Another Simple Hot Air Engine

Disassembled some of it and found a very small amount of oil in the long air passage (magenta coloured line) and between the faces indicated by arrows. If oil can get in there then air can most likely get out. The oil must have either run down from the displacer rod links and found its way through the bush or past the piston and down the air passage.

I'm not sure if gaskets would solve the issue (everything is now as tight as it can be) but even then it seems likely that cooling could be the main issue. I'm thinking about rejigging things, making it into a horizontal displacer attached to a much thinner and lighter piece of metal at the cold end and running a fixed tube to the power cylinder. Turning the standard through 90 degrees would totally disconnect it and the cylinder from the hot bits. That arrangement would be exactly the same as an engine I made ages ago (which also didn't run ) BUT I think most of the moving parts in that engine were too heavy and things probably didn't fit as well.

Quote from: PaulR

Quote from: Dave OttoAn insulator block between the hot and cold end would help, I use Linen Phenolic on mine with a thin gasket on each side. You would need to adjust the thickness of the cold end block to accommodate the thickness of the insulator. You could  also cut fins into the block, I think that it has way too much thermal mass to ever work the way you have it.

I was planning on putting a heat resistant sheet around the displacer near the upper end but hadn't worked out the means to hold it there. Re the thermal mass I figured that although heat would still spread throughout the engine it would radiate away and never get as hot as the hot end. Seems there's not enough difference as it stands though.

The insulator block goes between the metal parts not around it, to slow heat migration, see attached photos. All my engines 13 or so of them are happiest when they warm up and reach an equilibrium so to speak. The cold end gets warm but is able to maintain the proper differential for the engines to run well. I have also done as Jason pointed out and installed a heat deflector to also aid in keeping the heat away from the cold end. Some one commercially produced engines also incorporated this into their designs. Lake Breeze fans come to mind, along with many others.

Dave

Quote from: Dave OttoThe insulator block goes between the metal parts not around it, to slow heat migration, see attached photos.

It's worth trying that before I cannibalize it. The only material I have is 1/4" thick 'mill board' (or 1" thick vermiculite). I've sawn some off the mill board and it would probably drill ok but I'd need to thin it down otherwise it'll push the displacer cylinder back by 1/4" adding significant dead volume. I'll get the facemask on and experiment - it may be possible to file it thinner - I wouldn't want to put it in the lathe as it creates a fine white powder when sawn. There will still be some metal to metal contact though as the cylinder extends beyond the flange by about 1mm as I thought that would make it less likely to leak.

Nice engines BTW!

Hi Paul, I think mill board and vermiculite might be porous if used like a gasket, you can get heat resistent gasket material in various thicknesses either from a car spares shop or the infernalnet, not too expensive.
Andy

Quote from: Spinner21Hi Paul, I think mill board and vermiculite might be porous if used like a gasket, you can get heat resistent gasket material in various thicknesses either from a car spares shop or the infernalnet, not too expensive.

I tried a chunk of mill board between the displacer and the frame before reading your reply. I sanded down the rough side before using it which reduced the thickness a tad as well. It made a huge difference to the amount of heat being transmitted in the original and horizontal orientations but it still refused to run. I still think there could be an air leak between some of the mating faces. I'll have another play with it tomorrow but might resort to bypassing those potential leaks with a bit of copper pipe.

Anyway, useful to know that the materials might be porous so thanks for that.

Paul.
Drill a small hole from a surface that will connect to the inside cavities and gently pressurise the system. You could immerse the unit in water or use a soapy water solution spayed over the suspect joints. Any leaks will be detected immediately.
You can then plug it with a screw and PTFE to reseal it.
Regarding gaskets I have mentioned the use of general purpose Silicone sealant many times it is both heat resistant and easy to remove and replace.

  Graham.

Quote from: Alyn FoundryDrill a small hole from a surface that will connect to the inside cavities and gently pressurise the system. You could immerse the unit in water or use a soapy water solution spayed over the suspect joints. Any leaks will be detected immediately.
You can then plug it with a screw and PTFE to reseal it.
Regarding gaskets I have mentioned the use of general purpose Silicone sealant many times it is both heat resistant and easy to remove and replace.

Cheers Graham, why didn't I think of that?!! In fact I already have a hole that's temporarily plugged with a screw and ptfe tape - it's the just-visible black one resting on the base of the pillar drill (next to the yellow blob) in the last photograph. I'll give that a try later.

The silicone is also a good tip - it was one of the things that passed through my mind yesterday but I imagined the heat would cause problems. The other slight issue is that there's only a 2mm passage running up through the mating surfaces (green arrows in this image) which is very near an edge - I'd have to be very careful not to block that. That's the bit I'm considering bypassing with a pipe, from the temporarily plugged hole to the back of the cylinder.

EDIT second attachment shows current air passages in red, bypass route in green.

Made a little inlet, submerged it in water and blew some air through it. It was leaking around the piece of millboard but I was pleased to see not even a tiny bubble around any of the other joints or the displacer rod bush. I took that bit of insulation out and reassembled so it was in original form but there was still a leak in that area... it was coming out via the saw cuts shown in post #13 which I thought would be covered by the projecting displacer tube. I've filled them up as best I could with some JB Weld so it'll have to wait until tomorrow to see if that fixes it but I'll use some vaseline or silicone to help seal it when I reassemble.

Hi PaulR .
Based on my own experience:
No matter how simple the design, you must ensure a temperature difference between the hot and cold sections—and the greater that difference, the better.
Therefore, I would designate about one-third of the engine's hot section (  your engine ) to serve as the heat dissipation area. This is because, as the displacer moves forward, the hot (expanding) air travels to the rear; there, it needs to be cooled as rapidly as possible so that its volume contracts. The expansion of the air volume within the engine is crucial.

if you rely on the metal sections at the rear—the engine casing—they play a very minor role; even adding insulation won't make much of a difference. This is because the actual area allowing the hot air to cool accounts for only about one-third of the area behind the  hot cover.. That area really needs the fastest cooling or heat dissipation.

And yes, it can run; I built a similar engine a long time ago. However, it needs some fine-tuning—specifically the power piston stroke—to achieve optimal efficiency - That is a different battle.

Some good information from Minh Thanh. 

Your concern about blocking the transfer with sealant could be eliminated by using a tubular dowel  as used by most modern automotive manufacturers. 😉

Now you have a nipple attached to the system you could also test the action by using a cheap plastic syringe and tube, manually providing either a negative or positive pressure. 😉

  Graham.

As an addendum. A simple  “ U “ tube manometer made from clear Silicone tubing, filled with water could be connected to the nipple to see how many inches of water displacement is achieved by the heat applied.
GRC.

Quote from: Minh ThanhNo matter how simple the design, you must ensure a temperature difference between the hot and cold sections—and the greater that difference, the better.
Therefore, I would designate about one-third of the engine's hot section (  your engine ) to serve as the heat dissipation area.

Thanks for the detailed reply. This is the same situation I faced with the last engine - how to fit some cooling mechanism to that part of the displacer cylinder. The end cap of the cylinder is slightly larger in diameter than the body so I can't slide something on which makes adding fins tricky at this stage. Also, in a vertical orientation the fins will get the heat from the burner unless it's enclosed which doesn't leave much space. I think I'll rig up something temporary at the cold end just to give a bit of cooling and see if there's any life in it at all while on its side. If there is, I'll come up with a more permanent solution but I think the engine might end up living on its side rather than vertically as originally planned.

Hi PaulR .
I don't know why it's difficult to install the heatsinks... but one solution is to set up a simple water cooling system.
  Try to align your engine design with the true principles of the Stirling engine—specifically, the heating and cooling zones.

Quote from: Minh ThanhI don't know why it's difficult to install the heatsinks... but one solution is to set up a simple water cooling system.

Thanks, I'm thinking about it!

Hi !
With a Stirling engine, for instance, if it spins at around 500 rpm, the time the air spends in the hot zone (heating up) and the cold zone (cooling down) is extremely short; consequently, the resulting changes in air volume—whether expanding or contracting—are minute. Therefore, airtightness and friction are the two key issues you need to address. Airtightness is crucial to ensure that the changes in air volume effectively act upon the power piston. Regarding friction: since the volume changes are so small, the power output is also minimal, so you must minimize friction as much as possible (e.g., ensuring smooth movement between parts, reducing component weight, etc.).
Stirling and "flame-eater" engines may have few parts, but they are truly challenging to build.

        ""If you can do it, so can I. And I might even do it better.""

Quote from: Minh ThanhStirling and "flame-eater" engines may have few parts, but they are truly challenging to build.

Absolutely, I'm rememer the Scott flame licker I made and how many times I had to put it away (rather than throwing it away!  ) before finally getting it to run.

Paul I'm watching your perseverance with this build and hopefully one change, whatever it may be, and the engine will burst into life