Abstract: A supplemental vapor fuel injection system for internal combustion engines capable of utilizing numerous supplemental vapor type fuels such as propane, compressed natural gas (CNG), liquid natural gas (LNG), butane, ammonia, biogas, hydrogen, ammonia, and Hythane®. The system includes a vaporizer/pressure regulator that provides pressure regulated vapor fuel to two specially designed vapor fuel injectors and a controller unit capable of real-time control of the vapor fuel injectors. The injectors meter precise amounts of vapor fuel into a manifold that combines the vapor for delivery to a directional nozzle located in an airstream of the diesel engine.