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Multi-stage heat releases in homogenous charge compression ignition (HCCI) near the ignition threshold are analyzed in this study. Motored engine experiments are conducted with exhaust gas analysis by Fourier transform infrared spectrometer under hot ignition suppressed condition, in order to provide a deeper insight into the ignition mechanism of n-heptane. By increasing intake temperature from room temperature, heat release of low temperature oxidation (LTO) can be observed. Moreover, second heat release was observed after primary heat release of LTO, which increases rapidly with increasing intake temperature within narrow range below the high temperature oxidation (HTO) threshold. The mechanism of HTO preparation reaction is discussed.

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.

Chemical kinetic mechanism of compression ignition with PRF (iso-octane/ n-heptane) and NTF (toluene/ n-heptane) is investigated according to crank angle resolved in-cylinder sampling experiments. Profiles of two-stage consumption of fuel components in accordance with the timings of heat releases have been obtained. As well, production and consumption of intermediate species were observed. It was found that toluene consumption at the first stage is considerably less than that of n-heptane, whereas iso-octane consumption is comparable to that of n-heptane, which is accounted for by the smaller rate constant of toluene with OH. N-heptane and iso-octane are considered to produce formaldehyde; however, toluene has no or little contribution.

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.

The chemical mechanism responsible for controlling ignition timing by using additives in HCCI has been investigated. Dimethyl ether (DME) and methanol were used as the main fuel and the additive, respectively. Fuel consumption and intermediate formation in the first stage (cool ignition) were measured with crank angle resolved pulse-valve sampling and exhaust gas analysis, where HCHO, HCOOH, CO, H2O2 and other species were detected as the intermediate. The effect of methanol addition retarding ignition is represented by an analytical model in which the growth rate of the chain reaction is reduced by the methanol addition.

Autoignition in the homogeneous charge compression ignition (HCCI) process typically exhibits heat release in two stages called cool flame and thermal flame. The mechanisms governing these two stages were investigated using a DME-fueled HCCI engine and numerical simulations. Composition analysis after cool flame showed that the cool flame is explained by a chain reaction mechanism in which the chain terminator is the intermediate species formed in cool flame. In the case of thermal flame, although the chain reaction mechanism is complex, the behavior is clearly described by thermal explosion theory in which the rate-determining reaction is H2O2 decomposition.

Two different species measurements have been conducted for compression ignition of dimethyl ether in a motored engine. Crank angle resolved pulse-valve sampling with a resolution improvement scheme enabled to detect partial fuel consumption and formation of intermediate like H2O2 and HCHO at a cool ignition, as well as their total consumption at a hot ignition. FTIR analysis of exhaust gas in single cool ignition conditions confirmed formation of HCOOH and HCOOCH3 in cool ignitions. Results obtained in a range of equivalence ratio support the advantage of 2000 version of Curran et al. DME oxidation model.

Applicability of detailed chemical kinetic models to HCCI runs in terms of ignition timings and intermediate species composition has been investigated. An existed n-heptane model and its expansion to n-decane established in this study were particularly concerned. Exhaust gas analysis showing transient composition after cool flames indicated that the unmodified n-decane model overestimates fractions of various grade of aldehydes, whereas it represents experimental ignition timings. The aldehyde yield was found to be sensitive to reactions of aldehyde with OH rather than aldehyde formation reactions. Reactions of QOOH decomposition forming HO2 were also suggested as a candidate to be revised for the model improvement on ignition delays.

A simplified reaction model of dymethyl ether (DME) oxidation has been developed by extracting essential elementary reactions from a previous detailed mechanism. It consists of 23 reactions for 23 species without modification of rate coefficients in low temperature oxidation of the original model. Spatially non-dimensional calculations were conducted along with HCCI compression profiles using SENKIN code in CHEMKIN package. Good agreement with the detailed model was obtained in terms of ignition timing and profiles of species such as DME, HCHO, O2, H2O2, and CO as functions of intake gas temperature, equivalence ratio, and intake pressure. Adding a few reactions to the mechanism, the effective range of the model was extended to rich side, where CO emission is significant. Effect of methanol addition as an ignition suppressor was also properly described.