柴油机再循环废气分层的缸内形成机理及其对燃烧边界的调控机制研究

Exhaust gas recirculation (EGR) stratification is an effective measure to solve negative effects on diesel combustion and emissions caused by high EGR rate. However, some mechanisms about EGR stratification are still unresolved. This project focuses on two key scientific research problems, in-cylinder formation mechanism of EGR stratification in diesel engine and its regulation effects on combustion boundary. Quantitative characteristics of EGR spatial distribution and evolution in the directions of radial, axial and injection spray will be analysed, Large eddy simulation and advanced particle image velocimetry are adopted to investigate spatial distribution and evolution of in-cylinder flow and local in-cylinder flow respectively. The origin mechanism of EGR stratification related with in-cylinder flow will be identified. The field synergy principle between velocity field and EGR concentration field will be provided according to field synergy theory of convective mass transfer, the effects of field synergy relationship on EGR stratification formation will be clarified. In-cylinder equivalence ratio and temperature distribution will be extracted, the effects of EGR stratification on combustion boundary will be revealed combined with EGR distribution. The match mechanism between EGR stratification and injection strategy targeted on optimal in-cylinder equivalence ratio and temperature distribution will be proposed. Finally, EGR stratification will be organized based on mechanisms obtained from above research results to improve and optimize existing EGR stratification. The project will improve the application of EGR stratification in diesel engine.

再循环废气（EGR）分层是解决高EGR率对柴油机燃烧与排放性能不利影响的有效措施。然而柴油机EGR分层的缸内形成机理及其对燃烧边界的调控机制等基础科学问题尚不清楚，本课题拟针对上述问题开展研究。针对多缸柴油机应用EGR空间分布量化指标获得EGR分层轴向、径向、油束方向的演化规律，结合大涡模拟手段及光学可视化试验获得缸内流动细节演变规律与进气门处关键流场特性，确定缸内流动特性对EGR空间分布时空演化的影响机制；应用场协同理论获得速度场与EGR浓度场的协同关系，探明场协同关系对EGR分层形成的影响机制；获得不同EGR分层特征下缸内燃空当量比-温度分布，揭示EGR分层对燃烧边界条件的调控机制；以调控燃空当量比-温度分布为目标，建立EGR分层与喷油策略的匹配机制，并依据所获得的EGR分层理论，对已有的分层系统进行改进与优化。项目将为EGR分层在柴油机节能减排领域的应用提供理论支撑。

近年来，为了应对不断升级的环境污染问题，世界各国不断加严发动机燃油消耗及排放法规，推动内燃机高效、超低排放技术的发展。采用高EGR因具有突破传统柴油机氮氧化物和颗粒物排放极限的潜力而备受关注。然而，全局高EGR引起燃烧效率降低、碳烟排放升高。EGR分层被认为是解决高EGR不利影响的有效途径，展开废气分层形成及其对燃烧边界条件调制的基础研究对高效清洁发动机开发具有十分重要的意义。.本项目基于EGR分层台架试验数据，对缸内组分分层形成过程及燃烧过程进行大涡模拟，利用空间分层量化指标分析了EGR在气缸内气缸轴向、径向、油束方向的分层效果；分析了速度场、湍动能场、缸内拟序结构的演变规律及其对EGR分层效果的影响；分析了不同时序进气策略缸内速度场与组分质量浓度场的场协同特征；分析了废气分层引起的缸内温度-当量比燃烧边界变迁规律；分析了废气分层条件下，不同喷油规律对发动机热效率、碳烟和氮氧化物排放的影响规律。.所得主要结论如下：进气初期进气流速最高，湍动能最强，拟序结构贯穿整个气缸，气体组分混合强烈，不应在进气初期设置空气与废气时序交换；随着活塞下移，缸内气流运动衰减，混合变弱，有利于形成显著的废气分层；与废气早进和二次废气进入策略相比，废气晚进时序进气策略在1330、1650和1965r/min转速下均能获得更显著的废气分层，燃烧效率、氮氧化物和碳烟排放也得到显著改善。时序进气产生的废气分层特征由废气进气时段控制，提前喷油能够与晚进时序进气策略废气分层相匹配，同时降低氮氧化物和颗粒物排放并改善燃烧效率；时序进气策略使空气与再循环废气在时间和空间上最大限度分离，避免了气体在进气道内的相互混合，并缩短了气体在气缸内的混合时间，借助缸内流动衰减、拟序结构涡团分布及流场与组分质量浓度场的场协同特性，在压缩上止点附近形成良好的缸内组分分层，结合喷油规律调制燃烧过程中缸内温度和当量比分布，使燃料燃烧路径避开碳烟和氮氧化物生成区间，实现高效清洁燃烧的目的。.