# Stirling Engine Displacer Materials



## Nerdz (Nov 20, 2013)

Short and simple. Can I use Aluminum for the Displacer And the Displacer Cylinder in a Stirling Engine? Or do I have to use a Aluminum Displacer and Stainless Steel? I know the ideal material is graphite..But thats expensive as well!

The reason I ask is because Stainless steel is expensive and Ive never machined SS before. 

(I do know I have to use brass for the power piston though)


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## portlandron (Nov 20, 2013)

All the ones I have built use Graphite for the power piston. Found mine on ebay for a good price.

Stirling Engines are not the easy to make run. There needs to be as little friction as possible and still maintain good seals. Piston must be a good fit to cylinder, to tight then not enough power or to lose they will not hold pressure. Also the heat transfer is important.

As finicky as Sterlings are I would suggest staying with the material the designer calls for.


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## WOB (Nov 20, 2013)

Aluminum is fine for the displacer piston and cylinder as long as you don't exceed 500deg. F for the working fluid temp.  Make the piston hollow if possible to minimize inertia forces.

RWO


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## Goldflash (Nov 20, 2013)

I have used the Aluminium Tubes that Berocca  Fizzy Vitamins come in and I have also looked at Aluminium Cigar Tubes. 
The Berocca Aluminium Tubes are about 25.4mm OD and are thin wall so you can make a very light displacer piston running with about 1.5 mm clearance in the Hot Cylinder. I set mine with about 2 mm clearance at top Dead Center and Bottom Dead Center and with a 25.4 mm Diameter a Length of 75 mm for the Displacer Cylinder works well. Stroke can vary from about 25mm to 30 mm . I went for a 30mm stroke with this combination and it worked well.


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## Nerdz (Nov 20, 2013)

portlandron said:


> All the ones I have built use Graphite for the power piston. Found mine on ebay for a good price.
> 
> Stirling Engines are not the easy to make run. There needs to be as little friction as possible and still maintain good seals. Piston must be a good fit to cylinder, to tight then not enough power or to lose they will not hold pressure. Also the heat transfer is important.
> 
> As finicky as Sterlings are I would suggest staying with the material the designer calls for.



But why are they finicky? For example, if you stick with the ratio's given on this site, (which are stirling's original ratios)



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



Why wouldnt it work? After that its all just material science. From what Ive been reading You want the power piston and power cylinder to be as close as a fit as possible. The hot end has to be able to withstand the temperatures given, and the displacer has to be insulative as well as light. 

But how much space is needed between the displacer cylinder and the displacer itself? Ive read that 1mm to 2mm is sufficient (and even 0.0625'' is good enough as well).


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## Goldflash (Nov 20, 2013)

What I found crucial in getting the Sterling Engines to run is getting perfect perpendicular alignment between the Crankshaft and the Power Pistons bore and Displacer Piston Rod Bush. any misalignment will cause friction. [B]Any failure at this point will cause you a lot of Headaches[/B] 
Use Ball Bearings where possible and remove any bearing seals and flush with WD WD40 to remove any Grease. 
I went to a crosshead piston assembly to ensure that the Piston Rod and Displacer ran true in the Displacer Cylinder as if you run small clearances in the Hot Cylinder then there is a chance that the Displacer piston can droop and rub on the bore of the Hot Cylinder and again Friction Rears its ugly Head


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## portlandron (Nov 21, 2013)

Was going to respond to why Sterling Engines are "finicky" but Goldflash summed it up very will.


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