Typically, gasoline has a short shelf life (some months, maybe a year). This is handwaved in some post-apocalyptic settings where the writer wanted vehicles to play a major role (such as Mad Max — unless there was an explanation that I don't recall).
I would like to create a similar setting, but without having to handwave that detail. Are there plausible ways for modern-day passenger vehicles to remain operational in such a setting?
Modern gasoline has a shelf life problem because it contains ethanol, which, over time, absorbs water. If expired fuel is in a container, it is relatively easy to separate ethanol/water phase from "real gasoline" (hydrocarbons) and use it. If expired fuel is already in the tank, the task gets more tricky, but still doable by low-tech methods.
P.S. There was probably no E10 fuel yet in 1970/80s Australia, so gasoline was eternal :)
Several alternatives have been offered, including coal liquefaction, brewing your own ethanol, stripping water from old hydrocarbon fuels and so on. One alternative which hasn't been mentioned yet is "wood gas".
This was done during the Second World War when gasoline was being rationed, and used a process called pyrolysis to burn wood or charcoal in a low oxygen environment inside a "wood gas generator". The gasses emitted from the process were full of combustable products including Hydrogen, Carbon Monoxide and various aromantic hydrocarbons in low concentrations. This was fairly inefficient and low energy fuel, but with some adjustments 1930-40 era engines were able to use this.
A wide variety of different gassifier designs can be found here. The FEMA design is deliberately simplified so use in a post apocalyptic environment can be assured since it needs no expensive or hard to create parts, but can produce a lot of tars which gum up the engine. The Imbert designs is the classic WWII era one, while the Keith gassifier is considered the gold standard today.
wood gas generator attached to car
I advise anyone to carefully study the system before building your own, since Carbon Monoxide is one of the products of this process, and is very deadly. Don't play around if you are not sure what you are doing.
Cross section of a modified Keith gassifier.
The air enters at the bottom in blue, warms up as it flows through the heat exchanger, and warms further flowing past the warm gasifier itself. This preheated air adds substantial energy to the combustion, increasing the efficiency. Also notice the dual pipes exiting the gasifier; these are 3" pipes. They go into the settling chamber, expanding and slowing down. This causes them to release some of their energy to the heat exchanger, and some of the dust settles out. Gas exits near the top of this chamber, again in two 3" pipes, right into the cooling rack.
You can turn coal into oil; process is called liquefaction.
It is not currently economically useful, but it's not very complex and any steampunk technology can do it.
Note here still are many open-sky or otherwise not very deep coal mines that would be accessible to post-apocalyptic setting (not to mention coal can be manufactured from wood), where oil is much more difficult to reach.
Maybe Gastown was natural gas. Or biogas! I see gas powered vehicles going around town all the time now. Lots of buses.
Your postapocalyptics can use a fermenter to generate their own biogas out of mutant corpses, wilted flowers and their own bodily secretions. You might want some barbed wire on the sides of the Biogasbuss to make it properly MadMaxy. Also cover up that "grenare värld" heart with some skulls or something.
Different fuels last different lengths of time depending on different storage characteristics. But, in the end, no petroleum-based fuel lasts forever in common real-world storage environments.
Dump gasoline/petrol based cars. Those fuels, especially with today's use of ethanol-additives, won't hold up. Stick to diesel engines. Vegetable oils, animal fat, etc., can be burned in these engines. Sure, it's hard on them, fuel economy goes down, etc. but the engines can run, which is far easier to generate locally than any other modern engine fuel.
Even if you solve your gas problem, you have bigger worries. If you have modern engines, then you have to worry about parts that wear out, sensors that fail, electronics, batteries, rubber and plastic parts that degrade over time, belts, etc. For all practical purposes, none of these parts can be fabricated in your post-apocalypse world.
So your engine has a shelf-life. As do your tires, your fuel lines, your...
Yes, you can restore older engines to life with much care and feeding on the mechanics' part. But that still depends on scarce resources: parts, skilled labor, patience, and spare time.
Use the engines you can get for as long as you can, but don't get attached.
Instead, as soon as your world is stable enough to survive day-to-day on, begin working to decouple your survival from modern vehicles. Clear roads and bridges, dig your pits etc. with the construction machinery while it works. But assume that within the next 5 to 20 years, you will hit a wall where any given vehicle will fail beyond repair for some reason or other. Maybe because you can't scavenge a spark plug that fits. Or that rubber fuel line dry rots. Whatever. But you can't go back to the store and buy a new one.
So focus on phasing out the vehicles so they aren't required for survival. Start learning to breed and ride horses, for example.
Steam transport allows to burn anything. This is the reason why Russia still has steam locomotives that are conserved and waiting for the apocalyptic times when oil will not be easily accessible. During the great depression in the USA people burned grain in locomotives.
You can convert a lightweight internal combustion engine to steam, it is even easier with 2 cycle engines. You have to cut and weld the cylinders and making a new crankcase improves the efficiency a lot, but a decent mechanic could do it. Some engines (tall double acting cylinders) work better than others of course. You don't get the same power you do with petroleum fuels, but it will run around just fine. You can still get high speed you just do not accelerate as fast.
People have even converted trucks to run on firewood. And of course a mechanic with some fabrication skill could just make a steam engine. As a bonus if you design them right steam engines can be used to purify water as well, always a bonus in a post apocalyptic setting.
Also consider a liquid burning steam engine will burn expired fuels just fine.
Alternatively you can re-refine expired gasoline into usable gas, it's not to mechanically difficult. It will run out eventually but this will extend the time you can use unaltered cars.
Finally many vehicles are made to run on kerosene which is both easier to refine and has a far longer shelf life. Propane is even better with an unlimited shelf life. Besides existing engines that run on it converting a gasoline car to run of these is not that hard.
Lets go to Barter town! Barter town has methane! Barter Town has Pigs!
Pigs poop and methane cometh form pig-s#$t.
Ok, what is my point here? The internal Combustion engine, in theory, runs on a fuel being aerosolized, mixed with air, then detonated in a sealed chamber to generate lateral movement which is then turned into rotational movement which then, through a series of gears to produce work....That means that with a variety of fuels you can do this. It doesn't have to be petroleum based. In fact, there is a very wide range of options, including methane derived from pig-s#$t.
though if you chose that, you'd have to sing a top 40 hit while wearing a chain mail dress.
Anyway, what it really comes down to is how much work do you want your characters to have to do?
Biofuels are relatively easy, if not high yield. Diesel engines, according to legend, were built in the first place to run on peanut oil. Anyone who has abused themselves with flaming drinks knows about ethanol. You just have to remember that if you are using food crops for things other than just food, you have to hold back enough so you won't starve.
Wood Gassifiers are good. So is plain old steam.
Here is what you can do.
1) Pick an alternative fuel...any fuel.
2) Google "downside of ...."
3) bingo, you now have a pretty good idea of how much work your characters will have to do.
If you do it right, you get realism and some very good plot devices all in one shot.
Who runs Barter Town?
The lady singing a top 40 song while wearing the chain mail dress
What is preventing the brightest of the post-apocalyptic population from banding together and frugally using what yet-to-expire gasoline they have locally to journey to known petroleum repositories within their calculated travel distance and refining it---and the collected old gasoline---on demand? What has the proposed apocalypse wiped out? I mean my distant relatives made their own acetylene. How hard could it be to learn in a survival situation?
Post apocalyptic world we will naturally discover that the most abundant element on earth, hydrogen, H2 can be generated at the point of use from water using not electrolysis but harmonic frequencies, spontaneously releasing vast amounts of energy. to put this another way
"If energy fails, life fails so when the sun burns out it's lights our"
Now that is an apocalypse.
