Currently there's not really much to do with the Internal Combustion Engine cooling. I know it's very early game and various other reactors unlock not that far into the future, but the current state of the game as of July 2023 there isn't really any interesting play about the Aux module design in the early game.

Currently you slap 4 or 5 of the external or the internal cooling module onto the reactor, and one resistance multiplier vent (because it's numerically superior to 6 aux modules and they don't seem to have any cost I can determine).

This means that not only is it a place of potential failure if you design poorly (which is fine, there's lots of places in the game like this) but there's not really any interesting tradeoffs to be made. There's complexity here, and risk of failure, but no benefit to having this whole system - you can't make any interesting choices.

Right now, replacing the whole reactor aux part with an Internal/External cooling switch would be just fine, and probably better.

I'd rather it wasn't simplified that much, so some other aux modules available from the start of the game (or unlocked with the very first engine research perhaps, and maybe more for the other early ICE researches?) would be good. Here are some suggestions:

Internal Heat Sinks - does absolutely squat for cooling and resistance, but does increase the max temperature of the reactor by a significant chunk, allowing delaying the problem.

Emergency Exhaust Vents - Slight external cooling boost (making it a minor negative on internal cooling builds) but has a significant chance to negate damage when overheating - like 50% chance to ignore the damage (compounding so there can be multiple 50% chances, or whatever the best actual number turns out to be).

Secondary Cooling - Increases base cooling but reduces energy of the reactor? Unsure about this, but it would probably make a good upgrade item.

I've researched further in previous versions but my current campaign hasn't yet reached the second tier of reactors, so if some more Aux modules do show up and I've already thought of some, then my advice is please, push them earlier - much earlier - so that we're making choices that actually matter instead of having only three options: Fail/External/Internal.