Topological development of homogeneous-charge and spray-guided stratified-charge flames in an internal combustion engine

Abstract

In this work, engine flame topological development was studied to improve the understanding of flame growth in direct-injection engines, in particular when operated under homogeneous-charge and spray-guided stratified-charge combustion conditions. Flame wrinkledness is one of the most important and poorly understood engine combustion phenomena. Generally, a flame may wrinkle for two reasons: (1) its own natural instabilities and/or (2) through interaction with turbulent flow. The relative contribution of these two causes toward flame wrinkledness in the engine environment was unclear, so targeted experiments were performed to provide some clarity. The development of flame wrinkledness within an optically accessible engine was measured with a combination of planar laser–induced fluorescence and stereo particle image velocimetry under homogeneous-charge and stratified-charge conditions. Using the resulting data, equivalence ratio, charge velocities, and flame wrinkledness were quantified and analyzed. For the iso-octane–toluene mixtures studied, flame wrinkling was insensitive to thermo-diffusive flame front instabilities. The relative contribution of wrinkles of various spatial scales toward overall flame wrinkledness was also measured. Homogeneous-charge flames generally had lower wrinkling factors than stratified-charge flames. Overall, flame wrinkledness increased with flame size under both modes of engine operation. Large flames demonstrated an ability to maintain more large-scale wrinkles than small flames, which contributed to their overall higher levels of wrinkledness.