A supplementary slurry fuel atomizer of this invention, when used on a piston internal combustion engine, equipped with a common rail fuel injection system, improves slurry atomization, following injection, by expansion of gases, previously dissolved in the water phase of the slurry. This gas expansion separates the several fuel particles in each slurry droplet formed first during fuel injection. The thusly separated fuel particles subsequently burn more rapidly and completely, due to this fuel particle separation improving air access to the fuel. Apparatus is described for dissolving atomizing gas into the water phase of the slurry, in a contactor chamber at a pressure greater than the pressure prevailing in the engine cylinder volume, at the time of slurry injection.