alt.energy.renewable

On Sun, 6 Apr 2008 17:47:54 -0400, "daestrom"
wrote:


>Okay, but then here is another suggestion. When people look at the graphs,
>they may subconciously assume that the work of compression (3-4 on the PV
>diagram) is a lot less than the work extracted by the expansion (1-2). I
>say this because of the 'scale' you have. I know this diagram is similar to
>those I've seen in text books, so I can't really *fault* it. The line 3-4
>looks like it is only 1/2 the pressure (or even less) than the line 1-2.
>Similarly on the T-S, it *looks* like line 3-4 is very low compared to line
>1-2. This would be valid if the hot side was some 450-500F but I suspect
>that isn't true.
>
>Since the area under these lines is a measure of work and heat, it *looks*
>like it takes very little work to compress and that you get a lot of work
>from the expansion. But if you were to draw it 'to scale', you'd find that
>the line 3-4 is much closer to line 1-2 and that the work to compress is
>actually a pretty high fraction of the work extracted.

It's funny that some 30 years ago we spent many weeks in lectures and
with our noses in books studying thermodynamics and often looking at
PV diagrams for heat engines. Then some months later we came to test
a real engine coupled to a test indicator and dynamometer. The graph
roughly had the idealised shape that all the text books illustrated -
by roughly I really mean very, very roughly, some might suspect that
technical text book authors had never ever tested a real engine :)


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