Using Burnt Latke's fuel tool

Boom! The classic potato gun harnesses the combustion of flammable vapor. Show us your combustion spud gun and discuss fuels, ratios, safety, ignition systems, tools, and more.
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thejackal
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Mon Jun 29, 2020 8:17 pm

Greetings fellow spud chunkers, I've recently restarted a project (a golfballer similar to a BL-520 our friends in Hawaii used to make) after putting it on the shelf for two years. Most of the math I've become acquainted with, however, one input while using Burnt Latke's fuel tool to determine meter pipe length escapes me, fuel mixture (% by volume). Would someone be so kind as to explain the proper way to determine this value as minor changes in this number drastically changes the length of the meter pipe.
KannonKing
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Wed Jul 01, 2020 2:18 am

I think I came to the same conclusion about 10 years ago...

I assume you found this... it gives a good clue, but I still wasn't sure... I think he means volume of the propane (meter) is 4% and the volume of the air (chamber) is 100%
The pressure regulator proved to be the key element. A mid range base pressure of 37 PSI was established with the choice of meter volume. This combination gives a calculated 4% propane to air ratio by volume. The regulator is used to accurately adjust the mix up or down. When using Fuel Tool, we kept an eye on the required 41ml meter volume then adjusted the pressure until the desired 4% mix was reached. It took about 10 seconds and let us preflight the designs.
also
The flammable limit of propane is between 2.4% and 9.2% per volume when mixed with air. The ideal mixture is about 4.03%.
Note his default value for fuel air mix... 4.03%


http://www.burntlatke.com/ft_info.html
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Moonbogg
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Wed Jul 01, 2020 2:28 pm

I read spudblaster's hybrid fueling 101 which is stickied in the hybrid cannon forum. He told us how to do it, so I'll reiterate it here. I don't bother with any of this math myself because HGDT was thunk about so I don't have to do any thunking about fueling and I can just focus on my design. Here's what I noticed about what you said: it sounds like your meter pipe length is being driven by a fixed pressure. The meter pipe size should be fixed and the pressure is what should be allowed to change. In order to find the pressure, try this:

.042*chamber volume = propane volume required for meter pipe. Divide this number by the volume of the meter pipe and you get another number. Multiply that new number by 14.7 and that's the pressure your meter pipe should be at.

Example: 042*100 cubic inch chamber = 4.2 cubic inches of propane.
4.2/3 cubic inch meter pipe = 1.4 atmospheres
1.4*14.7 = 20.58 psi of propane in meter pipe.

This seems like a simple enough way to do it and it should work quite well.
KannonKing
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Wed Jul 01, 2020 3:04 pm

Correct, meter pipe volume needs to be approximate, with the fine tuning of moles of propane to moles of oxygen being done by adjusting pressure in the pipe. If you find that you can't get the right load, you can change the length of a nipple on the pipe to add/remove 1/2"+, much as Latke said here...

http://www.burntlatke.com/lp.html


Also...
Propane, Flammability and Combustion Requirements
In order for propane to burn, ignite or go through combustion, the criteria listed above must be met. Below are explanations of propane gas combustion characteristics.

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.
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