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Mini Hybrid~No Need To Measure!

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: SPG » Wed Nov 28, 2007 9:43 am

He doesn't even need a lathe, just get the right size tubing. it should be very easy to make in fact.
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Unread postAuthor: jimmy101 » Wed Nov 28, 2007 2:39 pm

Ragnarok wrote:The burst disc has little control over the peak pressure.

How do you figure that? A typical combustion peaks out at about 60 PSIG. With a burst disk you can get near 100 PSIG (closed chamber is about 120 PSIG). So the burst disk has a huge affect on the peak pressure in the chamber. Crap, that is the entire idea of a burst disk!

Ragnarok wrote:A 2x hybrid will produce roughly twice the pressure of a 1x combustion, regardless of burst disc. That double pressure is a very significant change.


You guys need a little basic physics and chemistry.

Yes, twice the pressure IN HALF THE VOLUME! Total energy is proportional to Pressure x Volume, so the energy in the two chambers is exactly the same for these numbers.

As the projectile moves down the barrel the pressure drops faster with the smaller chamber. Net affect is the pressure versus time (or force * distance) integral is basically the same.

The only benefit is the fairly minor affect of starting at 30 PSIA instead of 15 PSIA.
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Unread postAuthor: jimmy101 » Wed Nov 28, 2007 2:57 pm

FeLeX wrote:Jimmy really I dont want your dirty claws on my post.

get over it.

FeLeX wrote:You argue just for the sake of arguement. If you STILL dont unferstand how it works look up ICE. This gun is supposed to be a mini so if I make a chamber twice as big just to get the same performance (which it highly doubt I will) that would defeat the purpose of a mini. And since when coming up with something new is such a bad thing? Really Id like it if you wouldnt reply here anymore. You replied to my other posts and all it led to was a meaningless argument.

Edit: By the way Jimmy what ARE the advantages of standard hybrid?


FeLex, perhaps you should look at an ICE. The fuel is compressed only because you need to "recock" the piston for the next power stroke. Can you come up with an ICE design that does not have a compression stroke and that does not require a high pressure air source to charge the cylinder? Compressing the fuel does not significantly change the energy in the chamber (though it does make it burn a bit faster because of compression induced heating, which is irrelevant to the proposed gun design). Indeed, the boost in power from compressing the fuel comes from some of the energy produced in earlier combustion cycles. So the increase in power during the power stroke is exactly offset by the work required in the compression stroke.

Yes, you say you are making a mini but your chamber is twice (or whatever compression ratio you are going for) the size than the chamber in the firing configuration. Exactly zero net saving in the size of the gun. Indeed, the extra complexity and parts makes the gun bigger than it would be if it was a standard 1x configuration. Don't really see any advantage in that.

"By the way Jimmy what ARE the advantages of standard hybrid?"
It is clear you do not understand what a hybrid is, why they are built or how they do what they do. Listen (read) closely and learn. A standard hybrid has increased performance over a 1x gun because of the greater amount of fuel+oxidizer in the chamber. A 2x gun has roughly twice the energy in the chamber as a 1x gun. If I take a 1x gun and smash the volume of the chamber by two fold the gun will perform about the same. Same amount of energy (ignoring the small affect of 15 PSIG higher starting pressure).

A hybrid has increased performance because you are using much more fuel in the chamber than you would in the same gun operated at 1x.

It is as simple as that.

Nothing else you do to the fuel, look at it cross-eyed, wave a magic wand over it, pass a magnet around the chamber, or compress the chamber, will significantly change the performance of the gun.
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Unread postAuthor: SPG » Wed Nov 28, 2007 3:51 pm

Well I can't wait to see the thing built then, it'll be the perfect answer to all these questions. After all, you'll be able to simulate this scenario perfectly, fire it uncompressed and it'll be a normal combustion, fire it half compressed and it'll be a 2x Hyrbid (with a half size chamber) 3/4 compressed it'll be a 4x Hybrid and so on. If Jimmy's right we'll see no performance gains at all, if FeLeX is we'll see performance increases. It'll be highly repeatable and all we have to do is come up with an agreed method of testing performance - penetration tests perhaps?
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Unread postAuthor: FeLeX » Wed Nov 28, 2007 4:35 pm

Hey Jimmy YOUR A FAG.
Move out of your moms basement and get a job. Oh and retake highschool chemestry. ICE compresses only to recock? No ICE compresses the fuel to create higher pressures when ignited.
You are an imbicile my friend.

Finals coming up :<
Gonna go to Home Depot some time this weekend to get the smallest caulking gun and then the building begins.
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Unread postAuthor: mark.f » Wed Nov 28, 2007 7:09 pm

FeleX wrote:Hey Jimmy YOUR A FAG.
Move out of your moms basement and get a job. Oh and retake highschool chemestry. ICE compresses only to recock? No ICE compresses the fuel to create higher pressures when ignited.
You are an imbicile my friend.


Oh, very good way to counterpost. Simply insult him, in a thread designed to debate scientific theories. That'll put him in his place, it will. You sure showed him. :roll:

Basically, the point across is that a combustion of, say, 100 cubic inches, at a 1x mix, will contain the same potential chemical energy as a hybrid of 50 cubic inches, at a 2x mix. The point of whether or not the performance is the same is a little debatable, but testable.

So, in short, do not just say he's wrong and tell him to shut up. If you don't agree, think of your reason WHY, post it, in an intelligent and civilized manner, and hold an actual conversation on the matter.

No one's saying you can't build your mini. Go ahead and build it. Also, while you're at it, fire a mini 2x hybrid mix at a certain volume, (depending on your tube and initial volume), and then a 1x mix at double that volume, and compare the performances, that way we can LEARN something out of all of this.
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Unread postAuthor: Ragnarok » Wed Nov 28, 2007 8:36 pm

jimmy101 wrote:How do you figure that? A typical combustion peaks out at about 60 PSIG. With a burst disk you can get near 100 PSIG (closed chamber is about 120 PSIG). So the burst disk has a huge affect on the peak pressure in the chamber. Crap, that is the entire idea of a burst disk!

I said little, not none. I know the burst disc affects it, that's obviously true.
Still, what you've just said proves that the burst disc pressure is not the ultimate pressure the system can attain, and that's the important thing.

You guys need a little basic physics and chemistry.

Excuse me, I'm an engineering student at uni, with A levels in Physics, Maths and Chemistry. I am no idiot.

Yes, twice the pressure IN HALF THE VOLUME! Total energy is proportional to Pressure x Volume, so the energy in the two chambers is exactly the same for these numbers.

As the projectile moves down the barrel the pressure drops faster with the smaller chamber. Net affect is the pressure versus time (or force * distance) integral is basically the same.

JSR did these calculations (scroll down a little) with GGDT - all chamber have the same pV constant before triggering. I know it's modelling a pneumatic, but hell, it's proof that higher pressures and lower volumes do create higher muzzle energies.
And that is exactly what we are considering here.

Perhaps you should actually equate those integrals you refer to.
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Unread postAuthor: mark.f » Tue Dec 04, 2007 6:50 am

It all depends on acceleration throughout the barrel, (I believe I've said this before in another thread, but I'll repeat it here), this is a direct product of Force throughout the barrel. This is determined by the initial pressure and chamber volume. A simple way to try and verify a few of these things is to perform a simple calculation of average acceleration throughout the barrel. You could go so far as to obtain an equation relating acceleration to distance, and then integrate, but I challenge you to do that without a calculator, (the answer is not that significant either).

And that's all I got time for. :cry: Gotta finish that thought at school.
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Unread postAuthor: jimmy101 » Tue Dec 04, 2007 3:56 pm

Ragnarok wrote:JSR did these calculations (scroll down a little) with GGDT - all chamber have the same pV constant before triggering. I know it's modelling a pneumatic, but hell, it's proof that higher pressures and lower volumes do create higher muzzle energies.
And that is exactly what we are considering here.

Perhaps you should actually equate those integrals you refer to.

Look closely at the GGDT output that you linked to. GGDT says the flow is choked. The flow through a choked valve is a function of the gas velocity (which is a function of the pressure) and geometry of the valve. Are you sure the GGDT results are valid for a combustion gun, which typically do not have choked flow? Furthermore, in a compressed air gun the pressure starts at the max pressure and drops, in a combustion gun the pressure starts at atmospheric then rises, then drops. For a hybrid, the pressure starts out above atmospheric but a 2x mix is only 14.7 PSIG above atmospheric. The pressure still rises as the gun is fired before is starts to fall.

Excuse me, I'm an engineering student at uni, with A levels in Physics, Maths and Chemistry. I am no idiot.

Gee, me to, only I graduated about 25 years ago, then got a PhD in chem.

Since you are at uni, you should be able to post the integrals (my calculas is pretty rusty) instead of quoting results from GGDT, which are only marginally applicable to a combustion gun. GGDT will predict an optimal CB of about 0.2. Does that mean a combustion gun will work best at 0.2?

Besides, there is an error in Jack's calc's;
20 inch chamber at 50 psi - 406 feet per second
10 inch chamber at 100 psi - 537 feet per second
5 inch chamber at 200 psi - 684 feet per second
2.5 inch chamber at 400 psi - 830 feet per second
1.25 inch chamber at 800 psi - 958 feet per second
0.625 inch chamber at 1600 psi - 1006 feet per second
All the time, we're talking about the same quantity of air, because we're doubling the pressure but halving the volume. All the time, we're talking about the same quantity of air, because we're doubling the pressure but halving the volume.

GGDT works in gauge pressure. If Jack input the pressures as 100, 200, ... then the pressure in the chamber is not going up by a factor of two. He should have used 50 (65PSIA), 115 (130PSIA), 245 (260 PSIA), 505 (520 PSIA) to get the pressure going up by a factor of 2 each time.

So it comes down to someone explaining how pressurizing the fuel + air mixture results in more energy in the chamber, other than whatever work you put in in the pressurization step itself. In the proposed design the gun'll be pressurized to 2x, so there is 15 psi more pressure in the chamber. That is enough to give a measurable difference in the performance of the gun but that is the only boost in the performance that you'll get. You won't get significantly (measurably) more energy out of the combustion process itself. Where would that extra energy be coming from?
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Unread postAuthor: jimmy101 » Tue Dec 04, 2007 4:33 pm

FeLeX wrote:Hey Jimmy YOUR A FAG.
Move out of your moms basement and get a job. Oh and retake highschool chemestry. ICE compresses only to recock? No ICE compresses the fuel to create higher pressures when ignited.
You are an imbicile my friend.

Well put felex, your grasp of physics, chemistry, and the English language, is truly astounding.

You really should learn how an ICE works before you start playing with big kid toys like a spudgun.

Yes, compression causes higher pressure in the cylinder. But, where do you think the energy came from to do that compression? Engine fairies? Trolls? Gnomes?

Energy is consumed in the compression stroke, that energy (minus frictional and heat losses) is re-released in the firing stroke. No net energy is gained by compression.

I challenge you to find a single reference that says the heat of combustion of a fuel is significantly affected by pressurization. Since folks have been building ICEs for more than a hundred years, there will certainly be a large number of references, equations, rules of thumb, etc. that combustion engineers use to calculate the changes in the heat of combustion of a fuel as a function of pressure. Try to find a single formula that would allow an engineer to calculate the changes in the heat of combustion of a fuel as a function of compression ratio. (If you are good, you'll find references detailing how the efficiency changes with pressure, temp. etc., or how the burn rate changes with pressure, temp etc.)

Gee, can't find a single reference like that? I wonder why. Perhaps because the work done by an ICE is only modestly affected by the compression.

Most ICEs run at about 8:1 compression ratios. Your line of "thought" would suggest there is 8x more energy created because of the compression cycle in a typical ICE. Since most ICEs are about 30% efficient, all you would have to do is double the compression ratio again to 16:1, that would double the energy released during combustion, and double the power, and double the fuel efficiency of the ICE. Taadaa, you now have a 300HP eight cylinder that gets 60 MPG while moving a 2 ton vehicle.

Gee, why haven't combustion engineers ever thought of that before?
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Unread postAuthor: ShowNoMercy » Tue Dec 04, 2007 4:46 pm

Now I have never built a hybrid but I am going on the assumption that you put in double the amount of fuel into a regular chamber and to get that to explosive limits you inject double the atmosphere. So your mini hybrid would be taking a certain amount of fuel and the compressing the air but not adding more, so wouldn't that not achieve explosive limit?
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Unread postAuthor: windshrike » Tue Dec 04, 2007 4:51 pm

Anyway, this is a mini, so it will not be generating very much total power. This is a great idea for a small-bore gun(MINI) because the smaller the bore, the greater the pressure acting upon the projectile is needed to achieve a certain velocity. Now, I'll admit jimmy, that the TOTAL amount of energy is the same, is a 100" barrel really necessary? The higher pressure, despite the relatively lower volume, will transfer energy to the projectile at a faster rate, therefore allowing a smaller barrel and keeping true to being a mini.
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Unread postAuthor: Ragnarok » Tue Dec 04, 2007 11:42 pm

Although I can respect you are an educated person, I don't appreciate being told I'm a half wit. I have 10 GCSEs, three A-levels, I'm working hard for my degree in spite of illness and heavy depression, and last time my IQ was tested, it came out at above 150.

OK - into the discussion. I'm not quoting those GGDT figures as completely accurate, but there is a clear trend in the results, but as you asked me to do it, I'll run those integrals.

I'm going to make some assumptions in them though. I'm assuming flow is perfect with no losses, and also I'm assuming adiabatic conditions, but GasEq tells me the temperatures of both mixes are similar, so the losses are probably roughly the same. I will also assume that both burst discs are perfect, fracturing at the peak pressure of that mix.
Using GasEq again for the ratio of specific heats of a combusted mix, I get ~1.25 for both the 1x and 2x mixes.

To make it nice numbers to use, 100 cc chamber for the 1x mix, 50cc for the 2x mix, 1 metre barrel with 1 cm^2 cross sectional area (1.13cm dia)
Ok, it's past 4 in the morning, so I'll likely balls this up big time...

The volume increases linearly, so I'm handling that first.
If pV<sup>1.25</sup> is constant, then omitting the pressure term for the moment, the integral of volume is (V<sup>2.25</sup>/2.25)+C. As there are defined limits, the +C term is ignored in the calculations.

The integral of pressure can be found by dividing the pV<sup>1.25</sup> of the starting condition by the calculated integral of V<sup>1.25</sup> during firing, giving the numbers below:
1x mix has a pressure integral of 5.55 atm
2x mix has a pressure integral of 7.77 atm

For the barrel described, that's muzzle energies of 56J and 79J remembering to convert from atm to bar.
I may have left a horrible hole in my maths, but I think it's all right. It's nowhere near a doubling in energy, but the increase is a significant 40%.
Doubling again to a 4x mix, the integral is 9.35 atm, or in terms of energy 95J, a further increase of 20%, so the gains clearly taper off.

Assuming that's correct, those numbers are pretty convincing to me. I can't say much about the effects of using the same burst disc in all the situations, but I would assume that the higher mixes still had a slight edge.

To get in on the "torque" about combustion engines:
We know that these days, car engines do use higher compression ratios to get higher efficiencies, but as the maths I've just done shows, the gains are diminishing returns, so the increased work needed for compression will eventually eat the gains from the higher ignition pressure.

Also, the situation is slightly difficult. The extra compression does not just decrease "chamber" size, it also increases the volume of the "barrel", whereas here we are assuming the barrel is unaffected by the chamber compression. Actually, I went and did a nose in excel into the problem. There is a clear compression ratio where the engine is most efficient, in about the region that modern engines work at.
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Unread postAuthor: TwitchTheAussie » Fri Dec 07, 2007 9:20 pm

Question: How does a piston work in a hybrid?
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Unread postAuthor: psycix » Mon Dec 10, 2007 10:51 am

TwitchTheAussie wrote:Question: How does a piston work in a hybrid?

The suddenly created overpressure in the chamber caused by the combustion makes more pressure in front of the piston then behind it, wich moves it back.
Valve opened.
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