SE1650977A1 - Method and system for controlling change of gear ratio in a vehicle - Google Patents

Abstract

The present invention relates to a method for controlling change of gear ratio in a gearbox (103) of a vehicle (100), said gearbox (103) being arranged for transmitting power from a power source (101) to at least one drive wheel (113, 114) of said vehicle (100), said gearbox (103) comprising a plurality of selectable gear ratios, change of gear ratio being controlled by a vehicle control system, the vehicle further including a driver manoeuvrable means for requesting power from said power source. The method comprises the steps of:- detecting a reduced grip of at least one drive wheel of said vehicle,- determining whether driver manoeuvring of said driver manoeuvrable means for requesting power indicates a reduced grip, and- influencing a gear ratio changing operation in said gearbox (103) if driver manoeuvring of said driver manoeuvrable means for requesting power indicates a reduced grip.

Description

lO METHOD AND SYSTEM FOR CONTROLLING CHANGE OF GEAR RATIO IN AVEHICLE Field of the invention The present invention relates to vehicles, and in particularto a method and system for controlling a change of gear ratioin a vehicle. The present invention also relates to a vehicle,as well as a computer program and a computer program product that implement the method according to the invention.Background of the invention There exist various kinds of vehicle powertrains. For example,vehicle transmissions can be of an automatic kind, where avehicle control system completely controls gear ratio changingoperations. The gearboxes being used in these systems can, forexample, include conventional automatic transmissionscomprising torque converters, but may also include automated manual transmissions.

Automatic gear shifting in commercial (heavy) vehicles, inparticular long-haulage vehicles, often includes a controlsystem controlled gearshift of manual gearboxes, i.e.automatic control of gearboxes comprising a discrete number offixed gear ratios (i.e. gears). During operation of vehiclesof this kind, the gearbox is often controlled by the vehicle control system in a manner in which the gear ratio is selected in dependence of the current driving conditions.

In general, it is often desirable to propel the vehicle usingas high a gear as possible (i.e. the lowest possible gearratio given the circumstances) in order to reduce therotational speed of the power source, oftentimes a combustionengine, thereby reducing fuel consumption. Consequently, whenselecting gear ratio, fuel consumption is often a matter of concern. However, the vehicle should, in general, also be lO driven using a gear ratio that is capable of propelling thevehicle in a desired manner. This is in general the case, butwhen the vehicle is accelerating or decelerating, it is oftenrequired that the gearbox of the vehicle is set to a gearratio that is suitable, or capable, of providing enough powerto propel the vehicle without undesired retardation, e.g. whengoing uphill. Oftentimes it is also desirable that change ofgear is not performed in situations where a change of gear may have a negative effect on the propulsion of the vehicle.Summary of the invention It is an object of the present invention to provide a methodand system that improves control of changes of gear ratio, inparticular in situations when a change of gear ratio mayadversely affect the propulsion of the vehicle, e.g. byreducing the ability of a vehicle to overcome an uphillsection of road. This object is achieved by a method according to claim l.

According to the present invention, there is provided a methodfor controlling change of gear ratio in a gearbox of avehicle, said gearbox being arranged for transmitting powerfrom a power source to at least one drive wheel of saidvehicle, said gearbox comprising a plurality of selectablegear ratios, change of gear ratio being controlled by avehicle control system, the vehicle further including a drivermanoeuvrable means for requesting power from said power source, the method comprising steps of: - detecting a reduced grip of at least one drive wheel of said vehicle, - determining whether driver manoeuvring of said drivermanoeuvrable means for requesting power indicates a reduced grip, and lO - influencing a gear ratio changing operation in said gearboxif driver manoeuvring of said driver manoeuvrable means for requesting power indicates a reduced grip.

With regard to vehicles in general, gearboxes of the kind thathistorically has been used in vehicles having a manualtransmission, but where a change of gear is performedautomatically by means of the vehicle control system, arepresently often used. This applies, e.g., both to heavyvehicles and passenger cars. Such gearboxes are usuallyarranged to comprise a discrete number of distinct gear ratios(gears), where the gear ratios of gearboxes in heavy vehiclesoften are distributed such that one or more of the lowest gearratios (highest gears) are capable of propelling the vehicleat a constant speed when the vehicle is travelling on substantially level ground.

When vehicles of this kind, at least when being heavilyloaded, are travelling uphill, a change of gear to a highergear ratio (lower gear) is often required in order to provideenough power to properly overtake the uphill section of roadin a desired manner. In particular, when travelling uphill,there is often a minimum gear ratio below which the vehiclewill lose speed due to lack of power for propelling thevehicle. This imposes requirements on the manner in which gears are selected when a change of gear is to be performed.

If a change of gear is not performed at least to the highestgear (the lowest gear ratio) where this criterion isfulfilled, the vehicle will continue to decelerate. Thereexists strategies for performing such selection of gear toavoid situations in which the vehicle loses speed to a toohigh extent, and change of gear is often performed when one ormore criteria are fulfilled. For example, a change of gear to a lower gear (higher gear ratio) can be arranged to be lO performed when it is determined that the rotational speed ofthe combustion engine following the change of gear will notexceed some predetermined calibrated speed limit that is not to be exceeded.

There are, however, situations which may complicate a changeof gear. The present invention relates in particular tosituations where it is detected that at least one drive wheelof the vehicle suffers from a reduced grip, i.e. frictionbetween at least one drive wheel (tyre) and the surface uponwhich the vehicle is travelling may not be sufficient totransfer the required force to propel the vehicle. The reducedgrip may e.g. be due to slippery conditions, such as due tosnow, ice and/or loose gravel. Detection of reduced grip canbe performed, for example, by detecting speed of rotation ofthe vehicle drive wheels, and when one or more wheels startspinning or rotating at speeds exceeding speeds of non-drivenwheels of the vehicle, or other drive wheels, by more than apredetermined extent it can be determined that grip is reduced and one or more wheels are slipping.

A change of gear in such situations, in particular when thevehicle is going uphill, may be detrimental to the capabilityof the vehicle to actually climb the uphill section of road.For example, if a change of gear is performed when one or moredrive wheels are spinning, or when the drive wheels of thevehicle barely exhibit sufficient grip, the interruption inpropelling force on the drive wheels during a change of gear,and the associated increase in propelling force following thechange of gear, may often render the grip being lostaltogether. This may have the result that the vehicle losesspeed, thereby possibly requiring further changes of gear,which in the end may result in the vehicle standing still in an uphill section of slippery road. lO For this reason, a change of gear may be suspended when wheelslip is detected and one or more wheels are spinning to reducethe risk of situations of this kind arising. On the otherhand, once wheel slip no longer is detected, changes of gearare again controlled according to the general criteria thatprevailed prior to the detection of wheel spin. It has beenrealized, however, that the mere fact that the vehicle drivewheels no longer are spinning may not be sufficient toguarantee that slippery conditions no longer prevail, andhence situations of the above kind may still arise. Also, ifchange of gear is suspended as soon as a wheel slip isdetected, changes of gear may be suspended in situations wherethe reduction in grip is highly temporary and the suspended change of gear in reality is not motivated.

According to the present invention, gear ratio changingoperations may be influenced in situations where a reducedgrip of at least one drive wheel is detected, but only in dependence of the actually prevailing conditions.

According to the invention, when a reduced grip is detected,where this detection may comprise a step of detecting wheelslip of at least one drive wheel of said vehicle, this aloneis not sufficient to suspend or otherwise influence a changeof gear. Instead, according to the invention, it is alsodetermined whether driver manoeuvring of said drivermanoeuvrable means for requesting power also indicates areduced grip. That is, driver behaviour is taken into accountto determine whether the detected wheel slip can be consideredtemporary, and no further action needs be taken, or whether itis likely that slippery conditions still prevail. Hence, whendriver manoeuvring of the driver manoeuvrable means forrequesting power also indicates a reduced grip, gear ratio changing operations are influenced. lO According to the invention, gear changing operations areinfluenced only when driver manoeuvring of said drivermanoeuvrable means for requesting power has indicated areduced grip. That is, it can be determined from the manner inwhich the driver is manoeuvring the driver manoeuvrable meansfor requesting power whether it can be presumed that drivingconditions actually are slippery, and that change of gearshould be influenced accordingly. For example, if wheel slipis detected and the driver in connection to this, such asconcurrently or thereafter, reduces the request for apropelling power, e.g. by at least to some extent releasing anaccelerator pedal, this can be used as a confirmation thatconditions are still slippery and that a change of gear, ifpossible, should be avoided. At least, a change of gear to ahigher gear (lower gear ratio) should be avoided in situationsof this kind, since e.g. a possible loss in vehicle speed dueto the slippery conditions may substantially immediatelyrequire a further change of gear to a lower gear, the gearchanging operation only resulting in the vehicle ability of climbing the uphill section of road being reduced.

With regard to driver manoeuvring of the means for requestingpower, the step of determining whether driver manoeuvring ofsaid driver manoeuvrable means for requesting power indicatesa reduced grip may not only comprise the step of detecting a request for reduction of power from said power source.

Instead, it can also be determined e.g. a manner in which request for power is following some pattern, such as e.g. thatthe driver first reduces the request for power from the powersource, e.g. rapidly, to be followed by a comparatively slowerincrease in the request for power, which can be arranged to beinterpreted as driver manoeuvring of the means for requesting power in an attempt to search a highest possible tractive lO force. According to embodiments of the invention, it isdetermined whether following a detection of a decrease inrequested power from said power source, it is detected anincrease in requested power from said power source to arequest for less power than requested before said decrease inrequested power, in which case gear ratio changing operations are influenced according to the invention.

According to the invention, gear ratio changing operations maybe arranged to be influenced during at least a predeterminedperiod of time from when a reduced grip of said at least onedrive wheel no longer is detected, or from when drivermanoeuvring of the driver manoeuvrable means indicatesslippery conditions. That is, change of gear can be arrangedto be influenced at least for some suitable period of time, tothereby avoid situations of the vehicle again losing grip ifchange of gear is allowed too soon following a detection of reduced grip.

The step of influencing gear ratio changing operations canalso be arranged to be terminated when driver manoeuvring ofsaid means for requesting power no longer indicates a reducedgrip, such as e.g. if the driver requests a high power fromthe power source, which may be taken as an indication that the driver believes the conditions to no longer be slippery.

Furthermore, according to embodiments of the invention, it isnot required that wheel slip is detected while simultaneouslydriver manoeuvring of the means for requesting power of thepower source also indicates slippery conditions. Instead, itcan be determined whether the driver manoeuvring of the meansfor requesting power indicates slippery conditions within apredetermined period of time before and/or after the beginning and/or end of wheel spin, and if this is the case change of lO gear can be arranged to be influenced according to the invention once such indication is detected.

Furthermore, the driver manoeuvrable means for requestingpower from the power source may e.g. comprise an acceleratorpedal or other driver manoeuvrable means such as a lever orknob for regulating the power delivered by the vehicle powersource. The power source, in turn, may comprise an internalcombustion engine, one or more electrical machines or a combination thereof.

Change of gear can be influenced in various ways. For example,change of gear ratio can be delayed or suspended. Also, changeof gear ratio to a lower gear ratio can be prohibited, while,according to embodiments of the invention, change of gear ratio to a higher gear ratio may still be allowed.

Change of gear can be delayed, for example, by changing thedependency of the rotational speed of the power source whenchanging gear, so that propulsion on a current gear can bemaintained for longer periods of time by allowing the internalcombustion engine to operate at higher speeds of rotationprior to changing gear to a higher gear (lower gear ratio), orby allowing the internal combustion engine to reach lowerspeeds of rotation prior to changing gear to a lower gear(higher gear ratio) than the limitations prevailing whencommencing influence according to the invention. For example,the vehicle may be forced to maintain propulsion on a currentgear for as long as the rotational speed of the power sourcedoes not fall below a minimum speed limit. Change of gear mayalso be inhibited altogether for as long as continued propulsion on a current gear is possible.

According to embodiments of the invention it is also required that the driver takes at least one further action. For lO example, in addition, it may be required that the driverchanges load on the drive shaft, e.g. in order to increase thepressure of the drive shaft in an attempt to improve grip. Inaddition, or alternatively, it may be required that the drivere.g. locks a differential to thereby indicate that slippery conditions in fact are prevailing.

Further characteristics of embodiments of the presentinvention and advantages thereof are indicated in the detaileddescription of exemplary embodiments set out below and the attached drawings.Brief description of the drawingsFig. lA illustrates a powertrain of an exemplary vehicle; Fig. lB illustrates an example of a control unit in a vehicle control system; Fig. 2 illustrates an exemplary method according to an embodiment of the present invention; Fig. 3 illustrates an exemplary manner of influencing change of gear ratio.Detailed description of exemplary embodiments Fig. lA schematically depicts a powertrain of an exemplaryvehicle lOO. The powertrain comprises a power source, in thepresent example a combustion engine lOl, which, in aconventional manner, is connected via an output shaft of thecombustion engine lOl, normally via a flywheel lO2, to agearbox lO3 via a clutch lO6. An output shaft lO7 from thegearbox lO3 propels drive wheels ll3, ll4 via a final drivelO8, such as a common differential, and drive axles lO4, lO5 connected to said final drive lO8.

The combustion engine lOl is controlled by the vehicle control system via a control unit ll5. The clutch lO6 and gearbox lO3 are also controlled by the vehicle control system by means of a control unit 116.

The gearbox 103 can be set to a plurality of distinct(discrete) gear ratios, where a suitable gear ratio isselected for each driving situation by means of the vehicle control system.

In general, a change of gear is performed when the combustionengine 101 speed of rotation reaches a speed where conditionsfor a change of gear are fulfilled. When changing gear to alower gear (higher gear ratio) a condition for performing achange of gear may be that the speed of the combustion enginefollowing a completed change of gear is not allowed to exceedsome speed limit which often is a calibrated maximum enginespeed that is not to be exceeded. This maximum calibratedengine speed can be different for different gears and/orvehicle speeds. As a consequence, a change of gear to a lowergear is, in general, performed as soon as it is determinedthat the speed of the combustion engine on the new gear willnot exceed said speed limit. Alternatively, or in addition,the speed limits may be set for speed of rotation on a current gear.

As was discussed above, there exist situations where change ofgear solely according to criteria of this kind may have anegative impact on the propulsion of the vehicle, andembodiments of the invention provide a method for controllingchange of gear ratio in a manner that e.g. may facilitate avehicle to climb an uphill section of road when the gripbetween the vehicle drive wheels and the surface upon which the vehicle is travelling is reduced.

An exemplary method 200 according to embodiments of the present invention is shown in fig. 2, which method can be lO ll implemented at least partly e.g. in the control unit ll6 forcontrolling the clutch lO6 and gearbox lO3. As indicatedabove, the functions of a vehicle are, in general, controlledby a number of control units, and control systems in vehiclesof the disclosed kind generally comprise a communication bussystem including one or more communication buses forconnecting a number of electronic control units (ECUs), orcontrollers, to various components on board the vehicle. Sucha control system may comprise a large number of control units,and the control of a specific function may be divided between two or more of them.

For the sake of simplicity, Fig. lA depicts only control unitsll5-ll6, but vehicles lOO of the illustrated kind are oftenprovided with significantly more control units, as one skilledin the art will appreciate. Control units ll5-ll6 are arrangedto communicate with one another and various components viasaid communication bus system and other wiring, partly indicated by interconnecting lines in fig. lA.

Embodiments of the present invention can be implemented in anysuitable control unit in the vehicle lOO and hence notnecessarily in the control unit ll5. The control of thegearbox lO3 according to embodiments of the present inventionwill usually depend on signals being received from othercontrol units and/or vehicle components, and it is generallythe case that control units of the disclosed type are normallyadapted to receive sensor signals from various parts of thevehicle lOO. The control unit ll6 will, for example, receivesignals from the engine control unit ll5, where such signalsmay include signals regarding engine operation as well asdriver requests for power, and hence driver manoeuvring ofe.g. an accelerator pedal. The control unit ll6 will also deliver signals to the control unit ll5, e.g. to control 12 engine speed and delivered power during a change of gear. Thecontrol unit 116 will also receive signals, e.g. from one ormore other control units, relating to a detected wheel slip.Control units of the illustrated type are also usually adaptedto deliver control signals to various parts and components of the vehicle, e.g. to control the clutch 106 and gearbox 103.

Control of this kind is often accomplished by programmedinstructions. The programmed instructions typically include acomputer program which, when executed in a computer or controlunit, causes the computer/control unit to exercise the desiredcontrol, such as method steps according to embodiments of thepresent invention. The computer program usually constitutes apart of a computer program product, where said computerprogram product comprises a suitable storage medium 121 (seeFig. TB) with the computer program 126 stored on said storagemedium 121. The computer program can be stored in a non-volatile manner on said storage medium. The digital storagemedium 121 can, for example, include any of the groupcomprising: ROM (Read-Only Memory), PROM (Programmable Read-Only Memory), EPROM (Erasable PROM), Flash memory, EEPROM(Electrically Erasable PROM), a hard disk unit etc., and bearranged in or in connection with the control unit, whereuponthe computer program is executed by the control unit. Thebehaviour of the vehicle in a specific situation can thus be adapted by modifying the instructions of the computer program.

An exemplary control unit (the control unit 116) is shownschematically in Fig. 1B, wherein the control unit cancomprise a processing unit 120, which can include, forexample, any suitable type of processor or microcomputer, suchas a circuit for digital signal processing (Digital SignalProcessor, DSP) or a circuit with a predetermined specific function (Application Specific Integrated Circuit, ASIC). The 13 processing unit 120 is connected to a memory unit 121, whichprovides the processing unit 120, with e.g. the stored programcode 126 and/or the stored data that the processing unit 120requires to be able to perform calculations. The processingunit 120 is also arranged so as to store partial or final results of calculations in the memory unit 121.

Furthermore, the control unit 112 is equipped with devices122, 123, 124, 125 for receiving and transmitting input andoutput signals, respectively. These input and output signalscan comprise waveforms, pulses or other attributes that thedevices 122, 125 for receiving input signals can detect asinformation for processing by the processing unit 120. Thedevices 123, 124 for transmitting output signals are arrangedso as to convert calculation results from the processing unit120 into output signals for transfer to other parts of thevehicle control system and/or the component (s) for which thesignals are intended. Each and every one of the connections tothe devices for receiving and transmitting respective inputand output signals can include one or more of a cable; a databus, such as a CAN bus (Controller Area Network bus), a MOSTbus (Media Oriented Systems Transport) or any other bus configuration, or of a wireless connection.

Returning to the exemplary method 200 illustrated in fig. 2,the method starts in step 201, where it is determined whethera reduced grip with regard to one or more vehicle drive wheelsis detected, e.g. by detecting a wheel slip, which can bedetermined in any suitable manner, e.g. by detecting a suddenincrease in wheel speed of one or more drive wheels and/or corresponding increase in internal combustion engine speed.

The method according to the invention can be arranged to becarried out at all times when a reduced grip is detected, or e.g. only when the vehicle is travelling an uphill section of 14 road, where further criteria such as road inclination and/orcurrent vehicle weight may influence whether change of gear isto be influenced according to the invention or not. Forexample, a deceleration of the vehicle may be determined,where the method according to the invention may be arranged tobe carried out only when vehicle deceleration exceeds somedeceleration. When vehicle deceleration is low, the impact ofa change of gear is less detrimental to the propulsion of thevehicle than when vehicle deceleration is high, such as may bethe case when a heavily loaded vehicle travels an uphill section of road.

Hence, according to embodiments of the invention, the methodmay include the further step of determining a currentdeceleration of the vehicle 100, where vehicle decelerationcan be determined by means of any suitable means as is knownper se. According to such embodiments, when it is determinedthat the vehicle deceleration exceeds a deceleration limit,the method may be arranged to continue according to the following while otherwise the method may be ended.

When it is determined that the vehicle 100 is subject to areduced grip, such as by being subject to wheel slip, themethod continues to step 202, where the grip of the vehicle isfurther evaluated by determining whether driver manoeuvringof, in this example, the accelerator pedal also indicates areduced grip. This can be determined in various ways, e.g. bydetermining whether the driver, in connection with thedetection of e.g. wheel slip, reduces the request for powerfrom the internal combustion engine. In general, when a drivernotices that conditions of reduced grip prevail, a request forpower from the internal combustion engine is reduced in anattempt to regain grip. Conversely, it is highly unlikely that the driver maintains a request for a high power when wheels start spinning unless the driver realises that the reducedgrip is highly temporary, and that the grip immediately willrecover. Hence, according to embodiments of the invention, areduction in request for power from the internal combustionengine, e.g. detected by detecting a released acceleratorpedal, can be used as indication of the grip actually being reduced and still prevailing.

According to embodiments of the invention, it is not onlydetermined whether there is a reduction in the request forpower from the internal combustion engine, but it is alsodetermined whether the reduction is followed by an increase inthe request for power, where it can be determined whether theincrease at most is an increase e.g. to some percentage of therequest prevailing prior to the detection of reduced grip.Such behaviour can be taken as an indication that the driveris seeking grip and trying to maximise as much as possible theforce transmitted to the vehicle drive wheels. According toembodiments of the inventions, it can be determined whetherthe increase is followed by a further decrease in the requestfor power in order to determine that conditions are such that grip is still reduced and change of gear should be influenced.

When it is determined in step 202 that propulsion of thevehicle is subject to a reduced grip, the method continues tostep 203, where change of gear is influenced. Otherwise themethod can be arranged to return to step 201. According toembodiments of the invention, gear ratio changing operationsare influenced when driver manoeuvring of the acceleratorpedal indicates a reduced grip within a first period of timefrom said detecting a reduced grip of said at least one drivewheel. That is, it is not required that driver manoeuvring ofthe accelerator occurs simultaneously with the detection of wheel slip, but the driver manoeuvring of the accelerator may 16 take place within some period of time from the detection ofwheel slip. For example, some suitable number of seconds maybe utilised as time limit. In this way, the driver is giventime to react when one or more wheels start slipping, and foras long as the driver actually reacts according to the abovein a stipulated period of time change of gear is influenced according to the invention.

Hence, in step 203, control of change of gear, i.e. gearchanging operations, are influenced as a result of the dualindication of reduced grip of the vehicle. Gear changingoperations can be arranged to be influenced in various ways.According to embodiments of the invention, any changes of gearratio are delayed or suspended. According to embodiments ofthe invention, changes of gear ratio to lower gear ratios canbe prohibited while changes of gear ratio to higher gearratios may still be allowed. When it is determined, step 204,that slippery conditions no longer prevail, e.g. indicated bydriver manoeuvring of the accelerator pedal or no furtherindication of slippery conditions within a predeterminedperiod of time, influence according to the invention isdiscontinued and normal control of change of gear is resumed, step 205, and the method is ended in step 206.

As was mentioned above, according to embodiments of theinvention, when influencing gear ratio changing operations,this may include changing a criterion regarding when a changeof gear is to be performed. This criterion can be related towhen a change of gear is performed in relation to the speed ofrotation of the internal combustion engine, where a change ofgear can be influenced such that propulsion on a current gearratio is allowed at higher and/or lower speeds of rotation ofsaid power source in comparison to when a reduced grip is not prevailing. This may be necessary e.g. in situations where the 17 vehicle is decelerating due to insufficient power forpropulsion of the vehicle, or otherwise the vehicle speed ischanging so that normal gear changing criteria would otherwise trigger a change of gear ratio.

This is exemplified in fig. 3. Fig. 3 exemplifies a situationwhere the vehicle 100 is decelerating and being forced to bepropelled on a current gear for a longer period of time thannormally is the case. The x-axis represents time and the y-axis represents speed of rotation n of the internal combustion engine.

The solid line 301 represents the change in combustion enginespeed n for a situation where the vehicle 100 is being drivenin an uphill section and in a situation where the vehicle islosing speed. The dashed line 302 represents a combustionengine speed limit nmn that is not to be exceeded following a change of gear to a lower gear (i.e. change of gear ratio to a higher gear ratio).

Starting from time to, in the situation being represented bysolid line 301, the vehicle 100 is continuously losing speed(decelerating) and hence the rotational speed n of thecombustion engine 101 is also continuously reduced. Therequirements for a change of gear in the general case arefulfilled at time t1, e.g. that the rotational speed of theinternal combustion engine following the change of gear will not exceed the speed limit nmn, and/or reaches a minimum speed n When change of gear is not influenced according to the Change °present invention, a change of gear would normally be carriedout at this point in time, following dashed line 303. However,when influencing gear ratio changing operations according toinvention, the change of gear is not performed but delayed.

This can be accomplished e.g. by lowering the maximum allowed 18 combustion engine speed nmn following a change of gear to asecond speed limit nmmdw so that criteria for change of gear no longer are fulfilled at time tl. Alternatively, or in addition,the criterion can instead be set such that the vehicle isforced to continue on a current gear until the speed n of thewhich is combustion engine l0l has reduced to speed nmwßjwf lower than the combustion engine speed n at which change of changegear normally is carried out. In this way the criteria forperforming the gear changing operation will not be fulfilleduntil the vehicle 100 has decelerated further and the vehicle100 thereby being propelled for a longer period of time on the current gear.

Consequently, in the situation according to the solid line 301, when the vehicle 100 reaches time tl, no change of gear isperformed since the new gear changing criteria due to detectedwheel slip are not fulfilled. Instead, the vehicle 100 iscontrolled to continue on the current gear while vehicle speed, and thereby combustion engine speed n, may continue toreduce and change of gear is thereby at least delayed.

However, at some point the combustion engine speed at acurrent gear, if the vehicle continues to decelerate, mayreach a speed where a change of gear no longer can be avoided, e.g. due to the combustion engine speed reaching the minimumallowed speed nmwyjw, e.g. constituting, or being in the vicinity of, engine idling speed. According to the presentexample, this occurs at time tg, for which reason a change ofgear is carried out, although delayed according to the invention.

Furthermore, according to embodiments of the invention, thegear ratio changing operation is only influenced when the driver takes at least one further measure that indicates a 19 reduced grip. Such measures can, for example, be a detectionthat the driver changes load on wheel shafts, e.g. by reducingload on an auxiliary axle to thereby increase load on drivewheels in an attempt to improve possibilities to obtain asufficient grip. The drive may also take other actions, suchas e.g. lock a differential in order to increase the abilityto overcome a section of road where reduced grip isprevailing. Consequently, according to embodiments of theinvention, change of gear is only influenced when at least one such further indication of reduced grip is detected.

Finally, embodiments of the invention have been exemplifiedabove for a particular example of a vehicle, but is applicablefor any vehicle in which the vehicle control system controlsgear change. The invention also encompasses a vehicle comprising a system implementing the invention.

Claims (1)

1. Claims lJ Method for controlling change of gear ratio in a gearbox (103) of a vehicle (100), said gearbox (103) beingarranged for transmitting power from a power source (101)to at least one drive wheel (113, 114) of said vehicle(100), said gearbox (103) comprising a plurality ofselectable gear ratios, change of gear ratio beingcontrolled by a vehicle control system, the vehiclefurther including a driver manoeuvrable means forrequesting power from said power source, the methodcomprising steps of: - detecting a reduced grip of at least one drive wheel ofsaid vehicle, - determining whether driver manoeuvring of said drivermanoeuvrable means for requesting power indicates areduced grip, and - influencing a gear ratio changing operation in saidgearbox (103) if driver manoeuvring of said drivermanoeuvrable means for requesting power indicates a reduced grip. .Method according to claim l, wherein the step of detecting said reduced grip of at least one drive wheelof said vehicle comprises a step of:- detecting wheel slip of at least one drive wheel of said vehicle. .Method according to claim l or 2, wherein the step of influencing said gear ratio changing operation in saidgearbox (103) comprises a step of: - delaying or suspending a change of gear ratio. .Method according to any one of claims l-3, wherein the step of influencing a gear ratio changing operation in 21 said gearbox (103) comprises a step of:- prohibiting a change of gear ratio to a lower gear ratio. .Method according to any one of claims l-4, wherein the step of influencing a gear ratio changing operation insaid gearbox (103) comprises a step of: - changing a gear changing criterion related torotational speeds of said power source and gear ratios ofsaid gearbox such that propulsion on a current gear ratiois allowed at higher and/or lower rotational speeds ofsaid power source in comparison to when no reduced grip of at least one drive wheel is detected. .Method according to any one of claims l-5, wherein the step of influencing a gear ratio changing operation insaid gearbox (103) includes: - influencing a gear ratio changing operation in saidgearbox (103) if driver manoeuvring of said drivermanoeuvrable means for requesting power indicates areduced grip within a first period of time from saiddetecting a reduced grip of said at least one drive wheel. .Method according to any one of claims l-6, wherein the step of influencing a gear ratio changing operation insaid gearbox (103) includes: - influencing said gear ratio changing operation duringat least a second period of time from when a reduced grip of said at least one drive wheel no longer is detected. .Method according to any one of claims l-6, comprising a step of: - terminating said step of influencing said gear ratio 10. 11. 12 13 22 changing operation when driver manoeuvring of said means for requesting power no longer indicates a reduced grip. .Method according to any one of claims l-8, wherein the step of determining whether driver manoeuvring of saiddriver manoeuvrable means for requesting power indicatesa reduced grip comprises a step of: - detecting a request for reduction of power from said POWGI SOUKCG . Method according to any one of claims 1-9, wherein thestep of determining whether driver manoeuvring of saiddriver manoeuvrable means for requesting power indicatesa reduced grip comprises steps of: - detecting a decrease in requested power from said powersource, and - detecting an increase in requested power from saidpower source to a request for less power than requested before said decrease in requested power. Method according to any one of claims l-9, comprisingprior to the step of influencing a gear ratio changingoperation in said gearbox (103) a step of: - detecting at least one further measure taken by the driver indicating a reduced grip. .Computer program comprising program code that, when said program code is executed in a computer, enables saidcomputer to carry out the method according to any one of the preceding claims l-ll. .Computer program product comprising a computer-readable medium and a computer program according to claim 12,wherein said computer program is contained in said computer-readable medium. 14 15. 23 .System for controlling change of gear ratio in a gearbox (103) of a vehicle (100), said gearbox (103) beingarranged for transmitting power from a power source (101)to at least one drive wheel (113, 114) of said vehicle(100), said gearbox (103) comprising a plurality ofselectable gear ratios, change of gear ratio beingcontrolled by a vehicle control system, the vehiclefurther including a driver manoeuvrable means forrequesting power from said power source, the system beingcharacterised in that it comprises: - means adapted to detect a reduced grip of at least onedrive wheel of said vehicle, - means adapted to determine whether driver manoeuvringof said driver manoeuvrable means for requesting powerindicates a reduced grip, and - means adapted to influence a gear ratio changingoperation, if driver manoeuvring of said drivermanoeuvrable means for requesting power indicates a reduced grip. Vehicle, characterised in that it comprises a system according to claim 14.