Main Engineering is located on decks 15 and 16. Engineers will tell you that, if the Bridge is the nerve center of the ship, then engineering is the heart of the ship. The main engineering controls propulsion, energy distribution, sensors, navigation, environmental and other ship systems, usually located next to the warp core.
The warp nacelles in warp-capable shuttles and starships house the warp coils, necessary components of warp drive. The warp coils create a subspace displacement field, which “warps” the space around the vessel allowing it to “ride” on a spatial distortion, and travel faster than the speed of light. While not always present on starships, warp nacelles are the most common component of warp flight, dating as far back as Zefram Cochrane’s original warp ship, the Phoenix, circa 2063. They were also sometimes known as power nacelles or warp drive pod during the 23rd century.
Aboard Federation vessels of the 24th century, warp coils are fed by plasma conduits from the warp core reactor assembly. Nacelles are usually separated from the main structure of the ship because of radiation generated by the nacelles; when at optimal levels, the radiation could be deleterious to the safety of ship and crew. Nacelles are separated from the ship by large pylons and usually house a Bussard ramscoop at the fore end, primarily used for collecting hydrogen from space.