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Re: [Phys-L] Stirling engine efficiency



Okay, let's turn to considering other engines and forget about Carnot and
Stirling cycles for the moment. What is the efficiency of Otto, Brayton,
diesel, and other such cycles? Can you write them in terms of (Thot -
Tcold)/Thot? What can you say about their reversibility?

On Sun, Jul 31, 2016 at 12:28 AM, John Denker <jsd@av8n.com> wrote:

On 07/30/2016 07:09 PM, Carl Mungan wrote:

The issue is one of clearly defining the system. For example, consider a
Stirling cycle. The analysis of its efficiency *changes* depending on
whether the regenerator is included inside or outside of the system
boundary. If it is inside, the heat output during the isochoric
compression
is stored inside the system and re-input during the isochoric expansion.
It
thus does not count into Q_in or Q_out, whereas it does if the
regenerator
is outside the boundary.

I don't understand that, for multiple reasons. For starters, the
regenerator returns to its initial state at the completion of each
cycle, so it cannot possibly be a source or sink of energy or
entropy. It is not supposed to be connected to either reservoir.
So we don't care whether it's inside or outside the boundary of
"the system".

Also, let's focus on the /idea/ of efficiency. This has to do with
the amount of useful work you can obtain from a given amount of fuel.
That's been the idea since Day One (i.e. 1824, i.e. Réflexions sur
la puissance motrice du feu). Let's focus on that, rather than on
legalistic incantations about Qin and Qout.

The efficiency of an ideal Stirling engine is (Thot - Tcold) / Thot.
The laws of thermodynamics do not permit any greater efficiency.
There is nothing intrinsic to the Stirling cycle that limits it to
any lower efficiency.

From an engineering point of view, in a wide range of practical
situations, if you care about high efficiency, you can do better
with an almost-ideal Stirling-cycle engine than with an almost-
ideal Carnot-cycle engine. The Stirling engine is difficult to
build and difficult to analyze, but finesse and effort are rewarded.
In contrast, there's not much you can do to remedy the nonidealities
of a real-world engine that tries to follow a Carnot cycle.

Certain "modern PER-based" textbooks get this completely wrong. I
take responsibility for what *I* say; I am not responsible for what
other people say. Please do not shoot the messenger.

For details, see
https://www.av8n.com/physics/thermo/classical.html#sec-stirling

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Carl E. Mungan, Professor of Physics 410-293-6680 (O) -3729 (F)
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