A BattleMech is not allowed onto the battlefield without an engine, at least one weapon and at least 10 heat sinks. 'Mech movement is dictated by it's engine - the large the engine the faster it can move.
Engines are described by their rating - typically from 100 to 400. The higher the engine rating the faster the 'Mech can travel, however the higher the rating the more the engine weighs. Different 'Mech chassis and variants allow equipping different ranges of engines - no 'Mech can fit all engine ratings.
There are three types of engines - standard, light and extra light. Light and extra light engines weigh less for the same rating but offset that weight savings by having more volume. Engine technology differs between the Inner Sphere (IS) and Clans.
Standard Engine (STD)- a fusion engine and dense ceramic shielding thick enough to protect a 'Mech pilot from the fusion reaction, survive battle damage and serve as a heat sink. The baseline for engines, it takes six slots within the center torso (CT).
Light Fusion Engine (LFE) - Inner Sphere technology that reduces the weight of an engine at the cost of larger volume - in addition to the six slots within the CT the LE takes two slots within each side torso (ST).
Extra Light Engine (XL) - Engine technology even more light weight than LFE but has even larger volume. While occupying six slots in the CT the IS XL uses three slots within each ST, where the Clan XL only uses two slots within each ST (in addition to the six in the CT).
A BattleMech is destroyed if it loses three slots worth of its engine (see How to Destroy a 'Mech). Therefore, you will destroy a 'Mech that is using a STD engine by destroying its CT. An LFE or Clan XL equipped 'Mech is destroyed by the loss of its CT or both of its STs. An XL equipped 'Mech is destroyed by the loss of its CT or either of its ST.
While LFE and Clan XL equipped 'Mechs will survive the loss of a single ST, they are severely damaged. Their top speed is reduced by XX% and their heat capacity and dissipation is reduced by XX%. As an example, if you are close to your heat shut down (see Heat) and lose a ST, you may immediately shut down due to a lower total 'Mech heat capacity.
Special Cases • Urbie - The Urbanmech (UM, "Urbie") is stock equipped with a STD60, giving it the blistering top speed of over 32 KPH! The STD60 is unique to the Urbie and cannot be equipped onto a different 'Mech chassis • OmniMechs - While the Clan OmniMech allows for great flexibility with its hardpoint configuration by exchanging locations, its engine is fixed.
Travelling Up Grades - The heavier a 'Mech is the more it slows down when moving up a grade or slope. The "Hill Climb" skill tree node reduces this deceleration.