  Autoignition experiments of stoichiometric mixtures of s-, t-, and i-butanol
in air have been performed using a heated rapid compression machine (RCM). At
compressed pressures of 15 and 30 bar and for compressed temperatures in the
range of 715-910 K, no evidence of a negative temperature coefficient region in
terms of ignition delay response is found. The present experimental results are
also compared with previously reported RCM data of n-butanol in air. The order
of reactivity of the butanols is
n-butanol>s-butanol$\approx$i-butanol>t-butanol at the lower pressure, but
changes to n-butanol>t-butanol>s-butanol>i-butanol at higher pressure. In
addition, t-butanol shows pre-ignition heat release behavior, which is
especially evident at higher pressures. To help identify the controlling
chemistry leading to this pre-ignition heat release, off-stoichiometric
experiments are further performed at 30 bar compressed pressure, for t-butanol
at $\phi$ = 0.5 and $\phi$ = 2.0 in air. For these experiments, higher fuel
loading (i.e. $\phi$ = 2.0) causes greater pre-ignition heat release (as
indicated by greater pressure rise) than the stoichiometric or $\phi$ = 0.5
cases. Comparison of the experimental ignition delays with the simulated
results using two literature kinetic mechanisms shows generally good agreement,
and one mechanism is further used to explore and compare the fuel decomposition
pathways of the butanol isomers. Using this mechanism, the importance of peroxy
chemistry in the autoignition of the butanol isomers is highlighted and
discussed.
