高负荷下正丁醇/正辛醇双燃料内燃机内火焰扩散特性与爆震机理研究

Reactivity controlled compression ignition (RCCI) is one of the techniques to achieve high efficiency combustion and low NOx and soot formation in internal combustion engines. However, knocking phenomenon caused by high pressure rise rate and end-gas ignition tends to appear under high load conditions in dual-fuel engines. To address this issue, n-butanoal and n-octanol are selected as the port-injected fuel and direct-injected fuel, respectively. One of the reasons to choose alcoholic fuels is that their high latent heat of vaporization can ease the knock under high load. Then fundamental study is conducted on knock by considering flame propagation. Hence, methods on mitigating knock can be proposed which could finally benefit load extension. In this project, a multi-component reaction mechanism including n-butanoal and n-octanol is developed. This mechanism also works well under knocking conditions. The ignition characteristics and reaction pathway with different fuel blends can be investigated. Secondly, to detect the interaction of flame propagation and knock under high load conditions, the developed reaction mechanism, innovative adaptive knocking detection model and flame propagation model are coupled with KIVA4-CHEMKIN. Then, based on their interaction relation, the effects of the length of carbon bonds, oxygen concentration of fuels and the in-cylinder equivalence ratio on flame propagation and knock are studied under high load conditions in a dual-fuel engine. Based on above analysis, a theory of mitigating knock in dual-fuel engines can be proposed. In summary, the results of this project could contribute to improving the working stability of dual-fuel engines under full load range.

双燃料反应度控制燃烧是实现内燃机高效燃烧、低氮氧化物和碳烟排放的关键技术之一。但双燃料内燃机在高负荷下易发生燃烧粗暴及末端混合气自燃的爆震现象。为此，本项目选用正丁醇和正辛醇分别作为气道喷射及直喷燃料，利用醇类燃料高汽化潜热特性抑制高负荷下的爆震，并结合火焰扩散研究爆震机理，优化控制方法，实现负荷拓展。研究中，构建能够预测爆震并适用于双燃料燃烧的正丁醇/正辛醇多组分燃料模型，研究燃料掺混比对着火特性及化学反应路径的影响规律；基于KIVA4-CHEMKIN程序耦合燃料模型，创新性地开发自适应爆震区域探测模型，并结合火焰扩散燃烧模型，研究爆震工况下火焰扩散前锋面传播特性与爆震的相关性及其机理；由此研究醇类燃料碳链长度、含氧比例及缸内当量比分布对双燃料内燃机高负荷下火焰扩散传播特性和爆震强度的影响规律，最终提出避免爆震的有效控制方法理论。研究成果对双燃料内燃机实现全负荷稳定工作具有重要指导意义。

双燃料反应度控制燃烧是实现内燃机高效燃烧、低氮氧化物和碳烟排放的关键技术之一。但双燃料内燃机需要增加预混燃料比例实现高负荷工作，而高预混燃料比例使得气缸内易发生燃烧粗暴及末端混合气自燃的爆震现象。为此，本项目选用正丁醇和正辛醇分别作为气道喷射及直喷燃料，利用醇类燃料高汽化潜热特性抑制高负荷下的爆震，并结合火焰扩散研究双燃料内燃机爆震机理，最后通过调节控制参数达到缓解爆震的目的。研究中，首先构建了正丁醇/正辛醇多组分燃料模型，并从着火延迟、层流火焰速度、组分浓度等多个角度验证了该模型的可靠性，并揭示了正丁醇/正辛醇掺混比对着火特性的影响规律。然后将该燃料模型与三维CFD代码KIVA4-CHEMKIN耦合，实现了正丁醇/正辛醇双燃料内燃机数值模拟，通过与内燃机实验数据对比验证了该模型。为了研究爆震机理，选取正丁醇高预混比例工况为研究对象，建立了正丁醇/正辛醇双燃料火焰传播模型，以OH浓度锋面表示火焰锋面，并在燃烧室内选取了10个局部区域作为监控点，通过压力振荡法处理局部缸压，判定爆震强度，并结合自由基浓度等进行分析，结果表明在缸壁附近产生了较为强烈的爆震，主要是由正丁醇-空气混合气发生低温燃烧导致的，该研究揭示了高预混比例下双燃料内燃机爆震产生的机理。在此基础上，研究了燃料比例、废气再循环、进气门关闭角和双次喷射策略等边界条件对双燃料内燃机压力、热释放率、压力升高率、火焰锋面等燃烧特性及排放特性的影响规律，提出了缓解爆震的控制方法。本项目执行为实现正丁醇/正辛醇双燃料内燃机全负荷稳定工作具有重要指导意义。