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

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+O-2+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+O-2=OH+O, cannot overtake the rate of H+O-2+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+O-2+M= HO2+M rather than with the branching chain reaction. Conventional combustion control ways cannot be useful for activating low-temperature combustion.