Auto-ignition characteristics and kinetic modeling study of PODE3/n-heptane blends

Abstract

Polyoxymethylene dimethyl ethers (PODE_n) are promising oxygenated additives for diesel engines due to their high cetane number and low soot emissions. However, the auto-ignition characteristics of PODE₃ blended with diesel surrogate fuels like n-heptane remain unclear, especially under low-to-intermediate temperature conditions (600–1000 K). In this work, the ignition delay times (IDTs) of PODE₃/n-heptane blends (10–40 % PODE₃ molar fraction) were measured in a rapid compression machine (RCM) under stoichiometric conditions (φ = 1.0), pressure of 10 bar, and temperatures ranging from 600 to 1000 K. Results show that the IDT decreases significantly with higher PODE₃ content, particularly above 800 K. While below 700 K, the effect of PODE₃ addition on the IDTs was less pronounced. A merged kinetic model combining validated PODE₃ and n-heptane mechanisms accurately captured the IDT trends with varying fuel compositions and their negative temperature coefficient (NTC) behaviors. Kinetic analyses revealed that PODE₃ accelerates n-heptane's first-stage ignition by enhancing radical accumulation (e.g., ȮH) through H-atom abstraction. Sensitivity analysis identified HȮ2 radical dynamics as critical in controlling system reactivity, with PODE₃ exhibiting a stronger promotion effect than n-heptane at higher temperatures. Reaction pathway analysis further indicated that temperature elevation shifts fuel consumption toward PODE₃-dominated β-scission reactions, generating CH₂O and H₂O₂, which decompose to ȮH radicals and accelerate ignition. These findings provide critical insights into optimizing PODE₃-blended fuels for advanced engine designs.