BACKGROUND AND PRIOR ART

The invention relates to a method of controlling a synchronizing arrangement for a retarder. The invention also relates to a retarder provided with such a synchronizing arrangement. The invention also relates to a vehicle comprising such a retarder.

A hydrodynamic retarder device may be connected to a powertrain in a vehicle in order to brake the 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. When water or coolant is used as fluid in the work- space 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 evaporation point for fluid. However, the pressure will not reach the evaporation 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. 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.

Next time, when connecting the retarder to a powertrain, a gear wheel which drives the retarder is engaged and locked on a shaft of the powertrain by means of the coupling element. The coupling element may be an axially dis- placeable sleeve provided with internal teeth, which connects the gear wheel and the shaft. However, the sleeve, gear wheel and the shaft of the powertrain have different rotational speeds when the gear wheel should be locked on the shaft of the powertrain. Therefore, a synchronizing arrangement is used to synchronize the rotational speed between the sleeve, gear wheel and the shaft of the powertrain before the gear wheel is locked on the shaft. The synchroniz- ing arrangement comprises a latch cone ring and an inner cone ring arranged on the side of the gear wheel. The shaft of the powertrain may be a shaft in a gearbox.

In order to obtain good synchronization, the surface of peripheral latch teeth on the latch cone ring, which face the sleeve and are designed to engage internal teeth in the sleeve during synchronization, must be angled relative to the axis of rotation of the latch cone ring, said angle being balanced against the braking torque that the latch cone ring transmits to the sleeve in order to achieve synchronous speed. This means that said angle must be designed so that the latch teeth on the latch cone ring engage with that portion of the internal teeth in the sleeve that are at said angle and act on the sleeve sufficiently to achieve synchronous speed and then disengage from the portion of the internal teeth in the sleeve at said angle when the sleeve is to engage with the inner cone ring when synchronous speed has been obtained. In the engaged position between the sleeve and the inner cone ring, the internal teeth in the sleeve engage with peripheral coupling teeth of inner cone ring. The inner cone ring is attached to the gear wheel. To ensure that synchronous speed is reached before the sleeve passes the latch cone ring axially, the teeth of the latch cone ring must disengage from internal teeth at the right moment. This is achieved by a torque balance where the friction torque, also defined as the synchronizing torque, seeks to increase the overlap between the latch cone teeth and the inner cone teeth, while the torque arising from the teeth-teeth contact seeks to reduce the overlap between the teeth. When the peripheral latch teeth on the latch cone ring have disengaged from the internal teeth in the sleeve when synchronous speed has been obtained between the sleeve and the inner cone ring, the sleeve will be axially displaced so that the latch cone ring is moved inwards into the sleeve and stops in an axial position relative to the sleeve, said axial position being determined by the position at which the sleeve meets and engages with the inner cone ring on the gear wheel.

Pre-synchronization occurs before synchronous speed occurs. During pre- synchronization, the oil present between the conical surfaces must be evacuated so that a sufficiently high friction torque is developed, effectively blocking the engagement of the gear wheel during asynchronous speed.

During the pre-synchronization and the following synchronization process the axial position of the latch cone ring is defined because of the axial force from the sleeve acting on the latch cone ring. After the synchronization process the sleeve is coupled to the inner cone ring and also the gear wheel. In this posi- tion the sleeve, the latch cone ring, the inner cone ring and the gear wheel rotate together with the shaft of the powertrain as one unit.

When deactivating the retarder after braking the sleeve is returned to an initial position by means of a shifter fork, so that the rotor of the retarder is disconnected from the powertrain. Since no axial force from the sleeve is acting on the latch cone ring, it will be retracted from the inner cone and be ready for synchronization the next time the retarder should be activated. However, in some cases the the conical surface of the latch cone ring will stick to the coni- cal surface of the inner cone ring. This may happen during the first cycles of use, if incorrect oil is used or at the end of the life time of the synchronizing arrangement. Also, this problem may arise under other circumstances, especially if the coefficient of friction between the conical surfaces is larger than the tangent of the cone angle. If the conical surfaces of the latch cone ring and the inner cone ring have stuck together, the retarder will not be disconnected from the powertrain as intended. Therefore, the rotor of the retarder will rotate when the retarder is deactivated, which results in an increased fuel consumption. Also, if the conical surfaces of the latch cone ring and the inner cone ring have stuck together, the next time the retarder should be connected no synchroniza- tion process will take place between the gear wheel and the shaft of the powertrain.

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 DE102012007732 discloses a powertrain for vehicle, provided with a retarder, which is connected by means of a synchronizing arrangement to the output shaft of a gearbox in the vehicle. A sensor unit is provided to monitor the connection of the retarder to the output shaft of a gearbox. SUMMARY OF THE INVENTION

Despite prior art, there is a need to develop a method of controlling a synchronizing arrangement for a retarder, which detects a malfunction in the synchro- nizing arrangement and restores the synchronizing arrangement, so that it works properly.

The object of the invention is thus to provide a method of controlling a synchronizing arrangement for a retarder, which detects a malfunction in the syn- chronizing arrangement and restores the synchronizing arrangement, so that it works properly.

These objectives are achieved with a method of controlling a synchronizing arrangement for a retarder; a retarder; a vehicle; a computer program and a computer program product as set out in the appended claims.

According to the invention, an advantageously method of controlling a synchronizing arrangement for a retarder is achieved. The synchronizing arrangement comprises a sleeve, which is axially displaceable between a first and second position, a latch cone ring with a substantially circular first friction surface, an inner cone ring with a substantially circular second friction surface, which inner cone ring is attached to a transmission element, and a hub, which is attached to a first shaft. A first transmission element is connectable to the first shaft by means of the synchronizing arrangement and the first transmis- sion element is engaged with a second transmission element attached to the retarder.

The method comprises the step of:

a) displacing the sleeve from the first position to the second position for connecting the first transmission element to the first shaft,

b) measuring the time for synchronizing the rotational speed between the transmission element and the first shaft, c) comparing the measured time to an expected time for synchronizing the rotational speed between transmission element and the first shaft using the synchronizing arrangement, and

d) generating a failure mode if the measured time is substantially shorter than the expected time.

When knowing the expected time for synchronizing the rotational speed between transmission element and the first shaft, a possible failure due to lack of synchronization may be detected if the measured time is substantially shorter than the expected time. The reason why a lack of synchronization occures is that the friction surfaces of the latch cone ring and the inner cone ring has been stuck together. Thus, a malfunction in the synchronizing arrangement may be detected. According to an embodiment of the invention, the time for synchronizing the rotational speed between the transmission element and the first shaft in step b) is measured by monitoring the axial position of the sleeve during synchronization. Synchronization occurs when there is substantial no change in the axial position of the sleeve, but when there is a substantial change in rotational speed of the transmission element. However, a rapid displacement of the sleeve from the first position to the second position during no change in rotational speed of the transmission element indicates that no synchronization occurs. Thus, a malfunction in the synchronizing arrangement may be detected.

According to an embodiment of the invention, the expected time for synchronizing the rotational speed between transmission element and the first shaft is determined from synchronization torque, the rotational speed of the transmis- sion element and the first shaft, and inertia of the transmission element, the first shaft and the synchronizing arrangement. The expected time for synchronizing the rotational speed between transmission element and the first shaft using a specific synchronizing arrangement may be determined by calculating synchronization torque needed at the actual rotational speed of the transmission element and the first shaft, and also take notice of the inertia of the transmission element, the first shaft and the synchronizing arrangement.

According to an embodiment of the invention, the expected time is determined from a table of synchronization times for different rotational speeds of the first shaft stored in an electronic control unit.

As an alternative, a table may be created for a specific synchronizing arrangement. Each specific rotational speed of the first shaft has a specific synchronization time corresponding to the expected time.

According to an embodiment of the invention, the generated failure mode in step d) is indicated by means of a sound or visual signal.

If the retarder is arranged in a vehicle, the driver may receive a sound or visual signal if there is a synchronizing failure in the synchronizing arrangement. Thus, a malfunction in the synchronizing arrangement may be detected.

According to an embodiment of the invention, the method comprises the further steps of:

e) displacing the sleeve from the second position to the first position; and f) requesting a torque from the retarder if failure mode in step d) has been generated.

When requesting a torque from the retarder, fluid is supplied to the workspace. Since the sleeve and the latch cone ring rotates with the same rotational speed and torque is transferred between the sleeve and the latch cone ring, the requested torque from the retarder will act on the transmission element. The magnitude of the requested torque from the retarder is essentially the same as the synchronization torque, which leads to that the friction surfaces of the latch cone ring and the inner cone ring, which has been stuck together, will be released from each other. Thus the synchronizing arrangement may be restored, so that it works properly.

According to an embodiment of the invention, the method comprises the further step of:

g) measuring the ambient temperature, and requesting the torque from the re- tarder if failure mode in step d) has been generated and only if the ambient temperature is higher than a pre-determined temperature.

Since the retarder acts on the driving wheels in a vehicle, a sudden torque spike may cause a slip between the wheels and the road in slippery road con- ditions. Therefore, if the ambient temperature is low, as an example below the freezing point for water, it is preferable to perform the steps e) and f) right after start-up or launch of the vehicle.

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 retarder according to the invention,

Fig. 2 shows schematically a retarder provided with a synchronizing arrangement, which is controlled by the method according to the invention, Fig. 3 shows a sectional view of a synchronizing arrangement for a retarder, which is controlled by the method according to the invention, Fig. 4 shows a sectional view along line I - I in Fig. 3 of the latch cone ring in a synchronizing arrangement for a retarder, which is controlled by the method according to the invention,

Fig. 5 shows a sectional view of a synchronizing arrangement in Fig. 3 in a pre-synchronizing position,

Fig. 6 shows a sectional view of a synchronizing arrangement in Fig. 3 in a synchronizing position,

Fig. 7 shows a sectional view of a synchronizing arrangement in Fig. 3 in a position when the synchronizing process has ended,

Fig. 8a shows a graph over the axial position of the sleeve and the rotational speed of the inner cone ring in relation to time where a correct synchronization process has been detected,

Fig. 8b shows a graph over the axial position of the sleeve and the rotational speed of the inner cone ring in relation to time where a malfunction in the synchronization process has been detected, and

Fig. 9 shows a flow chart of a method of controlling a synchronizing arrangement for a retarder according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Fig. 1 shows a side view of a vehicle 1 , e.g. a truck, which comprises an engine 2 and a gearbox 4 provided with a retarder 6 and a synchronizing arrangement 8, which is controlled by the method according to the invention. The engine 2 is connected to the gearbox 4 and the gearbox 4 is further connected to driving wheels 10 of the vehicle 1 via a propeller shaft 12. Preferably, the engine 2 is an internal combustion engine but another type of engine 2 is also applicable, such as an electrical engine. The gearbox 4 may be a manual transmission, an automated transmission, an automated manual transmission, or a continuously variable transmission. The gearbox may be a single or a du- al-clutch transmission.

Fig. 2 shows schematically a retarder 6 provided with a synchronizing arrangement 8, which is controlled by the method according to the invention. A first shaft 14 is adapted to be connected to a powertrain 18 and a second shaft 16 is connected to a rotor 20 of the retarder 6. According to Fig. 2 the power- train 18 is arranged in the vehicle 1 , where the connection of the retarder 6 to the vehicle 1 is performed through the gearbox 4, which thus constitutes a part of the powertrain 18. In Fig. 2, the gearbox 4 is schematically presented. The first shaft 14 may therefore be a propeller shaft 12, which is both connected to the gearbox 4, and to the driving wheels 10 of the vehicle 1 .

A transmission 26 comprising a second transmission element such as a second gear wheel 24 arranged on the second shaft 16 engages with first transmission element such as a first gear wheel 40, which is releasably arranged on the first shaft 14. Preferably, there is an upshift through the transmission 26 in the order of 3:1 , but other ratios are possible such as 1 :1 . The second shaft 16 is preferably, by means of bearings, mounted in a retarder housing 22 and possibly also in a gearbox house 38. The rotor 20 is provided on the second shaft 16, which in an engaged state of the retarder 6 rotates at a speed propor- tional to the speed of the first shaft 14. A stator 28 is connected to the retarder housing 22 and will therefore not rotate.

The rotor 20 and stator 28 together form a workspace 30 having the form of a toroidal hollow space. The workspace 30 is filled with a fluid 32 such as water or coolant through an inlet opening 34 when the retarder 6 is requested to exercise a braking torque on the first shaft 14 connected to the gearbox 4 in order to brake the vehicle 1 and thus decrease or maintain the vehicle 1 speed. The braking torque is generated by the rotor 20 and stator 28 which are provided with blades or vanes 36, which creates a fluid flow in the workspace 30 when the rotor 20 rotates. The fluid flow forms, in conjunction with the vanes 36 of the rotor 20 and the stator 28, a reaction force, which results in the brak- ing torque. The higher the rotational speed of the rotor 20 and the greater the amount of fluid in the workspace 30, the larger is the reaction force and thus the braking torque. On occasions when the retarder 6 should not brake the vehicle 1 the workspace 30 is drained completely of the fluid 32 and the fluid is replaced in part by steam, causing the vanes 36 of the rotor 20 and stator 28 to create a steam flow in the workspace 30. However, the steam flow offers a reaction force on the second shaft 16, which generates an undesirable braking torque on the first shaft 14. The braking torque from the retarder 6 causes an increased fuel consumption of the vehicle 1 . Also, friction from bearings and seals in the retarder 6 generates a reaction force, which results in an in- creased fuel consumption. For this reason, the second shaft 16 can be disconnected from the first shaft 14 when the retarder 6 is not used to brake the vehicle 1 . Thus, the fuel consumption of the vehicle 1 is reduced. Filling and draining the workspace 30 with the fluid 32 is done via a fluid circuit 38. In order to reduce the fuel consumption a first gear wheel 40 of the transmission 26 can be disconnected from the first shaft 14 so that the second shaft 16 and thus the rotor 20 in the retarder 6 can be disconnected from the powertrain 18. As a result, the retarder 6 will not affect the vehicle 1 with a braking torque when the retarder 6 is deactivated. When the retarder 6 is to be activated, the retarder 6 must in a fast and efficient way be mechanically connected to the outgoing first shaft 14 from the gearbox 4. To accomplish this, a coupling element 42 is arranged between the second gear wheel 28 and the second shaft 22. The coupling element 42 comprises a synchronizing arrangement 8. When the retarder 6 is activated to brake the vehicle 1 the synchronizing arrangement 8 is thus activated so that the first gear wheel 40 is connected to the first shaft 14 by means of the synchronizing arrangement 8. Since the first shaft 14 rotates during engagement and the second shaft 16 is stationary, the synchronizing arrangement 8 will cause the second shaft 18 to rotate via the transmission 26. The synchronizing arrangement 8 is dimensioned to be able to transmit the large braking torque exerted by the retarder 6 on the first shaft 14.

Fig. 3 shows a sectional view of the synchronizing arrangement 8 for a retarder 6, which is controlled by the method according to the invention. The synchronizing arrangement 8 comprises a latch cone ring 46, an inner cone ring 48 arranged on the side of the first gear wheel 40 and a sleeve 50, which is axially displaceable by means of a shifter fork 52. The shifter fork 52 is axially dis- placeable by means of an actuating means 54. The latch cone ring 46 and the inner cone ring 48 are provided with interacting friction surfaces 56, 56', which preferably are of a conical design. The shifter fork 52 transmit axial force from the sleeve 50 to the latch cone ring 46 in order to bring about contact between the friction surfaces 56 on the latch cone ring 46 and the inner cone ring 48 during gear shifting. This means that an oil film formed between the friction surfaces 56 is displaced and an initial torque between latch cone ring 46 and the inner cone ring 48 builds up.

When connecting the retarder 6 to the powertrain 18 the first gear wheel 40 is engaged and locked on the first shaft 14 by means of the axially displaceable sleeve 50. A hub 58 provided with splines 60 on the periphery is attached to the first shaft 14 and allows the sleeve 50 to move axially. The hub 58 trans- mits torque between the first shaft 14 and the sleeve 50. However, the sleeve 50, first gear wheel 40 and first shaft 14 may have different rotational speeds when the gear should be shifted and when the first gear wheel 40 should be locked on the first shaft 14 by means of the sleeve 50. The synchronizing arrangement 8 is therefore used to synchronize the rotational speed between the sleeve 50, first gear wheel 40 and first shaft 14 before the first gear wheel 40 is locked on the first shaft 14. The shifter fork 52 transmits axial force from the sleeve 50 to the latch cone ring 46 in order to bring about contact between the friction surfaces 56, 56' on the latch cone ring 46 and the inner cone ring 48 during gear shifting. This means that an oil film formed between the friction surfaces 56, 56' is displaced and an initial torque between latch cone ring 46 and the inner cone ring 48 builds up.

In order to obtain good synchronization in the gearbox 4, the surface of latch teeth 62 on the latch cone ring 46, which face the sleeve 50 and are designed to engage internal teeth 64 in the sleeve 50 during synchronization, must be angled relative to the axis of rotation of the latch cone ring 46, said angle being balanced against the braking torque that the latch cone ring 46 transmits to the sleeve 50 in order to achieve synchronous speed. A number of balls 66, each loaded with a spring 68, are arranged in the sleeve 50, the purpose of these is to ensure that pre-synchronization occurs. The spring-loaded balls 66 act on abutment means 70 arranged on the latch cone ring 46 to ensure that the latch teeth 62 of the latch cone ring 46 are in the correct axial position relative to the internal teeth 64 of the sleeve 50 during pre- synchronization and the abutment means 70 press the spring-loaded balls 66 radially outwards when the sleeve 50 moves axially in relation to the latch cone ring 46 when the pre-synchronization has ended and when the synchronization or main synchronization should start. In fig. 3 the sleeve 50, latch cone ring 46 and the inner cone ring 48 are depicted on a distance to each other for clarity reason. In Fig. 3 the first gear wheel 40, first shaft 14 and hub 58 are schematically disclosed. The latch teeth 62 extend in a direction parallel to the centre line of the latch cone ring 46 and in a peripheral direction. The abutment means 70 extend in a direction parallel to the centre line of the latch cone ring 46 and in a peripheral direction. The abutment means 70 have a larger exten- sion than the latch teeth 62 in the direction parallel to the centre line. A circumferential groove 49 is arranged in the peripheral surface of the latch cone ring 46 adjacent to the abutment means 70. Fig. 4 shows a sectional view along line I - I of the latch cone ring 46. In the disclosed embodiment four abutment means 70 are arranged on a substantially equally distance on the periphery 72 of the latch cone ring 46. Also, the latch teeth 62 are arranged on a substantially equally distance on the periphery 72 of the latch cone ring 46.

Fig. 5 shows a sectional view of the synchronizing arrangement 8 in a pre- synchronizing position. The shifter fork 52 acts with an axial force on the sleeve 50 and displaces the sleeve 50 and also the latch cone ring 46 axially in relation to the hub 58 in direction towards the inner cone ring 48. The spring- loaded balls 66 are pressed into the direction of the circumferential groove 49 so that the axial position of the latch cone ring 46 in relation to the sleeve 50 is defined. For this reason the groove 49 has a design which interacts with the spring-loaded ball 40. As mentioned above, the spring-loaded balls 66 also act on the abutment means 70 arranged on the latch cone ring 46, so that the latch cone ring 46 will be displaced axially by the force from the spring-loaded balls 66 when the sleeve 50 is displaced by means of the shifter fork 52. As the sleeve 50 and the latch cone ring 46 are displaced axially the friction sur- faces 56, 56' on the latch cone ring 46 and the inner cone ring 48 will be brought to an adjacent position to each other. However, as the gearbox 4 is filled with oil a thin film 29 of oil is created between the friction surfaces 56, 56' on the latch cone ring 46 and the inner cone ring 48. The axial force from the latch cone ring 46 acting on the inner cone ring 48 means result in that the oil film formed between the friction surfaces 56, 56' is displaced.

The latch teeth 62 and the abutment means 70 are preferably situated at the side of each latch cone ring 46 that is closest to the inner cone ring 48 to allow the movement of the latch cone ring 46 in the sleeve 50 during the synchroni- zation process. The abutment means 70 have a smaller radial extension than that of distance between the internal teeth 64 in the sleeve 50. This allows the movement of the latch cone ring 46 in the sleeve 50 during the synchronization process.

Fig. 6 shows a sectional view of the synchronizing arrangement 8 in a syn- chronizing position in which the oil film formed between the friction surfaces 56, 56' has been displaced during pre-synchronization, the friction surfaces 56, 56' have contact with each other and an initial torque between latch cone ring 46 and the inner cone ring 48 is building up. The latch teeth 62 on the latch cone ring 46 engage with and rest against internal teeth 64 in the sleeve 50 during synchronization. Therefore, the surface of the latch teeth 62 on the latch cone ring 46 which engage with and rest against the surface of the internal teeth 64 in the sleeve 50 must be angled relative to the axis of rotation of the latch cone ring 46, and said angle must balance against the braking torque that the latch cone ring 46 transmits to the sleeve 50 in order to achieve syn- chronous speed. During the synchronization the sleeve 50, first gear wheel 40 and first shaft 14 have different rotational speeds. However, when a synchronous speed has been reached between the sleeve 50, first gear wheel 40 and first shaft 14 the angled surface of the latch teeth 62 on the latch cone ring 46 disengage from the angled surface of the internal teeth 64 in the sleeve 50, so that the sleeve 50 passes the latch cone ring 46 axially. To ensure that synchronous speed is reached before the sleeve 50 passes the latch cone ring 46 axially, the teeth 62 of the latch cone ring 46 must disengage from internal teeth 64 at the right time. This is achieved by a torque balance where the friction torque, also defined as the synchronizing torque, seeks to increase the overlap between the latch cone teeth 62 and the inner cone teeth, while the torque arising from the teeth-teeth contact seeks to reduce the overlap between the teeth. When the sleeve 50 moves axially in relation to the latch cone ring 46, the spring-loaded balls 66 are pushed radially outwards due to an radially directed force from an inclined surface on the abutment means 70. Since the surface on the abutment means 70 is inclined the spring-loaded balls 66 are gradually pushed radially outwards when the sleeve 50 moves in the axial direction. Fig. 7 shows a sectional view of the synchronizing arrangement 8 in a position when the synchronizing process has ended and when the peripheral latch teeth 62 on the latch cone ring 46 have disengaged from the internal teeth 64 in the sleeve 50 when the rotational speed is synchronous between the sleeve 50 and the inner cone ring 48. In this position the sleeve 50 has been axially displaced, so that the latch cone ring 46 has been moved inwards into the sleeve 50 and stopped in an axial position relative to the sleeve 50, said axial position being determined by the position at which the sleeve 50 meets and engages with the inner cone ring 48 on the first gear wheel 40. In this position the gear shifting operation has ended and the first gear wheel 40 is engaged on the first shaft 14. Also, in this position the spring-loaded balls 66 has been pushed even more radially outwards and rest on an outer surface of the abutment means 70, depicted as a forth surface 76 of the abutment means 70.

When deactivating the retarder 6 after braking, the sleeve 50 is displaced to the first, initial position by means of the shifter fork 52, so that the rotor 20 of the retarder 6 is disconnected from the powertrain 18. Since no axial force from the sleeve 50 is acting on the latch cone ring 46, it will be retracted from the inner cone ring 48 and be ready for synchronization the next time the retarder 6 should be activated.

However, in some cases the conical friction surface 56, 56' of the latch cone ring 46 will stick to the conical friction surface 56 of the inner cone ring 48. The risk of this happening is greater during the first cycles of use, if incorrect oil at the synchronizing arrangement 8 is used or at the end of the life time of the synchronizing arrangement 8. Also, this problem may arise under other circumstances, especially if the coefficient of friction between the conical friction surfaces 56 is larger than the tangent of the cone angle.

If the conical friction surfaces 56, 56' of the latch cone ring 46 and the inner cone ring 48 have stuck together, the retarder 6 will not be disconnected from the powertrain 18 as intended. Therefore, the rotor 20 of the retarder 6 will rotate when the retarder 6 is deactivated, which results in an increased fuel consumption. Also, if the conical friction surfaces 56 of the latch cone ring 46 and the inner cone ring 48 have stuck together, the next time the retarder 6 should be connected no synchronization process will take place between the first gear wheel 40 and the first shaft 14 of the powertrain 18.

Fig. 8a shows a graph over the axial position of the sleeve 50, the rotational speed of the inner cone ring 48 and the rotational speed of the first gear wheel 40 in relation to time where a correct synchronization process has been detected. The graph over the axial position of the sleeve 50 is shown with an unbroken line, the graph over the rotational speed of the inner cone ring 48 is shown with a broken line and the graph over the rotational speed of the first gear wheel 40 is shown with dots. In order to connect the the first gear wheel 40 to the first shaft 14 the sleeve is displaced from a first axial position, at A1 in fig. 8a, to a direction of a second axial position A3. If the retarder 6 has been deactivated for some time, the rotational speed of the first shaft 14 is zero when the sleeve 50 is in the first position A1 . The synchronizing process starts at a second point of time t2, when the sleeve 50 has been displaced to an in- termediate axial position A2. The first shaft 14 starts to rotate at the second point of time t2 since the torque and the rotational motion of the first gear wheel 40 are transferred to the first shaft 14 by means of the synchronizing arrangement 8. When the synchronizing process starts at the time t2 the axial displacement of the sleeve 50 stops so that the synchronizing process may take place under a certain synchronizing time TS. At a third point of time t3 the synchronizing process is completed and the first shaft 14 and the first gear wheel 40 have the same rotational speed n1 . Finally, the sleeve 50 is displaced to the second axial position A3, so that the internal teeth 64 in the sleeve 50 engage with external teeth 74 of the inner cone ring 48 at the point of time t4. Thus, fig. 8a represents a functional synchronizing process. However, after the synchronizing process, the conical friction surfaces 56, 56' of the latch cone ring and the inner cone ring may stick together, so that no synchronization occurs the next time the retarder 6 should be activated. The synchronizing time TS, for synchronizing the rotational speed between first gear wheel 40 and the first shaft 14 is measured and compared to an expected synchronizing time TE for synchronizing the rotational speed between first gear wheel 40 and the first shaft 14 using the synchronizing arrangement 8. A failure mode is generated if the measured synchronizing time TS is substantially shorter than the expected synchronizing time TE, since a possible malfunction in the synchronization process has been detected.

Fig. 8b shows a graph over the axial position of the sleeve 50 and the rotational speed of the inner cone ring 48 in relation to time where a malfunction in the synchronization process has been detected. The friction surfaces 56, 56' of the latch cone ring 46 the inner cone ring 48 has been stuck together, and therefore the retarder 6 is already connected to the powertrain 18. For this reason the first shaft 14 and the first gear wheel 40 have the same rotational speed n1 from the beginning. When the sleeve 50 is displaced from a first axial position, at A1 in Fig. 8a, to a direction of a second axial position A3 the internal teeth 64 of the sleeve 50 will engage with the external teeth 62 of the latch cone ring 46 at the point of time t2 and at the intermediate axial position A2 of the sleeve 50. At this point small indications in the axial movement of the sleeve 50 will occur. Thereafter the sleeve 50 is displaced to the second axial position A3, so that the internal teeth 64 in the sleeve 50 engage with the external teeth 74 of the inner cone ring 48 at the point of time t3. From the graph in Fig. 8 it is evident that no synchronization between the first shaft 14 and the first gear wheel 40 has been detected, because the synchronizing time TS is substantially zero. Thus a failure mode is generated because the measured synchronizing time TS is substantially shorter than the expected synchronizing time TE. The generated failure mode may be indicated by means of a sound or visual signal. Alternatively, the generated failure mode may be used to solve the problem with the stucked friction surfaces 56 of the latch cone ring 46 and the inner cone ring 48. This may be done by displacing the sleeve 50 from the second position A3 to the first position A1 and requesting a torque from the retarder 6. The torque from the powertrain 18 will be transferred through the friction surfaces 56, 56' of the latch cone ring 46 and the inner cone ring 48 and further to the retarder 6. However, the latch cone ring 46 and the inner cone ring 48 cannot transfer the high braking torque from the retarder 6 on the powertrain 18 and therefore the latch cone ring 46 and the inner cone ring 48 will be released from each other.

Fig. 9 shows a flow chart of the method for controlling the synchronizing arrangement 8 for the retarder, which synchronizing arrangement 8 comprises a sleeve 50, which is axially displaceable between a first and second position A1 , A3,

a latch cone ring 46 with a substantially circular first friction surface 56, an inner cone ring 48 with a substantially circular second friction surface 56', which inner cone ring 46 is attached to a transmission element 40, and a hub 58, which is attached to a first shaft 14;

the first transmission element 40 is connectable to the first shaft 14 by means of the synchronizing arrangement 8; and

the first transmission element 40 is engaged with a second transmission element 24 attached to the retarder 6.

The method comprises the step of:

a) displacing the sleeve 50 from the first position A1 to the second position A3 for connecting the first transmission element 40 to the first shaft 14, b) measuring the synchronizing time TS for synchronizing the rotational speed between the first transmission element 40 and the first shaft 14, c) comparing the measured synchronizing time TS to an expected time TE for synchronizing the rotational speed between first transmission element 40 and the first shaft 14 using the synchronizing arrangement 8, and

d) generating a failure mode if the measured synchronizing time TS is substan- tially shorter than the expected time TE.

When knowing the expected time TE for synchronizing the rotational speed between the first transmission element 40 and the first shaft 14, a possible failure due to lack of synchronization may be detected if the measured syn- chronizing time TS is substantially shorter than the expected time TE. The reason why a lack of synchronization occurs is that the friction surfaces 56, 56' of the latch cone ring 46 and the inner cone ring 48 has been stuck together.

Preferably, the synchronizing time TS for synchronizing the rotational speed between the first transmission element 40 and the first shaft 14 in step b) is measured by monitoring the axial position of the sleeve 50 during synchronization.

When the axial position of the sleeve 50 does not change or change very slow during a substantial change in rotational speed of the first transmission element 40, the synchronization occurs. However, a rapid displacement of the sleeve 50 from the first position A1 to the second position A3 during no change in rotational speed of the first transmission element 40 indicates that no synchronization occurs.

Preferably, the expected time TE is determined from synchronization torque, the rotational speed of the first transmission element 40 and the first shaft 14, and the reflected inertia of the first transmission element 40, the first shaft 14 and the synchronizing arrangement 8.

The expected time TE for synchronizing the rotational speed between the first transmission element 40 and the first shaft 14 using a specific synchronizing arrangement 8 may be determined by calculating synchronization torque needed at the actual rotational speed of the first transmission element 40 and the first shaft 14, and also take notice of the inertia of the first transmission element 40, the first shaft 14 and the synchronizing arrangement 8.

Preferably, the expected time TE is determined from a table of synchronization times for different rotational speeds of the first shaft 14 stored in an electronic control unit 76. As an alternative, a table may be created for a specific synchronizing arrangement 8. Each specific rotational speeds of the first shaft 14 has a specific synchronization time corresponding to the expected time TE.

Preferably, the generated failure mode in step d) is indicated by means of a sound or visual signal.

If the retarder 6 is arranged in a vehicle 1 , the driver may receive a sound or visual signal if there is a synchronizing failure in the synchronizing arrangement 8.

Preferably, the method comprises the further steps of:

e) displacing the sleeve 50 from the second position A3 to the first position A1 ; and

f) requesting a torque from the retarder 6 if a failure mode in step d) has been generated.

When requesting a torque from the retarder 6, fluid is supplied to the workspace 30. Since the sleeve 50 and the latch cone ring 46 rotates with the same rotational speed and torque is transferred between the sleeve 50 and the latch cone ring 46, the requested torque from the retarder 6 will act on the first transmission element 40. The magnitude of the requested torque from the retarder 6 is essentially the same as the synchronization torque, which leads to that the friction surfaces 56 of the latch cone ring 46 and the inner cone ring 48, which has been stuck together, will be released from each other.

Preferably, the method comprises the further step of:

g) measuring the ambient temperature d , and requesting the torque from the retarder 6 if failure mode in step d) has been generated and only if the ambient temperature d is higher than a predetermined temperature c2.

Since the retarder 6 acts on the driving wheels 10 in a vehicle 1 , a sudden torque spike may cause a slip between the wheels 10 and the road in slippery road conditions. Therefore, if the ambient temperature d is low, as an exam- pie below the frozen point for water, it is preferable to perform the steps e) and f) right after start-up or launch of the vehicle 1 .

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 the synchronizing arrangement 8 for the retarder 6, wherein said computer program P comprises program code for making the electronic control unit 76 or a computer 78 connected to the elec- tronic 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 76 or computer 78 connected to the electronic control unit 76. The computer program product comprises a program code stored on the electronic control unit 76 or computer 78 connected to the electronic control unit 76 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 76 or the computer 78 connected to the electronic control unit 76. Al- ternatively, the computer program product is directly storable in the internal memory M into the electronic control unit 76 or the computer 78 connected to the electronic control unit 76, 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 76 or the computer 78 connected to the electronic control unit 76.

The components and features specified above may within the framework of the invention be combined between the different embodiments specified.