Yoyodyne pulse fusion
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  starships in the early-2120s and allowed them to serve as early Earth colony ships. (, production art)

The propulsion system was also used in the late-23rd century on  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. ( production art)

Background information 
Based on , 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 , 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 's novels  and . 
Propulsion technology