SE540392C2

SE540392C2 - Method and system for controlling selection of gear ratio ina vehicle

Info

Publication number: SE540392C2
Application number: SE1551340A
Authority: SE
Inventors: Kourie Afram
Original assignee: Scania Cv Ab
Priority date: 2015-10-16
Filing date: 2015-10-16
Publication date: 2018-09-11
Also published as: DE102016011613A1; SE1551340A1

Abstract

The present invention relates to a method for controlling selection of gear ratio in a gearbox (103) of a vehicle (100), said gearbox (103) comprising a plurality of selectable gear ratios, a change of gear ratio being controlled by a vehicle control system and being dependent on a rotational speed ( n ) of said power source (101). The method being includes, when the driving resistance exceeds the available propelling force:- determining a reduction in speed of said vehicle (100) caused by the driving resistance exceeding the propelling force, and- controlling said dependency of the rotational speed ( n ) of said power source (101) when selecting gear ratio at least partly on the basis of said determined reduction in speed of said vehicle (100).

Description

METHOD AND SYSTEM FOR CONTROLLING SELECTION OF GEAR RATIO IN A VEHICLE Field of the invention The present invention relates to vehicles, and in particular to a method and system for controlling selection of gear ratio in 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 a vehicle control system completely controls gear changing operations. The gearboxes being used in these systems can, for example, include conventional automatic transmissions comprising torque converters, but may also include automated manual transmissions where the vehicle control system automatically controls gear shifting in “manual” gearboxes.

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

In general, it is often desirable to propel the vehicle using as high a gear as possible (i.e. the lowest possible gear ratio) given the prevailing conditions in order to reduce the rotational speed of the power source, oftentimes a combustion engine, to improve fuel efficiency. Consequently, when selecting gear ratio, fuel consumption is often a matter of concern, but at the same time the vehicle should, in general, be driven using a gear ratio that is capable of providing a propelling force on the vehicle drive wheels that is sufficient to meet the current driving resistance in a desired manner.

For example, when going uphill, in particular when the vehicle is heavily loaded, a change of gear ratio to a higher gear ratio it is often required to provide a high enough propelling force to propel the vehicle without undesired retardation.

Summary of the invention It is an object of the present invention to provide a method and system that controls one or more criteria for selecting gear ratio when changing gear from a lower gear ratio to a higher gear ratio in situations when the vehicle is subjected to a reduction in vehicle speed caused by the driving resistance exceeding the propelling force. This object is achieved by a method according to claim 1.

According to the present invention, it is provided a method for controlling selection of gear ratio in a gearbox of a vehicle, said gearbox being arranged for transmitting power from a power source to at least one drive wheel of said vehicle, said gearbox comprising a plurality of selectable gear ratios, a change of gear ratio being controlled by a vehicle control system and being dependent on a rotational speed of said power source. The method includes, when the driving resistance exceeds the available propelling force: - determining a reduction in speed of said vehicle caused by the driving resistance exceeding the propelling force, and - controlling said dependency of the rotational speed of said power source when selecting gear ratio at least partly on the basis of said determined reduction in speed of said vehicle.

With regard to vehicles in general, gearboxes of the kind that historically has been used in vehicles having a manual transmission, but where a change of gear is performed automatically by means of the vehicle control system, are presently often used. This applies, e.g., both to heavy vehicles and passenger cars. Such gearboxes are usually arranged to comprise a discrete number of distinct gear ratios (gears), where the gear ratios of gearboxes in heavy vehicles often are distributed such that one or more of the lowest gear ratios (highest gears) are capable of propelling the vehicle at a constant speed when the vehicle is travelling on substantially level ground.

When vehicles of this kind, at least when being heavily loaded, are travelling uphill, a change of gear to a higher gear ratio (lower gear) is often required. The change of gear increases the propelling force on the vehicle drive wheels that is generated by the power delivered by the power source, so that a propelling force can be obtained which meets the driving resistance of the vehicle in a desired manner to properly overtake the uphill section of road. In particular, when travelling uphill, there is often a minimum gear ratio below which the vehicle will lose speed due to insufficient propelling force for propelling the vehicle at a current speed.

As is known per se, the driving resistance is the resultant force that must be overcome in order to accelerate the vehicle and at least be met to maintain a current speed. The driving resistance may vary substantially from one situation to another, e.g. in dependence on vehicle speed, load (vehicle mass) and road inclination.

If a change of gear is not performed at least to the highest gear (the lowest gear ratio) where this criterion is fulfilled, the vehicle will continue to decelerate due to the propelling force on the vehicle drive wheels resulting from the power delivered by the vehicle power source being less than required for overcoming the driving resistance. The highest gear (lowest gear ratio) where enough power is obtained can, for example, be denoted as “Gear With Power to Accelerate”, in the following referred to as GWPA gear.

For this reason, a change of gear ratio to a gear ratio being equal to or above the GWPA gear ratio is common in order to be capable of overtaking the uphill section of road without losing vehicle speed other than what is required to fulfil criteria for engaging the GWPA gear. When a heavy vehicle enters an uphill section of road, the vehicle often decelerates due to insufficient propelling force and one or more changes of gear are in general carried out to maintain the rotational speed of the power source within set limits as the vehicle decelerates. The loss in vehicle speed that the vehicle undergoes when traversing an uphill section of road will depend e.g. on the length and inclination of the uphill section of road, and current load of the vehicle.

The deceleration, and thereby loss in speed, will also depend on the rotational speed at which a change of gear to a lower gear is carried out. If a change of gear is carried out such that the power source is being driven at higher rotational speeds, the deceleration, and thereby loss in vehicle speed, is in general lower than when the power source is being driven at lower rotational speeds, and hence a lower power being produced.

A change of gear is often performed when one or more criteria are fulfilled. For example, a change of gear to a lower gear (higher gear ratio) can be arranged to be performed when it is determined that the rotational speed of the combustion engine following the change of gear will not exceed some predetermined calibrated speed limit that is not to be exceeded, possibly by the further criterion that the vehicle is expected to be propelled on the new gear for at least a minimum period of time before the rotational speed of the combustion engine again has reached a rotational speed that requires a change of gear to a lower gear. The calibrated power source speed can, for example, be different for different gears and/or vehicle speeds.

Control system controlled changes of gear in situations where available propelling force on a current gear is not sufficient to maintain a vehicle speed, however, are not always performed in an optimal manner in view of the current driving conditions.

The present invention provides a method that adapts criteria for changing gear in dependence of prevailing driving conditions. This is accomplished by means of a system where a reduction in speed of the vehicle caused by the driving resistance exceeding the propelling force is determined, and where the dependency of the rotational speed of the power source when changing gear ratio is controlled at least partly on the basis of said determined reduction in speed of said vehicle.

This has the advantage that the loss in vehicle speed that the vehicle undergoes when travelling an uphill section of road can be controlled to a higher extent. For example, it may sometimes be advantageous to allow the vehicle to perform changes of gear at lower power source rotational speeds to improve fuel efficiency. This, however, increases vehicle deceleration and thereby loss in vehicle speed. If the vehicle is travelling an uphill section of road of some length, vehicle speed can be lost to an undesired extent even if the vehicle is only slowly losing speed.

According to embodiments of the present invention, such situations can be mitigated by determining the loss in vehicle speed that the vehicle undergoes e.g. from when the loss in speed begins. When it is determined that the vehicle e.g. has lost speed to some extent, where the loss in speed is caused by the driving resistance exceeding the propelling force, the dependency of the rotational speed of said power source when changing gear ratio is controlled at least partly on the basis of the loss in speed of the vehicle. The dependency can be arranged to be controlled on the basis of the loss in speed of the vehicle only when the loss in speed of the vehicle exceeds a predetermined speed difference.

For example, the rotational speed of the power source at which a change of gear is carried out, and/or the resulting rotational speed of the power source following the change of gear, can be increased. This allows the vehicle to be propelled at higher rotational speeds of the power source, and thereby higher power to reduce the deceleration and continued loss in vehicle speed. The increase in allowed rotational speed of the power source can be arranged to depend on the reduction in speed that the vehicle has suffered, and also on the deceleration of the vehicle.

Consequently, according to embodiments of the invention, the vehicle may be allowed to be driven e.g. at low rotational speeds of the power source when the loss in vehicle speed is below e.g. a predetermined change in speed, but where the vehicle is being driven at higher power source rotational speeds when the loss in speed since the loss in speed begun exceeds e.g. a predetermined reduction in speed.

Embodiments of the present invention may thereby provide a solution that adapts changes of gear to prevailing and changing conditions, where continued loss in vehicle speed can be reduced to some extent or even as much as possible.

The rotational speed of the power source at which a change of gear is carried out can be arranged to be continuously controlled on the basis of the loss in vehicle speed. In addition, the current vehicle deceleration can also be taken into consideration, so that the rotational speed of the power source at which a change of gear is carried out can be arranged to be dependent both on loss in vehicle speed and vehicle deceleration. For example, the maximum allowed rotational speed of the power source can be increased more for higher decelerations than for lower decelerations.

The control can be arranged to be performed only when changing gear ratio from a lower to a higher gear ratio, since it is oftentimes changes of gear ratios of this kind that are performed in situations when more power is desired.

The loss, or reduction, in speed of the vehicle can be the total reduction in speed of said vehicle from a speed where the reduction in speed commenced. The loss, or reduction, in speed of the vehicle can be e.g. the total reduction in speed that the vehicle has undergone since the reduction in speed due to an increase in driving resistance commenced. The reduction in speed can, for example, be arranged to be determined as a total reduction in speed of the vehicle from the time where the driving resistance exceeds the available propelling force.

According to one embodiment, the maximum allowed rotational speed of the power source can be arranged to subsequently be reduced if determined appropriate by changing driving conditions.

Further characteristics of embodiments of the invention and advantages thereof are indicated in the detailed description of exemplary embodiments set out below and the attached drawings.

Brief description of the drawings Fig. 1A illustrates a powertrain of an exemplary vehicle; Fig. 1B illustrates an example of a control unit in a vehicle control system; Fig. 2 illustrates an exemplary method according to embodiments of the invention; Figs. 3A-B illustrates the effect of embodiments of the invention; Fig. 4 illustrates another exemplary method according to embodiments of the invention.

Detailed description of exemplary embodiments Fig. 1A schematically depicts a powertrain of an exemplary vehicle 100. The powertrain comprises a power source, in the present example a combustion engine 101, which, in a conventional manner, is connected via an output shaft of the combustion engine 101, normally via a flywheel 102, to a gearbox 103 via a clutch 106. An output shaft 107 from the gearbox 103 propels drive wheels 113, 114 via a final drive 108, such as a common differential, and drive axles 104, 105 connected to said final drive 108.

The combustion engine 101 is controlled by the vehicle control system via a control unit 115. The clutch 106 and gearbox 103 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 is selected for each driving situation by means of the vehicle control system. As was mentioned above, there exist situations where the rotational speed of the combustion engine selected for performing a change of gear may not always propel the vehicle in a desired manner. Embodiments of the invention provide a method for adapting the rotational speed of the combustion engine at which a change in gear ratio is performed.

An exemplary method 200 according to embodiments of the invention is shown in fig. 2, which method can be implemented at least partly e.g. in the control unit 116 for controlling the clutch 106 and gearbox 103. As indicated above, the functions of a vehicle are, in general, controlled by a number of control units, and control systems in vehicles of the disclosed kind generally comprise a communication bus system including one or more communication buses for connecting a number of electronic control units (ECUs), or controllers, to various components on board the vehicle. Such a 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. 1A depicts only control units 115-116, but vehicles 100 of the illustrated kind are often provided with significantly more control units, as one skilled in the art will appreciate. Control units 115-116 are arranged to communicate with one another and various components via said communication bus system and other wiring, partly indicated by interconnecting lines in fig. 1A.

Embodiments of the invention can be implemented in any suitable control unit in the vehicle 100 and hence not necessarily in the control unit 116. The control of the gearbox 103 according to embodiments of the invention will usually depend on signals being received from other control units and/or vehicle components, and it is generally the case that control units of the disclosed type are normally adapted to receive sensor signals from various parts of the vehicle 100. The control unit 116 will, for example, receive signals from the engine control unit 115 and signals representing current vehicle speed and/or deceleration and/or loss in vehicle speed since the vehicle entered a state where the driving resistance exceeds the available propelling force. Control units of the illustrated type are also usually adapted to 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 programmed instructions. The programmed instructions typically include a computer program which, when executed in a computer or control unit, causes the computer/control unit to exercise the desired control, such as method steps according to embodiments of the invention. The computer program usually constitutes a part of a computer program product, where said computer program product comprises a suitable storage medium 121 (see Fig. IB) with the computer program 126 stored on said storage medium 121. The computer program can be stored in a non-volatile manner on said storage medium. The digital storage medium 121 can, for example, include any of the group comprising: ROM (Read-Only Memory), PROM (Programmable Read-Only Memory), EPROM (Erasable PROM), Flash memory, EEPROM (Electrically Erasable PROM), a hard disk unit etc, and be arranged in or in connection with the control unit, whereupon the computer program is executed by the control unit. The behaviour 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 shown schematically in Fig. 1B, wherein the control unit can comprise a processing unit 120, which can include, for example, any suitable type of processor or microcomputer, such as a circuit for digital signal processing (Digital Signal Processor, DSP) or a circuit with a predetermined specific function (Application Specific Integrated Circuit, ASIC). The processing unit 120 is connected to a memory unit 121, which provides the processing unit 120, with e.g. the stored program code 126 and/or the stored data that the processing unit 120 requires to be able to perform calculations. The processing unit 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 devices 122, 123, 124, 125 for receiving and transmitting input and output signals, respectively. These input and output signals can comprise waveforms, pulses or other attributes that the devices 122, 125 for receiving input signals can detect as information for processing by the processing unit 120. The devices 123, 124 for transmitting output signals are arranged so as to convert calculation results from the processing unit 120 into output signals for transfer to other parts of the vehicle control system and/or the component (s) for which the signals are intended. Each and every one of the connections to the devices for receiving and transmitting respective input and output signals can include one or more of a cable; a data bus, such as a CAN bus (Controller Area Network bus), a MOST bus (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 whether the propelling force on the vehicle drive wheels is less than the current driving resistance. This can be determined e.g. by estimating the current propelling force from the work produced by the combustion engine and the total gear ratio of the power train and drive wheel radius in a conventional manner. Further, the driving resistance can be arranged to be estimated in a conventional manner so that the two forces can be compared. The determination of step 201 can also be arranged to be determined e.g. by any other suitable method, and such determinations are often performed in existing vehicles, which methods can be used according to embodiments of the invention.

As was mentioned above, the driving resistance represents the combination forces that must be overcome in order to accelerate the vehicle, or equalled to maintain a current speed. The driving resistance is often estimated by one or more vehicle functions, and is therefore often already available on the vehicle communication bus system, and hence can be established in a straight-forward manner. Alternatively the driving resistance of the vehicle can be arranged to be determined by, or on demand of, the control unit in which embodiments of the invention are implemented. The driving resistance can be estimated in any suitable manner, and plural examples of estimating the driving resistance can be found in the prior art. For example, the driving resistance can be estimated as: FDrive_res= FAir Res+ FRoll Res+ FGrav(1) Where: FAir Resrepresents the air resistance, FRoll Resrepresents the rolling resistance, and FGravrepresents the force resulting from the influence of gravity.

These forces can be estimated in any known conventional manner. Further forces may also be included in the model, such as e.g. powertrain friction losses. Such losses can also be arranged to be included e.g. as part of the rolling resistance.

The determination in step 201 can, however, also be accomplished without determining the forces above. For example, it can be sufficient to determine if the vehicle involuntarily decelerates, since such deceleration may be construed as the available propelling force not being sufficient to maintain a current speed.

Consequently, it can be determined in step 201 instead, whether the vehicle is decelerating. The vehicle deceleration dec can be determined by means of any suitable means as is known per se, such as e.g. by the use of an accelerometer, or by determining a reduction in speed between two consecutive points in time.

In step 202 it is determined a loss in vehicle speed vlosssince the involuntary reduction in speed began, i.e. the difference between the current speed and the speed that the vehicle was being driven at when the involuntary deceleration began. This can be determined, e.g., by some suitable function in the vehicle control system. For example, vehicle speed, at least with regard to heavy vehicles, is, in general, logged throughout the journey and hence both present and past vehicle speed values can be obtained via the vehicle communication bus system. It is also possible to use a dedicated function that monitors the vehicle speed and generates a signal, e.g. when the vehicle speed starts reducing.

The method then continues to step 203, where it is determined if the loss in vehicle speed exceeds a first speed difference ??lim1. For as long as this is not the case, the method returns to step 201. In this case it is determined that the loss in vehicle speed vlossis not of a magnitude that triggers an increase of the limits regarding allowable combustion engine speed when changing gear. If it is determined that the loss in vehicle speed vlossexceeds the first speed difference ??lim1, the method continues to step 204, where one or more limits regarding the rotational speed of the combustion engine when changing gear are controlled.

The first speed difference ??lim1can, for example, be set to a speed difference where it is considered that further reductions in vehicle speed preferably should be avoided or at least be reduced. If the vehicle loses too much in speed, this may be stressful to the driver and also reduce the possibility to overtake an uphill section of road in a satisfying manner.

According to embodiments of the invention it can be determined, for example, whether the vehicle speed has been reduced some suitable percentage of the vehicle speed that prevailed when the reduction in speed began, or whether the speed has reduced to some particular speed. Other criteria can also be used. In general, a larger actual loss in speed can be arranged to be accepted in situations where the vehicle loses speed from comparatively higher speeds than when losing speed from comparatively lower speeds, and use of e.g. a reduction in speed measured in percentage automatically accounts for this.

For example, gear changing criteria regarding combustion engine speed n can be adapted such that an upcoming change of gear is arranged to occur earlier, i.e. a change of gear will occur at a higher combustion engine speed so that a higher power can be generated by the combustion engine to thereby reduce the ongoing deceleration of the vehicle, so that the vehicle will lose speed more slowly, or even stop the deceleration if the change of gear is a change to a GWPA gear.

This can, for example, be accomplished by increasing the highest rotational speed of the combustion engine 101 that is allowed following the change of gear, or by correspondingly increasing the rotational speed nlimon a current gear at which a change of gear is carried out. In this way, the criteria for performing the change of gear will be fulfilled earlier than otherwise would be the case. The actual change of gear can be performed in a conventional manner, but since the criteria for performing the change of gear has changed, the change of gear will be carried out earlier.

The effect of embodiments of the invention is explained further with reference to fig. 3A-B.

Fig. 3A exemplifies the difference between the inventive scenario and the normal scenario in a situation where the vehicle is losing speed. Fig. 3A shows the combustion engine 101 rotational speed n as a function of time t. Fig. 3A further discloses the change in combustion engine speed n for two different scenarios, the line 301 representing the difference in the change in combustion engine speed n according to embodiments of the invention in relation to a conventional situation represented by dash-dotted line 302. The dashed line 304 represents a combustion engine speed limit that in the general case 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, the vehicle 100 is being driven on a section of road where the vehicle speed v is maintained at a vehicle speed v0, see fig. 3B, which discloses the corresponding vehicle speed for the situation of fig. 3A. This corresponds to the combustion engine being driven at a combustion engine speed n0.

At time t1the vehicle 100 starts losing speed, i.e. begins to decelerate, and is continuously losing speed (decelerating) from time t1, and hence the rotational speed n of the combustion engine 101 is also continuously reducing.

In this case the deceleration is caused by the vehicle entering an uphill section of road where the inclination increases the driving resistance such that the propelling force is not capable of meeting the driving resistance and hence the vehicle starts to decelerate. According to the conventional solution, the combustion engine 101 is driven on the current gear until time t5, where the combustion engine speed n reaches a speed nchange, representing a time/speed where conditions for a change of gear to a lower gear (higher gear ratio) are fulfilled. As was mentioned above, a condition for performing a change of gear is often that the speed of the combustion engine following a completed change of gear is not allowed to exceed some speed limit nlim, indicated at 304, which often is a calibrated engine speed that is not to be exceeded. This calibrated engine speed can, for example, be different for different gears and/or vehicle speeds and/or driving modes such as eco/power. As a consequence, a change of gear to a lower gear is, in general, performed when it is determined that the speed of the combustion engine on the new gear will not exceed said speed limit nlim. This has the result that, according to the conventional solution, the speed of the vehicle combustion engine following the change of gear is increased to a speed limit equal to or less than nlimat time t6.

Further according to the conventional solution, this change of gear is performed irrespective of the rather substantial differences in driving conditions that in reality may prevail when a change of gear is to be carried out. For example, the change of gear is carried out at a determined combustion engine speed irrespective of whether the speed of the vehicle is dropping relatively fast or more slowly.

Flowever, as is illustrated by solid line 301, the situation according to embodiments of the invention is different. At time t2 the reduction in vehicle speed since the deceleration began has reached the relative speed difference ??limof step 203 above, and the allowed combustion engine speed following a change of gear is increased from the combustion engine speed nlimto the combustion engine speed nlim1. This, in turn, means that the following change of gear will occur at time t3 instead of at time t5 as in the conventional solution. This has the further effect that since the combustion engine 101 will be driven at a higher rotational speed following the change of gear, a higher power is available and the deceleration following the change of gear will be reduced. According to the present example, the change of gear is not to a GWPA gear, and hence there is still a deceleration, but lower than otherwise would be the case. That is, the vehicle is decelerating more slowly from time t4 according to line 304 than according to line 302.

Embodiments of the invention consequently, provides a solution that can adapt gear changing criteria to current driving conditions so that e.g. undesired loss in vehicle speed can be reduced. Furthermore, embodiments of the invention will only be utilized when required. If the vehicle is travelling uphill but the reduction in vehicle speed does not reach the speed difference ??lim1no change of the allowed combustion engine speed after change of gear is required.

Instead of, or in addition to, explicitly increasing the maximum allowed combustion engine speed following the change of gear, a further/alternative criterion can be set. For example, a criterion for change of gear can instead be set such that the vehicle is not allowed to perform a change of gear until the speed n of the combustion engine 101 has reduced to a speed nchange, see fig. 3A. In such situations, embodiments of the invention can be arranged to increase the speed nchangeat which change of gear is carried out to a higher combustion engine speed nchange1. This, however, has the same effect as the above since inherently the maximum allowed combustion engine speed following the change of gear is also increased. Therefore, according to an embodiment, the allowed rotational speed on a current gear is increased, whereby the maximum allowed combustion engine speed following the change of gear is also increased.

The determination in step 204 can be a specific combustion engine speed nlim1( nchange1) as above, or a relative increase ?nlimthat is added to the currently prevailing limit.

When a suitable determination has been made in step 204, the method can be arranged to return to step 201. According to one embodiment, the method can be continuously, or at suitable intervals, carried out to adapt the one or more from e.g. nlim1, nchange1, and/or ?nlimon the basis of current conditions. For example, any of , nchange1, ?nlimcan be arranged to be changed a number of times prior to a change of gear actually occurs, e.g. as the loss in vehicle speed progress. Alternatively, a determination can be performed e.g. only once between consecutive changes of gear. It is also to be understood that the loss in vehicle speed vlossis preferably determined from the vehicle speed where the reduction in speed commenced, i.e. v0in fig. 3B. Consequently the loss in vehicle speed vlosscan be determined as v0- v . The loss in vehicle speed vlosscan also be arranged to be determined from the time where the driving resistance exceeds the available propelling force, i.e. where the deceleration begins, from time t1 in figs. 3A-B.

This means that a number of changes of gears to lower gears can be arranged to be carried out as the vehicle decelerates, and the loss in vehicle speed vlosswill continue to increase. As a result, nlim1, nchange1, and/or ?nlimcan be arranged to increase between consecutive changes of gear, so that e.g. a change of gear following the change of gear at time t3 will be carried out such that e.g. an even higher combustion engine speed than Image available on "Original document" is allowed. According to one embodiment, when the loss in vehicle speed vlossexceeds some suitable speed difference ??lim2, the criteria for changing gear are set such that change of gear occurs at highest possible rotational speeds of the combustion engine to reduce deceleration as much as possible.

So far, only the reduction in vehicle speed has been contemplated when determining a gear changing dependency of combustion engine rotational speed. According to one embodiment, illustrated in fig. 4, the method further takes current deceleration into account. The method steps 401-403 of fig. 4 are similar to the method steps 201-203 of fig of fig. 2 and reference is made to the above. However, in step 404 the vehicle deceleration dec is determined, i.e. the rate at which the vehicle is losing speed. As was mentioned above, the deceleration can be determined e.g. by the use of an accelerometer, or by determining a reduction in speed between two consecutive points in time.

The deceleration when a vehicle is losing speed may vary substantially from one situation to another. For example, the vehicle may be losing vehicle speed, and hence combustion engine rotational speed n , relatively fast. This can be the case, for example, when the vehicle 100 is being driven in a steep uphill section of a road, in particular when being heavily loaded. Alternatively, the vehicle can be driven in a relatively less steep uphill section of road and/or being loaded to a lesser extent. Such differences in deceleration will result in the vehicle losing e.g. some specific number of km/h in relatively different periods of time. This can be accounted for according to embodiments of the invention. When the deceleration has been determined in step 404, the method continues to step 405, where similar to the above with regard to step 204 one or more limits regarding the rotational speed of the combustion engine when changing gear is controlled on the basis of both loss in vehicle speed and deceleration.

For example, a comparatively lower deceleration may be arranged to result in a lower increase in allowed rotational speed of the power source than a comparatively higher deceleration for a same loss in vehicle speed.

Furthermore, embodiments of the invention can be used in combination with the solution described in the parallel Swedish patent application 1551339-3, having the same filing date as the present application, the same inventor and the title “METHOD AND SYSTEM FOR SELECTING GEAR IN A VEHICLE”.

This parallel application relates to a method and system where an occurrence of undesired changes of gear can be reduced. As was mentioned above, a change of gear is in general performed when one or more criteria are fulfilled. As explained above, change of gear can be arranged to be performed when it is determined that the rotational speed of the combustion engine following the change of gear will not exceed some speed limit.

In contrast to the problems addressed above, it may sometimes be advantageous to lower the allowed rotational speeds of the combustion engine. Even if the vehicle is decelerating, there exist situations where the change of gear in reality may not be motivated. For example, if the deceleration of the vehicle on a current gear is relatively low, i.e. the vehicle may only slowly be losing speed, a change of gear may seem unnecessary. According to the parallel application, occurrence of such situations can be reduced. This is accomplished by means of a method where a deceleration of the vehicle is determined, and where the dependency of the rotational speed of the power source when changing gear is controlled at least partly on the basis of the determined deceleration. The maximum allowed rotational speed of said power source following the change of gear can be reduced, and be reduced more for lower decelerations than for comparatively higher decelerations. Consequently, the lower the deceleration, the longer the vehicle may be allowed to continue on a current gear before a change of gear is performed.

Although such solutions oftentimes work well, there exist scenarios where drawbacks regarding loss of vehicle speed discussed above may be increased. For example, if the vehicle has already suffered losses in vehicle speed, a reduction of the allowed combustion engine speed following a change of gear will further increase the rate at which speed is lost.

Embodiments of the invention can be used in combination with the solution described in the parallel application, so that changes of gear e.g. can be performed according to the parallel application for as long as the loss in vehicle speed does not exceed some suitable speed difference. When the vehicle has lost too much in speed, the vehicle can, instead be controlled according to the solution described herein to reduce negative impact of loss in speed.

According to the combined solution, the vehicle can be propelled at lower combustion engine speeds, and hence at a lower average combustion engine power, when possible, but where embodiments of the invention may ensure that the vehicle does not lose too much speed. Further according to the combined solution, e.g. the maximum allowed rotational speed may be arranged to increase and decrease according to the current driving conditions, so that the vehicle behaviour can be adapted to prevailing conditions.

Finally, embodiments of the invention have been exemplified above for a particular example of a vehicle, but is applicable for any vehicle in which the vehicle control system controls gear change. The invention also encompasses a vehicle comprising a system implementing the invention.

Claims

1. Method for controlling selection 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, a change of gear ratio being controlled by a vehicle control system and being dependent on a rotational speed (n) of said power source (101), the method being characterised in, when the driving resistance exceeds the available propelling force: - determining a reduction in speed of said vehicle (100) caused by the driving resistance exceeding the propelling force, and - controlling said dependency of the rotational speed (n) of said power source (101) when selecting gear ratio at least partly on the basis of said determined reduction in speed of said vehicle (100).

2. Method according to claim 1, wherein: - said reduction in speed of said vehicle (100) is a total reduction in speed of said vehicle (100) from a speed where the reduction in speed commenced.

3. Method according to claim 1 or 2, further including, following said control of said dependency: - initiating a change of gear ratio from a first gear ratio to a second gear ratio according to said controlled dependency.

4. Method according to any one of the preceding claims, further including, when controlling said dependency of the rotational speed (n) of said power source (101) when changing gear ratio: - on the basis of said first reduction in speed of said vehicle, determining a maximum allowed rotational speed (nlim_dec) of said power source (101) following a change of gear ratio.

5. Method according to claims 4, further including: - initiating a change of gear ratio from a first gear ratio to a second gear ratio such that the rotational speed of said power source (101) will correspond to said maximum allowed rotational speed (nlim_dec) upon engagement of said second gear ratio.

6. Method according to any one of the claims 1 -5, further including, when controlling said dependency of the rotational speed (n) of said power source (101) when changing gear ratio: - controlling a maximum allowed rotational speed (nlim_dec) of said power source (101) following said change of gear ratio in relation to a predetermined maximum allowed rotational speed (nlim_dec) of said power source (101) such that the maximum allowed rotational speed (nlim_dec) is increased to a first rotational speed for a first reduction in speed of said vehicle (100), and to a second rotational speed for a second reduction in speed of said vehicle (100), where said second rotational speed is higher than said first rotational speed when said second reduction in speed of said vehicle is higher than said first reduction in speed of said vehicle (100).

7. Method according to any one of the preceding claims, further including: - controlling said dependency of the rotational speed (n) of said power source (101) when to change gear ratio also on the basis of a speed of said vehicle (100) from which said reduction in speed commenced.

8. Method according to claim 7, further including: - controlling said dependency of the rotational speed (n) of said power source (101) such that a maximum allowed rotational speed (nlim_dec) of said power source (101) following a change of gear is increased more in relation to a predetermined maximum allowed rotational speed (nlim_dec) of said power source (101) the lower the vehicle speed from which the reduction in speed commenced is.

9. Method according to any one of the preceding claims, further including: - controlling said dependency of the rotational speed (n) of said power source (101) when changing gear ratio also on the basis of a deceleration ( dec ) of said vehicle (100).

10. Method according to claim 9, further including: - controlling said dependency of the rotational speed (n) of said power source (101) such that a maximum allowed rotational speed (nlim_dec) of said power source (101) following a change of gear is increased more the higher the deceleration (dec) is.

11. Method according to claim 9 or 10, wherein the maximum allowed rotational speed (nlim_dec) of said power source (101) following a change of gear ratio is reduced on the basis of the deceleration (dec ) of said vehicle when the reduction in speed of said vehicle (100) is below a first speed difference.

12. Computer program comprising program code that, when said program code is executed in a computer, enables said computer to carry out the method according to any of claims 1-11.

13. 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. System for controlling a selection 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, a change of gear ratio being controlled by a vehicle control system and being dependent on a rotational speed (n) of said power source (101), the system being characterised in, when the driving resistance exceeds the available propelling force: - means for determining a reduction in speed of said vehicle (100) caused by the driving resistance exceeding the propelling force, and - means for controlling said dependency of the rotational speed (n) of said power source (101) when selecting gear ratio at least partly on the basis of said determined reduction in speed of said vehicle (100).

15. Vehicle, characterised in that it comprises a system according to claim 14.