【作者】 段伟；

【导师】 洪伟；

【作者基本信息】 吉林大学 ， 动力机械及工程， 2011， 硕士

【摘要】 随着能源短缺和环境污染问题愈演愈烈,人们开始广泛关注实现汽车节能减排的相关技术。其中,汽车应用怠速-停止技术,即怠速时发动机熄火,行车时快速起动的技术,可以有效降低城市工况下燃油消耗量和整车排放,赢得了许多国家和地区政府的认可。传统进气道喷射汽油机从起动喷油到缸内着火做功最快也要2～3个冲程,致使重新起动产生较长时间延滞。而直喷汽油机可在1个冲程内着火做功,为快速起停技术提供了良好的载体。在缸内直喷汽油机上采用怠速-停止技术,实现快速起动,可以直接向处于压缩行程或膨胀行程的气缸喷入燃油。但是由于发动机热机停止,活塞停止位置的随机性,导致滞留在气缸中的气体的质量是不确定的,而且这些气体直接受到高温气缸壁、气缸盖的加热,温度要高于普通工况进入气缸内的气体温度,所以在起动时其着火的初始边界条件具有特殊性。本文针对这种条件研究了不同冷却液温度、活塞停止位置以及喷油参数对首次喷油气缸内着火情况的影响。并且基于CFD商用软件FIRE对活塞处于不同初始位置时,气缸内气液混合流的浓度场、温度场和速度场的动态分布特点进行了数值模拟,分析了不同喷油时刻和喷油量对火花塞附近混合气浓度的影响。本文的主要研究结论如下:1.当活塞从60°CA BTDC起动时,喷油后油束最前端出现卷吸现象,并在气缸内形成逆滾流,压缩上止点时缸内温度相对较低。喷油量较稀或适中时,均可点燃。喷油量较浓时,喷油时刻越早,失火的可能性越大。当活塞从90°CA BTDC起动时,喷油量适中时,喷油时刻对其燃烧情况影响不大。当活塞从120°CA BTDC或150°CA BTDC起动时,喷油量较多时,喷油时刻对其燃烧情况影响不大,但在150°CA BTDC起动时,喷油量较少时,自燃的可能性增加。当活塞从180°CA BTDC起动时,喷油时刻越晚,缸内温度越高,温度梯度越大。喷油量少或适中时,喷油时刻对其燃烧情况影响不大。喷油量较多时,喷油时刻过早或过晚时,自燃可能性增加。2.喷油时刻在90°CA BTDC附近时,缸内产生两个运动方向相反的滾流,在其作用下,油束向燃烧室两侧扩散。喷油时刻较早或较晚时,油束主要在壁面的引导下向火花塞附近运动。3.当冷却液温度在100℃,活塞初始位置距离压缩上止点小于90°曲轴转角时,在混合气偏浓和偏稀处出现失火区域,中间为点燃区域;随着活塞初始位置逐渐远离上止点,点燃区域逐渐增大,向偏浓失火区扩展。当活塞初始位置距离上止点大于120°曲轴转角时,在偏稀失火区和点燃区之间出现了自燃区,随曲轴转角逐渐增大,自燃区面积增加,向点燃区扩展。喷射时刻选择适中,处于压缩上止点前60°~90°曲轴转角并且采用较浓的混合气有助于避免自燃的发生。4.当发动机热机起动,直接向处于压缩冲程的气缸喷油时,冷却液温度对缸内混合气能否发生自燃影响较大。冷却液温度越高,自燃区越大。5.在点燃区内,对一定的喷油量,不同的喷油定时对其燃烧状况影响不大,燃烧始点一样,最高燃烧爆发压力相差不多。在自燃区内,不同的喷油定时对其燃烧状况影响较大,喷油定时越早,自燃始点随之提前,最高燃烧压力也相应增大。更多还原

【Abstract】 Because of increasingly attention on the energy shortage and environmental pollution, energy saving and emission reduction technology was caused widely attention. The start-stop technology (the vehicle was flameout in idling period and then could start quickly) which could reduce fuel consuming and emissions in urban conditions was widely accepted by many countries. There would be 2-3 strokes in the period between injection and ignition on port fuel injection engine. As a result, a long time delay existed in restart. However, it only needed one stroke on gasoline direct injection engine which provided favorable support for quick start-stop technology.For achieving quick start-stop technology on gasoline direct injection engine, fuels would be jetted into cylinder which is in the compression stroke or expansion stroke. However, in consider of hot stop and randomness of piston stop position, the mixture quality is uncertain. Moreover, these mixtures are heated by high-temperature cylinder wall and cylinder head. Therefore, the initial boundary conditions of ignition in start were special. In the paper, the research was carried out on the influence of different cooling liquid temperature, piston stopping position and injection parameters on ignition in cylinder of first injection. Additionally, based on the CFD commercial software FIRE, in different piston initial positions, numerical simulation was made on the dynamic distribution characteristics of mixture concentration field, temperature field and velocity field. Moreover the influence of different injection timing and qualities on mixture concentration near spark plug was analyzed. The main conclusions are as follows:a) When the engine started at 60°CA BTDC, the end of spray appeared entrainment phenomenon after injection and clockwise flow existed in cylinder. The temperature in cylinder was relatively low. Ignition was available near TDC when fuel quality was relatively more rarefied or proper. With fuel charge in cylinder being higher and injection timing being earlier, the possibility of misfire was lower. When the engine started at 90°CA BTDC, the influence of injection timing on combustion was little if the fuel charge was proper. When the engine started at 120°CA BTDC or 150°CA BTDC, the influence of injection timing on combustion was not obvious if the fuel charge was relatively more. However, to start at 150°CA BTDC, the possibility of self-ignition increased if the fuel charge was more rarefied. When the engine started at 180°CA BTDC, the later the injection timing was, the higher the temperature in cylinder was. The temperature gradient also increased if the injection timing was earlier. There was nearly no influence of injection timing on combustion when the fuel charge was proper or rarefied. For the fuel charge was relatively more, the possibility of spontaneous combustion increased if the injection timing was too early or too late.b) When the injection timing was at 90°CA BTDC, there were two adverse flows in cylinder which caused jets diffusing to chamber wall. Jets mainly moved to spark plug along the chamber wall if the injection timing was relatively early or late.c) When coolant temperature was 100°C, the initial piston position was less than 90°CA BTDC, it is available for ignition only based on the fuel charge being proper. Otherwise, there would be misfire phenomenon. As the crankshaft angle between the initial piston position and TDC was increasing gradually, the ignition zone expanded to the misfire zone of high fuel charge. When the angle was larger than 120°C, there was spontaneous combustion zone between misfire zone and ignition zone. Additionally, this zone expanded to ignition zone as the angle increased. The proper injection timing (60°CA BTDC --90°CA BTDC) and relatively high fuel charge were good for avoiding spontaneous combustion.d) In the process of hot engine start, the coolant temperature played significant roles on avoiding spontaneous combustion of mixture in cylinder if the injection timing was in compression stroke. The higher coolant temperature was, the larger spontaneous zone was.e) In ignition zone, for certain fuel charge, the influence of different injection timings on combustion was small. The pressure peak value of combustion was similar if the initial combustion positions were the same. In spontaneous combustion zone, the influence of different injection timings on combustion was large. As injection timing was earlier, the initial spontaneous combustion position was earlier, the pressure peak value also increased.更多还原