Niegemann Philipp, Herzler Jürgen, Fikri Mustapha, Schulz Christof
Institute for Combustion and Gas Dynamics-Reactive Fluids, University of Duisburg-Essen, 47048 Duisburg, Germany.
Rev Sci Instrum. 2020 Oct 1;91(10):105107. doi: 10.1063/5.0024614.
The interaction of fuel and lubricant droplets with gaseous fuel/air mixtures close to autoignition is relevant in the context of unwanted early autoignition in spark-ignition internal combustion (IC) engines. To study the influence of droplets on the ignition of fuel/air mixtures independent from the in-cylinder pressure/temperature history, the shock-tube technique in combination with an injection system was established, which enables the generation and injection of single droplets or droplet clusters of n-dodecane and lubricant base oil behind reflected shock waves at pressures and temperatures representative for the compression phase of IC engines. Injected droplets were imaged by high-repetition-rate laser-induced fluorescence. The ignition process was observed by imaging in the visible and UV simultaneously through the shock-tube end wall with a combination of color- and UV-sensitive high-repetition-rate cameras. It was found that the amount and composition of the injected liquid are important factors determining the extent of the interference with the ongoing autoignition of the premixed fuel/air bath gas. For a stoichiometric mixture of primary reference fuels (PRF95) in air, the droplets significantly accelerate ignition especially in the negative temperature coefficient regime at around 760 K. The comparison of the timing of local ignition and the occurrence of volumetric ignition indicates that only in cases where the surrounding gas is close to autoignition, the droplets can trigger early autoignition. This required temporal and spatial coincidence might explain the high level of randomness of early autoignition in engines.
在火花点火式内燃机中，燃油和润滑油液滴与接近自燃的气态燃料/空气混合物之间的相互作用与意外的早期自燃情况相关。为了独立于缸内压力/温度历史来研究液滴对燃料/空气混合物点火的影响，建立了一种将激波管技术与喷射系统相结合的方法，该方法能够在代表内燃机压缩阶段的压力和温度下，在反射激波后生成并喷射正十二烷和润滑油基础油的单个液滴或液滴簇。通过高重复率激光诱导荧光对喷射的液滴进行成像。利用对颜色和紫外线敏感的高重复率相机组合，通过激波管端壁同时在可见光和紫外线波段成像来观察点火过程。研究发现，喷射液体的量和成分是决定对预混燃料/空气浴气正在进行的自燃干扰程度的重要因素。对于空气中的一次参考燃料（PRF95）的化学计量混合物，液滴显著加速点火，尤其是在约760K的负温度系数区域。局部点火时刻与体积点火发生情况的比较表明，只有在周围气体接近自燃的情况下，液滴才能引发早期自燃。这种所需的时间和空间上的巧合可能解释了发动机中早期自燃的高度随机性。
