Interaction and ignition process of multiple injections of oxygenated fuels in an optical, heavy-duty diesel engine

Poly-oxymethylene ethers (OMEs) are a class of highly oxygenated synthetic fuels that offer promising pathways for decarbonizing transportation and enhancing energy security. Their favorable ignition properties, high cetane number, and soot-free combustion characteristics make them attractive alternatives to conventional diesel. However, their lower energy density, weaker negative temperature coefficient (NTC) behavior, and rapid mixing due to fuel-bound oxygen introduce complex interactions during combustion, particularly under multiple-injection strategies common in modern diesel engines. This study investigates the ignition and combustion behavior of OMEs compared to a conventional non-oxygenated surrogate fuel (n-dodecane) under various pilot-main injection configurations using a heavy-duty, optical single-cylinder engine. A suite of diagnostics, including apparent heat release rate (AHRR) analysis and simultaneous planar laser-induced fluorescence (PLIF) imaging of formaldehyde (HCHO) and hydroxyl (OH), was employed to capture the low- and high-temperature combustion phases. Experiments were conducted across a matrix of pilot injection durations, dwell times, and EGR dilution levels to evaluate their influence on ignition delay (ID), flame structure, and heat release dynamics. Results show that OME requires longer pilot injections to overcome rapid lean-out and achieve comparable ignition assistance due to its low stoichiometric air–fuel ratio (AFR_ST) and reduced LTHR contribution. A critical minimum injection duration was identified for OME below which the pilot fails to ignite, a behavior not observed with n-dodecane. Despite this, OME displays rapid, volumetric ignition once combustion initiates, owing to favorable mixture stratification from fuel-bound oxygen. A conceptual model is proposed to distinguish ignition regimes based on pilot duration and fuel oxygenation level, explaining the interplay between entrainment-driven mixing, HTHR suppression, and reactive zone formation. The findings enhance understanding of the underlying physics governing multiple injections and provide guidance for optimizing pilot strategies when adapting diesel engines to oxygenated fuels like OME.