[TheForge] The physics of a fire piston

Sat Jun 10 00:49:35 EDT 2006

What's interesting, is that the Aborigines have known this formula also, 
maybe not quite in these same terms.

----- Original Message ----- 
From: "Daniel T. Hayes" <dhayes at dthayes.com>
To: "'Sponsored by ABANA'" <theforge at mailman.qth.net>
Sent: Friday, June 09, 2006 8:36 PM
Subject: RE: [TheForge] The physics of a fire piston

> 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.
>>
>>
>> _______________________________________________
>> Manage membership or unsubscribe at:
>> http://mailman.qth.net/mailman/listinfo/theforge
>> theforge mail list group photo site is
>> http://www.photoaccess.com
>> Login:  blacksmithblacksmith at hotmail.com
>> password:  anvil
>> ___________
>>
>>
>>
> _______________________________________________
> Manage membership or unsubscribe at:
> http://mailman.qth.net/mailman/listinfo/theforge
> theforge mail list group photo site is
> http://www.photoaccess.com
> Login:  blacksmithblacksmith at hotmail.com
> password:  anvil
> ___________
>
>
>
> _______________________________________________
> Manage membership or unsubscribe at:
> http://mailman.qth.net/mailman/listinfo/theforge
> theforge mail list group photo site is
> http://www.photoaccess.com
> Login:  blacksmithblacksmith at hotmail.com
> password:  anvil
> ___________
>
>
> _______________________________________________
> Manage membership or unsubscribe at:
> http://mailman.qth.net/mailman/listinfo/theforge
> theforge mail list group photo site is
> http://www.photoaccess.com
> Login:  blacksmithblacksmith at hotmail.com
> password:  anvil
> ___________
>
>
>
> _______________________________________________
> Manage membership or unsubscribe at:
> http://mailman.qth.net/mailman/listinfo/theforge
> theforge mail list group photo site is
> http://www.photoaccess.com
> Login:  blacksmithblacksmith at hotmail.com
> password:  anvil
> ___________
>
>
> 