Internal-combustion engines – Poppet valve operating mechanism – Tappet
Reexamination Certificate
1999-06-17
2001-11-20
Denion, Thomas (Department: 3748)
Internal-combustion engines
Poppet valve operating mechanism
Tappet
C123S090160, C123S090570
Reexamination Certificate
active
06318323
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns a tappet for a valve train of an internal combustion engine according to the features of the preamble of claim
1
.
BACKGROUND OF THE INVENTION
A tappet of the pre-cited type is disclosed in DE-OS 41 11 610. An adjusting element extends through a bottom of the tappet and acts at one end on a closing member of a one-way valve of a clearance compensation element and cooperates at the other end with a groove on the outer peripheral surface of a contacting cam. This groove is arranged only on a small part of the outer peripheral surface of the cam and extends on both sides of the cam tip in the region of a lift-producing run-on and run-off flank of the cam. By an opening of the closing member outside of the region of the groove of the cam, it is intended to achieve any desired timing for valve intake and discharge. A more particular goal is to obtain an automatic shifting of the point of time of closing of the gas exchange valve concerned as a function of the engine speed.
However, this prior art document gives no incentive to the person skilled in the art as to how to relieve a high pressure chamber of the clearance compensation element during the cam base circle phase with the disclosed means in order to protect the gas exchange valve from camshaft displacements, torsional vibrations of the camshaft and other factors of influence such as circularity errors of the cam base circle which in the most unfavorable of cases can lead to the opening of the gas exchange valve concerned during the base circle phase.
OBJECT OF THE INVENTION
The object of the invention is therefore to create a tappet of the pre-cited type in which the aforesaid drawbacks are eliminated and, in particular, an undesired opening of its gas exchange valve during a base circle phase is avoided by the use of simple means.
SUMMARY OF THE INVENTION
The novel tappet of the invention for a valve train of an internal combustion engine, which tappet oscillates with its skirt in a bore of a cylinder head and is contacted on a top surface of its bottom by a cam of a camshaft, a hydraulic clearance compensation element being fixed against an undersurface of the bottom by a bushing which is in contact by one of its peripheral surfaces with a pressure piston that is axially displaceable relative to the bushing, a bottom of the pressure piston faces at least one gas exchange valve, a high pressure chamber (
13
) is arranged axially between the bottom and an annular shoulder of the bushing and a reservoir for hydraulic medium is arranged preferably axially between the annular shoulder and the bottom, the annular shoulder having at least one passage which is closed on a side nearer the high pressure chamber by a one-way valve which opens towards the high pressure chamber, a closing member of the one-way valve is biased towards the reservoir by a spring means, and the closing member can be displaced toward the bottom by an adjusting element which extends through an opening of the bottom and cooperates with a groove on the outer peripheral surface of the cam, is characterized in that
the groove extends over an entire outer peripheral surface of the cam with the exception of its base circle region &agr;, and the groove has a depth so that the adjusting element extending in the groove releases the closing member so that the closing member bears against the passage due to the force of its spring means, and
the force of the spring means has a magnitude so that for opening the closing member in the base circle phase of the cam, a hydraulic medium differential pressure of &Dgr;p≧0.4 bar is required at the cross-section A of the passage.
Due to the fact that the groove of the cam extends over the entire outer peripheral surface thereof with the exception of the base circle region, the one-way valve of the clearance compensation element is defined and positively opened. Due to this “opening”, the high pressure chamber and a reservoir of the clearance compensation element are in hydraulic communication with each other. As known, per se, to the person skilled in the art, possible displacements or torsional vibrations of the camshaft and circularity errors in the cam base circle no longer lead to an undesired stiffening of the high pressure chamber and an eventual relieving of the gas exchange valve loaded in closing direction, or even to an opening of the gas exchange valve. The invention, however, likewise covers solutions in which the groove of the cam also extends in parts of its base circle region.
During the aforementioned positive opening, clearance formed during the lifting phase of the gas exchange valve can be compensated in a known manner by a regulation of the quantity of hydraulic medium. At the same time, an undesired build-up of high pressure in the high pressure chamber during the base circle phase is prevented. Retroaction of additional cams, such as cams of fuel injection pumps, on the hydraulic clearance compensation during the base circle phase of the camshaft is likewise prevented. It is further known that relatively large dilatations of the exhaust valve occur particularly in transient operation, i.e. during a rapid acceleration of the internal combustion engine immediately upon its start. If it comes to the worst, it is also possible that the clearance compensation element cannot compensate for these length variations by its leak function and the gas exchange valve remains partly open during the base circle of the cam. This drawback, too, can be eliminated in a simple manner by the means provided by the invention.
According to a further feature of the main claim, the spring force of the spring means acting on the closing member is designed to be substantially larger than that of prior art spring means. In this way, the sealing action of the one-way valve is clearly enhanced also, and more particularly, at rapid accelerations of the tappet. It has been determined that no minimum hydraulic medium pressure from the reservoir is required for opening the closing member against the force of the spring means. Since this minimum pressure in prior art cup tappets is relatively low, for example in the idle running range, the force of the spring means for the one-way valve also had to be relatively small in order to assure a proper clearance compensating function of the clearance compensation element even in the idle running range. The proposed higher spring force of the spring means also assures a faster and more reliable closing of the one-way valve when cam lift starts. At the same time, it is also possible to realize a clearly larger stroke of the closing member compared to the prior art strokes so that, immediately upon a starting of the base circle phase, a relatively large flow cross-section is established on the valve by the adjusting element. At the same time, the hitherto required relatively close tolerances with regard to the stroke of the closing member can be relaxed and this is particularly favorable from the manufacturing point of view.
The proposed relatively high spring force of the spring means for the closing member necessitates a hydraulic medium differential pressure of &Dgr;p≧0.4 bar for opening the closing member in the base circle phase of the cam, while, due to the just described situation, the maximum differential pressures realized in the prior art were ≦0.2 bar. This pressure is measured at the passage for the closing member but without taking into account the mass forces and other factors of influence which need no further specification here. Thus, the larger the force of the spring means acting on the closing member relative to the dimensions of the tappet, the more reliable is the closing behavior of the closing member.
It is also conceivable to extend the non-recessed portion of the outer peripheral surface of the cam beyond the base circle region into the pre-cam region.
It is likewise possible to shorten a region of the closing ramp of the cam adjoining a run-off flank of the cam (seen in rotating direction of the cam). This is enabled by t
Bierman, Muserlian and Lucas
Denion Thomas
Ina Walzlager Schaeffler OHG
Nguyen Tu M.
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