This invention describes a method of increasing energy density of diesel and jet fuels by introducing extremely high energy density diamondoid components. The diamondoid components comprise a composition which has been processed to remove at least a portion of organics having fewer than 10 carbon atoms, the composition comprising at least about 65 weight percent alkyl-substituted diamondoid compounds which contain more than one quaternary carbon atom per molecule and less than about 35 weight percent of diamondoid compounds which contain less than two quaternary carbon atoms per molecule. The alkyl-substituted diamondoid compounds comprises adamantanes, diamantanes, triamantanes and tetramantanes. The additives of this invention improve the energy density of a middle distillate fuel while maintaining or improving the freeze point and pour points of the resulting mixture. Methods for producing the composition and for operating combustion drivers are also disclosed.
