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This paper presents results of studies investigating the reactivity of primary reference fuel blends in a motored research engine. Reactivity was quantified by measuring exhaust manifold carbon monoxide concentration [CO], cylinder pressure histories, and in-cylinder gas composition. The fuels used were n-heptane (0 PRF), isooctane (100 PRF), and blends of the two with octane values of 0, 25, 55, 63, 75, 87, and 100. A supercharged single-cylinder engine was motored at a constant engine speed and constant inlet pressure as the inlet gas temperature was increased by manifold heating until either the fuel/air mixture autoignited or the maximum temperature of the facility (490 K) was reached. In-cylinder gas samples were obtained and analyzed by gas chromatography for overall fuel reactivity and for the concentrations of light hydrocarbon (

This paper presents a comparison of the autoignition tendencies for straight chain hydrocarbons and mixtures of primary reference fuels in a motored engine. Minimum initial gas temperatures required to produce autoignition were measured as a function of fuel type, engine speed, and inlet manifold pressure. In-cylinder gas pressure vs. crank angle and exhaust gas carbon monoxide concentrations were also monitored during these experiments as indicators of chemical activity. Overall, the autoignition behavior of n-pentane, n-hexane, and their equivalent octane number primary reference fuel blends was found to be dissimilar in (i) the inlet temperatures at which autoignition occurred; (ii) the amount of CO formed prior to autoignition; and (iii) the effect of engine speed on minimum inlet autoignition temperatures. Possible causes for this behavior are discussed in the paper in terms of the Negative Temperature Coefficient behavior of large hydrocarbons.