Abstract:
A method for driving a hybrid vehicle during a load reversal includes: the application of a first torque on a first hybrid vehicle axle during an acceleration reversal; and the application of a second torque on a second hybrid vehicle axle during the acceleration reversal, a direction of action of the second torque being opposite to a direction of action of the first torque.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to hybrid drives, in particular axle hybrids. 
         [0003]    2. Description of Related Art 
         [0004]    Modern hybrid vehicles such as so-called axle hybrids are equipped with hybrid drives, which may include an electric motor and a combustion engine. Axle hybrids are characterized by the fact that one of the vehicle axles is driven by one drive source, for example a combustion engine, and the other vehicle axle is driven by another drive source, for example an electric motor. In this context, the electric motor may operate as a generator, in the event of a braking operation for example, in order to recuperate the braking energy and to store it in a vehicle battery. Furthermore, the electric motor may be used to drive a switchable drive axle in the event of an increased slip of the wheels, as disclosed in the printed document published German patent document DE 3542059 C1. 
         [0005]    To drive the vehicle, a propulsive torque is normally generated as a function of a gas or accelerator pedal position, which in an axle hybrid is always composed of a sum of the torques applied on the respective vehicle axles. Thus, at an accelerator pedal position of 0%, for example, a slight deceleration sets in, which is connected with a negative propulsive torque, a so-called overrun state of −20 Nm for example, a propulsive torque of 0 Nm sets in at an accelerator pedal position of approximately 10%, and a positive propulsive torque, a so-called traction state, sets in at an accelerator pedal position of above 10%. 
         [0006]    When the sign of the propulsive torque changes, which is connected for example with a change of the propulsive torque from an overrun to a traction state, or in an acceleration reversal of the vehicle, in which a sign of the accelerating force is changed, a load reversal occurs, which may result in a torque zero crossing. The torque zero crossing may cause oscillations in the affected drive train, which may impact the operating behavior of the vehicle negatively or may be disagreeably noticeable to the driver. One possibility of reducing the load reversal oscillations is to slow down the zero crossing artificially, which has the consequence, however, that the vehicle following an accelerator pedal position at less than 10% is only able to accelerate again after a delay. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The present invention is based on the realization that the oscillations produced by a load reversal may be reduced if during the load reversal, in which a change of the overall torque or the propulsive torque occurs, the different directions of action, i.e. the different signs of the axle torques, are maintained, for example by a reversal of a sign of the propulsive torque. Thus for example the sign of a torque produced by a combustion engine is maintained unchanged during a load reversal of the vehicle, which makes it possible to avoid the aforementioned zero crossing on the vehicle axle that is driven by the combustion engine. In order to brake the hybrid vehicle nevertheless, a negative torque may be set for example by the electric motor in such a way that the sum of the torques applied on the axles of the hybrid vehicle yields a negative overall torque or propulsive torque. In other words, only the zero crossing of the respective axle torque is permitted at which the least oscillations are to be expected, as may be the case for example in the torque produced by the electric motor. Alternatively, only the zero crossing of the respective axle torque is permitted, which is able to assume the lowest maximum value. 
         [0008]    Thus, due to a more direct mechanical coupling of the electric motor to an electromotively driven axle, lower vehicle oscillations are to be expected in the zero crossing of the electromotively generated torque than is the case in a zero crossing of the torque generated by the combustion engine. 
         [0009]    The present invention relates to a method for driving a hybrid vehicle using for example two drive sources acting on the respective hybrid vehicle axle in a load reversal including the application of a first torque on a first hybrid vehicle axle during the load reversal and the application of a second torque on a second hybrid vehicle axle during the load reversal, the direction of action of the second torque being opposite to a direction of action of the first torque. 
         [0010]    According to one specific embodiment, the load reversal includes an acceleration reversal, in particular an acceleration sign change, or an acceleration change or a drive load reversal or a change of a sign of a sum of all drive torques or a sign change of an overall drive torque, in particular a change from an overrun to a traction state or from a traction to an overrun state. 
         [0011]    According to one specific embodiment, the directions of action of the first torque and/or of the second torque are maintained during the load reversal such that none of the axles perform a load reversal. 
         [0012]    According to one specific embodiment, after a predefined time following the occurrence of the load reversal, torques of the same direction of action, i.e. the same sign, are applied on the first hybrid vehicle axle and on the second hybrid vehicle axle. For this purpose, the direction of action of the first or of the second torque may be changed such that the load reversal affects only one of the torques. Furthermore, this causes no additional oscillations. 
         [0013]    According to one specific embodiment, the load reversal is brought about by a braking of the hybrid vehicle, the direction of action of the first torque coinciding with a direction of rotation of the first hybrid vehicle axle during the load reversal, and the direction of action of the second torque being opposite to a direction of rotation of the second hybrid vehicle axle. The braking may therefore be effected by reversing a direction of rotation of a single torque. 
         [0014]    According to one specific embodiment, the first hybrid vehicle axle is driven by a combustion engine or by an electric motor and the second hybrid vehicle axle is driven by an electric motor or by a combustion engine. Preferably, only the electromotively generated torque is changed for the load reversal regulation. 
         [0015]    According to one specific embodiment, the respective torque is applied on the first hybrid vehicle axle and the second hybrid vehicle axle simultaneously so as to allow for a simple generation of an overall torque. 
         [0016]    According to one specific embodiment, a value of the first torque is kept constant during the load reversal, whereby its zero crossing is avoided. 
         [0017]    According to one specific embodiment, the first torque and/or the second torque are determined as a function of a difference between an actual overall torque, in particular a positive actual overall torque, and a setpoint overall torque, in particular a negative setpoint overall torque, the first and the second torques being generated in such a way that their sum corresponds to the setpoint overall torque. This allows for a simple determination of the total torque to be generated. 
         [0018]    According to one specific embodiment, the first torque or the second torque or the direction of action of the first or the second torque are selected on the basis of a user-dependent or operation-dependent parameter, in particular on the basis of an accelerator pedal position or a gear change or a motor speed, or a driving operation or a gear speed or an uphill gradient or a downhill gradient of a road to be traveled or a motor speed or a dynamics of accelerator pedal movements or as a function of a charge state of a battery or of more than one of the aforementioned parameters. 
         [0019]    The present invention further relates to a device set up in terms of program engineering, in particular a control unit, which is developed to run a computer program for implementing the method according to the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  shows a hybrid vehicle having two drive axles. 
           [0021]      FIG. 2  shows torque variations over time. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    The hybrid vehicle shown in  FIG. 1  includes a first axle  101 , which has a torque generated by a combustion engine applied to it for example, and a second axle  103 , which has a torque generated by an electric motor applied to it for example. 
         [0023]    In the event of a vehicle load reversal, in which a change of the overall torque occurs for example in a transition from a traction state to an overrun state, it is nevertheless necessary to obtain a negative overall torque. For this purpose, the sign of the torque on vehicle axle  101  is maintained as positive for example, while the torque on vehicle axle  103  undergoes a zero crossing and becomes negative. The negative torque is selected in such a way for example that, in an accelerator pedal position of 0% for example, the sum of the torques acting on the axles of the hybrid vehicle yields a negative overall torque, whereby the vehicle decelerates. In the sense of the present invention, a torque is positive if its direction of action coincides with a direction of rotation of the axle, and it is negative if its direction of action is opposite to the direction of rotation of the axle. 
         [0024]    The directions of action of the torques with respect to the vehicle axles in an accelerator pedal position of 0% are shown in exemplary fashion in  FIG. 1 . The directions of rotation of the two axles  101  and  103  are identical and are directed in such a way that the hybrid vehicle is moved forward. Thus the direction of action  105  of the torque, which is generated by the combustion engine for example, on first vehicle axle  101  coincides with a direction of rotation  107  of first vehicle axle  101  such that the sign of this torque is positive. By contrast, the direction of action  109  of the torque, which is generated by the electric motor for example, on second vehicle axle  103  is opposite to a direction of rotation  111  of second vehicle axle  103  such that the sign of the electromotive torque is negative. 
         [0025]    If, after a load reversal or a movement of the accelerator pedal to 50% for example, the vehicle is accelerated again, then the torque, which is generated by the combustion engine for example, on axle  101  may be increased without a zero crossing so as to allow for a quick reacceleration that is low in vibration. 
         [0026]    The respective operating mode or the load reversal to be expected is detectible on the basis of the accelerator pedal position for example. At an accelerator pedal position of 0% for example, it is thus possible to apply a braking torque of −30 Nm for example on the vehicle axle that is driven by the electric motor, while a torque of +15 Nm may be applied on the axle driven by the combustion engine in propulsion. This yields an overall torque of −15 Nm such that the vehicle is decelerated. At an accelerator pedal position of 0%, however, the propulsive torque on the axle driven by the combustion engine may also be +5 Nm or +10 Nm. The torque is preferably selected to be positive and of such a magnitude that the drive train of this axle is not brought into oscillation even when driving over bumps. 
         [0027]    At an accelerator pedal position of 50% for example, the axle driven by the electric motor has a braking torque of −15 Nm applied to it for example. At the same time, the axle driven by the combustion engine in propulsion has +100 Nm applied to it for example, resulting in an overall torque of +85 Nm. This operating mode may be selected in sporty driving for example. 
         [0028]    For further acceleration of the hybrid vehicle, however, a positive torque is applied to both axles. Thus, at an accelerator pedal position of 100% for example, the electromotively driven axle in propulsion is to have +100 Nm applied to it, while a propulsive torque of +300 Nm is to be applied on the axle driven by the combustion engine. If this operating mode follows upon an operating mode, in which the hybrid vehicle was decelerated, then the torque, which is applied on the electromotively driven axle, undergoes a change of sign, i.e. a zero crossing, while the torque generated by the internal combustion engine may be increased without a change of sign. This avoids a zero crossing of the torque generated by the combustion engine. 
         [0029]    If, in an accelerator pedal position of 0% for example, the axle torques are selected in such a way that on one axle a direction of action of the torque coincides with a direction of rotation of this axle and on another axle a direction of action of the torque is opposite to a direction of rotation of this axle, then this may result in an increased tire abrasion for example. For this reason, the directions of action of the torques are selected to be identical after a predefined time following the occurrence of the load reversal, for example after 10 s or 20 s. 
         [0030]    Furthermore, the torque or its direction of action may be selected for each axle as a function of a vehicle operating mode, for example a sport mode or an economical mode, of the selected gear, of an uphill gradient or of a downhill gradient of the road to be traveled, of a motor speed, of a dynamics of the accelerator pedal movements of the driver, for example in quick or frequent accelerator pedal movements, or as a function of a charge state of the battery or of more than one of the aforementioned parameters. 
         [0031]    The zero crossing of the torque that is generated electromotively for example may occur at a later time than the load reversal of the vehicle, for example the zero crossing of the vehicle propulsion, as is shown in  FIG. 2 . The zero crossing of the axle torque may also be performed at a time for example at which a gear speed change is performed for example so as to mask oscillations that possibly occur in the process. 
         [0032]      FIG. 2  shows a time characteristic of an overall torque  201 , a first torque  203 , which is applied on the first vehicle axle  101 , and a second torque  205 , which is applied on the second vehicle axle  103 . Starting for example from a decelerated travel following a previous load reversal (not shown in  FIG. 2 ), a new load reversal occurs at time  207 , in which the overall torque  201  undergoes a zero crossing and becomes positive for example. At this time, the directions of action of torques  203  and  205  differ such that at the time of the load reversal first torque  203  is positive and second torque  205  is negative. After the load reversal has occurred, both torques rise, second  205  undergoing a zero crossing at time  209  and becoming positive such that from this time onward the directions of action of both torques are identical. The time interval between times  207  and  209  defines for example a specified time, for example 1 s or 2 s, after the expiration of which the directions of action of the two torques are synchronized. 
         [0033]    According to the present invention, the degrees of freedom provided by the axle hybrid concept are thus utilized efficiently. Thus, in an overrun state for example, the drive torque on one of the axles, on the axle driven by the combustion engine for example, may remain positive. This achieves the result that the load reversal, e.g. the acceleration reversal of the vehicle, requires a zero crossing only on one axle.