An Experimental Study of Normal-Hexadecane and Iso-Dodecane Binary Fuel Blends in a Military Diesel Engine

The molecular composition of new hydrotreated renewable fuels consists of both straight chain and branched alkanes. These new fuels do not contain aromatic or cyclo-paraffinic hydro-carbon compounds which are regularly seen in conventional petroleum fuels. Both experimental and modeling work has shown that straight chain alkanes have shorter ignition delays (e.g. higher cetane number) as compared to branched alkanes. In order to better understand the effects of branched and straight chain alkanes fuels in diesel engines, an experimental study was pursued using binary blends of iso-dodecane (iC₁₂H₂₆ with abbreviation: iC12) and normal-hexadecane (nC₁₆H₃₄ with abbreviation nC16) in a military diesel engine (AM General HMMWV ‘Humvee’ engine). Mixtures of 50% iC12 with 50% nC16 as well as 25% iC12 with 75% nC16 were compared to 100% nC16 (cetane) fueled engine operation across the entire speed-load range. Higher nC16 fuel content operation resulted in modestly earlier fuel injection events and combustion phasing that delievered slightly worse engine brake performance (torque and fuel consumption). Interestingly, ignition delay and overall burn durations were relatively insensitive to the binary blends tested. The significantly different physical properties of iC12 relative to nC16 are believed to affect the fuel injection event leading to later fuel injection with increasing iC12 content. Later injection into a hotter chamber mitigates the lower cetane number of the higher iC12 content fuel blends.