Abstract:
A drive train ( 1 ) of a motor vehicle has an internal combustion engine ( 2 ) with a crankshaft ( 6 ). A transmission ( 3 ) is connected downstream of the internal combustion engine ( 2 ) to drive at least one axle ( 5 ) of the motor vehicle. A starter generator ( 23 ) is assigned to the internal combustion engine ( 2 ). The starter generator ( 23 ) is attached to the crankshaft ( 6 ) by two separate drive trains ( 13, 22; 24, 25 ). One ( 24, 25 ) of the drive trains ( 13, 22; 24; 25 ) has a shiftable clutch ( 29 ) and the other ( 13, 22 ) of the drive trains ( 13, 22; 24, 25 ) has a shiftable clutch ( 32 ) or a freewheel ( 11 ) that is active during a starter mode of the starter generator ( 23 ). The drive train ensures an optimum start capability of the cold internal combustion engine and additionally permits electric boosting of the internal combustion engine.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 USC 119 to German Patent Appl. No. 10 2012 109 852.2 filed on Oct. 16, 2012, the entire disclosure of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a drive train of a motor vehicle, having an internal combustion engine with a crankshaft, a transmission connected downstream of the internal combustion engine to drive at least one axle of the motor vehicle, and a starter generator assigned to the internal combustion engine. 
         [0004]    2. Description of the Related Art 
         [0005]    An electrically operated starter generator operates as a motor to start an internal combustion engine. The starter generator also can drive an axle of the motor vehicle together with the internal combustion engine during operation of the drive train as a mild hybrid. In the generator mode, the starter generator is driven by the internal combustion engine and/or via the axle of the motor vehicle. 
         [0006]    DE 10 2009 033 633 A1 discloses a drive train with a belt drive that has a belt that is guided over a pulley of the starter generator and functions to drive the generator. The generator can be operated temporarily as a starter motor and is coupled via a freewheel to the generator pulley or an engine crankshaft belt pulley. The freewheel allows the generator to overrun the generator pulley or the crankshaft during the motor mode. As a result, the power flow via the belt drive and the belt drive loading induced by the inherent inertia of the generator can be reduced during an engine start phase, and in phases with a high angular velocity rise of the crankshaft of the internal combustion engine. 
         [0007]    JP 58-116229 A describes the drive train of an oil pump of an internal combustion engine. The oil pump is attached either via a belt drive to the internal combustion engine or via a belt drive to an electric machine. To this end, a shiftable clutch is provided between the internal combustion engine and the belt drive that is assigned to the internal combustion engine. 
         [0008]    EP 1 079 085 A2 discloses the optional use of an electric machine as starter motor or as generator. The electric machine is attached by a shiftable clutch to a crankshaft of the internal combustion engine. The electric machine also is attached via an auxiliary shaft that is assigned a further shiftable clutch to a transmission that is connected downstream of the internal combustion engine. As a result, when the clutch that is assigned to the auxiliary shaft is closed, the electric machine can be operated in the generator mode via the transmission that is connected downstream of the internal combustion engine. 
         [0009]    It is an object of the invention to provide a drive train that ensures an optimum start capability in the case of a cold internal combustion engine and while also providing the possibility to boost the internal combustion engine electrically. 
       SUMMARY OF THE INVENTION 
       [0010]    The invention relates to a drive arrangement with a starter generator that is attached to a crankshaft by first and second separate drive trains. The first drive train has a shiftable clutch and the second drive train has a shiftable clutch or a freewheel that is active during the starter mode of the starter generator. The provision of the two separate drive trains of the starter generator enables the different operating states of the internal combustion engine and/or the drive train of the motor vehicle to be controlled in an optimum manner by attachment of one or the other of the separate drive trains. 
         [0011]    The clutch of one separate drive train is closed to achieve a favorable cold start of the internal combustion engine in the drive train via a particularly favorable transmission ratio that is relatively high. The transmission ratio can be realized in a simple way by a transmission that preferably is configured as a flexible drive mechanism, such as a belt drive. 
         [0012]    The other drive train of the starter generator makes it possible to drive auxiliary units of the internal combustion engine independently of the internal combustion engine. The auxiliary units are, for example, a coolant pump and/or a mechanical refrigerant compressor and can be operated by the starter generator before the internal combustion engine is started. 
         [0013]    If the first drive train has the shiftable clutch and the second drive train has the freewheel that is active during the starter mode of the starter generator, the internal combustion engine can be started only when the clutch of the first drive train is closed. In this case, the freewheel prevents the starter generator from introducing a starting moment into the crankshaft of the internal combustion engine via the second drive train. In this variant, the freewheel is overrun as the rotational speed of the started internal combustion engine increases. As a result, the crankshaft of the internal combustion engine drives the auxiliary units. 
         [0014]    In another variant, both separate drive trains have a shiftable clutch, and there is no freewheel. Thus both clutches are not closed at any time, since it is preferably to be assumed that the two separate drive trains have different transmission ratios. 
         [0015]    The auxiliary units preferably are driven via the starter generator by means of a transmission that is configured as a belt drive. The design of belt drives allows the transmissions to be structurally simple and with a desired transmission ratio. 
         [0016]    The use of clutches that are assigned to the separate drive trains permits optimization of the start of the internal combustion engine. Thus the separate drive train of the starter generator that interacts with the auxiliary units interacts with the crankshaft of the internal combustion engine with a lower transmission ratio than the other separate drive train. Accordingly, the other separate drive train is activated during cold starting. Restarting the internal combustion engine, that is to say warm starting, preferably takes place via the other separate drive train, which therefore has the lower transmission ratio and is assigned to the auxiliary units. Shifting of the drive trains can be brought about by simple shifting of the two clutches. 
         [0017]    The drive train of the invention makes boosting possible by operating the starter generator while driving with the internal combustion engine. The starter generator therefore is active as a motor and, when the freewheel in the second drive train is used, introduces an additional moment into the crankshaft via the second drive train at least when the clutch of the second drive train is closed. If two shiftable clutches are used in the two separate drive trains, the additional moment can be introduced into the crankshaft either via either drive train. 
         [0018]    The configuration of the drive train enables an increased power output of the internal combustion engine and therefore is particularly advantageous in the sport mode. The auxiliary units are not capable of being driven in the sport mode because the two clutches are open. Accordingly, both the starter generator and the auxiliary units are decoupled from the internal combustion engine. As a result, the internal combustion engine does not have to generate the power output for driving the auxiliary units and the full power output of the internal combustion engine is available for moving the vehicle. 
         [0019]    The drive train requires only a single electric machine in the manner of a starter generator that can be operated as a motor or as a generator. The auxiliary units can be driven with two transmission ratios, namely, one transmission ratio when driven by the starter generator, and the other transmission ratio when driven via the crankshaft of the internal combustion engine. 
         [0020]    Further features of the invention will become apparent from the drawings of exemplary embodiments and the detailed description, without being restricted hereto. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic illustration of a first embodiment of the drive train of the invention. 
           [0022]      FIG. 2  is a schematic illustration of a second embodiment of the drive train of the invention. 
           [0023]      FIG. 3  is a schematic illustration of a third embodiment of the drive train of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    A drive train  1  of a motor vehicle in accordance with a first embodiment of the invention is identified by the numeral  1  in  FIG. 1 . The drive train  1  has an internal combustion engine  2 , a transmission  3  connected downstream of the internal combustion engine  2  and a differential  4  connected downstream of said transmission  3  to drive an axle  5  of the motor vehicle. A crankshaft  6  of the internal combustion engine  2  is connected via a shiftable clutch  7  to a transmission shaft  8  that interacts with the differential  4 . 
         [0025]    An internal transmission  9  is provided on the side of the internal combustion engine  2  that faces away from the transmission  3 . The internal transmission  9  is assigned to the crankshaft  6  to drive an engine oil pump  10  of the internal combustion engine  2 . The crankshaft  6  is guided out of the side of the internal combustion engine  2  that faces away from the transmission  3  and receives a belt pulley  12  via a freewheel  11 . The freewheel  11  is designed to be overridden when the internal combustion engine  2  is in operation and the crankshaft  6  is rotating relatively quickly so that the belt pulley  12  is driven via the crankshaft  6 . 
         [0026]    The belt pulley  12  is a constituent part of a belt drive  13  that has a belt  14  and at least three further belt pulleys  15 ,  16 ,  17  about which the belt  12  is wrapped. Auxiliary units, including a mechanical refrigerant compressor  18  and a coolant pump  19 , are assigned to the internal combustion engine  2 . The auxiliary units are mounted on the side of the internal combustion engine  2  that faces away from the transmission  3 . The belt pulley  15  is connected fixedly to a shaft  20  of the coolant compressor  18  and the belt pulley  16  is connected fixedly to a shaft  21  of the coolant pump  19  so as to rotate with it. 
         [0027]    The belt pulley  17  is connected fixedly to a shaft  22  of a starter generator  23  so as to rotate with it. A further shaft  24  is connected to the side of the starter generator  23  opposite the shaft  22 . Accordingly, the starter generator  23  has two input or output shafts  22 ,  24 , depending on whether the starter generator  23  is driven or is the driver. 
         [0028]    The shafts  20 ,  21 ,  22  and  24  are mounted parallel to the bearing axis of the crankshaft  6  in a crankcase/cylinder head of the internal combustion engine  2 . 
         [0029]    A further belt drive  25  is arranged between the internal combustion engine  2  and both the transmission  3  and the clutch  7 . The belt drive  25  has a belt pulley  26  with a relatively small diameter, a belt pulley  27  with a considerably greater diameter, and a belt  28 . The belt pulley  26  is fixed to and rotates with the shaft  24 , while the belt pulley  27  is fixed to and rotates with the crankshaft  6 . The shaft  24  has a shiftable clutch  29  between the starter generator  23  and the belt pulley  26 . 
         [0030]    The starter generator  23  that is operated in the motor mode drives the belt drive  13 , which in turn drives the auxiliary units, such as the refrigerant processor  18  and the coolant pump  19 . The auxiliary units therefore can be driven even when the internal combustion engine  2  is at a standstill. 
         [0031]    To start the internal combustion engine  2 , it merely is necessary to operate the starter generator  23  in the motor mode and to close the clutch  29 . A very high moment for starting the internal combustion engine  2  therefore is introduced into the crankshaft  6  by the belt drive  25  on account of the relatively high transmission ratio. The high moment is particularly advantageous for starting the internal combustion engine  2  in a cold start. 
         [0032]    When the internal combustion engine  2  is started and the clutch  29  is open, the rotational speed of the crankshaft  6  increases. As a result, the freewheel is overridden and, as a consequence, a torque is introduced into the belt pulley  12 . Therefore, the belt pulley  12  drives the belt drive  13  and the auxiliary units. 
         [0033]    The diameter of the belt pulley  17  that interacts with the starter generator  23  via the shaft  22  is smaller than the diameter of the belt pulley  12  that interacts via the freewheel  11  with the crankshaft  6 . Accordingly, two different transmission ratios are possible in relation to the drive of the refrigerant processor  18  and the coolant pump  19 . The drive via the starter generator  23  therefore has a different transmission ratio than the drive via the crankshaft  6  when the internal combustion engine  2  is running. 
         [0034]    A boost function takes place when the clutch  29  is closed in the motor mode of the starter generator  23 , and the starter generator  23  introduces an additional torque to the torque of the internal combustion engine  2  into the crankshaft  6 . 
         [0035]    Two separate drive trains interact with the starter generator  23 , namely a first drive train formed by the belt drive  13  and the shaft  22 , and a second drive train formed by the shaft  24  and the belt drive  25 . 
         [0036]    An electric machine  30  and a hydraulic pump  31  that can be driven by the electric machine  30  are assigned to the transmission  3 . The electrically driven hydraulic pump  31  enables shifting of the transmission  3  even when the internal combustion engine  2  is stopped. 
         [0037]    The embodiment of  FIG. 2  differs from  FIG. 1  only in that the crankshaft  6  is not connected to the belt pulley  12  via the freewheel  11 . Rather, a shiftable clutch  32  is arranged between the crankshaft  6  and the belt pulley  12 . Parts that coincide with the embodiment of  FIG. 1  are denoted by the same reference numbers. 
         [0038]    The replacement of the freewheel  11  by the shiftable clutch  32  in the embodiment of  FIG. 2  enables the internal combustion engine  2  to be started via the belt drive  25 , as described with respect to  FIG. 1 , or alternatively by the belt drive  13  with a different and lower transmission ratio. In the first-mentioned start procedure, the clutch  29  is closed and the clutch  32  is open. In the last-mentioned start procedure the clutch  29  is open and the clutch  32  is closed. The last-mentioned start procedure is advantageous in the case of a warm start of the internal combustion engine  2 . 
         [0039]    The variant with the clutch  32  enables the refrigerant processor  18  and the coolant pump  19  to be driven when the clutch  32  is open and when the internal combustion engine  2  is at a standstill. However, the auxiliary units  18 ,  19  are driven by the internal combustion engine  2  via the crankshaft  6  when the clutch  32  is closed and the internal combustion engine  2  is started. The variant of  FIG. 2  also makes the boost function possible, by either one of the clutches  29  or  32  being closed. The different transmission ratios of the belt drives  13  and  25  ensure that both clutches  29 ,  32  are not closed at the same time. 
         [0040]    This also applies to the operation of the starter generator  23  in the generator mode. 
         [0041]    The variant with the two clutches  29 ,  32  also enables an increase in the power output of the differential  4  by neither the auxiliary units being driven by the internal combustion engine  2  nor the starter generator  23  being attached when both clutches  29 ,  32  are open. An increased power output of the internal combustion engine  2  therefore is available briefly. 
         [0042]    The embodiment of  FIG. 3  differs from  FIG. 1  in that the two belt drives  13 ,  25  are arranged on the side of the internal combustion engine  2  that faces away from the transmission  3 . This variant is advantageous when the spatial conditions make it necessary that the belt drive  25  is not arranged between the internal combustion engine  2  and the transmission  3 . 
         [0043]    Elements of the embodiment of  FIG. 3  that coincide with the embodiment of  FIG. 1  are denoted by the same reference numbers. 
         [0044]    In the embodiment of  FIG. 3 , the starter generator  23  has only one output shaft, namely the output shaft  22 . The belt pulley  14  of the belt drive  13  is fixed to and rotates with the output shaft  22 . The belt pulley  26  also is connected to the shaft  22  at a spacing from the belt pulley  14 , and on that side of the belt pulley  14  that faces away from the starter generator  23 . The crankshaft  6  is extended beyond the freewheel  11  that is assigned to the belt drive  13  and the belt pulley  12 , and is connected to the belt pulley  27  via the shiftable clutch  29 . 
         [0045]    In the embodiment of  FIG. 3 , as in the embodiment of  FIG. 1 , the auxiliary units are driven independently by the starter generator  23  via the belt drive  13  when the internal combustion engine  2  is at a standstill, and, after the clutch  29  is closed, the internal combustion engine  2  is started in the motor mode of the starter generator  23 . In accordance with the exemplary embodiment in  FIG. 1 , the generator mode and the boost function are realized in the embodiment of  FIG. 3 , as in the embodiment of  FIG. 1 . Reference is made to this extent to the comments made above with respect to  FIG. 3 . 
         [0046]    The embodiments of  FIGS. 1 and 3  therefore require only one element which is to be shifted actively, namely the clutch  29 , whereas the embodiment of  FIG. 2  requires two elements that are to be shifted actively, namely the clutches  29  and  32 .