【作者】 何旭；

【导师】 高希彦； 刘卫国；

【作者基本信息】 大连理工大学 ， 动力机械及工程， 2006， 博士

【摘要】 柴油机以其良好的动力性和经济性,得到日益广泛的应用,但它特有的燃烧方式决定了燃烧与排放之间的矛盾,微粒和NOx之间的矛盾。面临日益严格的排放法规和能源危机,为实现柴油机的清洁高效燃烧,国内外科研工作者都将HCCI燃烧作为研究的重点,力图有效解决HCCI点火控制、高负荷扩展等问题,以便尽快投入实际应用。针对燃油撞壁现象,近年来出现了有意引导油束撞击在指定位置的燃烧系统,可以获得良好的油气混合,从而达到改善燃烧的作用。 本文综述了当前广泛研究或应用的柴油机燃烧方式,总结出实现喷雾撞壁燃烧方式的四种主要措施:增设壁面油束反射凸缘,形成二次射流;燃烧室中央增设油束碰撞凸台,在燃烧室中心区域形成圆盘状油雾;喷嘴挡块碰撞,在喷嘴附近形成扩散状油雾;利用燃烧室收口及底部凸台,形成强烈的挤流和涡流。 基于喷雾撞壁的TR(Three-Rapidity)燃烧系统,以“快速燃油喷射、快速混合气形成和快速燃烧”为设计思想,采用收口尖底凸台ω燃烧室,壁面具有导向圆弧,配合锥面4×Φ0.36+垂直中心喷孔1×Φ0.20喷油器。在台架实验上取得了较好的碳烟排放效果。本文采用可视化研究和数值模拟的方式,对TR燃烧系统的混合气形成机理进行研究,并对其燃烧排放性能进行预测,主要内容如下: 首先组建了数字化测试系统,采用频闪照相方法对喷雾场进行测试,探求了用数字图像处理技术自动进行浓度场、喷雾锥角和贯穿度提取的方法。从图像的拍摄到数据的获取,整个过程实现了数字化在线实时测试。采用该系统对TR燃烧室和原ω型燃烧室的对比研究表明,TR燃油撞击壁面后在导向圆弧的作用下剥离,向燃烧室中心偏转,燃烧室底部凹坑内基本没有燃油分布,而原ω型燃烧室底部存在较浓的燃油分布。 对直喷式柴油机的气流运动、燃烧排放、油膜生成进行模拟,发现收口形尖底凸台燃烧室具有较好的混合气质量和燃烧性能,碳烟排放较好。燃油撞壁后在壁面附近形成油膜和较浓的混合气,容易生成有害物质。燃油撞壁可以加强燃油颗粒的蒸发和雾化。针对以上特点,提出喷雾撞壁燃烧系统柴油机的设计思想:采用收口燃烧室和尖底凸台,加强挤流运动,增强燃油雾化与蒸发。利用燃油碰撞加速燃油粒子粉碎和蒸发。减少壁面油膜的生成量,油膜分布薄而均匀。合理控制着火时刻,尽量在着火前将燃油全部喷入缸内,减少火包油的现象。 对TR燃烧室混合气形成进行研究,并和原ω型燃烧室对比,TR燃烧系统中的导向圆弧改变了缸内流场特性,壁面附近的气流运动得到加强,旋涡分布范围合理,加速了混合气的生成。在导向圆弧和中孔喷射的作用下,燃油空间分布趋于均匀,改变壁面燃油富集的现象,提高空气利用率,减少了燃烧室凹坑内未燃混合气的数量,燃烧更加充分。并且高温区域集中分布在燃烧室中部,外围存在温度较低的未燃空气。中孔喷射形成的混合气首先着火,并向四周扩展,时间上有序着火。分析表明中心喷孔直径不能过大。 对TR燃烧系统的性能实验表明TR燃烧系统具有良好的烟度排放特性,NOx排放在中小负荷低于原机排放,在高负荷则有所增加。随着转速的降低,TR燃烧系统的滞燃期延长,缸内温度升高,NOx排放增加,碳烟排放降低。TR燃烧系统对涡流的要求不高,四气门135柴油机的涡流可以较好地满足要求,不需要对原机气道做改动。当采用EGR时,TR燃烧系统在油耗变化不大的情况下,NOx排放明显改善,但是烟度也随之上升,需要确定最佳EGR率。随着喷油提前角向后推迟,TR燃烧系统的NOx排放下降,碳烟排放增加,上止点前11°CA喷油为最佳。对TR燃烧系统应用增压和增压中冷进行预测,采用增压可使碳烟排放降低6%,但NO升高75%。采用中冷措施碳烟排放降低15%,NO升高却只有16%,缸内平均压力大幅度增加。对TR燃烧系统应用分段喷射进行更多还原

【Abstract】 Diesel engine is used widely for its potential of promotion of power and lowing fuel cost. But the combustion mode of diesel engine embodies the conflicts between improving combustion and emission as well as PM and NO_x. Efforts of the researchers contribute to find the tradeoff between them. As a novel combustion mode, HCCI focuses on realizing highly efficient and clean combustion. The challenging problem it faces is how to control the ignition timing and how to get full-load performance. Another popular combustion mode is spray impingement combustion. Especially, impinging spray onto the fixed location, boosting the mixture and combustion with bounced fuel, was developed in recent few years.Reviewing current diesel combustion system with spray impingement, four strategies are adopted: reflex edge is fixed on the chamber wall, forming secondary jet; the raised plane is provided in the cavity, forming discal fuel spray; structure near nozzle can form diffused fuel spray; contracted chamber and sharp convexity can enhance air flow and promote mixture.Based on the design theoretics of diesel engine combustion system sumed up in the paper, TR combustion system with fuel impingement was set up and studied. It is modified from ω combustion chamber with an oriented arc at the chamber wall, specially and perfectly fit in a nozzle with 4× Φ0.36 orifices in conic section plus 1× Φ0.20 orifice in center. In the test study TR demonstrated an excellent performance with smoke emission. Optical investigation and CFD were used to study the mechanism of TR mixture formation and performance of combustion and emission. The main work is covered as the following:First a digital measurement was set up and spark photography of spray field was taken. The concentration field, cone angle and penetration were automatically got. The image processing software was developed to analyse the picture and obtain the digital results. The comparison between TR combustion chamber and the conventional ω combustion chamber show that, in the TR chamber, fuel was peeled off from the wall after impinging and turning to the center. So there is less fuel in the chamber bottom than in the ω chamber.The air flow, combustion, emission and fuel film of diesel was studied. It is found that contracted chamber has preferable mixture and combustion as well as smoke emission. Naturally, the much thicker film and richer mixture are formed after fuel impinging on the wall, which will deteriorate the combustion. On the other hand, fuel impingement is beneficial to droplet breakup and vapor. According to these, the theory of diesel combustion system with spray impingement is brought forward: contracted chamber and sharp convexity should be adopted to enhance air flow and promote mixture. Fuel impingement is used to accelerate fuel atomization. The film thickness should be reduced and have a thin distribution widely. The ignition time is reasonable. Fuel is injected into cylinder before ignition to avoid fuel surrounding fire.In TR combustion system, the oriented arc changes the flow field in the cylinder. The air movement near the wall is enhanced and eddy distribution is more reasonable, improving mixture formation. Central orifice spray and oriented arc make fuel distribute equably, which reduce the amount of rich fuel-air mixture near the wall. Then the air is sued effectively and unburned mixture cut down, which is useful to combustion. TR chamber has the high temperature in the center and the low temperature near the wall. The mixture formed by central orifice spray is ignited first and in turn ignites other mixture. The diameter of the central orifice could not be too large.The engine study of TR shows that it has very low smoke emission. At the low and part load, NOx emission of TR combustion system is less than original diesel, but more at high load conditions. When the speed decreases, ignition delay period is longer, temperature becomes higher,NOx emission increases and smoke reduces. The intake port swirl is not demanded by TR. In the four valves diesel, the intake port swirl is appropriate, so the intake port needn’t change. When EGR is applied, TR improves NOx emission obviously with fuel consumption changed little, but smoke emission increases. It is needed to make reasonable EGR ratio. With fuel supply advance angle delaying, the NOx emission of TR decreases and smoke emission increases. In die BTDC 5 0 CA, NOX reduces less than before. The BTDC 11° CA is the trade-off of fuel supply advance angle. The application of supercharge and intercooling in the TR is forecasted. The results show that supercharge reduces smoke by 6% but increases NO by 75%. However intercooling reduces smoke by 15% but increases NO only by 16%. Supercharge makes cylinder pressure enhanced, which means more output power. Split injection used on TR was studied. Comparing with conventional single injection, the split injection and its period as well as quantity can achieve better combustion in the same condition of fuel supply advance angle. But the fuel quantity of each injection needs to be optimized in order to get the tradeoff between PM and NOx. TR combustion system has great potential to improve emission.更多还原