Comparative and Optimization Study on Combustion and Flame Characteristics of PODE3 and Diesel in an Optical Engine

Polyoxymethylene Dimethyl Ether (PODEx), a novel electro-synthetic fuel (E-fuel), is being investigated for use in compression ignition engines to support the global transition towards carbon neutrality. This study investigates the combustion and flame development of PODE3 and diesel under single and dual injection strategies in an optical engine system, focusing on the partially premixed combustion (PPC) concept. It also investigates the impact of varying pre-injection energy ratios (PERs) on the compression ignition combustion characteristics of PODE3. Experimental results show that, under both injection strategies, PODE3 exhibits distinct combustion characteristics compared to diesel. Under a single injection strategy, PODE3 demonstrates shorter ignition delay, more advanced combustion phasing, higher peak cylinder pressure, and increased apparent heat release rate (AHRR), along with higher indicated mean effective pressure (IMEP) and reduced cyclic variability. PODE3′s combustion under single injection is characterized by early kernel formation and predominantly blue premixed flames, in stark contrast to the bright yellow soot radiation typical of diesel. It achieves a 75.9 % reduction in peak spatial integration of natural luminosity (SINL) and a 10.9 % increase flame area compared to diesel. Under dual injection with a PER of 60 %, PODE3 maintains superior performance metrics and demonstrates minimal soot formation with predominantly blue flame characteristics. Notably, dual injection facilitates earlier ignition and broader flame distribution, enhancing IMEP and merging flame fronts between adjacent sprays. An optimal PER of 40 % achieves peak performance in terms of cylinder pressure, AHRR, and IMEP, attributed to enhanced fuel atomization and improved air–fuel mixture homogeneity. In contrast, higher PERs (60 % and 80 %) lead to excessively lean mixtures, hindering flame propagation and reducing combustion performance. This study highlights PODE3′s superior combustion and flame characteristics as a promising alternative to diesel under PPC mode, with further enhancement achievable through optimal dual injection strategies.