First time designing sterling engine Looking for advice

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bacon9023e

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I am a Percision machining student designing a sterling engine for my senior project I have a rough design right out with A inch and a half bore And three and stroke plan to make the whole thing out of aluminum any advice would be helpful ie. Using O rings to seal it, Cylinder wall clearance, or Any general advice I have access to Haus CNC Mills And Lathes
 

Dalee

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Hi,

Not the worlds best expert on Stirlings. But I would not be real enthused about using aluminum for the cylinder or piston. The heat expansion would be awful for this application I think. Usually, you want material that doesn't soak up heat for those parts.

The one Stirling I have built, I used a 304 stainless steel, (material on hand), for the cylinder and graphite for the piston and valve. The stainless cylinder does not absorb heat fast and the graphite piston is lightweight and self-lubricating. And it has a super low heat absorption rate, preventing part growth and binding in the bore.

No rings of any sort be used. Sterlings make so little power that any drag in the powertrain will stop them dead. All you want is a very close running fit on the piston in the bore and that alone.

I would look at some Stirling models and pay close attention to the materials used in the construction of them.
 

bacon9023e

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Well I didn't really think about expansion I could harden some mild steel

As for sealing it I’m concerned that my pistons and cylinders won’t necessarily be perfect I’ll be boring and turning them on a CNC lathe so I was hoping to just give it a little more wiggle room using minimal contact hardened O-rings I’m not entirely sure what a good piston to cylinder clearance would be
 

minh-thanh

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bacon9023e !
Cylinder : you can make by cast iron ( I also make cylinders with 304 stainless steel, aluminum, brass: 304 stainless steel is difficult to process, aluminum conducts heat quickly )
Piston: you can make by cast iron, copper, brass, graphite
Pistons and cylinders: need almost "perfect"
Test pistons and cylinders: Pistons move freely from one end to the other of the cylinder - smoothly (without oil), when you seal one end of the cylinder: the piston cannot move.
With small stirling engine : do not use O ring (With large engine : I do not know)
 
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bacon9023e

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I’m using what’s on hand I have mild steel and aluminum and I could maybe get brass or copper
 

bacon9023e

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Yes I mean 10 Thousandths of an inch Piston to cylinder For instance I know a 350 Chevy small block the uses 30 thousandths But also uses cast-iron piston rings
 

fcheslop

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Ten thou is far too large more like 0.0001 that is for the power piston
The displacer can be around 40thou clearance
The hot cap is better made from stainless steel and machined to a wall thickness of about 20 thou then machine a section near its mounting to about 15 to 10 thou this makes a little heat dam and helps stop heat travelling into the engines frame
The ratio needs to be around 1.5 to 1
Have a look on Jan Ridders web site as he has a good read up on designing Stirling engines and has some good videos showing the power cylinder fits
What size are you thinking about
cheers
frazer
 

bacon9023e

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I’m doing a basic alpha configuration with two independent cylindersConnected with some kind of tube I’m not dealing with a displacer inside of a piston
 

bobhisey

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I use bronze for the cylinder, graphite for piston. Cast iron rusts causing drag.
 

a41capt

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For an easy solution, on my LTD Stirling engines I use Airpot Corp. glass cylinder/graphite piston dash pots. Here’s an example on ebay:

https://www.ebay.com/itm/airpot-Precision-air-dashpot-103530-1-/264525283594

The rubber boot comes off, and with a minor modification to the valve structure, you have an extremely low friction and tightly sealed power piston. The graphite is heat resistant, as is the cylinder, so these systems can be employed beyond the LTD designed engines.

there are several diameters and stroke combinations available, and you also get the enjoyment of watching the power piston cycling up and down!

When it’s completed, it may not be 100% shop built, but it will run, and with a Stirling engine, that’s a big plus!

John W
 

a41capt

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This is an example of a much smaller dash pot as described in my previous post on an LTD Stirling I built for my grandson. It’s running on the temperature differential between a cold pack and room air.

I have a couple dash pots that are 1 1/2“ and 2” also that will become the power cylinder/piston for a Stirling fan I’m designing.

John W
 

ignator

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Yes I mean 10 Thousandths of an inch Piston to cylinder For instance I know a 350 Chevy small block the uses 30 thousandths But also uses cast-iron piston rings
Just for correction, when you bore out large auto engines, the replacement pistons come in 0.010, 0.020, and 0.030 (inch) oversize increments. And typically 0.030 is the maximum size of overhaul available as a stock replacement. The running tolerance is much tighter then 10 thousands of an inch. And this depends upon bore diameter, as thermal expansion is the driver to initial fit.
 
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