Stirling "Hit 'N' Miss" Hot air engine

[Tom Booth]

I'm not making any promises, but, after some designing and thinking I have a kind of dead end displacer/+chamber with very little hot side air space and no real cold side, just the "air spring" probably pressurized, so, until there is some compression to push the displacer, it won't do much of anything at all.

But, at near full compression, near TDC the displacer moves, compressed air flows in to the hot side, air heats/expands and drives piston out.

So, what I'm thinking is, attached to a 4cycle IC engine, nothing happens during intake and exhaust, but during compression,.... Bang boom.

So, theoretically at least, this might allow an unmodified IC engine to be converted to external combustion.

Just take out the plug, attach the displacer, stick the hot end in a camp fire or whatever and pull the starter cord.

By "unmodified" I mean, it might not be necessary to convert it to two stroke or mess with the cam or valves. It would just coast through exhaust/intake then with compression, when the valves close, since the displacer is compression actuated, it only delivers a blast of heat at that point, then again coasts through the rest.

Since the displacer is basically pressed up against the hot heating cap through like 3 & 3/4 stroke and only fires on compression at TDC there is plenty of "dwell" gathering heat between "ignitions".

The same device should work attached to a 2 cycle as, again, it is pressure actuated, so will adapt itself to either 2 or 4 stroke.

Of course the carburetor, gas tank, and whatnot can just be tossed out, and a four stroke could be modified to two, but it would be interesting to see if it could run as 4.

I was thinking about taking the piston and cylinder out of an old lawnmower engine to build this thing, and thinking, now how do I need to modify this piston/cylinder to work with this displacer...

Then it dawned on me that it probably wouldn't require any modification.

If it works at all that is.

[matt brown]

Conventional theory is that a Stirling engine operates by first heating, then cooling the "working fluid" to cause expansion and contraction.

Conventional engine theory focuses on compression cycles with 4 processes wherein compression & expansion processes are 'interleaved' with heat and cooling processes. Assuming both compression & expansion have the same volume ratios (vs Atkinson cycle) we find that the same energy is req'd for 2:1 compression as 1:2 expansion, thus Win=Wout, and no Wnet. A sharp eye will note that Wnet=0 between compression & expansion regardless of whether these 2 processes are isothermal, isobaric, or adiabatic (but values vary between process prs). The heat engine 'trick' is simply separating the compression & expansion processes via interleaving heating & cooling, whereby, Win<Wout, since compression force at Tlow is less than expansion force at Thigh. The greater the spread between Thigh & Tlow, the greater Wnet; however, the thermal efficiency (Qin:Wnet) is a little more elusive, and requires careful study of process vs cycle values.

Most current heat engines use a 4 process compression cycle that starts with a compression process from the lowest energy state and then progresses thru the remaining cycle until returning to this state. If 'everyone' uses this scheme, there must be a reason...

[matt brown]

But, at near full compression, near TDC the displacer moves, compressed air flows in to the hot side, air heats/expands and drives piston out.

Common heat engine cycles require work at 2 different states where expansion force (Wpos) exceeds compression force (Wneg). Nearly always, any 'work' BETWEEN these 2 states will offset each other thruout any cycle akin a gas spring. So, all heat engines require a volumetric change, but a volumetric change does NOT guarantee a valid engine. Furthermore, most gains from a closed cycle and given up by back pressure on 'low side' of cycle.

Since there's only a few processes, what would this look like as PV plot ???

(1) isothermal compression--isochoric heating--adiabatic expansion
(2) isothermal compression--isobaric heating--adiabatic expansion
(3) ???

If you can plot it, it might be possible; if you can't plot it, it ain't possible...

[matt brown]

Common heat engine cycles require work at 2 different states where expansion force (Wpos) exceeds compression force (Wneg). Nearly always, any 'work' BETWEEN these 2 states will offset each other thruout any cycle akin a gas spring. So, all heat engines require a volumetric change, but a volumetric change does NOT guarantee a valid engine. Furthermore, most gains from a closed cycle and given up by back pressure on 'low side' of cycle.

Yikes, last line should read...

"Furthermore, most gains from a closed cycle are given up by back pressure on 'low side' of cycle."

kinda sad that edit feature shuts down after a few minutes...

[Tom Booth]

Conventional theory is that a Stirling engine operates by first heating, then cooling the "working fluid" to cause expansion and contraction.

Conventional engine theory focuses on....

Well, without getting into too much detail,... I've pretty much thrown all that out the window.

My basic theory is heat > in, work > out

If the engine starts in "condition A" and heat "X" is added so it changes to "condition B" then (or simultaneously) the work equivalent of "X" is taken out, then the "system" is back at "condition A".

Heat in, work out. Simple.

If that "cycle" starts out with the piston at TDC and ends with the piston at BDC, no worries, because having already returned to "condition A" energetically in terms of energy input and output, there will be a return to physical equilibrium regardless

That is;

Piston is at TDC. The addition of heat drives piston to BDC and there is simultaneously an equivalent amount of work output (to heat input).

How to get the piston back to TDC is not a problem.

There is an imbalance due to the piston having changed position. The imbalance will correct itself, that is, the piston is pushed back to its starting position at TDC by atmospheric pressure, or rather drawn back by a vacuum.

Basically isochoric heating - adiabatic expansion with work output. Then return to equilibrium

I don't know if there is a thermodynamic term for the condition or process where a vacuum draws the piston back, or atmospheric pressure pushes it back. Such a process does not seem to be recognized, afaik.

In other words, or to put it another way, with an illustration;

If I do work to push a car up an incline and then become exhausted and stop doing work, stop pushing, I don't have to worry about what energy is going to "push" the car back down the hill. It will roll back down the hill to where it started without any additional work input from me or anyone else, other than gravity.

In a similar way heat, pushing out a piston against atmospheric pressure is pushing "up hill" and when all the heat energy has been exhausted the piston will return without any additional heat or energy input, other than from the atmospheric pressure the work was acting on to begin with.

Infact, additional "work" energy can be extracted on the return journey. As the car rolls back down the hill, or the piston is driven back by atmospheric pressure.

So to say: "we find that the same energy is req'd for 2:1 compression as 1:2 expansion" well, actually, I don't think so.

Expansion takes all the work. So-called "compression" is just a return to equilibrium.

[Tom Booth]

I think the "mistake", if I may be so bold, generally made, in the thermodynamic analysis of heat engines is to imagine heat as a kind of entity in itself that if "added" to the working fluid must be "removed" for the gas to return to it's original state. But in a Stirling type heat engine in particular, there is no "intake" of any actual physical substance and no "exhaust". Only a fixed quantity of a gas sealed within a chamber with a movable wall/piston.

Energy excites the gas molecules in the chamber that do "work" on the piston.

When the source of that excitation is removed, the gas falls back to its previous state.

IMO, if energy has been taken out in the form of "work" and the heat/excitation is withdrawn, the "system" may actually rebound to a state with less energy, in the same way that a car pushed up hill, then no longer pushed up will roll back down the hill,... and beyond. It won't necessarily stop rolling just because it reached the bottom of the hill.

[Tom Booth]

My "Ringbom" type displacer "Hit-N-Miss" IC engine attachment idea is similar in concept to this:

https://youtu.be/4MPDmMaUc8o


Except, instead of the clank clank sounding atmospheric plunger, there would be a pressurized, adjustable air spring.

If it be imagined that the gold power cylinder (in the video) is a standard IC engine piston/cylinder/crank, and the silver side is the Ringbom displacer add-on, the pipe between them would tap right into the spark plug hole of the IC engine.

[Tom Booth]

I'm rather curious why there has been no response or comment on the last few posts on this thread.

Actually the other participants in the discussion Matt & Trevor have not returned (logged on) to the forum since April.

My reasoning took a departure from conventional Stirling engine theory, I think, yet, my conclusions appear to have a ready demonstration of sorts (the Ringbom engine)

The left (gold) side is a simple sealed piston/cylinder.

Any reason that this could not be ANY piston/cylinder? i.e. any conventional IC engine? The other side is just a pressure actuated displacer connected to the piston/cylinder.

Anyway, I found a local small engine repair guy with a large bone yard full of all kinds of lawn mowers, chain saws, weed eaters, generators - large and small, "junk" engine driven power equipment of all descriptions, power washers, wood chippers, power concrete trowels you name it. So I've already scrounged up, and taken to the workshop a collection of both 2 cycle and 4 cycle engines, and have various canisters, tanks to serve as the attached pressure/displacer Ringbom type add-on bit, heavy duty tap and die sets for pipe connections etc. so, am about ready to start experimenting with this concept, as soon as I finish up some other pressing projects unrelated to engines.

I had just been up reading this interesting website:

http://www.peterlynnhimself.com/stirling-engine

Which reminded me of this thread, due to some of the topic areas covered and ideas for solutions to various problems, trial and error experiments, etc.

I'm tentatively taking the lack of response as a good thing. No further objections or counter arguments to the theory or proposal of this concept, so far, at least.

Or, have people simply given up trying to talk sense into my hair brained schemes to set me straight?

Can anyone think of any reason why this could not possibly ever work?

[Tom Booth]

Fabricating a suitable high temperature displacer for the displacer side of such an engine is a related facet of this project. Powering a conventional IC engine with a Ringbom type displacer add-on, I imagine, would require very high input temperatures exceeding what the available or usual conventional displacer materials could withstand.

I have yet to manufacture a sample of DIY foamed glass/lava pumice rock displacer material, but I'm thinking that would be ideal for this application.

[Bumpkin]

Hey Tom, thanks for reminding me of Peter Lynn. I had visited that site quite a bit some years ago, partly for his attitude and realistic perspective. Looks like he’s added a lot since then.

Tom Booth said:
“Or, have people simply given up trying to talk sense into my hair brained schemes to set me straight?”

Some people don’t have enough self to allow for disagreements — which we’ve had and have, but so what? We share the notion that (possibly) an engine could make its own cold side. It has nothing to do with perpetual motion, it’s simply turning heat into power. We differ on how to get there, but I have to admit that I gave up on my own attempts some 40 years ago, so best wishes. I see your spark plug conversion as being a type of thermal-lag engine flow management. I think that in theory it could work, except that as a conversion of a donor I.C. engine it has the usual problem of minimal power overcoming the higher friction of an engine built for a different purpose. The 2-stroke donor would be difficult to lube without cooking the oil. The 4-stroke donor with the cam working would not only have twice the friction from two revolutions per power stroke, but more also from the valve-train — though it might benefit from the exhaust and intake strokes bringing in fresh cool air. (We probably disagree on that last bit, but in the least, the fresh air would help keep the engine from cooking itself.) In that regard too, it would be best to have the spark-plug replacement as thermally isolating as possible.

Bumpkin

[Tom Booth]

For some reason I completely forgot about 2 cycle engines running on a fuel + oil mix. Without the fuel intake, there would be no lubrication.

I'm surprised though, to see this engine run full throttle for nearly ten minutes without oil, so maybe just a coating of PTFE or a dry lubricant of some sort would allow a 2 stroke to run for a minute or two, just as a "proof of concept".

https://youtu.be/uS9YTzFHIFU

I didn't think it would last ten seconds.

I guess with a 4 stroke it would be better to just take out the cam shaft and disable the valves entirely and run it as a kind of two stroke.

I didn't have a problem with this engine overheating:

https://youtu.be/R_QB5amihko


I would imagine a converted 4 stroke IC with an actual oil bath would have no overheating issues.

Now I'm thinking a more conventional Stirling conversion, like the little red engine in the above video could be made from an old IC engine with just an attachment to the external part of the crankshaft, something like this:

Resize_20220704_010042_2021.jpg (134.01 KiB) Viewed 7352 times

[MikeB]

For the two-stroke engines, would it be possible to invert them, and effectively have a sump above the pistons?

[Tom Booth]

For the two-stroke engines, would it be possible to invert them, and effectively have a sump above the pistons?

I'm not sure I entirely understand your idea, but inverting the engine might actually make some kind of cylinder lubrication easier.

Likely the intake and exhaust ports would have to be plugged.

This engine conversion just has the crankcase cut open making manual lubrication possible.

https://youtu.be/wYXBMa8lAAU

4cycle, but I think the same could probably be done for 2.

Having it kind of inverted, or at least horizontal probably helps keep the oil in the cylinder.

[Bumpkin]

Hey Tom, considering the basic topic of the thread, your Gamma Stirling drawing brings to mind that making the displacer only connect the working fluid on one side like discussed earlier, would turn it into an Alpha Stirling. I don’t know if anyone has ever played with changing the dwell of an Alpha like has been done with so many Gamma variations, but there is that version with the cans in the water; as the cans (pistons) rise and fall, air passes through the connecting tube, but when there’s too much compression the water raises in the outer can (cylinder) delaying the full transfer in what I reckon could be called dwell. The pumping power used to raise the water is returned as the water falls again at the end of the transfer. I think the same thing could be done on a more traditional engine by using a spring on the crank or rod. Anyway if you could get it working, your collapsable displacer might do about the same thing without any linkage at all. A Ringbom Alpha of sorts. Dunno if that’s of any relevant inspiration, or just muddying the water, but thought I’d throw it in there.

Bumpkin

[Tom Booth]

Hey Tom, considering the basic topic of the thread, your Gamma Stirling drawing brings to mind that making the displacer only connect the working fluid on one side like discussed earlier, would turn it into an Alpha Stirling. I don’t know if anyone has ever played with changing the dwell of an Alpha like has been done with so many Gamma variations, but there is that version with the cans in the water; as the cans (pistons) rise and fall, air passes through the connecting tube, but when there’s too much compression the water raises in the outer can (cylinder) delaying the full transfer in what I reckon could be called dwell. The pumping power used to raise the water is returned as the water falls again at the end of the transfer. I think the same thing could be done on a more traditional engine by using a spring on the crank or rod. Anyway if you could get it working, your collapsable displacer might do about the same thing without any linkage at all. A Ringbom Alpha of sorts. Dunno if that’s of any relevant inspiration, or just muddying the water, but thought I’d throw it in there.

Bumpkin

Thanks.

Resize_20220319_212937_7532.jpg (115.58 KiB) Viewed 7290 times

I posted that image earlier to give a general idea of the concept, and yes, I guess it does have characteristics of both alpha and Ringbom.

I imagined some form of throttle that at low speed would restrict displacer movement to every other stroke or even less, but at higher speed would allow continuous displacer action to deliver a more continuous flow of heat, similar to the hit-n-miss type engines.

I've always wanted to build one of those two can engines, but haven't gotten around to it. I think I have some idea what you mean though.

Right now I'm having some issue with the adjacent business where my shop is being located. I've been bringing in equipment through the back entrance with my truck, but have to get to it through the parking lot of a used car dealer. He's apparently gotten it in his head that I don't have an easement to access the back door of my own building.

I hate having to deal with these kind of petty legal issues.

It is a row of commercial properties with an alley running behind the buildings. Our building is in the middle and his is on the end. So, he's been parking a truck in the alley to block access to the back of the other buildings, the others of which are currently up for sale.

But formerly the other stores received deliveries through the back alleyway, but now he has a bunch of used cars in the way. I guess he got used to having it to himself before we acquired the inside building.