本研究探討進氣閥入射角對缸內氣流滾轉運動的影響。針對一部二閥單缸四行程125 c.c.引擎，在引擎轉速1800 RPM及4000 RPM下，改變進氣閥入射角(19o至24o)，使用商用套裝計算流體力學(Computational Fluid Dynamics, CFD)軟體 CONVERGE，對進氣與壓縮行程期間進行計算模擬，探討並分析不同進氣閥入射角(θ)下，缸內氣流繞著汽缸徑向滾轉(Tumble)運動的變化。進氣閥入射角的改變會影響隨著曲軸角度變化的缸內流場模態、容積效率、缸內平均壓力與溫度。滾轉運動的強度可以藉由計算面平均渦度滾轉比、體平均渦度滾轉比等量化呈現。考慮缸內流場模態衍化、體平均滾轉比與容積效率，尋求進氣閥入射角之最佳化數值。缸內流場模態如果含有較完整的同向旋轉大渦流結構，且渦流旋轉中心較接近瞬間缸內垂直面的中心點，則體平均滾轉比數值會較大。容積效率在本研究所改變的進氣閥入射角範圍內，變化不明顯，最大值出現在進氣閥入射角19o，約為67.98%；最小值出現在進氣閥入射角22o，約為67.77%。最大值與最小值相差僅0.2%左右，所以，在本研究範圍內，進氣閥入射角對容積效率的影響不需考慮。所以體平均渦度滾轉比與缸內流場模態可以視為是決定最佳化進氣閥入射角的主要參數。以缸內流場模態來衡量，θ = 19o與20o時，渦流結構最完整且可延續至壓縮行程，θ = 21o時次之；θ ≧ 22o時，渦流結構的完整度不及θ ≦ 21o時。當θ ≧ 22o時，體平均滾轉比大約在0.11至0.12之間；當θ ≦ 21o時，體平均滾轉比大為提升到大於0.135，並隨著θ減小而增大。所以，θ在19o∼21o是較佳的選擇。當θ < 19o時，在閥的安排上有困難(小於此角度時，進、排氣閥有干涉的危險)，所以不予考慮。又因為滾轉比大於某些數值之後，所衍生的紊流與擴散增強對於燃燒的增強效應將趨近於飽和，其臨界滾轉比需由實驗獲得，所以進氣閥入射角不需太小。所以，最恰當的進氣閥入射角可使用實驗方法在19o與21o之間取得。

The in-cylinder flows in the axial planes of a motored two-valve, single-cylinder, four-stroke engine during the intake and compression strokes at engine speeds of 1800 RPM and 4000 RPM are diagnosed by using computational methods. Moderate and intense tumble motions are generated by changing the inclination angle (θ) of inlet valve from 19o to 24o. The computations are carried out by the computational fluid dynamic software CONVERGE. The ensemble averaged conservation equations for mass, momentum, and energy in transient state conditions with the k-ε turbulence model are solved. The orthogonal, structured grid which reproduces the geometry of the inlet port, exhaust port, combustion chamber, and real fluid system is automatically generated by CONVERGE. Quantified strengths of the rotating motions in the axial planes are presented by dimensionless tumble ratio, which is 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 indicate the correlation between strengths of tumble motion and inclination angle of inlet valve. The volumetric efficiency, instantaneous in-cylinder flow patterns, and cycle-averaged tumble ratio are considered to optimize θ. The results shows that the instantaneous flow patterns at θ ≧ 22o are not as coherent as those at θ ≦ 21o and therefore the cycle-averaged tumble ratios at θ ≧ 22o present small values than those at θ ≦ 21o. The optimized design for the inclination angle θ would be within the range of 19oand 21o.

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