本研究使用計算與實驗方法於一部歧管噴油的二閥單缸四行程250 c.c.引擎。在進氣與壓縮行程期間，使用計算流體動力學(Computational Fluid Dynamics, CFD)商用軟體STAR-CD，分析在未噴油時，缸內氣流滾轉(tumble)運動的衍化情形，並計算量化的滾轉比，用以評估滾轉強度是否恰當，同時探討轉速變化對於缸內滾轉(tumble)運動的影響；並在homogeneous charge模態下，計算歧管噴油時缸內的油滴粒徑與油氣濃度分佈。以油滴粒徑與油氣濃度分佈均勻度作為準則，找出較佳的噴油時機。實驗部分使用質點影像速度儀(particle image velocimeter, PIV)來進行缸內流場滾轉運動量測。藉由量測而分析得到的截面渦度滾轉比及循環渦度滾轉比等量化指標，評估流場滾轉運動的強度。實驗與計算結果比較，顯示實驗所量得的缸內渦旋產生與衍化過程與計算結果相類似，但是量化的對稱面渦度滾轉比及循環渦度滾轉比則約最大 81 %差異；非對稱面則有約最大 232 %差異。計算結果顯示，轉速對於流場結構衍化影響不大，但在較高轉速時滾轉比會較大。噴油時機若提早，則缸內平均粒徑SMD較小且混合較快、濃度分佈分佈較均勻。

A two-valve, four-stroke, port-injection motorcycle engine was studied by using the experimental and computational methods. The experimental investigation employed a particle image velocimeter (PIV) to measure the in-cylinder tumble flow motion in the symmetry and offset planes. The results was used to evaluate the appropriation of the strength of the in-cylinder tumble motion and to validate the computational results. The computational analyses used the commercial code STAR-CD to calculate the in-cylinder flow as well as the distributions of the fuel droplet diameter and fuel concentration. The computational results of flow evolution process in the cylinder were quite resemble to those measured by experiments. While the calculated tumble ratios on symmetry plane were significantly deviated from the measurement results by about 81 %, and offset plane were about 232 %. However, the computational results could be used for relative evaluation when the design mechanisms for tumble motion were conduced. The engine speed did not cause apparent flow evolution difference. But the calculated tumble ratio at high engine speed presented larger value than the low speed did. The calculation results of port injection showed that injection at advanced crank angle caused the fuel droplet diameter finer and the fuel concentration more uniform.

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