Clarification and Control of Ignition Mode Transition by Addition of Turbulence

本研究旨在阐明直喷预混压燃(DI-PCCI)的燃烧现象和控制策略，以实现着火和放热过程可控的低NOx(氮氧化物)和PM(颗粒物)排放。为此，我们试图利用强烈的湍流，迫使喷雾点火进入均匀燃料-空气混合物点火的两阶段点火模式。为了基本了解DI-PCCI的点火过程，在定容容器中研究了环境和喷射条件对正庚烷喷雾燃烧的影响。选择环境压力和温度来模拟类PCCI着火和放热过程。改变氧气浓度和喷射条件，在热焰起始点改变了混合气浓度的均匀性。建立了一个完整的气体采样系统，用于测量放热过程中NOx的浓度。基于随机湍流混合模型和简化的正庚烷氧化模型，建立了喷雾着火和燃烧模型，研究了混合气形成和化学反应过程之间的关系：喷嘴孔径较大、喷油压力较低时的弱混合可提前形成热火焰，而喷孔较小、喷油压力较高时的强烈混合可使热火焰延迟、放热速率降低。用全气体采样技术进行的实验表明，相当低的NOx生成量与延迟热火焰的强烈混合情况有关。利用点火和燃烧模型对燃料-空气混合物的当量比和温度历史进行了分析，结果表明，在整个混合气进入贫区时，反应速度随当量比的减小而减小，存在延迟着火和放热率降低的现象。

This study aims at clarification of combustion phenomena and obtaining control strategies for direct-injection Premixed Charge Compression Ignition(DI-PCCI) combustion to realize low NOx(nitrogen oxides) and PM(particulate matters) emissions with controllable ignition and heat release process. To this end, we attempted to utilize intense turbulence, which would force spray ignition into two-stage ignition mode found in ignition of homogeneous fuel-air mixture.To obtain fundamental understanding of ignition process in DI-PCCI, effects of ambient and injection conditions were investigated for n-heptane spray combustion in a constant volume vessel. Ambient pressure and temperature were selected so as to simulate PCCI-like ignition and heat release process. Changing oxygen concentration and injection conditions, homogeneity in mixture concentration was varied at the hot flame initiation. A total gas sampling system was constructed to measure NOx concentration during heat release. A spray ignition and combustion model was developed based on stochastic turbulent mixing model with a simplified n-heptane oxidation scheme to understand the relation between mixture formation and chemical reaction progress.Weak mixing with larger nozzle orifices and lower injection pressure provides earlier onset of hot flame, while intense mixing with smaller orifices and higher injection pressure gives delayed hot flame and reduced heat release rate. Experiments with total gas sampling technique reveals that quite low NOx production connects to the intense mixing case with delayed hot flame. Analysis of histories of equivalence ratio and temperature in fuel-air mixture with the aid of a ignition and combustion model suggests that delayed ignition and reduced heat release rate are given in the case that entire mixture falls into lean region where the reaction speed decreases with decreasing equivalence ratio.