【作者】 缪雪龙；

【导师】 黄震；

【作者基本信息】 上海交通大学 ， 动力机械及工程， 2009， 博士

【摘要】 节能和减排是当今内燃机研究的两大主题,柴油机与汽油机相比热效率高、经济性好,所以如何降低排放是柴油机研究的首要任务。通过对柴油机关键系统——进排气系统、燃烧系统的持续改进,特别是不断提高燃油系统的喷射压力,柴油机的排放水平已经有了明显的提高。但是柴油机的传统燃烧方式决定了NOx和碳烟颗粒排放的降低存在trade-off关系,单纯依靠完善缸内燃烧满足不了更高排放法规的要求,需要借助排气后处理技术才能达到。然而,现有的后处理系统复杂、价格高,因此内燃机研究者正在寻找一种既经济又有效的方法来降低柴油机的排放。低温预混合燃烧就是人们所希望的一种新的燃烧方式,它能够同时大幅度降低柴油机的NOx和碳烟颗粒排放。本文旨在一台批产的CA6DF柴油机上,通过技术改造来实现一种新的低温预混合燃烧方式,同时大幅度降低NOx和碳烟颗粒排放。对该柴油机的关键系统进行了改进设计。设计了具有低压缩比的大口径直口型燃烧室、具有高涡流比的进气道、能够实现高EGR率的冷却EGR系统;特别是设计了具有上下两层喷孔的超多喷孔油嘴,其特点是上下两束喷雾在燃烧室空间相互干涉,与传统油嘴相比具有大的喷孔流通面积,并采用高喷射压力的共轨燃油系统替代机械直列式燃油系统。在喷油泵综合性试验台上,试验研究了所设计的超多喷孔油嘴的喷油及喷雾特性;在发动机试验台上试验研究了原机燃烧和超多喷孔预混合燃烧特性。油嘴喷油特性试验研究表明:与传统油嘴相比,开发的超多喷孔油嘴具有高的喷油速率、短的喷油持续期,使燃油在着火前能够尽早喷入燃烧室,为实现均匀混合气的预混合燃烧提供保障。油嘴喷雾特性试验表明:与传统油嘴相比,超多喷孔油嘴具有较短的喷雾贯穿距离和较大的喷雾锥角。较短的喷雾贯穿距离可以避免喷雾与燃烧室壁面的干涉,从而控制HC排放;较大的喷雾锥角使喷雾能够卷吸更多的空气,促进燃油与空气的混合,有利于形成均匀混合气,改善燃烧性能。柴油机超多喷孔预混合燃烧试验研究表明:为了大幅度降低NOx排放,需要采用非常高的EGR率。小负荷时,由于过量空气系数高,可以采用高达80%的EGR率;高负荷时,采用相对低的约28%的EGR率。负荷愈大,则EGR率对降低NOx的作用愈大。为了消除EGR对碳烟颗粒排放的负面影响,必须使柴油机实现预混合燃烧。试验研究表明,低压缩比、冷EGR、推迟喷油等措施能够延长着火滞燃期;采用具有大流通面积的油嘴、以及提高喷射压力可以缩短喷油持续期。着火滞燃期的延长以及喷油持续期的缩短,在着火前有利于燃油与空气的充分混合,从而实现理想均匀混合气的预混合燃烧,使碳烟颗粒排放大幅度降低。过量空气系数调整试验研究表明,在实现NOx和碳烟颗粒排放大幅度降低的同时,为了不恶化CO、HC以及BSFC性能,过量空气系数存在一个最低的阈值1.5。当其低于该值时,CO、HC、以及BSFC性能明显恶化。为了在柴油机上实现理想的低温预混合燃烧,提出控制策略如下:低负荷采用大EGR率(甚至高达80%)、低喷射压力(约90MPa)、上止点前的喷油始点(约-1.5°CA ATDC);高负荷采用小EGR率(28%左右)、高喷射压力(约110MPa)、上止点后的喷油始点(约3°CA ATDC)。综合分析大量试验数据,得出的柴油机超多喷孔低温预混合燃烧区域是:转速从怠速到标定转速,负荷为各转速(1430r/min除外)对应外特性全负荷的50%。在该区域内试验柴油机实现了理想的低温预混合燃烧,使NOx和碳烟颗粒排放下降50%以上,很多工况下降80%-90%,同时保持BSFC性能基本不变。特别在低负荷由于过量空气系数高,实现了理想稀薄均匀混合气的预混合燃烧,因此BSFC即使在非常高的EGR率下还是与原机相当、有的工况还有下降。更多还原

【Abstract】 The energy saving and emission control are two issues in today’s study of the internal combustion engine. The challenge of the diesel engines with the high combustion efficiency, however, is how to reduce the exhaust emissions. The exhaust emissions of diesel engines have been greatly reduced by improving the key systems, especially continual increasing injection pressure. But these achievements haven’t yet met the future legislations because the reduction of NOx and soot exhibits the trade-off relationship. It is necessary to use the aftertreatment technology in order to meet the future emission legislations. But these aftertreatment devices are very complicated and expensive. Therefore, the researchers are seeking an efficient and economical method to simultaneously reduce NOx and soot emissions of diesel engines. The low-temperature premixed combustion, which can achieve this goal, has increasingly drawn the interest of researchers in the global.This study proposed a new low-temperature premixed combustion to achieve the simultaneous reduction of NOx and soot emissions in a volume production diesel engine of CA6DF by reconstructing the key systems. According to this concept, some developments are as follows. A toroidal combustion chamber of a low compression ratio was developed. Inlet ports of high swirl ratio were developed. A cooled EGR system was developed. Especially an ultramultihole (UMH) nozzle was developed. It has two layers of injection holes and a large flow area. Two sprays of the upper and lower layers meet in the space of the combustion chamber. The fuel injection and spray characteristics were measured in a fuel injection test bench. The combustion characteristics of the premixed combustion and the base engine were investigated in an engine test bench.The fuel injection test results showed that the UMH nozzle exhibited a higher injection rate, shorter injection duration than that of the conventional nozzle. In this way, the cyclic fuel can be completely injected into the combustion chamber prior to ignition, which is a prerequisite for premixed combustion. The spray characteristics test results showed that the UMH nozzle has a shorter spray penetration, and bigger angle than that of the conventional nozzle. The shorter spray penetration can reduce spray impingement on the wall, which is beneficial to HC emission control. The bigger spray angle results in more air entrainment into the spray, which can promote fuel-air mixing and help prepare a more homogeneous mixture.The test results showed that the heavy EGR is needed to greatly reduce NOx emission. The EGR rate can reach 80% when the load is lower due to the higher excess air ratio, while the higher load can use the lower EGR rate about 28%. The effect of the EGR on reducing NOx emission is enhanced with the increase of the load. It is necessary to achieve the premixed combustion to remedy the increase of the soot due to the heavy EGR. The test results showed that the ignition delay can be prolonged using the low compression ratio combustion chamber, the cooled EGR, and retarding the injection timing. The injection duration can be shortened using the UMH nozzle, and higher injection pressure. These characteristics can facilitate the better mixing of fuel and air before ignition to form the much more homogeneous mixture, and thus the soot was greatly reduced.The test results showed that the excess air ratioλhas a low threshold to keep the HC, CO, and BSFC from being deteriorated while greatly reduce NOx and soot emissions. This low threshold is about 1.5, which means the low-temperature premixed combustion was restricted to the low load.The control strategies for achieving the low-temperature premixed combustion are as follows. It is necessary to use high EGR rate ( 80%) , low injection pressure (90MPa), -1.5°CA ATDC of injection timing at low load. It is necessary, however, to use low EGR rate (28%), high injection pressure (110MPa), 3°CA ATDC of injection timing at high load.The results showed that the operation ranges of this diesel engine to achieve the low-temperature premixed combustion are as follows. The speed is from the idle speed to the rated speed. The load can reach to 50% of the full load of the corresponding speed (except for 1430r/min). The test diesel engine achieved the better low-temperature premixed combustion at these operation ranges, and thus NOx and soot emissions are simultaneously reduced by about 50%, even by 80%-90% at most test cases, while kept the BSFC from being significantly deteriorated. Especially the low load due to the high excess air ratio achieved much more homogeneous lean-mixture premixed combustion. Therefore the BSFC with the heavy EGR is comparable to the original level, even slightly reduced at some test cases.更多还原