The Carnot efficiency problem

[Tom Booth]

(...)
All he needed was an open mind and respect for cause and affect. Lack of that respect is, of course why we need laws, but if you have enough respect you can tactfully wait until nobody’s watching and do what you want.

Bumpkin

Maybe "cause and effect" is somewhat in question these days, but generally, assuming it exists, seems like a good rule of thumb, which is probably my biggest objection to the "Carnot Limit", as it has no identifiable cause.

Conservation of energy demands the energy put into a system, leaves the system in equal measure in one form or another including "work" output, friction, vibration, or whatever.

The Carnot formula insists, supposedly, that out of all the heat going into the engine only 20% or whatever can come out one way as "work" and the other 80% for some reason must come out some other way into the "cold reservoir" and does not account for friction loses and such at all.

The cause/effect relationship or reason is unclear.

How can the temperatures precisely limit engine efficiency?

Yes, it seems pretty obvious you can't get out more than you put in, but the Carnot limit says you can only get out, under the absolute best conditions, with zero friction loses, 20% of what you put in.

Is it coincidence that 20% is just the temperature difference on the kelvin scale? No, it's just a simple derivation. The high temperature is 20% higher than the low temperature. Of course this varies with temperature, but I'm talking LTD running on hot water type temperature.

With heat as "caloric" a cause can be imagined. A fluid flows down. So the height it can fall makes at least a plausible hypothesis.

With heat as energy, that half baked assumption falls apart.

But, it also does not hold up experimentally. If it did, it might be worth looking for a cause, but in my book, if it doesn't hold up experimentally that's where it ends. You can't assign a cause to a non-phenomenon.

[Tom Booth]

Goofy - the problem is this:
If for a moment, we ignore the "work done" on/by the piston, then what keeps the motor running? The simple answer is the alternating change in pressure of the working gas. What causes that change in pressure? The smaller reason is change of volume from the piston, but the larger reason (in most motors at least) is change of temperature due to moving the working fluid from hot area to cold.
If the cold end isn't colder than the hot end the pressure will never change, and the piston will never move.
Therefore, if the cold end doesn't remove heat from the working fluid, the cold end will heat up over time until equilibrium is reached...

Conversely, if we assume that cold end does nothing, then the only way for the motor to keep running, is if the work done by the piston removes exactly 100% of the energy that the hot end puts in.

This all is quite logical and fairly obvious, and I think true.

In particular the last sentence: "if we assume that cold end does nothing, then the only way for the motor to keep running, is if the work done by the piston removes exactly 100% of the energy that the hot end puts in."

Generally, though, that last logical conclusion is dismissed offhand as preposterous as a "violation of the 2nd Law of thermodynamics" rather than being subjected to experimental testing.

In every experiment I've been able to conceive and carry out so far over the past three years, the engines keep on running. Sometimes they seem to run better, sometimes maybe the same or not quite as well, but they all manage to keep on running.

https://youtube.com/playlist?list=PLpx2 ... OWvk1G2CQY

Of course, These engines are far from 100% efficient in actual fact, in practical terms. In many cases most of the heat supplied never even makes its way into the engine. Some is conducted away through the engine body to the air, and much of the heat that IS converted to mechanical motion or "work" turns almost immediately back into heat through friction, but none of these avenues for lost heat necessitate the engine having a "cold reservoir" or "sink".

[MikeB]

The corollary to that point, is that the hot end will tend to be self-regulating (though maybe not always sufficiently!) - heat transfers faster (apparently) between items that have a large temp difference than a small difference, so if the working fluid starts to heat up, presumably the rate of transfer will decrease?

[Tom Booth]

The corollary to that point, is that the hot end will tend to be self-regulating (though maybe not always sufficiently!) - heat transfers faster (apparently) between items that have a large temp difference than a small difference, so if the working fluid starts to heat up, presumably the rate of transfer will decrease?

True. Infact, at times, the likely scenario would be that the heat flow could temporarily reverse, at or near TDC when compression is at a maximum.