A Study of aeration effects on direct acting hydraulic valve train systems

The amount of research in the area of mass transfer of air as a bubble in hydraulic fluid or lubricant is very limited. There are only a few studies on the interaction of oils with air, especially knowledge of air solubility of the oil and diffusivity of dissolved air in oil, namely aeration. This paper will study the effect of aeration on a direct acting hydraulic valve lifter system in an internal combustion engine. Aeration is, primarily, influenced by hydrodynamic factors such as, design features of the oil system and the physicochemical characteristics of the oil. The direct acting hydraulic valve lifter (DAHVL) was introduced to the automotive industry because of some definite advantages, such as acoustic improvement and friction reduction. Although, the aforementioned, advantages of DAHVL are significant, the aeration effect due to hydraulics creates a challenge. Aeration stems from compressibility of air in oil. It is a fact that the incompressibility of the oil hydraulically maintains all components of the valve train in contact. Therefore, aeration tends to create separation between the valve train components. This separation creates certain effects on the dynamic performance. This paper has concentrated on 3 major specifics: how oil aeration affects engine operation (via a linear math model compared to experimental data), a brief discussion on what variables inherent in the design could significantly improve the system\u27s robustness to aeration effects (experimental only) and on what causes aeration (theory)