EP2226477B1 - Hydraulic unit for a cylinder head of a combustion engine with hydraulically variable gas exchange valve drive - Google Patents

Abstract

The hydraulic unit (5) has drive-side master and slave units (6, 8), low-and medium-pressure chambers (12, 16), and a controllable hydraulic valve (10) extending in a bottom part (22) of a hydraulic housing (21). A valve body blocks a throttle point (17') in a position corresponding to a hydraulic-flow from the medium-pressure chamber into a low-pressure chamber up to a throttling through-flow cross section. The body opens a low-throttle through-flow cross section in another position corresponding to a flow of hydraulic medium from the low-pressure chamber into the medium-pressure chamber.

Description

Field of the invention

• ■ zumindest eine antriebseitige Gebereinheit,
• ■ zumindest eine abtriebseitige Nehmereinheit,
• ■ zumindest ein ansteuerbares Hydraulikventil,
• ■ zumindest einen Mitteldruckraum,
• ■ zumindest einen Hochdruckraum, der im Übertragungssinn zwischen der zugehörigen Gebereinheit und der zugehörigen Nehmereinheit angeordnet und über das zugehörige Hydraulikventil mit dem zugehörigen Mitteldruckraum verbindbar ist,
• ■ zumindest einen als Hydraulikmittelreservoir dienenden Niederdruckraum, der über eine Drosselstelle mit dem zugehörigen Mitteldruckraum verbunden ist,
• ■ und ein Hydraulikgehäuse mit einem Gehäuseunterteil, einem Gehäusezwischenteil und einem Gehäuseoberteil,

wobei die Gebereinheit, die Nehmereinheit, der Hochdruckraum, das Hydraulikventil und der Mitteldruckraum im Gehäuseunterteil verlaufen, der Niederdruckraum im Gehäuseoberteil ausgebildet ist und die Drosselstelle im Bereich eines Hydraulikmitteldurchlasses durch das Gehäusezwischenteil verläuft.The invention relates to a hydraulic unit for a cylinder head of an internal combustion engine with hydraulically variable gas exchange valve train. The hydraulic unit comprises:

• At least one drive-side transmitter unit,
• At least one output-side receiving unit,
• At least one controllable hydraulic valve,
• ■ at least one medium-pressure space,
• At least one high-pressure space, which is arranged in the direction of transmission between the associated transmitter unit and the associated slave unit and can be connected to the associated medium-pressure chamber via the associated hydraulic valve,
• At least one low-pressure space serving as a hydraulic medium reservoir, which is connected via a throttle point to the associated medium-pressure space,
• ■ and a hydraulic housing with a housing lower part, a housing intermediate part and a housing upper part,

wherein the transmitter unit, the receiver unit, the high-pressure chamber, the hydraulic valve and the medium-pressure chamber run in the lower housing part, the low-pressure chamber is formed in the upper housing part and the throttle point extends in the region of a hydraulic medium passage through the intermediate housing part.

Background of the invention

Such a hydraulic unit is from the unpublished EP 2 060 754 A2 (State of the art according to Art. 54 (3) EPC). In the hydraulic unit proposed there are all essential, required for the hydraulically variable transmission of cam lobes on the gas exchange valves components and the pressure chambers in a common hydraulic housing in sandwich construction summarized. The lower housing part is formed very compact design, and in the intermediate housing part is also a substantially flat plate, so that each of the intermediate pressure chambers is limited to a correspondingly small volume.

As explained in the cited document, however, a small-volume medium-pressure chamber during the starting process of the internal combustion engine can be problematic, especially if it is a starting operation at low ambient temperatures and after a long period of stoppage of the internal combustion engine. This is due to the fact that the hydraulic fluid supply of the internal combustion engine still does not promote sufficient hydraulic fluid flow into the medium-pressure space during start-up and only the hydraulic fluid volume remaining in the medium-pressure space and shrunk at low temperatures is insufficiently large for a complete refilling of a then-expanding high-pressure space. This problem applies to a greater extent in a short time repetitive starting operations, since in this case the hydraulic fluid consumption from the medium-pressure space can be greater than the nachgeförderte from the hydraulic fluid supply to the internal combustion engine volume. Such multi-start operations are typical, for example, for taxi cabs at taxi stands.

To solve this problem, it is proposed in the cited document to form in the upper housing part serving as a hydraulic fluid reservoir low-pressure chamber, which has a throttle point in the intermediate housing part with the medium-pressure space connected is. With the help of the low-pressure chamber on the one hand during the starting process of the engine required hydraulic fluid reservoir for the medium-pressure space and thus expanded for the high-pressure chamber and on the other hand eliminates the risk of suction of gas bubbles as far as possible. The latter results from the intermediate housing part, which separates the low-pressure chamber from the medium-pressure chamber, so that during the stoppage phase of the internal combustion engine and thereby cooling and thus shrinking hydraulic fluid, the formation of gas bubbles in the medium-pressure space is prevented by suction of hydraulic fluid from the low pressure chamber.

The throttle point proposed in the cited document is designed as a stepped bore through the intermediate housing part with a small diameter amounting to only a few tenths of a millimeter. However, such a throttle may be disadvantageous in several respects. In particular, the rigid orifice has a flow rate independent flow characteristic with strong throttling in both directions, which is particularly pronounced in cold, i. highly viscous hydraulic fluid prevents rapid refilling of the medium-pressure space. In addition, in hydraulic fluid bores with a very small diameter, the increased risk of clogging with dirt particles in the form of manufacturing residues or abrasion during operation of the internal combustion engine. In addition, the production of small hydraulic fluid hole is associated with considerable effort. For example, in the case of a machined bore with high tool wear or frequent tool failure to be expected, while the production by means of laser beam leads to undesirably high shape and cross-sectional deviations of the desired geometry of the throttle point.

Another hydraulic unit is off DE 10 2006 008 676 A1 known.

Object of the invention

The present invention is therefore based on the object, in particular to further develop a hydraulic unit of the type mentioned above, that the medium-pressure chamber during cold start of the internal combustion engine is a both sufficiently large and sufficiently quickly available hydraulic fluid reservoir to the side.

Summary of the invention

The solution of this problem arises from the characterizing features of claim 1, while advantageous developments and refinements of the invention are the dependent claims can be removed. Accordingly, it is provided that the throttle point is formed by means of a displaceable relative to the hydraulic medium passage valve body and depending on the position of the valve body has different sized flow cross-sections. In this case, in its first position corresponding to the hydraulic fluid flow from the medium-pressure space into the low-pressure space, the valve body blocks the throttle point down to a throttling flow cross-section and releases a low-throttle flow cross-section in its second position corresponding to the hydraulic fluid flow from the low-pressure space into the medium-pressure space. In other words, the displaceable valve body allows a flow characteristic dependent on the flow direction, so that the hydraulic fluid transfer in the direction of the low-pressure chamber is still throttled, but in the opposite direction to the medium pressure space, but largely low resistance. In addition, eliminates the risk of clogging of the throttle point by dirt particles with the rigid and low-cross-sectional hydraulic fluid bore.

In a further development of the invention, the valve body should extend partially or completely in the hydraulic medium passage and be supported by stops on the housing intermediate part, which stops define the first and second position of the valve body. In the event that the valve body is a valve plate or has such, should be the first layer of the valve body defining stop a medium pressure chamber facing first surface on the intermediate housing part and form the valve plate together with the first surface a plate valve, wherein the throttling flow cross section through one or more bead-shaped depressions on the valve plate and / or the first surface is formed.

Compared with the stepped bore proposed in the cited prior art, whose throttling effect comes close to the properties of a viscous-independent diaphragm, the throttling action in bead-shaped depressions is much more dependent on the viscosity of the hydraulic fluid due to their relatively large length-cross-section ratio. This property is particularly advantageous if the upper housing part is provided with an overflow opening into the cylinder head. This not only serves to vent the low pressure chamber, but also the cooling of the hydraulic unit by escaping heated hydraulic fluid via low pressure chamber in the cylinder head and thus can be returned to the cooled hydraulic fluid circuit of the engine. Here, the viscosity-dependent throttling effect of the bead-shaped recesses causes a needs-based flushing of the hydraulic unit, which is ideally such that when hot hydraulic fluid flushing the largest possible and cold hydraulic fluid does not rinse the hydraulic unit.

In a constructive embodiment of the invention, one or two in the hydraulic medium passage attached and each end face of the stops for the valve plate forming jacks are provided. Alternatively, the valve body should have holding claws extending from the valve plate, starting through the hydraulic medium passage and extending over a low pressure chamber facing the second surface on the intermediate housing part. The second surface serves as the second position of the valve body defining stop. Such a valve body can be made particularly cost effective as an injection molded part made of plastic.

There is also the possibility that the valve body is a ball and the hydraulic medium passage has the shape of a spherical cap opening in the direction of the medium-pressure space. In this case, the throttling flow cross-section is by a bead-shaped extending in the axial direction of the spherical cap Groove formed on the inner circumferential surface of the hydraulic medium passage.

To hold the ball of the second position of the ball defining stop should be formed by one or more extending into the hydraulic medium passage material projections on the intermediate housing part. Preferably, three material projections distributed uniformly over the inner circumferential surface of the hydraulic medium passage are provided, which may also be produced by caulking the intermediate housing part.

Brief description of the drawings

Figur 1

eine schematische Darstellung eines hydraulisch variablen Gaswech- selventiltriebs;

Figur 2

die erfindungsgemäße Drosselstelle als Hydrauliksymbol;

Figur 3

eine Hydraulikeinheit in perspektivischer Gesamtdarstellung;

Figur 4

einen Querschnitt der Hydraulikeinheit gemäß Figur 3;

Figur 5

eine Drosselstelle mit Plattenventil gemäß Figur 4 in vergrößerter Schnittdarstellung (1. Seitenansicht);

Figur 6

die Drosselstelle gemäß Figur 4 in vergrößerter Schnittdarstellung (2. Seitenansicht);

Figur 7

den Ventilkörper gemäß Figur 4 in vergrößerter Perspektivdarstel- lung;

Figur 8

eine Drosselstelle mit Kugel in vergrößerter Schnittdarstellung;

Figur 9

den Schnitt A-A gemäß Figur 8;

Figur 10

eine Drosselstelle mit Plattenventil und Buchsen in vergrößerter Schnittdarstellung;

Figur 11

die obere Buchse gemäß Figur 10 in vergrößerter Perspektivdarstel- lung und

Figur 12

die untere Buchse gemäß Figur 10 in vergrößerter Perspektivdarstel- lung.

Further features of the invention will become apparent from the following description and from the drawings, in which embodiments of the invention are shown. Unless otherwise stated, the same or functionally identical features or components are provided with the same reference numbers. Show it:

FIG. 1

a schematic representation of a hydraulically variable gas exchange selventiltriebs;

FIG. 2

the throttle restriction according to the invention as a hydraulic symbol;

FIG. 3

a hydraulic unit in an overall perspective view;

FIG. 4

a cross section of the hydraulic unit according to FIG. 3 ;

FIG. 5

a throttle point with plate valve according to FIG. 4 in enlarged sectional view (1st side view);

FIG. 6

the throttle point according to FIG. 4 in enlarged sectional view (2nd side view);

FIG. 7

the valve body according to FIG. 4 in enlarged perspective view;

FIG. 8

a throttle point with ball in an enlarged sectional view;

FIG. 9

the section AA according to FIG. 8 ;

FIG. 10

a throttle point with plate valve and sockets in an enlarged sectional view;

FIG. 11

the upper bush according to FIG. 10 in an enlarged perspective view and

FIG. 12

the lower socket according to FIG. 10 in an enlarged perspective view.

Detailed description of the drawings

• ■ eine antriebseitige Gebereinheit 6, hier in Form eines vom Nocken 3 angetriebenen Pumpenstößels 7,
• ■ eine abtriebseitige Nehmereinheit 8, hier in Form eines das Gaswechselventil 4 unmittelbar betätigenden Nehmerkolbens 9,
• ■ ein ansteuerbares Hydraulikventil 10, hier in Form eines elektromagnetischen 2-2-Wege-Schaltventils,
• ■ einen zwischen der Gebereinheit 6 und der Nehmereinheit 8 verlaufenden Hochdruckraum 11, aus dem bei geöffnetem Hydraulikventil 10 Hydraulikmittel in einen Mitteldruckraum 12 abströmen kann,
• ■ ein an den Mitteldruckraum 12 angeschlossener Druckspeicher 13 mit einem federkraftbeaufschlagten Ausgleichskolben 14,
• ■ ein in Richtung des Mitteldruckraums 12 öffnendes Rückschlagventil 15, über das die Hydraulikeinheit 5 an den Hydraulikmittelkreislauf der Brennkraftmaschine angeschlossen ist,
• ■ und einen als Hydraulikmittelreservoir dienenden Niederdruckraum 16, der über eine Drosselstelle 17 in einer den Niederdruckraum 16 vom Mitteldruckraum 12 separierenden Trennwand 18 mit dem Mitteldruckraum 12 verbunden ist.

In FIG. 1 the basic structure of a hydraulically variable gas exchange valve drive 1 is schematically disclosed. Shown is a section of a cylinder head 2 of an internal combustion engine with a cam 3 of a camshaft and a spring-loaded gas exchange valve 4 which is essential for understanding the invention. The variability of the gas exchange valve drive 1 is generated by means of a hydraulic unit 5 arranged between the cam 3 and the gas exchange valve 4. the following components include:

• A drive-side transmitter unit 6, here in the form of a pump tappet 7 driven by the cam 3,
• ■ an output side slave unit 8, here in the form of a gas piston valve 4 directly actuated slave piston 9,
• ■ a controllable hydraulic valve 10, here in the form of an electromagnetic 2-2-way switching valve,
• ■ a high-pressure space 11 extending between the transmitter unit 6 and the receiver unit 8, from which hydraulic fluid can flow into an intermediate-pressure space 12 when the hydraulic valve 10 is open,
• ■ a pressure accumulator 13 connected to the medium pressure chamber 12 with a compensating piston 14 which is acted upon by a spring force,
• ■ a non-return valve 15 which opens in the direction of the medium-pressure chamber 12 and via which the hydraulic unit 5 is connected to the hydraulic fluid circuit of the internal combustion engine,
• ■ and serving as a hydraulic fluid reservoir low-pressure chamber 16 which is connected via a throttle point 17 in a the low pressure chamber 16 from the central pressure chamber 12 separating partition 18 with the central pressure chamber 12.

The known manner of operation of the hydraulic gas exchange valve drive 1 can be summarized to the effect that the high pressure chamber 11 between the transmitter unit 6 and the slave unit 8 acts as a hydraulic linkage, which - neglecting leaks - proportional to the stroke of the cam 3 displaced by the pump plunger 7 hydraulic volume as a function of the opening time and the opening duration of the hydraulic valve 10 is split into a first, the slave piston 9 acting sub-volume and in a second, in the medium pressure chamber 12 including pressure accumulator 13 effluent partial volume. As a result, the stroke transmission of the pump tappet 7 to the slave piston 9 and therefore not only the timing, but also the lifting height of the gas exchange valve 4 are fully variable adjustable.

FIG. 2 shows the throttle point 17 as a hydraulic symbol. Essential for the invention is the existence of a displaceable valve body 19 by which the flow of hydraulic fluid from the medium-pressure chamber 12 into the low-pressure chamber 16 is throttled significantly more strongly than in the opposite direction. As already mentioned in the introduction and implemented in the embodiments explained below, a viscosity-dependent throttling effect of the throttle body 17 is particularly advantageous if the low-pressure chamber 16 with a in the Cylinder head opening overflow 20 is provided (see FIG. 1 ). The overflow 20 serves not only to vent the low-pressure chamber 16, but also to cool the hydraulic unit 5, in that heated hydraulic fluid can escape via low-pressure chamber 16 into the cylinder head 2 and can therefore be returned to the cooled hydraulic fluid circuit of the internal combustion engine. The viscosity-dependent throttling action of the throttle point 17 causes a needs-based flushing of the hydraulic unit 5: in the ideal case theoretically, the highest possible flushing takes place with hot hydraulic fluid and no flushing of the hydraulic unit 5 with cold hydraulic fluid.

As it is described below FIGS. 3 and 4 becomes clear, the hydraulic unit 5 has a common hydraulic housing 21 in order to mount the hydraulic unit 5 as a preassembled and optionally already filled with hydraulic fluid assembly in the cylinder head 2 of the internal combustion engine. The executed for a 4-cylinder inline hydraulic unit 5 is in general view FIG. 3 out. The composite in sandwich construction hydraulic housing 21 consists of a lower housing part 22, formed as intermediate housing part 23 partition wall 18 and a housing top 24. While the housing parts 22, 23, 24 are screwed together at various Verschraubungspunkten 25 hydraulically sealing, the lower housing part 22 separate Verschraubungspunkte 26 Attaching the entire hydraulic unit 5 in the cylinder head 2 of the internal combustion engine.

The four transmitter units 6 each include a seated in the housing part 22 support member 27, a pivotally mounted thereon drag lever 28 with rotatably mounted roller 29 for a low-friction cam and the here actuated by rocker arm 28 and spring-loaded in the return stroke pump tappet 7 from the intermediate housing part 23 outgoing bracket 30 serve as a captive for the drag lever 28 when not mounted in the cylinder head 2 hydraulic unit 5. This is further designed so that each of the transmitter units 6 with two slave units 8 (see also FIG. 1 ) cooperates. In other words, for every pair gleichwirkender gas exchange valves 4, ie intake valves or exhaust valves of a cylinder of the internal combustion engine, only a cam 3 and a transmitter unit 6 is required, wherein the displaced by the pump plunger 7 hydraulic volume both slave units 8 simultaneously applied. On the side facing the transmitter units 6 side of the hydraulic unit 5 are each a transmitter unit 6 and the two slave units 8 associated hydraulic valves 10 can be seen with electrical connectors 31, wherein the open in the de-energized state hydraulic valves 10 in a known per se and not shown here manner are mounted in valve seats in the housing base 22.

The already in FIG. 3 On the basis of the bulges in the upper housing part 24 recognizable low pressure chambers 16 go clearly from the in FIG. 4 shown cross section through the hydraulic unit 5 before. In this cross section, the pressure accumulator 13 connected to the medium-pressure chamber 12 can be seen with the compensating piston 14, which is acted on by spring force. Although only one throttle point 17 'is shown, each of the medium-pressure chambers 12 can also communicate with the associated low-pressure chamber 16 via two or more throttle points 17'. Conversely, it would also be conceivable to associate each of two medium-pressure chambers 12 with two or more low-pressure chambers 16 separated from one another.

Both gas bubbles that pass through the throttling point 17 'from the medium-pressure chamber 12 into the low-pressure chamber 16 during operation of the internal combustion engine, as well as excess hydraulic fluid can be deposited in the interior of the cylinder head 2 via the overflow 20 extending in the upper housing part 24 and opening into the cylinder head 2 become.

In order to prevent a loss of hydraulic fluid from the low pressure chamber 16, in particular during the stoppage phase of the internal combustion engine, the upper housing part 24 is coated with a sealing material made of elastomer material, not shown here. In the illustrated embodiment, this coating is not limited only to the contact area to the intermediate housing part 23, but is located on the entire surface of here For sealing the joints between the lower housing part 22 and intermediate housing part 23 on the one hand and between intermediate housing part 23 and upper housing part 24 on the other hand, separate flat gaskets, such as single or multi-layered metal gaskets can be used in addition or as an alternative to E-lastomerbeschichtung ,

In the following explained FIGS. 5 to 12 three embodiments of the throttle restriction 17 according to the invention are illustrated. This runs in each case in the region of a per se low-throttle hydraulic fluid passage 32 through the intermediate housing part 23 and is formed by the partially or completely arranged in the hydraulic medium passage 32 and relative to this displaceable valve body 19. As already in FIG. 2 symbolically represented, the throttle body 17 depending on the position of the valve body 19 of different size flow cross sections, wherein the valve body 19 in its the hydraulic fluid flow from the medium pressure chamber 12 in the low pressure chamber 16 corresponding first position the throttle body 17 blocks up to a throttling flow cross section and in his the hydraulic fluid flow from the low pressure chamber 16 in the middle pressure chamber 12 corresponding second layer releases a low-throttle flow cross-section. The valve body 19 is respectively supported by stops, which define the first and second position of the valve body 19, on the housing intermediate part 23.

Both FIGS. 5 to 7 it is an enlarged view of the in FIG. 4 contained restrictor 17 'with valve body 19'. This is an injection-molded plastic part with a valve plate 33 and outgoing retaining claws 34, which are guided by elastic deformation through the hydraulic medium passage 32 therethrough. As the first position of the valve body 19 'defining stop is a medium pressure chamber 12 facing the first surface 35 on the intermediate housing part 23, here the underside, with which the valve plate 33 forms a plate valve (see FIG. 5 ). The throttling flow cross-section is formed by bead-shaped depressions 36 'on the valve plate 33. Depending on the desired viscosity dependence of the generated Throttling effect are compared to the straight recesses 36 'other geometries conceivable, such as a spiral depression of small cross-section and long in the case of a very high viscosity dependence. The holding claws 34 extend over a second surface 37 facing the low-pressure chamber 16 on the intermediate housing part 23, in this case the upper side thereof, which serves as stop which supports the retaining claws 34 and thus defines the second position of the valve body 19 '. As can be clearly seen in FIG. 6, the throttle point 17 'in this second position has a comparatively large, ie throttle-poor cross-section, due to the then open plate valve.

An alternative choke point 17 "goes out of the FIGS. 8 and 9 out. The valve body 19 "is a ball, and the hydraulic medium passage 32 in the intermediate housing part 23 has the shape of a spherical cap opening in the direction of the medium-pressure space 12. The throttling flow cross-section is formed by a bead-shaped recess 36" extending in the axial direction of the spherical cap on the inner lateral surface of the ball Hydraulic fluid passage 32 formed. While the spherical cap at the same time serves as the first position of the ball 19 "defining stop and the hydraulic fluid flow in the direction of the low pressure space can only be done via the bead-shaped recess 36", the second position of the ball 19 "defining stop by three material protrusions 38 on the intermediate housing part 23rd In this second position, the hydraulic fluid flow in the direction of the medium-pressure space is the entire surface of the ball 19 "with correspondingly low throttling available. The material protrusions 38 extending into the hydraulic medium passage 32 are formed by caulking the intermediate housing part 23 and are uniformly distributed over the inner circumferential surface of the hydraulic medium passage 32.

Another alternative restrictor 17 "'is in the FIGS. 10 to 12 shown. The valve body 19 "'is in this case designed as a disc-shaped valve plate, which is arranged in play-like manner between two bushings 39, 40 pressed into the hydraulic medium passage 32. The bushes 39, 40, each on the front side The upper bush 40 forms with the valve plate 19 "'a plate valve, wherein the throttling flow cross-section through four bead-shaped depressions 36"' on the medium pressure chamber 12 facing first surface 35 of the The lower bushing 39 is provided on its second surface 37, which faces the low-pressure chamber 16, with circular-arc interruptions 41, which provide a sufficiently low-throttle flow cross-section in the second position of the valve plate 19 "'.

List of reference numbers

1

Gas exchange valve train

2

cylinder head

3

cam

4

Gas exchange valve

5

hydraulic unit

6

transmitter unit

7

pump plunger

8th

receiver unit

9

slave piston

10

hydraulic valve

11

High-pressure chamber

12

Medium-pressure chamber

13

accumulator

14

balance piston

15

check valve

16

Low-pressure chamber

17

restriction

18

partition wall

19

valve body

20

overflow

21

hydraulic housing

22

Housing bottom

23

Intermediate housing part

24

Housing top

25

screw fixing

26

screw fixing

27

supporting

28

cam follower

29

role

30

hanger

31

Connector of the hydraulic valve

32

Hydraulic fluid passage

33

valve plate

34

Retaining claw

35

first surface on the housing intermediate part

36

bead-shaped depression

37

second surface on the housing intermediate part

38

Material projection on the housing intermediate part

39

Rifle

40

Rifle

41

interruption

Claims (10)

1. Hydraulic unit (5) for a cylinder head (2) of an internal combustion engine having a hydraulically variable gas-exchange valve timing mechanism (1), comprising

   ■ at least one drive-side transmitter unit (6),

   ■ at least one output-side receiver unit (8),

   ■ at least one actuable hydraulic valve (10),

   ■ at least one medium-pressure space (12),

   ■ at least one high-pressure space (11) which is arranged in the transmission direction between the associated transmitter unit (6) and the associated receiver unit (8) and can be connected to the associated medium-pressure space (12) via the associated hydraulic valve (10),

   ■ at least one low-pressure space (16) which is connected to the associated medium-pressure space (12) via a throttle point (17, 17', 17'', 17"'),

   ■ and a hydraulic housing (21) having a lower housing part (22) and an intermediate housing part (23),

   the transmitter unit (6), the receiver unit (8), the high-pressure space (11), the hydraulic valve (10) and the medium-pressure space (12) extending in the lower housing part (22), characterized in that the hydraulic housing (21) has an upper housing part (24), the low-pressure space (16) is formed in the upper housing part (24) and serves as hydraulic-medium reservoir, and the throttle point (17, 17', 17'', 17''') extends through the intermediate housing part (23) in the region of a hydraulic-medium passage (32), the throttle point (17, 17', 17", 17"') being formed by means of a valve body (19, 19', 19", 19"') which can be displaced relative to the hydraulic-medium passage (32) and having throughflow cross sections of different size depending on the position of the valve body (19, 19', 19", 19"'), the valve body (19, 19', 19'', 19''') shutting the throttle point (17, 17', 17", 17"') as far as a throttling throughflow cross section in its first position which corresponds to the hydraulic-medium flow from the medium-pressure space (12) into the low-pressure space (16), and the valve body (19, 19', 19", 19"') opening a low-throttling throughflow cross section in its second position which corresponds to the hydraulic-medium flow from the low-pressure space (16) into the medium-pressure space (12).
2. Hydraulic unit (5) according to Claim 1, characterized in that the valve body (19', 19'', 19"') extends partially or completely in the hydraulic-medium passage (32) and is secured by stops on the intermediate housing part (23), which stops define the first and second positions of the valve body (19', 19", 19"').
3. Hydraulic unit (5) according to Claim 2, characterized in that the valve body (19', 19"') is or has a valve plate (33, 19"'), the stop which defines the first position of the valve body (19', 19"') is a first surface (35) on the intermediate housing part (23), which first surface (35) faces the medium-pressure space (12), and, together with the first surface (35), the valve plate (19"', 33) forms a plate valve, the throttling throughflow cross section being formed by one or more crescent-shaped depressions (36', 36"') on the valve plate (33, 19"') and/or the first surface (35).
4. Hydraulic unit (5) according to Claim 3, characterized in that one or two bushes (39, 40) is/are provided which is/are fastened in the hydraulic-medium passage (32) and in each case forms/form one of the stops for the valve plate (19''') on the end side.
5. Hydraulic unit (5) according to Claim 3, characterized in that the valve body (19') has holding claws (34) which, starting from the valve plate (33), extend through the hydraulic-medium passage (32) and extend over a second surface (37) on the intermediate housing part (23), which second surface (37) faces the low-pressure space (16) and serves as the stop which defines the second position of the valve body (19').
6. Hydraulic unit (5) according to Claim 5, characterized in that the valve body (19') is an injection-moulded part made from plastic.
7. Hydraulic unit (5) according to Claim 2, characterized in that the valve body (19") is a ball and the hydraulic-medium passage (32) has the shape of a spherical cap which opens in the direction of the medium-pressure space (12), the throttling throughflow cross section being formed by a crescent-shaped depression (36") on the inner circumferential face of the hydraulic-medium passage (32), which crescent-shaped depression (36") extends in the axial direction of the spherical cap.
8. Hydraulic unit (5) according to Claim 7, characterized in that the stop which defines the second position of the ball (19") is formed by one or more material projections (38) on the intermediate housing part (23), which material projections (38) extend into the hydraulic-medium passage (32).
9. Hydraulic unit (5) according to Claim 8, characterized in that three material projections (38) are provided which are distributed uniformly over the inner circumferential face of the hydraulic-medium passage (32).
10. Hydraulic unit (5) according to Claim 8, characterized in that the material projections (38) are produced by caulking of the intermediate housing part (23).

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