Heater & Regenerator Design

[richjbarnett]

Hi Chaps,

Sorry for the slow reply, been on other things over Christmas.

Thanks very much for the information. Thats exactly what i was looking for on the tube heaters. I'm going to steer clear of using them and look to increase the surface area along the lines of the German guys engine.

What i have thought of is to drill 2 rings of fairly small diameter holes on 2 PCDs closely spaced. Then when i bore out in the inside and turn the outer to size i will break into the drillings, i havent done the maths but i'm thinking it should double my surface area from a simple smooth surface. The other advantage is it is quick and simple to do this, although care will be needed to deep drill the stainless!!

I've been looking at using some ceramics to thermally isolate the heater from the cooled side. Found a machineable ceramic that fits the bill but cost is a little high, so i think i may allow for its addition later in the design.

I have a question on the displacer piston. What material and wall thickness are you guys using?? Ive looked at machining some stainless end caps then use thin sheet (of some type and thickness) to form the tube. This wants to be kept to a minimum of weight but must be robust enough not to compress under the pressure of the engine. I have in mind using 0.5mm thick steel shim stock for this that is easily obtainable in 150mm wide strips. I can (i hope) carefully silver solder this together to form my tube.

What are your thoughts on this??

[Ian S C]

I'm using thin walled, about .010" stainless steel tube with a cap TIG welded on the hot end, the hot cap is made the same way. I have made hot caps from solid 316 stainless steel, expensive!
Silver solder may not be quite up to hot end temperatures, I run my motors at red heat, and the displacer turns blue to nearly half it's length, and above that at the end.
Ian S C
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[cbstirling2]

I have seen some builders thermally isolate the hot and cold halves of the displacer so the mass of the displacer acts as a regenerator.
So think 2 cans glued together with insulator in middle.

In order to allow for pressurized engines, I've read that some builders put a pin hole in the displacer. Compromise I suspect.

Just a thought: I agree that slots on inside is best, but I wonder if slots on the 3 part displacer would be nearly as good? On second thought, that would have lower performance on heat transfer from flame to inside of engine.

P.s. I think the drill holes with boring to be a clever idea!

[cbstirling2]

Rich, I hope you publish your results. Any thoughts on snifter design?

[richjbarnett]

I'm finding it hard to source really thin walled tube, so thats why i thought about making my own. I dont think i could weld 0.02" sheet by hand.

I was hoping the silver solder on the displacer piston would be good enough.

But you have made me think so im back looking at tube options.

[richjbarnett]

i've got the design covered for the internal pressure of the displacer. The shaft supporting the displacer is formed from tube, this is open in the displacer can and in the crank case, so it always sees engine pressure. I'm having a little balance pipe with a valve to allow crank / cylinder pressure to balance during pump up, 1/16th tube with rotary valve, so not adding to dead volumes by any great extent.

[Ian S C]

With atmospheric motors I often use a tubular displacer rod, so that the displacer's internal pressure stays at atmospheric pressure. Put a pin hole in the displacer, and power will go down, the motor might even decide not to go at all.
If you can get steel tube without a weld ridge down the inside you have a start. Next get a bit of steelthat you can turn down to a firm fit in the tube. Put some thread lock or similar on the bar, and push the tube on(don't take the bar out of the lathe. Now you are ready to turn the tube down to the thickness you want, I'v gone to .005" but that's too thin, .010"/.25 mm or even a little thicker is ok.
Hot caps are similar, or you can bore out a solid piece, fit a mandril as above then turn down the outside. Leave the end on the bored out bar and you have a cup shape.
Ian S C

[Aviator168]

this is open in the displacer can and in the crank case, so it always sees engine pressure

If the inside of the displacer sees engine pressure, it is dead space.

[Bumpkin]

Hey Aviator. It sounds like Rich is venting the displacer to the crankcase, where it wouldn't affect the working volume dead space.
Bumpkin

[Aviator168]

As long as there is no passage way to the engine working fluid, it is fine.

[richjbarnett]

Sorry didnt put that nicely. Yes im venting the displacer to the crank case. Its a pressurised cranks case engine. So my thinking was the displacer always has an internal pressure equal to the charge pressure.

[Aviator168]

Whatever you do. Just don't let the working fluid get inside the displacer, not even through a very small hole.

[richjbarnett]

The displacer is totally sealed to the working fluid. The pressure port i am talking about goes down the centre of the push rod and vents to the crank case, which is a sealed case and pressurised. So as i increase pressure the inside of the displacer sees this pressure so i do not get any crushing of the displacer (so i can keep it nice and thin).

Ive made a start and have been working on the displacer cylinder components.

Stainless heater with aluminium cooler with water jacket.

[Aviator168]

What pressure are you planning to run the engine in? The wall seems to be very thick.

[richjbarnett]

Looking to run at 15bar fairly quickly but then maybe more.

Yes the wall thickness is fairly thick, i went with the thought i can remove some more easier than add some back.

There is also some more material to come out of the bore. It will be having some vertical grooves put in to form an heat exchanger so will be loosing some of the wall thickness.

Im just a bit paranoid at having glowing red hot metal with lots of pressure inside !!