A portable on-demand hydrogen supplemental system is provided for producing hydrogen gas and injecting the hydrogen gas into the air intake of internal combustion engines for the purpose of increasing the combustion efficiency in the combustion chamber and lowering particulate emissions at idle. Hydrogen increases the laminar flame speed of diesel fuels, thus increasing combustion efficiency. Hydrogen and oxygen is produced by an electrolyzer from nonelectrolyte water in a nonelectrolyte water tank. The hydrogen gas is passed through a hydrogen gas collector. Nonelectrolyte water mixed with the hydrogen gas in the hydrogen gas collector is passed back thru the tank for distribution and water preservation. The system utilizes an an onboard diagnostic (OBD) interface in communication with the vehicle's OBD terminal, to regulate power to the system so that hydrogen production for the engine is adjusted based on the RPM level and operation conditions of the vehicle.