[TheForge] The physics of a fire piston

[Douglas C Wilson]

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. 
>