Diving into the Stirling world - fist engine isn't running

[Stirbert Robling]

Hi,

i'm new here. For a longer time i'm interested in the stirling engine principle and now i deceided to really deal with this topic.

My first contact with stirling was with a cryo pump at work, which is actually a stirling engine "running backwards", driven by an electrical actuator and pulling heat energy from one side to the other. Currently i bought some books and i'm reading lots of documentations, papers and what to find on the internet. As an engineer my goal is besides some hobby tinkering, to understand the calculations and design rules.

Driven by curiosity i build my first LTD stirling engine. All dimensions and ratios are designed on the basis of gut feeling

Well, it doesn't work!

So i hope you could give me a little help to understand whats wrong.

Some dimensions and ratios:

• Displacer diameter = 90 mm

• Displacer height = 16 mm

• Displacer travel = 16 mm

• Expansion cylinder height = 34 mm

• Circumferantial clearence = 1 mm

• Piston diameter = 30 mm

• Piston height = 40 mm

• Piston travel = 4 mm

• compression rato = ~1.2

I've tested it with glass of boiling water. The overall friction seems to be quite low, at least for me...

[Tom Booth]

There are a few things that seem, - well I wouldn't say "wrong" necessarily. Not in agreement with my own design parameters.

The power piston. Styrofoam. At least any styrofoam I'm familiar with might have a number of problems, semi-porous and not perfectly smooth as well as possibly rather compressible all those qualities of typical styrofoam would be something I'd want to avoid in a power piston. It is fine for the displacer but not the power piston.

I think styrofoam is also prone to static charges that might cause it to be attracted to or "cling" to the cylinder.

Maybe the extreme length of the power piston and short traveling distance is intended to compensate for those qualities mentioned?

Anyway, a long piston like that only makes some of the above problems, potentially that much worse.

Styrofoam is not something I would ever use as a power piston.

Coating it with something maybe might help to reduce the porosity and increase the rigidity, or maybe reduce the diameter and put a sleeve over it.

Thoroughly impregnating it with graphite powder might reduce some of the possible issues if the graphite can fill or plug any voids but that would be rather messy.

[Tom Booth]

There is also, I think, a disparity or incongruity between the displacer travel length and piston travel.

Usually it would be the other way around in an LTD style engine. The piston would usually travel a bit further linearly than the displacer.

[Tom Booth]

Take this little engine as a typical example:

Resize_20221123_131946_6868.jpg (156.96 KiB) Viewed 4949 times

The maximum distance the displacer can travel vertically is about 1/8 inch

The piston; about 3X that (3/8 inch)

At high speed, due to the magnetic lift mechanism, the displacer hardly lifts at all. Maybe 1/32 inch. This very slight lift on the displacer seems to make for a good running engine nevertheless.

[Stirbert Robling]

Thanks for your thoughts Tom!

The piston is so long, because initially it had a travel of 14 mm! In the meantime i've learned, that a LTD stirling can only work with a low compression rate = Vmax / Vmin, therefore i reduced the piston travel. Now the comp ratio is ~1.2

I think its a good idea to switch the values of travel for piston and displacer! At least reduce the one of the displacer. The piston needs enough force (by gas pressure) to lift the displacer against gravitiy and the gas squeezing past the displacer around its outer wall (friction). A shorter travel should ease that.
With the piston travel i'm not sure! The piston travel is short, which means a small radius on the wheel, resulting in a small leverage and thus a small attainable momentum (to drive the displacer). But for that the surface of the piston is large! Which results in more force = pressure x area.

First i will try to reduce the displacer travel.

[Goofy]

Hi Stirbert,

Another issue, could be your rate between swept volume of displacer and powerpiston. Your´s are 36.

"First, the swept volume of the displacer, which is the
product of the area of the displacer and its stroke, must be much higher than that
of the piston.
As a rule-of-thumb, the swept volume of the displacer must be
roughly fifty times that of the piston"

Source : Lemaire's thesis page 27, https://escholarship.mcgill.ca/concern/theses/4b29b997z

Second, I don´t think superglue is the best thing for sealing. Perhaps thermo-glue will do better.
Try to test if it is tight . . .

BR
Petter

[Stirbert Robling]

Thanks a lot Petter. That's a useful rule-of-thumb!

You're right about the superglue. For me it was a quick and dirty solution, to close and open again. It's sealing is not the best...

In the end when I gathered enough understanding and get my test engine to run, I will machine a beautiful one on my lathe and mill

[Tom Booth]

Swept volume may not be applicable to this type engine.

What I mean is, the primary heat exchanger (metal hot plate on the bottom) probably does not transfer heat to the plastic side walls of the chamber, so increasing the vertical traveling distance would not increase heat input as much as when the whole cylinder is heated metal.

It might be possible to increase the surface area without having to redesign the whole engine by replacing the flat plate with something with some kind of striations or groves.

Resize_20221126_135551_1988.jpg (100.95 KiB) Viewed 4894 times

Or some other texture, or means of increasing the surface area in the space available. Some people put metal BB's in the bottom or solder some metal screen on the bottom (inside that is).

I agree, super glue does not usually adhere well to plastic.