許多學者指出汽油缸內直接噴射式(gasoline direct injection,GDI) 引擎能達到高效率、低油耗及降低汙染等優點, 其中層狀燃燒為低油耗之關鍵技術, 層狀燃燒系統設計關鍵在點火位時, 混合氣以層狀分布,火星塞附近區域能聚集較濃之油氣,而其他區域油氣較稀薄,, 使得燃燒室內維持整體較高的空燃比, 因此獲得極低的油耗。但內燃機缸內流場極為複雜、不易掌握, 本研究針對目標GDI 引擎去探討燃燒室內流場現象, 分析使用的軟體為Ansys, 計算流體力學軟體(Computational Fluid Dynamics,CFD)利用Ansys Geometry進行幾何建構與修改, 接著進入Ansys Mesh對建置完成的幾何進行網格劃分, 最後來到Ansys Fluent選定數值模型即設定初始條件後開始計算, 以上設置完成後先從冷流場作分析,, 從中發現紊流動能與紊流強度與油氣的混合有直接關係, 因此在設計層狀系統的油氣分布時, 找尋壓縮行程中最大的紊流動能點當作噴油的結束時刻, 可使油氣集中於火星塞區域。最後本研究改變進氣流道與原始流道作比較, 發現只要紊流動能增強, 油氣的混合就會於火星塞區域更加均勻,如上可得知分析冷流場之重要性及有助於層狀模式系統的建立。

Many researchers have demonstrated that gasoline direct injection(GDI) engine can achieve high efficiency, low specific fuel consumption and low emissions.The key energy-saving technology is stratified combustion. With stratified combustion, at the ignition timing, the richer mixture should be gathered in spark plug region, the leaner air-fuel ratio is distributed in the other region. So that,the overall air fuel ratio in combustion chamber can keep in leaner air-fuel ratio condition, and decrease engine fuel consumption. The stratified combustion design is related to in-cylinder flow field, but flow field of internal combustion engine is extremely difficult to observe. Therefore, this research discusses the phenomenon of in-cylinder flow in combustion chamber by using ANASYS CFD software. First, the geometry module is applied for modifying target geometry and then generating mesh. After selecting numerical model and setting initial conditions, the cold-flow simulation can be started. From turbulent kinetic energy analysis, the mixture formation is significant related to turbulent kinetic energy, and finding maximum turbulent kinetic energy as the end of injection timing can concentrate richer mixture near spark plug.Furthermore, with proposed intake port design, higher turbulence intensity can be achieved than original one, and better stratified charge can be created. From above, the cold flow field simulation can give a well contribution to stratified combustion design.

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