Abstract: The internal combustion engine fuel system described includes a structure for mixing the alternative fuel, preferably hydrogen, with oxygen in ambient air to stratify the fuel. The system includes an adapter, and the adapter includes a housing mounted between spark plug and cylinder of the internal combustion engine. A plug is placed within the housing. The plug has ridges or grooves on its outer surface that act as mixing structures. Thus, when hydrogen is introduced into the adapter housing it is mixed with ambient oxygen within the chamber as it flows over the plug. The mixing structures in the housing creates a vortexing action as the hydrogen flows over the plug and towards the cylinder of the engine. An electrode protrudes from the plug towards the cylinder. The electrode is preferably platinum and generates the necessary spark to create combustion of the hydrogen/air mixture adjacent to the cylinder to thereby power the cylinder in the engine.