Tubing for cylinders and pistons

[Aviator168]

Just remember this place I usually get supplies for my aircraft. Interestingly, the tube can be used as cylinders and pistons too. For an 1" OD tube, I found a tube with 1.009 ID. I also found a 2.010" ID for a 2.0" OD.

http://www.aircraftspruce.com/catalog/m ... ng_un1.php

[Ian S C]

Aviator 168, Aluminium tube is made so that tubes of the same gage will fit inside the next size up. I started my working life as an aircraft maintainance engineer, specialising in piston engines up to P&W R 1830, and down to Continental, Lycoming and De Haviland Gypsy Major. But I did do a bit of airframe work too, Cessna, and Piper mainly, not a lot of aluminium tube there.
As engines go, aluminium sliding in or on aluminium is not a great idea, it galls up, and under friction tries to weld its self together.
In lower temperature engines aluminium will work although not efficienly, because of the high conductivity of the metal transfering the heat from the hot end to the cold end. In high temp engines aluminium loses its strength after about 450*C. Ian S C

[Aviator168]

Yes. You are correct. But the page linked to is for 4130 ALLOY STEEL tubes. Don't know much how this type works as engine parts.

[Ferraccio]

Aisi 4130 is a steel low alloy Chromium Moly
Is "structural steel", not particularly foreseen to high temperatures; however I think that at level of SE here treated is a good material, for stainless steel parts.
For performance see:
http://www.suppliersonline.com/propertypages/4130.asp

[Longboy]

DOM tubing is ready to go. No boring or honing, just fit your piston. Used it for I/C models and should work as a hot tube and power cylinder in Stirling. http://www.industrialmetalsales.com/DOM ... ubing.html

[Ian S C]

Aviator 168, 4130should make quite a good power cylinder, as supplied for aircraft use its seamless, the bore would need polishing, but it should be fairly round. As bought it is in the annealed state, but that should be OK.
The annealed ultimate tensile strength is 78,000lb/ sq inch, rising to a maximum of 200,000lb/ sq inch after heat treatment. Chrome Moly steel (4130), has been the standard steel for aircraft use since at least the 1920s.
The inside of the tube may be coated with Linseed oil to prevent rust (or some similar material). A displacer cylinder could be made out of light gage tube with a piece of 4130 welded on as a cap (It gas welds well, even quite thin). A suitable bit for the displacer, could be mounted on a mandril, and the thickness bought down to say .015", thinner if you dare, then braze a thin steel end on it at the hot end, glue an aluminium plug in the cold end with the shaft in it, its amazing how light it can be made. You can of cause use stainless steel instead, its a good idea to get that TIG welded. Ian S C

[Aviator168]

I did some calculations on the amount of heat that can be transferred into a system via a certain area size/thinness on certain material a while back. It should give you a good estimate of the max power of an external engine. I have misplaced the notes, and need to re-work it again. Contact area, thinness of the hot cap and pressure of the working gas have direct impact on the power density of the system. As far as stirling engines go. The large the displacement, the less favorable for heat transfer.

EDIT. Stainless steel is a terrible heat conductor. Tungsten is as good as aluminum as far as heat transfer is concerned.

[Ian S C]

aviator 168, poor conductivity of stainless steel is more an advantage than a disadvantage. With the displacer cylinder the metal should be .020" or less So the heat does not have far to go, but the resistance slows the transfer of heat in a lengthwise direction toward the cold end which is inches away. In the displacer its self, the slow conduction down its length is good. The strength under hot (red) is a major help, you can make it thinner, and maintain strength. Using mild steel you get a bit of corrosion that ends in a bit of flaking, but it will last quite a while, as long as its not over heated, too much time at red heat, and it ends up with bulges, or in horizontal motors it droops.
I tend to run my motors at their maximum (if you don't break some thing, your not trying hard enough). Ian S C

[Aviator168]

The thought of why stirling engines never reach the power of ICs has been bugging me for a long time. After all, ICs do running on 1 bar (except turbo/super charge). ICs has one big advantage that ECs can never dream of, in stirling term, the area of heat transfer surface is infinite. In order to increase the power of stirlings, companies have resorted to increase the pressure of the working gas. The general perception is that the large mass of the working gas, the more power a stirling can produce. That seems to be true on the surface. The reality is, pressurizing the engine increase of contact surface of the heat exchanger. In other words, if you can increase the pressure of the working gas without increase the mass, you can obtain the same additional power. Or if you can make heat transfer more effective, you can get more power out of the same engine.

[Ian S C]

Heres some of my ideas, I'll use rounded figures. Un pressurised 15psi, with luck you may nearly double that so 30psi, now pressurise to 150psi, and working on the same idea you then have a maximum pressure of 300psi. If you double the Kelven temperature of a gas you double either the volume, or the pressure of that gas, thats Boyles Law. So you can see that as motors go up in pressure into the thousands of pounds, so the power goes up, the only problem is heating the gas. Air is a poor gas to use, difficult to heat, Hydrogen is the best, but its small atom tends to leak through metal, its quite safe as long as theres no Oxygen. Helium is next best, and its inert- can't go bang. The other problem with high pressure engines is the cooling, with an IC engine a lot of the cooling is via the exhaust pipe, more is throgh the oil in the sump (some times through an oil cooler), the rest with the radiator or cooling fins. The hot air engine has just the water jacket/ radiator, or cooling fins. The main trick is getting the heat in and out, the rest is the quality of the design and build. Ian S C

[Aviator168]

Check this out. This implementation can pressurize the gas very high without pressurizing the system. It is a bit difficult to implement though.

http://www.youtube.com/watch?v=YvtekHPzbUw