive face comprises a ramp.

45. A transmission system, including: a first shaft; a first gear element rotatably mounted on the first shaft; a second gear element rotatably mounted on the first shaft; a first selector assembly arranged to selectively lock the first and second gear elements for rotation with the first shaft, the first selector assembly comprising a single engagement ring that includes a first side having a first set of engagement elements, wherein each engagement element has a drive face arranged to drivingly engage, in a first rotational direction, a first set of drive formations associated with the first gear element, and a non-driving face that is arranged to slip with respect to the first set of drive formations in a second rotational direction, thereby preventing driving engagement with the first set of drive formations, and a second side having a second set of engagement elements , wherein each engagement element has a drive face arranged to drivingly engage in the first rotational direction a second set of drive formations associated with the second gear element and a non-driving face that is arranged to slip with respect to the second set of drive formations in the second rotational direction, thereby preventing driving engagement of the second set of drive formations.

46. A drive train including a drive source, a friction clutch device and a transmission system according to any one of the preceding claims.

47. A vehicle including a transmission system according to any one of claims 1 to 45 or a drive train according to claim 46.