Role of Heat Accumulation by Reaction Loop Initiated by H2O2 Decomposition for Thermal Ignition

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.