POWERSHIFT TRANSMISSION

The present invention relates to a transmission system, an in particular, but not exclusively, to transmission layout that is suitable to enable substantially instantaneous gear shifts.

In instantaneous type transmission systems, for example of the type described in WO 2004/099654, WO 2005/005868, WO 2005/005869, WO 2005/024261 and WO 2005/026570. the arrangement is such that when drive is transmitted between the input and output shafts via a gear train, for example the first gear train, the gear selector mechanism can select the new (second) gear train under power without disengaging the first gear train, by locking the rotatably mounted gear wheel of the second gear train to its shaft. Thus momentarily, drive is transmitted between the input and output shaft via two gear trains. The selector mechanism subsequently disengages the first gear train and fully engages the second gear train. Drive is then transmitted between the input and output shafts via the new gear train only, thus providing uninterrupted power through a gearshift. The selector mechanism is arranged such that the gearshifts can take place under acceleration or deceleration.

The gear selector mechanisms of this type of transmission have four modes with respect to each adjacent gear trains:

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;

Disengaged in the forward toque direction while engaged in the reverse torque direction.

The last two of the above four modes enable a discrete ratio gearbox to have the ability to shift up or down ratios instantly under load without substantial torque interruption.

It is highly desirable to provide a compact transmission layout, at low cost, whilst still meeting Noise Vibration and Harshness (NVH) requirements and providing a large number of gear ratios.

Accordingly the present invention seeks to provide a transmission system that has at least one

of the above-mentioned characteristics by providing a new transmission layout.

According to one aspect of the invention there is provided a transmission system including an input shaft; first and second lay shafts; an output shaft; first and second torque paths for transferring drive between the input shaft and the first lay shaft, third and fourth torque paths for transferring drive between the input shaft and the second lay shaft, fifth and sixth torque paths for transferring drive between the first lay shaft and the output shaft, and seventh and eighth torque paths for transferring drive between the second lay shaft and the output shaft; and selector means arranged to selectively transfer drive along a selection of the first to eighth torque paths thereby transferring drive from the input shaft to the output shaft, wherein the first and second lay shafts are arranged substantially parallel to the input shaft and the distance between the longitudinal axis of the input shaft and the longitudinal axis of the first lay shaft is different from the distance between the longitudinal axis of the input shaft and the longitudinal axis of the second lay shaft. This provides a highly compact multi-path transmission layout having at least eight gear ratios that enables substantially instantaneous shifts to be achieved when used in conjunction with an appropriate selector assembly, for example ^' of the type described in WO 2004/099654. While the transmission layout is particularly suited for use with instantaneous selector assemblies, the layout can be used with other types of transmission such as dual clutch transmissions to provide seamless shifts.

Advantageously the output shaft is arranged substantially parallel to the input shaft and the longitudinal axis of the output shaft is offset from the longitudinal axis of the input shaft such that the input shaft and the output shaft are not co-axial. Having different shaft; centre distances and having the input and output shafts offset such that they are not co-axial gives the flexibility to select a suitable set of gear ratios for a vehicle having an internal combustion engine. Having a similar layout without different shaft centre distances and without the input and/or the output shafts being axially offset does not provide a suitable set of gear ratios for a vehicle with an internal combustion engine.

Advantageously the selector means can include at least one instantaneous selector assembly that is arranged to lock a gear element for rotation with one of the shafts from operational modes that including the following modes: lock the gear element for rotation with the shaft in the clockwise and anti-clockwise directions; lock the gear element for rotation with the input shaft in the clockwise direction and not lock in the anti-clockwise direction; and lock the gear element for rotation with the input shaft in the anti-clockwise direction and not lock in the

clockwise direction. This enables instantaneous shifts to be performed for the majority, if not all, of the gear ratios in the transmission, for at least some shift types, by actuation of a single instantaneous gear selector assembly and creating torque paths with the other selector assembly by pre-selecting gears. The arrangement also provides a common output which provides a well balanced and compact transmission. The layout also prevents transmission lockup which can occur in layouts having more than one instantaneous selector assembly because all instantaneous shifts are achieved by moving a single instantaneous selector assembly, which is inherently safe. The lockup problem only occurs when it is necessary to operate more than one instantaneous selector mechanism during a shift. Also, the layout can be easily scaled to provide the appropriate number of gear ratios by adding in additional gear trains and selector assemblies to create additional torque paths. A further advantage is that the layout is suitable for manufacture by existing dual clutch manufacturing facilities.

Advantageously the selector means includes at least one noR-instantaneous selector assembly to pre-select gear elements to create the torque paths. At least some of the non-instantaneous selector assemblies can include first and second selector parts that can be actuated independently of each other, wherein the first part is arranged to selectively engage a first gear to selectively lock it for rotation with one of the shafts and the second part is arranged to selectively engage a second gear to selectively lock it for rotation with the shaft. For example, the selector means can include at least one synchromesh selector assembly for creating torque paths that includes first and second synchromesh parts that can be actuated independently of each other, with the first part being arranged to selectively engage a first gear to selectively lock it for rotation with one of the shafts and the second part is arranged to selectively engage a second gear to selectively lock it for rotation with the shaft. Using a split synchromesh selector assembly provides the following operational modes: locking both the first and second gears for rotation with the shaft, one of the first and second gears is locked for rotation with the shaft and the other is not locked for rotation; and neither of the first and second gears are locked for rotation with the shaft (neutral).

The transmission can include a casing for housing the transmission components. Advantageously the first and second lay shafts extend substantially the full length of the casing. The first and second lay shafts overlap both the input and the output shafts.

Advantageously the torque paths can include a first gear train for transmitting drive between the input shaft and the first lay shaft and between the input shaft and the second lay shaft and

a second gear train for transmitting drive between the input shaft and the first lay shaft and between the input shaft and the second lay shaft. The first gear train can include a first gear element rotatably mounted on the first lay shaft and a second gear element rotatably mounted on the second lay shaft, and the selector means includes a first selector assembly for selectively locking the first gear element for rotation with the first lay shaft and a second selector assembly selectively locks the second gear element for rotation with the second lay shaft. The second gear train can include a third gear element rotatably mounted on the first lay shaft and a fourth gear element rotatably mounted on the second lay shaft, and wherein the first selector assembly is arranged to selectively lock the third gear element for rotation with the first lay shaft and the second selector assembly selectively locks the fourth gear element for rotation with the second lay shaft.

Advantageously the first selector assembly can be of the non-instantaneous type and can be used to pre-select gears. For example, the first selector assembly can include first and second parts that can be actuated independently of each other, with the first part being arranged to selectively engage the first gear element to selectively lock it for rotation with the first lay shaft and the second part is arranged to selectively engage the third gear element to selectively lock it for rotation with the first lay shaft, wherein the first selector assembly can select the first and second gear elements from operational modes that include the following modes: locking both the first and third gear elements for rotation with the shaft, locking one of the first and third gear elements for rotation with the shaft and not locking the other gear element for rotation; and not lock either of the first and third gear elements for rotation with the shaft

(neutral). Alternatively, the first selector assembly can be of the instantaneous type and can select the following operational modes with respect to each of the first and third gear elements: lock the gear element for rotation with the shaft in the clockwise and anti-clockwise directions; lock the gear element for rotation with the input shaft in the clockwise direction and not lock in the anti-clockwise direction; and lock the gear element for rotation with the input shaft in the anti-clockwise direction and not lock in the clockwise direction.

Advantageously the second selector assembly can be of the non-instantaneous type and can be used to pre-select gears. For example, the second selector assembly can include first and second parts that can be actuated independently of each other, with the first part being arranged to selectively engage the second gear element to selectively lock it for rotation the second lay shaft and the second part is arranged to selectively engage the fourth gear element

to selectively lock it for rotation with the second lay shaft, wherein the second selector assembly can select the second and fourth gear elements from operational modes that include the following modes: locking both the second and fourth gear elements for rotation with the shaft, lock one of the second and fourth gear elements for rotation with the shaft and not lock the other gear element for rotation; and not lock either of the second and fourth gear elements for rotation with the shaft (neutral). Alternatively, the second selector assembly can be of the instantaneous type and can select the following operational modes with respect to each of the second and fourth gear elements: lock the gear element for rotation with the shaft in the clockwise and anti-clockwise directions; lock the gear element for rotation with the input shaft in the clockwise direction and not lock in the anti-clockwise direction; and lock the gear element for rotation with the input shaft in the anti-clockwise direction and not lock in the clockwise direction.

The first gear train includes a fifth gear element mounted on the input shaft and the second gear train includes a sixth gear element mounted on the input shaft.

The fifth and sixth gear elements can be rotatably mounted on the input shaft and the selector means includes a third selector assembly that is arranged to selectively lock each of the fifth and sixth gear elements for rotation with the input shaft from operational modes that include the following modes: lock the gear element for rotation with the input shaft in the clockwise and anti-clockwise directions; lock the gear element for rotation with the input shaft in the clockwise direction and not lock in the anti-clockwise direction; and lock the gear element for rotation with the input shaft in the anti-clockwise direction and not lock in the clockwise direction; and damping means for damping engagement of the fifth and sixth gear elements by the third selector assembly. This provides a particularly compact arrangement.

Advantageously the third gear selector assembly is arranged to select the following operational mode with respect to the first gear element: the first gear element is not locked for rotation with the first shaft in the clockwise or anticlockwise directions (neutral). Alternatively the fifth and sixth gear elements can be fixed for rotation with the input shaft.

The transmission system may include a dual clutch arrangement for transferring drive from the engine to the input shaft. For example, the input shaft can include an inner part and an outer part that are arranged to rotate independently of each other, the fifth gear element is mounted on one of the inner and outer parts and is arranged to rotate with its part, and the

sixth gear element is mounted on the other of the inner and outer parts and is arranged to rotate with its part, and the transmission system includes first and second clutch devices, wherein the first clutch device is arranged to selectively transmit drive to the inner part of the input shaft and the second clutch device is arranged to selectively transmit drive to the outer part of the input shaft. Advantageously the selector means can include at least one instantaneous selector assembly.

pre-selectAdvantageously the torque paths can include a third gear train for transmitting drive between the first lay shaft and the output shaft and between the second lay shaft and the output shaft. The third gear train can include a seventh gear element rotatably mounted on the first lay shaft, an eighth gear element mounted on the output shaft and a ninth gear element rotatably mounted on the second lay shaft, and the selector means includes a fourth selector assembly for selectively locking the seventh gear element for rotation with the first lay shaft and a fifth selector assembly for locking the eighth gear element for rotation with the second lay shaft.

The torque paths can include a fourth gear train for transmitting drive between the first lay shaft and the output shaft and between the second lay shaft and the output shaft. The fourth gear train can include a tenth gear element rotatably mounted on the first lay shaft, an eleventh gear element mounted on the output shaft and a twelfth gear element rotatably mounted on the second lay shaft, and wherein the fourth selector assembly is arranged to selectively lock the tenth gear element for rotation with the first lay shaft and the fifth selector assembly is arranged to selectively lock the twelfth gear element for rotation with the second lay shaft.

In a preferred embodiment the first, second, fourth and fifth selector assemblies are non- instantaneous selector assemblies such as synchromesh, split synchromesh or dog type selector assemblies, and the third selector assembly is of the instantaneous type

In another preferred embodiment the first and second selector assemblies are of the instantaneous type and the fourth and fifth selector assemblies are of the non-instantaneous type. In this embodiment there is no third selector assembly, the fifth and sixth gear elements are fixed for rotation with the input shaft.

Advantageously the or each instantaneous selector assembly includes first and second sets of engagement members that are arranged to selectively lock the gear elements for rotation with

their respective shafts, and the gear elements that are selectable by the or each instantaneous selector assembly, such as the fifth and sixth gear elements, the first and third gear elements and second and fourth gear elements, each include first and second parts that are arranged to rotate relative to each other when initially engaged and a damping system for damping the relative rotational movement.

Alternatively, or additionally, the damping system can be included in the or each instantaneous selector assembly.

The "first part of each gear -element includes a first set of drive formations, the second part of each gear element includes a second set of drive formations, and when selecting the gear element with one of the first and second sets of engagement members, that set of engagement members is arranged to drivingly engage the second set of drive formations to cause relative rotational movement between the first and second parts of the gear element, the damping system is arranged to damp the relative rotational movement between the first and second parts of the gear element, and after some damping has occurred the engagement members are arranged to drivingly engage the first set of drive formations. This provides a hard drive surface after damping has occurred to prevent the engagement members from continuously loading the fluid damping system. That is, the second set of drive formations are engaged prior to the pistons reaching the ends of their respective piston chambers (see below).

Advantageously the third selector assembly is arranged to select one of the fifth and sixth gear elements while the other of the fifth and sixth gear elements is still engaged by the selector assembly. Each instantaneous selector assembly is arranged to select one gear from a gear pair while the other gear from the gear pair is still engaged.

Advantageously the or each instantaneous selector assembly can be arranged such that when a driving force is transmitted, one of the first and second sets of engagement members drivingly engages the engaged gear element, and the other set of engagement members is then in an unloaded condition. Preferably the selector assembly is arranged such that when a braking force is transmitted the first set of engagement members drivingly engages the engaged gear element, and the second set of engagement members is in an unloaded condition, and when a driving force is transmitted the second set of engagement members drivingly engages the engaged gear element, and the second set of engagement members is then in an unloaded condition.

The damping system is arranged to allow lost motion between the input shaft and at least one of the fifth gear element and the third selector assembly after the selector assembly engages the fifth gear element.

Advantageously the damping system can be a fluid damping system, and preferably a hydraulic damping system. The damping system can include first and second piston chambers located in the second part of the gear element and a first piston that is arranged to move with the first part of the gear element and to move into and out of the first and second piston chambers according to the relative rotational movement of the first and second parts of the gear element. And can further include third and fourth piston chambers located in the second part of the gear element and a second piston that is arranged to move with the first part of the gear element and to move into and out of the third and fourth piston chambers according to the relative rotational movement of the first and second parts of the gear element.

Advantageously the first and second pistons and each of their respective piston chambers are arranged to allow hydraulic fluid to leak from the chambers during a damping action. One of the first and second parts of the gear element includes meshing means for meshing with another gear element, and the other of the first and second parts of the gear element includes means for mounting the gear element on a shaft. When the first and second parts of the gear element are fitted together, the first and second sets of drive formations are located on the same side face.

The first set of drive formations includes n drive formations, wherein n is in the range 2 to 24, preferably 3 to 16 and more preferably 3 to 6, and the second set of drive formations includes n drive formations, wherein n is in the range 2 to 10, preferably 3 to 6. The drive formations in the first set of drive formations are distributed on the first part of the gear element such that they are substantially equally angularly spaced, and the drive formations in the second set of drive formations are distributed on the second part of the gear element such that they are substantially equally angularly spaced.

Advantageously the transmission system can include means for limiting the axial movement of the first and second sets of engagement members. The means for limiting the axial movement of the first and second sets of engagement members can include at least one of: a set of raised abutments, wherein the raised abutments are located on the second part of the gear element and are arrange alternately with the drive formations in the second set second set

of drive formations; and the first set of drive formations, the depth dimension being selected to determine the extent of axial limitation of the first and second sets of engagement members.

The damping means can include means for self-centring the relative rotational orientations of the first and second parts of the gear element. The means for self-centring includes resilient means, such as a spring element. When in an unloaded condition, the drive formations of the first set of drive formations are rotationally offset from the drive formations in the second set of drive formations. When in an unloaded condition, the drive formations of the first set of drive formations are rotationaily aligned with the drive formations in the second set of drive formations.

The damping fluid is supplied to the interior of the gear element via the first shaft.

The transmission system can include an electronically programmable control system for controlling operation of the or each gear selector assembly. For example, the control system may include a processing device that is programmed to control operation of the selector assemblies. This can prevent transmission lock up occurring by appropriate sequence control. Advantageously the control system can be arranged to move the unloaded set of engagement members out of engagement from the engaged gear element before actuating the other gear selector assembly to engage the new gear wheel. This is an important factor in preventing transmission lock up when torque reversals occur during a shift requiring the operation of more than one selector assembly since it removes the set of engagement elements out of engagement with the current gear wheel that would otherwise lock the transmission if a torque reversal occurred.

The control system can be arranged to control the operation of the or each selector mechanism to perform sequential gearshifts. The control system can arranged to bias the loaded set of engagement members towards the unengaged gear wheel until the loaded set of engagement members are free to move.

The transmission system may further include at least one of the following: means for determining the direction of torque in the transmission system when receiving a request for a gearshift; and means for preventing the direction of torque in the transmission changing during a gearshift.

The or each instantaneous selector assembly can be arranged to include first and second sets of engagement members that are moveable into and out of engagement with the first and second gear elements that are rotatably mounted on a shaft and an actuator system for actuating the engagement members, wherein the first gear selector assembly is arranged such that when a driving force is transmitted, one of the first and second sets of engagement members drivingly engages the engaged gear element, and the other set of engagement members is then in an unloaded condition and the actuator system is arranged to move the unloaded set of engagement members into driving engagement with the unengaged gear element to effect a gear change..

The selector assembly is such that when a braking force is transmitted the first set of engagement members drivingly engages the engaged gear wheel, and the second set of engagement members is in an unloaded condition, and when a driving force is transmitted the second set of engagement members drivingly engages the engaged gear wheel, and the first set of engagement members is then in an unloaded condition.

The actuator system includes a first actuator device for actuating the first set of engagement members and a second actuator device for actuating the second set of engagement members independently of the first actuator device. The actuator system for the first selector assembly can include a first actuator device for actuating the first set of engagement members and a second actuator device for actuating the second set of engagement members independently of the first actuator device. Preferably the actuator system includes a first actuator member for moving the first set of engagement members and a second actuator member for moving the second set of engagement members, which can be actuated by the first and second actuator devices respectively. Alternatively the first and second sets of engagement members can be actuated by a single actuator. The actuator assembly can include at least one resilient means arranged to move at least one of the first and second sets of engagement members into engagement with the first and second gear elements when the engagement members are in unloaded conditions. Preferably the or each resilient means is arranged to bias at least one of the first and second sets of engagement members towards the first or second gear element when the engagement members are drivingly engaged with a gear element.

Advantageously the or each instantaneous selector assembly is arranged such that when the first and second sets of engagement members engage one of the first and second gear

elements the backlash when moving between acceleration and deceleration is less than or equal to five degrees.

Advantageously the third selector assembly can be operated to select alternately the fifth and sixth gear elements when shirting between gear ratios, to provide substantially instantaneous sequential shifts.

According to another aspect of the invention there is provided a transmission system including: an input shaft; first and second lay shafts; an output shaft; wherein the first and second lay shafts are arranged substantially parallel with the input shaft and the distance between the longitudinal axis of the input shaft and the first lay shaft is different from the distance between the longitudinal axis of the input shaft and second lay shaft; a first gear train for transmitting drive between the input shaft and the first lay shaft and between the input shaft and the second lay shaft; a second gear train for transmitting drive between the input shaft and the first lay shaft and between the input shaft and the second lay shaft; a third gear train for transmitting drive between the first lay shaft and the output shaft and between the second lay shaft and the output shaft; and a fourth gear train for transmitting drive between the first lay shaft and the output shaft and between the second lay shaft and the output shaft.

The first gear train includes a first gear element rotatably mounted on the first lay shaft and a second gear element rotatably mounted on the second lay shaft; the second gear train includes a third gear element rotatably mounted on the first lay shaft and a fourth gear element rotatably mounted on the second lay shaft; the third gear train includes a fifth gear element rotatably mounted on the first lay shaft and a sixth gear element rotatably mounted on the second lay shaft; and the fourth gear train includes a seventh gear element rotatably mounted on the first lay shaft and an eighth gear element rotatably mounted on the second lay shaft.

The transmission includes at least one instantaneous selector assembly that is arranged to lock each gear element in a pair of the gear elements for rotation with one of the shafts from operational modes that including the following modes: lock the gear element for rotation with the shaft in the clockwise and anti-clockwise directions; lock the gear element for rotation with the input shaft in the clockwise direction and not lock in the anti-clockwise direction; and lock the gear element for rotation with the input shaft in the anti-clockwise direction and not lock in the clockwise direction; and at least one non-instantaneous selector assembly to pre-select gears to create torque paths between the input and output shafts.

The first gear train can include a ninth gear element rotatably mounted on the input shaft and the second gear train includes a tenth gear element rotatably mounted on the input shaft and the instantaneous selector assembly is arranged to selectively lock the ninth and tenth gear elements for rotation with the input shaft.

In another embodiment the transmission system can include first and second instantaneous selector assemblies, wherein the first instantaneous selector assembly is arranged to selectively lock the first and third gear elements for rotation with the first lay shaft and the second instantaneous selector assembly is arranged to selectively lock the second and fourth gear elements for rotation with the second lay shaft.

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

Figures Ia and Ib are schematic representations of a transmission system layout according to the invention;

Figure Ic is a schematic representation of a selector mechanism used in the transmission system of Figure Ia;

Figure Id is a schematic of a vehicle drive system including a transmission system in accordance with the invention;

Figures 2a to 2j show a gear wheel having a damping mechanism;

Figure 3 is a schematic that illustrates the interaction of a selector mechanism and the dogs on the side of a gear wheel;

Figure 4 is an isometric view of an engagement element from the selector mechanism;

Figures 5a-f illustrate diagrammaticaUy operation of the selector mechanism by showing movement of one engagement member from each set;

Figures 6a and 6b show a second embodiment of the invention, which is a variant of the first embodiment;

Figure 7 shows a third embodiment of the invention; and

Figure 8 shows a fourth embodiment of the invention that has a similar layout to that of Figures Ia and Ib, however a selector mechanism from the layout of Figures la-b has been replaced with a dual clutch arrangement and there are two input shafts.

Figures Ia and Ib show a transmission system 88 that includes comprises a single instantaneous selector mechanism 29 mounted on an input shaft 3 and synchromesh selector mechanisms 331a-d mounted on the first and second lay shafts 4,6, that are arranged to create torque paths between the input shaft 3 and an output shaft 1 via gears A\B\C\D\E\F',G\H\I\J\ and to make instantaneous shifts for each gear ratio for at least some shift types.

The gear trains are arranged as follows: a first gear train 308 including a gear A' mounted on the input shaft 3 via a bearing so that it can rotate relative to the input shaft 3, a gear C mounted on the first lay shaft 4 via a bearing so that it can rotate relative to the first lay shaft

4 and a gear E' mounted on the second lay shaft 6 via a bearing so that it can rotate relative to the second lay shaft 6; a second gear train 310 including a gear B' mounted on the input shaft

3 via a bearing so that it can rotate relative to the input shaft 3, and a gear D' mounted on the first lay shaft 4 via a bearing so that it can rotate relative to the first lay shaft 4 and a gear F' mounted on the second lay shaft 6 via a bearing so that it can rotate relative to the second lay shaft 6; a third gear train 312 including a gear G' mounted on the first lay shaft 4 via a bearing so that it can rotate relative to the first lay shaft 4, a gear K' mounted on the output shaft 1 so that it rotates with the output shaft 1 and a gear I' mounted on the second lay shaft 6 via a bearing so that it can rotate relative to the second lay shaft 6; and a fourth gear train 314 including a gear H' mounted on the first lay shaft 4 via a bearing so that it can rotate relative to the first lay shaft 4, a gear L' mounted on the output shaft 1 so that it rotates with the output shaft 1 and a gear J' mounted on the second lay shaft 6 via a bearing so that it can rotate relative to the second lay shaft 6.

The first and second lay shafts 4,6 are arranged substantially parallel with the input shaft 3 and are further arranged such that the distance Y between the longitudinal axis of the input shaft 3 and the longitudinal axis of the first lay shaft 4 is different from the distance Z between the longitudinal axis of the input shaft 3 and the longitudinal axis of the second lay shaft 6. The input and output shafts 3,1 are offset axially, that is the shafts are not co-axial. In Figures Ia and Ib, is the shafts are offset in the direction into the plane of the Figures. So if the Figures define the X-Y plane, the output shaft 1 is offset from the input shaft 3 in at least

the Z axis. The shafts can be arranged such that the input and output shafts 3,1 are offset in at least one of the three axes such that they are not co-axial. When looking end on at the shafts, the input and output shafts 3,1 are thus centrally offset, for example in a diamond or arrowhead shaped configuration. Having different shaft centre distances and having the input and output shafts offset such that they are not co-axial gives the flexibility to select a suitable set of gear ratios for a vehicle having a combustion engine. Having a similar layout without the different shaft centre distances and without the input and output shafts being axialiy offset does not provide a suitable set of gear ratios for a vehicle with an internal combustion engine.

Having the transmission system arranged in this manner helps to provide a highly compact arrangement with multiple torque paths. Furthermore, the gear ratios in this arrangement are typically in the form of a geometric spread.

It can be seen in Figures Ia and Ib that the input and output shafts 3,1 do not overlap longitudinally. The output shaft 1 starts substantially when the input shaft 3 ends. It can also be seen that the first and second lay shafts 4,6 run substantially the full length of the transmission casing 302 and are supported at each end by bearings. Preferably the casing 302 includes an internal partition 302a that includes bearings for supporting the first and second lay shafts 4,6 and the input and output shafts 3,1. The gears A',B',C'D\E',F' on the input side of the transmission are on one side of the partition 302a and the gears G',H',F,J',K',L' from the output side are on the other side of the partition 302a.

An instantaneous gear selector mechanism 29 is mounted on the input shaft 3 between the gears A' and B' and is arranged to selectively lock the gears A' and B' for rotation with the input shaft 3 in a manner described above for the first embodiment.

A first synchromesh selector mechanism 331a is mounted on the first lay shaft 4 between the gears C and D' and is arranged to selectively lock the gears C and D' for rotation with the first lay shaft 4. The first synchromesh selector mechanism is of a split type, that is, it has two synchromesh parts that can be actuated independently of the other, with each part being arranged to selectively engage one of the gears C and D' to selectively lock and unlock its respective gear for rotation with the first lay shaft. Using a split selector mechanism 331a provides the following operational modes: - both gear C and gear D' can be locked for rotation with the first lay shaft 4 simultaneously, one of the gears C and D' can be locked for rotation with the first lay shaft and the other is not locked for rotation; and neither gears C

and D' are locked for rotation with the shaft (neutral). A second synchromesh selector mechanism 331b is mounted on the second lay shaft 6 between the gears E' and F' and is arranged to selectively lock the gears E' and F" for rotation with the second lay shaft 6. The second selector mechanism 331b is also of the split type so that gears E' and F' can be selectively locked for rotation with the second lay shaft 6 in similar operational modes to the first selector mechanism 331a.

A third synchromesh selector mechanism 331c is mounted on the first lay shaft 4 between gears G' and H' and is arranged to selectively lock gears G' and H' for rotation with the first lay shaft 4. The third selector mechanism 33 Ic is of the conventional type that can only lock either gear G' or H' for rotation with the first lay shaft 4 at any one time. A fourth synchromesh selector mechanism 33 Id is mounted on the second lay shaft 6 between gears I' and J' and is arranged to selectively lock gears I' and J' for rotation with the second lay shaft 4. The fourth selector mechanism 33 Id is of the conventional type that can only lock either gear P or J' for rotation with the second lay shaft 6 at any one time.

By pre-selecting the gear paths it is possible to provide instantaneous shifts for at least eight gear ratios for at least some shift types using only a single instantaneous selector assembly and only five gear selector assemblies in total. A significant advantage of the first embodiment is that it can either be used as an eight-speed transmission or a sixteen-speed transmission according to the torque paths created by the selector mechanisms and the gears.

The structure of the instantaneous selector mechanism 29 and the gear A', and the way the selector mechanism 29 selectively engages the gears A' and B' will now be described. The structure of the gear B' is similar to the gear A'. Each of the gears C to M' is of the conventional type.

Figures 2a-j show the gear A' including a hydraulic damping mechanism 200 that is arranged to allow limited relative rotational movement between the gear A' and the input shaft 3 and / or the selector mechanism 29. The arrangement is such that the limited relative rotational movement softens the engagement of the new gear A' by the selector mechanism 29 thereby reducing the noise generated to acceptable levels. The relative rotational movement effectively increases the time that it takes for the gear A' to be locked for rotation with the input shaft 3 and thereby provides a longer period of time over which the energy generated by the collision is dissipated.

The gear A' comprises an outer annular part 202 and an inner annular part 204. The inner part

204 is arranged co-axially with the outer part 202 and is arranged for limited relative rotational movement therewith. The outer part 202 includes gear teeth 210 formed in a peripheral portion that are arranged to mate with teeth on a corresponding gear wheel fixed to the input shaft 3, and a set of drive formations in the form of a first set of dogs 212. The first set of dogs 212 includes six dogs 214 that are arranged to be engaged by the selector mechanism 29. The dogs 214 are preferably formed integrally with the outer part 202 of the gear wheel, but this is not essential, and are evenly circumferentially distributed about the side face, i.e. the angle subtended between the centres of a pair of dogs 214 is approximately 60° (see Figure 2a). Each dog 214 is arcuate, extends through an angle of approximately 30°, and includes two drive faces 216, one at each end, and a substantially planar upper surface 218.

The inner part 204 is rotatably mounted on the input shaft 3 via a bearing 203 and includes a second set of dogs 220 on a side face that are arranged to be engaged by the selector mechanism 29. The second set of dogs 220 is located on one side of the inner part of the gear wheel. The first set of dogs 220 comprises three dogs 222 evenly circumferentially distributed about the gear face, i.e. the angle subtended between the centres of a pair of dogs is approximately 120° (see Figures 2a and 3), and are preferably formed integrally with the first gear wheel, but this is not essential. Each dog 222 extends through angle of approximately 30°, and includes two drive faces 224, one at each end, and a substantially planar upper surface 226. Three dogs are used because this arrangement provides large engagement windows, that is, spaces between the dogs, to receive the engagement elements. Large engagement windows provide greater opportunities for the first gear selector mechanism 29 to fully engage the gear A' before transmitting drive thereto.

A set of raised abutments 228 is located on the same side face of the inner part of the gear wheel 204 as the second set of dogs 220 to prevent the selector mechanism 29 from engaging the first set of dogs 212 before the second set of dogs 220 is engaged. The set of raised abutments 228, includes three raised abutments 230 that are arranged alternately with the dogs 222. Each raised abutment 230 includes inclined end faces 232 and a substantially planar upper surface 234. Each raised abutment 230 extends through an arc of approximately 60°, is spaced by 15° from each adjacent dog 222, and has a depth that is substantially equal to the depth of the dogs 214 in the first set of dogs 212.

The inner gear part 204 includes two arcuate tracks 236 that are arranged about the

longitudinal axis of the inner gear part in the manner shown in Figure 2j. Each track includes two piston chambers 238: one located towards each end of the track 238. Each track 236 is arranged to house a piston 240. Each piston 240 is arcuate and includes a connector 242 that protrudes through a slot 244 in the curved surface 246 of the inner gear part that sits in a recess formed in the outer gear part 202, the arrangement being such that each piston 240 rotates with the outer gear part 202. Thus each piston 240 is arranged to move along its track 236 into and out of each of the piston chambers 238 according to the relative rotational orientations of the inner and outer parts 204,202 of the gear.

Preferably the inner gear 204 part is manufactured from first and second inner gear parts 205,207 that are welded together, for example by electron beam welding.

The piston chambers 238 are filled with a hydraulic fluid, which is fed through the input shaft 3 along an axial feed line 248 formed in the input shaft 3 along its central axis, and to the chambers 238 via radial feed lines 250 and a feed ring 252. The feed ring 252 includes an annular groove formed in its outer surface to enable it to continuously supply oil to the interior of the gear wheel. The oil supply system can be a closed system or an open system. For example, an open system can use the gearbox lubricating oil and include a system for pumping it from the sump of the gearbox to the interior of each gear wheel including the damping mechanism.

The movement of the pistons 240 along the tracks 236 is limited by hydraulic fluid being compressed within the piston chambers 238 and ultimately by the selector mechanism 29 drivingly engaging the first set of dogs 212 (see below).

The arrangement of the piston chambers 238, the pistons 240, and hydraulic fluid is such that there is a predetermined leakage rate under a given load as each piston 240 moves into one of the piston chambers 238. This is achieved by dimensioning the piston 240 such that there are gaps between it and the inside of the piston chamber 238 to enable a small amount of hydraulic fluid to escape. The leakage rate provides a means of designing into the gear A' the stiffness of the damping mechanism when the gear A' is selected by the gear selector mechanism 29. For example, a typical leakage rate is typically less than 10% and preferably around 5%.

The gear A' includes a circlip 254, which acts as a self centring spring. The circlip 254 sits in a groove 256 formed in the curved surface 246 of the inner part of the gear. A' lug 258

attached to the outer part of the gear 202 loads the circlip 254 when there is relative rotation between the inner and outer parts 204,202 of the gear. Thus when the load causing relative rotation between the inner and outer parts 204,202 reduces, the circlip 254 biases the inner and outer parts 204,202 to the neutral position. A' further advantage of using a circlip 254 is that it can be arranged such that it is rotationally balanced.

When the selector mechanism 29 engages the gear A', abutments 228 and first set of dogs 212 initially prevent full engagement from taking place. Thus the selector mechanism 29 is only able to drivingly engage the second set of dogs 220. When engagement takes place, the selector mechanism 29 drives the second set of dogs 220, which causes relative rotational movement between the inner and outer parts 204,202 of the gear wheel. The relative rotational movement causes each of the pistons 240 to move into one of its respective piston chambers 238 according to the direction of movement thereby loading the hydraulic fluid located therein and causing a quantity to be forced out of the chamber 238. The effect of this is to damp the engagement of the gear A'.

The first set of dogs 212 also acts as drive formations. When a predetermined amount of relative rotational movement occurs between the inner and outer parts 204,202 of the gear wheel, the selector mechanism 29 drivingly engages the first set of dogs 212. This prevents further relative rotational movement since the selector mechanism 29 then drives the outer part 202 of the gear wheel directly.

A similar effect occurs if the selector mechanism 29 engages the gear A' in the opposite torque direction. Thus damping takes place in both the clockwise and anti-clockwise directions.

Thus the damping system reduces the noise of the impact such that it is not audible by the driver of the vehicle or so that it is reduced to an agreeable level.

The first gear selector mechanism 29 includes a sleeve 34, first and second sets of engagement elements 35,36 and an actuator assembly 38.

The first gear selector mechanism 29 is mounted on the input shaft 3 between the gears A' and B'. The gear selector mechanism 29 is arranged to engage the first and second sets of dogs 212,220 located on the gears A\B\ The gears A\B' are mounted spaced apart on the input shaft 3 and are arranged such that the sides including the first and second dog groups

212,220 face each other.

The first and second sets of engagement elements 35,36 are mounted on the sleeve 34. The first set of engagement elements 35 comprises three elements 28 that are evenly distributed about the input shaft 3 such that their bases face inwards, and the axes of the elements 28 are substantially parallel with each other and the input shaft 3. The second set of engagement elements 36 comprises three elements 30, which are similarly arranged about the input shaft

3. The sets of engagement elements 35,36 are arranged to rotate with the input shaft 3 but are able to slide axially along the sleeve 34, and hence the input shaft 3, in response to a switching action of the actuator assembly 38. To facilitate this, the sleeve 34 includes six keyways 41 formed in its curved surface with each engagement element 28,30 having a complementary formation in its base. The keyways 41 may have substantially T-shaped profiles such that the elements are radially and tangentially (but not axially) restrained within the keyways 41 (see Figure 3). Alternatively, the keyways 41 can have slotted or dovetailed profiles to radially restrain the elements.

Preferably the elements are configured to be close to the input shaft 3 to prevent significant cantilever effects due to large radial distances of loaded areas thus reducing the potential for structural failure.

The arrangement of the engagement element sets 35,36 is such that elements of a particular set are located in alternate keyways 41 and the sets 35,36 can slide along the sleeve 34. The engagement elements in each set are rigidly connected to each other by an annular member

100 and move as a unit. Each set 35,36 can move independently of the other. The annular member 100 has a groove 102 formed in its outer curved surface that extends fully around the annular member. The engagement elements 28 in the first set of engagement elements 35 are preferably integrally formed with its annular member 100, though this is not critical. The engagement elements 28 are evenly distributed about the annular member 100. The second set of engagement elements 36 comprises three elements 30, which are held in a similar fixed arrangement by a second annular member 100. When there is relative movement between the first and second sets of engagement elements 35,36, the annular member 100 of the first engagement element set 35 moves over the second set of engagement elements 36 and the annular member 100 of the second engagement element set 36 slides over the first set of engagement elements 35.

Each engagement element 28 in the first engagement element set 35 has a first end 28a arranged to engage the first and second group of dogs 212,220 attached to the gear A' and a second end 28b arranged to engage the first and second groups of dogs 212,220 on the gear B'. The first and second ends 28a,28b typically have the same configuration but are opposite handed, for example the first end 28a is arranged to engage the first and second groups of dogs 212,220 during deceleration (reverse torque direction) of the gear A' and the second end 28b is arranged to engage the first and second group of dogs 212,220 during acceleration (forward torque direction) of the gear B'. Each engagement element 30 in the second engagement element set 36 is similarly arranged, except that the first end 30a is arranged to engage the first and second group of dogs 212,220 on gear A' during acceleration of the and the second end 30b is arranged to engage the first and second group of dogs 212,220 during deceleration of the gear B'.

When both the first and second sets of engagement elements 35,36 engage a gear wheel drive is transmitted between the input and output shafts 3,1 whether the gear is accelerating or decelerating.

The first and second ends 28a,30a,28b,30b of each engagement element include an engagement face 43 for engaging the first and second sets of dogs 212,220, a ramp 45, an end face 42 and may include a shoulder 44 (shown diagrammatically in Figure 4). The end faces 42 limit the axial movement of the engagement elements 28,30 by abutting the sides of the gear wheels and also the upper surfaces 2115,234 of the first set of dogs and abutments respectively. The engagement faces 43 may be angled to complement the drive faces of the dogs 216,224 so that as the engagement elements 28,30 rotate into engagement, there is face- to-face contact to reduce wear. Each ramp 45 is preferably helically formed and slopes away from the end face 42. The angle of inclination of the ramp 45 is such that the longitudinal distance between the edge of the ramp furthest from the end face 42 and the plane of the end face 42 is larger than the height of the dogs 212,220. This ensures that the transmission does not lock up when there is relative rotational movement between the engagement elements 28,30 and the dogs 212,220 that causes the ramp 45 to move towards engagement with the dogs 212,220. The dogs 212,220 do not crash into the sides of the engagement elements 28,30 but rather engage the ramps 45. As further relative rotational movement between the dogs 212,220 and the engagement elements 28,30 occurs, the dogs 212,220 slide across the ramps 45 and the helical surfaces of the ramps cause the engagement elements 28,30 to move axially

along the input shaft 3 away from the dogs 212,220 so that the transmission does not lock up.

The ramps 45 are also arranged to interact with the inclined end faces 232 of the abutments in order to move axially away from the gear A'.

The arrangement of the gear selector mechanism is such that it inherently prevents lockup of the transmission occurring when selecting a new gear.

Thus the selector assembly 29 is arranged to engage each of the gears A\B' from the following modes: 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 toque direction while engaged in the reverse torque direction. Since the first and second sets of engagement elements 35,36 can move independently of each other, it is possible to select a new gear A',B' while the current gear A',B' is still engaged and thus the selector mechanism 29 can perform instantaneous gear shifts since there is no loss of power when selecting a new gear, for at least some shift types.

When the engagement elements of the first and second sets 35,36 are interleaved, as in Figure 3, the engagement faces 43 of the first ends 28 a of the first set of engagement elements 35 are adjacent the engagement faces 43 of the first end 30a of the second set of engagement elements 36. When the first and second sets of engagement elements 35,36 are fully engaged with a gear, a dog 214 from the first set of dogs and a dog 222 from the second set of dogs is located between each pair of adjacent engagement faces 43. The dimensions of the dogs 214,222 and the ends of the elements are preferably such that there is little movement of each dog between the engagement face 43 of the acceleration element and the engagement face 43 of the deceleration element when the gear moves from acceleration to deceleration, or vice versa, to ensure that there is little or no backlash in the gear.

The actuator assembly 38 controls the movement of the first and second sets of engagement elements 35,36. The assembly 38 includes first and second actuators 46,64 and first and second actuator members 48,58. The first and second actuators 46,64 are force generator actuators and preferably part of an electrical system for example, an electro-mechanical system or an electro-hydraulic system. The first and second actuator members 48,58 are preferably in the form of independently controllable forks. Movement of the first set of engagement elements 35 is controlled by movement of the first actuator member 48, which is

controlled by the first actuator 46. Movement of the second set of engagement elements 36 is controlled by movement of the second actuator member 58, which is controlled by the second actuator 64. Thus the first and second sets of engagement elements move totally independently of each other unlike known systems, such as the system of WO 2004/099654, which only has a single actuator for controlling actuation of both sets of engagement elements. With the known systems the sets of engagement elements can move relative to each other however the actuation of each set of engagement elements is interdependent since there is only a single actuator for initiating movement.

Each actuator member 48,58 is arranged to extend approximately ISO degrees around the groove 102 of its respective set of engagement elements and includes a semi-annular part that is located within the groove 102 . Each set of engagement elements 35,36 can rotate relative to its respective actuator member 48,58 and is caused to move axially along the input shaft 3 by the actuator member 48,58 applying a force to the annular member 100.

Optionally the actuator assembly 38 may include resilient means, such as helical springs (not shown). The springs are arranged to bias the first and second sets of engagement elements to move in an axial direction when they are in driving engagement with a gear wheel and are unable to move. For example, the springs may be positioned between the first actuator 46 and the first actuator member 48 or between the first actuator member 48 and the first set of engagement elements 35,36.

Operation of the first and second actuators 46,64, and hence movement of the first and second sets of engagement elements is controlled by a transmission control unit 90. The transmission control 90 unit may include sensors for determining the operational conditions of selector mechanisms 29,31,33 in the transmission. Typically these monitor the positions of the actuator members 48,58 and hence the positions of the sets of engagement elements, for example whether they are engaged with a gear wheel or not. The sensors can be included in the actuators 46,64, and may be, for example, Hall effect type sensors.

The transmission control unit is preferably in the form of an electronic logic control system driven by a processor, which runs software that is arranged to control operation of the first and second actuators 48,64 and hence the first and second sets of engagement elements 35,36. The sequence programming is typically arranged to control movement of the gear selector mechanisms 29,3 la-d together with controlling the direction of torque in the transmission

such that it prevents conflict shifts occurring. Being able to control the actuation of the first and second sets of engagement elements 35,36 totally independently by use of first and second actuators 46,64 has the advantage that the magnitude and the timing of application of the biasing force applied by each actuator can be independently controlled. This means that even at low rotational gear speeds the engagement elements sets 35,36 do not accidentally disengage from the engaged gear wheel and thus no loss of drive is experienced.

Figure Ic is a schematic diagram of a drive system including a transmission system 88 in accordance with the invention. The drive system includes an engine 80, an engine control unit 82, a sensor system 84 for determining the direction of torque in the transmission, a clutch device 86 such as a friction clutch, a transmission system 88, and the transmission control unit 90.

The engine 80 is typically an internal combustion engine in a vehicle but may be an electric motor for electric vehicles or any other suitable drive source. The output of the engine 80 is largely determined by the driver loading a throttle input device 81 (typically a throttle pedal), which is connected to the engine via a throttle interface 83 and the engine control unit 82. The engine control unit 82 is arranged to monitor and adjust the output of the engine 80 in accordance with instructions received from the user and the transmission control unit 90. The engine control unit 82 may be a throttle potentiometer type system or alternatively an electronic control system (sometimes called a "drive by wire" system).

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

The torque value in the transmission is determined in part by the output of the engine 80 and in part by the operational condition of the clutch 86, which determines the maximum permissible torque that can be transmitted to the transmission (clutch torque limit) according to the clamp load between the input and output sides of the clutch. The clamp load between the input and output sides of the clutch is determined by the transmission control unit 90 via the clutch actuator 92. Reducing the clamp load between the clutch plates allows controlled relative rotational movement between the input and output sides of the clutch device 86 to control the value of torque transmitted. A' typical value for speed difference can be 25rpm when operating around 4000rpm (4000rpm on one side of the clutch to 4025rpm on the other side).

The input and output clutch sensors 93 detect the speeds of the input and output sides of the clutch 86 respectively. The readings from the sensors 93 are monitored by the transmission control unit 90, which determines whether relative rotational movement is occurring and the direction of torque according to the values received from the sensors 93. The transmission control unit 90 is arranged to control the clutch actuator 92 and select the clutch clamp load in order to transmit the desired amount of torque to the transmission 88.

The drive system may include one or more clutch clamp load sensors {not shown) in order to detect slip between the input and output sides of the clutch 86.

The optional sensor system 84 for determining the direction of torque in the transmission, may include an accelerometer for determining whether the vehicle is accelerating or decelerating such as a mercury switch, a pair of load cells arranged to detect strain in transmission components wherein from a comparison of the outputs of each load cell it is possible to determine the torque direction (see WO 2005/005869), a sensor for detecting throttle position and/or a sensor for determining the rate of change in velocity in a rotating transmission component, such as an output shaft. In each case, it is the transmission control unit 90 that determines the direction of torque based on signals received from the sensor(s) used.

Any other suitable way of determining the direction of torque in the transmission can be used.

Optionally, the system can include a speed sensor 98 for detecting the output speed of the transmission. This can assist the transmission control unit 90 to determine which gear is engaged, since it can be programmed with details of the gear ratios and knows the input speed from the output side of the clutch sensor 93. Also, the readings from the speed sensor 98 can be used by the transmission control unit 90 to take into account the effect of changing road conditions on the direction of torque in the transmission 88.

The operation of the first gear selector mechanism 29 will now be described with reference to Figures 5a-5e and 6 which for clarity illustrate diagrammatically the movement of the first and second element sets 35,36 by the relative positions of only one element from each set.

Figure 5a shows the first and second engagement element sets 35,36 in a neutral position, that is, neither engagement element set is engaged with a gear wheel. Figure 5b shows the first and second engagement element sets moving into engagement with the gear A' under the action of

the first and second actuators 46,64 in response to a gearshift request from the input device 94 or the engine management system 82. Preferably, the clutch is opened for the first gear shift.

Figure 5c shows a condition when the gear A' is fully engaged, that is, the engagement elements 28,30 are interleaved with the first and second sets of dogs 212,220. The first and second actuators 46,64 are arranged such that the actuator members 48,58 maintain the first and second engagement element sets 35,36 in engagement with the gear A'. Accordingly, drive is transferred through the gear A' to the input shaft 3 via the first engagement element set 35 when decelerating and via the second engagement element set 36 when accelerating.

Whilst accelerating in gear A' (rotating in the direction of arrow Y in Figure 5c), the engagement faces 43 of the engagement elements of the first engagement element set 35 are not loaded, whilst the engagement faces 43 of the engagement elements of the second element set 36 are loaded. When a user, or the engine control unit 82, wishes to engage gear B' an input signal is sent from the input device 94 or the engine control unit 82 to the processor. The processor instructs the transmission control unit to actuate the first actuator 46 to drive the first actuator member 48, which causes the engagement elements 28 of the first engagement element set 35 to slide axially along the keyways 41 in the sleeve 34 thereby disengaging the first engagement element set 35 from the gear A' (see Figure 5d).

The second actuator 64 is activated to move the second actuator member 58 and hence the second engagement element set 36 towards the gear B'. However, because the second engagement element set 36 is loaded, i.e. is driving the gear A', it cannot be disengaged from the gear A', and the second engagement element set 36 remains stationary, with the second actuator 64 biasing it towards the gear B'.

When the first engagement element set 35 slides axially along the input shaft 3, the gear B' is in the neutral position. In the neutral position the first set of dogs 212 is rotationally offset from the second set of dogs 222. The raised abutments 228 and the first set of dogs 212 are arranged to block full engagement of the second set of dogs 220. Each piston 240 is located outside of its respective piston chambers 238, substantially centrally along its track 236, and each chamber 238 is substantially filled with hydraulic fluid. When the gear B' is selected by the first engagement set the initial contact is with the end faces 42 on the upper surfaces 218,234 of the first set of dogs and the raised abutments. As the first set of engagement elements rotate relative to the gear B' their engagement faces 43 subsequently engage the

drive faces 224 of the second set of dogs 220, which causes relative rotational movement between the outer and inner parts of the gear 202,204. The relative rotational movement causes the pistons 240 to move along their tracks 236 into the piston chambers 238 in the direction of torque applied by the selector mechanism 29. As each piston 240 moves into its respective piston chamber 238 the hydraulic fluid located in therein is pressurised, which causes some of the hydraulic fluid to leak from the chamber 238. This action absorbs a significant proportion of the engagement energy thereby reducing the noise and Shockwave of the impact and hence damps the engagement. The relative rotational movement between the inner and outer parts 204.202 of the gear wheel also loads the circlip 254.

When sufficient relative rotational movement has taken place to enable the first set of engagement elements 35 to move past the dogs 214 on the outer part 202 of the gear that initially blocked axial movement, the first set of engagement elements 35 is able to move axially into the gaps between adjacent dogs 214 such that the drive faces 43 of the engagement elements 35 engage the drive faces 216 of the first set of dogs 212. Engaging the drive faces 216 of the first set of dogs prevents further relative rotation between the first and second parts of the gear 204,202 (see Figure 5e). Thus the first set of dogs 212 now takes the driving load, which prevents the continued loading of the piston chambers 238. This significantly increases the useful life of the gear element, while preserving its ability to reduce torque spikes.

When relative rotation is arrested, the engagement elements 28 drive the outer part 202 of the gear B' in the direction of Arrow Z in Figure 5e and wherein drive is transmitted between the input and output shafts 3,1 via the second gear train 7. As this occurs, the second engagement element set 36 ceases to be loaded, and is free to disengage from the first group of dogs 212 on the gear A' Since the second engagement element set 36 is biased by the second actuator 64 it slides axially along the keyways 41 in the sleeve 34 thereby completing the disengagement of the gear A' from the input shaft 3. The second engagement element set 36 slides along the keyways 41 until it engages the gear B', thereby completing engagement of the gear B' with the input shaft 3.

This method of selecting gear trains substantially eliminates torque interruption since the gear train B',C is engaged before the gear train A\D' is disengaged, thus momentarily, the gears A',B' are simultaneously engaged and locked for rotation with the input shaft 3, until the

newly engaged gear wheel overdrives the original gear wheel. This type of gearshift is said to be instantaneous since a new gear is selected before the existing gear is released.

When a gear wheel is engaged by both the first and second engagement element sets 35,36 it is possible to accelerate or decelerate using a gear wheel pair with very little backlash occurring when switching between the two conditions. Backlash is the lost motion experienced when the dog moves from the engagement face 43 of the acceleration engagement element to the engagement face 43 of the deceleration engagement element when moving from acceleration to deceleration, or vice versa. A conventional dog-type transmission system has approximately 30 degrees of backlash. A typical transmission system for a car in accordance with the current invention has backlash of less than five degrees.

Backlash is reduced by minimising the clearance required between an engagement member and a dog during a gearshift: that is, the clearance between the dog and the following engagement member (see measurement X in Figure 5b). The clearance between the dog and the following engagement member is in the range 0.5mm - 0.03mm and is typically less than 0.2mm. Backlash is also a function of the retention angle, that is, the angle of the engagement face 43, which is the same as the angle of the undercut on the engagement face of the dog 20a. The retention angle influences whether there is relative movement between the dog and the engagement face 43. The smaller the retention angle, the less backlash that is experienced. The retention angle is typically between 2.5 and 15 degrees.

Transition from the gear train B',D' to the gear train A\D' whilst decelerating is achieved by a similar process.

Whilst decelerating in the gear B' the engagement surfaces 43 of the elements of the first element set 35 are not loaded, whilst the engagement surfaces 43 of the elements of the second element set 36 are loaded. When a user, or an engine control unit wants to engage the first gear train 5 a signal is sent from the input device or the engine control unit to the processor. The processor instructs the transmission control unit to actuate the first actuator 46 to move the first actuator member 48 axially, causing the first engagement element set 35 to slide axially in the keyways 41 along the input shaft 3 in the direction of the gear A', thereby disengaging the first engagement element set 35 from the gear B'.

The transmission control system activates the second actuator 64 however 60 since the second engagement element set 36 is loaded, i.e. it is driving! y engaged with the dogs 212,220 on the gear B', it remains stationary but is urged towards the gear A'.

As the first engagement element set 35 slides axially in the keyways 41 the gear A' is in the neutral position. In the neutral position the first set of dogs 212 is rotationally offset from the second set of dogs 222. The raised abutments 228 and the first set of dogs 212 are arranged to block full engagement of the second set of dogs 220. Each piston 240 is located outside of its respective piston chambers 238, substantially centrally along its track 236, and each chamber

238 is substantially filled with hydraulic fluid. When the gear A' is selected by the first engagement set the initial contact is with the end faces 42 on the upper surfaces 218,234 of the first set of dogs and the raised abutments. As the first set of engagement elements 35 rotates relative to the gear A' its engagement faces 43 subsequently engage the drive faces

224 of the second set of dogs 220, which causes relative rotational movement between the outer and inner parts of the gear 202,204. The relative rotational movement causes the pistons

240 to move along their tracks 236 into the piston chambers 238 in the direction of torque applied by the selector mechanism 29. As each piston 240 moves into its respective piston chamber 238 the hydraulic fluid located therein is pressurised, which causes some of the hydraulic fluid to leak from the chamber 238. This action absorbs a significant proportion of the engagement energy thereby reducing the noise and Shockwave of the impact and hence damps the engagement. The relative rotational movement between the inner and outer parts

204,202 of the gear wheel also loads the circlip 254.

When sufficient relative rotational movement has taken place to enable the first set of engagement elements 35 to move past the dogs 214 on the outer part 202 of the gear that initially blocked axial movement, the first set of engagement elements 35 is able to move axially into the gaps between adjacent dogs 214 such that the drive faces 43 of the engagement elements 35 engage the drive faces 216 of the first set of dogs 212. Engaging the drive faces 216 of the first set of dogs prevents further relative rotation between the first and second parts of the gear 204,202 (see Figure 5e). Thus the first set of dogs 212 now takes the driving load, which prevents the continued loading of the piston chambers 238. Thus energy is transmitted between the input and second lay shaft 3,6 by way of the gear train A\D'.

As this occurs, the second engagement element set 36 ceases to be loaded and biasing of the second actuator 64 causes it to slide axially within the keyways 41 along the input shaft 3

towards the gear A', thereby completing disengagement of the gear B'. The second engagement element set 36 continues to slide within the keyways 41 along the input shaft 3 until it engages the gear A', thereby completing locking engagement of the gear A' with the input shaft 3.

Kick-down shifts, that is a gearshift from a higher gear train to a lower gear train but where acceleration takes place, for example when a vehicle is travelling up a hill and the driver selects a lower gear to accelerate up the hill, require a brief torque interruption to allow disengagement of the driving element set. For example, when accelerating in gear B', gear B' is niliy engaged by the first and second sets of engagement elements 35,36 and the first element set 35 drivingly engages the dogs. When a kick-down shift is requested by the user via the input device 94 or the engine control unit 82, the transmission control unit 90 reduces the clutch clamp load using the clutch actuator 92 until controlled relative rotational movement between the input and output sides of the clutch is detected by the transmission control unit 90 via the clutch sensor 93 readings. The engine speed is then adjusted to synchronise with the new gear speed, which typically involves increasing the engine speed.

The transmission control unit 90 is able to synchronise the speed since it is programmed with information relating to the gear ratios for each gear train and can determine the currently engaged gear and the new gear to be selected. Synchronising the engine speed in this manner has a smoothing effect when engaging the new gear and prevents the vehicle from lurching when the gear is selected. The clutch clamp load is then further reduced as is the throttle in order to maintain the new ratio speed. The loaded 35 and the unloaded element sets 36 are then disengaged from the gear B' by actuating the first and second actuators 46,64 such that loaded set disengages the gear B' prior to the unloaded set 36 engaging the gear A'. The torque spike caused by the engagement is minimised due to the speed synchronisation step.

However if a torque spike is generated its effect is mitigated by further relative rotation of the input and output sides of the clutch 86. In practice it is preferable to reduce the torque transmittable by the clutch to zero, or near zero, or at least sufficiently low such that the actuators 46,64 are able to move the sets of engagement before disengaging the loaded set of engagement elements 35. Although the shift is not entirely instantaneous, it is very quick and the power interruption is lower than previous methods and may not even be noticed by the driver. The torque is reinstated by the engine control unit 82, the clutch clamp load is restored by the clutch actuator 92 and control of the engine 80 is returned to the user.

When the unloaded second element set 36 is disengaged from the gear B', it can alternatively be held in the neutral position until after the loaded first element set 35 is disengaged from the gear B'. The second element set 36 can then be moved into engagement with the gear A', after which the torque and clutch are reinstated. This shift is not instantaneous.

Torque paths

Power enters the transmission 88 through the input shaft 3 and is transferred to the first and second lay shafts 4,6 according to the operational status of the selector mechanism 29. Drive is transferred to the output shaft 1 from the first and second lay shafts 4,6 according to the operational status of the synchromesh selector mechanisms 331a-d.

The transmission control unit 90 is arranged to pre-select the gears C',D',E',F',G',H\J' that are mounted on the lay shafts 4,6 by controlling the operation of the selector mechanisms

331a-d. Then by controlling the operation of the instantaneous selector mechanism 29, for example by alternately selecting the gears A' and B' on the input shaft 3, the transmission system can perform instantaneous shifts for each gear ratio, for at least some shift types, using only a single instantaneous selector mechanism 29. For example, the torque path for first gear with reference to Figure Ia is A',C',G',K\ that is the selector mechanism 29 locks gear A' for rotation with the input shaft 3, the synchromesh selector mechanism 331a locks gear C for rotation with the first lay shaft 4 and drive is transmitted to the output shaft via gears G', K'.

When there is a call for second gear, either by the engine management system 82 or user of the vehicle, the transmission control unit 90 pre-selects gear D' with the synchromesh selector mechanism 331a and then locks gear B' for rotation with the input shaft 3 in an instantaneous manner, for example see the description relating to accelerating up shifts and decelerating down shifts above. When the gear B' is fully selected by the selector mechanism 29, drive then flows to the output shaft 1 via gears B',D',G',K\ When there is a call for third gear, the transmission control unit 90 pre-selects gears E' and F with the synchromesh selector mechanisms 331b and 33 Id respectively, and then locks gear A' for rotation with the input shaft 3 in an instantaneous manner with the selector mechanism 29. When the gear A' is fully selected by the selector mechanism 29, drive then flows to the output shaft via gears

A',E',F,KL\ Fourth gear can be selected by pre-selecting gear F' with selector mechanism

331b and selecting gear B' with selector mechanism 29, provided by torque path B',F',F,K\

Fifth gear is provided by torque path A',C'H',L\ sixth gear by torque path B',D',H',L', seventh gear is provided by torque path A',E\J',L\ and eighth gear by B\F',J',L\

It will be appreciated by the skilled person that this is reversible and that instantaneous downshifts can be performed, for example by moving from sixth gear to fifth gear by preselecting gear C and subsequently selecting gear A'.

Thus the transmission system is sequential and can provide instantaneous shifts between the current gear and the previous gear, and the current gear and the subsequent gear, for at least some shift types by pre-selecting the gears mounted on the first and second lay shafts 5,6 and alternately selecting gears A' and B' with the instantaneous selector mechanism 29.

A transmission system arranged in this manner is compact in length, which is highly advantageous for front wheel drive applications. It provides for a significant amount of space for gears A' and B' to be relatively large in size to accommodate sufficiently large damping mechanisms 200 to accommodate the significant loads experienced during use. It also avoids transmission lockup problem since all instantaneous shifts are achieved by moving a single instantaneous selector mechanism 29, which is inherently safe. The lockup problem only occurs when it is necessary to operate more than one instantaneous selector mechanism during a shift. Furthermore,- using conventional non-instantaneous selector mechanisms to pre-select the gears mounted on the lay shafts 4,6 significantly simplifies the control capabilities required by the transmission control unit 90 since it is significantly easier to control engagement of a gear element with, for example a conventional synchromesh selector mechanism than it is an instantaneous mechanism 29. Also, instantaneous selector mechanisms typically require damped gears, or some other damping system, in order to meet

Noise, Vibration and Harshness (NVH) standards, therefore replacing instantaneous mechanisms with conventional synchromesh selector mechanisms obviates the need to damp all of the gears in the transmissions, which saves on space and cost, while still providing instantaneous shifts for each ratio.

The layout also provides the possibility of modularity, that is, it is easy to add additional gear trains to provide additional gear ratios and also multiple pathways without increasing the number of gear trains, as can be seen from the additional embodiments below.

Figures 6a and 6b shows a second embodiment, which is a variant of the first embodiment. The variant is similar to the first embodiment except that the first synchromesh selector mechanism 331a is not of the split type, but rather is of the conventional type that selects either gear C or D' at any one time.

Table 1 below illustrates one possible set of torque paths for each gear ratio.

Table 1

Table 2 below illustrates another possible set of torque paths for each gear ratio.

Table 2

In Tables 2 and 3, [AMT] represents a non-instantaneous shift. It is preferable to have this type of shift in the upper gears as illustrated in Table 3, when moving from seventh to eighth gear.

Figure 7 shows a third embodiment of the invention, which is similar to the first embodiment in terms of the layout of the shafts 1,3,4,6 and the gear trains, except that the there is no gear selector mechanism between the gears A' and B', the gears A' and B' are fixed for rotation with the input shaft 3, the split synchromesh selector assemblies 331a and 33 Ib between gears C-D' and E'-F' respectively are replaced with first and second instantaneous selector

assemblies 29, and gears C',D\E\F' include damper mechanisms 200. This arrangement reduces the number of selector assemblies to four while still being able to perform instantaneous shifts for eight gear ratios for at least some shift types. This reduces the cost of the transmission layout.

The distance Y between the input shaft 3 and the first lay shaft 4 is different from the distance Z between the input shaft 3 and the second lay shaft 6. This is an important characteristic of the transmission layout to achieve the desired ratios while at the same time producing a compact arrangement.

The output shaft 1 is offset from the input shaft 3 such that they are not co-axial. Thus the distance W between the output shaft 1 and the first lay shaft 4 can be different from the distance X between the output shaft 3 and the second lay shaft 6. Furthermore, the distance W can be different from the distance Y and the distance Z can be different from the distance X. In the situation where the distance W is substantially equal to the distance Y and the distance X is substantially equal to the distance Z then the shafts are offset in a direction that is substantially perpendicular to the plane of Figure 7.

Table 3 below illustrates one possible set of torque paths for«ach gear ratio.

Table 3

Figure 8 shows a fourth embodiment that is similarly arranged to the first embodiment however the instantaneous selector mechanism is replaced by a dual clutch arrangement 48 Ia- b. The input shaft 403 has first and second parts (inner and outer parts) 403a,403b. A gear A' is attached to the inner part 403a of the input shaft and is arranged to rotate therewith. A gear B' is attached to the outer part of the shaft 403b and is arranged to rotate there with. Sequential shifting is achieved by pre-selecting gears C\D',E\F',G',H\I\J' that are rotatably mounted on the first and second lay shafts 404,406 with split synchromesh selector

assemblies 431a,431b and standard synchromesh assemblies 431c,431d to create torque paths between the input shaft 403, the first and/or second lay shafts 404,406 and the output shaft 401 , and then to alternately transfer drive to the inner and outer parts 403a,403b of the input shaft by actuating the first and second clutch devices 481a,481b. Using dual clutches 481 a,481b can provide substantially instantaneous shifts for each gear ratio with the layout described.

It will be appreciated by the skilled person that the invention is not to be considered as strictly limited to the above embodiment and that modifications can be made that fall within the scope of the invention, for example the number of gear trains included and the specific type of selector assemblies used. Instead of using synchromesh assemblies on the first and second lay shafts conventional dog shift mechanisms or instantaneous selector mechanisms similar to mechanism 29 can be used, though this increases the complexity of the transmission control unit.

Alternatively, or additionally, to having an instantaneous selector mechanism mounted on the input shaft 3 it can be mounted on the output shaft 1, together with the rotatably mounted gears. This is less desirable than mounting on the input shaft 3 since it adds the inertia of the gears and lay shafts 4,6 to the shifting actions, however the concept falls within the scope of the invention.