Drive train and method for operating a drive train

Abstract

A drive train ( 10 ) for driving a motor vehicle has a combustion engine ( 12 ) for providing purely mechanical drive for the motor vehicle. A transmission ( 16 ) can be coupled to the combustion engine ( 12 ) via a shaft unit ( 14 ) for transmitting torque between the motor vehicle transmission ( 16 ) and the combustion engine ( 12 ). A reserve volume ( 18 ) is between the combustion engine ( 12 ) and the motor vehicle transmission ( 16 ) and is penetrated by the shaft unit ( 14 ). The reserve volume ( 18 ) can accommodate a high-voltage electric machine for purely electric drive of the motor vehicle or a low-voltage electric machine ( 32 ) for applying an electrically generated assistance torque to the motor vehicle transmission ( 16 ). As a result, a hybrid functionality can be made available to an at least limited degree, with the result that a cost-effective drive train ( 10 ) configured for a hybrid motor vehicle is made possible.

What is claimed is: 
     
         1 . A drive train for driving a motor vehicle, comprising:
 a combustion engine for providing purely mechanical drive for the motor vehicle;   a motor vehicle transmission for converting the rotational speed, the motor vehicle transmission being configured to be coupled to the combustion engine via a shaft unit for transmitting torque between the motor vehicle transmission and the combustion engine;   a reserve volume between the combustion engine and the motor vehicle transmission and penetrated by the shaft unit for positioning a high-voltage electric machine for providing purely electric drive for the motor vehicle; and   a low-voltage electric machine arranged in the reserve volume for applying an electrically generated assistance torque to the motor vehicle transmission.   
     
     
         2 . The drive train of  claim 1 , wherein the low-voltage electric machine has a rated operating voltage U below a direct contact limit, in particular of U<60 V. 
     
     
         3 . The drive train of  claim 1 , wherein the reserve volume is bounded in the axial direction on the engine side or transmission side by a torsional vibrational damper for damping torsional irregularities in the torque generated by the combustion engine. 
     
     
         4 . The drive train of  claim 3 , wherein the reserve volume is bounded in the radial direction by a clutch case that is attached to the combustion engine and to the motor vehicle transmission. 
     
     
         5 . The drive train of  claim 1 , wherein the low-voltage electric machine is a three-phase current machine. 
     
     
         6 . The drive train of  claim 5 , wherein the low-voltage electric machine is a three-phase current machine and power electronics assigned to the low-voltage electric machine are arranged in the reserve volume. 
     
     
         7 . The drive train of  claim 1 , wherein the low-voltage electric machine is connected electrically to a motor vehicle battery to also make available an on-board power system voltage. 
     
     
         8 . The drive train of  claim 1 , wherein the combustion engine can be uncoupled from the drive train by a switchable disconnecting element. 
     
     
         9 . A method for operating the drive train of  claim 1 , comprising using the low-voltage electric machine for providing purely electric drive for the motor vehicle in a power range below the rated maximum power of the combustion engine and for starting the combustion engine.