SE1551397A1 - A method for determining a desired gap between a motor vehicle and a lead vehicle - Google Patents

Abstract

A method for determining a desired gap between a present motor vehicle and a lead vehicle travelling ahead of the present vehicle, comprising the steps of: - identifying that a lead vehicle is travelling ahead of the present vehicle, - collecting data relating to a road gradient along an expected travelling route ahead of the present vehicle, - based on said data, simulating a future speed profile of the present vehicle for at least a present mode of operation of the present vehicle, - based on said simulated future speed profile, determining a desired gap to the lead vehicle.(Fig. 1)

Description

A method for determininq a desired aan between a motorvehicle and a lead vehicle TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for determining adesired gap between a present motor vehicle and a lead vehicletravelling ahead of the present vehicle according to the preambleof claim 1. The invention further relates to a computer program, acomputer program product, an electronic control unit, and amotor vehicle. By a motor vehicle is here intended a vehiclewhich is powered by an internal combustion engine and/or by anelectric motor. ln particular, but not exclusively, the method isintended for use in a heavy motor vehicle such as a truck or a bus.

By a gap is herein intended a gap in terms of either distance or time.

BACKGROUND AND PRIOR ART The cost of fuel for motor vehicles, e.g. cars, trucks and buses,represents a significant expense for the owner or user of thevehicle. A wide variety of different systems have therefore beendeveloped for e.g. fuel-efficient reducing fuel consumption, engines and fuel-economising cruise controls.

A driver of a motor vehicle with a cruise control usually selects a set speed v_set. The set speed v_set is the speed which the driver wishes the motor vehicle to maintain on a level road. Thecruise control then provides the engine system in the vehicle witha reference speed v_ref, where the reference speed v_ref is usedto control the engine. The set speed v_set may thus be seen asan input signal to the cruise control, while the reference speedv_ref may be seen as an output signal from the cruise control,which is used to control the engine, providing control of the vehicle's actual speed v_actual.

Traditional cruise controls (CC) maintain a constant referencespeed v_ref, which corresponds to the set speed v_set requestedby the driver. The value of the reference speed v_ref is herechanged only when the user himself adjusts the set speed v_set while driving.

Today there are also cruise controls, so-called economical cruisecontrols, such as Ecocruise controls and similar cruise controls,which try to estimate the current driving resistance and also haveknowledge about the historical driving resistance. Anexperienced driver who drives a motor vehicle without a cruisecontrol may reduce fuel consumption by adjusting the driving tothe characteristics of the road ahead, so that unnecessarybraking and/or fuel-consuming acceleration may be avoided. ln afurther development of these economical cruise controls, theambition is to mimic the experienced driver's adjustment ofdriving the motor vehicle based on knowledge about the roadahead, so that fuel consumption may be kept at a level as low as possible.

One example of such a further development of an economicalcruise control is a "Look Ahead" cruise control (LACC), i.e. astrategic cruise control using knowledge about road sectionsahead, i.e. knowledge about the appearance of the road ahead,in order to determine the appearance of the reference speedv_ref. Here, the reference speed v_ref is thus permitted, within apermitted speed interval Av_permitted, to differ from the setspeed v_set selected by the driver in order to achieve a morefuel-efficient driving. For example, by taking topographicinformation about the road section ahead of the vehicle intoaccount, the speed may be temporarily increased before e.g. anuphill slope, so that downshifting to a lower transmission modecan be avoided or delayed. ln this way, fuel consumption can bereduced. Also information about road curvature and legal speedlimits along the road section ahead of the vehicle can be taken into account.

One of the main factors affecting the energy consumption of avehicle, in particular at high speeds and for large motor vehicleshaving a large front area, is air resistance. A way to reduce theair resistance, and thereby the energy consumption, is thereforeto drive behind a lead vehicle and exploit the so called slipstreameffect. When two or more vehicles are involved in a so-calledconvoy, i.e. when trailing vehicles drive relatively proximate tolead vehicles, the fuel consumption of said vehicles can be reduced by, for example, 5-15%.

Modern motor vehicles can be equipped with radar technology tomeasure a distance to a lead vehicle. Some vehicles can also be equipped with a control system to automatically maintain a distance, chosen by a driver, to a lead vehicle. According to oneexample, such a system can comprise an actuating device withwhich the driver can manually set a position that corresponds toa given gap to a lead vehicle. Such an actuating device can e.g.that increments of distance to the lead vehicle between 10 and 75 have five different positions correspond to discretemeters, corresponding to time gaps within the range of 1-4seconds. This system is usually automated in the trailing vehicle.Alternatively, a driver of the trailing vehicle can choose to drive at a given distance to the lead vehicle.

WO2013/147682 discloses a method for adapting the speed of amotor vehicle such that it travels at a gap from a lead vehiclewhich is optimised for reducing the air resistance felt by thevehicle. Factors such as a front area and a load configuration ofthe lead vehicle, the present speed of the vehicle, and winddirection and wind force of the ambient air are taken into account However, air resistance is only one of many factors affecting thetotal energy consumption of the vehicle. Driving close to a leadvehicle in order to exploit the slipstream effect may give rise tothat other fuel cannot be fully utilised. For example, saving measures, such as described above, on a topographicallyvarying road, driving close to a lead vehicle may necessitatebraking more often than would otherwise be necessary, therebywasting kinetic energy and total increasing the energy consumption of the vehicle.

SUMMARY OF THE INVENTION lt is a primary objective of the present invention to achieve an, inat least some aspect, improved way of determining a desired gapto a lead vehicle travelling ahead of a present motor vehicle. lnparticular, it is an objective to achieve a way to determine adesired gap to a lead vehicle such that the total energyconsumption of the vehicle when driving along a travelling routeat a present mode of operation is minimised, not only the energy consumption resulting from air resistance felt by the vehicle.

According to a first aspect of the present invention, at least theprimary objective is achieved by means of the method initiallydefined, which is characterised in that it comprises the steps of: - collecting data relating to a road gradient along anexpected travelling route ahead of the present vehicle, - based on said data, simulating a future speed profile of thepresent vehicle for at least a present mode of operation ofthe present vehicle, - based on said simulated future speed profile, determining a desired gap to the lead vehicle.

The method according to the invention relies on a simulation ofthe future speed profile of the vehicle itself to determine adesired gap to the lead vehicle. The simulation is based on e.g.map data in combination with positioning information, so that thetopography along the road immediately ahead of the vehicle caninto account. if small variations in be taken Generally, topography are foreseen, it is probable that a fuel-economisingcruise control of the vehicle will want to keep the vehicle speedconstant or nearly constant to keep the fuel consumption at anoptimised level. lf instead large variations in topography areforeseen, the same cruise control may be set to let the actualspeed of the vehicle deviate from a set speed by e.g. reducingthe speed before a downhill road section on which the vehiclespeed is expected to increase, such that braking can be avoided.Such fuel-economising efforts may in a hilly terrain result in anoverall smaller energy consumption than exploiting a potential slipstream effect.

The method according to the invention thus takes a foreseenoptimised speed pattern of the vehicle into account whendetermining a desired gap to the lead vehicle. ln other words, thedesired gap to the lead vehicle is determined so that fuel-economising systems in the vehicle, such as a fuel-economisingcruise control, can be fully utilised without having to riskunnecessary braking of the vehicle. lf small speed variations areforeseen, the vehicle may be controlled to travel close to the leadvehicle, while as if the foreseen speed variations are large, thevehicle may be controlled to travel at a larger distance from the lead vehicle.

The determined desired gap may be used in the control of thevehicle so that the desired gap to the lead vehicle is attained andheld. This can, controlling the gap to the lead vehicle, either in time or in on one hand, be achieved by automatically distance, based on the determined desired gap. On the other hand, it can also be achieved by informing a driver of the vehicle about recommendable actions based on the determined desiredgap, such as increasing or decreasing a current gap to the leadvehicle. The driver can for example be prompted to select adesired gap based on a presented recommendation. ln order tobe able to control the vehicle such that a desired gap is reached,the actual current distance to the lead vehicle is continuouslymeasured and evaluated to learn whether this current distanceshould be decreased or increased. l\/leasuring the distance to thelead vehicle can be performed using any known method, such as by means of radar technology, Camera information, or similar.

Of course, also other data apart from data relating to the roadgradient along the expected travelling route ahead of the presentvehicle, such as traffic data, may be used to determine the futurespeed profile of the vehicle. Also parameters such as atransmission mode, a mode of operation, a current actual vehiclespeed, at least one engine characteristic, e.g. maximum and/orminimum engine torque, a vehicle weight, an air resistance, arolling resistance, a gear ratio in the gearbox and/or the powertrain of the vehicle, and a wheel radius, may be taken intoaccount According to one embodiment of the invention, it furthercomprises the step of: - determining whether the lead vehicle is configured so thatat a certain gap, a reduction in air resistance felt by thepresent vehicle is expected.

The step of determining a desired gap to the lead vehicle maythus be performed based on the configuration of the lead vehicle. ln this way, it is possible to determine a relatively large desired gap to the lead vehicle in case no energy savings throughslipstream effects can be expected, based on the configuration ofthe lead vehicle. lf the lead vehicle is so configured that energysavings can be expected, the desired gap to the lead vehicle isdetermined so that slipstream effects can be exploited asdiscussed above. For vehicles which are too small to generate aslipstream effect for the present vehicle, a relatively large gapcan be recommended or set, so that other fuel-economisingsystems or algorithms of the vehicle can be fully utilised. Thefront area and the load configuration of the lead vehicle may bedetected and evaluated using e.g. camera information and image processing.

According to one embodiment of the invention, said simulation ofan expected future speed profile is performed independently ofthe lead vehicle. ln other words, this simulation is performed alsowhen no lead vehicle is present in front of the present vehicle,e.g. at a predetermined frequency. No information about anexpected speed profile or similar of the lead vehicle is thusneeded.

According to one embodiment of the invention, the desired gap tothe lead vehicle is determined based on the expected futurespeed profile within a predetermined time period or within apredetermined distance interval. The foreseen speed variationwithin the nearest time period or distance interval is thereby usedto determine the desired gap, such as within the upcoming 2minutes or within the upcoming 2 km, or less. The interval maye.g. be set to 400 m. Speed variations foreseen to occur further ahead, i.e. more than 400 m ahead of the present vehicle position, are thereby not taken into account. Such a speedvariation is instead taken into account on a later occasion, whenbeing closer in time. The interva| should be chosen such thatthere is sufficient time to adjust the gap before a foreseen speedvariation.

According to one embodiment of the invention, the step ofsimulating a future speed profile comprises determining anexpected future speed interva| within which the vehicle speed isexpected to vary during travel along the expected travellingroute. Such a speed interva| can be obtained based on theexpected topography along the expected travelling route and canbe used to determine an accurate desired gap to the leadvehicle. The upper and lower limits of the expected speed interva| usually vary depending on topography. Largetopographic variations result in a relatively broad speed interva|,while as for a flat road, the speed interva| is expected to be nafTOW.

According to one embodiment of the invention, the desired gap tothe lead vehicle is determined based on the size of said speedinterva|. This is an accurate way of determining a suitable gap tothe lead vehicle. For a broad speed interva|, the desired gap ispreferably determined to be relatively large, and for a narrowspeed interva|, the desired gap is determined to be relativelysmall.

According to one embodiment of the invention, the step ofdetermining a desired gap to the lead vehicle comprises modifying a predefined preferred gap. Such a predefined preferred gap can be e.g. a preferred gap set by a driver of thevehicle or by a haulage company. lt may further be a smallestpossible gap, as determined e.g. by conditions set by aninsurance company or similar. The predefined preferred gap ismodified by taking the simulated speed profile into account, suchthat slipstream effects can be properly utilised. A driver of thevehicle may be asked to approve overriding of the predefinedpreferred gap before the gap is actually adjusted toward thedesired gap.

According to one embodiment of the invention, the step ofdetermining a desired gap to the lead vehicle comprisesselecting said desired gap from a predefined set of selectablegaps. This is a fast and sufficiently accurate way of determininga desired gap. For example, a set of selectable gaps cancomprise five gaps from 1.5 to 4 seconds, of which one can beselected or recommended in dependence on the size of anexpected future speed interval, using thresholds as a basis forselection or recommendation. Alternatively, the desired gap canbe calculated as a function of the size of the expected futurespeed interval.

According to one embodiment of the invention, the step ofsimulating an expected future speed profile is repeatedlyperformed with a predetermined frequency. ln this way, it isensured that the desired gap is always determined based on anup-to-date speed profile. The step of determining a desired gapcan be performed with the same said predetermined frequency,or with a different frequency. lt can also be performed irregularly, such as only when the expected future speed profile changes 11 significantly. For example, if the size of an expected future speedinterval deviates from the size of a previously simulated suchinterval by more than a predetermined threshold, the step ofdetermining a desired gap can be carried out. This step may alsobe conducted only when a lead vehicle is identified within a certain predetermined distance.

According to one embodiment of the invention, the speed of thevehicle in the present mode of operation is controlled by a cruisecontrol system. A cruise control system offers possibilities forsetting and keeping the determined desired gap to the leadvehicle, and also for fuel-economic driving of the vehicle whenthe determined desired gap is large.According to one embodiment of the invention, the step ofdetermining a desired gap to the lead vehicle comprisesfurnishing a basis to a driver of the vehicle for determining saiddesired gap. This gives a driver of the vehicle good control overparameters affecting the energy consumption of the vehicle. Thedriver can e.g. be presented with a recommended gap or with arecommended adjustment of the present gap and prompted toaccept it. The basis can e.g. be furnished by visual means, suchas on a display, or by audio means, such as by means of aloudspeakerAccording to one embodiment of the invention, it furthercomprises the step: - adjusting the speed of the present vehicle so that the desired gap is achieved. 12 The speed can be automatically controlled using e.g. a cruisecontrol, in which case a reference speed v_ref of the cruisecontrol is adjusted so that the desired gap is achieved. Thedesired gap is in this case used as an input signal to the cruisecontrol. The cruise control may be configured to automaticallyperform the adjustment of the speed based on the determineddesired gap. The speed may be adjusted in small increments,such that no abrupt changes of the speed and of the gapbetween the present vehicle and the lead vehicle occurs. Thespeed of the vehicle may also be controlled by means of acontrol system configured to automatically control the operationof the vehicle based on a measured gap to the lead vehicle sothat the desired gap is achieved and held. The cruise control isthereby temporarily inactivated and the speed is controlled based solely on the measured gap to the lead vehicle.

According to another aspect of the invention, at least the primary objective is achieved by a computer program comprisingcomputer program code for causing a computer to implement theproposed method when the computer program is executed in the computer.

According to a further aspect of the invention, at least theprimary objective is achieved by a computer program productcomprising a non-transitory data storage medium which can beread by a computer and on which the program code of the proposed computer program is stored.

According to a further aspect of the invention, at least the primary objective is achieved by an electronic control unit of a 13 motor vehicle comprising an execution means, a memoryconnected to the execution means and a data storage mediumwhich is connected to the execution means and on which thecomputer program code of the proposed computer program is stored.

According to a further aspect of the invention, at least theprimary objective is achieved by a motor vehicle comprising theunit. The motor proposed electronic control vehicle may preferably be a truck or a bus.

Other advantageous features as well as advantages of the present invention will appear from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will in the following be described with reference to the appended drawings, in which: Fig.1 is a flow chart showing a method according to theinvenüon, Fig. 2 schematically shows a control unit according to theinvenüon,and Fig.3 schematically shows a vehicle according to the invenüon. 14 DETAILED DESCRIPTION OF EMBODIMENTS OF THEINVENTION A method according to an embodiment of the present invention isWhen the method is initiated, a motor vehicle is travelling forward along a schematically shown in the flow diagram in fig. 1. road section. ln a step S1, data are co||ected relating to a roadgradient along an expected travelling route ahead of the vehicle.The road gradient may be obtained in various different ways. ltmay be determined on the basis of map data, e.g. from digitalmaps containing topographical information, in combination withe.g. GPS (globalThe positioning positioning information, positioning system) information. information may be used todetermine the location of the vehicle relative to the map data sothat the road gradient can be extracted from the map data.Various present-day cruise control systems use map data andpositioning information. Such systems may then provide the mapthe method thereby minimising the data and positioning information required for according to the present invention, additional complexity involved in determining the road gradient.

The road gradient may be obtained on the basis of a map inconjunction with GPS information, from radar information, fromcamera information, of information from another vehicle, frompositioning information and road gradient information storedpreviously on board, or from information obtained from trafficsystems related to the expected travelling route. ln systemswhere there is information exchange between vehicles, road gradients estimated by one vehicle may also be made available to other vehicles, either directly or via an intermediate unit such as a data base or the like. ln a subsequent step S2, a future speed profile of the vehicle is simulated based on the collected data. The simulation isperformed at least for a present mode of operation of the presentvehicle, such as for driving with a look-ahead cruise control(LACC) at a set speed v_set. ln this case, the LACC sets areference speed v_ref of the vehicle such that the energyconsumption of the vehicle is optimised with regard to thetopography of the expected travelling route. The LACC may forthis purpose be allowed to adjust the reference speed v_ref sothat it deviates from the set speed v_set, such as initiallydescribed. The deviation from the set speed may result in aforeseen speed variation within the nearest future within aninterval expected future speed Av_expected. This expected future speed interval Av_expected may of course differsignificantly from a permitted speed interval Av_permitted of theLACC. On a flat road, Av_expected is normally much smaller than the permitted speed the expected future speed intervalinterval Av_permitted, which sets the upper and lower limitswithin which the speed is allowed to be varied for a certain set speed v_set.

The simulation may be conducted on board the vehicle at apredetermined rate, e.g. at a rate of 1 Hz, which means that anew simulation result is provided every second. The section ofwhich the simulation is conducted road for represents a predetermined distance ahead of the vehicle, e.g. it might be 1-2 16 km long. The section of road may also be regarded as a horizon ahead of the vehicle, for which the simulation is to be conducted. ln addition to the aforesaid parameter of road gradient, thesimulation may also be based on one or more from among atransmission mode, a mode of operation, a current actual vehiclespeed, at least one engine characteristic, e.g. maximum and/orminimum engine torque, a vehicle weight, an air resistance, arolling resistance, a gear ratio in the gearbox and/or the power train of the vehicle, and a wheel radius.

At any point in time, independently of steps S1 and S2, it is in astep S3 identified that a lead vehicle is travelling ahead of thepresent vehicle. This identification can be conducted based one.g. information from a radar system or from camera information.When identified, information about the configuration of the lead vehicle may be such a lead vehicle is also additionalcollected. Such information includes a front area and a loadconfiguration of the lead vehicle, obtainable by means of imageprocessing. The additional information can be compared tocorresponding information of the present vehicle to evaluatewhether the lead vehicle is expected to cause a slipstream effectfor the present vehicle if the gap between the vehicles is sufficiently small. ln a step S4, a desired gap to the lead vehicle is determinedbased on the simulated future speed profile. This can beconducted automatically, for example each time that the futurespeed profile is simulated, at some other interval, or when someis fulfilled, such as when it is predetermined condition 17 determined that a distance to the identified lead vehicle is lessthan a predetermined distance. The desired gap is preferablydetermined based on the size of an expected future speedinterval Av_expected, in such a way that larger expected speedvariations also generates a larger desired gap. For example, the following conditions may be set: |Av| < a_1: Set the gap to 1 s;a_1s|Av| < a_2: Set the gap to 2 s;|Av| 2 a_2: Set the gap to 3 s, wherein |Av| is the size of the expected future speed intervalAv_expected and wherein a_1, a_2 and a_3 are predeterminedthreshold values. Of course, the number of thresholds used mayvary as well as the size of the gaps. A number of selectable gapsmay be available in a database, in which case a gap isautomatically selected based on the size of the expected futurespeed interval Av_expected. The gap may of course be specified in terms of distance instead of time.

The desired gap can also be calculated as a function of the sizeof an expected future speed interval Av_expected, in which casethe desired gap can be determined to be any value, e.g. a value within a predefined range. ln another embodiment, determining the desired gap comprisesmodifying a predefined preferred gap, such as a preferred gapset by a driver or an owner of the vehicle. A modification of thepreferred gap by a certain amount, as defined in time or in distance, may be suggested to the driver each time the expected 18 future speed profile is such that it would be advantageous to modify the predefined preferred gap.

As mentioned, the determined desired gap may be used tocontrol the speed of the vehicle, or it may be communicated tothe driver of the vehicle so that the driver may take anappropriate action. The radar system of the vehicle can bearranged so as to continuously determine a distance to the leadvehicle. The determined desired gap can thereby be set as adistance to the lead vehicle, whereupon a control system in thevehicle can automatically control the operation of the vehicle sothat the desired gap to the lead vehicle is achieved. While thevehicle is controlled so that it drives at a certain distance to thelead vehicle, the expected future speed profile is continuouslysimulated. On the basis thereof, it is determined whether or not to alter the gap to the lead vehicle. ln one example, a motor vehicle is travelling forward with anactivated Look Ahead cruise control. ln the motor vehicle, datarelating to the road gradient along an expected future travellingroute of the vehicle are continuously collected. The collecteddata are used to simulate an expected future speed profile of thevehicle during the next 1 km for at least a present mode ofoperation of the vehicle, i.e. with the cruise control activated witha constant set speed v_set. This is performed at a predeterminedfrequency of 1 Hz. As the motor vehicle approaches anothervehicle, it is identified that a lead vehicle is travelling ahead ofthe present motor vehicle. lt is checked whether the lead vehicleis expected to result in a slipstream effect. On the basis of the size of the variations in the expected future speed profile during 19 the coming 400 m, a desired gap to the lead vehicle isdetermined by selection from a database. The selected desiredgap is presented to a driver of the vehicle, who is prompted toaccept the presented gap. As the driver accepts the gap, thedesired gap is set and the vehicle is controlled such that the gapbetween the vehicles approaches the desired gap. ln another example, the vehicle may be configured toautomatically adjust the gap between the vehicles toward theselected desired gap without prompting the driver to accept suchan adjustment. lt is also possible to only prompt the driver incase the determined desired gap differs significantly from a predefined preferred gap, or based on some other condition. the art will determining a desired gap between a present motor vehicle and One skilled in appreciate that a method for a lead vehicle according to the present invention may beimplemented in a computer program which, when executed in acomputer, causes the computer to conduct the method. Thecomputer program usually takes the form of a computer programproduct which comprises a suitable digital storage medium onwhich the computer program is stored. Said computer-readabledigital storage medium comprises a suitable memory, e.g. ROM(read-only memory), PROM (programmable read-only memory),EPROM (erasable PROM), flash memory, EEPROM (electricallyerasable PROM), a hard disc unit, etc.

Fig. 2 depicts schematically an electronic control unit 400 of avehicle provided with an execution means 401 which may take the form of substantially any suitable type of processor or microcomputer, e.g. a circuit for digital signal processing (digitalsignal processor, DSP), or a circuit with a predetermined specificASIC). The execution means 401 is connected to a memory unit 402 which is function (application specific integrated circuit,situated in the control unit 400. A data storage medium 403 isalso connected to the execution means and provides theexecution means with, for example, the stored program codeand/or stored data which the execution means needs to enable itto do calculations. The execution means is also adapted tostoring partial or final results of calculations in the memory unit 402.

The control unit 400 is further provided with respective devices411, 412, 413, 414 for receiving and sending input and outputsignals. These input and output signals may comprisewaveforms, pulses or other attributes which the input signalreceiving devices 411, 413 can detect as information and whichcan be converted to signals which the execution means 401 canprocess. These signals are then supplied to the executionmeans. The output signal sending devices 412, 414 are arrangedto convert signals received from the execution means 401, inorder to create, e.g. by modulating them, output signals whichcan be conveyed to other parts of the vehicle and/or other systems on board.

Each of the connections to the respective devices for receivingand sending input and output signals may take the form of one ormore from among a cable, a data bus, e.g. a CAN (controllerarea network) bus, a MOST (media orientated systems transport) bus or some other bus configuration, or a wireless connection. 21 One skilled in the art will appreciate that the aforesaid computermay take the form of the execution means 401 and that the aforesaid memory may take the form of the memory unit 402.

Control communication systems in modern vehicles generally comprise a bus system consisting of one or more communication buses for connecting together a number ofunits (ECUs), components on board the vehicle. electronic control or controllers, and variousSuch a control system maycomprise a large number of control units and the responsibility for a specific function may be divided between two or more of them.ln the embodiment depicted, the present invention isimplemented in the control unit 400 but might also be implemented wholly or partly in one or more other control unitsalready on board the vehicle or a control unit dedicated to thepresent invention. Vehicles of the type here concerned are ofcourse often provided with significantly more control units than shown here, as one skilled in the art will surely appreciate.

The present invention according to one aspect relates to a motorvehicle 500 which is schematically shown in Fig. 3. The motorvehicle 500 comprises an engine 501 forming part of apowertrain 502 which drives driving wheels 503, 504. The motorvehicle 500 further comprises an exhaust treatment system 505,and a control unit 510, which corresponds to the above-mentioned control unit 400 in Fig. 2, and which is arranged to control the function in the engine 501. 22 The invention is of course not in any way restricted to the embodiments described above. On the contrary, many possibilities to modifications thereof will be apparent to a personwith ordinary skill in the art without departing from the basic idea of the invention such as defined in the appended claims.

Claims (15)

1. A method for determining a desired gap between a presentmotor vehicle and a lead vehicle travelling ahead of the presentvehicle, comprising the step of:- identifying that a lead vehicle is travelling ahead of thepresent vehicle, characterised in that it further comprises the steps of: - collecting data relating to a road gradient along anexpected travelling route ahead of the present vehicle, - based on said data, simulating a future speed profile of thepresent vehicle for at least a present mode of operation ofthe present vehicle, - based on said simulated future speed profile, determining a desired gap to the lead vehicle.

2. The method according to claim 1, further comprising thestep of: - determining whether the lead vehicle is configured so thatat a certain gap, a reduction in air resistance felt by thepresent vehicle is expected.

3. The method according to claim 1 or 2, wherein saidsimulation of an expected future speed profile is performed independently of the lead vehicle.

4. The method according to any one of the preceding claims, wherein the desired gap to the lead vehicle is determined based 24 on the expected future speed profile within a predetermined time period or within a predetermined distance interval.

5. The method according to any one of the preceding claims,wherein the step of simulating a future speed profile comprisesdetermining an expected future speed interval within which thevehicle speed is expected to vary during travel along the expected travelling route.

6. The method according to claim 5, wherein the desired gapto the lead vehicle is determined based on the size of said speed interval.

7. The method according to any one of claims 1-6, whereinthe step of determining a desired gap to the lead vehicle comprises modifying a predefined preferred gap.

8. The method according to any one of claims 1-6, whereinthe step of determining a desired gap to the lead vehiclecomprises selecting said desired gap from a predefined set of selectable gaps.

9. The method according to any one of the preceding claims,wherein the speed of the present vehicle in the present mode of operation is controlled by a cruise control system.

10. The method according to any one of the preceding claims,wherein the step of determining a desired gap to the lead vehiclecomprises furnishing a basis to a driver of the vehicle for determining said desired gap.

11. The method according to any one of the preceding claims,further comprising the step:- adjusting the speed of the present vehicle so that the desired gap is achieved.

12. A computer program comprising computer program code forcausing a computer to implement a method according to any oneof the claims 1-11 when the computer program is executed in the computer.

13. A computer program product comprising a non-transitorydata storage medium which can be read by a computer and onwhich the program code of a computer program according to c|aim 12 is stored.

14. An electronic control unit (400, 510) of a motor vehicle(500) comprising an execution means (401), a memory (402)connected to the execution means and a data storage medium(403) which is connected to the execution means and on whichthe computer program code of a computer program according to c|aim 12 is stored.

15. A motor vehicle (500) comprising an electronic control unit(400, 510) according to c|aim 14.