BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to an electric vehicle drive having an electric machine which can be driven alternatively as a motor or as a generator, and has a direct voltage connection to which a supply voltage can be applied, from a first energy source and a second energy source. The first energy source is designed for base load supply of the electric machine, while the second energy source comprises an energy accumulator designed for short-term peak load supply of the electric machine. Also, vehicle braking energy can be stored by way of the electric machine in its generator operating mode.
One problem associated with electric vehicle drives is that they must meet load profiles which fluctuate greatly with respect to time. However, conventionally used energy sources and energy accumulators can be optimized only with respect to a certain load profile. That is, accelerating operations require the provision of short-term power peaks; in contrast, for longer drives at a constant speed, a constant base load is required for a longer time period. Heretofore, there have been no electric energy accumulators or energy sources which are suitable both for the delivery of fairly large amounts of energy within a short time and for an unchangeable base load for a fairly long time period, and also have a power/weight ratio which is justifiable.
By a corresponding design of the electrodes, it is possible for a battery to provide high power for a short time (such as in the case of a lead starter battery, which is adapted in this manner). However, such a design leads to a reduced energy accumulating capacity of the battery. Battery systems on an alkaline base, such as nickel metal hydride batteries, also permit high power deliveries for a short time. However, they also have a limited energy accumulating capacity. Capacitors are also typical “power accumulators” with a limited accumulating capacity. On the other hand, metal/air batteries are known to have a high energy accumulating capacity, achieving gravimetric energy densities of up to 200 Wh/kg—more than twice as high as those achieved in the best conventional electrochemical energy accumulators. However, such batteries can be utilized only in the case of a discharge which is largely constant with respect to time.
When vehicles are braked, the braking energy converted into electric energy can be accumulated well in those electric energy accumulators which are also suitable for the input of high power peaks. However, a battery with a high internal resistance, which is incapable of delivering high power peaks for accelerations (to the extent achieved, for example, by nickel/metal hydride batteries) because of its characteristic, is also unable to absorb all energy occurring during such regenerative braking. Inversely, while the accumulating capacity is, however, limited, lead starter batteries as well as capacitor banks and fly wheels are capable of a high power input for a short time, such as occurs when braking energy is converted to electric energy.
In contrast to batteries, fuel cells provide electric energy sources in which no electric charge is accumulated. If electric driving energy is generated by means of a fuel cell, a reaccumulation of braking energy cannot take place without an additional accumulator of electric energy. In this case, braking energy converted into electric energy can be dissipated only without recoverability for the vehicle drive in an electric braking resistance.
An electric vehicle drive of the initially mentioned type is disclosed in German Patent Document DE 41 18 594 C1. Two electric energy accumulators with differing load profiles are assigned to the vehicle drive, and a discharge operation control device and automatic control device control, the supply of electric energy alternately from one, the other, or from both energy accumulators to the electric vehicle drive. In this case, the first energy accumulator is a high-performance battery which can provide a largely constant base load supply; while the second energy accumulator is a smaller battery which is connected to the energy accumulator when load peaks are present. Both energy accumulators are charged either by a connection with an electric supply network or during a braking or coasting operation of the motor vehicle assigned to the electric vehicle drive.
German Patent Document DE 195 22 563 A1 discloses an electric vehicle which has a traction battery and a capacitor accumulator, with braking being provided by way of the electric driving motor. Electric energy recovered in the process is supplied to the capacitor accumulator, and is intermediately stored there. As the result, a service-life-reducing exceeding of the maximal charging current of the traction battery is avoided.
German Patent Document DE 196 28 877 A1 discloses a motor vehicle having an electric machine and a battery constructed as a large-scale energy accumulator for providing a constant driving power. In addition, another high-performance energy accumulator is provided which is constructed as a power capacitor, a gyro accumulator, a spring-type accumulator or a pressure-type accumulator. During driving at a constant speed on a flat road, energy is taken only from the large-scale energy accumulator. If additional energy is required for acceleration operations or driving on slopes, energy can be taken from the high-performance energy accumulator. It is provided to brake the motor vehicle by means of the electric machine and to supply the energy released thereby at least partially back to the energy accumulators.
In British Published Patent Application GB 2 271 228 A, a power supply is free of interruptions, and under normal conditions a load is supplied by a main power supply, while a battery is uncoupled from the load and from a charging unit. In the case of an interruption of the main power supply, energy is provided to the load by the battery via a controllable voltage system converter and a transformer, whose winding is situated in the bridge arm of the voltage system converter. The battery can be charged by the main power supply via the charging unit, if defined conditions exist.
Japanese Patent Document JP 09-009417 A discloses a bidirectional DC/DC converter which consists of two voltage system converters, with one transformer disposed in-between. Via the transformer, a connection of a main battery connected with a direct voltage of an electric machine as well as another battery provided for feeding an auxiliary machine are supplied.
German Patent Document DE 198 10 467 C1, which is not prior art, discloses a hybrid driving concept for fuel cell vehicles, which contains a DC/DC converter arranged between an energy accumulator and an electric machine which can be driven alternately as a motor or a generator. Furthermore, a fuel cell connected to the electric machine, in addition to feeding the electric machine, can also cause the charging of the energy accumulator by way of the DC/DC converter.
It is an object of the invention to provide an electric vehicle drive of the initially mentioned type, in which one energy source is designed for the base load supply and the other energy source, constructed as an energy accumulator, is designed for short-term load peaks, and in which braking energy can be recovered and utilized efficiently.
This object is achieved by means of an electric vehicle drive in which the second energy source representing an energy accumulator is coupled by way of a bidirectional DC/DC converter to a supply connection of the first energy source connected with the direct-voltage connection of the electric machine. In this manner, braking energy can be supplied during braking operations to the second energy source constructed as an energy accumulator, and on the other hand it is also possible to operate the electric machine by means of energy from the second energy source when load peaks exist. In addition, it is possible to charge the second energy source by way of the DC/DC converter by means of energy of the first energy source.
In one embodiment of the invention, a power cut-off element is assigned to the first energy source, to avoid reverse currents. In this manner, the first energy source can be protected from excess voltage which may occur when recovering braking energy, or when load peaks are present during which the electric machine is operated by means of energy from the second energy source.
In another embodiment of the invention, the bidirectional DC/DC converter is constructed of two voltage system converters of the same type, which can be controlled in a timed manner, in a single-phase bridge with one transformer winding respectively in the bridge arm. In this manner, it is possible, by appropriate timing of the voltage system converters, to supply recovered braking energy to the second electric energy source constructed as an energy accumulator, and to operate the electric machine by means of energy from the first electric energy source as well as from the second energy source.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.