Aligning heat "vectors"

[Tom Booth]

"heat vectors" is probably not the best thing to call it, but I can't t really think of a better terminology.

Here is a first attempt to illustrate graphically what I'm talking about:

Resize_20230809_122630_0041.jpg (361.67 KiB) Viewed 9515 times

One of the main things to notice is that when the displacer moves down to the cold side there is a delay in heat increase. Once the hot plate is exposed it takes some time for the temperature of the working fluid to rise, for the heat to actually heat and expand the gas.

This delay is imperceptible to us, perhaps, but in an engine running at high RPM becomes significant, so the timing of the displacer movement needs to be "advanced" so that the actual peak heat input coincides with the other heat contributions or "vectors", or contributions to the "internal energy" if we want to avoid use of the term heat.

Work on the gas by atmospheric pressure contributes to "heat of compression". That's one vector.

Heat generated by the movement of the piston to compress the gas is another. This is potentially different from "external work" input from atmosphere. Essentially it represents the temperature increase due to the gas occupying less space and does not contribute to "internal energy" it only concentrates that energy, but that concentration of "heat" in a smaller area I'm still putting down as a "heat vector"

The other thing to note is the alignment of all these "heat vectors" on the vertical dashed lines, which basically represent time intervals.

[Tom Booth]

I've been trying to think of means and methods that could potentially increase the focus of concentration and alignment of these so-called "heat vectors" which theoretically should also increase both the power output and the "heat pump" action.

1. One method I've already tried with some apparent success is to increase the compression and expansion ratios by increasing the "throw" or traveling distance of the piston. This can be accomplished by attaching the power piston connecting rod at a radius further out from center on the crankshaft or flywheel.

Resize_20230817_003453_3686.jpg (131.09 KiB) Viewed 9288 times

2. Optimizing the displacer movement could make an additional contribution.

In my estimation, that means engineering the displacer to move in such a way that as far as possible, heat input is maximized at TDC with a sharp and early cut off of that heat input after TDC so that the heat is fully utilized and the working fluids internal energy is in deficit by the end of the power stroke so as to effect heat absorption by the "heat pump" action at BDC.


3. It might be interesting to in some way engineer some kind of energy storage where work output during the power stroke can be returned as work input during the compression stroke.

This is more or less passively implemented by either a flywheel as a temporary store of momentum, or by atmosphere acting as an "air spring", but I was thinking in terms of augmenting this passive "work output" storage with something more intentional.

For example, weighting the flywheel. Introducing a additional air spring or even an actual mechanical spring to absorb "work" on expansion and return some or all of that work during compression.

The purpose of this is not the conventional purpose of simply completing the cycle due to incomplete beat utilization, but rather to utilize the work output to effect cooling (removal of internal energy) at BDC and heating (returning "work" and therefore increasing internal energy, and therefore increasing the temperature at TDC).

Other means of effecting some such work phase transition might be through some work output in the form of electrical resistance, though it might not be practical to "return" an electrical load, perhaps this work output could be pulsed or intermittent so as to restrict output to the expansion stroke.

Another means might involve some placement of magnets to effect an attraction and repulsion. The work of moving magnets closer together with like poles facing each other on the power stroke would be returned on the compression stroke acting in the same manner as atmospheric pressure. A magnetic "spring".

The basic idea behind all this is to increase the amplitude of the 'heat vectors". So that the peaks are higher (hotter) and the troughs lower (colder), thus increasing ∆T without the application of an external cold source but instead increasing the ∆T by moving heat to the point where it is most useful for power production. And also creating cold to effect "contraction" for the "return power stroke". ("compression' by atmospheric pressure).

[Tom Booth]

The result then should be something like this:

Resize_20230817_014423_3516.jpg (143.43 KiB) Viewed 9279 times

Note, I am not attempting to follow any convention here by way of a "cycle" or formal PV diagram

At TDC the "heat vectors" combine to effect something like Isochoric, (constant volume) heat input, but really, heat is not so much 'input' from a heat source as CONCENTRATED at the focal point or apex of all the heat vectors in order to effect a kind of high pressure 'explosion.

The piston is then jettisoned towards BDC at high velocity. Work is extracted at this time adiabatically.

At BDC the velocity of the piston slows due to work output resulting in a temperature and pressure drop.

This results in a reversal due to an internal vacuum which allows atmospheric pressure to jettison the piston back towards TDC.

As the piston reaches TDC high heat is exposed by the displacer lifting, "work" from atmospheric pressure (and other "spring" action) results in heat of compression. Also as the piston slows down at TDC due to the rise in pressure, velocity is converted into heat.

[MikeB]

One experiment that I have yet to get around to, is de-coupling the displacer from the power piston, and therefore having two flywheels, one driven by the piston, and driving the load; the other driven by an electric motor and driving the displacer.

It seems likely to me, that this would automatically achieve the correct phase angle at any speed, but not sure how one would measure it.

[Tom Booth]

One experiment that I have yet to get around to, is de-coupling the displacer from the power piston, and therefore having two flywheels, one driven by the piston, and driving the load; the other driven by an electric motor and driving the displacer.

It seems likely to me, that this would automatically achieve the correct phase angle at any speed, but not sure how one would measure it.

There are a few "free piston" AND free displacer engines I've seen (running on Youtube). I haven't really thought much about how they stay synchronized.

That was also, almost the basic idea behind what I was attempting to do with the Arduino driven displacer, to operate the displacer independently with a servo in order to flesh out the best dwell, phase angle, lift height or whatever by just changing some numbers in the driver program.

It never got past the basic "proof of concept" stage though.

https://youtu.be/PMqtfqtXpRM

A simple speed control with a dial on a motor would be easier and more straightforward for sinusoidal motion, then "dwell" time, if it was of interest could be modified mechanically.

I THINK (could be wrong) that whatever speed the displacer moves the piston would necessarily have to follow the same rhythm. What else could it do? The displacer is in control of the heat input, the speed of heat input, the frequency of hesy input.

Control the displacer and you control the engine.

Maybe.

It would be interesting to find out anyway.

[MikeB]

I totally agree that the piston would have to remain "in rhythm" with the displacer, but the 'phase angle' would shift. If it shifted far enough, the engine is going to start to "skip a beat" and/or stop completely.
I think you might struggle to find a servo that will run fast enough to be able to modify the dwell time accurately enough to be useful. (I'd considered using an old-fashioned coil-based loudspeaker for that reason, long ago, but again not yet got round to doing so.)

[Tom Booth]

I totally agree that the piston would have to remain "in rhythm" with the displacer, but the 'phase angle' would shift. If it shifted far enough, the engine is going to start to "skip a beat" and/or stop completely.
I think you might struggle to find a servo that will run fast enough to be able to modify the dwell time accurately enough to be useful. (I'd considered using an old-fashioned coil-based loudspeaker for that reason, long ago, but again not yet got round to doing so.)

Yes, the inadequate servo is why that project never progressed.

Your idea of driving the displacer with a motor while attached to a flywheel should at least show something.

I think you are probably right about the phase angle shift. But that might be useful in controling engine speed, like a "Jake brake" in a diesel engine.

The back pressure would certainly cause the engine to "skip a beat" or two, like a hit or miss, but maybe it would return to synchronization at a higher or lower speed. Difficult to say without trying it out.

Certainly if the displacer were stopped altogether, the engine would have to stop as heat input would stop.

I think it would be well worth experimenting with. Maybe even lead to a breakthrough in Stirling engine speed control.

The relation between piston and displacer might act something like Jeff Muller's "magnetic gear"


https://youtu.be/rB0VGEV9KSs

Kept in sync by the force of pressure variations rather than magnets.

[MikeB]

I hadn't heard of a "Jake Brake" before - are they really a thing?
My driving instructor taught me how to use engine-braking, then told me never to use the technique unless necessary, as brakes are somewhat cheaper (and easier) to replace than cylinder linings!

[Tom Booth]

I hadn't heard of a "Jake Brake" before - are they really a thing?
My driving instructor taught me how to use engine-braking, then told me never to use the technique unless necessary, as brakes are somewhat cheaper (and easier) to replace than cylinder linings!

Yes, basically it just alters the timing or "phase angle" for the opening of the exhaust valve. In effect, turning the engine into an air compressor (without a tank). So the compressed air just blows out the exhaust without powering the engine, sounding like a pop gun. (Or 6 or 8 pop guns).

https://youtu.be/aX6kqv0Votk?si=r-CkVcOVeKCjeb6o

[Tom Booth]

I just came across this mechanical means of increasing expansion without increasing compression in a four stroke engine.

https://youtu.be/P68W5jOR0bA?si=8ttFJ_pW_CA6RayE

I mentioned earlier in the thread that in reality both the hot and cold "vectors" should be aligned and maximized.

Some extra expansion means more heat gets converted to work.

More compression however has questionable value, or has a negative impact. At best we can break even by utilizing "heat of compression" to get back the work that went into the compression.

But how to have more expansion without also having more compression?

It seems, in a four stroke IC engine there is a way and this apparently has a demonstrated benefit in improving efficiency.

But how could something like that be accomplished in a "one stroke" Stirling engine?

I've vaguely pondered on the problem before, settling on "increasing the throw" which increases both. Just increasing expansion without increasing compression seeming physically impossible, but seeing this video-animation makes me wonder. Is there some way to accomplish the same thing, or something similar in a Stirling engine?

Maybe some pressure relief port on the downstroke?

Is this even actually a good idea?

If the piston is pushed back by atmospheric pressure, would a relief valve actually relieve "pressure" or could it eliminate the "vacuum" that draws the piston back?

Would loosing "heat of compression" enhance or degrade performance. If you loose some air out a port, then there are fewer moles of gas that can be expanded. A Stirling engine doesn't have an intake to replenish the air, so loosing working fluid out a port during compression seems like a bad idea.

Something to think about anyway

[VincentG]

But how to have more expansion without also having more compression?

It's far simpler than any mechanical contraption. All you need to do is reduce the mass of air in the engine, and it will have more expansion than compression.

[Tom Booth]

But how to have more expansion without also having more compression?

It's far simpler than any mechanical contraption. All you need to do is reduce the mass of air in the engine, and it will have more expansion than compression.

I would think, though, that the air molecules are the force carriers. Is that not the reason for pressurization?

You just have fewer fewer molecules to both expand and contract. You can't reduce the .add of air during compression only and put it back during expansion without some "mechanical contraption", or maybe I don't understand.

Can you explain in more detail how this reduction in mass would work?

I was thinking a kind of spring loaded piston that would give during compression and return as the pressure dropped during expansion.

A modified Laminar Flow engine:

Resize_20230927_140242_2978.jpg (185.49 KiB) Viewed 8888 times

Not sure this would behave any different than some extra "dead air space", but the spring tension could be adjustable. So something to play around with. It would also prevent undesirable mixing of "air spring" air with working fluid air.

[VincentG]

Basically when given a certain engine displacement with a certain heat input, you can calculate the expansion ratio of the gas. If you maintain the displacement and the temperature, you can vary the expansion and compression ratio by adding or removing gas.

More gas will tend towards more compression and less gas will tend toward more expansion.

I usually find it helpful to think of extremes. So picture too much air in the engine, it would never have enough power to compress to TDC and the piston would be stuck at BDC. Conversely too little air, and it would never have enough power to expand to BDC.

You can easily test this by adding a test port in any Stirling engine. You then have to choose at what displacer and power piston position to close the test port. The engine will respond very differently to these different internal states.

[Tom Booth]

I think I could see, or imagine, how, since gas molecules do actually take up space, having too many too densely packed would make compression more difficult and expansion easier. You would be increasing the gases own natural molecular repulsive force, or rather, causing it to dominate.

Reducing the amount of gas would make compression easier and expansion more difficult as the attractive forces would tend to dominate.

You could find a happy medium.

This would tend to occur automatically as the engine is heated to "operating temperature".

This would all, of course, be "NON-IDEAL" gas behavior as "ideal gases" are supposed to have no volume of their own and no attractive or repulsive force of their own.

It might be interesting to see if barometric pressure has an influence on a model engines "operating temperature".

It seems the engine should require less heat during stormy weather as pressure is already low, and more heat on a clear day as pressure is relatively high.

If I'm out in left field somewhere, feel free to explain or give your own theory as to why this might be so.

For an "ideal gas" there really shouldn't be any difference as an ideal gas is "infinitely elastic".

As a matter of personal experience, I've found that my FORD pickup seems to always run better during rainy weather. I thought that might have something to do with the water vapor in the air, but maybe it has more to do with the barometric pressure.

[VincentG]

I don't think there is any reason to pick apart the ideal gas law. It works for most things, and none of us are about to personally redefine any gas constants.

It's openly admitted that real gases differ from the ideal. This was taken from the Wiki page on the ideal gas law.

In an isenthalpic process, system enthalpy (H) is constant. In the case of free expansion for an ideal gas, there are no molecular interactions, and the temperature remains constant. For real gasses, the molecules do interact via attraction or repulsion depending on temperature and pressure, and heating or cooling does occur. This is known as the Joule–Thomson effect.

Certain things I think should just be taken at face value. For instance, more molecules in the same engine will increase pressure. No need to complicate the basics.

This Joule-Thomson effect is what allows that Monotherm scheme to(potentially?) work.

I see it as icing on the cake for our external combustion engines. If used effectively it could increase power output, or reduce the energy input requirements.