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Conventional and alternative jet fuels, such as petroleum-derived Jet-A, coal-derived IPK, and natural-gas-derived S-8, display significant chemical and physical fuel property differences that influence their ignition characteristics. The current work addresses the need for surrogate mixtures capable of emulating the various properties of these fuels and their select blends, which are often used within compression ignited engines for acceptable ignition behavior. A six-component surrogate palette is proposed with species that are readily available within recent kinetic mechanisms, including ndodecane, n-decane, iso-cetane, iso-octane, decalin, and toluene. The use of these species allows for a seamless compositional transition between the neat target jet fuels and their blends. The surrogate optimizer, which includes various correlations and models to estimate properties of model mixtures, is used to determine the surrogate composition that best matches target fuel properties. For an accurate ignition quality prediction during the optimization, a non-linear Derived Cetane Number regression equation is generated from Ignition Quality Tester experiments of 76 surrogate component mixtures. The newly formulated surrogates and their blends successfully capture the wide range of properties present within the target fuels, including temperature-dependent physical properties such as density, viscosity, specific heat, and volatility, along with experimental ignition delays obtained from a constant volume spray chamber. Kinetic modeling with a detailed mechanism showed that predicted ignition delay times are in good agreement with shock tube and rapid compression machine ignition delay experiments. A sensitivity analysis with variations in the composition of the Jet-A surrogate showed that its calculated ignition delay times are most sensitive to the composition of n-dodecane among the four Jet-A surrogate constituents over the range of temperatures and pressures examined.

Biodiesel is a desirable alternative fuel for the diesel engine due to its low engine-out soot emission tendency. When blended with petroleum-based diesel fuels, soot emissions generally decrease in proportion to the volume fraction of biodiesel in the mixture. While comparisons of engine-out soot measurements between biodiesel blends and petroleumbased diesel have been widely reported, in-cylinder soot evolution has not been experimentally explored to the same extent. To elucidate the soot emission reduction mechanism of biodiesel, a single-cylinder optically-accessible diesel engine was used to compare the in-cylinder soot evolution when fueled with ultra-low sulfur diesel (ULSD) to that using a B20 biodiesel blend (20% vol.lvol. biodiesel ASTM D6751-03A). Soot temperature and KL factors are simultaneously determined using a novel two-color optical thermometry technique implemented with a high-speed CMOS color camera having wide-band Bayer filters. The crank-angle resolved data allows quantitative comparison of the rate of in-cylinder soot formation. High-speed spray images show that B20 has more splashing during spray wall impingement than ULSD, distributing rebounding fuel droplets over a thicker annular ring interior to the piston bowl periphery. The subsequent soot luminescence is observed by high-speed combustion imaging and soot temperature and KL factor measurements. B20 forms soot both at low KL magnitudes over large areas between fuel jets, and at high values among remnants of the fuel spray, along its axis and away from the bowl edge. In contrast, ULSD soot luminescence is observed exclusively as pool burning on the piston bowl surfaces resulting from spray wall impingement. The soot KL factor evolution during B20 combustion indicates earlier and significantly greater soot formation than with ULSD. B20 combustion is also observed to have a greater soot oxidation rate, which results in lower late-cycle soot emissions. For both fuels, higher fuel injection pressure led to lower late-cycle soot KL levels. The apparent rate of heat release (ARHR) analysis under steady skip-fire conditions indicates that B20 combustion is less sensitive to wall temperature than that observed with ULSD due to a lesser degree of pool burning. B20 was found to have both a shorter ignition delay and shorter combustion duration than ULSD.

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