A method of operating an internal combustion engine uses fuels having different reactivities obtained from the same fuel source. A first fuel having a first reactivity is stored in a fuel reservoir. A portion of the first fuel is converted to a second fuel having a second reactivity. The first fuel is introduced into a combustion chamber having a piston moving in a cylinder at a first time when the piston is relatively closer to a bottom dead center (BDC) position and the second fuel is introduced into the combustion chamber at a second time when the piston is relatively further from the BDC position. In an aspect, the convertor may be adjustable to alter the reactivity of the second fuel. In an aspect, the convertor may use a processing fluid associated with the engine to convert the first fuel to the second fuel.