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This paper examines the effect of charge cooling on the Research Octane Number (RON) of ethanol/gasoline blends. While gasoline is fully vaporized prior to entry into the engine in a standard RON test, significant charge cooling is observed for blends with high ethanol content, with the presence of a near-saturated and potentially two-phase air-fuel mixture during induction. Thus, the relative significance of the charge cooling and the autoignition chemistry cannot be determined from the standard RON test. In order to better delineate the effects of charge cooling and autoignition chemistry, a so-called ‘modified RON’ test is therefore devised in which the temperature of the air-fuel mixture entering the engine is fixed and representative of that observed for primary reference fuels (PRFs).

Burning velocities for unleaded conventional gasoline (CR-87) and air mixtures were determined experimentally over an extensive range of equivalence ratios at 353 K and 500 K and at atmospheric pressure. Nitrogen dilution effects on the laminar flame speed were also studied for selected equivalence ratios at these same conditions. Experimental measurements employed the stagnation jet-wall flame configuration and Particle Image Velocimetry (PIV). The laminar burning velocity was obtained using linear extrapolation of stretched flame data to zero stretch rate. The measured flame speeds were compared with numerical predictions using a minimized detailed kinetic model for primary reference fuel (PRF) mixtures, which was developed based on stirred reactor, shock tube and flow reactor data.