BETA Stirling Engine Design!

Discussion on Stirling or "hot air" engines (all types)
kennethkye
Posts: 17
Joined: Mon Jun 30, 2014 8:36 am

BETA Stirling Engine Design!

Post by kennethkye »

Hello everyone,

I think most of you guys heard about the design rule for GAMMA stirling engine as stated below.

i. length of displacer chamber L = 3 times its diameter.
ii. length of heater chamber = 2/3L
iii. length of cooler = 1/3L
iv. swept volume of displacer = 1.5 times swept volume of piston cylinder.
v. length of displacer = 2/3L and stroke = 1/3L.

I wish to know are those parameters applicable to BETA stirling engine also? If yes how do we calculate it since the displacer and power piston for BETA stirling engine are enclosed in a single cylinder which is different from GAMMA engine.
Thanks in advance!
Ian S C
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Location: New Zealand

Re: BETA Stirling Engine Design!

Post by Ian S C »

Yes, same, the two motors are the same, the only difference is the lay out. You would find out if you build a GAMA motor with parallel cylinders as close as you can, so that part of the power cylinder breaks into the displacer cylinder. Ian S C
kennethkye
Posts: 17
Joined: Mon Jun 30, 2014 8:36 am

Re: BETA Stirling Engine Design!

Post by kennethkye »

Hi Ian, I would like to ask how do you define the length of displacer chamber, power cylinder, length of heater chamber and length of cooler in BETA engine since it only consists a single cylinder? I really wish to know this. Thanks!
Ian S C
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Location: New Zealand

Re: BETA Stirling Engine Design!

Post by Ian S C »

In the BETA motor the cylinder is the length of the displacer plus stroke, plus end clearance, plus power piston length, and stroke. On most of my motors the bottom of the skirt of the piston comes out the end of the cylinder by a small amount(probably less than 1/4 the length of the piston). Ian S C
kennethkye
Posts: 17
Joined: Mon Jun 30, 2014 8:36 am

Re: BETA Stirling Engine Design!

Post by kennethkye »

Hi Ian!
It is possible the displacer is overlapping with power piston stroke? I have read some articles that says both pistons might overlap their stroke and create an effective working space.
Are both displacer and power piston allowed to overlap or they should not overlap each others at all.
If yes, what are the advantages and how should we apply the design parameters stated above? Thanks!
Ian S C
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Location: New Zealand

Re: BETA Stirling Engine Design!

Post by Ian S C »

Just as long as they only come within a mm or two of each other during the complete cycle, that's all you need worry about. Ian S C
flash3780
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Joined: Sun May 25, 2014 8:32 am

Re: BETA Stirling Engine Design!

Post by flash3780 »

Hi kennethkye,
The geometry of the traditional beta Stirling engine will somewhat prevent the displacer and the power piston from sharing the same portion of the cylinder. Remember that there is usually an annulus where the regenerator resides in a beta stirling engine. Of course there are ports in the compression space to let the air out of the cylinder, through the regenerator annulus, and into the expansion space. If the ports get blocked by the power piston or the displacer, you'll end up with an unwanted pressure differential across the displacer.

Chris
kennethkye
Posts: 17
Joined: Mon Jun 30, 2014 8:36 am

Re: BETA Stirling Engine Design!

Post by kennethkye »

Hi Flash!
Thanks for your reply! What if I'm going to design a beta engine with only cooling fins without the regenerator? Is it allowable to have a overlap stroke? I saw some articles from internet actually stated that there might be an overlapping of strokes in beta engine.
Ian S C
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Location: New Zealand

Re: BETA Stirling Engine Design!

Post by Ian S C »

There are a number of ways to see how the piston and displacer interact with each other. (A)by drawing the action in (say) 4 different stages of the cycle, you can do this manually, pencil and paper, or if you are more with it than I am, with a computer drafting program. (B) make a model from cardboard/wood, or what ever. (C) as you build the motor, with the piston and the displacer assembled to the crankshaft rotate the CS and see that the two parts just miss each other as they come toward each other, The latter method is what I use to check the correct length for the displacer rod, I try to get the gudgeon block on the end of the rod as close to the piston without touching it as possible.
Put the cooling fins on the bottom/cold end of the displacer, and down over the power cylinder(about 1/3 of the disp area). The motor should run as long as you keep the fuel to it, use LPG and it will go all day, or as long as you can afford the gas for. Ian S C
flash3780
Posts: 16
Joined: Sun May 25, 2014 8:32 am

Re: BETA Stirling Engine Design!

Post by flash3780 »

Hi kennethkye,
If you forgo a regenerator, I think it's possible to make an interference engine. As Ian S C said, you will need to lay out engine and make sure it all works. You'll want to slosh all of the working fluid into the expansion space somewhere near the point of maximum compression... so the power piston and displacer are going to be quite close to one another at that time: You'll need to make sure that they don't clash.

I'm working on my own engine right now, and I've found the resources on the Ohio University website to be invaluable. I think you may want to play around with the cycle analysis to determine the amount of benefit you get from adjusting the phase angle between the power piston and the displacer. From there, you can determine your optimal amount of interference.

Here's a great reference for said analysis:
http://www.ohio.edu/mechanical/stirling/index.html

I'd be interested to see what you come up with! :)

-Chris
Ian S C
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Re: BETA Stirling Engine Design!

Post by Ian S C »

The idea of using the long displacer is that it acts as a regenerator, particularly if you make it of stainless steel, or next best mild steel. These metals tend to stay hot at the hot end, and cool at the cool end, unlike aluminium that soon evens out the heat along its full length. This is not to say that an aluminium displacer wont work, and it works ok with a candle, or small meths burner. Some have made the displacer from stainless steel wool, I tried it once with no great improvement so I went back to a stainless steel tubular one, nothing lost, you'v got to try these things. Ian S C
flash3780
Posts: 16
Joined: Sun May 25, 2014 8:32 am

Re: BETA Stirling Engine Design!

Post by flash3780 »

Ian S C,
It's interesting to mention a displacer as a regenerator. Technically, I don't think that anything prevents you from making a regenerator integral to the displacer. I think that it's generally not approached that way because materials which make good regenerators don't like to wobble about. That said, I'm sure that's a design challenge that could be overcome.
As you mentioned, the displacer itself could act a bit like a regenerative heat exchanger. I wonder how this compares to a similar engine with a proper regenerator? You mentioned making a displacer from stainless steel wool with no major improvement: Do you have any details about that design? It sounds intriguing.

Chris
Ian S C
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Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: BETA Stirling Engine Design!

Post by Ian S C »

Chris, just rough description, the displacer about 3 3/4" long x 1 1/4" dia, was replaced with a similar size bundle of steel wool from the paint shop(think it was a medium grade), one problem is bits breaking off, and with this motor being vertical(hot at top), the bits dropped onto the piston. A better system would be dimpled stainless steel foil wrapped into a cylinder, and a very small gap down the side, could even be touching to get the air to pass through instead of around the diplacer (WARNING watch for increased friction if displacer touches the cylinder wall) Ian S C
kennethkye
Posts: 17
Joined: Mon Jun 30, 2014 8:36 am

Re: BETA Stirling Engine Design!

Post by kennethkye »

Hi guys, thanks for the information. I have actually done the the engine design on CAD drawing and it looks good. After I have assemble all the parts I found out the design of displacer connecting and CS rods were too long. So, I have make them to into shorter one instead and now I can play with the engine by turning the crankshaft On CAD drawing. From there I check the the relation for each part and change the dimensions to the desired ones. Currently my power piston and displacer are quite close to one another with only few mm gap and they both are overlapping each other. I use a 16mm stroke for power piston and 30mm stroke for displacer. Lets see how far I can get and I will keep you guys update.
flash3780
Posts: 16
Joined: Sun May 25, 2014 8:32 am

Re: BETA Stirling Engine Design!

Post by flash3780 »

Thanks kennethkye,
What software do you use for CAD? Did you end up making an interference engine?
If you'd feel comfortable posting your drawings, I'd be interested in seeing what you came up with.

-Chris
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