SE540963C2

SE540963C2 - A method for determining a change in air resistance felt by a motor vehicle

Info

Publication number: SE540963C2
Application number: SE1551395A
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: 2019-01-29
Also published as: SE1551395A1; DE102016012465B4; BR102016023026A2; DE102016012465A1

Abstract

A method for determining a change in air resistance felt by a motor vehicle as its distance to a lead vehicle travelling ahead of the motor vehicle changes, wherein said motor vehicle comprises an engine for transmission of a driving force to at least one driving wheel. The method comprisesa) detecting a distance to said lead vehicle and storing data relating thereto,b) determining the driving force transmitted by the engine and storing data relating thereto,c) estimating a driving resistance felt by the vehicle and storing data relating thereto,wherein steps a-c are carried out at least on a first occasion and on a second occasion, between which occasions the distance to the lead vehicle has changed. It further comprises the step: d) based on said stored data relating to the detected distance, the driving force, and the driving resistance, estimating a change in air resistance felt by the vehicle related to a change in the distance to the lead vehicle.

Description

A method for determining a change in air resistance felt bv a motor vehicle TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for determining a change in air resistance felt by a motor 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.

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.

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.

For a driver of a motor vehicle, or a haulage company owning the vehicle, it is of interest to know how the energy consumption of the vehicle is affected by the driving pattern. For example, it is of interest to know how the energy consumption can be influenced by varying the distance to a lead vehicle travelling ahead of the vehicle, so that the distance can be controlled to an optimum distance from an energy consumption point of view.

WO2013/147682 discloses a method for adapting the speed of a motor vehicle such that it travels at a distance 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, the method requires knowledge of many input parameters for determining the air resistance felt by the vehicle, which parameters may not always be readily available.

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 change in air resistance felt by a motor vehicle as a lead vehicle travelling ahead of the motor vehicle is detected and as its distance to said lead vehicle changes. In particular, it is an objective to achieve a simplified way of determining such a change, which can be performed also with few input parameters.

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: a) detecting a distance to said lead vehicle and storing data relating thereto, b) determining the driving force F_driving transmitted by the engine and storing data relating thereto, c) estimating a driving resistance F_res felt by the vehicle and storing data relating thereto, wherein steps a-c are carried out at least on a first occasion and on a second occasion, between which occasions the distance to the detected lead vehicle has changed, and wherein the method further comprises the step: d) based on said stored data relating to the detected distance, the driving force F_driving, and the driving resistance F_res, estimating a change in air resistance felt by the vehicle related to a change in the distance to the lead vehicle, wherein step d comprises comparing said stored data relating to the estimated driving resistance to said stored data relating to the detected driving force for each of said occasions, and, based on a difference between those, estimating said change in air resistance.

By first comparing the estimated driving resistance and the detected driving force, or the traction force, of the vehicle on a first occasion, and thereafter on at least a second occasion on which the distance to the lead vehicle has changed, it is possible to deduct a distance dependence of the driving resistance, and thereby estimate the slipstream effect. Thus, very few input parameters are needed to estimate the change in air resistance felt by the vehicle related to a change in distance to the lead vehicle.

The method according to the present invention relies on an estimation of the driving resistance F_res affecting the vehicle and a measurement of the driving force F_driving transmitted by the engine on at least two different occasions to determine a change in air resistance felt by the vehicle between those occasions, e.g. to estimate a slipstream effect. The change in air resistance that depends on a change in distance to a lead vehicle is hereinafter also referred to as an inter-vehicle distance dependent change in air resistance. This is a way of estimating a change in air resistance which requires few input parameters and which can therefore be applied in many different vehicles. The driving force is usually well known since this may, according to prior art, be calculated straightforwardly with the use of the torque emitted by the combustion engine. This torque is usually specified in the vehicle’s control system. With the use of gear ratio and wheel diameter, the torque can be converted into a driving force acting on the vehicle’s driving wheels.

The driving resistance felt by the vehicle during operation, i.e. the resultant of the forces impacting the vehicle, can be estimated in many different ways and taking different terms into account depending on the desired accuracy of the estimation. For example, a simple model of the driving resistance used in the method according to the invention may include terms describing a total force F_tot acting on the vehicle and a gravitational force F_grav, and an error term F_error. In this case, all forces that vary, such as roll resistance, air resistance etc., are included in the error term. Also a possible slipstream effect will be included in this term, and by determining how F_error varies with the inter-vehicle distance, it is possible to roughly estimate the slipstream effect. For better accuracy, the model may include terms describing e.g. a total force F_tot, a gravitational force F_grav, a rolling resistance force F_roll, an air resistance force F_air, and a force F_pt due to frictional losses in a powertrain of the vehicle. A separate model may be included for each of those terms. According to the invention, an inter-vehicle distance dependent air resistance may be included as a separate slipstream term F_slipstream in such a model, or included in a general air resistance term. A term F_error describing a model error can also be used to estimate the slipstream effect.

An advantage of the method according to the invention is that models for estimating a driving resistance of a vehicle, usually based on a force equation relating the forces acting on the vehicle, are already used in modern vehicles for different purposes. These purposes include, but are not limited to, predicting a behaviour of the vehicle as a road gradient changes, estimating a mass of the vehicle, and ensuring an adequate function of cruise controls, gear shifting systems and other systems used in the vehicle. Expanding of such existing models to also include an estimation of the slipstream effect, or the inter-vehicle distance dependent air resistance, can be easily implemented in a vehicle without the need to install additional sensors, etc.

The method according to the invention may be initiated as a lead vehicle is detected in front of the motor vehicle. Following this detection, steps a-c are conducted. If the motor vehicle detects another lead vehicle, e.g. as a result of an overtaking, the method should typically be reinitiated since the slipstream effect caused by the “new” lead vehicle may differ from that caused by the previous lead vehicle. If the motor vehicle loses track of the lead vehicle, such as in a curve or when the lead vehicle passes a top of a hill, the method may be reinitiated as the lead vehicle is detected again, unless it can be established that the detected vehicle is identical to the previously detected lead vehicle.

The method according to the invention can be used to give a driver of the vehicle a quick feedback as to how much fuel can be or has been saved by driving close to a lead vehicle. It can also be used as a basis for automatically controlling the vehicle to drive at a certain distance to a lead vehicle.

The distance to the lead vehicle can, according to prior art, be determined using e.g. radar technology, camera information, map data in combination with GPS (global positioning system) technology, or some other known technique.

According to one embodiment of the invention, steps a-c are repeated with a predetermined frequency. The frequency can be varied depending on the desired accuracy, wherein generally a higher frequency results in a better accuracy. For example, a frequency of 1 Hz can be used. If steps a-c are conducted also for other purposes in a motor vehicle, the frequency may be selected such that it is suitable also for those other purposes. For example, a frequency of 100 Hz may be suitable. Such a higher frequency reduces noise levels. Step d may e.g. be repeated with the same frequency as steps a-c or with a different frequency, or when a predefined condition is fulfilled.

According to one embodiment of the invention, steps a-c are repeated based on a detected distance to the lead vehicle. In this embodiment, computing power can be saved by only repeating steps a-c when needed for the accuracy of the estimation of the slipstream effect.

According to one embodiment of the invention, steps a-c are repeated each time the detected distance to the lead vehicle has changed by at least a predetermined distance from the previous occasion, wherein the predetermined distance is within a range of 5-50 m. Preferably, the predetermined distance is within a range of 10-20 m. A small change in distance between each occasion on which steps a-c are repeated results in a high accuracy. By increasing the amount that the distance is allowed to change between each occasion, computing power may be saved on the expense of accuracy. Exceeding the upper limit affects the accuracy of the estimation since the slipstream effect may differ significantly between each occasion and since the number of useful occasions for estimating the slipstream effect will be limited.

According to one embodiment of the invention, step d is repeated based on one of a predetermined frequency and a detected distance to the lead vehicle. As described above in connection with steps a-c, step d may be repeated each time the detected distance to the lead vehicle has changed by at least a predetermined distance from the previous occasion, wherein the predetermined distance is within a range of 5-50 m, preferably 10-20 m. By repeating step d based on a detected distance and steps a-c with a predetermined frequency, it is possible to adjust the method to optimise accuracy and/or resolution. A small change in distance between each occasion on which step d is repeated increases the resolution but reduces the accuracy. Step d may also e.g. be repeated more infrequently at large inter-vehicle distances and more often at small distances, where the slipstream effect is expected to be more important.

According to one embodiment of the invention, a model used in step c comprises at least a term F_roll relating to a rolling resistance of the vehicle and a term F_air relating to an air resistance felt by the vehicle. The air resistance and the rolling resistance are important factors affecting the total driving resistance of the vehicle and these terms are therefore useful to include in such a model in order to increase the accuracy in the determination of the slipstream effect. As already mentioned, the air resistance term F_air may be modelled to include an intervehicle distance dependence, or a separate term F_slipstream may be introduced. The model used is preferably based on a force equation relating the forces acting on the vehicle.

According to one embodiment of the invention, said model further comprises a term F_pt relating to frictional losses in a powertrain of the vehicle. Including this term increases the precision in the estimation of the change in air resistance.

According to one embodiment of the invention, the method further comprises determining a mass m of the vehicle, a current road gradient a, and an acceleration a of the vehicle. These parameters should all be included when modelling the total driving resistance felt by the vehicle since they have a large impact on the forces acting on the vehicle. The mass m can e.g. be determined from estimation based on the force equation, or from an estimation based on a suspension of the vehicle. The road gradient can also be obtained in many different ways with a large accuracy. The acceleration a is usually calculated from the velocity of the vehicle, which is given as a signal from e.g. a road speed sensor.

According to one embodiment of the invention, said mass m, said road gradient a, and said acceleration a are used in the estimation of the driving resistance F_res in step c. The total force F_tot = m<*>a and the gravitational force F_grav affecting the vehicle in this case does not need to be modelled, but can be introduced as known parameters in a model used in said estimation. This improves the accuracy in the later estimation of the change in air resistance.

According to one embodiment of the invention, step d comprises determining the air resistance felt by the vehicle as a function of the distance to the lead vehicle. By determining the air resistance felt by the vehicle as a function of the inter-vehicle distance, it is possible to predict changes in air resistance that will occur if the distance is changed by a certain amount, and thereby also to predict changes in energy consumption of the vehicle. Thus, this may include interpolating, extrapolating, curve fitting, etc. to obtain a function. The function may be used for giving feedback to a driver or an owner of the vehicle about fuel savings or potential fuel savings.

According to one embodiment of the invention, the method further comprises utilising said determined change in the air resistance felt by the vehicle as a function of the distance to the lead vehicle to control a speed of said motor vehicle such that said air resistance is reduced. This is a way of controlling the vehicle such that the energy consumption of the vehicle is minimised. The function can be used e.g. as an input parameter to a cruise control or to another control system of the vehicle.

According to one embodiment of the invention, the method further comprises utilising the determined change in air resistance as an input parameter in a model used for estimating a driving resistance felt by the vehicle. In this way, the model used for estimating the driving resistance F_res can be continuously improved.

According to one embodiment of the invention, the method further comprises communicating data related to said determined change in air resistance to a driver of the vehicle. In this way, the driver can, if desirable, adapt the driving of the vehicle such that air resistance is minimised. Data can e.g. be communicated visually using a display or similar, or orally using e.g. loudspeakers.

According to one embodiment of the invention, the method further comprises determining a reduction in energy consumption achieved by driving at a distance to a lead vehicle such that the air resistance felt by the vehicle is reduced. This can be during a certain time interval or during a certain distance interval, such as during the last 30 minutes or during the last 1 km. The energy consumption is easier to relate to and more relevant for a driver or a haulage company than the air resistance itself. Data relating to the reduction in energy consumption can be communicated to a driver as discussed above.

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 schematically shows a vehicle according to the invention, Fig. 2 is a flow chart showing a method according to the invention, Fig. 3 is a graph schematically showing a slipstream effect, and Fig. 4 schematically shows a control unit according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION A motor vehicle according to the present invention is shown in fig. 1. The motor vehicle 500 may e.g. be a passenger car, a truck or a bus, comprising an engine 501. The engine 501 is comprised in 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 is arranged to control the function in the engine 501 .

Generally, several forces act on the vehicle during operation. Those forces include a driving force F_driving, sometimes also referred to as a traction force, an air resistance force F_air, a rolling resistance force F_roll, a gravitational force F_grav, and a frictional force F_pt due to friction in the powertrain. The driving force F_driving acting on the driving wheels is the torque delivered by the engine, converted into a force using a present gear ratio and wheel diameter. The air resistance force F_air can be expressed as F_air = C_air*v<2>, wherein v is the vehicle velocity and C_air is a constant which depends on the air density, the vehicle’s area in the direction of travel, and the vehicle’s air resistance coefficient. This in turn depends on the design of the vehicle’s surfaces meeting the wind, where in principle all external details on the vehicle have an impact. The air resistance coefficient may therefore be difficult to calculate, and consequently there is a risk that the air resistance force is estimated incorrectly. The air resistance force is also strongly speed dependent, and consequently an incorrect estimation results in an increased impact with higher vehicle speeds.

The rolling resistance F_roll can be expressed as F_roll = C_roll*m*g cos ?, wherein m is the mass of the vehicle, g is the gravitational constant, a is the road gradient and C_roll is a rolling resistance coefficient, which primarily depends on the vehicle’s tyres/wheels. The rolling resistance coefficient may also be difficult to determine exactly.

The gravitational force F_grav may be expressed as F_grav = m*g sin ?.

The frictional force F_pt may in some cases be difficult to differentiate and may be partly or entirely included in the rolling resistance force F_roll or in the driving force F_driving in a model of the driving resistance affecting the vehicle.

A resultant force F_tot represents the force that is converted to an actual acceleration a of the vehicle according to F_tot = m*a.

A force equation describing the forces acting on a vehicle may be expressed as follows: F_tot = F_driving - F_air - F_roll - F_grav - F_pt - F_error, wherein F_driving is the driving force applied by the engine and wherein F_error is a model error accounting for the difference between the actual driving force F_driving and the modelled driving resistance F_res represented by F_res = F_tot F_air F_roll F_grav F_pt.

In other words, F_error = F_driving - F_res.

Ideally, the model error F_error = 0 N if the model coincides perfectly with reality, which is however often not the case.

In the case of a vehicle moving forward at a constant speed on a flat road, F_tot = F_grav = 0 N. All terms F_air, F_roll and F_pt will be negative forces acting to slow the vehicle down, while the driving force F_driving is positive, acting to keep the speed constant.

A method according to an embodiment of the invention is shown in the flow chart in Fig. 2. In this embodiment, a motor vehicle is travelling forward along a road section as it, in a step S0, encounters and detects another vehicle, a lead vehicle, travelling ahead of the vehicle. In a step S 1 , a distance to the lead vehicle is detected on a first occasion. This can be done using e.g. radar technology, camera information, map data in combination with GPS (global positioning system) technology, or the like. Data relating to the detected distance is stored on a data storage medium. In a step S2, which can be performed simultaneously with the step S1 , the driving force F_driving transmitted by the engine is determined, as described above, and data relating thereto are stored. In a step S3, which can also be performed simultaneously with the step S1 , the driving resistance felt by the vehicle is estimated using an estimation model, and data relating thereto are stored. The driving resistance may also be, at least in part, determined from measured data relating to e.g. road gradient and/or frictional losses in the power train.

The motor vehicle now moves closer to the lead vehicle and as the distance between the vehicles decreases, it is, in a step S4, checked whether steps S1-S3 have been carried out at least twice and at different detected distances from the lead vehicle. If not, steps S1-S3 are repeated on at least one more occasion. If steps S1-S3 have been carried out at least twice and at different detected distances from the lead vehicle, a step S5 is carried out. In this step, a change in air resistance felt by the vehicle related to a change in the distance to the lead vehicle is estimated based on the stored data relating to the detected distance, the driving force, and the driving resistance.

The estimation of the driving resistance is preferably based on the force equation discussed above. In one embodiment, the mass m, the acceleration a and the road gradient a are determined, which will be further discussed below. With those parameters wellknown, the total force F_tot and the gravitational force F_grav can be determined, and a model for estimating the driving resistance in step S3 thus has unknown terms F_air relating to air resistance, F_roll relating to rolling resistance, and F_pt relating to frictional losses in the powertrain. The model also includes an error term F_error, describing a difference between the determined driving force F_driving and the modelled driving resistance F_res: F_error = F_driving - F_res.

Each time step S3 is repeated, a value of F_error is generated and stored. In step S5, these generated values F_error can be mapped against the corresponding detected inter-vehicle distances, such that a distance dependence is revealed. Depending on the precision in the models used to describe the remaining terms F_roll, F_air, and F_pt, a revealed difference in F_error as the inter-vehicle distance changes can, at least in part, be attributed to the slipstream effect. This is very schematically shown in fig. 3, wherein the error term F_error is plotted as a function of distance to the lead vehicle. As can be seen, the error term F_error decreases as the motor vehicle approaches the lead vehicle, which can be attributed to the slipstream effect.

In a different embodiment, the slipstream effect is modelled as a separate term F_slipstream which is included in the model used to estimate the driving resistance. This term will have an opposite sign with respect to the air resistance term F_air, which in this case is not modelled as dependent on the inter-vehicle distance, and its absolute value will never be larger than the air resistance. For example, F_slipstream may be estimated as: F_slipstream = F_driving - F_tot - F_grav - F_air - F_roll - F_pt -F_error.

As described above, the term F_slipstream is repeatedly estimated for different values of the inter-vehicle distance in order to reveal a distance dependence. For some models, the previously estimated F_slipstream may be used as an input parameter in the subsequent estimation on the next occasion in order to improve the accuracy of the estimation.

Steps S1-S3 can be repeated at a predetermined frequency or as a certain condition is fulfilled, such as when the distance to the lead vehicle has changed by a predetermined amount, such as by a distance within a range of 5-50 m, preferably 10-20 m. Step S5 can be performed each time steps S1-S3 are performed, or only when some predefined condition is fulfilled. This can be e.g. when steps S1-S3 have been repeated a certain amount of times. Step S5 may include determining the air resistance, or the reduction in air resistance, felt by the vehicle as a function of the distance to the lead vehicle, i.e. fitting the estimated values to a function. The function may be used to present an achieved or a potential reduction in air resistance to a driver, e.g. on a display or by audio means, such as by means of a loudspeaker.

The determined change in air resistance as the distance to the lead vehicle changes may also be used to control the speed of the vehicle such that the air resistance is reduced. For example, a function describing the related change can be used as an input parameter to a cruise control or to another control system of the vehicle.

The change in air resistance can also be used to determine a reduction in energy consumption achieved by driving at a distance to a lead vehicle such that the air resistance felt by the vehicle is reduced. For example, the reduction energy consumption can be presented to a driver of the vehicle as a graph showing a reduction in fuel consumption achieved as a function of distance.

The vehicle mass is typically determined by a mass estimation algorithm based on either information from a suspension of the vehicle or on a measured or estimated moment of inertia of the vehicle. The acceleration a is typically determined from the velocity v of the vehicle, given by e.g. a road speed sensor.

The road gradient a 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.

One skilled in the art will appreciate that a method for determining a change in air resistance felt by a motor vehicle as its distance to a lead vehicle travelling ahead of the motor vehicle changes 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 (readonly memory), PROM (programmable read-only memory), EPROM (erasable PROM), flash memory, EEPROM (electrically erasable PROM), a hard disc unit, etc.

Fig. 4 depicts schematically an electronic control unit 400 of a vehicle, corresponding to the electronic control unit 510 of the vehicle 500 shown in fig. 1 , 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 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 change in air resistance felt by a motor vehicle (500) as its distance to a detected lead vehicle travelling ahead of the motor vehicle (500) changes, wherein said motor vehicle comprises an engine (501 ) for transmission of a driving force to at least one driving wheel (503, 504), characterised in that it comprises the steps: a) detecting a distance to said lead vehicle and storing data relating thereto, b) determining the driving force F_driving transmitted by the engine (501 ) and storing data relating thereto, c) estimating a driving resistance F_res felt by the vehicle (500) and storing data relating thereto, wherein steps a-c are carried out at least on a first occasion and on a second occasion, between which occasions the distance to the detected lead vehicle has changed, and wherein the method further comprises the step: d) based on said stored data relating to the detected distance, the driving force F_driving, and the driving resistance F_res, estimating a change in air resistance felt by the vehicle (500) related to a change in the distance to the lead vehicle, wherein step d comprises comparing said stored data relating to the estimated driving resistance F_res to said stored data relating to the detected driving force F_driving for each of said occasions, and, based on a difference between those, estimating said change in air resistance.

2. The method according to claim 1 , wherein steps a-c are repeated with a predetermined frequency.

3. The method according to claim 1 , wherein steps a-c are repeated based on a detected distance to the lead vehicle, preferably each time the detected distance to the lead vehicle has changed by at least a predetermined distance from the previous occasion.

4. The method according to any one of the preceding claims, wherein step d is repeated based on one of a predetermined frequency and a detected distance to the lead vehicle.

5. The method according to any one of the preceding claims, wherein a model used in step c comprises at least a term F_roll relating to a rolling resistance of the vehicle (500) and a term F_air relating to an air resistance felt by the vehicle (500), and preferably wherein said model further comprises a term F_pt relating to frictional losses in a powertrain (502) of the vehicle (500).

6. The method according to any one of the preceding claims, further comprising determining a mass m of the vehicle (500), a current road gradient a, and an acceleration a of the vehicle (500), preferably wherein said mass m, said road gradient a, and said acceleration a are used in the estimation of the driving resistance F_res in step c.

7. The method according to any one of the preceding claims, wherein step d comprises determining the air resistance felt by the vehicle (500) as a function of the distance to the lead vehicle, preferably wherein the method further comprises utilising said determined change in the air resistance felt by the vehicle (500) as a function of the distance to the lead vehicle to control a speed of said motor vehicle (500) such that said air resistance is reduced.

8. The method according to any one of the preceding claims, further comprising utilising the determined change in air resistance as an input parameter in a model used for estimating a driving resistance felt by the vehicle (500).

9. The method according to any one of the preceding claims, further comprising communicating data related to said determined change in air resistance to a driver of the vehicle (500).

10. The method according to any one of the preceding claims, further comprising determining a reduction in energy consumption achieved by driving at a distance to a lead vehicle such that the air resistance felt by the vehicle (500) is reduced.

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

12. 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 1 1 is stored.

13. 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 11 is stored.

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