[TheForge] The physics of a fire piston

[Garrick Peterson]

Actually, that formula is not constant with a change in Volume. If the
volume decreases, then the pressure must increase in order to remain
equal to the P1*V1 side of the equation. P1*V1 is when the piston is
at it's extended position, and P2*V2 is when the piston is in the
compressed position. V1 != V2, therefor P1 != V2.

You can do some algebra (which doesn't translate well to text
documents) to prove to yourself that as you reduce V2 by any factor,
you're increasing P2 by the same factor.

As the pressure goes up, the temperature goes up. Conversely, if you
take a piston and enlarge the volume (and thus reduce the pressure),
you cool the air down (which is a part of how they make Liquid O2 and
other gasses, btw).

Been quite some time since my last physics class, so I can't comment
on the ideal gas law any further (don't remember how absolute pressure
works in this case).

~Garrick

On 6/9/06, Bruce Freeman <FREEMAB at pt.fdah.com> 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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