Abstract:
A method and a device are provided for limiting the driving speed of a motor vehicle that allow a selectable, fixed fuel consumption per distance traveled to be maintained without limiting short-term acceleration. In this context, the driving speed is limited to a maximum value (vmaxbe), at which a predefined maximum fuel consumption (Besetpoint) for steady driving speed is not exceeded.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method and a device for limiting the driving speed of a motor vehicle.  
       BACKGROUND INFORMATION  
       [0002]     A method for limiting the driving speed of a motor vehicle is discussed in German Patent Application No. 102 01 160.  
       SUMMARY OF THE INVENTION  
       [0003]     In the method and the device of the present invention for limiting the driving speed of a motor vehicle, the driving speed is limited to a maximum value at which a predefined maximum fuel consumption for a fixed driving speed is not exceeded. A set, fixed fuel consumption can be maintained in this manner without limiting short-term acceleration. As a result, an economic manner of driving can be automated according to driver requirements.  
         [0004]     The maximum value for the driving speed may be determined from a maximum drive power of the vehicle, resulting from the relationship between the predefined maximum fuel consumption and a specific fuel consumption of the engine at a current operating point, via an inverse characteristic curve for a speed-dependent proportion of the driving resistance. In this manner, the maximum value for the driving speed can be determined particularly simply and easily. It is particularly advantageous when the maximum drive power is corrected by a speed-independent proportion of the driving resistance. The accuracy of the determination of the maximum value for the driving speed is increased in this manner.  
         [0005]     The speed-independent proportion of the driving resistance may also be filtered, in particular via a first order low pass filter. Noise influences in the calculation of the speed-independent proportion of the driving resistance can be largely equalized in this manner.  
         [0006]     Moreover, it may be useful to limit the driving speed as a function of the predefined maximum fuel consumption only when the gradient of a driving pedal position is less than a predefined value or a kick-down function is deactivated. In this manner, it is ensured that the fuel consumption-dependent limiting of the driving speed is invalidated in situations in which the driver is primarily concerned with achieving a driving speed that is as fast as possible. As a result, the driving safety, e.g., during passing operations, is ensured. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  shows a block diagram of an exemplary driving speed limitation using an embodiment of the device of the present invention.  
         [0008]      FIG. 2  shows an exemplary functional diagram for describing the structure of the device of the present invention and a flow of the method of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0009]      FIG. 1  shows an exemplary system for speed limitation in a motor vehicle in the form of a block diagram. A device  10  for limiting the driving speed of the motor vehicle is provided in this context. Device  10  includes an input/control unit  20 , which is connected to arrangement  15  for determining a maximum value vmaxbe, to which the driving speed is to be limited.  
         [0010]     Maximum value vmaxbe for the driving speed is transmitted by arrangement  15  to arrangement  30 , which perform the actual speed limiting function. In this context, arrangement  30  ensures that the current driving speed of the motor vehicle does not exceed maximum value vmaxbe for the driving speed. Input/control unit  20  allows the driver of the motor vehicle to define a maximum fuel consumption Besoll for a fixed driving speed. Moreover, the driver may activate arrangement  15  at input/control unit  20  such that the arrangement determines maximum value vmaxbe for the driving speed as a function of predefined maximum fuel consumption Besoll for a fixed driving speed. Input/control unit  20  generates an activation signal “ON” to activate arrangement  15 . Correspondingly, the driver may deactivate arrangement  15  at input/control unit  20  such that maximum value vmaxbe for the driving speed is not determined as a function of predefined maximum fuel consumption Besoll for a fixed driving speed. The input/control unit generates a deactivation signal “OFF” for arrangement  15  for this purpose.  
         [0011]      FIG. 1  also shows an engine control  25 , which provides device  10  and arrangement  15  with values for a transmission ratio factor üactual, an engine speed nmot, an actual engine torque Miactual, an actual acceleration aactual of the vehicle, an actual speed vactual of the vehicle, and a bit B_kdown, which indicates whether a kick-down of the vehicle was operated during use of an automatic transmission. The values for transmission ratio factor üactual, engine speed nmot, actual engine torque Miactual, actual acceleration aactual, and actual speed vactual are determined in a manner known from the related art. Bit B_kdown for operation of the kick-down is set during operation of the kick-down and is otherwise reset.  FIG. 2  shows a functional diagram to describe the structure of arrangement  15  as well as the flow of the method of the invention. An engine consumption map  35  to which engine speed nmot and actual engine torque Miactual are supplied as input quantities is provided in this context. A specific fuel consumption beENG of the vehicle engine results as the output quantity at the current operating point that is characterized by engine speed nmot and actual engine torque Miactual. Engine consumption map  35  is determined as a standard for every engine by the manufacturer and is therefore known from the related art. Specific fuel consumption beENG of the engine is multiplied by transmission ratio factor üactual at a first operation point  40 . The result of the multiplication operation is multiplied at a second operation point  45  by a wheel radius r wheel  of the wheels of the vehicle, wheel radius r wheel  being able to be known and stored in arrangement  15  or known in engine control  25  and transmitted to arrangement  15 . The result of the multiplication operation is designated as bePT in  FIG. 2  and supplied to a third operation point  50 . The quotient is formed from predefined maximum fuel consumption Besetpoint for the fixed driving speed, and multiplication result bePT at third operation point  50  so that a maximum drive power F BEsetpoint  of the vehicle results as follows: 
   F   BEsetpoint   =Be setpoint/ bePT   (1)  
         [0012]     Actual engine torque Miactual is also supplied to a fourth operation point  55  and multiplied there by transmission ratio factor üactual. The multiplication result is supplied to a fifth operation point  60  and multiplied there by wheel radius r wheel . The multiplication result is current drive power FAN of the vehicle.  
         [0013]     Actual acceleration aactual of the vehicle is multiplied by vehicle mass MFzg at a sixth operation point  65 . The multiplication result is inertial force T of the vehicle. Vehicle mass MFzg may be known and stored in arrangement  15 . Alternatively, vehicle mass MFzg may be known in engine control  25  and transmitted from there to arrangement  15 . Actual speed vactual of the vehicle is supplied to a first characteristic curve  85  as an input quantity. First characteristic curve  85  provides a speed-dependent proportion fv of the driving resistance of the vehicle from actual speed vactual. Inertial force T and speed-dependent proportion fv of the driving resistance are added together at a seventh operation point  70 . The addition result is subtracted from current drive power FAN of the vehicle at an eighth operation point  75 . The subtraction result is supplied to a filter  5  to equalize noise influences on the measured value for actual acceleration aactual and to prevent quick changes of maximum value vmaxbe for the driving speed. Filter  5  may be a first order low pass filter, for example. The equalized output signal of filter  5  is supplied as speed-independent proportion Fα of the driving resistance to a ninth operation point  80  and is subtracted there from maximum drive power F BEsetpoint . The subtraction result is supplied as an input quantity to a second characteristic curve  90 , which is inverse to first characteristic curve  85  and provides a speed value fv −1 , which is supplied to a first input  105  of a controlled switch  100 . A predefined absolute maximum speed VMAX, which is either preset in a fixed manner or may be defined by the user at input/control unit  20 , is supplied to a second input  110  of controlled switch  100 . Controlled switch  100  is controlled via an OR gate  95  to which deactivation signal “OFF” and bit B_kdown for the kick-down are supplied as input quantities. If one of the two input quantities “OFF” or B_kdown of OR gate  95  is set, i.e., the kick-down is operated or the driver used input/control unit  20  to deactivate the provision of maximum value vmaxbe for the driving speed as a function of predefined maximum fuel consumption Besetpoint for a fixed driving speed, the output of the OR gate is set and causes controlled switch  100  to connect second input  110  to output  115  of controlled switch  100 . In this case, predefined absolute maximum speed VMAX is supplied to arrangement  30  as maximum value vmaxbe for the driving speed. However, if the output of OR gate  95  is reset, i.e., neither the kick-down is operated nor the determination of maximum value vmaxbe for the driving speed as a function of predefined maximum fuel consumption Besetpoint for a fixed driving speed, is deactivated by the driver at input/control unit  20 , the reset output of OR gate  95  causes controlled switch  100  to connect first input  105  to output  115  so that output fv −1  of second characteristic curve  90  is supplied to arrangement  30  as maximum value vmaxbe for the driving speed.  
         [0014]     The speed limitation function performed by arrangement  30  is a function of the vehicle longitudinal motion that allows the driver to define a maximum value vmaxbe for the driving speed that may not be exceeded by actual speed vactual of the vehicle. This benefits driving safety as well as fuel consumption. If the driver would like to use the speed limitation function for limiting to a desired fuel consumption, this is not possible by only defining maximum value vmaxbe for the driving speed since fuel consumption also depends on speed-independent parameters, such as incline, engine operating point, or head wind. Fuel consumption is a fuel consumption per distance in this exemplary embodiment, i.e., fuel consumption per distance traveled, for example, the fuel consumption per 100 km of distance traveled.  
         [0015]     Arrangement  15  described here may be used to calculate maximum value vmaxbe for the driving speed as output fv −1  of second characteristic curve  90  so that predefined maximum fuel consumption Besetpoint for a fixed driving speed is not exceeded. Actual value Be for the fuel consumption per distance traveled may be calculated according to the following formula:  
             Be   =       [       beENG   ⁡     (     nmot   ,   Miactual     )       *     ∫       (       Froll   ⁡     (   vactual   )       +     Fair   ⁡     (   vactual   )       +     MFzg   *   aactual     +   Fbr   +     F   ⁢           ⁢   α       )     *   vactual   ⁢     ⅆ   t           ]     /     ∫     vactual   ⁢     ⅆ   t                   (   2   )             
 
         [0016]     The objective is to solve equation (2) for actual speed vactual. If actual value Be for the fuel consumption per distance traveled is replaced by predefined maximum fuel consumption Besetpoint for a fixed driving speed, maximum value vmaxbe for the driving speed valid for predefined maximum fuel consumption Besetpoint is obtained instead of the actual speed. In this context, the vehicle acceleration under maximum value vmaxbe for the driving speed need not be limited as a function of predefined maximum fuel consumption Besetpoint so that the vehicle agility is not impaired. Therefore, only maximum value vmaxbe for the driving speed is calculated, the value resulting in the fixed drive case, i.e., in which actual acceleration aactual of the vehicle is equal to zero and the braking force Fbr of the vehicle in equation (2) is also equal to zero. The driving resistance of the vehicle is made up of speed-dependent proportion fv, which is made up of rolling resistance Froll and air resistance Fair in equation (2), as well as of speed-independent proportion Fα, which is made up of the climbing resistance and the force of the headwind. Therefore, if actual acceleration aactual and braking force Fbr are each set to zero in equation (2), the following results: 
 
 Be=[beENG ( nmot,Mi actual)*[∫( F roll( v actual)+ F air( v actual))* v actual  dt+Fα*v actual  dt]/∫v actual  dt   (3) 
 
         [0017]     The following then results from equation (3): 
 
 Be/beENG ( nmot,Mi actual)− Fα=∫ ( F roll( v actual)+ F air( v actual))* v actual  dt/∫v actual  dt   (4) 
 
         [0018]     The right side of equation (4) corresponds with the average force generated by rolling resistance Froll and air resistance Fair. The quotient on the left side of the equation may be interpreted following multiplication with transmission ratio factor üactual and wheel radius r wheel  as a drive power F Be  of the vehicle, which results from actual value Be of the fuel consumption per distance traveled in relation to specific fuel consumption BeENG of the engine at the current operating point. The right side of equation (4) may be determined in a vehicle experiment as first characteristic curve fv (vactual) via actual speed vactual. The determination may be performed as follows:  
         [0019]     The vehicle is operated in windless conditions, on a flat driving surface, at a fixed actual speed vactual. Current drive power FAN of the vehicle is then determined from actual engine torque Miactual, as in  FIG. 2  using fourth operation point  55  and fifth operation point  60 , for any time period.  
         [0020]     The described vehicle experiment is conducted for a plurality of actual speeds vactual of the vehicle, which sufficiently cover the entire spectrum of possible actual speeds vactual of the vehicle. First characteristic curve  85  is formed in this manner. Actual speeds vactual of the vehicle then represent the data points of first characteristic curve  85 .  
         [0021]     The following relationship results from equation (4) on the basis of the described vehicle experiment: 
 
 Be/beENG ( nmot,Mi actual)− Fα=fv ( v actual)  (5) 
 
         [0022]     The driving resistances as a function increase strictly over actual speed vactual of the vehicle. As a result, first characteristic curve  85 , which corresponds with function fv (vactual), can be inverted. The inverting of first characteristic curve  85  then corresponds with function fv −1  (vactual) and second characteristic curve  90 . Second characteristic curve  90  allows equation (5) to be solved for actual speed vactual of the vehicle: 
 
 v actual= fv   −1 ( Be/beENG ( nmot,Mi actual)− Fα )  (6) 
 
         [0023]     If actual value Be for the fuel consumption per distance traveled is then replaced by predefined maximum fuel consumption Besetpoint for a fixed driving speed, equation (6) yields maximum value vmaxbe for the driving speed as output fv −1  of second characteristic curve  90  instead of actual speed vactual of the vehicle. This is represented in the following equation: 
 
 v  max  be=fv   −1 ( Be setpoint/ beENG ( nmot,Mi actual)− Fα )  (7) 
 
         [0024]     Engine consumption map  35  is described in equation (7) as BeENG (nmot, Miactual) and is determined as described as a standard for every engine by the manufacturer and is therefore provided in means  15  in a manner known from the related art. Predefined maximum fuel consumption Besetpoint for a fixed driving speed may be defined by the driver at input/control unit  20 . Therefore, only speed-independent proportion Fα of the driving resistance must still be calculated. Speed-independent proportion Fα of the driving resistance is able to be calculated according to  FIG. 2  from the force balance at the vehicle in that speed-dependent proportion fv of the driving resistance and inertial force T represented by MFzg*aactual are subtracted from current drive power FAN of the vehicle in accordance with the following equation: 
 
 Fα=FAN−fv ( v actual)− MFzg*a actual  (8) 
 
         [0025]     Current drive power FAN of the vehicle may be formed from actual engine torque Miactual, transmission ratio factor üactual, and wheel radius R wheel  in the manner described in  FIG. 2 . To prevent nervous vehicle behavior from quick changes of maximum value vmaxbe for the driving speed due to a distorted value for actual acceleration aactual, speed-independent proportion Fα of the driving resistance is filtered according to  FIG. 2 , e.g. by a first order low pass filter. As shown in and described with respect to  FIG. 2 , the driving speed is limited as a function of predefined maximum fuel consumption Besetpoint only when a kick-down function is deactivated, i.e., bit B_kdown is reset. Additionally or alternatively, it may be provided for the driving speed to be limited as a function of predefined maximum fuel consumption Besetpoint only when the gradient of a driving pedal position is less than a predefined value. If the gradient of the driving pedal position exceeds the predefined value, it is detected that the driver wants to accelerate as quickly as possible, similar to in the kick-down function, so that the limitation of the driving speed as a function predefined maximum fuel consumption Besetpoint is to be dispensed with also in this case and the driving speed is to be limited instead by predefined absolute maximum speed VMAX. Furthermore, as described with respect to  FIG. 2 , it may be provided for the driving speed to be limited as a function of predefined maximum fuel consumption Besetpoint only when the driver also activates arrangement  15  by setting the activation signal to “ON” at input/control unit  20 . In this case, the deactivation signal “OFF” is reset. For the case that the driver deactivates arrangement  15  at input/control unit  20  by setting the deactivation signal “OFF”, the driving speed is limited by predefined absolute maximum speed VMAX instead of as a function of predefined maximum fuel consumption Besetpoint as described in  FIG. 2 .