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Old 11-27-2006, 10:09 AM
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Question for your thermodynamicists out there.

Given 1 cubic inch of atmosphere and gasoline at the ideal mixuture, how much will it expand when burned? Or to put it another way, how many cubic inches would 1 cid expand to when burned, to achieve an equilibrium with the atmospheric pressure or 0 psi at sea level? If needed, assume a compression ratio of 10.00:1.

After the initial burn, what would the pressure be, before any expansion of the combustion chamber is allowed? If it's different, what is max combustion chamber pressure?
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Old 11-27-2006, 01:36 PM
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You need to clarify your question, there were numerous errors that make me question what you were interested in.

Sea level air pressure is 1 atmosphere, not 0 psi (which is a vacuum).
Equal parts air and fuel are not an ideal mixture for burning

Your question's answer is related to the formula pv=mrt (where m is a density factor)
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Old 11-27-2006, 02:11 PM
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I think what he's asking would be:

At sea level atmospheric pressure, when mixed at stoichiometric (14.7 parts air to 1 part fuel), how much would 1 cubic inch of air/fuel expand when burned?
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Old 11-27-2006, 05:50 PM
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no help, just wanna watch.
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Old 11-27-2006, 09:19 PM
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It's a non-trivial solution so far as I'm concerned.

A couple of things to consider. The stroke of the engine is important, that will limit the change in volume of the flue gasses and affect their tempreture.

I don't have my combustion engine manuals with me anymore, so I'm afraid I'm out of this one.
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Old 11-27-2006, 09:53 PM
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Well, thanks for the input anyway.


Don't let the stroke hang you up. The stroke is whatever length required to allow the pressure to drop to atmospheric.


Rusty, yes, you state it in simpler terms. Heck, take the engine out of it completely. How much would 1 ci compressed to ten times atmosphere expand when burned.

Last edited by 76supercab2; 11-27-2006 at 09:55 PM.
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Old 11-27-2006, 11:22 PM
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You have at least a couple more things undefined, with the most important being the temperature of the exhaust gases at the end.
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Old 11-28-2006, 12:28 AM
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He has also not defined the heat losses thru the combustion chamber walls and piston over time. The pressure at the bottom of the stroke is not anywhere near 1 atmosphere.
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Old 11-28-2006, 01:20 AM
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He wants to know what what hypothetical stroke length it would require to get the pressure to 1 atmosphere. though the question has been reworded a few times.

Shall we assume engine is operating temperature and standard EGT (no idea what that would be... guessing 700 - 1000F) and typical heat losses of an IC engine?

Gas cools rapidly as it expands. That's all I know.
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Old 11-28-2006, 06:08 AM
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Quote:
Originally Posted by aurgathor
You have at least a couple more things undefined, with the most important being the temperature of the exhaust gases at the end.
I would think that if the gasses expanded to the point that the pressure was atmospheric, the temperature would be similar to ambiant.
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Old 11-28-2006, 06:10 AM
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Quote:
Originally Posted by Torque1st
He has also not defined the heat losses thru the combustion chamber walls and piston over time.
Ignore losses

Quote:
Originally Posted by Torque1st
The pressure at the bottom of the stroke is not anywhere near 1 atmosphere.
Yes. Not part of this question.
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Old 11-28-2006, 06:21 AM
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Quote:
Originally Posted by furball69
He wants to know what what hypothetical stroke length it would require to get the pressure to 1 atmosphere. though the question has been reworded a few times.

Shall we assume engine is operating temperature and standard EGT (no idea what that would be... guessing 700 - 1000F) and typical heat losses of an IC engine?

Gas cools rapidly as it expands. That's all I know.

Assume standard combustion temperatures. Ignore engine operating temperature. I'm guessing that when it expands to the point that the pressure is equal to atmospheric, the temp will be around ambient.

Heck don't even put it in an engine. Have it in atmosphere for all I care.
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Old 11-28-2006, 06:24 AM
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Tell you what, if I've left anything else out, make an assumption, state what the assumption is, substitute some values for that assumption and post the results.
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Old 11-28-2006, 03:10 PM
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Hmm, my head hurts now.

Say you have your 14.6/1 ratio fuel mixture in a cylinder, basically a b0m-b, but one that is strong enough to contain the energy from the perfect fuel mixture, at atmospheric pressure. Ignite it, the fuel uses up the oxygen and converts to heat, increases the pressure and everything eventually cools back down to ambient temp. I'm thinking there'll be a slight vacuum because the volume/mass of fuel and oxygen that was expended and has been taken out of the cylinder.
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Old 11-28-2006, 04:20 PM
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If the mixture in the chamber is not compressed before ignition, then yes. However, if you compress the mixture first, you're putting energy into it and the temp will be higher than ambient. Ignite it and the temp is MUCH higher than ambient so it should occupy a much larger than original volume.

So if you took 10 ci and compressed it to 1 ci, ignite it and the temp goes to 1000 degrees F (pulling numbers from my sphinxter) and say ambient was 100 degrees F. Allowing the volume of the cylinder to change to allow the gas to expand as far as it could, what would the max volume of the cylinder be? I guess there would be some residual higer temperature when the cylinder is at max volume, because if the gas cooled completely back to 100 degrees, the volume should be the original 10 ci. But at max volume it seems the pressure internal to the cylinder should be back to atmospheric, just at a higher temp than ambient.


On edit, Furball, I reread your post and no, I don't think there would be a residual vacuum in your example. If your container does not change shape then upon ignition, the temperature and pressure rise, then as the temperature bleeds off, the pressure will drop until temp = ambient. Then the pressure inside the container should be the same as it was before ignition. Because nothing left the container. The chemical composition changed but all the atoms that were in there to begin with are still there. Unless H20 and CO2 occupy less space than HC and O2 (I'm simplifying here in the last sentence).

Last edited by 76supercab2; 11-28-2006 at 04:25 PM.
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