Hydraulic valve lash adjusters are used in valve trains of internal combustion engines in automotive vehicles to adjust a valve lash that results from thermal expansion, manufacturing tolerances and wear of the transmitting elements during a loading of a gas exchange valve by a cam. For this purpose, in the case of common lash adjusters, the respective mechanical transmitting element that transmits a cam lift of the cam to the gas exchange valve comprises a piston that is guided with sealing clearance for displacement in a housing and is elastically supported against the housing by a piston spring, so that a tensioning of the piston spring prevents any lash formation on the gas exchange valve.
The force transmission to the gas exchange valve via the lash adjuster during cam loading is regulated by a control valve that controls a flow of a hydraulic medium through an axial opening between a low pressure chamber of the piston serving as an oil reservoir and a high pressure chamber that is enclosed by the piston and the housing. The control valve includes a closing body, mostly a control valve ball that is arranged on a piston undersurface in the high pressure chamber, and a control valve spring that applies a spring force to the control valve ball. A fundamental distinction is made in this field between two types of constructions.
In a standard construction, the control valve spring is arranged as a closing operative element that presses the control valve ball with a biasing force against a valve seat on the axial opening configured as a piston bore on the piston bottom undersurface. Accordingly, the control valve of the lash adjuster is closed during a cam base circle phase when the cam of a rotating camshaft runs on the associated valve train member. During a subsequent displacement of the piston by a cam lobe, a corresponding adjusting stroke is transmitted with an adjusting force directly via the lash adjuster to the gas exchange valve that is directly actuated in opening direction. Because the oil in the high pressure chamber is incompressible, the lash adjuster then acts as a “rigid” adjusting member. It is only upon the subsequent expansion of the piston and housing relative to each other through the piston spring, when the cam re-reaches the base circle and the pressure in the high pressure chamber sinks, that the control valve opens against the force of the control valve spring and a pressure equalization takes place between the pressure chambers till the control valve spring closes the control valve again.
Because in this type of construction, especially during a commencing warming-up phase of the still cold engine, a so-called pumping-up of the lash adjuster via the control valve can take place, even to the extent of a negative valve lash that leads to high engine loading accompanied by increased wear, an alternative construction with an opening control valve spring has already been proposed.
Hydraulic valve lash adjusters of this type with an opening control valve spring are known as Reverse Spring Hydraulic Valve Lash Adjusters (referred to hereinafter as “RSHVA”) or Normally Open Lash Adjusters (NOLA), for example from EP 1 298 287 A2 and WO 2006 010 413 A1. In such constructions, the control valve spring is reverse-arranged, generally within the piston bore between the reservoir and the high pressure chamber, so that the control valve ball or the closing body is loaded in opening direction and the control valve is consequently open in the cam base circle phase. In this arrangement, the control valve ball is usually received in a closing body cap that is retained on the piston bottom undersurface. The closing body cap comprises openings that serve as an oil passage and a bottom for limiting the stroke of the control valve ball.
In the case of an RSHVA, a cam excursion at first causes a control oil stream to flow from the high pressure chamber to the low pressure chamber, i. e. in closing direction, as a result of which the lash adjuster collapses in axial direction, so that the piston and the housing are pushed together. The control oil flows around the control valve ball which, as a result, is then loaded both hydrostatically and hydrodynamically against the action of the control valve spring till a resultant axial force presses the control valve ball against the valve seat and the control valve closes. The collapsing movement of such an RSHVA manifests itself in a characteristic idle stroke before the actuation, properly speaking, of the gas exchange valve takes place. RSHVAs therefore act as “soft” adjusting elements that exclude a negative valve lash.
The idle stroke of the RSHVAs has an influence on the valve lift of the gas exchange valves and on the valve timing in the internal combustion engine. A corresponding idle stroke characteristic, that is speed-dependent due to the volume flow between the pressure chambers that varies with the cam speed, can be purposefully used in a valve or camshaft control for improving thermodynamic efficiency, for reducing pollutant emission and improving the quality of the idle stroke of the internal combustion engine as described, for example, in the not pre-published documents DE 10 2005 043 947.0 and DE 10 2005 054 115.1 of the applicant.
However, the closing time of the control valve and thus the idle stroke of RSHVAs may be subject to relatively large fluctuations. Tests have shown that a relatively high degree of dependence of the closing behavior of the control valve on the oil viscosity exists practically over the entire temperature range of the engine oil (−25° to +160° C.). Further important factors that influence the operation of an RSHVA include the flow geometry that is determined by the configuration of the individual control valve components, the closing displacement of the control valve closing body as also manufacturing and material tolerances. An important target in the further development of RSHVAs is therefore the minimization of these disturbing functional fluctuations and of the parameter-dependence. Different proposals for improvement have already been made in this connection.
In the RSHVA known from WO 2006 010 413 A1, temperature-sensitive means, for example, bimetal elements or memory metal elements, are arranged in the low pressure chamber or in the axial opening. With sinking temperature, these elements increasingly vary the oil stream flowing from the high pressure chamber to the low pressure chamber, so that a control valve closing time that is not sensitive to the temperature of the control oil or engine oil and, thus also, a great degree of equalization of the idle stroke at different operating temperatures can be obtained. Functional fluctuations from one temperature-sensitive element to another due to manufacturing tolerances and the relatively complex flow geometry that make an exact pre-determination of the hydrodynamics and hydrostatics of the control valve are hardly taken into account in this document.
The document 10 2004 018 457 A1 shows an RSHVA with different closing body geometries and guide aids for guiding the respective closing body in a correspondingly configured closing body cap. In particular, the valve closing body can be configured with a needle-shape, the needle being formed by a ball prolonged by a circular cylindrical intermediate member. The guide aids configured, for example, as guide surfaces surround the closing body with a guide clearance, so that the valve closing body is moved linearly during the closing operation, quasi in the manner of a piston.
The main concern of this document is to propose an RSHVA in which undesired eccentric dislocations and rotary movements caused in particular by unguided hydrodynamic loading can be prevented by a guided, predominantly hydrostatic axial loading of the valve closing body. This active closing body guidance eliminates operational fluctuations due to undesired closing body movements. Accordingly, the prior art RSHVA presents an improved service performance in this respect that is also less susceptible to fluctuation.
On the whole, the prior art publications on the improvement and further development of RSHVAs essentially concern the dependence of the closing behavior of the control valve, and thus of the idle stroke resulting therefrom, on flow geometry, temperature, speed and manufacturing tolerances. This has already made some improvements in this type of RSHVAs possible.
It would be further desirable to make available to the user, an RSHVA with an idle stroke that is matched to the particular application and/or the dimensions of the components of the RSHVA. An application-specific and/or construction-adapted idle stroke, in particular, in combination with the use of the speed-dependence, with a simultaneous reduction of the functional fluctuations, could lead to a further improvement of the efficiency and the advantages of RSHVAs and enlarge their scope of use. The prior are, however, contains no explicit suggestions in this regard.