SE541221C2

SE541221C2 - A method for determining a desired gap between a motor vehicle and a lead vehicle

Info

Publication number: SE541221C2
Application number: SE1551397A
Authority: SE
Inventors: Bas Oremus; Fredrik Roos; Mikael Ögren; Oscar Flärdh
Original assignee: Scania Cv Ab
Priority date: 2015-10-29
Filing date: 2015-10-29
Publication date: 2019-05-07
Also published as: DE102016012466A1; SE1551397A1

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.

Description

A method for determining a desired gap between a motor vehicle and a lead vehicle TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for determining a desired gap between a present motor vehicle and a lead vehicle travelling ahead of the present vehicle according to the preamble of claim 1. The invention further relates to a computer program, a computer program product, an electronic control unit, and a motor vehicle. By a motor vehicle is here intended a vehicle which is powered by an internal combustion engine and/or by an electric motor. In particular, but not exclusively, the method is intended 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 the vehicle. A wide variety of different systems have therefore been developed for reducing fuel consumption, e.g. fuel-efficient 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. The cruise control then provides the engine system in the vehicle with a reference speed v_ref, where the reference speed v_ref is used to control the engine. The set speed v_set may thus be seen as an input signal to the cruise control, while the reference speed v_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 reference speed v_ref, which corresponds to the set speed v_set requested by the driver. The value of the reference speed v_ref is here changed only when the user himself adjusts the set speed v_set while driving.

Today there are also cruise controls, so-called economical cruise controls, such as Ecocruise controls and similar cruise controls, which try to estimate the current driving resistance and also have knowledge about the historical driving resistance. An experienced driver who drives a motor vehicle without a cruise control may reduce fuel consumption by adjusting the driving to the characteristics of the road ahead, so that unnecessary braking and/or fuel-consuming acceleration may be avoided. In a further development of these economical cruise controls, the ambition is to mimic the experienced driver's adjustment of driving the motor vehicle based on knowledge about the road ahead, so that fuel consumption may be kept at a level as low as possible.

One example of such a further development of an economical cruise control is a "Look Ahead" cruise control (LACC), i.e. a strategic cruise control using knowledge about road sections ahead, i.e. knowledge about the appearance of the road ahead, in order to determine the appearance of the reference speed v_ref. Here, the reference speed v_ref is thus permitted, within a permitted speed interval ?v_permitted, to differ from the set speed v set selected by the driver in order to achieve a more fuel-efficient driving. For example, by taking topographic information about the road section ahead of the vehicle into account, the speed may be temporarily increased before e.g. an uphill slope, so that downshifting to a lower transmission mode can be avoided or delayed. In this way, fuel consumption can be reduced. Also information about road curvature and legal speed limits along the road section ahead of the vehicle can be taken into account.

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

Modern motor vehicles can be equipped with radar technology to measure 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 one example, such a system can comprise an actuating device with which the driver can manually set a position that corresponds to a given gap to a lead vehicle. Such an actuating device can e.g. have five different positions that correspond to discrete increments of distance to the lead vehicle between 10 and 75 meters, corresponding to time gaps within the range of 1-4 seconds. 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.

WO201 3/1 47682 discloses a method for adapting the speed of a motor vehicle such that it travels at a gap from a lead vehicle which is optimised for reducing the air resistance felt by the vehicle. Factors such as a front area and a load configuration of the lead vehicle, the present speed of the vehicle, and wind direction and wind force of the ambient air are taken into account.

However, air resistance is only one of many factors affecting the total energy consumption of the vehicle. Driving close to a lead vehicle in order to exploit the slipstream effect may give rise to that other fuel saving measures, such as described above, cannot be fully utilised. For example, on a topographically varying road, driving close to a lead vehicle may necessitate braking more often than would otherwise be necessary, thereby wasting kinetic energy and increasing the total energy consumption of the vehicle.

SUMMARY OF THE INVENTION It is a primary objective of the present invention to achieve an, in at least some aspect, improved way of determining a desired gap to a lead vehicle travelling ahead of a present motor vehicle. In particular, it is an objective to achieve a way to determine a desired gap to a lead vehicle such that the total energy consumption of the vehicle when driving along a travelling route at 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 the primary objective is achieved by means of the method initially defined, which is characterised in that it comprises the steps of: - 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.

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

The method according to the invention thus takes a foreseen optimised speed pattern of the vehicle into account when determining a desired gap to the lead vehicle. In other words, the desired gap to the lead vehicle is determined so that fueleconomising systems in the vehicle, such as a fuel-economising cruise control, can be fully utilised without having to risk unnecessary braking of the vehicle. If small speed variations are foreseen, the vehicle may be controlled to travel close to the lead vehicle, while as if the foreseen speed variations are large, the vehicle may be controlled to travel at a larger distance from the lead vehicle.

The determined desired gap may be used in the control of the vehicle so that the desired gap to the lead vehicle is attained and held. This can, on one hand, be achieved by automatically controlling the gap to the lead vehicle, either in time or in 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 desired gap, such as increasing or decreasing a current gap to the lead vehicle. The driver can for example be prompted to select a desired gap based on a presented recommendation. In order to be able to control the vehicle such that a desired gap is reached, the actual current distance to the lead vehicle is continuously measured and evaluated to learn whether this current distance should be decreased or increased. Measuring the distance to the lead 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 road gradient along the expected travelling route ahead of the present vehicle, such as traffic data, may be used to determine the future speed profile of the vehicle. Also parameters such as a transmission mode, a mode of operation, a current actual vehicle speed, at least one engine characteristic, e.g. maximum and/or minimum engine torque, a vehicle weight, an air resistance, a rolling resistance, a gear ratio in the gearbox and/or the power train of the vehicle, and a wheel radius, may be taken into account.

According to one embodiment of the invention, it further comprises the step of: - determining whether the lead vehicle is configured so that at a certain gap, a reduction in air resistance felt by the present vehicle is expected.

The step of determining a desired gap to the lead vehicle may thus be performed based on the configuration of the lead vehicle. In this way, it is possible to determine a relatively large desired gap to the lead vehicle in case no energy savings through slipstream effects can be expected, based on the configuration of the lead vehicle. If the lead vehicle is so configured that energy savings can be expected, the desired gap to the lead vehicle is determined so that slipstream effects can be exploited as discussed above. For vehicles which are too small to generate a slipstream effect for the present vehicle, a relatively large gap can be recommended or set, so that other fuel-economising systems or algorithms of the vehicle can be fully utilised. The front area and the load configuration of the lead vehicle may be detected and evaluated using e.g. camera information and image processing.

According to one embodiment of the invention, said simulation of an expected future speed profile is performed independently of the lead vehicle. In other words, this simulation is performed also when no lead vehicle is present in front of the present vehicle, e.g. at a predetermined frequency. No information about an expected speed profile or similar of the lead vehicle is thus needed.

According to one embodiment of the invention, the desired gap to the lead vehicle is determined based on the expected future speed profile within a predetermined time period or within a predetermined distance interval. The foreseen speed variation within the nearest time period or distance interval is thereby used to determine the desired gap, such as within the upcoming 2 minutes or within the upcoming 2 km, or less. The interval may e.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 speed variation is instead taken into account on a later occasion, when being closer in time. The interval should be chosen such that there is sufficient time to adjust the gap before a foreseen speed variation.

According to one embodiment of the invention, the step of simulating a future speed profile comprises determining an expected future speed interval within which the vehicle speed is expected to vary during travel along the expected travelling route. Such a speed interval can be obtained based on the expected topography along the expected travelling route and can be used to determine an accurate desired gap to the lead vehicle. The upper and lower limits of the expected speed interval usually vary depending on topography. Large topographic variations result in a relatively broad speed interval, while as for a flat road, the speed interval is expected to be narrow.

According to one embodiment of the invention, the desired gap to the lead vehicle is determined based on the size of said speed interval. This is an accurate way of determining a suitable gap to the lead vehicle. For a broad speed interval, the desired gap is preferably determined to be relatively large, and for a narrow speed interval, the desired gap is determined to be relatively small.

According to one embodiment of the invention, the step of determining 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 the vehicle or by a haulage company. It may further be a smallest possible gap, as determined e.g. by conditions set by an insurance company or similar. The predefined preferred gap is modified by taking the simulated speed profile into account, such that slipstream effects can be properly utilised. A driver of the vehicle may be asked to approve overriding of the predefined preferred gap before the gap is actually adjusted toward the desired gap.

According to one embodiment of the invention, the step of determining a desired gap to the lead vehicle comprises selecting said desired gap from a predefined set of selectable gaps. This is a fast and sufficiently accurate way of determining a desired gap. For example, a set of selectable gaps can comprise five gaps from 1.5 to 4 seconds, of which one can be selected or recommended in dependence on the size of an expected future speed interval, using thresholds as a basis for selection or recommendation. Alternatively, the desired gap can be calculated as a function of the size of the expected future speed interval.

According to one embodiment of the invention, the step of simulating an expected future speed profile is repeatedly performed with a predetermined frequency. In this way, it is ensured that the desired gap is always determined based on an up-to-date speed profile. The step of determining a desired gap can be performed with the same said predetermined frequency, or with a different frequency. It can also be performed irregularly, such as only when the expected future speed profile changes significantly. For example, if the size of an expected future speed interval deviates from the size of a previously simulated such interval by more than a predetermined threshold, the step of determining a desired gap can be carried out. This step may also be conducted only when a lead vehicle is identified within a certain predetermined distance.

According to one embodiment of the invention, the speed of the vehicle in the present mode of operation is controlled by a cruise control system. A cruise control system offers possibilities for setting and keeping the determined desired gap to the lead vehicle, and also for fuel-economic driving of the vehicle when the determined desired gap is large.

According to one embodiment of the invention, the step of determining a desired gap to the lead vehicle comprises furnishing a basis to a driver of the vehicle for determining said desired gap. This gives a driver of the vehicle good control over parameters affecting the energy consumption of the vehicle. The driver can e.g. be presented with a recommended gap or with a recommended adjustment of the present gap and prompted to accept it. The basis can e.g. be furnished by visual means, such as on a display, or by audio means, such as by means of a loudspeaker.

According to one embodiment of the invention, it further comprises the step: - adjusting the speed of the present vehicle so that the desired gap is achieved.

The speed can be automatically controlled using e.g. a cruise control, in which case a reference speed v_ref of the cruise control is adjusted so that the desired gap is achieved. The desired gap is in this case used as an input signal to the cruise control. The cruise control may be configured to automatically perform the adjustment of the speed based on the determined desired gap. The speed may be adjusted in small increments, such that no abrupt changes of the speed and of the gap between the present vehicle and the lead vehicle occurs. The speed of the vehicle may also be controlled by means of a control system configured to automatically control the operation of the vehicle based on a measured gap to the lead vehicle so that the desired gap is achieved and held. The cruise control is thereby 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 comprising computer program code for causing a computer to implement the proposed method when the computer program is executed in the computer.

According to a further aspect of the invention, at least the primary objective is achieved by a computer program product comprising a non-transitory data storage medium which can be read 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 motor vehicle comprising an execution means, a memory connected to the execution means and a data storage medium which is connected to the execution means and on which the computer 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 a motor vehicle comprising the proposed electronic control unit. The motor 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 the invention, Fig. 2 schematically shows a control unit according to the invention, and Fig. 3 schematically shows a vehicle according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION A method according to an embodiment of the present invention is schematically shown in the flow diagram in fig. 1. When the method is initiated, a motor vehicle is travelling forward along a road section. In a step S 1, data are collected relating to a road gradient along an expected travelling route ahead of the vehicle. The road gradient may be obtained in various different ways. It may be determined on the basis of map data, e.g. from digital maps containing topographical information, in combination with positioning information, e.g. GPS (global positioning system) information. The positioning information may be used to determine the location of the vehicle relative to the map data so that the road gradient can be extracted from the map data. Various present-day cruise control systems use map data and positioning information. Such systems may then provide the map data and positioning information required for the method according to the present invention, thereby minimising the additional complexity involved in determining the road gradient.

The road gradient may be obtained on the basis of a map in conjunction with GPS information, from radar information, from camera information, of information from another vehicle, from positioning information and road gradient information stored previously on board, or from information obtained from traffic systems related to the expected travelling route. In systems where 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.

In a subsequent step S2, a future speed profile of the vehicle is simulated based on the collected data. The simulation is performed at least for a present mode of operation of the present vehicle, such as for driving with a look-ahead cruise control (LACC) at a set speed v set. In this case, the LACC sets a reference speed v_ref of the vehicle such that the energy consumption of the vehicle is optimised with regard to the topography of the expected travelling route. The LACC may for this purpose be allowed to adjust the reference speed v_ref so that it deviates from the set speed v_set, such as initially described. The deviation from the set speed may result in a foreseen speed variation within the nearest future within an expected future speed interval ?v_expected. This expected future speed interval ?v_expected may of course differ significantly from a permitted speed interval ?v_permitted of the LACC. On a flat road, the expected future speed interval ?v_expected is normally much smaller than the permitted speed interval ?v_permitted, which sets the upper and lower limits within 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 a predetermined rate, e.g. at a rate of 1 Hz, which means that a new simulation result is provided every second. The section of road for which the simulation is conducted represents a predetermined distance ahead of the vehicle, e.g. it might be 1-2 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.

In addition to the aforesaid parameter of road gradient, the simulation may also be based on one or more from among a transmission mode, a mode of operation, a current actual vehicle speed, at least one engine characteristic, e.g. maximum and/or minimum engine torque, a vehicle weight, an air resistance, a rolling 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 a step S3 identified that a lead vehicle is travelling ahead of the present vehicle. This identification can be conducted based on e.g. information from a radar system or from camera information. When such a lead vehicle is identified, also additional information about the configuration of the lead vehicle may be collected. Such information includes a front area and a load configuration of the lead vehicle, obtainable by means of image processing. The additional information can be compared to corresponding information of the present vehicle to evaluate whether the lead vehicle is expected to cause a slipstream effect for the present vehicle if the gap between the vehicles is sufficiently small.

In a step S4, a desired gap to the lead vehicle is determined based on the simulated future speed profile. This can be conducted automatically, for example each time that the future speed profile is simulated, at some other interval, or when some predetermined condition is fulfilled, such as when it is determined that a distance to the identified lead vehicle is less than a predetermined distance. The desired gap is preferably determined based on the size of an expected future speed interval Av expected, in such a way that larger expected speed variations also generates a larger desired gap. For example, the following conditions may be set: | ?? | < a_1: Set the gap to 1 s; a_1? | ?? | < a_2: Set the gap to 2 s; | ?? | ? a_2: Set the gap to 3 s, wherein | ?v | is the size of the expected future speed interval ?v_expected and wherein a_1, a_2 and a_3 are predetermined threshold values. Of course, the number of thresholds used may vary as well as the size of the gaps. A number of selectable gaps may be available in a database, in which case a gap is automatically selected based on the size of the expected future speed interval ?v_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 size of an expected future speed interval ?v_expected, in which case the desired gap can be determined to be any value, e.g. a value within a predefined range.

In another embodiment, determining the desired gap comprises modifying a predefined preferred gap, such as a preferred gap set by a driver or an owner of the vehicle. A modification of the preferred gap by a certain amount, as defined in time or in distance, may be suggested to the driver each time the expected 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 to control the speed of the vehicle, or it may be communicated to the driver of the vehicle so that the driver may take an appropriate action. The radar system of the vehicle can be arranged so as to continuously determine a distance to the lead vehicle. The determined desired gap can thereby be set as a distance to the lead vehicle, whereupon a control system in the vehicle can automatically control the operation of the vehicle so that the desired gap to the lead vehicle is achieved. While the vehicle is controlled so that it drives at a certain distance to the lead vehicle, the expected future speed profile is continuously simulated. On the basis thereof, it is determined whether or not to alter the gap to the lead vehicle.

In one example, a motor vehicle is travelling forward with an activated Look Ahead cruise control. In the motor vehicle, data relating to the road gradient along an expected future travelling route of the vehicle are continuously collected. The collected data are used to simulate an expected future speed profile of the vehicle during the next 1 km for at least a present mode of operation of the vehicle, i.e. with the cruise control activated with a constant set speed v_set. This is performed at a predetermined frequency of 1 Hz. As the motor vehicle approaches another vehicle, it is identified that a lead vehicle is travelling ahead of the present motor vehicle. It is checked whether the lead vehicle is expected to result in a slipstream effect. On the basis of the size of the variations in the expected future speed profile during the coming 400 m, a desired gap to the lead vehicle is determined by selection from a database. The selected desired gap is presented to a driver of the vehicle, who is prompted to accept the presented gap. As the driver accepts the gap, the desired gap is set and the vehicle is controlled such that the gap between the vehicles approaches the desired gap.

In another example, the vehicle may be configured to automatically adjust the gap between the vehicles toward the selected desired gap without prompting the driver to accept such an adjustment. It is also possible to only prompt the driver in case the determined desired gap differs significantly from a predefined preferred gap, or based on some other condition.

One skilled in the art will appreciate that a method for determining a desired gap between a present motor vehicle and a lead vehicle according to the present invention may be implemented in a computer program which, when executed in a computer, causes the computer to conduct the method. The computer program usually takes the form of a computer program product which comprises a suitable digital storage medium on which the computer program is stored. Said computer-readable digital storage medium comprises a suitable memory, e.g. ROM (read-only memory), PROM (programmable read-only memory), EPROM (erasable PROM), flash memory, EEPROM (electrically erasable PROM), a hard disc unit, etc.

Fig. 2 depicts schematically an electronic control unit 400 of a vehicle 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 (digital signal processor, DSP), or a circuit with a predetermined specific function (application specific integrated circuit, ASIC). The execution means 401 is connected to a memory unit 402 which is situated in the control unit 400. A data storage medium 403 is also connected to the execution means and provides the execution means with, for example, the stored program code and/or stored data which the execution means needs to enable it to do calculations. The execution means is also adapted to storing partial or final results of calculations in the memory unit 402.

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

Each of the connections to the respective devices for receiving and sending input and output signals may take the form of one or more from among a cable, a data bus, e.g. a CAN (controller area network) bus, a MOST (media orientated systems transport) bus or some other bus configuration, or a wireless connection.

One skilled in the art will appreciate that the aforesaid computer may take the form of the execution means 401 and that the aforesaid memory may take the form of the memory unit 402.

Control systems in modern vehicles generally comprise a communication bus system consisting of one or more communication buses for connecting together a number of electronic control units (ECUs), or controllers, and various components on board the vehicle. Such a control system may comprise a large number of control units and the responsibility for a specific function may be divided between two or more of them.

In the embodiment depicted, the present invention is implemented in the control unit 400 but might also be implemented wholly or partly in one or more other control units already on board the vehicle or a control unit dedicated to the present invention. Vehicles of the type here concerned are of course 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 motor vehicle 500 which is schematically shown in Fig. 3. The motor vehicle 500 comprises an engine 501 forming part of a powertrain 502 which drives driving wheels 503, 504. The motor vehicle 500 further comprises an exhaust treatment system 505, and a control unit 510, which corresponds to the abovementioned control unit 400 in Fig. 2, and which is arranged to control the function in the engine 501.

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 person with ordinary skill in the art without departing from the basic idea of the invention such as defined in the appended claims.

Claims

1. A method for determining a desired gap between a present motor vehicle and a lead vehicle travelling ahead of the present vehicle, comprising the step 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, wherein said simulation of an expected future speed profile is performed independently of the lead vehicle, characterised in that it further comprises the steps of: - determining whether the lead vehicle is configured so that at a certain gap, a reduction in air resistance felt by the present vehicle is expected, - based on said simulated future speed profile and on the determined configuration of the lead vehicle, determining a desired gap to the lead vehicle.

2. The method according to claim 1, wherein the desired gap to the lead vehicle is determined based on the expected future speed profile within a predetermined time period or within a predetermined distance interval.

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

4. The method according to claim 3, wherein the desired gap to the lead vehicle is determined based on the size of said speed interval.

5. The method according to any one of claims 1-4, wherein the step of determining a desired gap to the lead vehicle comprises modifying a predefined preferred gap.

6. The method according to any one of claims 1-4, wherein the step of determining a desired gap to the lead vehicle comprises selecting said desired gap from a predefined set of selectable gaps.

7. 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.

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

9. 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.

10. A computer program comprising computer program code for causing a computer to implement a method according to any one of the claims 1-9 when the computer program is executed in the computer.

11. A computer program product comprising a non-transitory data storage medium which can be read by a computer and on which the program code of a computer program according to claim 10 is stored.

12. 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 which the computer program code of a computer program according to claim 10 is stored.

13. A motor vehicle (500) comprising an electronic control unit (400, 510) according to claim 12.