Fusion engines are the powerhouse of a BattleMech. They provide the power necessary to move and fire weapons. Unlike internal combustion engines, fusion engines generate power by fusing hydrogen isotopes (1H) much like the center of stars. Despite conventional thought, these engines are not potential mini-nuclear explosions. When damaged to a significant degree, these engines will automatically shut down all fusion reactions and power down the BattleMech.
Standard engines are mounted in the center torso and require six critical spaces. These engines cost less than XL Engines but weigh more. Additionally, 'Mechs mounting standard engines are more durable than those mounting XL Engines since the engine can survive the destruction of a side torso.
XL Engines are also mounted in the center torso but also extrude into each side torso due to their increased bulk. These engines weigh half as much as a standard engine of the same rating but cost much more. If the side torso of a 'Mech mounting an XL engine is destroyed, then 'Mech is disabled due to amount of damage the engine extruding into the side torso has taken.