双燃料缸内直喷发动机反应活性控制压燃(RCCI)混合过程对燃烧影响的机理研究

The concept of Reactivity Controlled Compression Ignition (RCCI) provides a new pathway for solving the control problem of low temperature combustion. The project based on the visible experiments of the dual-fuel direct injection mixing process in-cylinder and the RCCI combustion in a Rapid Compression Machine (RCM), combined with the numerical simulation of the computational fluid dynamics and the reaction kinetics. The pictures of velocity and concentration in the mixing process as well as the cylinder pressure and flame pictures in the combustion process will be measured, separately. The changing rules of fuel reactivity and its spatial distribution in the mixing process will be analyzed. The detailed reaction kinetics mechanism suitable for RCCI combustion of methane/n-heptane binary fuel will be developed, which will be further reduced for the CFD calculation. Based on the simulations, the influence mechanism of temperature, fuel concentrations and spatial of reactivity on the RCCI ignition and combustion will be explored. The influence rules of parameters controlled mixing process on the reactivity and its spatial distribution under the condition of the complicated turbulent in cylinder will be obtained from the above research. Moreover, the influence mechanism of charge reactivity, concentration and spatial distribution on the combustion process, as well as the coupling relation between the parameters controlled mixing process and combustion characteristics will be explained. A model will be established to numerically simulate the working process in an engine with direct injection natural gas/diesel. The control mechanism of RCCI combustion will be revealed and the combustion control strategy will be developed based on the parameters controlled mixing process. The implement of this presented project can provide a theoretical understanding of the low temperature combustion, and the achievement can be used to solve the real RCCI engine control problem, which has the important practical value.

反应活性控制压燃(RCCI)概念为解决低温燃烧控制难题提供了新思路。本项目以双燃料缸内直喷混合过程和快速压缩机RCCI燃烧可视化实验为基础，结合计算流体动力学和反应动力学数值模拟，测量混合过程速度及浓度场图像、燃烧过程压力和火焰图像，分析混合过程燃料反应活性及其空间分布的变化规律。发展甲烷/正庚烷RCCI燃烧详细反应动力学机理，利用其简化机理与CFD耦合计算，探究缸内温度、燃料浓度和活性空间分布对RCCI着火和燃烧的影响机理。通过以上研究获得缸内复杂湍流条件下混合过程参数对反应活性及其空间分布的影响规律。阐明充量反应活性、浓度及空间分布对燃烧过程的影响机制和混合过程控制参数与燃烧特性之间的耦合关系。建立缸内直喷天然气/柴油RCCI发动机工作过程数值计算模型。揭示RCCI燃烧控制机理，构建基于混合过程参数的燃烧控制策略。项目的开展将为解决低温燃烧的控制问题提供理论指导，具有重要的工程价值。

反应活性压燃（RCCI）可以满足理想低温燃烧对燃料化学反应活性的差异化要求。理论上可以通过对混合燃料化学反应活性的调控间接实现对低温燃烧过程的有效控制。然而在经典的RCCI模式（低反应活性燃料进气道喷射＋高反应活性燃料缸内直喷）下，低反应活性燃料始终以预混的形态参与混合，制约了充量的混合形态，实际的低温燃烧过程控制效果较差。本项目以内燃机低温燃烧过程的控制为背景，通过实验测量与理论分析相结合的方法，对双燃料缸内低压直喷RCCI发动机混合过程对燃烧的影响机理进行了深入的研究与探索，基本完成了项目计划书的内容。本项目采用空气辅助雾化喷嘴实现了柴油/天然气双燃料缸内低压直喷，为混合过程控制提供了新手段。通过定容燃烧弹双燃料喷雾可视化实验，测量了柴油/天然气低压直喷喷雾图像数据，证明了双燃料喷射参数与充量混合状态的强相关性，获得了双燃料直喷参数（喷射正时，脉宽，质量比等）对混合气喷雾特性（喷雾贯穿距、喷雾锥角及喷雾投射面积）的影响规律;开展了双燃料喷射策略的模拟研究，阐明了湍流条件下，喷射控制参数对充量反应活性和空间分布状态的影响规律，提出了通过控制双燃料直喷参数实现对缸内充量的混合形态及空间分布特征进行调控的方法；分析双燃料缸内低压直喷RCCI发动机燃烧过程调控实验结果，发现柴油喷射正时对CA50的影响最大，天然气喷射正时对火焰发展期有显著影响，替代率的变化主要影响双燃料混合充量的着火时刻，通过控制双燃料喷射策略，可以调控RCCI发动机的着火时刻、火焰发展期，CA50及燃烧持续期。揭示了双燃料混合过程控制对RCCI发动机燃烧特性的影响规律，构建了基于混合过程参数控制的燃烧调控策略，为改善低温燃烧过程控制提供了理论支撑。