New Theory of Hot Air Engine Operation

[Tom Booth]

This is fairly simple.

Ultimately, a hot air engine derives power from what we call an "expanding gas".

Gas, in a sealed chamber, if heated "expands". Or at least tries to, or observably, it looks that way.

Looked at microscopically, on the molecular level, though, are the gas molecules actually "expanding", getting larger or taking up more room?

I think it can be safely stated, no, the individual gas molecules do not get bigger or take up more room.

If kept confined in a strong enclosure, the heated gas cannot expand at all, but the "pressure" increases.

What is pressure?

The gas, when heated, gains kinetic energy and moves or vibrates faster, but does it actually expand physically? Not really, it would just like more room to move around and gets better at knocking other molecules out of the way, but it doesn't, as far as I know, actually grow any bigger.

So, if we took a close look inside at what is actually going on inside a Stirling engine as it is being heated, what would we see?

There would be a fixed number of gas molecules hitting the walls of the cylinder and bouncing off without loosing much if any energy, and also gas molecules hitting the piston, which in response could move and absorb the molecules energy, at which time the molecule striking the piston would move more slowly or even stop moving altogether.

So, when the gas inside the engine is exposed to "heat" or the hot surface exposed when an insulating "displacer" moves out of the way, trillions upon trillions of gas molecules begin to move faster nearly simultaneously and either strike the piston directly or knock other molecules that knock into the piston, or hit the cylinder wall then the piston, but all in all, the molecules do not lose the kinetic energy gained from exposure to the "heat" until they hit a MOVABLE object, which is the piston.

Generally, the temperature of the material that the engine and piston are made of is less important than the molecular arrangement of the material, or it's "heat capacity".

For example, a cold ACRYLIC chamber wall will absorb less kinetic energy (heat) than an aluminum or steel chamber wall.

So, if a Stirling engine has it's gas chamber constructed out of Acrylic, there will be less heat loss to the chamber walls reserving more energy for transfer to the piston.

In this scenario, which I think is the REAL scinario, is there any advantage to having a very large "cold" metal surface inside the engines air chamber that will easily absorb a whole lot of kinetic energy?

I would think, NO! We want all the heat added to all be transfered to the piston. Why could this transfer not be accomplished by constructing the entire engine out of some material that has virtually zero acceptance of kinetic energy/heat, while making the piston as free and easy to move as possible?

The heat input could be regulated in such a way, and the piston allowed to move freely in such a way that there is nowhere for the extra energy, introduced intermittently, to go, except into moving / transferring energy to - the piston, and NOWHERE ELSE.. No "sink", no cold side, no cold "heat exchanger".

IMO, if this were not actually a real possibility, no hot air engine in existence could operate at a high rate of speed without a flywheel, because heat cannot be conducted or transfered out of the engine fast enough. Heat removal by conduction, relatively speaking is quite slow. The only way "heat" can be transfered out of the gas instantaneously is by transfer of kinetic energy to the piston as "work" to power the load on the engine.

Further, IMO, the gas INSIDE the engine, does not "know" what conditions exist outside the engine. The gas will continue to "expand", transferring energy to the piston as long as the piston continues to move. It does not matter if the piston is in effect "running away" from the expending gas due to the momentum of the piston.

So, it is possible, in this way, for the gas "pressure" inside the engine to drop below the atmospheric pressure outside the engine.

By introducing a sudden "blast" of heat into the engine, the piston can be "knocked out", by a kind of sudden impact, like a baseball hit by a bat. The sudden impact transfers kinetic energy to the piston which continues to travel up the cylinder though the kinetic energy of the gas was exhausted by the initial impact.

The piston, continuing to travel outward by momentum due to the initial impact causes the gas to lose MORE energy than it had initially before heat was added.

If this were not a real possibility, we would see too, that there would be no possibility of a piston FULLY returning to it's starting point in a rapidly running engine, without any flywheel to force it back.

Yet, we DO SEE engines that DO operate at a high RPM without any flywheel, crankshaft, or other apparent means of completing a "compression stroke".

Without a flywheel, the only thing available that could effect "compression" is the atmospheric pressure outside the engine, which could not "compress" anything without the gas INSIDE the engine loosing ALL the energy that drove the piston out in the first place.

One example is enough, but there are many examples of very fast running heat engines operating without a flywheel. This alone, IMO validates this "New Theory", but most hot air engines are not optimized to take advantage of this reality. Instead, we have been told for more than a century that not only heat, but also cold or a "heat sink" is necessary and that it is "impossible" for a heat engine to operate without continuously "rejecting" heat to a "sink" or "cold reservoir".

This very long standing misconception has, IMO, resulted in poor engine design.

Heat inefficiencies and waste have been intentionally introduced because it was believed without question that such losses are not only unavoidable but absolutely necessary as a "LAW". It was long believed that not only most, but ALL of the heat had to be "let out" after being "let in" to the engine.

Concession was made, stubbornly, when it was found that some heat "disappeared" the energy represented by that heat having been converted to work.

OK, so it was finally admitted, after the passage of many years, when carful observation and measurement made it undeniable, SOME energy is converted to work by the engine.

In reality, there is no real reason why ALL the heat introduced into a hot air engine cannot be converted 100% into "work" output, and I would say that this is observably already the case in several examples of Stirling type hot air engines in existence today, in some cases, without modification, or by simply removing the unnecessary flywheel.

[Tom Booth]

Here are some videos of "thermo-acoustic" Stirling engines starting up WITH flywheels.

Now, IMO, what can be observed here in the start up of these types of engines SHOULD BE impossible, according to conventional theory regarding how heat engines are supposed to operate.

Heat is introduced into the engine and the expanding gas drives out the piston, in the process SOME (but NEVER ALL, as that would be "impossible") of the energy/heat is converted to work resulting in the gas cooling which allows the momentum or energy stored in the flywheel to push the piston back down the cylinder to compress the gas and complete the cycle.

Theoretically, ALL the added heat cannot ever be converted to work because that would be a violation of the second law of thermodynamics. (Conventional, well established heat engine theory, that is)

But viewing many, many, many videos of Stirling engines starting up, and observing a few of my own, there seems to be something contrary to established theory going on.

The piston is pushed out by the expanding gas, yes, and momentum is transfered to the flywheel, OK, fine, but then something unexpected happens.

SOMETHING causes not only the piston, BUT ALSO THE FLYWHEEL with all of it's stored energy to stop and reverse course. The flywheel fails to complete a full rotation and instead it is pushed? drawn? pulled back down the cylinder, resulting in a rocking back and forth motion.

Whatever the cause, pushed, pulled or whatever, the piston returns in direct opposition to the momentum of the flywheel.

Whatever force is causing the piston to return, it is strong enough to bring the heavy flywheel to a complete stop and push or pull it back down the cylinder COMPLETELY, and NOT with the assistance of the stored energy in the flywheel, but in direct opposition to that stored energy.

The piston and flywheel reverse course with such force that they then continue around the other way enough to not only fully compress the gas in the cylinder but all the way around and then again, the momentum of the flywheel is opposed by some force which again draws the piston, or pulls or pushes the piston back and this oscillation not only continues, but gradually gets stronger and stronger until finally the flywheel completes a complete turn and the engine is running "normally".


https://youtu.be/ygxMEPW-VQk


https://youtu.be/Xf0C2WUa9uE


https://youtu.be/7KPszsp8o9s


https://youtu.be/vEJhOTDrxhc


What exactly is going on here?


Shouldn't this kind of behavior from an engine be "impossible"? The piston is not supposed to be able to return all the way without some assistance from the flywheel, because all the heat cannot be removed.

It often takes these engines several minutes to get up to operating temperature. So how can the gas in the engine possibly cool off in a split second, reducing the internal pressure and allowing the piston and flywheel to be pushed/pulled back? Repeatedly, and with increasing force each time?

Well, this is some of the evidence in support of the above theory.

With each oscillation, the heated gas is doing more and more "work" to push the piston out, which results in an equivalent reduction in temperature and pressure so that the piston is pulled back in, even against the building pressure from continued heating and even against the stored energy/momentum of the flywheel.

[airpower]

Great post and in my opinion engines strokes are waaay to long. The less "pumping in", more efficient, bigger cylinder and not a single focused heat spot.
World is obsessed with flywheels and rotation but pendulums are way more efficient.
Veljko Milkovic world of Two-Stage Mechanical Oscillation with the pinky finger hammering with 25kg
https://www.youtube.com/watch?v=qktY1hDrRl8

[Nobody]

Gas is composed of molecules, energy, and space. The space expands, more distance between molecules, when heated. This space is larger because the motion of the molecules is faster. More collisions at higher velocities keeps them more distant.