Autoignition of n-Butanol/n-Heptane Blend Fuels in a Rapid Compression Machine under Low-to-Medium Temperature Ranges

Ignition delay times of n-butanol/n-heptane mixtures (0%, 20%, 40%, 60% n-butanol in moles) were measured using a rapid compression machine at compressed pressures of 15, 20, and 30 bar, in the compressed temperature range of 650-830 K, and with equivalence ratios of 0.4, 1.0, and 1.5. Sensitivity analysis and reaction fluxes analysis were performed using a detailed mechanism of blend fuels so as to evaluate the impact of n-heptane addition on the ignition and combustion process. Over the experimental conditions in this study, the blend fuels display apparent low and high temperature reactions and a negative-temperature-coefficient (NTC) behavior; with increasing n-butanol mole fraction in the mixtures, the ignition delay times increased; the pressure curves present two-stage heat release behavior at low temperature but one-stage behavior at high temperature. It is worth noting that the n-butanol mole fraction shows negligible effect on the ignition delay times for equivalence ratios of 1.5 at low temperature. Moreover, compressed pressure has a limited effect on the ignition of blend mixture at low temperature but certain influence at medium temperature range. The chemical analysis indicates that n-butanol also presents the NTC behavior because of the low temperature reactivity radicals pool produced by n-heptane. Sensitivity analysis shows that for the pure n-butanol fuel, H-abstraction from the gamma-carbon plays the dominant role in the reactions having the promoting effect on the low-temperature branching, while the major inhibiting reaction is H-abstraction from the a-carbon; for n-butanol/n-heptane mixtures, more reactions play the leading role. The major promoting reactions include some H-abstraction reactions from n-heptane, while the major inhibiting reactions add some elementary reactions of small radicals.