Re: [Phys-L] irreversible quasistatic

[Carl Mungan <mungan@usna.edu>]

> Consider the following system:  A heat engine drives a shaft which
> is connected to a mechanical brake.  The brake is inside the system
> boundary.  The subsystem consisting of the heat engine is reversible,
> but the overall system is grossly irreversible.
>
> The PV "indicator diagram" for the heat engine is the same as it
> would be without the brake.  It's just not the whole story.
>
> Area on the indicator diagram has dimensions of energy, but there
> is more to physics than dimensional analysis.  For a reversible
> system, the PdV work must be delivered across the system boundary,
> because there is nowhere else for it to go.  As soon as you add
> in the irreversible brake, there are additional terms in the
> equations.
>
> Using a Joule-Thompson plug makes the braking action internal
> to the gas, but it's the same idea.

Thanks, I fully agree with your thinking. But for pedagogical reasons (despite the engineering impracticality) I want to focus *only* on the engine subsystem. For specificity, you may assume I’m thinking of an ideal gas that undergoes an Otto-type cycle (two isochoric and two thermally-insulated processes). Note that I am using “Otto-type” and not “Otto” because many people define the latter to mean reversible; likewise I am using “thermally-insulated” instead of “adiabatic” to avoid anyone thinking I mean “isentropic.”

Standard analysis would be to analyze this engine assuming it’s reversible. I want to know what happens if one or more of the four processes are irreversible; however, all four remain quasistatic so that we can draw well-defined PV or TS diagrams. For specificity, assume one of the isochoric processes is as I described in my previous email.

Now what do you say?

-----
Carl E. Mungan, Professor of Physics  410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363 
mailto:mungan@usna.edu     http://usna.edu/Users/physics/mungan/