近年來隨著石化燃料逐漸枯竭, 尋找替代燃油維持動力機械的運轉成了燃眉之急。由於生質柴油與石化柴油的物性與化性相近, 一般的柴油引擎幾乎可以在不需要改裝的情形下, 便可直接使用生質柴油運作且生質燃油有著可再生利用、無虞耗竭的絕對優勢,是未來最有潛力能替代石化柴油的再生能源。本研究主要探討不同比例生質柴油在未經改裝的柴油引擎中的燃燒行為與其對引擎性能與排放之影響, 冀望本研究成果能對未來新一代生質柴油引擎的發展有棉薄之助益。
本研究以真實引擎尺寸及量測數據建立一柴油引擎(Mitsubishi 4M42-4AT2 共軌燃油噴射引擎) 之數值模型, 藉以進行不同比例生質柴油之研究, 研究對象包括了引擎中的流場、能量變化及不同比例生質柴油之燃燒行為。本研究使用的軟體為Ansys Fluent,以當中之In-cylinder 模組及吾人以凸輪軸所建構之閥門揚程-曲軸角關係來模擬
引擎作動, 並使用其內建之理論數學模型與經驗模型來建構柴油引擎之數值模型。例如使用RNG K-epsilon 紊流模型來進行流場運動行為之模擬, 其他還包含了控制液滴行為的Discrete Phase Model 及控制燃燒行為的Species Model 等。而初始條件、邊界條件皆以實驗數據進行設定, 如進排氣壓力、汽缸初始壓力、噴油相關之條件等。吾人期能利用此引擎數值模型探討實驗上實際引擎燃燒較不易觀測之現象與燃燒行為。
而透過本研究之實驗數據與數值模擬發現, 不同比例之生質柴油的燃燒行為(燃燒
延遲與燃燒速率) 相當一致, 這是由於當柴油中只要混入生質柴油, 其點火延遲時間將會被生質柴油所主導, 而由於柴油引擎中燃油不預混, 燃油化學反應遠快於控制擴散行為的紊流混合效應, 因此燃燒速率將由紊流混合決定。由於在相同轉速與負載下, 流場差異不大, 所以各不同比例之生質柴油之燃燒速率將會相近。
另外數值模擬顯示, 生質柴油的混入石化柴油可能造成燃燒時間點提前,間接導致較高的缸溫與缸壓, 而在柴油中與添加水一同噴注則可使燃燒時間點延後, 同時降低缸壓與缸溫。這似乎開啟了一道控制生質柴油燃燒延遲的門, 也許會是一個很值得探討的議題。

Biodiesel made from renewable resources and waste lipid is the most promising surrogate for diesel in the future. To assess the performance of an existing diesel engine fueled with various blends of biodiesels, a CFD model based on the real dimensions of a Mitsubishi 4M42-4AT2 diesel engine equipped with a turbocharger and a common-rail oil injection system is constructed in this work. This model will be used to investigate the combustion behaviors of biodiesels and the flow field inside the combustion chamber.
The In-cylinder module provided by Ansys Fluent together with the built-in turbulence model for solving fluid flow motion and various phenomenon models simulating the behaviors of oil droplets inside the combustion chamber will be used to simulate the interesting dynamics of a combustion engine. For effectiveness and convenience, the RNG k-epsilon turbulence model is adopted in this research.Discrete phase model (DPM) is used to simulate the oil injection. Species model is used to predict the combustion behaviors of injected fuels. Initial conditions and boundary conditions such as pressure and temperature at inlet and exhaust pressure at outlet will be given based upon the experimental data. We hope that the numerical simulation can provide us the data that were hard to obtain in experiment.
Revealed by both experimental data and numeric computation, the ignition delays of biodiesels of various blends are nearly the same. This may be attributed to the fact that biodiesel has a greater cetane number. This leads to the earlier ignition of biodiesel than the pure diesel, the heat released will in turn result in the ignition of the surrounding oils. Thus we propose that the cetane number of biodiesels is equal to the cetane number of pure biodiesel.
Since the oil is not premixed with air in diesel engines, the reaction rate of oil and oxygen is therefore controlled by the turbulence mixing effect of the eddy dissipation instead by the thermodynamic properties of oils. Under the same revolution speed and torque, we expect the flow fields will be quite similar. Consequently we can expect the reaction rates of biodiesels of various blends are almost identical, if the engine is running under the same revolution speed and torque.
Our simulations also indicate that oil injection with added water may result in longer ignition delay, which in turn results in the lower pressure and lower temperature in the cylinder. On the contrary, biodiesel tends to shorten the ignition delay of the mixed oils. This seems to open up a door of controlling the ignition delay of biodiesel of various blends by adding some extra amount of water during oil injection.

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