Another Atkinson Differential build

[Gordon]

JQ, curious to know if you measured the 'Ring Gap' at the 'Bore' diameter, if so what was it? As mentioned new rings were in the works, thought I'd share photos 1st is the Fixture - it has a .041" pin to establish the gap.
2nd is the HT Fixture simple Cast Iron Nipple two caps, the wood piece is to consume the oxygen, works splendidly-all that remains is a piece of charcoal.
3rd is the HT Fixture in the oven at 1400 F according to the analog temp meter.
All now cooling off in the oven - remove tomorrow when I finish the Pump Piston.

Regarding the use of some sort of gas, if anyone is interested I can provide the dimension for placing the 'oiler' hole such that it would be between the rings when the piston is at the bottom of the stroke (being that I redrew all the parts in Solidworks, it is not very hard to do a measurement), my thought is to make it a #4-40 at most, therefore able to be plugged with a set screw. Believe an oiler with a very small thru hole to be sufficient, anyone have thoughts on that?

I have been making rings using the same Trimbrel method except that I have read several times the the temp should be 1100° and I have been using Brownell anti scaling compound which seems to work well.

 

[Steamchick]

Lesson learned I hope! I assume you have manual advance? - Retard?
K

 

[bdolin]

Lesson learned I hope! I assume you have manual advance? - Retard?
K

Yes, lever on the handlebars. Compression release lever. Torque enough to move mountains or climb trees (dont ask!)

 

[Steamchick]

I remember it well... Good for may-be over 85-90MPH? But not on the UK's 70 mpH limited roads! A superb bike!
Have fun.

 

[jquevedo]

JQ, curious to know if you measured the 'Ring Gap' at the 'Bore' diameter, if so what was it? As mentioned new rings were in the works, thought I'd share photos 1st is the Fixture - it has a .041" pin to establish the gap.
2nd is the HT Fixture simple Cast Iron Nipple two caps, the wood piece is to consume the oxygen, works splendidly-all that remains is a piece of charcoal.
3rd is the HT Fixture in the oven at 1400 F according to the analog temp meter.
All now cooling off in the oven - remove tomorrow when I finish the Pump Piston.

Regarding the use of some sort of gas, if anyone is interested I can provide the dimension for placing the 'oiler' hole such that it would be between the rings when the piston is at the bottom of the stroke (being that I redrew all the parts in Solidworks, it is not very hard to do a measurement), my thought is to make it a #4-40 at most, therefore able to be plugged with a set screw. Believe an oiler with a very small thru hole to be sufficient, anyone have thoughts on that?

Ken:
Cylinder bore is 0.7490 in.
I made rings at 0.062 thick, inner diameter was as close as practically possible to 0.7490 - 0.80 in.
ring gap measured in bore as little as possible, just enough space for me to get the ring in the bore, I thin in practical measurable space of less than 0.004 in with a feeler gauge in a small bore is hard to get real accurate.
I use a very thin slit saw to split the rings.
I made the rings as the usual Trimble process with a lower OVEN temp 1100 F there are many discussions here and some other forums regarding the issues of softer rings created at 1400 F, not sure about this but i have always done mine at 1100.
Outside ring diameter was 0.010 in over desired finished diameter, then placed inside cylinder to adjust end gap, then machined in a custom made arbor for final diameter turning.

Engine ran OK for some time however not enough to call it a RUNNER, so I'M making new pistons and rings.
Will re-hone cylinder with one of those hones that look like bottle brushes, which are great for softening CROSS HOLES since they tend to round the sharp edges in this case the spark plug and ports.
Will make the pistons from Al 4032 (High Silica Aluminium), will use the same ring thickness but will make the piston grooves for rings LESS deep, I usually leave about 0.0010 - 0.0020 in smaller piston diameter than ring inner diameter, this allows easier insertion of pistons etc. This time I will go as close to ring inner diameter as practical.

 

[Steamchick]

Hi there jq.. Just a few odd comments.
I hope not to confuse you, but maybe add a bit of knowledge that will help you set-up the rings for correct machining "to size"?
First - having made a ring - slightly over-size - and slit it, you then need to clamp it in a fixture - compressed to the circle to get the gap you require in service. I assume you do not have a lazer to cut a 0.001" ring gap!
ALSO: IF I remember correctly from 50 years ago...The ring gap when installed should be about 0.004" per 1" of bore. - so you really want to achieve 0.0007 to 0.0008" ring gap when installed. So find some material that is the size of the ring gap you want when installed, cut a piece of that material that you can insert in the ring, and clamp it with a circular clamp (A piston-ring clamp, a hose clamp or piece of tin-plate and mole grips, or whatever you have to compress the ring without breaking it). THEN: When closed onto the finished ring gap insert (a piece of aluminuim foil, cigarette paper or whatever you find that is a suitable thickness) only THEN introduce the ring into your fixture for the finish machining of the diameter the few fractions of a thou less than bore size. (Linishing the last fraction of a thou so there are no machining marks on the ring).
THUS you will have made the ring with the required gap WHEN IN THE BORE. and as it was compressed from your ring made with a larger gap, the ring radial pressure will be a function of the difference between the gap as cut and the gap as installed.
Ring manufacture is a very complicated process in plants like Hepworth and Grandage - where I visited in Sunderland 30 years ago - as they were still making cast iron rings the only way that worked properly (after 75 years of ring manufacture). I was amazed at all the fixturing needed to get the metal from a cast circle to a machined circle - then reduced after slitting to get the periphery required as a perfect circle the size of the bore, with correct ring gap and radial pressure as installed. This was followed by re-boring the inside (in a sized cylinder fixture, so again the ring was in an "as installed" condition) to get the correct ID for the piston ring groove - minimal back clearance. Finally, the machined rings were in yet another fixture to be ground on the flat faces to get the exact thickness - to match the ring groove width.
I can't remember the clearances, but here are some dimensions from my Moto Guzzi works manual:
Bore: 74mm nominal: Actual:74.008 - 74.014mm
Piston OD: 74mm nominal: Actual 73.982 - 73.988

I'll let you work out the clearances in thousandths of inches so you can appreciate the small tolerances and clearances involved in MASS PRODUCTION of these parts. But these dimensions are for Aluminium pistons in chromed Aluminium cylinders, so no differential expansion required. These dimensions in a cast iron cylinder would seize. - But I'm sure you can find such dims and tolerances for an engine of your materials.

But for rings on pistons, the side clearances should be sensible (the Guzzi has cast iron rings in the aluminium piston).
Ring groove width in piston: to ring thickness - difference - 0.030 ~0.062mm.
Piston groove width: 1.52 ~1.54mm; Ring thickness: 1.478 ~1.49mm.
The fact you are 1/4 of the size doesn't matter as this dimension does not scale. (To the best of my knowledge).

ALSO: Ring gap (installed in cylinder) 0.25~0.45mm
Of course, your piston & bore are 1/4 of this size so the ring gaps will also be 1/4 of these gaps. But please check these dims for an engine of equivalent material to your engine - as the Guzzi's aluminium cylinder means these are following different differential expansions to you engine. (Al cylinder +cast iron rings + al piston) is not equal to (cat iron cylinder + cast iron ring + al piston). Hope this is clear?
I do not have dimensions for ring ID as installed, versus piston groove diameter. But understand in principle that this clearance between the CLOSED ring ID to piston groove diameter is only slightly larger than the side clearance. (0.030~0.062mm). My engine is assembled in the bike so I can't measure these for you... But I'm sure you'll find appropriate dimensions so you can get a better understanding of real mass production engines versus yours?
I hope this helps, but I'm not a piston & ring designer, just worked with some.
If anyone can correct this info, I'll be glad of your help/advice? (It's 30 years since I was doing that job, the memory isn't perfect!).
K

 

[peterl95124]

Ken, Jamie, since you guys live in the Bay Area I'd really like to see your Atkinson engines at a BAEM meeting some time (like after a vaccine is available and we re-start having meetings). Both the Atkinson Cycle and the Atkinson Differential engines are now on my bucket list.
Pete, still working on the Merlin the Cirrus and the Duesy, Lawrence.

 

[jquevedo]

K:

Thanks for the measurements, will try to make some correlation and hopefully make some sense on what tolerances are suitable for this engine.
As you mentioned, the key in manufacturing these rings after splitting has been the manufacturing of the machining mandrel and compression collar as well as the little piece that keeps the ring at that compressed state ( hopefully as close as to as installed measurements as possible) and then the very light machining of each individual ring.
You spend so much time and effort calculating the right measurements of the machining / holding tools and then you have to make sure your mandrel is accurate and centered as close to TRUE center as possible, then machining is not more than three seconds in the lathe, close to less than 3 seconds of polishing..

To tell you the truth, I have been using most of the tolerances that I use while building engines before, most of them in the 1 inch bore size and it worked fine for smaller engines like the Mastiff..

One major issue I have been facing is ring rotation, and the subsequent realignment of the ring gaps in the cross drilled holes ( ports or spark plug) and then the ring edges (closer to the gap) will wear off faster than the rest of the ring at this point the ring does not rotate any more but the ring stops sealing properly and the compression drops.

This is why my idea of producing a shallower ring groove to provide some drag in the ring to reduce the rotation. I know this is not best practice, we need ring rotation to make perfect circles on cylinder and rings to have a long life engine.
I understand this is more like the 2 stroke engines where a pin locates the ring and prevents the rotation, the only problem is how to practically do the same in a bore that is barely 0.750 in and rings which are 0.062 wide and about 0.040 thick..

Right now I have to finish replacing axles and bearings in one of those old Mercedes Benz Gwagen which is a nice distraction, having to make some tools to remove old bearings and install oil / grease seals..

 

[jquevedo]

Ken, Jamie, since you guys live in the Bay Area I'd really like to see your Atkinson engines at a BAEM meeting some time (like after a vaccine is available and we re-start having meetings). Both the Atkinson Cycle and the Atkinson Differential engines are now on my bucket list.
Pete, still working on the Merlin the Cirrus and the Duesy, Lawrence.

Peter, built an Atkinson Cycle engine which is a solid runner..idels at a very nice low speed and start with a couple of pulls on the flywheel..
The design is based in a couple of builders, Jan Ridders from the Netherlands and Frank Kurtz( sadly left us many years ago) but he made some very creative designs which inspired the design.

 

[peterl95124]

FYI, for a ring to seal it needs to be not only a perfect fit in the cylinder but also a perfect fit against the sides of the groove in the piston, IE the sides of the ring need to be perfectly smooth and the sides of the groove in the piston need to be perfectly smooth. Its easy to polish the sides of a ring to a mirror finish with a holder and 2000 grit wet-or-dry paper, but the groove in the piston is another matter, I always do this groove with a carbide tool and lots of tap-magic as a cutting fluid. Examination with a 10x magnifier should show a smooth machined surface, otherwise scrap and start over...

 

[Steamchick]

Lots of good information on this thread. I'm really enjoying reading! Thanks guys!
jq: You mention that you are going to reduce the clearance at the bak of the ring by making pistons with slightly larger diameter at the bottom of the ring groove. Well, I like your idea, but please be aware of how much the piston will expand, compared to the rings and (presumably cast-iron?) bore? The bore may only expand from room temp to 80 ~120 degrees C.... but the piston can get 100~150 degrees C hotter: And then the coefficient of expansion of the aluminium is so much greater than the cast iron, I think you may get to the point of seizure when you get a hot piston? (without exact dims I haven't done the sums). So please do the sums or post a picture of all the finished dims of the piston, rings and bore and we'll do the sums and let you know.
Peter, Text book perfect comments! Thanks.

 

[Steamchick]

On ring rotation...
As rings rotate, the end appears to be passing a port from your explanation. This is categorically a "No-no!" in my experience of engines. The radial pressure from and "endless" ring is constant, but the radial pressure of an end of a ring is higher (as far as I remember from 30 years ago - probably I'll find out from Google someday) and when it finds a port - as uncontrolled rings in a 2-stoke port - it will expand into the port. This will eventually cause a breakage and severe wear of the bore as the edge of the ring rubs on the edge of the port when it transfers from port to cylinder wall above or below the port. LOTS of extra heat will be generated, as the ring and bore machine each other. The expanded ring end will prevent further rotation to get past the port - as you mention - so the only solution (as on 2-strokes) is to peg the ring, where the ends are "mid-way" between ports. (On 2 strokes with transfer ports on 2 sides, and an extra transfer port opposite the exhaust port which is between the 2 main transfer ports, the pegs will be positioned between transfer port far away from the exhaust port.). As the ring gap is only 3~4 thou in in your case, a peg of that size isn't practical, but if you have (E.G.) a 0.030" peg, you can make the ring gap 0.033" wide when rings are fitted in the bore - which actually may make life easier? In fact, a peg the same size as the ring groove width is probably the best, with ring gap adjusted accordingly. Use brass for the peg and be sure it is below the surface of the piston, but only by a couple of thou.
Hope this is of some assistance?
K

 

[jquevedo]

K thanks for your comments, the issue as you described was, wear in ports area of the cylinder most likely caused by ring rotation and alignment to ports, some additional wear was maybe caused by metal residue in the middle section of the cylinder, causing the cylinder to have more wear where the rings rub that at the two ends.
Re-machined cylinder to new diameter and made pistons to fit with new dimensions and reduced ring groove depth, will use your idea of making a 0.0062 in pin and insert it where the ring end gap should be in relation to ports (power piston) and spark plug ( pump piston).
Thanks for the advise..
JQ

 

[Steamchick]

Hope it is a step forward anyhow.
Cheers!
K

 

[Ken Brunskill]

JQ, K & Peter,

Wonderful reasoning and information flow going on here! Heretofore I'd not been a follower of any online or web-based stuff of any description, yet I am finding that following this site most interesting and informative, akin to 'measuring twice cutting once! I was about to head out to the shop and finish up the rings for the Pump Piston, now with the information JQ has just provided I'm of the mind to do the pin concept as that just makes good logical sense! Will give it some serious consideration.

As to bringing the engine to BAEM meeting, will do! The next meeting is Sept 19th, since it is outdoors, maintaining a good social distance is easy, although not everyone wears a mask it is easy to maintain a safe distance and still participate. JQ where are you in the Bay Area, I am in Fremont.
Ken

 

[jquevedo]

Ken:

I used to attend some of the BAEM meetings when worked allowed it, my work was mostly international long term travel so I was pure luck if I was home and the meeting was scheduled.

Now I reside in Monterey near Carmel so the trip has gotten way too long specially when they moved the meetings up North from Palo Alto.
I also spend significant time in Baja CA Sur, near San Jose del Cabo where I have built my simple beach house, and sadly there is no Work shop there.
JQ

 

[peterl95124]

don't know how true this is but concerning ring groove depth in the piston, I've been advised that the ring should not be able to bottom out in the groove as the piston moves side to side in the cylinder, gas pressure needs to get behind the ring to create ring-to-cylinder sealing force. my guess is there needs to be as much depth clearance as there is side clearance, rule-of-thumb is at least 0.001" per inch of bore. also differential thermal expansion of the piston compared cast iron ring will increase the side clearance but decrease the depth clearance so depth should be even larger.

 

[Ken Brunskill]

Ken:

I used to attend some of the BAEM meetings when worked allowed it, my work was mostly international long term travel so I was pure luck if I was home and the meeting was scheduled.

Now I reside in Monterey near Carmel so the trip has gotten way too long specially when they moved the meetings up North from Palo Alto.
I also spend significant time in Baja CA Sur, near San Jose del Cabo where I have built my simple beach house, and sadly there is no Work shop there.
JQ

JQ,

To bad, would have been neat to have two of these show up at a BAEM meeting. With your place in San Jose del DCabo, do you fly fish? If so that would be an amazing coincidence, as that is my other significant avocation. I've not tried the message component of this site, has anyone? Is it 100% separate from these postings (presume so)?

 

[awake]

Peter, built an Atkinson Cycle engine which is a solid runner..idels at a very nice low speed and start with a couple of pulls on the flywheel..
The design is based in a couple of builders, Jan Ridders from the Netherlands and Frank Kurtz( sadly left us many years ago) but he made some very creative designs which inspired the design.

From the pictures, this looks like the other Atkinson design, not the differential - very different animals, from what I understand (which may not be much!)

 

[Ken Brunskill]

K, You have inspired me to make a new set of Pistons and Rings utilizing the 'Pinned' concept. Once the weather here has dropped (It is in the 100 F range at present) I will venture back out to my shop and make the fixtures shown in the two attachments. Your review and comments are encouraged, as I'd not thought about taking this path before you mentioned it.
In the meantime, I'll forge on with Dave Perrault's design, to see if I can get as far as JQ has. (Won't run it much as I suspect I'd damage the cylinder walls, I'll be satisfied if I get compression, and a "pop" for now.