Abstract: A mixture of air, oxygen and engine exhaust gases is added to a fuel and fed to an internal combustion engine in controlled amounts to reduce the amount of toxic material formed. The oxygen is supplied by a plurality of oxygen converters arranged in parallel so that one or more of the converters are charging while one or more of the remaining converters are discharging oxygen. Each converter includes an inner chamber having a fluidized bed of a metallic oxide surrounded by a heating chamber. Engine exhaust gases are fed to the heating chamber to raise the temperature of the metallic oxide to that at which a higher oxide is formed. To obtain oxygen the temperature of the fluidized bed is raised by means of the exhaust gases to that at which oxygen is released and the original metallic oxide is formed.