Another Atkinson Differential build

[Steamchick]

Ken, you are welcome to my knowledge. Just keep a big bucket of salt handy for those moments when you take my comments "with a pinch"! It can take me ages you Figure out when I was wrong -Often for the simplest mistake - so as usual, check, check again, doubt the result and check a different way before using my knowledge!
K

 

[Ken Brunskill]

A few weeks back there was mention of sharing detail/data, so in that vein here is some on my Atkinson Differential (Lets refer to them as just (AD). Anyway my AD build has the following:
Bore is .749"
Rings are .050" wide by .041" thick made via the HT Method (Trimble) with .0015" gap (At least a .001"feeler Gage passes but not a .002" when in a .749" ID Ring.
Compression was negligible on 1st try when I had (2) .025" wide rings in one slot on the PUMP piston, but achieved 20 PSI when I re machined it and put in .070" Cross section O'Rings (hand cranking with no run in of any sort).
Did a measurement of the max & min distances of the piston faces and determined that when the pistons are furthest apart the volume is .18 cu", at closest the volume is .047 cu" calculates to 3.83:1 compression ratio. (Measurements via Solidworks 3D model). Will do a compression test after every hr for 3 hrs to see if there is any change, since the O rings will be crossing the spark plug hole I anticipate a loss - will report findings.
Supposed to be cooler today so hope to get out in the shop, if it gets over 90 F, out there, that will put a stop to the efforts!

 

[Ken Brunskill]

Ken, you are welcome to my knowledge. Just keep a big bucket of salt handy for those moments when you take my comments "with a pinch"! It can take me ages you Figure out when I was wrong -Often for the simplest mistake - so as usual, check, check again, doubt the result and check a different way before using my knowledge!
K

K,

Looking for your expertise, please offer your opinion on sizing the rings for this little engine.
My bore is .749" Dia.
My rings were made to .7515" OD with .041" Wall, and are .051" wide. I have just lapped (.400 grit sandpaper) the gap to be .003" when in a .749" ID Ring. There is just barely a hint of light on the periphery.
Rereading your post of Sept. 4th, I interpret that your recommendation is for .002" (.052 mm) total clearance for the width, and .004" (.104 mm) between the ID of the ring (closed) and the OD of the Ring Groove.
Cheers, Ken

 

[dsage]

Ken if I read and you type correctly above I think you missed the first step in the Trimble method of making rings.
If your bore is 0.749 then the rings should also be 0.749 +/- zero OD i.e exactly the same as the cylinder bore. (to the best of your abilities) before they are split and heat treated for their end gap.
To split them I usually place one section of the circumference over a piece of #22 wire on a flat surface and press down with two thumbs close to the wire to snap the ring. That will give you a zero gap.
Optionally you can saw the gap but for a 1" piston that would require a 1thou saw blade. Never seen one.
After the heat treating I hold the snapped section securely over the edge of the table and put a piece of fine sand paper in the gap and carefully work the paper up and down to create the gap. Periodically put the ring squarely in the bore and check with a feeler gauge to get the required end gap. Rinse and repeat with the sandpaper to your end gap specification.
If you are seeing light around the circumference while in the bore, stop. The ring is no good.
While the ring is in the cylinder I make a cardboard mask to put over the inside diameter of the ring to block light. Shine a light from the top of the cylinder and look carefully up from below around the circumference. There should be no light visible except through the end gap.
My 2cents on the subject. Of course there are a hundred opinions on the same thing. I'm sure you'll see them all.

 

[Steamchick]

dsage - I agree with everything you say, except my experience is for the ring to be machined to the exact bore size when compressed to give the correct radial pressure ( and clamped between 2 collars to hold it at the correct compression when machining). At Hepworth and Grandage works in Sunderland - making cast iron rings - it was interesting to see a mandrel filled with a hundred or so rings, for final machining to bore diameter.
First the rings were cast - 6 around a central metal feeder - then broken into single pieces. Horizontal grinders set the ring thickness and they were clamped together to form a cylinder for machining the bore. Then the OD was machined after fitting on a different mandrel - a sliding fit for the bore. Also while held on this mandrel, the rings were split with a slitting saw. These rings were then fitted into a Bore-sized cylinder and clamped together on yet another mandrel. Then the finished bore size machined. So when released the rings had the correct amount of spring deflection for the required ring radial pressure to resist compression. Finally, rings were installed on a mandrel that was on a weird motion in a bore with very fine grinding paste to shape the barrel-shape required on the rings. On a model, the "running-in" should put the barrel honing onto the rings - providing you DON'T use molybdenum in the oil.
I don't know the Trimble method, but I hope that helps?
Ken

 

[Steamchick]

K,

Looking for your expertise, please offer your opinion on sizing the rings for this little engine.
My bore is .749" Dia.
My rings were made to .7515" OD with .041" Wall, and are .051" wide. I have just lapped (.400 grit sandpaper) the gap to be .003" when in a .749" ID Ring. There is just barely a hint of light on the periphery.
Rereading your post of Sept. 4th, I interpret that your recommendation is for .002" (.052 mm) total clearance for the width, and .004" (.104 mm) between the ID of the ring (closed) and the OD of the Ring Groove.
Cheers, Ken

Hi Ken, Let me try and understand your sizes.

• Ring OD 0.7515" to fit a bore 0.749" dia. gives 0.0025" "compression" of the ring when fitted. Sound Ok without consulting calcs for radial pressure... but I guess you are working to someone's proven design?
• Ring groove width: I guess you have 0.053" wide for your rings of 0.051" thick - That is a fine bit of machining! = OK. - as that is where most of the blow-by gas passes the piston.
• Back-gap (behind a closed ring) of 0.004" for a diameter 0.749" bore less the ring width of 0.041" (x 2) gives a ring groove diameter of 0.659" by my calculations. This should be Ok as the object is not to seize the ring radially, and permit some float for the piston to move (align with the various forces) within the bore. It accommodates more than potential "mis-alignment" of bore and piston line (perpendicularity of the crank line and bore), as the barrel shaped piston skirt allows a tiny rotation of the piston about the gudgeon pin, as the the piston oscillates between "suction" and "Compression" strokes..
• Hope this helps?
• Ken

 

[dsage]

Steamchick:
Not sure which side of the fence you're on. Make them over size and leave them as is - or make them over size and turn them on a mandrel after to size. Your two post are in conflict.

Certainly making them oversize and leaving them as-is they will NOT be round when jammed into a cylinder (even with an appropriate gap to get them to fit). The bore and ring are not the same circumference. SO that method is out.

The Trimble method does specify turning the last thou off on a mandrel but I've never had any luck being able to reliably turn a thou or so off the diameter after the fact on my lathe. There is more than a thou run-out in my lathe chuck and I assume most hobby lathes. So you end up with a ring with a non-round shape even though it might measure the right diameter here and there. This may work in a factory setting though as you point out. Not for me.

SO I turn them right from the get-go to the exact diameter and sometimes even polish them to get the last tenths.
Then the proper amount of expansion at the gap (Trimble specifies it) and the heat treating gives them their radial pressure. When they are inserted in the cylinder they will be round again and the exact diameter because they started out that way.

Certainly NOT leaving them oversized and making them fit with the gap. Which is what Ken's post was implying he was doing.

Again just my 2 cents for what has worked for me.

 

[Steamchick]

Sorry, probably mixed things up a bit, accidentally.
First you machine over-size to the required "spring" plus a couple of thou. Then compress in a bore that is the size for the correct amount of ring compression (actually they called it "spring tension" at H & G from what I remember?) and hold clamped axially (at the correct ring-gap) and machine to the BORE size while compressed. (That "couple of thou"). This means the COMPRESSED ring in the bore has a "perfect circle" and correct cylinder wall pressure. Perhaps this is just impossible for the rings as small as you are making? The smallest production process I saw was about 2" diameter. - But I was a visitor, not working there, so may be wrong with the process I remember? Also, I have no hard data of sizes etc. - which were their "secret" to get the required finished parameters and sizes.
I don't know the Trimble method. Is this a way of machining to size, then softening, expanding and hardening to give some ring to cylinder wall pressure? - This sounds like a heat treatment that could possibly distort the shape of the ring from the "perfect circle" that was the target at H & G. But they were dealing in cast iron rings that would not be heat treated. They did make steel wire rings as well, that were heat treated - but this was a time for soaking them in burning Ammonia gas at a very dull red heat to nitride the surface. Actually the steel ring shape came off a clever winder that gave the ring "tension" that was uniform when in the bore. Which meant it wasn't a perfect circle unloaded, as the end effect of the element of the ring close to an end was to give a different wall pressure when compared to a "middle" element that is supported by elements either side with the same wall pressure. There must be a mathematical paper on this somewhere, that is better than my imperfect memory from 35 years ago.
Sorry about confusion.
K

 

[dsage]

Sorry, probably mixed things up a bit, accidentally.
First you machine over-size to the required "spring" plus a couple of thou. Then compress in a bore that is the size for the correct amount of ring compression (actually they called it "spring tension" at H & G from what I remember?) and hold clamped axially (at the correct ring-gap) and machine to the BORE size while compressed. (That "couple of thou"). This means the COMPRESSED ring in the bore has a "perfect circle" and correct cylinder wall pressure. Perhaps this is just impossible for the rings as small as you are making? The smallest production process I saw was about 2" diameter. - But I was a visitor, not working there, so may be wrong with the process I remember? Also, I have no hard data of sizes etc. - which were their "secret" to get the required finished parameters and sizes.
I don't know the Trimble method. Is this a way of machining to size, then softening, expanding and hardening to give some ring to cylinder wall pressure? - This sounds like a heat treatment that could possibly distort the shape of the ring from the "perfect circle" that was the target at H & G. But they were dealing in cast iron rings that would not be heat treated. They did make steel wire rings as well, that were heat treated - but this was a time for soaking them in burning Ammonia gas at a very dull red heat to nitride the surface. Actually the steel ring shape came off a clever winder that gave the ring "tension" that was uniform when in the bore. Which meant it wasn't a perfect circle unloaded, as the end effect of the element of the ring close to an end was to give a different wall pressure when compared to a "middle" element that is supported by elements either side with the same wall pressure. There must be a mathematical paper on this somewhere, that is better than my imperfect memory from 35 years ago.
Sorry about confusion.
K

It's nice to hear the way they make them in industry. I've always wondered. I'm sure it could be duplicated by many model engineers. I've just followed the Trimble method for cast iron rings. He did a lot of research on the subject and came up with it. Of course it's probably not perfect but easy enough to accomplish except the skiming off a thou (for me).
As for the heat treating it's NOT done at high temp. The rings are heated only to dull red visible in a dark room. I guess that's maybe 1200 F or so (the way I do it anyway). Pretty low compared to "heat treating". Not even hot enough to create surface oxidization to any great extent (in my experience anyway). It think it's more of a stress relief of the stresses put in by opening the gap a prescribed amount. Then when they cool they are fixed in that position and the tension comes again when they are put in the cylinder.
Many rings are stacked in a jig and clamped. So I've never had them distort. They lay flat on a surface plate for lapping to exact thickness.
You should look up the procedure. It shouldn't be hard to find.

 

[dsage]

I guess we've sufficiently confused Ken now

 

[Ken Brunskill]

I guess we've sufficiently confused Ken now

Dave & Ken, enjoying the discourse and really appreciate the collective knowledge being shared here. Actually even though an octogenarian, I am not being confused in the least bit. 55 years ago, I was lucky enough to have gone through a Tool & Die Apprenticeship at two companies, at one of which I had to spend 6 months in the HT department, followed by another 1 1/2 years where we as apprentices had to heat treat our own work. Provided a great background for most of the rest of my career (Mechanical Manufacturing Engineer and sometimes Mechanical Design Engineer). Of course none of which had anything to do with gas engines. Having always worked in the SF bay area my experience was in audio visual, computers, & computer peripherals.

 

[dsage]

I meant confused as to the number of choices you have to ponder to make the rings.

 

[Ken Brunskill]

I meant confused as to the number of choices you have to ponder to make the rings.

Not even that! I am even contemplating an overlap gap. Hey, after all it is a hobby right?

 

[Steamchick]

I suspect that when you get the fine clearances of rings in grooves, and no light around the ring to bore, you'll have Ok compression. Even if the straight gap is a thou or 2 big.
K

 

[Ken Brunskill]

I suspect that when you get the fine clearances of rings in grooves, and no light around the ring to bore, you'll have Ok compression. Even if the straight gap is a thou or 2 big.
K

Made the Rings .003" under the Piston Ring Groove, width, assured that the Dia. of the Ring Groove was .003-.004" less than the Ring ID when in the Gap Checking/Setting ring. Installed the rings on the piston, and installed these in the engine, compression is the same as I had with the O-Rings (approx 20 psi. ,Feeling positive, thinking I might have success. Now to proceed with the Fuel & Water Tank.

 

[Steamchick]

Sounds good, but with all puddings, the proof is a satisfied full tum!
K

 

[dsage]

Ken before you move on.
Squirt a bit of fuel in it and see if you get a pop. Don't forget the oil mix.

 

[Ken Brunskill]

Thanks for that bit of advice Dave, trying to have it provide a 'Pop" before Saturdays BAEM meeting.

 

[Tim Wescott]

A detailed description of the Trimble method. I haven't made rings, so I can only pass it on 'cuz I read the article and remembered how to search it out: How To Make Piston Rings

 

[Ken Brunskill]

I suspect that when you get the fine clearances of rings in grooves, and no light around the ring to bore, you'll have Ok compression. Even if the straight gap is a thou or 2 big.
K

K,

Pursuant to the thought of split gap (overlap) or pinned rings:
1. Thinking about the split gap, or more correctly stated 'overlap', I am inclined to this over the pinned concept.
a) Seems that the overlap would provide a better seal, the logical question would be sizing the split. (Bottom view of the sketch.)
b) Theoretically this, it would appear provide the better seal of the (3) concepts.
c) Now to understand what the sizes should be, my thoughts are to have the overlap be such that the ring end up .000" to .0005" of width.
d) The length of the overlap will need to be such that each side can be cut with the ring expanded to almost breaking, trial & error I suppose.
e) The gap at both ends (each end),I suspect would be the same as for the traditional method (for a 1" ring .002").
2. The center concept appears to me to be the least likely to provide a good seal, as the gap is tangential to any pin size considered.
3. The upper most concept, seems as good as the traditional technique, while stopping ring rotation.