DE102022003151B3 - Method for operating an electric drive system - Google Patents

Abstract

The invention relates to an electric drive system for a motor vehicle (1) with an electrically driven front axle (2) and an electrically driven rear axle (3), wherein the electrically driven rear axle (3) comprises a first electric machine (4) which has a first rotor (5), which is designed to drive a first rear wheel (7), wherein the electrically driven front axle (2) comprises a second electric machine (10) which has a second rotor (11) which is designed to at least to drive a front wheel (16), the first rotor (5) being permanently torque-transmittingly coupled to the first rear wheel (7), the first electric machine (4) being a separately excited synchronous machine and the second electric machine (10) being a permanently excited synchronous machine is trained.

Description

The invention relates to a method for operating an electric drive system for a motor vehicle with an electrically driven front axle and an electrically driven rear axle.

Electric drive systems for motor vehicles are known from the general state of the art. For example, a front axle of the motor vehicle can be driven electrically just like a rear axle. The DE 10 2018 220 809 A1 describes such a vehicle with a focus of the description on the power electronic control of the electric drive system. The vehicle shown as an example essentially shows the generic structure of the electric drive system present here, in which a first electric machine has a first rear wheel, a second electric machine has a second rear wheel and a third electric machine via an axle drive, such as a differential, the front wheels of the vehicle shown.

An alternative to this is described DE 10 2011 080 236 A1 . This document describes a drive device for a single wheel of a motor vehicle, which could also be used for two or more wheels of the motor vehicle. An electric machine is provided for each driven wheel. The rotor of this electric machine is coupled to the respective driven wheel via a planetary transmission stage.

Furthermore, you can access the DE 10 2020 127 790 A1 to get expelled. This document describes a switchable electrical machine, which can be switched on via a planetary transmission stage with switchable freewheeling.

In the DE 38 01 351 A1 The basic option of switchable four-wheel drive is also described for a non-electrically powered motor vehicle.

Finally, it shows EP 2 567 859 A1 an electric drive system for a motor vehicle with two driven axles, each of which has 2 electric motors. The vehicle has at least one asynchronous motor and one synchronous motor. One method for operating this system provides, among other things, that a different rotor is used for starting the motor vehicle than for operation after starting.

The object of the present invention is to provide an improved method for operating an electric drive system, which enables energy-saving operation and at the same time allows an increase in driving performance, traction, dynamics and/or safety if necessary.

According to the invention, this object is achieved by a method for operating an electric drive system with the features in claim 1. Advantageous refinements and further developments of the method according to the invention result from the subclaims dependent on this claim 1.

The electric drive system according to the invention uses a basic structure, which is shown in principle in the first document mentioned at the beginning, but is not described further. It essentially consists of an electrically driven front axle and an electrically driven rear axle and can preferably be used in passenger cars, light commercial vehicles and the like. A further use, for example in heavy commercial vehicles or the like, such as construction machinery, military vehicles or the like, would also be fundamentally conceivable.

The electric drive system for a motor vehicle includes an electrically driven rear axle with a first electric machine with a first rotor which is designed to drive a first rear wheel. Furthermore, the electric drive system for a motor vehicle comprises an electric front axle, which has a second electric machine which includes a second rotor which is designed to drive a first front wheel.

In the drive system for the method according to the invention, the first rotor is permanently coupled to the first rear wheel in a torque-transmitting manner. According to the invention, the first electrical machine is designed as an externally excited synchronous machine, which is also abbreviated with the abbreviation EESM. According to the invention, the second electrical machine in the area of the front axle is designed as a permanently excited synchronous machine PMSM. The structure of the electric drive system for the motor vehicle makes it possible to ideally divide the tasks between the front axle and the rear axle with regard to the drive, with electrical decoupling being achieved by using the externally excited synchronous machine in the area of the rear axle by not energizing the electric machine can, which makes it possible to use a complex mechanical clutch including actuators in this area to avoid. This saves components, assembly and manufacturing effort, and thus costs, as well as installation space and weight.

Furthermore, it is provided according to the invention that in the area of the front axle a separating clutch is designed to couple or decouple the second rotor to the first front wheel in a torque-transmitting manner, as required. This separating clutch, which according to an extremely favorable development can be designed as a switchable freewheel, enables mechanical separation if necessary.

According to the invention, the method for operating such an electric drive system provides that at a vehicle speed which is greater than a first speed threshold, the second rotor is decoupled from the first front wheel by means of the separating clutch. Operation then takes place above the speed threshold exclusively with the externally excited synchronous machine and thus the rear wheel. This operating range is primarily used for fast driving with low to medium torques.

Furthermore, according to the invention, the speed threshold value depends on the direct current voltage or on a state of charge of a vehicle battery. The speed threshold at which the permanently excited synchronous machine on the front axle is decoupled is therefore correspondingly variable and can depend proportionally on the DC voltage. The DC voltage also decreases with the state of charge or the external temperature, which leads to a reduction in the speed threshold even if the dependence is assumed to be proportional. An exemplary speed threshold value could be, for example, approximately 130km/h, which is set accordingly for the design voltage, i.e. the voltage for which the direct current system of the electric drive is designed.

Furthermore, it is provided according to the invention that at a vehicle speed that is smaller than the first speed threshold value, the first electric machine is electrically decoupled if an accelerator pedal value is also smaller than an accelerator pedal threshold value. This is therefore an operation with loads that are not too high, which is an extremely common case when using a motor vehicle, and is typically even the most common case. In this situation, the front axle is primarily used, while the first electrical machine, i.e. the separately excited synchronous machine on the rear axle, is electrically decoupled by switching off the excitation current. In this situation, there are only mechanical losses from the rear axle; the electric machine does not have to be dragged along against electrical resistance.

The electric drive system with the different electric machines in the area of the front axle on the one hand and the rear axle on the other hand now enables an efficient increase in its drive efficiency through the possibility of an application-specific design, for example of the front wheel drive as an efficient drive and the rear wheel drive as a functional drive to meet high driving requirements. This applies in particular to the variant described with the clutch, which is designed in particular as a switchable freewheel.

In the sense of such a division and optimization of the front wheel drive for one task and the rear wheel drive for the other task, it is further provided, according to a very advantageous embodiment of the electric drive system according to the invention, that an overall rear axle ratio is greater than an overall front axle ratio. The overall rear axle ratio in the sense of the invention is to be understood as the overall ratio from a rear side shaft, which is connected to the driven wheel, to the first rotor shaft of the first electric machine.

Accordingly, the overall front axle ratio is the overall ratio from a first front side shaft to the second rotor shaft of the second electric machine in the area of the front wheel. It can preferably be provided that the overall rear axle ratio is greater than 6 and the overall front axle ratio is correspondingly smaller than 6.

A further very advantageous embodiment can provide that the first electrical machine is designed as an axial flux machine. In particular, such a system is ideal for designing the rear wheel drive as a functional drive.

According to a further very advantageous embodiment, the second electrical machine can be designed as a radial flux machine, which is correspondingly suitable as a permanently excited synchronous machine for the efficient drive in the area of the front axle.

According to a very advantageous embodiment, both the first rotor in the area of the rear axle and the second rotor in the area of the front axle can be coaxial with a first one Differential gears may be arranged in the case of the first rotor and a second differential gear in the case of the second rotor. Coaxial in the sense of the invention refers to rotating about the same axis of rotation.

A further very advantageous embodiment of the electric drive system for a motor vehicle according to the invention can further provide that the electric rear axle has a third electric machine, which can also be designed as an externally excited synchronous machine and in particular as an axial flux machine. This third electric machine has a third rotor which is designed to drive the first rear wheel and/or a second rear wheel. The drive for the rear axle can be provided by two motors. These can both drive the same wheel or, in particular, each can drive its own wheel on the rear axle, so they can be designed as a symmetrically divided drive for the right rear wheel on the one hand and the left rear wheel on the other, or alternatively they can both drive the rear wheel or wheels together.

A further embodiment of the invention can further provide that at a vehicle speed that is smaller than a second speed threshold and at an accelerator pedal value that is greater than the accelerator pedal threshold, both the first electric machine and the second electric machine are operated in a driving manner, wherein under these conditions an output drive power of the second electrical machine is greater than an output drive power of the first electrical machine. In this third operating range described, it is then the case that both electrical machines are operated, with the second electrical machine, i.e. the permanently excited synchronous machine of the front axle, being operated with a correspondingly higher power, i.e. the second electrical machine dominates as a power provider. This third operating range is at rather slow speeds but higher loads or higher required torques. The second speed threshold value is advantageously smaller than the first speed threshold value.

According to a further very advantageous embodiment of the method according to the invention, a fourth operating range can also be provided in which, at a vehicle speed that is smaller than the first speed threshold but greater than the second speed threshold, here too an accelerator pedal value is greater than the accelerator pedal threshold, both the first electric machine and the second electric machine are operated in a driving manner, the drive power output of the first electric machine, i.e. the externally excited synchronous machine of the rear axle, being greater than the drive power output of the second electric machine. In this fourth operating range, both electric machines are also used, with the first electric machine of the rear axle dominating the provision of drive power by being operated with greater drive power than the second electric machine. This fourth operating range represents higher loads and higher speeds, which, however, remain below the first speed threshold.

It is the case that the distribution of the power and ultimately also the distribution of the torque in the overlap area, i.e. the third and fourth operating range, in which both drives are operated, results from an efficiency-optimized operating strategy. According to the invention, it can be provided that the ideal torque distribution between the driven front axle and the driven rear axle is calculated for each operating point depending on the DC voltage of the electric drive system, wheel speeds and a requested total wheel torque, the ideal torque distribution being on the electric driven front axle on the one hand and the electrically driven rear axle on the other hand is calculated in such a way that a total output of the electric drive system is minimized.

The requested total wheel torque is proportional to the accelerator pedal position.

The mechanical power loss of the driven axles essentially consists of power loss from the gearboxes and power loss from the electrical machines. The mechanical power loss is advantageously calculated or determined depending on factors such as the speed, torque and gear ratio. Loss maps of electrical machines advantageously take into account the dependence on the direct voltage, the maximum direct current, the rotor speed and the rotor torque. All of this flows into the strategy optimized to minimize the performance of the overall drive.

Further advantageous embodiments of the electric drive system for a motor vehicle according to the invention and of the method for operating a variant of such an electric drive system also result from the exemplary embodiment described in more detail below with reference to the figures.

• 1 eine schematische Darstellung eines elektrischen Antriebssystems gemäß der Erfindung; und
• 2 verschiedene Betriebsbereiche in einem Diagramm der Fahrpedalstellung über der Fahrzeuggeschwindigkeit.

Show:

• 1 a schematic representation of an electric drive system according to the invention; and
• 2 different operating ranges in a graph of accelerator pedal position versus vehicle speed.

In the representation of the 1 a vehicle 1 is indicated schematically. The vehicle 1 is to be driven by an electric drive system, which includes an electrically driven front axle 2 and an electrically driven rear axle 3. The electrically driven rear axle 3 comprises a first electric machine 4 with a first rotor 5, which in the exemplary embodiment shown here is in permanent torque-transmitting connection to a driven rear wheel 7 or a side shaft 8 connected to it in a rotationally fixed manner via a first differential gear 6. A planetary gear 9 is indicated between the first differential gear 6 and the side shaft 8, which is to be understood as optional.

The electrically driven front axle 2 includes a second electric machine 10 in a second rotor 11, which drives front wheels 16 driven by a second differential gear 12 and two planetary gears 13, 14 or the side shafts 15 of these front wheels 16 which are connected to them in a rotationally fixed manner. Here too, the two planetary gears 13, 14 could also be dispensed with. In the area of the side shafts 15 of the driven front axle 2, separating clutches 17 in the form of switchable freewheels are provided.

In the area of the electrically driven rear axle 3, a structure with a further third electric machine 18 with a third rotor 19 can optionally be provided, which drives a second driven rear wheel 20 or its side shaft 21 via the first differential gear 6 via an optional planetary gear 22. Both electric machines 4, 18 could also drive the two rear wheels 7, 20 together via the first differential gear 6.

The electric drive system for the motor vehicle 1 now provides that the first electric machine 4 and the third electric machine 18 - if present - are each designed as externally excited synchronous machines EESM, preferably as axial flux machines. In contrast, the second electrical machine 10 of the electrically driven front axle 2 is designed as a permanently excited synchronous machine PMSM, and here in particular as a radial flux machine.

The two electrically driven axes 2, 3 have different tasks for operating the electric drive system. The electrically driven front axle 2 serves as an efficient drive. Its task is to meet the most common customer-relevant driving requirements and to predominantly drive the vehicle in everyday use. The requirement extends over a comparatively large speed range, but a relatively low torque. This suits the principle of the permanently excited synchronous machine. The drive of the electric front axle 2 can be switched off if necessary via the switchable freewheels 17 as separating clutches 17. Ideally, a parking lock is not required here to minimize losses. The total gear ratio between the second rotor 11 of the second electric machine 10 and the two side shafts 15 of the electrically driven front axle 2 is ideally less than 6.

The electrically driven rear axle 3, on the other hand, is designed for the so-called functional drive. Their task is to meet high driving requirements beyond everyday operation, for example special load cases such as kickdown, starting with a trailer on an incline and the like. The first electrical machine 4 or the two electrical machines 4, 18 are designed as externally excited synchronous machines, in particular as axial flux machines. There is no mechanical clutch in the area of the electrically driven rear axle 3, but rather the first rotor 5 or the first rotor 5 and the third rotor 19 are permanently connected to the side shafts 8 or 8 and 21. This means that no mechanical coupling elements or actuators are required. In the event that the driven rear axle 3 is not driven, it remains mechanically connected, the drive is simply not energized, which in the installed externally excited synchronous machines means that they rotate electrically without loss, so that there are no electrical losses, only mechanical losses the friction in the electrically driven rear axle 3 occurs accordingly. It is the case that the selection of the electrically driven rear axle 3 is ideally suited for the functional drive, since the positioning means that the usability of the drive is advantageous due to the increased traction due to the driving dynamics.

The various conceivable operating ranges for such an electric drive system for the motor vehicle 1 are now shown in the schematic diagram 2 shown. An accelerator pedal position or an accelerator pedal value S is plotted against a speed v of the vehicle 1. The accelerator pedal value S is the value, for example as a percentage by which the accelerator pedal is pressed.

This accelerator pedal value S is essentially proportional to the sum of the drive torques, i.e. to a requested total drive torque, on the driven vehicle wheels.

A first operating range I can be seen in the diagram at the bottom left. It ranges from an accelerator pedal value with the accelerator pedal essentially not actuated to an accelerator pedal threshold value S1 shown with a dashed line in the direction of the accelerator pedal value S. In the direction of the vehicle speed v, the first operating range I ranges from zero to a first speed threshold value v1. This area, which is in the 2 is shown somewhat schematically, the area on the right in the direction of the vehicle speed would actually end with a hyperbolically downward sloping curve. However, this is no longer relevant to the explanation here, so it is not shown.

A second operating range II adjoins the first operating range I on the right for higher vehicle speeds v, i.e. vehicle speeds v which are greater than the first speed threshold value v1. It projects above the accelerator pedal threshold value S1 in the direction of the accelerator pedal value S. In the direction of the accelerator pedal value S above the accelerator pedal threshold value S1, the first operating range I is followed by a third operating range III, which extends in the direction of the vehicle speed v up to a second speed threshold value v2, which is smaller than the first speed threshold value v1. A fourth operating range IV lies in the area above the accelerator pedal threshold value S1 and between the first speed threshold value v1 and the second smaller speed threshold value v2.

Operation in the first operating range I now takes place, for example, up to a total drive torque of approximately 3000 Nm on the wheel, which is essentially proportional to the accelerator pedal value S, in exclusive operation of the permanently excited synchronous machine 10, i.e. efficient operation with the electrically driven front axle 2. This Operation takes up a significant part of the everyday operation of the motor vehicle 1. It is characterized by comparatively low total drive torque and, accordingly, an accelerator pedal that is not fully depressed.

The second operating range II at higher speed v of the motor vehicle 1 and up to a maximum total drive torque occurring at these speeds is the operating range I, in which only the electrically driven rear axle 3 is used, i.e. the pure functional drive.

The two operating areas III and IV, on the other hand, are mixed areas or transition areas in which both the electrically driven front axle 2 and the electrically driven rear axle 3 are used, optimized for the efficiency of the overall drive. In this mixed operation of the services it is the case that in the third operating range III the electrically driven front axle 2 and thus the permanently excited synchronous machine 10 is dominant, the externally excited synchronous machine 4 or the externally excited synchronous machines 4, 18 of the electrically driven rear axle 3 only deliver small power parts added. Above the second speed threshold value v2, at higher required torques and thus at an accelerator pedal value S above the accelerator pedal threshold value S1, a drive with both driven axles 2, 3 occurs, in which, however, the first electric machine 4 or the first electric machine 4 and the third electric machine 18 of the electrically driven rear axle 3 dominates in terms of providing drive power.

Claims (11)

Method for operating an electric drive system for a motor vehicle (1) with an electrically driven front axle (2) and an electrically driven rear axle (3), wherein the electrically driven rear axle (3) comprises a first electric machine (4) which has a first rotor (5) which is designed to drive at least a first rear wheel (7), the electrically driven front axle (2) comprising a second electric machine (10) which has a second rotor (11) which is designed to to drive at least one first front wheel (16), the first rotor (5) being permanently torque-transmittingly coupled to the first rear wheel (7), characterized in that the first electrical machine (4) is a separately excited synchronous machine and the second electrical machine (10 ) is designed as a permanently excited synchronous machine, wherein a separating clutch (17) is designed to couple and decouple the second rotor (11) with the at least one first front wheel (16) in a torque-transmitting manner, wherein at a vehicle speed (v) which is greater than is a first speed threshold value (v1), by means of the separating clutch (17), the second rotor (11) is decoupled from the at least one driven front wheel (16), the first speed threshold value (v1) being dependent on a direct voltage or a charger level of a vehicle battery depends, whereby at a vehicle speed (v) which is smaller than the first speed threshold value (v1), the first electric machine (4) is electrically decoupled if an accelerator pedal value (S) is also smaller than an accelerator pedal threshold value (S1). Procedure according to Claim 1 , characterized in that the separating clutch is designed as a switchable freewheel (17). Procedure according to Claim 1 or 2 , characterized in that an overall rear axle ratio is greater than an overall front axle ratio. Method according to one of the preceding claims, characterized in that the first electrical machine (4) is designed as an axial flux machine. Method according to one of the preceding claims, characterized in that the second electrical machine (10) is designed as a radial flux machine. Method according to one of the preceding claims, characterized in that the first rotor (5) is arranged coaxially to a first differential gear (6). Method according to one of the preceding claims, characterized in that the second rotor (11) is arranged coaxially to a second differential gear (12). Method according to one of the preceding claims, characterized in that the electrically driven rear axle (3) additionally has a third electrical machine (18) which is designed as a separately excited synchronous machine and which has a third rotor (19) which is designed for this purpose to drive the first rear wheel (7) and/or a second rear wheel (20). Method according to one of the preceding claims, characterized in that an ideal torque distribution between the driven front axle (2) and the driven rear axle (3) is calculated as a function of the DC voltage of the vehicle battery, wheel speeds and a requested total drive torque, such that a Total power delivered by the electric drive system is minimized. Method according to one of the preceding claims, characterized in that at a vehicle speed (v) which is smaller than a second speed threshold value (v2) and at an accelerator pedal value (S) which is greater than the accelerator pedal threshold value (S1), both the first electric machine ( 4) as well as the second electrical machine (10) are operated in a driving manner, wherein a driving power of the second electrical machine (10) is greater than a driving power of the first electrical machine (4) and wherein the second speed threshold value (v2) is smaller than the first speed threshold value (v1) is. Procedure according to Claim 10 , characterized in that at a vehicle speed (v) which is smaller than the first speed threshold (v1), but greater than the second speed threshold (v2), driving both the first electric machine (4) and the second electric machine (10). are operated, the drive power of the first electrical machine (4) being greater than the drive power of the second electrical machine (10).

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