Wikipedia:Reference desk/Archives/Science/2017 June 29

= June 29 =

Physics in different engine designs
2 engines have the same number of cylinders, engine displacement, stroke ratio (etc). One is shaped like a V with 2 cylinder heads while the other is shaped like an I with one cylinder head. Why is it that the I shaped engine has more power output in the same measured timeframe as the power output of the V shaped engine? 64.170.21.194 (talk) 02:22, 29 June 2017 (UTC)
 * More cylinder heads implies more mass to accelerate and decelerate with each stroke, wasting power. (Even if the design allows most of the kinetic energy to be reused, there's still going to be losses in the process, due to friction, etc.)  StuRat (talk) 02:26, 29 June 2017 (UTC)
 * The cylinder head is bolted to the engine block and does not accelerate. Please ignore the incompetent advice of the above editor. As to the Op's question, it is far too hard to explain in a short thread, and plenty of counter examples can be found. Greglocock (talk) 03:59, 29 June 2017 (UTC)
 * I believe sturat may have been talking about the moving components in the cylinder head (such as the valve train). "Please ignore the incompetent advice of the above editor" that's rather rude and hypocritical. You claim it's too hard to explain and that there are plenty of counter examples yet you fail to post a single reference. I'd say greglocock's advice is more incompetent.. at least sturat was contemplative. 64.170.21.194 (talk) 04:35, 29 June 2017 (UTC)
 * Well i'll let the unicorn and rainbow specialists sort this out, as an engineer I don't do much with either. However, I'll give you two bits of actual help. In the example you have specified the BMEP of the V engine is less than that of the I engine. The reasons why BMEP varies are many, far too many for an answer like this, but include friction, pumping losses, volumetric efficiency, valve timing, exhaust back pressure, ignition timing, and mixture. John B Heywood wrote a book that is both readable and useful "Internal Combustion Engine Fundamentals". Greglocock (talk) 06:20, 29 June 2017 (UTC)
 * There is credence to the inertia of the number of valves in the cylinder head. Hemi engines have two valves per cylinder. They have a reputation for high horsepower, but they aren't the best out there. There are cylinder heads with many smaller valves. The valves barely open, allowing them to open and close faster than the two-valve Hemi engines. Because the valves can open and close faster, the engine can run faster and more efficiently. It doesn't make a difference for a consumer vehicle, but if you are trying to maximize HP per cylinder, you will want to go for 32-valve cylinders like the AMG (and double the HP of a HemiV8 with a little inline 4). 209.149.113.5 (talk) 15:04, 29 June 2017 (UTC)
 * Hemi engines can have any number of valves they like. Four is just as common as two, and has been for decades. Five and six aren't that unusual either. Andy Dingley (talk) 15:52, 29 June 2017 (UTC)
 * I have seen 4-valve hemi (all lower case) engines from Europe, but I've never seen a 4-valve Hemi (upper case Dodge TM) engine. I did see a 4-valve mod for the 5.7L Hemi, but the heads weren't hemispherical. You were basically modding a hemispherical head into a wedge head. 209.149.113.5 (talk) 12:13, 30 June 2017 (UTC)
 * see Single-cylinder engine: it only says that it "will often deliver the maximum power possible within a given envelope", not that "the I shaped engine has more power output than as the V shaped engine"
 * The latter may be true for some ideal engine, but for a number practical reasons multicylinders are still the common real thing.
 * Now simple geometry (ratio of volume against surface) says that for the same displacement the 2-cylinders (and even more so with more cylinders) has


 * more surface of the cylinder, meaning
 * more leaking heat, needing to be cooled instead of contributing to gas pressure and work
 * more cylinder mass to stand the same pressure
 * more piston mass
 * more piston friction perimeter
 * twice as much valvetrain to move, twice as much pipes in and out etc. each being smaller but MORE than half those of the 1-cylinder, the combine volume and mass are bigger
 * It seems to me that this explains the claim
 * Gem fr (talk) 10:41, 29 June 2017 (UTC)


 * What do you mean by "an engine" here? A conceptual engine, differing only in one detail?  Or a particular model of engine, built and running?
 * In the first case, there is no difference. Certainly no fixed saying that V engines are more or less powerful, as a law of nature. In the other case, there are a great many other factors that will differ between the two, and we've no information on those.
 * Otherwise, what Greglocock said. Andy Dingley (talk) 11:09, 29 June 2017 (UTC)


 * How can two engines have the same number of cylinders if one has two cylinder heads (two cylinders) and one has one cylinder head (one cylinder)? 209.149.113.5 (talk) 14:38, 29 June 2017 (UTC)
 * The first clause says, "same number of cylinders", although isn't clear how many this is. It's common for V engines to have two cylinder heads, inline engines to have one head, and for this head to serve as many cylinders as needed. Andy Dingley (talk) 15:01, 29 June 2017 (UTC)


 * I think this is just blurring the definition of the "cylinder head." I have a V8. So, I can take the long block with four big bumps on it and call it a cylinder head, but I can also take that off. Under it is a valve cover, coil, and spark plug mount for each cylinder on that side of the engine. I can call each of those a cylinder head. So, I could say that my V8 has two cylinder heads - one on each side, or I could say that it has 8 cylinder heads - one for each cylinder. Because the number of valves per cylinder, which you'd expect to find in the cylinder head assembly, is pertinent to this discussion, I immediately think of the cylinder head as being the assembly independently mounted to each cylinder, not the cover that goes over all the cylinders. But, I can easily see why someone would use a different definition, which is perfectly valid, especially on engines where you can't separate the cover and independent assemblies. 209.149.113.5 (talk) 15:09, 29 June 2017 (UTC)


 * There is a surprising number of Engine configuration. A cylinder can have no head (Opposed-piston engine ). 2 cylinders can share the same cylinder head, as Blooteuth illustrated above. And, while it seems pointless to use several cylinder heads on a single cylinder, it doesn't seem impossible (as per the joke :"according to feasibility test, it can done; according to idiotability test, it would be stupid"). So the number of cylinders and the number of cylinder heads are not necessary the same (plus the pertinent point raised by 209.149.113.5 above). Gem fr (talk) 15:17, 29 June 2017 (UTC)


 * Yes, it is a blurring of what is meant by the word "head". It can mean anything from "the part of the cylinder where the valve(s) and spark plug(s) are" to "the entire assembly of wires and hoses bolted to the top of the cylinder block." I personally shouldn't have jumped in with what I was thinking of by "head." 209.149.113.5 (talk) 15:36, 29 June 2017 (UTC)


 * Monobloc engine and junk head might both be of interest. We don't have an article on narrow-angle V engine, but there are some on Lancia V4 engine (a series of barely related engines, thrown into one WP article) and also VR6 engine.
 * For an engine of n cylinders, there are usually either 1, n/2 and n cylinder heads used, depending on design. These days it is usual to have one head per cylinder bank, i.e. one for an inline engine, two for a V and almost never any more than this. Large diesels (ships and railway locos) still often use a single head per cylinder. Pre-war engines (and US fire engines) with non-monobloc cylinder blocks used to use n/2 or n/3 heads (one head to two or three cylinders), but that's pretty rare nowadays.
 * "Cylinder head" does have a clear definition, albeit a broad one. You can't have two per cylinder. Andy Dingley (talk) 15:52, 29 June 2017 (UTC)
 * Again: Cylinder head. Blooteuth (talk) 15:58, 29 June 2017 (UTC)
 * There is a design (that I don't think is used) in which the piston slides back and forth in a cylinder with spark plugs on each end (with associated valves). Depending on your definition of "head", you could call that a two-head cylinder. I believe we've beat the vagueness of "cylinder head" to death by now. Some people have their own definition and violently disagree with any other definition. Some people (like me) prematurely jump to the conclusion that others are using the same definition, but agree that there are other definitions. Others smartly ask exactly what meaning is being used before continuing the discussion. 209.149.113.5 (talk) 12:13, 30 June 2017 (UTC)
 * Sorry, yes, you can have a double-acting cylinder, although these are unsuccessful in medium- or high-speed internal combustion engines. I should have stated "you can't have two heads per combustion chamber". Andy Dingley (talk) 14:55, 30 June 2017 (UTC)


 * What hasn’t be pointed out here clearly, is that V engines have a greater 'power to weight ratio' because they are more compact than a straight inline, thus weigh less for the equivalent horsepower. So the engines don't produce more power per displacement (whether it be cubic centimeters  or cubic inches) but more power per unit mass of the engine. Aspro (talk) 16:02, 29 June 2017 (UTC)
 * Why do V engines implicitly have lower mass? There is no evidence for this. For water-cooled engines, it's more likely the opposite. Andy Dingley (talk) 17:41, 29 June 2017 (UTC)
 * It must be clear from the illustration that the "compactness" of the V engine is the reduced length of a line of cylinders at the cost of increased width. The duplication of surrounding hardware (manifolds, valve gear, cooling fins or channels, etc.) inevitably decreases 'power to weight ratio' but the squarer engine shape enables a more compact vehicle design than an inline engine with the same cylinders. Blooteuth (talk) 12:32, 30 June 2017 (UTC)


 * The OP specifically stated the following condition: "One is shaped like a V with 2 cylinder heads while the other is shaped like an I with one cylinder head." Therefore all of the nonsense above about a separate cylinder head for each piston and multiple heads per piston is irrelevant.
 * The OP asked the following question: "Why is it that the I shaped engine has more power output in the same measured timeframe as the power output of the V shaped engine?" All of the nonsense above about the weights and dimensions of the engines is irrelevant. He doesn't care about that. His comparison is simply on the basis of two different engines, of which one may be several times the weight of the other but the displacement is the same. He is not making the comparison on the basis of power to weight ratio.
 * Now, what do we make of the comparison that he did actually state, "more power output in the same measured timeframe"? I don't know what it means, since power is the rate of doing work. Is he asking why the I has better acceleration?
 * If we assume that he might mean simply "more power output", is it really a fact that an I-engine always has more power output than a V for "the same number of cylinders, engine displacement, stroke ratio (etc).", or has he posed a question that's based on a fallacy? This is the question that must be answered first. If it can be shown that even one production V-type engine develops more power than an I-type of the same number of cylinders, displacement, stroke etc, then the premise for his question breaks down and we need proceed no further. Akld guy (talk) 01:51, 1 July 2017 (UTC)
 * Blooteuth already, far as i know correctly, pointed out that "there is no fundamental difference in Engine efficiency" between the different design layouts. That is true with one exeption! Opposed-piston engines can be more effective because they are ideally suited to reach a much higher Compression ratio and thereby a higher combustion efficiency. However they are also in general more difficult to design and operate and that is why other common combustion engine concepts, even the likely exotic Wankel engine, are more commonly build and used. --Kharon (talk) 05:25, 1 July 2017 (UTC)

Changing the refractive index of air (without heating it).
Hello, again!

I've recently been studying optics as it relates to the translucency of various materials (water, air, etc.), and something has now crossed my mind, namely, how the visible spectrum reacts to changes in the state of said material. Permit me to elaborate.

Distilled water, for instance, remains perfectly clear when still. But when agitated, its optical properties distort, and it becomes considerably less transparent. Also, when air is heated considerably (say, in the vicinity of a pizza oven) it also tends to appear `blurry` from a distance.

I cannot help but wonder, though, does there exist any known method for changing the refractive index of air without heating it? Sc., can one—somehow—make otherwise regular air distort light rays between 400 nm and 700 nm in wavelength, and make whatever stands behind it appear `blurry` to an observer.

Thank You. Pine (talk) 08:41, 29 June 2017 (UTC)
 * You could add water vapour, or some other gas to air. Another way is to vary the pressure with sound waves. Turbulence due to strong wind would also cause variations in refractive index. see Schlieren photography and Astronomical seeing. Graeme Bartlett (talk) 09:56, 29 June 2017 (UTC)


 * ———Another way is to vary the pressure with sound waves.———


 * Now that really intrigues me! What kind of device, in your expert opinion, would most adequately do such (in a space approximately 30 cm X 15 cm X 10 cm), would said sound waves be audible/infrasonic/ultrasonic, and—lastly—would this in any way harm people physiologically?


 * Please forgive my impetuosity, but this has been gnawing at me for a good long while, now. :)
 * Pine (talk) 10:19, 29 June 2017 (UTC)
 * look at Acoustic resonance, i think it will answer your questions since it give you the formulas you need. Ordinary audible sound device can do the trick, however i don't know how much power is needed for the effect to be seen without special apparatus. May be it requires harmful power, or even too much power. Gem fr (talk) 13:31, 29 June 2017 (UTC)
 * Given the speed of sound in air 345 m/sec, I speculate the possibility that a strong longitudinal ultrasonic emission over 200 kHz (inaudible) could by modulating the air's refractive index create a virtual Diffraction grating with 1.5mm spacing that when illuminated with very far Infrared light (invisible, equivalent to millimeter radio wavelengths or EHF near 300 GHz) could bend the unseen light into a diffraction pattern. Blooteuth (talk) 22:18, 29 June 2017 (UTC)


 * Heat in itself induce very small change in the refractive index of air. Main effect of heat is changing the density of air.
 * You want to produce a mirage, you should look at the article. It has an artificial mirage example, much easier to do with water than with air. Gem fr (talk) 13:31, 29 June 2017 (UTC)
 * I also omitted to mention that changing wavelength changes refractive index a bit. This explains the green flash. Graeme Bartlett (talk) 21:45, 29 June 2017 (UTC)

Dual SIM
I'm thinking of getting a dual sim Android phone to avoid having to swap SIMs at the border. I only ever need one SIM active at any given time. I don't need the "dual standby" feature.

1. Is this even possible on Android?

2. Does the phone consume extra power if I do it this way?

3. Can I switch the SIMs in software? As in, at the border can I just go through the menus and choose "change primary SIM" and the active SIM will switch? If I had to pop the SIM tray out and physically swap the SIMs then it's no better than my current routine. Scala Cats (talk) 17:24, 29 June 2017 (UTC)
 * If you don't get an answer here, try: Computing desk. —2606:A000:4C0C:E200:0:0:0:3 (talk) 20:30, 29 June 2017 (UTC)
 * 1 Yes - many Android phones have this feature.
 * 2 Yes - it will need to power the extra SIM and use some radio power to contact the cell sites to see if the sim can connect to any network or not.
 * 3 yes go to the configuration section and select the SIM for making calls, texting, and data. You should be able to take calls on either SIM if the network allows it. Graeme Bartlett (talk) 21:42, 29 June 2017 (UTC)
 * For point 1, a lot depends on whether the OP is just saying they don't need but don't mind having dual standby, or they refuse to use a phone with dual standby. If it's the later, I don't think it's accurate to say "many" have the feature. The vast majority of dual SIM phones for Android are dual standby as that's in much more demand than switchable only/single standby dual SIM. Switchable only/single standby dual SIM Android phones do exist, but your selection will be a lot more limited. As for 2, if it's not a dual standby phone I'm not convinced it does consume more power, or at least not more in a measurable way. (The actual chips used for such phones may make a different but that's a lot more complicated. Not that a single standby phone really needs a special chip other than some part to enable SIM switching but different phones have difference chips.) If you are using a dual standby phone then yes it will use more power when both networks are enabled, although for most use cases on Android the extra power usage isn't really that significant. I'd also note that per my earlier point, comparing a dual SIM dual standby phone to one without is actually complicated by the fact that at a minimum, the chips used tend to differ. In reality many phones don't have single and dual SIM options so the differences are likely to be more than just the chops. Power usage differences will probably often depend on these other factors much more than having two networks. (You can of course simply not put in a 2nd SIM, or disable it to compare power usage from having dual standby and single standby on a specific dual standby phone.)  Note that in any case, probably most dual standby phones will also allow you to selectively disable the either SIM or network. However this may not change the preferred network for various features, you may have to do that separetely. I just tried and that's how my Alcatel Flash 2 functions, and I believe. (By comparison if you actually take out the SIM it's generally smart enough to only try and use the SIM that's in the phone.) This is most significant for the data since it's simply not available which you may not appreciate especially if you aren't actively using the internet or you're using an app with some offline features. (Of course if you actively use a lot of data, you also should consider the difference between dual standby as are most phones, and dual active. Same too if you're calling a lot. But if you're not interested even in dual standby then this wouldn't matter.) By comparison at least on my phone, I get a specific warning I need to enable the SIM if I try to SMS or call with a disabled SIM.  While disabling one SIM or network may save power, as I said above it's not likely to be that significant. In fact depending on how often and how long you take to switch I wouldn't be surprised if you could easily end up using more power by disabling one SIM given LCD screen power usage. So unless you have some good reason why you don't want dual standby other than power, I'm not convinced there's a good reason to go around disabling the other SIM.  Also it may not be necessary to choose a preferred SIM for calls or SMS, with both phones I've owned you have the option to be asked everytime. There may also be some smarts where it tries to call back via the same number for a missed call and likewise for a SMS.  One final point note that many dual standby phones with 4G/3G especially those using the various cheap MTK chipsets only have one radio for the 4G/3G part. For obvious reasons, this is generally tied to the SIM used for data. This is fine and likely reduces power usage slightly if the both of network supports 2G with equal coverage but if at least one either doesn't have 2G at all or has poorer or no coverage in some areas then the dual standby can be a lot more limited for obvious reasons. Since the OP isn't interested in dual standby this doesn't seem that important.  Nil Einne (talk) 03:04, 30 June 2017 (UTC)

On what size and time scale can undisturbed air's refraction index appreciably vary?
A specific tiny region of air can have x molecules now and x+1 molecules the next picosecond, no xenon atoms now and 1 the next picosecond.. Sagittarian Milky Way (talk) 17:42, 29 June 2017 (UTC)
 * If the volume dV is small the number of particles in it will obey Poisson distribution with its parameter $$\lambda=ndV$$, where n is the average particle number density. The root-mean-square deviation will be $$\sqrt{\lambda}$$. The characteristic time will be $$\sim\sqrt[3]{dV}/v$$, where v is the average particle velocity. Ruslik_ Zero 20:21, 29 June 2017 (UTC)


 * Air at STP contains approximately 2.5 x 1022 molecules per liter. Air's refractive index for visible light of wavelength of, say, 430 nm is largely determined by the cubical wavelength volume (430 nm)3 = 8 x 10-17 liter containing about 2 million molecules. Blooteuth (talk) 21:40, 29 June 2017 (UTC) edited Blooteuth (talk) 12:15, 1 July 2017 (UTC)


 * The density of air is 2.5 x 1022 molecules per liter, not 2.522. (430 nm)3 is 8 x 10-17 liter, not 8-20 liter, and that volume contains about 2 million molecules at STP, not 2000. CodeTalker (talk) 22:51, 29 June 2017 (UTC)

Bioluminescent fungus
What is the biological/evolutionary advantage for bioluminescence in fungi? They certainly don't need to "find each other" in the dark, and don't rely on insects to spread their spores (or do they?). 2606:A000:4C0C:E200:0:0:0:3 (talk) 20:26, 29 June 2017 (UTC)
 * I'm not sure if there are advantages, although there may be (possibly someone who knows more about fungi can expand). As long as this trait is not harmful enough to prevent enough organisms from reaching maturity and reproducing to keep the species alive, it can persist.  — Paleo  Neonate  - 21:54, 29 June 2017 (UTC)
 * It is a rather expensive evolutionary advancement, and shouldn't have happened without an appreciable benefit. 2606:A000:4C0C:E200:0:0:0:3 (talk) 22:27, 29 June 2017 (UTC)


 * From the article: "The physiological and ecological function of fungal bioluminescence has not been established with certainty. It has been suggested that in the dark beneath closed tropical forest canopies, bioluminescent fruit bodies may be at an advantage by attracting grazing animals (including insects and other arthropods) that could help disperse their spores. Conversely, where mycelium (and vegetative structures like rhizomorphs and sclerotia) are the bioluminescent tissues, the argument has been made that light emission could deter grazing."  Dragons flight (talk) 22:51, 29 June 2017 (UTC)
 * Thanks. (Don't know how I missed that -- skimmed too fast, I guess). 2606:A000:4C0C:E200:0:0:0:3 (talk) 23:20, 29 June 2017 (UTC)


 * If a pigment can absorb light at a certain frequency, it can also transmit light at the same frequency. I only just read that in certain cases bioluminescence may be an accidental byproduct of metabolic "leakage" where pigments that absorb light during the day leak light at night when enough free energy is present.  I'll see if I can find the source.  Basically, the pigment is useful enough during the day that its radiating a little cool light at night does not harm the organism sufficiently to select against the trait. μηδείς (talk) 01:44, 30 June 2017 (UTC)
 * The source is Raven, Biology of Plants 6th Ed. p 122.
 * μηδείς (talk) 14:16, 30 June 2017 (UTC)


 * As an aside: For those living in the northern hemisphere. This is a good time of year to go looking for bioluminescents. Tip. Pick a very hot day and for two hours before sunset, sit in total darkness; in order that ones eyes can become fully dark adapted...   (OK, if one has red-goggles to hand you can  you use  them to preserve your night vision but who carries around a pair of red googles?). For the first time one witness it -it is is awesome.  It is just like walking into Santa’s grotto (but without the the little elves). The other   bioluminescent phenomena that is easy to witness is shell-fish et. al. on a hot night.  The bacteria on prawns and such like, can glow surprisingly brightly. The bacteria producing this bioluminescent is not pathogenic, so don't panic - as I am still here to tell the tale. Aspro (talk) 19:09, 30 June 2017 (UTC)


 * Yes but look what it's done to your ability to construct sentences with proper grammar, punctuation and proofreading. Akld guy (talk) 21:17, 1 July 2017 (UTC)


 * We used to camp in the early fall regularly in Atlantic County NJ, and the firewood that was left over from the previous year (i.e., rotten logs) always had bioluminescent bracket fungi. You could even see it when the fire was down to embers in the cabin. μηδείς (talk) 03:16, 1 July 2017 (UTC)

Study that measures response time and cognitive perception
I remember a specific study that measures the response time of participants. Basically, the conclusion of the study is that, when the delay between the participant's pushing-on-the-button and the light (signal?) giving off is very short, then the participant will perceive that he/she is not causing the light to turn on. In other words, there must be some time between the action and response to be perceived as the effect of a cause. What is this study? 50.4.236.254 (talk) 22:50, 29 June 2017 (UTC)
 * The relevant Wikipedia article is: Mental chronometry. A Google Scholar search of your title finds several such studies. 2606:A000:4C0C:E200:0:0:0:3 (talk) 23:25, 29 June 2017 (UTC)
 * Just so you know, mental chronometry is too broad to be a relevant answer to the question. And I don't see how a Google scholar search of the title will lead to a precise result. You have to read the description in the OP to understand fully the question. I'm afraid that I may have misused some words in the title, so I don't even trust my own writing. However, I try to describe my recollection in the description, which is more important. 50.4.236.254 (talk) 00:49, 30 June 2017 (UTC)


 * I don't think you are describing the findings correctly, but I think you may be referring to . That paper is not freely accessible, but you can read an account of the methods and findings in https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4614377/. Our article on the illusion of control also discusses these studies to some degree. Looie496 (talk) 17:31, 30 June 2017 (UTC)


 * I believe you are probably thinking of Benjamin Libet's immensely influential work on voluntary action; although your description does not perfectly match the methodology of his most famous iteration of the test for volition, nor the conclusions taken from it, the description is close enough to suggest these are probably the experiments you are remembering.  S n o w  let's rap 01:31, 2 July 2017 (UTC)


 * Here's an additional description of the experiment, from a talk by Susan Blackmore:  (and with the greatest possible irony, the video is going to lag a little bit before taking you to the right moment in the presentation, despite my adding the time code!).  Does that sound like what you were looking for?  S n o w  let's rap 01:56, 2 July 2017 (UTC)


 * That video is about an experiment that measures the time it takes for someone's will to become movement, or something like that. No, I swear my memory was about clicking on a button. If the button is clicked, then it will light on. If it's not clicked, then it's not on. The experiment tests the length of time on perceived cause, meaning that if the length of time between the action and response is short enough, then the person will think that she/he is not causing the response. But your video sounds equally interesting, though. 50.4.236.254 (talk) 16:37, 2 July 2017 (UTC)

Longevity of fluorsecent lights
Does turning a fluorsecent light off once a day for a few minutes and then on again shorten its life compared with leaving it on all the time? Thank you bunnies!!! Anna Frodesiak (talk) 23:15, 29 June 2017 (UTC)
 * Yes, it incurs wear on the filaments and the ballast. Turning it off if you know you're going to turn it on again in a few minutes is best avoided. --47.138.161.183 (talk) 00:20, 30 June 2017 (UTC)
 * Thank you! :) :) :) Anna Frodesiak (talk) 00:31, 30 June 2017 (UTC)
 * Now you are entitled to say: "See, I told you so!" - 2606:A000:4C0C:E200:A975:997:5261:F444 (talk) 20:14, 30 June 2017 (UTC)