The present invention relates to a multi-speed transmission unit and to a shaft or gear engagement mechanism suitable for use in a multi-speed transmission unit.

The present invention will be discussed with reference to automobile multi-speed transmission units or "gearboxes", but should not be considered limited to such use. The multi-speed transmission unit of the invention could be used to transmit rotation from an input gear to an output shaft in any application or machinery.

In usual constant mesh gearboxes two parallel shafts are used with one gear of a meshing pair of gears fixed to one shaft and the other gear normally free to rotate on the second shaft and fixed only to its shaft when drive from one shaft to the other through the meshing pair of gears is required.

Normally engagement of the rotatable gear with its shaft is achieved by engagement of teeth provided on one flat side of the gear with the teeth of a sliding hub mounted slidably on exterior splines on the shaft. The hub is moved axially along the splined shaft to engage the gear and lock the gear to the shaft.

In the transmission units of the prior art the shafts used must be of a length sufficient to accommodate all of the gears and the axially sliding hubs, with room for the hubs to slide.

The present invention in a first aspect provides a

shaft and gear engagement mechanism comprising a hollow rotatable shaft, a gear rotatably mounted on the hollow shaft, a locking member movable between a first position in which the locking member engages the gear and locks the gear to the shaft such that the gear and hollow shaft rotate together and a second position in which the locking member is out of engagement with the gear and the gear and shaft are free to rotate relative to each other, and actuating means for moving the locking member between the first and second positions, which actuating means has a movable actuating member connected to locking member, the actuating member being located within the shaft wherein the locking member and the actuating member are connected together so that the locking member is positively moved between the first and second positions by movement of the actuating member.

Due to the forces generated by rotation it is very important to have a locking member which is moved positively between positions. This gives certainty of gear engagement and disengagement. A preferred feature of the first aspect of the invention is given in claim 2.

The present invention in a second aspect provides a shaft and gear engagement mechanism for locking a gear rotatable on a hollow shaft to the hollow shaft comprising: a locking member located in a radially extending slot provided in the hollow shaft, a body slidable axially within the hollow shaft having an axially extending profiled slot, a follower member connected to or part of the locking member slidably located in the profiled slot, and actuating means for sliding the body axially along the shaft, wherein on axial motion of the body along the shaft the

follower member follows the profile of the slot and moves the locking member radially of the shaft to engage the gear and lock the gear to the shaft.

The present invention further provides multi-speed transmission units having the shaft and gear engagement mechanism of the first and second aspects of the invention such units being detailed in claims 4, 4 and 6. The present invention additionally provides methods of operating the transmission units as detailed in claims 7, 8 and 9.

The present invention provides in a third aspect a multi-speed transmission unit comprising: a first gear drivable to rotate at a first speed, a second gear drivable to rotate at a second speed different from the first speed, and a shaft, wherein the first and second gears are mounted on the shaft for rotation relative thereto and engagement means is provided operable to lock each gear separately to the shaft, characterised in that the engagement means comprises a locking member which is moveable radially of the shaft and actuating means operable to move the locking member radially outwardly from the shaft to engage one of the first and second gears and radially inwardly towards the shaft to disengage an engaged gear.

By providing engagement means movable radially of a shaft, rather than axially as in the prior art, the multi-speed transmission unit of the invention can be made with much smaller axial dimensions, since the shaft of the transmission unit does not have to accomodate axial

movement of a hub along the shaft. This decreased axial length results in a reduced span of shaft between the bearings which decreases bearing loads and increases the efficiency of the transmission unit since frictional losses in the bearings are decreased. Furthermore, shorter shaft are stiffer than longer shafts and are less likely to deflect hence maintaining a better tooth mesh resulting in longer life and quieter gears. The housing of the multi-speed transmission unit also can be made much stiffer since the length is decreased.

The invention has the further advantages that it is less heavy than the multi-speed transmission units of the prior art and also costs less to manufacture.

Preferred features of the multi-speed transmission unit are given in Claims 2 to 10.

The present invention additionally provides vehicles having the aforementioned multi-speed differential units, the preferred features of the vehicles being given in claims 9 to 27.

The present invention in a fourth aspect provides a multi-speed transmission unit comprising: a first gear which in use is driven to rotate at a first speed, a second gear which in use is driven to rotate at a second speed different to the first speed, and a shaft of circular external cross-section, wherein the first and second gears are mounted on the shaft and are rotatable relative thereto, the first and second gears each having a generally circular aperture defined by an inwardly facing surface, the shaft passing through the generally circular apertures in the first and second gears, first engagement means is provided which is operable to engage the inwardly

facing surface of the first gear and thereby lock the first gear to the shaft and which is operable to disengage the said surface to allow the first gear to rotate relative to the shaft, and second engagement means is provided which is operable to engage the inwardly facing surface of the second gear and thereby lock the second gear to the shaft which is operable to disengage the said surface to allow the first gear to rotate relative to the shaft. Additional preferred features of the fourth embodiment of the invention are given in claims 29 to 31.

The present invention will now be described with reference to a preferred embodiments illustrated in the accompanying drawings, in which,

Figure 1 is an isometric cutaway view of first embodiment of a shaft and gear engagement mechanism according to the present invention.

Figure 2 is an isometric cutaway view of a second embodiment of a shaft and gear engagement mechanism according to the present invention.

Figure 3 is a cross-sectional view of a first embodiment of a multi-speed transmission unit according to the invention.

Figure 4 is a schematic representation of a control system for a multi-speed transmission unit accords to the invention.

Figure 5 is a cross-sectional view of a second embodiment of a multi-speed transmission unit according to the invention.

Figure 6 is a cross-sectional view of a third embodiment of a multi-speed transmission unit according to the invention.

In figure 1 there can be seen a bobbin 1 located inside a hollow shaft 2. The bobbin 1 can move axially inside the shaft 2.

Gears 5 and 9 can be seen mounted on the shaft 2 and these gears are normally rotatable relative to the shaft 2.

Two sets of teeth 3 and 6 are slidable radially of the shaft 2. The two sets of teeth 3 and 6 are slidable within slots provided in the shaft 2. Each set of teeth is a set of teeth slidable in three separate slots spaced equally (ie. every 120 ) around the shaft 2 (although any number of teeth could be used as required) .

The ends of the teeth 3 and 6 are provided with follower members in the form of pegs 4 which are located in profiled slots 8 provided in the bobbin 1.

The bobbin 1 is connected to a rod 7 which is connected to actuating means (not shown in figure 1) . The rod 7 extends axially within the shaft 2 and the actuating means is used to move the bobbin 1 axially within the shaft 2.

The rod 7 is either fixed to the bobbin 1 to rotate therewith or, as shown in figure 1, bearing 10 is provided so that the bobbin 1 is rotatable with respect to the rod 7, but can only move axially with movement of rod 7.

In operation, the rod 7 is moved axially of shaft 2

by the actuation means. The axial movement of rod 7 causes axial movement of bobbin 1 and the two sets of teeth 3 and 6 slide in the profiled slots 8 in the bobbin

1. The profiled slots 8 in the bobbin 1 are of a profile chosen such that axial movement of the bobbin relative to the shaft 2 can cause the teeth 3 and/or 6 to move radially of the shaft 2.

Movement of the rod 7 in the direction of the arrow shown in figure 1 causes the teeth 3 to move radially outward from the shaft 2 to engage with the gear 5. The teeth 3 engage with the walls of grooves machined on the inside of the gear 5 to lock the gear 5 to the shaft 2. Thereby, drive can be transmitted via the gear 5 to the shaft 2.

Movement of the rod 7 in the direction opposite to the arrow shown in figure 1, starting with the teeth 3 and 6 in the position shown in figure 1, causes the set of teeth 3 to be retracted within the shaft 2 so that the gears 5 and 9 are both free to rotate relative to shaft

2. Further movement of the rod 7 in the direction opposite to the arrow shown in figure 1 causes the set of teeth 6 to be forced radially outward of the shaft 2 to engage with slots provided in the gear 9.

The profiles of the profiled slots 8 provided in the bobbin 1 are chosen such that only one of the gears 5 and 9 can be engaged at any one time, but both can be disengaged at the same time. In fact, considering the motion of the set of teeth 3 from a starting position shown in the accompanying figure, when the rod 7 is moved in the direction opposite the direction of the arrow shown in figure 1 then the set of teeth 3 move down to disengage gear 5 and at such a point neither the set of teeth 3 nor

the set of teeth 6 engage either of the gears 5 and 9. With further motion the set of teeth 6 are then extended to engage gear 9.

It is very important to have a mechanism which provides positive withdrawal of two sets of teeth 3 and 6 into the shaft 2 since the rotation of the shaft 2 will bring about a resultant radially outwardly acting force on the teeth which must be counteracted to withdraw them. However, it is not necessary to completely withdraw the teeth within the shaft; the applicant can envisage a transmission unit in which only partial withdrawal is necessary to allow free rotation of a gear relative to a shaft.

It can be seen from figure 1 that the gears 5 and 9 abut along opposed faces. There is no need for axial spacing between the gears 5 and 9 to allow for axial movement of a hub provided on the exterior of the shaft 2 as would be common in the prior art. A multi-speed transmission unit incorporating the illustrated shaft and gear engagement mechanism can therefore be of much shorter length than the equivalent multi-speed transmission units of the prior art. The shorter length is advantageous in several ways apart from the obvious advantages of more compact packaging.

First, it will be appreciated that the shaft 2 must be mounted in bearings at each end. By reducing the length of the shaft 2 the length of shaft unsupported by bearings is decreased and the load on the bearings is therefore decreased. In this way, frictional losses of the transmission unit are lessened.

The shorter length of shaft 2 also enables the shaft

to be much stiffer and this avoids problems of warping and whipping of the shaft. The housing of the multi-speed transmission unit can also be made much stiffer and the advantage of this is that the gears and shafts can be securely kept in alignment without deformation of the housing.

The shaft and gear engagement mechanism of the invention has further advantages in that the mechanism is of lower weight than the sliding hub engagement mechanism of the prior art transmission systems and costs less.

The actuating means for the rod 7 could be mechanical actuating means (e.g. a gear shift stick in an automobile) or they could be hydraulic, electrical or electro-mechanical actuation means.

Whilst in the preferred embodiment of the invention shown described above only two gears 5 and 9 are provided on the shaft 2, any number of gears could be provided on the shaft and the shaft and gear mechanism means altered accordingly.

In figure 2 there is shown a shaft and gear engagement mechanism for three gears 5, 9, 13. Many parts of the mechanism of figure 2 are identical to parts of the figure 1 and thus have been given identical reference numerals.

In the figure 2 mechanism a bobbin 14 is used which has profiled slots 11 designed to control three sets of teeth 3, 6 and 12. The sets of teeth 3 and 6 can engage respectively gears 5 and 9 as in the figure 1 embodiment. The set of teeth 12 can engage the gear 13 to lock the gear 13 to the shaft 2.

The profiled slots 11 have a profile designed to radially extend only on the sets of teeth 3, 6, 12 at a time. In the figure 2 the teeth 9 are extended to engage the gear 13. If the rod 7 is moved in the direction of the arrow shown in figure 2 then the set of teeth 12 will be withdrawn into the shaft 2 so that all of the sets of teeth are withdrawn in the shaft 2 and the gears 5, 9 and 13 are free to rotate relative to shaft 2. If the movement of the rod 7 is continued then the teeth 6 will be extended radially to engage the gear 9. If the movement is further continued then the teeth 6 will be withdrawn into the shaft 2 and then the teeth 3 will be extended radially of the shaft 2 to engage the gear 5.

Figure 3 shows a multi-speed transmission unit which uses the shaft and gear engagement mechanism of figure 2. The transmission unit has a housing 20 in which there are rotatably mounted various gears and shafts as will be described later.

The power input to the transmission unit is received at the flywheel 21 which is mounted on a spindle 22 rotatably supported in the housing 20 by bearings. The spindle 22 has a splined portion 23 which engages the hub of a clutch plate 24.

A first clutch arrangement 25 is provided adjacent the flywheel 21 to act between the flywheel 21 and a hollow shaft 26 which is coaxial with the spindle 22. Three gears 27, 28, 29 are mounted on the shaft 26 to rotate therewith.

The clutch plate 24 is part of a second clutch arrangement 30 which is provided to act between the

spindle 22 and a hollow shaft 31 coaxial with both the shaft 26 and spindle 22. Three gears 32, 33 and 34 are mounted on the shaft 31 to rotate therewith.

A hollow shaft 35 is mounted in the housing 20 in parallel spaced apart relationship with the shaft 31. The hollow shaft 35 is connected to a bevelled gear 36 which engages a bevelled output gear 37 which is arranged to transmit drive to the wheels of a vehicle. Six gears 40, 41, 42, 43, 44, 45 are mounted on the shaft and are rotatable with respect to the shaft 35. Two gear and shaft engagement mechanisms 47 and 48 (identical to the shaft and gear mechanism shown in figure 2). are provided within the hollow shaft 35.

The first gear and shaft engagement mechanism 47 comprises a bobbin 50 which is moveable axially within the hollow shaft 36 by an actuating rod 51. The bobbin 50 has a profiled slot 55 which controls the motion of three teeth 56, 57, 58, one tooth for each of the gears 40, 41 and 42 (the teeth shown are each one of a set of three teeth and there are other profiled slots identical to slot 55 - this is illustrated in figure 2). The teeth 36, 57,

58 are slidable radially to engage the gears 40, 41 and 42 and lock them to the shaft 35.

The second shaft and gear engagement mechanism 48 comprises a bobbin 52 which is moveable axially along the hollow shaft 35 by an actuating rod 53. The actuating rod 53 is tubular and is coaxial with rod 51, the rod 51 being slidable within rod 53. The bobbin 52 has a profiled slot

59 which controls the motion of the three teeth 60, 61, 62, one tooth for each of the gears 43, 44, 45 (it will be appreciated that each tooth is one of a set of three teeth and each bobbin has three identical profiled slots for the

three teeth of each set). The teeth 60, 61, 62 are slidable radially in slots in shaft 35 to engage the gears 43, 44 and 45 and lock them to the shaft 35.

The changing of the gears is controlled by an electronic controller. Illustrated in figure 4 and described later in a controller suitable for controlling the multi-speed transmission unit in use in an automobile. The controller controls two three-position hydraulic actuators 70,71 for moving the actuator rods 51 and 53 and also actuators (not shown) for engaging and disengaging the clutches 25 and 30.

The actuators 70 and 71 are identical and thus only one will be described in detail. The actuator 70 comprises a piston 72 connected to the rod 51. The piston

72 is moveable in a cavity 73 defined between the housing 20 and an end plate 78. The piston 72 has two cylindrical portions 72A and 72B of different diameters. The cavity

73 also has two cylindrical portions of different diameters. The larger cylindrical portion 72B of the piston 72 is of a diameter which matches the diameter of the smaller cylindrical portion of the cavity 73. The piston portion 72B defines in the cavity a chamber 73B which is connected to a hydraulic feed port 76. An annular ring member 77 is provided on the exterior of the smaller portion 72A of piston 72 and is slidable relative to the piston 72. The annular ring member 77 and piston portion 72A define in the cavity a chamber 73A which is connected to a hydraulic feed port 75.

The controller 500 will control valves 450, 451 to regulate the hydraulic fluid supply to the hydraulic feed ports 75 and 76 from and to a source of fluid pressure 452 and fluid return 453. When pressurised hydraulic fluid is

supplied- to both feed ports 75 and 76 the piston 72 will assume a mid-position since the fluid pressure in chamber 73A will force the annular ring member 77 against a step defined by the differing diameters of the cavity 73 and the fluid pressure in chamber 73B will force the portion 72B of piston 72 into abutment with the annular ring portion 77.

When pressurised fluid is supplied to feed port 75 and the feed port 76 is connected to return (to allow fluid flow from chamber 73B) then the piston 72 will move to a far right position (towards end plate 78) under the influence of pressure in chamber 73A.

When pressurised fluid is supplied to the feed port 76 and the feed port 75 is connected to return (to allow fluid flow from chamber 73A) then the piston 72 will move to the position shown in figure 3 under the influence of pressure in chamber 73B. The annular ring member 77 moves with the piston 62 and a drilling will be provided in the ring member 77 to allow fluid flow through without the formation of a vacuum and/or the creation of a hydraulic lock.

In figure 4 there is schematically shown a control system for the multi-speed transmission unit of figure 3 in use in a vehicle. The controller 500 shown in the figure is networked with the controller 501 with an engine management system of the vehicle.

A sensor 502 is provided to measure the engine rotational speed and provide a signal indicative thereof to the engine management controller 501 and the transmission controller 500. A sensor 503 is provided to measure the rotational speed of the shaft 26 ie. the

output speed of clutch 25 and provide a signal indicative thereof to a transmission controller 500. There is also provided a sensor 504 for measuring the rotational speed of the shaft 31, ie. the output of the clutch 24, the sensor providing a signal to the controller 500.

Actuator position sensors 505 and 506 provide the transmission controller 500 which signals respectively indicative of the positions of the actuators 70 and 71 (ie. signals indicating which of the three possible positions each actuator is occupying) . Clutch position sensors 507 and 508 are also used to provide the transmission controller 500 with signals indicative of the positions of the clutches 24 and 25 respectively.

An accelerator pedal position sensor 509 is used to provide a signal indicative of acceleration pedal position which is relayed to engine management controller 501 and to transmission controller 500. Also a throttle position sensor 510 is used to provide the engine management controller 501 with a signal indicator of throttle position. A crank position sensor 523 is further used to provide the engine management controller 501 with a signal indicative of crank position.

A vehicle speed sensor 511 is used to provide the transmission controller 500 with a signal indicative of vehicle speed. A longitudinal acceleration sensor 512 is used to provide the transmission controller 500 with a signal indicative of the longitudinal acceleration of the vehicle and a lateral acceleration sensor 513 is used to provide the transmission controller 510 with a signal indicative of the lateral acceleration of the vehicle.

A brake pedal sensor 514 is used to provide the

transmission controller with a signal which indicates when brakes are applied.

A sensor 518 provides the transmission controller with a signal indicative of the speed of a non-driven vehicle wheel.

A selector lever 525 is provided to enable the driver to select neutral, drive, reverse or park. The selector lever 525 can also allow the driver to select a particular gear ratio. The selector lever generates a signal for the transmission controller 500.

The transmission controller processes all the signals it receives in a manner to be described later and then generates several electrical signals. Control signals are provided to the valves of actuators 70 and 71. Control signals are also provided to the actuators controlling clutches 24 and 25.

The transmission controller 500 provides a signal to a display 519 in the vehicle which indicates to the driver the gear ratio selected. The controller 500 also provides a signal to a warning buzzer 520 to indicate system failure. The controller can also provide information to a diagnostics computer via a diagnostics port 516.

The controller 500 provides a control signal to the starter solenoid 521 of the vehicle and also to the power supply 524 for the vehicle transmission.

The controller 500 has a memory in which a suitable mapping table is programmed. Using the mapping table, the controller chooses an appropriate gear ratio for the various input signals it receives.

When the user of the transmission unit selects neutral using the lever 525 then both of the clutches 25 and 30 (see figure 3) are disengaged by the controller 500. The controller 500 at the same time controls the actuator 70 to move shaft and gear engagement mechanism 47 to lock gear 40 to the shaft 36, the combination of gear 46 and gear 27 providing first (lowest) gear ratio of the transmission unit. The controller 500 also controls the actuator 71 to move gear engagement mechanism 48 to lock gear 43 to the shaft 36 to rotate therewith; the combination of the gears 43 and 32 provide the second gear ratio.

As drive is selected the controller 500 engages the clutch 25 so that drive is transmitted from flywheel 21 through shaft 26 via clutch 25 and then to gear 27, gear 40, shaft 35 and gear 36 to output gear 37. Once a first certain engine speed is detected by the sensor 502 , the controller will recognise from the mapping table stored in the memory that the second gear ratio is required and will select the higher ratio by gradually disengaging clutch 25 until the engine flares in speed by 30 or 40 rpm (this being detected by sensor 502) and then the controller 500 gradually engages clutch 30 whilst continuing to disengage clutch 25. When the clutch 25 is disengaged and clutch 30 engaged then the second gear ratio is selected and drive is transmitted from flywheel 21 through spindle 22, clutch 30, shaft 31, gear 32, gear 43, shaft 35 and gear 36 to output gear 37.

Initally after selecting the second gear ratio the controller may keep the first gear ratio preselected by shaft and gear engagement mechanism 47 (ie. the gear 40 will remain locked to shaft 35) . However, when the

controller 500 notes by comparing the signals generated by the engine speed sensor 502 and the vehicle longitudinal acceleration sensor 502 with the mapping table memory that the engine has reached speed for a change of preselected gear ratio and that the vehicle is still accelerating then the controller 500 will control actuator 70 to cause bobbin 50 to move to withdraw the tooth which locks gear 40 to shaft 35 and to extend a tooth to lock gear 41 to shaft 35. Then when the engine speed reaches the speed for selecting the third gear ratio, for the controller 500 will select the third gear ratio by disengaging clutch 30 and engaging clutch 25.

The same procedure occurs throughout the range of gear ratios with the vehicle accelerating and decelerating; the controller 500 selecting a gear ratio on the disengaged clutch whilst another gear ratio is driven by the engaged clutch and the controller disengaging one clutch and engaging the other to change gear.

It should be appreciated that the controller 500 in some instances may swap from one preselected gear ratio to another several times before there is a swap over between clutches. For instance if gear 32 is selected and clutch 30 is engaged (i.e. the vehicle is in second gear) and the driver keeps acceleration and decelerating then the controller may swap between preselecting gear 27 and gear 28 several times before clutch 30 is disengaged and clutch 25 engaged.

At all times the transmission controller 500 knows the gear ratio currently selected and the gear ratio currently preselected by the position of the actuators 70 and 71, sensed by the sensors 505 and 506. The sensors 507, 508 for the clutches 24 and 30 also tell the

controller 50 the state of engagement of the clutches 25 and 30.

The transmission controller 500 is connected to the sensor 515 for detecting the opening of a vehicle door so that the controller 500 can bring the transmission to full power before the driver of the vehicle starts the engine. Obviously, the engine of the vehicle cannot be started with any of the gears engaged and the transmission controller 500 is connected to the starter solenoid 521 to ensure that this cannot happen. To prevent delay for the driver of the vehicle, the transmission controller controls the power supply 524 to the transmission, so that the transmission is fully operational before the driver attempts to start the engine of the vehicle.

The controller 500 at all times displays to the driver via the display 519 the gear ratio selected.

Since the transmission controller knows the engine speed from sensor 502 and also knows the output speeds from the clutches from sensors 503 and 504 the controller can quite easily provide cruise control in co-operation with the engine management controller 501. Methods of cruise control are well known and therefore will not be described in detail in the specification. Cruise control can be selected by the driver using a selector 516 which sends a signal to transmission controller.

The transmission controller 500 can also provide traction control since it can detect wheel slip by a comparison of the non-driven wheel speed (provided by sensor 518) and the output speeds of the clutches 25 and 30 (as measured by sensors 503 and 504) . The controller 500 can provide traction control by varying the gear ratio

selected to alter the torque transmitted to the driven wheels and/or by co-operation with the engine management controller 501 to vary the torque generated by the engine.

The controller 500 can also deal with problems created by usual automatic transmission systems. With normal automatic transmission systems a driver can face difficulties when driving down an incline with his foot off the throttle. Clearly, in this situation the driver would like some engine braking, but the vehicle will in fact keep accelerating and a normal automatic transmission will keep changing up gears. For this reason, most usual automatic transmissions are provided with a manual selector by which the driver can select a lower gear ratio to provide some engine braking. The present invention can do this automatically. If the controller 500 notes from the signal generated by accelerator pedal position sensor 509 that the driver has his foot off the accelerator and if the controller notes from the vehicle longitudinal accelerometer 502 that the vehicle is still accelerating then the transmission controller 500 will select a lower gear ratio than the gear currently selected to provide engine braking. The driver can also be provided with a facility to use the selector 525 to override the normal mode of operation of the transmission controller 500 so that a specific gear ratio can be selected by the driver.

With the standard automatic transmissions the driver can face problems when driving round a corner. If a driver is driving round a corner and finds that he needs to back off the accelerator, then he can find that the standard automatic transmission suddenly changes up a gear ratio and this can cause steering problems for the driver and also grip problems for the vehicle. The transmission controller 500 of the present system will note from the

lateral accelerometer 513 when a vehicle is cornering and will hold a vehicle in gear when the driver backs off the throttle, rather than changing up the gear as would be normal with the automatic transmissions of the prior art. The controller can do this by comparing the output of the lateral acceleration with the map held in memory to determine when it is appropriate to hold a particular gear ratio.

With standard automatic gear boxes a driver can sometimes find difficulties when heading up an incline. If the driver is heading up an incline on half throttle and then comes to an obstruction, for instance a slow moving truck, then he will back off the throttle and a standard automatic transmission will change up a gear. When the driver then tries to pull around the obstruction and overtake, he finds that he is in too high a gear. The controller 500 of the new transmission unit can detect using the map stored in memory when the accelerator pedal position is more open than the vehicle performance measured by the sensors would suggest. This indicates that a vehicle is heading up hill. The controller will then hold a low gear when the driver backs off the throttle, rather than changing up a gear as is standard in the present day automatic transmissions. The driver then is able to accelerate more quickly when passing an obstruction. Nevertheless, when the driver reaches the top of the incline and starts heading downhill then the controller will note the vehicle acceleration downhill and will change up a gear for the same throttle position.

A mode switch 530 can be provided by which the driver can select a mode of vehicle operation. The controller can have several different mapping tables programmed into memory which each give different vehicle

performance. This enables a driver, for instance, to select a sports mode in which the controller will use a "sports" mapping table so that gear ratios will be changed at higher engine speeds to normal. Alternatively, he could select an "inexperienced driver" mode, for instance, by use of a key code on a key pad. The transmission controller would then alter the gear change operation of the vehicle to reduce the performance of the vehicle. Indeed, the vehicle could be provided with a "valet mode" which provides dramatically reduced vehicle performance, so that the owner of the vehicle knows that the valet parking his car will not race his car around the streets. By providing a key pad as selector 516 and using entry codes, the driver can ensure that only he can select the modes he requires by entering a number code.

The transmission unit of the present invention can also provide an effective anti-theft device, because the transmission controller 500 can cause both clutches 25 and 30 to be simultaneously engaged, with the shaft and gear engagement mechanism 47 engaging one gear to shaft 36 and the second shaft and gear engagement mechanism 48 engaging a second gear shaft 36. In this way, the vehicle transmission is locked and the vehicle cannot be moved. Again, the driver could select such an anti-theft mode by selector 516 (eg. a key pad) .

In normal operation of the transmission controller 500, the controller 500 will be programmed with a mapping table designed to obtain the best fuel consumption, minimum emissions and optimum performance. The transmission controller 500 will select a gear ratio best suited to meet requirements. The controller will perform three basic functions;

i. It will determine which gear ratio should be selected and when a change in gear ratio should be made; ii. The controller will determine which gear ratio should be preselected and when the gear ratio should be preselected. iii. The controller will control the engagement of one clutch and the disengagement of the other clutch to ensure a smooth transition from one ratio to the other.

It is possible that the controller could also be provided with means to monitor how a driver drives a vehicle, to determine whether the driver likes to drive in a "sporty" fashion or whether the driver likes to drive more sedately. This can be done by monitoring the driver's use of the accelerator pedal and also the vehicle acceleration. The transmission controller 500 can then select an appropriate mapping table to suit the driver's driving style.

Whilst a separate controller can be provided for controlling the transmission unit, the functions of the controller can be carried out by suitable programming of an existing engine management system. The engine management system would preferably control the torque generated the engine driving the transmission unit so that the torque is decreased when the controller disengages one clutch and engages the other; this aids smooth swap over and minimises clutch wear.

The transmission unit is very easy to control and requires little processor capacity. The gear changeover conditions will need only a very simple mapping table of conditions stored in memory. The processor need only

control preselection of gears (for which it has a relatively long time) or swap over between clutches; never both at the same time.

The transmisson unit can change gear with little power interrupt. Thus, for instance, when the transmission unit is used in an automobile the driver will feel no drop in power as gears are changed. This is achieved by carefully controlling the two clutches 25 and 30 so that one is engaged as the other is disengaged. The use of the clutch position sensors facilitate a smooth changeover.

Although reverse gear is not shown in figure 2 it will be provided in the transmission unit and will be driven by one of the clutches. The controller will only allow selection of reverse gear from a transmission unit neutral condition and if the vehicle is stationery.

A second embodiment of a transmission unit according to the invention is shown in figure 5. The gear ratio selection procedure is identical with the figure 2 gearbox and this will not be described. However, the gearbox layout is different.

A flywheel 100 is provided which receives drive to the gearbox from a shaft 101. A clutch arrangement 103 is provided to separate flywheel 100 from a shaft 102. An actuator (not shown) is controlled by a controller (similar or identical to controller 500) to act on the levers 104 to engage and disengage the clutch 103. When the clutch 103 is engaged the shaft 102 rotates with the flywheel 100, when the clutch 100 is disengaged the flywheel 100 and shaft 102 are free to rotate relative to one another.

A hollow shaft 105 is permanently connected to shaft 101 in the clutch arrangement 103, the shaft 103 rotating with the shaft 101. The shaft 105 provides the input to a clutch 106 which separates shaft 105 from a shaft 107, the shaft 107 being of larger diameter than and coaxial with shaft 102.

Gears 113, 114 and 115 are mounted on the shaft 107 and rotate therewith. Gears 110, 111 and 112 are mounted on the shaft 107 and rotate therewith.

A second hollow shaft 116 is journalled in the transmission unit in parallel spaced apart relationship to shafts 102 and 107. Slidable inside the shaft 116 are two bobbins 117 and 118 of a shaft and gear engagement mechanism identical to the mechanism disclosed in figure 2.

Six gears 119, 120, 121, 122, 123 and 124 are provided on the shaft 116 and are rotatable with respect to the shaft 110. The gears 119, 120 and 121 can be locked to the shaft 116 by teeth actuated by bobbin 118. The bobbins 117 and 118 are slidable in the shaft 116 and are controlled by actuators (not shown) connected to the rods 125 and 126 which are respectively connected to bobbins 117 and 118. Rod 125 is slidable within coaxial hollow rod 126.

The shaft 116 is connected to a gear 127 which engages a gear 128 which drives an output shaft 129. The gear 128 is journalled in bearings 130 and 131 and does not engage shaft 102.

The first (lowest) ratio of the transmission unit is provided by the engagement of gears 110 and 119; the

second gear ratio is provided by the engagement of gears 113 and 122; the third gear ratio is provided by the engagement of gears 111 and 120; the fourth gear is provided by the engagement of gears 114 and 123; the fifth gear ratio is provided by the engagement of gears 112 and 121; the sixth gear ratio is provided by the engagement of gears 115 and 124.

Similar to the figure 3 transmission unit the figure

5 transmission unit is controlled by a controller which changes gear by swapping from one clutch to the other, whilst preselecting a gear to be driven on engagement of the disengaged clutch.

A third embodiment of transmission unit is shown in figure 6. The method of gear selection used in the figure

6 transmission unit is the same as the previously described methods used for the figures 3 and 5 transmission units and will thus not be described in detail. The layout of the transmission unit is different and will now be described in detail.

A flywheel 200 is provided which is driven by an input shaft 201. The flywheel 200 is separated from a hollow shaft 203 by a clutch arrangement 204. Mounted on the hollow shaft 203 are two gears 205 and 206 which rotate with the hollow shaft 203.

A shaft 202 coaxial with and located inside hollow shaft 203 is engaged by the flywheel 200 and rotates therewith. A clutch mechanism 205 is provided to separate the shaft 202 from a hollow shaft 207 which is coaxial with and surrounds the two shafts 202 and 203. Two gears 208 and 209 are mounted on shaft 207 and rotate therewith.

A hollow shaft 210 is provided in parallel spaced apart relationship with shafts 202, 203 and 207. In shaft 210 there are located two shaft and gear engagement mechanisms of the type previously described with reference to figure 1, the mechanisms respectively having two bobbins 215A and 215B. Four gears 211, 212, 213, 214 are mounted on the shaft 210 which are rotatable with respect to the shaft. Teeth controlled by the bobbin 215A are extendable to individually lock the gears 211 and 212 to the shaft 210. Teeth controlled by the bobbin 215B are extendable to individually lock the gears 213 and 214 to the shaft 210. The shaft 210 is connected to an output gear 216 to drive the gear 216.

A hollow shaft 220 is provided in spaced apart parallel relationship with the shafts 202, 203, 207 and 210. In shaft 220 there is provided a shaft and gear selection engagement mechanism of the type previously described with reference to figure 1 (but with a slot profile different to figure 1) . The bobbin 221 of the shaft and gear engagement mechanism is shown in the figure 4.

Two gears 222 and 223 are provided on the shaft 220 and are rotatable with respect to the shaft 220. The gears 222 can be individually locked to the shaft 220 by teeth controlled by the bobbin 221. The shaft 220 is connected to a gear 224 to drive the gear.

The two output gears 216 and 224 both engage a final drive wheel (not shown) which provides the output from the gearbox.

The engagement of gears 205 and 211 provides the first (lowest) gear ratio; the engagement of gears 209 and

214 provides the second gear ratio; the engagement of gears 206 and 212 provides the third gear ratio; the engagement of gears 208 and 213 provides the fourth gear ratio; the engagement of gears 222 and 206 provides the fifth gear ratio; the engagement of gears 223 and 208 provides the sixth gear ratio. Since gear 222 provides the fifth gear ratio and 223 the sixth gear ratio, there is no need for the profile of the slot in bobbin 221 to provide for disengagement of the gear 222 when the gear 223 is engaged; fifth gear will always be the next gear required when sixth gear is selected.

As with the previously described gearboxes the selection of consecutive gears is achieved by disengaging one clutch and engaging the other and gear is selected to be driven by a disengaged clutch whilst the clutch is disengaged.

The figure 4 is compact in axial length and thus can be used when packaging constraints do not allow use of the figure 2 or the figure 3 gearbox.

The actuators used for the gearboxes could be linear electrical motors or hydraulic or mechanical or any other suitable device.