Method for operating a drive with an internal combustion engine and an electric machine

This is a national stage of PCT application No. PCT/EP01/09474, filed on Aug. 16, 2001. Priority is claimed on that application and on Application No. EP 00118614.7, filed in Europe on Aug. 28, 2000.

1. Field of the Invention

The invention relates to a method for operating a drive with an internal combustion engine and an electric machine.

2. Description of the Prior art

DE 197 52 661 A1 describes an on-board power system for a motor vehicle with a combined drive including an electric machine and an internal combustion engine which act on the same drive train. On the one hand, the electric machine supports, for example, the starting process of the internal combustion engine as a motor and, on the other hand, outputs charge power to a vehicle operating battery, and possibly to an additional short-term starting capacitor, as a generator. In order to be able to operate with the highest possible efficiency and thus with the lowest possible loading of the energy stores or energy supply means in the generator operating mode, the electric losses both in the converter and in the asynchronous machine which is provided for the starter-generator are advantageously kept low. This is accomplished by virtue of the fact that, in a field-oriented control, the internal torque of the asynchronous machine of the start-generator is set according to the principle of controlling or regulating the rotor flow with optimum loss by correspondingly adapting the magnetizing current as a function of an active current which is determined in accordance with the respectively required drive torque.

The object of the present invention is to reduce further the environmental stress of a combined drive with mixed electric motor/internal combustion engine, in particular of a motor vehicle with a corresponding hybrid drive, using a simple cost effective device.

Taking as a starting point the fact that a shortened warming-up phase can reduce the fuel consumption and the stressing of the environment by exhaust gases, the electric machine is used, preferably in the cold-starting operating mode, as an additional load on the internal combustion engine in order to excessively increase its heat loss selectively in comparison to a customary driving operating mode. In this way the internal combustion engine more quickly reaches its optimum operating point which is characterized by low consumption and low stressing of the environment by exhaust gases. When there is a correspondingly cool external temperature, the interior of a motor vehicle can at the same time be heated up more quickly while dispensing with a separate additional heater.

Given selective operation of the internal combustion engine outside its optimum operating point, the heat loss which occurs in the electronics which correspondingly control the electric machine can also be used in particular to heat up the interior of a motor vehicle which is driven by the drive.

According to the method of the present invention, the additional load can be obtained by a corresponding charge power which is output by the electric machine operating in the generator operating mode to an operating battery, preferably to a main battery which ensures the basic electric power supply in a motor vehicle.

If such an additional load cannot be conducted away in the form of a battery charge load, for example owing to the state of charge of the battery, the method according to the invention also provides for the electric machine to be operated with a respectively degraded level of efficiency in comparison with the optimum level of efficiency on the basis of the set point drive torque and rotational speed. The additional heat which is produced here in the electric machine itself can advantageously be used to heat up the internal combustion engine and/or for additional heating of the interior of the motor vehicle.

A setting of a level of efficiency of the electric machine which is intentionally degraded can be achieved, for example in an asynchronous machine with field-oriented control of the magnetic current, known per se from DE 197 52 661 A1, by selectively changing the magnetizing current from the optimal value which is aimed at to minimize the electrical losses in the known case.

In the Drawing:

FIG. 1 is a schematic diagram of an on-board power system of a vehicle.

The figure shows a drive based on the vehicle on-board power system known from DE 197 52 661 A1. The drive for a motor vehicle is a hybrid drive with an internal combustion engine VB and an electric machine in the form of an asynchronous machine as a starter-generator SG with field-oriented control.

The vehicle on-board power system illustrated in the figure can basically be divided into a first vehicle on-board power system component U 1 with a voltage level of, for example, 12 volts corresponding to the voltage of the battery B, a second vehicle on-board power system component U 2 for supplying high-power loads with a voltage level of, for example, 180 volts, and a third vehicle on-board power system component U 3 with a voltage level of for example 400 volts for charging a starting capacity K for starting an internal combustion engine VB using a starter-generator SG which can be fed via a converter U. The third vehicle on-board power system component U 3 is also used in the generator operating mode of the starter-generator SG to adapt its output voltage of approximately 180 volts, set by means of the converter U, to the voltage level of the second vehicle on-board power system component U 2 . A control component is used for power-optimum operation of the asynchronous machine, which is provided as a starter-generator SG, preferably with a squirrel-cage rotor.

During preparation for starting of the internal combustion engine VB by means of the starter-generator SG, which then operates as an asynchronous motor, for example after the ignition key is activated, a starter capacitor K in the third on-board power system component U 3 is charged to a voltage of approximately 400 volts via the connection V 1 using step-up controller (DC/DC converter) HSST. Switches S 1 and S 2 in the connection V 2 or V 3 , respectively, are opened during this charging phase of the starting capacitor K. The connection V 1 is blocked in the opposite direction to the battery B by a diode D 1 in the connection V 1 .

During the actual starting process, the internal combustion engine VB can be accelerated to a rotational speed of approximately 400 to 700 rpm using the asynchronous motor of the starter generator SG. If the internal combustion engine VB includes electronic valve control, the valves are opened during the acceleration and thus set to a compressionless state. As soon as the internal combustion engine VB has reached an anticipated rotational speed or the necessary angle of rotation of its crankshaft, the internal combustion engine VB can be fired with the normal valve position and the starter-generator SG can be switched off at the drive end. If the energy which has been previously stored in the starter capacitor K is not sufficient for the starting process, energy can be additionally drawn from the battery B via the step-up controller (DC/DC converter) HSST and the connection V 1 for the starter-generator SG fed via the converter U if the voltage level of the third vehicle on-board power system component U 3 falls below the voltage level of the second vehicle on-board power system component U 2 .

After the firing process of the internal combustion engine VB, the starter-generator SG may switch to the generator operating mode. Here, the voltage level of the third vehicle on-board power system component U 3 is adjusted, as intermediate circuit voltage, to the voltage level of the second vehicle on-board power system component U 2 of for example 180 volts by the converter U and the starter capacitor K which is expediently used as an intermediate circuit capacitor.

After the voltage level of the vehicle on-board power system component U 2 has been reached, switches S 1 and S 2 are arranged so that the switch S 2 is opened and the connection V 2 thus disconnected and the switch S 1 closed so that the connection V 3 from the vehicle on-board power system component U 3 to the vehicle on-board power system component U 2 is brought about. The step-up controller (DC/DC converter) HSST is expediently deactivated. The starter-generator SG then operates in the generator operator mode and supplies the vehicle on-board power system component U 2 , and the high-power loads connected to it, via the regulated intermediate circuit voltage of the vehicle on-board power system component U 3 .

To operate in the generator operating mode of the starter-generator SG with the highest possible level of efficiency and thus with the lowest possible loading of the energy stores or energy supplying means, the electrical losses both in the converter U and in the asynchronous machine which is provided for the starter-generator SG are advantageously kept low by virtue of the fact that in a field-oriented control FOR the internal torque M i1 of the asynchronous machine of the starter-generator SG is set according to the principle of controlling or regulating the rotor flow with optimum loss by correspondingly adapting the magnetizing current as a function of an active current which is determined in accordance with the respectively required drive torque.

According to the present invention, an additional load of the internal combustion engine VB can be set in order to increase the feed loss of internal combustion engine in a selective fashion, in particular in order to shorten the warming-up phase. This may be accomplished by setting the magnetization current differently from the way previously provided according to DE 197 52 661 A1 in order to achieve an optimum level of efficiency. For this purpose, an optimizer OPT for switching over between a known operating mode with optimum level of efficiency on the one hand and an operating mode which differs from it according to the invention in order to excessively increase the heat loss is provided. A corresponding setpoint value for the field-oriented control FOR can be predefined by setting a corresponding setpoint torque M setp or a correspondingly magnetizing current Flow setp . For the purpose of respectively controlling, in accordance with the rotational speed, the rotational speed of the internal combustion engine VB is transmitted both to the optimizer OPT and to the field-oriented control FOR via a rotational speed sensor S. The setpoint torque M setp may be determined based on the speed indicated by speed signal n. The setpoint torque M setp may also be determined from a database PI based on a difference between the actual voltage U 2 act of the second vehicle on-board power system U 2 and the desired voltage U 2 set up thereof.