直喷汽油机冷起动和暖机工况壁面油膜对未燃碳氢和微粒排放的影响

With the increasingly stringent emission regulations on the engine-out HC (Hydrocarbon) and particulate emissions from GDI (Gasoline Direct Injection) engines, widespread attentions have been attracted to solve this problem. This project is focusing on investigating the combustion chamber wall fuel films formation, desorption and distribution discipline and their effects on HC and particulate emissions using experimental analysis, numerical simulation and chemical analysis methods in the cold-start and warm-up conditions of GDI engines. This project will reveal the combustion chamber wall fuel film formation mechanism at low wall temperature; determine the spray impingement and evaporation models for GDI engines in cold-start and warm-up operations; clarify effects of operating parameters on the HC and particulate emission amount, and their effects on engine-out particle size distribution discipline and chemical compositions; explore the profound mechanism of fuel film forming HC and particulate emissions; determine the contribution rate of fuel films on the whole HC and particulate emissions at low wall temperature; propose synergetic control algorithms based on fuel films and explore technical measures to reduce the HC and smoke emissions from GDI engines in cold-start and warm-up operations. This project will be certain to provide important theoretical basis and technical support for GDI engines to achieve high-efficiency combustion and ultra-low emission targets.

随着排放法规的日益严格以及人们对微粒危害的深入认识，直喷汽油机未燃碳氢和微粒排放的问题受到广泛关注。本项目针对直喷汽油机未燃碳氢和微粒排放较高的冷起动和暖机工况，利用试验研究、模拟计算和化学分析等手段系统研究燃烧室壁面油膜成膜、脱附和分布规律及其对未燃碳氢和微粒排放的影响。揭示直喷汽油机在壁面温度较低情况下壁面油膜的形成机理及分布规律，确定直喷汽油机冷起动和暖机工况喷雾撞壁模型和油膜蒸发模型，明确运行参数对未燃碳氢及微粒排放数量、质量、粒径分布和化学组分的影响规律，探索壁面油膜形成未燃碳氢和微粒排放的内在机理，确定壁面温度较低时油膜对未燃碳氢和微粒排放的贡献率，提出基于油膜的未燃碳氢和微粒排放的协同控制方法，探索直喷汽油机冷起动和暖机工况降低未燃碳氢和微粒排放的技术措施。为直喷汽油机实现高效燃烧和超低排放目标提供重要的理论依据和技术支持。

本项目研究了直喷汽油机未燃碳氢和微粒排放较高的冷起动和暖机工况。建立了定容弹喷雾撞壁实验平台，进行了喷雾撞壁特性实验，研究了喷油压力、背压、环境温度、表面粗糙度、喷雾撞壁距离和壁面倾斜角度等对喷雾撞壁后的蒸发和雾化的影响规律；搭建了缸内直喷汽油机实验台架，改进了二级稀释系统，针对暖机工况进行实验，研究过量空气系数、点火正时、喷油正时、喷油压力、喷油次数、喷油比例和冷却液温度等参数对直喷汽油机暖机过程的未然碳氢和微粒排放的影响；建立实验用直喷汽油机燃烧室三维仿真模型，利用喷雾撞壁特性实验校验喷雾和蒸发模型，利用台架实验数据校验燃烧模型，仿真研究了冷起动工况的转速和喷油策略对缸内湍流动能、油膜量、浓度场、未燃碳氢和微粒排放的影响。结果表明：.1. 根据定容弹喷雾撞壁实验可知：减小壁面粗糙度，升高环境温度，降低环境压力，适当减小撞壁距离，以及选取合适的壁面倾斜角度，均有助于喷雾撞壁后的蒸发和雾化，改善缸内的油气混合状态，减少未燃碳氢和微粒排放。.2. 根据暖机实验可知：在暖机过程中，冷却液温度的升高，有助于减少微粒排放。压缩行程单次喷射时，采用理论空燃比和微浓混合气，适当推迟点火，选择合适的喷油正时和喷油压力，有利于减少未燃碳氢排放。进气行程多次喷射控制策略、进气压缩两次喷射策略中提高第二次喷射比例和升高冷却水温度均有利于改善发动机的燃烧，降低未燃碳氢和微粒排放。.3. 仿真分析冷起动工况可知：提高转速，可增强缸内气流运动，降低油膜质量，促进混合气的形成。推迟喷油时刻，总油膜量先降低后升高，存在合适的喷油时刻使未燃碳氢排放最低；减小喷油持续期，活塞顶油膜质量先减少后增加，气缸壁和进气门油膜质量逐渐降低，气缸盖油膜质量先增加后减少，未燃碳氢排放减少，缸内soot质量分数降低。两次喷射策略能有效降低油膜总量，改善燃烧状态，降低soot排放。