GB2508833A

GB2508833A - A method of controlling stop-start operation of an engine of a motor vehicle

Info

Publication number: GB2508833A
Application number: GB1222301.2A
Authority: GB
Inventors: Themi Philemon Petridis
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2012-12-12
Filing date: 2012-12-12
Publication date: 2014-06-18
Anticipated expiration: 2032-12-12
Also published as: GB201222301D0; DE102013225701A1; GB2508833B; CN103863299B; CN103863299A

Abstract

A method for controlling the stop-start operation of an engine 10 of a motor vehicle 5 is provided in which a lower limit Ll is changed between an upper value LlR to a lower value LiS when the operating state of the engine 10 changes from running to stopped. The lower limit Ll defines, in combination with an upper limit Lu, an operating range for a moveable clutch pedal position threshold Thl. Traversing of the moveable threshold Thl by movement by a driver of the motor vehicle 5 of a clutch pedal 25 will cause a change in operating state of the engine 10 depending upon the direction of motion of the clutch pedal 25.

Description

A Method of Controlling Stop-Start Operation of an Engine of a Motor vehicle This invention relates to motor vehicles and in particular to a method for improving the performance of an engine stop-start system a motor vehiole.

Vehicles equipped with start-stop systems and manual transmissions use thresholds based on clutch pedal position to determine when a driver has released the clutch pedal from a pressed position to a released position. This transition from a pressed condition to a released condition is used by the start-stop system to stop the engine. In addition, when the driver moves the clutch pedal from a released position past a threshold towards a pressed position this is used by the stop-start system as a trigger to re-start the engine.

Previously these thresholds have been fixed and their value set so as to accommodate drivers who habitually rest their foot on the clutch pedal (UK patent application number 1206179.2 filed 5th April 2011 by the same applicants describes in detail such a system and method) . The method proposed UK patent application number 1206179.2 has the benefit of increasing the availability of stops across the fleet of all vehicles so equipped and thereby increasing the overall fuel savings. Analysis of real world customer data indicates that up to 35% of customers rest their foot on the clutch pedal.

However, there are a further group of foot resting' drivers who fail to fully benefit because of a particular driving habit. These drivers rest their foot on the clutch pedal sometimes referred to as "riding the clutch" producing 15% of pedal travel, but as the vehicle is stopping, briefly fully release the clutch pedal and then rest their foot back on the clutch pedal again to once again ride it so as to press it back to say 15% pressed. This foot adjustment' often occurs when the driver re-positions their heel into a more comfortable position.

A consequence of allowing a clutch Released/Pressed threshold (Thi) to vary within a fixed range such as, for example, 10% to 30% to accommodate fcot resting' (as in UK patent applicaticn number 1206179.2) is that a driver who adjusts their foot in the manner described above may miss engine stop opportunities as follows.

As the driver fully releases the clutch pedal during a foot adjustment which typically oocurs as the vehicle is stopping, the Released/Pressed threshold Thl will follow' the driver's foot all the way to the most released limit of its range (say 10%) . At this point the clutch pedal state indicates Released. Then if the driver once again rests his foot on the clutch pedal (back to 15%) , the pedal position passes the Released/Pressed threshold at 10% and so the clutch pedal state changes from Released to Pressed.

If, while stopping the vehicle, the driver rests his foot back on the clutch pedal in this way before the start-stop system has shut down the engine, then the start-stop system will not shut down the engine because the clutch pedal is in the Pressed state and the start-stop system requires that it is Released before commanding an engine stop. In such a case the driver did not intend to prevent an engine stop and, as a consequence, the engine will use fuel unnecessarily.

Alternatively, if the driver rests his foot back on the clutch pedal by more than 10% after the start-stop system has already shut down the engine, then a change in pedal state from Released to Pressed will oocur causing the start-stop system to re-start the engine. This is undesirable as the driver did not intend to re-start the engine and the engine will use fuel unnecessarily.

To reduce the occurrence of such missed engine stop opportunities it is possible to reduce the range of thresholds from 10 to 30% to 20 to 30% so that the minimum limit of the range (20%) exceeds the typical foot resting position (15%) . However, while this benefits foot resting' and foot adjusting' drivers, those drivers who fully release the clutch pedal (0%) must press the pedal further (e.g. to the 20% limit) to reach the PRESSED state and trigger an engine start. They perceive this as a less eager' start and it will result in an increase in start time.

It is an object of the invention to provide a method of controlling stop-start operation of an engine of a motor vehicle that minimises the above referred to problems.

According to a first aspect of the invention there is provided a method of controlling the operation of an engine stop-start system of a motor vehicle in which a change in engine operating state is effected in response to the traversing of a clutch pedal position threshold wherein the method comprises defining upper and lower limits for the value clutch pedal position threshold and reducing the lower limit from a first value of clutch pedal position when the engine is running to a second, less pressed, value of clutch pedal position when the engine is stopped.

The first value may correspond to a clutch pedal position that is more pressed than an expected clutch pedal position due to a driver of the motor vehicle riding the clutch.

The first pressed value may be in the range of 18 to 25% pressed.

The second value may correspond to a clutch pedal position that is less pressed than an expected clutch pedal position due tc a driver of the motor vehicle riding the clutch.

The second pressed value may be in the range of 6 to 12% pressed.

The method may further ccmprise changing the lower limit from the second pressed value to the first pressed value when the state of the engine changes from stopped to running without causing a change in running state of the engine back to the stopped state.

The lower limit may be changed so that the lower limit is equal to the minimum of the first pressed value and the maximum of the previous lower limit pressed value or the current value of the clutch pedal position threshold.

The method may further comprise determining the engine running state and selecting the first or second pressed value based upon the determination of engine running state.

The method may further comprise incrementally increasing the lower limit from the second, less pressed, value of clutch pedal position to the first value of clutch pedal position when the engine state is determined to have changed from stopped to running.

Incrementally increasing the lower limit may comprise using the algorithm Rl(t) = MIN(Ll-, F4ax(Rl(t-l), Thi)) to increment the lower limit. Where Rl(t) is a transitional lower limit to be set, Rl(t-1) is the last value of transitional lower limit set, Ihi is the current value of the moveable threshold and R is a predefined value chosen as the value for the lower limit when the engine is running.

According to a second aspect of the invention there is provided an engine stop-start system for a motor vehicle comprising an electronic control unit and a clutch pedal position sensor to measure the position of a clutch pedal, the electronic control unit being operable to effect a change in engine operating state in response to a measured position of the clutch pedal changing so as to traverse a clutch pedal position threshold defined within the electronic control unit and is further operable to define upper and lower limits for the value of the clutch pedal position threshold wherein the electronic control unit is further operable to reduce the lower limit from a first pressed value of clutch pedal position when the engine is running to a second, less pressed, value of clutch pedal position when the engine is stopped.

The first pressed value may correspond to a clutch pedal position that is more pressed than an expected clutch pedal position due to a driver of the motor vehicle riding the clutch.

The second pressed value may correspond to a clutch pedal position that is less pressed than an expected clutch pedal position due to a driver of the motor vehicle riding the clutch.

The electronic control unit may be further operable to change the lower limit from the second pressed value to the first pressed value when the state of the engine changes from stopped to running without causing a change in running state of the engine back to the stopped state.

The lower limit may be changed so that the lower limit is equal to the minimum of the first pressed value and the maximum of the previous lower limit pressed value and the current value of the clutch pedal position threshold.

According to a third aspect of the invention there is provided a motor vehicle having an engine, a clutch pedal to control the engagement state of a clutch and an engine stop-start 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.l Is a schematic view of a motor vehicle aocording to a third aspect of the invention having a stop-start system according to a second aspeot of the invention; Fig.2 is a schematic diagram showing the operating range of movement of a clutch pedal showing three defined zones when the engine is running; Fig.3 is a schematic diagram similar to Fig.2 but showing the three defined zones when the engine is not running; Figs. 4A to 4D are clutch position versus time diagrams showing four stop-start scenarios; and Fig.5 is a high level flow chart showing a method for adjusting thresholds forming part of a method of controlling stop-start operation of an engine of a motor vehicle according to a first aspect of the invention.

With reference to Fig.l there is shown a motor vehicle having an engine 10 and a stop-start system including an electronic control unit 16 having an electronic clutch pedal position processor (EC3P) l6C and a clutch pedal position sensor 26 arranged to sense the position of a clutch pedal 25. It will be appreciated that the clutch pedal position sensor 26 could measure the position of the clutch pedal 25 in any convenient manner and need not be directly connected tc the clutch pedal 25 as shown. It could, for example and without limitation, alternatively monitor the position of a piston of a clutch master cylinder. In the case of the example being described the output from the clutch pedal position sensor 26 is oonverted into a percentage clutch pedal position where 0% pressed represents a fully released clutch pedal 25 and 100% pressed represents a fully pressed clutch pedal 25. However, it will be appreciated that in other cases the same techniques described herein could be applied to other representations of clutch pedal position such as, for example, clutch pedal position sensor output signal level or magnitude.

It will also be appreciated that the EC3P 160 could be a separate unit and need not be formed as part of the electronic control unit 16. Furthermore, the EC3P 160 could, as described herein, carry out all of the steps required to assess clutch pedal position and provide an output to the electronic control unit 16 indicative of whether the engine 10 is to be stopped or started or could merely convert the signal from the clutch pedal position sensor 26 into a filtered, conditioned signal for use by the electronic control unit 16 to carry out all of the steps required to assess clutch pedal position and determine whether the engine 10 is to be stopped or started.

In most respeots the electronic control unit 16 and the EC3P 160 are constructed and operate exactly as the same components described in UK patent application number 1206179.2 and so will not be described again in detail.

The primary difference between this invention and that described in UK patent application number 1206179.2 is that the boundary between a transition zone "T" and a released zone R" is set in this case based upon whether the engine is or is not running whereas in UK patent application number 1206179.2 this boundary is fixed.

Referring now to Figs. 2 and 3 there is shown in a schematic form the range of motion of the clutch pedal 25 and in particular three predefined zones used in controlling the stopping and starting of the engine 10.

Referring firstly to Fig.2 there is shown in a diagrammatic manner how the FC3P 160 processes the output signal from the clutch pedal position sensor 26 into zones of operation based upon a percentage of the fully pressed position for the case where the engine 10 is not stopped, that is to say, the engine 10 is running.

In the zone "El" the control output from the EC3P 160 will indicate that the clutch pedal 25 is released' that is to say, a clutch operated via movement of the clutch pedal will be in an engaged state.

In a zone "P", in which the bite point of the clutch is located, the control output from the EC3P 160 will indicate that the clutch pedal 25 is being pressed' In the transition zone "T", the control output from the FC3P 160 will be dependent upon the location and motion of the clutch pedal 25.

In the case shown where the engine 10 is not stopped but running, the released zone "El" is defined by a lower limit of 0% and an upper predefined fixed limit of Ll which in this case is 20%.

The transition zone "I" is defined by the lower predefined limit Ll5 and by an upper predefined fixed limit Lu which in this case is 30%. The upper predefined fixed limit Lu is chosen such that it has a positional value less than an expected bite point of the clutch operated by the clutch pedal 25 but greater than the positional value due to displacement of the clutch pedal 25 due to a driver riding the clutch pedal 25.

The pressed zone P" is defined by the upper predefined fixed limit Lu and by the maximum pressed position (1-00%) A further, moveable, tracking threshold Thl operates within the transition zone T' and is constrained by the lower limit Ll and by the upper limit Lu. That is to say, Lu »= Thi »= L1R.

Referring now to Fig.3 there is shown in a diagrammatic manner how the EC3P 160 processes the output signal from the clutch pedal position sensor 26 into zones of operation based upon a percentage of the fully depressed position for the case where the engine 10 is stopped that is to say it is not running.

As before, there are three zones "R", "P" and "T" which have the same characteristics as previously described.

However, in this case, where the engine 10 is stopped, the released zone "R" is defined by a lower limit of 0% and an upper predefined limit of Lb which is 10%.

The transition zone T" is defined by the lower predefined limit L13 and by the upper predefined limit Lu which in this case is set at 30%. The upper predefined fixed limit Lu is, as before, chosen such that it has a positional value less than an expected bite point of the clutch operated by the clutch pedal 25 but greater than the positional value due to displacement of the clutch pedal 25 due to a driver riding the clutch pedal 25.

-10 -The pressed zone "P" is defined by the upper predefined fixed limit Lu and by the maximum pressed position (100%) The moveable tracking threshold Thi operates as before within the transition zone I' and is constrained by the lower limit Ll and by the upper limit Lu. That is to say, in this case Lu »= Thl »= L15.

Operation in the transition zone is as described in UK patent application number 1206179.2 with the moveable threshold Thi tracking the clutch pedal position (CP) so as to always lie slightly to the side of the current clutoh pedal position opposite the direction the clutoh pedal 25 is moving in what is termed a transition window. Therefore, if the clutch pedal 25 is moving towards the fully released position 0%, then the moving threshold Thl will lie to the pressed side of the current clutch pedal position.

Similarly, if the clutch pedal 25 is moving towards the fully pressed position (100%) then the moving threshold Thl will lie to the released side of the current clutch pedal position.

Fig.6 and the description relating thereto in UK patent application number 1206179.2 describes such a tracking process in more detail.

Referring now to Figs.4A to 4d there are shown four exemplary scenarios of variation in clutch pedal position versus time. In all of these scenarios the clutch pedal 25 has previously been fully pressed (100%) so as to allow a transmission (not shown) of the motor vehicle 5 to be placed into a neutral state, the vehicle 5 has been braked to a halt and the clutch pedal 25 is now being released from a clutch pedal position greater than the upper limit Lu by the driver.

-11 -In Fig.4A the driver operates the clutoh pedal so as to fully release it (0%) and then after a small amount of time has elapsed presses on the clutch pedal again to move it towards the fully pressed position (100%) Because the clutch pedal history is that the clutch pedal has been pressed more than the upper limit Lu (30%) the moveably switching threshold Thi will have been dragged up to the upper limit Lu and so will be sitting at 30%.

Therefore, when the driver moves the clutch pedal to a position of slightly less than 30% pressed, as indicated by point P1, the threshold Thl will be crossed and the engine will begin to stop because traversing the moveable threshold from pressed to released constitutes a trigger for an engine stop. While the clutch pedal 25 continues to be further moved in the release direction through the transition zone "T" the moveable threshold Thi will track the current clutch position so as to lie on the pressed side of the current clutch position by in this case 3%.

However when the moveable threshold Thl reaches the lower limit Ll, as indicated by point P2, the moveable threshold Thl can no longer follow the clutch pedal position because it is constrained by the value set for the lower limit L1T< which in this case is 20%. The moveable threshold Th therefore remains held constant at 20% as indicated by the horizontal dotted line between the points P2 and P3.

At time t', corresponding to point P3, the engine speed (N) has reached zero and so the engine 10 has stopped and, because the engine 10 has stopped the lower limit is changed from the limit for engine running Ll to the limit for engine not running Ll. The lower limit for the engine not running state is, in this case, set at 10% and so, as soon as the engine 10 stops running at time t, the moveable threshold Thl will be released and moves rapidly to a new -12 -value of 10% as indicated by the transition from point P3 to point P4.

There is shown a time delay of several seconds during which the clutch pedal 25 is fully released, the engine 10 is stopped and the moveable threshold Thi remains at 10% and then, at time t', the driver begins to press the clutch pedal 25 so as to move it towards the fully pressed position. The engine 10 will not start immediately because the moveable threshold TH1 is sat at 10% and the clutch pedal position is initially less pressed than this but, as soon as the moveable threshold Thl is traversed in the released to pressed direction, which in this case is at point P5 occurring at time t2', the engine 10 is started and the lower limit would like to move from the engine stopped lower limit LII (10%) to the engine running lower limit R (20%) . However, if the lower limit is allowed to switch immediately back to the engine running lower limit L1 then the engine 10 would stop again because the current clutch pedal position would then fall on the released side of the moveable threshold Thl which would of course also be moved to 20% because the moveable threshold Thl can never be below the lower limit irrespective of which lower limit is in effect.

To overcome this problem, the moveable threshold Thl and the lower limit are arranged to trail the clutch pedal position by approximately 3% from the engine start-up until the clutch pedal position reaches 23%, the moveable threshold Thl then has a value of 20% as indicated by point P6. At point P6, the lower limit is then reset to 20% conforming to the requirements for the lower limit Ll-used for an engine running state.

As the clutch pedal 25 is pressed further in the pressed direction from point P6 the moveable threshold Thl will continue to trail the clutch pedal position by -13 -approximately 3% until the clutch pedal positicn reaches 33% and the moveable threshold has a value of 30%, as indicated by the point P7. The moveable threshold Thl is then constrained by the upper limit Lu so that it can no longer trail the clutch pedal position and will remain sat at 30%, as indicated by the horizontal line extending away from the point P7. The moveable threshold Thi will remain sat at 30% pressed until the clutch pedal 25 once more traverses the upper limit from the pressed side.

Referring now to Fig.4B the driver operates the clutch pedal 25 so as to move it towards the fully released position but only moves it to a 15% pressed position and not fully released (0%) thereby riding the clutch. Then, after a small amount of time has elapsed, the driver presses on the clutch pedal 25 to move it towards the fully pressed position (100%) Because the clutch pedal history is that the clutch pedal has been pressed more than the upper limit Lu (30%) the moveable threshold Thl will have been dragged up to the upper limit Lu and so will be sitting at 30%.

Therefore, when the driver releases the clutch pedal 25 to a position of slightly less than 30% pressed as indicated by the point P1', the threshold Thl will be crossed and the engine 10 will begin to stop. While the clutch pedal 25 continues to be released in the transition zone "T" the moveable threshold Thl tracks the current clutch position so as to lie on the pressed side of the current clutch position.

However when the lower limit Ll is reached as indicated by point P2' the moveable threshold Thl can no longer follow the clutch pedal position because it is constrained by the value set for the lower limit Ll, which in this case is 20%.

The moveable threshold Thl therefore remains sat at 20%, as -14 -indicated by the horizontal dotted line between the points P2' and P3'.

At point P3' corresponding to time tO' the engine speed (N) has reached zero and so the engine 10 has stopped and, because the engine 10 has stopped the lower limit is changed from the limit for engine running L1R to the limit for engine not running Ll,. The lower limit Ll for the engine not running state is in this case set at 10% and so as soon as the engine 10 stops running the moveable switching threshold Th will be released and moves rapidly to a new value of 3% more than the current clutch pedal position as indicated by the transition from point P3' to point P4' . The lower limit Ll is moved to 10% but the moveable threshold Thi cannot follow it all the way because to do so would make it cross the current clutch pedal position. Therefore, although the lower limit LII is set at 10%, the moveable threshold Thl is constrained to stay on the pressed side of the current clutch pedal position (15%) to prevent an immediate restart.

This is similar to the situation from points P1' to P2' where it trails the clutch pedal position and a similar technique is used.

In the example shown, there is then a time delay of several seconds due perhaps to a vehicle halt at traffic lights. During this delay the clutch pedal 25 remains at the 15% pressed position and the engine 10 remains stopped.

Then at time tl' the driver begins to press the clutch pedal 25 so as to move it towards the fully pressed position. The engine 10 will not start immediately because the moveable threshold Thl is set at 18% but, as soon as the mcveable threshold Thl is traversed, which in this case is at point P5', the engine 10 is started. The lower limit now needs to move from the engine stopped lower limit Ll?, (10%) to the engine running lower limit L1R (20%) . However, if the lower limit were to be switched back immediately to the -15 -engine running lower limit 1R then it would overtake the clutch pedal positicn and the engine 10 would stop again.

This is because the current clutch pedal position would then fall on the released side of the moveable threshold Thi which would also be moved to 20% because it can never be below the lower limit irrespective of which lower limit is in effect.

To overcome this problem the moveable threshold Thl and the lower limit trail the clutch pedal position by a small amount (in this example 3%)from the engine start-up until the clutch pedal pcsition CP reaches 23%, at which point in tine the moveable threshcld Thl then has a value of 20% indicated by point P6' . The lower threshold is incrementally adjusted using a transition algorithm discussed later.

At point P6' , the lower limit is then reset to the lower limit value for engine running (L1R = 20%) so as to conform to the requirements for an engine running state.

As the clutch pedal 25 is pressed further beyond point P6' the moveable threshold Thl will continue to trail the clutch pedal position by approximately 3% until the clutch pedal position reaches 33% when the moveable threshold Thl has a value of 30% as indicated by the point P7' . The moveable threshold Thl is then constrained by the upper limit Lu so that it can no longer trail the clutch pedal position and will remain sat at 30%, as indicated by the horizontal line extending away from the point P7' . The moveable threshold Thl will remain sat at 30:1 pressed until the clutch pedal 25 once more traverses the upper limit from the pressed side.

Referring now to Fig.4C the driver operates the clutch pedal 25 so as to move it to the fully released 0% pressed position. The driver then releases the clutch pedal 25 to -16 -move it from the fully pressed position to a position where it is 15% pressed before the engine has stopped.

Because the clutch pedal history is that the clutch pedal has been pressed more than the upper limit Lu (30%) the moveable threshold Thi will have been dragged up to the upper limit Lu and so will be sitting at 30%.

Therefore when the driver releases the clutch pedal 25 to a position of slightly less than 30% pressed, as indicated by point Fl", the threshold Thi will be crossed and the engine 10 will begin to stop. While the clutch pedal 25 continues to be released in the transition zone T" the moveable threshold Thl tracks the current clutch position so as to lie on the pressed side of the current clutch position.

However when the lower limit R is reached as indicated by point P2" the moveable threshold Thi can no longer follow the clutch pedal position CP because it is constrained by the value set for the lower limit Llp. which in this case is 20%. The moveable threshold Thl therefore remains sat at a value of 20%, as indicated by the horizontal dotted line between the points P2" and P3".

At time t2 corresponding to point P3" the engine speed (N) has reached zero and the engine 10 is now stopped.

Because the driver has moved the clutch pedal immediately from the fully released position (0% pressed) at time point tl to a 15% pressed position before the engine 10 has stopped at time t2 the effect is similar to that previously described with reference to Fig.4B.

At point P3" because the engine 10 has now stopped the lower limit is changed from the limit for engine running L1R to the limit for engine not running L13. The lower limit L13 -17 -for the engine not running state is in this oase set at 10% and so as soon as the engine 10 stops running the moveable switching threshold Thl will be released and moves rapidly to a new value of, in this case, 3% more than the current clutch pedal position as indicated by the transition from point P3" to point P4". The lower limit Ll is moved to 10% but the moveable threshold Thl cannot follow it all the way because to do so would make it cross the current clutch pedal position. Therefore, although the lower limit Ll:. is set at 10% the moveable threshold TH1 is constrained to stay on the pressed side of the current clutch pedal position to prevent an immediate restart.

In the example shown there is then a time delay of several seconds where the clutch pedal 25 remains at the 15% pressed position and the engine 10 remains stopped. The driver then begins to press the clutch pedal 25 at time t3 so as to move it towards the fully pressed position. The engine 10 will not start immediately beoause the moveable threshold Thl is set at 18% but, as soon as the moveable threshold Thl is traversed, which in this case is at time t3' corresponding to point PS", the engine 10 is started.

As previously discussed, the lower limit Ll now needs to move from the engine stopped lower limit Ll:, (10%) to the engine running lower limit Ll (20%) . However, if the lower limit Ll were to be switched back immediately to the engine running lower limit 1R then it would overtake the clutch pedal position and the engine 10 would stop again. This is because the current clutch pedal position would then fall on the released side of the mcveable threshold Thl which would also be moved to 20% because it can never be below the lower limit irrespective of which lower limit is in effect.

To overcome this problem the moveable threshold Thi and the lower limit trail the clutch pedal position by a small amount (in this example 3%) from the engine start-up until the clutch pedal position reaches 23%, at which point in -18 -tine the moveable threshold Thl then has a value of 20% indicated by point P6". The lower limit is transitioned using an algorithm as discussed hereinafter. At point P6", the lower limit is then reset to the lower limit value for engine running (L1R = 20%) so as to conform to the requirements for an engine running state.

As the clutch pedal 25 is pressed further the moveable threshold Thl will continue to trail the clutch pedal position by approximately 3% until the clutch pedal position reaches 33% when the moveable threshold Thi has a value of 30% as indicated by the point P7". The moveable threshold ThI is then constrained by the upper limit Lu so that it can no longer trail the clutch pedal position and will remain at 30%, as indicated by the horizontal line extending away from the point P7". The moveable threshold Thl will remain sat at 30% pressed until the clutch pedal 25 once more traverses the upper limit from the pressed side.

Referring now to Fig.4D, the driver operates the clutch pedal so as to move it to a fully released (0% pressed) position. The driver then presses the clutch pedal 25 at tine t2'''and moves it to a position where it is at 15% pressed but in this case after the engine has stopped at tine to.

Therefore when the driver releases the clutch pedal 25 to a position of slightly less than 30% pressed, as indicated by point El", the threshold Thl will be crossed and the engine 10 will begin to stop. While the clutch pedal 25 continues to be released in the transition zone T" the moveable threshold Thl tracks the current clutch -19 -position so as to lie on the pressed side of the current clutch position.

However, when the lower limit Ll is reached, as indicated by point P2", the moveable threshold Thl can no longer follow the olutoh pedal position because it is constrained by the value set for the lower limit Ll, which in this case, is 20%. The moveable threshold Thl therefore remains at a constant value of 20%, as indicated by the horizontal dotted line between the points P2" and P3".

At point P3" the engine speed (N) has reaohed zero as indicated by point tO and the engine 10 is now stopped.

At point P3", because the engine 10 has stopped, the lower limit is changed from the limit for engine running 1R to the limit for engine not running L15 indicated by point P4". The lower limit Ll for the engine not running state is in this case set at 10% and so as soon as the engine 10 stops running the moveable switching threshold Thl will be released and moves rapidly to a new value of 10% as indicated by the transition from point P3" to point P4".

The lower limit Ll is moved to 10% and the moveable threshold Thl follows it all the way because the clutch pedal position is at 0% pressed at this point in time.

The driver after a very short time delay then begins to press the clutch pedal 25 at time t2''' so as to move it towards a pressed position of 15% due to a foot position readjustment having taken place thereby riding the clutch.

Because the lower limit Id is now set to the engine not running or engine stopped level Ll, the engine 10 will be started when the clutch pedal position crosses the lower limit Ll3 at the point PB" because the moveable threshold ThI is also set at 10%.

-20 -Because the engine 10 is now running, the lower limit now needs to move from the engine stopped lower limit Ll[, (10%) to the engine running lower limit Ll (20%) However, if the lower limit were to be switched back immediately to the engine running lower limit fiR then it would overtake the clutch pedal position and the engine 10 would stop again. This is because the current clutch pedal position would then fall on the released side of the moveable threshold Thl which would also be moved to 20% because the moveable threshold Thi can never be below the lower limit irrespective of which lower limit (L1R or Ll) is in effect.

To overcome this problem, the moveable threshold Thl and the lower limit are set lower than the clutch pedal position by a small amount (in this example 3%) from the engine start-up until the clutch pedal position CF reaches 15% at point PlO''', at which point in time the moveable threshold Thl then has a value of 12% indicated by point P9'''.

The driver in this example then maintains the clutch pedal 25 pressed at the 15% pressed position for a short period of time as indicated by the horizontal line PlO" to PS",.

During the period from PlO" to PS''' the moveable threshold Thi remains at 12% along with the lower limit which is constrained by the fact that it cannot overtake the clutch pedal position.

At point PS" the driver begins to move the clutch pedal towards the fully pressed position and the lower limit along with the moveable threshold Thi now follows the clutch pedal position by 3% to the engine running lower limit fiR (20%) as indicated at point P6" where, the clutch pedal -21 -position is 23% beoause the moveable threshold Thi is trailing the clutch pedal position by 3%.

At point P6", the lower limit is reset to the lower limit value for engine running (L15 = 206:) so as to conform to the requirements for an engine running state.

As the clutch pedal 25 is pressed further in the pressed direction the moveable threshold Thl will continue to trail the clutch pedal position by approximately 3% until the clutch pedal position reaches 33% when the moveable threshold Thi has a value of 30% as indicated by the point P7". The moveable threshold Thl is then constrained by the upper limit Lu so that it can no longer trail the clutch pedal position and will remain sat at 30%, as indicated by the horizontal line extending away from the point P7". The moveable threshold Thl will remain sat at 30% pressed until the clutch pedal 25 once more traverses the upper limit from the pressed side.

Therefore in this case the driver is given feedback that their foot repositioning action is reducing the opportunities for stopping the engine 10 to save fuel by the fact that the engine 10 stops but then restarts as soon as they rest their foot on the clutch pedal.

The stop-start system could be arranged to detect such a situation and provide a visual warning to the driver not to ride the clutch pedal 25.

With reference to Fig.5 there is shown in outline form a method of controlling stop-start operation of the engine of the motor vehicle 5 according to the invention that is enabled as various algorithms and operating procedures in the electronic control unit 16 and/or the electronic clutch pedal position processor 160.

-22 -The method commences at step 310 where it is determined whether the engine 10 is stopped or not stopped.

If the engine 10 has stopped the method advances to step 320 where upper and lower limits (Lu and Ll) for the transition zone T" are set to 30% pressed and 10% pressed respectively.

The method then advances to step 325 where it is checked whether the engine 10 is running that is to say it is not stopped. If the engine 10 is not running, that is to say it is stopped, the method continuously cycles through step 325 with the same upper and lower limits Lu and Ll until the state of the engine 10 changes to not stopped' If the engine 10 is found in step 325 to be not stopped that is to say it is starting or has started then the method advances to step 328. In step 328 the lower limit has to be incremented from the level set for the engine not running case (Li3 = 10% pressed in this example) to the level to be set for the engine running case (fiR = 20% pressed in this

example)

It is important that the lower limit does not overtake the clutch pedal position as this would result in an immediate transition back to the engine stopped state.

Therefore to achieve this aim a transitional lower limit is set until the lower limit L1R is reached using an algorithm such as:-P1(t) = MIN(R11<, l4ax(Rl (t-1) , Thl) Where: - 111(t) = a transitional lower limit to be set; Rl(t-1) = the last value of transitional lower limit set; -23 -Thl = the current value of the moveable threshold; and Llk = a predefined value chosen as the value for the lower limit when the engine is running.

If 20% is used as the predefined value chosen as the value for the lower limit L1R when the engine is running, then:-Rl(t) = F4IN(20, Max(Rl(t-1), Thl)) That is to say, Rl(t) is equal to the minimum of 20% or the maximum value of either the previous value of Rl(t-1) from the last iteration or the current value of the moveable threshold Thl whichever is the greater.

For example if:-Thi = 14% and Rl(t-1) = 12% Rl(t) = Mm (20% and Max( 14% or 12%)) Rl (t) = Mm (20% or 14%) P1(t) = 14% As a further example, if:-Thi =22% and Rl(t-l) = 19% Then P1(t) = Min(20 and Max (22 or 19)) Therefore in this case P1(t) = 20% It will be appreciated that the values used by way of example are not limiting and that other upper and lower limits could be used depending upon the construction and characteristics of the respective clutch actuation system and the values could be signal levels rather than percentages.

It will be appreciated that the engine running lower limit L15 must be set higher that is to say more pressed than an expected clutch pedal position due to a driver of the -24 -motor vehicle riding the clutch. Therefore in the examples given lower limit L1R is set at 20% because the driver clutch riding position is assumed to be 15% pressed.

It will be appreciated that the engine not running lower limit Ll must be set lower that is to say less pressed than an expected clutch pedal position due to a driver of the motor vehicle riding the clutch. Therefore in the examples given lower limit LII is set at 10% because the driver clutch riding position is 15% pressed.

Although in the examples given the value of the lower limit for the engine running case is 20% it will be appreciated that this need not be the case and the lower limit LL for the engine running case could be in the range of 18 to 25% pressed if an upper limit Lu of 30% is used.

Similarly although the value Ll for the engine stopped case has been given as 10% this need not be the case and the lower limit value L1H could be in the range of 6 to 12% pressed.

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

-25 -Claims 1. A method of oontrolling the operation of an engine stop-start system of a motor vehiole in whioh a ohange in engine operating state is effeoted in response to the traversing of a clutch pedal position threshold wherein the method comprises defining upper and lower limits for the value clutch pedal position threshold and reducing the lower limit from a first value of clutch pedal position when the engine is running to a second, less pressed, value of clutch pedal position when the engine is stopped.
2. A method as claimed in claim 1 wherein the first value corresponds to a clutch pedal position that is more pressed than an expected clutch pedal position due to a driver of the motor vehicle riding the clutch.
3. A method as claimed in claim 1 or in claim 2 wherein the second value corresponds to a clutch pedal position that is less pressed than an expected clutch pedal position due to a driver of the motor vehicle riding the clutch.
4. A method as claimed in any of claims 1 to 3 wherein the method further comprises changing the lower limit from the second pressed value to the first pressed value when the state of the engine changes from stopped to running without causing a change in running state of the engine back to the stopped state.
5. A method as claimed in claim 4 wherein the lower limit is changed so that the lower limit is equal to the minimum of the first pressed value and the maximum of the previous lower limit pressed value or the current value of the clutch pedal position threshold.

-26 -
6. A method as olaimed in any of claims 1 to 5 wherein the method further comprises determining the engine running state and selecting the first or second pressed value based upon the determination of engine running state.
7. A method as olaimed in olaim 6 wherein the method further comprises incrementally increasing the lower limit from the second, less pressed, value of clutch pedal position to the first value of clutch pedal position when the engine state is determined to have changed from stopped to running.
8. An engine stop-start system for a motor vehicle comprising an electronic control unit and a clutch pedal position sensor to measure the position of a clutch pedal, the electronic control unit being operable to effect a change in engine operating state in response to a measured position of the clutch pedal changing so as to traverse a clutch pedal position threshold defined within the electronic control unit and is further operable to define upper and lower limits for the value of the clutch pedal position threshold wherein the electronic control unit is further operable to reduce the lower limit from a first pressed value of clutch pedal position when the engine is running to a second, less pressed, value of clutch pedal position when the engine is stopped.
9. A system as claimed in claim 8 wherein the first pressed value corresponds to a clutch pedal position that is more pressed than an expected clutch pedal position due to a driver of the motor vehicle riding the clutch.
10. A system as claimed in claim 8 or in claim 9 wherein the second pressed value corresponds to a clutch pedal position that is less pressed than an expected clutch pedal position due to a driver of the motor vehicle riding the clutch.

-27 -
11. A system as claimed in any of claims 8 to 10 wherein the electronic control unit is further operable to change the lower limit from the second pressed value to the first pressed value when the state of the engine changes from stopped to running without causing a change in running state of the engine back to the stopped state.
12. A system as claimed in claim 11 wherein the lower limit is changed so that the lower limit is egual to the minimum of the first pressed value and the maximum of the previous lower limit pressed value and the current value of the clutch pedal position threshold.
13. A motor vehicle having an engine, a clutch pedal to control the engagement state of a clutch and an engine stop-start system as claimed in any of claims 8 to 12.
14. A method of controlling the operation of an engine stop-start system of a motor vehicle substantially as described herein with reference to the accompanying drawing.
15. An engine stop-start system substantially as described herein with reference to the accompanying drawing.
16. A motor vehicle substantially as described herein with reference to Fig.1 of the accompanying drawing.