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Method for capturing combustion pressure peak

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Method for capturing combustion pressure peak

Unread postAuthor: Moonbogg » Tue Oct 06, 2009 2:25 am

Hey,
Sorry if this has been posted, but I found it while using "the google". Attach a check valve and a high pressure gage to a chamber and voila, combustion peak pressure becomes TRAPPED in the gage, assuming the gage can handle the fast spike.
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Re: Method for capturing combustion pressure peak

Unread postAuthor: jackssmirkingrevenge » Tue Oct 06, 2009 2:42 am

Moonbogg wrote:Attach a check valve and a high pressure gage to a chamber and voila, combustion peak pressure becomes TRAPPED in the gage, assuming the gage can handle the fast spike.


Since the pressure generated by combustion is a function of its high temperature, wouldn't the pressure go down instantly as the gas cool?
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Re: Method for capturing combustion pressure peak

Unread postAuthor: Moonbogg » Tue Oct 06, 2009 4:31 am

jackssmirkingrevenge wrote:
Moonbogg wrote:Attach a check valve and a high pressure gage to a chamber and voila, combustion peak pressure becomes TRAPPED in the gage, assuming the gage can handle the fast spike.


Since the pressure generated by combustion is a function of its high temperature, wouldn't the pressure go down instantly as the gas cool?


Oh BALLS! I think you are right. Maybe you could video tape it and see where it stops., but even then the needle might over swing due to acceleration and give a false reading. Maybe a digital gage that records peak pressure? Damn, I thought this would work. I want a way to actually measure it. I want DATA!
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Unread postAuthor: jmadden91 » Tue Oct 06, 2009 4:41 am

I have definitely seen the check valve + gauge system used by somebody on here to measure the peak pressure. I thought it was larda but i had a look and it doesnt seem so. But someone definitely has, Ill have a better look later
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 06, 2009 5:06 am

Why not fit a pencil gauge and attach an actual pencil to it so that it records maximum movement.
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Unread postAuthor: psycix » Tue Oct 06, 2009 5:18 am

Indeed. Igniting a mix in a closed chamber spikes up to a certain pressure, but drops back to near-atmospheric within a second.

Some readable material for those interested:
http://www.spudfiles.com/forums/closed- ... 15594.html
http://www.spudfiles.com/forums/more-te ... 13501.html
http://www.spudfiles.com/forums/studies ... 12807.html
http://www.spudfiles.com/forums/tempera ... 12849.html


EDIT: whoa you guys post fast.
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Unread postAuthor: Ragnarok » Tue Oct 06, 2009 7:34 am

jackssmirkingrevenge wrote:Why not fit a pencil gauge and attach an actual pencil to it so that it records maximum movement.

I wouldn't count on that working. What would probably happen is that the gauge's inertia would mean it would head past the peak pressure reading.

But how about this for a theory. Burst disks - fit a cannon with a burst disk that vents to atmo. Adjust until you find a thickness that bursts on about half of shots (or alternatively, the thinnest thickness that never bursts, and the thickest one that always does.)
Rig up the same dimensions of burst disks in some kind of test set-up, and pressurize with a pump until they burst. Record those pressures, and then you have your answer(s).

It'd be some work, but it'd get you the data.
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Unread postAuthor: Hubb » Tue Oct 06, 2009 7:35 am

Jimmy101 did it. Check here.

As far as how I would do it if I ever desired to do so - I would make a tiny chamber in front of the check valve and fill it with some type of oil. As the gases heat up, it will push the oil into the gauge and set the pressure. When the gases return to atmospheric pressure, the oil will still remain in the gauge until dumped. This should compensate for the needle bounce and should provide a much more accurate reading than using a pencil gauge.
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Unread postAuthor: drac » Tue Oct 06, 2009 7:43 am

If you're good with stoichiometry and chemistry, you could find the moles of air/propane and calculate volume before combustion versus after combustion. We did this in my chem class 2 semesters ago, but I can't remember it.
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Unread postAuthor: Moonbogg » Tue Oct 06, 2009 8:03 am

Hubb wrote:Jimmy101 did it. Check here.

As far as how I would do it if I ever desired to do so - I would make a tiny chamber in front of the check valve and fill it with some type of oil. As the gases heat up, it will push the oil into the gauge and set the pressure. When the gases return to atmospheric pressure, the oil will still remain in the gauge until dumped. This should compensate for the needle bounce and should provide a much more accurate reading than using a pencil gauge.


The oil may diesel won't it?
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Unread postAuthor: D_Hall » Tue Oct 06, 2009 8:31 am

It won't help in the immediate future but....

Vera will be fully instrumented.

That includes high accuracy pressure gages, high speed pressure gages(*) and strain gages.

I can record data at 100,000 samples per second.

Yes, I will have data.



(*) High accuracy and high speed are rarely the same thing. So yes, I'll have both.
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Unread postAuthor: Hubb » Tue Oct 06, 2009 8:36 am

The oil may diesel won't it?

Not sure. According to Jimmy's testing, he was only able to demonstrate around 100psi with the combustion. I'm not sure, but I wouldn't think that would be enough pressure to cause the oil to diesel (but it may combined with the heat).

I saw a diagram of this somewhere, but I don't know where. The way that builder had it set up probably wouldn't allow it to diesel. I'll see if I can find it, but I may have to end up drawing one out instead.
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Unread postAuthor: jimmy101 » Tue Oct 06, 2009 9:56 am

I wouldn't think the oil would diesel. The peak pressure is 120 PSIG or less which is too low to ignite diesel type fuels. A typical diesel engine is what, at least 15:1 compression, which would be ~225 PSI and an increase in density of 15x as well.

Heck, if the oil did diesel then just use water. The fairly small surface area of the water exposed to the heat probably wound't heat up at all.
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Unread postAuthor: psycix » Wed Oct 07, 2009 4:54 am

Yes, I will have data.

Does "having" equal "sharing" here?




Oil vapor in an environment with oxygen can diesel. Liquid oil won't.
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Unread postAuthor: jimmy101 » Wed Oct 07, 2009 10:28 am

The cheapo pencil type tire gauges, which are peak pressure recorders, have been used to monitor for lightning induced explosions in sealed off mines.

http://www.cdc.gov/niosh/mining/pubs/pdfs/tn490.pdf

The gauges are checked every few months to see if there has been an explosion.
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