compression ratio/displacement ratio

[sequesteredsun]

Greetings to all my fellow heat engine enthusists, I am wanting to build a stirling engine powered fan for the top of my wood stove with a temp. range of 200-600*F . I was wondering if there is any of you out there that , I guess first off , help me understand the importance of the correct compression ratio in an engine such as this and what some workable specs for this engine of this description wood be such as power piston dia and stroke , and dislpacer dia , length and stroke. And secondly the relationship between compression ratio and displacement ratio in the stirling engine. Thanks

[Ian S C]

Greetings,I,v built two of these motors, stove top fans. The first a Ringbom motor was far too noisy with the free running displacer, the second a GAMMA type is most successful, it uses the normal ratio of swept volumes, ie the swept volume of the displacer is 1,5 that of the power cylinder, giving the ratio of 1.5 :1, this is the same ratio used by Stirling in his engines, and is the ratio recomended by the modern designers for high temperature motors. Ian S C

[sequesteredsun]

Thanks for the reply Ian, I 'm assuming that swept volume is bore x stroke, is this correct?
Thanks, seqsun

[Ian S C]

seqsun, thats right, you'v got it. Ian S C

[sequesteredsun]

Hi Ian, this 1.5:1 ratio , is it assuming that both cylinders are the same diameter? If this is the case , then what are the best considerations when the displacer is 2.25"dia. and the power cyl. is 1.25"dia.?
Regards , Seqsun.

[Ian S C]

The ratio is by volume,ie., if the displacer cylinder is 1.5 cubic inch, the power cylinder should be approximatly 1 cubic inch. the other ratio to look out for is the displacer its self, the length should be about three times the diameter. You can work out the volume with the calculator on your computor, mine has ways of doing it easy. If worst comes you can fill the space to be measured with water, and measure that. Ian S C

[sequesteredsun]

Hi Ian, Are you able to tell me ,on your fan stirling what the power piston dia and stroke are and the displacer dia , length, and stroke are?
Thanks Seqsun

[Ian S C]

The power cylinder; bore 35 mm, stroke 1/2". Displacer cylinder bore 35 mm, stroke 3/4". The clearance of the displacer in its cylinder is 1.5 mm, the displacer is therefore 32 mm dia X 96 mm long, I think theres about 2 mm clearance at each end. The fan is 12" dia, with 3 wooden blades, the pitch of the blades may be adjusted, if the motor is going too fast the pitch can be increased to create more drag. Ian S C

[sequesteredsun]

Hi Ian, I really appreciate you getting back with some real life specs , this , with the previous explaination cements the principles that are in play . I guess your engine would be what they call a"square" engine where the bores are similar and the strokes reflect the 1.5:1 magic ratio?

Many thanks, Seqsun

[Ian S C]

I went and looked at the Ringbom vertion, The displacer cylinder is 45 mm dia X about 150 mm long, the displacer has a stroke of 7/8" maximum, being free running it's usually about 1/2" to 3/4". The displacer rod is 10 mm, that seems to work, its pure guess work to get the size right, there must be a formula, but I,v built three motors and they all work ( well four actually, but the other one has a free piston as well). The power piston is identical to the other motor (interchangable) at 35 mm, but the stroke is 1". The fan is 14" diameter, and has 3 metal blades, keep your fingers out, it works like a circular saw. I'm sure that with propper design, a Ringbom motor can be made at least as quiet as any other Stirling engine, I just have'nt struck the right dimentions yet. Ian S C

[sequesteredsun]

Hi Ian, What is the stove temp. that your stove top fan runs on?
Seqsun

[Ian S C]

Sorry ,you know as much about the stove temp as I do, al I know it runs at about half way between 0 and 1 (the control goes 0 to 5), its an electric stove, with radiant hot plates. I do not have a log burner or the like, come the winter I'll try and get it into town so my nephew can try it on hi log burner. If you turn the hot plate up to 5, the fan just about jumps off the stove. Ian S C

[sequesteredsun]

When calculating the 1.5:1 ratio does one have to consider the "dead" volume between the displacer and cylinder wall?
Thanks Seqsun

[Ian S C]

Maybe one should, but I'v never bothered, often the ratio is only approximately 1.5:1, the space being so small(hopefully), it does'nt really matter too much in my motors, I'll have to think a bit more about it. Ian S C

[Ian S C]

If ypur going to advertise cars, could you find one with a Stitling Engine please. Ian S C