Abstract:
A method of controlling the operating response of a hybrid drive of a vehicle is described, the hybrid drive having an internal combustion engine and at least one electric motor as the drive motors, and the drive shafts of the drive motors being mechanically linkable to a drive train of the vehicle. A drag torque characteristic curve for the hybrid drive is established by targeted activation of the at least one electric motor.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method for controlling the operating response of a hybrid drive of a vehicle, the hybrid drive including an internal combustion engine and at least one electric motor as the drive motors, and the drive shafts of the drive motors being functionally connectable to a drive train of the vehicle. 
   BACKGROUND INFORMATION 
   Hybrid drives for vehicles are known. In the hybrid drives claimed here, an internal combustion engine is combined with at least one electric motor so that multiple drive sources are provided for the vehicle. According to the requests specified by a vehicle driver, the drive sources can optionally supply their drive moments to a drive train of the vehicle. Depending on the actual driving situation, this results in various drive configuration possibilities for the drive, in a manner known as such, which in particular are used for improving driving comfort and reducing energy use as well as pollutant emissions. 
   In hybrid drives for vehicles, serial configurations, parallel configurations, and mixed configurations of an internal combustion engine and electric motors are known. Depending on the configuration, the electric motors may be directly or indirectly coupled to the drive train of the internal combustion engine. For the mechanical linkage of the internal combustion engine and/or the electric motors, it is known to provide these with a mechanically linkable configuration via gearing, for example planetary gears or the like, and clutches. 
   It is known that the internal combustion engine functions as the main drive source for the vehicle, while the electric motors, depending on the actual driving situation, may be engaged or disengaged. In drive designs it is known that for a negative torque request by a vehicle driver the internal combustion engine delivers a drag torque which is determined by the internal losses in the internal combustion engine as a function of an instantaneous engine rotational speed. If the gears are changed during the negative torque requests by the vehicle driver, for example in the operation of a manual transmission or actuation of a hydrodynamic converter (automatic transmission), abrupt variations in the output torque of the drive result on account of the abrupt change in the rotational speed acting on the drive train (corresponding to the gear position selected). This is caused by the fact that the drag torque of the internal combustion engine is a function of the rotational speed. 
   SUMMARY OF THE INVENTION 
   The method according to the present invention, in contrast, offers the advantage that the drag torque is independent of the engine rotational speed of the internal combustion engine when the vehicle driver requests a negative torque (drag torque). By establishing a drag torque characteristic curve for the hybrid drive through a targeted activation of the at least one electric motor of the hybrid drive, it is advantageously possible to influence in a targeted manner a drag torque characteristic curve of the hybrid drive as a function of the vehicle speed. By activating the at least one electric motor, the drag torque of the internal combustion engine may be compensated for in whole or in part, or an additional drag torque may be generated by operating the electric motor in generator mode. 
   In particular, by disengaging the internal combustion engine from the drive train, by engaging the clutch, for example, drag torques may be applied solely by the at least one electric motor, thereby establishing a drag torque response for the vehicle to which the vehicle driver is accustomed. In each case, an abrupt response of the drag torque characteristic curve of the internal combustion engine, and thus of the hybrid drive, may be compensated for by the resulting variations in the activation of the electric motor. 
   In one preferred embodiment of the present invention, a so-called coasing operation of the hybrid drive is achieved without disengaging the internal combustion engine from the drive train. The drag torque applied by the internal combustion engine is compensated for by activating the at least one electric motor in such a way that the drag torque of the internal combustion engine is exactly compensated for by a positive torque of the at least one electric motor. The vehicle is then decelerated solely by the drive resistance acting on the vehicle. 
   In addition, in one preferred embodiment of the present invention it is possible to vary the drag torque characteristic curve as a function of a braking request of a vehicle driver by the targeted activation of the at least one electric motor. This enables braking support to be easily provided by increasing a drag torque of the hybrid drive. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a torque characteristic curve of an internal combustion engine. 
       FIG. 2  shows the variation of an output torque of a vehicle. 
       FIG. 3  schematically shows a hybrid drive of a vehicle. 
       FIG. 4  shows a block diagram of the influence on the drag torque of the hybrid drive. 
       FIG. 5   a  shows a first view of the drag torque characteristic curve of drive sources for the hybrid drive. 
       FIG. 5   b  shows a second view of the drag torque characteristic curve of drive sources for the hybrid drive. 
       FIG. 6  shows various drag torque characteristic curves of the hybrid drive. 
       FIG. 7  shows a block diagram of the coordination of drag torque influencing as a function of a braking request for the vehicle. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows for illustrative purposes a torque characteristic curve of an internal combustion engine as a function of the rotational speed. A maximum torque characteristic curve  10  for a power request by a vehicle driver is illustrated, as well as a drag torque characteristic curve  12  for a negative power request by the vehicle driver. A negative power request is present when the vehicle driver does not actuate the gas pedal of the vehicle; i.e., the gas pedal is at or approximately at the zero position. Based on characteristic curve  12  in  FIG. 1 , it is apparent that in this operating mode of the vehicle, drag torque characteristic curve  12  is established as a function of engine rotational speed n. The higher the engine rotational speed n, the higher the drag torque. The drag torque is negative with respect to the output torque of the internal combustion engine, and in particular is a function of internal losses in the internal combustion engine, in particular friction and the like. Above drag torque characteristic curve  12  a region  15  is designated which cannot be adjusted by activating the internal combustion engine, for example, for non-Diesel internal combustion engines, a region which cannot be adjusted by positioning a throttle valve or specifying an ignition angle, and for Diesel engines, a region which cannot be adjusted by a fuel injection quantity. It is clear that an operating point of the internal combustion engine is defined by engine rotational speed n and torque M, the upper limit being bounded by maximum torque characteristic curve  10  and the lower limit being bounded by drag torque characteristic curve  12 . 
     FIG. 2  illustrates an output torque M A  as a function of time t. It is assumed here that at a time t 1  a transmission mechanically linked to the internal combustion engine is switched, for example by manual or automatic shifting, resulting in an abrupt change in output torque M A  when a negative torque on the internal combustion engine is requested by a vehicle driver. This abrupt change in output torque M A  acts on the drive wheels of the vehicle and results in impaired driving comfort. 
     FIG. 3  schematically illustrates a hybrid drive  14  of a vehicle which includes an internal combustion engine  16  having the characteristic curves illustrated in  FIGS. 1 and 2 .  FIG. 3  illustrates a parallel hybrid drive in which a drive train  18  includes at least one, in this case two, electric motors  20  and  22  in addition to internal combustion engine  16 . Drive train  18  includes clutches  24 ,  26 , and  28  and transmission  30 . By engaging clutches  24 ,  26 , and/or  28  and activating internal combustion engine  16  or electric motor  20  and/or  22 , it is possible to establish a selectable output torque on drive shaft  32  of drive train  18 . The structure and operating principle of such parallel hybrid drives  14  are generally known, so that further detail is not provided within the scope of the present description. 
     FIG. 4  shows once more, in a block diagram, the possibility of influencing the drag torque of the configuration shown in  FIG. 3 . It is assumed that a negative torque request by a vehicle driver is present; i.e., the gas pedal is at or approximately at the zero position. This negative torque request  34  is fed to a drag torque coordinator  36 . Drag torque coordinator  36  may be formed from a circuit system, not shown in detail, which for example is a component of a control unit (motor control unit) of hybrid drive  14 . Drag torque coordinator  36  sends control signals  38 ,  40 , and  42 . Control signals  38 ,  40 , and  42  activate internal combustion engine  16  or electric motors  20  and  22  in such a way that these signals request a defined torque M 16 , M 20 , or M 22  which act on drive train  18 . In the example shown, two electric motors  20  and  22  are assumed, it being clear that the number of electric motors may be greater or smaller, so that a corresponding number of control signals for electric motors  20  and  22  is provided by drag torque coordinator  36 . A control signal  44  is also fed to drag torque coordinator  36  which corresponds to the instantaneous gear ratio of transmission  30 , and thus to its gear position. 
     FIG. 5   a  illustrates once again drag torque characteristic curve  12  for internal combustion engine  16 , clearly showing the dependency of the characteristic curve on rotational speed n. 
     FIG. 5   b  shows the maximum and minimum possible torque characteristic curves  46  and  48 , respectively, of an electric motor as a function of rotational speed n E  of the electric motor. Rotational speed n E  of the electric motor is a function of the supply voltage, for example, so that rotational speed n E  of electric motors  20  and  22 , and thus their torque M E , may be readily controlled by signals  40  and  42  ( FIG. 4 ). 
   Based on  FIGS. 5   a  and  5   b , it is apparent that it is possible to establish a resulting drag torque characteristic curve for hybrid drive  14  by superimposing the torque characteristic curve of internal combustion engines  16  and the torque characteristic curves of electric motors  20  and  22 . When clutches  24 ,  26 , and  28  are engaged, the sum of superimposed torques M V  or M E  is the maximum achievable drag torque of hybrid drive  14 . The dependency of the torques on rotational speed n V  or n E  results in a dependency on instantaneous vehicle speed v. 
     FIG. 6  clarifies once again a superimposition of drag torque characteristic curve  12  for internal combustion engine  16  on the respective maximum or minimum torque characteristic curves  46 ,  46 ′ and  48 ,  48 ′ of electric motors  20  or  22 , respectively. In each case the torques are plotted as a function of vehicle speed v. Drag torque characteristic curve  12  is plotted as a characteristic curve family (dash-dot-dash-line), the various drag torque characteristic curves  12  resulting from the instantaneous engaged mode of transmission  30 . Five drag torque characteristic curves  12  are plotted in  FIG. 6 , assuming a 5-speed manual transmission  30 . The maximum torque characteristic curves of electric motors  20  and  22  are represented by  46  and  46 ′, respectively, and the minimum torque characteristic curves of electric motors  20  and  22  are represented by  48  and  48 ′, respectively. 
   Corresponding to the engagement of clutches  24 ,  26 , and  28 , various possibilities are selected for influencing the overall drag torque of hybrid drive  14  by internal combustion engine  16  or electric motors  20  and  22 . The following possibilities result, for example:
     1. Clutch  28  is disengaged, so that the drag torque is determined solely by electric motor  22 .   2. Clutches  28  and  26  are engaged and clutch  24  is disengaged, so that the drag torque of hybrid drive  14  is determined by electric motors  20  and  22 .   3. Clutches  24 ,  26 , and  28  are each engaged, so that the drag torque of the hybrid drive is determined by electric motors  20  and  22  as well as by internal combustion engine  16 .   

   It is clear that various superimpositions of drag torque characteristic curve  12  of internal combustion engine  16  are possible as a function of the activations of clutches  24 ,  26 , and  28  and of electric motors  20  and  22 . When clutch  24  is disengaged, a so-called coasing operation of hybrid drive  14  is initiated, in this case it still being possible to influence the drag torque by electric motors  20  and  22 . 
   In  FIG. 6 , the maximum possible drag torque characteristic curve of hybrid drive  14  is denoted by reference number  50 . This results when drag torque characteristic curve  12  of the internal combustion engine is superimposed on minimum torque characteristic curves  48  and  48 ′ of electric motors  20  and  22 . As shown in  FIG. 6 , this maximum possible drag torque characteristic curve  50  exhibits discontinuities as a function of vehicle speed v. For this reason a continuous drag torque characteristic curve  52  is selected by drag torque coordinator  36 , which is maximally matched to maximum possible drag torque characteristic curve  50 . This selected drag torque characteristic curve  52  is obtained by storing control signals  38 ,  40 , and  42  for internal combustion engine  16  or electric motors  20  and  22  in drag torque coordinator  52  for each vehicle speed v as a function of the gear position of transmission  30  (signal  44 ). Electric motors  20  and  22  may be operated in engine mode or generator mode within their maximum or minimum characteristic curves  46 ,  48 , respectively, so that it is possible to set drag torque characteristic curve  52  for hybrid drive  14  by the resulting superimposition of characteristic curves. 
   Of course, other drag torque characteristic curves  52  may be selected by executing appropriate routines. Drag torque coordinator  36  may optionally be provided with a characteristic map control which enables various drag torques to be set as a function of additional operating parameters of the vehicle for the same vehicle speeds v. 
   A flow diagram for variation of drag torque characteristic curve  52  as a function of an actuation of the vehicle brakes is illustrated as an example in  FIG. 7 . Initially there is no request for a negative torque (no drag torque by the vehicle driver) according to field  60 . A query  62  continually checks whether a drag torque request is present. This can be achieved by monitoring the position of the gas pedal, for example. If the position of the gas pedal is below a specifiable limit, drag torque coordinator  36  is activated by signal  34  (negative torque request). If the position of the gas pedal is not below the specifiable limit, the request remains “no drag torque” (field  60 ). 
   Drag torque coordinator  36  generates signals  38 ,  40 , and  42 , resulting in the establishment of drag torque characteristic curve  52 . As shown in  FIG. 6 , this drag torque characteristic curve  52  is defined as a function of speed. 
   If the braking device is actuated during the drag torque request by the vehicle driver, so that detection may be performed, for example through a signal applied to a brake light switch, when the braking device is actuated a query  64  sends a signal to drag torque coordinator  36 , which then increases the drag torque one time (field  52 +). If the vehicle driver actuates the brake again (query  66 ), a new control signal is generated at drag torque coordinator  36  which then further increases the drag torque, either to a higher level or up to the maximum drag torque (field  52 ++). A query  68  constantly monitors whether a further positive torque request by the vehicle driver, for example by actuation of the gas pedal, is made. If this positive torque request is made, control of the drag torque characteristic curve is transferred in field  60  so that the drag torque is reduced by a transition function, and hybrid drive  14  is controlled corresponding to the positive torque request. 
   The explanations make it clear that many different drag torque characteristic curves for hybrid drive  14  may be established by drag torque coordinator  36 . This may be achieved as a function of vehicle speed v and/or as a function of an actuation of the vehicle brakes. As discussed, this may involve two or more steps. Gas pedal dynamics, for example, may be taken into account as an additional control variable. The gas pedal dynamics may be derived from a gradient of a gas pedal signal, for example. Additional influencing variables may be, for example, a given roadway grade such as when traveling downhill or the like. In this case the drag torque may be adjusted in such a way that a constant speed v of the vehicle is maintained. 
   In summary, it is apparent that a negative torque for the operating points of hybrid drive  14  may be specified, within the physical limits of hybrid drive  14 , by influencing the drag torque characteristic curve via drag torque coordinator  36 . In particular, drag torque characteristic curves may be established which are independent of the rotational speed. In addition, sudden changes in torque during downshifting of transmission  30  are prevented by regulation of speed-dependent drag torque curves. Furthermore, the drag torque response of hybrid drive  14  may be taken into account during changes of the operating mode, for example internal combustion engine mode, electric motor mode, or combined internal combustion engine and electric motor mode. Lastly, a so-called coasing operation is also possible when internal combustion engine  16  is engaged in drive train  18  by electric motors  20  and/or  22  compensating for the drag torque of internal combustion engine  16 . 
   The explanation of the exemplary embodiment was based on a parallel hybrid drive  14 . Of course, the operating response of the hybrid drive may be controlled by influencing a drag torque characteristic curve for serial hybrid drives and mixed (parallel and serial) hybrid drives as well.