A method for reducing NOx emissions during operation of an internal combustion engine in commerce which, when burning hydrocarbon fuel as a primary fuel, in the absence of any secondary fuel, has a characteristic stoichiometric ration. The method includes the following: in the absence of electrolytic activity, providing and entraining a quenching species in a gaseous medium and then interacting the quenching species with constituents present during oxidation of the primary fuel in a combustion chamber of the engine.