i love the option to have different engines with different efficiencies. but some things could improve alot. as it is, there are many possibilities, but you don't need more than few of them.
1. when i use a HS 50 engine instead of 2 HS 25 engines i need 33 % less fuel. that's cool. but when i use a HS 2 engine instead of 2 HS 1 engines, i save only 1 % fuel. that's negligible. i'd never use a HS 2 engine, i just design a HS 1 one and be more flexible. the efficiency increase should be similar to have efficiency considerations also for small engines.
2. on the other hand, the saved fuel from HS 25 to 50 is so much, that it rarely makes sense to build a HS 25 engine, and certainly even less anything inbetween.
even if i don't need a HS 50 engine, i'll just take it instead of the HS 25 and have a faster ship that needs less fuel. because for ships with 2,500 tons and more, the ship becomes FASTER and needs LESS fuel if i use one HS 50 engine instead of one HS 25 engine. the ship will be a bit more expensive to build and a bit larger. but that's rarely an issue compared to the advantages. the larger engine does needs more fuel per hour of course, but since it's faster you need less total fuel to travel the same distance (in less time!).
3. it's also sad that engines are capped at HS 50. it usually makes no sense to build large ships (as long as they have a HS 50 engine) that need many engines, because you can as well make a bunch of small ships without losing anything. some components are cheaper if you build them larger, but that doesn't outweigh the inreased flexibility of multiple smaller ships. smaller ships are also better from a tactical point of view.
i would like to see huge commercial ships, bulky and slow, but efficient. part of this could come from "logistic times", but in many cases the most important factor is fuel usage. we have less powerful engines that need less fuel, which make "slow and efficient", but they don't make "large".
having said all that, here's my suggestions:
1. to make decisions more interesting, increase the efficiency differences between small engine sizes. if the 1-50 range should be kept, a root function like efficiency = root(HS-1) x 7 would do nicely. this results in values like (rounded)
HS / fuel used in %
50 / -50
35 / -42
25 / -35
15 / -30
10 / -20
5 / -15
2 / -7
1 / -0
that's a small change and there's a bit more reason to use different sized engines. i also think from HS 25 to 50 it's sufficient to have only every 5th HS as option (25, 30, 35, 40, 45, 50). i can't imagine anyone would ever build a HS 47 engine.
i'd like that alot better, but this does not solve the problem, that a HS 50 engine will be faster and needs less fuel than a HS 25 engine beginning with a certain ship size (in this case a bit higher, like 5000 tons). nor does it give a reason to build large ships.
2. for that, the calculation for fuel usage needs to be different. where "E" is the efficiency (currently HS / 100), the used calculation is:
fuel usage(HS) = fuel usage x (1 - E)
a calculation for a larger range of possible HS would have to look like:
fuel usage (HS) = fuel usage / (1 + E)
this calculation could be open-ended, but it also needs a root function to scale nicely. if i use E = root(HS-1) x 7 (like above), values then look like this
HS / fuel used in %
1 / 0
2 / -3
3 / -5
5 / -8
10 / -13
15 / -17
20 / -20
25 / -22
35 / -26
50 / -30
75 / -35
100 / -39
125 / -42
150 / -44
200 / -48
250 / -51
300 / -53
400 / -57
500 / -60
750 / -65
1000 / -68
1500 / -73
2000 / -75
now if you build a huge engine, it can make a big difference and you'll probably still use the ship a few engine techs later. note that for this, the build cost should scale with sze, but the research cost for the engine designs should scale with the saved fuel, not with size.
there is another problem, which is that for bigger ships it's often better (faster, less fuel needed, cheaper to research and build) to use multiple lower power engines instead of one higher power engine. the only solution (besides completely changing the system) i find for this is to restrict all ships to one engine each. currently that's not possible because of the maximum engine size. with my suggestion it would be possible.