Geometric and Positional Isomer Effects on Ignition Behavior of Cycloalkanes: Implications for Sustainable Aviation Fuels

This study investigates the ignition behavior of six dimethylcyclohexane (DMCH) isomers and cis-/trans-decalin, focusing on the effects of molecular geometry and substitution patterns. Ignition delay and derived cetane number (DCN) were measured using a CFR ignition quality tester under ASTM D6890 conditions. Among the DMCH isomers, cis-1,3-DMCH exhibited the highest reactivity (DCN = 37.4), while cis-1,2-DMCH showed the lowest (DCN = 21.8). The most significant stereochemical effect was observed between cis- and trans-1,3-DMCH, with a 12-unit DCN difference. Temperature- and pressure-dependent tests of decalin isomers revealed that cis-decalin consistently ignited faster and exhibited stronger pressure sensitivity, supporting a mechanistic interpretation based on 1,5- versus 1,6-H shift pathways. Additionally, the impact of trace polar degradation products on ignition delay (ID) was assessed, revealing significant ID reduction in trans isomers after storage. These findings highlight the need to consider geometric isomerism in kinetic modeling, sustainable aviation fuel formulation, and surrogate design, particularly for fuels rich in cycloalkanes.