[TheForge] The physics of a fire piston

[Demon Buddha]

The relevant relationship is Gay-Lussac:

	P1/T1 = P2/T2 or, equivalently T1/P1 = T2/P2

and is derived directly from the ideal gas law.

Rearranging, we get:

	T2 = (T1*P2)/P1

As pressure rises, temperature rises as well.  So, if we start at, say 
25*C (298*K, or 77*F) with 1 atm, and compress some volume by 15:1, we get:

	(298 * 15)/1 = 4470 = 4197*C = 7586*F

Oh, BTW, Dan Hayes was right and I was wrong.  I did the wrong 
conversion, which I discovered yesterday.  Actually, I got 4197*C, which 
= 7586*F.  I don't think that the inside of the cylinder is reaching 
that temperature, but I do wonder at how great the real losses are. 
Temperature, whatever it may be, isn't maintained for any appreciable 
time and I'm sure there are pressure losses around the piston.  I'd be 
curious to know just how much loss, though.

As was mentioned, the rise in temperature is called "adiabatic" in 
thermodynamics.  It describes the rise in T with no energy added to the 
system, i.e., T rises as the sole effect of compression.

Bruce Freeman wrote:
> I'm not following this discussion.
> 
> The ideal gas law is PV=nRT
> 
> where P=absolute pressure, V=gas volume, n=amount of gas (in
> gram-molecular weight, aka "moles"), R=the gas constant (e.g., 0.082
> liter-atmospheres/mole-degreeK), and T is degrees Kelvin.
> 
> However Boyle's law states that P1 x V1 = P2 x V2.   I.e., PV is
> constant with changes of volume.
> 
> Now if Boyle's law holds, then a fire piston would produce no
> temperature change at all.  What's going on in a fire piston is not
> encapsulated by the ideal gas law. 
> 
> So what's REALLY going on has little to do with the ideal gas law and
> instead relates to conservation of energy.  I.e., we do all that work
> shoving down on the piston, and that work is expressed as heat.  The
> trick is to convert mechanical work to heat and to localize that heat at
> the tinder.  
> 
> A fire drill is one way of doing that.  A fire piston is another. 
> Beating on a cold piece of steel between a hammer and anvil is a third. 
> Actually, flint and steel is a forth, but this is less obvious because
> what's going on there is that the steel is the primary tinder as well as
> the object upon which the flint does work.
> 
> In the case of a fire piston, we have one more advantage - the oxygen
> pressure goes up, making the tinder more likely to combust.
> 
> Bottom line:  Come up with a "fire piston" that you can hit with a big
> hammer, and it should be even more effective.
> 
> Bruce
> NJ
> 
> 
> 
>>>>"Daniel T. Hayes" <dhayes at dthayes.com> 6/8/2006 8:23:14 PM >>>
> 
> Actually, the "T" in the Ideal Gal Law is absolute temperature (i.e.
> Kelvin
> or Rankine). 70 Farenheit = 528.67 Rankine. Compressed adiabatically
> 15:1,
> the air temperature would go up to 7930 Rankine or 7471 Farenheit.
> 
> -----Original Message-----
> From: theforge-bounces at mailman.qth.net 
> [mailto:theforge-bounces at mailman.qth.net] On Behalf Of Demon Buddha
> Sent: Thursday, June 08, 2006 1:01 PM
> To: Sponsored by ABANA
> Subject: Re: [TheForge] Re: fire piston
> 
> nah...  if you compress air at 70*F to 1/15th its volume, the temp
> rises 
> to 1050*.  That should be enough to ignite tinder.  I know it sounds 
> odd, but the ideal gas law doesn't lie.
> 
> Freddie Warner wrote:
> 
>>You're right, Robert, the more I think of it the more I believe there
> 
> 
>>has to be some kind of vapor in the cylinder. You have to be able to
> 
> 
>>compress whatever ignites and fine wood shavings or dried grass won't
> 
> do 
> 
>>that. I bet they add something like some sort of vegitable oil or 
>>something that vaporizes.
>>
>>
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