本研究針對機車單缸125 c.c.引擎，進行球形燃燒室幾何設計最適化研究。主要探討的設計參數是穹頂半徑與活塞頂部幾何；次要探討的設計參數是Squish區域的圓角半徑及穹頂與缸壁之夾角。使用套裝計算流體力學(Computational Fluid Dynamics, CFD) 軟體CONVERGE，探討引擎在固定轉速6000 RPM及節氣門開度全開時，對汽缸缸內未噴油之冷流場進行計算模擬。分析引擎燃燒室幾何結構改變時，缸內氣流徑向滾轉(Tumble)運動的值化變化，並計算容積效率、缸內平均壓力與溫度、紊流動能及滾轉比在不同設計條件下的量化變化。以面平均渦度滾轉比、體平均循環渦度滾轉比等數值來呈現缸內滾轉運動的量化強度。燃燒室幾何結構改變會使缸內流場模態隨著曲軸角改變，而造成缸內氣流滾轉強度有所差異。缸內氣流滾轉比改變會影響紊流強度及油氣混和的優劣，過去研究顯示，最終會影響引擎之燃燒效率與污染物排放濃度。由於缸內氣流滾轉運動的強度會影響最終引擎之性能，所以欲設計較佳的燃燒室幾何時，缸內流場模態衍化、體平均循環渦度滾轉比是必須考慮的重要參數之一。本研究共進行六十四個燃燒室幾何設計分析計算。結果顯示：原始引擎的體平均循環渦度滾轉比為0.158，而最佳化的穹頂與活塞幾何改變案例之體平均循環渦度滾轉比為0.183，兩者相差將近16%。分析各研究參數之重要性，顯示穹頂半徑是影響缸內滾轉運動的主要參數之一。然而，本研究未進行燃燒計算、燃燒分析及性能測試實驗，所以，體平均循環渦度滾轉比增加16%對引擎最終性能之影響尚待探討。

The in-cylinder flows in the axial planes of a motored two-vavle, single-cylinder, four-stroke engine during the intake and compression strokes at an engine speed of 6000 RPM and throttle opening of 100% were diagnosed by using computational methods. Moderate and intense tumble motions were generated by changing the combustion chamber geometric design. The computations were carried out by the computational fluid dynamics (CFD) software CONVERGE. The ensemble averaged conservation equations for mass, momentum, and energy in transient state conditions with the k-ε thubulence model were solved. The orthogonal, structured grid which reproduced the geometry of the inlet port, exhaust port, combustion chamber, and real fluid system was automatically generated by CONVERGE. Quantified strengths of the rotationg motions in the axial planes were represented by a dimensionless variable tumble ratio, which was defined as the ratio of mean angular velocity of the vortices in the target plane at a certain crank angle to the average angular velocity of the crank. The quantitative results of cycle-averaged tumble ratio indicated the correlation between strengths of tumble motion and combustion chamber geometric design. The results showed that the engine with an optimized combustion chamber presented an significant increase in cycle-averaged tumble ratio by about 16% when compared with that of the original engine.

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