Clarification and Control of Ignition Mode Transition by Addition of Turbulence

本研究旨在澄清燃烧现象，获得直喷预混装药压缩点火（DI-PCCI）燃烧的控制策略，实现点火和放热过程可控的低NOx（氮氧化物）和PM（颗粒物）排放。为此，我们尝试利用强湍流迫使喷雾点火进入均质燃料-空气混合气点火时的两级点火模式。为了从基础上了解DI-PCCI的点火过程，研究了环境条件和喷射条件对正庚烷在定容容器中喷雾燃烧的影响。选取环境压力和温度，模拟类似pcci的点火和放热过程。改变氧浓度和注射条件，会改变热爆时混合液浓度的均匀性。构建了全气体采样系统，用于测量放热过程中的NOx浓度。基于简化正庚烷氧化方案的随机湍流混合模型，建立了喷雾点火燃烧模型，以了解混合物形成与化学反应过程的关系。较大喷嘴孔和较低注射压力的弱混合可以使热火焰提前发生，而较小喷嘴孔和较高注射压力的强混合可以使热火焰延迟并降低热释放速率。用全气取样技术进行的实验表明，较低的NOx产量与延迟热火焰的强烈混合情况有关。利用点火燃烧模型对燃料-空气混合物等效比和温度的变化历史进行了分析，结果表明，当整个混合物处于反应速度随等效比减小而减小的稀薄区时，燃料-空气混合物的点火延迟，放热速率降低。

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.