In this study, a single-cylinder direct-injection spark-ignition research engine with full optical access was used to investigate the influence of the flow field and fuel/air mixing on cyclic variability, in particular in the early flame propagation. The engine was operated under lean-burn conditions at 1500 rpm. Two different injection strategies were compared, port-fuel injection (PFI) and direct injection (DI), the latter with early and late injection split about 2:1 in fuel mass. High-speed particle image velocimetry captured the flow in the tumble plane in the compression stroke. The velocity fields and the movement of the tumble vortex are analyzed. Simultaneously, a second camera detected the chemiluminescence of the flame, and the projected area of the line-of-sight-integrated flame luminosity was extracted through morphological image processing. By combining pressure-based combustion analysis and high-speed optical diagnostics, the early flame propagation and the flow field are correlated. In separate experiments the equivalence ratio was imaged for the DI at selected crank angles and correlated with CA10 to learn about the influence of mixture inhomogeneity on early flame propagation. With PFI, the flow near the spark plug just before ignition is closely related to the subsequent speed of combustion. The combustion-relevant flow features can be traced back in time to about –90 °CA. In contrast, the chosen DI scheme results in a decorrelation of flow and flame, and the equivalence ratio distribution at ignition becomes more important. For both flow and mixture fields, regions of high correlation with early-combustion metrics are typically associated with gradients in the multi-cycle average fields.

在这项研究中，使用具有全光学通路的单缸直喷火花点火研究发动机来研究流场和燃料/空气混合对循环变化的影响，特别是在早期火焰传播中。发动机在 1500 rpm 的稀薄燃烧条件下运行。比较了两种不同的喷射策略，端口燃料喷射 (PFI) 和直接喷射 (DI)，后者的早期和晚期喷射在燃料质量上的比例约为 2:1。高速粒子图像测速仪捕获了压缩冲程中翻滚平面中的流动。分析了滚涡的速度场和运动。同时，第二台摄像机检测到火焰的化学发光，并通过形态学图像处理提取视线综合火焰光度的投影面积。通过结合基于压力的燃烧分析和高速光学诊断，早期火焰传播和流场相互关联。在单独的实验中，当量比在选定的曲柄角处为 DI 成像，并与 CA10 相关，以了解混合物不均匀性对早期火焰传播的影响。使用 PFI，点火前火花塞附近的流量与随后的燃烧速度密切相关。与燃烧相关的流动特征可以追溯到大约 –90 °CA。相比之下，选择的 DI 方案导致流量和火焰的去相关性，并且点火时的当量比分布变得更加重要。