S-2 graf unit

From Memory Alpha, the free Star Trek reference.

The S-2 graf unit is a type of warp engine used on Klingon starships in the 23rd century.

In 2269, the D7-class IKS Klothos suffered damage to its graf unit during transition into the Delta Triangle domain of space, and was unable to escape to normal space without the help of the USS Enterprise. (TAS: "The Time Trap")

It is not known if the same type of graf unit was used on all Klingon ships, or just the class of the Klothos.