本研究針對一1.5 L之缸內直噴汽油引擎其中一單缸，進行HCCI操作模式之缸內流場動態、燃氣製備過程、燃燒特性及引擎性能分析。透過三維暫態計算流體力學模擬結合簡化化學動力反應模型，模擬條件為引擎轉速2000 rpm及指示平均缸壓為4 bar 部分負載。而進行解析HCCI燃燒特性前，首先針對GDI引擎之冷流場噴霧動態特性進行模擬。分別探討活塞表面幾何及噴油正時，對於缸內流場及燃氣混合過程之影響。在HCCI燃燒模擬方面，則透過流場、壓力、溫度場及反應中間產物生成情形探討HCCI之燃燒機制。研究中也針對噴油時機對於HCCI引擎之點火時點與引擎性能影響進行分析。歸納模擬結果得知，引擎之活塞表面構造可導引缸內氣流，影響燃氣分佈及混合情形。當引擎於均質燃燒模式時，碗狀活塞可藉由較強之漩流使燃氣集中於火星塞位置以利點火，且在分層燃燒模式時亦較能達到較佳之燃氣分層效果。噴射時機對於燃氣混合及液滴霧化情形有明顯影響；過於延後噴油時機對於燃氣混合效果有不良影響。HCCI燃燒機制受到缸內流場、溫度以及燃氣分布影響甚大，當燃氣分布不均或燃燒室壁面有熱點產生，恐造成引擎產生爆震及燃燒不完全之現象。OH自由基之產生與HCCI引擎的點火時點有關，藉由分析其產量變化可知HCCI引擎之點火時點變化趨勢。本文中分別針對噴油時機為-270、-225、-180、-135 ˚ATDC進行探討HCCI引擎之操作特性，發現引擎之點火時點會隨著噴油時機而提前。比較MEP及廢氣排放結果，發現噴油時機為-270 ˚ATDC引擎性能表現較佳。

The objectives of this study are to investigate the mixture preparation process in a GDI engine combustion chamber and the characheristics of HCCI combustion using 3D transient reacting flow simulations with reduced chemical mechanisms. The engine intake flow, fuel spray and combustion characteristics were simulated at a 2000 rpm partial load condition. The emphasis has been put on the effects of the injection timing on the mixture preparation, the ignition timing, the engine performance, and the exhaust emissions. Before studying the characterics of the HCCI engine, the effects of the piston crown geometry and the injection timing on interaction between the in-cylinder flow and mixture preparation were investigated. The results of cold flow simulation showed that, the fuel rich zones were distributed in the core of the cylinder with the bowl-shaped pistion due to the more intense swirl. The mixture stratification was also better with the bowl-shaped piston under stratified mode. The mixture preparation and the droplet atomization were significantly influenced by the injection timing. Adverse effects on the uniforming of the charge mixture were found when the injection timing was too late. The results of HCCI simulations showed that, the combustion mechanism of HCCI was controlled by the in-cylinder flow fields, temperauture and mixture distributions. Knock and incomplete combustion would occur with an inhomogeneous mixture or when hot spots were generated on the combustion chamber wall. The effects of injection timing on HCCI engine performance were investigated. With earlier injection timing, the ignition timing of the HCCI engine was advanced. In this study, the better engine performance was found with the injection timing at -270 ˚ATDC.

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