本研究探討改變一部二閥單缸四行程125 c.c.引擎之進氣埠流道的幾何形狀對於缸內滾轉運動之流動型態與強度的影響，並以量化的體平均循環滾轉比為指標，從幾種幾何形狀中找出較佳的設計。設計四個具有不同進氣埠幾何形狀的汽缸頭 (分別命名為F8A、VJ3、M1-1、M1-5)，安裝於引擎，並將原來的鋁合金汽缸改裝為透明壓克力缸體。使用質點影像速度儀 (particle image velocimetry, PIV)量測汽缸內的瞬時流場；利用商業套裝計算流體動力學 (Computational Fluid Dynamics, CFD)軟體CONVERGE分析在進氣與壓縮行程期間缸內氣流的滾轉（tumble）運動。以旋度 (vorticity)為基礎定義一個無因次的體平均循環渦度滾轉比 (‾("T" _"v,CA" ))；另採用CONVERGE的滾轉比定義，以圍繞質心的軸向速度為基礎定義一個CONVERGE滾轉比 (‾("T" _"v,CA" )’)。依據這兩個滾轉比的定義計算出量化的缸內氣流滾轉強度，並配合缸內滾轉運動之流動型態以及CFD計算得出的容積效率判斷四個汽缸頭流動的優劣。實驗與計算結果顯示，進氣埠流道形狀修改後容積效率幾乎沒有變化 (約1%左右)，而體平均循環滾轉比卻上升許多 (6.5 ~ 34.3%)。其中尤以M1-1的體平均循環渦度滾轉比增強最多 (27.3% @ 4000 RPM)。與引擎性能測試結果相比較，在空燃比13 ~ 18之間，M1-1的制動馬力燃料消耗量 (BSFC)比原引擎 (F8A)降低許多 (約6%)。滾轉運動的增強造成紊流強度的增加與擴散的增強，有助於空氣和燃料的混和，因此可以提升引擎性能，降低油耗與有害廢氣排放之濃度。改變空然比測試之結果顯示，修改過後的汽缸頭M1-1可以更稀油燃燒，並保以原性能輸出，同時抑制汙染物之生成。

The in-cylinder flows of a motored two-valve, single-cylinder, four-stroke engine were studied by experimental and computational methods. Moderate and intense tumble motions were generated by changing the inlet port configurations. Experimental were carried out by the particle image velocimeter (PIV). The engine cylinder, piston, and accessories were modified to meet the requirements of laser-light sheet shooting and camera viewing when the particle image velocimeter was applied. Conditional sampling technique was employed to acquire the instantaneous velocity data at predetermined crank angles. Ensemble average of large amount of the instantaneous velocity maps obtained at various crank angles presented clear pictures of the evolution processes of the tumble motions in the engine cylinder. Sectional streamlines and the velocity vectors showed the topological flow structures. The computations were carried out by the fluid dynamics code CONVERGE. The ensemble averaged conservation equations for mass, momentum, and energy in transient state conditions with the turbulence model standard k-Ɛ were solved. The grid, reproducing the geometry of the inlet port, exhaust port, combustion chamber, and real fluid system, was made by using CONVERGE STUDIO module. The CONVERGE STUDIO was designed to facilitate the moving grid, transient analysis of internal combustion engines. The boundary conditions prescribed to the computation domain were the same as the experimental ones. The inception, establishment, evolution, and destruction processes of the tumbling vortical structures during the intake and compression strokes were presented and discussed. Quantified strengths of the rotating motions in the axial planes were presented by dimensionless tumble ratios, which was defined as the mean angular velocity of the vortices in the target plane at a certain crank angle divided by average crank angle velocity. The quantitative results of the cycle-averaged tumble ratios showed that the modified inlet ports induced large tumble intensity and turbulence intensity. By comparing the quantified non-dimensional parameters of the in-cylinder flow motions with the engine performance, the correlation between the flow characteristics and the engine output was closely correlated. It was found that the engine performance was apparently increased and the lean limit air-fuel ratio tends to increase toward lean side with higher tumble intensity for lean burn engine. The CFD results quantitatively followed the measurements of experiments. However, the quantitative comparisons showed large differences.

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