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Title:
HYDRODYNAMIC RETARDER DEVICE
Document Type and Number:
WIPO Patent Application WO/2016/209148
Kind Code:
A1
Abstract:
The invention relates to a hydrodynamic retarder device, comprising a rotor (20) and a stator (42) which together form a workspace (44) connected to a fluid circuit (49), and an expansion vessel (72) connected to the fluid circuit (49). A vacuum chamber (76) is connected to the workspace (44), which vacuum chamber (76) is arranged to remove fluid (46) from the workspace (44). The invention also relates to a vehicle (1) comprising such a hydrodynamic retarder device (2). The invention also relates to a method of controlling a hydrodynamic retarder device (2).

Inventors:
FÄRM, Johnny (Fruängsgatan 29, Hägersten, 129 51, SE)
Application Number:
SE2016/050596
Publication Date:
December 29, 2016
Filing Date:
June 21, 2016
Export Citation:
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Assignee:
SCANIA CV AB (151 87 Södertälje, 151 87, SE)
International Classes:
F01P11/00; B60T1/087; B60T10/02
Domestic Patent References:
WO2015038052A12015-03-19
Foreign References:
DE1605977A11970-03-19
DE2236483A11974-02-07
DE19860706A12000-07-06
US2498572A1950-02-21
US6167993B12001-01-02
Attorney, Agent or Firm:
WALLIN, Ulf (Scania CV AB, Södertälje, 151 87, SE)
Download PDF:
Claims:
Claims

1 . A hydrodynamic retarder device, comprising

a rotor (20) and a stator (42) which together form a workspace (44) connected to a fluid circuit (49) and an expansion vessel (72) connected to the fluid circuit (49), a vacuum chamber (76) connected to the workspace (44), wherein the fluid circuit (49) comprises a first controllable direction valve (64) for connecting and disconnecting the fluid circuit (49) to and from the workspace (44), characterized in that the workspace (44) and the vacuum chamber (76) are connected to a vacuum circuit (78), the vacuum chamber (76) is arranged to remove fluid (46) from the workspace (44) and the vacuum circuit (78) comprises a second controllable direction valve (84) for connecting and disconnecting the vacuum chamber (76) to and from the workspace (44). 2. A hydrodynamic retarder device according to claim 1 , characterized in that the fluid circuit (49) comprises a first controllable direction valve (64) for connecting and disconnecting the fluid circuit (49) to and from the workspace (44).

3. A hydrodynamic retarder device according to any of claims 1 and 2, charac- terized in that the vacuum chamber (76) has a fixed volume.

4. A hydrodynamic retarder device according to any of claims 1 - 3, characterized in that a vacuum pump (63) is arranged in the vacuum circuit (78) for generating a negative pressure in the vacuum chamber (76).

5. A hydrodynamic retarder device according to claim 4, characterized in that the vacuum pump (63) is a reciprocating piston pump arranged between at least two, and preferably four check valves (80, 82, 85, 86) in the vacuum circuit (78).

6. A hydrodynamic retarder device according to any of claims 1 - 5, characterized in that the vacuum circuit (78) is connected to the fluid circuit (49).

7. Vehicle (1 ), characterized in that the vehicle (1 ) comprises a hydrodynamic retarder device (2) according to any of claims 1 - 6.

8. A method of controlling a hydrodynamic retarder device, comprising a rotor (20) and a stator (42) which together form a workspace (44) connected to a fluid circuit (49), and

an expansion vessel (72) connected to the fluid circuit (49),

characterized in that the method comprises the step of:

a) controlling torque on the rotor (20) by generating a negative pressure in the workspace (44). 9. The method according to claim 8, characterized in the further step of:

b) controlling torque on the rotor (20) by regulating a volume of fluid (46) supplied to the workspace (44).

10. The method according to any of claims 8 and 9, characterized in that in step a) said negative pressure is generated by means of a vacuum circuit (78) connected to the workspace (44).

1 1 . The method according to claim 10, characterized in that said negative pressure in the vacuum circuit (78) is generated by means of a vacuum cham- ber (76) connected to the vacuum circuit (78).

12. The method according to any of claims 10 and 1 1 , characterized in that said negative pressure in the vacuum circuit (78) is generated by means of a vacuum pump (63) connected to the vacuum circuit (78).

13. The method according to any of claims 8 - 1 1 , characterized in the further step of: c) connecting the rotor (20) to a powertrain (4) of a vehicle (1 ) when the negative pressure has been generated in the workspace (44).

14. The method according to any of claims 8 - 13, characterized in the further step of:

d) generating the amount of negative pressure in the workspace (44) in relation to the lowest braking torque levels of the rotor (20).

15. A computer program product comprising a program code stored on a me- dia readable by a computer (74) for performing the method steps according to any of the claims 8 - 14, when said program code runs on an electronic control unit (16) or computer (74) connected to the electronic control unit (16).

Description:
Hydrodynamic retarder device

BACKGROUND AND PRIOR ART The present invention relates to a hydrodynamic retarder, a vehicle, which comprises such a hydrodynamic retarder device and a method for controlling a hydrodynamic retarder device according to the preambles of the appended independent claims. A hydrodynamic retarder device is arranged to brake a driving source, such as a propeller shaft in a vehicle. Often the retarder is used as an auxiliary brake, which complements the wheel brakes of the vehicle. Thus excessive wear of the wheel brakes is avoided. The retarder comprises a rotor and a stator, which together form a workspace having a toroidal geometrical form. The workspace must be filled with a fluid, such as water, coolant or oil as quickly as possible when a braking torque from the retarder is requested. A slow filling initially leads to a lack of braking torque from the retarder, which leads to an exaggerated use of the wheel brakes of the vehicle, since the wheel brakes are used to brake the vehicle before the retarder delivers the required torque. This may result in unnecessary wear of the wheel brakes.

A hydrodynamic retarder device is typically used to brake the vehicle at high braking torques or during long duration of the braking, e.g. when travelling down a slope. When the retarder is activated, the wheel brakes are normally not activated. Therefore, the wheel brakes of the vehicle are not exposed to unnecessary wear. When water or coolant is used as fluid in the workspace the braking torque is controlled by means of the volume of water or coolant filled in the workspace. High braking torques exerted by the retarder are achieved when the work- space is completely or substantially completely filled with water or coolant. The volume of fluid in the workspace is controlled by means of one or a number of restriction valves arranged in a fluid circuit connected to the workspace. The pressure within the workspace increases when the flow of fluid is restricted from the workspace. When the restriction valve in an outlet channel from the workspace is opened the volume of fluid within the workspace will decrease, which in turn results in a reduced pressure within the workspace. The fluid or a part of the volume of fluid in the workspace will then evaporate, due to the decrease of the static pressure in the workspace to a level which coincides with the vaporizing point for fluid. However, the pressure will not reach the vaporizing point for the fluid in all parts of the workspace and therefore a small volume of fluid may be left in the workspace despite the preferred evacuation. This small volume of fluid left in the workspace will contribute to a considerable braking torque on the vehicle. For this reason it is difficult to control the retard- er at lower braking torques.

During certain driving conditions it would be helpful for the driver to use the retarder to brake the vehicle, for example at gently sloping downhills and when the cruise control is activated. Under such driving conditions, controllable low braking torques from the retarder would be useful.

When the workspace is filled with fluid the rotor starts to rotate and a torque is exerted on the powertrain. This torque is used as braking torque when the retarder is coupled to the powertrain in the vehicle.

The fluid is evacuated from the workspace when no braking torque should be provided. However, when fluid has been evacuated from the workspace the powertrain of the vehicle still rotates the rotor, which results in a residual torque acting on the powertrain. The residual torque results in an increased fuel consumption of the vehicle. In order to reduce the fuel consumption the rotor is disconnected from the powertrain by means of a coupling element when the retarder is deactivated and should not brake the vehicle. Thus, the rotor will substantially stand still and not rotate when the rotor is disconnected from the powertrain.

However, if a small volume of fluid is left in the workspace the torque on the rotor will be too high the next time the retarder is activated and the rotor is connected to the powertrain. The reason for this is the small volume of fluid left in the workspace. The high torque at the moment of connecting the rotor to the powertrain results in a substantial stress on the mechanical coupling element between the rotor and powertrain. In order to solve this problem an evacuation pump may be installed in the fluid circuit which pumps the residual fluid out from the workspace before connecting the rotor to the powertrain. Depending on the amount of fluid left in the workspace at the moment of connecting the rotor to the powertrain the capacity of the vacuum pump must be considerably high to ensure that all fluid can be evacuated fast enough. Also, if the evacuation pump fails, fluid may be left in the workspace when the rotor is connected to the powertrain. As a result, a substantial stress on the mechani- cal coupling element between the rotor and powertrain will occur due to too high torque at the moment of connecting the rotor to the powertrain, which may lead to a failure in the mechanical coupling element.

The document EP1251050 A1 shows a retarder for vehicles with a rotor and stator, wherein the rotor is arranged to be connected and disconnected to and from the propeller shaft of the vehicle by a clutch device which is pneumatically controlled.

The document US20070102251 A1 shows a water retarder provided with a leakage pump for discharging fluid from the workspace of the retarder.

SUMMARY OF THE INVENTION Despite prior art, there is a need to develop a hydrodynamic retarder device, which facilitates the connection of the rotor in the retarder to a powertrain in a vehicle and which facilitates the controlling of braking at the lowest torque levels.

The object of the present invention is thus to provide a hydrodynamic retarder device of the type defined in the introduction, which facilitates the connection of the rotor in the retarder to a powertrain in a vehicle. Another object of the present invention is to provide a hydrodynamic retarder device of the type defined in the introduction, which facilitates the controlling of braking torque at the lowest torque levels.

These objects are achieved with a hydrodynamic retarder device, a vehicle which comprises such a hydrodynamic retarder device, a method of controlling such a hydrodynamic retarder device and a computer program product, which are characterized by the features specified in the appended independent claims. These objectives are also achieved with a computer program for controlling such a hydrodynamic retarder device.

According to the invention, an advantageous hydrodynamic retarder device is achieved, comprising a rotor and a stator which together form a workspace connected to a fluid circuit, and a fluid chamber connected to the fluid circuit. A vacuum chamber is connected to the workspace, which vacuum chamber is arranged to remove fluid from the workspace. This facilitates the connection of the rotor of the retarder to a powertrain in a vehicle and also facilitates the controlling of braking torque at the lowest torque levels. The vacuum chamber al- ways has a pressure below atmosphere pressure. During activation of the retarder the torque on the rotor will be at an acceptable level when the rotor is connected to the powertrain. Before connecting the rotor to the powertrain, any small volume of fluid left in the workspace is removed from the workspace by means of suction force by means of the vacuum chamber. The mechanical coupling element between the rotor and the powertrain will then be subjected for torques at acceptable levels. During certain driving conditions, such as gen- tly sloping downhills, the driver may use the retarder to brake the vehicle. Also, under such driving conditions the cruise control may be activated. This is possible since any residual fluid in the workspace can be removed by means of the vacuum chamber. Since a pressure under atmosphere pressure has been established in the workspace also vaporized fluid may be removed by means of the vacuum chamber. Thus, it may be possible to control the retarder at lower braking torques since the vacuum chamber will control the remaining volume of fluid or vaporized fluid to a very low level in the workspace

The invention the fluid circuit comprises a first controllable direction valve for connecting/disconnecting the fluid circuit to/from the workspace. This facilitates the connection of the rotor of the retarder to the powertrain in the vehicle, because no fluid may enter the workspace from the fluid circuit when the first controllable valve disconnects the fluid circuit from the workspace. The vacuum chamber then removes all fluid from the workspace, before the rotor of the retarder is connected to the powertrain. Also, controlling of braking torque at the lowest torque levels may be facilitated because the vacuum chamber removes fluid from the workspace in order to decrease the retarder torque and the first controllable valve may connect the fluid chamber to the workspace in order to increase the retarder torque.

According to another embodiment of the invention the vacuum chamber has a fixed volume. Such a vacuum chamber will have no movable parts which may fail or which may cause a leakage. Therefore, the pressure below atmosphere pressure in the vacuum chamber will always be available in order to facilitate the connection of the rotor of the retarder to the powertrain in the vehicle and also to facilitate the controlling of braking torque at the lowest torque levels. According to invention the workspace and the vacuum chamber are connected to a vacuum circuit. It is useful to arrange the vacuum chamber in a separate vacuum circuit, so that other components that cooperate with the vacuum chamber can be connected to the vacuum circuit.

According to still another embodiment of the invention a vacuum pump is arranged in the vacuum circuit for generating vacuum or a negative pressure in the vacuum chamber. The vacuum pump ensures that the pressure in the vacuum chamber will always be below atmosphere pressure in order to facilitate the connection of the rotor of the retarder to the powertrain in the vehicle and also to facilitate the controlling of braking torque at the lowest torque levels.

According to still another embodiment of the invention the vacuum pump is a reciprocating piston pump arranged between two or more check valves in the vacuum circuit. The reciprocating piston pump may also be used in combination with the vacuum chamber to remove any residual fluid in the workspace.

According to the invention the vacuum circuit comprises a second controllable valve for connecting/disconnecting the vacuum chamber to/from the work- space. This facilitates the connection of the rotor of the retarder to the power- train in the vehicle, because when the second controllable valve connects the vacuum chamber with the workspace the vacuum chamber removes all fluid from the workspace, before the rotor of the retarder is connected to the power- train. Also, the controlling of braking torque at the lowest torque levels may be facilitated because the vacuum chamber removes fluid from the workspace in order to decrease the braking torque and the second controllable valve may connect the vacuum chamber to the workspace in order to increase and decrease the braking torque at the lowest torque levels. According to still another embodiment of the invention the vacuum circuit is connected to the fluid circuit. Thus, the vacuum circuit and the fluid circuit may be arranged as a closed common circuit. BRIEF DESCRIPTION OF THE DRAWINGS

Below is a description of, as examples, preferred embodiments of the invention with reference to the enclosed drawings, in which:

Fig. 1 shows schematically a vehicle in a side view, with a hydrodynamic retarder device according to the invention, Fig. 2 shows a sectional view of a hydrodynamic retarder device according to a first embodiment of the invention,

Fig. 3 shows a sectional view of a hydrodynamic retarder device according to a second embodiment of the invention, and

Fig. 4 shows flow chart according to a method of controlling a hydrodynamic retarder device according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE IN- VENTION

Fig. 1 shows a schematic side view of a vehicle 1 , which is equipped with a hydrodynamic retarder device 2 according to the present invention. The vehicle 1 is also equipped with a powertrain 4 comprising a gearbox 6, which is connected to a combustion or/and electric engine 8, which provides a driving torque to the driving wheels 10 of the vehicle 1 via the gearbox 6 and a propeller shaft 12. The driving wheels 10 are provided with wheel brakes 1 1 . An electronic control unit 16 is arranged for controlling the retarder 2. Fig. 2 shows a sectional view of a hydrodynamic retarder according to a first embodiment of the invention. A first shaft 18 is connected to a rotor 20 of the retarder 2 and a second shaft 22 is adapted to be connected to a driving source. According to Fig. 2 the driving source is provided in the vehicle 1 , where the connection of the retarder 2 to the vehicle 1 is performed through the gearbox 6, which thus constitutes the driving source. In Fig. 2, the gearbox 6 is schematically presented. The second shaft 22 may therefore be a propel- ler shaft 12, which is both connected to the gearbox 6, and to the drive wheels 10 of the vehicle 1 . The second shaft 22 may also be an output shaft in the gearbox 6.

A transmission 26 in the form of a first gear wheel 24 arranged on the first shaft 18 engages with a second gear wheel 28, which is releasably arranged on the second shaft 22. Preferably, there is an upshift of the rotational speed of the second shaft 22 through the transmission 26 in the order of 3:1 , but other ratios are possible such as 1 :1 . The first shaft 18 is preferably, by means of bearings 36 and 37, mounted in a retarder housing 40 and possibly also in a gearbox housing 38. The rotor 20 is provided on the first shaft 18, which in an engaged state of the retarder 2 rotates at a speed proportional to the speed of the second shaft 22. A stator 42 is connected to the retarder housing 40 and will therefore not rotate. The rotor 20 and stator 42 together form a workspace 44 having the form of a toroidal hollow space. The workspace 44 is filled with a fluid 46 such as water or coolant through an inlet opening 47 when the retarder 2 is requested to exercise a braking torque on the second shaft 22 connected to the gearbox 6 in order to brake the vehicle 1 and thus decrease or maintain the speed of the vehicle 1 . The braking torque is generated by the rotor 20 and stator 42 which are provided with blades or vanes 48, which creates a fluid flow in the workspace 44 when the rotor 20 rotates. The fluid flow forms, in conjunction with the vanes 48 of the rotor 20 and the stator 42, a reaction force, which results in the braking torque. The higher the speed of the rotor 20 and the greater the amount of fluid in the workspace 44, the larger is the reaction force and thus the braking torque. On occasions when the retarder 2 should not brake the vehicle 1 the workspace 44 is drained completely of the fluid 46 and the fluid is replaced in pari by steam, causing the vanes 48 of the rotor 20 and stator 42 to create a steam flow in the workspace 44. However, the steam flow offers an undesirable reaction force on the first shaft 18, which generates an undesirable braking torque on the second shaft 22. The braking torque from the retard- er 2 causes an increased fuel consumption of the vehicle 1 . Also, the friction from the bearings 36 and 37 and seals 83 of the first shaft 18 generates a reaction force, which results in an increased fuel consumption. For this reason, the first shaft 18 can be disconnected from the second axis 22 when the re- tarder 2 is not used to brake the vehicle 1 . Thus, the fuel consumption of the vehicle 1 is reduced. Filling and draining the workspace 44 with the fluid 46 is made via a fluid circuit 49.

When the retarder 2 should be activated the workspace 44 must be filled with the fluid 46 as quickly as possible to achieve braking torque from the retarder 2. A slow filling leads to an initial loss of braking torque from the retarder 2, resulting in an excessive use of the wheel brakes 1 1 and thus unnecessary wear of the wheel brakes 1 1 .

The second gear wheel 28 which can be disconnected from the second shaft 22 causes the first shaft 18 and thus the rotor 20 in the retarder 2 to be disconnected from the transmission 6, so that the retarder 2 is not affecting the vehicle 1 with a braking torque when the retarder 2 is deactivated. When the retarder 2 is to be activated, the retarder 2 must in a fast and efficient way be mechanically connected to the outgoing second shaft 22 from the gearbox 6. To accomplish this, a coupling element 54 is arranged between the second gear wheel 28 and the second shaft 22. The coupling element 54 preferably comprises a synchronization device provided with synchronizing rings (not shown). Such a synchronizing device is common in gearboxes. It is also possible to design the coupling element 54 as a friction clutch, such as a disk clutch.

When the retarder 2 is activated to brake the vehicle 1 the coupling element 54 is thus activated so that the second gear wheel 28 is connected to the second shaft 22 by means of the coupling element 54. Since the second shaft 22 rotates during engagement and the first shaft 18 is stationary, the coupling element 54 will cause the first shaft 18 to rotate via the transmission 26. The coupling element 54 is dimensioned to be able to transmit the large braking torque exerted by the retarder 2 on the second shaft 22.

Preferably, the fluid 46 supplied to the workspace 44 is water or coolant. The fluid 46 is supplied from the cooling system 55 of the combustion or/and electric engine 8. Thus, the fluid circuit 49 of the retarder 2 is interconnected with the combustion or/and electric engine's cooling system 55. The braking torque of the retarder 2 is controlled by the volume of fluid 46 that is active in the workspace 44. The fluid 46 volume is controlled by a control valve 60 which is arranged in fluid connection with an outlet channel 62 from the workspace 44. By restricting the outflow of fluid 46 from the workspace 44 by the control valve 60 the pressure p in the workspace 44 increases. When the control valve 60 opens the volume of fluid 46 in the workspace 4 will decrease, which in turn causes the pressure p in the workspace 44 to decrease. The fluid 46 or a partial volume of the fluid 46 contained in the workspace 44 will evaporate due to the fact that the static pressure p in the workspace 44 drops to the vaporizing point for the fluid 46.

When the retarder 2 is deactivated inflow of fluid 46 to the workspace 44 is terminated and the control valve 60 is opened so that fluid 46 can be evacuated from the workspace 44. However, the pressure p in parts of the workspace 44 and the exhaust duct from the workspace does not fall below the vaporizing point. Therefore, the fluid 46 in the workspace 44 may linger and stay in the workspace 44 despite the desired evacuation of fluid 46. Any residual fluid 46 in the workspace 44 contributes to the unwanted residual torque, which contributes to increased fuel consumption. However, when disconnecting the sec- ond gear wheel 28 from the second shaft 22 any transfer of residual torque from the retarder 2 to the powertrain 4 is prevented. Problems may arise when the retarder 2 should be connected to the power- train 4 at the next occasion when braking the vehicle 1 , because of the fluid 46 present in the workspace 44. At the engagement an unacceptably high torque, which causes high stresses in the coupling element 54, may arise.

Therefore, a vacuum chamber 76 is connected to the workspace 44, which vacuum chamber 76 is arranged to remove fluid 46 from the workspace 44 in order to facilitate the connection of the rotor 20 of the retarder 2 to the power- train 4 in the vehicle. The vacuum chamber 76 is arranged to always have a negative pressure or a pressure below atmosphere pressure. When the retarder 2 should be activated the torque on the rotor 20 will be at an acceptable low level when connecting the rotor 20 to the powertrain 4. Before connecting the rotor 20 to the powertrain 4 any small volume of fluid 46 left in the workspace 44 is removed from the workspace 44 by means of suction force gener- ated by the vacuum chamber 76. As a result, the mechanical coupling element 54 between the rotor 20 and powertrain 4 will then be subjected to torques at acceptable levels.

Preferably, the vacuum chamber 76 has a fixed volume. Such a vacuum chamber 76 will have no movable parts which may fail or which may cause a leakage. Therefore, the negative pressure in the vacuum chamber 76 will always be available in order to facilitate the connection of the rotor 20 to the powertrain 4 in the vehicle 1 and also to facilitate the controlling of braking torque at the lowest torque levels which will be explained in detail below.

The workspace 44 and the vacuum chamber 76 are connected to a vacuum circuit 78. It is useful to arrange the vacuum chamber 76 in a separate vacuum circuit 78, so that other components that cooperate with the vacuum chamber 76 also can be connected to the vacuum circuit 78.

In the vacuum circuit 78 a vacuum pump 63 is arranged for generating the negative pressure in the vacuum chamber 76. The vacuum pump 63 ensures that the pressure in the vacuum chamber 76 always will be below atmosphere pressure. The vacuum pump 63 is, according to the first embodiment, a reciprocating piston pump arranged between two check valves 80, 82 in the vacuum circuit 78. The vacuum pump 63 comprises a reciprocating piston 87 which is controlled by means of a power means 88. A spring 89 may be arranged within the vacuum pump 63 in order to push the piston in one direction of the reciprocating movement. The vacuum pump 63 may also be of a design other than a reciprocating piston pump. The vacuum pump 63 may also be used in combination with the vacuum chamber 76 to remove any residual fluid 46 in the workspace 44.

The vacuum circuit 78 comprises a second controllable valve 84 for connecting/disconnecting the vacuum chamber 76 to and from the workspace 44. When the second controllable valve 84 connects the vacuum chamber 76 with the workspace 44 the vacuum chamber 76 removes all fluid 46 from the workspace 44, before the rotor 20 of the retarder 2 is connected to the powertrain 4.

The vacuum circuit 78 is connected to the fluid circuit 49, so that the vacuum circuit 78 and the fluid circuit 49 may be arranged as a closed circuit.

If a component in the fluid circuit 49 fails, the fluid 46 will however remain in the workspace 44 at the engagement of the retarder 2, which leads to that the torque becomes too large when connecting the retarder 2 to the powertrain 4. As a precautionary measure, to prevent the coupling element 54 from being subjected to an excessive torque the control unit 16 monitors the engagement of the retarder 2 so that the engagement torque does not exceed the maximum torque which the coupling element 54 is designed for. A pressure sensor 19 is provided at the workspace 44 which measures the pressure p in the work- space 44 or downstream of the workspace 44. The pressure p in the workspace 44 is substantially proportional to the torque generated by the rotor 22. Thus, the torque of the rotor 20 and the first shaft 18 can be calculated in order to determine the torque when connecting the retarder 2 to the powertrain 4 by means of the coupling element 54.

The control unit 1 6 is coupled to the pressure sensor 1 9, the control valve 60 and the speed sensor 9. A first controllable direction valve 64 is also connected to the control unit 16. The first controllable direction valve 64 is included in the fluid circuit 49 and is opened and closed by signals from the control unit 1 6. During activation of the retarder 2 the first controllable direction valve 64 is opened to supply fluid 46 to the workspace 44. When the retarder 2 is deac- tivated the first controllable direction valve 64 is closed so that no fluid 46 is supplied to the workspace 44. The control unit 1 6 is also connected to a power element 66 which engages and disengages the coupling element 54. A position sensor 77 monitors the position of the coupling element 54 and the position sensor 77 is connected to the control unit 1 6. The power element 66 may be a hydraulic or pneumatic cylinder or an electric motor that controls the coupling element 54.

The first controllable direction valve 64 may disconnect the fluid circuit 49 from the workspace 44 so that no fluid 46 may enter the workspace 44 from the fluid circuit 49. The vacuum chamber 76 then removes all fluid 46 from the workspace 44, before the rotor 20 of the retarder 2 is connected to the powertrain 4. Also, the controlling of braking torque at the lowest torque levels may be facilitated because the vacuum chamber 76 removes fluid 46 from the workspace 44 in order to decrease the braking torque. If the braking torque thereaf- ter must be increased the first controllable direction valve 64 may connect the fluid circuit 49 with the workspace 44 in order to supply fluid 46 to the working space 44.

A coolant pump 68 is arranged to provide fluid 46 in the cooling system 55 and the fluid circuit 49 to flow through the first controllable direction valve 64 and into the workspace 44. The rotation of the rotor 20 also pumps and circulates fluid 46 through the fluid circuit 49. The cooling system is also provided with a heat exchanger 70 and an expansion vessel 72. A thermostat valve 71 arranged in the cooling system 55 directs fluid 46 in a direction through the heat exchanger 70 or by-pass the heat exchanger 70 depending on the temperature of fluid 46.

The vacuum chamber 76 connected to the workspace 44 also facilitates the controlling of braking torque at the lowest torque levels. Under certain driving conditions such as gently sloping downhills the driver may use the retarder 2 to brake the vehicle. Also, under such driving conditions the cruise control may be activated. This is possible since any residual fluid 46 in the workspace 44 is removed by means of the vacuum chamber 76 so that a negative pressure is established in the workspace 44. As a result, it is possible to control the retarder 2 at the lowest braking torques. The first and second controllable direction valves 64, 84 are coordinated and connected to the electronic control unit 16 so that they together can control the braking torque at the lowest torque levels. The vacuum chamber 76 removes fluid 46 from the workspace 44 in order to decrease the braking torque and the first controllable direction valve 64 connects the fluid circuit 55 to the work- space 44 in order to increase the braking torque. The second controllable direction valve 84 may connect the vacuum chamber 76 to the workspace 44 in order to decrease the braking torque at the lowest torque levels.

Fig. 3 shows a sectional view of a hydrodynamic retarder device according to a second embodiment of the invention. In the second embodiment of the invention the vacuum pump 63 is arranged between four check valves 80, 82, 85, 86 in the vacuum circuit 78. When using four check valves 80, 82, 85, 86 the reciprocating movement of the piston 87 in the vacuum pump 63 can be used in in both directions. In this embodiment the return spring 89, which is acting on the piston 87, can be removed and the power means 88 for controlling the piston 87 may be arranged to control the piston 87 in both directions. Using the reciprocating movement of the piston 87 in both directions for pumping fluid 46 the pumping effect increases and the retarder 2 according to the invention may respond faster. The vacuum pump 63 ensures that the pressure in the vacuum chamber 76 always will be below atmosphere pressure. According to the second embodiment the vacuum pump 63 may also be used in combination with the vacuum chamber 76 to remove any residual fluid 46 in the workspace 44.

Fig. 4 shows a flow chart of the method for controlling a hydrodynamic retarder device 2, comprising a rotor 20 and a stator 42 which together form a workspace 44 connected to a fluid circuit 49, and an expansion vessel 72 connect- ed to the fluid circuit 49.

The method comprises the step of:

a) controlling torque on the rotor 20 by generating vacuum or a negative pressure in the workspace 44.

The method further comprises step of:

b) controlling torque on the rotor 20 by regulating a volume of fluid 46 supplied to the workspace 44. Preferably, said vacuum or negative pressure is generated by means of a vacuum circuit 78 connected to the workspace 44.

Preferably, said vacuum or negative pressure is generated in the vacuum circuit 78 by means of a vacuum chamber 76 connected to the vacuum circuit 78.

Preferably, said vacuum or negative pressure is generated in the vacuum circuit 78 by means of a vacuum pump 63 connected to the vacuum circuit 78.

The method further comprises step of:

c) connecting the rotor 20 to a powertrain 4 of a vehicle when the vacuum or negative pressure has been generated in the workspace 44. The method further comprises step of:

d) generating the amount of vacuum or negative pressure in the workspace 44 in relation to the lowest braking torque levels of the rotor 20. The present invention also relates to a computer program P and a computer program product for performing the method steps. The computer program P controls the method of controlling of a hydrodynamic retarder device 2, wherein said computer program P comprises program code for making the electronic control unit 16 or the computer 74 connected to the electronic control unit 16 to performing the method steps according to the invention as mentioned herein, when said computer program P is run on the electronic control unit 16 or computer 74 connected to the electronic control unit 16.

The computer program product comprises a program code stored on the elec- tronic control unit 16 or computer 74 connected to the electronic control unit 16 readable, media for performing the method steps according to the invention as mentioned herein, when said computer program P is run on the electronic control unit 16 or the computer 74 connected to the electronic control unit 16. Alternatively, the computer program product is directly storable in the internal memory M into the electronic control unit 16 or the computer 74 connected to the electronic control unit 16, comprising a computer program P for performing the method steps according to the present invention, when said computer program P is run on the electronic control unit 16 or the computer 74 connected to the electronic control unit 16.

According to the above, the retarder 2 may be provided at a vehicle 1 for braking the vehicle 1 , but it is also possible to use the retarder 2 according to the invention for other applications. According to the above, the vehicle 1 , the combustion or/and electric engine 8, the transmission 6 or propeller shaft 12 constitute a drive source, which directly or indirectly is coupled to the retarder 2. Other power sources can however be connected to the retarder 2. The components and features specified above may within the framework of the invention be combined between the different embodiments specified.