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hybrid mix-air amount question

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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hybrid mix-air amount question

Unread postAuthor: JDP12 » Thu Sep 20, 2007 11:10 am

I get that for a 2x, 3x, et cetera you add 2x or whatever amount as much fuel as air. But I have a question. Say hybrid has two meters- one for air, one for fuel, usual setup.

For easy math purposes, let's set chamber volume to 100mL. You would need 4mL of fuel, propane for example. (Right now I am discussing a 1x mix) BUT, unless you created a vacuum inside your chamber prior to injecting the fuel, wouldn't there already be 100mL of air inside? This would correspond to about 20.95 mL of oxygen.

So then moving on to a 2x mix. You would inject 8mL of propane, BUT then wouldn't you only measure out another 100mL of air into your meter, then inject that 100mL into the chamber? Because there is already 100mL of air inside it?

So basically wouldn't you only have to INJECT FROM YOUR METER, 100mL of air for a 2x, 200mL for a 3x, 300mL for a 4x, and so on.

Does everyone get my question? I saw boilingleadbath's post on some topic about hybrids, and he said that for a 100mL chamber, you would have 4mL propane, and 96mL of air, but how is that possible? Isn't there 100mL of air inside it already, and you are adding 4mL of air? If I'm wrong, please let me know why and how to properly calculate amount of air.

Oh here is what boilingleadbath's post says.

As far as hybrids go, here is my attempt at explaining what is meant by the mix numbers:

Let's say you have a chamber with a volume of 100ml. The size does not matter, I just chose this volume because it makes the math easy.

Now, if you fill this chamber with 96 ml of air (measured at Standard Pressure and Temperature - STP) and 4 ml of propane (measured at STP), we find that the internal pressure will be about 1 bar absolute.
We would call this a 1x mix, because 1 bar is about equal to the pressure of the atmosphere (often, in english, we express we express this by saying "1 atmosphere of pressure").

However, if one where to fill the chamber with 192 ml of air and 8 ml of propane, the internal pressure would be about 2 bar.
Because 2 bar is about 2 atmospheres, we would call this a 2x mix.

A 3x mix would be 288 ml of air and 12 ml of propane; a 4x mix 384/16... and so on.
*********************************************

I am not sure how most users vent their ("clear the air") hybrids, but I know that some just inject clean air into the chamber until they have displaced most of the combustion products.
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Unread postAuthor: silverdooty » Thu Sep 20, 2007 11:38 am

From the wiki
A hybrid is first injected with fuel, then with pressurized air. The amount of air depends on how much fuel was added, and is calculated to achieve a stoichiometric fuel/air ratio in the chamber. A hybrid using twice as much fuel and air as a comparable combustion gun is said to be using a 2X mix, higher mixtures can be used and will produce even higher pressures. The fuel and air needs to be measured and matched carefully to ensure reliable operation, pressure gauges and fuel meters are used for this.


my basic understanding is you can't burn more fuel without more air. so having 3x more fuel than air/oxygen is not going to work.
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Unread postAuthor: dongfang » Thu Sep 20, 2007 1:12 pm

Hi,

For easy math purposes, let's set chamber volume to 100mL. You would need 4mL of fuel, propane for example. (Right now I am discussing a 1x mix) BUT, unless you created a vacuum inside your chamber prior to injecting the fuel, wouldn't there already be 100mL of air inside? This would correspond to about 20.95 mL of oxygen.


Quite right. In fact, if you take a 100 mL chamber at atmospheric pressure, and add 4 mL of some gas, then ... hmm well, it depends on the pressure of those 4mL. If you open a valve between a chamber with 100 mL or air and a chamber of 4 mL of fuel gas, both at the same pressure, then after some time, you have 104 mL of mixture.
If the pressure in the tow chambers is not the same before valve opening, well, then the pressure will rise in one and drop in the other....

Maybe you really wanted a 4:96 mixture, not a 4:100 mixture. Or was it the other way around...?

It soon gets complicated. I think, after having gotten stuck in the math, most spudders just make a general estimate, like needing 10 shots of fuel for 3x, and then try with a little more and a little less till the best result is seen :)

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Soren
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Unread postAuthor: Pete Zaria » Thu Sep 20, 2007 1:15 pm

@ilovetoblowthingsup,

You're right, when calculating in hybrid mixtures, you have to factor in the air already in the chamber, so your first hypothesis was right - for a 2x mix you'd inject 100ml of air and 8ml of propane - because there's already 100ml of air in the chamber.
No air is added for a 1x mixture, 1 atmosphere of air is already present in the chamber. With a non-sealed chamber (as in most combustion guns) it is assumed that when you inject the propane, it will raise the internal pressure of the chamber slightly, and some air (and possibly a teeny bit of propane, too) will leak out. For hybrids with sealed chambers this is not an issue.

Yes, venting is usually done by purging the chamber with fresh air for long enough to assure all the exhaust gases from the last shot have been vented.

I'm not sure I exactly understand your question.

@silverdooty,
Every flammable gas has an upper and lower explosive limit (UEL and LEL). Propane's explosive limits are 2.37–9.5%. So, if there's less than 2.4% propane, or more than 9.5% propane, it won't ignite, regardless of pressure. Make sense? There has to be AT LEAST 90% air in the mixture, or there's not enough oxygen to support combustion.
This situation changes dramatically when you start to add in oxidizers such as pure oxygen or nitrous oxide.

But this UEL and LEL are why it's impossible to put too much fuel in a normal combustion gun - it won't ignite. In a hybrid, the general idea is to put in 2x or 3x more fuel, and then balance out the mix by adding an appropriate amount of air to bring the mixture back to about 4% propane (supposedly 4.2% propane is the ideal mixture).

Peace,
Pete Zaria.
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Unread postAuthor: JDP12 » Thu Sep 20, 2007 4:07 pm

yea Pete, my question was if you needed to account for the air already in chamber, thanks, just wanted to check
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