本研究針對一部單缸二閥四行程125 c.c.引擎，進行球形燃燒室幾何設計最佳化研究。使用商業套裝計算流體力學(Computational Fluid Dynamics, CFD) 軟體CONVERGE，探討引擎在固定轉速5000 RPM時，對汽缸缸內冷流場及熱流場進行計算模擬。分析引擎燃燒室幾何結構改變時，缸內氣流繞著汽缸徑向滾轉(Tumble)運動的變化，並計算容積效率、缸內平均壓力與溫度、壓縮比、滾轉比在不同幾何設計條件下的數值變化。面平均渦度滾轉比、體平均循環渦度滾轉比等數值是用以呈現缸內滾轉運動的量化強度。燃燒室幾何結構變化會導致缸內流場模態隨著曲軸角度而改變，因而造成缸內氣流滾轉比有強度差異。缸內氣流滾轉比改變會造成紊流強度、油汽混合的優劣性質，進而影響引擎缸內燃燒情況的良窳。考慮缸內流場模態衍化、體平均循環渦度滾轉比、容積效率，可以判斷出最佳化的燃燒室幾何設計。本研究發現引擎於壓縮行程期間流場模態的衍化模態，可作為設計引擎燃燒室最佳化幾何之重要參考，以減少設計引擎燃燒室時所需要之成本與時間。本研究共進行五十八個球形燃燒室幾何設計分析計算，結果顯示原始引擎(F91)的體平均循環渦度滾轉比為0.406，本研究改變不同的穹頂幾何結構、活塞頭形狀案例中，最高的體平均循環渦度滾轉比為0.578，兩者相差將近42%，顯示穹頂幾何設計參數是影響缸內滾轉運動的主要參數。將體平均循環渦度滾轉比最佳化模型進行歧管噴油及燃燒分析，結果顯示：當引擎於高轉速運轉時，最佳模型比最差模型之引擎輸出功略微提昇；高強度缸內滾轉運動能大幅降低廢氣中汙染物的排放量。

The in-cylinder flows in the axial planes of a motored two-vavle, single-cylinder, four-stroke engine at an engine speed of 5000 RPM were diagnosed by using computational methods. Moderate and intense tumble motion were generated by changing the combustion chamber geometric design. The computations were carried out by the computational fluid dynamic (CFD) software CONVERGE. The ensemble averaged conservation equations for mass, momentum, and energy in transient 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 42%, an insignificant increase in power output by about 0.3%, and drastic reductions in exhaust of CO and NOx when compared with those of the original engine.

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