MercTech: Engines

General overview
Unlike in the base game, where a mech's engine is inaccessible, Merctech adds engines as actual physical items that can be bought and sold, damaged and repaired, and replaced with others to permit new build possibilities. During combat in non-hardcore mode, an engine can take up to 3 critical hits, and will be destroyed on the fourth hit. In hardcore, this is reduced to 3 hits to destroy, which means that an IS-spec XL engine will always be destroyed with the loss of a side torso.

Standard Fusion Engine:
In use since before the Star League, the standard fusion engine has formed the core of every battlemech up until the development of a more advanced version by SLDF scientists. While the fall of the Star League and the coming of the Succession Wars had wiped out many of the technologies the Star League had created, the simple and rugged standard fusion engine would return to being the sole engine in use on battlemechs until the technological renaissance of the mid-31st century.

Mechlab rules: The Standard Fusion Engine takes 6 slots in the center torso.

Extra-Light Fusion Engine (XL Engine):
An advanced version of the fusion engine designed by Star League scientists, the Extra-Light Fusion Engine makes use of various advanced materials and technologies to reduce the fusion engine's weight with significantly lighter, but bulkier engine shielding. Being half the weight of a normal fusion engine, the XL engine can free up a significant amount of tonnage for a larger engine and/or more armor and equipment. While these weight savings allow for the creation of far more potent battlemechs, the XL engine's shielding protrudes into the side torsos, allowing it to be more easily damaged. In fact, the loss of a side torso will lead to significant damage to the engine, which can lead to a greater loss of performance than it would for a standard engine.

The Clans had improved the Extra-Light Fusion Engine, reducing the size of the shielding in the side torsos and improving the redundancies in case a side torso is destroyed, reducing the loss of performance.

Mechlab rules: The XL engine takes 6 slots in the center torso and 3 slots in each side torso. Bare in mind that with Hardcore mode enabled, destruction of the side torso guarantees a combat kill.

Light Fusion Engine (LFE):
A former member of the Clans, Wolf's Dragoons, now a legendary mercenary company, wanted an engine produced with Inner Sphere technology that had similar weight savings to their ClanTech XL engines with the same overall durability. Enlisting the help of their close equipment manufacturing associate Blackwell Heavy Industries, they successfully created an engine that fit this criteria, being 75% the weight of a standard engine, while having smaller shielding in the side torsos than an Inner Sphere-built XL engine.

This technology would begin to trickle throughout the Inner Sphere after Lyran spies had stolen the plans for it.

Mechlab rules: An LFE takes 6 slots in the center torso and 2 slots in each side torso.

Compact Fusion Engine (CFE):
A rare fusion engine variant that would appear in the 3060s, the Compact Fusion Engine goes the opposite direction of the Light and Extra-Light engines, using heavier, but more compact shielding to reduce the engine's size. Being half the size of a standard fusion engine, the Compact Engine not only frees up more space in the center torso for more equipment, but is less likely to be hit once the armor is breached, which can allow a mech to continue fighting after suffering significant damage. The drawback for this reduction in size is its weight, weighting half-again that of a comparable standard engine, which limits its use to units that are willing to give up tonnage for more equipment space.

Mechlab rules: A CFE take 3 slots in the center torso.

Extra Extra-Light Fusion Engine (XXL):
Taking the fusion engine weight savings of the Extra-Light Fusion Engine to its extreme, XXL engines weigh only a third that of a comparable standard engine, offering even more free tonnage than the Star-League designed XL engine. These weight savings come with the drawback of significantly bulkier heat shielding, taking twice the space in the side torsos that an XL engine does, significantly increasing the chance of the shielding taking damage.

Mechlab rules: The XXL engine takes 6 slots in the center torso and 6 slots in each side torso.

Additional notes:
Early battlemechs used a number of less sophisticated components before the development their more advanced, and now ubiquitous successors. One of these obsolete components included an early fusion engine. Referred to as the Primitive Fusion Engine, it is heavier than a comparable standard fusion engine for the same performance. Although it is no longer produced within the Inner Sphere, there are rumors of mechs retrofitted with these engines appearing in the Periphery, although if they're being manufactured there or just functioning relics is still unknown.