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I am in the process of designing a 4 cylinder double acting radial stirling engine. To simplify fabrication I hope to use square structural steel tubing for the pistons and cylinders. The pistons will be sealed with teflon "overlays" on the four sides that contact the cylinder walls and the cylinder walls will be well polished where the PTFE and cylinder walls contact.

I know why square pistons are not practical in an IC engine but is there any reason that they would not work in a Stirling engine?

The cylinder of a Stirling Engine not having piston rings must be more accurate than an IC motor or steam engine, while it is possible to make square power units(you can't call a square a cylinder), there have been steam engines made that way. You don't really want steel on steel in rubbing contact. Are you going to get the piston coated with something like Xylon, this is Teflon that is baked on. You would be more likely to get a good fit with round tube that you could hone and lap to fit. The best metal for a piston is cast iron, and the cylinder, best CI, but steel quite good. For ready made cylinders the inside tube of an automotive shock absorber, get an old one at the car wreckers. Opening it is a bit messy, go careful and let the oil drain out.

Structural steel is rarely square in precision engineering terms.

Ian S C

I agree that square steel tube is never actually "square" but machining it to true dimensions is pretty simple.
I have some experience with Stirling engines and prototype design and fabrication having owned a prototype fab shop for about a decade in my 30's.

Only the interior of the "cylinder" will have to be perfectly true and polished.
The sides of the piston will be fully sheathed and so the sheathing will be machined to true rather than the underlying piston.
There will be no steel to steel contact.

Teflon sheet can be machined to sheath the pistons lower section..with graphite sheathing the upper 1/3 for heat protection and sealing.

In the past 30 years I have seen a great deal of fabricated (welded) steel replace what had traditionally been cast iron components with very good results.

The engine contemplated is fairly large so salvage shock absorbers won't suffice.
8 double acting cylinders w 3.5" bores.
But I appreciate the suggestion.

What I am really asking is are there any reason that "square pistons" are not practical in a Stirling engine.

Thanks.

The main problem that I see is getting a good seal specially at the corners, even if left with a radius, round holes are always easiest( no corners). I think it sounds like an interesting design, a lot bigger than my biggest. Bore 2 1/4", power stroke 1 3/8".

Ian S C

I agree that trueing the radiused corners inside the "cylinders" is more difficult since they cannot simply be bored true.
Reciprocating rather than rotating tools must be used.

I am depending on the piston sheathing (graphite for the top third and Teflon for the bottom third) which can be easily machined true to provide an adequate seal. The graphite will be machined a few thousands larger than the bore and should "self lap" when mated with the cylinder prior to engine assembly. I plan to test a prototype piston/cylinder assembly for longevity by driving the assembly with the same reciprocating tool base used to polish the cylinder by monitoring pressure retention over a period of a few weeks using the initial prototype piston/cylinder assembly.

If the sealing capability of the piston sheathing degrades quickly the concept will have to be modified or scrapped,

There is always the possibility of using a block of graphite as the piston. Over all an interesting project. A similar motor I have read of was a rotary engine in which the cylinders and crankcase rotate(as in the rotary aircraft engines of WW1), I imagine your motor as horizontal with the crankshaft vertical.

Ian S C

Which steak engines had square pistons?

With Stirling engines being over 100 years old there is no reason to reinvent them. I recommend sticking to proven designs. I've have a handful of engines and I tend to sell off bad runners. I've noticed that alpha engines give the best power followed by beta and gamma in third place.

The best designed engines I have is a Japanese KYG of alpha Ross linkage. From what I can tell, KYG followed Andy's designs and the power and run time prove it.

https://global.rakuten.com/en/store/ove ... -se-905hp/

https://youtu.be/DGgkTB6Pd_M

A strong second place design is a L shaped Alpha. You pay a price for more dead space due to distance between the hot and cold space but vastly simple linkage.

https://youtu.be/ieAXiSZcS90

With the L shape ALPHA, what you loose to dead space you gain on less mechanical parts, although with the ALPHA motor there is less friction on the pistons as there is virtually no side movement with the con rods not moving side to side as they do with a crank. I changed the bell crank guide to a cast iron vertical fork. To the right of the bell crank you can see the hole that supported the stabilizer arm, I think the motor performs better now, I also changed to air cooling as I tended not to use the radiator.

Ian S C

In order to represent the basic overall performance of an inner combustion engine, over its working variety, we can use some parameters and geometrical relationships of the piston and combustion chamber. Engine overall performance relates to each fuel efficiency and dynamic output (energy and torque), that are influenced directly via the simple parameters of the engine.

To bear in mind the running ideas of an internal combustion engine works examine the thing How an inner combustion engine works.

The predominant geometric parameters of the cylinder, piston, connecting rod and crankshaft are depicted within the photo below.
Wherein:

IV – intake valve
EV – exhaust valve
TDC – top dead center
BDC – bottom lifeless middle
B – cylinder bore
S – piston stroke
r – connecting rod length
a – crank radius (offset)
x – distance among the crank axis and the piston pin axis
θ – crank angle
Vd – displaced (swept) volume
Vc – clearance extent

The piston movements in the cylinder among TDC and BDC. In order to finish a full combustion cycle, the piston executes 4 strokes and the crankshaft makes two whole turns. The displaced volume is the volume in which the piston movements, the clearance extent is the extent left in the cylinder while the piston reaches TDC.

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Basic geometric parameters of the ICE’s piston and cylinder
ICE Components & Systems
Basic geometric parameters of the ICE’s piston and cylinder

In order to signify the basic performance of an inner combustion engine, over its running variety, we can use a few parameters and geometrical relationships of the piston and combustion chamber. Engine overall performance relates to each fuel efficiency and dynamic output (energy and torque), which are stimulated at once by the simple parameters of the engine.

To don't forget the working standards of an internal combustion engine works examine the thing How an inner combustion engine works.

The foremost geometric parameters of the cylinder, piston, connecting rod and crankshaft are depicted in the photograph underneath.
Basic piston and cylinder geometry parameters of inner combustion engines

Image: Basic piston and cylinder geometry parameters of internal combustion engines

in which:

IV – consumption valve
EV – exhaust valve
TDC – top useless middle
BDC – backside useless middle
B – cylinder bore
S – piston stroke
r – connecting rod period
a – crank radius (offset)
x – distance between the crank axis and the piston pin axis
θ – crank perspective
Vd – displaced (swept) quantity
Vc – clearance extent

The piston movements inside the cylinder among TDC and BDC. In order to complete a full combustion cycle, the piston executes 4 strokes and the crankshaft makes two complete turns. The displaced quantity is the quantity wherein the piston movements, the clearance extent is the quantity left in the cylinder while the piston reaches TDC.

In this educational we are going to look into the way to calculate the volumetric ability of the engine , what the compression ratio is and which can be the principle geometric parameters of the engine.

For one cylinder, the displaced extent Vd is the product between the stroke of the piston and the region of the cylinder (almost the same with the area of the piston):

For one cylinder, the displaced volume Vd is the product between the stroke of the piston and the area of the cylinder (nearly the same with the area of the piston):
Vd=SAc

The area of the cylinder is:
Ac=πB24

This gives the volumetric capacity of one cylinder which is equal with the displaced volume:
Vd=SπB24

To find the total volumetric capacity (displacement) of the engine, we only have to multiply the volumetric capacity of one cylinder with the number of cylinders Nc:
Vd=NcSπB24

Let’s take an example of engine and calculate the volumetric capacity. In the article BMW’s iPerformance plug-in hybrid electric vehicle (PHEV) powertrain architecture we have the technical specification of the internal combustion engine:

S = 94.6 mm
B = 82 mm
Nc = 4

Replacing the values in the expression of Vd, gives:
Vd=1998336.9mm3=1998.3369cm3

The technical specification states that the engine capacity is 1998 cm3, which is the same with the calculated volume.

Engine displacement is usually given in liters L, cubic centimeters cm3 (SI) or cubic inches in3 (US). The bore and stroke are given in mm so we need to apply a conversion in order to get the requested unit for the volume:
1L=10−3m3=103cm3=106mm3=61in3

The displacement of the modern internal combustion engines varies between 1.0 L and around 6.0 L, with the average of around 1.5 – 2 L. There is a clear tendency of decreasing the volumetric capacity of an engine (downsizing) in order to fulfill the more stringent fuel emission standards.