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Stirling engine displacer design? Answered

I am designing a gamma Stirling engine (this type) and I am unsure how long the piston should be and how long its stroke length should be the specs for the cylinder and piston pipes are below.

The pipe for the cylinder is stainless steel.
Wall thickness: 1.56mm
Outer diameter: 31.7mm
Inner diameter: 28.6
Length: 200mm (can be cut smaller)
(Stainless steel cap will be welded to one end, the other end where the shaft comes out will be plugged with this nylon/graphite stuff)

The pipe for the displacer piston is aluminium pipe. 
Wall thickness: 1mm
Outer diameter: 27.88mm
Inner diameter: 25.88mm
Length: ?
(Two aluminium plugs will be used to seal its ends)

How long should the piston, cylinder and stroke lengths be based on the specs?


The Piston it's self can be as long or short as you like as long as it is a near perfect fit.

It must stay sealed in the cylinder at max travel and not hit the end at minimum travel.

Making full scale drawings ideally in a CAD system for speed should let you assess these dimensions.

The seal at the displacer shaft needs to be good but not tight. LOW FRICTION is the key to getting a model Stirling to work well. In general this seal is long and well oiled.

The displacer should be as light as possible whilst able to withstand the heat. i have in the past experimented by compressing aluminum foil into a suitable tube with a hammer/vice to produce a sort of aluminum foam slug. this is robust but very light.

Just by looks 1.25 inches seems to be the best stroke length does this sound about right?

I noticed that your stroke length is really large, won't this mean that the engine is slow and it won't reach as higher pressure inside the engine?

The displacer is generally much larger than the cylinder to heat/cool as large a volume as possible.

In very low heat differential (coffee cup) Stirling the displacer chamber is HUGE.

I mad the volume about 4 x the cylinder vol. As I said it's eye ball and not calculated but the speed will be a function of how fast you can heat/cool the air.

I have a Working Stirling model in the garage and can give you actual measurements tomorrow if your interested.

I would appreciate the measurements.

Might I ask, why do most Stirling engines of this type and size have a displacer stroke length of about 25-64mm?

Working with your figures (almost) The displacer should be considerably longer than the cylinder.

These dimensions seem to look OK (no calcs done just eyeball)


Not read that before I think. Is it still considered gospel ?


NASA is a reputable source so I guess the essential principles are still good. I haven't found anything more recent or detailed. (so far)

I wonder how much things have changed ? Have the microCHP people made a breakthrough, or have they just bitten the bullet and managed to engineer the highly sealed high pressure environment you need inside an efficient engine ?

Very hard to say without being able to inspect the system over time. Most authorities seem to quote efficiency up to 40% for Stirling as being easily achievable (in large sizes) .

Exotic filling gases help heat transfer as well as an excellently engineered system.

Generally weight and the need for a goof temp differential are the biggest detraction.

The FORTRAN program looks to have been produced through an ASR 33
There are more constants and variables then electric motor designs.
The program even accounts for losses in a tube bend for three gases.
Concluding Sterling power able to propel small air planes very quietly.

Thanks for the cool ( and hot ) pointer.