A deuterium injector is a component installed in the warp core of a starship. Shortly after Lieutenant Lee Kelso transferred aboard the USS Enterprise from the USS Potemkin, Lieutenant Commander Montgomery Scott caught Kelso making slight adjustments to the injectors. While Kelso was trained as a helmsman, he also had vast amounts of engineering knowledge, which sparked a friendly rivalry with Commander Scott. (TOS - My Brother's Keeper novel: Constitution)