Manson Hot Air Engine Build

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rmac
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Manson Hot Air Engine Build

Post by rmac »

Well, well. I thought I was done building hot air engines for a while after I finished the Moriya pair a few weeks ago. But then I stumbled on a video of something called the "Manson" hot air engine and it looks really interesting. It's similar to the Stirling engine, except that the displacer piston is connected directly to the power piston (so they move together), and it has only three moving parts--the piston assembly, the connecting rod, and the crankshaft. What could be simpler?

I decided to try to make an engine similar to the one in this video, but a little bigger in order to use up some of the material left over from the Moriya project. The video includes a sketch of some of the parts, but it is more or less illegible, so I combined what I could figure out from the video with information from other sources to create a plan I hope will work. Here's some of what I was thinking along the way, including links to those other sources:
  • The displacer OD will be 1" to match some stainless steel tubing left over from the Moriya project.
  • Based on the 1" displacer OD, the working piston OD will be 0.46" according to the guidelines given here: http://techref.massmind.org/techref/mem ... anson.html
  • The stroke will match the working piston diameter, according to the guidelines given here: http://www.w-haag.de/mansonmotor.htm
  • The displacer piston length will be 3 times the stroke, so the air is forced away from the temperate middle portion of the displacer cylinder and into the hot and cold ends. This is sort of a guess on my part, but it follows the Moriya design.
  • The displacer piston cap will look about like the equivalent Moriya part.
  • The displacer cylinder ID will be 1-1/16", according to the guidelines given here: http://techref.massmind.org/techref/mem ... anson.html
  • The displacer cylinder length will be the displacer piston length, plus the stroke, plus a little extra to make sure the displacer piston doesn't bump against the ends of the cylinder.
  • The power cylinder will be bushed with bronze or brass.
  • An insulating spacer will provide thermal isolation between the hot cap and the cold end. The spacer will be centered on the displacer cylinder length.
  • The hot end will be generally modeled after the Moriya hot end.
  • The cold end fins and such will be generally modeled after the Moriya cold end.
  • The bearing supports, crankshaft, and connecting rod will be generally modeled after the corresponding Moriya parts.
  • The connecting rod must be long enough (relative to the stroke) to limit sideways forces on the piston and also to result in (somewhat) sinusoidal piston motion. According to internet folklore, the connecting rod length should be somewhere in the neighborhood of twice the stroke, or longer. Longer is better (up to a point), so let's say it's three times the stroke.
Armed with all that and a little help from Fusion 360, I came up with this:

1.png

The following pictures show the internals and make it easier to describe how the engine works. The first thing to notice is the power piston. It's hollow, and has four little holes in the bottom such that the pressure inside the piston is always the same as that inside the displacer cylinder. Then on the left side of the power piston there are two openings. These line up with holes in the power cylinder when the piston is at top dead center and bottom dead center.

2.png

To understand how the engine works, consider the picture immediately above. Imagine that heat is being applied to the bottom of the displacer cylinder, and that the crankshaft is rotating clockwise. The pressure inside the displacer cylinder and the piston will increase, driving the piston and displacer upward. At the same time, more and more of the air will be displaced into the hot end, where it will get heated and increase the pressure even more. Until ...

3.png

Boom. When the piston reaches top dead center, one of the holes in the piston will line up with the vent hole in the cylinder and the pressure inside the engine will immediately fall to that of the surrounding atmosphere. As the crankshaft continues to rotate clockwise (due to inertia), the piston and the displacer will be forced downward. This will in turn force the air from the hot end of the displacer cylinder into the cold end. As the air cools, the pressure inside the engine will drop, creating a suction that will help further to pull the piston downward. Until ...

4.png

Boom (again). When the piston reaches bottom dead center, the other hole in the piston will line up with the cylinder vent hole and the pressure inside the engine will immediately rise to that of the surrounding atmosphere. Inertia will then cause the piston to start rising again and the cycle will repeat.

That's the theory, anyway. If anybody sees something that looks wrong or missing from all this, or something that could be improved, I'd love to hear about it. I still need to make some working drawings before I can actually cut up any metal. So it's definitely not to late to make changes.

-- Russell Mac
Last edited by rmac on Fri Aug 13, 2021 9:32 pm, edited 1 time in total.
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NP317
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Re: Manson Hot Air Engine Build

Post by NP317 »

Interesting design concepts. The operating description does not make sense to my overheated brain today.
Maybe I will better understand it on a cooler day...
I will be watching with interest.
RussN
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

I'm just getting back to this after a week (or two!) of distractions. I want to shrink fit a 3/4" OD brass plug into the aluminum block that forms the cold end. You can kinda see this in the pictures I posted earlier. Then I'll bore out the brass later to form the power cylinder. I've not tried a shrink fit before, so I'm wondering how big to make the hole in the aluminum.

Here's what I'm thinking at the moment:

The coefficient of thermal expansion for aluminum is about 13x10-6 inches/(inch-degree F). If I heat the aluminum to about 400F above room temperature, a 3/4" hole should expand by 13x10-6 * 400 * 3/4", or about 0.004". It seems reasonable to split that in two and make the hole about 0.002" smaller than the brass plug at room temperature. Sticking the brass in the freezer should give even more wiggle room.

So two questions:

Is this even a good idea?

Does the 0.002" number sound about right?

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

Post by Harold_V »

I applaud your ability to deal with the numbers.
One thing to think about is what happens when you install a bushing in a bore. Thin walls collapse quite easily, and the coefficient (of expansion) of aluminum is greater than the coefficient of copper alloys, if memory serves.

That said, if you don't have enough press (or compression), you run the risk of the bushing coming loose when heat is applied.

I'm of the opinion that you don't need heat to install the liner. Don't install the sleeve as a solid. Rough it, leaving adequate stock for finish machining after it's installed. Assuming you have an arbor press, simply provide two or three thou press fit and push it home. By providing a short lead on the liner (a snug slip fit in the bore) you can get it started without issue. The heavy press will collapse the bushing, providing more than enough holding power. That's assuming the aluminum in which it gets pressed has an adequate wall thickness.

When doing something like this, I like to leave stock on both ends, so they can be machined after the insert is in place. Makes it a lot easier, as you don't have to fight the depth of the install.

I'm not a fan of preheating aluminum, especially when it has been aged, or otherwise hardened. You run the risk of losing the aged condition, which, once gone, can not be recovered without solution annealing the aluminum. That may or may not be an issue for your situation, but it's something to keep in mind for the future.

H
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

Harold wrote: I applaud your ability to deal with the numbers.
Thanks!
Harold wrote: the coefficient of aluminum is greater than the coefficient of copper alloys, if memory serves.
Yes, about 20% greater.
Harold wrote: Assuming you have an arbor press
Ay, there's the rub. I don't have an arbor press. That's one reason I'm thinking about the heat-then-shrink idea. (The other reason is just that it sounds like something interesting to try.)
Harold wrote: When doing something like this, I like to leave stock on both ends, so they can be machined after the insert is in place. Makes it a lot easier, as you don't have to fight the depth of the install.
I have that covered, for exactly the reason you mention.
Harold wrote: I'm not a fan of preheating aluminum, especially when it has been aged, or otherwise hardened. You run the risk of losing the aged condition, which, once gone, can not be recovered without solution annealing the aluminum. That may or may not be an issue for your situation, but it's something to keep in mind for the future.
I'll admit to being 100% clue-free on this subject or whether it even matters for what I'm doing. But my gut tells me that there must be some minimum temperature required in order to change the characteristics of the material. Am I really going to mess things up with temps in the kitchen oven range?

-- Russell Mac
John Hasler
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Re: Manson Hot Air Engine Build

Post by John Hasler »

.002" seems like a lot of interference on .75". I'd go with .001". There are tables and calculators on the Web if you want to get it exact.

Note that the assembly has to be done quickly and in a single motion. It's best to use jigs and fixtures to be sure the part goes in straight, square, and all the way even though it may seem like it should just drop in. I always set it up on my arbor press even though I often don't actually need to use it. In your case you will want to be ready with a drift and a hammer. Get every thing ready before you start heating the aluminum and perhaps even do a dry run. It's frustrating to get the insert cocked and scrap both parts.


What is the highest temperature the assembly will be subject to in operation? The aluminum will expand more than the brass so you need to be sure you will still have interference. The difference is small for brass and aluminum, though.
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

John Hasler wrote: Note that the assembly has to be done quickly and in a single motion. It's best to use jigs and fixtures to be sure the part goes in straight, square, and all the way even though it may seem like it should just drop in. I always set it up on my arbor press even though I often don't actually need to use it. In your case you will want to be ready with a drift and a hammer. Get every thing ready before you start heating the aluminum and perhaps even do a dry run. It's frustrating to get the insert cocked and scrap both parts.
Thanks for the extra hints, John. I didn't really think about getting everything set up ahead of time. I'll do that for sure. For alignment, I took Harold's suggestion and turned down the sleeve for a short distance so it will slide into the undersized hole. That seems like it ought to work fine.
John Hasler wrote: What is the highest temperature the assembly will be subject to in operation? The aluminum will expand more than the brass so you need to be sure you will still have interference. The difference is small for brass and aluminum, though.
I don't really have a guess on that. It's the "cold" side of a hot air engine, but the fins on the my other, similar engine get plenty hot. I guess we'll see.

-- Russell Mac
John Hasler
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Re: Manson Hot Air Engine Build

Post by John Hasler »

>I took Harold's suggestion and turned down the sleeve for a short distance so it will slide into the undersized hole.

I do that as well when possible. I've found that it is still best to have a fixture to guide the insert in.
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

The shrink fit exercise turned out to be a good news, bad news story.

Bad news first. I failed to get the bushing pushed into the aluminum part as far as it was supposed to go. Almost, but not quite. The reason could have been one or more of any of a number of things:
  • I had the parts all made before I read John's suggestion that .002" of interference might be too much. So maybe .001" would have worked better.
  • My toaster oven might not get as hot as it says it does.
  • My expansion calculations might have been incorrect.
  • My measurement of the parts might have been off.
  • Maybe I wasn't quick enough with the assembly.
  • Etc.
  • Etc.
  • Etc.
I can definitely see where an arbor press would be a huge help. Anyway, the good news is that there's enough extra material on both parts that I can still finish the assembly according to the print.

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

Post by Harold_V »

In a pinch you can use your mill vise. Center the object on the screw (by eye) and tighten the vise. So long as you don't use a cheater, there's no harm done.

Shrink fits work very well when you have abundant clearance and you can move fast enough to get the parts properly assembled. You're better off cold pressing items like this, due in part to the limited mass and rapid heat transfer. Even with a serious amount of press, so long as the insert isn't solid (has a reasonably thin wall), it will collapse to accommodate the excessive material.

H
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

After another week-long distraction from this project, I got the cold end done today, but not without some problems. Somewhere along the line as I was milling the 5/16" x 1/8" slots where the uprights attach, the end mill I was using pulled out of the collet a little bit without my noticing and the slots came out too deep.

I'm not sure exactly why that happened. Maybe I didn't crank down on the drawbar enough? Or maybe I was cutting too aggressively? With a dull cutter? Or maybe a chip or something was stuck between the collet and the cutter? Who knows!

Anyway, I had enough time invested in the part that I didn't want to start over, so I cheated by JB Welding some aluminum strips into the errant slots and redoing just the slots. The end result is kind of ugly, but it will work even if the epoxy fails because everything is held together by the screws that attach the uprights to the cold end.

I'm hoping to have more time to work on this thing from here on out.

-- Russell Mac

cold_end.jpg

upright.jpg
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NP317
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Re: Manson Hot Air Engine Build

Post by NP317 »

Nice Save.
RussN
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