Info on piston ring pressure requirements?

[Gordon]

There is heaps of comments around about these particular engines (2 piston type)and reference to the flaw in the design of the model engine.
Basically the timing of the cycles is way off.
Mate of mine was in the process of redesigning it as he can watch the process in a cad "open cage" view, just other projects have taken over.
We have both built the cycle engine with success.

I started using the Gingery design and have since used another design which is better but still very marginal. There is just never a real compression stroke. Both pistons are moving toward the firing point but at different speed so compression is low no matter how good the parts are made. The engine runs just enough that you keep thinking that just a few tweaks it will run. I also built the Atkinson Cycle and that runs.

 

[bluejets]

Like #10, I use 1mm x 1mm and a 3mm piece jammed in between the ring ends while heat treating bolted in between two large mass steel plates.
After natural cooling, rings are fitted to a jig for re-trueing to round and correct diameter in the lathe.
Ring ends are then filed to a bore-fitted end clearance of 2 thou per inch of bore.
Rings are then sized flat on glass with wet/dry lubed up to fit the ring groove.

 

[petertha]

A more updated write up in post #54 that speaks to some of these matters (specifically the attachment Making Piston Rings. docx)

http://www.modelenginemaker.com/index.php?topic=3794.msg68928#msg68928

 

[johwen]

Thanks. That pretty much verifies my calculations.
Is there any reason for making the rings that narrow? .03375 and .0225 are really narrow and smaller than any cut off blade so it requires a special ground bit. The rings I have made have been 1/16 or 3/32 wide. A .0225 wide ring would seem to be pretty fragile.

Johwen Here again. I use the Trimble formula for making rings and to get great compression from the get go I make the ring groove twice the width and put two rings in the groove with the gaps set at 180 degrees apart. Works fantastic using the trimble formula exactly Ring gap set at .oo1 per inch of bore diameter give it ago Cheers. John

 

[Gordon]

Johwen Here again. I use the Trimble formula for making rings and to get great compression from the get go I make the ring groove twice the width and put two rings in the groove with the gaps set at 180 degrees apart. Works fantastic using the trimble formula exactly Ring gap set at .oo1 per inch of bore diameter give it ago Cheers. John

I never thought about putting two rings in the same groove. I could use my existing pistons with .093 groove and put two .046 rings in each groove.

 

[Gordon]

That width and thickness are what Trimble recommends. Going wider does not make for a better seal and going thicker would probably give greater wall pressure and wear the cylinder faster. Just my opinion so take it for what its worth. I have cutoff blades as small as .019 width so a ring of this size I would use the dimensions given or bump it to the next cutoff size or two which is .025 or .031. The bump would be to a .0235 or a .0285 wide ring.

If 1/16 or 3/32 is what you are comfortable with and seem to work well then I see no problem doing that. The numbers Trimble cites are more inline with a conventional engine. We all know that there is not a lot about model engines that is conventional so go with what works.

Remember that his method is supported by lots of math and research that seems to make a lot of sense to me. Also the fact that many builders over the years have made successful rings if they follow the method closely. That's why I use it and my Rings have worked very well the last few times I have used it.

If you try to make rings, other than the width of the ring, follow the method exactly as laid out. It's kind of a pain in the rump but easier than making rings 2 or 3 times.

Use the right cast iron
Machine everything to the correct dimension
Stress relieve them correctly
Fit them in the ring correctly.

Do all that and you will be quite happy.

How important do you think dowel diameter would be. For instance for 1.5" bore the theoretical dowel dia is .225. Would it make a great deal of difference if the dowel were made to a standard size for example .1875 or .25 as long as the mandrel dia were calculated to reflect the dowel size. Making a dowel at .225 is not impossible but certainly a lot more work. As long as the OD and ID are correct the dowel would seem less critical since it is just to hold the gap during heat treat.

 

[stevehuckss396]

Everything about the fixture is critical to hold the ring to a specific shape and put even stress around the ring. I do not change anything about the entire process and they work great. Just saying.

 

[Gordon]

It's just visual basic. See if it works

Sorry to be a PITA but I am having a problem with your calculator.
Timbrel says that M/B = 1.0072 + .1569D/B -2T/B
That makes M= (1.0072 +.1569D/B -2T/B) x B

For instance on your sample of 1.5 Bore your calculator says M = 1.370253 with D = .225
Using the formula I come up with M = 1.4171025

M= (1.0072 +.1569x.225/1.5 -2x.0645/1.5) x 1.5

Any idea what I am missing?

 

[stevehuckss396]

DowelDiameter = CylinderBore * 0.15
MandrelDiameter = CylinderBore * (1.0072 + (0.1569 * DowelDiameter) - (2 * RingThickness))
DowelOffset = CylinderBore * (0.4927 + (0.1832 * DowelDiameter) - (RingThickness / 2))
RingGrooveDepth = RingThickness + 0.003
RingGrooveOD = CylinderBore - (RingGrooveDepth * 2)
InsideDiameter = CylinderBore - (RingThickness * 2)

I don't have the magazines in front of me but here is the math I think you are trying to figure out. If this doesn't help I can dig out the article and see if I have made an error somewhere.

 

[petertha]

How important do you think dowel diameter would be. For instance for 1.5" bore the theoretical dowel dia is .225. Would it make a great deal of difference if the dowel were made to a standard size for example .1875 or .25 as long as the mandrel dia were calculated to reflect the dowel size. Making a dowel at .225 is not impossible but certainly a lot more work. As long as the OD and ID are correct the dowel would seem less critical since it is just to hold the gap during heat treat.

I entered your 2 dowel dimensions & you can see how it (black triangle) moves vs. (red dot) ideal.
Maybe someone else can validate,its been a while since I looked at my spreadsheet. I assumed you maintained the recommended ring thickness, otherwise that changes things again.

 

[petertha]

Question for you engineering types. If the ring is made thinner than the recommendation, all else equal, is the result (black triangle) saying it will not meet min 30 psi minimum because its below that threshold line. Does that make sense?

 

[Gordon]

DowelDiameter = CylinderBore * 0.15
MandrelDiameter = CylinderBore * (1.0072 + (0.1569 * DowelDiameter) - (2 * RingThickness))
DowelOffset = CylinderBore * (0.4927 + (0.1832 * DowelDiameter) - (RingThickness / 2))
RingGrooveDepth = RingThickness + 0.003
RingGrooveOD = CylinderBore - (RingGrooveDepth * 2)
InsideDiameter = CylinderBore - (RingThickness * 2)

I don't have the magazines in front of me but here is the math I think you are trying to figure out. If this doesn't help I can dig out the article and see if I have made an error somewhere.

My math is rusty and old but:
Timbrel says M/B=1.0072 + .1569D/B - 2T/B
I think M = (1.0072 + .1569D/B - 2T/B) B
you say M = B(1.0072 + .1569D -2T)

I may well be wrong. It happens frequently.

 

[Gordon]

I entered your 2 dowel dimensions & you can see how it (black triangle) moves vs. (red dot) ideal.
Maybe someone else can validate,its been a while since I looked at my spreadsheet. I assumed you maintained the recommended ring thickness, otherwise that changes things again.

OK Thanks. I was not actually relating it to the graph and charting T/B and D/B

 

[Rustkolector]

I have an Excel Trimble ring design spreadsheet that I found somewhere in the forums that agrees with what Petertha has shown. However, in it all three curves intersect forming a closed triangle. In this spreadsheet altering either the thickness or the wall pressure changes the dowel and mandrel diameter. I tend to stay in the middle and away from installation stress.

 

[stevehuckss396]

My math is rusty and old but:
Timbrel says M/B=1.0072 + .1569D/B - 2T/B
I think M = (1.0072 + .1569D/B - 2T/B) B
you say M = B(1.0072 + .1569D -2T)

I may well be wrong. It happens frequently.

I have re-read the article in the 3 issues. I recreated the examples that were given by him, there were 2. All my numbers were exact to his. I may carry out the decimal a little farther.

 

[Gordon]

I have re-read the article in the 3 issues. I recreated the examples that were given by him, there were 2. All my numbers were exact to his. I may carry out the decimal a little farther.

I am calculating for a 1.125 bore
B= 1.125
T=.048
D=.16875
If I enter that into the original formula I get M= 1.0656 and O = .5608
If I use your formula and your Visual Basic calculator it ends up M= 1.054886 and O= .5620

M/B=1.0072 + .1569*.16875/1.125 -2*.048 = .945405
.945405 *1.125=1.06358

Am I doing something wrong?

 

[editor123]

I have an Excel Trimble ring design spreadsheet that I found somewhere in the forums that agrees with what Petertha has shown. However, in it all three curves intersect forming a closed triangle. In this spreadsheet altering either the thickness or the wall pressure changes the dowel and mandrel diameter. I tend to stay in the middle and away from installation stress.

Passing around that spreadsheet really violates the StrictlyIC copyright. Posting it on this forum does too. Frances Washburn, Robert's widow still sells back issues of the magazine to make a living. Making a spreadsheet for oneself from ones owned copies of the magazine is OK, it just should not be passed around.

 

[Rustkolector]

My apology for any copyright infringement of the spreadsheet screen capture posted here. I was not aware of the origin of the spreadsheet when I downloaded it. That is definitely is an SIC issue I need. Thank you for the mention.
Jeff

 

[petertha]

Terry wrote up a very clear example with a corresponding diagram
https://www.homemodelenginemachinist.com/threads/another-knucklehead-build.27584/page-14#post-324137

 

[editor123]

My apology for any copyright infringement of the spreadsheet screen capture posted here. I was not aware of the origin of the spreadsheet when I downloaded it. That is definitely is an SIC issue I need. Thank you for the mention.
Jeff

Not a problem. Frances has listed the three issues that contain the Trimble articles on the SIC Website. www.strictlyic.com will get you there. The only thing Trimble got wrong is the temperature for heating the rings. His recommendation is too high and at that temperature, the characteristics of the cast iron are altered. 1100 degrees F for one hour is what our club members use for the supercharged V-8's they build.
BTW, Trimble was the guy loaned to NASA who got Apollo to the moon. He was working on anti-gravity when he died.