The system is used with an internal combustion engine, is mounted on a vehicle and is operable primarily when the vehicle is at rest for generating and storing hydrogen gas on the vehicle. The system includes a gas cylinder, an electrolyzer connected to the gas cylinder, a D.C. power supply connected to the electrolyzer and including electrical apparatus for converting A.C. current to D.C. current and a control circuit connected to the D.C. power supply, to the electrolyzer, and to the gas cylinder, all of which are mounted within the vehicle. The control circuit for controlling generation and storage of hydrogen gas is operable from and connectible to a conventional A.C. source.