NOVELTY - The reformer reactor for converting hydrocarbon fuels (20) into a hydrogen rich gas for fuel cells and/or exhaust treatment applications by auto-thermal reaction, comprises a cylindrically shaped housing (2) with an inner wall (4) and an outer wall (6) forming a space between them and two side faces (8a, 8b), a fuel inlet (22) provided in one of the two side faces (8a), and a ceramic monolith or metal grid catalyst (28) for the auto-thermal reaction provided in the reaction chamber. The inner wall and the two side faces form a reaction chamber of the reformer reactor.    USE - The reformer reactor is useful for converting hydrocarbon fuels e.g. diesel into a hydrogen rich gas for fuel cells and/or exhaust treatment applications by auto-thermal reaction.    ADVANTAGE - The reformer reactor prevents the formation of soot by burning the fuel particle in contact with the inner wall of the reactor without need of hydrogen storage thus easily and economically converting hydrocarbon fuels into the hydrogen rich gas.    DETAILED DESCRIPTION - The reformer reactor for converting hydrocarbon fuels (20) into a hydrogen rich gas for fuel cells and/or exhaust treatment applications by auto-thermal reaction, comprises a cylindrically shaped housing (2) with an inner wall (4) and an outer wall (6) forming a space between them and two side faces (8a, 8b), a fuel inlet (22) provided in one of the two side faces (8a), and a ceramic monolith or metal grid catalyst (28) for the auto-thermal reaction provided in the reaction chamber. The inner wall and the two side faces form a reaction chamber of the reformer reactor. The inner wall is charged with a first electric charge. The fuel inlet is charged with the first electric charge, and formed as a fuel injector. The catalyst is charged with a second electric charge, which is opposite to the second electric charge. The space is designed as oxidizing agent passage between an oxidizing agent inlet provided in the outer wall and an oxidizing agent outlet provided in the inner wall. The oxidizing agent outlet is located in vicinity of the fuel inlet, and designed as holes for introducing air and/or steam into the reactor chamber. The holes have various size and/or shape, and are located in a variety of different locations. An INDEPENDENT CLAIM is included for a method for converting hydrocarbon fuels e.g. diesel into a hydrogen rich gas for fuel cells and/or exhaust treatment applications by auto-thermal reaction.    DESCRIPTION OF DRAWING(S) - The diagram shows a reformer reactor.    Housing (2)    Inner wall (4)    Outer wall (6)    Sides faces (10) Reaction chamber (8a, 8b)    Hydrocarbon fuels (20)    Fuel inlet (22)    Catalyst. (28)