The hydraulic valve lifter provided on internal combustion engines to automatically compensate for dimensional changes in the valve train must be matched closely to the characteristics of the valve on which the hydraulic lifter is installed, and the close match should be maintained over the useful life of the combustion engine.
The fluid reservoir that forms part of most hydraulic lifters have two inherent disadvantages; they are open to the atmosphere and they form natural traps.
Exposing the hydraulic fluid to the atmosphere allows oxidation and evaporation of the hydraulic fluid and in addition, an entranceway for foreign material which then accumulates in the trap structure. Oxidation and/or evaporation cause viscosity shifts in the hydraulic fluid as, for example, by formation of compounds that alter the original fluid viscosity properties. Foreign material, such as dust suspended in the hydraulic fluid, will eventually interfere with the intended operation if circulated through the various moving and non-moving parts of the hydraulic valve lifter.
All open reservoir lifters have the problems alluded to above, regardless of whether they are the flood type or manual fluid make-up type. The obvious solution is to seal the fluid reservoir. Sealing, however, may present still a different set of problems. Pressure may build up in the fluid reservoir which will reduce the differential across the orifice in the hydraulic lifter and reduce the lifter collapse rate. Without adequate control, the collapse rate may be reduced to zero and the engine valve spring then may experience excessive unloading. Still, in principle, a factory assembled presealed hydraulic valve lifter assembly should facilitate installation of the lifter on the engine and, moreover, the hydraulic valve lifter should undergo essentially no changes either in crucial dimensions or in operating characteristics during its normal use life.
Such a hydraulic valve lifter assembly forms the subject matter of this invention. The possibility of pressure buildup in the fluid reservoir has been made into an advantageous feature of the invention hereof.
The object of this invention is to provide a reliable gas filled sealed unit hydraulic valve lifter.