Page 5 of 8
Posted: Wed Feb 10, 2010 10:25 pm
I was reading over this sticky as it is my first time building a hybrid, and I thought I'd like to point out what I think is missing information.
Regarding the chamber pressure method of measuring mixes, you would first calculate the correct amount of propane for say a 10x mix:

Vp = ((100/.958)-100)*10
Vp = 43.84 in^3

to find the pressure in the chamber after adding the propane, you figure out how much volume you are adding, and multiply that percentage by the pressure in the chamber (14.7 psi)

Pp = (43.84/100)*(14.7)
Pp = 6.44 psi

The next step is what is missing. From what I've seen it is assumed that you can now pump up the chamber to 132.3 psi (14.7*9), but this assumption neglects the volume of propane you just added. In order to get the correct mix, you must add in the pressure of the propane, thus upping the pressure to 138.7 psi. I know this is a minor error at small mixes, but with a 10x mix as above, that's over 6 psi.

Just thought I'd clarify, because I know this was bugging me, so it might have confused others as well

Posted: Mon Mar 08, 2010 7:16 pm
i've been planning on building a hybrid for some time and i've been studying on proper fuel then strength of the gun as these are the two most important things ie mainly - PROPER FUEL MIXTURE. in searching the web i've come across this wich i found useful because the write up at the top is awesome and top notch but im not math smart. so i hope this helps out some people as it has me.

Propane Limits of Flammability - The lower and upper limits of flammability are the percentages of propane that must be present in an propane/air mixture. This means that between 2.15 and 9.6% of the total propane/air mixture must be propane in order for it to be combustible. If the mixture is 2% propane and 98% air, there will not be combustion. If the mixture 10% propane and 90% air, combustion will not occur. Any percentage of propane in a propane/air mixture between 2.15% and 9.6% will be sufficient for propane to burn. However, an improper air/gas mixture can produce Carbon Monoxide (CO) that is a deadly product of incomplete combustion

Posted: Mon Mar 08, 2010 10:36 pm
Correction: strength of the gun is the most important aspect of a hybrid. Otherwise it is known as either a pipe bomb, or an accident waiting to happen. Any fuel ratio between the limits will work, but it will be less powerful, which sort of defeats the whole point of hybrids.

Posted: Mon Mar 08, 2010 11:20 pm
A bit of topic but I don't think it deserves a whole new thread.
What would be the best way to get the most accurate reading for a small chamber volume.. are the only 2 ways to measuere chamber volume water and sand or something similar? This is crucial beacause my chamber is very small and I need a precise number so I can get the mixes right...

Posted: Tue Mar 09, 2010 2:44 am
theBOOM wrote:A bit of topic but I don't think it deserves a whole new thread.
What would be the best way to get the most accurate reading for a small chamber volume.. are the only 2 ways to measuere chamber volume water and sand or something similar? This is crucial beacause my chamber is very small and I need a precise number so I can get the mixes right...
I have an idea. If it is THAT small, you could make sure it is good and dry, then weigh it. Then fill it with water and weight it again.

Posted: Tue Mar 09, 2010 6:10 am
10 ml Syringe, fill with water. Fill chamber from syringe, not how much is missing. Or use a larger syringe if it's bigger.

On these scales, it's a good idea to use a premix (such as from a venturi setup), then pressurize that. There's a how to use a shock pump for reliable 20x mixes in minis using this.

Posted: Tue Mar 09, 2010 5:11 pm
The only problem is my "main" chamber is a cross made of forged steel... I believe that if I measure the volume with water it might cause oxidation of the cross...I could paint it or something but that would just be too much trouble imo. Is there any way I could go around this without using water yet still getting an accurate result?

Posted: Tue Mar 09, 2010 5:13 pm
hmm... what about other liquids, like for example oil?

Posted: Tue Mar 09, 2010 8:32 pm
theBOOM wrote:The only problem is my "main" chamber is a cross made of forged steel... I believe that if I measure the volume with water it might cause oxidation of the cross...I could paint it or something but that would just be too much trouble imo. Is there any way I could go around this without using water yet still getting an accurate result?

Posted: Tue Mar 09, 2010 9:14 pm
Thanks Dr that should be easy to work out

Posted: Thu Mar 25, 2010 2:59 am
Sorry to correct the thread again , but there is one more thing I thought I should add: the proper ratio of air to propane is actually 15.6:1, or 93.9% air. In the first post it is listed as 95.8% air which is a bit lean (too much air). The ratio can be calculated using the method detailed on this site: http://www.brighthub.com/engineering/me ... 15235.aspx
and by knowing that C3H8 + 5 O2 → 3 CO2 + 4 H2O + heat.

Also does anyone have any tips for proper fuel MIXING? I've tried fueling using the chamber pressure method (no meter pipe) and have had little success with 2x mixes. I was thinking that maybe adding a venturi to the inlet, or adding the air first might help. Either that or my gauge isn't accurate enough at such low mixes....

Posted: Thu Mar 25, 2010 4:22 am
a fan or put something in the chamber and shake will mix fuel/air

Posted: Thu Mar 25, 2010 5:21 am
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Cras nec placerat erat. Vivamus dapibus egestas nunc, at eleifend neque. Suspendisse potenti. Sed dictum lacus eu nisl pretium vehicula. Ut faucibus hendrerit nisi. Integer ultricies orci eu ultrices malesuada. Fusce id mauris risus. Suspendisse finibus ligula et nisl rutrum efficitur. Vestibulum posuere erat pellentesque ornare venenatis. Integer commodo fermentum tortor in pharetra. Proin scelerisque consectetur posuere. Vestibulum molestie augue ac nibh feugiat scelerisque. Sed aliquet a nunc in mattis.

Posted: Thu Mar 25, 2010 3:28 pm
C<sub>3</sub>H<sub>8</sub> + 5O<sub>2</sub> ----> 3CO<sub>2</sub> + 4H<sub>2</sub>O

Meaning that there is five times as much oxygen as there is propane.

Take a sample of air. Its oxygen content is 21%. Imagine that as 21 parts. So, we need 21/5 parts propane, which amounts to 4.2 parts, or 4.2% of the initial air volume added in propane. After that, there are 104.2 parts present, 4.2 of which are propane, so the final mix is 4.2/104.2 = 4.03% propane. This is 95.97% air. Some confusion may arise because the injected propane has 4.2% the volume of the starting air volume, but the final mix consists of 4.03% propane. Either way, whatever method is detailed on that site is inaccurate.

As to fuel mixing, did you try with the proper mix, or the mix that the site told you to use? 6% propane would be more difficult to ignite than a stoichiometric mix. Small chamber dimensions or low gas flow will also contribute to the problem, as will poor ignition placement. With manometric metering, I'd recommend high flow for oxidizer addition after fuel is added, or using a nozzle to increase flow speed (I've not tested this method though).

Fans fare poorly in hybrids - the gases have sufficient density for heat transfer melting plastic parts to be a problem. Chains and such are only feasible for very small launchers, and increase surface area in the chamber for heat loss.

One doesn't need an incredibly accurate gauge for manometric metering to work; a +/- 1% 15psi gauge is a good choice for low mixes. It's easy to calculate the acceptable margin of error for a gauge of given pressure rating based on propane's flammable limits in your oxidizer.

Posted: Thu Mar 25, 2010 4:25 pm
I have used a nozzle in my piston hybrid. It works okay, but the ignition placement is terrible. It could be my mix, but bleeding the mix out through the fill valve usually corrects the issue.