Stirlingengineforum.com

Tom,
"You are like No No No... there is always "positive pressure" not a "vacuum"."

All vacuums we create are imperfect and have a positive pressure. Not all positive pressures are vacuums. The vacuum of space is a mere few partials per cubic meter. At that miniscule positive pressure many solids sublime. Meaning they evaporate. That fact has very little if any use for the Stirling Engines we are studying here.

You brought up the phenomenon of a barrel being crushed by atmospheric pressure when the pressure gets low enough.

In a heat engine the pressures are much much higher. Because of the pressure being higher the resistance to compression from the build up of temperature and pressure, is greater. It takes work to compress a gas. It is the same amount of work that the expansion provides, outputs, shaft work, etc... . The gas doesn't care what is compressing it so a PV diagram works for all engines. If both expansion and compression is adiabatic.

If you think one pressure measurement is insufficient, you may have a group of the spread around the different chambers and tubes. It would be interesting to see data from several sensors plotted on one or several diagrams.

Here is a PV diagram for a four stroke ICE:
It comes from this link:

https://x-engineer.org/pressure-volume-pv-diagram/

Notice that there is an enclosed loop labeled +W and one labeled -W. One is a work providing/output cycle, the other is a work absorbing cycle. The identity can also be discerned by the direction of the paths. Clockwise power out. Counter clockwise power in measured by the area inside the cycle. I think the diagram is an artist/engineers rendering of a real engine's PV curve.

The -W cycle is the two strokes, exhaust and intake. They are irreversible as the work input to the gas volume doesn't increase internal energy and is considered a heat loss. It is smaller than the +W cycle.

I have provided it as a demonstration of how work can be measured using a PV diagram. The areas enclosed are a direct measurement of the work from and to the gas volume, and before any other losses except thermodynamic ones.

It clearly shows the differences from the ideal cycle. It clearly will have lower efficiencies than an ideal otto.

Fool wrote: ↑Sat Aug 26, 2023 11:58 pm Tom,
"You are like No No No... there is always "positive pressure" not a "vacuum"."

All vacuums we create are imperfect and have a positive pressure.
(...)

So what?

What matters is the relative pressure.

Below atmospheric pressure is a "partial vacuum".

Debating the issue is nothing more than semantic hair splitting, irrelevant and a waste of time.

Does an "imperfect" vacuum result in a crushed steel drum?

Does an "imperfect vacuum" result in the piston returning in a Stirling engine?

Does your "positive pressure" prevent the steel drum from "imploding". (Which you say is actually an explosion?)

Yes. Yes. No. Last three questions.

The steel drum crush is not an explosion. There is no bounce. The drum just collapsed inward. Most people don't discern a difference between that and a submarine implosion where the inward rush and energy density is sufficient to cause a bounce that then blows the debris outwardly destroying the vehicle. It's that outward motion that makes it become an explosion. Again the steel drum lacks the explosion process.

It is a little like the difference between a balloon popping and a stick of dynamite.

Fool wrote: ↑Mon Aug 28, 2023 6:29 am Yes. Yes. No. Last three questions.

(...)

Good.

Case closed.