Abstract: The invention provides an internal combustion engine, such as a diesel engine, set to run on an emulsion of fuel and water, supplied to an emulsifier through separate fuel and water lines. The water feed preferably is under gravity, and the water line has a metering orifice to control the flow of water. If the engine is provided with emulsion overspill, this is returned through a return line for the overspilled emulsion, and said return line has a surge tank with a vent enabling the venting of air and/or the breaking of vacuum in the return line.