GB2517816B

GB2517816B - A method for limiting the amount of energy dissipated in a friction clutch during engagement of the clutch

Info

Publication number: GB2517816B
Application number: GB1407234.2A
Authority: GB
Inventors: D Crisp Nicholas; Philemon Petridis Themi
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2013-08-30
Filing date: 2014-04-24
Publication date: 2019-06-26
Anticipated expiration: 2034-04-24
Also published as: GB201407234D0; GB2517816A; GB201315505D0; GB2517753A; DE102014216973A1; CN104417534A; RU2014135484A; DE102014216973B4; RU2675294C2; CN104417534B

Description

A Method for Limiting the Amount of Energy Dissipated in a Friction Clutch during Engagement of the Clutch

This invention relates to motor road vehicles and in particular to a method for limiting the amount of energy dissipated in a friction clutch drivingly connecting an engine to a transmission during a period of time when the clutch is being engaged.

It is known that whenever a friction clutch is disengaged a speed differential can subsist between the engine and an input to the transmission that is eliminated when the clutch is fully engaged. The synchronising of the engine and transmission input speeds while a gear is engaged in the transmission will generate energy which is dissipated as heat in the clutch.

This is a particular problem if the driver is demanding a high level of torque from the engine during the engagement phase of the clutch. The high level of torque will normally result in 'engine flare' when the clutch is partially engaged due to the difficulty is accurately synchronising throttle pedal and clutch pedal movements. 'Engine flare' is when the speed of the engine rises rapidly due the presence of a high level of output torque and no significant load to resist acceleration of the engine as is the case when the clutch is disengaged or partially engaged.

Heat generation within the friction clutch is becoming an increasing problem due to the fact that clutch torque capacities are being reduced to meet modern packaging demands .

Heat generation is a particular problem during a power-downshift because the engine speed has to rise during the shift in order to allow it to be synchronised with the input to the transmission at the end of the downshift. If the rate of change of engine speed is too rapid towards the end of the clutch engagement then driveline shuffle and acceleration disturbance will be felt by the driver.

It is desirable, particularly in the case of a power-downshift, if the engine speed is slightly higher than the transmission speed at the end of the clutch engagement because this gives the impression of urgency to the driver and improves acceleration.

It is an object of the invention to provide a method of limiting the amount of energy dissipated in a friction clutch during engagement of the friction clutch.

According to a first aspect of the invention there is provided a method for limiting the amount of energy dissipation in a friction clutch of a motor road vehicle drivingly coupling an engine to a transmission during engagement of the clutch while the transmission is in gear, the method comprising producing a target engine speed and controlling the engine based on the target engine speed wherein the target engine speed is a target clutch slip speed based upon a combination of a current input speed to the transmission and a transition speed based upon the engagement state of the clutch.

The transition speed may vary as a function of clutch engagement state between a maximum value when the clutch engagement state is disengaged and a minimum value when the clutch engagement state is fully engaged.

The engine may be controlled to match the current engine speed to the target engine speed.

The method may further comprise producing an engine launch speed for use in launching the motor road vehicle from rest and the target engine speed may be the maximum of the target engine launch speed and the target clutch slip speed.

The target engine launch speed may be the lowest engine speed predicted to produce a vehicle launch with low energy dissipation in the clutch.

Alternatively, the target engine launch speed may be one of a range of engine speeds predicted to produce a vehicle launch with low energy dissipation in the clutch.

The engagement state of the clutch may be determined based upon the position of a clutch pedal.

According to a second aspect of the invention there is provided a system for limiting the energy dissipation in a in a friction clutch of a motor road vehicle drivingly coupling an engine to a transmission during engagement of the clutch while the transmission is in gear wherein the system comprises an electronic controller for controlling the engine and a clutch slip controller for producing a target engine speed for use in controlling the engine based on the target engine speed wherein the target engine speed is a target clutch slip speed based upon a combination of a current input speed to the transmission and a transition speed based upon the engagement state of the clutch.

The electronic controller may control the speed of the engine to match the target clutch slip speed.

The transmission may have an input driven by the clutch, the clutch may be operated by a clutch pedal, a clutch pedal position sensor may be used to determine the engagement state of the clutch and the target clutch slip speed may be based upon a combination of a current speed of the input to the transmission and a transition speed based upon the position of the clutch pedal.

The transition speed may vary between a maximum value when the clutch pedal is fully depressed and a minimum value when the clutch pedal is fully released.

The system may further comprise a launch controller to produce a target engine launch speed for launching the vehicle from rest and the target engine speed is the maximum of the target engine launch speed and the target clutch slip speed and the engine is controlled by the electronic controller based upon the target engine speed.

The target engine launch speed produced by the launch controller may be the lowest engine speed predicted to produce a successful vehicle launch with low energy dissipation in the clutch.

Alternatively, the target engine launch speed produced by the launch controller may be one of a range of engine speeds predicted to produce a successful vehicle launch with low energy dissipation in the clutch.

According to a third aspect of the invention there is provided a motor road vehicle having a system constructed in accordance with said second aspect of the invention.

The invention will now be described by way of example with reference to the accompanying drawing of which:-

Fig.la is a schematic diagram of a motor road vehicle according to a third aspect of the invention having a system according to a second aspect of the invention;

Fig.lb is a schematic diagram of a torque controller forming part of the system shown in Fig.la;

Fig.2a is a chart showing the change in clutch engagement state during a power-downshift

Fig.2b is a chart showing for the same time line as Fig.2a the gear state during the power-downshift;

Fig.2c is a chart showing for the same time line as Fig.2a unregulated engine speed, transmission input speed and target engine speed for the power-downchange;

Fig.3a is a chart showing the gear state during a vehicle launch;

Fig.3b is a chart showing for the same time line as Fig.3a the change in clutch engagement state during the launch;

Fig.3c is a chart showing for the same time line as Fig.3a unregulated engine speed, transmission input speed, target clutch slip speed and target engine speed for the launch;

Fig.4 is a chart showing for the same time line as

Fig.2a unregulated engine speed, transmission input speed and target engine speed for a power upchange;

Fig.5 is schematic representation of various clutch pedal positions and the resulting clutch engagement states;

Fig.6 is a high level flow chart of a first method for limiting the amount of energy dissipated in a friction clutch according to a first aspect of the invention;

Fig.7 is a high level flow chart of a second method for limiting the amount of energy dissipated in a friction clutch according to a first aspect of the invention; and

Fig.8 is a method for combining the first and second methods shown in Figs. 6 and 7.

With reference to Fig.l there is shown a motor road vehicle 5 having four road wheels 6 and an engine 10 driving a manual transmission 12 via a friction clutch 13. The clutch 13 is operated as is well known in the art by a clutch pedal (not shown) via an actuation mechanism (not shown) of any known type. An input to the clutch 13 rotates at a speed NE equivalent to the rotational speed of the engine 10 and an output from the clutch 13 rotates at a speed tp equivalent to the rotational speed of an input shaft of the manual transmission 12. When the clutch 12 is fully engaged there will be substantially no slip across the clutch 13 and so the input and output speeds of the clutch are the same and the engine speed is equal to the input speed to the transmission 12 (NE = ifi) .

The transmission 12 drives in this case the front wheels 6 of the motor vehicle 5 via a driveline 16 however it will be appreciated that the invention is equally applicable to all wheel drive and rear wheel drive motor vehicles .

An electronic controller 20 is provided to control the operation of the engine 10 in response to a number of inputs 14, 15, 17, 18, 19. A first input is an engine speed sensor 14 which provides a signal to the electronic controller 20 indicative of engine speed (NE) . A second input is a selected gear sensor (SGS) 15 which provides an input to the electronic controller 20 indicative of at least a currently engaged gear and in some cases also an indication of a yet to be engaged gear. A third input is a clutch pedal position sensor 17 which provides an input indicative of current clutch pedal position (CP) . Clutch pedal position is used in the case of this example to infer the engagement state of the clutch 13 however it will be appreciated that other methods to infer clutch engagement state could be used such as for example a release bearing displacement sensor or system pressure sensor in the case of a hydraulically actuated clutch 13.

The preferred method is the use of clutch pedal position sensor 17 because it is cost effective and because such sensors are commonly already present for other control functions . A fourth input is a road speed sensor 18 which in this case is a common sensor used by an antilock brake system but could be any type of sensor used to sense the rotational speed of the driveline downstream from the transmission 12. A fifth input is an accelerator pedal position sensor 19 used to provide an input of requested torque TD from a driver of the motor vehicle 5.

In the case of this example the rotational speed NT of the input shaft to the transmission 12 is inferred based upon selected gear ratio and road speed but in other embodiments a separate rotational speed sensor could be provided.

During normal use the electronic controller 20 will control the engine 10 in response to a torque demand from the driver as communicated by the accelerator pedal sensor 19. In this case the engine 10 is a diesel engine and so if more torque is required the amount of fuel supplied by a fuel injection system 11 and the timing of the injection of the fuel to the engine are varied to meet the requested demand. In the case of a spark ignited engine various methods can be employed to vary the torque output from the engine as is well known in the art.

The electronic controller 20 includes a clutch slip controller (CSC) 25 the function of which is to limit the amount of energy dissipated in the clutch 13 during an engagement phase of the clutch 13.

The electronic controller 20 is operable to modulate the torque demand to the engine 10 during engagement of the clutch 13 so that engine flare is reduced thereby limiting the amount of energy that will be dissipated in the clutch 13. In one embodiment the electronic controller 20 includes a torque controller shown diagrammatically in Fig.lb the effect of which is to clip the torque demand TD from the driver as derived from the accelerator position sensor 19 if the demand torque TD produces a higher than desirable engine speed NE. The target for the engine speed is set by the CSC 25 and the torque demand clipping function could also be incorporated as part of the CSC 25.

In the torque clipper shown in Fig.lb the current level of torque demand TECur is reduced by δ if the current engine speed is greater than the engine speed target NT set by the CSC 25. If the current engine speed is not greater than the engine speed target NT set by the CSC 25 then the driver demanded torque TD is used. The value of δ can be a fixed or variable value. In the case of a variable value it could be based on the difference between current engine speed and target engine speed.

Operation of the CSC 25 is as follows, when the signal from the clutch pedal sensor 17 indicates that the clutch 13 is disengaged and the signal from the SGS 15 indicates that a gear is selected the CSC 25 is operable to set an engine speed target NT for the engine 10 for the current clutch pedal position CP.

Fig.5 shows in a schematic form various clutch engagement states as they relate to clutch pedal position CP.

In a first zone of clutch pedal positions the clutch pedal 23 is termed released (R). In the released zone the state of the clutch 13 is always engaged.

In a second zone of clutch pedal positions the clutch pedal is termed pressed (P). In the pressed zone the state of the clutch 13 changes from an engaged state to a disengaged state. The 'bite point' of the clutch 13 always occurs in the pressed zone. It is in the pressed zone that the majority of heat generating slip will occur due to the partially engaged state of the clutch 13.

In a third zone of clutch pedal positions the clutch pedal is termed depressed (D). In the depressed zone the driver has moved the clutch pedal a large amount from its normal resting position and in the depressed zone the clutch 13 is always disengaged. In the depressed zone no heat will be generated in the clutch 13 because it is disengaged.

In one example the relevant clutch pedal percentage limit for the released zone was 0 to 20% of total clutch pedal travel, the relevant clutch pedal percentage limits for the pressed zone were 20% to 85% of total clutch pedal travel and the relevant clutch pedal percentage limit for the depressed zone was 85% to 100% of total clutch pedal travel. The bite point occurred at a clutch pedal position of 75%. It will be appreciated that between the bite point and the start of the depressed zone clutch slip is occurring but the torque transferred is insufficient to move the motor vehicle 5.

The zones "R", "P" and "D" are set as part of a calibration process for the clutch position sensing system and the values given are only examples of possible calibrated values.

Therefore when the clutch pedal position CP is indicated from the clutch pedal sensor 17 to be depressed and the SGS 15 indicates that a gear is engaged it can be inferred that heat will be dissipated when the clutch 13 is subsequently engaged and so the CSC 25 is active to limit the amount of energy dissipated in the clutch 13.

The CSC 25 determines a target engine speed NT for the engine 10 during the engagement process. This is done by using the current road speed and the selected gear ratio to produce a predicted value for the input speed th of the input shaft of the transmission 12. The predicted input speed tp is then used in combination with a transition speed NLSl based upon current clutch pedal position CP to produce a value for a target clutch slip speed NTSL. The transition speed NLSl varies based upon the clutch pedal position CP.

Therefore, target clutch slip speed NTSl = (th + NLSl)

It will be appreciated that the relationship between clutch pedal position CP and transitional speed NLSl can be of any desired relationship.

The value for NLSl can be stored as a relationship between clutch pedal position CP and transitional speed NLSl in the form of a look-up table or could be repetitively calculated using an algorithm.

If the system only has a CSC 25 then the value of target clutch slip speed NTSl is used as the target engine speed NT .

Once a value for NT has been produced by the CSC 25 the electronic controller 20 uses this value to control the torque demand TE to the engine 10 so as to drive the engine speed NE towards the target engine speed NT. In a situation where the torque demand TD from the driver is producing an engine speed NE lower than the target engine speed NT this will be directly used to control the engine 10. However, if the current driver torque demand TD is producing an engine speed higher that the target engine speed NT, the driver torque demand TD is modified or clipped so as to allow the engine speed to converge with the target engine speed NT.

It will appreciated that the engine speed NE is not necessarily equal to the target engine speed NT because it may not be possible for the engine 10 to slow quickly enough to follow the changes in target engine speed NT but nevertheless the engine speed NE will be constrained by the target engine speed NT thereby limiting the amount of energy to be dissipated in the clutch 13 by reducing the speed differential across the clutch 13.

While the CSC 25 is active, the engine 10 will be unresponsive to the excessive torque demands from the driver that would cause the engine speed NE to exceed the target engine speed NT. In this way flare of the engine speed NE during the clutch engagement is avoided and so the energy dissipated will be less than if flare was allowed to occur.

The relationship between clutch pedal position CP and engine transitional speed NLSl may vary continuously for the whole range of clutch pedal travel. However, it is preferred if a small difference between target engine speed NT and input shaft speed th such as, for example, 50 Rpm is provided even when the clutch pedal position lies in the released zone. It will be appreciated that eventually the engine speed NE will equal the input shaft speed tp and this will occur when the clutch pedal position CP is in the released zone. This is because the CSC 25 only provides a target engine speed NT and does not set an actual engine speed NE.

It will be appreciated that the target engine speed NT is not a fixed value but is cyclically being updated based upon clutch pedal position CP and the current input speed tp to the transmission 12.

The electronic controller 20 further includes in this example a launch controller 28 but in other embodiments may include only the CSC 25.

The function of the launch controller 28 is to produce a target launch speed NTL for the engine 10 designed to provide a good launch with low energy dissipation in the clutch 13. The target launch speed NTL set by the launch controller 28 is such that using an engine speed below the target launch speed NTL will likely result in a failed launch either due to poor pick-up or stalling of the engine 10.

It will be appreciated that the structure shown is exemplary in nature and that the CSC 25 and the launch controller 28 could be separate units and need not be part of a single electronic controller and that the functionality of these controllers could be produced in some other manner. It will further be appreciated that the functionality of the CSC 25 and the launch controller 28 could be provided by way of software and that they may not be physical entities.

Figs.2a to 2c show a typical power-downshift and how the slip controller CSC 25 operates to limit the energy dissipated in the clutch 13.

In Fig.2c the line "A" is that for a representative engine speed with no speed control, line "B" is the target engine speed NT set by the CSC 25 (NT = NTSl) and line tp is the input speed of the transmission 12. The actual engine speed NE will be close to but not necessarily co-incident with the line "B". It will be appreciated that Figs.2a to 2c are diagrammatic in nature and do not necessarily represent an actual power-downshift.

At time "0" the clutch pedal 23 is moved away from its resting position towards the fully depressed position and the clutch state changes from fully engaged to fully disengaged.

At time "1" a lower gear is selected while the clutch 13 is fully disengaged (in depressed zone) and the CSC 25 becomes active and, in this case, sets the target engine speed NT = Ph + 300Rpm.

Between time "1" and time "2" the clutch pedal 23 is being released from the depressed zone entering the pressed zone at time "2" and the clutch state changes from disengaged to a partially engaged state reaching the 'bite point' (BP) at time "3". In the case of the example shown, the value of target engine speed NT will remain constant during this period of time at NT + 300Rpm. In other examples the relationship between the input speed NT and the target engine speed NT will vary continuously once the clutch pedal pressed zone is entered.

At time "4" the clutch pedal 23 is still in the pressed zone but the clutch 13 is nearly fully engaged. The engine transitional speed NLSl is set by the CSC 25 from this clutch pedal position until the clutch pedal is fully released at a constant speed (50 Rpm) above the current input shaft speed Ph of the transmission 12 and so the target engine speed NT tracks the input speed Ph but is set 50 Rpm higher.

Between time "2" and time "4" the clutch pedal position Cp is in the pressed zone and the clutch 13 is being engaged.

At time "5" the clutch 13 enters the released zone and at time 6 the clutch 13 is fully engaged and the engine speed NE is synchronised with the input shaft speed th.

Note that, as time passes from time "2" to time "4", the difference between the target engine speed NT and the input shaft speed NT of the transmission 12 is gradually reduced to provide a controlled and smooth gear change.

Because the energy dissipated in the clutch 13 is related to the area enclosed by the lines "A" and "NT" for the non-slip controlled case and "B" and "NT" for the slip controlled case assuming the engine speed NE is equal to the target engine speed NT at all times, which it will not necessarily be as the actual engine speed is not controlled directly. Therefore the energy dissipated in the clutch 13 will be reduced by an amount substantially equivalent to the area above the line "B" and bounded by the lines "A" and "B" (in fact bounded by the true engine speed NE (not shown) and line A).

Therefore a significant reduction in the energy dissipated in the clutch 13 is obtained compared to the unlimited engine speed situation and this lower energy dissipation which will result in lower clutch temperatures and lower clutch wear.

Figs.3a to 3c' show a typical launch from rest and how the slip controller CSC 25 can be used to reduce or limit the energy dissipated in the clutch 13.

In Figs.3c and 3c' the line "A" is that for a representative engine speed with no slip control, line "B" is a target engine speed NT for the engine 10 during the launch, line "C" is a target engine speed NTSl produced by the clutch slip controller 25 and line NT is the transmission input speed. It will be appreciated that the Figs.3a to 3c' are diagrammatic in nature and do not necessarily represent an actual launch.

At time "0" the clutch pedal 23 is in the released zone and begins to move away from its resting position towards the fully depressed position. Because the vehicle speed is zero at this point in time, the electronic controller 20 deduces that the required functionality is that required to launch the vehicle 5 from rest.

Between time "0" and time "1" the clutch state changes from engaged to disengaged and clutch pedal 23 moves to a fully depressed position at a point time lying between time "0" and time "1".

At time "1" a starting gear such as first gear is selected while the clutch 13 is fully disengaged and operation of the clutch slip controller 25 and the launch controller 28 commence.

The CSC 25 sets a target engine speed NTSl equal to tp + 300 Rpm and because tp = 0 during this time period the value of NTSl = 300 Rpm. The launch controller 28 sets a target launch speed NTL which in this case is 1200 Rpm but in practice will be a range of speed values. The target engine speed NT for the engine 10 is set to the maximum of NTL or NTSl which in this case results in a target engine speed NT = 1200 Rpm. The engine 10 will begin to accelerate to reach this target engine speed which it achieves in this case at time "3" which corresponds in this case to the time the bite point of the clutch 13 is reached.

At time "2" the clutch pedal 23 enters the pressed zone and the clutch 23 is partially engaged but a bite point (BP) has not been reached and the values for NT, NTL and NTSl remain the same from time "1" up to time "3".

At time "3" the clutch pedal 23 has been moved from the depressed zone into the pressed zone and the clutch state changes from disengaged to a partially engaged state referred to as the 'bite point' where drive begins to take place .

Between time "3" and time "4" the value of target launch speed NTL remains the same but the value for target clutch slip speed NTSl increases with increasing transmission input speed Ph but not as quickly as the rate of increase of input speed Ph because the operation of the CSC 25 is such as to cause the target clutch slip speed PhsL to converge with the input speed Ph during this time period. The target engine speed NT is still equal to NTL because NTL > NTSl-

At time "4" the clutch 13 is virtually engaged and the target engine speed NT is still based upon the target launch speed NTL set by the launch controller 28 at a speed determined to be an optimum value for the launch.

At time "4" the clutch pedal 23 is still in the pressed zone but the clutch 13 is nearly fully engaged. The engine transitional speed PTLsl is set by the CSC 25 from this clutch pedal position until the clutch pedal is fully released at a constant speed (50 Rpm) above the current input shaft speed Ph of the transmission 12 and so the target engine speed PhsL tracks the input speed Ph but is set 50 Rpm higher.

The engine speed NE is not fully synchronised with the input shaft speed Ph and the target clutch slip speed NTSl produced by the slip controller 25 is still less than the target engine speed NT set by the launch controller 28.

Therefore, at time "4" the relationship between input speed Ph and clutch pedal position CP changes so that for clutch pedal positions CP equal to or less displaced than this a constant speed difference of 50 Rpm is set for the clutch slip speed target NTSl- It will be appreciated that this change could occur at another clutch pedal position and is not related to the fact that, in this case, the driver begins riding the clutch pedal 23 at this point in time using the same clutch pedal position.

At time "5" the clutch 13 is moved to the released zone and is fully engaged and the engine speed NE is synchronised with the input shaft speed tp at time "6".

In the region between time "4" and time "5" the target engine speed NT changes from a launch controller 28 set target NTL to a CSC 25 set target speed NTSl because the target speed NTSl derived from the CSC 25 exceeds the target speed NTL derived from the launch controller 28 at some point during this time period.

The operation of the launch controller 28 is to try and maintain the engine speed NE between upper and lower limits chosen to provide an optimum launch with minimum excessive energy production.

If the CSC 25 were to be used alone for a vehicle launch then the engine 10 would likely stall or would pick up very slowly because the target speed NTSl based on engine transitional speed NLSl and tp would be lower than the engine speed NE required to successfully launch the motor vehicle 5.

However, if a launch controller 28 is used alone then this would inhibit engine speed NE at the end of the launch when Ne = tp thereby limiting the acceleration of the motor vehicle 5.

The effect of combining the two controllers 25, 28 can best be understood with reference to Fig.3c' which is an enlargement of the area "X" shown on Fig.3c.

The control of the engine target speed NT is based upon using the higher value of the engine speed NTL derived from the launch controller 28 and the engine target speed NTSl derived from the slip controller 25.

Therefore to the left of point "P" on Fig.3c' the higher of the two speed limits NTSl< Ntl is NTL and so the engine speed target speed NT is set at this level.

The engine speed launch limit NTL can either be a predefined fixed value for the motor vehicle 5 or can be determined based upon the current situation of the motor vehicle 5 such as, for example, its weight and whether it is on an up-slope a down-slope or level ground.

To the right of point "P" the value of engine speed limit NTSl produced by the CSC 25 is greater than the launch speed limit NTL from the launch controller 28 and so this is used as the target engine speed NT instead of the target launch speed NTL.

It will be appreciated that if only the launch speed limit Ntl is used then the engine 10 cannot accelerate above speed NTL if the launch controller 28 is active and it remains active while there is a positive speed difference between the engine speed NE and the input speed tp to the transmission 12.

Therefore, if there is slip occurring due to the driver riding the clutch pedal 23 the engine speed NE will never be equal to the input speed tp and there will be energy dissipated in the clutch 23 until the driver releases the clutch pedal 23 fully. However, by using the CSC 25 it is possible to continue with the acceleration while maintaining only a small amount of slip thereby providing a smooth end to the launch phase with good acceleration without excessive energy dissipation in the clutch 23. Therefore in this case the driver riding the clutch does not limit the acceleration of the vehicle 5 to the same extent as it does when the target launch speed NTL is used.

As before the transition speed NLSl produced by the CSC 25 varies with clutch pedal position CP .

As before a significant reduction in the energy dissipated in the clutch 13 is obtained which will result in lower clutch temperatures and lower clutch wear.

Fig.4 shows a representative chart for a power-upshift. The clutch position and gear shift charts have been omitted but would be similar to those shown in Figs.2a and 2b except that, in the case of Fig.2b, the gear change is an upshift not a downshift.

In Fig.4 the line "A" is that for a representative engine speed with no speed control, line "B" is a target engine speed NT for the engine 10 during the change and line Ph is the transmission input speed. It will be appreciated that Fig.4 is diagrammatic in nature and does not necessarily represent an actual gear change.

At time "0" the clutch pedal 23 is moved away from its resting position towards the fully depressed position and the clutch state changes from engaged to disengaged.

At time "1" a higher gear is fully selected while the clutch 13 is disengaged, the input shaft speed PL drops due to the higher gear ratio and because the vehicle begins to slow.

The CSC 25 becomes active at time "1" when the gear is selected and begins to try and drive the engine speed NE to the target engine speed NT.

While the clutch pedal 23 transitions from the depressed zone to the pressed zone that is to say, between times "1" and "2", the transition engine speed limit NLSl is set to 300 Rpm and so the target engine speed NT is 300 RPM above current input shaft speed th and so tracks the input shaft speed tp as shown.

At time "3" the clutch pedal 23 is in the pressed zone and the clutch state is a partially engaged state referred to as the 'bite point' where drive begins to take place.

The transitional speed NLSl is now varied based upon clutch pedal position CP until, at time "4", the clutch 13 is virtually engaged and the transitional speed NLSl is then held by the CSC 25 at a constant value above the current input shaft speed tp of the transmission 12 until the end of the engagement.

At time "5" the clutch 13 is engaged and the clutch pedal position enters the released zone and, at time "6", the clutch pedal 23 is fully released and the engine speed NE will synchronise with the input shaft speed ip.

Between times "1" and "6" the CSC 25 is active and will reduce the engine speed Ne towards the target engine speed NT indicated by line "B".

As before, the energy dissipated in the clutch 13 is directly related to the area enclosed by the lines "A" and "Np" for the non-slip controlled case and "B" and "Np" for the slip controlled case. Actually it is the area bounded by the actual engine speed NE and Np but because the engine speed NE approximates to the target engine speed NT the area bounded by "B" and "Np" is a good approximation.

Therefore the energy dissipated in the clutch 13 is reduced by an amount equivalent to the area above the line "B" and bounded by the lines "A" and "B" which is a significant reduction in the energy dissipated in the clutch 13 and will result in lower clutch temperatures and lower clutch wear.

It will be appreciated that the transition speed NLSl can vary in any way with respect to clutch pedal position CP provided that the difference between the target clutch slip speed NTSl and the input speed tp reduces as the clutch pedal is moved from the depressed to the released position.

With particular reference to Fig.6 there is shown a first embodiment 100 of a method for the amount of energy dissipated in a friction clutch during engagement of the clutch that is particularly suitable for use in the case of a power-on gear shift.

The method starts at box 110 and then advances to box 120 where it is checked whether a gear is engaged in the transmission 12 and whether the clutch 13 is disengaged.

If the clutch pedal position CP indicates that the clutch 13 is not disengaged or there is no gear currently engaged then the method cycles around box 120. It will be appreciated that a gear must be engaged for an input speed tp of the transmission 12 to be determined unless a separate speed sensor is provided and it will be further appreciated that substantially no energy can be dissipated in the clutch 13 while the transmission 12 is in neutral.

If the conditions of box 120 are met, the method advances to box 130 where the current input speed tp of the transmission 12 is determined. It will be appreciated that this could be direct measurement using a sensor or be derived from vehicle speed using the current gear ratio selected and the effective gearing between the transmission 12 and the road.

From box 130 the method advances to box 140 where the current clutch engagement state is determined based upon clutch pedal position CP .

Then, in box 150, the value of CP is used to determine a current value of target engine speed NT . As previously discussed, the target engine speed NT is based upon a combination of the current input speed Ph and a value NLSl generated by the CSC 25 based upon the clutch pedal position CP.

Therefore target engine speed NT = NTSl = (Ph + NLSl)

That is to say, in this case because only a target clutch slip speed NTSl is generated this is used as the target engine speed NT .

Moving on from box 150 to box 160 it is checked to see whether the current engine speed NE as sensed by the engine speed sensor 14 is greater than the target engine speed NT .

If the current engine speed NE is not greater than the target engine speed NT the method advances to box 180 where it is checked whether the current engine speed NE is equal to the current input speed Ph and, if it is, the method 100 ends at box 190 but, if it is not, the method returns to box 130.

If in box 160 the current engine speed NE as sensed by the engine speed sensor 14 is determined to be greater than the target engine speed NT, the method advances to box 170.

In box 170 the electronic controller 20 controls the engine 10 towards the target engine speed NT. In most cases this will involve the driver demand for torque being attenuated or reduced so as to permit the engine 10 to slow in a passive manner but may also include active engine braking such as applying a load to the engine 10 via an attached electrical generator or compressor or by closing a throttle valve or exhaust brake.

From box 170 the method returns to box 130 and the subsequent boxes are executed again.

It will be appreciated that the target engine speed NT is not constant during an engagement but rather is cyclically updated as is the value of target clutch slip speed NTSl changes. A cycle time of approximately 10ms is possible for this cyclic updating.

With particular reference to Fig.7 there is shown a second embodiment 200 of a method of limiting the amount of energy dissipated in a friction clutch during engagement of the clutch that is particularly suitable for use in the case of a vehicle launch.

The method starts at box 210 and then advances to box 215 where it is checked whether a gear is engaged in the transmission 12 and whether the clutch 13 is disengaged.

If the clutch pedal position CP indicates that the clutch 13 is not disengaged or there is no gear currently engaged then the method cycles around box 215. It will be appreciated that a gear must be engaged for an input speed ip of the transmission 12 to be determined unless a separate speed sensor is provided and it will be further appreciated that substantially no energy can be dissipated in the clutch 13 while the transmission 12 is in neutral.

If the conditions of box 215 are met, the method advances to box 220 where a target engine launch speed NTL is set based upon either stored parameters or by direct calculation. A range of target launch speeds may be set to provide a successful launch with low energy dissipation in the clutch 13.

From box 220 the method advances to box 230 where the current input speed tp of the transmission 12 is determined. It will be appreciated that this could be direct measurement using a sensor or be derived from vehicle speed using the current gear ratio selected and the effective gearing between the transmission 12 and the road. The current clutch pedal position CP is also determined in this case in box 230 based upon the output from the clutch pedal position sensor 17 however this could be determined separately.

From box 230 the method advances to box 240 where the current clutch pedal position CP is used in combination with the current input speed tp to produce a value of target clutch slip speed NTSL. NTSL = (th + NLSL)

Then in box 250 the value of NTL from the launch controller 28 is compared with the value of NTSL obtained from the CSC 25.

If the value of NTL is greater than the value of NTSl then the value of NTL is used for the target engine speed NT as indicated in box 260 otherwise the value of NTSl is used for the target engine speed NT as indicated in box 270.

From box 260 the method advances to box 265 to check whether the current engine speed NE is substantially equal to the current input speed to the transmission 12 and, if it is, the method 200 ends in box 290. It will be appreciated that a very small difference between engine speed NE and input speed tp is possible and this test is to check whether the engine 10 and the transmission 12 are synchronised meaning that slip control is no longer required.

If in box 265 it is determined that the current engine speed NE is not equal to the current input speed NT to the transmission 12, the method returns to box 230 and the subsequent boxes are executed again.

Returning to box 270, the method advances from box 270 to box 280 to check whether the current engine speed NE is substantially equal to the current input speed Ph to the transmission 12 and, if it is, the method 200 ends in box 285.

If in box 280 it is determined that the current engine speed PTE is not equal to the current input speed Ph to the transmission 12, the method returns to box 230 and the subsequent boxes are executed again.

In the case of this example, the target launch speed NTL once set is retained for the duration of the launch.

However, in other embodiments the value for target launch speed may be cyclically updated as is the value of target clutch slip speed PhsL· A cycle time of approximately 10ms is possible for this cyclic updating.

With particular reference to Fig.8 there is shown how the methods shown in Figs.6 and 7 can be combined to provide a method usable for either a vehicle launch or a power on gear change.

The method 300 determines in box 310 whether the vehicle 5 is moving and if it is advances to box 320 which is a transfer to box 110 on Fig.6 and if it is not moving the method advances from box 310 to box 330 which is a transfer to box 210 on Fig.7.

It will be appreciated that the methods shown and described are exemplary in nature and that the invention is not limited to the exact combination of method steps or sequence shown and described.

In summary the method provides a target engine speed NT for the engine 10 that is based at least partially on the engagement state of the clutch 13.

When the clutch 13 is disengaged a large difference (NE-fh) in speed is permitted between the engine 10 and the input to the transmission 12 but, when the clutch 13 approaches full engagement, the allowable difference in speed (Ns-NU is reduced.

Preferably, a small positive difference between the target engine speed NT and the input speed tp is retained even when the clutch 13 is fully engaged to provide a positive feel to the final stages of the engagement.

By reducing the allowable difference in speed between the engine 10 and the input to the transmission 12 (Ns-NU based upon clutch engagement state, a smoother transition is provided and there is less risk of driveline shuffle.

It will be appreciated by those skilled in the art that although the invention has been described by way of example with reference to one or more embodiments it is not limited to the disclosed embodiments and that alternative embodiments could be constructed without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method for limiting the amount of energy dissipation in a friction clutch of a motor road vehicle drivingly coupling an engine to a transmission during engagement of the clutch while the transmission is in gear, the method comprising producing a target engine speed and controlling the engine based on the target engine speed wherein the target engine speed is a target clutch slip speed based upon a combination of a current input speed to the transmission and a transition speed based upon the engagement state of the clutch.

2. A method as claimed in claim 1 wherein the transition speed varies as a function of clutch engagement state between a maximum value when the clutch engagement state is disengaged and a minimum value when the clutch engagement state is fully engaged.

3. A method as claimed in claim 1 or in claim 2 wherein the engine is controlled to match the current engine speed to the target engine speed.

4. A method as claimed in any of claims 1 to 3 wherein the method further comprises producing an engine launch speed for use in launching the motor road vehicle from rest and the target engine speed is the maximum of the target engine launch speed and the target clutch slip speed.

5. A method as claimed in claim 4 wherein the target engine launch speed is the lowest engine speed predicted to produce a vehicle launch with low energy dissipation in the clutch.

6. A method as claimed in claim 4 wherein the target engine launch speed is one of a range of engine speeds predicted to produce a vehicle launch with low energy dissipation in the clutch.

7. A method as claimed in any of claims 1 to 6 wherein the engagement state of the clutch is determined based upon the position of a clutch pedal.

8. A system for limiting the energy dissipation in a in a friction clutch of a motor road vehicle drivingly coupling an engine to a transmission during engagement of the clutch while the transmission is in gear wherein the system comprises an electronic controller for controlling the engine and a clutch slip controller for producing a target engine speed for use in controlling the engine based on the target engine speed wherein the target engine speed is a target clutch slip speed based upon a combination of a current input speed to the transmission and a transition speed based upon the engagement state of the clutch.

9. A system as claimed in claim 8 wherein the electronic controller controls the speed of the engine to match the target clutch slip speed.

10. A system as claimed in claim 8 or in claim 9 wherein the transmission has an input driven by the clutch, the clutch is operated by a clutch pedal, a clutch pedal position sensor is used to determine the engagement state of the clutch and the target clutch slip speed is based upon a combination of a current speed of the input to the transmission and a transition speed based upon the position of the clutch pedal.

11. A system as claimed in claim 10 wherein the transition speed varies between a maximum value when the clutch pedal is fully depressed and a minimum value when the clutch pedal is fully released.

12. A system as claimed in claim 10 or in claim 11 wherein the system further comprises a launch controller to produce a target engine launch speed for launching the vehicle from rest and the target engine speed is the maximum of the target engine launch speed and the target clutch slip speed and the engine is controlled by the electronic controller based upon the target engine speed.

13. A system as claimed in claim 12 wherein the target engine launch speed produced by the launch controller is the lowest engine speed predicted to produce a successful vehicle launch with low energy dissipation in the clutch.

14. A system as claimed in claim 12 wherein the target engine launch speed produced by the launch controller is one of a range of engine speeds predicted to produce a successful vehicle launch with low energy dissipation in the clutch.

15. A motor road vehicle having a system as claimed in any of claims 8 to 14.