A device that injects hydrogen gas into an engine's air intake or intake manifold that is demand controlled by the vehicle's throttle linkage in an approximately linear manner. When the throttle is depressed, hydrogen generation can start or increase, and when the throttle is released, hydrogen generation can stop of decrease. The device of the present invention uses the vehicle's own vacuum to control the production of hydrogen by forcing a liquid to rise in a chamber and into contact with metal in response to increasing vacuum thus producing an increasing amount of hydrogen gas with increasing throttle depression.