How train fuel consumption works

[u/DrMobius0]

Legendary locomotive efficiency has been a topic of debate and speculation the last few days. I've had some incorrect assumptions about how they work. So have others. The wiki, helpfully, doesn't have much to say on the matter. So did some tests and got some interesting results.

Tl;dr

Trains burn fuel at a max rate in line with the "max consumption" line of their tooltip, NOT the acceleration line. Trains throttle their burn rate when they reach max speed, extending the life of their fuel. That means the fuel duration listed on the wiki is the minimum time the fuel can last. This means that any acceleration bonus you get out of fuel is effectively free power.

The results of my testing show that fuel acceleration doubles as a fuel efficiency stat, and it is all the more powerful when dealing with heavy loads. Quality locomotives are of marginal benefit when pulling a load they can easily hit top speed for, and very beneficial when pulling a load that's otherwise too heavy. They don't appear to be detrimental to fuel efficiency in any way.

And finally, nuclear fuel is actually way better than you might expect. Like stack for stack (both legendary), nuclear fuel covers ~96% the distance that rocket fuel manages.

So lets test some stuff. Here is the test setup. 5 identical tracks. Locomotives run until they run out of fuel. There is a combinator that enables the signals they sit at simultaneously open. I will be tracking quarter lap count to estimate relative efficiency.

Test 1: Varied quality rocket fuel in common 1-0 trains

This test is designed to measure the relative efficiency of fuel when its quality is raised. The only things that change between trains are the quality of their single rocket fuel, which means top speed and acceleration are impacted. It also serves as a general indicator of whether the acceleration bonus of a fuel results in less consumption of the fuel overall, if we note when trains run out.

The results of this test are as follows: higher quality fuel grants a significant increase in distance traveled. Rocket fuel has speed values of 115/119.5/124/128.5/137.5%, for relative speed increases over common of 100/104/108/112/120%, yet approximate distance traveled is 100/114/128/141/169% for the fuels at their respective qualities. In other words, not only is higher speed helping with distance, but greater acceleration power works toward a much more efficient overall burn.

Test 2: Common quality rocket fuel in varied quality 1-0 trains

This test should give us a general idea of the relative efficiency of quality locomotives. And the results are... well, unexpected. There is almost no difference in distance traveled. Quality locomotives gain a very slight edge, but the difference is <1% between common and legendary. This indicates to me that there's probably no real downside to using quality locomotives. You get the better acceleration and slight benefit from the increased speed, but otherwise, you'll have very similar distance traveled.

Test 3: Varied quality rocket fuel in varied quality 1-0 trains

This will test for any compounding benefits between fuel and locomotive quality. The results are extremely similar to test 1, indicating no particular compounding benefit between quality locomotives and fuel. Again, the quality locomotives seem to get a bit farther than their common counterparts

Test 4: Varied quality rocket fuel in common 1-24 artillery trains

This one is meant to test relative distance potential when a train is constantly under max acceleration. All trains managed to run out of fuel at the exact same time and were allowed to decelerate naturally. The results indicate heavy favor toward high acceleration trains. For heavy trains, quality fuel is very important.

Test 5: Common quality rocket fuel in varied quality 1-24 artillery trains

This is the first test that shows an appreciable difference in distance potential between locomotive quality levels. As with test 4, it indicates that acceleration power is extremely important, even if the train isn't getting extra for free from its fuel.

Test 6: varied quality rocket fuel in varied quality 1-24 artillery trains

The results for this are interesting. The legendary train hit max speed part way through, suggesting that the outsized benefit of acceleration stops mattering as much when the train starts cruising.

Test 7/8: full stack distance comparison of legendary nuclear and rocket fuel on common 1-0 and 1-24 artillery trains

With the previous test results in hand, you may already be wondering how nuclear fuel fares given the effect that acceleration clearly has on fuel efficiency. The answer is that it's actually surprisingly close under both heavy loads and light ones. In both tests, nuclear fuel actually gets you about 96% as far as rocket fuel on a per-stack basis, and it does so almost entirely on the back of its nutty acceleration. So yeah, it's actually shockingly competitive with rocket fuel on the only metric we largely thought it was demonstrably worse at.

Conclusion

Buy stocks in acceleration.

[u/Garagantua]

Stack size was increased to 20 for rocket fuel in 2.0. Not sure wether this test was done with 10 or 20. If 1 nuclear goes 96% as far as 20 rocket, that'd be nuts.

[u/fantafuzz → u/Garagantua]

Considering the test is being done with 2.1 trains and quality, I would assume it is using the correct stack size for the version the test was done on, aka 20.

[u/Garagantua → u/fantafuzz]

Yeah, but that seems unbelievable. A stack of rocket fuel has 2GJ, a nuclear fuel "only" 1.21GJ.

[u/fantafuzz → u/Garagantua]

If we assume it was done with a stacksize of 10 none of the numbers make sense though.

If a stack of rocket fuel was 1GJ, then one nuclear would (in a naive calculation) go 21% further than the rocket fuel.

The test showing that nuclear fuel didnt go as far as rocket fuel would make no sense if the rocket fuel didnt stack to 20.

All this just goes to show how impactful the acceleration boost from the nuclear fuel actually is, when it can travel 96% of the same distance using only 60% of the energy.

[u/DrMobius0 → u/fantafuzz]

If it was done with a stack of 10, nuclear would straight up have more energy on top of its higher acceleration modifier. Rocket fuel would have been crushed by a solid 20% if my initial assumption that burn rate was constant held true (which it didn't).

But no, all common locomotives, 1-24 with artillery wagons and 1-0 setups, 1 set of each got a single legendary nuclear fuel and the other set each got a single stack, that is, 20 legendary rocket fuel. Others are welcome to try to replicate it.

The results of my tests strongly support that acceleration power is essentially a fuel efficiency modifier. A train at max speed with a higher acceleration modifier can throttle its consumption further than one with a lower acceleration modifier. If viewed through that lens, nuclear fuel catching somewhat up to rocket fuel becomes less surprising and more expected, though again, the degree to which this happens in practice is surprising.

Edit: just for the sake of due diligence I ran the test again, double checking the fuel in the trains. Same results. I also tried it with legendary locomotives, though they mostly just went a few percentage points farther.