D7 engine room

The engine room on a D7-class battle cruiser

Elysian ships 4

The Klothos, a ship equipped with a S-2 graf unit, trapped in Elysia

The S-2 graf unit was a type of engine used as the warp system on Klingon starships in the late 2260s.

The S-2 graf unit was a rough equivalent of the warp drive used by Starfleet ships such as the Constitution-class in the same era. The engine normally allowed a D7-class battle cruiser to catch up with a ship traveling at the speed of warp 9. (TAS: "The Time Trap"; TOS: "The Enterprise Incident")

In 2269, the D7-class IKS Klothos suffered damage to its graf unit during the transition into the small alternate universe Elysia. The ship was unable to escape to normal space without the help of the USS Enterprise. The starships connected their warp systems and together they reached warp 8 to escape the time warp vortex in the time barrier between the universes. (TAS: "The Time Trap")

According to the Star Trek: Starship Spotter, other Klingon starships, at least the Bird-of-Prey, K't'inga-class and the Vor'cha-class all use different types of dilithium conversion graf units as warp nacelles.
The term "Graf" was also seen in TNG-R: "Redemption II", where an article by a scientist called Bormanis suggested "that the action of null-field tachyons and gapless energy spectra remain invariant with respect to Lee Graf generation in boson fields."

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