Patent Publication Number: US-8972138-B2

Title: Method and module for controlling a vehicle&#39;s speed

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a 35 U.S.C. §§371 national phase conversion of PCT/SE2011/050808, filed Jun. 21, 2011, which claims priority of Swedish Application No. 1050666-5, filed Jun. 23, 2010, the contents of which are incorporated by reference herein. The PCT International Application was published in the English language. 
     FIELD OF THE INVENTION 
     The present invention relates to a method and a module for controlling a vehicle&#39;s speed on the basis of its predicted speed. 
     BACKGROUND TO THE INVENTION 
     Today&#39;s traditional cruise controls maintain a constant reference speed which is usually set by the driver. The value of the reference speed is only altered when the driver him/herself adjusts it while driving. The reference speed is then passed to a control system which controls the vehicle in such a way that, if possible, the vehicle maintains the reference speed. If the vehicle is equipped with an automatic gear change system, its gears change so that it can maintain desired speed. 
     When cruise control is used in hilly terrain, the cruise control system will try to maintain set speed both uphill and downhill. This sometimes results in the vehicle accelerating over the crest of a hill and potentially into a subsequent downgrade on which it has then to be braked to avoid exceeding the set speed, which is a fuel-expensive way of running the vehicle. Even in cases where it does not accelerate over the crest, the vehicle may of course need to be braked downhill to avoid exceeding the set speed. 
     To reduce fuel use on undulating roads in particular, economical cruise controls such as Scania&#39;s Ecocruise® have been developed. The cruise control tries to estimate the vehicle&#39;s current speed and also has knowledge of the historical running resistance. The economical cruise control may also be provided with map data with topographical information. The vehicle is then located on the map, e.g. by means of GPS, and the running resistance along the route ahead is estimated. The vehicle&#39;s reference speed may thus be optimised for different types of road in order to save fuel. 
     An example of a cruise control which uses topographical information is described in the document entitled “Explicit use of road topography for model predictive cruise control in heavy trucks” by Erik Hellström, ISBN: LiTH-ISY-EX-05/3660-SE. The cruise control here is by real-time optimisation, and a cost function is used to define the optimisation criteria. A large number of different solutions are calculated and evaluated, and the solution resulting in least cost is employed. The considerable number of calculations involved makes it necessary to use for the purpose a processor with a suitably large capacity. 
     In other forms of cruise control the number of possible solutions is reduced by choosing instead to iterate from one solution along the vehicle&#39;s intended itinerary. However, the topography of the carriageway, the weight of the vehicle and the performance of the engine may result in differences in demand for processor capacity for determining the reference speed. More calculations are required in the case, for example, of a heavily laden truck with medium-high engine power travelling on an undulating road than in that of a lightly laden truck with a high engine power travelling on a relatively level road. This is because the truck in the first case is likely to accelerate on each downgrade and slow down on each upgrade, whereas in the second case the truck will run quite evenly. 
     The processor incorporated in the system will therefore be subject to relatively large demands, since the load upon the processor may vary greatly from situation to situation. For example, its capacity needs to be large enough to cope quickly with situations where a large number of calculations have to be done. It therefore needs to be dimensioned to cope with such situations despite the fact that they occur during only a limited proportion of the processor time used. 
     The object of the present invention is to propose an improved system for controlling a vehicle&#39;s speed so that the amount of fuel used can be minimised and, in particular, in such a way that the load upon the processor becomes lighter and more even. A further object of the invention is to propose a simplified cruise control which has predictable behaviour compared with other economical cruise controls. 
     SUMMARY OF THE INVENTION 
     At least one of the above objects is achieved by a method for controlling a vehicle&#39;s speed which comprises:
         adopting a desired speed v set  for the vehicle;   determining by means of map data and location data a horizon for the intended itinerary which is made up of route segments with at least one characteristic for each segment;   effecting the following during each of a number of simulation cycles (s) each comprising a number N of simulation steps conducted at a predetermined frequency f:
           making a first prediction of the vehicle&#39;s speed v pred     —     cc  along the horizon with conventional cruise control when v set , is presented as reference speed, which prediction depends on the characteristics of said segment;   comparing the predicted vehicle speed v pred     —     cc  with v min  and v max , which demarcate a range within which the vehicle&#39;s speed is intended to be;   making a second prediction of the vehicle&#39;s speed v pred     —     Tnew  along the horizon when the vehicle&#39;s engine torque T is a value which depends on the result of said comparison in the latest preceding simulation cycle (s−1);   determining at least one reference value which indicates how the vehicle&#39;s speed is to be influenced on the basis of at least one of said comparisons in that simulation cycle (s) and the predicted vehicle v pred     —     Tnew ;   
           sending to a control system in the vehicle said at least one reference value with respect to which the vehicle is thereupon regulated.       

     The object or objects are achieved according to another aspect by a module for controlling a vehicle&#39;s speed which comprises an input unit adapted to receiving a desired speed v se , for the vehicle, a horizon unit adapted to determining by means of map data and location data a horizon for the intended itinerary which is made up of route segments with at least one characteristic for each segment, and a calculation unit adapted to effecting the following during each of a number of simulation cycles (s) each comprising a number N of simulation steps conducted at a predetermined frequency f:
         making a first prediction of the vehicle&#39;s speed v pred     —     cc  along the horizon with conventional cruise control when v set , is presented as reference speed, which prediction depends on the characteristics of said segment;   comparing the predicted vehicle speed v pred     —     cc  with v min , and v max , which demarcate a range within which the vehicle&#39;s speed is intended to be;   making a second prediction of the vehicle&#39;s speed v pred     —     Tnew  along the horizon when the vehicle&#39;s engine torque T is a value which depends on the result of said comparison in the latest preceding simulation cycle (s−1);   determining at least one reference value which indicates how the vehicle&#39;s speed is to be influenced on the basis of at least one of said comparisons in that simulation cycle (s) and the predicted vehicle v pred     —     Tnew . The module is further adapted to sending to a control system in the vehicle said at least one reference value with respect to which the vehicle is thereupon regulated.       

     The method described results in a substantially constant load upon the processor, since the same number of predictions are made during each simulation cycle at a constant frequency. The processor load is independent of the power output of the vehicle&#39;s engine, the weight of the vehicle and the topography of the route. The processor concerned thus knows how much processor capacity will be needed, which is therefore consistently allocated. The processor load will thus be the same in different situations of changing topology and be independent of the vehicle&#39;s engine torque. It is thus also possible for the processor concerned to be dimensioned without having to cater for any extreme worst scenarios but rather to cater for uniform processor load. The processor cost can thus be reduced. 
     Speed variations ahead along the horizon are predicted for only two different modes of driving the vehicle, resulting in a relatively light processor load. The reference value to be aimed at is then chosen on the basis of the result of the predicted modes of driving. By predicting the vehicle&#39;s speed at some other torque, e.g. minimum or maximum torque, it is possible to evaluate whether or not to aim at a certain torque for the vehicle. According to an embodiment, what is presented is v set , maximum torque or minimum torque. A reference value aimed at is not altered if the speed v pred     —     cc  stays within certain limits. 
     Preferred embodiments are described in the dependent claims and the detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS 
       The invention is described below with reference to the attached drawings, in which: 
         FIG. 1  depicts a module according to an embodiment of the invention. 
         FIG. 2  is a flowchart for the method according to an embodiment of the invention. 
         FIG. 3  illustrates a predicted speed during a simulation cycle according to an embodiment of the invention. 
         FIG. 4  illustrates a number of simulation cycles according to an embodiment of the invention. 
         FIG. 5  illustrates the vehicle&#39;s predicted speeds according to an embodiment of the invention. 
         FIG. 6  is a flowchart for the method according to another embodiment of the invention. 
         FIG. 7  illustrates an alternative embodiment according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
       FIG. 1  depicts a module for controlling a vehicle&#39;s speed according to an embodiment of the invention. The module comprises an input unit adapted to receiving a desired speed v set  for the vehicle. The driver may for example set a desired speed v set  which he/she wishes the vehicle to maintain. The module comprises also a horizon unit adapted to determining a horizon for the intended itinerary by means of map data and location data. The horizon is made up of route segments with at least one characteristic for each segment. An example of a segment&#39;s characteristics might be its gradient a in radians. 
     In the description of the present invention, GPS (Global Positioning System) is indicated for determining location data for the vehicle, but it should be appreciated that other kinds of global or regional positioning systems are also conceivable to provide the vehicle with location data, e.g. systems which use a radio receiver to determine the vehicle&#39;s location. The vehicle may also use sensors to scan the surroundings and thereby determine its location. 
       FIG. 1  illustrates how the module assimilates information about the intended itinerary via maps (map data) and GPS (location data). The itinerary is conveyed bit by bit to the module, e.g. via CAN. The module may be separate from or form part of the control system or systems which are to use reference values for regulating. An example of a control system is the vehicle&#39;s engine control system. Alternatively, the map and positioning system unit may also be part of a system which is to use reference values for regulating. In the module, the bits are put together in a horizon unit to form a horizon and are processed by the processor unit to create an internal horizon with respect to which the control system can regulate. The horizon is then continually supplemented by new bits from the GPS and map data unit to maintain a desired length of horizon. The horizon is thus updated continuously when the vehicle is in motion. 
     CAN (controller area network) is a serial bus system specially developed for use in vehicles. The CAN data bus makes digital data exchange possible between sensors, regulating components, actuators, control devices, etc. and ensures that two or more control devices can have access to the signals from a given sensor in order to use them to control components connected to them. 
     The module comprises also a calculation unit adapted, during each of a number of simulation cycles (s) each comprising a number N of simulation steps conducted at a predetermined frequency f, to making a first prediction of the vehicle&#39;s speed v pred     —     cc  along the horizon with conventional cruise control when v set  is presented as reference speed, which prediction depends on said segment&#39;s characteristics. The calculated vehicle speed v pred     —     cc  is compared with v min  and v max , which demarcate a range within which the vehicle&#39;s speed is intended to be, and a second prediction of the vehicle&#39;s speed v pred     —     Tnew  is made along the horizon when the vehicle&#39;s engine torque T is a value which depends on the result of said comparison in the latest preceding simulation cycle (s−1). At least one reference value is determined which indicates how the vehicle&#39;s speed is to be influenced on the basis of at least one of said comparisons in that simulation cycle (s) and the predicted vehicle speed v pred     —     Tnew . The module is further adapted to sending to a control system in the vehicle said at least one reference value with respect to which the vehicle is thereupon regulated. How the predictions are made is explained in more detail below. 
     The module and/or the calculation unit comprise at least a processor and memory adapted to performing all of the calculations, predictions etc. herein described. 
       FIG. 2  is a flowchart of the steps involved in the method for controlling the vehicle&#39;s speed according to an embodiment of the invention. The method comprises as a first step A) adopting v set  as a desired speed for the vehicle to maintain, and as a second step B) determining by means of map data and location data a horizon for the intended itinerary made up of route segments with at least one characteristic for each segment. According to the method, a number of simulation cycles (s) are then conducted during the length of the horizon. A simulation cycle (s) comprises C) a number N of simulation steps conducted at a predetermined frequency f and entails C 1 ) making a first prediction of the vehicle&#39;s speed v pred     —     cc  along the horizon with conventional cruise control when v set  is presented as reference speed, which depiction depends on said segment&#39;s characteristics, C 2 ) comparing the predicted vehicle speed v pred     —     cc  with v min  and v max , which demarcate a range within which the vehicle&#39;s speed is intended to be, C 3 ) making a second prediction of the vehicle&#39;s speed v pred     —     Tnew  along the horizon when the vehicle&#39;s engine torque T is a value which depends on the result of said comparison in the latest preceding simulation cycle (s−1), and C 4 ) determining at least one reference value which indicates how the vehicle&#39;s speed is to be influenced on the basis of at least some of said comparisons in that simulation cycle (s) and the predicted vehicle speed v pred     —     Tnew . As a further step D) said at least one reference value with respect to which the vehicle is to be regulated is sent to a control system in the vehicle. This method achieves a constant and predetermined processor load when the vehicle&#39;s reference speed is determined and regulated. 
     During a simulation cycle (s) the prediction according to C 1 ) and the prediction according to C 3 ) are made in parallel, as illustrated in  FIG. 2 . The result of the comparison of v pred     —     cc  with v 1  and v max  in the latest preceding simulation cycle (s−1) determines the torque T at which v pred     —     Tnew  is to be predicted in simulation cycle (s). If for example each simulation cycle comprises 100 simulation steps at a frequency of 100 Hz, each simulation cycle takes 1 second. 
     v set  is therefore the driver&#39;s input signal of desired cruise control speed, and the reference value is the value with respect to which the vehicle is regulated. The reference value is preferably any from among a reference vehicle speed v ref , a reference torque T ref  or a reference engine speed ω ref . v ref  is presented to the engine control unit&#39;s speed regulator. 
     For traditional cruise controls, v ref =v set . The speed regulator then controls the vehicle&#39;s speed according to v ref  by demanding necessary torque from the engine&#39;s torque regulator. When the reference value is instead a reference torque T ref , T ref  may be sent directly to the engine&#39;s torque regulator, and when the reference value is a reference engine speed ω ref , ω ref  may be sent directly to the engine&#39;s speed regulator. If v pred     —     cc  when compared with v min  and v max  does not go beyond these limits, v set  is presented, according to an embodiment, as reference value. The reference value then depends entirely on this comparison. Which reference value will be aimed at for other cases is explained below. 
     How the various predicted speeds are arrived at will now be explained. 
     The total force F env  acting upon the vehicle from the environment is made up of rolling resistance F roll , gravitation F and air resistance F air . Gravitation is calculated as
 
 F=m·g·α   (1)
 
where m is the weight of the vehicle and a the gradient of the road in radians. Since mostly only small angles are concerned, sin(α) is approximated to α. Air resistance is calculated as a factor k multiplied by the square of vehicle speed, as follows:
 
                     F   env     =       F     roll   ,   present       +     m   ·   9.82   ·   α     +     k   ·     v     i   -   1     2                 (   2   )                   F     roll   ,   present       =     F     roll   ,   est         ,     
     ⁢     m   =     m   est       ,     
     ⁢     k   =       1   2     ⁢     ρ   ·     C   d     ·   A                 (   3   )               
where A is the vehicle&#39;s estimated frontal area, C d  the resistance coefficient which depends on the streamline shape of the object, ρ the density of the air and pi the weight of the vehicle estimated by its weight estimation system as m est . Current rolling resistance F roll,present  is also estimated in the vehicle continuously as F roll,est . For calculation of m est  and F roll,est  please refer to the dissertation entitled “Fuel Optimal Powertrain Control for Heavy Trucks Utilizing Look Ahead” by Maria Ivarsson, Linköping 2009, ISBN 978-91-7393-637-8. v i−1  is the vehicle&#39;s predicted speed at preceding simulation step.
 
     The force F drive  which propels the vehicle forwards depends on which prediction is made, It is taken according to an embodiment as either maximum possible power (maximum torque) or least possible power (minimum torque, e.g. drag torque). It is possible, however, to take any desired force within the range
 
F min ≦F drive ≦F max   (4)
 
and the vehicle&#39;s speed v pred     —     Tnew  may therefore be predicted at torques other than maximum or minimum torque. F max  is calculated as maximum available engine torque, a function of engine speed, multiplied by the total transmission ratio and divided by the effective tyre radius r wheel . F min  is arrived at in the same way as F max  but with minimum torque instead:
 
                     F   max     =           T   max     ⁡     (   ω   )       ·     i   tot         r   wheel               (   5   )                 F   min     =           T   min     ⁡     (   ω   )       ·     i   tot         r   wheel               (   6   )               
where n is the vehicle&#39;s engine speed and i tot  the vehicle&#39;s total transmission ratio.
 
     The vehicle&#39;s acceleration Acc is given by:
 
Acc=( F   drive   −F   env )/ m   (7)
 
     According to an embodiment, simulation steps during a simulation cycle of N steps have a constant step length which depends on the vehicle&#39;s speed. The length dP of each simulation step is given by:
 
 dP=K·ν   init   (8)
 
where K is a time constant, e.g. 0.9 s, and ν init  is prevailing vehicle speed at the beginning of the simulation.
 
     The time dt for a simulation step is given by:
 
 dt=dP/v   i−1   (9)
 
where v i−1  is predicted speed at preceding simulation step i−1.
 
     The speed difference dν is:
 
 d ν=Acc· dt   (10)
 
     Energy consumed dW for a simulation step is given by:
 
 dW=dP· ( F   drive   −F   min )  (11)
 
     The speed ν i  at current simulation step becomes:
 
ν i =ν i−1   +dν   (12)
 
     Total time t i  for current simulation step is:
 
 t   i   =t   i−1   +dt   (13)
 
     Total energy consumed W i  for current simulation step is:
 
 W   i   =W   i−1   +dW   (14)
 
       FIG. 3  illustrates how a vehicle speed is predicted during a simulation cycle of N simulation steps with a horizon which is L meters long. The prediction is discontinued after N simulation steps, i.e. after a simulation cycle. A fresh simulation cycle then begins in the next time sample. Each simulation cycle has a predetermined frequency f. At a frequency of 100 Hz, for example, 100 simulation steps are conducted per second. As the length of each simulation step depends on the vehicle&#39;s speed v init  at the beginning of the prediction, the length of the predicted section of the horizon varies with the vehicle&#39;s speed. For example, at 80 km/h (22.22 m/s) the horizon becomes 2 km long if f=100 Hz and K=0.9 s, since each simulation step dP then becomes 20 m long and over 100 steps the horizon then becomes 2 km.  FIG. 3  shows a fresh speed v i  predicted at each simulation step i. Since the same number of predictions (v pred     —     cc  and v pred     —     Tnew ) is made at each simulation step, the processor load depends entirely on the number of simulation steps within the horizon. The number of simulation steps is determined by the frequency f, which is a predetermined value. The maximum processor load can therefore always be determined in advance, which is advantageous in that the processor can accordingly be dimensioned in advance. The processor load is therefore independent of road topography, vehicle weight and engine type. v pred     —     cc  and v pred     —     Tnew  may be vectors with N values, or alternatively only maximum and minimum values for v pred     —     cc  and v pred     —     Tnew  are saved in each simulation cycle, as explained in more detail below.  FIG. 4  illustrates three simulation cycles (s−1), (s) and (s+1) and the predictions made during each cycle. In each cycle, a prediction of v pred     —     cc  and a prediction of v pred     —     Tnew  are made. v pred     —     cc  is compared with v min  and v max  after each cycle, and the result of this comparison serves as a basis for determining preferably the torque T for v pred     —     Tnew  for the next cycle, as explained previously and as illustrated in  FIG. 4  by arrows marked “CHOSEN T”. This is illustrated at step C 2  in the flowchart in  FIG. 2 . Two different vehicle speeds are thus predicted during each simulation cycle. According to an embodiment, v pred     —     Tnew  is compared with v min  and v max  after a simulation cycle, and if v pred     —     Tnew  is below v min  or above v max  the vehicle has to be controlled with respect to v set . If v pre     —     Tnew ≧v min  and ≦v max  the vehicle has instead to be controlled with respect to the torque determined for v pred     —     Tnew . Each simulation cycle is thus followed by determining whether it is v pred     —     cc  or v pred     —     Tnew  which is to influence the vehicle&#39;s speed. A control unit is then presented with reference values representing speeds determined which may therefore be either a reference vehicle speed, a reference torque or a reference engine speed. 
     Reference values which the vehicle&#39;s control system is to aim at are thus determined continuously when the vehicle is in motion. They are preferably determined as from a certain predetermined distance ahead of the vehicle and are then synchronised in the control unit so that the calculated reference value for a given situation is set at the right time. An example of such a distance is 50 meters, which the control unit therefore caters for in regulating the vehicle. 
     An embodiment according to the invention will now be explained with reference to  FIGS. 5 and 6 . The upper part of  FIG. 5  illustrates the predicted speed v with conventional cruise control along a horizon with a road profile which is depicted in the lower part of the diagram. The flowchart in  FIG. 6  shows how v pred     —     cc  is predicted as a first step S 1 . After v pred     —     cc  has been predicted clearly during N steps, it is compared with v min  and v max , as illustrated at the next step S 2 . If v pred     —     cc  is below v min , an upgrade is identified. If v pred     —     cc  is above v ma , a downgrade is identified. If an upgrade is identified and v pred     —     cc  is therefore lower than v min , as at P 1  in  FIG. 5 , the vehicle&#39;s engine torque T in the second prediction v pred     —     Tnew  is taken as a maximum torque in the next simulation cycle. This is illustrated at step S 21  in  FIG. 6 , see also formula (5), and is represented by a broken line in  FIG. 5 . This presupposes, however, that v pred     —     cc  was below v min  before potentially going above v max . If a downgrade is identified and v pred     —     cc  is therefore higher than v max , as at P 2  in  FIG. 5 , the vehicle&#39;s engine torque T in the second prediction v pred     —     Tnew  is taken as a minimum torque in the next simulation cycle. This is illustrated at step S 31  in  FIG. 6 , see also formula (6), and is represented by a broken line in  FIG. 5 . This presupposes, however, that v pred     —     cc  was higher than v max  before potentially going below v min . According to an embodiment, the calculation unit described with reference to  FIG. 1  is adapted to performing the calculations etc. described. 
     If v pred     —     cc  is higher than or equal to v min  and lower than or equal to v max , v ref  is taken as a reference value which represents v set , as illustrated at step S 3  in  FIG. 6 . The reference value is then preferably taken as v set . This means for example that no hill is identified and hence that v pred     —     cc  is outside the speed range or that the vehicle will be travelling on a level road. v set  therefore then becomes the reference speed with respect to which the vehicle&#39;s control system is to regulate. v pred     —     Tnew  is preferably still predicted at maximum or minimum torque in order to achieve uniform processor load. 
     If a hill is identified, a preferred embodiment applies rules for determining the reference value with respect to which the vehicle is to be regulated. The calculation unit is then adapted to applying rules for determining the reference value. One rule comprises comparing the vehicle speed v pred     —     Tnew  in the second prediction with v min  and v max , which demarcate a range within which the vehicle&#39;s speed is intended to be. Another rule is that if v pred     —     Tnew  in the second prediction is between v min  and v max  or equal to v min  or v max  the vehicle&#39;s aimed-at reference value is determined as a value which represents the predicted speed v pred     —     Tnew . It is thus possible to ensure that the vehicle&#39;s speed will not go above or below the speed limits if maximum or minimum torque is presented. In  FIG. 5 , three dotted lines in the upper diagram illustrate how v pred     —     Tnew  is predicted at maximum torque and is then predicted to exceed v max . As this scenario is undesirable, v set  is presented as reference value. Only when v pred     —     Tnew  is predicted to not go above v max  may a reference value representing that speed be presented. 
     This is illustrated in the flowchart in  FIG. 6 , so if v pred     —     Tnew  is predicted at maximum torque it is compared with v max  at a step S 22 . If v pred     —     Tnew  is lower than or equal to v max , the vehicle&#39;s maximum torque has to be presented as reference value (step S 23 ). A maximum torque is preferably presented at the time when v pred     —     cc  goes below v min , provided that v pred     —     Tnew  is predicted not to exceed v max . Similarly, if v pred     —     Tnew  is predicted at a minimum torque, it is compared with v min  at a step S 32 . If v pred     —     Tnew  is higher than or equal to v min , the vehicle&#39;s minimum torque has to be presented as reference value (step S 33 ). A minimum torque is therefore preferably presented at the time when v pred     —     cc  exceeds v max , provided that v pred     —     Tnew  is predicted to not go lower than v min . 
     For the vehicle to reach minimum or maximum torque, the reference speed v ref  may be presented with an offset. Minimum torque may for example be presented by presenting the reference speed v ref  as a low value below v min , e.g. v min −k 1  where k 1  is within the range 1-10 km/h. The engine&#39;s control unit will then present drag torque for the engine. 
     Maximum torque may be reached by the reference speed being presented as a high speed above v max , e.g. v max +k 2 , where k 2  is within the range 1-10 km/h. 
     Another rule is that if v pred     —     Tnew  in the second prediction is above v max  or below v min , the vehicle&#39;s reference value is taken as v set . This is illustrated by step S 3  in  FIG. 6 . 
     At the next sample, e.g. at each second, the method starts again from S 1 . The time taken to perform the calculations is adapted so that the whole method is run through during a simulation cycle. 
       FIG. 7  illustrates three different speed profiles A, B and C and the amount of fuel injected A 1 , B 1  and C 1  during corresponding periods, when a vehicle is predicted to travel down a hill. Speed profile A illustrates an alternative embodiment of the invention whereby v ref  continually drops as far as possible, but not below v min , to save extra fuel at the expense of longer run time. The amount of fuel injected is thus reduced or throttled at an earlier stage than in the other driving modes illustrated. It is however desired to revert to v set  after the downgrade, as illustrated in the diagram. Reduced fuel consumption is thus prioritised in that lowerings take place so long as the vehicle&#39;s speed can thereafter revert to v set . This embodiment may for example be implemented in the method illustrated in the flowchart in  FIG. 6  by v max  at step S 2  being taken as v set  and/or by v ref  being taken as v min  at step S 3 . This results in no overspeed nor any increased losses due, for example, to air resistance. As the lowering of the vehicle&#39;s speed takes place earlier and to a lower speed, the result is reduced fuel consumption. This also makes it possible for lighter vehicles with a gross weight of less than about 30 tonnes to save fuel to a greater extent. Speed profile B in  FIG. 7  illustrates a driving mode according to earlier embodiments of the invention, and speed profile C a driving mode whereby the vehicle is run with traditional cruise control. v kfb , represents the speed limit for driving with traditional cruise control. B 1  and C 1  illustrate amounts of fuel injected during corresponding periods. K is a percent value for the amount of fuel injected. 
     According to an embodiment, the highest speed v pred     —     cc,max  and the lowest speed v pred     —     cc,min  during the first prediction of the vehicle&#39;s speed v pred     —     cc  with ordinary cruise control are determined and are then used in the comparison with v max  and v min  to determine the vehicle&#39;s engine torque T in the second prediction. The calculation unit is then adapted to performing these calculations. It is therefore only necessary to save scalars instead of whole vectors, thereby saving memory space. Nor need the values be saved after they have been used in the simulation cycle, since no adjustment of reference values takes place rearwards in the horizon and they will not be used again for calculations. This difference compared with some other calculation algorithms saves processor capacity and helps to achieve constant processor load. In the same way it is also possible for the highest and lowest speeds for the prediction of v pred     —     Tnew  to be determined during a simulation cycle. 
     According to an embodiment, a hysteresis is added to the reference value, e.g. v ref , in order to avoid jerky regulation and to avoid missing some fuel saving. The calculation unit is then adapted to adding this hysteresis to the reference value v ref . To avoid the speed reference v set  aimed at being altered at each simulation cycle, e.g. at each second, v ref  is allowed to go somewhat outside the speed range bounded by v min  and v max . This deviation may for example be 1 or 2 km/h beyond v min  or v max . When for example the vehicle&#39;s torque is determined as minimum torque, v ref  may be allowed to go somewhat beyond v min , as illustrated in the following example: 
     
       
         
           
               
               
            
               
                   
                   
               
               
                   
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                 hysteresis: 
               
               
                   
               
            
           
         
       
     
     In this example it may be seen that v pred     —     cc,min  goes below the speed limit v min  of 70 km/h at 5 seconds. v pred     —     cc,min  is then 69 km/h. v ref  should then really be presented at maximum torque, causing v ref  at 5 seconds to be 80 km/h. In the next time sample, however, v pred     —     cc,min  goes back above the v min  limit, which means that a different v ref  is presented. To avoid v ref  varying greatly as short intervals, a v ref  value which is somewhat below v min  is presented instead. 
     According to another embodiment, v min  and v max  are aimed at as reference values. This embodiment is an alternative to using hysteresis or to adding a constant k 1  or k 2  to the reference value and may therefore be used to avoid jerky regulation or to reach a certain low or high torque. The embodiment results in less sensitivity to any errors in the speed predictions and avoids driver irritation by not going below v min  or above v max . 
     An embodiment takes into account the engine&#39;s efficiency and comfort/drivability when choosing which control strategies to predict (i.e. that which results in v pred     —     Tnew ). Taking as magnitude and/or time the torque which results in v pred     —     Tnew  differently depending on the engine&#39;s efficiency or on the basis of comfort requirements makes it possible to achieve comfortable and economical cruise control. This may be implemented on the basis of rules whereby for example there is a certain torque at a certain engine speed, or whereby torque which results in more acceleration than a certain limit value is never allowed. 
     The invention comprises also a computer programme product comprising programme instructions for enabling a computer system in a vehicle to perform steps according to the method described when the instructions are run on said computer system. The invention comprises also a computer programme product in which the instructions are stored on a medium which can be read by a computer system. 
     The present invention is not restricted to the embodiments described above. Sundry alternatives, modifications and equivalents may be used. The above embodiments therefore do not limit the scope of the invention, which is defined by the attached claims.