Abstract: An apparatus for feeding hydrogen into an intake (11) of an engine (1), including: an air inlet (23); a hydrogen inlet (21); a mixing chamber (20) for generating a fuel mixture from hydrogen and air introduced from the inlets, the chamber being adapted to couple to the intake of the engine, for introducing the fuel mixture thereto; and control means (26) for controlling airflow through the mixing chamber. The control means may include an air speed control for varying an effective cross-section of air flow through the apparatus. The air speed control is preferably in the form of an annular collar arranged in the chamber for radial expansion or contraction.