Analysis of a Lost-Motion-Type Hydraulic System for Variable Valve Actuation 930822
A general model for the analysis of lost-motion type hydraulic devices used in variable valve actuation is presented. The devices covered by the model include those featuring passive (semi-active) or active bleed control, with external damping. In the case of semi-active bleed control, the bleed-orifice area was fixed during a cam event. However, different orifice areas were considered at different cam-shaft speeds yielding a wide variety of phase, duration, and maximum-lift values. With a fully active bleed control, where the bleed-orifice area was varied as a function of cam-shaft angle during the cam event, independent control of phase, duration, and maximum lift yielded more drastic variations in valve-lift behavior. Governing equations based on dynamic and hydraulic characteristics of the system include Coulomb and hydrodynamic friction forces, inertia forces, an external damping force, and fluid compressibility.
Valve lift near the valve-closing point, where hydraulic damping is most effective, is found to be particularly sensitive to variations in oil viscosity. Performance of hydraulic damper is affected by viscosity variations and cam-shaft speed. Over a wide range of viscosity variations, the onset of cavitation in the system is speed dependent and can be eliminated by proper sizing of flow areas. However, at higher viscosities, the system is prone to cavitation even at moderate speeds regardless of the size of flow areas. Also presented are the results of cam force and camshaft torque at different operating conditions.