A Study on Fuel Design for Controlling Ignition and Initial Combustion of Transient Fuel Sprays

为了最大限度地减少直喷发动机产生的颗粒物和一氧化氮的排放，需要更均匀和更清洁的燃烧。为此目的，进行了一项基础研究，以澄清燃料的选择和设计策略。从恒容容器中正庚烷喷雾点火的实验结果来看，在燃料-空气混合先于化学反应进行的情况下，发现了两级点火。在这种情况下，通过促进混合而延迟了有效热释放的开始，这与柴油条件下的高度非均质点火不同。结合随机燃料-空气混合模型和Schreiber简化化学动力学格式建立了点火模型。分析了预测结果的细节，对上述测量趋势进行了定性解释。研究了两种结构相同或不同的烃类混合物的着火特性。将i-辛烷加入到正庚烷中导致了两级点火的出现，这是在自然吸气柴油发动机的典型压缩温度和压力下纯正庚烷点火中从未观察到的特征。结果表明，可燃性较低的燃料可作为另一种燃料组分反应的稀释剂。从实验中获得的知识表明，如果可以使用足够的燃料成分，则可以通过优化燃料成分和燃料-空气混合来实现均匀稀薄燃烧。虽然在定量精确预测方面还需要进一步改进，但所建立的点火模型将有助于采用更详细的动力学模型来获得喷雾点火和燃烧的控制策略。

More homogeneous and leaner combustion is needed to minimize the emissions of particulates and nitric oxides from direct-injection engines. For this purpose, a fundamental study was carried out to clarify the strategy for selection and design of fuels.From the experimental results on ignition of a n-heptane spray using a constant-volume vessel, two-stage ignition was found in the case that fuel-air mixing progressed in advance of chemical reactions. In this condition, the start of an effective heat release was delayed by promoting the mixing unlike in highly heterogeneous ignition at diesel conditions.A ignition model was developed combining a stochcastic fuel-air mixing model with a Schreiber's reduced chemical kinetic scheme. Analyzing the details of the predicted results, a qualitative explanation was made on the above measured tendencies.Ignition characteristics were investigated on mixtures of two hydrocarbons with the same or different structures. Addition of i-octane to n-heptane resulted in the appearance of the feature of two-stage ignition that was never observed in pure n-heptane ignition at a typical compression temperature and pressure in naturally aspirated diesel engines. It was suggested that the fuel with lower-ignitability acted as a diluent in the reactions of another fuel component.Knowledges obtained from the experiments indicate the possibility of homogeneous lean combustion by optimizing the fuel composition and fuel-air mixing if adequate fuel components can be utilized. Although further improvement is needed for quantitatively precise predictions, the developed ignition model will be useful to obtain the strategies for control of spray ignition and combustion employing a more detailed kinetic model.