SS Mariposa, a colony ship with a Yoyodyne pulse fusion drive
Yoyodyne pulse fusion, or YPS pulse fusion, was a type of warp drive propulsion system and drive type for starships that used nuclear fusion reactors.
The Yoyodyne Propulsion Systems propulsion technology was utilized in DY-500-class starships in the early-2120s and allowed them to serve as early Earth colony ships. (TNG: "Up The Long Ladder", production art)
The propulsion system was also used in the late-23rd century on Whorfin-class starships utilized by El-Aurians. The YPS pulse fusion propulsion system allowed the transports to travel at a maximum warp speed of warp factor 4. (Star Trek Generations production art)
Background information[]
Based on ENT: "First Flight", the DY-500 pulse fusion drive was presumably limited by the warp 2 barrier, as that speed was first surpassed in 2143 according to the episode.
According to the Star Trek: The Next Generation Companion, 2nd ed., p. 88, the use of the term "Yoyodyne" is an in-joke reference to the 1984 cult classic science fiction film The Adventures of Buckaroo Banzai Across the Eighth Dimension!. The film in turn took the name from Thomas Pynchon's novels The Crying of Lot 49 and V..