摘 要
引擎汽缸頭與缸壁溫度的均勻度取決於汽缸頭與汽缸水套的幾何形狀與流場狀態，而入水口、出水口與墊片開口的位置對溫度均勻分佈的影響更為深鉅。因此，本研究利用商業套裝計算流體力學(computational fluid dynamics, CFD)軟體STAR-CD，針對一部四行程單缸水冷式引擎之散熱機構，進行模擬計算並結合實驗方法，改良汽缸水套的設計，以提升其散熱效果。設計過程先以數值計算的方式，給定一個均勻熱通量，模擬計算原始汽缸頭與汽缸水套的速度、溫度以及熱傳係數的分佈，將此熱傳係數當成不變量。然後實驗量測原始汽缸的缸頭與缸壁溫度分佈，進而得知水套欲修改的方向。將熱對流係數與實驗量得的缸頭與缸壁溫度輸入數值計算程式，得到缸頭與缸壁的局部熱通量。由於汽缸頭的溫度非常不均勻，且在排氣埠附近特別高。因此，將原引擎由汽缸壁入水的模式改為由汽缸頭入水，再進入汽缸水套，並在汽缸頭流道中以數個擋板作為調整冷卻水流動路線的結構體，在汽缸水套中增加一片擋板，以引導流體行經較完整的缸壁流道，一共進行12種流道的調整模型。將原引擎的局部熱通量與熱傳係數輸入修改引擎的模型進行計算模擬，得出修改引擎汽缸頭與缸壁的溫度分佈，最後得出一個最佳化的設計。將此最佳化設計做成實體模型，在引擎測試台上進行溫度量測。結果顯示，缸頭外圈的最高溫度由173 oC降為93oC；內圈由220oC降為143oC。外圈平均溫度由106 oC降為92 oC；內圈由143 oC降為121 oC。缸壁的溫度則沒有太大變化，呈現可接受的型態。由實際引擎性能實驗結果，缸頭的溫度調整，造成引擎馬力與扭矩些微提升，油耗幾乎沒有變化，HC的排放在低轉速時下降，在高轉速時稍微升高。顯示此一溫度調整幅度很大，可以解決原始引擎熱應力集中的問題，且不影響引擎性能。

Abstract
A methodology is developed to improve the heat dissipation performance of a single-cylinder, four-stroke-cycle, water-cooled engine by employing the experimental method in conjunction with the numerical simulation technique. At first, the commercial code, STAR-CD, is employed to calculate the velocities of cooling water, temperatures of cylinder head and cylinder wall, and heat transfer coefficient distributions of the engine walls by providing an uniform heat flux to the cylinder head and cylinder. The heat transfer coefficients are obtained by using a empirical formula imbedded in the numerical code. The calculated heat transfer coefficients are assumed to be unvaried for the modified engine. The real temperature distributions around the engine cylinder and the cylinder head are measured experimentally by attaching 40 T-type thermocouples with 250 m bead diameter to circumferential positions of the engine cylinder and cylinder head. The measured temperature data and the calculated heat-transfer coefficients are used to compute the local heat flux by using the fundamental conduction heat transfer formula. With the distributions of heat flux, the STAR-CD is employed to calculate the flow field and the temperature distributions of the fluids and the cylinder walls of the modified engine. Totally 12 modified engine designs are studied. The water inlet for the original engine is moved to the cylinder heat and few blockage structures are added to guide the path of the cooling water in the water jacket of the cylinder head and the cylinder. A optimized design is obtained numerically. The new design is mockup and the temperature distributions are experimentally measured. The results show that the maximum temperatures of the outer and inner circumferential of the original engine are 173oC and 220oC, respectively. In the modified engine, they decreased by a large quantity of about 80 oC to 93oC and 143oC, respectively. The average temperatures of the outer and inner circumferential of the original engine are 106oC and 143oC, respectively. In the modified engine, they decreased by a large quantity to 92oC and 121oC, respectively.

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