Electric drive system for a motor vehicle

Abstract

Electric drive system (1) for a motor vehicle comprising a housing (2), an electric machine (3) comprising a rotor (5) non-rotatably connected to a rotor shaft (6), a planetary gear set (7) and an axle drive (13), wherein the planetary gear set (7) comprises exactly one sun gear (8) and exactly one planet carrier (9), wherein the planet carrier (9) carries a set of first planet gears (10) and a set of second planet gears (11), wherein the first planet gears (10) have a larger diameter than the second planet gears (11), wherein one of the first planet gears (10) and one of the second planet gears (11) are non-rotatably connected to each other, wherein the sun gear (8) is in mesh with the first planet gears (10), wherein the rotor shaft (6) is non-rotatably connected to the sun gear (8), and wherein the planet carrier (9) is non-rotatably connected to an input shaft (15) of the axle drive (13).wherein exactly one ring gear (12) is provided which engages with the second planet gears (11), wherein a first freewheel (18) is provided between the ring gear (12) and the housing (2), by means of which the ring gear (12) is coupled to the housing (2) in such a way that rotation of the ring gear (12) in a forward direction of rotation of the rotor (5) is blocked, characterized in that a second freewheel (19) is provided between the ring gear (12) and the housing (2), which is switchable, by means of which the ring gear (12) is coupled to the housing (2) in such a way that rotation of the ring gear (12) against the forward direction of rotation of the rotor (5) is blocked in the engaged state.

Description

The invention relates to an electric drive system for a motor vehicle comprising a housing, an electric machine, a planetary gear and an axle gear of the type defined in more detail in the preamble of claim 1. From DE 10 2020 109 112 A1 an electric drive system with an electric machine, a planetary gear and an axle gear is known. Another electric drive system for a vehicle is essentially known from DE 10 2013 225 519 A1. This patent describes a system comprising an electric machine, a planetary gear set, and an axle drive, wherein the planetary gear set has a planet carrier with two rotationally fixed planet gears for each planet. These mesh with two different ring gears, which can be braked against a housing of the electric drive system by means of braking elements. This results in two different gear ratios between the electric machine on the one hand and the axle drive on the other. A similar electric drive system is known from the generic patent DE 10 2016 223 110 B3, in which, however, only one ring gear is provided, which meshes with stepped planetary gears, and a freewheel is provided between the ring gear and the housing. The disadvantage of the described two-speed system lies particularly in the switching and braking elements for the two planetary gears. These must be designed as multi-plate shift elements, which leads to increased losses during open-circuit operation, i.e., when the vehicle is coasting. Ultimately, this results in a higher power requirement for the vehicle itself and thus reduces its range. The object of the present invention is to provide an improved electric drive system which in particular avoids or at least reduces the aforementioned disadvantages. According to the invention, this problem is solved by an electric drive system with the features in claim 1, and in particular in the characterizing part of claim 1. Advantageous embodiments and further developments are described in the dependent claims. The electric drive system according to the invention comprises, similar to the drive system in the prior art mentioned above, an electric machine with a rotor and a rotor shaft. Furthermore, the electric drive system comprises a planetary gear set with exactly one sun gear and exactly one planet carrier. The rotor shaft is non-rotatably connected on one side to the rotor of the electric machine and on the other side to the exactly one sun gear of the planetary gear set. “Transversely fixed connection” within the meaning of the present invention means that the elements or components connected in a rotationally fixed manner are arranged coaxially to each other and are connected to each other in such a way that they rotate at the same angular velocity. The planet carrier also carries a set of first planet gears and a set of second planet gears. The first and second planet gears are each rotatably mounted on the planet carrier in a manner known per se. The first and second planet gears have different diameters, with the first planet gears meshing with the sun gear and the second planet gears meshing with a ring gear of the planetary gear set. The planetary gear set comprises the ring gear as its sole ring gear. A first freewheel is also provided between the ring gear and the housing, by means of which the ring gear is coupled to the housing in such a way that rotation of the ring gear in a forward direction of rotation of the rotor of the electric machine is blocked.Only the traction torque of the electric motor in its positive direction of rotation can be transmitted via the first freewheel between the ring gear and the housing. During deceleration, automatic disengagement occurs; when switching back from deceleration to traction, automatic re-engagement takes place. The design is exceptionally simple and efficient, and without the need for any actuators, it enables the electric motor to transmit torque to the axle drive in the forward direction. In the reverse direction, i.e., when the power flows from the axle drive towards the electric motor, the motor does not need to be dragged along. Instead, it is automatically decoupled via the first freewheel, allowing coasting without additional power losses in the electric drive system. This increases the overall performance and efficiency of the electric drive system. As a result, a vehicle equipped with such an electric drive system as its primary or auxiliary drive has a greater range. Due to the decoupling during thrust via the first freewheel, it is inherently impossible to reverse drive with motor operation of the electric machine in the negative direction of rotation and to recuperate energy with generator operation of the electric machine. According to the invention, a second freewheel is arranged between the ring gear and the housing, by means of which the ring gear is coupled to the housing in such a way that rotation of the ring gear against the forward direction of rotation of the rotor is blocked. This freewheel therefore operates during deceleration and not during traction. During traction operation of the electric machine, it thus operates via the first freewheel, which opens during deceleration. Simultaneously, a connection is established during deceleration via the second freewheel, which opens automatically during traction, so that both reverse travel and energy recuperation are possible. To retain the aforementioned advantage of lossless or low-loss sailing, the invention provides that the second freewheel is switchable. Switchable in the context of the invention means that it can be turned on and off. In the engaged state, it acts like a non-switchable freewheel, blocking rotation in one direction and allowing free rotation in the opposite direction. In the disengaged state, the blockage is released, the freewheel is deactivated, and the connection can rotate freely in both directions. By switching off the second freewheel, the blockage it creates during overrun operation is released, thus enabling lossless sailing with a vehicle equipped with the electric drive system during overrun operation.If recuperation is required or a reverse drive is necessary, such operation can be enabled simply and efficiently by switching the second freewheel so that its blocking function is switched on again. According to a further highly advantageous embodiment of the electric drive system according to the invention, the second planet gears can be arranged to overlap axially with the axle drive. Such an axially overlapping arrangement with the axle drive, so that the second planet gears lie at least partially within the same area in the axial direction of the electric drive system, enables a very compact design in the axial direction. For the purposes of this invention, the axial direction shall always be understood as the axial direction along or parallel to the main axis of rotation of the electric drive system, here parallel to the axis of rotation of the rotor shaft. The radial direction, as described herein, is perpendicular to this axial direction. Another exceptionally advantageous design of the electric drive system involves the axle drive having a bevel gear differential with a differential cage. This differential cage, which can also be referred to as the differential housing, forms the differential input shaft. Particularly with this axle drive design, a significant space saving can be achieved through axial overlap with the second planet gears, i.e., the smaller diameter planet gears on the planet carrier. A particularly advantageous design of the electric drive system incorporates a first axial bearing, which axially supports the ring gear relative to the planet carrier. This also contributes to the most compact design possible, both axially and radially. According to a further highly advantageous embodiment, a first radial bearing can also be provided by means of which the planet carrier is radially supported relative to the housing, wherein a planet carrier-side bearing half of the first radial bearing is arranged radially outside a housing-side second bearing half. The planet carrier is thus supported via the first radial bearing from the radial outside on a radially more inwardly arranged part of the housing, e.g., an axially projecting collar of a housing partition. A further embodiment of the electric drive system can also include a second axial bearing, by means of which the ring gear is axially supported against either a parking lock gear or the housing. Such support, for example on the parking lock gear, which is typically rotationally fixed to the input shaft of the axle drive, or alternatively directly on this input shaft or a flange of this input shaft, allows for a very compact design. However, because the relative movements occurring between the input shaft and the ring gear are comparatively large, it can also be advantageous from a loss perspective to support the ring gear on the housing with the second axial bearing, so that both design variants have their advantages, depending on the optimization objective. The parking lock wheel, which is typically fixed to the input shaft of the axle drive, can then be braked or locked to the housing via a switching element. This allows the axle drive to be blocked and prevents rotation of the output shaft and the vehicle's wheels. According to a particularly advantageous embodiment of the electric drive system of the invention, the rotor shaft can be supported by combination bearings. For this purpose, a first combination bearing and a second combination bearing are provided, each configured to provide axial and radial support for the rotor shaft relative to the housing. In both the first and second combination bearings, the respective rotor shaft-side bearing half is arranged radially within the respective housing-side bearing half. The rotor shaft is thus supported against parts of the housing, such as radially extending intermediate walls of the housing, which lie radially outside the rotor shaft. Both freewheels can be integrated into a single assembly thanks to a remarkably efficient further development of the electric drive system. Such an assembly can be positioned between the axle drive and the housing and would therefore be easily accessible from the outside of the housing, particularly for integrating the actuators in the case of the switchable freewheel. In principle, a switching function for the first freewheel would also be possible. However, this is not relevant in practice, so it is typically omitted for cost reasons. The electric machine is particularly preferably implemented as an axial flux machine. Further advantageous embodiments of the electric drive system also result from the exemplary embodiment, which is described in more detail below with reference to the figure. The only accompanying Fig. 1 shows a schematic sectional view of an electric drive system according to the invention. Figure 1 schematically depicts an electric drive system 1 for a motor vehicle (not shown). The electric drive system 1 comprises a housing 2 and an electric machine 3 with a stator 4 fixed against rotation relative to the housing 2 and a rotor 5 rotatable relative to the stator 4, which in turn is fixed against rotation to a rotor shaft 6. The electric drive system 1 also comprises a planetary gear set 7 with a sun gear 8, which is non-rotatably connected to the rotor shaft 6, and a planet carrier 9, which carries a set of first planet gears 10 and a set of second planet gears 11. The first planet gears 10 have a larger diameter than the second planet gears 11. The adjacent first and second planet gears 10 and 11 are each non-rotatably coupled to each other. The second planet gears 11, with a smaller diameter than the first planet gears 10, mesh with a ring gear 12 of the planetary gear set 7. In addition, the electric drive system 1 includes an axle drive 13, which is designed here as a bevel gear differential. An output shaft 14 of the axle drive 13 forms the output, indicated by the arrows, in particular to the wheels of the vehicle. The differential cage 15 forms the input shaft 15 of the axle drive 13 and is rotationally fixed to the planet carrier 9. The differential cage 15 can also be rotationally fixed relative to the housing 2 by means of a parking lock gear 16 and a corresponding switching element 17 in order to implement a parking lock for the vehicle. The assembly shown here with the single ring gear 12 of the planetary gear 7 ultimately represents a single-gear system for driving or at least partially driving the motor vehicle via the electric drive system 1. The ring gear 12 of the planetary gear 7 is coupled to the housing 2 of the electric drive system 1 via a first freewheel 18 in such a way that rotation of the ring gear 12 in a forward direction of rotation of the rotor 5 is prevented. Thus, only traction torques of the electric machine 3 are transmitted to the output shaft 14 via the first freewheel 18, while in overrun mode the first freewheel 18 automatically rotates freely, thereby disengaging the electric machine 3 by releasing the ring gear 12 of the planetary gear 7. This allows overrun mode without dragging the electric machine 3, enabling efficient and low-loss operation of the electric drive device 1 during so-called coasting, i.e., when the vehicle is coasting without propulsion. When switching back from overrun to traction, the first freewheel 18 engages automatically, and the power of the electric machine 3 can again be transmitted to the output shaft 14.The first freewheel 18 alone would not allow for recuperation, nor would it be possible to reverse the vehicle using the electric motor 3, as this would require power transmission between the output 14 and the electric motor 3 in the opposite direction, i.e., the direction of thrust. To enable this, an additional second freewheel 19 is provided between the ring gear 12 and the housing 2. This freewheel 19 is designed to transmit torque during deceleration but not during acceleration. It thus complements the first freewheel 18, making recuperation and reverse driving possible. The second freewheel 19 is switchable. By switching this second freewheel 19, as shown in the illustration in Fig.As indicated by the arrow 1, the locking function can be switched on or off, so that the second freewheel 19 either operates in the described manner as a locking freewheel 19 in thrust or can be switched to a freewheel 19 that is free in both thrust and traction, so that depending on the switching position of this second freewheel 19 either recuperation or reverse driving is possible, while with the locking function switched off, sailing without dragging the electric machine 3 is made possible. The combination of the two freewheels 18, 19, of which the second freewheel 19 is switchable, thus enables a permanent connection in one direction and a switchable connection in the other direction of rotation or force. The two freewheels 18, 19 can preferably be combined into an integrated unit 28, which is arranged, in particular, radially R outside the differential cage 15 but axially overlapping it. The unit is preferably easily accessible from the outside of the housing 2, especially to allow simple and efficient actuation of the switching actuator of the second freewheel 19, for example, by means of a hydraulic line, an electrical signal line, and / or the like. Overall, the assembly can be realized in an exceptionally compact manner. Not only can the integrated unit 28 with the two freewheels 18, 19 be designed to overlap the axle drive 13 or its differential cage 15 in the axial direction A of the output shaft 14 or the rotor shaft 6, but also, in particular, the set of the second planet gears 11 and preferably the ring gear 12. All of this results in a very compact assembly, especially in the axial direction A. The electric machine 3, which can be designed in particular as an axial flux machine, can be supported on the housing 2 with respect to its rotor shaft 6 by a first combination bearing 20 and a second combination bearing 21, wherein the rotor shaft 6 is arranged radially inside and the housing is arranged radially outside the two combination bearings 20, 21. The combination bearings can transmit both axial and radial forces, and are thus a combination of axial and radial bearings. In particular, they can be angular contact ball bearings, four-point contact ball bearings, or the like. The planet carrier 9 of the planetary gear 7 can then also be supported on the housing 2 via a first radial bearing 22, with the housing 2 being arranged radially inside and the planet carrier 9 radially outside this first radial bearing 22. A second radial bearing 23 can then be located in the area of ​​the planet carrier 9 to support the two first and second planet gears 10, 11 of the respective set of planet gears, which are connected to each other in a rotationally fixed manner. The ring gear 12 can preferably be supported in the axial direction A by a first axial bearing 24 against the planet carrier 9. A second axial bearing 25 can serve to support the ring gear 12 and the freewheels 18, 19 connected to it in the other axial direction A. This axial bearing 25, which is shown in Fig. 1 without an actual counterpart, can, depending on the design, optionally be supported either on the parking lock gear 16, which results in a very compact design, or alternatively on the housing 2, which results in a somewhat larger design, but is advantageous with regard to losses. The axial bearings 24, 25 can in particular be designed as axial needle bearings, the radial bearings 22, 23 in particular as cylindrical roller bearings or deep groove ball bearings, as is generally known and customary. Further relevant bearing points in the assembly of the electric drive system 1 are located in the area of ​​the output shaft 14, where, in the illustration of Fig. 1 on the left, a radial bearing 26 is arranged and, in the illustration of Fig. 1 on the right, a combination bearing 27, which can transmit axial and radial forces, is arranged. In these, the radially inner output shaft 14 is supported on the radially outer housing 2.

Claims

Electric drive system (1) for a motor vehicle comprising a housing (2), an electric machine (3) comprising a rotor (5) non-rotatably connected to a rotor shaft (6), a planetary gear set (7) and an axle drive (13), wherein the planetary gear set (7) comprises exactly one sun gear (8) and exactly one planet carrier (9), wherein the planet carrier (9) carries a set of first planet gears (10) and a set of second planet gears (11), wherein the first planet gears (10) have a larger diameter than the second planet gears (11), wherein one of the first planet gears (10) and one of the second planet gears (11) are non-rotatably connected to each other, wherein the sun gear (8) is in mesh with the first planet gears (10), wherein the rotor shaft (6) is non-rotatably connected to the sun gear (8), and wherein the planet carrier (9) is non-rotatably connected to an input shaft (15) of the axle drive (13).wherein exactly one ring gear (12) is provided which engages with the second planet gears (11), wherein a first freewheel (18) is provided between the ring gear (12) and the housing (2), by means of which the ring gear (12) is coupled to the housing (2) in such a way that rotation of the ring gear (12) in a forward direction of rotation of the rotor (5) is blocked, characterized in that a second freewheel (19) is provided between the ring gear (12) and the housing (2), which is switchable, by means of which the ring gear (12) is coupled to the housing (2) in such a way that rotation of the ring gear (12) against the forward direction of rotation of the rotor (5) is blocked in the engaged state. Electric drive system (1) according to claim 1, characterized in that the second planet gears (11) are arranged axially overlapping with the axle drive (13). Electric drive system (1) according to claim 1 or 2, characterized in that the axle drive (13) has a bevel gear differential with a differential cage (15) as the input shaft. Electric drive system (1) according to one of the preceding claims, characterized in that a first axial bearing (24) is provided by means of which the ring gear (12) is axially supported relative to the planet carrier (9). Electric drive system (1) according to one of the preceding claims, characterized in that a first radial bearing (22) is provided, by means of which the planet carrier (9) is radially supported relative to the housing (2), wherein a planet carrier-side bearing half of the first radial bearing (22) is arranged radially outside a housing-side second bearing half. Electric drive system (1) according to one of the preceding claims, characterized in that a second axial bearing (25) is provided, by means of which the ring gear (12) is axially supported against a parking lock gear (16) or the housing (2). Electric drive system (1) according to one of the preceding claims, characterized in that a first combination bearing (20) and a second combination bearing (21) are provided, each of which is arranged to provide axial and radial support for the rotor shaft (6) against the housing (2), wherein in both the first combination bearing (20) and the second combination bearing (21) the respective rotor shaft-side bearing half is arranged radially within the respective housing-side bearing half. Electric drive system (1) according to one of the preceding claims, characterized in that the two freewheels (18,19) are combined in an assembly (28). Electric drive system (1) according to one of the preceding claims, characterized in that the electric machine (3) is designed as an axial flux machine.

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