Stirling "Hit 'N' Miss" Hot air engine

[Tom Booth]

As probably nearly everybody here already knows, a Hit-N-Miss engine is generally an internal combustion engine that regulates its own speed by only firing the ignition when required, usually every other cycle of rotation, similar to a four cycle engine.

The basic Idea being that enough power is delivered to the engine to carry it through several rotations of the crank. If the engine is too slow and needs more power the frequency of the ignition can be increased, if too fast, decreased

In a Stirling engine the "ignition" or heat delivery, to the engine is controlled by the displacer. But, with most hot air engines, the displacer is connected to the crank.

Were there ever any Hit-N-Miss type Steam engines or Hit-N-Miss hot air engines of any kind? If so, I have not been able to find a reference to any with a brief, casual search.

Anyway, I have an idea that a Hit-N-Miss type Stirling (or other type) Hot Air engine is possible.

The displacer for such an engine would operate in conjunction with a speed regulator and would only lift or move from the hot heat exchanger intermittently, when additional power is needed. When less power is required, the engine could "coast" along on residual heat, more or less like a thermal lag engine that continues to run on residual heat after the heat source is removed, slowing down the engine.


https://youtu.be/oDM-DCPFYLs


https://youtu.be/oZDnewTck8A


I have an idea for a new (as far as I know) kind of displacer mechanism that only has displacement on the hot side, without any functional cold side.

This thread is to document my efforts to design and build such a Hit-N-Miss hot air engine.

Of course comments & suggestions welcome.

[Nobody]

I like it.

[Bumpkin]

I can’t find the reference now, but I am certain I have seen text/diagrams pertaining to a (mid 1800s?) Stirling that governed by varying the displacer stroke. Not hit and miss, but sorta the same premise.
Bumpkin

[Tom Booth]

The main problem that has been occupying my mind for some time is, or was: If the "displacer" / displacer chamber only has, or in theory, only requires a hot side for heat input, where would the air to fill the hot side come from?

The displacer needs some empty air space to move into.

An Alpha type engine has a sealed piston to move that introduces air into the heated chamber, but there is no method for adjusting the stroke or the timing.

I had an idea a long time ago:

hot_potato_engine.gif (72.02 KiB) Viewed 4965 times

Discussed here:

viewtopic.php?f=1&t=488

That was back in 2010.

Not much has changed really, except I'm thinking that instead of a "regenerator" of sorts, being bumped by the power piston to release heat, the movable regenerator could be, instead of a porous regenerator, a simple piston could be used, perhaps with a "rolling sock" type seal to minimize friction, or it could be attached to a kind of planar spring, or perhaps the "piston" /displacer thing could just be attached to a diaphragm backed up with an "air spring".

To simplify and facilitate heat input, I think maybe an arrangement similar to this common type heat engine would work well:

Resize_20220319_201013_3213.jpg (172.93 KiB) Viewed 4965 times

But with some significant differences.

The connecting pipe that brings the power piston and displacer chamber into association would be moved forward, in front of, or nearer to the front (hot cap) of the displacer chamber and the back, normally cold end of the displacer chamber would consist of some form of sealed "air spring" perhaps assisted or augmented by an actual spring for pressure adjustments.

The displacer connecting rod could be eliminated, as I imagine that the "displacer" would be actuated by simple compression of the air by the power piston. Similar to the gif animation, but the push rod could be eliminated. Instead of a push rod, the displacer would be pushed back by increasing air pressure, which would simultaneously allow the introduction of more heat.

This would increase the compression ratio and substantially reduce "dead air space".

In a way I imagine this operating very much like an internal combustion engine, but without any combustible fuel/air mixture, so with no need for intake and exhaust.

So, you would have compression. The increase in pressure during compression forces the "displacer" to react, exposing the compressed air to the hot cap. The heated compressed air then expands, driving the piston back out.

As the piston moves out, the air pressure decreases and the displacer returns, also returning pressure/power.

The "work" of driving out the piston against the resistance of a load, cools the expanded air so it can again be compressed.

The displacer movement depends, to one degree or another on the air, and/or mechanical spring tension behind it, which could be adjustable "on the fly" by various means, from near zero movement to "full" motion, with corresponding heat input, from near zero, just to introduce enough heat to overcome friction under no-load conditions to full power with maximum heat introduction each cycle under heavy load conditions.

Perhaps the "air spring" tension could be regulated using a reserve air tank with a more or less ordinary pressure regulator, similar to what is used on any ordinary compressed air tank, something like one of these:

Resize_20220319_204307_7991.jpg (60.21 KiB) Viewed 4965 times

Then a simple automatic control mechanism could be used to control air spring pressure which in turn would control displacer motion/heat input and therefore power output

Many different arrangements might be possible, but that is the general concept

Basically, replace the cold side displacer chamber air space with a sealed "air spring". There could be several different ways of implementing that so that there is no actual "displacement" of air between a hot and a cold space, but rather the hot space expands and contracts against an air spring.

[Tom Booth]

Top view conceptual sketch.

Resize_20220319_212937_7532.jpg (115.58 KiB) Viewed 4961 times

Various possible means of controlling spring tension are shown; from a simple hand lever to a centrifugal regulator.

[matt brown]

Tom - I think your hot potato design is a clever improvement over common thermal lag design. Years ago, i spent a lot of time on similar 'bash valve' schemes, but they don't solve an 'invisible' problem. What both are trying to achieve is simple heat addition into gas after compression, but what really happens in more like (1) a gas spring style adiabatic compression & expansion of main gas, and (2) oscillation of heater gas. Simply wanting heat transfer to occur between these 2 gas masses does not effect transfer. Sure, some will occur, but not enough to write home about. In my favorite scheme of this, I used a large flat plate diaphragm affair where the bash valve exposed a veined cavity near TDC...a goofy Otto Cycle scheme that could be open, closed, or pressurized. I love the simplicity of Otto Cycles due to distinct processes, fast adiabatics, and a non specific dT input. But a close study will reveal (1) Otto favors no dead volume, and (2) as dead volume increases, Otto approaches Brayton.

Modern model gamma SE have their transfer tube to power piston as far away from heater head as possible which reduces thermal short and engine stall. There's no advantage to having this transfer tube near heater head, since the temperature of the gas is NOT what drives the engine. No, it's dP that drives the engine where MEP is manipulated thruout cycle via (relatively) small hot high pressure volume vs large cold low pressure volume.

[Tom Booth]

...

There's no advantage to having this transfer tube near heater head, since the temperature of the gas is NOT what drives the engine. No, it's dP that drives the engine where MEP is manipulated thruout cycle via (relatively) small hot high pressure volume vs large cold low pressure volume.

I wondered a lot about this. My thinking was that a fast moving air molecule IS also a high pressure molecule, which made me wonder about the actual benefit, if any, of a regenerator, which captures, or rather absorbs energy from all the hottest molecules.

So, why would it not be an advantage to have the power piston as close as possible to the actual heat source, rather than at the end of a long pipe stuffed with steel wool?

So, I actually spent a lot of time recently, and even after posting that sketch, thinking on how could the power piston be brought closer?

I think the Essex engine endeavors to accomplish this by having the power piston and displacer piston "butt heads" so to speak.

Since the design actually has the displacer/displacer chamber and power piston/cylinder working independently, I think an Essex type arrangement is possible, but, I wanted to be able to stick the hot cap into one side of a wood or coal fire box, but a more conventional burner heating method would allow this.

Anyway, my conclusion, after giving it a lot of thought, is that, on a molecular level, high heat and high pressure are just two different terminologies for the same kinetic energy.

Maybe that's wrong, but I'm not sure what reason there is to think so.

I'm not aware of any other hot air engine design that ever managed to actually eliminate the cold side "dead air space" as this one is at least attempting to do. So, has it ever been tried?

In an internal combustion engine, heat and pressure are more or less synonymous and the ignition spark is, I think, universally situated directly above the power piston, with very little clearance.

The main good reason I can think of for situating the power piston far away and keeping it cold is lubrication. Oil can burn or even explode.

That is OK in an internal combustion engine, they are designed to handle internal petrol explosions, so burning a little oil is not much of a problem.

[Nobody]

Essentially what you have drawn is a Ringbom with adjustable lift pressure. Since you have shown a governor it will tend to be a constant speed variable power process.

As shown, the piston and connecting tube will become the cold space.

[Tom Booth]

....

As shown, the piston and connecting tube will become the cold space.

Well, yes. Though my design includes material considerations intended to prevent rather than facilitate incidental heat loss.

For example the power piston, cylinder and connecting tube (if any) could be made of low-conductivity ceramic, or perhaps titanium. And insulated.

So the power cylinder would become the "cold space" with expansion and conversion of heat into work rather than by conducting heat away to a sink, which from my research and experiments, I've come to the conclusion is unnecessary

[Tom Booth]

Continuing from this:

In an internal combustion engine, heat and pressure are more or less synonymous and the ignition spark is, I think, universally situated directly above the power piston, with very little clearance.

The main good reason I can think of for situating the power piston far away and keeping it cold is lubrication. Oil can burn or even explode.

That is OK in an internal combustion engine, they are designed to handle internal petrol explosions, so burning a little oil is not much of a problem.

As a mater of fact, one thing I've been considering seriously is using this design to power an ordinary internal combustion engine, converted to run on a displacer/expanding hot air, in much the same way an IC engine can be converted to run on compressed air or steam.

It would mostly just be a matter of removing the spark plug, gas tank, intake and exhaust, disable the valves, and thread the connecting pipe from the displacer chamber into the spark plug hole.

[Tom Booth]

Essentially what you have drawn is a Ringbom with adjustable lift pressure. Since you have shown a governor it will tend to be a constant speed variable power process.

...

The governor was an unlikely addition, meant only to be suggestive of the engine being capable of self governing in some way.

The mere presence of a governor does not mandate that the engine is constant speed. All internal combustion engines have governor's that work in conjunction with a throttle.

In my drawing, the hand lever is the effective "throttle", which might just as well be a foot pedal.

The governor simply maintains the average speed set by the throttle lever.

The drawing is about as simplistic as I could make it. As indicated, it is a "concept drawing" not an engineering blueprint.

[Tom Booth]

Essentially what you have drawn is a Ringbom with adjustable lift pressure.

Not really.

A Ringbom, though the displacer movement is pressure actuated, still has cold side dead air space, a cold side cooling jacket/heat exchanger, i.e. providing for conductive heat removal to a "sink". Air flow between the hot and cold sides is unrestricted in the Ringbom. Also, the pressure actuation of the displacer is effected in a different way. In the Ringbom the entire displacer chamber is either high or low pressure acting through a shaft that communicates with atmosphere.

My design incorporates an actual seal between the hot space and "air spring", so no exchange of air takes place between them.

The power piston/cylinder is not intended to be any sort of "cold space" or "sink".

[Nobody]

The adiabatic power piston and cylinder will heat up when hot air comes back out of the "potato" displacer chamber when the piston retracts. As expansion takes place the gas will cool. The walls of the cylinder will begin heating, as the gas cools and expands. Higher pressure in the "potato" displacer chamber will be pushing air out. Eventually the air will cool below the cylinder walls. The walls will now be cooling and the air heating. Eventually the piston will reverse.

The compressing air still cooler than the cylinder walls will heat from both compression and the walls. Eventually the air will become hotter than the cylinder walls. The air will heat the cylinder walls as the piston pushes the air back into the "potato" displacer chamber. If the temperature of compression is as hot or hotter than the "potato" displacer chamber no heat will transfer.

The heat converted to work will come back when work is converted to heat on the return stroke.

The same will happen to the air in the "potato" displacer chamber. Air will heat when compressed, cool when expanded. Heat will transfer from hotter to colder. The heat will move out of the "potato" displacer chamber into the cylinder and piston. Cause that is what heat does. It will progress until there is an equilibrium between the cylinder and "potato" displacer chamber. Then the piston will become an air spring.

I could be wrong. Just a peer review. Be skeptical, daring, and learn.

[Nobody]

I liked the idea of the adjustable displacer. And the governor.

[Tom Booth]

The adiabatic power piston and cylinder will heat up when hot air comes back out of the "potato" displacer chamber when the piston retracts. As expansion takes place the gas will cool. The walls of the cylinder will begin heating, as the gas cools and expands. Higher pressure in the "potato" displacer chamber will be pushing air out. Eventually the air will cool below the cylinder walls. The walls will now be cooling and the air heating. Eventually the piston will reverse.

The compressing air still cooler than the cylinder walls will heat from both compression and the walls. Eventually the air will become hotter than the cylinder walls. The air will heat the cylinder walls as the piston pushes the air back into the "potato" displacer chamber. If the temperature of compression is as hot or hotter than the "potato" displacer chamber no heat will transfer.

The heat converted to work will come back when work is converted to heat on the return stroke.

The same will happen to the air in the "potato" displacer chamber. Air will heat when compressed, cool when expanded. Heat will transfer from hotter to colder. The heat will move out of the "potato" displacer chamber into the cylinder and piston. Cause that is what heat does. It will progress until there is an equilibrium between the cylinder and "potato" displacer chamber. Then the piston will become an air spring.

I could be wrong. Just a peer review. Be skeptical, daring, and learn.

I have some difficulty following all the "hotter than" and "colder than" the walls... etc.

Generally though, If you begin talking about "adiabatic" (a change in the working fluid too fast for heat transfer with the surroundings) talking about "eventually" this and "eventually" that, assumes a slow process. So, that seems contradictory.

Your turn of phrase in many instances makes no sense. How can a power piston itself be "adiabatic"? How can a piston "become an air spring"?

Sorry but I'm not going to waste time trying to untangle whatever it is you might be trying to say.