Characteristics of auto-ignition in a stratified iso-octane mixture with exhaust gases under homogeneous charge compression ignition conditions

Ignition and propagation of a reaction front in a counterflow system of an iso-octane/air stream mixing with an exhaust gas stream is computationally investigated to understand the fundamental characteristics of homogeneous charge compression ignition (HCCI) auto-ignition. Various mixing rates are imposed on the system and the effects of dissipation rates on auto-ignition are studied. Ignition delay and front propagation speed across the mixing layer are determined as a function of a local mixture fraction variable. The results show that mixture inhomogeneity and dissipation rate have a significant influence on ignition. Diffusive transport is found to either hamper or advance ignition depending on the initial reactivity of the mixture. Based on the relative importance of diffusion on ignition front propagation, two distinct ignition regimes are identified: the spontaneous ignition regime and the diffusion-controlled regime. The transition between these two regimes is identified using a criterion based on the ratio of the timescales of auto-ignition and diffusion. The results show that ignition in the spontaneous regime is more likely under typical HCCI operating conditions with iso-octane due to its high reactivity. The present analysis provides a means to develop an improved modelling strategy for large-scale engine simulations.