Comparative Autoignition Trends in Butanol Isomers at Elevated Pressure

Abstract

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≈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, offstoichiometric experiments are further performed at 30 bar compressed pressure, for t-butanol at = 0.5 and = 2.0 in air. For these experiments, higher fuel loading (i.e. = 2.0) causes greater pre-ignition heat release (as indicated by greater pressure rise) than the stoichiometric or = 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.

The influence of n -butanol blending on the ignition delay times of gasoline and its surrogate at high pressures

“…The volume traces were determined from the measured pressure trace of the non-reactive experiment using isentropic core relations and a temperature-dependent mixture specific heat ratio [31]. Details on compositions and volume histories can be found in Supplementary…”

Using rapid compression machines for chemical kinetics studies

“…In this approach, continuous stirring is needed to ensure a homogeneous mixture, for instance by using a magnetic stirrer [cf. 44,65,[70][71][72][73][74][75]. In addition, the possibility of fuel cracking/pyrolysis within the mixture preparation vessel over an extended time period needs to be avoided, and characterization experiments should be conducted to confirm the assumptions of minimal/negligible fuel loss, mixture homogeneity, and non-decomposition of fuel in the mixture preparation vessel, as carried out in [e.g., 65,73,74].…”

Intermediate species measurement during iso-butanol auto-ignition

“…Experiments have also been performed in rapid compression machines, although the acquired ignition delay times [20] reaction network for iso-butanol oxidation, especially the pathways involving oxygen addition to fuel radicals [22]. No speciation studies have been reported at high pressures and low to intermediate temperatures.…”

“…No speciation studies have been reported at high pressures and low to intermediate temperatures. Weber and Sung [20] commented that none of the existing models can predict the ignition delays at high pressure (30 bar) and low to intermediate temperature (754-857 K) well, and highlighted the importance of the peroxy chemistry in the low temperature pathways of iso-butanol oxidation based on sensitivity analysis of the ignition delay time using the Sarathy mechanism [17]. Without intermediate species information, the competing ratios between β-scission reactions and fuel propagation into the low temperature chain branching pathway of oxygen addition to fuel radicals were not investigated directly.…”