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Intermediate species formed in the cool ignition stage of autoignition were evaluated by exhaust gas analysis with FT-IR in a test engine at hot ignition suppressed conditions. PRF (iso-octane/n-heptane) and NTF (toluene/n-heptane) were used as the fuels. The fuel consumption rate decreases with increasing iso-octane content in PRF and toluene content in NTF. HCHO generation rate increases with increasing iso-octane content in PRF but the opposite trend was found in NTF. These tendencies correspond to the difference in the detail reaction mechanism for PRF and NTF oxidation.

Exhaust gas analysis has been conducted for a test engine operated in HCCI mode at hot ignition suppressed condition, to detect intermediate species formed in low temperature oxidation (LTO). PRF (isooctane/ n-heptane) and NTF (toluene/ n-heptane) were used as fuel mixtures. The LTO fuel consumption decreases with increasing iso-octane content in PRF and toluene content in NTF, but only NTF showed a nonlinear effect. These tendencies were reproduced by O-D and 3-D simulations with detailed chemistry; however, quantitative differences were found between chemical models. The essential mechanism of high octane number fuel affecting the ignition property of n-heptane is discussed by developing a simplified model summarizing chain reaction of LTO, in which OH reproduction and fuel + OH reaction rate play important roles.