Development of a reduced polyoxymethylene dimethyl ethers (PODEn) mechanism for engine applications

Polyoxymethylene dimethyl ethers (PODEn) features high oxygen content (> 45 wt%) and cetane number (> 60) and is a promising alternative fuel for diesel engines. In the current study, a reduced primary reference fuel (PRF)-PODEn chemical kinetic mechanism consisting of 145 species and 585 reactions is proposed for combustion and soot formation prediction. The PODEn sub-mechanism is reduced from a comprehensive mechanism using an iterative reduction procedure with different mechanism reduction methods (directed relation graph with error propagation, sensitivity analysis and isomer lumping), and is then incorporated into the base reduced PRF-PAH mechanism. This reduced PRF-PODKn mechanism has been extensively validated against experimental results including ignition delay times, laminar flame speeds, species concentration profiles and homogeneous charge compression ignition (HCCI) combustion. At specific operating conditions, PODE3HCCI combustion exhibits a three-stage heat release, which is found to be caused by PODE 3 decomposition, CH2O-to-CO reactions and CO-to-CO2 oxidation, respectively. Furthermore, the present mechanism is further applied in three-dimensional simulations of the engine direct injection compression ignition (DICI) combustion fueled with pure diesel (D100), gasoline/diesel blends (GD50) and gasoline/diesel/PODKn blends (GDP30). Compared to D100, the soot emissions are reduced with GD50 and GDP30 by 75% and 96%, respectively, due to the higher ratio of premixed combustion and higher fuel oxygen content. Additionally, 0-D modeling investigations with the present mechanism are conducted to clarify the chemical effects on soot formation. The A(4) formation of PODE3 is three orders of magnitude smaller than that of PRF since no C-C bonds exist in PODE3's molecules. The decline of A(4) formation in the n-heptane/iso-octane/PODE3 blend illustrates that oxygenated fuels like PODEn are beneficial to the soot reduction.