內燃機的性能及污染物之生成取決於燃燒特性，為了增進其燃燒效果，降低污染物之排放，並進而提升整體性能輸出。因此利用增強缸內氣流滾轉運動的方式，以達成上述之目的，並同時增進稀薄燃燒之極限，使其進一步達到稀油燃燒，促使污染排放合乎環保標準，並保有原空燃比下之性能輸出。本研究透過改變進氣埠流道幾何形狀，以增進缸內氣流滾轉運動。研究過程中使用質點影像速度儀(particle image velocimetry, PIV) 並搭配商業套裝計算流體動力學(computational fluid dynamics, CFD)軟體STAR-CD，針對一部四閥單缸四行程引擎，於進氣埠流道形狀修改前後，進氣和壓縮行程期間，分析缸內氣流繞著缸徑軸上的滾轉(tumble)運動和繞著汽缸軸上的旋轉(swirl)運動。並藉由循環渦度滾轉比、循環渦度旋轉比及循環絕對紊流強度等量化指標，定量分析進氣埠流道幾何形狀修改前後的流場結構與引擎性能間的相關性。實驗與計算結果顯示，進氣埠流道形狀修改後之循環渦度滾轉比較修改前之進氣埠提升，代表缸內滾轉強度增強。搭配性能測試驗證後發現，滾轉運動的增強造成紊流強度的增加，有助於空氣和燃料均勻混合，並提升引擎性能，降低油耗與廢氣之產生。改變空燃比測試之結果顯示，修改後的缸頭可以更稀油燃燒，並保有原性能輸出，同時抑制汙染物之生成。透過計算與實驗結果的比較發現，計算所得到的整體趨勢與實驗相近，因此可利用計算模擬方法幫助車輛設計研發人員於引擎開發過程中更有效率。

The in-cylinder flows in the axial and diametral planes of a motored four-valve, single-cylinder, four-stroke engine during the intake and compression strokes are diagnosed by using experimental and computational methods. Moderate and intense tumble motions are generated by changing the inlet port configuration. Experiments are carried out by the particle image velocimeter. The engine cylinder, piston, and accessories are modified to meet the requirements of laser-light sheet shooting and camera viewing when the particle image velocimeter is applied. Conditional sampling technique is 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 present clear pictures of the evolution processes of the tumble and swirl motions in the engine cylinder. Sectional streamlines and the velocity vectors show the topological flow structures. The computations are carried out by the fluid dynamic code STAR-CD. The ensemble averaged conservation equations for mass, momentum, and energy in transient state conditions with the turbulence model k-ε are solved. The grid, reproducing the geometry of the inlet port, exhaust port, combustion chamber, and real fluid system, is made by using es-ice module. The es-ice is designed to facilitate the moving grid, transient analyses of internal combustion engines and is used in conjunction with PROSTAR and STAR-CD. The boundary conditions prescribed to the computation domain are the same as the experimental ones. The inception, establishment, evolution, and destruction processes of the swirling and tumbling vortical structures during the intake and compression strokes are presented and discussed. Quantified strengths of the rotating motions in the axial and diametral planes are presented by dimensionless tumble and swirl ratios, which is defined as the ratio of 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 number show that the modified of inlet port can induce large tumble motion 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 and the engine output is closely linked. It is found that the engine performance can be apparently increased and the pollutant emissions are drastically decreased when the tumble ratio is raised by the re-configured inlet-port geometry. The relation between the lean limit air-fuel ratio and the tumble intensity is also studied to seek for an indication of the combustion characteristics under part-load condition. The lean limit air-fuel ratio tends to increase toward lean side with higher tumble intensity. It is therefore desirable to increase the tumble intensity for lean burn engine. The CFD results qualitatively follow the measurements of experiments. However, the quantitative comparisons show large differences.

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