In connection with the use of hydrodynamic brakes, such as re- tarders in vehicles, a certain brake action always arises during the propulsion of the vehicle as a consequence of the rotation of the rotor with a driving shaft of the vehicle. This brake action is caused by the air, which is located in the toroid-shaped space, being forced to circulate. The circulating air provides in a corre- sponding way as the introducable medium when it circulates in the toroid-shaped space a brake action. The supplied medium is usually an oil having suitable properties. However, the density of the air is only a fraction of the density of the oil but the brake action arising hereby is not entirely negligible. The fuel con- sumption of a vehicle having a retarder therefore becomes un- necessarily high.

In order to reduce this circulation of air in the toroid-shaped space between the stator and the rotor, a plurality of different solutions have been proposed. A so-called dazzling-screen is for example used which is intended to be moved into the gap be- tween the stator and the rotor for preventing said circulation of air, when no brake action is desired. The use of dazzling- screens includes a plurality of movable mechanical parts, which results in a complicated construction.

From EP 233 331 it is known to supply a blocking medium to the toroid-shaped space of a retarder in order to reduce the circula-

tion of air and thus to reduce the brake action obtained by the circulating air. The blocking medium is supplied as a radially di- rected yet in the toroid-shaped space. The intention is that the supplied blocking medium has to form a blocking medium curtain in the area between the stator and the rotor in such a way that the circulation of air between the stator and the rotor is reduced.

If the insertion of the medium here does not take place with a sufficiently high velocity, the heavy circulation of air, which may amount to 100 m/s in the toroid-shaped space, will relatively quickly move the supplied blocking medium towards the walls of the toroid-shaped space.

SUMMARY OF THE INVENTION The object of the present invention is to provide a hydrodynamic brake comprising means, which in a simple and effective way reduces the circulation of air in the toroid-shaped space, which arises in the hydrodynamic brake when no brake action is de- sired to be obtained.

This object is achieved by the hydrodynamic brake of the initially mentioned kind, which is characterised by the features men- tioned in the characterising part of claim 1. By supplying the second medium with a high pressure and in a direction partly towards the circulating air stream, a retardation of the velocity of the circulating airflow is obtained, when the medium hits the circulating air. By the supply of the medium with a suitably high pressure, it penetrates through the outermostly located air layer in the toroid-shaped space, which has the highest velocity. This is necessary for preventing that the medium, substantially im- mediately after it has been supplied, is moved towards the walls of the toroid-shaped space by the outermostly located air stream. The second medium, which is supplied with a pressure, is finely dispersed in the toroid-shaped space to a medium mist, which is retarding the velocity of the circulating air.

According to a preferred embodiment of the present invention, the second medium is arranged to be supplied in form of at least one jet. Said jet may have a suitable shape and a pressure, which actively reduces the direction of motion and the velocity of the air. Such jets ought to be supplied with a velocity of above 10 m/s in order to guarantee that they will be able to retard and penetrate the outermost air layer in the toroid-shaped space.

Preferably, the velocity ought to be at least 20 m/s. Alterna- tively, the second medium may be supplied as a number of jets in order to obtain a more spread influence on the circulating air in the toroid-shaped space. Advantageously, the second medium is identical with the first medium. The first medium is usually an oil. Such an oil is, in the most cases, also suitable as a medium for reducing the circulation of air in the toroid-shaped space. By using the same medium for preventing said circulation of air as for providing a brake action, the handling of the second medium is simplified. No special collecting containers for the medium need to be provided, for example.

According to another preferred embodiment of the present in- vention, said means comprises at least one injection member, which is arranged to inject the second medium into the toroid- shaped space. Such an injection member is arranged to supply the second medium into the toroid-shaped space with a suitable pressure. The injection member may also comprise a suitable nozzle, which provides one or several jets with a suitable shape for reducing the circulation of air. The injection member may supply the second medium from a radially outwardly located portion of the toroid-shaped space. Thereby, the second medium may be injected towards the circulating air in an area when the air flows from the rotor to the stator and has its highest velocity.

Advantageously, the injection member supplies the second me- dium, in this case, via an orifice in a bottom surface of the re- cess of the stator. Since the stator is immobile, it is relatively uncomplicated to provide an injection member in the stator for the supply of the second medium in said area. In order to obtain

an effective reduction of the velocity of the circulating air, the second medium ought to be supplied in a direction which has an angle of 15° to 45° to a radial direction, which extends from the orifice of the injection member to a centre in the toroid-shaped space. The circulating air is presumed to circulate round about in the toroid-shaped space along the bottom surfaces of the re- cesses of the stator and the rotor. Hereby, the principal flow di- rection of the air becomes substantially perpendicular to such a radial direction. The more the supply direction of the medium deviates from said radial direction the more contrary the medium hits the circulating air stream. Advantageously, the second me- dium is supplied in one or several jets in a direction with an an- gle of 15° to 45° to said radial direction. An angle of about 30° seems to be the most favourable in order to retard most effec- tively the velocity of the circulating air stream at the same time as a considerable part of the second medium may pass through this air stream and form a medium mist in the toroid-shaped space.

According to another preferred embodiment of the present in- vention, said injection member may be arranged to inject the second medium from a radially inwardly located portion of the toroid-shaped space. Hereby, the velocity of the circulating air is reduced in the passage from the stator to the rotor. Advanta- geously, the injection member is, in this case, provided in a shaft portion of the rotor and comprises an orifice in the bottom surface of the recess of the rotor. Advantageously, the second medium is supplied in a direction with an angle of 15° to 45° to a radial direction, which extends from the orifice of the injection member to a centre in the toroid-shaped space. Supplying jets with a high pressure in an angle range of about 30° seems to be the most favourable for in the most efficient manner retarding the circulating air stream at the same time as an effective me- dium mist is obtained in the toroid-shaped space. Suitably, said means comprises a plurality of injection members located both at radially outwardly and inwardly located portions and with con-

stant intervals along the circular extension of the toroid-shaped space. Thereby, the circulation of air may be counteracted ef- fectively in the whole toroid-shaped space.

BRIEF DESCRIPTION OF THE DRAWINGS In the following a preferred embodiment of the invention is de- scribed as an example with reference to the attached drawings, in which: Fig 1 shows a cross section through a portion of a hydro- static brake according to the present invention, Fig 2 shows separately the stator showed in Fig 1 and Fig 3 shows separately the rotor showed in Fig 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT INVENTION Fig 1 shows a cross section through a portion of a hydrodynamic brake in form of a retarder of a motor driven vehicle. The re- tarder comprises a stator 1 and a rotor 2. The stator 1, which is shown separately in Fig 2, comprises a body having an inner ra- dial surface 3, which defines a circular opening. A rotatable shaft 4 is arranged to extend through the circular opening of the stator 1. The rotatable shaft 4 is, via a transmission device, in connection with a driving shaft of the vehicle. The stator 1 is fix- edly provided in the vehicle in a suitable way. The body of the stator 1 comprises an annular recess 5, which in the mounted state of the retarder, extends around the rotatable shaft 4. The annular recess 5 is restricted in a radial direction outwards by an outer edge 6 and in a radial direction inwards by an inner edge 7. A number of vanes 8 are provided with uniform intervals along the circular extension of the annular recess 5. The vanes 8 have a substantially radial extension through the recess 5 from its inner edge 7 in a direction towards its outer edge 6. The

lateral surfaces of the vanes 8 have an angle of about 45° in relation to the bottom surface of the recess 5.

The rotor 2, which is shown separately in Fig 3, comprises a body with a shaft portion 9, which is fixedly connected to the rotatable shaft 4. Thereby, the rotor 2 will rotate synchronously with the rotatable shaft 4. The rotor 2 moreover comprises a substantially corresponding construction as the stator 1 and comprises an annular recess 10, which extends around the ro- tatable shaft 4 in the mounted state of the rotor 2. The annular recess 10 is restricted in a radial direction outwards by an outer edge 11 and in a radial direction inwards by an inner edge 12. A plurality of vanes 13 are provided with uniform intervals along the circular extension of the annular recess 10. The vanes 13 have a substantial radial extension through the recess 12 from its outer edge 11 in a direction towards its inner edge 12. Each of the lateral surfaces of the vanes 12 has an angle of about 45° in relation to the bottom surface in the recess 10.

The stator 1 and the rotor 2 are coaxially provided in rotation to each other around the shaft 4 in such a way that the annular re- cesses 5,10 of the stator 1 and the rotor 2 together form a toroid-shaped space, which extends around the rotatable shaft 4. The stator 1 comprises a plurality of openings 14 through which a medium, in form of an oil, is introduceable to the toroid- shaped space when the vehicle is intended to be braked. The supplied oil is guided, during the rotation of the rotor 2, by the vanes 13 in the direction of rotation and radially outwards in the toroid-shaped space along the bottom surface of the recess 10 and is thrown with a high velocity from the outer edge 11 of the recess 10 of the rotor 2 over to the outer edge 6 of the recess 5 of the stator 1. The direction of rotation of the oil in the toroid- shaped space is shown by the arrows a in Fig 1. The oil hits the vanes 8 of the stator 1 and the motion of the oil in the direction of rotation of the rotor 2 is retarded and the oil is guided by the vanes 8 radially inwards along the bottom surface of the recess

5 until it reaches the inner edge 7 of the recess 5. Here, the oil again is thrown over to the rotor 2 and hits the rotor 2 at the in- ner edge surface 12 of the recess 10. The oil hits the rotating vanes 13 of the rotor 2 at an angle in such a way that a sub- stantially optimal brake action is obtained by the rotor 2. There- after the oil is guided by the rotating vanes 13 at the same time as it is guided radially outwards along the bottom surface of the recess 10. The brake action obtained by the rotor 2 depends partly on the quantity of oil which is supplied and is circulated in the toroid-shaped space and partly on the rotary speed of the rotor 2. During the braking process, the kinetic energy of the oil is transformed to heat energy. The stator 1 comprises a number of openings 15 through which the oil is arranged to be removed from the toroid-shaped space, when a brake action is not any longer desired.

In a corresponding way as the circulating oil supplies a brake action to the rotor, a circulation of the existing air in the toroid- shaped space supplies a brake action. Said circulation of air is a problem when no brake action is desired. Since the air has a much lower density than oil, the brake action, which is supplied by the circulating air, becomes considerably lower than the one provided by the oil. However, the supplied brake action is not entirely negligible and results in, for example, at the propulsion of a motor vehicle, an unnecessarily high fuel consumption.

In order to counteract such a circulation of air, a first injection member 16 has been provided at an outer portion of the stator 1. The first injection member 16 is arranged to supply a second medium, which with advantage is identical with the oil used for obtaining a brake action in the retarder. The first injection mem- ber 16 supplies the second medium by one or several jets, which are supplied with a high pressure in such a way that the medium obtains an initial velocity of about 20 m/s. The jets are arranged to be injected in a direction which deviates with an an- gle v1 of about 30° to a radial direction r1, which extends from

the orifice of the injection member 16 and through a centre c in the toroid-shaped space. The principal air stream obtains a cor- responding flow direction a as the oil. Consequently, the air stream follows substantially the bottom surfaces of the toroid- shaped space in the stator 1 and the rotor 2. Thus, the substan- tial flow direction a of the air becomes in the injection area sub- stantially perpendicular to such a radial direction ri. The larger angle, at which the second medium is supplied to the toroid- shaped space, to said radial direction, the more contrary the medium hits the circulating air stream. However, the injection angle v1 may be varied within a range from 15° to 45°. The in- jected second medium counteracts here the passage of the air from the rotor 2 to the stator 1 before it forms a medium mist in the toroid-shaped space.

A second injection member 17 is provided in a shaft portion 9 of the rotor 2 and comprises an orifice in the bottom surface of the recess 10 of the rotor 2. The second injection member 17 is ar- ranged to supply the second medium by a plurality of jets with a high pressure. The jets are arranged to be injected in a direction which deviates with an angle V2 to a radial direction r2, which extends from the orifice of the injection member 17 and through said centre c in the toroid-shaped space. However, the injection angle v2 may be varied within a range from 15° to 45°. The in- jected medium counteracts here the passage of the air from the stator 1 to the rotor 2 before it forms a medium mist in the toroid-shaped space.

The first 16 and the second 17 injection members are provided so that the injected second medium first of all has to reduce the velocity of the air during the passage between the stator and rotor in one of the directions. Here, the velocity of the air may be substantially considerably reduced by supplying the medium with a high velocity in a direction partly towards the flow direc- tion a of the air. By supplying the jets with a high velocity, the medium passes principally through at least the outermost air

layer and obtains a spreading such that a medium mist of small finely dispersed medium droplets are formed, which further counteract the circulation of air between the stator and the rotor in the toroid-shaped space. However, the formed oil droplets are gradually moved towards the bottom surface of the recess 5,10 of the stator 1 or the rotor 2, whereafter the oil is guided out.

The first 16 and second 17 injection members are with advan- tage provided at constant intervals along the extension of the toroid-shaped space for counteracting the circulation of air be- tween the stator 1 and the rotor 2 along the whole extension of the toroid-shaped space.

The present invention is not in any way restricted to the above embodiment described in the drawings but may be modified freely within the scopes of the claims.