Disclosed is a fuel additive system for internal combustion engines in which the fuel additive system is disposed on-board a vessel such as marine vessel or locomotive. The fuel additive system includes a fuel additive reservoir containing a fuel additive in fluid communication with an internal combustion engine. An electronically controlled injector, such as an eductor, is in fluid communication with the internal combustion engine and a fuel level meter or sensor, fuel efficiency meter or sensor, or other meter or sensor which, upon sensing certain pre-set conditions, e.g., increase of fuel level in a fuel storage tank or decrease in fuel efficiency, the injector injects an appropriate amount of fuel additive from the fuel additive reservoir and into the fuel stream that ultimately enters the internal combustion engine.