高功率密度柴油机混合气形成和燃烧过程基础研究

In order to meet more and more stringent requirements in toxic emissions and fuel consumption, Diesel eninges for vehicle are more and more downsized. As a result, power density of diesel engines for passenger car has been increased in recent years from 40kW/L to 100kW/L. This research proposal focuses on basic sytudy into mixture formation and combustion process of high-power-density (HPD) diesel engines so as to overcome key technical barriers in combustion system development during the product development of HPD diesel engine with a power density output of greater than 90kW/L. Main research contents of this proposal are that: (1) dense fuel spray characteristics and their effects on air/fuel mixing process under conditions of high injection pressure (>180MPa), high air density (>60kg/m3) and high average fuel injection rate; (2) multi in-cylinder air flow patterns and characteristics, turbulence structures and their influences on air/fuel mixing process under the condition of high intake pressure (>0.45Mpa) and high air intake flow rate; (3) fast-burn and high-efficiency combustion process, mixing rate control and heat release rate control, as well as parametric optimization across engine cycles. These researches are carried out in a constant volume bomb with high background pressure and air density, an optical engine with quartz windows and a single-cylinder HPD diesel engine. Advanced laser diagnostics such as PIV and PLIF are used to visualize fuel injection spray, in-cylinder air flow, and air/fuel mixing concentration. More detailed analysis of fuel injection, in-cylinder flow and their effects on air/fuel mixing and combustion process are conducted by virtue of multi-dimensional numerical computation. Through these research activities it is expected to reveal initial mechanisms in and between fuel spray, air flow, air/fuel mixing and combustion for HPD diesel engines. These research results can be used to guide HPD diesel engine combustion system development.

为了满足日益严苛的有害排放和燃油经济性法规，车用柴油机正向高功率密度(Downsize)方向发展，但是其混合气形成和燃烧过程与传统柴油机有很大差别。本项目针对升功率90kW/L以上高功率密度柴油机燃烧技术瓶颈，开展高功率密度柴油机混合气形成和燃烧过程基础研究。采用先进激光诊断技术和多维数值模拟技术，在高温高压定容弹、光学发动机和高功率密度单缸柴油机上开展下列研究内容：（1）在高喷射压力、高工质密度、大燃油喷射速率条件下，密集燃油喷雾特性及其对油气混合过程的影响；（2）在高进气压力、大进气速率条件下，缸内空气运动特性及其对油气混合过程的影响；（3）在低过量空气系数、短主燃期、大燃料燃烧速率条件下，快速高效燃烧过程组织、放热速率控制以及多参数热力循环优化。揭示高功率密度柴油机喷雾、流动、混合和燃烧的内在规律，为我国先进高功率密度柴油机燃烧技术开发提供理论指导。

随着汽车排放法规和油耗法规的日益严格以及军用装备高机动性的要求，无论民用发动机还是军用发动机都在朝高强化方向发展。当前国际上车用柴油机的功率密度已达90kW/L，未来10年还将进一步提高到120kW/L；我国发动机的功率密度与国外差距较大，应该迎头追赶。高强化柴油机的开发首先遇到混合气形成和燃烧过程组织的技术挑战，本项目围绕高强化柴油机混合气形成和燃烧过程的关键科学问题，在燃油喷雾特性、缸内空气运动、油气混合和燃烧放热三方面开展了大量基础研究工作，取得了如下创新性研究成果：1）在高强化柴油机喷雾、流动、混合和燃烧方面获得了一批基本规律，积累了大量基础数据，丰富了燃烧系统设计和匹配经验，在一台高强化单缸柴油机上成功实现了90kW/L功率密度强化指标。2）发现了高强化柴油机进气和缸内流速、速度场分布、涡流比、湍流强度及湍流场分布等不随增压压力提高而变化的规律，揭示了进气密度提高才是高强化柴油机进气量增加的主要贡献和几乎唯一的原因；推导了压缩过程中涡流比预测模型；揭示了涡心进动现象造成切向脉动速度的变化是燃烧室中心区湍流能量生成的来源。3）在燃油喷射过程中，高压高速喷雾及其诱导的空气卷吸运动是促进缸内燃油蒸发和油气混合的主要因素，其中喷油压力、喷孔直径及燃烧室结构对喷雾和混合起重要作用；在停止喷油的中后期燃烧过程中，需要组织一定的涡流或挤流促进混合和燃烧；挤流能显著加强燃烧室中心的湍动能；低涡流比和多喷孔数配合能够获得良好的混合和燃烧过程。4）采用进气门晚关的米勒循环技术，可以在功率输出不变的情况下显著降低最大燃烧压力和温度，有效降低高强化发动机的机械负荷和热负荷。这些研究成果已经在国防装备动力的重大研发中得到应用，将为我国高强化柴油机燃烧系统设计开发提供理论依据。已发表论文18篇，其中国际期刊及会议论文10篇，在全国内燃机学术会议上做特邀报告1次，邀请内燃机行业专家20多人召开“高强化柴油机燃烧技术研讨会”1次。