Experimental and numerical study of ignition and combustion processes of primary reference fuels for advanced compression-ignition applications

Abstract

The ignition and combustion processes of primary reference fuels must be well-understood in a “beyond-research octane number” regime at temperature and pressure conditions relevant to compression-ignition engines compared to their spark-ignition counterparts. This study explores the low- and high-temperature ignition and combustion processes in high-pressure spray flames of primary reference fuel 80 (80 % iso -octane, 20 % n-heptane) using simultaneous 50 kHz formaldehyde planar laser-induced fluorescence and 100 kHz schlieren imaging. Experiments were conducted in a constant-volume pre-burn vessel with an Engine Combustion Network Spray A-3 injector, complemented by 0D closed-homogeneous reactor calculations and 3D computational fluid dynamics simulations under matched conditions. The advanced diagnostics provided detailed insights into cool flame development, formaldehyde consumption, high-temperature ignition, and the onset of polycyclic aromatic hydrocarbons formation. Simulations for primary reference fuel 80 combustion were conducted using two distinct chemical mechanisms, effectively capturing key ignition characteristics, though some discrepancies remain, particularly under low-reactive conditions due to model simplifications. The results emphasize the critical role of cool-flame chemistry and two-stage ignition dynamics in spray combustion of primary reference fuels under elevated pressures and temperatures. These findings advance our understanding of ignition processes in primary reference fuels, guiding the development of improved combustion models and fuel formulations for advanced compression-ignition applications. © 2025 Elsevier Ltd.