Regenerator and cylinders in a beta stirling engine

[RenewableEnergy]

Hi, I'm new in this forum and I would like to have some suggestion regarding the material to use for making the regenerator and the hot and cold cylinders in a beta stirling engine.
My idea is to use stainless steel for hot cylinder and aluminum for cold cylinder; for the regenerator I think to use a double layer aluminum-copper mesh similar to thoso used in filters.
There is somebody that can give me some more suggestion?

RenewableEnergy

[Ian S C]

Hi, you'v got it right for the hot end with stainless steel, use it for the hot cap, and the displacer. For the power piston, I recomend cast iron, and for the cylinder either cast iron, or steel. if the motor is air cooled, aluminium fins could be fitted at the cold end. For the regenerator, best would be stainless foil, next stainless mesh, or fine stainless wire. You don't want copper in there as it conducts the heat too well, the regeneratorneeds to absorb, and give up heat at the hot end, and the reverse at the cold end.
We all will be interested on how you get on, ask all the questions, and we'll try and not confuse you too much with different answers. Ian S C

[RenewableEnergy]

Hi Ian S C,

thank you so much for your prompt reply.
Using the specifications found on an italian book I designed four different possible rhombic drive beta SE with different displacements and powers.
Now I have to define the working pressure in order to fix the wall thickness of each cylinders.
I would like to introduce two external regenerators, one from cold to hot end and the other form hot to cold end using on each one way valves.
Furthermore I'd like to use one safety valve for rapid discharge of the gas in case of over pressure and one valve to load air, helium or hydrogen gas.
Do you have some drawing and/or picture on hot end cap. I found some video on YouTube, but they does not work properly.
Do you think that these ideas can work?

[Ian S C]

What was the Italian book?
The regenerator does not have valves, to work as a regenerator the air must pass through from hot to cold, leaving some of its heat on the way down and picking it up on the way back. You don't need a safty valve as such, but a valve can be incorperated to bleed gas to control speed, but things are getting complex, so forget it until you get a motor running. If you have not made a pressurised engine before, start with air as your gas, its cheaper, and more simple to use.
Don't know what you are using for a hot cap, but for a plain atmospheric motor its just a cup shape, preferablty stainless steel so that it will with stand the heat. This can be made a number of ways, I'll go through a list of the ways I'v done it. The first ones I made were machined out of solid stainless bar, later ones are made of stainless tube (thin as you can get, old vaccume cleaner pipes good), with an end TIG welded on, another way is stainless kitchen containers, or I'v used the steel case of a NiCad battery. If you look in my gallery, you'll see a 2.5 cc GAMMA motor, it uses an AA size battery case for the displacer, and the little beam engine uses one for the displacer cylinder/hot cap. Ian S C

[RenewableEnergy]

The italian book is"Guida pratica al motore Stirling" by authors F. Immovili, G. Ascari, F. Molinari.
Reading it I knew in details the operating principle of the SE and the differences between all the types of engine.
It gives suggestions on how to right size SE and all its components.
Regarding the use of valves in the regenerator, you are right, normally it is not requested. I read an american patent application where in order to increase the performances of a free piston SE it disclosed the use of valves to reduce the free flow of the gas between hot and cold cylinders.

[Ian S C]

I was wondering about the book, as to whether it was a text book, but from the little I'v seen on google, it looks as if its a hobby type book. The book I started on was "The Stirling Engine Manual" by James G. Rizzo from Malta. I have the 1 st volume, but if you have built a few simple motors, I think vol 2 would be a good one to give you a bit of help, James is a most respected builder of Stirling Engines. Another book worth looking at (it's a free down load), "Making Stirling Engines" by Andy Ross it's 68 pages.
Out of interest what sort of machinery do you have, and do you have welding / brazing gear. Ian S C

[RenewableEnergy]

Thank you for your suggestion on the books.
At the moment I'm only collecting information and then I'll start with the design.
Using this italian book I found infos about the sizing of a rhombic drive beta SE.
Surfing on www.solarheatengines.com I tried so simulate the four configurations I'm thinking to make.
Of course I'll start from the smallest and then I'll scale up.
My first target is a solar dish concentrator of two/four meters of diameter with double axis tracking and a 80cm3 rhombic drve beta SE with external regerator and helium working gas at a pressure of maximum 50 atm.
The only doubt I have is the noise.
I also saw the Whisper Gen site and I don't think that it is so noisy due to the fact that is installed inside a kitchen.
Which kind of experience do you have?

[Ian S C]

I know the bloke who designed, and built, and is one of the owners of the Whispertech company, I live in a small town about 40Km from Christchurch New Zealand, where the Whispergen originated, its now built in Spain. The whisper gen is far quieter than your refrigerator/ freezer, about the only way, other than the indicator light to tell its going is to put your hand on it, you can feel a slight vibration. It is putting out 1000W, now think of how much noise a 1 1/4hp electric motor makes. I can't rememberbut I think they ended up using Argon gas, Within the next very few years Helium may not be easily available. I think they tried Nitrogen as well, These gasses at 1200psi.
If your motor makes a lot of noise, you'v got something wrong in the mechanical build of the motor, ie.,too much tollerence in the bearings, or something like that. One of mine had a problem with noise, the aluminium displacer used to melt down during a good run, I cured that after the 2nd time, I changed to stainless steel, now that motor is in bits for a rebuild, to upgrade it further, it's a GAMMA free piston motor.
Ian S C

[RenewableEnergy]

Hi Ian,

one of the most important aspects in desinging SE is the sealing. I would like to use round rings made by Viton (Dupont) or Vulkollan (Bayer). These two plastic materials have high resistance to mechanical stress and are often used to make gaskets and o-ring.
What do you think about them? Do you have more suggestion?

[theropod2]

Hi Ian,

one of the most important aspects in desinging SE is the sealing. I would like to use round rings made by Viton (Dupont) or Vulkollan (Bayer). These two plastic materials have high resistance to mechanical stress and are often used to make gaskets and o-ring.
What do you think about them? Do you have more suggestion?

I think that if one wants a pressurized engine the crank and flywheel need to be encased in a sealed chamber. The flywheel could have a dual disc axial flux permanent magnet 3 phase alternator built in. The power could be then sent to the outside world. If the hot end was made well the heat input could be well isolated from the pressure chamber. This arrangement could eliminate the need to use seals on moving parts.

R

[Ian S C]

As theropod2 says if you pressurise your motor, its best to seal the generator/alternator within the engineand have no moving parts comming out of the engine case. the Whispergen has an alternator in the bottom of the engine, driven by a swash plate there is no crank shaft, the engine is started by motoring the alternator. The alternator was designed by Whispertech, and is similar to the motor used in washing machines by Fisher and Pykel of New Zealand, you can find refrences to these motors on google, and how to modify them for use as alternators.
I would recomend that you start with not much more than 100psi for the first pressurised motor, even that can cause injury if things go wrong, but will give a usefull indication of the effect of pressurisation, The Philips gen set used about 200psi of air, its not until you get to the really high pressures that you have to move to an inert gas, there is some small advantage at lower pressures, but cost tends to out weigh the advantage. Ian S C

[RenewableEnergy]

Hi theropod2,

thank you for your suggestions. As I wrote at the beginning of this thread I'd like to design a rhombic drive beta SE.
In this configuration the flywheel consist of the third gear moved by of the counter rotating gears linked to the arms and to the pistons.
See also the following video http://www.youtube.com/watch?v=Evjbvqpw ... w&index=17
I also read the suggestion from Ian to use a Fisher&Paykal washing machine motor. It is a good idea, but here in Italy is not so easy to find this motor.

[Geoff V]

RenewableEnergy

Regarding your proposed design, I must ask why you favour the three gear Rhombic as depicted in the video. Most of the noise generated in a Rhombic is from the syncronizing gears and adding a third gear will simply increase the noise and the friction, I see no point. I also fail to understand the benefits of installing a generator/starter inside the crankcase for two reasons. Firstly it is very difficult to design a motor with sufficient torque to turn the engine through compression when starting yet efficient enough to be of use when functioning as a generator. Secondly, the loads on the crankcase through pressurisation increase dramatically because the casing has to be enlarged to accomodate the starter/generator.
There are many examples of good pressurised engines with external flywheels, Philips MP1002CA, GM GPU3, et al. Sealing the output shaft is easily achieved with a Rulon lip seal or an O ring, either way it is far easier than embeding a starter/generator inside the crankcase.
Next may I ask why you have chosen the Rhombid drive? On paper it seems to be the perfect choice, in practice it leaves alot to be desired, true, it can be perfectly balanced but only if you can achieve building a displacer and piston with identical masses (not easy). Furthermore the geometry of a Rhombic causes a very rapid compression due to the angularity of the conrods as they approach TDC. This rapid rise in adiabatic heating will overload the compression heat exchanger in all but the very best examples.
Clearly it is essential to pressurise a Stirling cycle engine to realise any usefull power output, (0.2w/cc/bar abs) and good sealing piston rings with low friction is the hardest 'nut to crack', so before you start to cut metal I would recommend you seriously consider the double acting gamma.

http://www.starspin.com/stirlings/jimd6.html

GeoffV

[RenewableEnergy]

My first idea of rhombic drive beta SE has the following features:

displacement: 80cm3
diameter of the gears: cm 13,542
pin radius: cm 3,386
distance between connecting rod: cm 7,038
length of connecting rod: cm 8,464
power displacement: cm3 20
work displacement: cm3 60
bore: cm 3,2
power stroke: cm 2,4
work stroke: cm 7,4
power piston diameter: cm 3,1
displacer piston diameter: cm 3,0
power piston length: cm 6,4
displacer piston length: cm 9,6
total power cylinder length: cm 8,8
total work cylinder length: cm 17,00
length of thermal insulation: cm 0,5

Do you think that it could work at ambient pressure and 80°C/450°C temperature gap?

[RenewableEnergy]

In order to improve the performance of my rhombic drive beta SE I would like to use air or helium pressure at the range of 10-50 bar and to tranfer and storage heat I'm checking if it is safe to use NaK alloy.
Anybody has some infos about that?