A Study on Fundamental Combustion Properties of Oxymethylene Ether-2

Oxymethylene ethers (OMEn, n=1-5) are a promising class of synthetic fuels that have the potential to be used as additives or substitutes to diesel in compression ignition engines. A comprehensive understanding of their combustion properties is required for their safe and efficient utilization. In this study, the results of a combined experimental and modeling work on oxidation of OME2 are reported: (i) Ignition delay time measurements of stoichiometric OME2/synthetic air mixtures diluted 1:5 with nitrogen using the shock tube method at pressures of 1, 4, and 16bar, and (ii) laminar flame speeds of OME2/air mixtures using the cone angle method at atmospheric and elevated pressures of 3 and 6bar. The experimental data sets obtained have been used for validation of three detailed reaction mechanisms of OME2 obtained from literature. The results of ignition delay time measurements showed that ignition of OME2 is characterized by pre-ignition activity at the low temperature side of the measurements regardless of the pressure. Regarding the performance of the different reaction mechanisms, the model from Cai et al. (2020, "Auto-Ignition of Oxymethylene Ethers (OMEn, n = 2-4) as Promising Synthetic e-Fuels From Renewable Electricity: Shock Tube Experiments and Automatic Mechanism Generation," Fuel, 264, p. 116711) best predicted the temperature and pressure dependence of ignition delay times. For laminar flame speeds, the experimental data were well matched by the mechanism from Ren et al. (2019) at p=1, 3, and 6bar and for all equivalence ratios considered. From sensitivity analyses calculations, it was observed that chain reactions involving small radicals, i.e., H, O, OH, HO2, and CH3 control the oxidation of OME2. The comparison of the results of this work and our previous work (Ngugi et al. (2021)) on OME1 show that these two fuels have similar oxidation pathways. The results obtained in this work will contribute to a better understanding of the combustion of oxymethylene ethers, and thus, to the design and optimization of burners and engines as well.