A portable, on-demand hydrogen generation system is provided for producing hydrogen and injecting the hydrogen as a fuel supplement into the air intake of internal combustion engines, more particularly to vehicles. Hydrogen and oxygen is produced with a fuel cell at low temperatures and pressure from water in a supply tank. The hydrogen and oxygen is passed back thru the supply tank for distribution and water preservation. The gases are kept separate by a divider in the tank and the water level in the tank. In the case of gasoline engines, the hydrogen is directed to the air intake of the engine while the oxygen is vented to the atmosphere. The device is optionally powered by the vehicle battery, a stand alone battery, waste heat of the internal combustion engine or solar energy. The system utilizes a vacuum switch or other engine sensor that permits power to the device and therefore hydrogen production only when the engine is in operation. Therefore, as the hydrogen is produced it is immediately consumed by the engine. No hydrogen is stored on, in or around the vehicle.