Cyclic variability is investigated in an optically accessible single-cylinder spark-ignition research engine by introducing artificial exhaust gas in controlled amounts to the homogenous air–fuel mixture before ignition. A skip-fire scheme ensures the absence of internal exhaust gas recirculation (EGR) and allows the engine to be fired continuously for acquisition of large statistics. Four operating conditions ranging from a stable 0% EGR case up to a highly unstable extreme EGR case are analyzed to examine the increasing effects of homogeneous EGR on the cycle performance. To that end, high-speed measurements of the velocity field via particle image velocimetry and flame imaging in the tumble plane allow the determination of phenomena leading to various flame positions and sizes as well as faster and slower combustion cycles. Through extensive conditional statistical and multivariate correlation techniques, flames are found to be heavily influenced by large-scale velocity motion, especially with the presence of greater EGR which leads to lower flame speeds. The greater sensitivity of slower flames to variations in the velocity field manifests itself in an exponential increase in cyclic variability of the maximum in-cylinder pressure and causes misfire cycles where the flame is blown off or quenched at the cylinder roof. In the most extreme cycles at the highest EGR level, the state of the large-scale velocity structures at the time of ignition determines whether the flame propagates towards the center of the cylinder (and is blown off or quenched) or if the flame sustains growth by propagating within the lingering tumble vortex.

在光学可访问的单缸火花点火研究发动机中研究循环变异性，方法是在点火前将受控量的人造废气引入均匀的空气 - 燃料混合物。跳火方案确保不存在内部废气再循环 (EGR)，并允许发动机连续点火以获取大量统计数据。分析了从稳定的 0% EGR 情况到高度不稳定的极端 EGR 情况的四种运行条件，以检查均质 EGR 对循环性能的增加影响。为此，通过粒子图像测速和滚动平面中的火焰成像对速度场进行高速测量，可以确定导致各种火焰位置和大小以及更快和更慢燃烧循环的现象。通过广泛的条件统计和多元相关技术，发现火焰受到大尺度速度运动的严重影响，特别是存在较大的 EGR 会导致较低的火焰速度。较慢的火焰对速度场变化的更大敏感性表现为最大缸内压力的循环变化呈指数增加，并导致火焰在缸顶处被吹灭或熄灭的失火循环。在最高 EGR 水平的最极端循环中，点火时大尺度速度结构的状态决定了火焰是否向气缸中心传播（并被吹灭或熄灭）或火焰是否持续增长通过在挥之不去的翻滚漩涡中传播。