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Old 07-08-2008, 01:03 AM   #11
joe d
 
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Default Re: My first Hot Air (Stirling) Engine

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Originally Posted by NickG


I haven't heard of sterno to be honest! I did a quick search on google and it seems it is some sort of gel? Don't know what an equivalent would be.

Nick
Nick: I believe it's labelled as "Chafing" fuel for the UK market. Look under the warming trays next time you're dining up-scale, and swipe the tin!

Presumably available where camping/caravaning supplies are to had?

Joe


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Old 07-08-2008, 01:31 AM   #12
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Default Re: My first Hot Air (Stirling) Engine

An Esbit-type tablet might work well too, if they still have those.

On my sterling fan (seen above, for now), the two biggest pain points were cylinder fit for the piston and frictional drag in the displacer & con-rod for it. The piston had to drop through the cylinder on it's own, yet 'pop' when pulled out.. Graphite made a big help here.

The displacer body and rod were dragging a tiny bit, which caused all sorts of troubles as well. They need to have minimal clearance, but some clearance. You probably ought to be able to disconnect the power cylinder and free-spin it for tens of seconds.





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Old 07-08-2008, 04:59 AM   #13
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Default Re: My first Hot Air (Stirling) Engine

Here are some quotes from James Rizzo's book, The Stirling Engine Manual.

First, he says the displacer piston and cylinder should be made from stainless steel as first choice, mild steel as a second choice. For an optimal regeneration effect, you want the hot end of the displacer to be hot and the temperature tapering off toward the cold end. The problem with aluminum is, it conducts heat so well, that it will soon be hot over it's entire length and loose all regenerative effect. Stainless, and to a lesser degree, mild steel will absorb and give up heat pretty well, but do not conduct head as efficiently.

Also, minimizing the amount of dead space anywhere in the engine will maximize the pressure differential between hot and cool air. I recall talking with Rudy Kouhoupt before his death about both the dead space and the materials for the displacer and displacer cylinder, and he pretty much agreed with Rizzo. You want the clearance between the displacer piston and cylinder as small as you can get it without touching. And, according to Kouhoupt, you want the displacer piston walls as thin as you can get them.

Here are some more quotes in Rizzo's book, which was taken from a study made by R. Sier and published in Model Engineer, Sep 3, 1976:

1. The length of the displacer chamber = 3 times is diameter.

2. The length of the heated chamber = 2/3 of the length of the displacer chamber (cylinder).

3. The length of the cooling chamber = 1/3 of the length of the displacer cylinder.

4. Swept volume of the displacer = 1 1/2 times the swept volume of the power piston.

5. Length of the displacer = 2/3 of the length of the displacer cylinder.

6. Stroke of the displacer = 1/3 of the length of the displacer cylinder.

Rizzo also acknowledges that although these seem to be optimal dimensions, and followed by many successful stirling engines, there is room for considerable departure as required by the specific model.

Chuck F.
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Old 07-08-2008, 05:57 AM   #14
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Default Re: My first Hot Air (Stirling) Engine

Thanks Joe, will have a look for that in the local camping shop.

Nice engine Shred. My piston does drop through and pop when pulled out, that's why I was quite impressed with myself, however, maybe the pop isn't a loud enough one!

The displacer rod doesn't seem to drag, it will spin for maybe 10 seconds with power piston disconnected, however, there may be slight leakage around it.

Cfellows, thanks for all the useful information, I will try to build those parameters into my next design!

Nick
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Old 07-08-2008, 03:29 PM   #15
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Default Re: My first Hot Air (Stirling) Engine

Sterno is sold as a small (4 oz. and 7 oz. cans here) tin containing a pinkish, highly flammable gel which, when ignited, puts out a prodigious amount of heat. Its primary uses are for heating chafing dishes and the like and as a convenient small heat source for backpackers and such.

In the USA we have a supplier chain, Smart & Final, which caters to the needs of commercial caterers, etc.. They carry it. I've never checked camping supply stores but they certainly might have it.

Incidentally, Sterno is an ideal way to heat a small vertical firetube boiler. In fact, it's called out in the design of the boiler for the steam roller I'm building.
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Old 07-10-2008, 02:16 AM   #16
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Default Re: My first Hot Air (Stirling) Engine

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Originally Posted by NickG
Thanks Joe, will have a look for that in the local camping shop.

Nice engine Shred. My piston does drop through and pop when pulled out, that's why I was quite impressed with myself, however, maybe the pop isn't a loud enough one!

The displacer rod doesn't seem to drag, it will spin for maybe 10 seconds with power piston disconnected, however, there may be slight leakage around it.

Cfellows, thanks for all the useful information, I will try to build those parameters into my next design!

Nick
I don't think slight leakage is a big deal. Another way to check the piston is to close one end of the cylinder and push on it-- it should feel like there's a spring under it.

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Old 07-23-2008, 01:03 PM   #17
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Default Re: My first Hot Air (Stirling) Engine

Hi all,

A quick update on this engine... I am happy to say to coin a phrase 'the engine has surrendered'! Well, after a fashion anyway, whilst being tortured with an ice cube and a butane pencil torch!

I bored out the displacer cylinder to 9/16" and made a new hot cap and displacer piston 1/2" diameter. This now gives a ratio of swept volumes of 1.78:1 instead of 1:1 (it was never going to work with 1:1!)

The engine nearly ran with the small butane torche but wouldn't quite keep spinning. However, if I also put an ice cube on the cold end the engine did run, only at a couple of hundred rpm though.

So I think the main problems now are lack of cooling on the cold end and heat transfer between the hot cap and the cold end, there may be an excess of friction still in the system also.

The fins are far too small, I think water cooling would be the way to go but since the displacer cylinder is aluminium it would not be easy to make a water jacket. I always mess things like that up, however, maybe I could make one up from brass sheet and glue it on with some sort of epoxy resin?

I should also make a new hot cap with a flange and some insulating material between it and the displacer cylinder (cold end).

Once the ice cube has gone, the temperature difference between the cold end diminishes too much and the engine quickly comes to a stop. Not sure how long it would run with cold water surrounding the displacer cylinder, maybe 5 mins. I might try to make this jacket and a spirt burner and at least I can do short demonstrations then. Now that I have made the larger displacer though, the engine looks a bit out of proportion though, as I also took the opportunity to make it 1/2" longer.

I may just make the burner and a base then put this one to bed, remembering always to carry ice cubes with me if I take the engine haha! I am pretty confident I could design a working engine straight off now but want to move onto other things. I want to do a flame licker and an i.c. engine before I go on to my main project which is a sweet pea 5" narrow gauge loco.

Will post up pics and video soon.

Thanks for your help.

Nick
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Old 07-29-2008, 12:26 AM   #18
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Default Re: My first Hot Air (Stirling) Engine

Hi all,

Another update on my first stirling engine. I've now made a burner and a wooden base. You will be able to see that woodwork is not one of my strong points but it is OK. Also made a burner, I was quite pleased with the way that turned out, however, it could do with having a larger capacity. It runs out too quickly, I realised this as I was making it! Also, I should have offset the hole for the wick as I've found it prefers the heat to be pretty much at the end of the hot cap when running, which means the burner is right at the edge of the wood and doesn't look right! This is also because I extended the length of the hot cap as stated in previous post, this means the hot cap is now too long relative to the engine base!

I also started on a sheet brass water jacket which I think may allow the engine to work by covering the displacer cylinder in cold water. However, I'm in two minds whether to put it on, I would have to glue it somehow onto the aluminium, then I'd need to make some bits that slot down from the top and glue them in place also, sounds messy! I was thinking I could make a square aluminium plug for the top with a slot in it to make it look prettier. What are peoples thoughts on this? I could just leave it air-cooled as a static model and put this one down to experience, but on the other hand it would be nice if I could at least demonstrate it working for a few minutes!

Please see pictures and videos below,

Nick
















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Old 07-29-2008, 12:32 AM   #19
chuck foster
 
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Default Re: My first Hot Air (Stirling) Engine

hey nick that sure is a nice running engine

one day after i get all the current projects finished i would like to build one (i will be about 300 years old by that time )

seriously though very nice workmanship

chuck
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Old 07-29-2008, 02:04 PM   #20
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Default Re: My first Hot Air (Stirling) Engine

Thanks for the kind words Chuck, however, about half the workmanship is not good at all! There are quite a few mistakes, I changed lathes about 1/2 way through the project though and that improved things dramatically. I can say that I've learnt a lot from this and used many new techniques so my next one should see a marked improvement, fingers crossed!

Nick


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