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Hybrid fuel mixing and cold weather

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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Hybrid fuel mixing and cold weather

Unread postAuthor: PeteS » Sun Dec 08, 2013 11:38 am

Ignition seemed a little flaky in my tests this morning. It was a moderately cold 28 F. It always fired but sometimes took a few button pushes over a 30-40 second period. I could hear that it was arcing in the chamber with each button push, so I assume that it was a fuel/air mixing issue. I guess something to act as an agitator in the chamber is the solution since fans do not seem like the answer for hybrids.

This morning's shoot raised some questions that I am hoping someone might be able to comment on:
1. Does just waiting with no agitation before ignition cause better or worse mixing? I would have guessed worse, but results today seemed to indicate better. I could shake the gun, tip it end for end numerous times and it would still act up a bit and after setting still for a while it would work.
2. Once mixed how quickly does propane settle back out and pool in the bottom of the chamber if at all?
3. How much and in what way will cold temperatures affect both mixing and performance.
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My wife upon seeing my latest hybrid and hearing an explanation of it:
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Re: Hybrid fuel mixing and cold weather

Unread postAuthor: jackssmirkingrevenge » Sun Dec 08, 2013 12:45 pm

PeteS wrote:1. Does just waiting with no agitation before ignition cause better or worse mixing? I would have guessed worse, but results today seemed to indicate better. I could shake the gun, tip it end for end numerous times and it would still act up a bit and after setting still for a while it would work.


Waiting makes no difference. As you're pressurising the mix, agitation is not an issue either.

2. Once mixed how quickly does propane settle back out and pool in the bottom of the chamber if at all?


It won't - Brownian motion trumps gravity

Image

3. How much and in what way will cold temperatures affect both mixing and performance.


Cold gasses are denser, so injecting fuel by syringe at low temperature means that you are probably injecting a greater proportion of fuel. If this is the case your mixture could have been a bit rich, try injecting slightly less fuel or pressurising slightly more.
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Re: Hybrid fuel mixing and cold weather

Unread postAuthor: PeteS » Sun Dec 08, 2013 1:13 pm

That makes sense. I bet the mix was a bit on the rich side. Thanks.
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My wife upon seeing my latest hybrid and hearing an explanation of it:
"That really isn't a potato cannon anymore, is it?"
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