Numerical study of the combustion mechanism of a homogeneous charge compression ignition engine fuelled with dimethyl ether and methane,with a detailed kinetics model

Abstract

The auto-ignition and combustion mechanisms of dimethyl ether (DME) in a fourstroke homogeneous charge compression ignition (HCCI) engine were investigated using a zero-dimensional thermodynamic model coupled with a detailed chemical kinetics model. The results indicate that DME displays two-stage auto-ignition, and heat release with a low-temperature reaction and a high-temperature reaction (HTR). Heat release with the HTR can be separated into two stages: blue flame and hot flame. HCCI ignition is controlled by hydrogen peroxide (H₂O₂) decomposition, and OH plays a very important role in HCCI combustion. Formaldehyde (CH₂O) is the main source of H₂O₂. Based on the sensitivity analysis of chemical reactions, the major paths of the DME reaction occurring in the engine cylinder are clarified. The major paths of the DME reaction is H-atom abstraction from DME, followed by the first addition of O₂ and second addition of O₂, and then oxidation to formaldehyde (CH₂O), the formyl radical (HCO), and finally carbon monoxide (CO). CO oxidation occurs at the hot flame by the elementary reaction CO + OH = CO₂ + H. At leaner DME concentrations, CO cannot be completely converted to carbon dioxide (CO₂), and the process will result in high CO emissions.