GB2522870A

GB2522870A - Traction control method

Info

Publication number: GB2522870A
Application number: GB1402046.5A
Authority: GB
Inventors: Markus Hose
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2014-02-06
Filing date: 2014-02-06
Publication date: 2015-08-12
Anticipated expiration: 2034-02-06
Also published as: GB201402046D0; GB2522870B

Abstract

Traction control method using two selectable gear ratios TP1, TP2. The method defines desired vehicle speed and at least partially simultaneously engages first and second clutches 9B, 9C with gear ratios to provide, respectively, vehicle drive speeds which are equal / higher and equal / lower than the desired speed, and modulates clutch engagement to provide an effective gear ratio suitable for the desired speed. Clutches 9B, 9C may be friction clutches, possibly regulated to vary torque to driven wheel(s) in response to changing torque demand. Gear ratios may correlate to particular engine speeds (e.g. for turbo activation / engine torque provision), and clutch engagement may be varied to maintain said speed. Clutches 9B, 9C may slip to dissipate engine power as heat, in dependence on terrain type, ambient temperature / pressure, vehicle component temperature, or traction control mode. A gearbox, transmission, powertrain, and powershift or dual clutch may be used to implement the method.

Description

TRACTION CONTROL METHOD

TECHNICAL FIELD

The present disclosure relates to a traction control method and apparatus and particularly, but not exclusively, to a method of traction control for a vehicle, a control system for implementing a traction control method, a transmission and/or powertrain equipped with such a control system, and a vehicle equipped with such a transmission/powertrain or control system.

BACKGROUND OF THE INVENTION

It is known to provide a traction control system, particularly but not exclusively for 4x4 vehicles, and particularly but not exclusively for use in such vehicles that may be required to operate off-road and/or in challenging conditions, said conditions being challenging either as a result of terrain type or of weather conditions. Such conditions may include one or more of Grass, Gravel and Snow (GGS), but may also include rocky terrain, mud and/or ruts, ice, sand and/or steep gradients.

One aspect of known traction control methods is to control the amount of wheel slip' that is allowed by any one wheel or set of wheels at any particular time. If a wheel or set of wheels is seen to be unduly speeding up, then a known method of traction control relies first on the use of engine braking' or de-rating to provide negative or reduced torque from the engine via the driveline, and additionally the supplementary use of wheel braking in order to provide a negative or retarding torque in order to maintain vehicle speed and/or traction.

There are however limitations to this methodology, particularly as regards the use of wheel braking and especially where a vehicle is traversing surfaces with a low friction coefficient and at very low speeds, where fine control would be preferable.

Wheel brake usage tends to suffer limitations, one such limitation being due to the possible resolution of speed measurement that is available. When a wheel is turning very slowly, inputs to a speed measurement device can be intermittent -or the calculation of wheel speed may be comparatively slow, due for example to a low rate of pulses coming from a pickup sensor. This problem of low-speed measurement will be readily understood by the person skilled in the art. This makes control via braking of the wheels, based on such speed calculation, difficult, and can result in judde or other undesirable characteristics with the effect that that wheel speeds may vary in an unpredictable manner -potentially varying in speed quite suddenly and fundamentally resulting in sub-optimal control of the target wheel speed.

Another issue with brake use in traction control is that the braking friction available at any particular moment via the wheel brakes is not necessarily constant. Brake discs and pads, for example, can become covered in water or mud or other substances which can at least momentarily alter the friction coefficients of the braking surfaces, and also as they are subsequently wiped clean in use. This makes the modelling of braking in a traction control module problematic and may necessitate the use of a feedback loop in order to provide sufficient braking control on a real time basis. Such feedback systems themselves suffer intrinsically from lag.

A further issue with wheel braking is that wheel brake system actuators have a limited degree of fineness in control -variance in brake on' pressure is not necessarily very sensitive and it is possible for wheels to be brought to a total halt when in fact it would be preferable or was intended to simply slow them down. Brakes that are locked on' must then be released in order to allow the wheel to move again, which has its own problems -first there is the problem of stiction' to overcome to allow release of (for example) brake pads from brake discs, which may give rise to sudden wheel movement, and secondly brake off' pressure is typically produced, as a rule, by means such as the spring effect of the piston seals or other passive' means -there is rarely, if ever, a controlled actuator for providing brake off movement, and so control of braking in a situation where the amount of braking is being reduced is even less precise. This latter problem is an issue both in a situation where a given amount of braking on a moving wheel is being reduced as well as where a brake is being released from a stationary wheel.

A wheel may be in a situation where it is being retarded due to engine braking. It may then additionally be slowed or halted by a wheel brake. A situation may arise where the engine may momentarily apply positive torque whilst the wheel is slowed or halted -the backlash is crossed and torque is applied against the action of the brake, and this will result in wind up' in the driveline as a result. Should there then be a request for positive torque from the wheel, or simply brake release, via a traction control system, the brake will be released -and the wound up torque in the driveline will then also be released, resulting in a sudden impulse of torque at the wheel. This has the potential to cause the wheel to break traction.

This may be exacerbated if the request for positive torque from the traction control system has also resulted in a request for additional torque from the engine.

Prior art systems may also be limited in that they do not tend to make positive torque demands other than those required to re-match a driver demanded torque. A traction control system will have one input which correlates to a driver torque demand, such as a position of an accelerator pedal. At a given position, an engine will be required to produce a certain torque. In a situation where a wheel is detected as slipping or overspeeding, prior art systems will reduce the torque demand on the engine (such that the engine torque demand is lower than that which is otherwise apparent from accelerator pedal position) and the engine will be de-throttled accordingly, perhaps even such that engine braking occurs. If this is insufficient, wheel braking will also be applied. When the slip or overspeed event has ended, the traction control system will make a torque demand on the engine which is once again concomitant with the accelerator pedal position. As such, prior art systems do not make increased positive torque demands on an engine other than as such to re-match an originally driver-demanded torque demand.

Another issue is that the engine is required to be responsive to changes in torque demand (albeit in a limited fashion as described elsewhere herein). It may well be required to increase or decrease torque in response to torque demands from a traction control system.

This is an issue in that engine response times are not necessarily particularly fast in comparison with the potential demands of a traction control scenario. An engine may take for example up to lOOms to respond significantly to a change in demanded torque, and in a situation where a wheel is beginning to slip or bog down', this is a comparatively long time.

This situation is significantly further aggravated where an engine may be provided with a turbo boost system, which is a more and more common situation due to engine downsizing' which is driven by modern legislative demands. Where an engine has been allowed or controlled so as to drop to a low torque level, it may also have dropped to low engine speed, say under 2,000rpm in a petrol engine or even lower in a diesel engine (these will be understood as mere examples and the skilled man will be familiar with the types of arrangements and typical rev ranges common in the art), and in such a situation the turbo may be inactive. A demand for extra torque from an engine may then require the turbo to spool up' before the requisite level of torque is produced, and this can introduce a further undesirable delay into the responsiveness of a traction control system.

The use of brakes in traction control is further undesirable in that their use is generally readily heard and felt by a driver and passengers, with concomitant reduced ride quality and a potential feeling of passenger insecurity or reduced vehicle composure, particularly where the brake use results in sudden or jolting decelerations and accelerations.

It is desirable to make improvements to traction control systems, preferably improvements addressing at least some of these problems.

SUMMARY OF THE INVENTION

Modern vehicle transmissions may include multi-speed gearboxes for example power shift' or dual clutch' gearboxes with the ability to pre-select a multiplicity of gear ratios and shift between such selected ratios.

It is known to provide, and it is envisaged that embodiments of the invention will provide for, a multi-speed constant mesh gearbox having at least one input and an output, two or more forward speed ratios and one or more reverse speed ratios between said input or inputs and output, further being provided with a plurality of clutches, at least two of said clutches preferably being friction clutches, which may be engaged in various combinations with a plurality of gears and gearbox shaft components in order to enable the various speed ratios, said gearbox being adapted for pre-selection of a next required speed ratio in use, and for changing from a current speed ratio to said next required speed ratio; wherein said speed ratios are arranged for pre-selection to provide a shift between two forward speed ratios, a shift between two reverse speed ratios, and/or a shift between forward and reverse speed ratios.

The arrangement of gear wheels, shafts and clutches within the transmission according to embodiments of the invention provides for a shift (with continuous transmission of torque to the vehicle driving wheels) between forward speed ratios, between reverse speed ratios, and between forward and reverse speed ratios. Such an arrangement ensures versatility in a transmission suitable for both on-highway and off-highway use.

Surprisingly, it has been found that a transmission as described above can provide advantages with regards to the problems of traction control as also herein described.

According to an aspect of the invention there is provided a method of traction control for a vehicle provided with: a transmission comprising at least two selectable gear ratios and; at least one wheel driven via the transmission, the method comprising: defining a desired speed of the vehicle; at least partially engaging a first transmission clutch with a selected gear ratio suitable for a vehicle speed equal to or higher than the desired speed; at least partially engaging a second transmission clutch with a selected gear ratio suitable for a vehicle speed equal to or lower than the desired speed; and modulating the engagement of the clutches so that the effective gear ratio is suitable for the desired speed.

In an embodiment there may be provided a method of traction control for a vehicle provided with: a transmission comprising at least two selectable gear ratios and; at least one wheel driven via the transmission, the method comprising: defining a desired speed of the vehicle; at least partially engaging a first transmission clutch with a selected gear ratio suitable for a vehicle speed higher than the desired speed; at least partially engaging a second transmission clutch with a selected gear ratio suitable for a vehicle speed lower than the desired speed; and modulating the engagement of the clutches so that the effective gear ratio is suitable for the desired speed.

In an embodiment there may be provided a method of traction control for a vehicle provided with: a transmission comprising at least two selectable gear ratios and; at least one wheel driven via the transmission, the method comprising: at least partially engaging a first transmission clutch with a selected gear ratio suitable for a vehicle speed equal to or higher than a given speed; at least partially engaging a second transmission clutch with a selected gear ratio suitable for a vehicle speed equal to or lower than a given speed; and modulating the engagement of the clutches so that the effective gear ratio is suitable for the given speed.

In embodiments, a given or desired speed may be a predetermined value, the present speed of the vehicle, or a target speed of the vehicle. If the speed is a predetermined value, said value may be one appropriate to: a given terrain or surface over which the vehicle may be or may be about to traverse; a traction control mode of the vehicle; ambient pressure and/or temperature; temperature of a vehicle component or fluid; a driver-selected value. Said value may be calculated during motion of the vehicle by a control module, system, electrical or computing circuit or program of the vehicle.

In an embodiment, it may be arranged so as to modulate the engagement of the clutches so as to provide varying torque to at least the at least one driven wheel. This may allow the provided torque to meet a calculated torque resistance encountered by a driven wheel or wheels.

In an embodiment, the clutch modulation is managed so that the controlled response speed of an engine supplying torque to the transmission is not required to be immediate upon a change in torque demand requested by a traction control system. Upon a requirement for a change in torque supply to the wheels, the first elements in the driveline to be subject to a variance or control input will be the clutches noted. For example, if the torque requirement at the wheels increases, the engine will, if not subject to control inputs, begin to slow. With a controller in place, this slowing will be noted and a control input made to increase fuelling or throttle or change ignition timing so as to speed the engine back up again. However, there is a finite delay before such a control system will begin to take effect and before the engine responds to the commanded changes. The intercession of the clutch control means that the actual torque delivered to the wheels during this finite delay time better meets the requirement.

Advantageously, such changes to clutch engagement are relatively quick. As the clutch engagement is varied, this will affect the speed of the engine. For example, there may be an increase in torque demand to the wheels. The first clutch may then be more closely engaged to add torque to the driveline. In effect a flywheel of the engine will then momentarily be acting as a torque reservoir' which sheds rotational inertia in order to fulfil the torque demand. The engine speed will then naturally tend to decrease. However, the engine speed may be controlled to compensate by a change to the engine throttle setting.

Advantageously, the intervention of the clutch control mitigates the fact that engine speed control (such as by throttle) is itself relatively slower. Further, it is generally recognised in the art that engine speed is relatively easier to measure than engine torque, whereas it is generally recognised that torque may be measured comparatively well in clutches. It will be understood that this may at least in part relate to the relative speeds of elements of these items. By this methodology, the measurements required at engine and clutches conveniently accord with the relative ease of making those measurements, whilst the required speed of response of these items conveniently accord with the relative speeds at which adjustments to the items can be made. In effect, torque applied, ultimately to the wheels, can be monitored and controlled at the clutches, whilst speed is monitored and controlled at the engine, where speed is most easily measured due to the relatively high rotational velocities of elements of the engine (such as the flywheel or input shaft), particularly when a high numerical gear ratio (i.e. 1st) is selected.

The opportunity, according to aspects of the invention, to fill' the time lag between a torque increase request and the ability of an engine to fulfil it by throttling up is also of particular use in the scenario where an engine may be turbocharged and may be subject to a boost response delay -being able to moderate clutches so as to provide extra torque during the time period in which an engine responds to the torque demand, including where time is taken for a turbo to spool up, offers a clear advantage.

Advantageously it will be recognised that this methodology tends to ensure that the speed of an engine may be kept relatively constant, with lesser requirement to increase or decrease revs. An engine may then for example also be kept advantageously at a speed where, for example, boost from a turbine is constantly available, and/or where available torque of the engine is known to be at a maximum or other known advantageous level. The gear ratios selected for the method of the invention may be selected advantageously in relation to a requirement to maintain a particular engine speed. This ability to maintain an engine at a more constant speed also adds to the perceived composure of a vehicle, as persons in the vehicle will not then be made aware of constantly changing engine speed. Advantageously the vehicle may be perceived to be more comfortable to travel in and also may be perceived to cope' better with demanding situations than other prior art vehicles. Further, if an engine is kept on boost' then it will be able to respond more rapidly to demands for more speed and/or torque and will also be able to respond more quickly to a given demand increment than the same engine when off boost'.

Immediate changes to torque demand may also be satisfied more quickly by the methodology of using the two clutches rather than attempting to modulate torque by engine control, and it will be recognised that this is an advantage in traction control scenarios. It should also be recognised that applying torque to an additional clutch, associated with a lower wheel torque, offers a faster response than reducing the torque of a clutch of a fixed ratio.

Further, this methodology will obviate to some degree the requirement for torque modulation by use of wheel brakes -the use of the clutches is comparable in speed of response and is much more predictable as against the disadvantages of wheel braking as noted herein. For vehicles with more than one driven wheel, the envelope of effect of the clutch control on the individual wheel slip will be dependent on how much the driveline normal degrees of freedom (i.e. differentials) can be locked or limited in speed difference. The greater this limiting the less variation in individual wheel speeds will be from the clutches and the greater the clutch control envelope will be. The wheel speed may in effect be controlled by the use of the clutches in the transmission, shifting in bias between the two pre-selected gears, rather than by control of the wheel speed by use of brakes. The further advantage is that various elements of the transmission driveline to the wheels will be rotating at higher speeds than the wheels themselves, and effective measurement of wheel speed can thus be taken by inference or calculation from a speed reading taken at such an element -the speed and resolution of such measurement can then be quantitatively superior to a speed measurement taken at the actual wheel with the limitations as discussed for prior art systems, allowing for better measurement and control of the resultant wheel speed.

Another advantage of using methods according to the invention is potentially prolonged brake life. Use of transmission components to control torque at the wheels, instead of using friction braking elements, means that where in previous scenarios brake components may have been used when covered in mud, dirt, sand, slurry, etc, their use will be lessened or obviated. This lessens the wear on braking components caused by such contaminants.

Use of the transmission via the clutches to control the wheel speed will also advantageously avoid the wind up' of transmission elements otherwise inherent in a system in which wheels are retarded or even stopped by brakes, with the disadvantages discussed herein. The likelihood of sudden impulses of torque coming down the drivetrain to the wheels, causing undesirable jerking effects and possible loss of traction, is reduced. Similarly, the problems of brake release, stiction and so on are also reduced as much lower use is made of the wheel brakes to affect wheel speed control.

A further advantage is a reduction in noise. Clutch control is relatively quiet but brake control, normally implemented by an ABS or anti-lock braking system, can generate noise.

This impacts on the composure of a vehicle and is undesirable, potentially reducing driver/occupant confidence, particularly in a hi-spec or luxury vehicle. Aspects of the invention provide for reduced incidence of driver/passenger awareness of brake use, generally smoother progress with less and fewer sudden accelerations/decelerations due to brake use, and an increased resultant ride quality and composure.

Predictability of a wheel speed control system as per an aspect of the invention, wherein control is enacted by two clutches controlling drive bias between two pre-selected gear ratios in the driveline, is also enhanced. The problems of predicting the effects of a wheel brake, wherein the momentary effects of dirt, grease, mud etc may be present, are avoided.

Control of clutches will be well understood by the person skilled in the art as being comparatively precise, and many clutches in driveline/powertrain systems comprising a gearbox as described will have springs or actuators for disengagement as well as actuators for engagement of the clutches, in contrast to wheel brakes, as discussed elsewhere herein.

In an embodiment, the clutches are deliberately run with a degree of slip' sufficient to dissipate a given amount of engine power. One method of allowing an engine to respond quickly to torque/power demands is to alter the amount of ignition advance or retardation within the engine. This allows a very quick response, as ignition can be advanced or retarded on a per cylinder per event' basis. However this method is not without its drawbacks.

For example, an engine may be deliberately run with a certain amount of ignition retardation.

In the event an increased torque is demanded, say from a stability control module, the ignition can be quickly advanced so that the engine runs more efficiently. The drawback of this is that whilst the engine is running with retarded ignition, it is less efficient, and this has a negative effect on emissions, which may in many cases require systems elsewhere in the vehicle to deal with exhaust gases in order to fulfil legislative emissions criteria. It may also result in an increased fuel usage.

Where, in accordance with aspects of the current invention, two clutches are simultaneously engaged, and, as noted in the present embodiment, at least one of these is run with a certain amount of slip, an alternative approach is possible. An engine can be run more consistently in a more efficient mode with ideal ignition timing so as to generate a given amount of power/torque -some of this power/torque can be dissipated by the clutches. By way of example, 1, 2, 5 or 10% of the maximum engine power could be dissipated in this manner. This may equate to a dissipation of, for example, 5kW on a continuous basis.

Other figures may be achievable depending on characteristics of the vehicle (such as cooling capacity) or the environment (such as ambient temperature or pressure). It may also be possible to achieve larger dissipation figures if it can be foreseen that the requirement for the power dissipation will only be for a relatively short period of time -by way of example, for up to 30 seconds or up to 60 seconds until such time as the vehicle is predictably going to be, for instance, returning to relatively normal speed travel, ie: 50mph, on a level road surface. Such a prediction may be made by a driver or other occupant, or a system within a vehicle which takes as an input information from a GPS system, or some other mapping or route navigation system, prediction system, or sensory package which gives an indication of the upcoming terrain and/or driving situation the vehicle is likely to encounter within such a timeframe.

In the event then that there is a requirement for a change in torque from the powertrain to the wheels, one or both of the clutches can be modulated in order to fulfil it by changing the amount of engagement or slip and thus the amount of power/torque so dissipated. This can be done more quickly than a throttle change to the engine itself would allow, and also without a requirement to alter the engine ignition timing, with its concomitant disadvantages.

It will be appreciated that the amount of engagement of each clutch can be controlled so as to dissipate a given determined or predetermined amount of power. This may be useful where it is calculated that a particular amount of power, for example 2kW, is deemed to be the necessary reserve' power which is desirable in a given driving environment (for example, dependent on the roughness of a surface on which the vehicle may be travelling).

Such a calculation may be made on the basis of empirical data, which may be held in a memory store of an electronic system, module, circuit or computer program of a vehicle, or may be made dynamically as a vehicle is traversing a particular surface -again, such dynamic calculation may be enabled by an electronic system, module, circuit, algorithm or computer program of the vehicle.

An amount of power dissipated by the method may be dependent on a selected vehicle mode, selectable either by a driver or automatically. Such modes, by way of example, may typically comprise a mud and ruts' mode, a grass/gravel/snow' mode, a road' mode, an ice/snow' mode, a hill climb/descent' mode, andlor a rocky terrain' mode, and certain other characteristics or responses of a vehicle may alter in dependence on which of these modes are selected.

In embodiments, by way of example, the clutch associated with the higher ratio gear may be effectively fully or substantially engaged and driving the vehicle, and the clutch associated with the lower ratio gear may then be engaged to the degree necessary to dissipate a determined amount of power, which is then releasable on demand in accordance with aspects and other embodiments of the invention. It will be appreciated that other ranges of clutch engagement will be possible (for example, equal engagement of both clutches), dependent on the characteristics of the vehicle and the requirement of the vehicle as it traverses various surface types. It will be appreciated that in general, the higher ratio clutch will tend to add positive (driving) torque as it is more engaged, whilst the lower ratio clutch will tend to add negative (retardant or overrun') torque as it becomes more engaged.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a schematic representation of a vehicle 1, equipped with an engine 3 and a transmission 4, driving at least one wheel 6 via further powertrain elements 5, traversing a terrain 2 upon which are obstacles stones 7 and muddy water 8; Figure 2 shows a diagram illustrating in schematic a OCT gearbox having: end of crankshaft 9 with a twin clutch mechanism 9A having clutches 9B and 90 on driven input shafts having, respectively, pinion gears 9R2, 901, 903, 905 and 9R1, 902, 904, and 906; layshafts lOB and 100 carrying, respectively, gears R2, Fl, F3, F5 and Ri, F2, F4 and F6; idler reverse gears 91, and output shaft 10. The skilled man will recognise the operation of such a gearbox and in particular that various gear ratios may be selected and/or preselected in such a gearbox by action of the various shaft elements, clutches 98 and 90 and further clutches, normally dog clutches (not shown), to provide any of a variety of routes for torque transmission through the gearbox.

Figure 3 shows a diagram illustrating the gearbox of Figure 2 in which components 98, 901, Fl and lOB provide a first torque path (TP1) from crankshaft 9 to output shaft 10 (effectively consistent with first' gear), whilst components 90, 902, F2 and 100 provide a second torque path (TP2) (consistent with second' gear).

In normal use in such a gearbox, clutches 9B and 90 would be actuated in such a manner that torque transfer from crankshaft input 9 to output 10 would occur such that torque would be transmitted either via the first torque path or via the second torque path, or a transfer from the first path to the second torque path would occur by synchronised clutch 9B de-actuation and clutch 9C actuation (or vice versa to transfer the other way) within as short a period of time as possible, for example in the order 500 milliseconds or less. By contrast, aspects and embodiments of the present invention require both clutches to be at least partially engaged simultaneously for extended periods (for example, greater than 500 milliseconds or greater than 1 second) so that torque is transmitted via both torque paths for extended periods.

DETAILED DESCRIPTION

A method in accordance with an embodiment of the present invention is described herein with reference to the accompanying Figures 1 to 3.

With reference to Figure 1, the vehicle 1 is traversing the terrain 2 in direction of arrow 11.

Said suiface 2 may be generally, for example, covered in a layer of mud or ice (not shown) which means that the grip available for the wheels including wheel 6 is limited. Other obstacles such as rocks/stones 7 and/or mud or muddy water puddles 8 may also be present.

As the vehicle traverses the surface 2 in the direction of arrow 11, it is preferable for the comfort and convenience of the driver that a steady speed be maintained. In order to maintain a steady speed, a varying amount of torque from the powertrain (comprising the elements engine 3, gearbox 4, further elements 5 (comprising for example a propshaft, driveshafts, a differential) and the driven wheel 6 will be required as the driven wheel 6 is subject to varying amounts of resistance as it traverses the surface 2. For example in Figure 1, wheel 6 is shown completing a descent over stone 7, during which descent reduced or even negative torque may have been required to prevent an acceleration and maintain steady vehicle speed. Conversely, wheel 6 is about to ascend local slope 12, during which ascent a greater torque will be required. In the prior art, the reduced or negative torque would typically be enabled by engine braking, potentially requiring reduced fuelling of the engine, or by wheel braking. Either of these approaches has disadvantages, as discussed elsewhere herein. The required greater torque for the ascent of slope 12 would be enabled by an increased opening of the throttle and/or increased fuelling of the engine, again, with the concomitant delay disadvantages discussed. Alternatively the engine may have been running with retarded ignition and ignition timing may be advanced in older to fulfil the greater torque demand -again, with disadvantages as discussed.

In accordance with an embodiment, gearbox 4 is controlled by a controller (not shown) so as to simultaneously engage the first gear' and second gear' torque paths as shown in Figure 3-from crankshaft 9 to output 10 via components 98, 901, Fl and 108 for the first torque path and via components 90, 902, F2, 100 for the second torque path. The first and second gears may be arranged with components such that when they are fully connected via their respective clutches in normal use, an engine speed of 2000rpm would equate to a vehicle speed of 5mph and 15mph respectively. In an embodiment, the desired speed of the vehicle may be 10mph. First and Second gears of the gearbox are simultaneously engaged, in accordance with an embodiment, via the aforementioned first and second torque paths, with both clutches 98 and 9C partially engaged. The engine is kept running at a steady 2000rpm. The resultant speed of wheel 6, with clutches 93 and 90 both partially engaged, is such as to drive the vehicle at the desired 10mph on a level surface. In the situation where wheel 6 was descending stone 7, modulation of speed and torque of the wheel 6 was achieved by the controller decreasing the engagement of clutch 9C and increasing the engagement of clutch 93, resulting in an effective greater engagement of first gear relative to second gear. This results in increased engine braking and a minimisation of wheel brake use. As wheel 6 begins to ascend slope 12, the relative engagement of the clutches is modulated by the controller in the converse, so that there is an effective greater engagement of second gear relative to first gear. Momentarily, an extra torque requirement is fulfilled by a reduction in speed of the engine flywheel, which gives time for an allied request for extra torque from the engine to take effect. This results in an effective modulation of the speed and torque delivery of wheel 6, by the controller, by means of control of the clutches and torque paths within the gearbox. Conveniently, demands for engine torque changes are made less frequently due to the modulation properties of aspects of the invention, which will tend to smooth out' relatively frequent but minor perturbations, as will be understood.

Further, it will be understood that the 2000rpm of the engine may represent an engine speed at which a maximum torque may be available, or at which a turbo boosting mechanism might be fully active and spooled up' so that in the event there is a demand for increased engine power, there is minimal delay in engine response caused by turbo lag.

It will be appreciated that this will be particularly efficacious in the circumstance, for example, that slope 12 is particularly steep and requires a swift and large increase in torque from the engine in order to power wheel 6 up the slope.

It will also be seen that at the point in time at which wheel 6 passes through muddy puddle 8, it can be expected that wheel brakes (not shown) associated with wheel 6 may become covered with muddy water. The efficiency of said brakes may then become impaired, and/or the exact brake pressure required to produce a particular braking torque may then be difficult to predict. In accordance with aspects of the invention when such method aspects are in use, the likelihood of a requirement to use the wheel brakes will in fact be lessened, and so this contributes to the predictability and controllability of the traction control mode of the vehicle.

It will of course be appreciated that the embodiment as described in relation to the Figures is purely by way of example. Speeds noted in relation to a gearbox first' and second' gears are purely illustrative. Other gears may be selected as required. It will further be appreciated that whilst aspects of the invention are described in embodiments where a gearbox is specified, it would be possible to enable methods of the invention in any transmission system where at least two torque paths between a power source (nominally an engine) and at least one driven wheel may be simultaneously engaged. It will be appreciated that nominally the clutches will be friction clutches, and may be dry' or wet'.

In other embodiments the gearbox provided may have an epicyclic gear arrangement, or any other variant may be provided whereby more than one clutch is used to control power flow through different paths and speed ratios.

In an embodiment, the engine may be running at a power level x and power may be transmitted via two torque paths in the transmission as described. The total amount of torque z transmitted to the driven wheels however may be less by a specific amount y (as well as normal driveline losses). The amount of power y not transmitted to the wheels is deliberately lost' in the transmission clutches as heat generated by slipping of the clutches.

In embodiments, specific means are provided (with which the skilled person will be familiar, such as cooling systems) for dissipating, using, or otherwise managing that heat. The amount y represents power which is available to the traction control system that can be quickly taken up by closer engagement of one of the clutches in a torque path -this power is more readily available to fulfil a request from a traction control system than a request for additional power from the engine. As y tends to zero, a request for additional power can be sent to the engine prior to y reaching zero. Accordingly as z approaches x, x increases so that y then tends to remain at a positive value. In essence the slower response time of the engine to requests for additional power can be buffered' by use of the powertrain power reserve y so that the response time to put extra power to the wheels is at the quicker clutch engagement rate. At least an amount of power y is always available for swift addition to the wheels via the transmission, at a rate quicker than making a request direct to the engine.

The engine may then always be allowed to run at a power level at which, for example, a turbo is always engaged, with advantages of quicker engine response time as elsewhere discussed. In effect the powertrain is able to act as a readily and quickly accessible power reserve for the traction control system.

It will be recognised that a controller or control system suitable for implementing any method according to any aspects of the invention may take any one of a number of possible forms, such as a vehicle or gearbox or transmission ECU or a specific traction control' module or any other suitable module, electronic circuit, computer chip, computer programme or other similar system or device within a gearbox, transmission, powertrain, or vehicle.

It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. Further aspects of the present invention will now be set out in the accompanying numbered paragraphs: 1. A method of traction control for a vehicle provided with: a transmission comprising at least two selectable gear ratios and; at least one wheel driven via the transmission, the method comprising the steps: defining a desired speed of the vehicle; at least partially engaging a first transmission clutch with a selected gear ratio suitable for a vehicle speed equal to or higher than the desired speed; at least partially engaging a second transmission clutch with a selected gear ratio suitable for a vehicle speed equal to or lower than the desired speed; modulating the engagement of the clutches so that the effective gear ratio is suitable for the desired speed.

2. A method as described in paragraph 1 wherein the clutches are friction clutches and comprising the step of modulating the engagement of the clutches so as to provide varying torque to the at least one driven wheel.

3. A method as described in paragraph 2 wherein the clutch modulation occurs to vary torque to the at least one driven wheel during the period of time which an engine of the vehicle requires to respond to a change of torque demand.

4. A method as described in paragraph 1 comprising the step of selecting the gear ratios in relation to the desired speed so as to correlate to a particular desired engine speed.

5. A method as described in paragraph 4 wherein clutch engagement is modulated so as to maintain said particular engine speed.

6. A method as described in paragraph 4 wherein said engine speed equates to: An engine speed at which a turbo unit is active, or; An engine speed at which a particular torque is available from the engine.

7. A method as described in paragraph 1 wherein at least one of the transmission clutches is allowed to slip such that an amount of engine power is dissipated as heat.

8. A method as described in paragraph 7 wherein the amount of engine power dissipated is predetermined.

9. A method as described in paragraph 7 wherein the amount of engine power dissipated is calculated in dependence upon one or more of the following: The terrain type over which the vehicle is travelling; The surface over which the vehicle is travelling; Ambient temperature; Ambient pressure; The temperature of a component or fluid of the vehicle; A selected traction control mode of the vehicle.

10. A method as described in paragraph 7 wherein the amount of engine power dissipated as heat is modulated by engagement of at least one of the clutches.

11. A method as described in paragraph 10 wherein the engine power dissipated as heat is modulated to vary the amount of engine power transmitted to the at least one driven wheel 12. A controller for implementing the method of paragraph 1 13. A gearbox or transmission comprising a controller as described in paragraph 12.

14. A powertrain suitable for a vehicle, comprising a controller as described in paragraph 12.

15. A vehicle comprising a controller as described in paragraph 12.

16. A method as described in paragraph 1 in which a powershift or optionally a dual clutch gearbox is provided.

17. A gearbox, transmission, powerirain or vehicle comprising a controller as described in paragraph 12, comprising a powershift or optionally dual clutch gearbox.

Claims

CLAIMS: 1. A method of traction control for a vehicle provided with: a transmission comprising at least two selectable gear ratios; and at least one wheel driven via the transmission, the method comprising the steps: defining a desired speed of the vehicle; at least partially engaging a first transmission clutch with a selected gear ratio suitable for a vehicle speed equal to or higher than the desired speed; at least partially engaging a second transmission clutch with a selected gear ratio suitable for a vehicle speed equal to or lower than the desired speed; modulating the engagement of the clutches so that the effective gear ratio is suitable for the desired speed.
2. A method as claimed in claim 1 wherein the clutches are friction clutches and comprising the step of modulating the engagement of the clutches so as to provide varying torque to the at least one driven wheel.
3. A method as claimed in claim 2 wherein the clutch modulation occurs to vary torque to the at least one driven wheel during the period of time which an engine of the vehicle requires to respond to a change of torque demand.
4. A method as claimed in any preceding claim comprising the step of selecting the gear ratios in relation to the desired speed so as to correlate to a particular desired engine speed.
5. A method as claimed in claim 4 wherein clutch engagement is modulated so as to maintain said particular engine speed.
6. A method as claimed in claim 4 or 5 wherein said engine speed equates to: an engine speed at which a turbo unit is active; and/or an engine speed at which a particular torque is available from the engine.
7. A method as claimed in any preceding claim wherein at least one of the transmission clutches is allowed to slip such that an amount of engine power is dissipated as heat.
8. A method as claimed in claim 7 wherein the amount of engine power dissipated is predetermined.
9. A method as claimed in claim 7 orB wherein the amount of engine power dissipated is calculated in dependence upon one or more of the following: the terrain type over which the vehicle is travelling; the surface over which the vehicle is travelling; ambient temperature; ambient pressure; the temperature of a component or fluid of the vehicle; a selected traction control mode of the vehicle.
10. A method as claimed in any of claims 7 to 9 wherein the amount of engine power dissipated as heat is modulated by engagement of at least one of the clutches.
11. A method as claimed in claim 10 wherein the engine power dissipated as heat is modulated to vary the amount of engine power transmitted to the at least one driven wheel
12. A controller for implementing the method of any of claims ito 11.
13. A gearbox or transmission comprising a controller as claimed in claim 12.
14. A powertrain suitable for a vehicle, comprising a controller as claimed in claim 12 or a gearbox or transmission as claimed in claim 13.
15. A vehicle comprising a powertrain as claimed in claim 14, a gearbox or transmission as claimed in claim 13 or a controller as claimed in claim 12.
16. A method as claimed in any of claims ito ii in which a powershift or optionally a dual clutch gearbox is provided.
17. A gearbox, transmission, powertrain or vehicle as claimed in claims 13 to 15 respectively comprising a powershift or optionally dual clutch gearbox.