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 generated.
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.
With state-of-the-art retarders featuring a common oil supply, oil is sucked in from a transmission sump by a pump and directed through a filter, a heat exchanger and a switch valve during a brake-free phase, i.e. during accel¬eration. A pressure-limiting valve arranged after the heat exchanger controls the system pressure in such a manner that sufficient hydraulic pressure is available for control of the valves and for activation of the retarder. There are also retard¬ers featuring a reservoir from which additional oil can be supplied to the re-

tarder. For this, the reservoir is air-pressurized, by means of which the oil con¬tained in the reservoir can be supplied to the retarder via a control valve, for example.
The drawback here is that hydraulic controls always entail system-inherent activation lags. These are composed of various dead times, with a major factor being the delay required for the hydraulic displacement of piston valves. Owing to the required volume flow, especially the control valve for the cooling circuit preferably features a large cross section and therefore a large piston diameter and piston stroke. Such a control valve consequently needs a large control oil volume flow, and it takes a relatively long time for the piston to reach the "ON" position. During this time, oil from the oil volume flow flows into the transmission sump, with this oil volume not being available to fill the re¬tarder. If a reservoir is pressurized during this phase, this drawback is even aggravated because also part of this oil volume exits the retarder again. Activa¬tion of the reservoir is therefore delayed and takes place only after completed changeover of the hydraulic valves. A delay is thus deliberately accepted.
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.

Here, too, there is the disadvantage that oil flows into the transmission sump during retarder activation and is not available to fill the retarder. As filling takes place here exclusively via the pump, that is without a reservoir, a rela¬tively large quantity of oil can flow into the transmission sump during the initial retarder activation phase.
This invention is based on the task to improve the control behavior of a hydrodynamic retarder without extra effort and expenditure 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 an oil supply. Rotor and stator are preferably arranged in a retarder hous¬ing connected to the hydraulic circuit. The hydraulic circuit features a filter, a hydraulic pump, a heat exchanger, a conrol valve, and hydraulic ports. The volumetric flow of the hydraulic pump can be adjusted in such a manner that a volumetric flow independent 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. Retarder and transmission can be arranged in such a manner that they share a common oil supply. In transmissions featuring a common oil supply, the intake is ideally arranged in the front area of the housings to ensure smooth oil exchange 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 combination 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 valve is provided which activates or deactivates the control valve in the hydraulic circuit, and thus the retarder. As a result of the design and control of this valve according to the invention, a significant improvement of retarder control behavior is realized. This valve is connected to a return line. During a brake-free phase, the volumetric flow is directed through this valve and returned to the oil supply after the valve. When this valve is activated, for example by the control unit, the volumetric flow in the return line can be partly or completely blocked by this valve. This makes sure that oil can no longer flow back to the oil supply immediately after response of the valve, and that the entire supply of the hydraulic pump is available for the control of the hydraulically activated valves and adjustment of the hydraulic pump. This significantly improves the dynamics of the system. The valve may be designed as a 5/2-way valve or a 5/3-way valve. It may be controlled by the existing control unit or by a separate unit. Furthermore, it may be designed in such a manner that it can be controlled hydraulically, pneumatically, electrically or electromagnetically, or via a servomotor.
System leakages can be equalized by the hydraulic pump. Ideally, lea-gages and oil volumes released after retarder deactivation are returned to the oil reservoir, by means of which enough oil is again available immediately after retarder deactivation for the next braking action, and the response time of the system equals the initial response time. The oil reservoir may be connected to a transmission venting system or a transmission breather vor ventilation, or it may feature its own venting device. The electrohydraulic system according to the invention can be implemented with primary as well as secondary 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.
Fig. 2 A control diagram of a hydrodynamic retarder according to a first embodiment of the present invention.
Fig. 3 A control diagram of a hydrodynamic retarder according to a second embodiment of the present invention.
Fig. 4 A diagram showing a valve position over time.
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, control valves 4, 8,10, 11, and an 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 a control pressure for the retarder 9. The control unit 17 may ideally be connected to a Controller Area Network (CAN) 25. The valve 11 is preferably designed as a proportioning valve controllable by the control unit 17. It is used to control the hydraulic pump 1 and preferably also for activation of the filling valve 10. The oil supply 23 is composed of an oil reservoir 12 of the retarder 9 and a transmis¬sion sump 15 of a transmission. The oil reservoir 12 and the transmission sump 15 are separated from each other by an overflow edge 16. The hydraulic pump 1 provided for in the electrohydraulic system 18 is envisaged 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 reservoir 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. The level of braking action is set via the pump pressure of the hydraulic pump 1, which can be controlled by the valve 11. Initiation of a braking action requires fastest-possible filling of the retarder 9. For this purpose, a control pressure is generated via the 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. This filling valve 10 may, for exam¬ple, be designed as a 2/2-way valve. A connection between the oil reservoir 12 and the intake port 13 of the hydraulic pump 1 can be established by the filling valve 10. 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. Con¬sequently, not all of the oil quantity needs to be sucked in via the transmission intake port 14, as a result of which the response time of the electrohydraulic system 18 is improved. Leakages in the electrohydraulic system 18 can be equalized by the hydraulic pump 1. Ideally, leakages and the oil volume re¬leased after deactivation of the retarder 9 are returned to the oil reservoir 12 so

that enough oil is available again immediately after deactivation for the next braking action, and the response time of the system 18 equals the initial re¬sponse 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 trans¬mission 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. 1 shows the control diagram of a hydrodynamic retarder 9 at the beginning of a braking phase. The electromagnetic control valve 8 is shown here as a 3/2-way valve in its activated state. The control valve 8 is controlled by the control unit 17. The subsequent hydraulic control valve 4, however, is still in the OFF position. The hydraulic pump 1 is set towards maximum displace¬ment already by the valve 11. It becomes obvious in this phase that an oil vol¬ume flow via the hydraulic pump 1, the heat exchanger 2, the control valve 4, the orifice 6 and the return line 24 continues to flow into the oil reservoir 12 or the oil supply 23 and not into the retarder 9. The oil can be directed into the retarder 9 by the hydraulic pump 1 only when the control valve 4 is in the ON position. The filling valve 10 can preferably be controlled by means of hydraulic pressure. Ideally, the hydraulic pump 1 and the filling valve 10 are controlled by the same valve 11, for exampe a proportioning valve. The filling valve 10 is shown in its non-activated state.
Fig. 2 shows an embodiment of a control diagram for a hydrodynamic retarder 9 according to the invention. In contrast to the embodiment described in Fig. 1, the control valve 8 is not designed as a 3/2-way vavle but as a 5/2-way valve 8a, and the return line 24 is connected to the control valve 8a. Con¬sequently, a volumetric flow during a brake-free phase is also directed through the control valve 8a. If the control valve 8a is activated by the control unit 17,

the return line 24 will immediately be blocked by the control valve 8a. This ma¬kes sure that during the activation phase of the retarder 9 no more oil can flow back into the oil supply 23 immediately after response of the control valve 8a, and that the entire supply of the hydraulic pump 1 is available for control of the respective valves 4,10 and for adjustment of the hydraulic pump 1. As a result, the dynamics of the system 18 can be significantly improved. Suitable ar¬rangement of the hydraulic ports in the electrohydraulic system 18 permits connection of the return line 24 to an already existing valve and integration of a closure into this valve to block the return line 24. For example, the function of the control valve 8a can be integrated into the control valve 8 of Fig. 1, or the control valve 8 of Fig. 1 can be changed to a 5/2-way valve 8a. As a result, the electrohydraulic system 18 can be realized at low cost and without additional space requirement. This electrohydraulic system 18 can be used for retarders 9 with separate oil supply as well as retarders 9 with common oil supply 23. This embodiment of the electrohydraulic system 18 can also be realized without the filling valve 10.
Fig. 3 shows another embodiment of a control diagram for a hydrody-namic retarder 9. In contrast to the embodiment described in Fig. 2, the control valve 8a is not a 5/2-way valve but a 5/3-way valve 8b. The return line 24 is connected to the control valve 8b. Controlled by the control unit 17, the control valve 8b can be set to its center position in which a volume flow in the return line 24 is blocked by the control valve 8b, for example by means of a slide valve or a piston. This, however, does not yet result in activation of the retarder 9. The advantage here is that the volume flow through the return line 24 is block¬ed, but valve 4 can be connected through only after complete changeover of the filling valve 10 by means of the third position of the control valve 8b, that is after establishment of the connection to the oil reservoir 12. This makes sure that oil can only be sucked in from the oil reservoir 12, by means of which suc¬tion problems with transmissions with unfavorable transmission intake ports 14 can be avoided. The connecting-through time of the control valve 8b can be determined by a time function. Such a time function can be stored in the control

unit 17, or in a separate control unit. It is also conceivable that the connecting-through time of the control valve 8b is determined by any other information available in the electrohydraulic system 18, such as a pressure signal in the hydraulic circuit 5.
Fig. 4 shows the valve position of the control valve 8b over time. The control valve 8b is designed as a 3-position valve, for example as a 5/3-way valve as described in Fig. 3. At the time instant retarder OFF 26, the control valve 8b is not controlled, and the retarder 9 is consequently not activated. If the control valve 8b is in its valve center position 27, the return line 24 is blocked but the retarder 9 has not yet been activated. At the time instant re¬tarder ON 28, the control valve 8b is fully active, with the return line 24 still being blocked and the control valve 4 being controlled. With activated control valve 4, the retarder 9 is filled with oil via the hydraulic circuit 5. Furthermore, the control valve 8b can also be designed in such a manner that the oil volume flow in the valve center position 27 of the control valve 8b is not fully but only partially blocked, which is advantageous with respect to pressure surges occur¬ring during activation of the retarder 9.

Pate nt Claims
1. Hydrodynamic retarder (9) for a motor vehicle, with the retarder featur¬ing a rotor and a stator and being controllable by an electrohydraulic system (18), equipped 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 re¬tarder speed, and circulation of oil being ensured by the pump (1), character¬ized in that the electrohydraulic system (18) features a valve (8a, 8b), by means of which a volume flow in a return line (24) to an oil supply (23) is con¬trollable.
2. Hydrodynamic retarder (9) according to claim 1, characterized in that the valve (8a, 8b) is designed as an electromagnetic control valve.
3. Hydrodynamic retarder (9) according to claim 1 or 2, characterized in that the valve (8a) is designed as a 5/2-way valve.
4. Hydrodynamic retarder (9) according to claim 1 or 2, characterized in that the valve (8b) is designed as a 5/3-way valve.
5. Hydrodynamic retarder (9) according to one of the claims 1 thru 4, characterized in that the volume flow in the return line (24) to the oil supply (23) can be partially blocked by the valve (8a, 8b).
6. Hydrodynamic retarder (9) according to one of the claims 1 thru 4, characterized in that the volume flow in the return line (24) to the oil supply (23) can be completely blocked by the valve (8a, 8b).

7. Hydrodynamic retarder (9) according to one of the preceding claims, characterized in tliat the function of the valve (8a, 8b) can be integrated into an already existing valve (8).
8. Hydrodynamic retarder (9) according to one of the preceding claims, characterized in that the valve (8a, 8b) can be controlled by the control unit (17).
9. Hydrodynamic retarder (9) according to one of the claims 1 thru 7, characterized in that the valve (8a, 8b) can be controlled by a separate unit.

10. Hydrodynamic retarder (9) according to one of the preceding claims, characterized in that the valve (8a, 8b) can be controlled via a time function.
11. Hydrodynamic retarder (9) according to one of the claims 1 thru 9, characterized in that the valve (8a, 8b) can be controlled via a signal available in the electrohydraulic system (18), for example via a pressure signal in the hydraulic circuit (5).
12. Hydrodynamic retarder (9) according to claim 1, characterized in that the retarder (9) and a transmission feature a separate oil supply.

13. Hydrodynamic retarder (9) according to claim 1, characterized in that the retarder (9) and a transmission share a common oil supply (23) being composed, for example, of an oi reservoir (12) and a transmission sump (15).
14. Hydrodynamic retarder (9) according to claim 13, characterized in that the common oil supply (23) features an overflow edge (16) between the oil reservoir (12) and the transmission sump (15).
15. Hydrodynamic retarder (9) according to one of the preceding claims, characterized in that the electrohydraulic system (18) features a filling valve (10) by means of which a connection can be established between the oil reser¬voir (12) and an intake port (13) of the pump (1).
16. Hydrodynamic retarder (9) according to claim 15, characterized in that the filling valve (10) is designed as a 2/2-way valve.
17. Method for the control of a hydrodynamic retarder (9) for a motor vehicle, with an electrohydraulic system (18) featuring a control unit (17) and a hydraulic circuit (5), and with oil circulation being ensured by a pump (1), char¬acterized in that a volume flow in a return line (24) to an oil supply (23) is con¬trolled by a valve (8a, 8b).
18. Method according to claim 17, characterized in that the valve (8a, 8b) is controlled electromagnetically, for example via the control unit (17).

19. Method according to claim 17 or 18, characterized in that the valve
(8a, 8b) is controlled via a time function.
20. Method according to claim 17 or 18, characterized in that the valve
(8a, 8b) is controlled via a signal available in the electrohydraulic system (18),
for example via a pressure signal in the hydraulic circuit (5).
21. Method according to one of the claims 17 thru 20, characterized in
that the valve (8a, 8b) is controlled in such a manner that the volume flow in the
return line (24) to the oil supply (23) is completely blocked.
22. Method according to one of the claims 17 thru 20, characterized in
that the valve (8a, 8b) is controlled in such a manner that the volume flow in the
return line (24) to the oil supply (23) is partially blocked.