Valve drive with additional lift in the cam base circle

A valve drive of an internal combustion engine, having a finger lever which acts at one end on a gas exchange valve and which is seated at the other end on a head of a housing of a support element, which housing runs in an axially movable manner in a reception bore of a cylinder head, the finger lever is acted on by a cam from the base circle from which there projects in a fixed manner an additional cam piece for generating an additional lift of the gas exchange valve, a bore extends from a bottom of the housing in the direction of the head, in the lower portion of which bore there runs a pressure piston which is axially movable relative to the housing and via the lower face of which the support element is mounted on a base of the reception bore of the cylinder head, a high-pressure chamber for hydraulic medium is formed between an upper face of the pressure piston and a ring part situated above said upper face and which is fixed with respect to the housing, and to generate the additional lift, the high-pressure chamber is flooded with hydraulic medium such that the housing is deployed relative to the pressure piston, supported on the hydraulic medium column accumulated in the high-pressure chamber, and to deactivate the additional stoke, the hydraulic medium in the high-pressure chamber is discharged such that the housing is retracted.

BACKGROUND

The invention concerns a valve train of an internal combustion engine, said valve train comprising a cam follower acting on at least one gas exchange valve, said cam follower being loaded by a cam from whose base circle an optionally actuable additional cam piece for generating an additional lift of the gas exchange valve projects.

DE 30 03 566 A1 discloses a valve train whose camshaft comprises a control shaft that is axially displaceable in the camshaft. An additional cam projects out of the base circle of a cam of the camshaft illustrated inFIG. 1. This additional cam is received in a pocket of the cam for a radially outward movement out of the pocket. For obtaining an additional lift on the outlet valve, the control shaft can be displaced in axial direction such that a radial collar of the control shaft causes an outward movement of the additional cam.

A drawback of the aforesaid device is the immense structural complexity that leads to high costs. The camshaft must have a hollow configuration and comprise the additional control shaft. A complex and expensive operating device becomes necessary. In addition, it must be assured that the additional cam is not lost during phases of high rotational speed of the camshaft.

SUMMARY

It is therefore an object of the invention to provide a valve train comprising an additional lift in the cam base circle as stated above, but without the aforesaid drawbacks. In particular, it must be possible to create the additional lift in the cam base circle for realizing a decompression or an internal exhaust gas return without complicated modifications to the existing valve train and cylinder head designs.

The invention achieves the above object by the fact that the cam follower is a finger lever that acts at on end on at least one gas exchange valve and is seated at another end via a contact piece on a head of a housing of a support element, which housing extends for axial movement in a reception bore of a cylinder head, wherein a bore extends from a bottom of the housing in direction of the head, a pressure piston that is axially movable relative to the housing extends in a lower section of the bore, the support element being mounted through a lower front end of the pressure piston on a base of the reception bore of the cylinder head, wherein a high pressure chamber for hydraulic medium is formed between an upper front end of the pressure piston and a ring part that is fixedly arranged on the housing and is situated above the upper front end, wherein for generating the additional lift, the high pressure chamber can be flooded with hydraulic fluid such that the housing moves outwards relative to the pressure piston and is supported on the accumulated hydraulic medium column, and wherein for deactivating the additional lift, the high pressure of the hydraulic medium in the high pressure chamber can be reduced such that the housing retreats relative to the pressure piston.

Thus, a valve train particularly for realizing a decompression or an exhaust gas return is created that is free of the aforesaid drawbacks. Preferably it is intended to use the valve train in connection with an outlet valve but its use with an inlet valve is likewise imaginable and intended. For example, in the case of currently used start-stop devices of internal combustion engines, it is required that, after a stop phase, the engine reaches a high rotational speed adequately fast and with a minimum of resistance. In this connection, a decompression during the compression stroke on the outlet valve proves to be advantageous. It is likewise possible to transport residual gas into the combustion chamber (internal exhaust gas return (AGR)) through an additional opening of the outlet valve during the suction stroke, and this creates advantages for the combustion engine such as a lowering of the combustion temperature which leads to a reduction of nitric oxides. If necessary, it is also possible to use the proposed device for pressing residual gas into the suction tract. The valve train can be used particularly advantageously in a quality-controlled internal combustion engine, but not exclusively.

According to a preferred embodiment of the invention, the high pressure chamber is to be closed in direction of a reservoir situated above the high pressure chamber by using a non-return valve, known from hydraulic lash adjusters. It is of course also imaginable to obtain a forced opening of the non-return valve for deactivating the additional lift while creating a zero lift for the housing of the support element by using a slide that is operated, for instance, electromagnetically by an external means. However, the invention preferably proposes to provide, in the reservoir, a locking slide that can be moved away from (or towards the non-return valve) by hydraulic medium pressure, which locking slide is guided in the bore of the housing and forcedly opens the non-return valve through a compression spring force at a reduced hydraulic medium pressure, so that the high pressure chamber almost collapses and the additional lift function is thus deactivated so that an oscillating zero lift of the housing relative to the pressure chamber is created.

For a simple flooding of the reservoir of the housing of the support element, according to one proposition, the support element comprises at least one passage in the form of a bore that is fed out of a pressure oil duct that extends in the cylinder head. As a pressure oil duct it is possible, e.g. to use that duct that was provided in hitherto used cylinder heads for supplying pressure oil to the hydraulic support elements.

Further advantageous developments of the invention concern a configuration of a head region of the housing. Thus, according to one variant, the head region can comprise a semi-circular recess into which a spherical head of a contact piece of the finger lever engages. The contact piece can extend integrally from the finger lever or, for example, form a part of a lash adjusting screw.

As an alternative to the above, the invention proposes to configure the head itself with a semi-circular-like shape, so that the finger lever bears against the head through a semi-circular recess. In this case, but also in the aforesaid development, the head can also be a part of a separate cylinder piece that is connected fixedly to the housing, for example, by pressing-on or welding.

The invention finally proposes providing in the head region at least one venting opening leading out of the reservoir. In this way, undesired air accumulated in the support element can escape. At the same time, this results in a good lubrication of the mounting region to the finger lever.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1illustrates a valve train1of an internal combustion engine. The valve train1comprises a finger lever2which acts at one end on a gas exchange valve3and which is seated at another end via a contact piece4, configured in the present case as a lash adjusting screw31, on a head5(see alsoFIG. 3) of a housing6of a support element7, said head5comprising a semi-circular recess30.

The housing6extends for axial movement in a reception bore8of a cylinder head9. It can be seen further that the finger lever2is loaded by a cam10from whose base circle11an additional cam piece12for generating an additional lift of the gas exchange valve3projects.

A stepped bore14extends from a bottom13of the housing6in direction of the head. A pressure piston16which is axially movable relative to the housing6extends in a lower section15of the bore14, and a leak gap for hydraulic medium is formed between an outer wall of the pressure piston16and the bore14. Through a lower front end17, the pressure piston16is permanently mounted on a base18of the reception bore8of the cylinder head9.

A high pressure chamber21for hydraulic medium is formed between an upper front end19of the pressure piston16and a ring part20that is fixedly arranged on the housing6while being situated above the upper front end19. The ring part20comprises a non-return valve23configured as a ball valve that opens into the high pressure chamber21. A compression spring34that is braced between the ring part20and the pressure piston16extends in the high pressure chamber21.

The non-return valve23is forcedly open in direction of the high pressure chamber21through a locking slide22. The locking slide22extends in a reservoir24formed above the ring part20and constitutes a piston that is guided in the bore14of the housing6. The locking slide22is biased through the force of a compression spring means25in a direction towards the non-return valve23. The compression spring means25is supported on a head side base26of the bore14of the housing6.

The aforesaid reservoir24serves to supply hydraulic medium to the high pressure chamber21. For generating the additional lift of the gas exchange valve during a cam base circle phase11, the locking slide22can be brought out of contact with the non-return valve23by flooding the reservoir24through a pressure duct29situated in the cylinder head9, so that the housing6, in its extended state and with closed non-return valve23, is supported on the pressure medium column in the high pressure chamber21.

For deactivating the additional lift, the prevailing hydraulic medium pressure is reduced so far that the piston-like locking slide22, via its lug27, opens the non-return valve23through the force of its compression spring means25. This leads so to say to a collapse of the high pressure chamber21and the housing6thus performs a zero lift relative to the pressure piston16.

FIG. 2discloses an alternative configuration of the head5of the housing6. The head5comprises a ball-like vaulting and is a part of a separate cylinder piece32that is fixedly connected to the housing6. Thus, the counterpart in the finger lever2is configured as a semi-circular recess (not shown).

FIG. 2further shows two vent openings33in the head5. Through these openings, the undesired air accumulated in the reservoir24can escape to the exterior. A lubrication of the side of the mounting point directed towards the head is also realized through these openings.

LIST OF REFERENCE NUMERALS