Ignition and Combustion Characteristics of OME₃₋₅ and N-Dodecane: A Comparison Based on CFD Engine Simulations and Optical Experiments

Abstract

Synthetic fuels derived from renewable power sources, so-called e-fuels, will play a crucial role in achieving climate-neutral future mobility because they can be used in the existing fleets and in hard-to-decarbonize applications. In particular e-fuels that contain oxygen in their chemical structure can also burn more cleanly in terms of soot formation. For compression-ignition engines, polyoxymethylene dimethyl ethers (PODEs or OMEs) are among the most promising candidates for such oxygenated e-fuels. Here, we investigated the characteristics of injection and combustion of OME3-5 mixture compared to n-dodecane, a reference diesel-like fuel. Both single and multi-injection, comprising a short pilot injection, is used. Experiments were performed in a single-cylinder optically accessible Bowditch-type engine, injecting with 1500 bar pressure with a 3-hole injector (Spray B of the Engine Combustion Network). Liquid and vapor penetration were measured by imaging the spray illuminated by a pulsed light-emitting diode (LED). Ignition delay, lift-off length and flame morphology were investigated based on multi-spectral high-speed imaging of chemiluminescence. For simulations, a 3D CFD engine model was developed. The combustion simulation was performed on a 120° sector mesh onto which flow and turbulence fields from a gas exchange simulation are mapped prior to fuel injection. The model accounts for piston-ring blow-by. For the combustion of both fuels, detailed reaction mechanisms were used. In general, quite good agreement between model predictions and experimental results was achieved. In particular the consideration of blow-by losses by the CFD model produced a realistic behavior during the high-pressure cycle. Both CFD simulation and optical experiments, reveal significant differences between the two fuels. For OME, the liquid phase penetrates further into the combustion chamber, the ignition delay is shorter compared to n-dodecane and the equivalence ratio of OME during combustion is significantly leaner.