In a serial hybrid drive, the drive wheels are driven by an electric drive. The electric drive, and the associated converter, is supplied by an electrical energy accumulator. A controller is configured to determine the motor power corresponding to a predefined driving situation (e.g., gas pedal position). To extend the travel range, as a range extender a combustion engine is provided which charges an electrical energy accumulator and a battery, via an alternator. The combustion engine is controlled depending on the driving situation, the power demand predefined by the driver and the actual power demand.
In a parallel and combined hybrid drive, the combustion engine works on the drive wheels, for example, via a converter and step-less gears, the electric motor acting in parallel on the drive train as a function of the power demand and acts as support. With use of a suitable coupling by way of which the combustion engine can be separated from the drive, a purely electric driving mode is also possible, the combustion engine supplying the electrical energy accumulator via an alternator and in this operating mode acts as a range extender for the serial hybrid drive described.
DE 100 07 136 A1 describes a drive power control system for a motor vehicle which uses a parallel hybrid system. A controller acting on the combustion engine is connected with various sensors detecting the driving situation, such as in particular a gas pedal sensor, travel speed sensor, battery sensor, engine rotation speed sensor. The combustion engine is controlled as a function of the torque demand required following a predefined strategy, in particular to operate said engine in an optimum efficiency range.
DE 10 2009 054 839 A1 describes a range extender for a hybrid motor vehicle. The amount of DC voltage converted from the AC voltage generated by the alternator is set by controlling the engine rotation speed of the combustion engine.
In the known hybrid systems or range extenders, the working noise of the combustion engine is often perceived by the driver as not matching the driving situation, and thus, is regarded as a disadvantage. This is the case, for example, if the combustion engine of the range extender is operated constantly at its best point, i.e., in a range in which the engine generates energy most efficiently. This best point often lies in the region of a relatively high rotation speed. Particularly, when the vehicle has stopped and at low speeds, however, it is unpleasant and unusual for the combustion engine to be operated at a higher rotation speed and hence with a loud engine noise.
Also changing between several operating points is not always acceptable to the driver. Thus it can occur that while driving at constant speed, the operating point is changed since, for example, the charge state of the battery requires this. Furthermore, from conventional vehicles (vehicles with combustion engine and manual or stepped automatic transmission), it is expected for the engine noise of the combustion engine to increase according to the fixed ratios of the transmission as the speed rises, and to diminish as the speed falls. Operating the combustion engine as a range extender with constant operating points, irrespective of speed, leads to a monotonous sound of the combustion engine, in particular without a typical noise change on acceleration and deceleration.
A further problem is the energy consideration. A range extender serves to extend the range of the vehicle when the vehicle is operated primarily electrically, which means that the combustion engine is active when the battery reaches a low charge state. To avoid further discharge of the battery, the energy generated by the range extender must be greater than the energy required to drive the vehicle and operate the ancillaries. If static operating points are allocated to the range extender in certain driving situations, e.g., full vehicle load or long uphill climbs, it may be that the power demand of the vehicle is greater than the power generated by the range extender in its allocated operating point. To compensate for this power difference, necessarily the power of the vehicle drive or drives must be reduced/limited.