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HYBRID FUELING 101

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: randompkguy » Fri Mar 26, 2010 4:07 pm

Thanks for clearing that up. So I guess the method detailed on that other site gives you mass percent, which is useless in this case since we are measuring by volume. (stupid mistake on my part)
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Unread postAuthor: Gun Freak » Thu Apr 08, 2010 7:24 pm

I am planning on building a hybrid in the future, just have one quesiton. When you say like a 5x or 10x mix, that means 5 or 10 times the amount of air as fuel right?
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Unread postAuthor: Heimo » Thu Apr 08, 2010 7:38 pm

Gun Freak wrote:I am planning on building a hybrid in the future, just have one quesiton. When you say like a 5x or 10x mix, that means 5 or 10 times the amount of air as fuel right?


it would be 5X or 10X the fuel that would go in a normal combustion along with the correct amount of air to burn all that fuel
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Unread postAuthor: Gun Freak » Thu Apr 08, 2010 7:43 pm

Oh so the mix isn't proportional to the amount of air and fuel in the chamber?
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Unread postAuthor: Heimo » Thu Apr 08, 2010 8:39 pm

let me put it a different way... I read somewhere that the correct amount of propane in a combustion is 4% of chamber volume IIRC. 4% propane in a chamber would make for a 1X mix say you put 8% in the chamber that would make a 2X mix and along with the extra propane you put in extra air to allow for correct combustion, so say say you want a 2X mix then you would put in 8% propane and pressurize the chamber to 2X normal atmospheric pressure

anyone who knows more about the subject please correct me if I am wrong anywhere
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Unread postAuthor: SpudBlaster15 » Thu Apr 08, 2010 9:30 pm

Gun Freak wrote:I am planning on building a hybrid in the future, just have one quesiton. When you say like a 5x or 10x mix, that means 5 or 10 times the amount of air as fuel right?


No, the ratio of air to fuel remains constant, regardless of how much you pressurize the chamber.

Imagine taking 100 cubic inches of stoichiometric fuel mixture at atmospheric pressure, and compressing it to 1/5 of that volume. You now have a 20ci 5X hybrid mixture. The ratio of air to fuel remains the same, but you're packing a relatively large amount of it into a smaller space.
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Unread postAuthor: SpudFarm » Fri Apr 09, 2010 5:42 am

Heimo wrote:let me put it a different way... I read somewhere that the correct amount of propane in a combustion is 4% of chamber volume IIRC. 4% propane in a chamber would make for a 1X mix say you put 8% in the chamber that would make a 2X mix and along with the extra propane you put in extra air to allow for correct combustion, so say say you want a 2X mix then you would put in 8% propane and pressurize the chamber to 2X normal atmospheric pressure

anyone who knows more about the subject please correct me if I am wrong anywhere


It is much easier to use bars when talking about hybrid fueling IMO. 1bar=14.7psi if I remember correctly.

Lets say the perfect ratio of fuel/air is 4:96 as above, for a 10x mix you inject 40% of the chamber volume of propane into the chamber and 9 bars of air (not 10 since there are one atmosphere already) and ignite the mix.
It really is that simple.

Manometric metering is the way to go. Then it is as simple as opening the propane valve to 0.4bar and then the air valve to 9 bars and fire for a 10x mix.

Hope it is understandable, otherwise tell me.
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Unread postAuthor: Gun Freak » Fri Apr 09, 2010 4:06 pm

SpudFarm wrote:
Heimo wrote:let me put it a different way... I read somewhere that the correct amount of propane in a combustion is 4% of chamber volume IIRC. 4% propane in a chamber would make for a 1X mix say you put 8% in the chamber that would make a 2X mix and along with the extra propane you put in extra air to allow for correct combustion, so say say you want a 2X mix then you would put in 8% propane and pressurize the chamber to 2X normal atmospheric pressure

anyone who knows more about the subject please correct me if I am wrong anywhere


It is much easier to use bars when talking about hybrid fueling IMO. 1bar=14.7psi if I remember correctly.

Lets say the perfect ratio of fuel/air is 4:96 as above, for a 10x mix you inject 40% of the chamber volume of propane into the chamber and 9 bars of air (not 10 since there are one atmosphere already) and ignite the mix.
It really is that simple.

Manometric metering is the way to go. Then it is as simple as opening the propane valve to 0.4bar and then the air valve to 9 bars and fire for a 10x mix.

Hope it is understandable, otherwise tell me.


Yes I think understand... you mean 4% though right? you said 40. I don't want to blow myself up, but I don't want to be critical either :P
So the "X" mix is related to the amount of fuel you inject to the chamber. But say for a 10x mix, the air pressure will always be 9 times the atmospheric pressure, so 9 bars in a 100 cu (is it cu or ci?) chamber?
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Unread postAuthor: SpudFarm » Fri Apr 09, 2010 4:50 pm

1x: 4.2%=0.042bar of propane and no extra air.
2x: 8.4%=0.084bar of propane and 1bar of air.
3x: 12.6%=0.126bar of propane and 2bar of air.
see the pattern?
so the 40% (since I used a 4% mix) came from me using a 10x mix as an example, like this:
10x: 42%=0.42bar of propane and 9bar of air.

you should start to see a pattern now. It is a piece of cake once you just get it.
Remember this is the manometric metering method wich measure the chamber pressure instead of measuring the pressure in a small pipe.
This way metering is simpler and faster. Depending on your design it can be cheaper to.
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Unread postAuthor: MrCrowley » Fri Apr 09, 2010 5:13 pm

Yes I think understand... you mean 4% though right? you said 40.

He said 40% of the chamber volume is injected with fuel (for a 10x mix), then 9 bars of air which brings down the fuel:air percentage to 4.2% fuel and 95.8% air.

It really is pretty simple to understand, the hard part is getting the cannon to work. BTW, the fuel is measured in the meter but the air is injected straight in to the chamber.

Go search through a couple dozen (yes, dozen) topics in the hybrid discussion and showcase sections. It is good to learn things for yourself, after you have done that i'm sure you'll have about 10 questions, start a topic, list the questions and we can help. It is a bit unorganised if you hijack threads and ask them there.
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Unread postAuthor: Gun Freak » Fri Apr 09, 2010 5:17 pm

Oh sorry. Didn't think it was hijacking if I was asking a question on it, but ok.
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Unread postAuthor: MrCrowley » Fri Apr 09, 2010 5:19 pm

Well you're asking less about the actual fueling methods and calculations and more about how to fuel a hybrid and the basics of a hybrid.
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Unread postAuthor: saefroch » Thu Apr 15, 2010 10:11 am

I know that this is a quite old thread, but being a sticky I hope I'll be okay posting on this. I did some stoichiometry by myself and came up with a mix for fueling a hybrid; pressurize with oxygen until meter reads 100 psi, then pressurize with propane until meter reaches 122.5 psi.

Is this accurate? I'm pretty sure I corrected for the original chamber contents- 21% oxygen, 79% useless gases.

Also just another question, very random, and I hope that it isn't hijacking the thread, but has anyone ever put so much pressure into a hybrid that it ignites by itself like a diesel engine?
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Unread postAuthor: POLAND_SPUD » Tue Nov 30, 2010 2:43 pm

I don't have a gauge in 0-1bar range and I am not really into ordering one now....
but I realized that old analogue type blood pressure units have a gauge.. It took me a while to find mine but I think I might have found something useful

The one I have (and most of them) is in 0-300 mmHg range (0-5.8 psi). That seems almost ideal for low mix manometric fuelled hybrids, don't you think ?


Sure I know it isn't anything special, it's not that you can't get good gauges in that range...
However, analogue blood pressure gauges are quite cheap -> http://cgi.ebay.com/BLOOD-PRESSURE-GAUG ... 35ae422c21

and

http://cgi.ebay.com/NEW-BLOOD-PRESSURE- ... 53e4f61347

EDIT
ohh and here you can convert mmHg -> psi
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Unread postAuthor: DYI » Tue Nov 30, 2010 4:43 pm

@saefroch:
That's not quite a valid mix. Assuming stoichiometric oxy/propane, 122.5psi pre-ignition would be a 33.4X mix. However, 100psi oxygen (plus the air already in the chamber) indicates a 33.9X mix. It's close though. It would certainly ignite, and the issue with your calculations could have easily been a rounding error. Also, you'll find that those "useless" gases improve performance considerably, and avert potentially disastrous detonations. :wink:

As to your other question: You mean filling with some arbitrary mix, then compressing with a piston or similar to cause heating and ignition? No, I don't think that's ever been done. It's normally more practical to simply use a higher mix, unless you can't with whatever fuel you're using (for example, the 200X "limit" with propane).

@POLAND: Although manometric meters are nice, they're not the most practical with lower mixes, especially if you're using a small chamber.
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