基于天然气高压直喷与EGR协同的双燃料低速机性能优化基础研究

Natural gas dual-fuel technology is an important development direction of low-speed marine engine in the future. In view of the high NOx emission of natural gas high pressure direct injected dual-fuel low-speed engines, this proposal plans to carry out fundamental research on performance optimization of dual-fuel low-speed marine engine based on coordination of natural gas high pressure direct injection and Exhaust Gas Recirculation (EGR) with the introduction of EGR technology. This proposal takes high pressure natural gas (10~30Mpa) as the main research object. Based on an optical constant volume vessel system, the high-speed schlieren technique, combustion and emission analyzing, and three-dimensional CFD simulation method will be used to study:1) the formation process and mechanism of high pressure direct injected natural gas jetting mixture; 2) the influence mechanism of EGR on jetting combustion of high pressure direct injected natural gas and its optimization measure; 3) the method for performance optimization of natural gas high pressure direct injected dual fuel low-speed engine under EGR synergistic conditions. This proposal intends to illuminate the formation mechanism of high pressure direct injected natural gas jetting mixture, reveal the influence path and inherent mechanism of EGR on high pressure direct injected natural gas jetting combustion, and propose the optimization method for high pressure direct injected natural gas jetting combustion and low-speed engine performance under EGR synergistic conditions. This proposal is the basic theory for performance optimization of natural gas high pressure direct injected dual-fuel low-speed engines. It has important theoretical significance and engineering application value for promoting and guiding the efficient and clean utilization of natural gas on dual-fuel low-speed engines.

天然气双燃料技术是低速机未来的一个重要发展方向。针对天然气高压直喷式双燃料低速机存在的高NOx排放问题，本项目拟导入EGR，开展基于天然气高压直喷与EGR协同的双燃料低速机性能优化基础研究。本项目以高压天然气（10~30Mpa）为主要研究对象，基于光学定容弹系统，借助高速摄影纹影技术、燃烧与排放分析和三维CFD仿真手段，重点研究：1）高压直喷天然气射流混合气的形成过程和机理；2）EGR对高压直喷天然气射流燃烧的影响机制及优化方法；3）天然气高压直喷式双燃料低速机EGR协同条件下性能优化方法；最终阐明高压直喷天然气射流混合气形成的机理，揭示EGR对高压直喷天然气射流燃烧影响的详细路径和机制，提出EGR协同条件下高压直喷天然气射流燃烧及低速机性能的优化方法。本研究是天然气直喷式双燃料低速机性能优化的理论基础，对于推进和指导天然气在双燃料低速机上的高效清洁利用，具有重要的理论意义和工程应用价值。

天然气双燃料船舶发动机技术是未来的一个重要发展方向。针对天然气高压直喷式双燃料发动机存在的高NOx排放问题，本项目导入EGR，开展了基于天然气高压直喷与EGR协同的双燃料发动机性能优化基础研究。基于光学定容弹系统，利用高速摄影纹影技术，研究了双燃料低速机天然气直喷压比范围内的天然气射流过程；建立了高压直喷天然气射流三维仿真模型，研究了高压直喷天然气射流混合气的形成与发展过程。发现高压直喷天然气与空气混合会经历4个过程，其中冲击混合在促进混合气体积的快速增长方面具有优势，并给出了高压直喷天然气射流体积、体积变化率及过量空气系数随射流时间的变化规律；单束柴油引燃单束高压直喷天然气射流的研究表明，高压直喷天然气射流火焰中心存在的内陷区是碳烟的主要生成区域，柴油引燃距离对高压直喷天然气射流的火焰浮起长度和内陷长度有明显影响。基于天然气高压直喷式双燃料重型发动机模型，研究了EGR对高压直喷天然气射流燃烧的影响机制，在此基础上从引燃柴油和天然气本身角度研究了EGR作用下高压直喷天然气射流燃烧的优化方法。发现EGR通过影响引燃柴油着火、燃烧及天然气本身燃烧来对高压直喷天然气射流燃烧过程造成间接和直接影响；缩小柴油喷孔直径、增加引燃油量可增强对直喷天然气的引燃持续时间和引燃强度，都可有效促进天然气射流燃烧过程，一定程度上缓解EGR对燃烧的滞后效应，而天然气预喷可大幅削弱甚至消除EGR对缸内燃烧的负面影响。从引燃柴油、天然气本身及引燃柴油与天然气相互作用角度研究了EGR协同条件下天然气高压直喷式双燃料发动机性能的优化方法。发现较高比例（超过30%）的天然气预喷协同中等比例EGR的策略，可使天然气高压直喷式双燃料发动机获得较好的综合性能；40%天然气预喷比例协同30%EGR率，可将原机（无EGR）NOx、soot和指示比油耗分别降低82.26%、11.15%和2.2%，但未燃甲烷排放大幅升高。