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I am writing a scifi story in which Earth's orbit has been altered, making the planet generally colder. This has made the northern hemisphere, as well as most of the southern, practically inhospitable.

Due to this, most of humanity died and took the global distribution chains with them. A couple centuries later, people have all the technological knowledge of the 21st century available to them - at least what can be salvaged from books - but no access to the materialized technology itself. You will hardly be able to use gasoline if you aren't able to drill for oil and send it to a refinery. Even if you had the materials to rebuild the infrastructure, most people live in settlements of no more than a few thousand at most, struggling with basic agronomy - so using oil for energy isn't economically viable.

One settlement in my world, though, has a few hundred thousand people in it, with many of them being scholars and scientists. They should be able to build at least a few machines powered by internal combustion or electricity. I am considering options for a viable source of fuel.

I know that automotive ethanol can be extracted and refined from plants. The most important sources in our own world are sugar cane in Brazil and corn in the US. Sugar cane is not an option in my worldbuilding due to a lack of tropical climates though.

Then there is biodiesel, which can be made from some oily plants, and I've read it can also be made out of animal fat.

So, supposing the following conditions:

  1. A settlement in a climate with temperatures varying from approximately 0C (32F) to 15C (59F) throughout a ~400 day-long year, at latitude 3 degrees South - very little variation in day length, but the distance from the Earth to the Sun varies from 1 AU to 1.3 AU.

  2. There are enough arable lands and enough people to work on them to keep everyone fed. Plants and cattle could be grown and raised for energy sources.

  3. The settlement needs to have between 20 and 60 car-sized vehicles moving, possibly a small train - no need to make this comfortable, they might be running in experimental and/or spartan conditions.

What would be the best biological source for fuel? Best not in terms of power density or performance, but in terms of "a post-apocalyptic settlement would be able to make it and use it".

Or - frame challenging myself here - are sustainable, biological sources just not viable, making steam power fueled by coal or wood preferrable?

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    $\begingroup$ How does humanity degrade so much in technological development ? You dont need to be a genius to make an ICE if you have billions of examples laying around. Any first semester Engineering student can build you a Wood-Gas Generator and make electricity with an efficiency of 1%. Similarly, Oil and Natural gas dont degrade the same way Gasoline does. And all the GMOs we use for agriculture are still there. We wouldnt start from square one. $\endgroup$
    – ErikHall
    Jul 11, 2023 at 11:00
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    $\begingroup$ A primitive-ish (or just large) diesel engine can work on vegetable oil just fine, no need to refine anything. I foresee a brisk market in 1950s or 1960s tractor engines. $\endgroup$
    – AlexP
    Jul 11, 2023 at 11:36
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    $\begingroup$ The obvious answer is biodiesel, which is reflected in the current answers. The problem isn't the fuel, it's whether or not your civilization has the ability to build an engine that can use biodiesel, which requires stronger metals and greater tolerances than a steam engine. $\endgroup$
    – JBH
    Jul 11, 2023 at 15:12
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    $\begingroup$ Back to the future: whale oil! $\endgroup$
    – Jon Custer
    Jul 11, 2023 at 18:49
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    $\begingroup$ "people have all the technological knowledge of the 21st century - at least what can be salvaged from books" and "most people live in settlements of no more than a few thousand at most, struggling with basic agronomy" seem largely at odds. A huge part of having all of that knowledge is having a bunch of people that don't have to worry about where their next meal is coming from. $\endgroup$ Jul 11, 2023 at 23:07

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Given the conditions you describe, I would expect their first vehicles to be external combustion vehicles: steam cars. Simpler to build, and they can run on anything that burns.

Internal combustion replaced steam because it provides more power while using less fuel. The tradeoff is that the fuel has to be pretty standardized and reasonably clean; you are burning it inside the engine and you don't want a lot of crud to build up in your engine. External combustion doesn't have that problem. If a bunch of soot builds up on the underside of the boiler, nobody really cares.

There would likely be several generations of steam car. The oldest ones are inconvenient: you have to shovel stuff into the furnace yourself. Newer ones might have a feed for powdered coal, or vegetable oil, or whale oil, or crude oil, or whatever the community can spare. (But maybe there would be an option to manually shovel in charcoal or something in case there's an emergency and the primary intended fuel is unavailable.)

The cars might also be designed so that the steam pipes route through the passenger section of the car, so that a little bit of heat will leak into the car to warm the passengers.

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You can easily adapt any diesel engine to run on vegetable oil.

https://www.caranddriver.com/features/a15129612/how-to-convert-your-diesel-to-run-on-vegetable-oil/

so if your people have all the modern knowledge and are able to build a car, they can adapt it to run on vegetable oil or any other fat. The pipes and motor have to be warmed up (even more than with normal diesel) and some parts would have to be adapted in size to handle the different viscosity. Apart from that, they should not have any problems.

Of course, it's less efficient than more refined fuels, but it will work.

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Short answer: Rapeseed and sunflower oil

Rapeseed and sunflower oil are common biological energy sources. Palmoil and soybeans work great as well, but probably don't agree with the climate you describe. One hectare of agricultural land produces roughly 800 kg of oil (might be less if your people don't have enough fertilizer), so a comparatively small area should cover the need of your vehicles and also be enough to run a few tractors to increase the agricultural yield. The oil can be burnt in modified diesel engines or refined into biodiesel.

I would avoid animal fat (except if it is a waste product) since the additional step in the food chain usually means a loss of ~90 % of the energy, but the residue from the oil press could be used for feeding animals.

Steam engines with coal might scale better if there is coal nearby and your town needs more than the few vehicles you describe, but steam engines are more complicated to operate than a diesel engine, so I would go with the vegetable oil.

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    $\begingroup$ can the whole world start calling it canola yet? $\endgroup$ Jul 11, 2023 at 13:51
  • $\begingroup$ @user253751 If only the Canadians were a little less polite and more pushy. :) $\endgroup$
    – Glen Yates
    Jul 11, 2023 at 19:56
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    $\begingroup$ @user253751: They're technically not the same thing. Canola has been genetically modified to have low erucic acid content. So, while all canola is rapeseed, not all rapeseed is canola. Still, "rapeseed" is a very unfortunate name, like something you'd expect to find between the robberyseed and murderseed oils. $\endgroup$
    – dan04
    Jul 12, 2023 at 15:46
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Assuming ethics are not an issue in the new society, render the blubber of marine mammals to get oil. Whales might be difficult, but natives managed with small boats and hand launched harpoons. However seals on the beach were often hunted with only clubs. With a large percentage of the world in arctic conditions, areas to grow plants are at a premium and will be needed for food.

You might also see more steam powered vehicles than IC. It takes less material and the machining does not need to be as precise. It will be be a large investment in time and resources.

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    $\begingroup$ If food is short and your fuel is edible then it is less likely it will be used for non-food purposes. But steam powered by (inedible) wood is a good idea. $\endgroup$ Jul 12, 2023 at 3:02
  • $\begingroup$ After a couple of centuries, whale populations in the ocean should be doing very well. I'm not sure how smart whales are, but I think they might be smart enough that I would prefer not to hunt them. $\endgroup$
    – steveha
    Jul 13, 2023 at 8:35
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Coal or natural gas is a lot better.

Coal or natural gas is going to have 2 huge advantages. You don't need mechanized agriculture to make it in quantity and it is not competing for arable land. plus there are a lot of existing powerplants that can run off of them.

Existing car can easily be converted to running on wood gas or coal gas, it was done in the past and can be done at a small home shop. You can even use crop waste for this.

Or you can run them on gasoline, the Fisher-Tropsch process turns coal into gasoline and is fairly easy to do.

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Do keep in mind with populations so small they are not making machines, they are repairing and reusing.

The major problem biological sources have is you are competing with your food producing land and labor, which is a big problem in a small community, with limited technology. Mining coal is really easy if you don't care about the environmental impact.

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    $\begingroup$ To be fair, in a world that has gone icy, warming from fossil fuels might even be welcome. $\endgroup$ Jul 11, 2023 at 19:47
  • $\begingroup$ Unless there's a cartel that benefits from the control that the icy, inhospitable status-quo gives them over everyone else. Then you might have problems. $\endgroup$
    – aroth
    Jul 12, 2023 at 2:55
  • $\begingroup$ @TheSquare-CubeLaw: Assuming there are still enough people to make a global impact. $\endgroup$ Jul 12, 2023 at 3:05
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Earlier in history saw mills were placed on rivers to take advantage of the rivers raw power. No reason some factories can't take advantage of that.

I would suggest electric cars/machinery, because the electric motor is far simpler than a car with internal combustion engine.

No spark plugs, high heat,pressure, carburetor,air filters, mufflers, cylinders need to be machine to something like 0.001 inches and more.

Electric motors/generators are relatively simple to build.

Yes, lead-acid batteries aren't great, but they have been used a long time. Also potentially recycling old car batteries.

Also you could potential use smallish (or whatever they choose to build) hydro-electric dams.

Solar power might be an option, but a bit more complex to make. Wind power is also a viable option, they don't need to be the 100' tall versions we have today.

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  • $\begingroup$ Some futurists such as Tony Seba think that solar power (plus wind and batteries) will be the most common energy source in the future. The best thing about solar and wind is that you don't need to come up with fuel for them. If you can get a plant running to make solar panels, and you can make enough solar panels to run the plant, then you can keep cranking out more panels. Current technology solar panels are good for a minimum of 25 years, so the energy they make in their useful lifetime should be much in excess of the energy to build them. $\endgroup$
    – steveha
    Jul 13, 2023 at 8:47
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My solution would be to compost waste material to generate methane. Store it in gas bags rather than cylinders. Gas bag vehicles were used when other fuels were scarce. Composting methane can provide a pure fuel source in one go. If I was to set about building something myself, that is where I would start.

However, I feel I am answering the wrong question here. If you have an internal combustion engine, then you have the technology to make it. The Industrial Revolution was largely built on coal and iron. If you have a small community, and coal resources for a blacksmith's forge, then you can probably build a steam-engine. You may want that power the machines you use to make better engines. To get more coal and iron, you make the steam-truck. This ran on roads. Railways are more efficient but making a flat, smooth railway may be beyond your manpower and metal resources to start with. In the UK, Foden steam-powered trucks were more common than petrol driven trucks until WW-II in the UK. I believe some coal mines in Venezuela about 1900 used steam trucks to deliver coal to the coast, using a third of their load as fuel. I don't know if the equator is still in the same place on your world, but that might be a good place.

There is no need to replay the Industrial Revolution exactly. There are places where oil can be found close enough to the surface. The first oil wells were simple enough. You can use hydro-electricity and overhead power lines for electric trams. But you still have to make the bits for these.

Once you have made an internal combustion engine of some sort, these can run off fluidised coal. If coal is what you have, then this is what you do. The Churchill tank could run off coal (not a success at the time, but works).

Once you have iron and coal, you get choices. You can expand your iron industry (bigger plants are generally more efficient) or you can use the technology to make other things (copper, aluminium).

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    $\begingroup$ "Who. Run. Bartertown." $\endgroup$
    – Mazura
    Jul 13, 2023 at 12:43
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What would be the best biological source for fuel? Best not in terms of power density or performance, but in terms of "a post-apocalyptic settlement would be able to make it and use it".

Use grass as fuel. Your society is better off using the resources they already have. Horse and cart works fine. Oxen and cart as well. Dogs and sleds. Slaves and palanquins etc,. (although these last two won't eat grass).

Vehicles pose a problem quite separate from making them work, after 200 years where are you getting your rubber tyres from etc? What condition are the roads in? There's a lot more to vehicles than fuel. So if you must have an engine, go electric and solar with batteries.

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You may also need lubricant, rubber for tires, etc. There was a book called 'Cross Time Engineer' and several sequels that dealt with these issues. The plot was a modern engineer in 13'th century Poland who wanted to ready Poland with high tech to fight off the impending Mongol hordes. The series hasn't aged well on a social level (misogony) but the tech is plausible. IIRC he didn't build automobiles, he built rails and boats.

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All depends on how you really wan to structure your world.

Given your starting point I would guess there would be a sizeable stress to get food enough for everyone; taking away the amount needed to power a "industrial" economy doesn't seem a good idea (rapeseed and beetroot would provide biodiesel and ethanol otherwise).

OTOH you should have a lot of "ancient scraps" laying around (possibly in inhospitable, too cold, areas where scavenging expeditions could provide additional narrative material); electrical motors and wind/hydraulic generators could be found/repaired/regenerated.

All batteries would be hopelessly corroded, but trains and static apparatus don't need them and could provide bootstrap power.

If you insist to get biodiesel or similar crap (presumably to increase CO2 and thus warm up again your world) you can also think about reinventing whale hunting (if any left) as in colder climates fatty fishes/mammals should thrive.

Notice any "sensible" post-apocalyptic setup won't be able to follow "old development routes" as a lot of readily available materials have been mined out; some can be scavenged, (e.g.: copper), but others would have been diluted and scattered beyond recovery (e.g.: tungsten, gallium, tantalum, ...)

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If you want to go the ethanol route, I would suggest sugar beet, which is pretty hardy and prefers cool climates. Its also pretty easy to process with methods from the 1800's. Sugar beets also make good fodder for farm animals Presumably there could be a lot of small operations growing a few acres of sugar beet, distilling alcohol for personal use, and feeding the leftovers to their farm animals.

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