WO2006037408A1

WO2006037408A1 - Variable valve drive for an internal combustion engine

Info

Publication number: WO2006037408A1
Application number: PCT/EP2005/009488
Authority: WO
Inventors: Valeria Vacarescu; Eduard Golovatai-Schmidt
Original assignee: Schaeffler Kg
Priority date: 2004-10-02
Filing date: 2005-09-03
Publication date: 2006-04-13
Also published as: DE102004048072A1

Abstract

The invention relates to a variable valve drive comprising a device for producing an operation-dependent secondary stroke and a gas exchange valve (36) for exhaust gas return intake. Said valve is actively connected to a cam follower (2a, 2b, 2c) which is exposed to a cam (4) action and is supported by a hydraulic cushion (23) connected to an intermediate hydraulic line (19a, 19b) closable by a locking system (22). The cam (4) comprises a cam lobe (17) and a cam recess (18) with respect to a base circle (16), wherein said came lobe (17) is disposed outside of the base circle (16) and produces the primary stroke of a gas exchange valve (36), the cam recess (18) is disposed inside said base circle (16) and the cam follower (2a, 2b, 2c) penetrates into said cam recess while the elongation of the hydraulic cushion (23). In the operating mode of the internal combustion engine, the primary and secondary stroke of the gas exchange valve (36) are carried out when the cam follower (2a, 2b, 2c) emerges from the cam recess (18) on the base circle (16).

Description

Name of the invention

Variable valve train of an internal combustion engine

description

Field of the invention

The invention relates to a variable valve train of an internal combustion engine having a device which serves for a mode-dependent secondary stroke wenigs¬ least a gas exchange valve for an internal exhaust gas recirculation. A der¬ like trained valve train preferably forms the mechanical basis for the implementation of homogeneous auto-ignition combustion.

Background of the invention

A valve train with a mode-dependent secondary stroke of the gas exchange valve is already proposed for a device which is to serve as an engine brake in air-compressing internal combustion engines. Motor brakes are used as a safety-relevant supplement to the service brake usually as Dau¬ erbremse in commercial vehicles and are based on the principle that the drag torque of the notch engine in overrun and not befeuer¬ th by increasing the charge exchange work can be significantly increased and the vehicle is braked. For this purpose, the exhaust valve is opened again during the compression phase, so that the cylinder charge is not compressed in the manner of a gas spring, but is pushed into the outlet channel while being pushed out. Thus, DE 40 07 287 A1 describes an engine brake in which a roller tappet taps off a cam which, in addition to a conventional cam elevation and a base circle, has a cam indentation which, viewed in the cam direction, adjoins the cam elevation. The roller tappet can be hydraulically extended by having a chamber which fills with hydraulic fluid while the roller tappet passes through the cam recess. The inflow and outflow of hydraulic fluid to and from the chamber is controlled by a hydraulic valve, which is closed when the engine brake is activated in the phase in which the cam follower dives out of the cam indentation onto the base circle. In this case, the roller tappet generates an additional stroke during the compression phase. In the fired operation of the internal combustion engine, however, the chamber can be emptied again via the then open hydraulic valve, so that the roller tappet shortens when retracted from the cam indentation back to its original length and generates no additional stroke on the outlet valve.

Object of the invention

Compared with the known prior art, the invention has the object to provide a valve train, which allows an internal Rücksaugen of exhaust gas in sufficient quantity and with sufficiently accurate and fully variable adjustable dosage, in particular to operate an internal combustion engine with so-called homogeneous self-ignition ,

Summary of the invention

The subject of the present invention is therefore a valve drive which serves for exhaust gas recirculation. This is in particular the prerequisite for a Be¬ operation of the internal combustion engine with homogeneous and self-igniting La¬ training. Such a combustion method, also referred to as HCCI (Homogeneous Charge Compression Ignition), can be used primarily for self-ignited diesel internal combustion engines as well as for spark-ignited internal combustion engines, at least in partial load operation of the internal combustion engine, primarily for the purpose of reducing emissions. The combustion process, characterized by start of combustion, gravity and duration, maximum cylinder pressure rise and peak pressure, is determined in the HCCI process essentially by controlling the charge composition and the charge temperature profile. It turns out that in this Brenn¬ process a high charge temperature for controlling the ignition timing is desired. A very effective means for increasing the charge temperature is to increase the residual gas content, ie to increase the content of untrushed or purged exhaust gas returned to the cylinder of the previous combustion cycle in the cylinder charge for the next combustion cycle. In this case, the residual gas content must be able to be fully variably adjusted to the operating point of the internal combustion engine, whereby residual gas quantities of 60% of the cylinder charge and more may be required. Residual gas components can no longer be provided at this altitude via internal exhaust gas recirculation through conventional valve overlap or via an arrangement for external exhaust gas recirculation. Moreover, the HCCI process reacts with unacceptable combustion processes extremely sensitively to changes in the charge properties, so that in addition to the provision of residual gas in the required quantity, a combustion cycle-faithful, highly precise and cylinder-specific metering of the residual gas fraction is also required.

The features according to the invention provide a variable valve drive with at least one inlet and at least one outlet valve, of which at least one gas exchange valve is in operative connection with a cam follower loaded by at least one cam. This therefore also includes a valvetrain, in which the cam follower is designed switchable by coupling means and selectively transfers strokes of a plurality of cams depending on its coupling state to the gas exchange valve. But also valve trains, which continuously vary the stroke of the gas exchange valve by means of a cam and other Ver adjusting elements should be included in the scope of protection.

The cam follower is supported on a hydraulic cushion, which is connected to at least one closable by means of shut-off hydraulic fluid path. The cam has a Nocken¬ elevation and a Nockeneinformung relative to a base circle, wherein the cam lobe is outside the base circle and generates a primary stroke of the gas exchange valve and the cam indentation lies within the base circle into which the cam follower immerses when the hydraulic pad is hydraulically elongated.

In an operating mode in which the internal combustion engine is operated with the exhaust gas recirculation according to the invention, the primary stroke and the secondary stroke of the at least one gas exchange valve take place. The prerequisite for this is that the blocking means is closed as soon as the cam follower emerges from the cam recessing on the base circle. In this case, the hydraulic medium remains enclosed in the hydraulic cushion, and the base circle raised relative to the cam formation constitutes an additional cam lobe which generates the secondary stroke.

In another operating mode of the internal combustion engine, only the primary stroke of the at least one gas exchange valve takes place. In this case, the blocking means is opened as soon as the cam follower emerges from the cam indentation onto the base circle. In this operating mode, the gas exchange valve does not undergo a secondary stroke, since the hydraulic cushion is emptied via the opened shut-off means until the valve drive has a hydraulic length which corresponds to the base circle of the cam.

In particular, for an operation of the internal combustion engine in the homogeneous auto-ignition combustion method, it is necessary that the duration of the secondary stroke and thus the amount of recirculated exhaust gas fully variable with an arbitrary selectable time at which the shut-off is opened adjustable. Moreover, it is expediently provided according to claims 8 and 9, that the duration of the Sekundärhubs for each cylinder of Brenn¬ engine individually and recalculated for each revolution of the cam of a control unit and is newly adjustable.

In a variant according to claim 3, it is proposed that the secondary stroke takes place at an outlet valve which lies partially or completely within the stroke of the inlet valve. In this preferred embodiment, exhaust gas already exhausted into the exhaust passage during the intake stroke of Internal combustion engine sucked back into the combustion chamber via the then opened exhaust valve again.

However, according to claim 4, it may also be expedient that the secondary stroke takes place at an inlet valve which lies partially or completely within the stroke of the outlet valve. In this alternative embodiment, exhaust gas is expelled in Ausschiebetakt the internal combustion engine in the inlet channel and sucked back into the combustion chamber during the intake stroke.

A combination of these aforementioned possibilities is proposed in claim 5. Accordingly, to adjust the amount and temperature of the residual gas, it can be advantageous to suck both exhaust gas out of the inlet channel and out of the outlet channel.

A further advantageous embodiment of the invention provides according to claim 6, that closable by shut-off hydraulic fluid path serves as a drain of the hydraulic fluid from the hydraulic cushion and another hydraulic fluid path as an inlet of the hydraulic fluid to the hydraulic cushion. Due to the separation of inlet and outlet of the hydraulic medium, undesired pressure fluctuations, as they can occur in a hydraulic path which simultaneously serves as inlet and outlet, especially when a plurality of hydraulic pads are supplied, are effectively reduced.

According to claim 7, another expedient embodiment provides for the use of an electrohydraulic 2/2-way valve as a simple and inexpensive shut-off means for the hydraulic fluid path. However, it is also thought of the use of equally effective shut-off devices such as electrohydraulic Proportionalventile or electro-hydraulic 3/2-way valves thought.

In claim 10 it is proposed that the force of a spring means acts on the cam follower in the direction of the cam indentation. This supports the hydraulic elongation of the hydraulic cushion. The valve drive according to the invention can be found in various designs. It is thus clear from claim 11 that the cam follower is guided linearly and directly on the hydraulic cushion in a manner comparable to cup or roller ram drives. Alternatively, it is possible, according to claim 12, to use a lever-type cam follower and to arrange the hydraulic pad below a support bearing, which is located at an end of the cam follower remote from the gas exchange valve. In this case, this is preferably a low-friction roller rocker arm, which is rotatably mounted on a support element. According to claim 13, the hydraulic cushion but also in a plunger, which is located at a the gas exchange venti I zuge¬ facing end of the lever-like cam follower, be arranged.

In an expedient development of the invention, it is provided according to claim 14, that the at least one hydraulic fluid path with a Hydraulikmittel¬ conveying device whose output pressure level is higher than that of a Schmieröl¬ pump for the lubricating oil circuit of the internal combustion engine, is in communication. In this way, a supply of the hydraulic cushion is ensured with sufficient volume flow of hydraulic fluid, so that in the operating mode with exhaust gas recirculation even at higher speeds of the internal combustion engine, a fast and complete filling of the hydraulic cushion is guaranteed.

As hydraulic means, the lubricating oil of the internal combustion engine is used according to claim 15 for the sake of simplicity. Conversely, however, it is also conceivable to use any other suitable fluids in a hydraulic fluid circuit, which would then be separable from the lubricating oil circuit of the internal combustion engine.

The invention will be explained in more detail in the following embodiments.

Brief description of the drawings In accompanying drawings, the valve train according to the invention is shown schematically in various embodiments and by way of example for the Sekundär¬ stroke of an exhaust valve. Show it:

FIG. 1 shows a linearly moved cam follower at the time of

Closing the valve after passing through the cam elevation, wherein the hydraulic fluid path serves as inflow and outflow of hydraulic fluid to or from the hydraulic cushion,

Figure 2 shows the valve gear of FIG. 1 at the time in which the

Cam follower is completely immersed in the cam indentation,

FIG. 3 shows the valve drive from FIG. 1 at a time during the secondary stroke,

FIG. 4 shows the valve drive from FIG. 1 when the secondary stroke has ended,

FIG. 5 shows the linearly moved cam follower according to FIG. 1 with a hydraulic medium feed path and a hydraulic medium drainage path,

FIG. 6 shows a lever-like cam follower, the hydraulic pole being arranged below a support bearing, which is located at an end of the cam follower facing away from the gas exchange valve,

FIG. 7 shows a lever-like cam follower, the hydraulic pole being arranged below a support bearing, which is located at an end of the cam follower facing the gas exchange valve, FIG. 8 shows a control diagram of the internal combustion engine with a secondary stroke at an outlet valve.

Detailed description of the drawings

FIG. 1 shows a cam follower 2a in the form of a tappet 1a, which is arranged between a driver 3 and a cam 4 and is driven in a manner known per se by the cam 4 in the direction of actuation of a gas exchange valve 36. The plunger 1a and the driver 3 are mounted linearly movable in a trained as a passage opening 5 guide 6a of an internal combustion engine. Of course, can be provided as a cam contact surface instead of a rigidly connected to the Stö¬ ßel 1a and just, spherical or preferably cylindrically aus¬ formed cam contact surface 7a arranged on the plunger 1a, rotatably mounted.

The plunger 1 a has an end face 9 a on a side facing away from the cam 4, and a hollow-cylindrical recess 10 a on the end 8 a. An end face 12a and a hollow cylindrical recess 13a are located on an end 11 of the driver 3 facing the plunger 1a. Within the cavity 14a formed by the recesses 10a, 13a, there are hydraulic means as well as a spring means 15, which acts on the plunger 1a and the driver 3 at a distance and keeps the plunger 1a in contact with the cam 4.

The cam 4 rotates clockwise in the exemplary embodiment and has a cam elevation 17 and a cam formation 18 relative to a base circle 16. The cam elevation 17 lies outside the base circle 16 and generates a primary stroke of the gas exchange valve 36. The cam information 18 lies within the base circle 16.

In the angular position of the cam 4 shown in Fig. 1, the plunger 1a is located on the base circle 16 of the cam 4 and the gas exchange valve 36 is closed. In this position, the end face 9a of the plunger 1a and the end face 12a of the driver 3 are at the level of one in the guide 6a opening hydraulic fluid path 19a. This serves as an inflow and outflow of hydraulic fluid and has a hydraulic fluid delivery device 20 and a shut-off device 22 designed as a hydraulic valve 21.

In Fig. 2, the plunger 1a is completely immersed in the cam indentation 18 to form a hydraulic pad 23. The hydraulic cushion 23 consists of the plunger 1a and driver 3 as well as the in the hydraulic medium path 19a between the guide 6a and the hydraulic valve 21 located Hydraulik¬ medium. Amongst other things, it is essential for the rigidity of the hydraulic pad 23 that an inner circumferential surface 24 of the guide 6a comprises an outer lateral surface 25 of the plunger 1a and an outer lateral surface 26 of the driver 3 in a sealing-gap-like manner in order to prevent leakage of hydraulic fluid from the hydraulic pad 23 and On the other hand, to prevent penetration of gas bubbles in the hydraulic cushion 23 or at least to keep low.

After removal of the plunger 1a from the cam indentation 18, the cam contact surface 7a of the plunger 1a is located on the base circle 16, wherein in the illustration according to FIG. 3, a secondary stroke of the gas exchange valve 36 takes place with the hydraulic valve 21 closed. The secondary stroke is, as shown in Fig. 4, terminated when the hydraulic pad 23 has emptied so far about the then opened hydraulic valve 21, that the gas exchange valve 36 closes.

In FIG. 5, a hydraulic fluid path 19b which can be closed by the hydraulic valve 21 serves merely as a drain of the hydraulic fluid from a hydraulic pad 23. The supply of hydraulic fluid to the hydraulic pad 23 takes place via a further hydraulic fluid path 19c, in which the hydraulic fluid delivery device 20 and a throttle point 27 and to the hydraulic cushion 23 opening check valve 28 are arranged. The throttle point 27 ensures that the pressure of the hydraulic fluid downstream of the hydraulic fluid delivery device 20 does not collapse. In the embodiment according to FIG. 5, the hydraulic cushion 23 consists of between the plunger 1a and the driver 3 as well as in the hydraulic fluid path 19b between the guide 6a and the hydraulic valve 21 and located in the hydraulic fluid path 19c between guide 6a and check valve 28 hydraulic means.

In alternative embodiments of the valve gear according to the invention according to Figures 6 and 7, a cam follower 2b, 2c as a follower lever 29a, 29b with rotatably mounted roller 30 as cam contact surface 7b formed. In FIG. 6, the hydraulic pad 23 is arranged below a support bearing 32a designed as a support element 31, which pivotally supports the drag lever 29a and is located at an end 34 of the drag lever 29a facing away from the gas exchange valve 36. The support element 31 is arranged to be linearly movable in a guide 6b of the internal combustion engine designed as a blind bore 33 and has an end face 9b on an end 8b facing away from the follower lever 29a, which serves as an axial stop for supporting the support element 31 end face 12b of the blind bore 33 faces. Within a cavity 14b formed by a hollow-cylindrical recess 10b of the support element 31 and a hollow-cylindrical recess 13b of the blind bore 33 are hydraulic means as well as the support element 31 and the end face 12b of the blind bore 33 spaced-apart spring means 15, which is the roller 30 of the finger lever 29a in contact with the cam 4 stops.

FIG. 7 shows a further embodiment variant of the inventive valve drive with a cam follower 2c designed as a drag lever 29b. This variant corresponds functionally to the design of the valve drive according to FIG. 5, whereby the hydraulic pad 23 is arranged below a plunger 1b, which is located at an end 35 of the drag lever 29b facing the gas exchange valve 36. The drag lever 29b is pivotally supported on a support bearing 32b of the internal combustion engine.

The formation of the secondary lift of the gas exchange valve 36 depends on angular positions of the cam 4, to which the hydraulic valve 21 opens and closes. This relationship is illustrated in FIG. 8 on the basis of valve strokes "VL" in a schematic timing diagram of the internal combustion engine. for a designated with "SL-EX" secondary stroke on an exhaust valve. The primary lift of the exhaust valve designated PL-EX ends at "EXC" after the top dead center "TDC" of the internal combustion engine, while an intake valve is already opened with a primary stroke "PL-IN". A bottom dead center of the internal combustion engine is designated "BDC".

"HVC-L" is the latest possible angular position of the cam 4 at which the hydraulic valve 21 closes to enclose the hydraulic pad 23 and to produce a secondary lift of the exhaust valve. This corresponds to the angular position of the cam 4 at which the cam follower 2 a, 2 b, 2 c begins to dive out of the cam indentation 18 onto the base circle 16. A minimal secondary stroke in terms of duration and lifting height results when the hydraulic valve 21 reopens as soon as possible at "HVO-E", whereby in the limiting case the angular positions "HVC-L" and "HVO-E" are identical. The secondary stroke of the outlet valve ends at the latest at the angle position of the nozzle 4 in the case of "SLC", whereby termination of the secondary stroke is initiated at the latest at the angle position "HVO-L" by opening the hydraulic valve 21. With regard to a complete closing of the gas exchange valve 36 in "SLC", the hydraulic valve 21 may close as soon as possible again at "HVC-E" to ensure a sufficient shortening of the hydraulic pad 23.

In the event that no secondary stroke is to take place on the outlet valve, the hydraulic valve 21 is preferably permanently open in order to avoid sucking air into the hydraulic cushion 23, as might occur with the hydraulic valve 21 closed.

FIGS. 1-7 and the timing diagram of FIG. 8 also apply to a secondary stroke at the inlet valve or to a combination of secondary strokes at the inlet and outlet valve, given a corresponding modification of the angular position of the cam indentation 18.

The hydraulic fluid delivery device 20 shown in FIGS. 1-7 can be identical to the lubricating oil pump of the internal combustion engine. The hydraulic But likmittelfördereinrichtung 20 may also be connected in parallel to the lubricating oil pump or in series with this. In all of these cases, the hydraulic means for supplying the hydraulic cushion 23 is identical to the lubricating oil of the internal combustion engine. Another alternative embodiment is that the hydraulic fluid is a different fluid from the lubricating oil of the internal combustion engine. In this case, then the supply of the hydraulic pad 23 takes place in a hydraulic medium circuit separated from the lubricating oil circuit of the internal combustion engine. Unless the hydraulic fluid delivery device 20 is identical to the lubricating oil pump of the internal combustion engine, it is advantageous that the output pressure level of the hydraulic fluid delivery device 20 is higher than that of the lubricating oil pump. Thus, in the operating mode with exhaust gas recirculation, a quick and complete filling of the hydraulic pad 23 is ensured even at higher rotational speeds of the internal combustion engine.

List of reference numbers and symbols

a plunger 19c hydraulic fluid path b plunger 20 Hydraulikmittelför¬ a Nockenf ger ger of the device b cam follower 21 hydraulic valve c cam follower 22 shut-off

Driver 23 hydraulic pads

Cam 24 inner circumferential surface

Through hole 25 Outer jacket surface a Guide 26 Outer jacket surface b Guide 27 Throttle a Cam contact surface 28 Check valve b Cam contact surface 29a Cam follower a End 29b Cam follower b End 30 Roller a End surface 31 Support element b End surface 32a Support bearing 0a Recess 32b Support bearing 0b Recess 33 Blind hole 1 End 34 End 2a end face 35 end 2b end face 36 gas exchange valve 3a recess 3b recess 4a cavity 4b cavity 5 spring means 6 base circle 7 cam elevation 8 Nockeneinformung 9a hydraulic fluid path 9b hydraulic fluid path VL valve lift

TDC top dead center

BDC Bottom dead center PL-EX Primary lift of the exhaust valve

PL-IN primary lift of intake valve

SL-EX secondary lift of the exhaust valve

EXC exhaust valve closing time

HVO-E Earliest opening angle of the hydraulic valve HVO-L Latest opening angle of the hydraulic valve

HVC-E Earliest closing angle of the hydraulic valve

HVC-L Latest closing angle of the hydraulic valve

Claims

claims

1. A variable valve train of an internal combustion engine having a device which serves for a mode-dependent secondary stroke of at least one gas exchange valve (36) for an exhaust gas recirculation, preferably in a homogeneous auto-ignition combustion method, with the following features:

^■ the valve drive has at least two gas exchange valves (36), one of which is an intake valve and at least one of which is at least one of an exhaust valve;

^■ at least one of the at least two gas exchange valves (36) is operatively connected to a cam follower (2a, 2b, 2c) acted upon by at least one cam (4), which cam follower (2a, 2b, 2c) can be supported on a hydraulic pad (23) which is connected to at least one hydraulic medium path (19a, 19b) which can be closed by blocking means (22);

^■ the at least one cam (4) has, relative to a base circle (16), a cam lobe (17) and a cam recess (18), the cam lobe (17) lying outside the base circle (16) and having a primary stroke of the gas exchange valve (36) generated and the No¬ ckeneinformung (18) within the base circle (16), in which the cam follower (2a, 2b, 2c) dips in hydraulic elongation of the Hydraulik¬ pad (23);

^■ In an operating mode of the internal combustion engine, the primary stroke and the secondary stroke of at least one of the at least two gas exchange valves (36), wherein the blocking means (22) is closed as soon as the cam follower (2a, 2b, 2c) from the Nockeneinformung (18) on the Bases (16) emerges and wherein ^■ in another mode of the internal combustion engine, only the

Primary stroke of at least one of the at least two Gaswechsel¬ valves (36) takes place, wherein the blocking means (22) is opened as soon as the cam follower (2a, 2b, 2c) from the Nockeneinformung (18) the base circle (16) emerges.

2. Valve drive according to claim 1, wherein a duration of the Sekundärhubs with arbitrary selectable time of opening of the shut-off (22) is fully variable adjustable.

3. Valve gear according to claim 1, wherein the at least one of the at least two gas exchange valves (36) is an exhaust valve whose secondary stroke is partially or completely within the stroke of the intake valve.

4. Valve gear according to claim 1, wherein the at least one of the at least two gas exchange valves (36) is an inlet valve, the secondary stroke is partially or completely within the stroke of the exhaust valve.

5. The valve train according to claim 1, wherein both the intake valve and the exhaust valve perform the secondary stroke, wherein the secondary lift of the intake valve is partially or fully within the Primärhubs the exhaust valve and the secondary lift of the exhaust valve partially or completely within the Primärhubs the intake valve.

6. Valve drive according to claim 1, wherein the closing means (22) ver¬ closable hydraulic fluid path (19b) as a drain of the hydraulic fluid from the hydraulic cushion (23) and a further hydraulic fluid path (19c) as an inlet of the hydraulic fluid to the hydraulic cushion (23), wherein the Feed has at least one throttle point (27) and a ter to the Hydraulikpols¬ ter (23) opening check valve (28).

7. Valve drive according to claim 1, wherein the shut-off means (22) is an electro-hydraulic 2/2-way valve.

8. Valve drive according to claim 2, wherein the duration of the Sekundärhubs for each revolution of the cam (4) re-calculated by a control unit and is newly adjustable.

9. Valve drive according to claim 8, wherein in the case of a multi-cylinder internal combustion engine, the duration of the Sekundärhubs cylinder individually calculated by the control unit and is adjustable.

10. Valve drive according to claim 1, wherein the cam follower (2a, 2b, 2c) by the force of a spring means (15) in the direction of the cam (4) beauf¬ beat.

11. Valve drive according to claim 1, wherein the cam follower (2a) is guided linearly and directly on the hydraulic cushion (23) supportable.

12. Valve drive according to claim 1, wherein the cam follower (2b) is lever-like and the hydraulic cushion (23) below a Abstützlagers (32a), which at a gas exchange valve (36) facing away from the end (37) of the

Cam follower (2b) is arranged.

13. Valve drive according to claim 1, wherein the cam follower (2 c) is lever-like and the hydraulic cushion (23) below a plunger (1 b), which at a gas exchange valve (36) facing the end (34) of the cam follower (2 c) , is arranged.

14. Valve drive according to claim 1, wherein the internal combustion engine has a lubricating oil circuit with a lubricating oil pump whose output pressure level is lower than that of a separately provided hydraulic fluid conveying device.

15. Valve drive according to claim 13, wherein the hydraulic fluid is lubricating oil of the internal combustion engine.