本研究針對一部二閥單缸四行程125 c.c.機車引擎(在此稱為基本引擎)，固定引擎轉速於4000 RPM，使用商用套裝計算流體力學(Computational Fluid Dynamics, CFD)軟體CONVERGE對引擎進行計算模擬。藉由改變噴油時機，尋找最佳的油氣濃度分佈、平均燃油粒徑與燃油蒸氣率。並針對引致較差與較佳濃度分佈之噴油時機進行燃燒計算，分析引擎之性能。若噴油時機在上一行程之進氣行程開始上死點後20º至爆炸行程結束下死點前30º (亦即目標行程之進氣行程開始上死點前700º至210º)，可使大部分噴射之燃油停滯於進氣歧管之閥背上，並在目標行程進氣閥打開前預先蒸發得到較多之蒸氣量，而在目標行程進氣閥開啟後，以蒸氣形式進入缸內，使汽缸內之燃油濃度分佈較均勻，混合效果較佳。引擎性能計算結果顯示：缸內油氣濃度較不均勻的噴油時機導致較低之缸內溫度、壓力與引擎輸出馬力與較高之排氣污染物值；反之，油氣濃度均勻度高之噴油時機導致較高之之缸內溫度、壓力與引擎輸出馬力與較低之排氣污染物值。最佳與最差之噴油時機可導致引擎輸出馬力相差8.8%。另外對基本引擎的冷流場進行計算模擬，計算缸內滾轉運動的流動型態、截面渦度滾轉比與循環渦度滾轉比等量化指標。並使用實驗方法，針對基本引擎以及另外一部參考引擎，以質點影像速度儀(Partical Image Velocimeter, PIV)進行缸內流場滾轉運動瞬間速度量測。基本引擎的計算與實驗結果顯示，兩者之缸內流場結構衍化相似，滾轉比變化趨勢也大致相似。基本引擎與參考引擎實驗結果顯示，參考引擎在進氣行程的渦旋結構較明顯，因此滾轉比也較高：參考引擎的體平均循環渦度滾轉比較基本引擎高6%。

The fuel injection timing of a two-valve, four-stroke, port-injection motorcycle engine was studied by using the experimental and computational methods. The computational fluid dynamic software CONVERGE was used to caculate the best distributions of the fuel droplet diameters, fuel concentrations and fuel evaporation propornality by optmizing the fuel injection timing. The calculation results showed that the best injection timing occurred within a period of that 700º~210º crank angle before top dead center (TDC) of the target intake stroke. Injection the fuel within the aformetioned period could cause most of the injected fuel stays in the port, evaporates, and distributes uniformly in the cylinder during the target intake stroke, and therefore induces higher combustion efficiency. The calculated engine power output of injecting the fuel within the appropriate period can be increased by approximately 8.8% when compare with that injecting the fuel out of the appropriate period. The tumble ratio of the in-cylinder flow was measured by a particle image velocimeter (PIV) to compare with the computational results. The calculated flow evolution processes in the cylinder were quite similar to those by experiment.

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