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Propane fueling question

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Propane fueling question

Unread postAuthor: Combustion Monkey » Fri Jan 30, 2009 4:45 pm

I was doing some testing today with odd results. The day started off with shooting the cannon with golf balls and spuds. Testing went well and the cannon fired flawlessly. The cannon started off at around 65 deg f. As time went on the propane tank cooled to the point were it would not put out 60 psi (very cold here). Firing stoped and the cannon was put into a very warm truck with the heater running for a half hour or so. When testing resumed, I would guess the tank was around 80 deg f. The gun ran like crap. misfires, low power fires, you name it, it happened. No amount of messing with pressure up or down solved the problem. The question is. Does the volume of propane change as temperature changes even though the pressure in the meeter tube remains constant? And are there other variables due to temperature change that I am not accounting for?

EDIT: changed title of topic
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Last edited by Combustion Monkey on Fri Jan 30, 2009 6:39 pm, edited 1 time in total.
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Unread postAuthor: sputnick » Fri Jan 30, 2009 6:15 pm

Well if the pressure is constant in the same volume, then there is no way the amount has changed, that's not right... Maybe it was that the propane has a harder time igniting at the lower temperatures of your gun, it was sitting out for a half hour, and you say it is very cold there... that would be my guess, but hey, I am certainly no expert on combustion guns.
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Unread postAuthor: Combustion Monkey » Fri Jan 30, 2009 6:28 pm

What struck me as odd was that the gun ran worse when it was WARMER.
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Unread postAuthor: chenslee » Mon Feb 02, 2009 8:54 am

PV=nRT.

P=pressure
V=volume
n=number of molecules of fuel in moles.
R=gas constant
T=temperature.

If the pressure and volume are constant and the temperature of your gas goes down, the equation is only able to balance itself by increasing the number of molecules of fuel.

I.e, when the temperature drops, the density of the fuel increases, and you end up running rich.
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Unread postAuthor: Combustion Monkey » Mon Feb 02, 2009 3:32 pm

chenslee wrote:
If the pressure and volume are constant and the temperature of your gas goes down, the equation is only able to balance itself by increasing the number of molecules of fuel.

I.e, when the temperature drops, the density of the fuel increases, and you end up running rich.


Thank you very much. I was guessing it was something like that, but I don't have the science background to figure that sort of thing out. Is there a way to calculate the change in pressure for say every ten deg of temperature change? It would save a lot of guess work and frustration. We get 10-20 deg temperature changes in the afternoons here fairly regularly and i've seen as much as 60 deg changes over the course of a day.
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Unread postAuthor: starman » Mon Feb 02, 2009 3:51 pm

It's common problem with propane fueling. However, a decent regulator should compensate for it. When it's warm the regulator will be turned down further. When it's cooler, you may have to turn it all the way up just to get just to get 60 psi. If it's really cold you won't get even that much out of it. That's one reason to use higher volume/lower pressure meters, especially in colder weather.

I kept my meter and propane/regulator in an insulated ice chest (at roughly room temp) while shooting outside during New Years Eve. Also there might be some coozies that would fit a propane tank...might be worth a try as long as the tank is at a good temp already.
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Unread postAuthor: Combustion Monkey » Wed Feb 04, 2009 4:25 am

I am looking for an insulated camping water bottle holder that will work for me. I got one of my stuck regulators working so I at least I had pressure control. And fortunately I built the meeter to run at 55-65 psi so i'd have some time as the tanks cool down from temp( it was 27deg yesterday morning, warmest in weeks!) I need to make a fueling cheat sheet for the temperature fluctuations and stick a magnetic thermometer on the tank. Oh and just for reference this year out at a New Years Eve bonfire I had a bottle of beer freeze in my gloved hand and start slushing out the top!
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Unread postAuthor: Fnord » Wed Feb 04, 2009 10:25 am

A question to everyone:

Maybe I'm totally wrong about this, but here is my idea.

Wouldn't it be easier to have a meter volume that needs to be run (at 70F) at ~115 psi for a stiochiometric mix? Wouldn't that give a constant amount of propane and forgo the need for a regulator, regardless of temperature?
On the first shot in a very cold environment you'd be running lean due to cold air, but the residual heat from the chamber would counteract that quickly.
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Unread postAuthor: mark.f » Wed Feb 04, 2009 10:42 am

That's the principle behind the default meter setup on burntlatke's site, but I also like control, so I use a regulator.
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Unread postAuthor: starman » Wed Feb 04, 2009 11:18 am

_Fnord wrote:Wouldn't it be easier to have a meter volume that needs to be run (at 70F) at ~115 psi for a stiochiometric mix? Wouldn't that give a constant amount of propane and forgo the need for a regulator, regardless of temperature?
On the first shot in a very cold environment you'd be running lean due to cold air, but the residual heat from the chamber would counteract that quickly.


Since you'll be drawing cold fresh air into your chamber, I don't see any real heating advantage there. The propane tank and meter is still cold. I've found high pressure/low volume meters get to where they just won't work at all in cold enough environment. The only way to compensate is double up on the meter shots and use 1/2 pressure each.

In any case, a regulator is a requirement to be able to meet these demands in any consistant way.
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Unread postAuthor: Combustion Monkey » Wed Feb 04, 2009 3:48 pm

starman wrote: Since you'll be drawing cold fresh air into your chamber, I don't see any real heating advantage there. The propane tank and meter is still cold.


I was thinking that one of the reasons the gun may have been firing spotty after it was brought up to 80deg was the difference in temperature. Would the gun being at 80deg and the fuel being at around 30deg would cause the fuel to not mix correctly even with a fan? With the 30 deg fuel being heavier than at room temp perhaps my fan just blew the charge to the back of the chamber and away from the spark. Any of you science guys have a theory one way or the other on that?
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Unread postAuthor: trigun » Wed Feb 04, 2009 4:32 pm

Well propane is heavier than air.

So when you inject proane in any chamber it technically sinks to the bottom. The fan would begin circulating inside and move the propane and air together spreading said propane in the chamber.
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Unread postAuthor: chenslee » Thu Feb 05, 2009 2:38 pm

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Unread postAuthor: Radiation » Thu Feb 05, 2009 3:00 pm

trigun wrote:Well propane is heavier than air.

So when you inject proane in any chamber it technically sinks to the bottom. The fan would begin circulating inside and move the propane and air together spreading said propane in the chamber.


Uh... No. Propane will not "sink to the bottom" it will disperse evenly over the course of several minutes possibly hours. The molecular weight doesn't matter with gases as they tend towards randomness. A fan simply speeds up this process.
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Unread postAuthor: chenslee » Thu Feb 05, 2009 3:17 pm

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