The Reactivity Behavior of n-Heptane and Isooctane Blends in a Motored Knock Research Engine

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 (<C₄) and product species. In-cylinder measurements for each blend examined two factors: (1) the effect of inlet gas temperature on the net reactivity as measured late in the cycle; and (2) the reactivity as a function of crank angle at the highest, non-autoigniting inlet temperature. Results showed that all PRF blends greater than 55 PRF exhibited Negative Temperature Coefficient behavior, with in-cylinder measurements indicating that as much as 40-50% of both n-heptane and isooctane were consumed during the cycle. Also, increasing the amount of n-heptane in a PRF blend increased the level of fuel consumption for both n-heptane and isooctane as well as the magnitudes of stable intermediates. Heat release calculations indicated that the energy release for the 63 PRF was 6.9% of the total energy available when 48% of the fuel had reacted.