According to one or more embodiments, an internal combustion engine may be operated by a method including one or more of the steps of passing a first fuel and a second fuel into a combustion chamber of an engine cylinder to form a fuel mixture, and combusting the fuel mixture with a spark plug to translate the piston housed in the engine cylinder and rotate a crank shaft coupled to the piston. The engine cylinder may include a cylinder head and cylinder sidewalls, and the combustion chamber may be defined at least partially by the cylinder head, the cylinder sidewalls, and the piston. The first fuel may include a greater octane rating than the second fuel. The combustion chamber may include an end gas region and a central region, the central region more near to the spark plug than the end gas region. The first fuel and second fuel may be passed into the combustion chamber such that the end gas region has a greater concentration of the first fuel than the central region, and the central region has a greater concentration of the second fuel than the end gas region.