Pneumatic Piston Efficiency: Understanding Losses & Maximizing Work Output

[kandelabr]

hi.
say we have a pneumatic cylinder with a piston and seals that cause a lot of friction, and the whole system is well isolated, so we could assume expansion is isentropic.
friction causes loss of energy, so the cylinder warms up.

since we add heat to expanding air, the expansion goes polytropic, and with that kind of expansion we get more work from gas.

if I'm right, where do the losses come from?

 

[TVP45]

What made it expand?

 

[kandelabr]

it's a pneumatic cylinder. i guess compressed air.

 

[TVP45]

So, can you create a PV diagram of the process? Or can you write an equation relating change in internal energy, heat flow, and work done on and work done by the cylinder?

 

[kandelabr]

sure i can.
in an idealised model, all heat transfers to gas, thus it expands polytropically with n = ... (an equation where heat = work done by friction)

it's simple, it's just a bit confusing because seals are always a problem, and there's always a lot of yada yada around friction. in this case, if I'm correct, it shouldn't matter., and a seal like that would be quite easy to make.

 

[TVP45]

OK. I really don't understand your question. What were you looking for?

 

[EngineerAhmad]

Maybe I couldn't understand your question well, but the main reason for losses is the friction that you talk about,because we can't completely get rid of it, also the expansion of the cylinder is less than the expansion of the piston.


http://www.arb-up.com/files/arb-up-2008-6/39s66849.jpg

 

[TVP45]

What sort of seals are you using?

 

[kandelabr]

EngineerAhmad: if they're made out of the same material, their expansion is the same.
anyway, I'm not talking about expanding cylinder and piston, but air.

you get most work from compressed air if expanded isothermally and least if expanded isentropically.

in this case we have an isolated system, so the expansion is isentropic.
and we also have friction between the piston seal and the cylinder, and that causes warming of cylinder and work losses.
cylinder warms up air as it expands, so expansion isn't isentropic anymore, so we get more work from this expansion.

therefore, it doesn't matter how much friction the seal causes. more friction means more heat, and more heat means more work extracted from compressed air.

theoretically, I'm right, but i know it's not that simple.
what am i missing?

 

[TVP45]

EngineerAhmad: if they're made out of the same material, their expansion is the same.
anyway, I'm not talking about expanding cylinder and piston, but air.

you get most work from compressed air if expanded isothermally and least if expanded isentropically.

in this case we have an isolated system, so the expansion is isentropic.
and we also have friction between the piston seal and the cylinder, and that causes warming of cylinder and work losses.
cylinder warms up air as it expands, so expansion isn't isentropic anymore, so we get more work from this expansion.

therefore, it doesn't matter how much friction the seal causes. more friction means more heat, and more heat means more work extracted from compressed air.

theoretically, I'm right, but i know it's not that simple.
what am i missing?

I'm sorry that I haven't been asking clearer questions. So, please allow me to try again.

As I understand your situation, you allow compressed air to enter a cylinder. The cylinder then expands. You believe that process to be isentropic; yet, you notice the cylinder warms from friction and transfers heat to the air. You then believe this causes the expanding air to do even more work and you are puzzled by what seems to be a paradox.

Is this correct?

If so, you have not identified the correct boundary(s) for work and heat. The thermodynamic energy formula is straightforward, but the boundary question can be tricky. Further, I believe you are confusing the terms isentropic and adiabatic.

I do wish you would provide a diagram or equations.

 

[FredGarvin]

There is nothing that dictates all, or even ANY of that extra heat due to friction goes to the working gas. The thermal conductivity of metals is going to be much higher than that of air.

 

[kandelabr]

There is nothing that dictates all, or even ANY of that extra heat due to friction goes to the working gas. The thermal conductivity of metals is going to be much higher than that of air.

so that's the reason seals have to be as low on friction as possible?
what if i insulated the cylinder really well (could be vacuum or aerogel), make the crank plastic or of some similar low thermal conductivity material, i guess i'd get some energy back as work from friction heat, right?