So, in my setting, there is a certain kingdom, which is capable of producing a special sort of liquid, that is seemingly not compressible, however, it actually "charges" up when compressed, and when exposed to some form of trigger, the liquid will then violently expand outwards with the work of the "stored" compression.

Said liquid runs on magic, so no need to think too hard about how it works. What sort of implications might such a liquid have on the general economy and industrialization of the kingdom?


  • The Liquid Potential Energy (LPE) has the same density as water, of 1 g / ml
  • LPE is completely chemically inert, and and does not react to anything chemically. IE rusting, tenderizing etc.
  • LPE is found naturally in pools in underground caverns, and is not particularly expensive to collect
  • LPE is clear when uncharged, and becomes completely opaque when fully charged.
  • LPE is capable of storing up to 10 kilo Joules of work per milliliter
  • To discharge the energy in LPE, you need to jolt it with some electricity, usually by magic
  • LPE discharges completely and absolutely, it cannot be half-discharged, and will discharge all energy from connecting bodies of LPEF when triggered.
  • LPE discharges itself by expanding its volume rapidly, not unlike an explosion.


  • LPE is only charged when it is actively compressed, standard atmospheric pressure is not enough to start the charging process
  • The LPE is initially at 10 kJ/ml, however, feel free to hypothesize scenarios if it were higher, as many pointed out diesel has a higher energy density
  • LPE after discharging, returns to it's clear uncharged state, and can be readily recharged and discharged infinitely
  • LPE discharges 100% efficiently into kinetic energy that expands outwards. It is visually similar to an explosion, minus the ensuing fireball
  • LPE when first collected from the underground is already fully charged.
  • $\begingroup$ Does air pressure count as compression? $\endgroup$
    – Samuel
    Mar 27, 2015 at 7:42
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    $\begingroup$ According to en.wikipedia.org/wiki/Energy_density, petroleum has an energy density of 32kJ/ml, which is triple that of your phlebotinium energy source. It would for all intents and purposes be inferior to petroleum, except in ease of collection and reduction of environmental impact. $\endgroup$
    – March Ho
    Mar 27, 2015 at 8:10
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    $\begingroup$ I don't quite understand. You say it is incompressible, just storing energy by when triggered it explodes violently outward. Without some compression, there is no energy storage (or expansion when pressure is released). Unless you are simply treating it as an ideal battery, in which case, just treat it as an ideal battery. $\endgroup$ Mar 27, 2015 at 9:05
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    $\begingroup$ "Yes, a simple heavy rock on top of a piston could charge this up" - if the rest of the world uses real-world physics, the rock has not lost any energy in this scenario, but the LPE has gained it. Very simply you could put a heavy rock on top, charge it, set it off, and blast the rock up 100s of feet, gaining potential energy of the rock for free. That is what gives you perpetual motion. A closed heavy piston arrangement could just keep going with a strong push each time it was triggered. You will need to either accept perpetual motion, or block or hand-wave this solution away as not practical $\endgroup$ Mar 27, 2015 at 10:01
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    $\begingroup$ Looks like you invented watered-down gasoline $\endgroup$ Mar 27, 2015 at 10:30

3 Answers 3


The LPE is a energy carrier fuel that is better than compressed hydrogen, but worse than diesel.

Cold engines Expansion of the fuel is what drives both steam engines and internal combustion engines. Since you "ignite" it with electricity, it closely resembles a combustion engine with a spark plug. The stated energy density of 10 kJ per milliliter equals 10 Megajoules per liter, which is a comparable to the 15 MJ/l of Methanol fuel, but only has a third of the energy density of diesel. Compressed hydrogen is about 5 MJ/L. What's special about your magic fuel is that you don't say it's producing heat.

Expensive to produce The LPE is similar to hydrogen fuel in that it's not an energy source, but a carrier. The energy has to be produced elsewhere. Assuming you're working in a medieval setting, there's really not that many powerful energy sources, so your LPE can't revolutionize society. Sure you can retrofit windmills and watermills, but they won't deliver enough power to fuel say, an army of siege engines or a transport network of trains.

Everything is better with a boom Discounting large-scale transport, you can always use the LPE in place of dynamite and gunpowder. The rapid "combustion" of LPE makes it a good choice in this regard.

  • $\begingroup$ Well, this is sort of disappointing for me, I had set the bar at 10 kJ/ml, and it turns out to be so low. However, i can not quite just edit the question to make it higher, I did however ask about if it was higher. Be that as it may, I like the idea that it does not produce heat. If it does not produce heat, would it be more effective than conventional fuel? $\endgroup$ Mar 27, 2015 at 9:27
  • $\begingroup$ Well, in say a piston, we have that pressure * volumes / temperature stays constant, so if the LPE is ignited in a closed chamber and can't expand, the temperature would increase. But in a piston like we have in an engine, I think the LPE just causes a volume increase and drives the piston, so it might be more efficient than ordinary fuel, where a lot is wasted on heat. Diesel engines have an efficiency of 50%, so maybe we can double the usefulness of the LPE with regards to conventional fuels. $\endgroup$
    – Abulafia
    Mar 27, 2015 at 9:40
  • $\begingroup$ Still, the major "problem" with LPE is not the density, but how you're going to produce a lot of charged LPE in a medieval setting. You need some sort of energy source. Why not stipulate compression in those underground caverns? real-world oil and gas is under tremendous pressure and you could have the LPE seep up from great depths where it has been pre-compressed. $\endgroup$
    – Abulafia
    Mar 27, 2015 at 9:42
  • $\begingroup$ @grimmsdottir: Its hard to have an intuition for energy densities. In an answer on scifi stackexchange, I showed that the time-travel flux capacitor in the movie "back to the future" spends only a gallon of diesel's worth of fuel for every trip through time. scifi.stackexchange.com/questions/14812/… $\endgroup$
    – Abulafia
    Mar 27, 2015 at 9:47
  • $\begingroup$ Huh, I had forgotten that real life oil and gasses were created under great pressures. I think I will modify my question so that most gathered LPE is already fully charged from the great pressures underground. $\endgroup$ Mar 27, 2015 at 9:51

No fossil fuels
So this could work as a way to power an engine that would be almost indistinguishable from a perpetual motion machine. I'm picturing an engine with pistons, but instead of aerosolizing fuel, the pistons would just compress one set of LPE's. A spark is applied, the LPE violently expands, which drives the piston and compresses another set of pistons, and turning a shaft at the same time. I don't know if your energy density is high enough, but it could be tweaked a little if need be. Since it's magic it could expand with enough force to compress the next set, while having enough to move the crank shaft.
For a spark you could use a pizo electric element.
Barring wear and tear on the pistons/gaskets/shaft the engine could run constantly without any fuel source. This also means engines could be really really small, or pretty big, depending on the need. This would have a pretty big effect on industry and economy.

  • $\begingroup$ -1. You clearly do not understand the meaning of "perpetual motion machine" means. Perpetual means "Forever" and not "until I run out of gas". $\endgroup$
    – Aron
    Apr 3, 2015 at 7:53
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    $\begingroup$ @Aron - I just noticed your nonsensical comment here. AndyD273 doesn't describe the use of any consumable. This design might not work (because of charging time), but if it did it would satisfy normal over-unity-without-consuming-anything definitions for a perpetual motion machine. $\endgroup$
    – Rex Kerr
    Apr 5, 2015 at 19:35
  • $\begingroup$ @Aron I also said "almost indistinguishable from a perpetual motion machine". Obviously forever is impossible. Things wear out. So the closest you can get is a machine that needs minimal input of materials and little to no outside energy. If you want to get really anal about it, you can claim that it uses magic as fuel and say that disqualifies it. The real point you missed is that it would be an engine that would be much more sustainable than our fossil fuel based technology. Seriously, if you want to play smartypants, at least try to understand the whole thing, not just the first line. $\endgroup$
    – AndyD273
    Apr 6, 2015 at 13:13

This only works if you don't have Newtonian physics.

Energy (Work) = Force x Distance

Your liquid doesn't move, so no matter how much force you apply, no energy goes into it.

Since Newtonian physics is out the window, you can build a perpetual energy machine. Get a massive metal pillar, as tall as you can support. Put your liquid under it, let the metal rest on it for a while; it charges up. Then, transfer the pillar to support struts and pour the liquid off. You've moved essentially nothing (maybe a few millimeters), but now you have charged-up liquid. Take a tiny bit of the liquid to "burn" to push the metal pillar up on struts next time, and the rest is free energy. If you use a gravity/hinge-based strut system, just lifting the pillar up and down should be almost free.

Repeat, over and over and over. Woohoo!!

  • $\begingroup$ Yes...because moving a massive pillar is easy... $\endgroup$
    – Aron
    Apr 3, 2015 at 7:54
  • $\begingroup$ @Aron - Not sure if that is supposed to be facetious or not, but yes, moving a massive pillar a cm or so is easy. Pile-driving machines do it routinely. It's a simple exercise in mechanics with many solutions (e.g. long levers would work well in this application). $\endgroup$
    – Rex Kerr
    Apr 3, 2015 at 21:25
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    $\begingroup$ @Aron - You are arguing with someone who did postdoctoral research in biophysics, but what does it matter? Reality doesn't care what degrees we might have. It does care if we put an incompressible liquid column underneath a counterbalanced weight and then detatch the counterbalance. Then the force will "all" (all save for friction) be transmitted through the column of liquid. You do still need to move things vertically a tiny bit to overcome the compression in the load-bearing side, but that's it. $\endgroup$
    – Rex Kerr
    Apr 5, 2015 at 19:01
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    $\begingroup$ @Aron - Enough energy to overcome frictional forces (and lift enough to overcome compression in the attachment mechanism). These are small compared to the amount of energy in a fluid that charges up to an energy density of 10 kJ/mL. $\endgroup$
    – Rex Kerr
    Apr 5, 2015 at 19:27
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    $\begingroup$ @Aron - The whole point of this hypothetical fluid is that it's not Newtonian (not in the normal sense of a Newtonian fluid: it doesn't obey the relationship between work and force). $\endgroup$
    – Rex Kerr
    Apr 5, 2015 at 19:38

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