[TheForge] The physics of a fire piston

[Daniel T. Hayes]

Bruce,

Boyle's Law (PV = constant) tells you nothing about temperature. As applied
to a Fire Piston, you can, however, use it to predict the pressure at the
end of the stroke. For example, if your piston starts out 1-7/8" from the
bottom and you ram it down to within 1/8"of the bottom, you will raise the
pressure 15-fold. P1 x V1 = P2 x V2, or alternatively P2 = P1 x (V1 / V2).
Therefore P2 = P1 x (1-7/8 / 1/8) = 15 x P1.

To predict the rise in temperature, you use Charles's Law (VT = constant),
not Boyle's Law. By Charles's Law T1 / V1 = T2 / V2 or T2 = T1 (V1 / V2).
Revisiting the above example, T2 = T2 x (1-7/8"/ 1/8") = 15 x T1.

The point of my prior post was simply that in using the gas laws (Boyle's
Law, Charles's Law, The Ideal Gas Law, and their various and sundry
derivations) you need to use absolute temperature.

The term "adiabatic" refers to an idealized process where there is no heat
loss or gain.

Dan


-----Original Message-----
From: theforge-bounces at mailman.qth.net
[mailto:theforge-bounces at mailman.qth.net] On Behalf Of Bruce Freeman
Sent: Friday, June 09, 2006 8:45 AM
To: theforge at mailman.qth.net
Subject: [TheForge] The physics of a fire piston

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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