matt brown wrote: ↑Sun Apr 14, 2024 8:16 pm
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...I assume your current PV beef is that PV plots disregard load vs no load conditions.
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My "beef" is, basically;
Carnot based his assumption that heat goes THROUGH a heat engine in tact, to power the engine just like water through a turbine, (that is, there is no material change in the "substance" of either the water or the "heat", both being some kind of fluidic substance), on Caloric theory.
Joule later proved experimentally that heat is a form of energy. "Heat" being one form of energy, "Work" another form of energy. One could be converted completely into the other. Rather than passing through a "heat engine" the heat is "consumed", or "disappears" entirely having been transformed completely into a different form of energy.
Carnot's Caloric based theories were completely incompatible with Joule's experimental results. Even Carnot himself (in his private journals written shortly before his death) concluded that Caloric theory had to be abandoned and that "heat" and "work" were interconvertible, one into the other.
In the Carnot/Caloric theory heat RUNS THROUGH a heat engine like water, in one side and out the other.
In Joules findings, heat enters the engine and completely "disappears" or is "consumed", or "goes out of existence" and is entirely replaced by "work" and vice versa. In other words, a "heat engine" is not unlike a refrigerator in that it PRODUCES a lowering in temperature. Heat goes in resulting in work along with a reduction in heat. A reduction in heat = cold. Cold is the absence of heat. The transformation of heat into work results in an absence or reduction in heat, a lowering in temperature.
This is a natural consequence of conservation of energy. "Heat" is one form of energy, "work" another. When one increases the other decreases and vice versa. The transformation of heat into work results in LESS heat. A lowering in temperature.
More than 20 years after Canots passing we find Kelvin commenting on Carnot's treatise on heat:
As quoted near the start of the thread:
In referring to heat engines generally Kelvin wrote:
if it has absorbed any heat during one part of the operations, it must have given out again exactly the same amount during the remainder of the cycle. The truth of this principle is considered as axiomatic by Carnot, who admits it as the foundation of his theory; and expresses himself in the following terms regarding it, in a note on one of the passages of his treatise:
"In our demonstrations we tacitly assume ... that the quantities of heat lost by the body under one set of operations are precisely compensated by those which are absorbed in the others. This fact has never been doubted; it has at first been admitted without reflection, and afterwards verified, in many cases, by calorimetrical experiments. To deny it would be to overturn the whole theory of heat, in which it is the fundamental principle."
(...) I shall refer to Carnot's fundamental principle, in all that follows, as if its truth were thoroughly established
The entire field of thermodynamics developed by Kelvin then, is a return to Caloric theory.
Yes, PV indicator diagrams were Watts invention. It was a simple mechanical tracing taken directly from the engine. Pressure inside the engine pushed a plunger or gauge attached to a pen, which was also attached in some way to the piston. Probably this was nothing more than a method for monitoring the engine, to regulate the heat input and avoid explosions.
With Kelvin it became abstract nonsense imbued with his reserected version of the Carnot/caloric theory.
Yes, the current PV diagrams not only don't differentiate between load and no-load conditions they are entirely abstracted from any real engine or actual physical mechanics, leading to such nonsense statements as:
...Zero area inside the cyclic path equals zero energy output. Zero input too....
If the gas is expanding and contracting and driving the piston, the engine is running tracing out a PV diagram with the pen attached to the piston, at a minimum the "working fluid" or gas, expanding and contracting is doing "work" to overcome the inertia and friction of the engine. Heat is going into the engine being converted to mechanical motion.
But, because it appears there is "Zero area inside the cyclic path" supposedly nothing is happening. No energy input???
How then do we have a continuous change in volume? How do we have a cycle at all? How do we have a running engine?
If the same engine is doing shaft work turning an electric generator that is putting out 10,000 kilowatts does the abstracted idealized PV so-called diagram represent the work being done by the expanding gas to overcome this mechanical resistance on the shaft?
Nope. No energy in, no energy out.
Joule's results were experimentally demonstrated.
Thermodynamics based on Kelvin and others resurrection of Caloric theory is shot through and through with hypothetical abstract pie in the sky nonsense that has no correspondence with the real world or any real engine.
These abstract PV diagrams of "reversible" engine cycles do not represent anything real.
There are no "reversible" engines, there is no "Carnot engine". Thermodynamics is garbage, in particular the so-called "Second Law" and specifically the so-called "Carnot Limit" equation as generally interpreted and applied which can be easily demonstrated experimentally.
Just try to find the "caloric" flowing through a Stirling engine and out to the "cold reservoir".
As Joules experiments demonstrated, it isn't there.
The "heat" DISAPPEARS!
The "heat" (molecular kinetic energy) is converted to, or transformed into the mechanical motion of the engine.
In the process of heat conversion there is a corresponding "fall" in temperature. No heat passes through the engine. Not through the "working fluid" anyway.
In a steam engine, if steam passes through the engine and comes out as steam, you have latent heat carried by the steam to the condenser, there is mass carrying latent heat passing through the engine. In an IC engine you have air and fuel with combustion gasses.
There is no mass to carry any latent heat through a Stirling engine. Heat goes in and "disappears" resulting in "work" output or mechanical motion.