Another Atkinson Differential build

[dsage]

The o-rings are green. One to seal the adapter to the hose end and one to seal the adapter into the model cylinder head.
The black o-ring does nothing in this application . It's for the original seal to a standard plug hole. Bronze because it's what I had around and the original end was brass.

 

[Ken Brunskill]

Elegant! Thanks Dave.

 

[Ken Brunskill]

Well, I fabricated all the parts as Dave P. had designed them, assembled everything less the rings and all operated smoothly, then observed a fair amount of stiffness with the spanking new rings installed, so the thought expressed by strangemitsku might turn out to be true. (BTW, Fishing was OK lastnight, catching not so much). Will be doing some E motor run in starting Monday, with the suggestions and comments here, think I'll try to power it on its own after each hr. of E motor breaking in. Will keep posting as progressing.

OK, ran into an issue doing the E-Motor run-in, as mentioned here the stiffness has caused the pivot pins (which were a light press fit in the 'ARMS'), and being catilevered enough to cause binding, (I think caused by the stiffness) which after 10 min. of relatively low speed worked one of the the pins out of it's 'ARM'. Analyzing options & fix to this. My suggestion to anyone thinking about building to this design, make the components out of steel vs. aluminum.
I know my holes were dead straight/square and to size, feel the angular pressure in soft material (6061 Aluminum) is a major factor. The mechanism operated smoothly until I installed the rings, which were built to Dwight Giles methodology.

 

[dsage]

By chance did you fit a ring into the cylinder (without the piston) to ensure you have enough ring end-gap?
And also that there is enough space behind the rings in the piston groove?

Of course if either of those is not sufficient you probably wouldn't have got the piston into the cylinder.
What are the ring dimensions? Did you use two thin rings per piston ring groove as you once said you were going to do. Or just wide one?

 

[stragenmitsuko]

The pins are a press fit in the arms and they pivot in the frame ?
I thought it would be the other way around .

How about pressing / bolting / loctiteing the pins in the frame .
I would thread them , add a small shoulder and loctite them in place .
And th use an extended bronze bushing in the arms wich could also be bolted or locktited to the arm .

 

[Ken Brunskill]

By chance did you fit a ring into the cylinder (without the piston) to ensure you have enough ring end-gap?
And also that there is enough space behind the rings in the piston groove?

Of course if either of those is not sufficient you probably wouldn't have got the piston into the cylinder.
What are the ring dimensions? Did you use two thin rings per piston ring groove as you once said you were going to do. Or just wide one?

Dave, made a dummy cylinder exactly the same I.D. as the cylinder (.750"), made sure I had .001"-.002" end gap, rings on the Power Piston are .050" wide (1/groove), Pump Piston rings had same end gap, but since there are no side ports, just a cylindrical surface I'd gone with (2 rings/groove) as my 1st try. Did a relatively fair measurmemt of the load required to get the rings to close, near as I could measure they were consistent, and about as expected, the .025" wide rings came in about 1/2 of their .050" wide cousins. The angular load on the ARM pins in aluminum, & related losening, has set me back a bit, before I remake the ARM'S I am going to try cross through pins - as perpindicular to the implied load as I can make them. Might just be a simple enough solution.

 

[Ken Brunskill]

The pins are a press fit in the arms and they pivot in the frame ?
I thought it would be the other way around .

How about pressing / bolting / loctiteing the pins in the frame .
I would thread them , add a small shoulder and loctite them in place .
And th use an extended bronze bushing in the arms wich could also be bolted or locktited to the arm .

Like your suggestion 'strangemitsuko', the ARM's piviot on bearings, that are mated to posts in the FRAME, they are secure there being a light press fit to a shoulder and tightened to that shoulder from the far side of the frame, a good design there! The culprit pin is a 3/16" diameter pin press fit into the 1/4" thick ARM. So I think I will invest in a 7/32 -32 Tap & Die (Sure that will come in handy in the future - or that will be my explanation to my lady ) then do as you suggest including the loctite, since this particular pin requires a 3/8" Dia. spacer I'll use that as the shoulder.
I'll make drawings of the new pins and have them available for anyone interested in building this model.

 

[stragenmitsuko]

(Sure that will come in handy in the future - or that will be my explanation to my lady )

Now that sounds familiar

 

[Ken Brunskill]

Now that sounds familiar

Ended up making the 7/32-32 Tap, will post a picture and details if anyone is interested.

 

[jquevedo]

Ken:
I made a small change in the design of same engine, made the arms a one piece element, started from a 0.500 thick Al plate and machined all critical dimensions ( holes) then machiend profile and top surface to allow for the two ARM and Small ARM pieces to be made as one piece, then made the groove/space for the piston arm to be inserted, the pin that will hold the arm to piston Arm is a 0.125 in with external retaining rings.
Engine has very good compression, no binding and seems like it should run..
Working on ignition and water and fuel tanks now.

 

[jquevedo]

sorry forgot the side view

 

[stragenmitsuko]

Ended up making the 7/32-32 Tap, will post a picture and details if anyone is interested.

Please do , I'm always interested how others approach this .

@jquevedo
what are the two brass pipes for on the side view ?
Coolant maybe

 

[jquevedo]

Coolant entry and exit, they connect to a water tank and fill a void pocket behind the main cylinder body, thermal circulation is expected to happen, top one is hot / exit and is at a 30 degree angle

 

[Ken Brunskill]

Ken:
I made a small change in the design of same engine, made the arms a one piece element, started from a 0.500 thick Al plate and machined all critical dimensions ( holes) then machiend profile and top surface to allow for the two ARM and Small ARM pieces to be made as one piece, then made the groove/space for the piston arm to be inserted, the pin that will hold the arm to piston Arm is a 0.125 in with external retaining rings.
Engine has very good compression, no binding and seems like it should run..
Working on ignition and water and fuel tanks now.

Very nice looking work, you are to be congratulated!
I have stripped the paint from mine, and am tumbling them in just plain gravel, to provide a matt finish. Then back to understanding the compression issue.

 

[Ken Brunskill]

Please do , I'm always interested how others approach this .

@jquevedo
what are the two brass pipes for on the side view ?
Coolant maybe

Here is a photo of the tap, I used (3) .024 wires for making the tap, carefully making sure to maintain ZERO on the cross feed and note the reading on the compound, then made the screw thread of the mating part imediately afterward, ended up with a Class 1 fit. The chip relief was milled with a . 125" Ball End Mill deep enough to achieve a flat face or .030" for this 32 TPI thread. Then the tricky part was back to the lathe to pick up the thread (done with dykem, 10X Optivisor) and just barely remove the dykem & burrs from milling the thread relief.

 

[Ken Brunskill]

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

Dave, Looked for my reply without success, do think I replied via email. Belatedly, realizing others may be equally interested. So, in the Perrault design the Power Piston (on the LH side, from the front) has the chamfers.

 

[Ken Brunskill]

An update, way back I mentioned that I was going with (2) rings/groove, which I did only for the PUMP piston, well that did not work out so well, I ended up with an unmeasurable compression, as in it did not even budge the needle on the gage. Next I modified that piston to accomodate some .070" cross section O-Rings, that while knowing it would not be suitable long term (the O-ring crosses the sparkplug hole, and would not hold up.) This resulted in acheiving a 20 psi jump in compression, so a remake of the PUMP piston, and full width rings are being made.

 

[jquevedo]

Ken:
I've been experiencing compression issues as well, first tried the 1 ring per grove at 0.035 thickness, at start compression was over 20-25 psi before breakin-in, then it went down to less than 5 psi.

Cleaned and re honed cylinder, made new pistons with very close fitting and rings, this time 1 per grove at recommended 0.062 in thickness. Compression was above 20 psi but quickly went down to around 10 psi, seems like the challenge here is balancing friction vs compression.

Have made rings using Trimble method ( heated to 1100 F), forming and machining (turning on a mandrel to final size after parting ) as described by Tom Schwartz in Model engine builder, as well as a combination of both methods.
So far the best rings I think I have been able to make are using BOTH methods. Trimble heat treatment to 1100 F with final sizing using methods and machining as described in Schwartz approach. While compression is low arlound 10-12 psi, engine kind of start, sputters but refuse to run for considerable time, close inspection of rings shows they have not seated all the way around, some more time on assisted cranking may be needed hoping the rings will eventually seat all the way around.

Start attempts are better when engine is cold (i.e. overnight) and when the spark is closer to TDC..

Best regards.
Jaime

 

[Steamchick]

Just a point on combustion... Of course many old timers will remember flooding carburettors.... and using a manual choke - all necessary on a cold engine - or one with lower compression at start when the slow cranking speed allows much more blow-by (time) past the ring grooves and end gaps. (Modern engines with computer controlled fuel injection simply don't have "human applied" choke or other fuel enrichment! Younger folk (e.g. my daughter) simply don't know what the choke is for! - last seen on her Ford Fiesta in the 1980s.
Often a drop of oil helps cold starting as is closes some of the gap around the top, bottom and inside of the rings, as well as surface roughness of the bore, (Although microscopic gas dynamics define the machining grooves in the bore as microscopic labyrinth seals, so "aerodynamically" they develop a back-pressure to resist gas flow.). So running-in the rings in a bore will help develop compression, but ideally this should be done by idling the engine or running on a light load, so that the bore is heated (and heating distortion partly compensated) and therefore the rings will bed-in "more truly circular" and the bore likewise. But stop periodically and check for high spots - as these are where any engine will seize. Hand cranking the engine (especially with a torque wrench - gauge type - will enable monitoring of running-in. When the torque isn't reducing (from an initial hand wind) then the running-in is OK from an initial point of view and examination of high and low spot witness marking is the best monitor, until you get a good fit.
The reason you need more fuel at low compression at start is simply that the cold engine causes some fuel "spray" to condense, and therefore the mixture is effectively leaner than when hot and running - when the compression at firing will be naturally higher du to adiabatic heating of the intake charge - which doesn't happen at low speed cranking. That is more or less isothermal compression. But Models exacerbate the starting problem, as a 1 thou ring gap of a 500cc cylinder lets through a smaller fraction of the intake charge (proportionately) than the same 1 thou ring gap of a 5cc engine.
On ring size and fitting, I assume you are closely following drawings for fit of the ring width and inside diameter sizes and tolerances, as also the ring groove width and bottom diameter tolerances. These are critical to minimise the gas leakage past the rings and WILL NOT change with running-in. Often this is where rings don't work well, even when properly bedded-in to the bore. Blow-by gas goes around the back of the ring to get past the ring-groove quite effectively. On Manufactured engines, the piston groove width and depth tolerances are very closely controlled, and rings are ground to 1/10th thou in. tolerances for thickness, so they JUST don't seize in the piston ring-grooves. Also, most of the pressure of the ring on the bore comes from gas blowing past the top surface of the ring to the back of the ring on the compression and firing strokes, so the actual "static" ring pressure should be "just enough" to make the initial sealing contact of ring to bore.
Sorry if I'm teaching Grandma to play Rachmaninov again.... But I can also talk about being Brahms and Liszt! (She was a concert pianist anyway).
K

 

[Steamchick]

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).

Just comments for interest? => When engine testing at the NIS### factory where I worked, the oil consumption had periodic spikes where the 3 rings on any piston aligned during long term testing. And on 4 cylinder engines this happens 4 times more often than a single cylinder. It was monitored on 200 hour durability tests, as on "your car" you don't want to be topping-up the oil every week!
But the rotating ring syndrom is necessary for the "perfect circle" of a ring to fit the same circle of the bore. As the ring pressure is constant for most of the ring, but changes close to the cut ends, the radial variation of pressure (hence wear) needs the rotation to maintain longevity of the ring to bore seal by the rotation causing uniform wear around the bore.
2-strokes with pegged rings have much shorter lives....
K