Devices, methods and system for combustion of an intimate emulsion of water-in-fuel without the use of chemical additives in order to reduce emissions and improve fuel economy with little or no engine or boiler de-rating or modifications. Apparatus and method for producing, combusting, storing and reducing viscosity of a fuel comprised of water and oil or other hydrocarbons, which is constituted as an emulsion without the need for chemical stabilizers by exposure to constant cavitation.