| Summary: | 碩士 === 中原大學 === 機械工程研究所 === 89 === Internal combustion (I. C.) engines are still the primary power sources for cars and motor cycles; thus, the performance of those engines is one of the key items studied in their design. It is noted that the thermofluid phenomena play an essential role in the performance of an engine. In addition, the combustion effectiveness inside a cylinder depends on the mixing of the fuel-air mixture. Furthermore, the structure of the fluid flow field immediately before combustion has a major impact on the intake and exhaust of gases and the propagation of flame. As a result, many researchers performed the studies on the flow fields in various engines.
The study of the flow fields in an I. C. engine has two approaches — the experimental technique and the numerical simulation. Even though the former can measure some of the parameters of the engine under study, the measurement can only focus on a specific area of the engine; it can not measure the overall flow field occurring inside the engine. Besides, the experimental apparatus is expensive to set up and the resources needed for time and manpower is tremendous and is prohibitively high for most of the research organizations. In recent years, within a limited amount of time for computation and using computational fluid dynamics (CFD) methodologies for numerical calculations, the high-speed computers have the capabilities to simulate the overall behavior of the flow field in the engine. They can also predict the performance of an engine by adjusting the engine operational parameters.
This study used KIVA-3V CFD methodology to analyze the flow field occurring in an engine — with varying angles of the intake port. The turbulence model used is the Renormalization Group (RNG) k-e turbulence model which provides a relatively higher turbulence than the Standard k-e turbulence model. The results of this study indicate that changing the angle of the intake port has a profound effect on the flow field during the intake stroke of the engine cycle. For the case with the intake-exhaust valves canted at zero degree (the so-called vertical valves), the tumble ratio, swirl ratio, turbulent intensity, and turbulent kinetic energy in the direction perpendicular to the bisecting plane of the cylinder are at their highest levels than those obtained for the other cases. This effect is most pronounced after the compression stroke of the cycle. As a result of this study, it is observed that the behavior of the overall flow field from the intake to the completion of compression stroke has a major impact on the effectiveness of the mixing of fuel-air prior to the ignition of the engine cycle.
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