LIGHTWEIGHT ICEs & FCEs
The standard, most common Internal Combustion Engine in use in the 31st century has hardly changed from the ICE in the 21st century. However, alternatives to these engine types using more advanced (and more expensive) designs and materials have existed for some time.
The Light Internal Combustion Engine replaces the regular steel alloys with other alloys, the three most common being nickel-based, cobalt-based, and titanium-aluminide based superalloys, whose strength is retained better than most steels at high temperatures, allowing the pistons to operate at a higher temperature and resulting in higher thermal efficiencies, and a modestly lighter engine for the same power. Light ICEs are also far more likely than standard ICEs to use more exotic thermodynamic cycles than the common Otto cycle like the Atkinson cycle, so they're usually slightly more fuel efficient than regular ICEs.
The Light ICE is somewhat uncommon, especially compared to the regular ICE, precisely due to the exotic material inputs. The materials are more difficult to find replacement parts for, and the acquisition cost increases the cost of the engine near that of fuel cells, which lessens the primary advantage of the ICE. It's still fairly cheap compared to everything else, so that combined with most model's flexible fuel consumption finds the Light ICE used in mid-range combat vehicles in between the capabilities of regular ICEs and fusion vehicles.
The XL ICE is rare. The XL ICE was developed around the same time that Endo-Steel was invented and replaces the regular steel alloys with them, allowing for a nearly 1-to-1 replacement of standard ICEs with half the weight. Unfortunately, the cost of an XL ICE is equivalent to a Fusion engine while it "only" matches the performance of one, making the XL ICE's suitability on battlemechs or industrialmechs dubious at best. It's no surprise that the limited quantities of endo-steel produced was prioritized for Battlemech skeletons and LB-X autocannons.
The XL ICE, with only few examples produced, temporarily went extinct during the Succession Wars with the destruction of the orbital endo-steel factories. After the rediscovery of endo-steel, very limited examples of XL ICEs were produced, mostly as experimental designs. The first mass-produced examples of XL ICEs were produced by Clan Hell's Horses, followed by Clan Diamond Shark's XL ICE vehicles built for export.
A similar story exists with Fuel Cell engines, with the light-weight version taking advantage of the fuel cell's low operating temperatures to use light, high-strength titanium alloy frames to cut weight, and the XL fuel cells jumping into endo-steel structures. While not really used for Battlemechs, these do find a niche on high-end combat vehicles.
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Light ICE
Cost: (3125*Engine Rating*Unit Tonnage)/75 C-Bills
Tech Level: C/D-E-D-C
Weight: 1.5x Equivalent Fusion Engine
Crits:
Battlemech- 6 center torso, 2 side torsos (1 Clan)
Vehicle- 1 slot (0 Clan)
XL ICE
Cost: (5000*Engine Rating*Unit Tonnage)/75 C-Bills
Tech Level: D/F-X-F-E
Weight: 1x Equivalent Fusion Engine
Crits:
Battlemech- 6 center torso, 3 side torsos (2 Clan)
Vehicle- 2 slots (1 Clan)
Light FCE
Cost: (7500*Engine Rating*Unit Tonnage)/75 C-Bills
Tech Level: D/E-F-E-D
Weight: 0.9x Equivalent Fusion Engine
Crits:
Battlemech- 6 center torso, 2 side torsos (1 Clan)
Vehicle- 1 slot (0 Clan)
2 "free" heat sinks
XL FCE
Cost: (14000*Engine Rating*Unit Tonnage)/75 C-Bills
Tech Level: E/F-X-F-E
Weight: 0.6x Equivalent Fusion Engine
Crits:
Battlemech- 6 center torso, 3 side torsos (2 Clan)
Vehicle- 2 slots (1 Clan)
3 "free" heat sinks
