Air suspension system of a motor vehicle and method for operating an air suspension system

Abstract

Air suspension system (1) of a motor vehicle, comprising: - a multiplicity of pneumatic springs (5, 6, 7, 8), by means of which a level position of the motor vehicle is variable by supply and removal of compressed air, wherein at least two of the pneumatic springs (5, 6) are assigned to a first axle (A) of the motor vehicle, and wherein two further pneumatic springs (7, 8) are assigned to a second axle (B) of the motor vehicle, wherein each of the pneumatic springs (5, 6, 7, 8) is assigned a pneumatic spring valve (21, 22, 23, 24), - a compressed air supply unit (2, 3), which provides the compressed air by intake of ambient air or compression of system air, - a first compressed air reservoir (11), which is preferably designed as a high-pressure reservoir, and a second compressed air reservoir (12), which is preferably designed as a low-pressure reservoir, - wherein the pneumatic springs (5, 6) of the first axle (A) are pneumatically connected to a first reservoir port (14) of the second compressed air reservoir (12) with the interconnection of a switchover valve (25), wherein - the compressor (3) is pneumatically connected on the suction side to a second reservoir port (28) of the second compressed air reservoir (12) with the interconnection of a further switchover valve (26), and a method for operating such an air suspension system (1).

Description

[0001] Air suspension system of a motor vehicle and method for operating a

[0002] Air suspension system

[0003] The invention relates to an air suspension system of a motor vehicle according to the preamble of patent claim 1 and a method for operating an air suspension system according to the preamble of patent claim 5.

[0004] Electronically controlled air suspension systems for leveling passenger cars have been around for some time. The main components of the air suspension system are height-adjustable air springs, which cushion the vehicle body, and a compressed air supply unit, which provides compressed air for height adjustment. The air springs are connected to the air supply unit via pneumatic lines. In addition, various height and pressure sensors are provided, as well as an electronic control unit for evaluation and control. Various switching valves are provided in the pneumatic lines, which are controlled by the control unit and assume various switching states (open / closed). It goes without saying that the sensors and the switching valves are connected to the control unit via electrical lines.

[0005] The air suspension system allows the vehicle body to be actively controlled in terms of height / level relative to a vehicle axle. Depending on requirements, the air springs are filled or emptied by compressing compressed air via the compressed air supply unit and switching certain valves to adjust the vehicle's ride height. This allows for leveling after loading the vehicle, or, for example, the vehicle can be lowered while driving to save fuel.

[0006] There is a growing desire to make it easier for vehicle occupants to get in and out. This can be particularly necessary when the vehicle body is in a high normal position so that the vehicle occupants can get in more easily. This means that the vehicle must be lowered when stationary. With a closed air supply system, the vehicle is regulated by releasing the compressed air from the air springs into a pressure accumulator. With axle-by-axle regulation, the compressed air from the air springs of one axle is first released into the pressure accumulator and then the compressed air from the air springs of the other axle is released into the same pressure accumulator. Due to the pressure differences between the air springs and the pressure accumulator and the associated flow rate, the control speed is low. Axle-by-axle regulation also causes the vehicle body to rock, which must be avoided because it is associated with a low control speed.

[0007] The axle-by-axle control also applies to a lifting, ie the raising of the vehicle body, in which in the closed system compressed air from the pressure accumulator is transferred directly or via the compressor into the air springs or in the open system, in which the compressed air from the pressure accumulator or from the environment is transferred via the compressor into the air springs.

[0008] In the current state of the art, the air springs are controlled axle-by-axle when adjusting the vehicle body height. This results in a rocking effect, which is undesirable and negatively impacts comfort. Furthermore, connecting the axle control in series lengthens the control time within which a desired level adjustment is achieved.

[0009] From DE 10 2019 201 444 A1 an air suspension system of a motor vehicle is known, comprising a plurality of air springs by means of which a level position of the motor vehicle can be changed by supplying and discharging compressed air, wherein at least two of the air springs are assigned to a first axle of the motor vehicle, and at least two further air springs are assigned to a second axle of the motor vehicle, wherein each air spring is preceded by an air spring valve, a compressed air supply unit which provides the compressed air by sucking in ambient air or compressing system air, a main accumulator and an additional accumulator which are designed to store the system air, wherein the air springs of the second axle are connected to a changeover valve device of the air suspension system via a connecting valve and can be connected directly to the additional accumulator via an additional accumulator valve.Because the air springs on the second axle are directly connected to the additional accumulator, they can be separated from the rest of the system by closing the connecting line. As a result, a direct compressed air path can be opened between the air springs on the second axle and the additional accumulator, without any pressure connection to the rest of the system. This makes the additional accumulator available for the air springs on the second axle. By simultaneously opening all air spring valves and two valves on the changeover valve device of the additional accumulator valve, with the connecting valve closed, the compressed air from the air springs on each axle is exchanged with the associated accumulator. If the pressure differences between the air springs and the accumulator are sufficiently high, the compressed air flows over from the chamber with the higher pressure level into the chamber with the lower pressure level.The exchange of compressed air continues until the pressure level has equalized or balanced. Because the air springs on each axle each have their own accumulator, the control process takes place simultaneously on both axles.

[0010] The object of the invention is to provide an air suspension system with two compressed air reservoirs, which enables an improved, uniform and simultaneous height change of the vehicle body on two vehicle axles and to provide an improved height change process which takes place evenly and simultaneously on two vehicle axles.

[0011] This object is achieved by the features of the independent device claim and the features of the independent method claim. Preferred embodiments are set forth in the respective subclaims. The invention and further preferred embodiments are explained in the following description with reference to the figures.

[0012] Shown are: Fig. 1 a pneumatic circuit diagram of a first exemplary air suspension system,

[0013] Fig. 2 is a pneumatic circuit diagram of a second exemplary air suspension system, and

[0014] Fig. 3 is a pneumatic circuit diagram of a third exemplary air suspension system.

[0015] Figure 1 shows a pneumatic circuit diagram of a first exemplary electronically controllable air suspension system 1 of a motor vehicle, which operates in closed air supply mode. This system comprises a compressor 3, which is driven by an electric motor 2 and is designed as a dual-piston compressor, and a plurality of air springs 5 ​​to 8, with air springs 5 ​​and 6 being assigned to a first axle A of the vehicle (e.g., the rear axle) and air springs 7 and 8 being assigned to a second axle B of the vehicle (e.g., the front axle). An air spring valve 21 to 24 is connected upstream of each air spring 5 to 8.

[0016] In addition, air suspension system 1 comprises a dryer 4, which is designed to dry the air sucked in from the environment by compressor 3, and a throttle check valve device 13 connected downstream of dryer 4. In order to store the sucked-in air as system air in air suspension system 1, a first compressed air reservoir 11 and a second compressed air reservoir 12 are provided. The first compressed air reservoir 11 is preferably designed as a high-pressure reservoir, and the second compressed air reservoir 12 is preferably designed as a low-pressure reservoir. Furthermore, a switching valve device is provided, which connects compressor 3, first and second compressed air reservoirs 11 and 12, as well as air springs 5 ​​to 8. The switching valve device enables the system air to be pumped back and forth between the individual components by appropriately switching the valves. This meansThe compressed air is shifted between the individual air springs 5 ​​to 8 and the two compressed air reservoirs 11 and 12 by opening certain valves, either due to a sufficient pressure difference or with the assistance of compressor 3, in order to adjust the height of the vehicle body. The switching valve device comprises switching valves 17 to 20, 25, and 26, which are implemented as electronically controllable 2 / 2-way valves and designed as normally closed valves.

[0017] The pneumatic circuitry of air suspension system 1 is implemented as follows. Compressor 3 is connected on the suction side to an inlet line 9 and a check valve located therein, through which air is drawn in from the atmosphere. On the pressure side, dryer 4 and throttle check valve device 13 are pneumatically connected downstream of compressor 3 to remove moisture from the drawn-in air. Between the outlet side of compressor 3 and dryer 4, a drain line 10 branches off with an interposed drain valve 16, through which system air from air suspension system 1 can be discharged into the atmosphere.

[0018] Compressor 3 is connected on the pressure side via a first compressed air line 31 to air springs 5 ​​to 8 and to the first compressed air reservoir 11. Thus, air springs 5 ​​and 6 of the first axle A of the vehicle are pneumatically connected to the compressor outlet via a first switching valve 17, and air springs 7 and 8 of the second axle B of the vehicle are pneumatically connected to the compressor outlet via a second switching valve 18. Thus, a first line section 31a branches off from the first compressed air line 31 to the first switching valve 17, and a second line section 31b branches off to the second switching valve 18.

[0019] Furthermore, compressor 3 is connected on the intake side to air springs 5 ​​to 8 via a second compressed air line 32. Thus, air springs 5 ​​to 8 are pneumatically connected to the compressor inlet via a third switching valve 19. Air springs 5 ​​and 6 of the first vehicle axle A are pneumatically connected to the third switching valve 19 via a first line section 32a of the second compressed air line 32, and air springs 7 and 8 of the second vehicle axle B are pneumatically connected to the third switching valve 19 via a second line section 32b of the second compressed air line 32. At the same time, the first line section 32a leads to the first switching valve 17, and the second line section 32b leads to the second switching valve 18.At the pneumatic connection point of air springs 5 ​​to 8 with the first, second and third switching valves 17, 18 and 19, a pressure sensor 15 is also arranged to determine the air pressure in the system or in the various components.

[0020] The first compressed air reservoir 11 is pneumatically connected to the compressor outlet by a fourth changeover valve 20 arranged upstream thereof, so that a third line section 31 c branches off from the first compressed air line 31 to this valve.

[0021] According to the example, the second pressure medium accumulator 12 is pneumatically connected to air springs 5 ​​and 6 of the first vehicle axle A via a first accumulator connection 14 with the interposition of a fifth switching valve 25. This is done via a third compressed air line 33, which is connected downstream of air spring valves 21 and 22 of the first vehicle axle A and branches off from the first line section 32a of the second compressed air line 32 and leads to the first accumulator connection 14 of the second pressure medium accumulator 12.

[0022] In addition, the second pressure medium accumulator 12 is pneumatically connected to the compressor inlet via a second accumulator connection 28, for example, with the interposition of a sixth switching valve 26. This occurs via a third line section 32c of the second compressed air line 32.

[0023] Not shown in the figure, but of course part of the electronically controlled air suspension system, an electronic control device is provided by means of which the switching valves and the motor are controlled as required.

[0024] The following describes various control processes performed by the air suspension system 1. The example air suspension system 1 is characterized by the fact that it operates in closed-loop air supply mode. This means that compressed air is shifted back and forth between the first compressed air reservoir 11, the second compressed air reservoir 12, and the air springs 5 ​​to 8 as required. Examples of such processes include lowering the vehicle while driving, adjusting the body to a desired level after loading, or lowering the vehicle while stationary to facilitate entry and exit of the vehicle's occupants.

[0025] To fill the system, compressor 3 first draws in air from the atmosphere via inlet line 9 and fills the first compressed air reservoir 11 with the compressed air. This occurs via the first compressed air line 31 with the fourth changeover valve 20 open and the changeover valves 17 and 18 closed. Filling the second compressed air reservoir 12 with air from the atmosphere is also possible. For this purpose, the second, third, and fourth changeover valves 18, 19, and 20 are closed, while the first and fifth changeover valves 17 and 25 are open, creating a pressure path from compressor 3 to the second compressed air reservoir 12. The air spring valves 21 and 22 are, of course, closed. For filling, the electric motor 2 is controlled by the control device, so that compressor 3 draws in and compresses the air.

[0026] According to the example, the second compressed air reservoir 12 has two reservoir connections 14 and 28. The second compressed air reservoir 12 is pneumatically connected to air springs 5 ​​and 6 of the first vehicle axle A via the first reservoir connection 14 and an intermediate fifth changeover valve 25, and the second compressed air reservoir 12 is pneumatically connected to air springs 7 and 8 of the second axle B via the second reservoir connection 28 and an intermediate sixth changeover valve 26 and a third changeover valve 19. Because the second compressed air reservoir 12 is designed as a low-pressure reservoir and has a lower pressure than in air springs 5 ​​to 8, a parallel and simultaneous lowering of the vehicle body on both axles A and B can be carried out if the pressure difference is sufficiently large.This is done in such a way that compressed air with a high pressure level from air springs 5 ​​and 6 of the first axis A flows via the fifth switching valve 25 at the first storage connection 14 into the second (low) pressure air storage 12, and that compressed air with a high pressure level from air springs 7 and 8 of the second axis B flows via the third and sixth switching valves 19 and 26 at the second storage connection 28 into the second (low) pressure air storage 12, whereby compressor 3 is not operated.

[0027] The second compressed air reservoir 12, with its two reservoir connections 14 and 28, therefore offers the advantage that, when the air spring pressures differ in both axles, the different air pressures only meet in the second compressed air reservoir 12, rather than mixing beforehand in a common line to the reservoir. This has a positive effect on the control speed on the individual axles.

[0028] For example, in order to lower the vehicle body, it is also possible, in addition to utilizing a pressure difference between the air spring pressures and the low-pressure accumulator, to additionally operate compressor 3 so that it continuously empties the second compressed air reservoir 12 and transfers the compressed air to the first compressed air reservoir 11. In other words, if compressed air flows from air springs 5 ​​to 8 into the second compressed air reservoir 12 due to a sufficiently large pressure difference, compressor 3 is operated so that it sucks out the compressed air conveyed into the second compressed air reservoir 12 and transfers it to the first compressed air reservoir 11. For this purpose, the first, second and third changeover valves 17, 18 and 19 are closed, while the fifth, sixth and fourth changeover valves 25, 26 and 20 are open. For example, the sixth changeover valve 26 can be opened and closed as needed so that the second compressed air reservoir 12 can be emptied in a targeted manner.In this way it is ensured that the pressure difference between the air spring pressures and the pressure in the second.

[0029] Compressed air reservoir 12 remains as large as possible, which leads to a faster control time when lowering the vehicle to the target level because the pressure from air springs 5 ​​to 8 is reduced more quickly into the second compressed air reservoir 12. As a result, pressure equalization of the compressed air from air springs 5 ​​to 8 to the compressed air in the second compressed air reservoir 12 is delayed as long as possible, which results in a faster change in height. Furthermore, it is possible, for example, to use compressor 3 to lower the vehicle body by directly reducing the air pressure from air springs 5 ​​and 6 of the first axle A into the second compressed air reservoir 12 and to reduce the air pressure from air springs 7 and 8 of the second axle B into the first compressed air reservoir 11 via compressor 3, which compresses the compressed air from air springs 7 and 8 in the first compressed air reservoir 11.For this purpose, the first and second switching valves 17 and 18 are closed, while the fifth switching valve 25 is open to reduce the air pressure from air springs 5 ​​and 6 of the first axle A into the second compressed air reservoir 12, wherein the sixth switching valve 26 is closed and the third switching valve 19 and the fourth switching valve 20 are open to reduce the air pressure from air springs 7 and 8 of the second axle B into the first compressed air reservoir 11.

[0030] The simultaneous adjustment of the vehicle body on both axles A and B can be achieved without operating compressor 3 if there is a sufficient pressure difference from the first compressed air reservoir 11 in air springs 5 ​​to 8. For this purpose, the fourth, first, and second switching valves 20, 17, and 18, as well as the air spring valves 21 to 24, are opened.

[0031] Simultaneous adjustment of both axes A and B can also be achieved by operating compressor 3, which is fed with compressed air from the second compressed air reservoir 12, further compresses it, and delivers it to air springs 5 ​​to 8 via open switching valves 17 and 18. In addition, the second switching valve 20 can be opened, and the compressed air from the first compressed air reservoir 11 can be used to further raise air springs 5 ​​to 8.

[0032] Figure 2 shows a pneumatic circuit diagram of a second exemplary electronically controllable air suspension system 1 of a motor vehicle, which corresponds in essential aspects to the first exemplary air suspension system, so that only the differences will be discussed, and reference is made to the description of Figure 1. The first compressed air reservoir 11 is additionally pneumatically connected to the suction side of compressor 3 via a seventh switching valve 27. Thus, a fourth line section 32d branches off from the second compressed air line 32 to the seventh switching valve 27, and the seventh switching valve 27 is then pneumatically connected to the first compressed air reservoir 11.

[0033] The first compressed air reservoir 11, designed as a high-pressure accumulator, is connected to the compressor inlet by a seventh changeover valve 27. This enables the pressure energy present in the first compressed air reservoir 11 to be used to increase the air pressure in air springs 5 ​​to 8 by operating compressor 3. Air stored under high pressure in the first compressed air reservoir 11 is therefore directly available to compressor 3 when it comes to filling air springs 5 ​​to 8 and raising the vehicle body. This means that in order to simultaneously regulate the vehicle body on both axles A and B, the seventh changeover valve 27 is opened so that the compressed air contained in the first compressed air reservoir 11 is fed to compressor 3, and the compressed air further compressed by compressor 3 flows into air springs 5 ​​to 8 via open changeover valves 17 and 18.

[0034] Figure 3 shows a pneumatic circuit diagram of a third exemplary electronically controllable air suspension system 1 of a motor vehicle, which corresponds in essential aspects to the first exemplary air suspension system, so that only the differences are discussed and otherwise reference is made to the description of Figure 1.

[0035] The first compressed air reservoir 11 comprises a first reservoir connection 29, by means of which the compressor 3 is pneumatically connected on the pressure side to the first compressed air reservoir 11. According to the example, the first compressed air reservoir 11 also comprises a second reservoir connection 30, by means of which the compressor 3 is pneumatically connected on the suction side to the first compressed air reservoir. This is achieved via a fourth compressed air line 34, which leads from the second reservoir connection 30 to the inlet line 9, with a seventh changeover valve 27 interposed. In this exemplary embodiment, the direct connection of the first compressed air reservoir 11 to the compressor 3 via the second reservoir connection 30 also serves to enable the more rapid use of highly compressed air in the first compressed air reservoir 11 to fill the air springs 5 ​​to 8 with compressed air via the compressor 3.

[0036] In general, it can be noted for all three embodiments that with this configuration the compressor can adjust the pressures in the two compressed air reservoirs to an optimum level both during and after altitude change processes. As a basis, the pressure in the second compressed air reservoir should always be kept as low as possible and in the first compressed air reservoir as high as possible. A high pressure difference compared to the air spring pressures offers the greatest possible overflow potential, so that the compressor needs to be used as rarely as possible for altitude change processes. For example, during a load equalization process when the vehicle is stationary, operation of the compressor as a disruptive noise source is undesirable. In this system, the compressor can therefore be operated more during off-peak times after the altitude change process and can also be designed to be smaller and simpler in terms of performance, as it is subject to less stress.

[0037] List of reference symbols

[0038] 1 air suspension system

[0039] 2 electric motor

[0040] 3 Compressor

[0041] 4 dryers

[0042] 5 air spring

[0043] 6 air spring

[0044] 7 air spring

[0045] 8 air spring

[0046] 9 Inlet line

[0047] 10 Drain line

[0048] 11 first compressed air storage

[0049] 12 second compressed air storage

[0050] 13 Throttle check valve device

[0051] 14 first memory connection

[0052] 15 Pressure sensor

[0053] 16 Drain valve

[0054] 17 first changeover valve

[0055] 18 second changeover valve

[0056] 19 third changeover valve

[0057] 20 fourth changeover valve

[0058] 21 first air spring valve

[0059] 22 second air spring valve

[0060] 23 third air spring valve

[0061] 24 fourth air spring valve

[0062] 25 fifth changeover valve

[0063] 26 sixth changeover valve

[0064] 27 seventh changeover valve

[0065] 28 second storage connection

[0066] 29 first memory connection

[0067] 30 second storage connection 31 first compressed air line

[0068] 31a first line section

[0069] 31 b second line section

[0070] 31c third line section 32 second compressed air line

[0071] 32a first line section

[0072] 32b second line section

[0073] 32c third line section

[0074] 32d fourth line section 33 third compressed air line

[0075] 34 fourth compressed air line

Claims

Patent claims 1. Air suspension system (1) of a motor vehicle, comprising: - a plurality of air springs (5, 6, 7, 8) by means of which a level position of the motor vehicle can be changed by supplying and discharging compressed air, wherein at least two of the air springs (5, 6) are assigned to a first axle (A) of the motor vehicle and wherein two further air springs (7, 8) are assigned to a second axle (B) of the motor vehicle, wherein each of the air springs (5, 6, 7, 8) is assigned an air spring valve (21, 22, 23, 24), - a compressed air supply unit (2, 3) which provides the compressed air by sucking in ambient air or compressing system air, - a first compressed air reservoir (11), which is preferably designed as a high-pressure reservoir, and a second compressed air reservoir (12), which is preferably designed as a low-pressure reservoir, - wherein the compressor (3) is pneumatically connected on the pressure side with the interposition of a first switching valve (17) to the air springs (5, 6) of the first axle (A) and is pneumatically connected with the air springs (7, 8) of the second axle (B) with the interposition of a second switching valve (18), - wherein the compressor (3) is pneumatically connected on the suction side to the air springs (5, 6, 7, 8) of both axles (A, B) via a third changeover valve (19), - wherein the compressor (3) is pneumatically connected on the pressure side to the first compressed air reservoir via a fourth changeover valve (20), - wherein the air springs (5, 6) of the first axis (A) are pneumatically connected to a first accumulator connection (14) of the second compressed air accumulator (12) via a fifth changeover valve (25), characterized in that - the compressor (3) is pneumatically connected on the suction side to a second storage connection (28) of the second compressed air storage device (12) via a sixth changeover valve (26).

2. Air suspension system (1) according to claim 1, characterized in that the compressor (3) is pneumatically connected on the suction side to the first compressed air reservoir (11) with the interposition of a seventh changeover valve (27).

3. Air suspension system (1) according to claim 2, characterized in that the compressor (3) is pneumatically connected on the suction side to the first compressed air reservoir (11) exclusively via the seventh changeover valve (27) and without the use of a further changeover valve (27).

4. Air suspension system (1) according to claim 1 or 2, characterized in that the first compressed air reservoir (11) comprises a first reservoir connection (29) by means of which the compressor (3) is pneumatically connected on the pressure side to the first compressed air reservoir (11), and in that the first compressed air reservoir (11) comprises a second reservoir connection (30) by means of which the compressor (3) is pneumatically connected on the suction side to the first compressed air reservoir (11) with the interposition of a seventh changeover valve (27).

5. Method for operating an electronically controllable air suspension system (1) of a motor vehicle comprising: a plurality of air springs (5, 6, 7, 8), by means of which a level position of the motor vehicle can be changed by supplying and discharging compressed air, wherein at least two of the air springs (5, 6) are assigned to a first axle (A) of the motor vehicle and wherein two further air springs (7, 8) are assigned to a second axle (B) of the motor vehicle, wherein each air spring (5, 6, 7, 8) is preceded by an air spring valve (21, 22, 23, 24), a compressed air supply unit (2, 3), which provides the compressed air by sucking in ambient air or compressing system air, a first compressed air reservoir (11), which is preferably designed as a high-pressure reservoir, and a second compressed air reservoir (12), which is preferably designed as a low-pressure reservoir, wherein the compressor (3) is arranged on the pressure side with the interposition of a first switching valve (17) is pneumatically connected to the air springs (5, 6) of the first axis (A) and is pneumatically connected to the air springs (7, 8) of the second axis (B) with the interposition of a second switching valve (18), wherein the compressor (3) is pneumatically connected on the suction side with the interposition of a third switching valve (19) to the air springs (5, 6, 7, 8) of both axes (A, B), wherein the compressor (3) is pneumatically connected on the pressure side with the interposition of a fourth switching valve (20) to the first compressed air reservoir, wherein the air springs (5, 6) of the first axis (A) are pneumatically connected to a first reservoir connection (14) of the second compressed air reservoir (12) with the interposition of a fifth switching valve (25), characterized inthat the compressor (3) is pneumatically connected on the suction side with the interposition of a sixth switching valve (26) to a second accumulator connection (28) of the second compressed air accumulator (12), and that for the simultaneous adjustment of the level of the motor vehicle on the two axles (A, B), the air spring valves (21, 22) of the air spring (5, 6) of the first axle (A) and the fifth switching valve (25) are kept open so that compressed air from the air springs (5, 6) of the first axle (A) flows via the first accumulator connection (14) into the second compressed air accumulator (12), and that the air spring valves (23, 24) of the air spring (7, 8) of the second axle (B) are kept open so that compressed air from the air springs (7, 8) of the second axle (B) flows via the second accumulator connection (28) into the second compressed air accumulator (12).

6. The method according to claim 5, characterized in that a pressure level in the air springs (5, 6, 7, 8) is greater than a pressure level in the second compressed air reservoir (12), so that a pressure difference between the air springs (5, 6, 7, 8) and the second compressed air reservoir (12) exists, wherein the simultaneous adjustment of the level of the motor vehicle on both axles (A, B) initially takes place without operation of the compressor (3).

7. The method according to claim 6, characterized in that the compressor (3) is switched on and the fourth switching valve (20) is opened, so that compressed air is transferred from the second compressed air reservoir (12) to the first compressed air reservoir (11) by means of the compressor (3).

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