Wikipedia:Reference desk/Archives/Science/2012 January 30

= January 30 =

Mustard seed
Is Mustard seed the smallest seed in the world? roscoe_x (talk) 09:15, 30 January 2012 (UTC)


 * No, of course not. The bible is full of claims that are not true, because the people who wrote and rewrote and edited and censored and translated the bible were often incorrect. Read this. Von Restorff (talk) 09:24, 30 January 2012 (UTC)


 * N.B. the actual Parable of the Mustard Seed is prone to various translations.  Many say the smallest seed on "the earth", but is that the earth you're planting it in, or the Earth as a planet?  Anyway, I've moved this under the collapse because it is also irrelevant. Wnt (talk) 18:06, 30 January 2012 (UTC)

Collapsed discussion over appropriateness of a religious response to a secular question, as well as the response which provoked it. Please continue any such discussion on WT:RD. -- ToE 12:25, 30 January 2012 (UTC) -- In reality this is a religious question, but not everyone can accept the fact the bible contains errors.Von Restorff (talk) 12:58, 30 January 2012 (UTC)


 * After reading your userpage: read this... Did you know God prefers atheists? Von Restorff (talk) 09:32, 30 January 2012 (UTC)

Is it really necessary to turn a simple scientific question into a religious issue? Roger (talk) 11:01, 30 January 2012 (UTC)
 * I assume you were probably not aware that this is a religious issue and not a simple scientific question but it is. I wish it wasn't, and I assume you do too. It is 2012, people should've stopped believing this kind of nonsense a long time ago. Von Restorff (talk) 11:10, 30 January 2012 (UTC)


 * I am fully aware of the origin of the mistaken idea that the mustard seed is the smallest. I am questioning the propriety of turning the question into an opportunity to post links to ranty pages about religion. Just answer the question with unembellished facts. OK, 'nuff said. Roger (talk) 11:37, 30 January 2012 (UTC)
 * Well, the unembellished fact is that this one of the many incorrect claims made in the bible. Are you claiming I posted "links to ranty pages about religion" here? I posted 3 links here. 1 is a link to a cartoon, not a "ranty page about religion". 1 link points to our article Ignosticism; if you are saying that page is "ranty" I will reply with . The third link is to a blogpost, EverGreg used the same link, and if you truly believe that page is "ranty" you and I have different ideas of what the word "ranty" means. If you are claiming I posted links to "ranty pages about religion" here would you please be so kind to point them out? Von Restorff #talk# 11:48, 30 January 2012 #UTC#


 * (ec)The point is is is not necessary to refer to religion at all #particularly in a way apparently intended to belittle or insult another editors presumed beliefs# when answering a botanical question. This is the Science Refdesk, not Religion or Mythology Refdesk. Doing so is simply gratuitously trying to provoke controversy. Can we please take this to the Talk page if you feel it's necessary to continue the discussion. Roger (talk) 12:05, 30 January 2012 (UTC)
 * Of course you did not answer my question. You still claim this was a botanical question; I am not sure why you still want to pretend it is even though you said you are aware of the origin of the mistaken idea that the mustard seed is the smallest. Was it my intention to belittle or insult another editors presumed beliefs? Thanks for letting me know! I was under the impression my intention was to help a misinformed person who believed an unreliable source, but obviously I am sometimes not aware of my own intentions, thanks for correcting me. Maybe we can both spend our time better than having this metadiscussion. Von Restorff (talk) 12:18, 30 January 2012 (UTC)


 * Von Restorff—the question asks if mustard seeds are the smallest seeds. Your first two posts reference the Bible, God, and atheism. It is just my opinion, but I think your response strays from that which is strictly asked. Bus stop (talk) 12:15, 30 January 2012 (UTC)
 * After an insane amount of editconflicts : I had written a response, but then you added the word "strictly" and I got an editconflict. My response to the current version is: "I agree". Von Restorff (talk) 12:36, 30 January 2012 (UTC)

No, the smallest seeds are found with certain orchids The mustard seed is about 15 times bigger than the smallest orchid seeds. EverGreg (talk) 11:31, 30 January 2012 (UTC)


 * That link is to the blog of a LASC biology professor which mentions the seed size of several plants:
 * Mustard (family Brassicaceae): about 1/20th of an inch
 * Petunia: about 1/50th of an inch
 * Begonia: about 1/100th of an inch
 * Certain epiphytic orchids: 1/300th of an inch.
 * These tiny orchid seeds have no endosperm and contain underdeveloped embryos. As EverGreg says, that's 1/15th the size of a mustard seed, but volume scales as length cubed, meaning that those orchid seeds have less than 1/3000th the volume!
 * This would seem to be a common question, and that information should be in either our mustard seed or parable of the mustard seed article. While the link appears to give reliable information, as a blog it does not satisfy WP:RS.  We certainly should be able to find these seed sizes sourced elsewhere. -- ToE 14:19, 30 January 2012 (UTC)
 * While I am no expert on flowers, those really-really small seeds strike me as being not so likely to have been native to the Middle East. So a question more to the point might be, "Would the people of the Biblical region of Palestine likely have been aware of any seed smaller than the mustard seed?" ←Baseball Bugs What's up, Doc? carrots→ 02:28, 31 January 2012 (UTC)


 * Yes, they would have. Many species of orchids are found natively in Palestine.  Jesus was not well informed about scientific issues, even compared to the intellectuals of his time.  --140.180.15.97 (talk) 05:18, 31 January 2012 (UTC)


 * Since the smaller seeds are decorative flowers, and probably not many people were doing intensive agriculture with decorative flowers back then, it's entirely possible that nobody ever figured out how small those seeds were. Also keep in mind that they wouldn't have known that all flowering plants even have seeds. StuRat (talk) 05:32, 31 January 2012 (UTC)


 * Seeds of many fruits are small. Figs come to mind, and my impression is that they were part of that culture. Berries like raspberries and blackberries have tiny seeds. HiLo48 (talk) 03:31, 31 January 2012 (UTC)


 * ToE: As for a reputable source on seed size, I think the Science report "A brief history of seed size" (DOI: 10.1126/science.1104863) would be sufficient. The supporting online material (found at ) contain an impressive wealth of data on seed size. While the authors study seed mass, they also find that mass and volume is approximately proportional for all species, with seed mass (mg) = 1.1 * seed volume(mm^3)^0.96. Quoting the paper:


 * Present-day species have seed masses ranging over 11.5 orders of magnitude, from the dust-like seeds of orchids (some of which weigh just 0.0001 mg) to the 20-kg seeds of the double coconut.


 * this implies a smallest seed volume of 0.000062 cubic milimetres. Assuming a spherical shape, that gives a diameter of 0.049 millimetres, or about 1/500th of an inch. (about 1/600 if you assume a cube). Half their raw data came from the Kew gardens seed database which could also be consulted. Meanwhile, "Global patterns in seed size" (Moles et.al.) find that seeds are a lot bigger in the tropics. It's really the story of how seed size relate to evolutionary strategies that's the big story here. — Preceding unsigned comment added by EverGreg (talk • contribs) 15:50, 31 January 2012 (UTC)

Colder on high mountains
why is it colder on high mountains — Preceding unsigned comment added by Toshad Salwekar (talk • contribs) 10:48, 30 January 2012 (UTC)
 * Please see Lapse rate. -- ToE 10:58, 30 January 2012 (UTC)


 * The lapse rate article, while correct, is an unusually hard read for a wikipedia article. It's about a meteorological term and the physics is only mentioned in passing. To summarize, the article makes two points:
 * The sun heats the ground and the air closest to the ground absorb this heat, the air further up do not
 * Pressure decreases with height. As hot air rises, it therefore expands. With reference to the ideal gas law, its temperature must then decrease. Therefore, the air is not mixed to equal temperature but maintains a temperature gradient.
 * I've been unable to find a wikipedia article which presents this clearly. By comparision, why is the sky blue is explained in detail. :-/ EverGreg (talk) 12:19, 30 January 2012 (UTC)


 * The OP may also be interested in reading our temperature inversion article for a description of the opposite situation. -- ToE 16:06, 30 January 2012 (UTC)

Does HIV can live on alcohol?
if a scientist takes some HIV (no matter what type of HIV) infected blood, and puts it in a sterile Petri-dish that contains Chlorhexidine (or pure alcohol), will it kill the Virus?, if not, what similar preparation could in fact do so?, interesting indeed. thanks. — Preceding unsigned comment added by 79.182.45.12 (talk) 13:09, 30 January 2012 (UTC)


 * If you are actually in a situation where you might need to handle or dispose of HIV-contaminated blood or blood products (or blood and blood products of unknown HIV or other infectious disease status), then you should consult with your school or workplace's health and safety or infectious disease control staff. The US EPA has produced a list of commercial antimicrobial agents that are – when used properly – effective against HIV: .  TenOfAllTrades(talk) 14:29, 30 January 2012 (UTC)

I'm glad I looked this up. It does indeed look like HIV can survive in dried blood for days, even weeks. I have to say, I didn't know this, and there is evidence that even large forum discussions of people online don't know this either! This may be a point where a little public awareness would go a long way. Diluted bleach will kill HIV, but some other products that "kill HIV" are labelled to do so with an unrealistic 10-minute exposure. An interesting "Primary Surgery Wiki" describes chlorhexidine as ineffective, though the 70% ethanol in which it is usually made up might be effective - anything less than 70% apparently is not. They sound skeptical of all methods of chemical disinfection due to the chance that the blood will kill the disinfectant rather than the other way around. And they say 10 minutes. Wnt (talk) 18:35, 30 January 2012 (UTC)

Skepticism and learning
how can a student who is in the process of learning be "skeptic" about what he/she is being taught by his/her professors? It may seem stupid at first, but when do students exactly learn to think in a skeptical way?--Irrational number (talk) 15:23, 30 January 2012 (UTC)


 * Salam chetori :) (the OP speaks Farsi). You might be interested in critical thinking, which explains the process in general. As for self-teaching skepticism, my only advice is that if you choose to think for yourself, you are well on your way. I don't think there's any special time when this happens, but I learnt more when I had to fend for myself, when I realised my life wasn't going to be as cosy as I had expected. You learn a lot by realising you are kind of on your own, so you have to learn to think for yourself. IBE (talk) 16:38, 30 January 2012 (UTC)


 * Skepticism has many definitions. When most people use it, it generally means refusing to believe in anything without evidence--such as paranormal phenomena or religion.  Philosophical skepticism is the refusal to assert that it's possible to have any certain truths about the world.  Usually, skepticism doesn't mean refusing to believe your calculus professor when he proves the mean value theorem, or refusing to believe your history professor when he says WWI started in 1914.  --140.180.15.97 (talk) 20:03, 30 January 2012 (UTC)
 * Actually, skepticism can be useful even in math or especially in history.


 * A high school math teacher of mine said that if you tossed a coin five times and it came up heads every time, then you were likely to get more tails than heads during the next five tosses. The same teacher told the class that it was apparently impossible to integrate x-1 dx, because it would result in a division by 0 if you use the formula for integrating xn dx.  Math teachers are sometimes wrong.


 * And skepticism can be very important when learning history. As a general rule, in whatever country you're learning history in, the history books in that country are at risk for having a bias towards describing historical events from a perspective that makes the country you're learning history in, and its current leadership, look good.  As an example, when I was young, I was taught that Christopher Columbus was a heroic explorer who "discovered" the Americas, rather than being taught that he essentially was the beginning of a European invasion into a continent that had some fairly large civilizations for the time already living there, who arguably had more of a right to be there than did the invading newcomers.  Red Act (talk) 22:34, 30 January 2012 (UTC)


 * My physics teacher told us a completely messed-up version of Schroedinger's Cat, which demonstrated only that observing a thing can involve changing it as in the whole "photon hits the particle" thing (he said that the poison was in a vial which was broken when you opened the box, killing the cat when you observed it. Especially puzzling as he'd included a radioactive substance for no apparent purpose). When I carefully phrased the correction as a question ("I'd heard it was... Is that something different?") he claimed there were "lots of different versions". Needless to say, that physics class didn't get to learn about anything especially interesting in quantum mechanics. He also forgot to teach us large portions of the syllabus, and I was the only one to pass the exams first time (because I checked the syllabus). At least I had good maths and chemistry teachers, or I'd have had to learn a lot more of the physics syllabus myself. Having carefully tested him, I learnt to ignore his teaching and use the time to study the textbook. Less effective than studying it outside the classroom, though, as his classes ended up feeling scarily unsafe to me and the other girl students. Is that scepticism? 86.166.41.126 (talk) 23:14, 2 February 2012 (UTC)


 * I don't believe they need to learn specifically to be skeptical. They learn by doing and by example being in the company of other people. If you spend time with people with an inquisitive mind, you'll end up being inquisitive. The same applies for religious people.88.26.74.157 (talk) 21:39, 30 January 2012 (UTC)


 * One caution: While I encourage you to be skeptical, it's not always wise to voice that skepticism. For example, some teachers will get mad and retaliate if you question them.  The same is true of bosses and friends, spouses, etc.  So, be sure you are prepared for the consequences, before you decide to announce your skepticism. StuRat (talk) 00:48, 31 January 2012 (UTC)
 * There are also some people who never become skeptics, so I don't think that there is a "usual age" when most people become skeptics. --Lgriot (talk) 11:01, 31 January 2012 (UTC)
 * The OP might be confusing "skepticism" with "critical thinking". And Stu makes an important point. It's funny how often those who overtly express skepticism get angry when their own comments are viewed with skepticism. ←Baseball Bugs What's up, Doc? carrots→ 13:31, 31 January 2012 (UTC)

Compass points South instead of North
Whenever I go near a compass, it switches polarity and points South instead of North, and generally won't return to pointing North. Why is this so? 122.149.61.46 (talk) 15:56, 30 January 2012 (UTC)
 * On these occasions, do you have about your person any significant pieces of metal (e.g. a belt buckle) or electronic equipment containing magnets (such as a cassette recorder or headphones) that might be overpowering the relatively weak terrestrial magnetic field? This would explain temporary deflections (see Compass, last paragraph), though not a permanent change in a compass needle's polarity, which would however be possible with stronger magnets or electric circuitry. {The poster formerly known as 87.81.230.195} 90.197.66.38 (talk) 16:19, 30 January 2012 (UTC)
 * The answer is no. This has occurred since childhood. I first noticed it when I buggered up my grandfather's compass just by holding it. Since then I have affected many compasses and now warn people to keep them away from me. Most recently today! Somebody let a compass near me and it started to turn South! They took it away and it went back to North. I'm just wondering why this would happen. 122.149.61.46 (talk) 16:42, 30 January 2012 (UTC)
 * Our Uri Geller article describes him similarly moving a compass; perhaps you can be The Next Uri Geller! What you are describing does  sounds like a paranormal phenomenon.  If you are not pulling our collective legs (and unless someone else here offers a more mundane explanation) then you should present yourself to a physics professor at a local university.  Please let us know when something gets published. -- ToE 17:34, 30 January 2012 (UTC)
 * I agree with this, let a university research lab independently measure and verify. Then we can refer to that report later on. Electron9 (talk) 00:54, 31 January 2012 (UTC)
 * We do have Bioelectromagnetism and Biomagnetism articles but they do not describe your particular power. The latter states, "The present scientific definition took form in the 1970s, when an increasing number of researchers began to measure the magnetic fields produced by the human body. The first valid measurement was actually made in 1963, but the field began to expand only after a low-noise technique was developed in 1970." These magnetic fields are presumably much less than what would be necessary to swing your average compass. -- ToE 17:53, 30 January 2012 (UTC)
 * This claim is definitely extraordinary, and you'll face some skepticism. If it is true, it is worth documenting in detail.  First, unless you happen to have been struck with a magnetic monopole (not inconceivable, but again, extraordinary), the compass should not simply point "south" no matter how you hold it - at any given time you should have a north pole and a south pole.  (Humorously we all have a south pole, and you might need an understanding fellow researcher to read the compass there!)  A normal magnetic field might conceivably be explainable by some kind of lost magnetic surgical implement, an encysted swallowed magnet, even a biochemical abnormality that would somehow allow some magnetic substance to precipitate in your tissues with a biologically ordered domain orientation.  But all of those things are, again, truly bizarre - it's hard to get a field strong enough.
 * Realistically, until proven otherwise, I'm going to assume either hoax or something the matter with the compass. But again, if you're telling the truth, keep up your investigation.  Who knows what you might come up with. Wnt (talk) 18:02, 30 January 2012 (UTC)
 * Wouldn't it be pointing toward the IP user, not simply toward the south? That in itself doesn't readily make sense to me. – Kerαu noςco pia ◁ gala xies 18:07, 30 January 2012 (UTC)
 * Oops! Yeah, that's what I meant. Wnt (talk) 18:36, 30 January 2012 (UTC)


 * I think the OP should make a video of the phenomenon and post it here. --140.180.15.97 (talk) 19:57, 30 January 2012 (UTC)


 * Anecdotally, I used to sit next to a girl who made my wind-up wristwatch run fast. We could sit and watch the second hand whizz round while she was holding the watch! I wonder if this is a similar phenomenon? --TammyMoet (talk) 20:33, 30 January 2012 (UTC)
 * I used to sit next to a girl who made time fly as well, though I assumed it was simply a matter of perception, and you seem to be describing the second hand actually moving faster. A long long time ago I owned either a wind-up wristwatch or stopwatch (can't remember) which would do the same thing when it was shaken very rapidly, and I assumed it was the watch mechanism's escapement slipping.  The shaking I would do was at a very low amplitude -- almost more inducing a vibration than displacing the watch back and forth any significant distance -- but the shaking action would still have been visible to an observer.  Perhaps your friend either had a more practiced hand, or was able to put pressure on the case in such a way that caused the escapement to similarly slip. -- ToE 00:52, 31 January 2012 (UTC)
 * Time flying when sitting next to a pretty girl is actually a well-known consequence of relativity, as described by Albert Einstein at least as early as 1948, and expanded on in the 1950s. TenOfAllTrades(talk) 03:13, 31 January 2012 (UTC)
 * No I actually noticed the phenomenon when sitting next to her in class with the watch on my wrist. It got worse when she held the watch. And I'm not sure why time would seem to fly when a straight girl sits next to another straight girl... --TammyMoet (talk) 12:09, 31 January 2012 (UTC)
 * Assuming the OP is not pulling our ref desk leg, this is different from the usualj alleged "paranormal" occurrence, in that it's something that can easily be tested with scientific rigor. Maybe contact Mythbusters, or failing that, your high school science teacher. ←Baseball Bugs What's up, Doc? carrots→ 02:26, 31 January 2012 (UTC)

It's interesting that the OP geolocates to Australia. From my reading, compasses that are designed for the northern hemisphere don't work very well in the southern hemisphere, because of the inclination of the magnetic field lines relative to the surface of the earth. Therefore there are special southern hemisphere compasses, which point south rather than north. Looie496 (talk) 02:34, 31 January 2012 (UTC)
 * Links: Magnetic dip and Compass. (I think that it is more accurate to say that they are balanced to work properly in the southern hemisphere, not so much that they point south instead of north.) -- ToE 03:00, 31 January 2012 (UTC)


 * I am very skeptical of the claim. It is as credible as "I can levitate." Edison (talk) 05:03, 31 January 2012 (UTC)
 * Being able to levitate at will is a bit more unlikely I guess. But I can make a video in which I appear to be levitating in no time. I would recommend claiming you achieve the effect in a paranormal way so you can earn 1 million dollar in James Randi's paranormal challenge. Von Restorff (talk) 08:44, 31 January 2012 (UTC)

OP: If you have a macbook or other laptop with a built-in camera, point it so that you can see a table with a compass on it, with the compass clearly visible, and the background. Start recordering, move away from the compass (wearing as little clothing as decent) and then move close to it. Upload this video somewhere where we can see, such as http://www.mediafire.com/. It should take you seconds to set up making the video, and another 3-5 minutes to make and uplload it. If you're on windows you might start with downloading some free software to make a video using the built-in web cam. Let us know if you need any more help, and we look forward to seeing the effect!! 188.6.73.215 (talk) 09:24, 31 January 2012 (UTC)
 * Even I can make a video like that, by asking someone else to hold a strong magnet under the table, so that is not very convincing evidence. Von Restorff (talk) 23:43, 1 February 2012 (UTC)

Culm power
I approve of many of the Occupy movement's sentiments, but after looking at one of their latest cluster fucks I'm thinking it would be nice to have a more responsible organization that would find abandoned buildings to administer with the consent of local authorities. To do this, they'd have to bring something to the table, and heat is such a gift - both to preserve the building from humidity etc., and to fight the Occupiers' frosty nemesis. As it happens, Pennsylvania is littered with piles of "culm", a sort of fine-grained coal residue, and/or other spoil tip material. Sometimes large piles, sometimes very small, often in lands that are since abandoned or even turned into state forests and gamelands. While the smaller deposits might not yet have become economical for any company to exploit, I wonder if a few people with a pickup truck and a plan could do something with it.

Now a full century ago, using culm for heat was a hot topic - apparently its main problem was that as finely ground carbon it could create too much heat for any known fire brick to survive. (Other samples were much less pure, however, producing much more ash) Apparently though the Wooten firebox was such a way. In more modern times some plants have been built to burn the stuff, e.g.. There was also a much-vaunted proposal by Ed Rendell to use culm to produce synthetic liquid fuel, which has produced a now-venerable sign proclaiming the future site of the plant on scenic route 61. Something about this is explained at. There are a lot of things up about a "smokeless culm gasification furnace" from China, e.g., which I think must be smaller and less complex - not sure.

Anyway, what I'm wondering is, is it conceivable for a progressive group to set up a practical, safe, and acceptably clean way to burn small amounts of culm for local heating purposes? Wnt (talk) 19:59, 30 January 2012 (UTC)


 * Generically, an organisation that uses profit-making for social ends is called a social enterprise. It's possible that such an entity would be able to do this.


 * I'm completely unclear as to what the progressive group has to do with anything. Are you just asking if burning culm is a good idea? That seems like a hard enough question in and of itself to think about whether progressive protest groups would think it is a good idea. (It certainly has nothing to do with the purpose of their protest. It may even be quite poor from the point of view of CO2 emissions). In any case, I would do a little more research before trusting Glenn Beck's account of anything the Occupy movement allegedly does — the guy is a total loon and his take on things is always slanted if not totally made up. --Mr.98 (talk) 01:40, 31 January 2012 (UTC)


 * I was trying to frame the parameters; I know, after all, that culm can be burned at an industrial scale. True, the political thing was a distraction... Wnt (talk) 02:14, 31 January 2012 (UTC)


 * An apropos: While the material in Spoil tips are combustible, a discussion on processing may confuse this 'coal dust' with activated carbon (produced in e.g. the Kværner-process) or even with carbon black. The activated carbon can be burned profitably and cleanly, but these materials are more interesting as storage mediums for hydrocarbons or uses unrelated to combustion. EverGreg (talk) 10:28, 31 January 2012 (UTC)

Why is there a lot of soot in most diesel exhaust but not a lot in most gasoline exhaust?
Topic says it all. ScienceApe (talk) 21:07, 30 January 2012 (UTC)


 * Exhaust gas is the main article, with a specific diesel exhaust subarticle that talks about the origin of the visible soot. DMacks (talk) 21:10, 30 January 2012 (UTC)


 * I read diesel exhaust, didn't find anything about why it produces soot in the first place, or rather why diesel produces more than gasoline. ScienceApe (talk) 21:24, 30 January 2012 (UTC)


 * This page says (in slightly fractured English): "The fuel in diesel engine is ignited by the heat of the compressed air. It results in that fact that fuel had no time to fully mix with the air and it produces CH, NOX and carbon black during the combustion process. The carbon black is particularly visible then it colors the exhaust in black." Alansplodge (talk) 22:00, 30 January 2012 (UTC)


 * So a gasoline engine results in a more complete combustion of the fuel, and that's why it produces less soot? ScienceApe (talk) 22:31, 30 January 2012 (UTC)


 * In the US, at least, they recently switched to a cleaner diesel, and we no longer see black clouds pouring out of trucks as a result. So, apparently, use of low quality fuel is also an important factor. StuRat (talk) 00:38, 31 January 2012 (UTC)


 * The phenomena are unrelated. The diesel standard adopted across the United States in 2007 was for ultra-low-sulfur diesel; it's 'cleaner' because of the very low sulfur content (resulting in reduced emissions of sulfur oxides), not because it's less sooty.  Reduced soot is due to the increasing use of diesel particulate filters in modern vehicles, encouraged by more stringent regulation of particulate emissions. TenOfAllTrades(talk) 01:49, 31 January 2012 (UTC)

The causes of soot (carbon particles) in diesel exhaust is not completely understood, as physical chemists have yet to produce complete data on reaction rates for the multitude of chemical reactions taking place simulataneously within the cylinder. So we can't yet do an accurate computer simulation. However, the following is clear:- Keit124.182.165.61 (talk) 02:13, 31 January 2012 (UTC)
 * 1) It is NOT because of combustion less complete than in gasoline engines - generally, diesel engines have more complete combustion.
 * 2) Gasoline engines do not (unless maladjusted) produce soot because they operate stoichiometrically - there is always just enough air to burn the fuel - this keeps combustion temperature high throughout the throttle range and even when idling.  Gasoline is easily vaporised - why this matters is explained below.
 * 3) Diesel engines always draw in a full quantity of air, well above stoichiometric level, regardless of throttle position - so combustion temperature is low, particularly at part thottle settings.  It takes high temperatures to avoid precipitating out carbon, and diesel combustion temperatures are not much above this temperature.
 * 4) Diesel engines tend to produce vsisbile soot when starting up from cold, because the cold cylinder temperatures pull the combustion below the soot limit temperature.
 * 5) Diesel engines may produce visible soot at full load thottle settings because the volume of fuel injected may be above the capacity of the injectors to handle while "atomising/misting" the fuel.  If the fuel particles are too large, the liquid in each particle holds local combustion temperature below the soot limit.  This often occurs with cheap fuel and/or worn injectors.  In other words, injector capacity sets an upper throttle limit to operating without soot.  It doesen't happen with petrol engines because gasoline is easily vaporised.
 * 6) Even if injector capacity is increased, a diesel engine will still produce soot at a (still higher) power setting.  I've been trying to figure out why this is.


 * 1 & 5 and so on. The fuel -as you say- is not taken into the cylinder as a vapour. The injector 'squirts' out a liquid – which hopefully mists – but at high delivery rates, it goes in as a jet. Note: Some oil compression engines have an over-fueling setting for starting. Fuel from the injector  relies on the 'swirl' (created by the funny shape on top of the  piston ) to mix it with the compressed charge of air. This means that there are areas of compressed air, that are very rich in  fuel, which in a petrol engine doesn't exist. The oxygen in these areas get depleted faster than gaseous diffusion can bring in more oxygen to combust the remaining fuel. So, just like an Oxyacetylene torch with the oxygen off – the incomplete combustion produces shoot. When the engine is cold, the compression cycle looses too much of its  heat to the cold surfaces of piston, cylinders wall and head, thus preventing  the  completely combust the fuel because much the heat from the  compression is lost. This is now understood,  because a well maintain 'modern' compression  ignition  engines are designed (regardless of if  they used fuel oil or 'diesel' spec fuel)  to   forgo the  clouds of smoke. Lastly, no modern engines use the Diesel Cycle any more- WP need up dating badly on this point to make this clear.  There are engines that run on  gasoline/petrol, oil (of various sec's), diesel grade, kerosene/paraffin, vegetable oil, methane, etc., but non are Diesel Cycle Engines which WP articles seems to suggest. --Aspro (talk) 21:52, 31 January 2012 (UTC)
 * this sound interesting; Aspro, could you tell us more?Staticd (talk) 04:44, 2 February 2012 (UTC)

Aspro is almost, but not quite right. (a) injectors are designed to inject droplets so small that conversion to a vapour occurs within the cylinder - sufficient liquid to interfere with combustion remains only at excessive throttle settings as I said. (b) A excess fuel rich portion of a charge need not result in soot. If the temperature is high enough, the carbon is evolved as a gas and can forthwith be burnt when it mixes with air from a lean portion. If not it is precipitated out as solid carbon, which won't burn. Carbon can exist for short periods (picoseconds??) as a gas below the sublimation temperature (>4000 K) but once it goes solid, the sublimation temperature is required to reconvert it back to gas for combustion. (c) I've long been amused that virtually every college physics text book talks about the diesel cycle (constant pressure combustion), supported by the math, with only the better ones hinting that actual engines don't work that way. Real "diesel" engines all basically approximate constant volume combustion the same as a gasoline engine (combustion is lamost complete before the piston moves very far from top dead centre). This is easily shown by taking "card diagrams" (plots of pressure vs crank angle). You do find that engineering textbooks on diesel engines talk about mixed cycle combustion (part constant volume & part constant pressure). The only engines to use a diesel cycle were the first couple of prototypes built under the direction of Rudolf Diesel himself. He thought that constant pressure with extremely high compression was the way to go, but it's not, as 1) the power output is abysmally low, 2) he did not factor into his calculations the loss of heat to the cylinder walls etc, and 3) he did not take into account the big end bearing friction loss, which rises alarmingly with compression. Engineers pointed this out at the time, his prototypes performed poorly, and he quietly changed to constant volume combustion.  This caused much ridicule of R Diesel and legal argument over his patents at the time.  Engine designers and engineers have always known all this, but the authors of college textbooks haven't noticed, and have kept on regurgitating diesel cycle. I guess the WP article is in the same league. Keit 58.169.241.117 (talk) 07:42, 2 February 2012 (UTC)


 * Keit 58.169.241.117. How about getting setting up a WP account to knock these articles into shape. Beware though. Some of your edits might get reverted by editors that want to believe the over-simplistic explanations that they have just read in the popular press.  You however, appear to have the necessary background to such  argue it out.
 * @Staticd my previous post was just what came off the top of my head from what I learn many decades ago (and mostly forgotten). Keit has rounded it off much better.--Aspro (talk) 19:10, 2 February 2012 (UTC)


 * I've been thinking of setting up a WP account, but knocking the WP Diesel Engine and Diesel Cycle articles into shape
 * is quite a big ask. There's a lot of words there.  There's a lot of good stuff, but a lot of common misconceptions too.  The main thing is: did OP ScienceApe get from us what he needed? Keit124.178.181.41 (talk) 04:33, 3 February 2012 (UTC)


 * Well ScienceApe ! Have we gone off on a skewed tangent, or did you find some the above responses helpful?--Aspro (talk) 16:16, 3 February 2012 (UTC)