Massimiliano Muccillo

Study of a new mechanical variable valve actuation system: Part II—estimation of the actual fuel consumption improvement through one-dimensional fluid dynamic analysis and valve train friction estimation

It is commonly recognized that one of the most effective ways to improve Brake-Specific Fuel Consumption (BSFC) in a spark-ignition engine at partial load is the adoption of VVA strategies, which largely affect the pumping work. Many different solutions have been proposed, characterized by different levels of complexity, effectiveness and costs. VVA systems currently available on the market allow for variable valve timing and/or lift (VVA). The design of a new mechanical VVA system has been discussed in Part I of this article. That study led to the development of a four-element VVA mechanism. Now, to estimate the potential advantages of the studied system on engine performances, one-dimensional thermo-fluid dynamic analyses were conducted, considering both full load and partial load operating conditions. For this reason, this article addresses the definition of the one-dimensional model of a 638-cm3 single-cylinder engine under development, which will be equipped with the four-element VVA system. The findings from the one-dimensional study will be discussed in detail. In particular, the parametric analyses, which concern the engine power at wide open throttle and the SFC at partial load, will be presented. These results, however, are only theoretical results because the one-dimensional simulation is not able to take into account the increased friction losses due to the complexity of the VVA system. Therefore, to correctly quantify the actual fuel consumption allowed by the studied system (net of the generally increased power dissipated by friction when compared to a conventional valve train), a specific methodology, discussed in Part I, has been adopted.

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

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.