Selection Of Stirling Engine Flywheel

[mechengr2801]

I am final year mechanical Engineering student at University of Engineering and Technology Peshawar. I am working on Stirling Engine for my Final Year Project. I have selected gamma-type for that purpose. Other specs are;
> Displacer Dia= 72mm
>Displacer stroke= 72mm
>Piston Dia= 36mm
>piston stroke=36mm
target power=30 watts @ atmospheric pressure
Now I am confused in the selection of flywheel. Is there any general relation b/w the power ans size/mass of flywheel? Can anyone suggest me flywheel size/radius for my engine?

[Ian S C]

With an engine that size, if one flywheel, I'd be thinking of about 8" or so, but it depend on each individual engine. Its the weight at the rim that counts, and the diameter, ie., the flywheel may be 6" dia and 5lb, or 10" and 5lb, the 10" one will be more effective, steel is a good material for making flywheels, I'v got one 14" one waiting for me to get on with building an open crank IC motor. My main method is to take a rough guess, get the motor running, thats not too critical, then change the flywheel for one of a different size (bigger or smaller), note the difference in torque, and revs, change the other way, and do the same checks, theres proberbly a mathimatical way of doing it, but practical trials work.
Just looking at your dimentions, I would revise them; the ratio between the dispacer swept volume, and the power piston swept volume should be 1.5 : 1, I'll leave you to work that out. Keep one or other, and change the other one to get the correct ratio. Disp 72 mm stroke X 72 mm bore. Power P 36 mm X 36. To get the required power I would tend to enlarge the bore of the power cylinder, and possibly the stroke also. Ian S C

[mechengr2801]

@ IAN S C
Thankyou very much for your support. you are absolutely right that swept volume of displacer= 1.5 times swept volume of piston cylinder as described in
http://www.cnccookbook.com/CCStirlingModels.htm
and some other books.This point slipped out of my mind while watching to one of the "John Rajos"(greenpowerscience.com) video in which he says that piston dia is generally 1/2-1/3 times the displacer dia but probably he is talking about model stirling engine for hobby or demonstration purposes. thanks again
I want to discuss some other points also.(1) I am thinking of using Graphite for making Power piston while the machinist is insisting upon "Teflon mixed with graphite"(70%:30%). Whats your suggestion?
2) As I will be testing my engine @ different pressures(project requirement), so obviously I will face leakage problem @ high pressures. A teacher told me to use stuffing box(chamber containing viscus oil through which connecting rod moves ) to avoid any leakage? what do you think?
(3) The clearance b/w displacer cylinder dia and displacer dia is 2mm i.e 72mm and 70 mm respectively, is it ok?

[Ian S C]

As the temperature decreases at the hot end, the displacer volume increases, note the large displacer used in the low temp motors.
I'v not used graphite, but only because I'v not had any to use. I use light weight cast iron, running in (usually) steel, but some times cast iron cylinders (latter best), graphite is often used in glass cylinders, but steel is good. Keep away from plastics, unless you can design out the expansion problems, but it is possible to use the graphite impregnated teflon as a piston ring (thes rings can be found in automotive shock absorbers), or if you put ring grooves in the piston, you can wrap plumbers teflon tape in the groove. I prefer plain pistons.
If the motor is pressurised, the crank case is pressurised too. one way around leaks is to build the motor with an inbuilt generator that can be used as a starter motor as well, no shaft coming out of the motor at all. I have the Ross Yoke motor in my gallery to pressurise to may be 30 / 40 psi, I'v yet to design a seal for the crankshaft. If I can get a seal for it I think 10 or even 20 watts is possible, I would then need to use the radiator for cooling, it's not required on that motor at atmopheric pressure. I would keep away from oil especially in a pressurised motor. A stuffing box with teflon tape in it should do.
Your clearance sounds OK, the important thing is an unobstructed air flow, but not too much dead space. If you make the space too small, you run the risk of the displacer touching the cylinder wall, this creates friction (and makes unwanted noise). I have not studied it much, but I think that at certain speeds, a vibration can be set up in the displacer, which if the clearance is too small can cause contact in the cylinder, it may happen more with a horizontal displacer, on a bigger (in my sizes) motor, with a light displacer rod. Ian S C

[mechengr2801]

As for graphite, you can easily find it with SUBMERSIBLE PUMPS mechanics. I have got mine from there.

[Ian S C]

Its also used in quite large lumps in foundries. Ian S C

[mechengr2801]

@IAN S C
keeping in view my engine size, can u suggest me the Channel size(Diameter) for transferring the working fluid b/w cylinders.?

[Ian S C]

Its hard to be specific, it depends on the exact lay out of the motor, I would suggest not less than 6 mm would be about right, but if the motor can be designed so that the bores of the two cylinders over lap, it can be arranged that the ends of the cylinders meet leaving a hole shaped like this (). If you have to drill from one cylinder to the other, two or more holes could be used, a short fat hole/ holes is good, the dead space does not have great effect. When you look at a BETA motor you realise that no restriction is needed, as the bores are equal. The effects of dead space in connecting tubes is discussed in Model Engineer 18 Feb 1977, by W.D. Urwick, he attached a 6ft rubber hose between the cylinders with little or no ill effect. Ian S C