Though inefficient and under-powered from the standpoint of the 24th century, over a thousand years before the Lang Cycle fusion engine was a mainstay of Promellian design for several generations and performed well in that role.
In 2366, the derelict battle cruiser Cleponji was discovered by the USS Enterprise-D near the remains of Orelious IX. The ship's engines were still intact, a fact that Captain Jean-Luc Picard was quite enthused about. Later, during an away team mission, Data was able to restore minimal power to the engines, however, they had been largely drained by the presence of aceton assimilators which had been deployed as a booby trap by the Menthar. (TNG: "Booby Trap")