This invention is related to an additive composition comprising metal-based quantum clusters (QCs) dispersed in a hydrocarbon medium. The additive composition is useful as a fuel additive, as it acts as a combustion improver for liquid and gaseous fuels. The invention describes a process for the synthesis of the additive composition comprising metal-based materials in atomic cluster form in hydrocarbon dispersible medium. The stable liquid dispersion of the QC has been doped into the hydrocarbon fuels at required concentrations. The measurable flame temperature of the fuels, e.g., commercial LPG on burner has been observed to increase by at least 60-80° C. The flame with high heat through put can be used for efficient cooking, heating, annealing and other high thermal applications. The additive composition may also be used to improve the fuel economy of the liquid hydrocarbon fuels.