Electric axle drive train and method for operating an electric axle drive train

Abstract

The invention relates to an electric axle drive train (1) for a motor vehicle (2) with an electric machine (3) which has a stator (6) through which a hydraulic fluid (5) can flow for cooling the stator, a rotor (7) being mounted so as to be rotatable relative to the stator (6), and with a heat exchanger (24) through which the hydraulic fluid (5) flows. In order to reduce undesirable pressure losses during operation of the electric axle drive train (1), in particular at low temperatures, the heat exchanger (24) is combined with a bypass for the hydraulic fluid (5), wherein the bypass for the hydraulic fluid (5) has a permanently open hydraulic resistance (25), wherein the hydraulic resistance (25) is an orifice plate, wherein the heat exchanger (24) with the bypass and the permanently open hydraulic resistance (25) is arranged between a pump arrangement (11) and the stator (6) of the electric machine (3).

Description

The invention relates to an electric axle drive train for a motor vehicle having an electric machine which has a stator through which a hydraulic fluid can flow for cooling the latter, with respect to which stator a rotor is arranged in a relatively rotatably mounted manner, and having a heat exchanger through which the hydraulic fluid flows. The invention further relates to a method for operating such an electric axle drive train.Electric motors are increasingly used for the drive in motor vehicles in order to create alternatives to internal combustion engines that require fossil fuels. In order to improve the suitability of electric drives for all days and in addition to be able to offer users the usual riding comfort, considerable efforts have already been made.A detailed illustration of an electric drive is given in an article of the journal ATZ 113. Vol. 05 / 2011, pages 360-365 of Erik Schneider, Frank Fickl, Bernd Cebulski and Jens Liebold, with the title: High-Integration and Flexible Electric Drive Unit for E-vehicles. In this article, a drive unit for an axle of a vehicle is described, which drive unit comprises an electric motor which is arranged concentrically and coaxially with a bevel gear differential, wherein a shiftable 2-gear planetary gear set is arranged in the power train between electric motor and bevel gear differential, which is also positioned coaxially with the electric motor or the bevel gear differential or spur gear differential. The drive unit is of very compact construction and, owing to the shiftable 2-gear planetary gear set, allows a good compromise between climbing capability, acceleration and energy consumption. Drive units of this type are also referred to as E-axles or electrically operable axle drive train.From DE 10 2010 048 837 A1, such a drive device with at least one electric motor and at least one planetary differential drivable with a rotor of the electric motor is known, wherein the planetary differential has at least one planet carrier operatively connected to a rotor of the electric motor, first planetary gears and second planetary gears rotatably mounted on the planetary carrier, as well as a first sun gear and a second sun gear, each of which is operatively connected to an output shaft of the planetary differential. The first planetary gears mesh with the first sun gear and each of the second planetary gears mesh with the second sun gear and with one of the first planetary gears. Furthermore, the sun gears are arranged coaxially with a rotation axis of the rotor.In the development of electric machines and gear units provided for E-axes, there is a continuing need to increase their power densities, so that the cooling necessary for this purpose, in particular of the electric machines and the gear units, is becoming increasingly important. Due to the necessary cooling capacities, hydraulic fluids, such as cooling oils, have become established in most concepts for transporting heat away from the thermally exposed regions of an electric machine and / or a transmission.The transmissions usually provided in the designated E-axles are generally lubricated with a transmission oil, wherein this is frequently also used as cooling oil for the electric machine. In order to reliably deliver the lubricant or cooling oil to the various lubricating or cooling points, it is known to form a corresponding hydraulic fluid circuit in the corresponding E-axes.It is therefore quite common to form within an E-axis an oil sump for storing the amount of oil necessary for lubrication and cooling, from which sump the above-mentioned hydraulic fluid circuit is fed with oil.The volume required for an oil sump is, however, limited by two essential factors: on the one hand, by the available installation space in the electric axle drive train and, on the other hand, by the maximum permissible fill level in the transmission for achieving the efficiency specifications, in particular by avoiding splashing losses of rotating toothing components.WO 2021 / 246 088 A1 discloses an electric axle drive train for a motor vehicle having an electric machine which has a stator through which a hydraulic fluid can flow for cooling the latter, and having a heat exchanger through which the hydraulic fluid flows.Furthermore, German laid-open specification DE 10 2019 117 893 A1 discloses a drive train for a motor vehicle having a directly cooled electric machine, wherein a cooling circuit for conducting a coolant through the electric machine and a transmission are provided. The coolant serves to lubricate and cool the transmission and to directly cool electrical lines of the stator.The publication EP 4 199 326 A1 shows a further drive train of a vehicle and a cooling method for an electric motor.It is the object of the invention to reduce undesirable pressure losses during operation of an electric axle drive train according to the preamble of claim 1, in particular at low temperatures.The object is achieved in an electric axle drive train for a motor vehicle having an electric machine which has a stator through which a hydraulic fluid can flow for cooling the latter, a rotor being arranged in a relatively rotatably mounted manner with respect to said stator, and having a heat exchanger through which the hydraulic fluid flows, characterized in that the heat exchanger is combined with a bypass for the hydraulic fluid, the bypass for the hydraulic fluid having a permanently open hydraulic resistance, the hydraulic resistance being a diaphragm. The heat exchanger, which is also referred to as a heat exchanger, serves during operation of the electric axle drive train to temperature control, in particular to cool, the hydraulic fluid. Depending on the design of the heat exchanger or heat exchanger, undesirable pressure losses via the heat exchanger or heat exchanger can be kept small. This is the case in particular at high temperatures when the hydraulic fluid has a low viscosity. The hydraulic fluid is, for example, a hydraulic oil which, for short, is also referred to as oil and whose viscosity increases greatly at low temperatures. During operation of a conventional electric axle drive train, this leads, among other things, to high pressure losses occurring via the heat exchanger. These high pressure losses can be avoided by the bypass with the permanently open hydraulic resistance. This is also in particular not critical at low temperatures because no cooling is required in the stator at low temperatures. The permanently open hydraulic resistance in the bypass is advantageously designed such that during normal operation of the electric axle drive train, in particular at rising temperatures, a sufficient flow through the heat exchanger or heat exchanger is ensured. The permanently open hydraulic resistor is designed as a diaphragm. The claimed solution is more advantageous than conventional solutions both in terms of installation space requirement and in terms of cost requirement. In addition, the electric axle drive train with the bypass and the permanently open hydraulic resistor is advantageously maintenance-free.In this case, the heat exchanger with the bypass and the permanently open hydraulic resistor is arranged between a pump arrangement and the stator of the electric machine. In this way, effective cooling of the stator during operation of the electric axle drive train can be ensured in a simple manner.A further preferred exemplary embodiment of the electric axle drive train is characterized in that the heat exchanger with the bypass and the permanently open hydraulic resistor is incorporated into a cooling system which additionally serves for cooling power electronics. The cooling system comprises, for example, a coolant circuit, in particular a water circuit, which is combined with a hydraulic fluid circuit. By cooling the power electronics, the efficiency during operation of the electric axle drive train can be further increased.A further preferred exemplary embodiment of the electric axle drive train is characterized in that the cooling system additionally serves for cooling at least one climate component. It is accepted that the electric axle drive train becomes structurally and production-related more complex.A further preferred exemplary embodiment of the electric axle drive train is characterized in that the hydraulic fluid within the electric axle drive train is further provided for cooling and / or lubricating a wet-running transmission arrangement which is coupled to the electric machine. The wet-running transmission arrangement serves in the electric axle drive train in particular to improve the driving comfort. The wet running transmission assembly is lubricated and / or cooled with the hydraulic fluid during operation of the electric axle drive train.A further preferred exemplary embodiment of the electric axle drive train is characterized in that the wet-running transmission arrangement is arranged within a transmission chamber, on which a first sump for storing the hydraulic fluid is provided below the transmission chamber in the direction of gravity and hydraulically connected thereto, wherein the first sump is hydraulically connected to a second sump formed in the axle drive train for storing the hydraulic fluid, wherein one or the pump arrangement is coupled to the second sump, by means of which first the hydraulic fluid can be conveyed from the first sump into the second sump and secondly the hydraulic fluid can be supplied from the second sump through the heat exchanger with the bypass and the permanently open hydraulic resistance to the stator for cooling thereof. The intake volume of hydraulic fluid of the first sump is advantageously smaller than the intake volume of hydraulic fluid of the second sump.A further preferred exemplary embodiment of the electric axle drive train is characterized in that the heat exchanger is designed to be rather small in terms of power, preferably with a power of less than twenty kilowatts. The rather small heat exchanger has, for example, a power of approximately ten kilowatts. In conventional electric final drive trains, this can lead to undesirably high pressure losses when providing the required volume flows, particularly at low temperatures. Due to the permanently open hydraulic resistance in the bypass, which bypasses the heat exchanger, the pressure losses can be significantly reduced.A further preferred exemplary embodiment of the electric axle drive train is characterized in that the heat exchanger in combination with the bypass for the hydraulic fluid having the permanently open hydraulic resistance is designed by a fluidic and thermal simulation in such a way that the combination is thermally intrinsically safe. It can thus be ensured in a simple manner that the heat exchanger combined with the bypass and the permanently open hydraulic resistor functions permanently correctly in the electric axle drive train.The invention further relates to a heat exchanger, a bypass and / or a hydraulic resistor in the form of a diaphragm for an electrical axle drive train described above. The parts mentioned can be purchased separately.In a method for operating an electric axle drive train described above, the object indicated above is alternatively or additionally achieved in that the hydraulic fluid flows through both the heat exchanger and the bypass for the hydraulic fluid having the permanently open hydraulic resistance. Thus, the above-described undesirably high losses at low temperatures during operation of the electric axle drive train are significantly reduced. At normal temperatures, for example at an ambient temperature of approximately sixteen to twenty-four degrees Celsius, it is ensured by appropriate design that a sufficient amount of hydraulic fluid per unit time flows through the heat exchanger. The same applies to higher temperatures at which the cooling function provided with the heat exchanger is more important.An increase in efficiency is, among other things, the arrangement and efficient interconnection of the volumes (sumps) accommodating different hydraulic fluids within the axle drive train. For this purpose, the first sump assigned to the transmission arrangement can set the lowest possible fill level of hydraulic fluid, that is to say superfluous hydraulic fluid can drop, for example, directly from the transmission chamber into the first sump via drain openings of the transmission housing provided for this purpose and is pumped from there via the pump arrangement into the downstream second sump.This achieves the advantage that, on the one hand, the splashing losses in the transmission arrangement can be kept low and, at the same time, a sufficiently large amount of hydraulic fluid can be provided for cooling the stator of the electric machine.The pump arrangement is preferably designed such that a volume flow of hydraulic fluid for cooling the stator of the electric machine of 5-25 l / min, most preferably 5-20 l / min, can be provided. To represent these volume flows, the hydraulic fluid present in the axle drive train and stored in the stubs must generally be pumped through the cooling circuit of the stator of the electric machine several times per minute. In order to avoid too strong foaming of the hydraulic fluid, it is preferable to limit the circulation of the hydraulic fluid in the axle drive train to at most 5 circulations / minute, preferably at most 3 circulations / minute. In this context, it is then further preferred that the amount of hydraulic fluid stored in the drive train is between 1-8.5 liters, preferably between 1.5-6.5 liters.By dividing into two sumps separated from one another, it is also possible to function second, larger sumps as a settling chamber for the hydraulic fluid, in that sufficient time can be given to the hydraulic fluid for defoaming.The hydraulic fluid is a flowable, liquid medium, preferably an oil. In principle, it would also be conceivable for the hydraulic fluid to be based on a water base and in particular to be formed as an emulsion which preferably contains an oil.First, the individual elements of the claimed subject matter are explained in the sequence in which they are stated in the set of claims and particularly preferred embodiments of the claimed subject matter are described below.An electrically operable axle drive train includes an electric machine and preferably a transmission assembly coupled to the electric machine. The transmission arrangement and the electric machine form a structural unit. This can be formed, for example, by means of a drive train housing in which the transmission arrangement and the electric machine are accommodated jointly.Alternatively, it would of course also be possible for the electric machine to have a motor housing and / or the transmission to have a transmission housing, wherein the structural unit can then be effected by fixing the transmission with respect to the electric machine. The transmission housing is a housing for receiving a transmission. The object is to guide shafts present over the bearings and to provide the wheels (possibly cam disks) with the degrees of freedom under all loads that they require without hindering them in the rotational and possibly path movement, as well as to absorb bearing forces and supporting torques. A transmission housing can be formed in one or more shells, that is to say in an unshared or divided manner. The housing should also be able in particular to damp noise and vibrations and also to absorb lubricant securely. The gear housing is preferably formed from a metallic material, in particular preferably from aluminum, grey cast iron or steel cast iron, in particular by means of a primary molding process such as casting or die casting.The motor housing encloses the electric machine. A motor housing can also accommodate the control and power electronics. The motor housing can furthermore also be a component of a cooling system for the electric machine and can be designed in such a way that cooling fluid can be supplied to the electric machine via the motor housing and / or the heat can be dissipated to the outside via the housing surfaces. In addition, the motor housing protects the electric machine and the optionally present electronics from external influences.A motor housing can be formed in particular from a metallic material. Advantageously, the motor housing can be formed from a metallic cast material, such as aluminum die casting, magnesium die casting, grey casting or steel casting.The electric machine serves for converting electrical energy into mechanical energy and / or vice versa, and it generally comprises a stationary part referred to as a stator, stator or armature, and a part referred to as a rotor or rotor and arranged movably, in particular rotatably, with respect to the stationary part.The electric machine of the electric axle drive train is preferably designed as an axial flow machine. The magnetic flux in an electric axial flux machine (AFM) is directed in the air gap between stator and rotor axially to a direction of rotation of the rotor of the axial flux machine. There are different types of axial flow machines. One known type is a so-called I-arrangement, in which the rotor is arranged axially next to a stator or between two stators. Another known type is a so-called H-arrangement in which two rotors are arranged on opposite axial sides of a stator.In principle, it is also possible for a plurality of rotor-stator configurations to be arranged axially next to one another as an I-type and / or H-type. In this context, it would also be possible to arrange both one or more rotor-stator configurations of the I-type and one or more rotor-stator configurations of the H-type next to one another in the axial direction. In particular, it is also preferable for the rotor-stator configuration of the H-type and / or of the I-type to be each of substantially identical design, such that they can be joined together in a modular manner to form an overall configuration. Such rotor-stator configurations can be arranged in particular coaxially with respect to one another and can be connected to a common rotor shaft or to a plurality of rotor shafts.In particular, the electric machine is dimensioned such that vehicle speeds greater than 50 km / h, preferably greater than 80 km / h and in particular greater than 100 km / h can be achieved. Particularly preferably, the electric motor has a power greater than 30 kW, preferably greater than 50 kW and in particular greater than 70 kW. It is furthermore preferred that the electric machine provides rotational speeds greater than 5,000 U / min, particularly preferably greater than 10,000 U / min, very particularly preferably greater than 12,500 U / min.Motor vehicles in the sense of this application are land vehicles which are moved by machine power without being bound to railway tracks. A motor vehicle can be selected, for example, from the group of passenger cars (passenger cars), trucks (trucks), small-size trucks, light-weight trucks, bicycles, buses (COM) or tractors.The transmission arrangement can be coupled in particular to the electric machine which is designed to generate a drive torque for the motor vehicle. The drive torque is particularly preferably a main drive torque, so that the motor vehicle is driven exclusively by the drive torque. The transmission arrangement is preferably designed as a planetary transmission, very particularly preferably as a shiftable, in particular two-speed planetary transmission.Advantageous embodiments of the invention are specified in the dependent claims. The features listed individually in the dependent claims can be combined with one another in a technically expedient manner and can define further embodiments of the invention. In addition, the features specified in the claims are more precisely described and explained in the description, further preferred embodiments of the invention being presented.According to an advantageous embodiment of the invention, it can be provided that the pump arrangement is arranged in the second sump, whereby a particularly short routing with correspondingly low drag losses results.According to a further preferred development of the invention, it can also be provided that the pump arrangement has a pump with a first channel for extracting the hydraulic fluid from the first sump into the second sump and a second channel for delivering the hydraulic fluid from the second sump for cooling the stator. It can be achieved in this way that a corresponding pump can have only one pump motor, which is to be preferred for cost reasons.Furthermore, according to a likewise advantageous embodiment of the invention, it can be provided that the second channel is designed as a high-pressure channel, so that the hydraulic fluid can be conveyed safely and with high efficiency through the cooling circuit of the stator.According to a further particularly preferred embodiment of the invention, it can be provided that the first channel has a riser pipe which is configured such that the hydraulic fluid opens into the second oil sump during operation of the axle drive train above a maximum fill level of hydraulic fluid. This can achieve, in particular, the effect that improved defoaming can be achieved. Furthermore, the invention can also be further developed to the effect that a swirl separator is arranged at the free end of the riser pipe, which further improves the defoaming of the hydraulic fluid.In a likewise preferred embodiment variant of the invention, it can also be provided that the first sump has / has a discharge screw on the bottom with a magnet and / or the second sump has / has a discharge screw on the bottom with a magnet, as a result of which a separation or immobilization of metallic particles from the hydraulic fluid can be effected.It can also be advantageous to further develop the invention to the effect that a heat exchanger is arranged in the second channel, by means of which heat exchanger the hydraulic fluid guided through the second channel is cooled before entering the stator, which improves the cooling capacity that can be provided for the stator.According to a further preferred embodiment of the subject matter of the invention, it can be provided that a suction filter is arranged in the first channel upstream of the pump arrangement in the conveying direction, as a result of which particles can likewise be separated out of the hydraulic fluid, which can likewise contribute to an increased service life as a result of reduced wear in the axle drive train.Finally, the invention can also be advantageously embodied such that the transmission chamber and the second sump are connected to one another at the top in the direction of gravity and a venting means is provided in the top region of the second sump, by means of which venting means both the transmission chamber and the second sump can be effected. The advantage which results from this is in particular that a compact design of the axle drive train is supported.The invention will be explained in more detail below with reference to figures without limiting the general concept of the invention.The following are shown: FIG. 1 is a hydraulic schematic diagram of the electric axle drive train; and FIG. 2 shows two motor vehicles each having an electric axle drive train in a schematic block diagram.FIG. 1 shows a hydraulic circuit diagram of an electric axle drive train 1 for a motor vehicle 2, as is also shown by way of example in FIG. 2.The axle drive train 1 comprises an electric machine 3 and a wet-running transmission arrangement 4 coupled to the electric machine 3. The electric machine 3 has a stator 6, through which a hydraulic fluid 5 can flow for cooling the latter, and to which a rotor 7 is arranged in a relatively rotatably mounted manner. The hydraulic fluid 5 is furthermore used within the axle drive train 1 for cooling and / or lubricating the wet-running transmission arrangement 4, wherein the wet-running transmission arrangement 4 is arranged within a transmission chamber 8, at which a first sump 9 for storing the hydraulic fluid 5 is provided below the transmission chamber 8 in the direction of gravity and hydraulically connected thereto.The first sump 9 is hydraulically connected to a second sump 10 formed in the axle drive train 1 for storing the hydraulic fluid 5, wherein the receiving volume of hydraulic fluid 5 of the first sump 9 is smaller than the receiving volume of hydraulic fluid 5 of the second sump 10.In the exemplary embodiments shown in FIGS. 1 to 2, the pump arrangement 11 is arranged in the second sump 10. It can also be seen from FIG. 1 that the pump arrangement 11 has a pump 12 with a first channel 13 for extracting the hydraulic fluid 5 from the first sump 9 into the second sump 10 and a second channel 14 for delivering the hydraulic fluid 5 from the second sump 10 for cooling the stator 6. The second channel 14 is designed as a high-pressure channel. For this purpose, the pump 12 has a pump motor, not designated in any more detail. In particular, a suction filter can also be provided before and / or within the second channel 14. Only the second channel 14 designed as a high-pressure channel is then connected upstream of the second sump 10 arranged in the second sump 10, i.e. the pump 12 draws in the hydraulic fluid 5 from the second sump 10 via this suction filter and transports the filtered hydraulic fluid 5 via the high-pressure channel in the direction of the heat exchanger and subsequently into the stator 6 of the electric machine 3.A heat exchanger 24 is arranged in the second channel 14, by means of which the hydraulic fluid 5 guided through the second channel 14 is cooled before entering the stator 6. In the first channel 13, a suction filter 23 is arranged upstream of the pump arrangement 11 in the conveying direction in order to prevent particles and contaminants from entering the pump 12. This suction filter 23 can also be designed, for example, as a screen.The transmission chamber 8 and the second sump 10 are connected to one another at the top in the direction of gravity, wherein a venting 22 is provided in a top region 21 of the second sump 10, by means of which venting both the transmission chamber 8 and the second sump 10 can be effected.As can also be seen from FIG. 1, the first sump 9 has a bottom discharge screw 17 with a magnet 18 and the second sump 10 has a bottom discharge screw 19 with a magnet 20, which can likewise contribute to improved deposition of metallic abrasion.FIG. 2 finally shows two motor vehicles 2 each having an electric axle drive train 1. In the lower embodiment, a vehicle 2 is shown in which the electric powertrain 1 is comprised of two electric machines 3 and two transmission assemblies 4. In this case, each electric machine 3 then drives a vehicle wheel of a vehicle axle assigned to it. In this case, the two electric machines 3 and the two transmission arrangements 4 are preferably of substantially identical design.The invention is not limited to the embodiments shown in the figures. The foregoing description is, therefore, not to be considered as limiting, but illustrative. The following claims should be understood to mean that a said feature is present in at least one embodiment of the invention. This does not exclude the presence of further features. If the patent claims and the above description define "first" and "second" features, this designation serves to distinguish two features of the same type without specifying a ranking.The heat exchanger or heat exchanger 24 shown in FIG. 1 is designed to be rather small. That is, the heat exchanger or heat exchanger 24 has a power of about ten kilowatts, for example. Such heat exchangers exhibit too high pressure losses especially at low temperatures and the necessary volume flows.By the combination with the bypass and a permanently open hydraulic resistor 25, the pressure losses which are undesirable per se at low temperatures can be reduced to a tolerable extent. The permanently open hydraulic resistor 25 is designed as a diaphragm and can be produced cost-effectively. In addition, the bypass with the permanently open hydraulic resistor 25 can be implemented cost-effectively and in a structurally simple manner in the electric axle drive train 1 with regard to the required installation space.The low pressure losses that can be achieved with the permanently open hydraulic resistor 25 can significantly increase the efficiency of the hydraulic circuit for the hydraulic fluid 5 in the electric axle drive train 1 that is illustrated in FIG. 1. In addition, the permanently open cover 25 is maintenance-free.Before a practical use of the permanently open hydraulic resistor 25 in the electric axle drive train 1, it is advantageously proven via a fluidic and thermal simulation that the system shown in FIG. 1 with the heat exchanger 24 and the bypass with the permanently open hydraulic resistor 25 is thermally intrinsically safe.List of reference characters1 Axle drive train 2 Motor vehicle 3 Electric machine 4 Transmission arrangement 5 Hydraulic fluid 6 Stator 7 Rotor 8 Transmission chamber 9 Sump 10 Sump 11 Pump arrangement 12 Pump 13 Flood 14 Flood 17 Discharge screw 18 Magnet 19 Discharge screw 20 Magnet 21 Head region 22 Vent 23 Suction filter 24 Heat exchanger, heat exchanger 25 Permanently open hydraulic resistor

Claims

Electric axle drive train (1) for a motor vehicle (2) having an electric machine (3) which has a stator (6) through which a hydraulic fluid (5) can flow for cooling the latter, to which stator a rotor (7) is arranged in a relatively rotatably mounted manner, and having a heat exchanger (24) through which the hydraulic fluid (5) flows, wherein the heat exchanger (24) is combined with a bypass for the hydraulic fluid (5), wherein the bypass for the hydraulic fluid (5) has a permanently open hydraulic resistor (25), wherein the hydraulic resistor (25) is a diaphragm, characterized in that the heat exchanger (24) having the bypass and the permanently open hydraulic resistor (25) is arranged between a pump arrangement (11) and the stator (6) of the electric machine (3).Electric axle drive train (1) according to Claim 1, characterized in that the heat exchanger (24) with the bypass and the permanently open hydraulic resistor (25) is incorporated into a cooling system which additionally serves for cooling power electronics.Electric axle drive train (1) according to Claim 2, characterized in that the cooling system additionally serves for cooling at least one air conditioning component.Electric axle drive train (1) according to one of the preceding claims, characterized in that the hydraulic fluid (5) within the electric axle drive train (1) is further provided for cooling and / or lubricating a wet-running transmission arrangement (4) which is coupled to the electric machine (1).Electric axle drive train (1) according to one of the preceding claims, characterized in that the wet-running transmission arrangement (4) is arranged within a transmission chamber (8), on which a first sump (9) for storing the hydraulic fluid (5) is provided below the transmission chamber (8) in the direction of gravity and hydraulically connected thereto, wherein the first sump (9) is hydraulically connected to a second sump (10), formed in the axle drive train (1), for storing the hydraulic fluid (5), wherein one or the pump arrangement (11) is coupled to the second sump (10), By means of which, on the one hand, the hydraulic fluid (5) can be conveyed from the first sump (9) into the second sump (10) and, on the other hand, the hydraulic fluid (5) can be supplied from the second sump (10) through the heat exchanger (24) with the bypass and the permanently open hydraulic resistor (25) to the stator (6) for cooling the latter.Electric axle drive train (1) according to one of the preceding claims, characterized in that the heat exchanger (24) is designed to be rather small in terms of power, preferably with a power of less than twenty kilowatts.Electric axle drive train (1) according to one of the preceding claims, characterized in that the heat exchanger (24), in combination with the bypass for the hydraulic fluid (5) having the permanently open hydraulic resistor (25), is designed by a fluidic and thermal simulation in such a way that the combination is thermally intrinsically safe.Heat exchanger (24) combined with a bypass for the hydraulic fluid (5), wherein the bypass for the hydraulic fluid (5) has a permanently open hydraulic resistance (25) and wherein the hydraulic resistance is a diaphragm for an electric axle drive train (1) according to one of the preceding claims.Method for operating an electric axle drive train (1) according to one of Claims 1 to 8, characterized in that the hydraulic fluid (5) flows through both the heat exchanger (24) and the bypass for the hydraulic fluid (5) having the permanently open hydraulic resistor (25).

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