Gemon-4 ion engine

The Gemon-4 was a large model of ion engine manufactured by Cygnus Spaceworks for use on Imperial I-class Star Destroyers.

Behind the scenes
In Star Wars Legends, Gemon-4 ion engines were first identified in The Galactic Empire—Ships of the Fleet pop-up book, however, with their canon identification in Ultimate Star Wars, they appear to be the Imperial-I's primary and only ion drive, supplanting the Destroyer-I ion engine.