Investigation into auto-ignition characteristics and kinetics modeling of PODE₂/n-heptane mixtures using an RCM

Polyoxymethylene dimethyl ethers (PODE_n) have emerged as promising oxygenated synthetic fuels for compression ignition engines applications. While extensive research has focused on the combustion and emissions of PODE_n/diesel-fueled engines, fundamental understanding of their ignition behavior remains limited, despite its critical importance for computational fluid dynamics simulations in fuel/engine co-optimization. Therefore, this study measured ignition delay times (IDTs) of stoichiometric PODE₂/n-heptane blends in a rapid compression machine at 10 bar and temperatures ranging from 600 to 1000 K. A merged kinetic mechanism for n-heptane/PODE₂ blends was developed and validated, showing good agreement with the experimental IDTs. Results demonstrate that blending PODE₂ into n-heptane reduces IDTs and enhances mixture reactivity, particularly between 800 and 950 K. When the blending ratio of PODE₂ is relatively high, the negative temperature coefficient (NTC) behavior is progressively attenuated, leading to a slight increase in IDTs within the NTC region due to the absence of NTC characteristics in neat PODE₂. Kinetic modeling analyses reveal that at 700 K, 40 % PODE₂ addition advances the onset of first-stage ignition through enhanced OH radical generation and heat accumulation, though the total IDT is slightly extended. At 900 K, PODE₂ promotes earlier ignition, driven by increased OH formation via the CH₃ + HO₂ → CH₃O + OH pathway despite a reduction in OH production from H₂O₂ decomposition. Reaction pathway and rate-of-production analyses indicate that H-abstraction and secondary O₂-addition reactions of n-heptane dominate ignition chemistry at low PODE₂ content, while PODE₂ oxidation becomes increasingly influential with rising blend ratio or temperature. These findings provide mechanistic insights into the combustion behavior of PODE₂/n-heptane blends and inform strategies for fuel/engine co-optimization.