NOVELTY - Diesel fuel composition comprises a fuel component and an additive component comprising at least one epoxy-ester species (I) (0.1-60 wt.% of the diesel fuel composition).    USE - The composition is useful in vehicle engines.    ADVANTAGE - The composition has: improved combustion efficiency; higher cetane value; and low sulfur content. The composition is renewable.    DETAILED DESCRIPTION - Diesel fuel composition comprises a fuel component and an additive component comprising at least one epoxy-ester species of formula (I) (0.1-60 wt.% of the diesel fuel composition).    R1 = 1-3C alkyl, where at least 80% of (I) have R1 as CH3;    R3 = 1-10C alkyl or 1-10C alkenyl, where at least 50% of (I) have R3 as n-octyl;    R2 = molecular linkage comprising subunits of formula (-S5-S4-S3-S2-S1-);    S1, S3, S5 = 1-6C alkyl, 1-6C alkenyl and/or 1-6C alkoxy or absent; and    S2, S4 = epoxy, 1-4C alkyl, 1-4C alkenyl and/or 1-4C alkoxy or absent    INDEPENDENT CLAIMS are also included for:    (1) a method (II) for improving combustion efficiency, and correspondingly cetane rating, in a diesel fuel, comprising: generating epoxy-ester species, comprising esterifying the 8-22C unsaturated fatty acids with an alcohol to form the unsaturated esters, and epoxidizing the unsaturated esters to form the epoxy-ester species; and combining the epoxy-ester species with the diesel fuel to provide a diesel fuel composition with increased combustion efficiency;    (2) a method (III) for improving combustion efficiency, and correspondingly cetane rating, in a diesel fuel, comprising: generating epoxy-ester species comprising extracting the unsaturated esters from an ester-based biodiesel, and epoxidizing the unsaturated esters to form the epoxy-ester species; and combining the epoxy-ester species with diesel fuel to provide a diesel fuel composition with increased combustion efficiency;    (3) a method (IV) comprising: providing the biodiesel fuel comprising the unsaturated ester species; and epoxidizing at least a portion of the unsaturated ester species in the biodiesel fuel to form an biodiesel fuel composition comprising a quantity of epoxy-ester species, where the biodiesel fuel composition has enhanced combustion efficiency relative to the biodiesel fuel from which it was derived; and    (4) a method (V) comprising: providing the triglyceride-based oil comprising triglyceride species bearing regions of unsaturation; transesterifying the triglyceride-bearing oil to yield an esterified composition comprising unsaturated esters; epoxidizing the esterified composition to yield an epoxidized composition comprising epoxy-ester species; and blending at least a portion of the epoxidized composition with diesel fuel to form an enhanced diesel fuel composition.