Ion-Beam Accelerators were powerful engines used to drive USOs and X-Com Hybrid Subs in the Second Alien War. At least one of these, along with a Magnetic Navigation module, are required to construct an Alien Sub.
The Ion-Beam Accelerator drives an Alien Sub by using Ion power generated from Zrbite, a precious alien power source, to create thrust powerful enough to travel underwater or through the air at unrivalled speeds. It has also been observed that similar devices appear in Alien Colonies and Artefact Sites, presumably used to power the base facilities. Like the Elerium-powered antimatter reactors from the previous war, Ion-Beam Accelerators are prone to explode with enough force to blow open a sub's hull when under fire. While exploding an engine can injure, if not kill, nearby aliens, the explosion can also destroy nearby modules and its Zrbite power cells, reducing the score of successful recovery and taking a good chunk out of the salvaged parts. Often, a sunken alien sub will lose its engines in the crash, and the damage from the explosion alters the mission somewhat, from creating dust clouds to destroying componets to blasting the sub's hull open.
Ion-Beam Accelerators are manufactured from Aqua Plastic and Zrbite, and the Hybrid Subs they are used on are fueled by Zrbite.
If a wrecked engine is not burning at the start of a recovery mission, there's a chance that a lump of Zrbite can be found in the wreck, much like how destroyed Alien Power Sources (except for those destroyed by the crash or explosives) yielded Elerium.