Using more than one engine will never be efficient due to the additional weight, wha I would prefer is if the currently existing option to change a task groups speed would also allow more efficient engine operation.
Obviously an engine will be at its best operating condition when running at it's designed speed, just like a real engine, but there should be some advantage of running slower.
At the moment halving a ships speed just makes it take twice as long which uses just as much fuel.
You shouldn't be able to throttle back an FAC's engines to suddenly make them as efficient as a freighter, but going slower should save fuel, the question is how much? Related yo this perhaps you could throttle above 100% but at severe risk of failure?
Let's consider the normal power curve, going from 100% to 300% increases fuel use by 15 times, would it be acceptable to allow any engine the ability to do this again, also increasing failure rate by the same amount?
What about the opposite, I don't recall the exact mechanic but I think you can go down to .2 power and get the a 20th fuel consumption. If this was enacted as well then it would be trivial to lower an engines power even further for stupidly efficient freighters, or military vessels with anemic engines of negligible failure rate.
I think a good compromise would be to allow any engine to throttle up or down towards the currently existing limits, .2 or x3, but with an efficiency curve that's altered. This way you wouldn't get more powerful engines since the current max power already push the limit of what's possible, or more efficient engines as the same applies, but you gain flexability with median powered engines.
The question is how should this be balanced? Say you're using an engine with 1.0 power multiplier and have researched a max power of x3.
Obviously an engine actually designed to operate at that power is more efficient, perhaps increasing fuel use and failure rate by an additional 50% per level, and for reduced speed the opposite by decreasing the extra efficiency by 50% per level.
After designing the ship you send it out to battle and come across an enemy armada, you're outgunned and outclassed. The captain orders the overdrive setting. Which will enable it to outrun the enemy.
What happens?
Let's imagine the ship was designed for decent range, maybe 60 billion kilometres, not particularly good for a scout but fine for a warship.
If normal engine consumption was used we find that after going to overdrive the ship now has 3 times the speed and 15 times consumption. Range has dropped to 12 billion kilometres, which is still ample to outrun an enemy, even through multiple systems. But it's unfair to allow any engine to do this since the engine isn't running at its designed power, it should be less efficient.
What if we increase fuel consumption by the proposed 50% per level? Power levels increase at a rate of 25% more per level up to 3 times so from 1.0 to 3.0 is 8 levels, that's 4 times additional fuel consumption. A total of 60 times what the standard engine uses, but only 4 times less efficient as the properly designed engine of that power.
That range of 12 billion drops to 3 billion.
What if the inverse was true?
I can't remember the vanilla efficiency gains for less power so I'll leave this untill later, but if you start with a 3.0 power engine and decide to throttle it back using the above mechanics the instead of getting a 1/15th fuel consumption for a third the speed instead you multiply the vanilla efficiency change by 1.5 per level as before. So 8 levels gives an increase of 4 times of the vanilla 1/15 efficiency of an engine running at 1.0.
A third the speed gives you 26% the fuel use. Actual fuel efficiency gain isn't much in this example, you'll use 86% as much fuel to get there 3 times longer. But reducing power even further would give better efficiency gains.
