Smaller walking beam

[picnic]

Is a smaller version of the tin can walking beam engine likely to work or is a minimum volume of air required? I was thinking of the displacer cylinder being perhaps 1 inch diameter and 2 to 3 inches long. I'd be making the parts using traditional model engineering practices rather than using actual tin cans etc.

Has anyone made such an engine?

TIA

picnic

[Ian S C]

Hi Picnic, that size should work OK, a 1" dia displacer should be about 3" long, and the displacer cylinder the legth of the stroke longer, plus the gap at each end. The ratio of volume between the displacer and the power cylinder should be someware about 1.5:1, thats fairly univesal for high temperature motors. There is a photo of a smaller motor on my gallery site, its a GAMMA motor with a 5/8" bore. Ian S C

[picnic]

Thanks Ian... do you mean a ratio of 1.5:1 between the displacer cylinder and power cylinder or displacer itself?

I'm going to make some drawings as I go and upload them, if you wouldn't mind looking them over that would be great :D

EDIT: By my calculation the power cylinder would be around 9.5mm diamter making the piston about 7.5mm diamter... is this likely to be too small?

[Ian S C]

Yes you'v got the ratio right.
I imagine that the 9.5 mm is the outside dia of the power cylinder.
If the two- power and displacer have the same stroke, I think (my maths is a bit weak) that the bore of the power cylinder should be about 11/16". It would be possible to use the 7.5 mm dia but the stroke would be quite long. Find out the stroke that you will use for the displacer and work out the volume of that, you can then select a diameter for the power cylinder, and work out the volume required. Just use pi r sqr x the length, and just keep changing the length part until you get to the ratio of 1.5: 1 or near enough (theres proberbly an easier way that you may know). Sorry if I ramble on a bit.
A simple example would be Power stroke 1/2" displacer stroke 3/4", equal bores. Or there is one here in my collection of articals, this one by Stan Bray in Model Engineer, it is a V type with a 3/4" stroke, a 1" dia displacer,and a 1/2" dia power piston, I think the piston could well be made 5/8" dia at least, but the 1/2" would work OK. The smaller you go the more accurate you have to make the bits. Ian S C

[picnic]

Thanks again. Just need to finish off my first "full size" engine and I'll get on to those drawings!

[Ian S C]

What is the full size engine? Ian S C

[picnic]

Oh, the 'full size' is just my take on the tin can beam engine - http://stirlingengineforum.com/viewtopic.php?f=1&t=880

[picnic]

Am I right in thinking the displacer itself should be about 2/3 the volume of it's cylinder for this type of engine? I was thinking of a 20mm diameter displacer in a 25mm tube (air gap of 2.5mm) and making the displacer about 50mm in length.

TIA

picnic

[Ian S C]

picnic, 50 mm length will work, 60 mm would be better, and 1.5 to 2 mm would be a better gap, erring on the small size. The length of the cylinder is the length of the displacer, plus the stroke, plus the gap at each end. As far as the thirds go, the one third gets heated the one at the other end gets cooled. and the middle bit actually serves to an extent as a regenerator, this is also part of the function of the long displacer. Ian S C

[picnic]

Here's the first part of the drawings....comments welcome!

[Ian S C]

picnic, one thing I would suggest is that the displacer rod bush should be made longer, 20 mm would be some were near what I would think of, a little more or less, no matter. In a short bush the rod can tend to move sideways, and sort of wedge its self a bit, longer the better, also better as sealing goes. I'v used brass, bronze and cast iron, also mild steel with a carbon impregnated teflon bush for this item. Oops, The bush I'v described is for the diplacer cylinder. The displacer cylinder needs to be a little longer, to alow a little gap at each end, say a total of 2 mm - 3 mm, giving a total length of 77 to 78 mm, the threaded bush on the end of the displacer would be better if it were flush with the end. I make minewith a plain hole, and a disc with a thread about half way down the displacer, the end of the displacer is held in with high temp shaft lock glue, and the rod is held in with glue, I used to braze the rod to the displacer end plug. Ian S C

[picnic]

Thanks Ian, as it stands it would have (75-55-10)/2 = 5mm clear at each end, isn't that enough? Yes, the displacer cylinder bush will be much longer!

[Ian S C]

Yep, thats OK. When I build, I design on the go, no drawings, can be a bit difficult when someone wants some details. Ian S C

[Bidwell]

Why only a 4 mm hole? Wouldn't a "full size" hole allow less restriction?