Abstract:
A motor vehicle having a braking device and drive engine which can be used independently of one another for decelerating the vehicle. A method of controlling the vehicle during a thrust operation includes steps of detecting a requirement to change a currently engaged first gear in a transmission that transfers the braking force from the engine; determining a first deceleration brought about by the engine in the first gear; determining a second deceleration brought about by the engine in a second gear to be engaged; and changing the gear by disengaging a clutch that couples the engine to the transmission, the first gear is disengaged, the second gear is engaged, and the clutch is again engaged. In this case, the braking device is controlled, during the gear change, in such manner that any jerk experienced by the vehicle is below a predetermined value.

Description:
[0001]    This application claims priority from German patent application serial no. 10 2015 221 156.8 filed Oct. 29, 2015. 
       FIELD OF THE INVENTION 
       [0002]    The invention concerns a motor vehicle with a drive-train having a change-speed transmission. In particular, the invention concerns the control of the change of a gear step currently engaged in the transmission during thrust-mode operation of the motor vehicle. 
       BACKGROUND OF THE INVENTION 
       [0003]    A motor vehicle comprises a drive-train with a drive engine, a clutch, a transmission and a drive wheel. In the transmission various gear steps can be engaged. The gear steps give rise to different step-down ratios between the rotational speed of a drive engine and that of a drive wheel when the clutch is closed. During thrust operation, for example when driving downhill, the motor vehicle is decelerated by means of the drive engine. If the transmission is not designed to operate without traction force interruption, then in order to change the gear step currently engaged the drive-train has to be temporarily disconnected by means of the clutch, and the drive engine cannot then exert any braking force on the motor vehicle. When the clutch is closed a jerk of the motor vehicle can occur, which is unpleasant for a person on board or may cause damage to cargo. Moreover, a component in the drive-train may be affected by more severe wear due to the jerking. 
         [0004]    DE 102 16 546 A1 concerns a motor vehicle which can be slowed by an engine braking force. It is proposed to compensate the change of the engine braking force as much as possible during a gear step change of a transmission, by activating another braking device in order to maintain the speed of the motor vehicle. 
         [0005]    DE 103 62 004 A1 concerns a similar technique, wherein the braking device is in particular controlled in such manner that a transition between braking forces of different strengths before and after the gear step change takes place in a predetermined manner. 
       SUMMARY OF THE INVENTION 
       [0006]    The purpose of the present invention is to indicate a method for the improvement of gear step changes. The invention solves that problem by virtue of the objects of the independent claims. The subordinate claims describe preferred embodiments. 
         [0007]    A motor vehicle comprises a braking device and a drive engine which can be used independently of one another to decelerate the motor vehicle. A method for controlling the motor vehicle during thrust operation comprises steps in which a requirement is detected to change a gear step currently engaged in a transmission which transmits the braking force of the drive engine; a first deceleration is determined, which is brought about by the drive engine in a first, currently engaged gear step; a second deceleration is determined, which is brought about by the drive engine in a second gear step that is to be engaged; and the gear step is changed by opening a clutch that couples the drive engine to the transmission, disengaging the first gear step, engaging the second gear step and then closing the clutch. During the gear step change the braking device is controlled in such manner that a jerk undergone by the motor vehicle is below a predetermined threshold value. 
         [0008]    By controlling the braking force as a function of the jerk, a harmonious and material-protecting gear step change can be carried out. A passenger on board the motor vehicle can perceive the gear step change as more pleasant. Since the jerk is less severe, the loading and therefore the wear of components of the motor vehicle are reduced. 
         [0009]    The braking device can be controlled in such manner that the jerk is virtually eliminated and the deceleration of the motor vehicle is therefore as constant as possible. The deceleration, i.e. the negative acceleration of the motor vehicle, depends on the braking force applied by the braking device. By controlling the gear step change so that there is no jerk, the deceleration of the motor vehicle does not change and thus the speed of the motor vehicle can for example increase or decrease at a constant rate. 
         [0010]    Furthermore, the braking device can be controlled in such manner that the deceleration is almost zero so that the speed of the motor vehicle is as constant as possible. Accordingly the gear step change can be carried out under constant conditions, in order for example to redistribute the braking forces between the drive engine and the braking device. 
         [0011]    Changing the gear step can also take place slowly. In that way the jerk can be kept below the predetermined threshold value when a relatively large deceleration is associated with the gear step change. The gear step change is usually carried out as quickly as possible, whereby a limit is set on the maximum possible deceleration change for a predetermined maximum jerk. By prolonging the duration of the gear step change, with the same maximum jerk a greater deceleration change is produced. For this, in various embodiments one or more steps of the gear step change can be carried out slowly or one or more pauses can be interposed between the steps. 
         [0012]    A computer program product comprises program code means for carrying out the above-described method when the method is carried out on a programmable implementation device or when it is stored on a computer-readable data carrier. 
         [0013]    A control device for the above-described motor vehicle comprises a first interface with a clutch for connecting or separating the transmission to or from the drive engine; a second interface with the transmission for controlling the engagement or disengagement of a gear; and a processing device designed to carry out the above-described method. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The invention will now be described in more detail with reference to the attached figures, which show: 
           [0015]      FIG. 1 : A schematic representation of a drive-train in a motor vehicle; 
           [0016]      FIG. 2 : A flow chart of a method for controlling the motor vehicle of  FIG. 1 ; and 
           [0017]      FIG. 3 : Graphs of parameters in the motor vehicle of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]      FIG. 1  shows a schematic representation of a drive-train  105  arranged in a motor vehicle  100 . The motor vehicle  100  can in particular be a utility vehicle or a passenger car. The drive-train  105  comprises a drive engine  110 , a clutch  115 , a transmission  120 , a braking device  125  and a drive wheel  130 . Other components too can be comprised in the drive-train  105 , for example a cardan shaft and/or a cardan gear system between the transmission  120  and the drive wheel  130 . A decelerating or braking force of the drive engine  110  can depend on its rotational speed. 
         [0019]    The braking device  125  can be a permanent brake or a service brake or a combination of both, and preferably acts upon the motor vehicle  100  independently of the opening condition of the clutch  115 . The drive-train  105  is designed to enable the drive engine  110  to act upon the drive wheel  130  in order to be able to accelerate or decelerate the motor vehicle  100 . In the transmission  120  various gear steps can be engaged in order to produce various step-down ratios between the drive engine  110  and the drive wheel  130 . The transmission  120  is preferably a transmission that cannot be powershifted, so that in the transmission  120  usually at any one time at most one gear step is engaged. The individual gear steps can be synchronized or unsynchronized. In particular, the transmission  120  can be an AMT transmission (AMT=Automatic Manual Transmission), which in principle can be actuated manually by a driver or automatically by means of one or more actuators. 
         [0020]    To control the transmission  120 , in particular a control unit  135  can be provided, which acts upon the actuator(s). 
         [0021]    The control unit  135  can in addition be connected to the drive engine  110 , the clutch  115  or the braking device  125  in order to detect the condition of the respective components  110 ,  115 ,  125  so that the values detected can be taken into account for the control of the transmission  120 . One of the components  110 ,  115 ,  125  can also be actively controlled or influenced by the control unit  135 . Thus, in one embodiment a rotational speed of the drive engine  110  can be influenced, the clutch  115  can be opened or closed, or the braking device  125  can be gradually applied or released. 
         [0022]    The control unit  135  can be made integrally with the transmission  120 . The clutch  115  as well, or the braking device  125  can be integrated with the transmission  120 . When it is designed as a permanent brake the braking device  125  preferably comprises a retarder or intarder, which is designed to apply a preferably measurable braking force on the output side of the transmission  120 , which is connected to the drive wheel  130  in a torque-transmitting manner. If the braking device  125  is in the form of a service brake, then it can also act directly on one of the wheels of the motor vehicle  100 , in particular a drive wheel  130 . 
         [0023]    During thrust operation, for example when driving downhill, the motor vehicle  100  is at least partially slowed down by a braking force applied by the drive engine  110 . However, this braking force is not available when the drive-train  105  is interrupted by means of the clutch  115 , for example when a gear step engaged in the transmission  120  is being changed. During the gear step change the braking device  125  can be controlled so that it too applies the braking force of the drive engine  110 , for example so that the motor vehicle  100  does not accelerate further. The braking force of the drive engine  110  depends on its rotational speed, which in turn, when the clutch  115  is closed, depends on the gear step engaged. After a downshift the braking force of the drive engine  110  is usually greater than before (because the rotational speed is higher), and after an upshift it is lower than before (because the rotational speed of the engine is lower). Particularly when interrupting or connecting the drive-train  105 , when the braking device is activated or deactivated, the motor vehicle can undergo a jerk, which can bring about increased wear on elements of the drive-train  105  or be perceived as unpleasant by a passenger on board the motor vehicle  100 . 
         [0024]    It is proposed to gauge the braking force applied by the braking device  125  during a gear step change in such manner that a change of the acceleration of the motor vehicle  105 , the so-termed jerk, does not exceed a predetermined threshold value. The jerk can act positively or negatively, depending on whether the acceleration of the motor vehicle increases or decreases. Only the positive, only the negative, or both types of jerk can be kept quantitatively below the threshold by appropriately influencing the braking device  125 . It is also possible to provide dedicated, equal or different thresholds for the positive and negative jerk. 
         [0025]      FIG. 2  shows a flow chart for a method  200  for controlling a transmission such as the transmission  120  of  FIG. 1 . The control unit  135  in  FIG. 1  is preferably designed to carry out the method  200 . For this, it can comprise a programmable microcomputer and the method  200  can be in the form of a computer program product. In one embodiment the method  200  is carried out by only one control unit  135 , which controls the drive engine  110 , the transmission  120 , the clutch  115  and the braking device  125 . In other embodiments, for one or more of the components  110 ,  115  and  125  it is also possible to provide one or more other control units, with which the control unit  135  designed to control the transmission  120  can communicate. 
         [0026]    It is assumed that the motor vehicle  100  is in thrust operation, so that the drive engine exerts a braking force on the motor vehicle  100 . 
         [0027]    In a step  205  a requirement is detected for a gear step change from a first, currently engaged gear step to a second gear step that is to be engaged. Consequently, in a step  210  a first deceleration of the motor vehicle  100  brought about by the drive engine  110  while the first gear step is engaged, and a second deceleration that occurs when the second gear step is engaged, are determined. During this a rotational speed change of the drive engine  110  and a change of the braking force applied by it as a result are preferably taken into account. 
         [0028]    Thereafter, in sequence, in a step  215  the clutch  115  is opened, in a step  220  the first gear step is disengaged, in a step  225  the second gear step is engaged and in a step  230  the clutch  115  is closed. In the present case this sequence is called the change of the gear step or also the gear step change. In parallel with the gear step change, in a step  235  the braking device  125  is actuated and the size of the braking force applied by the braking device  125  is controlled in such manner that a harmonic transition between the gear steps takes place. In particular, the braking device is controlled in such manner that that the jerk undergone by the motor vehicle  100  remains quantitatively below a predetermined threshold value. For this, a variation of the actuation of the braking device  125  can be predetermined in step  210  and implemented in step  235 , or an adjustment can be made which ensures that the acceleration change of the motor vehicle remains below the threshold value. It is also possible to predetermine a variation and adapt it on the basis of a determination of the jerk. 
         [0029]    A gear step change when downshifting the transmission  120  usually lasts for only a predetermined time, during which the transition from a low to a high braking action of the drive engine  110  (or the reverse) cannot take place in some circumstances without giving rise to a more severe jerk than specified. In such a case the gear step change can be prolonged in duration, by carrying out one of the steps  215  to  230  more slowly or by interposing a pause between consecutive steps of the gear step change. In that way the predetermined jerk severity can be maintained, so that even in this case a harmonious gear step change can be carried out 
         [0030]    In yet another embodiment it can in addition be monitored whether a rotational speed of the input side or of the output side of the transmission  120  is higher than a value specified for a safe engagement of the second gear step. If so, then in particular the braking device  125  can be actuated more firmly or for a longer time, in order to reduce the rotational speed on the output side, or the drive engine  110  can be controlled so as to adapt its rotational speed. 
         [0031]      FIG. 3  shows graphs of parameters in the motor vehicle of  FIG. 1 . The time is plotted horizontally. In the vertical direction variations of a number of parameters are shown, which take place parallel with one another in time. A first variation  305  shows the speed of the motor vehicle  100 , a second  310  variation shows the total braking force acting on the motor vehicle  100 , a third variation  315  shows the jerk that acts on the motor vehicle  100  a fourth variation  320  shows a braking force from the drive engine, and a fifth variation  325  shows the braking force applied by the braking device  125 . 
         [0032]    The representation is to be understood as qualitative and relates, as an example, to a situation in which the motor vehicle  100  is driving downhill, whereby the motor vehicle, at first in a segment t 1 , accelerates more rapidly than can be compensated by the braking force of the drive engine  110  alone. Accordingly a downshift gear step change is carried out in order to bring the drive engine  110  to a higher rotational speed so that it can produce a larger braking force. 
         [0033]    For this, in a segment t 2  the braking device  125  is activated and the clutch  115  is opened, so that the braking force of the drive engine  110  decreases. In parallel with this, the braking force of the braking device  125  is increased in order to compensate for the braking force loss. In the present case the braking device  125  is actuated firmly enough to reduce the speed  305  of the motor vehicle  100 . Owing to the change of the total braking force  310  acting, the motor vehicle  100  undergoes a jerk  315 , but the braking device  125  is controlled so that the jerk  215  remains quantitatively below a predetermined threshold value. 
         [0034]    In the next segment t 3  the braking force  310  remains, for example, constant and the speed  305  falls at a constant rate, and during this there is no jerk  315 . In this segment the first gear is disengaged and the second gear is engaged. 
         [0035]    In a following segment t 4  the clutch  115  is closed again, so that the braking force applied by the drive engine  110  increases. At the same time, the force of the braking device  125  is slightly reduced in order to take the increased braking force into account. Preferably, this adjustment is again done in such manner that the jerk  315  produced remains quantitatively below a predetermined threshold value. 
         [0036]    In the embodiment described, in a segment t 5  the braking action of the drive engine  110  is sufficiently large for the meanwhile reduced speed of the motor vehicle  100  to remain the same when the braking device  125  remains inactivated. 
         [0037]    Other situations too can be imagined, in which a gear step change of the transmission  120  can be carried out with assistance from the braking device  125  in such manner that a deceleration of the motor vehicle  100  is changed only so slowly that the jerk produced, or its severity, remains below a predetermined threshold value. In one embodiment the braking device  125  is controlled in such manner that the deceleration is as constant as possible, so that the jerk  315  is as close to zero as possible. In yet another embodiment the deceleration of the motor vehicle is as close to zero as possible so that the speed of the motor vehicle  100  is as constant as possible. 
       INDEXES 
       [0000]    
       
           100  Motor vehicle 
           105  Drive-train 
           110  Drive engine 
           115  Clutch 
           120  Transmission 
           125  Braking device 
           130  Drive wheel 
           135  Control unit 
           200  Method 
           205  Requirement for a gear step change 
           210  Determination of the decelerations 
           215  Clutch opened 
           220  First gear step disengaged 
           225  Second gear step engaged 
           230  Clutch closed 
           235  Braking device adjusted 
           240  End 
           305  First graph: Speed 
           310  Second graph: Total braking force acting on the motor vehicle 
           315  Third graph: Jerk 
           320  Fourth graph: Braking force from the drive engine 
           325  Fifth graph: Braking force from the braking device 
         t 1  to t 5  Time segments