引擎性能的優劣良窳取決於燃燒特性，而燃料與空氣之混合程度對燃燒特性影響更為深鉅。因此，探討缸內氣流的型態、結構與演化格外顯得特別重要。本研究使用質點影像速度儀(particle image velocimetry, PIV)，分別針對一部四閥雙缸四行程引擎與一部四閥單缸四行程引擎，在進氣道內裝設氣流偏折閥(inlet-deflection valve)以調制缸內氣流的流場模態。於進氣和壓縮行程期間，進行缸內氣流運動的速度量測，診測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。為了解決循環變異所產生的變動性，使用樣本平均方法取得固定在某一曲軸角度下的流場平均速度，進而描繪出整體流場演化過程，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，定量分析氣流偏折閥在全開與全關下的流場結構與引擎性能間的相關性。實驗結果顯示，不論是將氣流偏折閥設置在進氣歧管內的引擎或是安置在進氣埠的引擎，當氣流偏折閥全關時，缸內均能有效形成旋轉運動，使量化後的循環渦度旋轉比明顯增強，並牽引紊流強度的提升。也由於旋轉運動的衍生，造就缸內燃料與空氣得以混合較均勻，改善引擎在怠速與低速下的燃燒，在引擎性能輸出方面與抑制廢氣生成量均有較佳表現，並於稀薄燃燒下呈現更佳效果。欲有效調制全域之引擎性能，可先調整滾轉比，使之大於0.4以上，再使用氣流偏折閥於低油門開度與中低轉速下，使缸內氣流旋轉比大於0.1。

The evolution processes of 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 a particle image velocimeter. A device, which is called the “inlet-deflection valve”, being capable of deflecting the inlet flow is installed to the inlet port. 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 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 swirl number show that the installation of the inlet-deflection valve can induce large swirl motion and turbulence intensity, particularly, in the intake stroke. 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 at low throttle opening and low engine speed can be apparently raised if the cycle-averaged swirl ratio is higher than about 0.1. The limitation of applying the swirl-generating technique to the ranges of the high throttle opening and high engine speed is because that the means of generating the swirl motion in the cylinder would generally cause the volumetric efficiency to deteriorate when the inlet-deflection valve is functioning at large throttling and high engine speed. At mediate or high throttle opening and high engine speed, tumble ratio higher than approximately 0.4 is recommended.

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