本研究以計算與實驗方法探討機車引擎缸內直噴技術的可行性。標的設備是一部四閥四行程250cc之進氣歧管噴油引擎，若要改裝為缸內直噴引擎，需要修改許多參數，並考慮引擎操作模態是在homogeneous charge或者stratified charge。其中較重要的參數包括: (一)引擎壓縮比、(二)進排氣渦流強度、(三)噴油嘴安裝位置與角度、(四) 噴油壓力、噴油時機、噴油時程與噴油量、(五)火星塞安裝位置與點火時機、(六)活塞頭形狀。本研究所要探討的參數是噴油壓力、噴油時機、噴油時程與噴油量以及活塞頭形狀。使用STAR-CD模擬軟體計算引擎安裝平面活塞頭並噴油時，汽缸內在不同曲軸角度的油氣濃度分佈以及油滴粒徑大小。經由判斷較佳的油氣分佈均勻度以及較小的油滴粒徑，得到最佳化的噴油時機為CAinj = 80°，此一最佳化的噴油時機可以給homogeneous charge操作模態使用。另外設計16種不同形狀的活塞頭，計算油氣的濃度分佈是否於火星塞處。此一形狀的活塞頭可以供給 stratified charge使用。對一個多孔式汽車引擎用的噴油嘴，進行缸外噴油特性實驗研究，將所獲得的噴油時機延遲數據輸入機車引擎的計算。進行安裝噴嘴以及平面活塞頭時的homogeneous charge引擎性能實驗。結果顯示，在非怠速時，若噴油壓力Pf = 18 bar且CAinj = 81°，引擎的扭力、馬力、油耗、排氣均可達到最佳值。在怠速時，CAinj必須提前至66°才能得到較低的廢氣排放值。由於引擎設計的其他參數並未修改，所以怠速時的廢氣排放值(HC = 326 ppm、CO = 1.07 %)稍高於原來的進氣歧管噴油引擎的廢氣排放值(HC = 262 ppm、CO = 0.16 %)，但已經遠低於第五幾期法規的上限值(HC≦1200 ppm、CO≦2.5 %)。在非怠速(轉速為3000 RPM)的固定扭力(13.9 N-m)實驗結果顯示，單位制動油耗(bsfc)可以達到比進氣歧管噴油引擎更低的數值。例如噴油壓力30 bar時，缸內噴油引擎以及進氣歧管噴油引擎的bsfc分別是410 g/kw×h及653 g/kw×h，相差37%，在缸內直噴引擎廢氣排放值(HC = 372 ppm、CO = 1.56 %)稍高於進氣歧管噴油引擎的廢氣排放值(HC = 415 ppm、CO = 0.77 %)。

Computational and experimental approaches were used to study the feasibility of applying the gasoline direction injection (GDI) technique to a motorcycle engine. The target device was a four-valve, four-stroke, port-injection engine with a displacement volume of 250cc. Several important differences existed between a GDI engine and a port-injection engine, for examples, (1) compression ratio, (2) vortex strengths of intake port and exhaust port, (3) injector location and angle, (4) injection pressure, injection timing, injection period, and injected fuel mass flow rate, (5) spark plug ignition timing and location of spark plug, (6) configurations of piston head, etc. The parameters of injection pressure, injection timing, injection period, and injected fuel mass flow rate as well as the configurations of piston head were considered in this study. The STAR-CD CFD code was used to calculate the fuel distribution and droplet size to obtain the optimized injection timing for the homogenous charge operation. The optimized injection time was 80o of crank angle. Totally sixteen piston head configurations were designed and optimized to obtain the best fuel concentration around the spark ignition location or the stratified charge operation. The engine test showed that the optimized ignition timing was 81o of crank angle for the homogeneous charge operation using a flat piston head. At idle, the optimized injection time was 66o of crank angle so that low exhaust concentrations (HC = 326 ppm, CO = 1.07%) were detected. These values were significantly lower than the thresholds of the latest regulations. The results of constant-torque engine tests at the engine speed of 3000 RPM showed that the break specific fuel consumption (bsfc) of the GDI engine was significantly smaller that of the port-injection engine by about 37%. The exhaust concentrations of HC and CO of these two engines were similar. Should the compression ratio, injector location & angle, vortex strength of in-cylinder flow, spark plug ignition timing and location of spark plug are further modified, drastic improvements on the power output, fuel consumption, and exhaust may be expected.

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