Engine - XL Engine
The Extralight Fusion Engine (often shortened to XL Fusion Engine) is mechanically similar to a Fusion Engine but uses much lighter radiation shielding. Replacing the dense tungsten carbide of standard fusion engines with a crystalline polymer similar to that of double heat sinks, the XL fusion engine is half the mass for the same performance.
There is a penalty in bulk, however. The Inner Sphere XL fusion engine is twice as large as the standard fusion engine, while the Clan model is about two-thirds larger. The larger volume of the engines makes them much more likely to end up in the path of penetrating hits. Further, these bulkier engines intrude into the side torso bays of a BattleMech, exposing the 'Mechs to catastrophic engine damage from structural damage that would be otherwise survivable.
There is also a penalty in cost; an XL fusion engine is four times as expensive as a standard, a component that is already usually the largest cost in a BattleMech. Despite this, the allure of huge weight reduction means that the technology sees wide use (as evidenced by the long list of manufacturers below).
Description provided by Sarna.Net
In MW:PR 3051 the XL engine resides in the center torso of the mech. Additionally, the XL engine takes up slot space in the left torso and right torso. When moving, the mech's engine will generate a slight amount of heat (No more than 5 pts). Destroying the center torso of the mech, will destroy the engine and therefore the mech itself. Destroying the left torso or right torso of a mech mounting an XL engine will also destroy the mech.