WO2022243101A1

WO2022243101A1 - Passenger car having an electric rear-wheel drive

Info

Publication number: WO2022243101A1
Application number: PCT/EP2022/062562
Authority: WO
Inventors: Tobias DIECKHOFF; Martin Hertel; Benedikt Cöster; Daniel Lorenz; György Müller; Arthur Schröder; Erwin Michels; Benedikt Ohlig; Rene BRENNER; Thomas Zettelmeyer; Christian HAERTER; Matthias Winkel; Kim Führer; Mathias Kopp; Frank Schmidt
Original assignee: Zf Friedrichshafen Ag
Priority date: 2021-05-19
Filing date: 2022-05-10
Publication date: 2022-11-24
Also published as: DE102021205073A1

Abstract

The invention relates to a passenger car having an electric rear-wheel drive, comprising a drive system that includes power electronics (4), an electric motor (5), transmission gearing (6) and differential gearing (7), which together form an electric axle (2), and connected to the differential gearing (7), two axle drives (8, 18) on each free end of which a rear wheel (3) is mounted, and a brake system. The brake system has a central brake (9) arranged such that it is effective within the electric axle (2) between the electric motor (5) and the differential gearing (7).

Description

Passenger cars with electric rear-wheel drive

The invention relates to a passenger car with electric rear-wheel drive according to the independent claim.

Passenger car according to the present invention is, in the broadest sense, a multi-lane vehicle with its own drive, which is approved in particular for use on public roads, with the primary purpose of transporting people. This can also be an autonomously driving shuttle vehicle, for example.

In passenger cars (car), according to the prior art, each wheel is coupled with its own brake. A car that usually has four wheels has four individual brakes. The braking system and the drive system are systems that are designed separately from one another. This applies to both conventional and electrically powered cars. A special feature of electrically powered passenger cars is that the electric motor (e-motor) in generator mode can contribute a significant and situation-dependently variable braking torque in addition to the wheel brakes.

It is the object of the invention for an electrically driven car, in which the electric motor acts on the rear axle, to reduce the manufacturing costs by simplifying the braking system of the driven rear axle.

This object is achieved according to claim 1 for a passenger car with electric rear-wheel drive. Advantageous designs are described in the dependent claims.

The invention includes a passenger car with electric rear-wheel drive, with a drive system containing power electronics, an electric motor, a transmission gear and a differential gear, together forming an E axis, and two side shafts connected to the differential gear at their free ends Rear wheel is attached, and a braking system. By the braking system has a central brake, which is arranged to act within the E axis between the electric motor and the differential gear, an electrically powered passenger car is provided in which the electric motor acts on the rear axle with a simplified braking system on the driven rear axle.

The central brake is preferably a disc brake with a brake disc and a brake caliper.

Advantageously, the brake disc is formed by a gear of the transmission gear.

Alternatively, the brake disc is arranged on a rotor shaft of the electric motor, an intermediate shaft of the transmission or a differential input of the differential gear.

A variant provides that the central brake is a multi-disc brake that is arranged on a differential input of the differential gear.

Advantageously, the braking system has two braking circuits, each including one of two front wheel brakes and the central brake.

According to an alternative, the brake system has two brake circuits, both of which include two front wheel brakes and the central brake, the front wheel brakes and the central brake each having two brake pistons.

The brake system preferably has two brake circuits, each of which includes one of two front wheel brakes and another brake circuit is present, with the central brake and a rear axle brake actuator, which is integrated into the E axis.

The first side shaft and the second side shaft preferably have the same stiffness. The central brake is preferably an eddy current brake, which is arranged either on the rotor shaft of the electric motor or on an intermediate shaft of the transmission gear.

Furthermore, the e-axis preferably contains an internal control unit which is designed to control a division of a necessary braking power between engine braking by the e-motor and friction braking by the central brake. In the event that not only the rear axle, but also the front wheels are equipped with electromechanical brake-by-wire brakes, the control of the entire braking system of the vehicle, including the driving stability control, can advantageously run on the control unit of the electric axle.

The invention is explained in more detail below using exemplary embodiments with reference to drawings. For this show:

Fig. 1 - 10 different advantageous arrangements of the central brake along the E-axis,

Fig. 11 - 13 different versions of the division of the brake system into brake circuits,

14 shows an embodiment with an integrated parking brake function,

15 shows a first cooling concept for cooling the central brake, and FIG. 16 shows a second cooling concept for cooling the central brake.

1 shows a rough schematic representation of a passenger car 1 according to the invention with the features that are essential for understanding the invention.

A car 1 with electric rear-wheel drive according to the invention contains, like gattungsglei surface vehicles, an electric drive system and a braking system to drive the two rear wheels 3 and, independently, to brake the two front wheels and the rear wheels 3 in them. The drive system contains power electronics 4, an electric motor 5, a transmission gear 6, a differential gear 7 and two side shafts 8, 18, which define the rear axle and are connected to the differential gear 7, and on whose free ends one of the rear wheels 3 is attached . the The components of the drive system that transmit the drive torque of the electric motor 5 to the rear wheels 3 together form a so-called electric axle 2.

In contrast to the prior art, in which each of the two rear wheels 3 is assigned a brake, the braking system contains a central brake 9, which is arranged acting within the E axis 2 between the electric motor 5 and the differential gear 7. It is not possible to simply omit the brakes that are usually applied to one rear wheel 3, since the electric motor 5 in generator mode alone cannot generate the braking power required for emergency braking on the side shafts 8, 18 of the rear axle and, in particular, depends on the state of charge and the temperature of the battery, in extreme cases, no effective recuperation torque is generated. The central brake 9 advantageously forms a functional and constructive unit with the E axis 2, which is shown in FIG.

Depending on the design of the E -axis 2 , that is to say in particular the design of the transmission gear 6 and the differential gear 7 , the central brake 9 can advantageously be arranged to act at different positions along the E -axis 2 .

Fig. 2 shows a first embodiment for the arrangement of the central brake 9 within the E -axis 2. The housed in an E-axis housing 10 E -axis 2 has a transmission gear 6 in the form of a two-stage, axis-parallel gear bes. The transmission gear 6 connects a rotor shaft 11 of the electric motor 5 via an intermediate shaft 12 to the differential gear 7, which divides the torque acting on the differential gear 7 on the side shafts 8, 18 of the rear axle. The central brake 9 is arranged on the rotor shaft 11, outside of the E-axle housing 10. An arrangement of the central brake 9, represented here by a brake disc 13 and a brake caliper 14, outside of the E-axle housing 10 has the advantage of direct cooling by the ambient air and that no brake dust is released inside the E-axle housing 10, which would soil the E-axle 2. According to a second exemplary embodiment, which is not shown in the drawings, the central brake 9 is also arranged on the rotor shaft 11 , but in contrast to the previous exemplary embodiment, it is arranged inside the E-axle housing 10 . The central brake 9 is protected here against external influences, brake wear is held back from the environment and it can be actively cooled by the lubricant and the cooling circuit of the e-axis 2 .

In a third advantageous exemplary embodiment, shown in FIG. 3, the central brake 9 is on an end of the rotor shaft remote from the transmission gear 6

11 arranged.

A fourth exemplary embodiment, shown in FIG. 4, differs from the previous one in that the central brake 9 is arranged in a separate brake housing 15 outside of the E-axis housing 10 . In this way, on the one hand, brake wear can be kept away from the E axis 2, and the central brake 9 can be encapsulated from external influences and brake wear can be retained.

In a fifth exemplary embodiment, illustrated in FIG. 5, the central brake 9 is arranged on the intermediate shaft 12 of the transmission gear 6. The intermediate shaft

12 represents a good compromise between an arrangement on the rotor shaft 11 and an arrangement on a differential input 16 with regard to the level of the operating speed of the central brake 9 and the braking torque to be provided by it. Due to the gear ratio step to the differential gear 7 or to the side shafts 8, 18 it has to apply less torque here than there. A smaller friction radius is therefore possible with the same actuation force, which simplifies integration into the E axis 2. The central brake 9 is in turn arranged within the housing 10 of the E-axis.

A sixth exemplary embodiment, not shown in the drawings, differs from the previous one only in that the central brake 9 is arranged outside of the E-axle housing 10 . In a seventh embodiment, shown in Fig. 6, the central brake 9 is more integrated into the E -axis 2 by a seated on the intermediate shaft 12 toothed wheel 17 of the transmission 6, on which friction surfaces are formed, as a brake disc 13 is used. Alternatively, other components that transmit the rotational movement of the rotor shaft 11 to the differential gear 7 can be used as a brake disc 13 in other designs that are not shown in the drawings.

According to an eighth exemplary embodiment, shown in FIG. 7, the central brake 9 contains a separate brake disk 13 connected to the differential input 16.

In contrast to the aforementioned exemplary embodiments, the transmission gear 6 of the E axis 2 in a ninth exemplary embodiment shown in FIG to get gears. This gear 17 is used as a brake disc 13 here.

Instead of using the gear wheel 17 as a brake disc 13, a separate brake disc 13 can alternatively be attached to the differential input 16 parallel to the gear wheel 17.

In the tenth embodiment shown in FIG. 9, instead of the brake disk 13 at the differential input 16, a wet-running multiple-disc brake 19 is arranged. Such a concept is particularly advantageous with regard to the dissipation of frictional heat through the volume flow of a cooling liquid. The heat can also be transported particularly cheaply to positions further away in the car, where it can be released into the environment via a heat exchanger or used to heat the interior, for example.

An exemplary embodiment shown in FIG. 10 differs from the aforementioned exemplary embodiment in that the E axis 2 is designed coaxially with the two side shafts 8, 18. Here the transmission gear 6 is designed as a planetary gear, wherein a web 20 is connected to the differential input 16 . With these parts, the brake disc 13 is in turn connected to the central brake 9.

An arrangement of the central brake 9 on the rotor shaft 11 analogous to FIG. 3 would of course also be possible.

Regardless of whether, according to the prior art, a brake acts on a rear wheel 3 or, according to the invention, a central brake 9 acts indirectly on both rear wheels 3, high demands are placed on the brake system in terms of functional reliability and redundancy. Splitting the braking system into two braking circuits is necessary and common, so that sufficient braking is still available even if one subsystem fails.

The following exemplary embodiments show advantageous variants of how such a division of the braking system with a central brake 9 can take place. For the sake of clarity, components of the brake system known from the prior art, such as brake pedals or an ABS actuator, are not detailed.

In the following exemplary embodiments, the central brake 9 is shown as an example on the rotor shaft 11 analogously to FIG. 1, but it can be arranged at any other location on the E axis 2, as shown in the previous exemplary embodiments.

In Fig. 11 a possible division of the brake system is shown schematically, in which the two front wheel brakes and the central brake 9 are hydraulically actuated, since the two front wheel brakes are actuated via a first brake circuit 21 and the central brake 9 via a second brake circuit 22 with a brake booster 23 connected. If the first brake circuit 21 fails, however, only a significantly reduced braking torque generated by the central brake 9 is available.

According to the embodiment in FIG. 12 , the two front wheel brakes are each assigned to separate hydraulic brake circuits 21 , 22 . Both brake circuits 21, 22 close also in each case the central brake 9, which for this purpose has two independent brake pistons. As a result, if one of the two brake circuits fails, a higher residual braking torque is available than if, as in the previous variant, the brake circuit on the front axle fails. Disadvantages of this solution are an increased outlay on parts and, if a brake circuit fails, an asymmetry of the braking torque about the longitudinal axis of the vehicle, so that rotation of the vehicle may have to be counteracted by steering intervention.

An asymmetrical braking torque in the event of a brake circuit failure is avoided if the front wheel brakes each have two brake pistons, which are each assigned to the two independent brake circuits. However, this is an expensive solution.

Fig. 13 shows a further embodiment in which the front wheel brakes are controlled via a separate hydraulic brake circuit 21 22 egg each. In this case, the central brake 9 is not connected to the brake booster 23 connected to the front wheel brakes via a hydraulic line, but is connected in a further separate brake circuit to a self-sufficient rear axle brake actuator 26 which is integrated into the E axis 2 . This can be designed as an electro-hydraulic or electro-mechanical brake actuator, which advantageously receives the control information via a data bus 25 of the car 1 .

It is particularly advantageous if the control electronics of the rear axle brake actuator 24 are integrated into the power electronics 4 . This enables a cost-effective design, since no separate housing is required for the control electronics. In addition, optimal coordination of the longitudinal dynamic functions of driving and braking, in particular the combination of regenerative braking of the electric motor 5 and supporting friction braking of the central brake 9, is particularly easy to implement. Analogously to the scheme shown in FIG. 13, it is of course also possible to equip the two front wheel brakes with a separate electro-hydraulic or electromechanical actuation. A complete "brake by wire" concept would then be implemented.

There are two sensible variants for implementing a parking brake function in a braking system with a central brake 9 .

The two front wheel brakes can each be equipped with an electric parking brake, which is not shown in the drawings.

As shown in FIG. 14, it is more advantageous if the parking brake function is also integrated into the E axis 2 . The connected to the central brake 9 rear axle brake actuator 26 is designed so that it provides a necessary braking torque even when the vehicle is stationary for a long time. The function of the electric parking brake is thus integrated into the central brake 9. However, since the differential gear 7 is arranged between the central brake 9 and the rear wheels 3, the car could roll away if, for example, one of the rear wheels 3 is lifted to change a tire. For this reason, this variant only makes sense in combination with a differential lock 27. In order to be able to represent the parking brake function, this differential lock 27 can be very simple, for example as a switchable positive connection.

The arrangement of the central brake 9 in the E -axis 2 also places special demands on the transmission path between the central brake 9 and the rear wheels 3, which is significantly longer than with conventional rear wheel brakes.

The design of the components for the maximum possible braking torques with corresponding safety goes without saying. For good system performance, driving comfort and safety, it is particularly advantageous if the two side shafts 8, 18 have a similar, in particular the same, rigidity. With different lengths of the side shafts 8, 18, this can be compensated for by different cross sections. be ted. In addition, it is advantageous for side shafts 8, 18 to be as stiff as possible, in order to keep the angle of rotation low during emergency braking and to limit dynamic effects due to high torque gradients on central brake 9, especially during ABS intervention. The side shafts 8, 18 represent the main rigidity of the drive train in electric vehicles. The electric motor 5 and in the present case the central brake 9 form the main mass moment of inertia in relation to the natural frequency of the drive system. In order to avoid the stimulation of resonances by the central brake 9, particularly with ABS brakes, the drive system and the brake system should advantageously be designed in such a way that the quotient of the natural frequency of the drive system and the ABS control frequency does not exceed the values 0.5, 1, 0 .75 or 2 assumes. In particular, a distance of at least 0.05 from these ratio values must be observed. It is particularly advantageous, for example, if a ratio of drive system natural frequency and brake control frequency in the range of 0.6-0.7 is set.

Various versions of the central brake 9 are possible, in particular as a dry-running drum or disk brake or as a wet-running multi-disk brake 19. The version as a dry-running disk brake is particularly advantageous, since this is an optimum of low drag torque and high torque transmission capacity in relation to on the necessary radial installation space and control represents.

The design of the central brake 9 as an eddy current brake occupies a special position. The maximum torque of an eddy current brake depends on the engine speed or vehicle speed. It is therefore particularly advantageous to arrange the eddy current brake on a shaft with the highest possible speed, i.e. the rotor shaft or an intermediate shaft, in order to obtain a comparatively better effect. An advantage of the combination of electric motor 5 and eddy current brake is that in the low speed range in which the eddy current brake can provide less braking torque, electric motor 5 can bring in its maximum torque for braking or recuperation. In the high speed range it is exactly the opposite. This combination also represents the currently possible charging capacity the battery does not limit the generator output, since the energy converted by the electric motor 5 operating in the generator mode can be implemented directly in the eddy current brake.

In order to be able to build the central brake 9 as small as possible and thus to facilitate its integration into the electric axle 2, it makes sense to provide the highest possible proportion of the braking torque for the rear wheels 3 through a negative torque of the electric motor 5 as engine braking .

The topology according to FIG. 13 offers an optimal prerequisite for optimal coordination of the two braking variants, in which the E axis 2 receives the braking request from the vehicle electronically via the data bus 25 . An internal control unit on the e-axis 2 then adjusts the optimum split between engine braking and friction braking by the central brake 9, depending on the situation, with factors such as the battery charge level, temperature of the battery, electric motor 5, power electronics 4 and brake disc 13 (recorded using sensors or calculation models ), vehicle speed, previous driver behavior and route information are evaluated.

In situations in which the condition of the vehicle battery does not permit a sufficiently high charging capacity, a minimum braking component of the electric motor 5 set for the design of the central brake 9 can also be ensured by converting the current generated into heat via a load resistor is converted, or the electric motor 5 is deliberately put into poor efficiency by the type of control and thus itself absorbs an increased proportion of thermal energy.

In order to protect the various system components, it is proposed to use a calculation model to monitor the thermal state of the electric motor 5, the power electronics 4 and the central brake 9 and to implement a model-based forecast for the temperatures. An intervention in the drive torque can then prevent the car from being accelerated into a speed range from which it could not be safely braked again without exceeding a critical temperature limit. A particular challenge is the provision of adequate driving stability control when the rear wheels 3 of the rear axle cannot be braked wheel-selectively.

In the case of ABS control, a so-called “select low” control strategy is therefore preferably used, in which the optimum slip of the rear wheels with the lowest adhesion value to the road for the side shafts 8, 18 is set. According to the prior art, the slip speed is measured by speed sensors on the respective wheels. This is also possible with the present system with a central brake 9 . Alternatively, the speed sensors of the side shafts 8, 18 can also be dispensed with and the brake control can be carried out on the basis of the sensor data of the front wheels and the speed of the electric motor 5, which is precisely known via the rotor position sensor.

In the concept presented, traction control is only possible to the extent that the drive torque of the electric motor 5 is limited to a value that does not cause the driven rear wheel 3 with the most unfavorable friction conditions to spin. Due to the good and very nimble controllability of the electric motor 5, this is better possible than with an internal combustion engine. Alternatively, a differential lock 27 can also be used at low speeds to start off with a higher torque when the friction coefficients on the drive wheels are very different.

Another challenge when designing brakes is dissipating the braking energy that has been converted into heat. Various concepts for air and liquid cooling are known in the prior art. If a central brake 9 is integrated into the E axis 2, it can be advantageous to use the same cooling medium as for the electric motor 5. This is particularly useful with an air-cooled electric motor 5, since the cooling medium air does not circulate here , but is released into the environment. The idea is therefore the central brake 9 at the include the last point in the cooling air path of the electric motor 5. As a result, the cooling system, which primarily has to cool the components of the electric drive train, is not additionally burdened by the braking heat.

A cooling concept is shown schematically in FIG. 15 in which the cooling air of the electric motor 5 is also used for the active cooling of the central brake 9 . From a fan 28, the cooling air is first fed into the electric motor 5 via corresponding guide channels, since the maximum permissible temperature of the electric motor 5 is lower than that of the central brake 9. The cooling air is discharged from the electric motor 5 led to the central brake 9 to cool it before it escapes into the environment.

A further implementation of a cooling concept is shown schematically in FIG. Here the cooling air sucked in by the fan 28 is first directed into a branch 29 in which the air flow can be divided into two paths as required. The first path leads through the electric motor 5 and the second path leads directly to the central brake 9, so that, compared to the aforementioned cooling concept, cooler air can be made available that has not already been preheated by the electric motor 5 . Behind the central brake 9 a particle filter 30 is arranged to reduce fine dust emissions. Behind this is another branch 31 , from which part of the air is branched off in the direction of the battery 32 , which means that it is heated more quickly to operating temperature using the waste heat from the electric motor 5 and the central brake 9 . Instead of or in addition to the battery 32, the passenger compartment can also be heated with the heated air.

Analogously to air cooling, the basic idea can also be applied to liquid cooling systems, only here the cooling medium circulates in the system.

The various designs presented for the central brake 9, the possibilities of their arrangement along the E axis 2, the variations in the design of the brake circuits 21, 22, the different concepts of cooling, the possibilities of combining with a parking brake and the possibilities of controlling the Braking performance of the central brake 9 can be combined with each other and are not limited to the solutions specifically shown. Reference car e-axis rear wheel power electronics e-motor transmission gear differential gear first side shaft central brake e-axis housing rotor shaft intermediate shaft brake disc brake caliper brake housing differential input gear wheel second side shaft multi-disc brake bridge first brake circuit second brake circuit brake booster data bus rear axle brake actuator differential lock fan branch particle filter further branch battery

Claims

patent claims

1. Passenger car (1) with electric rear-wheel drive, with a drive system containing power electronics (4), an electric motor (5), a transmission gear (6) and a differential gear (7), together with an electric axis (2nd ) forming, and two side shafts (8, 18) connected to the differential gear (7), at the free ends of which a rear wheel (3) is attached in each case, and a braking system, characterized in that the braking system has a central brake (9) which is inside the E axis (2) between the electric motor (5) and the differential gear (7) is arranged to act.

2. Passenger car (1) with electric rear-wheel drive according to claim 1, characterized in that the central brake (9) is a disc brake with a brake disc (13) and a brake caliper (14).

3. Passenger car (1) with electric rear-wheel drive according to claim 2, characterized in that the brake disc (13) is formed by a gear of the transmission gear (6).

4. Passenger car (1) with electric rear-wheel drive according to claim 2, characterized in that the brake disc (13) on a rotor shaft (11) of the electric motor (5), an intermediate shaft (12) of the transmission (6) or a differential input ( 16) of the differential gear (7) is arranged.

5. Passenger car (1) with electric rear-wheel drive according to claim 1, characterized in that the central brake (9) is a multi-plate brake (19) which is arranged on a differential input (16) of the differential gear (7).

6. Passenger car (1) with electric rear-wheel drive according to claim 1, characterized in that the braking system has two brake circuits (21, 22), each including one of two front wheel brakes and the central brake (9).

7. Passenger car (1) with electric rear-wheel drive according to Claim 1, characterized in that the braking system has two brake circuits (21, 22), both of which include two front wheel brakes and the central brake (9), the front wheel brakes and the central brake each have two brake pistons.

8. Passenger car (1) with electric rear-wheel drive according to Claim 1, characterized in that the brake system has two brake circuits (21, 22), each of which includes one of two front wheel brakes and a further brake circuit is present, with the central brake (9th ) and a rear axle brake actuator (26), which is integrated into the E -axis (2).

9. Passenger car (1) with electric rear-wheel drive according to claim 1, characterized in that the first side shaft (8) and the second side shaft (18) have the same rigidity.

10. Passenger car (1) with electric rear-wheel drive according to claim 1, characterized in that the central brake (9) is an eddy current brake, which is either on the rotor shaft (11) of the electric motor (5) or an intermediate shaft (12) of the transmission ( 6) is arranged.

11. Passenger car (1) with electric rear-wheel drive according to claim 8, characterized in that the E -axis (2) contains an internal control unit which is designed to distribute a necessary braking power to engine braking by the electric motor (5th ) and to control friction braking by the central brake (9).