Role of Heat Accumulation by Reaction Loop Initiated by H2O2 Decomposition for Thermal Ignition 2007-01-0908
Detailed reaction path analyses of DME (dimethyl ether, CH3OCH3) and n-heptane (n-C7H16) were performed computationally with the “contribution matrix” showing the contribution ratios of important elementary reactions to formation or removal of every species or heat release at transient temperatures. It was found that the “H2O2 reaction loop” defined by the authors plays an important role in the initiation of thermal ignition. This is a reaction loop composed of four reactions, H2O2 + M → 2OH + M, OH + CH2O → HCO + H2O, HCO + O2 → HO2 + CO and 2HO2 → H2O2 + O2. The overall reaction is 2CH2O + O2 → 2H2O + 2CO + 473 kJ. This loop begins to be active, when the OH formation by H2O2 + M → 2OH + M becomes dominant against those by cool-flame reactions with NTC's (negative temperature coefficient) at about 950 K. The loop releases a significant amount of heat without consuming H2O2. When the temperature increases by this loop up to about 1500 K, branching chain reactions in the hydrogen-oxygen reaction mechanism begin to be dominant so that thermal ignition takes place. It is considered that the heat release rate of the loop is determined mainly by the H2O2 concentration, and the possible amount of heat release by the loop is determined mainly by the CH2O concentration. In an internal combustion engine, the robustness of the thermal ignition is improved when the heat release rate by H2O2 loop prevails the internal energy removal rate by an adiabatic piston expansion. Therefore, the H2O2 concentration at the end of NTC range is a key factor controlling the HCCI (homogeneous charge compression ignition) combustion.