CH712781A1 - Displacer unit and hydraulic start-stop system with such a displacer unit. - Google Patents

Abstract

The present invention relates to a displacer unit [1], in particular axial piston machine, wherein a one-way / non-return valve [3] in the housing [2] of the displacer unit [1] is housed. The displacer unit [1] comprises a housing [2], an engine [39] for converting a rotor power into a hydraulic power, a suction port [4] for supplying a fluid to the engine [39] and the said one-way / non-return valve [3].

Description

Description: The present invention relates to a displacement unit and a hydraulic start-stop system with such a displacement unit.

It is known that mobile machines have hydraulic devices and these can be used to perform a job. The hydraulic drive system comprises a hydraulic pump which is preferably controllable or regulable in the delivery volume and which is typically mechanically coupled to a primary drive designed as an internal combustion engine. The internal combustion engine generally comprises an electric starter for starting the internal combustion engine, which is fed by a battery. This battery is charged by a generator driven by the internal combustion engine.

In the prior art it is known to start an internal combustion engine, in particular a diesel engine, by electrical starter systems, by electrical starter generators, which represent a combination of starter system and generator in one component, and by hydraulic start-stop systems. When an internal combustion engine starts, it is necessary to rotate its output shaft in order to carry out the work cycles of a piston arranged in the internal combustion engine.

The prior art provides for this a hydraulic start-stop system in which a mechanically coupled to an internal combustion engine displacement unit is not only controlled by the internal combustion engine in "pump mode", but starting the internal combustion engine by using the displacement unit in " Engine operation ». If hydraulic power is required for the working hydraulics, the displacement unit is driven to the internal combustion engine with the aid of the mechanical coupling and sucks in a fluid, typically an oil or the like, from a fluid reservoir and passes this on to the working hydraulics on the high-pressure side. This makes it possible for the working hydraulics, for example, to move an excavator arm or to raise the sign of a caterpillar.

In "engine operation" it is provided that a stopped internal combustion engine is started with the help of the displacement unit. For this purpose, a branching line is connected to the inlet opening of the displacement unit, which connects a fluid in the direction of its engine, which can convert a hydraulic power into a rotary power, which is connected at one end to a fluid reservoir via a one-way valve and to the other of its ends is connected to a pressure accumulator via a switchable valve. If the pressure accumulator has a sufficient amount of fluid under pressure, this fluid can be used to start a stationary internal combustion engine. For this purpose, the shut-off valve arranged in the path between the pressure accumulator and the engine of the displacement unit is opened in order to allow the pressurized fluid to flow into the displacement unit.

If the pressurized fluid reaches the engine, this leads to a rotation of the output shaft of the internal combustion engine coupled to the displacement unit. This rotation of the output shaft, which is necessary when starting an internal combustion engine, replaces the typical activity of an electric starter system.

It is the object of the present invention to further develop a displacement unit suitable for the hydraulic start-stop system in an advantageous manner.

The displacement unit according to the invention, which can in particular represent an axial piston machine, comprises a housing, an engine for converting the rotary power into a hydraulic power and a suction port for supplying a fluid to the engine. The displacement unit is characterized by a one-way valve which is arranged in the housing of the displacement unit.

The arrangement of the one-way valve in the housing of the displacement unit leads to a reduction in the flow resistance between the hydraulic oil storage tank (also: fluid reservoir) and the pump drive engine of the displacement unit. A lower total flow resistance ensures a higher suction limit speed, which characterizes an operating state of a pump in which there is a lack of filling in a suction stroke due to vacuum formation, which can lead to a reduction in the flow rate of the pump and damage to the pump due to cavitation erosion in the subsequent delivery stroke. For a high suction limit speed, it is advantageous to keep the flow resistance as low as possible.

Since the total flow resistance between the hydraulic oil storage tank (also: fluid reservoir) and the engine of the displacement unit results from the supply line from the storage tank to the one-way valve and the supply line from the one-way valve to the engine, the total flow resistance decreases when the section of the highest Flow resistance, the area around the one-way valve, is located near the engine. This is due to the fact that the cross section of the suction channel from the suction connection of the displacement unit to the engine is typically reduced. If the pressure drop (or flow resistance) remains the same, the passage cross section of the check valve can be made smaller the closer it is to the engine. In addition, a geometrically smaller check valve is cheaper to implement and requires less installation space. It is therefore possible to achieve the positive effects listed above for the check valves (also: one-way valves) typically arranged outside the housing of the displacement unit in the prior art.

In addition, there is a high pressure level of the fluid in a motorized use of the displacement unit between the engine and the one-way valve. It follows that the oil connection between these two components

CH 712 781 A1 must be high pressure tight. If the one-way valve were located outside the housing of the displacement unit, the corresponding hydraulic hose would have to withstand the high pressure load. According to the displacement unit according to the invention, in which the one-way valve is arranged in the housing itself, a hydraulic hose led to the suction connection of the displacement unit only has to be able to withstand the low pressure.

According to an optional modification of the invention, the one-way valve is arranged in the flow path between the suction connection and the engine.

[0013] The one-way valve is preferably integrated in the housing of the displacement unit.

According to a further modification of the invention, it is possible that the one-way valve is arranged in an intake duct of a connection plate of the displacement unit or cooperates with the connection plate.

According to a preferred variation of the invention, the cross section of the flow path from the suction port towards the engine is reduced, preferably this reduction is carried out continuously and the one-way valve is arranged at a position in the flow path at which the cross section of the flow channel is not maximum.

[0016] The one-way valve is preferably arranged in the flow path at a distance from the suction connection in the direction of the engine.

According to an optional modification of the invention, the one-way valve is designed to allow the fluid to flow only in the direction of the engine and to minimize or prevent a flow of the fluid in the opposite direction.

According to a development of the invention, the one-way valve is a check valve or a slide valve, preferably a high-pressure operated slide valve.

According to a development of the invention, the displacement unit further comprises a second connection for supplying a fluid to the engine, the flow paths emanating from the first suction connection and the second connection merging into a common flow path and the one-way valve in the area between the first suction connection and the united common flow path is arranged.

By providing the second connection for supplying a fluid to the engine, the displacement unit is particularly suitable for use in a hydraulic start-stop system. As already explained in the introductory part of the description, it is not the pump operation but the motor drive that is used for starting an internal combustion engine mechanically coupled to the displacement unit. For this purpose, a fluid stored under pressure in a pressure accumulator is fed to the engine. The one-way valve is provided in order to prevent the pressurized fluid from flowing into the storage tank. The inflow of a fluid at the second connection and the outflow from the first suction connection is therefore not possible.

According to a preferred embodiment of the invention, the one-way valve flow is optimized and cooperates with a flow-optimized valve seat. A flow-optimized one-way valve and a flow-optimized valve seat minimize the formation of eddies when fluid flows in along a fluid passage opening at the suction connection.

Preferably, the flow path that can be closed by the one-way valve is a Venturi contour that can be closed by the one-way valve at the point of the smallest flow cross section in the region of the valve.

When inflowing the fluid, e.g. a hydraulic oil, in the displacement unit working in pump operation, the maximum flow velocity is reached at the point of the smallest flow cross section, which causes a suction effect which supports the pump operation.

According to a further development of the invention, the venturi contour has an optimized curvature in order to minimize the formation of eddies of a fluid flowing through the one-way valve.

The suction connection can preferably be connected to a flange plate which is designed as part of a venturi contour formed with the flow path of the suction connection.

The invention further relates to a hydraulic start-stop system which comprises a displacement unit according to one of the variants presented above.

Preferably, the hydraulic start-stop system further comprises an internal combustion engine which is mechanically coupled to the displacement unit and is designed to drive the displacement unit and to be driven by the displacement unit, and a pressure accumulator which is connected to an outlet connection of the displacement unit and is connected to a second connection for supplying a fluid to the engine of the displacement unit, the first suction connection being connected to a fluid reservoir.

According to an optional modification of the hydraulic start-stop system, a pressurized fluid in the fluid reservoir is used to start an internal combustion engine in which a fluid flowing from the fluid reservoir drives the displacement unit and via the mechanical coupling between the displacement unit and Internal combustion engine exercises a starting movement for the internal combustion engine, preferably a first for guiding the fluid

CH 712 781 A1

Valve between the fluid reservoir and the second connection and a second valve between the outlet connection and a connection to the fluid reservoir are provided.

Furthermore, the present invention also relates to a mobile work machine, in particular a mobile excavator, a crawler excavator, a mobile crane, with a hydraulic start-stop system according to one of the variants described above.

[0030] Further features, details and advantages of the invention will become apparent from the detailed discussion of the following figures.

Show:

[0031]

1: a schematic representation of a displacement unit according to the invention with an integrated check valve,

2: a schematic representation of a part of the displacement unit according to the invention,

3: a schematic representation of the displacement unit according to the invention according to a further embodiment,

4: an enlarged detailed view of the further embodiment, and

Fig. 5: a circuit diagram for a hydraulic start-stop system according to the invention.

Fig. 1 shows a schematic representation of a displacement unit according to the invention, for example an axial piston machine, in which the section around a suction port 4 of the displacement unit 1 is carried out in detail. It can be seen that a check valve 3 is arranged in the housing 2 of the displacement unit 1. In regular pump operation of the displacer unit 1, a fluid flows through the suction connection 4 into the interior of the displacer unit 1, and is therefore sucked in by the engine (not shown). The engine is driven via a mechanical coupling with an internal combustion engine, not shown. Due to the suction effect of the engine, the check valve 3 moves into its open position and enables a flow connection between a reservoir and the engine.

Fig. 2 shows an enlarged view of the displacer unit 1 in the area of the suction port 4. The housing 2 has a suction port 4 and a second port 7, both of which are each designed to draw a fluid in the direction of the engine of the displacer unit 1 respectively. The non-return valve 3 prevents a fluid from flowing into the second connection 7 and flowing out of the suction connection 4. Therefore, a fluid flowing into the second connection 7 is supplied to the engine of the displacement unit 1. The flow path of the suction connection 4 and the flow path of the second connection 7 merge downstream to form a common flow path 8, via which a flow connection to the engine is established.

The check valve 3 is slidably mounted in a recess, so that at a pressure coming from the direction of the engine or the second connection 7 of the check valve passes into a closed position. However, if a suction pressure is exerted on the check valve 3 due to the movement of the engine, it moves away from its closed position into its open position and a fluid can flow through the suction connection 4 and the fluid flow bores 13 in the direction of the engine. It can also be seen that a flange plate 6 is arranged on the housing 2 of the displacer unit and has an intake duct 5. It can be seen that the intake duct 5 reduces its flow cross-section to the position that can be closed by the check valve. If the displacement unit 1 designed in this way is used in a hydraulic start-stop system, the intake duct 5 is connected to a fluid reservoir or a storage tank of fluid. A flow from the storage tank through the check valve 3 in the direction of the engine then takes place during a pump operation of the displacement unit 1. However, if the displacement unit 1 is used to start an internal combustion engine or to assist in a starting process, a fluid under high pressure is applied to the flowed to the second connection 7 and the displacement unit is used in motor operation. The check valve 3 is forced into its closing position by the fluid flowing in at the second connection 7 in order to prevent the fluid from flowing into the storage tank. Instead, it flows in the direction of the engine, where it causes movement of the engine, which in turn converts the output shaft of the internal combustion engine, which is mechanically firmly coupled to the engine, into a starting movement.

Fig. 3 shows a flow-optimized representation of the check valve 3 and a flow-optimized valve seat 9. You can recognize the flow-optimized check valve 3 on the beveled surface of the valve piston 16. This comes into contact with the valve seat 9 with the wall of a flow path in such a way that a Sealing for the fluid takes place. The flow path is provided with an optimized Venturi contour shape, which causes an increase in flow velocity and a reduction in eddy formation and thus a lowering of the flow resistance. This Venturi contour shape is only interrupted at the valve seat in order to obtain a satisfactory sealing effect in a closed state of the check valve 3. You can also see that it is in the transition area

CH 712 781 A1 of the flange plate 6 to the housing 2 gives an O-ring 14 in order to achieve tightness. Another O-ring 14 is also arranged on the circumferential side in the flow path between the Venturi contour shape projecting into the housing 2.

FIG. 4 shows an enlarged detail from FIG. 3 in the area of a valve seat 9. This clearly shows the optimized, chamfered contour of the venturi shape 17, 18, which closes in a sealing manner when the valve piston 16 is placed on the valve seat 9. The element shown with reference number 19 corresponds to an undercut.

5 is a schematic illustration of the hydraulic start-stop system 30. An internal combustion engine 31 can be seen, which is coupled to the engine 39 of the displacement unit 1 via a mechanical coupling 35. In addition, one recognizes the one-way valve 3, here a non-return valve, which is characteristically arranged for the invention within the housing 2 of the displacement unit 1. Furthermore, the displacement unit 1 is connected to a fluid reservoir 34 at its suction connection 4. This fluid reservoir 34 serves as a storage tank for the fluid.

[0038] In addition, the displacement unit 1 has a second connection 7, which is designed to guide a fluid in the direction of the engine 39. The second connection 7 is connected to a pressure accumulator 32 via a first valve 36. The fluid reservoir 32 is designed to receive a fluid under a high pressure and to store it therein. The working hydraulics are connected to the outlet connection 33 of the displacement unit 1. Furthermore, a second valve 37 and a third valve 38 are connected to the outlet connection 33. The third valve 38 can close or open a flow path between the outlet port 33 and the fluid reservoir 34. The second valve 37 can close or open a connection to the pressure accumulator 32.

The system shown in FIG. 5 is explained on the basis of the various operating states which the system can assume. In a first operating state, the working hydraulics are supplied with fluid. This corresponds to normal operation. Herein the valves 38, 37 and 36 are in a closed state. In normal operation, the displacement unit 1 is driven by the diesel engine 31. The displacement unit 1 sucks in the working fluid, for example a hydraulic oil, via the check valve 3. The check valve 3 is designed in such a way that the pressure drop is as small as possible along the direction of passage. Depending on the volume flow, i.e. The speed and displacement volume of the displacement unit 1 must not fall below a certain suction pressure. As a result, the line pressure is limited by the suction pressure. The displacement unit 1 conveys the fluid from the outlet connection 33 (also: working connection) to the valve slide located in the working hydraulics. The displacement unit 1 is subjected to a load-sensing control, which ensures that the displacement unit 1 adjusts the delivery volume flow in accordance with a volume flow requirement. With an axial piston machine, this is done by adjusting the helix angle.

A further operating state of the system shown in FIG. 5 loads the pressure accumulator 32. In this state, the valves 38 and 36 are closed. The valve 37 is in its open position. To load the pressure accumulator 32, which can also be designed as a hydraulic accumulator, the fluid conveyed by the displacement unit 1 is conveyed into the pressure accumulator 32.

The starting of the diesel engine can be seen as a further operating state. To start the diesel engine, it must be in a stopped state. Then all valves 36, 37 and 38 must be closed. Furthermore, the hydraulic pressure accumulator 32 must be sufficiently charged when the diesel engine 31 is switched off in order to be able to fulfill a start requirement. To initiate the starting process, valves 36 and 38 are opened, whereas valve 37 remains closed. The pressurized fluid passes from the pressure accumulator 32 via the second connection 7 to the suction channel of the displacer unit 1. The check valve 3 closes the connection between the pressure accumulator 32 and the collecting tank 34 of the fluid in accordance with the pressure ratio present there. The intake duct 7 of the displacement unit 1 is thus under pressure and the displacement unit 1 works by a motor. The drive train is accelerated, causing the engine's starting speed to be exceeded, which means that the engine that is started immediately afterwards is towed up to its operating speed. Since the valve 38 is open, the fluid gets from the working connection 33 of the displacement unit 1 back into the collecting tank 34. During this starting process, the working hydraulics cannot be supplied with hydraulic power.

The hydraulic start-stop system described above is particularly suitable for use in a mobile machine. It is possible for the working machine, in which one or more hydraulic displacement units on the internal combustion engine may drive via a pump distributor gear. It is possible that the output of the internal combustion engine is converted into hydraulic power by means of an adjustable displacement unit. It should be noted that the at least one displacement unit is mechanically connected to the internal combustion engine at all times. It can also be provided that the at least one displacement unit controls the working hydraulics of the mobile working machine, e.g. a stick cylinder, a bucket cylinder, a boom cylinder and a travel drive. Typical applications would be mobile excavators, crawler excavators, mobile cranes etc.

Preferably, the use of the hydraulic start-stop functionality should only be used when the operating temperature of the drive system has been reached, i. H. As is known in the case of car start-stop systems, the primary drive should only be switched off when the operating temperature, in particular that of the engine oil, has been reached. In a mobile machine, this affects not only the operating temperature of the internal combustion engine, but also the hydraulic oil.

CH 712 781 A1 The hauling-up of the internal combustion engine required for starting should preferably always take place with the hydraulic system both during a cold start and during a warm start. In order for the latter to be possible, the internal combustion engine should only be switched off and possibly delayed until the hydraulic pressure accumulator is fully charged. There are two exceptions, the first being the detection of an emergency stop and the second the detection of a leak in the pressure accumulator and the hydraulic line which is only available for accumulator operation.

If a shutdown of the internal combustion engine is suppressed due to a pressure accumulator which is not yet fully charged, the internal combustion engine should not be switched off only when the pressure accumulator is already fully charged, but rather the masses rotating in the drive system (flywheel, gearwheels in the pump distributor gear) are taken into account and the amount of energy that can be extracted from them is fed to the pressure accumulator instead of continuing to charge it with continued fuel consumption.

The rotational energy that is still present on the flywheel of the internal combustion engine or a separate rotational speed sensor is used to determine the rotational energy that is still present. To take into account the proportion of the rotational energy that can be supplied to the pressure accumulator, the hydraulic oil temperature present in the pressure accumulator and the pressure present in the pressure accumulator are measured and taken into account. These three sizes are read into a control unit. From this, the point in time is determined at which there is sufficient rotational energy to fully charge the pressure accumulator, taking into account the losses. At this time, the requested shutdown of the internal combustion engine is released. The calculation is based on a map stored in the control unit with the input variables: speed, oil temperature, pressure level in the accumulator. This results in the energy that can be supplied to the memory until the drive comes to a standstill. The characteristic curve is defined for each drive configuration by the results of functional tests.

It is also conceivable that conventional starting devices are still available as back-ups for warm starts and for "cold starts".

Furthermore, the hydraulic start-stop system can support the electrical starting process.

[0049] A torque to the drive train by means of the displacer unit is particularly advantageous when - in relation to the execution of the internal combustion engine accelerates as a diesel engine at least to its starting speed, and advantageously to its operating speed in the range of 1200 min ^-1 to 2200 min ^-1, preferably from 1400 min ^-1 to 1900 min ^-1, particularly preferably to 1500 min ^-1 to 1800 min ^-1 .

Torque can be applied to the drive train with the help of the displacement unit of an internal combustion engine operated in downspeeding mode, at least to its starting speed. Based on the execution of the internal combustion engine as a diesel engine is preferably up to 1500 min ^-1, a working speed is in the range of 1200 min ^-1. If the speed was reduced further, the hydraulic pumps required to convert mechanical to hydraulic power would have a correspondingly large design. That would be uneconomical.

One could also apply a torque to the drive train already under load with the help of the displacement unit in order to accelerate an internal combustion engine at least to its starting speed.

In the displacement unit used for towing up the internal combustion engine, the output torque is virtually proportional to the swivel angle of the swash plate of an axial piston machine at quasi-constant high pressure.

[0053] A hydraulic accumulator, in particular a bladder accumulator or a piston accumulator, can be used as the pressure accumulator.

The maximum pressure level in the pressure accumulator is between 100 and 450 bar, preferably 150 to 300 bar and especially 200 bar.

The pressure accumulator can be loaded directly via a displacement unit coupled to the internal combustion engine on the power take-off. The displacement unit charging the pressure accumulator can also be located on the primary side of the pump distributor gear. The displacement unit charging the pressure accumulator can also be located on the secondary side of the pump distributor gear.

The displacement unit charging the pressure accumulator can be provided exclusively for charging the pressure accumulator. The displacement unit charging the pressure accumulator can be provided for charging the pressure accumulator and for operating at least one secondary consumer. In this case, it is possible that the displacement unit in question in a certain time interval a) charges only the pressure accumulator, b) is only used to operate the at least one secondary consumer, or c) supplies the pressure accumulator and simultaneously at least one secondary consumer.

The displacement unit charging the pressure accumulator can be provided for charging the pressure accumulator and for the operation of at least one main consumer, e.g. B. the travel drive. In this case, it is possible that the displacement unit in question in a certain time interval a) charges only the pressure accumulator, b) is only used to operate the at least one main consumer, or c) supplies the pressure accumulator and simultaneously at least one main consumer.

CH 712 781 A1 The displacement unit charging the pressure accumulator can be identical to the displacement unit dragging up to start the internal combustion engine. The displacement unit charging the pressure accumulator can be different than the displacement unit which takes over the towing of the latter when the internal combustion engine is started.

Warp-free starting of the internal combustion engine immediately after a torque request or a work request by the driver requires a high torque in order to accelerate the drive train in a short time. As a result, a hydraulic starting process of the proposed system or the availability of a working function of the mobile working machine is faster than with today's electrical starting systems.

[0060] Fuel savings can also be achieved by switching off the internal combustion engine if no power output is required to use the primary functions. The translational movement of the mobile working machine and the actuation of the working hydraulics are referred to as the primary function. The secondary functions, such as (e.g. the operation of the air conditioning system, the lighting should continue to be performed by auxiliary consumers if necessary during the stop operation, should preferably be supplied from the conventional electrical system.

The invention enables an increase in comfort and safety by temporarily lower noise pollution at the site during the shutdown of the internal combustion engine.

The loading of the pressure accumulator is also through recuperation z. B. possible during braking.

Loading of the memory is also possible by raising the load point. The target output of the internal combustion engine is deliberately chosen to be higher than the drive power required. The pressure accumulator is charged with the “excess” output.

A load point lowering is also conceivable. The power output of the internal combustion engine is then briefly reduced. The “performance deficit” required to supply consumers is taken from the pressure accumulator.

The invention also enables an active boost for a brief increase in power made available to the drive. Protection against stalling of the internal combustion engine can also be obtained.

Claims (15)

Claims 1. displacement unit (1), in particular an axial piston machine, comprising: a housing (2), an engine for converting the rotary power into hydraulic power, and a suction port (4) for supplying a fluid to the engine, characterized by a one-way valve (3 ), which is arranged in the housing (2) of the displacement unit (1). 2. Displacement unit (1) according to claim 1, wherein the one-way valve (3) is arranged in the flow path between the suction connection (4) and the engine. 3. displacement unit (1) according to any one of the preceding claims, wherein the one-way valve (3) in the housing (2) of the displacement unit (1) is integrated. 4. displacement unit (1) according to any one of the preceding claims, wherein the one-way valve (3) in a suction channel (5) of a connecting plate (6) of the displacement unit (1) is arranged. 5. displacement unit (1) according to any one of the preceding claims, wherein the cross section of the flow path from the suction port (4) towards the engine is reduced, preferably continuously reduced, and the one-way valve (3) is arranged at a position in the flow path at which the Cross section of the flow channel is not maximum. 6. displacement unit (1) according to any one of the preceding claims, wherein the one-way valve (3) in the flow path from the suction port (4) is arranged spaced in the direction of the engine. 7. displacement unit (1) according to any one of the preceding claims, wherein the one-way valve (3) is designed to allow the fluid to flow only in the direction of the engine and to minimize or prevent a flow of the fluid in the opposite direction. 8. displacement unit (1) according to any one of the preceding claims, wherein the one-way valve (3) is a check valve or a slide valve, preferably a high pressure operated check valve. 9. displacement unit (1) according to one of the preceding claims, further comprising a second connection (7) for supplying a fluid to the engine, wherein the flow paths emanating from the first suction connection (4) and from the second connection (7) converge into a common flow path (8) combine and the one-way valve (3) is arranged in the area between the first suction connection (4) and the combined common flow path (8). 10. displacement unit (1) according to any one of the preceding claims, wherein the one-way valve (3) is flow-optimized and cooperates with a flow-optimized valve seat (9). 11. Displacement unit (1) according to one of the preceding claims, wherein the flow path that can be closed by the one-way valve (3) is a Venturi contour and / or the one-way valve (3) is designed to close the flow path at the point of the smallest flow cross section. CH 712 781 A1 12. displacement unit (1) according to one of claims 10 or 11, wherein the Venturi contour has an optimized curvature in order to minimize vortex formation of a fluid flowing through the one-way valve (3). 13. displacement unit (1) according to one of claims 10 to 12, wherein the suction connection (4) with a flange plate (6) is connectable, which is designed as part of a Venturi contour formed with the flow path of the suction connection (4). 14. A hydraulic start-stop system (30) comprising: a displacement unit (1) according to one of the preceding claims, an internal combustion engine (31) which is mechanically coupled to the displacement unit (1) and is designed to drive the displacement unit (1) and to be driven by the displacement unit (1), and a pressure accumulator (32) which is connected to an outlet connection (33) of the displacement unit (1) and to a second connection (7) for supplying a fluid to the engine of the displacement unit (1), the first suction connection (4) of the displacement unit (1) is connected to a fluid reservoir (34). 15. The hydraulic start-stop system (30) according to claim 14, wherein a pressurized fluid in the fluid reservoir (32) is used to start an internal combustion engine (31) by a fluid flowing from the pressure reservoir (32) Displacement unit (1) drives and exerts a starting movement for the internal combustion engine (31) via the mechanical coupling (35), preferably a first valve (36) between the fluid reservoir (32) and the second connection (7) and a second valve for guiding the fluid (38) are provided between the outlet connection (33) and a connection to the fluid reservoir (34). CH 712 781 A1