Stirling "Hit 'N' Miss" Hot air engine

[Nobody]

Simplified:

Adiabatic expansion followed by adiabatic compression means zero taxable work. The system temperature will equalize until it mimicks an air spring system.

No heat in, zero work out.

[matt brown]

Simplified:

Adiabatic expansion followed by adiabatic compression means zero taxable work. The system temperature will equalize until it mimicks an air spring system.

No heat in, zero work out.

And exactly what nixed Peter Tailer's bogus thermal lag scheme. I never could grasp Tailer chasing a patent for such a lame scheme unless he had a 'hole card' that seemed plausible at filing time. With most patents, you have to carefully read between the lines and (these days) avoid a bunch of needless misdirection.

[matt brown]

Hot Potato.png (8.13 KiB) Viewed 7809 times

Heads up, behold the B&B (Booth & Brown) Hot Potato engine...

Tom - your gif had me harking back to my similar diaphragm scheme where I was mostly transfixed on feeding cylinder gas to heater from below. Gadz, I dreamt up sooo many lame schemes with double-diameter opposed piston and double diameter cylinders (eyes roll)...but I think I see how to solve the problem.

Take a look at your gif (static shot here) and consider a tube at top of cylinder space connected to top of heater space near end cap, where a one-way snifter admits gas into tube, and a one-way snifter valve admits gas into heater space. All 3 volumes (cylinder, heater, & tube) would require some study based on temperature, but the basic scheme appears xlnt if most of Qin is absorbed passing thru hot potato vs 'heater space' proper.

Then, as you suggested somewhere, simply modify 2 stroke ICE and viola !!!

OK, Nobody, time to chime in...

[Tom Booth]

I was thinking more along the lines of one of these, two or four stroke(converted to two stroke) except with a displacer, no valve of any kind would be needed.

https://youtu.be/gy8K-DmP4UU

https://youtu.be/xmapdIx3vWA

https://youtu.be/wYXBMa8lAAU


And, a other difference being; air does not enter or leave the overall system.

[matt brown]

I have a horde of various stuff plucked from ebay over many years, but long stroke stuff is hard to find. If short stroke would suffice, I'd start with RC engine around .60 before moving up to modified ICE. Yeah, valveless would be nice, but I think a snifter valve in transfer tube at heater head is necessary to isolate heat flow (within reason) from intake flow AND keep heater space gas from backflowing into transfer tube. However, with small scale model, it might be possible to dispense with both valves...something I'm still pondering. The trick here appears to be getting the 3 volumes sized correctly.

[Tom Booth]

I have a horde of various stuff plucked from ebay over many years, but long stroke stuff is hard to find. If short stroke would suffice, I'd start with RC engine around .60 before moving up to modified ICE. Yeah, valveless would be nice, but I think a snifter valve in transfer tube at heater head is necessary to isolate heat flow (within reason) from intake flow AND keep heater space gas from backflowing into transfer tube. However, with small scale model, it might be possible to dispense with both valves...something I'm still pondering. The trick here appears to be getting the 3 volumes sized correctly.

Not sure what you have in mind exactly, or why any valves would be necessary.

What I have in mind would be little different from many classic heat engines, like this:

https://youtu.be/zXlm0hqWs9Q

Except the displacer would be "free", and actuated by pressure changes and with an "air spring" and no connecting rod.

The main idea is just to eliminate the cold dead air space behind the displacer. Well, the space would still be there, for the displacer to move into, but sealed off. Rather than cooling the working gas by shunting it to a "sink", that same "cold side" air space would just act as an air spring, without mixing with the heated air at all.

There really doesn't need to be any transfer tube either. The cylinders could be side by side with just a port between them.

The air flow would be the same as any Stirling; basically just simple expansion and contraction.

This is a "free displacer" Stirling:

https://youtu.be/OQfh5exmT3U

Similar to the "hot potato" engine.

Or this:

https://youtu.be/kcbQGug8Obg

Less complicated not more.

Except those two have cold side + dead air space, which it is my intention to eliminate as unnecessary.

Not only unnecessary, but power robbing.

[matt brown]

I'm pondering your hot potato eng per gif, but with transfer tube per previous comm. My current focus is gaming these volumes correctly, otherwise (1) compression will stall due to Wneg>Wpos, or (2) intake to heater space will stall due to reservoir effect (inter-related PVT issue). Then arrange heating such that Qin is mostly from gas passing thru potato vs heater space. I'm gonna write off spring issues and consider Wneg during compression = Wpos during expansion.

[matt brown]

slingshot.JPG (368.5 KiB) Viewed 7758 times

I have a few old sketches on this computer, so behold the 'slingshot'. Consider this a single cylinder Stirling engine with 2 opposed single-acting pistons, A & B, running 1:1. Note TDC & BDC limits at top, hot-regen-cold at bottom, AND no dead volume. Drawn here as a Stirling, the phasing & dead volume issues disappear, but there's various other realities that would tax this simple scheme: (1) hot piston seal (2) meager regen (3) working volume could be more effective as annular space if pistons were stepped and sleeved, but add massive complexity (4) cylinder would be more effective if finned, fluted, etc., but more complexity.

Another variant has 2 paralllel cylinders with conduits connecting each pair of cylinder ends, and cylinder diamters and 'inter' phasing to suit your persuasion (various cycle potentials), but this bugger gets heady fast vs previous version.

[matt brown]

pinwheel.png (250.71 KiB) Viewed 7757 times

Here's another simply scheme, and one of my favorites, behold the 'pinwheel'. Most rotary schemes fall for 2 rotors, as if one ain't good enough. But that's what happens when all you see is Gast air pumps vs Hydrovane compressors. OK, so what we have here is simplicity on steroids where the fast adiabatics and geometry allow 2 flavors: Otto or Atkinson via 2 different inlet apertures (note clockwise rotation). The main issue here is that the HP res at top will further compress the adiabatically compressed gas once the res is exposed, so a small clearance volume at TDC is preferred, along with a wider spread between 'outlet' & 'inlet' than shown. A couple yrs back, some guy tried to patent such, but I doubt he'll get anywhere. Heck, I blew thru this obvious scheme many years ago due to (1) dP across vanes (2) low Pmax vanes (3) low Tmax everything (4) iffy seals. Still, another good study where mech matches cycle, open or closed cycle a nobrainer, no thermal lag issue like dreamy eyed isothermal whatever, no dead volume, and no throttle/regulation issues (the 'senior class' will know how that works).

Then there's the guys at Star Rotor (et al) who are making a career out of reinventing the wheel...

[Tom Booth]

..., I'd start with RC engine around .60 before moving up to modified ICE. ...

I've been experimenting with small models long enough I think

I've been contemplating displacer chambers made from something more along this line lately:

Resize_20220322_105320_0165.jpg (71.22 KiB) Viewed 7748 times

Or maybe one of these pressure tanks. The diaphragm is already built right in. Not sure how much heat it could take though.

Resize_20220322_110542_2851.jpg (42.23 KiB) Viewed 7747 times

The rubber diaphragm I mean, intended for water not scorching hot air.

[Trevor]

Hi Tom
Maybe the hit & Miss engine is a little difficult in an hot air engine but I believe the Bailey Engine was pretty close though it did use a governor of sorts which kind of does the same thing.

[Trevor]

Here is a pic of the engine. One of these were used at the Mossman Mill near Cairns.

[Tom Booth]

I think this "NASA" type engine has a displacer that is actuated by pressure changes.

https://youtu.be/SJsuwIUb0wE

When pressure in the outer can increases (by the "power piston" diaphragm moving down/inward) the lower pressure inside the smaller displacer can lifts the displacer, this adds heat, driving the power piston out, which lowers pressure so the displacer drops down. Heat addition stops, the piston returns etc.

The NASA arrangement still has a cold side and usually water cooling I guess. So, cold side dead air space.

What I'm thinking is that eliminating cold side / dead air space would increase power output enough that a "four stroke" type heat engine might be possible, where "ignition"/heat input is only required every other cycle more or less as needed, similar to Hit-N-Miss.

I haven't actually studied up on what mechanism is used in a Hit-N-Miss type governed engine.

[Trevor]

Tom - Not sure what you are looking at might work. It appears to be a thermo acoustic engine and these need to be well balanced.
Not sure they can be made to variant pressure.
Tibsim is very good at making these and he is member of this group.
Below is a link to one of his engines.
https://www.youtube.com/watch?v=F4XylZS7XH0

Regards Trevor

[Tom Booth]

Yes, I'm familiar with Tibsim's work and thermoacoustic engines generally, in theory at least.

I don't think what I'm working on is thermoacoustic though.

In theory, I think, thermoacoustic engines require considerable "dead air space" for "waves" to propagate.

This engine I'm working on, during the compression stroke in particular, will have almost no air space. Compression should be considerably higher than what would be typical of a more conventional Stirling engine.

I'm not counting the "air spring" as dead air space, though, perhaps it could or should be.

It would, or will be, something like the "hot potato" engine (above) but with a diaphragm behind the displacer with an air pocket behind the diaphragm. Instead of a mechanical metal spring, an "air spring". And a solid displacer rather than a regenerator. And no rod to poke the displacer, it will be forced to move back against the air spring by compression pressure.

The main reason for using a diaphragm is to prevent the mixing of hot and cold air, which, theoretically, reduces power output by continually diluting the hot air with cold air.

The air spring should probably also be thermally isolated from the environment to reduce heat loss from secondary compression within the air spring itself.

I also plan on experimenting with pressurization and diaphragm tension.

It's all pretty theoretical at this point and may not work at all.

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

I'm trying to eliminate all cooling, on the theory that a perfectly efficient engine would utilize all of the heat, converting the heat to mechanical motion, or power output, making active cooling unnecessary.

Instead, "cooling" would result from converting heat to power output.

Last night in the shop, I again ran a "high temperature" Stirling engine glowing red hot with a propane torch and the transfer tube and power cylinder remained quite cool.