Fuel is activated prior to combustion in an engine by an electrical arc established across an annular gap between two electrodes, and sustained for a significant portion of the injection cycle. The vaporized or gaseous fuel is directed through a spiral recess to be caused to flow vortically and cause the arc to migrate about the annular gap to reduce electrode erosion and increase exposure of the fuel to the activating effects of the arc. The arc is formed by separating contacting electrodes which applying a current to the electrodes. One of the electrodes is movable under the influence of the pressurized fuel to cause separation from the other fixed electrode, so that the arc is established as fuel begins flowing. The electrodes have opposing rimmed cavities, with fuel flow shifting the arc radially inwardly, migrating to extend between the inner wall of each cavities, and around which the arc rotates under the influence of the vortical fuel flow.