Patent Application
20060201466
The invention relates to a hydraulic valve play compensation element for the control drive of an internal combustion engine, which is provided as a reverse-spring/free-ball element, with a cylindrical housing, a cylindrical piston guided therein with sealing play, and a control valve, which is arranged between a low-pressure chamber of the piston and a high-pressure chamber of the housing on a piston head and which has a closing body, which can contact a valve seat surrounding an axial bore hole of the piston head when carrying out a stroke in the closing direction against the action of a control-valve spring and which can contact a contact surface of a valve cap surrounding the closing body in the opening direction.
Hydraulic valve play compensation elements are used for compensating for the play formed due to wear and tear or heat expansion when the cam stroke is transferred between transmission elements from a camshaft to a gas-exchange valve of the internal combustion engine. Through the use of the compensation element, a low-noise and low-wear operation of the valve drive and the best possible matching of the cam lift with the stroke of the gas-exchange valve should be achieved.
Such compensation elements each have a control valve, which is provided as a non-return valve and which has a closing body, for example a ball, and a control-valve spring applying a force onto the closing body. In the standard construction of the control valve, the control-valve spring applies a force on the closing body in the closing direction. Therefore, the control valve is predominantly closed and the return stroke of the valve lay compensation element is eliminated. In this configuration, there is the risk of pumping the compensation element upwards and producing a “negative valve lash.”
These disadvantages are avoided with control valves, whose control-valve spring applies a force on the closing body in the opening direction, or control valves, in which a spring is completely eliminated. Compensation elements with such a control valve are designated as reverse-spring elements due to the inverted arrangement of the control-valve spring or as free-ball elements due to the lack of a spring. These exert a positive influence on the thermodynamics, the pollutant emissions, and the mechanical stress on the internal combustion engine and are therefore being used increasingly.
In the standard construction, the control valve is predominantly closed in the base-circle region of the cam due to the spring force of the control-valve spring. In a reverse-spring element, however, the control valve in this region is held open by the force of the control-valve spring. In a free-ball element, the closing is not forced. Because such an element can be closed only by hydrodynamic and hydrostatic forces from the lubricating-oil flow set at the beginning of the cam lift and flowing from the high-pressure chamber to the low-pressure chamber, the element always has a return stroke before the beginning of the valve stroke of the gas-exchange valve. The size of the return stroke depends on the length of the closing time of the control valve at each engine rpm and this depends, in turn, on the viscosity/density of the lubricating oil, which here is used in a known way as hydraulic medium.
To close the control valve of a reverse-spring/free-ball element, a so-called critical lubricating-oil speed is necessary. This depends on the lubricating-oil viscosity and thus on the lubricating-oil temperature. For a high lubricating-oil viscosity/density, i.e., for a low lubricating-oil temperature, the critical lubricating-oil speed is lower and is therefore reached more quickly than for a lower lubricating-oil viscosity, thus a high lubricating-oil temperature. For a cold start, this leads to a shorter closing time of the control valve and thus to a smaller return stroke than for a warm-running engine. However, a small return stroke means a large valve overlap. This results in a high internal exhaust-gas recirculation, which causes noisy, low idle running. This can be improved by raising the idling rpm, but leads to costs in terms of pollutant emissions and fuel consumption.
Reverse-spring/free-ball elements of the type named above are known, for example, from EP 1 298 287 A2, JP 61-185607, and U.S. Pat. No. 4,054,109. These publications present compensation elements, for which the control valve has a ball bearing as a closing body. In these known publications, the closing body is guided in bore holes. However, because lubricating oil must flow around the closing body for closing the control valve, the guide gap cannot be selected arbitrarily small. Therefore, the closing body is not guided ideally, which leads to deviations in the closing behavior. On the other side, surrounding the closing body with oil is necessary to be able to define the closing behavior. Here, the closing behavior exhibits considerable thermal dependence.
Thus, for reverse-spring/free-ball elements, the closing body of the control valve is open in the base circle of the cam. For closing the control valve, a volume flow must flow past the closing body, which causes a pressure difference on the closing body, whereby this closes the control valve. To keep the tolerances of the generated return stroke small, the stroke of the closing body should fluctuate as little as possible. In addition, an eccentric gap between the closing body and the guide wall surrounding it has a negative effect on the flow, whereby the closing behavior is also influenced.
The invention is based on the objective of achieving improved guidance of the closing body, so that the closing body cannot deviate from its movement track. In this way, the stroke tolerance should be clearly limited.
According to the invention, this objective is met in that, in the region of the axial bore hole for the closing body, a narrow guidance surface is formed with gaps, which enable a volume flow of hydraulic medium between the high-pressure chamber and the low-pressure chamber. So that the closing behavior is not negatively affected, despite the better guidance, the closing body according to the invention is guided with discontinuous geometries.
The gaps can be formed by groove-shaped channels, which are incorporated into the inner surface of the piston head located on the axial bore hole. The closing body can have outwardly projecting guidance ribs on its outer surface, which form gaps, which are spaced apart one behind the other in the peripheral direction, and which are directed towards the valve cap. On its inner surface, the valve cap can also have guidance ribs, which form the gaps. The guidance ribs project from the valve cap in the peripheral direction with spacing one behind the other towards the closing body.
Due to the channels or guidance ribs, a good guidance of the closing body is achieved, wherein the control valve can pass sufficient hydraulic medium through the gaps, so that the closing behavior is not worsened. If the gaps, through which the hydraulic medium flow, are located on the closing body itself, this is a useful configuration in that it cannot tilt about its height axis, so that a clean seal is realized on the valve seat of the control valve.
Corresponding advantages can be achieved if gaps, which enable a volume flow of hydraulic medium between the high-pressure chamber and the low-pressure chamber, are provided in the region of the axial bore hole on a narrow guidance surface for the control-valve spring. In this way, the gaps can be formed by groove-shaped channels, which are incorporated into the inner surface of the piston head located around the axial bore hole.
A tight guidance of the control-valve spring has the effect that this cannot change direction. In this way, spring force tolerances and thus deviations in the closing behavior of the control valve are reduced. Through the gaps in the guidance of the control-valve spring, the flow of hydraulic medium is not impaired, but instead it is improved even more. However, the gaps may not extend into the valve seat, so that the control valve can be sealed securely.
Preferred embodiments of the invention are shown in the drawing and are described in more detail below. In the drawings:
The valve play compensation element shown in
The high-pressure chamber 3 is connected to the low-pressure chamber 8 by a central axial bore hole, which is arranged in the bottom piston head 5. It is part of a control valve 11 provided with a control-valve spring 10. This extends into the high-pressure chamber 3 underneath the bottom piston head 5. A compression spring 12 is supported in a central recess 13 on the base 14 of the high-pressure chamber 3. It acts on the piston 4 and thus the entire valve drive with its compressive force. The top piston head 6 has on its outer surface 15 a central conical recess 16 for guiding, for example, the ball-shaped end 17 of a not-shown tappet push-rod. Another central axial bore hole 18, which is located in the top piston head 6, creates the connection of the low-pressure chamber 8 with the lubricating-oil supply of the valve drive. The closing body 19 of this compensation element is a ball.
The control valve 20 according to the invention shown in
The compensation element according to
In the similar configuration according to
Finally,
| Number | Date | Country | Kind |
|---|---|---|---|
| 102005010711.7-01 | Mar 2005 | DE | national |
