Binary Mixtures of Branched and Aromatic Pure Component Fuels as Surrogates for Future Diesel Fuels

Future synthetic diesel fuels will likely involve mixtures of straight and branched alkanes, possibly with aromatic additives to improve lubricity and durability. To simulate these future fuels, this study examined the combustion characteristics of binary mixtures of 50%, 70%, and 90% isododecane in hexadecane, and of 50%, 70%, and 80% toluene in hexadecane using a single-cylinder research diesel engine with variable injection timing. These binary blends were also compared to operation with commercial petroleum diesel fuel, military petroleum jet fuel, and five current synthetic Fischer-Tropsch diesel and jet fuels. The synthetic diesel and jet fuels showed reasonable similarity with many of the combustion metrics to mid-range blends of isododecane in hexadecane. Stable diesel combustion was possible even with the 80% toluene and 90% isododecane blends; in fact, operation with 100% isododecane was achieved, although with significantly advanced injection timing. As the concentration of toluene in hexadecane increased, combustion was either stable or progressed quickly to misfire; 80% toluene in hexadecane resulted in stable combustion but 85% toluene blends did not combust at all. With either blend, there was not a progressive change in peak pressure, maximum rate of pressure rise, or combustion phasing leading to extreme values. Instead, only modest changes in these metrics occur as blend fraction changes across a wide range. Increasing blend fraction of either component does steadily increase ignition delay, although 50% mixtures of either component cause only a modest change in ignition delay. Increasing blend fraction also increases the amount of fuel consumed in the rapid premixed combustion phase, although estimation of this fraction showed that it increases even more slowly than ignition delay with increasing concentration. Only when ignition delay is longer than approximately 1.8 ms does the amount of energy released from the premixed-phase burn show significant increase. The increasing fraction of the branched or aromatic component causes changes to the mixture properties that can reduce the rate of entrainment and mixing in the diesel jet, partially compensating for small increases in ignition delay. Mixture properties for each blend were measured using standardized testing procedures, including tests for density, viscosity, surface tension, and cetane number. The results suggest that blends of up to 50% of branched or aromatic components could be utilized in a diesel engine with only modest impact to combustion characteristics or performance metrics; higher concentrations may be utilized with increasing effects.