Autoignition dynamics of DME/air and EtOH/air homogeneous mixtures

The autoignition kinetics of DME/air and EtOH/air stoichiometric mixtures are compared with the use of algorithmic tools from the CSP method at a range of initial conditions that refers to the operation of reciprocating engines. DME and EtOH are two isomer fuels, with the potential for production from renewable sources, that have virtually identical thermochemistry; i.e. very closely equal heat of combustion and adiabatic flame temperature. These isomer fuels have drastically different ignition delays because of their different kinetics. In particular, the first and largest part of the ignition delay in the DME and EtOH cases is dominated by two different sets of components of carbon chemistry, while the last and shortest part is dominated by the same hydrogen chemistry. Considering sufficiently large initial temperatures, in the DME case the time scale that characterizes autoignition in the first part is promoted by single-carbon chemistry and is opposed mainly by recombination of CH3 radicals. On the contrary, in the EtOH case the two-carbon chain retains its bond in that part. Therefore, the hydrogen chemistry plays an important role in promoting the generation of the time scale that characterizes autoignition from the start of the process, while the reactions that oppose the generation of this time scale involve HO2 and H2O2 and they are not as effective as the reactions opposing ignition for DME. These features generate a substantially shorter ignition delay for EtOH. This situation is reversed for sufficiently low initial temperatures due to the shift in relative importance between internal and external H-abstraction that occurs as temperature increases