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Autoignition of the five distinct isomers of hexane is studied experimentally under motored engine conditions and computationally using a detailed chemical kinetic reaction mechanism. Computed and experimental results are compared and used to help understand the chemical factors leading to engine knock in spark-ignited engines and the molecular structure factors contributing to octane rating for hydrocarbon fuels. The kinetic model reproduces observed variations in critical compression ratio with fuel structure, and it also provides intermediate and final product species concentrations in much better agreement with observed results than has been possible previously. In addition, the computed results provide insights into the kinetic origins of fuel octane sensitivity.

The chemistries controlling autoignition of primary reference fuels (n-heptane/isooctane binary mixtures) and binary olefin/paraffin mixtures have been inferred from experimental motored-engine measurements. For all n-heptane/isooctane and olefin/paraffin mixtures, each component of the mixture reacted via parallel intramolecular mechanisms with the only interactions being via small labile radicals. The octane qualities of the neat components appears to be dictated not by the initial reaction rate of the fuel, but by the reaction rate of the subsequent fuel-product reactions. In contrast, the blending octane quality of a component appears to be dictated more by the rate of the initial fuel reactions. The abnormally high blending octane qualities of olefins result from them having high rates of initial fuel reaction combined with slow rates of subsequent fuel-product reactions.

The differing chemistries controlling autoignition of octane-enhancing ethers and of cyclic ethers have been inferred from stable intermediate species measurements using the motored-engine technique. Species measurements for three octane-enhancing ethers and three cyclic ethers are presented, compared with literature studies, and contrasted with those of the other ethers. The chemical mechanisms responsible for autoignition of both ether classes are detailed, compared, and used to explain the differences in octane qualities of the two ether classes. Chemical interactions resulting from the addition of MTBE to the paraffin 2,3-dimethylbutane were measured, and are discussed in terms of the chemical mechanisms controlling the autoignition of the neat ether and paraffin.

Autoignition chemistries of several paraffins, olefins, and aromatics were examined in a motored engine at different engine conditions. Paraffin chemistry was dominated by “negative-temperature coefficient” (NTC) behavior which became more pronounced at lower pressures, higher temperatures, and shorter reaction times. In contrast, olefin and aromatic chemistries did not exhibit NTC behavior. Measured pressures and calculated temperatures at fired octane rating conditions showed slightly lower pressures, higher temperatures, and lower reaction times at Motor octane rating conditions when compared to Research conditions. Therefore, paraffins would have a more pronounced NTC behavior under Motor rating conditions than under Research conditions.

The autoignition chemistries of the olefins 1-butene, 2-butene, isobutene, 2-methyl-2-butene, and 1-hexene and their corresponding paraffins were examined in a motored, single-cylinder engine by measuring stable intermediate species and performing heat-release analyses. The same engine conditions were used for each olefin-paraffin pair, and compression ratio was varied to affect different levels of chemical activity. Paraffin autoignition chemistry is dominated by hydrogen abstraction from the fuel, followed by the intramolecular alkylperoxy isomerization mechanism. Olefin autoignition chemistry differs markedly being controlled by radical addition to the double bond. Hydroxyl radical addition is followed by oxygen addition to the adjacent radical site, followed by scission forming two carbonyls. Hydroperoxyl radical addition yields an epoxy directly. Experimental measurements for each olefin-paraffin pair are compared with each other and with literature values.

Isobutane autoignition chemistry was examined in a motored, single-cylinder engine by measuring stable intermediate species, performing heat release analyses, and measuring visible emissions. The engine was motored at speeds of 600 and 1600 r/min, and compression ratio was varied to affect different levels of chemical activity. At 1600 r/min, species measurements, heat release and visible emissions all exhibited a “negative-temperature” coefficient region; thus, “intermediate-temperature” chemistry controls autoignition at 1600 r/min. By implication, classic “low-temperature” chemistry controls at 600 r/min. Experimental measurements are compared with isobutane literature values, with previous n-butane results, and specific isobutane autoignition chemistry is discussed an light of the measurements.

n-Butane autoignition chemistry was examined in a single-cylinder engine by measuring intermediate species, performing heat release analyses, and measuring visible emissions. The motored engine technique was employed at engine speeds of 600 and 1600 r/min, and compression ratio was varied to affect different levels of chemical activity. At 1600 r/min, species measurements, heat release and visible emissions all exhibited a wide “negative-temperature” coefficient region; thus, “intermediate-temperature” chemistry controls autoignition at 1600 r/min. By implication, classic “low-temperature” chemistry controls at 600 r/min. Experimental measurements are compared with literature values, and specific n-butane autoignition chemistry is discussed in light of the measurements.