Autoignition of dimethyl ether and dimethoxy methane sprays at high pressures

Recent studies suggest that the use of ethers as fuels or fuel additives may be a key to the simultaneous reduction of both particulate and NO _x emissions from Diesel engines. The present study is directed towards understanding the chemical kinetics of autoignition of ethers under Diesel-like conditions. Autoignition experiments were performed in a constant volume apparatus (CVA), that allowed independent control of temperature, pressure, and oxidizing gas composition. Hollow cone sprays of methanol, dimethyl ether (DME), CH₃OCH₃, and dimethoxy methane (DMM), CH₃OCH₂OCH₃, were created in quiescent air with a standard Diesel injector, and autoignition delays were inferred from pressure-time histories. A detailed chemical kinetic mechanism was developed to describe the pyrolysis, oxidation, and autoignition of methanol, DME and DMM at high pressures. The mechanism predicts autoignition delay time under Diesel-like conditions. The kinetic modeling illustrated the following: 1) peroxide intermediates play an important role in the autoignition mechanism, and are responsible in part for the excellent autoignition quality of DMM and DME at lower temperatures, 2) it is important in modeling high pressure autoignition to use high pressure limiting rate constants for uni-molecular reactions, and 3) adiabatic mixing of the injected fuel with oxidizer (air) is a viable approach for determining optimum conditions of temperature and stoichiometry for the autoignition of stratified fuel-air mixtures.