Another Atkinson Differential build

[Ken Brunskill]

This thread via the suggestion of Dave Sage;

I bought a set of Atkinson Differential plans from Dave Perralt, and proceeded to draw them up in Solidworks, for two reasons, the primary reason was to learn the Solidworks program, the second was to understand the design, and do a check of the plans. I will say that his design was very good, I found only one error, which I pointed out and was thanked for catching that. I have built other models from plans and found his to be the best I've encountered.

Being a Tool Designer and Mechanical Manufacturing Engineer, I probably am a bit picky - hats off to Dave Perralt! Having read Dave Sage's thread, and seeing all the problem he and others have had with designs & their errors, as well as not getting their model to operate, I am hoping that this model will not be in the same category. Mr Perrault has an operating model built from this design, I hope I can have another.

So with that said here goes in somewhat chronological order: (Have not been taking a lot of photos, or following HMEM all that faithfully - actually don't care to spend much time surfing or posting - will try to do better if people are interested.)

The photo 'Christmas Present from Son.jpg Was some what of a joke for our nurse son, who has zero idea of what is involved in making anything, I said "watch this space'".
The photo 'Base-Frame-Flywheel-Cylinder.jpg ' was the 2nd photo to him, It shows the Base, Vertical Frame & Purchased Fly Wheel.
The photo 'Machining Left Arm.jpg ' is the set up for machining the Left Arm, yep no CAD-CAM equipment in this old mans shop.
The photo 'Mid April Progress.jpg' was take mid April to show my kids and friends what I am doing to stave off 'Cabin Fever'
The photo 'Partially complete Atkinson Differential.jpg" was for the record to show our son and family what that block of Cast Iron i.e., Christmas Present has become.

I am now awaiting some Aluminum Round and Brass Hex Bar stock, so I can fabricate the pistons, and Hex screws, one of my friends at the BAEM is going to provide the ignition components. In the meantime I'll build the gas and water tanks. Then hopefully this Covid-19 will allow me to visit my friend in his Sr Living's nice wood shop to make the box to house this whole thing.

St that's where it stands at the moment.

 

[Gordon]

As one of the folks who have spent far too much time trying to get one of these to run I hope that at some point we can figure our why some of these engines run and others do not even come close. Different designs seem to have only minor differences.

 

[Ken Brunskill]

As one of the folks who have spent far too much time trying to get one of these to run I hope that at some point we can figure our why some of these engines run and others do not even come close. Different designs seem to have only minor differences.

Maybe it will be worthwhile to analyze the volume, compression, and pressure holding capability of the various designs. (Just food for thought.)

 

[stragenmitsuko]

Great another atkinson thread .
I'm following along .
Hope all problems will be solved .

pat

 

[Ken Brunskill]

At his point I am ready to make the pistons and rings, made the 1st Pump Piston, Atkinson Differential's have two (2) a Pump & a Power piston. The plans I have are from Dave Perralt, his plans were created from his working engine, with two (2) so anyone else but me would just follow that.
But no; I have to question whether the compression might be improved from his two .062" T X .050" W rings (remember this is working), by using three (3) .031" T X .045" W rings. The three rings would in theory produce 25% less friction by my calculations. However the ring gap increases three fold (DUH, three gaps vs two), maybe laying it out like this serves to provide the answer, still worth putting it out to the collective wisdom and experience of others.
Then again maybe four rings, two. .031' rings in the same .062" ring groove, that seems to cancel out or reduce the affect of the gap issues.

Regardless of which way, the rings need to be made, I've heard from our club president that some have successfully made their rings with the Trimble method while others have had success with making them oversize cutting the gap, heat treating, then squeezing down on a mandrel and finish machining them to size. Need to understand that a bit better, I think I can visualize that process but want to see photos of the fixturing before start to make sure I fully understand that process.
So would appreciate any and all comments and photos of fixtures.

 

[Peter Twissell]

Hi Ken,

I don't understand why three rings would have more leakage through the ring gaps. The gaps are in series - any leakage would have to pass through the first gap, then find its way round to the second and if you have three rings, then find its way round the third gap.
Two rings in one groove can eliminate leakage through the ring gaps, provided you can stop the rings from rotating and aligning the gaps - possibly with pins like 2 strokes use.
In general, more rings should result in less leakage, as the pressure across each ring is reduced.

 

[Ken Brunskill]

Thanks Peter, your comment makes perfect sense to this old man, I guess I was not thinking enough.

 

[Ken Brunskill]

Hi Ken,

I don't understand why three rings would have more leakage through the ring gaps. The gaps are in series - any leakage would have to pass through the first gap, then find its way round to the second and if you have three rings, then find its way round the third gap.
Two rings in one groove can eliminate leakage through the ring gaps, provided you can stop the rings from rotating and aligning the gaps - possibly with pins like 2 strokes use.
In general, more rings should result in less leakage, as the pressure across each ring is reduced.

Further thought on this is; seems that if initially placed 180 deg. apart the two rings in one groove would take quite awhile to become aligned, then with two set's the chance of both sets becoming aligned would be even more unlikely, could happen though. Think since I am in the mood, I'll try both and measure the compression as well as the decompression time (Anyone ever done this? Any suggestions on how?)

 

[Peter Twissell]

Piston rings can rotate faster than you might think. Observation link shows up to 1rpm at 1000 engine rpm.
You might need to be quick with your compression measurements!
Sod's law (Murphy's law, law of universal cussedness) clearly states that the gaps will align (because you don't want them to).

 

[Ken Brunskill]

Piston rings can rotate faster than you might think. Observation link shows up to 1rpm at 1000 engine rpm.
You might need to be quick with your compression measurements!
Sod's law (Murphy's law, law of universal cussedness) clearly states that the gaps will align (because you don't want them to).

Very well acquainted with Murphy! Seems to be ever present in my shop, the smaller the object that hits the floor the further and more difficult to retrieve.

 

[Ken Brunskill]

Followed some of the Ring Creation threads, great resource! Thought the idea of turning the OD 1st, then cutting the Thickness +.002" & depth .010" deeper than the finished ID, was a good idea, worked like a champ, just as described.
Now to bring the ring blanks to the desired Thickness. Then cut the ring gap (Bought a .006" X 2" Slitting saw for this [only $13].)
As mentioned earlier, Going to try (1) ring/groove & (2) rings in (1) groove, thats why there are so many ring blanks of differing witdths, being turned.

 

[Ken Brunskill]

Thought I'd post pictures of the simple method used to deburr the rings (they were turned to final size for the OD, .030" under size for the ID, width cut with narrow parting tool .010" deeper than final ID, finally bored to final ID.
Photo is a Dremel stone and one of the rings being deburred. Took but a minute or two for each ring.

 

[Ken Brunskill]

Getting close to completion of the engine, finishing the pistons, installing the rings, expecting to have all that accomplished by weeks end. Have all the electronics except the Spark Plug (It's in route). Now for the fuel and coolant system, then a stand/box.

 

[ALEX1952]

it is quite common to put a pin in the gap of the ring to stop rotation, of course this has to be allowed for when "gapping" the ring, it also makes it more of a pig to fit the ring.

 

[Ken Brunskill]

Alex, thanks for that thought, these are pretty small pistons, so the pins would be super tiny.
Next task is to final polish the ring gap to acheive final OD, install the rings the final assembly - should be finished assembling the engine tomorrow.

 

[Tim Wescott]

Goodness gracious those are itty bitty pistons!

 

[stragenmitsuko]

Yes indeed , what dimensions are those pistons ?

Pat

 

[Ken Brunskill]

Pistons are .750" Diameter. The photo posted on 6/2/20 is 1st photo of the engine partially assembled. Right now awaiting delivery of hex Stainless to make the screws to hold the Intake/Exhaust Valve Manifold (The Brass part in front.)

 

[Tim Wescott]

Pistons are .750" Diameter.

My eyes deceived me -- 3/4" isn't too bad. Somehow I thought the were 3/8 or smaller.

 

[dsage]

Ken:
As you look at the front of the engine and the oscillating arms, which piston (left or right) has the chamfers on it?