S-2 graf unit
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  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. (; ) 

In 2269, the D7-class  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 . 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. ()

See also
 Tritium intermix

de:S-2-Graf-Einheit
Warp
Propulsion technology