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Can you make this addititive from common (perhaps household) materials?

  1. May 22, 2008 #1
    Here's one for the McGyvers amongst you.

    Let's suppose we want to power a model-aeroplane that runs on a mixture of gasoline, plus nitromethane. Leaving the gasoline aside because we have plenty, the challenge is:

    Can the nitromethane be home-made? Ie from household materials, like stuff you can buy from a supermarket?
  2. jcsd
  3. May 23, 2008 #2
    Hmm, you can use somewhat of 9 times more nitromethane than gasoline in one stroke. And the energydensity är like a factor 4 on gasoline/nitromethane.

    How does nitromethane work in a RC-model?

    I think that you can probably NOT do it at home. But hey, I wouldn't mind being wrong on this one.
  4. May 23, 2008 #3
    Dunno but I found a 0.8 hp one that takes 5-10% nitromethane (the rest is actually not gasoline, but let's keep things simple in this thread, I'm only stuck for nitromethane).
  5. May 23, 2008 #4


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    We probably shouldn't discuss the home manufacture of this material in this forum. It is available for purchase. I believe there are some papers to fill out, though.

    Happy hunting!
  6. May 23, 2008 #5
    You are gonna blow yourself up.
  7. May 23, 2008 #6
  8. May 23, 2008 #7


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    As chemisttree said, we are not going to allow a discussion of synthsizing nitromethane at home. The stuff is dangerous. Do you know where you can buy it? What does it cost?
  9. May 23, 2008 #8
    Read what you just quoted. It says "instead of nitromethane"
  10. May 23, 2008 #9


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    Ah. What do you mean instead of nitromethane? Are there other booster-type substances that work? Even VP racing fuel is just higher octane gas, I think.
  11. May 23, 2008 #10
    Dunno, I just want to make a ultra-lightweight electricity generator powered by diesel or other fuel that I produce myself. And I thought the 210 grams / 0.8 hp for 200 dollars of that engine are absolutely amazing and possibly a market with great potential too. Diesel engines are preferred because they are so much simpler and easy to maintain, and possibly more reliable too.

    More details on my thread "The lightest diesel engine known to man".
  12. Jun 3, 2008 #11
    Forget nitromethane. Like most other nitro-somethings, it's made by nitrating methane-- nothing you want to fool with. Unless nitric acid is a 'common household ingredient' around your house... and I doubt it is. No, even then; this is not something you want to do.

    But happily, there are two reasons you don't need to! First, you can just buy yourself a whole gallon of 100% pure, grade-A nitromethane at the same hobby shop that sells the engine. Second, considering your stated purpose, the whole nitromethane thing is in the wrong direction--- you want max energy density in your fuel and max efficiency in extracting it. Exotic fuels like nitromethane are as far from "produce myself" and "market potential" as you can get.

    Consider: a titanium can with feed lines delivering pure hydrazine and either nitric acid or pure hydrogen peroxide will give you the most incredible power-to-weight ratio ever seen by man, I'm sure; but all three of these fuels are incredibly expensive, can only be made by sophisticated chemical plants as a practical matter... and all three will eat your face off.

    Stick with 'make yourself' and 'market potential'; leave off the exotic fuel thinking--- I think you'll get farther, and almost assuredly live longer.

    That's my advice, anyway.
  13. Jun 4, 2008 #12
    I've enjoyed your post, thank you. :smile:

    Since you really seem to know chemistry, what about this one: Why won't plain diesel oil burn in those tiny engines? (they work by just compressing the fuel, which produces heat in an adiabatic change, which combined with an optional heating coil ignites the fuel - diesel cycle).
  14. Jun 4, 2008 #13
  15. Jun 6, 2008 #14
    Do you know what the compression is in these model-size diesel-cycle engines?

    Also, whether these are direct-injection or indirect-injection would tell me more...

    You see, I'm not sure that these engines flat-out will not run on diesel, but there's probably a specific problem these mixes of fuel are supposed to resolve. My best guess, especially with all that alcohol involved, is that it's there to thin the fuel enough to spray it properly in a very small injector. The other big reason alcohol is added to diesel, though, is to make it ignite at a lower compression.

    Both of those would amount to overcoming problems with getting straight diesel fuel to ignite or burn properly in such a small engine. What I meant by saying I'm not sure they can't do it is that you could almost certainly engineer out whatever the problem is without modifying the fuel, if that were important to you... If the problem is too-low compression, then I would guess it's not higher because of the metals they're using-- if the cylinders and heads are aluminum, for example, they might think normal diesel compression is too much for such small bits of aluminum. If that is the case, one could always make the parts out of titanium, let's say, or tool steel--- they did what they did because of the compromises that were important to them (I would guess light weight and high power over everything else; sacrifice anything neccesary for power-to-weight ratio).

    If the injector were the crux of the matter, or the fuel pressure, those could be engineered around as well; but you can see why the model people might not be willing to have a fuel system as big as the motor itself, if that's what's required--- but you might be completely comfortable with that in your own application.

    It should be noted that when it comes to a simple, typical recriprocating-piston engine, especially a single-cylinder or twin, that there's not a lot of difference between diesel, gasoline, alcohol, straight vegetable oil, producer gas... the motor itself is largely the same-- for some you would require a spark to fire them; for some you would not. Other than that, it's mostly what you'd call 'tuning' that determines which fuel will burn--- questions of timing, advance, compression, and fuel-air mixture. A diesel, for example, can be run on mixtures of gasoline, petroleum diesel, kerosene, alcohol, biodiesel, waste vegetable oil, straight vegetable oil, or waste petroleum oil--- a very simple diesel engine won't care much which of these are present or how much; so I'm sure you can imagine how fuels can be messed around with and still work. I know of a guy who uses untreated sunflower oil to power every diesel engine in his life, and he cuts it about 10% with regular pump gasoline to thin it out to the consistency of #2 diesel; and apparently, he gets great results with it.

    All that's neccesary to make a gasoline engine run alcohol, on the other hand, are modifications to compression, timing (when the spark fires), and possibly fuel-air mixture. Specifically, you'd increase the compression, advance the spark, and I would guess run a bit leaner than you would with gasoline.

    My point here is that engines and fuels are more of an open playground than a set of rigid pigeonholes. One thing to remember about gas turbines, for example, is the fact that a turbine will run on almost any flammable liquid, from diesel to Jack Daniels to Chanel #5. I think a lot of people, when they hear "jet fuel" think that it's super-sophisticated in some way... but most jet fuels are kerosene-based, and one of the biggest reasons the Germans were so enthusiastic about the prospects for the gas turbine in WWII was the fact that they could run on low-grade diesel fuel, while normal piston aircraft required the highest-octane fuel available for high-performance engines. High octane is neccesary for high compression without detonation; and that same high compression is neccesary for wringing the most power out of those aircraft engines.

    So, do keep your own mind open about what to run in what; don't be afraid of the conventional wisdom which might tell you this-or-that is 'not recommended.'

    Hope you don't mind the length....
  16. Jun 8, 2008 #15
    I found they vary between 7.5:1 and 9:1. Also found normal diesel engines compress at 14:1, and can go as high as 25:1.

    The one I am considering because it is so powerful at a rated 0.8 hp, seems to be direct-injection:


    Would heating the fuel tank somehow be an equivalent alternative to this thinning?

    Would heated diesel ignite at a similarly lower compression?

    That sounds familiar, I did see somewhere a titanium engine, can't remember where.

    Alright! I don't mind weight that much, anything up to 10 kg is probably fine for me (I just want to make a very portable generator out of it).

    That sounds great. I wonder if it can be done with a small engine.

    Will a gas turbine engine work if miniaturized?
  17. Jun 8, 2008 #16

    So it would certainly seem as if the low compression is the big thing they're trying to work around. Roughly 17:1 is considered the old-school (meaning primitive engines; widest range of acceptable fuels) compression for diesels. So the alcohol and nitro are probably there mostly to ensure that ignition will take place.

    Now, something else that came up since I last posted was that I looked at photos of some of these engines, and they're pretty clearly two-stroke. This limits the effective compression one can get, because the classic four strokes needed are paired up in twos and 'sharing' strokes. Anyway, it's worth bearing in mind-- you might have better results shooting for this small size, yes; but not limited yourself to the two-stroke design. It will be more complicated, but the modellers aren't avoiding it because it's too hard to pull off (in my opinion), but rather because of what I said before--- they're willing to trade off anything and everything for small size, and that argues strongly for no moving valves, etc.

    Back to our fuels... In any engine, the goal is to get the fuel to vaporize--- liquid fuel plus air does not equal the kind of combustion we're looking for here. In indirect-injected engines, you get the benefit of simpler injectors, because the fuel is vaporizing in a chamber before the cylinders. In direct injection, as the name implies, fuel is injected directly into the cylinders--- so you need a more sophisticated injector (all things being equal) and higher pressure to achieve the atomization of the fuel. This also becomes more and more difficult as the compression increases... So the thinking may have been, we direct-inject in these tiny engines because it's fewer parts, but it limits the compression we can use before our little injector can't overcome the cylinder pressure. Keep in mind, with a traditional diesel, fuel is injected at the top of the compression stroke (or just before the top), against the highest pressure the cylinder can offer, and the combination of fuel and air compresses instantly. The classic 'tick' or knock that you hear in diesel engines is the sound of the injector firing; combustion takes place immediately thereafter. I suppose these little engines can't have very high pressure fuel systems, so the way they're currently designed is to balance the highest compression possible (within the limitations of two-stroke and the small size) against needing a more complex fuel system to create the needed pressure. Meaning, if we took one of these engines as currently conceived and just upped the compression to 14:1 or higher, the first thing we'd notice is that no fuel at all could make it into the chamber. Let me say, though, I know less about the two-stroke diesels, so another possibility is that, since fuel is being injected early in the compression stroke, it can't be any higher or else burning would start before the stroke was at the top, and some of the energy of the burning mixture would actually be opposing our desired direction of rotation. I'm not sure if that's accurate thinking, though, because we started this thought based on the idea that compression was too low for effective combustion--- if higher compression would suddenly start the burn early; I don't know, the two seem to contradict one another. The nature of two-stroke engines carries a lot of penalties, as you see, while providing the benefit of simplicity. It seems to me that what you need is the most efficient production of energy from your fuel, and the most complete extraction of that energy, bar none--- so I think you should be willing to be more complex if there will be vast benefits in that efficiency and extraction.

    Looking at the engine you referenced, my suppositions only go so far because it's a two-stroke... but I also noticed that Enya does offer four-stroke engines, including a four-stroke diesel! It makes no more power than the engine you linked (0.8 hp), so I don't infer anything clearly by its existence, except for the fact that: a., miniature valvetrains, camshafts, and injectors are certainly doable, and b., that you will be able to reverse-engineer the already-existing engines out there to get towards what you'd like to have.

    As to your question of fuel heating, very well done! Heating the fuel has a very direct relationship to how thin it will be, all the way up to and including vaporizing the fuel with heat alone! Some who run on straight vegetable oil simply preheat the fuel instead of thinning it chemically. The guy I mentioned also feels he gets a benefit by having gasoline present--- better combustion and soforth, so it's not entirely just to make the fuel thinner. But this is definitely a valuable direction to be thinking in. You can make a waste oil burner with nothing more than a blower and a drip-valve for the fuel--- the interesting part is, as soon as the combustion is happening at very high temperatures (and it will be; it's part of the nature of the thing), and if you're preheating the fuel to the point where it is vaporising even before it comes out of the fuel line, then you can burn the darkest, nastiest old-oil sludge (like used motor oil) so cleanly there are effectively no noxious emissions! I think that's incredible. Check out the Association of Backyard Metalcasters--- most of these guys use this type of burner to heat their casting furnaces... temps of 6,000 deg. F are possible, and at those kind of temperatures almost anything will burn, and burn so completely that there's just nothing left to make emissions... just the basics you'd expect, CO, CO2, etc. Thier thinking is often based on maximum energy density of fuel (because they quickly see how much fuel they have to burn to melt a given amount of metal)... some of them started with propane, because it's clean... but realized that fuel oil, heating oil, diesel oil; these have the highest density of common liquid fuels (over 135,000 BTU's per gallon), so they use less for the same output of heat. They soon realized that at the temperatures they're putting in there, once it's burning well you could pour roofing tar in there and it will still combust very completely. Exciting work, in my opinion.

    Finally (for the moment), gas turbines will certainly work if miniaturized, but of course there are limitations. RPMs and heat both tend to be very high in turbines, which is what leads to their short service lives, and demand for sophisticated and expensive materials. There are many metal alloys and soforth that are so expensive turbines are the only place I know of that they're even used. In that application, they are neccesary and/or justified... what's interesting to consider is that some of these would probably have never been developed, or perhaps created in a lab but never ever used, if it weren't for thier practical neccesity in gas turbines. Turbines are an exciting subject to a lot of people, so there's quite a significant body of amateur turbine enthusiasts. They do very interesting stuff, and have establised a lot of what can or will work, and what is realistically impossible (for a backyard tinkerer, anyway). For your own effort, there would be a benefit to sticking with what you can do yourself--- see, if it's too complex to build at home (or, let's say, just farming out the machine work), then it's going to be expensive to build, and that will be a major concern as you get closer and closer to a usable product. Consider: an engine the size of a quarter, that extracted 100% of the energy in whatever fuel put into it (which is, of course, a practical impossibility), while producing no emissions and no noise at all... no maintenance of any kind required, ever. Sounds pretty good, huh? Let's say I have them, and can make more... but the price tag is one trillion dollars per copy. It might as well not exist, for all the good it would do--- the cost/benefit analysis would always be saying, in effect, don't bother building it. Low cost brings its own benefit, though... so much of one, in fact, that if I just offered you a ten-horsepower engine, and it had to weigh, let's say, 2,000 pounds (so it's basically a stationary engine), but was more-or-less emission-free and was more efficient (in terms of power extracted from the fuel) than any other engine in history--- and we can produce them for $300 a copy. That's a world-changer right there.

    So, in summation, although I earlier advised you to not be afraid to go for more complexity, if the benefit is there; do always keep in mind that the final thing produced has to be producable, and at a cost that is bearable for the benefits it offers. Happily, I think you can achieve that effect without having to think about it too much--- just focus on your experimenting. Take the example of hydrazine; we considered at the start that it's simply too dangerous to work with. Well, that brings with it that it's expensive, and that it can't ever be in the hands of Joe and Jane Average. So even if we had our quarter-sized super-engine I hypothesized earlier, if it would require constant handling and use of hydrazine, it's basically a nonstarter. So if your own experiments are ever precluded by being too expensive or too difficult to carry out yourself, that's a great hint that, even if you overcame the difficulty in the short term (like, I have a friend with a chemical lab, let's say), it will come up again in the long run, and might be enough to kill the whole project. If you can experiment with it at home, then you'll probably stay in territory that will lend itself to humanly-priced production and final products.

    I'm glad that this discussion seems to be helping to move your efforts along...
    Last edited: Jun 8, 2008
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