A numerical investigation of the combustion kinetics of reactivity controlled compression ignition (RCCI) combustion in an optical engine

This work numerically investigates the detailed combustion kinetics occurring in an optical, reactivity controlled compression ignition (RCCI) engine. Experimental data from the engine combustion network (ECN) relating to spray H and optical RCCI engine spray/combustion were used for model validation. It was found that the RCCI combustion ignition occurred in the squish, bowl rim edge, and downstream of the spray periphery. To provide insight into key reaction pathways, an in-cylinder reaction pathway analysis method was used, and four characteristic RCCI combustion features were selected: (1) initial low temperature heat release (LTHR) from the high-reactivity fuel (n-heptane) on the spray periphery; (2) intense LTHR, where both iso-octane and n-heptane were converted to intermediates (e.g., CH2O) through oxygen-related reactions; (3) early stage high temperature heat release (HTHR) with CH2O as the core source species; (4) and intense HTHR, characterized by a substantial energy release. Further analysis of the reactive combustion surfaces demonstrated that the interior flame structure was controlled by OH-CO-O2 combustion kinetics and the exterior was controlled by CH2O-HCO (formyl radical) combustion kinetics. In addition, surrounding the reactive surfaces, iso-octane was consumed primarily through decomposition reactions, forming CH2O, which further fueled the following high temperature combustion.