BACKGROUND AND PRIOR ART The present invention relates to a hydrodynamic retarder device according to the preamble of patent claim 1 , a vehicle, which comprises such a hydrodynamic retarder device according to the preamble of patent claim 15, and a method for controlling a hydrodynamic retarder device according to the preamble of patent claim 16.

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 a 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 sufficient 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 process, e.g. when travelling down a slope. When the retarder is activated, the wheel brakes are 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 workspace 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 the fluid. However, the pressure will not reach the va- porizing 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 braking torque on the vehicle. For this reason it is difficult to control the retarder at the lowest braking torques.

The cooling system for the engine in a vehicle is provided with an expansion vessel. Since the temperature and pressure in the cooling system varies, also the volume of coolant in the expansion vessel varies. When providing a vehicle with a hydrodynamic retarder device having coolant fluid or water as working fluid, the variations of the coolant volume in the expansion vessel are enhanced. For this reason, the expansion vessel in the cooling system must be replaced with a larger expansion vessel which contains a larger volume of coolant fluid. However, in some cases there will not be enough space for a larger expansion vessel in the vehicle. Also, there are standardization sizes of expansion vessels, which make it difficult to replace a specific expansion vessel with another, larger expansion vessel.

Also, when the cooling system for the engine is connected to the fluid circuit of the retarder, pressure pulses occur in the cooling system when the retarder is activated and deactivated. This will result in that the coolant volume in the expansion vessel varies. Under some driving conditions it would be helpful for the driver to use the re- tarder 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.

The retarder is connected to the powertrain of the vehicle via a retarder transmission. In order to reduce energy losses and thus reduce the fuel consumption of the vehicle, the retader transmission is disconnected so that the retarder is disconnected from the powertrain when the retarder is deactivated and not braking the vehicle.

When the workspace is filled with fluid the rotor starts to rotate and a torque is exerted on the powertrain via the rotating rotor. 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 small amount of 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. A too 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 into 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 con- necting 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 mechanical coupling element between the rotor and powertrain will occur due to a 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 the propeller shaft of the vehicle by a clutch device which is pneumatically controlled.

The document US2012222633 A1 shows a cooling system for a vehicle comprising a compensation chamber, which is provided with a working medium for a hydrodynamic coupling. However, such compensation chamber is designed to be adapted to both the cooling system and the characteristics of the hydro- dynamic coupling.

SUMMARY OF THE INVENTION

Despite prior art, there is a need to develop a hydrodynamic retarder device, which prevents changes in the amount of liquid in the coolant system connected to the hydrodynamic retarder device. Also, 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 torque 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 prevents changes in the amount of liquid in the coolant system connected to the hydrodynamic retarder device.

Another object of the present invention is 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.

Still 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, and a method of controlling such a hydrodynamic retarder device, which is 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.

These objectives are also achieved with a computer program product. 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 first fluid circuit, and a first expansion vessel connected to the first fluid circuit. The first expansion vessel is a part of the cooling system of the vehicle. A second expansion vessel is connected to the workspace, which second expansion vessel is arranged to supply fluid to the workspace. When supplying fluid from the second expansion vessel to the workspace in order to generate braking torque on the rotor, the cooling system of the vehicle will not be affected, because the volume of fluid in the first expansion vessel will primarily not be used to fill the workspace. When water or coolant is used as fluid for the retarder, some of the water or coolant is evaporated into steam and some water or coolant remains in liquid phase. In the first fluid circuit substan- tially all water or coolant will remain as liquid.

According to the invention, 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 always has a pressure below atmosphere pressure. When the retarder should be activated, the torque on the rotor will be at an acceptable level when connecting the rotor 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 generated by the vacuum chamber. The mechanical coupling element between the rotor and power- train will then be subjected to torques at acceptable levels. Under certain driving conditions, such as gently 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 cham- ber will control the remaining volume of fluid or vaporized fluid to a very low level in the workspace

According to another embodiment of the invention, the second expansion vessel and the vacuum chamber are arranged in a common housing and separat- ed by means of a partition wall. Such housing can be designed and arranged at any suitable place in the vehicle. According to another embodiment of the invention, the second expansion vessel is arranged in fluid connection with the vacuum chamber. In order to evacuate fluid from the workspace the fluid is first led to the vacuum chamber before it is transferred to the second expansion vessel. The vacuum chamber will accumulate any pressure peaks in the first fluid circuit and in the first expansion vessel when the fluid is pushed out of the workspace due to the positive pressure created in the workspace by the vanes of the rotor when the rotor is rotated. 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 still another embodiment of the invention, the workspace and the vacuum chamber are arranged in fluid connection with 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 inside or outside of the vacuum circuit for generating vacuum or nega- tive 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. Other types of pumps than reciprocating piston pumps may be used. According to still another embodiment of the invention, the vacuum pump is arranged in the vacuum chamber. When the vacuum pump is arranged in the vacuum chamber no additional space is needed in the vehicle for the vacuum pump.

According to still another embodiment of the invention, the first fluid circuit comprises a first controllable valve for connecting/disconnecting the first fluid circuit and the first expansion vessel to/from the workspace. The first controllable valve connects the first fluid circuit and the first expansion vessel to the workspace when the workspace has been filled with fluid from the second expansion vessel. The fluid will then circulate in the first fluid circuit and pass a cooler arranged in the first fluid circuit. When the first controllable valve disconnects the first fluid circuit and the first expansion vessel from the workspace no fluid may enter the workspace from the first fluid circuit and the first expansion vessel. The vacuum chamber may then remove all fluid from the workspace, before the rotor of the retarder will be connected to the powertrain, or for controlling braking torque at the lowest torque levels.

According to still another embodiment of the invention, the vacuum circuit comprises a second controllable valve for connecting/disconnecting the vacu- urn chamber to/from the workspace. This facilitates the connection of the rotor of the retarder to the powertrain in the vehicle, because when the second controllable valve connects the vacuum chamber with the workspace the vacuum chamber removes substantially all fluid from the workspace, before the rotor of the retarder will be connected to the powertrain. Also, the controlling of braking torque at the lowest torque levels may be facilitated since the vacuum chamber removes fluid from the workspace in order to decrease the braking torque when the second controllable valve is connected the vacuum chamber with the workspace.

According to still another embodiment of the invention, the workspace and the second expansion vessel are connected to a second fluid circuit. The second fluid circuit may be isolated from the first fluid circuit, so that the cooling system of the vehicle will not be affected when fluid is supplied from the second expansion vessel to the workspace in order to generate braking torque on the rotor. Also, pressure pulses may be avoided in the cooling system when the retarder is activated and deactivated.

According to still another embodiment of the invention, the second fluid circuit comprises a third controllable valve for connecting/disconnecting the second expansion vessel to/from the workspace. When the third controllable valve connects the second expansion vessel with the workspace, fluid is flowing from the second expansion vessel into the workspace. Depending on requested torque from the retarder, the third controllable valve will be controlled to deliver a predetermined volume of fluid to the workspace. According to still another embodiment of the invention, the vacuum circuit is connected to the first and second fluid circuits. Thus, the vacuum circuit and the first and second fluid circuits may be arranged as a common closed 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 re- tarder 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 a flow chart according to a method of controlling a hydrodynamic retarder device according to the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

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 vehi- cle 1 is also equipped with a powertrain 4 comprising a gearbox 6, which is connected to a combustion and/or 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 retarder 2 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 by 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 propeller 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 is filled with a fluid 46, such as water or coolant through a first and second inlet opening 47, 51 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 thereby decrease or maintain the speed of the vehicle 1 . Primary, fluid 46 will enter the second inlet opening 51 and secondly, fluid 46 will enter the first inlet opening 47 when the workspace 44 has been filled with fluid 46 from the second inlet opening 51 .

The braking torque is generated by the rotor 20 and stator 42 which are pro- vided 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 stator 42 a reaction force, which results in the braking effect. The higher the speed of the rotor 20 and the greater the amount of fluid 46 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 wholly or partly of the fluid 46 and the fluid is replaced in part 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 retarder 2 thus causes an increased fuel consumption of the vehicle 1 . Also, the friction from the bearings 36 and 37 and seals 94 of the first shaft 18 generate a reaction force, which results in an increased fuel consumption. For this reason, the first shaft 18 may be disconnected from the second shaft 22 when the retarder 2 is not used to brake the vehicle 1 . Thus, the fuel consumption of the vehicle 1 is reduced. Filling the workspace 44 with the fluid 46 is made via a second fluid circuit 83 and draining is made via a vacuum circuit 78. However, small changes in torque are made by means of controlling the control valve 80.

When the retarder 2 should be activated the workspace 44 must be filled with 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 , which will be subjected to unnecessary wear.

The disconnectable second gear wheel 28 on 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 operating with a braking torque on the vehicle 1 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 in the gearbox 6. To accomplish this, a coupling element 54 is arranged between the second gear 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 coupled 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 ele- ment 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. A second expansion vessel 73 is connected to the workspace, which second expansion vessel 73 is arranged to supply fluid 46 to the workspace 44. This is possible since the expansion vessel 73 is pressurized by the pressurized air 91 at the top of the expansion vessel 73 above the fluid surface 93. The expansion vessel 73 is completely filled with air when the retarder 2 is assembled. When supplying fluid 46 from the second expansion vessel 73 to the workspace 44 in order to generating braking torque on the rotor 20, either at the beginning of the braking or when increasing the braking torque, a cooling system 55 of the vehicle 1 will not be affected, because the volume of fluid 46 in a first expansion vessel 72 will primarily not be used to fill the workspace 44. The first expansion vessel 72 constitutes a part of the cooling system 55 of the vehicle 1 . Also, 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 reduce the braking torque on the rotor 20 or to empty all the fluid 46 from the workspace 44. A small decrease of the braking torque on the rotor 20 can be adjusted by the control valve 60. This facilitates the connection of the rotor 20 of the retarder 2 to a powertrain in a vehicle 1 and also facilitates the controlling of braking torque at the lowest torque levels. The vacuum chamber 76 is arranged to always have a negative pressure or a pressure below atmosphere pressure. When the retarder 2 is to be activated, the torque on the rotor 20 should be at an acceptable 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 is therefore removed from the work- space 44 by means of suction force generated by the vacuum chamber 76. As a result the mechanical coupling element 54 between the rotor 20 and power- train 4 will then be subjected to torques at acceptable levels. Under certain driving conditions, such as gently sloping downhills, the driver may use the retarder 2 to brake the vehicle 1 . Also, under such driving conditions the auto- matic cruise control of the vehicle 1 may be activated. This is possible since any residual fluid 46 in the workspace 44 has been removed by means of the vacuum chamber 76 and since a pressure under atmosphere pressure has been established in the workspace 44. Thus, it may be possible to control the retarder 2 at the lowest braking torques.

The second expansion vessel 73 and the vacuum chamber 76 are arranged in a common housing 77 and separated by means of a partition wall 79. Such housing 77 may be designed and arranged at any suitable place in the vehicle 1 . The second expansion vessel 73 is connected to the vacuum chamber 76 through a connection pipe 81 . The workspace 44 and the vacuum chamber 76 are connected to a vacuum circuit 78.

A vacuum pump 63 is arranged in the vacuum circuit 78 for generating the negative pressure in the vacuum chamber 76. Preferably, the vacuum pump 63 is a reciprocating piston pump 63, and according to the first embodiment the pump 63 is arranged between two check valves 80, 82 in the vacuum cir- cuit 78. The vacuum pump 63 may also be used in combination with the vacuum chamber 76 to remove any residual fluid 46 in the workspace. Preferably, but not necessarily, the vacuum pump 63 is arranged in the vacuum chamber 76 so that no additional space is needed in the vehicle 1 for the vacuum pump 63. The vacuum pump 63 comprises a reciprocating piston 98 which is con- trolled by means of a power means 99. A spring 100 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. Water or coolant, and steam within the vacuum chamber 76 are transferred to the second expansion vessel 73 through a pump inlet 87 via the vacuum pump 63 and the two check valves 80, 82. The two check valves 80, 82 are so arranged that the fluid 46 may not enter the vacuum chamber 76 from the second fluid circuit 83 or from the second expansion vessel 73. The vacuum circuit 78 comprises a second controllable direction valve 84 for connecting/disconnecting the vacuum chamber 76 to/from the workspace 44. A first fluid circuit 49 comprises a first controllable direction valve 64 for con- necting/disconnecting the first fluid circuit 49 and the first expansion vessel 72 to/from the workspace 44. The workspace 44 and the second expansion vessel 73 are connected to a second fluid circuit 83, which is connected to the workspace 44 by means of the second inlet opening 51 . The second fluid cir- cuit 83 comprises a third controllable direction valve 86 for connecting/ disconnecting the second expansion vessel 73 to/from the workspace 44. The vacuum circuit 78 is connected to the first and second fluid circuits 49, 83. The second fluid circuit 83 is connected to the second expansion vessel 73 via the connection pipe 81 .

Each of the second and third controllable direction valves 84, 86 has three positions. A first open position, a second closed position and third throttle position 101 , 102. In the third position the fluid 46 can pass the valve 84, 86 with a restricted flow and thus it is possible to adjust the flow of fluid 46 in order to ad- just the torque of the retarder 2.

A pressure sensor 88 connected to the second expansion vessel 73 receive information about the pressure within the second expansion vessel 73. Thus, it is possible to achieve information about the fluid 46 level and the volume of fluid 46 inside the second expansion vessel 73. As an alternative the pressure sensor 88 may be replaced by a level sensor, which can detect the fluid level in the second expansion vessel 73. The pressure sensor 88 is also connected to the control unit 16, so that it is possible to control the first, second and third controllable valves 64, 84, 86 together with the vacuum pump 63 in order to control the fluid 46 volume and pressure within the second expansion vessel 73.

An external conduit 90 is connected to the vacuum chamber 76. Through the external conduit 90, fluid 46 must be added to the vacuum chamber 76 in order to remove air from the vacuum chamber 76 when assembling the retarder 2 and to achieve the correct total volume of fluid 46 within the circuits 55, 78, 83 and expansion vessels 72, 73 of the hydrodynamic retarder device 2. An open- ing and closing valve 92 is arranged in fluid connection with the external conduit 90 to open and close the external conduit 90.

As mentioned above, the fluid 46 supplied to the workspace 44 is preferably water, such as cooling water or coolant. When the workspace 44 has been filled with water or coolant from the second expansion vessel 73, water or coolant is added to the workspace 44 from the cooling system 55 of the combustion and/or electric engine 8. Thus, the fluid circuit 49 of the retarder 2 is interconnected with the cooling system 55 of the combustion and/or electric engine 8 in order to cool the fluid 46 flowing through the workspace 44. 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 flow is controlled by a control valve 60 which is disposed 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 44 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 deactivating the retarder 2, inflow of fluid 46 in to the workspace 44 is terminated by closing the first and third controllable direction valves 64, 86. Thereafter the second controllable valve 84 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 the increased fuel consumption. However, when disconnecting the second gear wheel 28 from the second shaft 22 any residual torque is eliminated. However, problems arise when the retarder 2 should be connected to the powertrain 4 at the next occasion when braking the vehicle 1 , because of the fluid 46 present in the workspace 44. At the connection of the retarder 2 to the powertrain 4 an unacceptably high torque, which causes great stresses in the coupling element 54, may arise. Therefore, the 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 powertrain 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 is activated the torque on the rotor 20 is desired to 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 therefore removed from the workspace 44 by means of suction force generated 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 and also to facilitate the controlling of braking torque at the lowest torque levels which will be explained in detail below. If the pressure within the vacuum chamber 76 increases to a predetermined level a third check valve 85 will open and fluid 46 may flow out to the first fluid circuit 49 through a bypass pipe 89, which connects the vacuum chamber 76 with the first fluid circuit 49.

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. 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 will be connected to the power- train 4, 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 that the coupling element 54 is designed for. A pressure sensor 19 is provided at the workspace 44 which measures the pressure p in the workspace 44 or downstream of the workspace 44. The pressure p in the workspace 44 is proportional or almost proportional to the torque generated by the rotor 20. 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. A position sensor 95 measures the position of the coupling element 54, and the position sensor 95 is connected to the control unit 16.

The control unit 16 is coupled to the pressure sensor 19, 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 first fluid circuit 49 and is opened and closed by signals from the control unit 16. During activation of the retarder 2 the controllable direction valve 64 is opened to supply fluid 46 to the workspace 44 and when the retarder 2 is deactivated the controllable direction valve 64 is closed so that no fluid 46 is supplied to the workspace 44. The control unit 16 is also connected to a power element 66 which engages and disengages the coupling element 54. 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 first fluid circuit 49 and thus the first expansion vessel 72 from the workspace 44 so that no fluid 46 may enter the workspace 44 from the first fluid circuit 49. Also the third controllable valve 86 is closed. 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. The third controllable direction valve 86 may connect the second expansion vessel 73 to the workspace 44 in order to increase the braking torque from the retarder.

The cooling system is also provided with a heat exchanger 70 and the first 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 the fluid 46. 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 valve 64 and in to the workspace 44 for cooling the fluid 46 heated by the increased pressure in the workspace 44. The rotation of the rotor 20 also pumps and circulates fluid 46 through first fluid circuit 55.

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 1 . Also, under such driving conditions the cruise control may be activated. This is possible since any residual fluid 46 in the workspace 44 can be removed by means of the vacuum chamber 76 and since a negative pressure has been established in the workspace 44. As a result, it is possible to control the retarder 2 at the lowest braking torques.

The second and third controllable direction valves 84, 86 are coordinated and connected to the electronic control unit 16 so that they together can control the retarder 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 third controllable valve 86 connects the second expansion vessel 72 to the workspace 44 in order to increase the braking torque. The second controllable valve 84 may connect the vacuum chamber 76 to the workspace 44 in order to increase and 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 inven- tion the vacuum pump 63 is arranged between four check valves 80, 82, 96, 97 in the vacuum circuit 78. When using four check valves 80, 82, 96, 97 the reciprocating movement of the piston 98 in the vacuum pump 63 can be used in both directions. In this embodiment the return spring 100, acting on the piston 98, can be removed and the power means 99 for controlling the piston 98 may be arranged to control the piston 98 in both directions. Using the reciprocating movement of the piston 98 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 sec- ond 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 the rotor 20 and the stator 42 which together form the workspace 44 connected to the first fluid circuit 49, and the first expansion vessel 72 connected to the first fluid circuit 49.

The method comprises the step of:

a) generating braking torque on the rotor 20 by supplying fluid 46 from the second expansion vessel 73 to the workspace 44 through the second fluid circuit 83. When supplying fluid 46 from a second expansion vessel 73 to the workspace 44 through a second fluid circuit 83 in order to generate braking torque on the rotor 20 the cooling system of the vehicle 1 will not be affected, because the volume of fluid 46 in the first expansion vessel 72 is not used to fill the work- space 44.

The method further comprises the step of:

b) supplying fluid 46 from the second expansion vessel 73 to the workspace 44 by means of a positive air pressure in the second expansion vessel 73.

The energy from the existing positive air pressure 91 inside the second expansion vessel 73 is used to push the fluid 46 out of the second expansion vessel 73 and further to the workspace 44. Thus, when the third controllable valve 86 is opened the fluid 46 flows out of the second expansion vessel 73 due to the positive air pressure 91 inside the second expansion vessel 73.

The method further comprises the step of:

c) evacuating fluid 46 from the workspace 44 to the second expansion vessel 73 through a vacuum chamber 76, which is connected to the workspace 44 and the second expansion vessel 73.

In order to deactivating the retarder 2 the fluid 46 is evacuated from the workspace 44 and is first led to the vacuum chamber 76 before it is transferred to the second expansion vessel 73. The vacuum chamber 76 will accumulate any pressure peaks in the first fluid circuit 49 and in the first expansion vessel 72 when the second controllable valve 84 is opened and the fluid 46 is pushed out of the workspace 44 due to the positive pressure created in the workspace 44 by the vanes 48 when the rotor 20 is rotated. Preferably, fluid 46 is pumped from the vacuum chamber 76 to the second expansion vessel 73 by means of the vacuum pump 63 arranged inside or outside of the vacuum chamber 76. When or during the vacuum chamber 76 is filled with fluid 46 the vacuum pump 63 arranged inside or outside of the vacuum chamber 76 pumps the fluid 46 out of the vacuum chamber 76 and into the second expansion vessel 73. The air 91 within the second expansion vessel 73 is then compressed by the fluid 46 entering the second expansion vessel 73.

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 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 com- puter 74 connected to the electronic control unit 16.

The computer program product comprises a program code stored on the electronic 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 brak- ing 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 and/or 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.