Chemical Kinetics Study on Two-Stage Main Heat Release in Ignition Process of Highly Diluted Mixtures 2013-01-1657

Some experimental data indicate that an HCCI process of a highly diluted mixture is characterized with a two-stage profile of heat release after the heat release by low-temperature oxidation, and with slow CO oxidation into CO₂ at a low temperature. In the present paper, these characteristics are discussed using a detailed chemical kinetic model of normal heptane, and based on an authors' idea that an ignition process can be divided into five phases.

The H₂O₂ loop reactions mainly contribute to heat release in a low-temperature region of the TI (thermal ignition) preparation phase. However, H+O₂+M=HO₂+M becomes the main contributor to heat release in a high-temperature region of the TI preparation phase. H₂O₂ is accumulated during the LTO (low-temperature oxidation) and NTC (negative temperature oxidation) phases, and drives the H₂O₂ loop reactions to increase the temperature during the TI preparation phase. When the heat capacity of a mixture increases by dilution, H₂O₂ is consumed in a lower-temperature region. Thus, the heat release by the H₂O₂ loop reactions stagnates at a lower temperature, causing a gap of heat release between the low-temperature and high-temperature regions of the TI preparation phase.

When a mixture is diluted to a considerable extent, the rate of a branching chain reaction, H+O₂=OH+O, cannot overtake the rate of H+O₂+M=HO₂+M to the end of an ignition process. Thus, the CO oxidation into CO₂, CO+OH=CO₂+H, slowly proceeds along with H+O₂+M=HO₂+M rather than with the branching chain reaction. Conventional combustion control ways cannot be useful for activating low-temperature combustion.