Design dimension for stirling engine...Need help!

[kennethkye]

Hi I'm new in this forum.
I have a project which is to design and fabricate a beta-type solar stirling engine to generate electricity for a small bulb. However, I'm facing some difficulty in deciding the dimensions for each part of the engine for example the size of bore/stroke of the power and displacer pistons, swept and dead volume, length of connecting rod etc... This is my first time doing this type of designing question and I hope to get some advises and help from you guys. Can you guys share some of your experience and design with me. Really appreciate for your help. Thank you!

Regards
kennethkye

[Ian S C]

Hi, what sort of heat gathering are you thinking of using, parabolic reflector, large freznel lens? Solar can give you good heat, if you look in my gallery, you'll see my second engine, normally fuelled with LPG, gained a bit of power when heated with a freznel lens that was designed to be put in front of a TV screen.
Ian S C

[kennethkye]

Hi Ian, I'm planing to use parabolic reflector as the heat collector and the purpose is to power a small bulb. Actually I'm in the early stage of the designing process and I need to decide the dimensions of each part of my beta stirling engine before i can start to draw out my design on CAD. However, I have no clue on what should be the correct dimension ratio for each component of the engine so that the engine can be run after fabricated...could you please help me on this. Thanks!

[Ian S C]

A friend of mine bought a little Solar powered Stirling Engine that was advertised in Popular Mechanics, at least 40 years ago, it has a round parabolic reflector about 18" diameter, it sits on a camera tripod, and even runs quite well on a cloudy day. He was at the time developing, and selling solar hot water heating systems, and when he went to shows he would set up the little motor, and let it run, it didn't, but could drive a little generator. A small low voltage DC motor works as a generator, look for one that runs the slowest.
You would do ok with an engine with a power cylinder of an inch or so bore, and a stroke of 3/4" to 1". You will find on reading that the swept volume of the displacer should be 1.5, and the power piston should be 1, the ratio of 1.5 : 1, that is the ratio that Robert Stirling used on his motors. The other ratio is with the displacer, it should be approximately three times as long as the diameter.
As far as materials go, the hot cap is best made of stainless steel, mild steel will do quite well. The displacer should be made in a similar fashion. The power piston, if metal is best made of cast iron, the cylinder also of cast iron, but steel is just about as good. Ian S C

[kennethkye]

Thanks a lot for the useful information Ian! Can you explain on how the hot cap is connect to the cylinder?
Also how to seal the power piston as the design for beta stirling engine requires the displace connecting rod to pass through the power piston.
Besides, which type of strokes is more preferable, longer or shorter?(in my case to generate electricity by a generator)

[Ian S C]

I'd tend to go for a fairly short stroke, this keeps the con rod angle to a minimum there by reducing friction, it also helps to have the con rod as long as possible for the same reason.
The displacer rod passes through the piston in the BETTA motor, I'v made these glands out of brass, cast iron, and steel with a Carbon impregnated Teflon bush, they all work OK. Make the gland as long as you can, one inch is a good round figure for any rod from 1/8" up to 1/4", along gland does two things, it guides the displacer accurately, and it reduces the leakage of air.
The hot cap can be connected to the power cylinder by a number of means, my first few motors I carved the hot cap out of solid bars of stainless steel, at the open end I cut an internal thread, on the end of the power cylinder I cut a matching external thread, and the two parts screw together, the method produces miles of swafe, and take quite a time, but is well worth all the work. The way I'v been doing it of late is using very thin stainless tube (some of it was vacuum cleaner extension tube) with the end TIG welded on, and the other end Loctited into a flange, the flange bolted to another one on the other half of the cylinder.
I have used the steel case of a NiCad battery. It is possible to make the complete cylinder from one piece, as long as you can hone, and polish the part of the bore occupied by the power piston. Ian S C

[kennethkye]

Thanks for the great info Ian! One more question I wish to know is did the displacer piston passing through the fins cylinder part in BETA type engine when it is displaced to its lowest position? Also did the power piston passing through the hot cap cylinder part when it reaches its highest point?

[Ian S C]

I think now the easiest way to under stand the movement of the displacer and piston, is to get your drawing instruments, and some paper, and draw some diagrams, draw it at bottom of the power stroke, then 90*, then top of stroke, then 270*, you will then see the relationship of the parts. Ian S C

[kennethkye]

Hi Ian! Can u explain how the displacer piston rod is support in BETA engine as the connecting rod pass through the power piston? I understand one end of the displacer connecting rod is connected to the crankshaft and the other end is connected to the displacer piston, but what is supporting the rod? Thanks!

[Ian S C]

The usual way to support the displacer rod, is the gland through the piston, it's quite easy if you have a cast iron piston, hollowed out so it has thin walls/skirt, and the crown thick enough to put a thread in, I use 3/8" UNF, you could go to 1/2" UNF, or a fine metric thread, 10 mm or 12 mm. A fine thread is easier to seal, and give you more treads in the piston crown. The gland, maybe an inch long, and at the side an attachment point for the power piston con rod. Takes a bit of imagination, but not too difficult, mostly lathe work, a bit of work with a hacksaw, and file, and a hole at right angles to the bore for the con rod pin. Ian S C

[kennethkye]

Hi Ian!
I have decided to fabricate my beta engine with the parameters as stated below:

Cylinder ID=20mm
Hot cylinder length=81mm
Cold cylinder length(fins length)=56mm
Displacer piston diameter=18mm
Displacer piston length=54mm
Displacer piston stroke=40mm
Power piston diameter=20mm
Power piston length=12mm
Power piston stroke=20mm

It is ok with these design parameters? Is the displacer stroke too big? I saw someone stated that the displacer itself should be about 2/3 the total length of the cylinder. Is this apply to beta engine also because my length of displacer is not even half of my cylinder length? Please help on this thanks!

[Ian S C]

With a power stroke of 20 mm the displacer should be 30 mm, not 40 mm. Ratio 1:1.5. The length of the displacer is ok, but you need room in the cylinder to take the displacer plus the length of the stroke, plus the end clearance, 54 + 30 + ?3= 87.
I think your heading the right way.
Ian S C

[kennethkye]

Hi Ian!

The reason I make the displacer stroke to be 40mm is because is has a smaller diameter than power piston diameter.
I have a 20mm diameter piston with 20mm stroke will displace 6283 millimeter cube of air.
If the swept volume ratio is 1:1.5, the displacer needs to displace 9424 millimeter cube of air.
So my 18mm diameter displacer need a 38mm stroke to displace 9669 millimeter cube of air to maintain the ratio.
If with a 30mm stroke, the displacer displace 7634 millimeter cube which is equal to ratio of 1:1.2 only?

Do I have it right? or we just ignore the diameters and consider the stroke ratio only? Thank you!

[Ian S C]

Sorry your right. A bit either way is not to much trouble. If you have lots of time, and can be bothered, it's worth doing some experimental work, altering the ratio, and after doing that trying different crankshaft angles, to do that you need a CS that one of the cranks can be adjusted. There can be more to these motors than just looking at it run. Yesterday I was playing with the little beam engine in the gallery (10 mm bore, 12 mm stroke), getting it to run an alternator to light up an LED, it did after I put a bigger burner under it.
Ian S C

[kennethkye]

Hi Ian, I have changed my power piston stroke to 16mm so that I have a shorter displacer stroke which is 30mm. The first thing I want to do is to make sure the engine can be run and hopefully it will do so. If I have time I will try to play around with the engine configuration for improvement work. I will keep you update.