Experimental and Numerical Study on Emission in an HCCI Engine Operated with Neat Dimethyl Ether

This paper presents a new detailed chemical kinetic model for dimethyl ether (DME) combustion that consists of 97 species and 457 elementary reactions. Simulation results with this model were compared with experimental DME ignition data and good agreement was obtained.

A new reduced chemical kinetic model for DME involving 36 species and 73 reactions is proposed to improve prediction capabilities of the kinetics models and to identify key reactions and important species for Homogenous Charge Compression Ignition (HCCI) engine applications. Results from this model adequately predict observed results. The kinetic model using this reduced chemical kinetic mechanism includes three sub-models: a low temperature and negative temperature coefficient region sub-model, a pyrolysis and oxidation sub-model for high temperature, and a sub-model for oxides of nitrogen (NO_X). The simplified chemical kinetic model correlates well with the thermodynamic model and can be used to simulate combustion process of DME as well as formation of formaldehyde (CH₂O), formic acid (HCO₂H) and NO_X in HCCI engines, and calculated results also agree well with those from the new detailed model.

In order to gain further insights of CH₂O and HCO₂H formation, the kinetic model was coupled with CFD software FLUENT to predict the combustion and exhaust emissions of DME from the start of compression to the end of the expansion process. The results indicate that the latter part of the expansion process significantly influences engine-out formaldehyde, and high concentration areas of partial oxidation products of CH₂O and HCO₂H were found in the flame quenching layer and piston crevices. Comparisons between calculated and experimental results show that the present simulation methodology is capable of predicting unconventional hydrocarbon emissions of a HCCI engine fueled with DME.