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Butane is the simplest alkane fuel for which more than a single structural isomer is possible. In the present study, n-butane and isobutane are used in a test engine to examine the importance of molecular structure in determining knock tendency, and the experimental results are interpreted using a detailed chemical kinetic model. A sampling valve was used to extract reacting gases from the combustion chamber of the engine. Samples were withdrawn at different times during the engine cycle, providing concentration histories of a wide variety of reactant, olefin, carbonyl, and other intermediate and product species. The chemical kinetic model predicted the formation of all the intermediate species measured in the experiments. The agreement between the measured and predicted values is mixed and is discussed. Calculations show that RO2 isomerization reactions are more important contributors to chain branching in the oxidation of n-butane than in isobutane.

Experiments were conducted using a single cylinder, spark ignition research engine to examine the effect of the branched chain alkane isobutane (RON 102) on the autoignition chemistry of straight chain n-butane (RON 94), Isobutane was added in increasing quantities from 10 to 48% by volume. Measurement of cylinder pressure and the analysis of end gas species were used as diagnostics to determine the knock point and ascertain the end gas reactivity. Increasing the amount of isobutane suppressed the chemical reactivity, decreased the knock intensity, and delayed the time to autoignition. Examination of the chemical species in the end gas suggests that (i) low temperature chemical reactions are occurring; and (ii) increasing the amounts of isobutane in the fuel mixture leads to higher yields of propene and chain terminating methyl radicals.

Experiments were conducted in a modified single cylinder research engine to study the correlation between heat release and knock. A skip-fire strategy was used to isolate the heat release due to chemical preignition reactions from the exothermicity from the propagating flame front. n-Butane, isobutane and mixtures of these two were used as the fuels in the study. Pressure profiles of n-butane and isobutane mixtures indicate that there is a decrease in the heat release occurring during the second skip cycle, upon increasing the percentage of isobutane in the mixture. Analysis of chemical species indicate that there are two competing chemical pathways occurring during the heat release. The role that these chemical pathways play during autoignition is discussed.