Exploring the Geometric Effects of Turbulence on Cyclic Variability

Cyclic variability in spark ignition engine combustion, especially at high dilution through lean burn or high EGR rates, places limits on in-cylinder NOx reduction and thermal efficiency. Flame wrinkling, resulting from interactions with turbulence, is a potential source of cyclic variations in turbulence. Previous studies have shown that flame kernels are subject to significant distortions when they are smaller than the integral length scale of turbulence. With the assumption that flame development is not subject to noticeable variations, after it reaches the integral length scale, the authors have shown that turbulent-burning-caused combustion variability can be successfully modeled as a function of laminar flame speed and turbulence intensity. This paper explores the contributions of flame wrinkling to flame kernel growth variation. As the kernel growth problem is complex, this study only explores one of the many aspects of the problem. The complete description of the phenomenon requires consideration of additional effects, which are discussed here. However it is shown that geometric variations of the flame kernel can potentially constitute a significant portion of variability. These variations can be considered to be present at all operating conditions at almost constant magnitude. Although the problem is non-linear, studying individual phenomena can provide useful information in understanding variability.