Displacer size% of pressure cylinder on can engines?

[Carlos_A_M]

I have been making a few gamma and beta engines out of cans and I have been wondering something for a while, what is the best size for the displacer for a "soda can" engine? I have seen sizes all over the place, some say 66.6% of the pressure cylinder, others 50%, others 30%, etc. I have seen videos like Leandro Wagner's where he uses a relatively small displacer and gets up to 4 digit rpm and even power multiple leds at once with it. How does a smaller displacer like his get so much rpm using that heavy of a flywheel? I have been searching an answer for quite a while and have seen none apart from the usual 60%, but if so then why does his generate so much torque?

[skypupbob]

Hello, You are right, the sizes are all over the place. It seems the power cylinder to displacer cylinder ratio is related to how much temperature difference there is.
The low temperature LTD engines can have a ratio of 50:1 and will operate with a temp. difference of 5 degrees.
Then there are the engines with propane burners turning the displacer red hot with a water jacket cooling the other end. Those are a 1:1 ratio.
The LTD engines turn slowly, and the high temp. ones run at a much higher RPM.
You can't use soda cans for a high temp engine. You need to use stainless steel. The stainless water bottles work well.
There are formulas for calculating the ratio based on heat differential.

[Carlos_A_M]

Do you know any of those formulas? Or a site that explains how to use them in detail? Jbl's sim gave 60%. Multiple equations from different sites range wildly in size too.

[skypupbob]

I do not know the formulas, but I think I saw them on these forums. Try doing a search for them.
I tend to want to experiment with trial and error than spend my time with math problems. The formulas would get me closer to the desired results but I don't use them and just go ahead and build without much calculations involved. As a result, the last one I built won't run. Got the power cylinder too big. OH WELL ! I'll rebuild with a larger displacer and see what happens.

[Tom Booth]

I concluded a long time ago that the size of the displacer itself is irrelevant. The internal space within the displacer serves no real purpose. "Displacer" is, infact (IMO) probably something of a misnomer.

I personally think the main purpose of a so-called "displacer" is just to isolate, and/or insulated the working gas from the heat source intermittently. Anything that accomplishes that, even just a flat disk or diaphragm with zero internal volume can work.

See for example, some of the "thermo-acoustic" type engines that use a diaphragm "displacer".

The main function of the displacer is just to bring the working fluid (air/gas) into contact with the heat source and away again.

Take a look at TK Motors engine designs:

https://m.youtube.com/channel/UCkjcPbaP ... Uuw/videos

This, apparently, quite powerful "tin can" engine has a diaphragm displacer.

https://youtu.be/r9lYsW0Df08