Seite - 44 - in Advanced Chemical Kinetics

understand the reaction activity for the test fuels, the autoignition delay times are exam- ined by conducting the numerical calculation using ‘Closed Homogeneous Batch Reactor’ in CHMEKIN-Pro with the chemical-kinetic mechanisms explained in the section describing the chemical-kinetic mechanism for test fuels. The initial pressure was set as 3.0 MPa to represent the maximum in-cylinder pressure for motored operation of engine modeled for this work. In addition, the initial fuel/air equivalence ratio (φo) was set as 0.5 since HCCI engines are generally operated with lean in-cylinder charge mixture. Figure 3 compares the results of autoignition delay times between test fuels. As can be observed, methane does not exhibit any of the low-temperature reaction (LTR) or negative temperature coefficient (NTC) behavior typical of larger paraffinic fuels such as n-heptane. (The term ‘negative temperature coeffi- cient’ is used to denote the temperature regime where the rate of fuel consumption decreases with increasing temperature, rather than increases as in all other regimes.) This indicates that methane is very resistant to autoignition and correspondingly has a very high octane number (=120). In contrast, for DME, the highest fuel autoignition reactivity (i.e. the shortest autoigni- tion time) is observed until the initial temperature of 1170 K. In addition, DME displays NTC Figure 3. Comparison of autoignition delay times for methane, DME, n-heptane and iso-octane. Advanced Chemical Kinetics44