Hydrodynamic Retarder
This invention concerns a hydrodynamic retarder for a motor vehicle of a kind as defined in more detail in the preamble of claim 1.
In addition to the service brakes of a vehicle, in particular of a commer¬cial vehicle, which usually are wearing frictional brakes, vehicle manufacturers offer nonwearing deceleration systems. Such deceleration systems, like retard-ers or engine brakes, can also be used to maintain a constant downhill speed.
Retarders include additional hydrodynamic, hydrostatic or electrody-namic braking equipment arranged on the transmission or the engine as well as intarder systems integrated within the transmission housing. A distinction is also made between primary retarders depending on the engine speed and secon¬dary retarders depending on the vehicle speed. The rotor used in hydrodynamic retarders for vehicle deceleration is usually directly linked to a transmission shaft. In primary retarders this is the drive shaft or transmission input shaft; in secondary retarders it is the transmission output shaft. The stator is usually housing-fixed. The retarder is normally not active if rotor and stator are not fluid-filled.
Retarders are used especially in commercial vehicles, for example in order to absorb the kinetic braking energy resulting from deceleration at high speed and to convert it into heat; but they are also well-suited for continuous braking performance, such as during prolonged downhill driving.
The hydraulic energy of a fluid is used for braking in hydrodynamic re¬tarders, with the physical principle corresponding to that of a hydrodynamic clutch with fixed turbine. A hydrodynamic retarder thus features a rotor ar¬ranged in the power flow and a retarder housing-fixed stator. Activation of the

retarder results in the amount of oil corresponding to the desired braking power being fed into the blade chamber, with the turning rotor entraining the oil, which is supported on the stator, by means of which a braking effect acting on the rotor shaft is produced.
There are essentially two different state-of-the-art principles used for retarder control.
On the one hand, there are pneumatic control systems involving retarder control pressure acting via air pressure on a working medium or oil level. Lim¬ited control response, the need to separate working medium and air pressure as well as the existence of a vehicle-based pneumatic supply are the draw¬backs implied here.
On the other hand, there are control systems with hydraulic pressure being provided by a permanently driven hydraulic pump, preferably a gear¬wheel pump. This concept implies the drawback of the speed-dependent vol¬ume flow of the pump, which necessitates a filling reservoir especially for low speeds and large valve and flow cross sections for high speeds.
The disadvantages of the described retarder control systems are, among others, poor response times and additional costs for a reservoir and for control valves, for example. Moreover, considerable installation space is needed.
In DE 101 41 794 A1 a hydrodynamic retarder for a motor vehicle is disclosed which is controlled by a hydraulic circuit. The hydraulic circuit features a heat exchanger and a hydraulic pump, with the volumetric flow of the hydrau¬lic pump being adjustable in such a manner that a volumetric flow independent of the vehicle speed or the propshaft speed or retarder speed can be set. Ac¬cording to an embodiment, the hydraulic pump is controlled in such a manner that a constant volumetric flow is realized for a wide speed range, with this flow

being adequate for retarder filling and control. The advantage of this design is that no filling reservoir is needed. Moreover, the response time of the hydraulic pump equals the initial response time, which results in a robust temporal behav¬ior. Initiation of a braking action requires fastest-possible filling of the retarder. For this purpose, a proportioning valve is used to generate a control pressure which sets the hydraulic pump to maximum displacement. A large oil volume fow is necessary to ensure the required response time of the system.
A drawback here is that a large oil volume flow cannot always be sup¬plied via a transmission intake port because cavitation and air aspiration can occur in an oil supply. Cavitation is defined as bubbling in fluids at low pressure. A specified system response time can therefore not always be ensured.
This invention is based on the task to improve a response time of a sys¬tem for the control of a hydrodynamic retarder and to remove the drawbacks of the state of the art.
The task on which this invention is based is solved by a generic hydro-dynamic retarder possessing also the characteristic features of the main claim.
This invention concerns a hydrodynamic retarder with a rotor and a sta-tor for a transmission in a motor vehicle. The retarder is controlled by an elec-trohydraulic system comprising a control unit, a hydraulic circuit, control valves, and a common oil supply shared by the transmission and the retarder. Rotor and stator are preferably arranged in a retarder housing connected to the hy¬draulic circuit. The hydraulic circuit features a filter, a hydraulic pump, a heat exchanger, a conrol valve, and hydraulic ports. The volumetric flow of the hy¬draulic pump can be adjusted in such a manner that a volumetric flow inde¬pendent of the vehicle speed or the propshaft speed or the retarder speed can be set.

During a brake-free phase, oil is sucked in from a transmission sump by the hydraulic pump and directed through a heat exchanger, by means of which the oil temperature adapts to the temperature of a cooling medium in a vehicle cooling system. In transmissions featuring a common oil supply, the intake is ideally arranged in the front area of the housings to ensure smooth oil ex¬change and consequently effective cooling of the transmission. The control valve and an orifice representing a defined hydraulic cross section are arranged in the hydraulic circuit and generate an appropriate pressure drop in combina¬tion with the hydraulic ports. This pressure acts on a recirculation surface on the hydraulic pump, by means of which the pump independently controls the necessary volume flow irrespective of the pump speed. In the electrohydraulic system, another control valve is provided whose valve stroke can be generated by an electromagnet. A force generated by activation of the electromagnet acts on the piston of the control valve, which causes displacement of the piston and consequently activation of the control valve in the hydraulic circuit, and there¬fore retarder activation or deactivation. The level of braking action is set via the hydraulic pump pressure which, in turn, can be varied by means of a further valve in the system, such as a proportioning valve. Initiation of a braking action requires fastest-possible filling of the retarder. For this purpose, the proportion¬ing valve generates a control pressure which sets the hydraulic pump to a pre¬ferred maximum displacement. A large oil volume flow is required to ensure a specified system response time.
For such an oil volume flow to be guaranteed in spite of a relatively long transmission suction port and cavitation or air intake, the electrohydraulic sys¬tem for the control of the hydrodynamic retarder according to the invention features a filling valve. This filling valve may be a check valve, a 2/2-way valve, or a 3/2-way valve. The check valve, for example, can be a ball-type check valve, a slide-type check valve, or a lamellar valve. By means of the filling valve, a connection can be established between a retarder-arranged oil reservoir and an intake duct of the hydraulic pump. This ensures that the retarder can also be

filled from this oil reservoir within a sufficiently short time. Consequently, the total oil quantity does not have to be sucked in from the transmission sump via the transmission intake duct. As a result, cavitation and air intake are reduced and the response time of the system is significantly improved. The oil reservoir can be realized by the retarder housing or by an external reservoir preferably arranged close to the retarder. The oil reservoir can take any shape. A cylindri¬cal form as for a pneumatic reservoir is not required. Any leak occurring in the system can be equalized by the hydraulic pump. For ventilation of the oil reser¬voir it can be connected to a transmission venting system or a transmission breather, or feature its own venting device. The filling valve can preferably be designed to be hydraulically actuatable and controllable by the same valve as the hydraulic pump. This will not cause any extra costs or additional production expenditure. It is also conceivable that the filling valve is controlled by the al¬ready existing control unit or by a separate own unit. The filling valve can be designed in such a manner that it can be controlled hydraulically, pneumati¬cally, electrically, electromagnetically, or by a servomotor. The electrohydraulic system according to the invention can be implemented with primary or secon¬dary retarders.
The basic principle of the invention, which permits several versions, is illustrated in more detail below by means of an exemplary drawing. Illustrations:
Fig. 1 A control diagram of a hydrodynamic retarder according to a first embodiment of the present invention.
Fig. 2 A control diagram of a hydrodynamic retarder according to a second embodiment of the present invention.
Fig. 3 A control diagram of a hydrodynamic retarder according to a third embodiment of the present invention.

According to Fig. 1, an electrohydraulic system 18 for the control of a hydrodynamic retarder 9 features a control unit 17, a hydraulic circuit 5, valves 4, 8,10,11, and a common oil supply 23. The hydraulic circuit 5 comprises a hydraulic pump 1, a filter 19, a heat exchanger 2, and the control valve 4. The control unit 17 is also connected to temperature sensors 20, 21 and a pressure sensor 22. The first temperature sensor 20 monitors the oil temperature; the second temperature sensor 21 monitors the temperature of a vehicle cooling system 3. The pressure sensor 22 may be provided for the monitoring of the control pressure for the retarder 9. The control unit 17 may ideally be connected to a Controller Area Network (CAN) 24. The valves 8,11 can be controlled via the control unit 17. The first valve 8 can serve as a safety shutoff valve, and the second valve 11 is preferably designed as a proportioning valve and used to control the hydraulic pump 1. The common oil supply 23 consists of an oil reservoir 12 of the retarder 9 and a transmission sump 15 of a transmission. The hydraulic pump 1 provided for in the electrohydraulic system 18 is envis¬aged as a pump whose displacement volume is adjustable. The filter 19 is arranged in an intake port 13 between the hydraulic pump 1 and the oil reser¬voir 12 or the transmission sump 15.
During a brake-free phase, oil is sucked in from the transmission sump 15 by the hydraulic pump 1 and directed through the heat exchanger 2. This results in the oil temperature being adapted to the temperature of a cooling medium in a vehicle cooling system 3. The control valve 4 and an orifice 6 ar¬ranged in the hydraulic circuit 5 generate an appropriate pressure drop in com¬bination with the hydraulic ports. This pressure acts on a recirculation surface 7 on the hydraulic pump 1, as a result of which the pump can control a required volumetric flow independent of the pump speed. Activation of the control valve 8 causes activation of the control valve 4 in the hydraulic circuit 5, and thus activation or deactivation of the retarder 9. The level of braking action is set via

the pump pressure of the hydraulic pump 1 which, in turn, can be varied by the valve 11, for example by a proportioning valve. Initiation of a braking action requires fastest-possible filling of the retarder 9. For this purpose, a control pressure is generated via the proportioning valve 11, by means of which the hydraulic pump 1 is set to maximum displacement. A large oil volumetric flow is required to ensure the necessary response time of the system.
The electrohydraulic system 18 for the control of the hydrodynamic re¬tarder 9 features the filling valve 10 to ensure this oil volumetric flow in spite of a relatively long transmission intake port 14 and cavitation or air intake. By means of the filling valve 10 a connection can be established between the oil reservoir 12 and the suction port 13 of the hydraulic pumpi. The oil reservoir 12 is preferably arranged close to the retarder 9 to make sure that the retarder 9 can also be filled from this oil reservoir 12 within a sufficently short time. Con¬sequently, not all of the oil quantity needs to be sucked in via the transmission intake port. The filling valve 10 is designed as a check valve which will open as soon as the suction vacuum exceeds a prescribed level, and thus open the connection between oil reservoir 12 and hydraulic pump 1. The filling valve 10 will open as soon as a predefined volume flow is exceeded in the hydraulic pump 1. Consequently, this is an automatic demand-controlled opening and closing function requiring no additional external intervention. It is a robust and easy-to-arrange design. Cavitation and air intake are reduced, and the re¬sponse time of the system 18 is significantly improved. Any leakage occurring in the electrohydraulic system 18 can be equalized by the hydraulic pump 1. Ide¬ally, leakages and the oil volume released after deactivation of the retarder 9 are returned to the oil reservoir 12 so that enough oil is available again immedi¬ately after deactivation for the next braking action, and the response time of the system 18 equals the initial response time. During brake-free phases, the oil can be sucked in from the transmission sump 15 by the hydraulic pump 1, and directed to the oil reservoir 12 via the heat exchanger 2. The oil reservoir 12 features an overflow edge 16 towards the transmission sump 15. Excessive oil

will only return to the transmission sump 15 when the oil reaches a specific fluid level, i.e. the oil reservoir 12 is full. Consequently, enough oil can be stored near the retarder 9 even during extreme downhill driving conditions.
Fig. 2 shows a second embodiment of the invention. It only differs from the embodiment described in Fig. 1 in that the filling valve 10 is not a check vavle but a 2/2-way valve preferably controlled by means of hydraulic pressure. Ideally, the arrangement chosen is such that the hydraulic pump 1 and the 2/2-way valve can be contolled by the same valve 11, for example a proportioning valve. A permanent connection to the transmission permits oil to be sucked in from the transmission sump 15 via the transmission suction port 14, and thus equalization of any leakage occurring in the hydraulic circuit 5.
Fig. 3 shows a third embodiment of the invention. It only differs from the embodiments described in Fig. 1 and Fig. 2 in that the filling valve 10 is a 3/2-way valve arranged between the suction port 13 and the oil reservoir 12 as well as the transmission sump 15. This 3/2-way valve can preferably be controlled by means of hydraulic pressure. Ideally, the arrangement chosen is such that the hydraulic pump 1 and the 3/2-way valve can be controlled by the same valve 11, for example a proportioning valve. This arrangement permits filling of the retarder 9 exclusively from the oil reservoir 12. Consequently, there is a virtually complete separation between the retarder 9 and the transmission sump 15. Suction vacua in the area of the transmission suction port 14 can thus be prevented during an activation phase of the retarder 9. In Fig. 3 the filling valve 10, designed as a 3/2-way valve, is shown in its non-activated state, which permits oil to be sucked in from the transmission sump 15. Actuation of the filling valve 10 results in the connection between the oil reservoir 12 and the suction port 13 being established and the connection between the transmission sump 15 and the suction port 13 being interrupted. Consequently, the retarder 9 can be filled exclusively from the oil reservoir 12.

References

1 Hydraulic pump
2 Heat exchanger
3 Vehicle cooling system
4 Control valve
5 Hydraulic circuit
6 Orifice, throttle
7 Recirculation surface on the hydraulic pump
8 Control valve
9 Retarder
10 Filling valve
11 Control valve
12 Oil reservoir
13 Intake port
14 Transmission intake port
15 Transmission sump
16 Overflow edge
17 Control unit, ECU
18 Electrolhydraulic system
19 Filter
20 Temperature sensor
21 Temperature sensor
22 Pressure sensor
23 Common oil supply
24 Controller Area Network (CAN)

Patent Claims
1. Hydrodynamic retarder (9) for a motor vehicle, with the retarder featur¬ing a rotor and a stator and sharing a common oil supply (23) with a transmis¬sion, with a control unit (17) and a hydraulic circuit (5) featuring a pump (1), a heat exchanger (2) and a valve (4), with a volume flow of the pump (1) being adjustable independent of the vehicle speed or the propshaft or retarder speed, characterized in that the common oil supply (23) consists of an oil reservoir (12) and a transmission sump (15) and that the oil reservoir (12) can be con¬nected to a suction port (13) of the pump (1) via a filling valve (10).
2. Hydrodynamic retarder (9) according to claim 1, characterized in that the filling valve (10) is designed as a 2/2-way valve.
3. Hydrodynamic retarder (9) according to claim 1, characterized in that the filling valve (10) is designed as a 3/2-way valve.
4. Hydrodynamic retarder (9) according to one of the claims 1 thru 3, characterized In that the filling valve (10) can be hydraulically controlled by means of an already existing control valve (11), for example by a proportioning valve.
5. Hydrodynamic retarder (9) according to one of the claims 1 thru 3, characterized in that the filling valve (10) can be controlled by the control unit (17).
6. Hydrodynamic retarder (9) according to one of the claims 1 thru 3, characterized in that the fillling valve (10) can be controlled by a separate unit, for example electronically, electromagnetically, hydraulically, pneumatically, or by a servomotor.

7. Hydrodynamic retarder (9) according to claim 1, characterized in that the filling valve (10) is a check valve.
8. Hydrodynamic retarder (9) according to claim 1, characterized in that the shared oil supply (23) features an overflow edge (16) between the oil reser¬voir (12) and the transmission sump (15).
9. Hydrodynamic retarder (9) according to one of the preceding claims, characterized in that the oil resen/oir (12) is arranged in such a manner that it can be filled by any leakage and by any oil volume released after deactivation of the retarder (9).

10. Hydrodynamic retarder (9) according to one of the preceding claims, characterized in that the oil reservoir (12) can additionally be filled by the pump (1) with oil from the transmission sump (15).
11. Hydrodynamic retarder (9) according to one of the preceding claims, characterized in that the oil reservoir (12) can be realized in the retarder hous¬ing as well as by a separate external reservoir.
12. Hydrodynamic retarder (9) according to one of the preceding claims, characterized in that the oil reservoir (12) is arranged as close as possible to the retarder (9).
13. Hydrodynamic retarder (9) according to one of the preceding claims, characterized in that the oil reservoir (12) features a connection to a transmis¬sion venting system or a transmission breather.
14. Hydrodynamic retarder (9) according to one of the claims 1 thru 12, characterized in that the oil reservoir (12) features its own venting device.

15. Method for the control of a hydrodynamic retarder (9) for a motor vehicle, with the retarder being controlled via an electrohydraulic system (18) with a control unit (17) and sharing an oil supply (23) with the transmission, with a volume flow of a pump (1) being adjustable independent of the vehicle speed, or propshaft speed or retarder speed, characterized in that the shared oil sup¬ply (23) consists of an oil reservoir (12) and a transmission sump (15), and that the oil resen/oir (12) is connected to an intake port (13) of the pump (1) via a filling valve (10).
16. Method according to claim 15, characterized in that the filling valve (10) is hydraulically controlled, preferably by an already existent valve (11), for example a proportioning valve.
17. Method according to claim 15, characterized in that the filling valve (10) is electrically or electromagnetically controlled via the control unit (17).
18. Method according to claim 15, characterized in that the filling valve (10) is controlled via a separate unit, for example electrically or electromagneti¬cally, hydraulically, pneumatically, or via a servomotor.
19. Method according to one of the claims 15 thru 18, characterized in that the oil resen/oir (12) is permanently filled by leakage and by an oil volume released after deactivation of the retarder (9).
20. Method according to one of the preceding claims, characterized in that the oil reservoir (12) is additionally filled with oil from the transmission sump (15) by the pump (1).

21. Method according to one of the preceding claims, characterized in
that the oil reservoir (12) is vented via a transmission venting system or a
transmission breather.
22. Method according to one of the claims 15 thru 20, characterized in
that the oil reservoir (12) is vented via its own venting device.