Driving mechanisms of high-cetane number, synthetic fuel autoignition: The case of OME 2−4

Oxymethylene dimethyl ethers (CH 3 O(CH 2 O) x CH 3 or OME x ) are potential synthetic substitutes of diesel and jet fuel that can be acquired through CO 2 recycling and have the unique “clean” feature that they do not contain C-C bonds in their atom chain, but only C-O, and therefore can combust without soot. Also, they present favourable ignition characteristics and high cetane numbers. In order to determine ignition control strategies, the dominant oxidation pathways must be identified. To this end, autoignition of OME 2−4 /air mixtures is studied using the tools of Computational Singular Perturbation algorithm. It is shown that as the length of the atom chain x increases, ignition delays become shorter. This is because, at the very early stages of the process, H-abstraction and fuel break-up reactions realize in a larger number of ways for larger x. The process develops first through a chemical runaway that tends to slow-down autoignition and then through a thermal runaway that tends to speed-up autoignition at a progressively stronger rate. It is shown that the total heat release is independent of x, although the ignition delay becomes shorter with increasing x. The two stable intermediates H 2 O 2 and CH 2 O are shown to shorten ignition delays, when used as additives in the initial mixtures. It is demonstrated that OME x synthetic fuels offer the capability of soot-less combustion, combined with ignition control.