Wikipedia:Reference desk/Archives/Science/2019 July 23

= July 23 =

Self-fuelling vehicle
Is it or will it be possible to design a vehicle (land, aircraft or spacecraft) which extracts and collects the fuel or energy from its own exhaust or jet in a recycled, closed-loop pattern for self-fuelling and continous movement, possibly based on the mass-energy equivalence? 212.180.235.46 (talk) 12:00, 23 July 2019 (UTC)
 * wot??? This would negates the whole point of an exhaust; and, of course, I suspect you are on the trail of perpetual motion Gem fr (talk) 12:18, 23 July 2019 (UTC)


 * Read Exhaust gas and think about how something like that would have to work. ←Baseball Bugs What's up, Doc? carrots→ 14:27, 23 July 2019 (UTC)
 * If you burn a liquid fuel, I think it's possible to conceive a condensation-like chemical process that converts the exhaust vapour backward to fuel. 212.180.235.46 (talk) 17:02, 23 July 2019 (UTC)
 * No matter how much you condense nitrogen, carbon dioxide and water, how likely are you to get petroleum back? ←Baseball Bugs What's up, Doc? carrots→ 17:32, 23 July 2019 (UTC)
 * Ye, well, of course it is possible. Photosynthesis does just that. Or you can do it with, say, a nuclear reactor and Fischer–Tropsch process on board; you would just need a vehicle the size of an oil refinery. We know how to do it, and it can be done; it would just be so stupid and moronic, only people ready to spend other people money to make a fortune out of subsidies consider doing it Gem fr (talk) 18:06, 23 July 2019 (UTC)
 * A car the size of an oil refinery. Imagine a highway filled with such cars. As for photosynthesis, I don't think they produce petroleum. Maybe if the car could run on simple sugars instead of gasoline. ←Baseball Bugs What's up, Doc? carrots→ 19:09, 23 July 2019 (UTC)
 * Ethanol is simple to produce from sugars, and is a good fuel. (Although the fuel system has to be designed to handle it, since ethanol is also a good solvent.) --47.146.63.87 (talk)
 * Yes, you could take the exhaust and convert it back to fuel and air (or other oxidizer), by just performing the reverse reactions. This follows from conservation of mass. The reactants don't vanish into nothingness; they just change form. But, the energy for these reactions has to come from somewhere, and it will always take at least the same amount of energy to run the reaction in reverse (conservation of energy). The point of burning the fuel in the first place was to liberate energy to do work on the surrounding environment—that is, moving the vehicle. If you then take all that energy and use it to reverse the reaction, you've just made a useless machine (unless the point is to demonstrate scientific principles). Or, if the vehicle has some other source of energy you can tap for reversing the combustion, why not just use that other source to propel the vehicle in the first place? Doing so will generally be more efficient because when energy changes form (such as from electrical energy to chemical energy), in any real-world process some is inevitably lost (second law of thermodynamics). Also, you mentioned spacecraft, and this would not work for reaction engines like rockets, since in those the exhaust is what performs the work of moving the vehicle. If the exhaust is captured and not allowed to escape, the engine can't propel the vehicle. You mention mass–energy equivalence in your original post. Mass–energy equivalence doesn't invalidate the laws of thermodynamics. It just says mass and energy are interconvertible (in a sense, they're the same thing). In fact, any time energy is released, including in chemical reactions, the mass of the reaction products is less than that of the reactants. (And of course, if the reaction absorbs energy, the mass of the products is greater.) The "missing" mass becomes energy. The quantities are just smaller and harder to measure in chemical reactions than in, say, nuclear reactions. --47.146.63.87 (talk) 23:54, 23 July 2019 (UTC)
 * Addendum: for clarity, taking all the energy from the engine and using it to regenerate the reactants would only even work with a fictional, perfectly efficient engine. All real engines are less than 100% efficient, and thus irretrievably lose some energy. --47.146.63.87 (talk) 23:59, 23 July 2019 (UTC)
 * Even fictional engines require extreme assumptions for perfect efficiency. No engine that extracts energy from a thermodynamic cycle may exceed the efficiency of the carnot cycle, and for that to be 100% efficient, you need either an infinitely hot energy source or a heat sink at absolute zero. And for the carnot cycle, you're already assuming the heat sink is infinitely vast, and that friction doesn't exist. Someguy1221 (talk) 04:41, 26 July 2019 (UTC)


 * Converting exhaust back into fuel would require energy. More energy than you could get from burning the fuel.
 * So sure, you could, in theory use a solar panel to collect the energy you needed to convert exhaust back into burnable fuel. (Hydrogen, maybe?) But at that point what you'd have would not be a "self fueling car", what you'd have is "The world's most absurdly complicated solar powered car".
 * The only value there would be in creating such a beast would be as an art installation to make fun of free-energy enthusiasts. ApLundell (talk) 07:52, 24 July 2019 (UTC)


 * There's turbocharging so the exhaust gas isn't useless, but that's not "self-refueling". You're limited by the second law of thermodynamics according to which any fuel has a limited, finite use, whether we're talking about internal combustion or a perfect E=mc^2 conversion. You may circulate the fuel several times to maximize efficiency, and you might even be able to invent a turbocharging process that extracts more energy per cycle than the internal combustion (as that has <50% efficiency), but you can't power the vehicle on the same canister of fuel indefinitely. 93.136.81.26 (talk) 23:23, 24 July 2019 (UTC)

Cedrus libani and droughts
I have come across different scientific papers quoting the loss of forest resources in the East Mediterranean, especially of the Cedrus libani forests, as having contributed to what currently are some of the worst droughts in 900 years in the region. Is the drought-forest exploitation relationship NOT unanimously accepted by the scientific community though, as it is not mentioned on wiki? The fact that the cedars/forests in that region have been widely exploited in antiquity is well-known and documented, but not many seem to link it to desertification or soil erosion. Theory that I have heard #1: Is it because fertility in the region was associated with human interference anyways, and its 'loss' is only apparent because humans stopped cultivating? Theory #2: The other way around. Region was fertile, humans liked it, but now after such long exploitation it's less fertile and hotter, with desertification occurring faster than if all or a part of the original flora was still intact? Theory #3: Neither; the area is to keep becoming hotter by default due to macroclimatic trends, and the forest wouldn't play a pivotal role in this.

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https://www.sciencedirect.com/science/article/pii/S0378112700003753 https://www.sciencedirect.com/science/article/pii/S2352409X16304928 https://www.sciencedirect.com/science/article/pii/S0169555X1300370X https://www.sciencedirect.com/science/article/pii/S0277379115300603 — Preceding unsigned comment added by Anbu95 (talk • contribs) 14:57, 23 July 2019 (UTC)


 * You can discard theory #3. macroclimatic trends are down when you start in antiquity (global warming is most recent). Besides, it is wrong to associate hot and dry. You get a drought when the air is dry, and the colder, the drier (which is why it is raining so much at the equator, and so little near the pole.
 * by "fertility" you probably mean quality and thickness of the soil: check this for the reason it can grow or be destroyed. Forest are not always good for soil-making, I don't know if the cedar tree is of the soil-making or soil-destroying kind.
 * Gem fr (talk) 18:45, 23 July 2019 (UTC)

Anbu95 (talk) 11:10, 24 July 2019 (UTC)But is drought unanimously accepted as a side-effect of forest cutting? We see it happening in the Amazon and it is speculated to have happened/be happening in Africa and the East Mediterranean.
 * Each case is different. Depends on where the water comes from, if there is a mountain or not, what kind of vegetation replace the forest... Also depends on what type of drought you are talking about. In the case of Lebanon, water mainly comes from the Mediterranean sea, and rainfalls are a product of the existence of Lebanon mountain range, so I don't think rainfalls are significantly affected (don't take my word for it, this is just a prior). But without the forest, water may run faster with little benefit for local plants. Again, this depends on what vegetation replace the trees, if the soil increases or is destroyed... Gem fr (talk) 09:33, 25 July 2019 (UTC)


 * The presence of beavers can keep the water from running away, as they dam it up and create wetlands. Of course, beavers need some trees, but not an entire forest. SinisterLefty (talk) 14:11, 25 July 2019 (UTC)
 * what? beavers in Lebanon??? Oh. Actually this is Lebanon, Illinois. Joke aside, and while I am not sure that beavers are important (or even living) in Lebanon, yes, wildlife too has an effect Gem fr (talk) 17:34, 25 July 2019 (UTC)


 * Wikipedia does have an article Desertification in Lebanon. Some causes of desertification specific to Lebanon are mentioned in the "Affected areas" section.  (Only general causes of desertification are listed in the "Causes" section, which could use some improvement.)
 * Sometimes hypotheses are not included in Wikipedia because no one has gotten around to including them yet, even if the goal is for the articles to both reflect scientific consensus and mention significant disagreements with the consensus.--Wikimedes (talk) 06:01, 26 July 2019 (UTC)


 * Note that the loss of the cedar forests is not a recent event; until the 20th century, the only known survivors were a small clump of trees at Bsharri, known as the Cedars of God. Writing of a visit in 1550, Pierre Belon was told that there were only 28 trees in the grove "though it is difficult to count them". See The Beshare Cedars of Lebanon As Seen By Travellers. So if the lack of cedars really is influencing recent climate events, the effect must be a very gradual one.
 * BTW, our article on the Eurasian beaver says that "Austen Layard, the British explorer of Assyrian ruins, notably Babylon and Nineveh, reported finding beavers during his visit to the Kabur River in Syria the 1850s, but noted they were being rapidly hunted to extirpation". Alansplodge (talk) 18:37, 26 July 2019 (UTC)


 * Interesting. Does this roughly correspond with the pattern of desertification there ? If so, we may have a new theory. SinisterLefty (talk) 01:31, 27 July 2019 (UTC)