Sterling(beta) refuses to work at all after 7 builds.. Help!

Discussion on Stirling or "hot air" engines (all types)
Hopper
Posts: 54
Joined: Fri Feb 08, 2013 6:54 pm

Re: Sterling(beta) refuses to work at all after 7 builds.. H

Post by Hopper » Thu Feb 28, 2013 6:41 pm

There are a few rules of thumb floating around, based on what the original Mr Stirling wrote and what has been found since, BUT many working engine plans do not follow those rules of thumb but were found by trial and error by guys who developed their own plans.
The "Tin Can" engines seem to vary from these rules of thumb by a huge degree.

Some rules of thumb for "real" Stirling engines made in a machine shop:

Power piston diameter is about the same as the stroke. IE, a 1" power piston will have a 1" stroke.

Swept volume of the displacer piston is 1.3 to 1.5 times the swept volume of the power piston. (ie area of the piston X stroke, ie 3.14 X Piston Radius X Piston Radius X stroke)

Displacer piston length is about three times the displacer diameter.
Displacer piston has a small clearance in the displacer cylinder and has a small end clearance at each end of its stroke.
Displacer chamber length = length of displacer + stroke + small clearance each end.

Most engines are built with the same stroke for both pistons, but the displacer piston is made larger to get the extra volume. Eg a typical engine might have a 1" stroke for both cylinders, with a .750" diameter power piston and 1" diameter displacer piston that is 3" long and a displacer chamber that is just over 4" long.

BUT your tin can engines with rubber diaphragms seem to throw this out the window.
Your thread here has got me interested in tin can engines ( I usually make my models from brass etc on a lathe) so I am now making Myfordboy's Travel Sweets tin engine.
It uses a displacer about 95mm diameter and 18 mm thick in a can that is 100mm diameter and 40mm long.
Displacer stroke is 12mm.

Power "piston" latex glove diaphragm is 30mm diameter and has a 6mm stroke.

All these dimensions are on Myfordboy's templates that he will email if you contact him, and is clearly explained on his blog page and excellent video.


So you can see there is no magic forumla that fits all applications and types of engines. That is why it is best to stick with an establised design for your first build.

If you really want to learn the finer principles of how the Stirling works so you can do your own design work, Google around for Andy Ross's book on Stirling Engine Design and the NASA book on Stirling Engine Design Manual.

Insainhouserecords
Posts: 47
Joined: Fri Feb 15, 2013 3:18 pm

Re: Sterling(beta) refuses to work at all after 7 builds.. H

Post by Insainhouserecords » Thu Feb 28, 2013 6:56 pm

I got to tell you man, that was an awesome answer! Lord knows it seems the sudo "rules of thumb" where nowhere to be found! If this was yahoo answers you would take the cake. Thank you to the bearer of knowlage.

Hopper
Posts: 54
Joined: Fri Feb 08, 2013 6:54 pm

Re: Sterling(beta) refuses to work at all after 7 builds.. H

Post by Hopper » Thu Feb 28, 2013 7:32 pm

Insainhouserecords wrote:I got to tell you man, that was an awesome answer! Lord knows it seems the sudo "rules of thumb" where nowhere to be found! If this was yahoo answers you would take the cake. Thank you to the bearer of knowlage.
Like I said though, don't think you can take those rules of thumb and apply them to tin can engines. It seems like all the working tin can engines use a displacer diameter that is huge compared to the power piston or diaphragm.

Insainhouserecords
Posts: 47
Joined: Fri Feb 15, 2013 3:18 pm

Re: Sterling(beta) refuses to work at all after 7 builds.. H

Post by Insainhouserecords » Thu Feb 28, 2013 7:58 pm

Actually I've been thinking of using a Tin lamp oil can that has a screw top but is much like the (sweet can). I've also been swapping crank types around also seem to be ok if I can just calculate the crank throw and displacer size right. By types I've really seen two main types, the displacer in the middle and pushrods on sides type. This crank is made by making full bends in the wire. The other is a crank not actually bent outward on its crank slots but rather it has a slight bend for the displacer slot and another slight bend for the pushrod slot. The first type can be seen on scraptopower and the second on Jim larson's "simple can engine".

Hopper
Posts: 54
Joined: Fri Feb 08, 2013 6:54 pm

Re: Sterling(beta) refuses to work at all after 7 builds.. H

Post by Hopper » Thu Feb 28, 2013 9:01 pm

Yeah the scrap to power crankshaft looks more complex because is has two connecting rods to the diaphragm.
He says in there that the crank throw for the displacer should be about 20mm and for the power diaphragm about 5mm.
So that gives a stroke of 40mm for the displacer and 10mm for the diaphragm.
There are your measurments you need right there.

Then you make your displacer the length required so that it has a small gap of say 2mm top and bottom.
So displacer length = length of can minus 40mm stroke, minus 4mm clearance.
Displacer diameter should be a few millimeters less than the inside of the can.

When bending the crank, the crank throw for the displacer is 90 degrees from the crank throw for the power diaphragm.
So when the displacer is at the very top, the diaphragm should be in about the middle of its stroke, ie flat diaphragm.

Hopper
Posts: 54
Joined: Fri Feb 08, 2013 6:54 pm

Re: Sterling(beta) refuses to work at all after 7 builds.. H

Post by Hopper » Thu Feb 28, 2013 9:11 pm

PS
If you are building an engine with the small diaphragm off to the side from Scrap To Power, did you see this template:
https://037b0921-a-62cb3a1a-s-sites.goo ... edirects=0

Looks like 15mm cramk throw for the displacer and 5mm for the small diaphragm mounted on the elbow off to the the side.
That is 30mm displacer stroke and 10mm for the power.
Then make your displacer to fit the can you have, ie length of outercan minus 30mm stroke minus 4mm clearance.
Make displacer a sloppy loose fit several millimetres smaller diameter than the tin.

Careful bending of the wire crankshaft is important so everythign lines up with minimum friction.

Insainhouserecords
Posts: 47
Joined: Fri Feb 15, 2013 3:18 pm

Re: Sterling(beta) refuses to work at all after 7 builds.. H

Post by Insainhouserecords » Thu Feb 28, 2013 10:40 pm

Thanks man. I'm going to try again tomorrow using some of these figures. Hopefully things will workout!

Hopper
Posts: 54
Joined: Fri Feb 08, 2013 6:54 pm

Re: Sterling(beta) refuses to work at all after 7 builds.. H

Post by Hopper » Fri Mar 01, 2013 12:10 am

Insainhouserecords wrote:Thanks man. I'm going to try again tomorrow using some of these figures. Hopefully things will workout!
You'll get there. It just takes patience

Insainhouserecords
Posts: 47
Joined: Fri Feb 15, 2013 3:18 pm

Re: Sterling(beta) refuses to work at all after 7 builds.. H

Post by Insainhouserecords » Fri Mar 01, 2013 6:58 am

Thank all of you that didn't give up in my cause. The future is bright! Maybe this post will help a lot of people that can find the information listed here and use it for the incline of man!

Insainhouserecords
Posts: 47
Joined: Fri Feb 15, 2013 3:18 pm

Re: Sterling(beta) refuses to work at all after 7 builds.. H

Post by Insainhouserecords » Sat Mar 02, 2013 1:17 pm

Should the diaphragm only swell when the displacer falls down or when its pulled up?

Insainhouserecords
Posts: 47
Joined: Fri Feb 15, 2013 3:18 pm

Re: Sterling(beta) refuses to work at all after 7 builds.. H

Post by Insainhouserecords » Sat Mar 02, 2013 1:30 pm

Also, does the action of the diaphragm push the crank or does it pull the crank? I seem to get pressure both ways but with the connections I'm using for the pushrods to connect to the crank might not be suitable. I see a lot if people saying to use "electric terminal blocks" but my hardware store here doesn't have them. So, I am using hooped wire ends. I don't have much slack and they roll rather smoothly themselves. Is this ok or might this be one of my problems? What are substitutes that you guys have can up with?

Ian S C
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Location: New Zealand

Re: Sterling(beta) refuses to work at all after 7 builds.. H

Post by Ian S C » Sat Mar 02, 2013 7:47 pm

A place to get electrical fittings, You could try an electrical contractor or similar for old used stuff, they change stuff all the time, and it just gets put it the rubbish, he might even let you look through the rubbish. Another place could be a demolition yard. If you can't find old stuff, an end could be made by taking about 2" of stiff wire, bend it round a nail or something about the same size as the crank so that half way along you form a small loop, maybe 2 turns, and bring the 2 ends parallel, these can then be attached to the con-rod with glue, or solder. Hope thats semi inteligent. Ian S C

Insainhouserecords
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Joined: Fri Feb 15, 2013 3:18 pm

Re: Sterling(beta) refuses to work at all after 7 builds.. H

Post by Insainhouserecords » Sat Mar 02, 2013 8:38 pm

Thanks man, that will work. I have a similar setup on my engine. I'm actually remaking the displacer now with new calculations. My next worry is that my (simple style) rear diaphragm PVC pipe isn't large enuf compared to my displacer.

Hopper
Posts: 54
Joined: Fri Feb 08, 2013 6:54 pm

Re: Sterling(beta) refuses to work at all after 7 builds.. H

Post by Hopper » Sat Mar 02, 2013 8:47 pm

Insainhouserecords wrote:Should the diaphragm only swell when the displacer falls down or when its pulled up?
What causes the diaphragm to swell upwards is the expansion of the air trapped inside the can when it is heated up by the burner or candle.
The displacer itself does not make any direct difference to the air pressure inside the can.

What happens indirectly is when the displacer is at the bottom of its stroke, the bottom of the can is occupied by the displacer and most of the air is at the top of the can, where it is cool. So the air cools off and and contracts, so the diaphragm will return to its lower position.

When the displacer is at the top of the stroke, the air in the can has to move down to the bottom of the can to get out of the displacer's way.
This puts the air in contact with the hot end of the can by the burner/candle. The heated air expands. The expanding air increases the pressure in the can. The increased pressure pushes the rubber diaphragm upwards.

So you can check the sealing of your can if you do not have the crank or rods in position by holding the displacer in the upper position and putting a flame under the can. The rubber diaphragm should swell upwards as the can heats up. If it does not, you have an air leak somewhere that needs fixing before the the engine will work.
Last edited by Hopper on Sat Mar 02, 2013 9:10 pm, edited 1 time in total.

Insainhouserecords
Posts: 47
Joined: Fri Feb 15, 2013 3:18 pm

Re: Sterling(beta) refuses to work at all after 7 builds.. H

Post by Insainhouserecords » Sat Mar 02, 2013 9:02 pm

Thank for the info, I was thinking maybe my displacer had too much loose room and was letting the air pass by without this cycle taking effect. I was thinking piston but I guess that why its called a displacer. I was going to even try a solid arm displacer but its more complex for sure. So, Are you saying that even with the "pipe on side" unit it should still work the same way? My PVC opening is 3/4in and when I apply heat on the bottom it does swell but when the displacer and everything is intact it only swells when I turn it over but the "swell" isn't as great of a "swell".

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