BACKGROUND

1. Field of the Invention

The invention relates to a drive train and to a method for operating a drive train, by means of which a motor vehicle can be driven.

2. Description of the Related Art

DE 102 46 839 A1 discloses a drive train for a hybrid motor vehicle. The drive train has an electric machine for providing purely electric drive of the hybrid motor vehicle. The electric machine can be coupled to a transmission input shaft of the motor vehicle transmission between a torsion damper connected to a crankshaft of an internal combustion engine and a motor vehicle transmission.

There is constant need for cost-effective manufacture of a drive train that is configured for a hybrid motor vehicle.

The object of the invention is to provide a cost-effective drive train for a hybrid motor vehicle.

SUMMARY

According to the invention, a drive train is provided for driving a motor vehicle. The drive train has a combustion engine for providing purely mechanical drive for the motor vehicle and a motor vehicle transmission for converting the rotational speed. The motor vehicle transmission can 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 is provided between the combustion engine and the motor vehicle transmission and is penetrated by the shaft unit for positioning a high-voltage electric machine for providing purely electric drive for the motor vehicle. Additionally, a low-voltage electric machine is arranged in the reserve volume for applying an electrically generated assistance torque to the motor vehicle transmission.

The reserve volume of the drive train is configured to provide a high-voltage electric machine that is connected to a high-voltage traction battery via correspondingly dimensioned power electronics. As a result, the drive train can be used for a hybrid motor vehicle in which an intended engine power can be made available even in a purely electric fashion by the high-voltage electric machine. This drive train can also be used in a motor vehicle that is not configured as a hybrid motor vehicle, in which case the high-voltage electric machine, a clutch, the high-voltage traction battery and the associated high-voltage power electronics can be eliminated, thereby reducing costs. In this context, the same drive train can be used comprehensively for different motor vehicle models with and without a hybrid functionality in that the high-voltage electric machine is or is not positioned in the reserve volume depending on the motor vehicle model. Thus, the number of identical parts that are common to different models is increased, with the result that the drive train can be manufactured more cost-effectively by mass production.

In the case of a hybrid motor vehicle, the reserve volume is taken up essentially by the high-voltage electric machine and the clutch. In contrast, in the case of a motor vehicle that is not configured as a hybrid motor vehicle, the reserve volume is essentially empty and unused, apart from a shaft or shaft unit.

In the inventive refinement, the reserve volume need not remain empty and unused, but instead is used for the low-voltage electric machine. The low-voltage electric machine is positioned in the reserve volume that is not used by the high-voltage electric machine and essentially requires no additional installation space. In particular, the low-voltage electric machine can make available a significantly lower maximum rate of power compared to the high-voltage electric machine that is provided for supplying purely electric drive for the motor vehicle, with the result that the low-voltage electric machine can be manufactured more cost-effectively. In addition, power electronics dimensioned for relatively low power and a battery that is given smaller dimensions can be provided so that costs can be reduced as compared to the high-voltage electric machine. At the same time, it is possible for the low-voltage electric machine to output power to the motor vehicle transmission, for example to make available additional torque for a suddenly occurring request for power peak (“torque boosting”). Likewise, it is possible, given a sufficiently low requested power, for the motor vehicle to be driven in a purely electric fashion by the low-voltage electric machine, with the result that electric driving of the motor vehicle is then possible, at least to a limited degree, without expensive high-voltage components. This permits a type of cost-effective “entry hybridization” as a further option in a drive train, which is configured as a kit system. The reserve volume provided for the high-voltage electric machine is used for the low-voltage electric machine, thereby enabling a cost-effective hybrid functionality at least to a limited degree, with the result that a cost-effective drive train configured for a hybrid motor vehicle is made possible.

Given the use of a low-voltage electric machine, the shaft unit can be connected to a transmission input shaft of the motor vehicle transmission or can be embodied in one piece with the transmission input shaft of the motor vehicle transmission. The combustion engine is an internal combustion engine. The shaft unit can include an isolating element, in particular a clutch, via which a motor shaft of the combustion engine, e.g. a crankshaft, can be connected to the shaft unit to transmit torque. Part of the shaft unit can be connected to the motor vehicle engine by the disconnecting element, for example via a torsional vibrational damper, while the low-voltage electric machine is connected by the other part of the shaft unit to drive the motor vehicle in a purely electric fashion and/or recuperate brake energy. The drive train may be configured as a parallel hybrid. The shaft unit may be configured to be separately removable. As a result, it is basically possible in the removed state of the shaft unit to connect the combustion engine to the motor vehicle transmission without a low-voltage electric machine connected intermediately, and in the process to eliminate the reserve volume. The drive train can as a result easily be used in the manner of a kit for other designs. The low-voltage electric machine can be connected to power electronics configured for the low-voltage electric machine, and the power electronics can be arranged completely or partially in the reserve volume or outside the reserve volume.

The low-voltage electric machine preferably has a rated operating voltage U below a direct contact limit. This permits the use of conventional components and tools as well as the use of customary workshop personnel, i.e. it permits expensive and costly high-voltage-related measures in terms of material and personnel to be avoided. Such a direct contact limit can be assumed, for example, given a rated operating voltage U <60 V, wherein somewhat higher or lower values are also possible. In any case, the low-voltage electric machine is more powerful than a 12 V starter motor. At the same time, the voltage of the low-voltage electric machine is low enough for a cost-effective energy supply and/or cost-effective power electronics to be made possible.

The reserve volume may be 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. The torsional vibrational damper can have a flywheel that bounds the reserve volume in the axial direction. When the torsional vibrational damper is arranged in the axial direction on the engine side, the reserve volume can be bounded in the axial direction on the transmission side by a cover or torque converter of the motor vehicle transmission.

The reserve volume may be bounded in the radial direction by a clutch case that is attached to the combustion engine and to the motor vehicle transmission. The low-voltage electric machine can have a stator that can be connected to the clutch case, while a rotor of the low-voltage electric machine can be connected directly or indirectly to the transmission input shaft.

The low-voltage electric machine may be connected electrically to a motor vehicle battery to make the necessary voltage available. Thus, the low-voltage electric machine can be fed by the motor vehicle battery that battery already is present for supplying energy to further components of the motor vehicle, for example the 48 V battery. As a result, a separate traction battery is not needed for the low-voltage electric machine.

The combustion engine may be capable of being coupled to the motor vehicle transmission via a switchable disconnecting element. As a result, the combustion engine can be decoupled easily from the shaft unit, and the combustion engine does not cause any unnecessary drag torque. At the same time this makes a purely electric drive for the motor vehicle possible.

The invention also relates to a method for operating a drive train that, such as the drive train described above, in which a low-voltage electric machine is used for providing purely electric drive of the motor vehicle in a power range below the rated maximum power of the combustion engine and/or for starting, or cold starting, the combustion engine. Using the low-voltage electric machine in the reserve volume that is provided for the high-voltage electric machine enables, at least to a limited degree, a cost-effective hybrid functionality and/or cold start functionality, thereby enabling a cost-effective drive train for a hybrid motor vehicle.

The invention will be explained by way of example with reference to the drawings of exemplary embodiments. The features presented below can constitute an aspect of the invention, either individually in each case or in combination.

