Study of a new mechanical variable valve actuation system: Part I—valve train design and friction modeling

Abstract

This article addresses the design of a new mechanical Variable Valve Actuation (VVA) system. The basic scheme consists of three main elements, which enable valve lift variation. Although VVA systems could reduce the specific fuel consumption due to an important de-throttling of the intake system, the systems can lead to higher friction losses due to the increased number of components. For this reason, a specific numerical algorithm was implemented to determine either the cam profile or the kinematic and dynamic characteristics of the entire system. In this way, it was possible to estimate the instantaneous and average power dissipated by the frictions for the actuation of each valve. These evaluated frictions will be used in Part II for the estimation of the actual improvement in terms of specific fuel consumption at part load net of the increased mechanical power dissipated when compared to a conventional valve train. A preliminary thermo-fluid dynamic analysis revealed that the proposed variable valve actuation system is unable to significantly reduce the specific fuel consumption because of the inability to carry out valve actuation strategies that reduce the pumping work. A more flexible mechanical VVA system has been thus developed, which is able to allow intake valve deactivation, as well as variation in valve lift, timing and duration. Finally, in Appendix 1, an analytical procedure aimed at the determination of the geometry of the conjugate profiles of a generic mechanism has been described with the aim of obtaining a general methodology for the design of a mechanical VVA system.