Filling and venting device for a supplementary tank

The fuel tank in particular if containing diesel accommodates a smaller container for an additive reducing the temperature created in the combustion of small particles present in the fuel. The container can be filled by removing a plug (12), lifting the upper end of the supply duct (8) from the inlet (9) and connecting it to a hose. The air escaping from the container when it is filled is guided along a second duct (13) enveloping the supply duct (8) and released from the upper area of the unit.