  Despite the high technical relevance of early flame kernel development for
the reduction of cycle-to-cycle variations in spark ignition engines, there is
still a need for a better fundamental understanding of the governing
in-cylinder phenomena in order to enable resilient early flame growth. To
isolate the effects of small- and large-scale turbulent flow motion on the
young flame kernel, a three-dimensional DNS database has been designed to be
representative for engine part load conditions. The analysis is focussed on
flame displacement speed and flame area in order to investigate effects of
flame structure and flame geometry on the global burning rate evolution. It is
shown that despite a Karlovitz number of up to 13, which is at the upper range
of conventional engine operation, thickening of the averaged flame structure by
small-scale turbulent mixing is not observed. After ignition effects have
decayed, the flame normal displacement speed recovers the behavior of a laminar
unstretched premixed flame under the considered unity-Lewis-number conditions.
Run-to-run variations in the global heat release rate are shown to be primarily
caused by flame kernel area dynamics. The analysis of the flame area balance
equation shows that turbulence causes stochastic flame kernel area growth by
affecting the curvature evolution, rather than by inducing variations in total
flame area production by strain. Further, it is shown that in local segments of
a fully-developed planar flame with similar surface area as the investigated
flame kernels, temporal variations in flame area rate-of-change occur.
Contrasting to early flame kernels, these effects can be exclusively attributed
to curvature variations in negatively curved flame regions.
