The relationship between force and work and how it applies to heat engines

[VincentG]

...from the layman's point of view.

I may have stirred up some controversy in this thread titled "Let's beat up Carnot" viewtopic.php?f=1&t=5549&p=20664#p20664 . I received some PM's that attempted to point out my confusion of force, work, and perhaps torque.

While there are errors made in that post, it was not a confusion, so much as a reapplication or different way of looking at things. At the basis of the concept is the established fact that without movement, force does not equal work. If we place a 50lb weight on the ground, the ground is acting on the weight with an equal 50lbs of force, but no work is done. To restore potential, just move the earth. Now if one were to hold the same weight in a static curl for a minute or so, no work is done. But I bet your arm would get tired. To restore potential, just let go.

The difference is the potential of the 50lbs of weight to continue to do work. The ground is acting as an immovable object and so the weight has less potential to do work than it is given by your arm.

Let's scale this up a bit and pick up a 5000lb weight with a hydraulic ram and then shut the valve. The valve acts as the ground and removes all potential of the weight to do work. Open the valve and the weight comes crashing down until we start the engine, run the hydraulic pump and apply its pressure(force) to hold the weight back up. Still, we have not done any work beyond lifting the weight in the first place, despite all the fuel burned to run the pump and hold it there.

A 5000lb force is holding back the 5000lb potential of the weight. Now, to do work, all we have to do is apply .0000000000(you get the idea)00001lbs of extra force and the magic of work happens.

Now, in the real world we live in, it is impossible to apply force to anything and not cause movement. "Everything is rubber" is a phrase often used in the high precision machining world. Put a .0001" resolution dial indicator on even a large steel framed object and you can make it move with your pinky finger.

Now how does that apply to heat engines? The pressure of heated gas in a vessel will distort the vessel. The vessel then acts as the ground and limits the potential of the gas to do any further work. To restore potential, just cool the gas. The rapid oscillation of the pressure does continuous work on the vessel in the form of vibration. And in this case, pressure, not volume, is all that is needed.

I was listening to a podcast about nuclear fusion. With all the energy potential it promises, they were still talking about the Carnot cycle as the limiting factor to utilizing the heat energy.

[Tom Booth]

....The pressure of heated gas in a vessel will distort the vessel. The vessel then acts as the ground and limits the potential of the gas to do any further work. To restore potential, just cool the gas. ...

I don't know the reason or motive behind this post, or anything about the private debate you say has gone on and I've pretty much avoided the "let's beat up Carnot" thread as the subject has been pretty much talked to death already and just seems to lead to endless unresolvable arguments that go nowhere. But anyway...

From the portion of your post quoted above, applying that to my shop compressor.

Are you suggesting that the "pressure" or "work potential" in a tank of compressed air is due to the elastic deformation of the air tank? Because that's what it sounds like.

A tank of compressed air does not do any work. '"The vessel then acts as the ground and limits the potential of the gas to do any further work."

Then you say: "To restore potential, just cool the gas".

I'm not sure what situation you have in mind, but for my air tank full of compressed air, cooling the air would reduce the pressure reducing the potential to do work. For the gas at least, that is.

Are you maybe thinking of the deformation of the tank?

If there was, say, some kind of clockwork attached to the "inflated" tank, cooling the compressed gas inside the "contracting" tank would then act like a weight on the clockwork causing it to run. In other words, cooling the gas n the tank would restore the pressure equilibrium so the outside atmospheric pressure could do "work" restoring the tank to its original shape.

To one degree or another, I think also the elastic deformation of the metal itself wanting to return to its original shape also plays a role, as well as the elastic nature of the gas, that is, there are temperature dependent forces of molecular attraction and repulsion that cause the gas particles to "want" to maintain a certain "comfortable" distance from one another.

This seems to be bringing a "private" debate out into the open for wider discussion but I don't really know what it's all about or what the other side of the argument might be, or what we are REALLY talking about, presumably something to do with Carnot and hot air engines.

It reminds me, at least, of my theories regarding how a Free Piston is able to return to TDC in a "thermal lag" type engine when the connecting rod is detached from the flywheel.

This experiment for example:


https://youtu.be/e-7DFp_B0y4?si=7Ilsw0vfulnkeL78


There is a "vessel". The air inside is heated causing pressure that pushes the piston out, in this case DOWN.

I am completely mystified that the piston can and does return back UP to TDC, probably ten times per second not only against gravity applying a downward force on the piston but with a weight hanging on the piston many times bevier than the piston. That is a rather big heavy magnet, the kind used to hang clipboards to a metal cabinet or whatever. All the while heat is being applied to the "vessel" which SHOULD logically be causing the gas to expand and push the piston all the way out of the cylinder.

You say, "to restore potential just cool the gas". Well, I agree, I think. But in this case, what is cooling the gas?

[VincentG]

There was no such private debate, I only received an offer to correct errors I made in the first post of that thread. That just sparked my thoughts further on the subject.

I am only suggesting that the oscillation of plastic deformation causes a vibration of the vessel that can be used one way or another as talked about in my heat powered mechanical amplifier thread.

It's a bit of a stretch I admit, but the thought experiment has led me to what I see as a breakthrough in the more practical nature of a standard atmospheric pressure gamma engine. I will make another post soon to detail this, and also offer insight as to what can cool the gas.