Abstract:
A method of operating a hybrid drive system with a main driving machine  11 —more particularly an internal combustion engine—and a supplementary driving machine  12 )—more particularly an electric machine—for a motor vehicle, comprising
       a first gear changing partial drive  15  with an input shaft  17  and an output shaft  23  and a first group of gear changing pairs of gearwheels each having a gearwheel connected in a rotationally fast way to its shaft and a switching gearwheel which can be switchably coupled to its shaft and whose input shaft ( 17 ) can be coupled to the main driving machine  11;      a second gear changing partial drive  16  with an input shaft  18  and an output shaft  24  and a second group of gear changing pairs of gearwheels each having a gearwheel connected in a rotationally fast way to its shaft and a switching gearwheel which can be switchably coupled to its shaft and whose input shaft  18  is connectable in respect of drive to the supplementary driving machine  12  at a fixed ratio; wherein the pairs of gear changing gearwheels, in the gear changing sequence, are alternately associated with one of the gear changing partial drives  15, 16  and wherein the two input shafts  17, 18  can be connected to one another in a rotationally fast way via a coupling unit  29 , wherein, during operation by means of the supplementary driving machine  12  only there is effected a gear change between two gears adjoining one another in the gear changing sequence by changing the torque flow from one gear changing partial drive  15  to the other gear changing partial drive  16.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European Patent Application No. EP07005675.9 filed Mar. 20, 2007. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a method of operating a hybrid drive system and to a hybrid drive system with a main driving machine, more particularly an internal combustion engine, and a supplementary driving machine, more particularly an electric machine, for a motor vehicle. Other types of driving machines are not excluded. For example, it is possible to provide two electric machines as the main driving machine and the supplementary driving machine or, in addition to an internal combustion engine as the main driving machine, a hydraulic machine as the supplementary driving machine. 
       BACKGROUND OF THE INVENTION 
       [0003]    Vehicles with a hybrid drive system in the different embodiments have, in certain driving cycles, a more advantageous exhaust gas behaviour than vehicles which are driven entirely by an internal combustion engine. They therefore become more and more important on the market. 
         [0004]    If an electric machine is used as the supplementary driving machine, it can be used as an engine and a generator. As far as the engine function is concerned, wherein it is necessary to provide a battery for power supply purposes, it is possible to use it to start an internal combustion engine or to use it as a driving motor. When used as a generator, the electric machine is used for charging the battery, wherein the energy is obtained from the internal combustion engine or from the recovery of the kinetic vehicle energy. 
         [0005]    Hybrid drive systems are described for example in WO 2005/073005 A1, DE 100 49 514 A1 and DE 198 18 108 A1. 
         [0006]    From DE 199 60 621 A1 there is known a hybrid drive for vehicles with a manual gearbox which comprises a first switchable partial drive which, optionally, can be connected in respect of drive to an internal combustion engine and/or an electric machine, as well as a second switchable partial drive which, in respect of drive, can be connected to the electric machine which can be operated as an electric motor or a generator. The first partial drive comprises a first lay-shaft and an output shaft as well as six transmission stages; the second partial drive comprises a second lay-shaft and the same output shaft and comprises three transmission stages. 
       SUMMARY OF THE INVENTION 
       [0007]    It is therefore the object of the present invention to provide a method of operating a hybrid drive system which allows a simplified design, as well as a hybrid drive system which is characterised by a simplified design relative to the number of available transmission stages. More particularly it is desirable to provide a compact design for transverse installation in motor vehicles. 
         [0008]    The objective is achieved by providing a method of operating a hybrid drive system with a main driving machine—more particularly an internal combustion engine—and a supplementary driving machine—more particularly an electric machine—for a motor vehicle, comprising 
         [0009]    a first gear changing partial drive with an input shaft and an output shaft and a first group of gear changing pairs of gearwheels each having a gearwheel connected in a rotationally fast way to its shaft and a switching gearwheel which can be switcbably coupled to its shaft and whose input shaft can be coupled to the main driving machine; 
         [0010]    a second gear changing partial drive with an input shaft and an output shaft and a second group of gear changing pairs of gearwheels each having a gearwheel connected in a rotationally fast way to its shaft and a switching gearwheel which can be switchably coupled to its shaft and whose input shaft can be connected in respect of drive to the supplementary driving machine; 
         [0011]    wherein the two input shafts can be connected to one another in a rotationally fast way via a coupling unit, wherein, during operation by means of the main driving machine only, there is effected a gear change between two gears adjoining one another in the gear changing sequence, by changing the torque flow from one gear changing partial drive to the other gear changing partial drive. 
         [0012]    Furthermore it is proposed according to a preferred embodiment that, during operation by means of the supplementary driving machine only there is effected a gear change between two gears adjoining one another in the gear changing sequence, by changing the torque flow from one gear changing partial drive to the other gear changing partial drive. In this way it is possible to reduce the number of gear changing pairs of gearwheels in both gear changing partial drives, combined, to the number of required gears. 
         [0013]    Furthermore, the objective is achieved by providing a hybrid drive system with a main driving machine—more particularly an internal combustion engine—and a supplementary driving machine—more particularly an electric machine—for a motor vehicle, comprising 
         [0014]    a first gear changing partial drive with an input shaft and an output shaft and a first group of gear changing pairs of gearwheels each having a gearwheel connected in a rotationally fast way to its shaft and a switching gearwheel which can be switchably coupled to its shaft and whose input shaft can be coupled to the main driving machine; 
         [0015]    a second gear changing partial drive with an input shaft and an output shaft and a second group of gear changing pairs of gearwheels each having a gearwheel connected in a rotationally fast way to its shaft and a switching gearwheel which can be switchably coupled to its shaft and whose input shaft is connectable in respect of drive to the supplementary driving machine; 
         [0016]    wherein the pairs of gear changing gearwheels, in the gear changing sequence, are alternately associated with one of the gear changing partial drives and wherein the two input shafts can be connected to one another in a rotationally fast way via a coupling unit. 
         [0017]    The essential part of the solution consists in providing the drive in the form of two partial drives whose gear stages are distributed so as to alternate, i.e. the first, the third and the fifth gear are associated with the partial drive which is connectable to the main driving machine, i.e. the internal combustion engine, and the second, the fourth and the sixth gear are associated with the partial drive which is firmly connected to the supplementary driving machine, i.e. the electric machine. 
         [0018]    The first gear is thus available for starting by means of the internal combustion engine and for starting electrically, there is available the first gear or the second gear. If the two input shafts are firmly connected to one another, it is possible to use gears one to six, and optionally, a reverse gear when using the internal combustion engine. In such a case, in gears one, three and five, the electric machine can remain disconnected from the drive and in gears two, four and six, the electric machine can remain torque-free. Furthermore, if the two input shafts are firmly coupled, when operating with the electric motor, it is possible to use gears one to six for gear changing purposes, while the internal combustion engine is disconnected by the friction coupling. When the input shafts are disconnected, gears two, four and six are available for driving the vehicle by the electric motor only. By coupling the two input shafts (gears two, four and six) and, respectively, by disconnecting the two input shafts (gears one, three and five) a boost operation is possible in all gears, i.e. operation by internal combustion engine with an additional electric drive. 
         [0019]    By selecting an appropriate sequence of opening and closing the friction coupling of the internal combustion engine and the coupling unit between the two input shafts, followed by a suitably adapted sequence of operating the switching units for the different gears, a traction-force-interruption-free method of switching between the gears is possible. Prior to switching the coupling unit, it is advisable to synchronise the speeds of the input shafts. A decisive feature of this kind of operation is that with the inventive drive assembly, the element to be switched (manual clutch, switching unit) can always be disconnected, while at least one drive, either the electric machine or the internal combustion engine remains in a torque transmitting connection with the drive output, i.e. with an output gearwheel. During the switching process, the speeds of the elements to be switched can be adapted by controlling the electric machine and the internal combustion engine, so that at least said coupling unit for connecting the two input shafts can be provided in the form of a simple switching coupling (synchronising unit). The friction coupling of the internal combustion engine permits a slipping connection of the internal combustion engine, such as it is common practice. 
         [0020]    Due to the inventive arrangement of the electric machine and the internal combustion engine, torque will be added up when both machines are operated. As will be explained below, it is possible to use the electric motor for starting purposes and to operate it as a generator in a recuperation mode. 
         [0021]    The drive system is designed in such a way that under full load conditions and under permanent load conditions, only the internal combustion engine is used. However, the desired functions of a hybrid drive have been put into effect at low cost and without any limitations. 
         [0022]    According to an advantageous first embodiment which permits a radial compact design it is proposed that both input shafts are arranged coaxially, especially in-line with one another, and are connectable to one another by a coaxially arranged coupling unit. More particularly, it is proposed that the two output shafts are in-line with one another and integrally connected to one another. 
         [0023]    According to a second design embodiment which permits a short length for a transverse installation in the motor vehicle it is proposed that the two input shafts are arranged parallel to one another and that the coupling unit is arranged coaxially on one of the input shafts and acts on a switching gear wheel which is arranged on said input shaft and which forms a pair of gearwheels with a gearwheel firmly arranged on the other input shaft. Furthermore, it is proposed that the two output shafts are arranged so as to extend parallel to their input shaft and are each coupled by fixed gears to an individual output gear. 
         [0024]    According to a third design embodiment which permits a short length for a transverse installation in the motor vehicle too it is proposed that the two input shafts are arranged parallel to one another and that the coupling unit is arranged coaxially on one of the input shafts and acts on a switching gear wheel which is arranged on said input shaft and which forms a geartrain with a gearwheel firmly arranged on the other input shaft. This geartrain especially can comprise an intermediate gearwheel being firmly connected to the supplementary driving machine. Furthermore, it is proposed that the two output shafts form one integral shaft member. 
         [0025]    Further advantageous embodiments are described in the sub-claims to the contents of which reference is hereby made. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The different operating conditions which, above, were indicated only, are described in greater detail in the following description of the drawings. 
           [0027]    Three preferred embodiments of the invention are illustrated in the drawings and will be described below. 
           [0028]      FIG. 1  shows the drive concept of an inventive hybrid drive system in a first embodiment in a three-shaft-design in a neutral position. 
           [0029]      FIG. 2  shows the drive concept according to  FIG. 1  when starting and driving the motor vehicle, using the electric machine EM. 
           [0030]      FIG. 3  shows the drive concept according to  FIG. 1  when starting the internal combustion engine CE by the electric machine EM in the stationary condition of the vehicle. 
           [0031]      FIG. 4  shows the drive concept according to  FIG. 1  when starting the internal combustion engine CE by the electric motor EM while the vehicle is driven electrically. 
           [0032]      FIG. 5   a  shows the drive concept according to  FIG. 1  when changing up from the second gear to the third gear during a first phase and during the boost mode. 
           [0033]      FIG. 5   b  shows the drive concept according to  FIG. 1  when changing up from the second gear to the third gear during a second phase. 
           [0034]      FIG. 5   c  shows the drive concept according to  FIG. 1  when changing up from the second gear to the third gear during a third phase. 
           [0035]      FIG. 5   d  shows the drive concept according to  FIG. 1  when changing up from the second gear to the third gear during a fourth phase. 
           [0036]      FIG. 6   a  shows the drive concept according to  FIG. 1  when changing down from the third gear to the second gear during a first phase. 
           [0037]      FIG. 6   b  shows the drive concept according to  FIG. 1  when changing down from the third gear to the second gear during a second phase. 
           [0038]      FIG. 6   c  shows the drive concept according to  FIG. 1  when changing down from the third gear to the second gear during a third phase. 
           [0039]      FIG. 6   d  shows the drive concept according to  FIG. 1  when changing down from the third gear to the second gear in a fourth phase and in the boost mode. 
           [0040]      FIG. 7   a  shows the drive concept according to  FIG. 1  in the recuperation mode (second, fourth or sixth gear). 
           [0041]      FIG. 7   b  shows the drive concept according to  FIG. 1  in the recuperation mode (first, third or fifth gear). 
           [0042]      FIG. 8  shows the drive concept according to  FIG. 1  when the vehicle is the stationary condition and driving a compressor. 
           [0043]      FIG. 9  shows the drive concept of an inventive hybrid drive system in a second embodiment in a four-shaft-design in a neutral position. 
           [0044]      FIG. 10  shows the drive concept according to  FIG. 9  when starting and driving with the electric machine EM. 
           [0045]      FIG. 11  shows the drive concept according to  FIG. 9  when starting the internal combustion engine CE by the electric machine EM with the vehicle driving. 
           [0046]      FIG. 12  shows the drive concept according to  FIG. 9  when starting the internal combustion engine CE by the electric machine EM in the stationary condition of the vehicle. 
           [0047]      FIG. 13   a  shows the drive concept according to  FIG. 9   a  when changing up from the fourth gear to the fifth gear during a first phase. 
           [0048]      FIG. 13   b  shows the drive concept according to  FIG. 9  when changing up from the fourth gear to the fifth gear during a second phase. 
           [0049]      FIG. 13   c  shows the drive concept according to  FIG. 9  when changing up from the fourth gear to the fifth gear in a third phase. 
           [0050]      FIG. 14   a  shows the drive concept according to  FIG. 9  in the boost mode (internal combustion engine CE in the first, the third or the fifth gear). 
           [0051]      FIG. 14   b  shows the drive concept according to  FIG. 9  in the boost mode (internal combustion engine CE in the second, the fourth or the sixth gear). 
           [0052]      FIG. 15  shows the drive concept according to  FIG. 9  in the recuperation mode. 
           [0053]      FIG. 16  shows the drive concept according to  FIG. 9  in the stationary condition of the vehicle when driving a compressor. 
           [0054]      FIG. 17  shows the drive concept of an inventive hybrid drive system in a third embodiment in a three-shaft-design in a neutral position. 
           [0055]      FIG. 18  shows the drive concept according to  FIG. 17  when starting and driving the motor vehicle, using the electric machine EM. 
           [0056]      FIG. 19  shows the drive concept according to  FIG. 17  when starting the internal combustion engine CE by the electric motor EM while the vehicle is driven electrically. 
           [0057]      FIG. 20  shows the drive concept according to  FIG. 17  when driving with the internal combustion engine CE. 
           [0058]      FIG. 21  shows the drive concept according to  FIG. 17  in the boost mode (internal combustion engine CE in the first, the third or the fifth gear). 
           [0059]      FIG. 22   a  shows the drive concept according to  FIG. 17  when changing up from the second gear to the third gear during a first phase. 
           [0060]      FIG. 22   b  shows the drive concept according to  FIG. 17  when changing up from the second gear to the third gear during a second phase. 
           [0061]      FIG. 22   c  shows the drive concept according to  FIG. 17  when changing up from the second gear to the third gear during a third phase. 
           [0062]      FIG. 22   d  shows the drive concept according to  FIG. 17  when changing up from the second gear to the third gear during a fourth phase. 
           [0063]      FIG. 23  shows the drive concept according to  FIG. 17  in the recuperation mode (first, third or fifth gear). 
           [0064]      FIG. 24  shows the drive concept according to  FIG. 17  when the vehicle is the stationary condition and driving a compressor. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0065]      FIG. 1  shows an inventive hybrid drive system in a first embodiment. The subsequent description of  FIG. 1 , in principle, also applies to  FIGS. 2 to 8  which merely show different switching conditions of the drive concept. 
         [0066]    There is shown a hybrid drive system which comprises a main driving machine  11 , here in the form of an internal combustion engine CE, a supplementary driving machine  12 , here in the form of an electric machine EM, and an auxiliary driven machine  13 , here in the form of a compressor for an air conditioning system A/C. The internal combustion engine  11  is connectable by a friction clutch  14  (Cl) which can be provided in the form of a wet or dry clutch. The drive comprises two gear changing partial drives  15 ,  16  (stepped gear changing boxes) which are characterised in that they each comprise their own input shafts  17  and  18 . The input shaft  17  of the first partial drive carries the gearwheels of gears  1 ,  3  and  5  and is connectable by the friction clutch  14  to the internal combustion engine  11 . The input shaft  18  of the second partial drive  16  carries the gearwheels of gears  2 ,  4  and  6  as well as an input gearwheel  19  which, by means of a gearwheel  20 , is in a stepped down driving connection with the electric machine  12  and, by means of a gearwheel  21 , with the air conditioning compressor  13 . In this embodiment, the output shafts  23 ,  24  of the two partial drives  15 ,  16  are firmly connected to one another; more particularly, they are provided in the form of a one-piece shaft. The switching gearwheels of the individual gears are positioned on the output shaft  23 ,  24 , and there is provided a common switching unit  25  for gears  3  and  5  and a further common switching unit  26  for gears  1  and  6 , as well as a switching unit  27  for gears  2  and  4 . This concept does not include a reverse gear. Reversing can take place by reversing the direction of rotation of the electric machine  12 . In addition, by using a reversing gearwheel on an intermediate shaft and a further switching unit, it is also possible to reverse the vehicle in the usual way when it is operated by the internal combustion engine  11 . The output shaft  23 ,  24  acts via a gearwheel  22  on an output gearwheel  28  of the drive, from which output can be taken, Between the input shafts  17 ,  18 , in accordance with the invention, there is arranged a coupling unit CU which, more particularly if the speeds of the two input shafts are synchronised, can be switched so as to be suitable for various operating conditions which will be described below with reference to further figures. 
         [0067]    The fixed gearwheels of gears  1  to  6  which are arranged in a rotationally fast way on the input shafts  17 ,  18  have been given in the gear sequence the reference numbers  41 ,  42 ,  43 ,  44 ,  45 ,  46 , and the respective switching gearwheels which are loose gearwheels suitable for being coupled to the output shafts  23 ,  24 , have been given in the gear sequence the reference numbers  51 ,  52 ,  53 ,  54 ,  55  each in  FIG. 1  only. The fixed gearwheels and the loose gearwheels could also be interchanged between the input and output shafts. 
         [0068]    In  FIG. 2 , the coupling unit  29  is disengaged, so that the input shafts  17 ,  18  are separated from one another. Of the drive gears, only the second gear is engaged by the switching unit  27 . In this switched condition, electric starting the vehicle—depending on the direction of rotation for forward driving or reversing—can be effected by the electric machine, with driving the vehicle also being possible with the electric machine. It is conceivable to change up into the fourth or sixth gear, in which case the traction force would be interrupted. A darker line indicates the torque flow from the electric machine  12  to the output gearwheel  28 . 
         [0069]    The following switched conditions apply: 
         [0070]    torque from EM 
         [0071]    clutch Cl open 
         [0072]    coupling CU disengaged 
         [0073]    second gear engaged. 
         [0074]      FIG. 3  shows the electric machine  12  in the starter function for the internal combustion engine. For this purpose, the coupling unit  29  is engaged and the function clutch  14  is closed. All gears are disengaged by the switching units  25 ,  26 ,  27 . A darker line shows the torque flow from the electric machine  12  to the internal combustion engine. 
         [0075]    The following switched conditions apply: 
         [0076]    torque flow from the electric machine EM 
         [0077]    coupling unit CU engaged 
         [0078]    clutch Cl closed
       starting of the internal combustion engine CE.       
 
         [0080]      FIG. 4  shows the internal combustion engine  11  being started by the electric machine during electric driving of the vehicle. The second gear is engaged by the switching unit  27 , so that torque flows from the electric machine  12  via the pair of gearwheels of the second gear to the output gearwheel  28  of the drive, whereas at the same time the coupling unit  29  is engaged and the friction clutch  14  is closed in order to start the internal combustion engine  11  in the torque flow via the two input shafts  18 ,  17  and the friction clutch  14 . Dark lines show the torque flow from the electric machine  12  to the internal combustion engine  11  and to the output gearwheel  28 . 
         [0081]    The following switched conditions apply: 
         [0082]    torque from the electric machine EM
       switching unit CU engaged       
 
         [0084]    clutch Cl closed 
         [0085]    second gear engaged. 
         [0086]    The illustrations of  FIG. 5  show different phases of changing up from the second into the third gear. 
         [0087]    In  FIG. 5   a  the friction clutch  14  is closed and the coupling unit  29  is engaged. Furthermore, the second gear is engaged by the switching unit  27 . Torque flows from the internal combustion engine  11  via the input shafts  17 ,  18  and the pair of gearwheels of the second gear to the output shaft  23 ,  24 , so that the vehicle can be driven by the internal combustion engine. There is indicated an additional torque flow from the electric machine via the pair of gearwheels  20 ,  19  to the input shaft  13 . This is the so-called boost mode in which additional torque is applied by the electric machine. The latter could also run in a torque-free condition. However, in the present case, the boost mode forms part of the switching process which follows. Covered lines indicate the torque flow from the internal combustion engine  11  and from the electric machine to the output gearwheel  28 . 
         [0088]    The following switched conditions apply: 
         [0089]    torque from the internal combustion engine CE 
         [0090]    clutch Cl closed 
         [0091]    coupling unit CU engaged 
         [0092]    additional torque from the electric machine EM 
         [0093]    second gear engaged. 
         [0094]    In  FIG. 5   b , the second gear is still engaged, but the friction clutch  14  is opened in order to separate the internal combustion engine  11  from the input shaft  17  and render it torque-free. Hereafter, the coupling unit  29  is disengaged in order to separate the input shaft  18  driven by the electric machine  12  from the input shaft  17 . A thickened line indicates the torque flow from the electric machine to the output gearwheel  28 . 
         [0095]    The following switched conditions apply: 
         [0096]    torque from the electric machine EM 
         [0097]    clutch Cl open 
         [0098]    coupling unit CU disengaged 
         [0099]    second gear engaged. 
         [0100]      FIG. 5   c  shows that the second gear continues to be engaged by the switching unit  27 , but at the same time, the third gear is engaged by the switching unit  25 . The input shaft  17  continues to be torque-free because the friction clutch  14  continues to be open. The torque flow takes place from the electric machine  12  via the fixed connection of the shafts  18 ,  24  to the output gearwheel  28 . 
         [0101]    The switched conditions are as follows: 
         [0102]    torque from the electric machine EM 
         [0103]    clutch Cl open 
         [0104]    coupling unit CU disengaged 
         [0105]    second gear still engaged 
         [0106]    third gear already engaged. 
         [0107]      FIG. 5   d  shows how the switching process is concluded in that the second gear is disengaged by the switching unit  27 , whereas at the same time, by closing the friction clutch  14 , the gearwheels of the already engaged third gear are incorporated in the torque flow from the internal combustion engine  11  via the input shaft  17  into the torque flow to the output shaft and to the output gearwheel  28 . A thickened line indicates the torque flow from the internal combustion engine to the output gearwheel  28 . 
         [0108]    The switched conditions are as follows: 
         [0109]    torque from the internal combustion engine CE, 
         [0110]    clutch Cl closed 
         [0111]    coupling unit CU disengaged 
         [0112]    second gear disengaged 
         [0113]    third gear engaged. 
         [0114]    The individual illustrations of  FIG. 6  show different phases of changing down from the third to the second gear. 
         [0115]    In  FIG. 6   a , the friction clutch  14  is closed and the coupling unit  29  is disengaged, so that the input shafts  17 ,  18  are separated from one another. The switching unit  25  couples the switching gearwheel of the third gear to the output shaft  23 , i.e. the third gear is engaged. Torque flows from the internal combustion engine  11  via the input shaft  17  and the pair of gearwheels of the third gear to the output shafts  23 ,  24  and to the output gearwheel  28 . The thickened line symbolises the torque flow from the internal combustion engine  11  to the output gearwheel  28 . 
         [0116]    The switched conditions are as follows: 
         [0117]    torque from the internal combustion engine CE
       friction clutch Cl closed       
 
         [0119]    coupling unit CU disengaged 
         [0120]    third gear engaged. 
         [0121]      FIG. 6   b  shows that for preparing the gear change, the second gear is engaged by means of the switching unit  27 , with no torque flowing yet via the pair of gearwheels of the second gear because the coupling unit  29  continues to be open, with the input shaft  18  being disconnected from the input shaft  17 . The electric machine  12  thus does not receive any power. The thickened line symbolises the torque flow from the internal combustion engine  11  to the output gearwheel  28 . 
         [0122]    The switched conditions are as follows: 
         [0123]    torque from the internal combustion engine CE, 
         [0124]    friction clutch Cl closed 
         [0125]    coupling unit CU disengaged 
         [0126]    third gear engaged 
         [0127]    second gear engaged. 
         [0128]    In  FIG. 6   c , the friction clutch  14  is now open, so that the input shaft  17  is separated from the torque flow. The electric machine  12 , whose speed has already been synchronised, now takes over the torque for driving the vehicle in the second gear. The switching gearwheel of the third gear, which is torque-free, is separated by the switching unit  25  from the output shaft  23 . The thickened line symbolises the torque flow from the electric machine  12  to the output gearwheel  28 . 
         [0129]    The switched conditions are as follows: 
         [0130]    friction clutch Cl open 
         [0131]    coupling unit CU disengaged 
         [0132]    third gear disengaged 
         [0133]    second gear engaged. 
         [0134]      FIG. 6   c  shows the final phase of the gear change from the third to the second gear, with the coupling unit first being engaged, so that the input shafts  17 ,  18  are coupled. Thereafter, the friction clutch  14  is closed, so that additional torque can flow from the combustion engine  11  via the input shafts  17 ,  18  and the pair of gearwheels of the second gear to the output gearwheel  28 . Thereafter, the electric machine  12  can be taken out of the torque flow by disengaging the second gear. Thickened lines symbolise the torque flow from the internal combustion engine  11  and from the electric machine  12  to the output gearwheel  28 . 
         [0135]    The switched conditions are as follows: 
         [0136]    torque from the internal combustion engine CE 
         [0137]    torque from the electric machine EM (optional) 
         [0138]    friction clutch Cl closed 
         [0139]    coupling unit CU closed 
         [0140]    second gear engaged. 
         [0141]    When the second gear is newly engaged or remains engaged, this represents the boost mode of the first gear. For each gear of the first partial drive  15  connected to the internal combustion engine  11  (first, third, fifth gear), there are four different stages of the boost mode which can be activated by engaging the second, forth or sixth gear of the second partial drive  16  or by engaging the coupling unit  29 . 
         [0142]      FIG. 7   a  shows a switched condition for energy recuperation when the vehicle is being pushed. The friction clutch  14  is open and the coupling unit  29  is disengaged. Via the engaged second gear, torque flows from the output gearwheel  28  via the output shaft  24  to the input shaft  18 , so that the electric machine  12  is generator-operated. A thickened line symbolises the torque flow from the output gearwheel  28  to the electric machine  12 . 
         [0143]    The switched conditions are as follows: 
         [0144]    torque to the electric machine EM
       friction clutch Cl open       
 
         [0146]    coupling unit CU disengaged 
         [0147]    second gear engaged. 
         [0148]    When the coupling unit  29  is disengaged, the recuperation mode can alternatively being used in the second, forth, or sixth gear of the second partial drive  16 . 
         [0149]      FIG. 7   b  shows the switched condition in the recuperation mode, i.e. when the vehicle is being pushed, using the third gear. The switching gearwheel of the third gear is coupled by the switching unit  25  to the output shaft  23 , so that torque flows from the output gearwheel  28  via the output shaft  24 ,  23 . Just as in the case when the first and the fifth gear are used, the coupling unit  29  has to be engaged so that torque is transmitted from the input shaft  17  to the input shaft  18  and from there to the electric machine  12  which is generator-operated. A thickened line symbolised the torque flow from the output gearwheel  28  to the electric machine  12 . 
         [0150]    The switched conditions are as follows: 
         [0151]    torque to the electric machine EM
       friction clutch Cl open       
 
         [0153]    coupling unit CU engaged 
         [0154]    third gear engaged. 
         [0155]    When the coupling unit  29  is engaged, the recuperation mode can be alternatively being used in the first, third or fifth gear of the first partial drive  15 . 
         [0156]      FIG. 8  shows the auxiliary output machine, i.e. the air conditioner compressor  13 , being operated by the electric machine  12  when the vehicle is stationary. The friction clutch  14  is open and the coupling unit  29  is also disengaged. All the switching units  25 ,  26 ,  27  are in the neutral position. The thickened line symbolises the torque flow from the electric machine  12  to the air conditioner compressor  13 . 
         [0157]    The switched conditions are as follows: 
         [0158]    torque from the electric machine EM
       friction clutch Cl open       
 
         [0160]    coupling unit CU disengaged
       all switching units in the neutral position.       
 
         [0162]      FIG. 9  shows an inventive hybrid drive system in a second embodiment. The description of  FIG. 9 , in principle, also applies to that of  FIGS. 10 to 16  which show different switched conditions of the same drive concept which is shown in  FIG. 9  in the neutral position. Identical components and assemblies have been given the same reference numbers as in  FIGS. 1 to 8 . In this case, too, there is shown a hybrid drive system with a main driving machine  11  in the form of an internal combustion engine CE with a supplementary driving machine  12  in the form of an electric machine EM and an auxiliary output machine  13  in the form of an air conditioner compressor A/C, which comprise two gear changing partial drives  15 ,  16 . The input shaft  17  of the first partial drive  15  is connectable by a friction clutch  14  to the internal combustion engine  11 . Furthermore, the partial drive  15  comprises an output shaft  23 ′ which carries the switching gearwheels of gears  1 ,  3  and  5  as well as of the reverse gear R. The associated output shaft  23 ′ drives an output gearwheel  28  via a gearwheel  221 . The second partial drive  16  comprises an input shaft  18  which, via an input gearwheel  19  and a gearwheel  20 , is in a driving connection with the electric machine  12 . Via a farther gearwheel  21 , the input shaft  18  is also in a driving connection with the air conditioner compressor  13 . The input shaft  18  is connectable to the input shaft  17  by a coupling unit  29  via a pair of gearwheels  45 ,  50 , with the gearwheel  45  being firmly positioned on the input shaft  17 , whereas the gearwheel  50  arranged on the input shaft  18  is a switching gearwheel which is switched by the coupling unit  29 . The coupling unit is part of the switching unit  32  which, at the same time, switches the sixth gear. A further switching unit  27  for the second and the fourth gear is positioned on the associated output shaft  24 ′ of the partial drive  16 . The output shaft  24 ′ acts via a gearwheel  222  also on the output gearwheel  28  which, for drawing reasons, is shown twice in this figure because, in actual fact, the shafts  17 ,  18 ,  23 ′,  24 ′ are not positioned in one plane. In this embodiment, the partial drive  15  also comprises a reverse gear which, via a switching unit  33 , is switched jointly with the first gear. The set of gearwheels of the reverse gear R comprises a reversing gearwheel  34 . 
         [0163]    The fixed gearwheels of gears  1  to  5  and of the reverse gear R which are arranged in a rotationally fast way on the input shafts  17 ,  18  are designated in said gear sequence with the reference numbers  41 ,  42 ,  43 ,  44 ,  45 , 47  and the corresponding switching gearwheels of the gears, which switching gearwheels are loose gearwheels connectable to the output shafts  23 ′,  24 ′ are designated in said gear sequence with reference numbers  51 ,  52 ,  53 ,  54 ,  55 ,  57  each in  FIG. 9  only. In contrast hereto, the fixed gearwheel  46  of the sixth gear is arranged on the output shaft  24 ′ in a rotationally fast way, whereas the respective switching gearwheel  56  is arranged on the second input shaft  18  in the form of a connectable loose gearwheel. 
         [0164]      FIG. 10  shows the driving condition of the vehicle being effected electrically. The friction clutch  14  of the internal combustion engine  11  is open and the coupling unit  29  is also disengaged, whereas the second gear is engaged by the switching unit  27 . There occurs the torque flow, shown by thickened lines, from the electric machine  12  via the input shaft  18  to the output shaft  24 ′ and from there to the output gear  28 . 
         [0165]    The switched conditions are as follows: 
         [0166]    torque from the electric machine EM
       friction clutch Cl open       
 
         [0168]    coupling unit CU disengaged 
         [0169]    second gear engaged. 
         [0170]      FIG. 11  shows the starting process of the internal combustion engine  11  when the vehicle is driven by the electric machine  12 . The second gear is engaged by the switching unit  27 . In addition, the coupling unit  29  is engaged and for starting the friction clutch  14  is closed. The torque flow symbolised by thickened lines takes place from the electric machine  12  to the internal combustion engine and to the output gearwheel  28 . 
         [0171]    The switched conditions are as follows: 
         [0172]    torque from the electric machine EM 
         [0173]    coupling unit CU engaged 
         [0174]    friction clutch Cl closed 
         [0175]    second gear engaged. 
         [0176]      FIG. 12  shows the starting process of the internal combustion engine  11  by means of the electric machine  12  when the vehicle is stationary. The coupling unit  29  is engaged for coupling the input shafts  18  and  17 . The friction clutch  14  is closed. All gears are disengaged. The torque flow symbolised by thickened lines takes place from the electric machine  12  via the input shaft shafts  18 ,  17  to the internal combustion engine  11 . 
         [0177]    The switched conditions are as follows: 
         [0178]    torque from the electric machine EM 
         [0179]    coupling unit CU engaged
       friction clutch CU closed.       
 
         [0181]    The illustrations of  FIG. 13  show different phases of the switching process from the fourth to the fifth gear. 
         [0182]    In  FIG. 13   a , the friction clutch  14  is closed and the coupling unit  29  is engaged. Torque flows from the input shaft  17  to the output shaft  18 ; the switching units  33 ,  25  on the output shaft  23 ′ are in a neutral position, whereas the fourth gear is engaged by the switching unit  27 . Torque flows from the internal combustion engine  11  via the input shaft  24 ′ to the output gear  28 , as indicated by thickened lines. 
         [0183]    The switched conditions are as follows: 
         [0184]    torque from the internal combustion engine CE 
         [0185]    friction clutch Cl closed 
         [0186]    coupling unit CU engaged 
         [0187]    fourth gear engaged. 
         [0188]      FIG. 13   b  shows that by disengaging the coupling unit  29 , the input shaft  17  is disconnected from the input shaft  18 . At the same time, the electric machine  12  takes over the task of transmitting torque via the gearwheels of the fourth gear to the output shaft  24 ′. The internal combustion engine  11  is also uncoupled from the input shaft  17  by opening the friction clutch  14 . The fifth gear is engaged by the switching unit  25 . As indicated by thickened lines, the torque flow takes place from the electric machine  12  via the shafts  18 ,  24 ′ to the output gearwheel  28 . 
         [0189]    The switched conditions are as follows: 
         [0190]    torque from the electric machine EM 
         [0191]    coupling unit CU disengaged
       friction clutch Cl open   fourth gear engaged.       
 
         [0194]    In  FIG. 13   c , the friction clutch  14  is closed again, so that the internal combustion engine  11  transmits torque to the output gearwheel  28  via the input shaft  17  of the first partial drive  15 , the gearwheels of the fifth gear and the output shaft  23 ′. This corresponds to the so-called boost mode. However, the provision of power by the electric machine  12  can also be cancelled. The torque flow takes place from the combustion engine  11  via the shafts  17 ,  23 ′ to the output gearwheel  28  and from the electric machine  12  via the shafts  18 ,  24 ′ to the output gearwheel  28 , as shown by thickened lines. 
         [0195]    The switched conditions are as follow; torque from the internal combustion engine CE friction clutch Cl closed torque from the electric machine EM (optional) coupling unit CU disengaged fifth gear engaged fourth gear engaged. 
         [0196]    The illustrations of  FIG. 14  show the vehicle driving in the boost mode, i.e. both driving machines  11 ,  12  provide torque. 
         [0197]    In  FIG. 14   a , the combustion engine  11 , with the clutch  14  being in the closed condition, is coupled to the output gearwheel  28  by means of the gearwheels of the third gear which is engaged by the switching unit  25 . At the same time, the electric machine  12  is coupled to the output gear  28 , with the fourth gear being engaged by the switching unit  27 . The coupling unit  29  necessarily has to be disengaged. The boost mode shown here can be set in the same way for gears  1  and  5  of the first partial drive  15 . The torque flow takes place from the internal combustion engine  11  via the shafts  17 ,  23 ′ to the output gearwheel  28  and from the electric machine  12  via the shafts  18 ,  24 ′ to the output gearwheel  28 . 
         [0198]    The switched conditions are as follows: 
         [0199]    torque from the internal combustion engine CE 
         [0200]    friction clutch Cl closed 
         [0201]    torque from the electric machine EM 
         [0202]    coupling unit CU disengaged 
         [0203]    third gear engaged 
         [0204]    fourth gear engaged. 
         [0205]    With the coupling unit  29  being disengaged, the recuperation mode can be alternatively being used in the second, forth or sixth gear of the second partial drive. 
         [0206]      FIG. 14   b  shows the friction clutch  14  of the internal combustion engine  11  in a closed condition, but the switching units  33 ,  25  of the first partial drive  15  are in the neutral position. However, the coupling unit  29  is engaged and the fourth gear is engaged by means of the switching unit  27 , so that torque is introduced into the input shaft  18  both by the electric machine  12  and by the internal combustion engine  11  and transmitted to the output shaft  24 ′. The switched condition shown here can also be used for gears  2  and  6  of the second partial drive  16  for the boost mode. The torque flow takes place from the internal combustion engine  11  via the shafts  17 ,  18 ,  24 ′ to the output gearwheel  28 . 
         [0207]    The switched conditions are as follows: 
         [0208]    torque from the internal combustion engine CE 
         [0209]    friction clutch Cl closed 
         [0210]    torque from the electric machine EM 
         [0211]    coupling unit CU engaged 
         [0212]    fourth gear engaged. 
         [0213]      FIG. 15  shows the recuperation mode, i.e. the recovery of energy when the vehicle is being pushed. The friction clutch  14  of the internal combustion engine  11  is open or closed; in each case, however, the switching units  33 ,  25  of the first partial drive are in the neutral position and the coupling unit  29  is disengaged. In the second partial drive, the fourth gear is engaged by the switching unit  27 . Torque flows from the output gearwheel  28  via the output shaft  24 ′ and the pair of gearwheels of the fourth gear to the input shaft  18  and thus to the electric machine which is generator-operated. The second and the sixth gear of the second partial drive  16  can be used in the same way for the recuperation mode. 
         [0214]    The switched conditions are as follows: 
         [0215]    friction clutch Cl open 
         [0216]    coupling unit CU disengaged 
         [0217]    fourth gear engaged. 
         [0218]      FIG. 16  shows the air conditioner compressor  13  being driven by the electric machine  12  with the vehicle being in the stationary condition. All the switching units and the coupling unit  29  are in the neutral position. The friction clutch  14  can be open or closed. Torque flows from the electric machine  12  to the air conditioner compressor  13 , as indicated by a thickened line. 
         [0219]    The switched conditions are as follows: 
         [0220]    friction clutch Cl open 
         [0221]    coupling unit CU disengaged 
         [0222]    all switching units in the neutral position. 
         [0223]      FIG. 17  shows an inventive hybrid drive system in a third embodiment. The subsequent description of  FIG. 17 , in principle, also applies to  FIGS. 18 to 24  which merely show different switching conditions of the drive concept. 
         [0224]    There is shown a hybrid drive system which comprises a main driving machine  11 , here in the form of an internal combustion engine CE, a supplementary driving machine  12 , here in the form of an electric machine EM, and an auxiliary driven machine  13 , here in the form of a compressor for an air conditioning system A/C. The internal combustion engine  11  is connectable by a friction clutch  14  (Cl) which can be provided in the form of a wet or dry clutch. The drive comprises two gear changing partial drives  15 ,  16  (stepped gear changing boxes) which are characterised in that they each comprise their own input shafts  17  and  18 . The input shaft  17  of the first partial drive carries the gearwheels of gears  4 ,  6  and  2  and is connectable by the friction clutch  14  to the internal combustion engine  11 . The input shaft  17  is in direct driving connection with the air conditioning compressor  13 . The input shaft  18  of the second partial drive  16  carries the gearwheels of gears  5 ,  1  and  3  as well as of the reverse gear R and an input gearwheel  19  which, by means of a gearwheel  20 , is in a stepped down driving connection with the electric machine  12  and, by means of a gearwheel  21 . In this embodiment, the output shafts  23 ,  24  of the two partial drives  15 ,  16  are integral with one another; more particularly, they are provided in the form of a one-piece shaft. The switching gearwheels of the individual gears are positioned on the input shaft  17 ,  18 , and there is provided a switching unit  25  for gear  4  and a common switching unit  26  for gears  6  and  2 , as well as a switching unit  27  for gears  5  and  1  and another common switching unit  30  for gear  3  and the reverse gear R. The switching gear of the reverse gear acts via a reversing gearwheel on an intermediate shaft upon a fixed gearwheel on the output shaft  23 ,  24 . Between the input shafts  17 ,  18 , in accordance with the invention, there is a coupling unit  29  (CU) effective which, more particularly if the speeds of the two input shafts are synchronised, can be switched so as to be suitable for various operating conditions which will be described below with reference to further figures. The coupling unit  29  comprises a loose gearwheel  50  on the input shaft  17  which is switchable by the switching unit  25 , which gearwheel  50  is engaged with a drive gearwheel of the electric machine  12 . 
         [0225]    The fixed gearwheels of gears  1  to  6  and R which are arranged in a rotationally fast way on the output shaft  23 ,  24  have been given in the gear sequence the reference numbers  41 ,  42 ,  43 ,  44 ,  45 ,  46  and  47  and the respective switching gearwheels which are loose gearwheels suitable for being coupled to the input shafts  17 ,  18 , have been given in the gear sequence the reference numbers  51 ,  52 ,  53 ,  54 ,  55 ,  56  and  57  each in  FIG. 1  only. The fixed gearwheels and the loose gearwheels could also be interchanged between the input and output shafts. 
         [0226]    In  FIG. 18 , the coupling unit  29  is disengaged, so that the input shafts  17 ,  18  are separated from one another. Of the drive gears, only the first gear is engaged by the switching unit  27 . In this switched condition, electric starting the vehicle—depending on the direction of rotation for forward driving or reversing—can be effected by the electric machine, with driving the vehicle also being possible with the electric machine. It is conceivable to change up into the third or fifth gear, in which case the traction force would be interrupted. A darker line indicates the torque flow from the electric machine  12  to the output shaft  23 ,  24 . 
         [0227]    The following switched conditions apply: 
         [0228]    torque from EM 
         [0229]    clutch Cl open 
         [0230]    coupling CU disengaged
       first gear engaged.       
 
         [0232]      FIG. 19  shows the internal combustion engine  11  being started by the electric machine during electric driving of the vehicle. The first gear is engaged by the switching unit  27 , so that torque flows from the electric machine  12  via the pair of gearwheels of the first gear to the output shaft  23 ,  24  of the drive, whereas at the same time the coupling unit  29  is engaged and the friction clutch  14  is closed in order to start the internal combustion engine  11  in the torque flow via the input shafts  17  and the friction clutch  14 . Dark lines show the torque flow from the electric machine  12  to the internal combustion engine  11  and to the output shaft  23 ,  24 . 
         [0233]    The following switched conditions apply: 
         [0234]    torque from the electric machine EM 
         [0235]    switching unit CU engaged 
         [0236]    clutch Cl closed 
         [0237]    first gear engaged. 
         [0238]      FIG. 20  shows the state of driving with the internal combustion engine. The fourth gear is engaged by the switching unit  25 . A darker line shows the torque flow from the internal combustion engine to the output shaft  23 ,  24 . The coupling unit  29  is to be engaged and the friction clutch  14  is to be closed. To use the gears  1 ,  3  and  5  by means of the switching units  27 , whereas the coupling unit  29  is to be disengaged and the friction clutch is to be closed to use the gears  2 ,  4  and  6  by means of the switching units  25 ,  26 . 
         [0239]    The following switched conditions apply: 
         [0240]    torque flow from the internal combustion engine CE 
         [0241]    coupling unit CU engaged 
         [0242]    clutch Cl closed 
         [0243]    forth gear engaged. 
         [0244]      FIG. 21  show the vehicle driving in the boost mode, i.e. both driving machines  11 ,  12  provide torque. In  FIG. 21 , the combustion engine  11 , with the clutch  14  being in the closed condition, is coupled to the output gearwheel  28  by means of the gearwheels of the second gear which is engaged by the switching unit  26 . At the same time, the electric machine  12  is coupled to the output shaft  23 ,  24  with the first gear being engaged by the switching unit  27 . The coupling unit  29  is disengaged. The boost condition shown here can be set in the same way by engaging gears  4  and  6  of the first partial drive  15  or by engaging the coupling unit  29 . The torque flow takes place from the internal combustion engine  11  via the shaft  17  to the output shaft  23 ,  24  and from the electric machine  12  via the input shaft  18  to the output shaft  23 ,  24 . 
         [0245]    The switched conditions are as follows: 
         [0246]    torque from the internal combustion engine CE 
         [0247]    friction clutch Cl closed 
         [0248]    torque from the electric machine EM 
         [0249]    coupling unit CU disengaged 
         [0250]    second gear engaged 
         [0251]    first gear engaged. 
         [0252]    In all modes of the first partial drive  15  mentioned above the second partial drive  16  can alternatively being used for the boost mode in the first, third or fifth gear. 
         [0253]    The illustrations of  FIG. 22  show different phases of changing up from the second into the third gear. 
         [0254]    In  FIG. 22   a  the friction clutch  14  is closed and the coupling unit  29  is engaged. Furthermore, the second gear is engaged by the switching unit  26 . Torque flows from the internal combustion engine  11  via the input shaft  17  and the pair of gearwheels of the second gear to the output shaft  23 ,  24 , so that the vehicle can be driven by the internal combustion engine. There is indicated an additional torque flow from the electric machine via the pair of gearwheels  20 ,  19  to the input shaft  18 . The third gear is already engaged by means of the switching unit  30  This is the so-called boost mode in which additional torque is applied by the electric machine. The latter could also run in a torque-free condition. However, in the present case, the boost mode forms part of the switching process which follows. Covered lines indicate the torque flow from the internal combustion engine  11  and from the electric machine to the output gearwheel  28 . 
         [0255]    The following switched conditions apply: 
         [0256]    torque from the internal combustion engine CE 
         [0257]    clutch Cl closed 
         [0258]    coupling unit CU disengaged
       additional torque from the electric machine EM       
 
         [0260]    second gear engaged, 
         [0261]    third gear engaged. 
         [0262]    In  FIG. 22   b , the second gear is still engaged, but the friction clutch  14  is opened in order to separate the internal combustion engine  11  from the input shaft  17  and render it torque-free. Hereafter, the second gear is disengaged by the switching unit  26 . A thickened line indicates the torque flow from the electric machine to the output shaft  23 ,  24 . 
         [0263]    The following switched conditions apply: 
         [0264]    torque from the electric machine EM 
         [0265]    clutch Cl open 
         [0266]    coupling unit CU disengaged 
         [0267]    second gear disengaged. 
         [0268]      FIG. 22   c  shows that the coupling unit  29  is engaged by the switching unit  25 , to connect the input shaft  17  via the input shaft  18  to the output shaft  23 ,  24 . The input shaft  17  continues to be torque-free because the friction clutch  14  continues to be open. The torque flow takes place from the electric machine  12  via the input shaft  18  to the output shaft  23 ,  24 . 
         [0269]    The switched conditions are as follows: 
         [0270]    torque from the electric machine EM 
         [0271]    clutch Cl open 
         [0272]    coupling unit CU engaged 
         [0273]    second gear disengaged 
         [0274]    third gear already engaged. 
         [0275]      FIG. 22   d  shows how the switching process is concluded by closing the friction clutch  14 . The gearwheels of the already engaged third gear are incorporated in the torque flow from the internal combustion engine  11  via the input shaft  17  and the input shaft  18  to the output shaft  23 ,  24 . A thickened line indicates the torque flow from the internal combustion engine to the output shaft  23 ,  24 . 
         [0276]    The switched conditions are as follows: 
         [0277]    torque from the internal combustion engine CE, 
         [0278]    clutch Cl closed 
         [0279]    coupling unit CU engaged 
         [0280]    second gear disengaged 
         [0281]    third gear engaged. 
         [0282]      FIG. 23  shows the switched condition in the recuperation mode, i.e. when the vehicle is being pushed, using the third gear. The switching gearwheel of the third gear is coupled by the switching unit  30  to the output shaft  23 ,  24 , so that torque flows from the output shaft  24 ,  23  to the input shaft  18 . The coupling unit  29  has to be disengaged so that torque is transmitted from the input shaft  18  to the electric machine  12  which is generator-operated. A thickened line symbolises the torque flow from the output shaft  23 ,  24  to the electric machine  12 . 
         [0283]    The switched conditions are as follows: 
         [0284]    torque to the electric machine EM 
         [0285]    friction clutch Cl open 
         [0286]    coupling unit CU disengaged 
         [0287]    third gear engaged. 
         [0288]    At the recuperation mode can be alternatively being used in the first, third or fifth gear of the second partial drive  16 , when the coupling unit  29  is disengaged, or in the second or sixth gear of the first partial drive  15 , when the coupling unit  29  is engaged. 
         [0289]      FIG. 24  shows an auxiliary driven machine, i.e. the air conditioner compressor  13 , being operated by the electric machine  12  when the vehicle is stationary. The friction clutch  14  is open whereas the coupling unit  29  is engaged. The switching units  26 ,  27  and  30  are in the neutral position. The thickened line symbolises the torque flow from the electric machine  12  to the air conditioner compressor  13 . 
         [0290]    The switched conditions are as follows: 
         [0291]    torque from the electric machine EM 
         [0292]    friction clutch Cl open 
         [0293]    coupling unit CU engaged. 
         [0294]    With the coupling unit  29  being disengaged, any combination of one of the second, forth or sixth gear of the first partial drive  15  and any one of the first, third or fifth gear of the second partial drive  16  can be used to drive the compressor  13  from the electric machine  12 .