Alpha Stirling with dwell

[Tom Booth]

Talking about Alpha type Stirling and looking at animations, trying to figure out if there was ever any "constant volume" anywhere, I had a sudden brainstorm

What if the hot piston had "dwell". The cold power piston could just have the normal sinusoidal motion.

Imagine the top hot piston connected to the crankshaft by a Scotch yoke with dwell at both ends

https://youtu.be/I5ZwxzSrrO0?si=BqEfL3WD0KGOaZzU

ezgif.com-gif-maker.gif (289.92 KiB) Viewed 7451 times

This could just go in the "perpetual ideas" thread, but, I might just try building an engine of this sort so maybe it is worth a thread of its own so progress updates on a possible build can be posted at some future date.

[Tom Booth]

Or, going with the theory that a little heat input goes a long way, dwell just one end to prevent heat input at inopportune times (during contraction).


https://youtu.be/3SCbTB50csM?si=baFLNk2wWHPQnOGd


Watching how an alpha appears to.operate, the hot side piston mostly just serves as a kind of "displacer" exposing alternately exposing the hot cylinder wall surfaces to the working fluid to expand the gas, pushing the cold power piston while the hot piston is "straight arm", not really producing power. Then covering the heat source during "contraction". Again the hot piston is "straight arm" while the cold power piston returns.

As in the gamma, a sinusoidal motion for the displacer or "heat input valve" is not optimal, so also, I think, adding a "dwell" to the effective "displacer" in the alpha would allow a number of timing optimizations.

It does not appear to me that the hot side piston in an alpha actually serves any power producing function. In fact, due to the sinusoidal motion it is, at times moving in opposition to the expansion or contraction and just robbing power, or at best, being dragged along by the cold power piston making no real contribution.

An advantage of the alpha is that the entire hot cylinder provides considerable surface area for heat exchange when all added together, and the "displacer" (hot piston) is ALL HOT. The hot working fluid is never actually transfered to a cold heat rejection plate as in an LTD. Heat transfer to the cold power piston is prevented by the regenerator.

I think with a Scotch yoke, dwell and some timing adjustment, the regenerator could be dispensed with as more heat could be utilized (converted) for power production, so the gas would naturally cool on expansion into the cold side power piston. This should, I think, transfer more direct kinetic force to the power piston during heating/expansion, rather than heat being absorbed into the regenerator

[matt brown]

Sq cycle.jpg (67.12 KiB) Viewed 7413 times

As this diagram indicates, an alpha would require 2 dwells: (1) TDC dwell for hot piston while cold piston compresses (2) TDC dwell for cold piston while hot piston expands.

An alpha is similar an ICE where Wnet varies by Wpos/Wneg where Wpos is hot piston MEP and Wneg is cold piston MEP. Despite no displacer for an alpha, some refer to the cold piston as a displacer since the hot piston is the power piston.

Assuming a simple alpha with 2 identical cylinders, 90deg phasing and 90 deg dwell would be a home run for DIY, but this hasn't happened...no one has found a dwell mech that solves both driving and driven forces like a common slider-crank. A Scotch Yoke might work for a model, and a swashplate might work for a small engine, but the forces involved in any real power will require something more substantial. Another issue for any dwell mech is inertia/momentum loss which has likely scared off most serious research.

[Tom Booth]

(...)
Despite no displacer for an alpha, some refer to the cold piston as a displacer since the hot piston is the power piston.
(...)

IMO it's the other way around, as described in my previous post.

Generally the displacer acts as a "heat valve". In an Alpha, that role is played by the hot piston/cylinder.

Further, when heating is at its maximum, the hot piston is fully extended, which I described above as "straight arm". That is, the hot piston connecting rod is perpendicular to the crankshaft.

Resize_20230928_015544_4365.jpg (81.45 KiB) Viewed 7407 times

In that "straight arm" or perpendicular position, the hot piston cannot impart any work to the crankshaft. The expanding gas can only effect the cold piston to push the crank. The hot piston is driven by the cold piston through the crank.

Likewise during the "contraction" phase, though the hot piston is at the opposite perpendicular angle.

Resize_20230928_020355_5198.jpg (80.83 KiB) Viewed 7407 times

Again, in such a position the hot piston is incapable of imparting any angular turning force to the crankshaft because it is exactly perpendicular. Locked in.

You could say: in such a position, atmospheric pressure can only act on the cold piston to push it in, the hot piston is held almost stationary, it can't go anywhere.

Now I don't necessarily disagree that "some refer to the cold piston as a displacer since (they think) the hot piston is the power piston."

Intuitively, perhaps, it may seem to make sense that the hot piston would be the power piston, but in an LTD, which "piston" is in direct contact with the heat source effectively controlling heat input? The displacer.

[Tom Booth]

At this position:

Resize_20230928_020355_5198.jpg (80.83 KiB) Viewed 7403 times

The cold piston has not yet completed the "contraction" stroke, but because of the sinusoidal attachment of the hot piston to the crank it is begining to be pulled outward exposing the working fluid to heat. This works against contraction.

If the hot piston were held stationary a bit longer the cold power piston could better complete the cycle.

"Dwell" on the cold piston would not be desirable at all because it is doing the actual shaft work. Any loss of momentum for the cold piston would be detrimental.

[Tom Booth]

At the start of the "contraction" stroke:

Resize_20230928_024347_7091.jpg (82.29 KiB) Viewed 7398 times

There is considerable air volume still remaining in the hot side which would tend to work against atmospheric pressure being able to drive either piston further inward.

I think it would be beneficial if the timing could be adjusted so that the heat input would be discontinued sooner. A scotch yoke with an extended dwell for the hot piston would allow that. There might be other mechanical means of accomplishing the same thing but in actuality, I don't think there is much force associated with the hot piston, it is just being carried by the cold power piston much like a displacer in an LTD.

Actually any force involved with the hot piston is negative work. A dwell mechanism would take the hot piston out of the power train altogether reducing negative work.

[Goofy]

"Again, in such a position the hot piston is incapable of imparting any angular turning force to the crankshaft because it is exactly perpendicular. Locked in."

- And this is a common problem of (almost) all piston engines, incl. steam and ICE. The ignition or steam/air inlet is a few deg. before
or after TDC, where the crank is "locked".
If we look at a piston engine as an oscillating pendulum or a swing, then where shall the energy input be ?

This relates to the "Girl on a swing problem", when to push with a minimum of effort ?

It can be mathematical shown, that the best time is when the girl has the highest velocity, that is "closest to the ground" or in the middle between the two DC´s
But (of cause) every grandpa push the girl in the dead center, where the velocity is zero, even though it´s the most inefficient time to the energy input.
There is obvious relation between the "timing" in girl on a swing and the piston engine . . .

[Tom Booth]

All true, though if the hot piston is only acting as a kind of displacer, being "locked in" is not really any problem. The problem is it isn't locked in (aka "dwell") long enough. Due to the sinusoidal motion, heat is being added when the engine is in the compression phase.

Another way I think the Alpha is working against itself is when the hot piston pulls back, exposing the hot cylinder walls, it is not just allowing heat in, but it is also drawing a lot of air into the hot cylinder. I think at that point the hot gas being generated should be expanding out to drive the cold piston. The hot piston then has to push all of this just heated up hot air back out for the heat to hopefully be absorbed into the regenerator. Quite a volume of air is heated to no effect, it just stays in the hot cylinder.

Maybe instead of a piston there could just be a sleeve of sorts that could be withdrawn to expose the working fluid to the hot surfaces without pulling such a large volume of air into the hot cylinder. Something like this perhaps?

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