Manson Hot Air Engine Build

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NP317
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Re: Manson Hot Air Engine Build

Post by NP317 »

I think the short length of the hot end on your new project places the heat source too close to the cold end.
That was my immediate reaction to the design when you first presented it.
RussN
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

RussN wrote: I think the short length of the hot end on your new project places the heat source too close to the cold end. That was my immediate reaction to the design when you first presented it.
You may be right. That very thing is what made me think it might work better with the parabolic mirror than with the alcohol burner, since the mirror concentrates the heat right where it needs to be without a bunch of hot gas enveloping the cold end. But it didn't work out that way. I've since been obsessing about why it works so much better when I squirt water into the cylinder vent.

I played around with the engine some more yesterday and I'm slowly abandoning my guess that the performance is improved when water is introduced because the water is boiling and making steam at just the right time. This for a few reasons:
  1. After I get some water on/in it, it will run well for a fairly long time before slowing down again. I don't think the effect of the water would last as long if it was boiling away.
  2. If I spray some water on the hot end when it is running well, the water does not sizzle. This says that even the hot end is below the boiling point.
  3. Some water comes shooting out the cylinder vent, and it doesn't feel anywhere near boiling temperature to my uncalibrated hand.
  4. It seems highly unlikely that water inside the engine would magically boil and generate steam at just the right time within the cycle.
My new theory is that the water inside the engine is just helping the cold side stay cold by cooling off the power piston. One difference between this engine and the Moriya engine I made earlier is that this one has a direct thermal path from the displacer piston--which spends a lot of time in the hot end of the displacer cylinder--and the power piston--which is thermally (and otherwise) adjacent to the cold end. So it may be that the piston assembly is heating the cold end from the inside to the point where the cooling fins just can't cope.

So the next experiment is to somehow thermally isolate the displacer piston from the power piston. One way would be to make the end of the displacer piston that screws onto the power piston out of some (thermally) insulating material. Another would be to make the entire displacer piston out of something less thermally conductive. Not sure what material to use for either idea, though, since it needs to tolerate some heat. Maybe Teflon?

-- Russell Mac
Last edited by rmac on Fri Aug 13, 2021 2:32 pm, edited 1 time in total.
pete
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Re: Manson Hot Air Engine Build

Post by pete »

I'm obviously no materials expert, but it's not hard to do a bit of research with Google. Teflon and heat? Afaik that's not great for the fumes it produces.They make what is loosely called machinable graphite, no idea if it would stand up to the loads as a piston though. It would also lower friction as well. Ceramic is generally a pretty good insulator, a bit tough to add threads to it so some careful redesign would be needed. And higher tech materials usually come with a hefty price tag.
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

I surrender.

I fooled around with this thing some more during last week, never got it to work right, and mostly just confused myself with different guesses as to what was going on. Most of the ideas I had for fixing it involved quite a bit of redesign and rework that I don't have time for right now. So I called it a partial success (since it did sorta run), put the SHCSs and bearings back into inventory, and the rest of it in the scrap bucket.

One thing that's fairly common among the Manson models you see for sale is that they use glass test tubes for the displacer cylinder and the displacer piston itself. One reason of course is that it's interesting to watch the piston moving back and forth, but I'm starting to think that glass also just happens to be a good material for those parts because it doesn't conduct heat very well. With any luck I'll get to try another one someday with that in mind.

-- Russell Mac
tscoupland
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Re: Manson Hot Air Engine Build

Post by tscoupland »

Hi Russell
NO don't give up. I just read your whole build log and it has really inspired me. Very impressed by your skills.
I too watched the Latheman video and wanted to build one like he built with a copper hot end and solid plastic displacer, but unless you have some lying around, ouch ! that plastic is expensive.
There where one or two details missing from the video which you have clarified for me, thanks
There was another build by Plani on the MEM website https://www.modelenginemaker.com/index. ... 243.0.html
his seems to be based on the Walter Haag design. He used the glass tubes for the hot end and displacer.

I am only just at the design stage, playing with ideas in Fusion 360.

Not sure which route to take now.

Tim Coupland
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

rmac wrote: Most of the ideas I had for fixing it involved quite a bit of redesign and rework that I don't have time for right now. With any luck I'll get to try another one someday.
Well, a couple of months later, "someday" is here at last and I've got some time to make another attempt at building a Manson engine.

This time I wanted to work from a set of published, known-to-operate plans instead of my own guessed-at design. Although the drawings on
this site (warning, it's in German) are kind of sketchy, they seemed complete enough to run with. So I started with the basic dimensions given, guessed at a few missing details, and came up with another Fusion 360 model. Besides maybe having more "correct" dimensions, this version has the displacer piston and its cylinder made from glass test tubes. I'm hoping they will be better thermodynamically than the metal parts I had before. Maybe.

Regardless, here's a section through the Fusion model:

cutaway.png

And here are the first two parts:

piston_and_displacer_holder.png

The Fusion model shows a hole through the threaded end of the piston. It's there to keep pressure from building up inside the displacer as the engine heats up. The as-built piston has two holes there. Why? Because the axial one has the broken-off piece of a piece of a drill stuck in it. Rats. Fortunately, I was able to save the part by drilling a second hole alongside the first. I'm calling it the "Plugged Displacer Vent Bypass Port" because that sounds better than "Quick and Dirty Fix #1".

-- Russell Mac
Wolfgang
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Re: Manson Hot Air Engine Build

Post by Wolfgang »

For Stirling engine hot ends, austenitic stainless steel is hard to beat, at least for the model builder. I have experience with the following types:
1) Type 304L austenitic. Better than carbon steel, copper, brass, etc. Relatively short life if run at red-hot temperatures.
2) Type 316L austenitic. Better that 304L even at red-hot temperatures. More difficult to machine. Use very sharp tools plus cutting fluids.
3) Type 310 stainless steel. This stuff may be tricky to get but is by far the most heat and scale resistant of the austenitic stainless steels. And it machines fairly easily. This is my preferred material for hobby type hot air engines.
4) Inconel 600. I have a piece but never used it. Suitable if engine is run at bright red heat for prolonged periods. Then there is inconel X 750...

I make the displacer piston 3 diameters long and of the same material as the hot cap. This helps to keep the cold end cool, to such an extend that an epoxy joint fastening the stainless steel displacer cap to the aluminum base has NEVER failed during thousands! of hours of run time.

Only the top half of the hot cap is heated, with external fins. The wall thickness between the heated part and the mounting flange is kept very thin, .010" or less, to minimize axial conduction and loss of heat.

Go to Youtube, BasementEngineer channel, and look for emergency home generator for a video clip. It has had over 375,000 visits and some interesting commentary. w
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

Thanks for the hints, Wolfgang. I looked at your video, and I think your engine is a (heavily!) modified version of the one published in Popular Science back in the 1960s. Is that correct?

I'm not especially interested in lots of output power, so I don't run my engines red hot!

-- Russell Mac
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

Well, gee whiz. I spent all day yesterday drilling one hole.

The hole in question is the 5/8" diameter, 1/4" deep cavity where the cylinder block attaches to its support post. I didn't want to just plunge in with an end mill because doing that in the past has given me lots of chatter and an oversize hole. For whatever reason, I remembered an ancient thread on this board where Harold suggested roughing out an undersized hole and then cleaning it up with a boring head. Great!

So I dug out my never-used-before-by-me boring head and discovered that I didn't have a suitable boring bar for it. So I backed up and made a boring bar out of some drill rod. That's actually what took most of the day. The roughing and boring went pretty quickly once I got everything set up. And the result was good.

Then today I cut some fins.

The other thing to note here is that there is a brass cylinder (not visible in the picture) pressed into the axial hole. I pressed it in cold this time (again per Harold's suggestion) and that worked much better than when I tried to do a shrink fit on the previous Manson attempt.

-- Russell Mac

cb_and_bb.jpg
pete
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Re: Manson Hot Air Engine Build

Post by pete »

Really nice looking parts rmac.Those boring heads are a pretty powerful tool. Imnsho a mill is only half the machine without one. I've seen a few on Youtube making complex and very time consuming rotary table set ups just to cut a partial but constant internal radius. You have to know the coordinates anyway to position the C/L of that radius, so those, some edge finding and a pre set boring head will do exactly the same a whole lot quicker. And a cross hole boring bar is just as useful for larger holes and radii. Fwiw Criterion makes a good one for heads that take 1/2" shank tools.
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

Thanks, Pete, for the compliment, and also for all the thoughtful and comprehensive help you routinely dispense on this board. It's much appreciated.

My boring head doesn't have the cross hole, but that does indeed look like a fine tool for scraping away at a larger inside radius. I'll keep that in mind. I think the few times I've needed to do something similar, I wound up mounting the part on a faceplate and boring it on the lathe.

By the way, are sets of (I guess) HSS boring bars like these as common as (I think) they used to be? Or has the world gone mostly to carbide?

bb.png

-- Russell Mac
Wolfgang
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Re: Manson Hot Air Engine Build

Post by Wolfgang »

rmac wrote: Sat Oct 23, 2021 1:00 pm Thanks for the hints, Wolfgang. I looked at your video, and I think your engine is a (heavily!) modified version of the one published in Popular Science back in the 1960s. Is that correct?

I'm not especially interested in lots of output power, so I don't run my engines red hot!

-- Russell Mac
Yes indeed Russell, it started out as that engine in Popular Science as I acknowledged in the video, including the cigar tube displacer piston. Many, MANY modifications later it looks as in the video, capable of producing 1 Watt electrical output. The radio requires about 1/2 Watt or so, thus the engine is only loafing when powering that radio.

Even an engine built for occasional running would benefit from stainless steel hot end components because ordinary steel will corrode very quickly in that application because all protective oils are burned off and the steel oxidizes. Perhaps if the hot parts were nickel or chrome plated it would help but I have never done this.
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