自從Mitsubishi Motor 於1995年開始採用缸內燃油噴射(gasoline direct injection, GDI)技術於汽車引擎之後，全世界均廣為認知此項技術的優點，包括節省油耗、降低排氣污染、增加燃燒效率，進而提升整體性能輸出。通常缸內燃油噴射技術必須配合缸內氣流的滾轉與/或旋轉運動以達到所需求的空氣/燃料混合型態(homogeneous或 stratified charge)，才能達成上述之目的。因此，探討缸內流場以及噴霧流場與氣流運動交互作用後的結構與衍化有其必要性。本研究利用商業套裝計算流體動力學(computational fluid dynamics, CFD)軟體STAR-CD，針對一部四行程單缸二閥機車引擎，探討在有、無加入燃油噴射的狀況下，於進氣和壓縮行程期間分析缸內氣流繞著缸徑軸上的滾轉運動和繞著汽缸軸上的旋轉運動以及缸內燃油噴射之噴霧流場與氣流運動交互作用後的結構與衍化，並使用質點影像速度儀(particle image velocimetry, PIV)量測噴油嘴將燃料射出於一大氣壓力下之霧化情形，並藉由循環渦度滾轉比、循環渦度旋轉比、噴霧穿透長度及霧化液滴粒徑，了解缸內氣流及噴霧特性。計算結果顯示，引擎的缸內流場為極佳之滾轉運動，缸內燃油噴射隨時間衍化的軌跡與氣流的交互作用將可作為設計缸內直噴引擎的重要參考。噴霧實驗結果發現，經噴油嘴射出的燃料，在短時間內具有高澎脹、高穿透以及高霧化，並使用粒徑分析儀量測50 bar結果顯示，在不同距離下，霧化的液滴平均直徑約為30~50 μm。

Abstract
The gasoline direct injection (GDI) has been engaged to the combustion system of an automotive engine since 1995 by the Mitsubishi Motor. It has been well recognized that the fuel consumption, exhaust emission, and engine performance can be drastically increased by application of the GDI technology. The GDI technology usually must be combined with the in-cylinder flow motion in order to regulate the mixture concentration distribution in the cylinder to a target pattern. By using this technology, the mixture preparation in the engine cylinder can be homogeneous or stratified charge and therefore the combustion can proceed under control. The GDI technology, however, is not popularly used in the small motorcycle engine. In this study, the feasibility of application of the GDI technology to a single cylinder, two-valve, 249 cm3 displacement motorcycle engine is studied numerically and experimentally. The engine is installed by a fuel injector and is hooked up to a modified engine test bench. The in-cylinder flows with/without fuel injections are calculated by a commercial software package of CFD code, the STAR-CD. The in-cylinder flow shows the evolution process of the tumble motion and strong tumble intensity. Coherent tumbling structure starts to appear after 40o crank angle from the top dead center during the intake stroke. It persists even to the final stage of the compression stroke. Therefore, taking the strategy of injecting the fuel during the intake stroke (about 90o after top dead center) to form the homogeneous charge for the high-load and high-speed operation as well as injecting the fuel during the compression stroke (about 60o before the top dead center) to form the stratified charge for the low-load and low-speed operation becomes possible. The tumble flow motion and its interaction with the atomized fuel droplets are also studied. The position, direction, timing, and pressure of injection are found to be the most important parameters. In order to characterize the injection features, the atomization characteristics of fuel injection in the atmosphere are measured by a particle image velocimetry. The spray penetration and droplet size distribution are presented.

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