Auto-ignition and low temperature combustion in HCCI for nonhomogeneous DME/air mixtures

Homogeneous compression charge ignition engines (HCCI) in theory compromise the low emissions characteristics of spark ignition engines and the high efficiency of diesel engines by compressing a homogeneous mixture at high pressure and low temperatures (< 1000 K). At such low temperatures ignition is controlled by low chemical kinetics, local flow field, mixture composition, temperature and diffusion/thermal transport. Detailed chemical kinetic model is, therefore, essential to capture the required auto-ignition physics. The HCCI concept is faced by many challenges as controlling the rate of heat released to avoid severe in-cylinder pressure rise at high loads and CO emissions at low engine loads. One method to resolve these problems is mixture stratification by introducing temperature and mixture composition fluctuations. Therefore, the highly compressed premixed charge is in reality nonhomogeneous and is characterized by high turbulence level. With different fuels and initial temperatures, this affects the autoignition characteristics at different engine loads. A parametric homogeneous ignition study is performed to investigate the effect of initial charge temperature (T=764, 833,1000K). Sensitivity analysis is used to assess the important reactions and species at each ignition stage. Three ignition stages are observed, low temperature ignition stage, intermediate ignition stage, and high temperature ignition stage. The first ignition stage is controlled by low temperature chemistry (LTC). LTC is found to be initiated by DME oxidation that is followed by a sequence of isomerization steps at low temperatures. H₂O₂ is also produced prior to LTC ignition and is consumed only during the intermediate temperature ignition stage. For T=1000K single stage high temperature ignition is observed, where CO is produced and consumed in a very narrow window in time. A single case (T=764K) is compared with a high fidelity two dimensional Direct Numerical Simulation (DNS). The DNS case is for a stratified Dimethyl Ether (DME)/air mixture under typical low-load HCCI conditions. Inhomogeneity prior to autoignition is introduced in the initial temperature field and the fuel/air mixture compositions. Preliminary results are presented here for the 2D case. The hot/cold spots and the fuel-rich/lean pockets are found to be uncorrelated. The 2D results show that the LTC chemistry is initiated at the locations where the hot spots coexist with the fuel-rich pockets and that the autoignition delay is expected to be shorter at the locations where the temperature of the hot spot is higher and coexist with a fuel-rich pocket.