[TheForge] The physics of a fire piston

[Bruce Freeman]

Of course the ideal gas law applies (to the extent that air approximates
being an ideal gas).  That's not the issue.

The issue is whether the ideal gas law >explains< the temperature rise
in a fire piston.  I assert it cannot.  The very premise by which some
folks were arguing that it >does< actually shows that it cannot.

To whit:  Initial conditions of pressure, volume and temperature:  P1,
V1 & T1.  Final conditions:  P2, V2 & T2.

Ideal gas law:  PV=nRT  (where n is amount, a constant, and R is the
ideal gas constant)

Rewrite this as PV/T = nR and clearly PV/T is a constant for a system
that does not gain or lose mass.

Hence P1*V1/T1 = P2*V2/T2

Now if, as suggested, P1=1 atm and V1 is (for instance) 15 cc, and
P2=15 atm and V2 = 1 cc, then we have

(1 atm *15 cc)/T1 = (15 atm * 1 cc)/T2

or T1=T2 for no change in temperature.

Now the power of the ideal gas law is such that it will remain
(approximately) correct even if there IS a change in temperature.  But
the ideal gas law doesn't >explain< that temperature change.

We can step aside from "ideal gases" for a moment and consider
non-ideal gases, like hydrogen.  Open the valve on a hydrogen tank and
you risk blowing yourself up.  Hydrogen HEATS UP on rapid decompression.
 This is not accounted for by any ideal gas relationship, only by
intermolecular interactions irrelevant to ideal gas relationships.  Now,
this is an unusual property of hydrogen, and I don't mean it as
representative.  I give it only as an example of how the ideal gas law
>does not< explain all gas behavior.  Another gas might heat up on
compression.

In case someone is coming in late to this discussion, I assert that the
heat produced by a fire piston is from the work done upon it - a direct
analogy to frictional heat of a fire drill.


Bruce
NJ

>>> dcwilson at mindspring.com 6/9/2006 10:09:41 PM >>>
Bruce,

Apply your experience and chemical intuition. You know that on 
compression, temperature rises which is how diesel engines function.
You 
also know that to a first approximation the ideal gas law *does* apply

to all gases. This suggests that there is a flaw in your argument. What

does not apply in the fire piston example is Boyle's law. Boyle's law 
only applies at constant T and n.

Regard,
Doug

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. 
>   

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