Another Atkinson Differential build

[jquevedo]

Ken:

I've had good luck stopping ring rotation by controlling the amount of clearance between piston groove and ring.
I've done my set of rings using trimble method, then machining them in a very accurate and well centered spigot with the right dimensions based on the ring and bore and ran my engine for several hours to the point of having some pops but no sustained run, so I opened the pistons to replace rings and they were in the same location where they were installed..both pistons had decent be in of rings but compression was low (1round 10-12 PSI) I think we need to achieve over 20 PSI's to get a decent pop..
Funny thing is most times I start with 20 plus PSI and then trying to bed in the rings, compression goes down to almost 5 PSI and then slowly start to come back up when the bed in the cylinder.
At this point in time, I think the need for pinned rings is not necessary. Good rings and controlling the ring groov gap to ring to be under 0.005 in would be sufficient to stop the rotation, my rings are 0.062 thick once polished on the sides and the groove in the piston is 0.063 in.

Kind regards

 

[Steamchick]

Hi Ken,. I last saw overlap piston rings back in the early 1960s. (I think IJ fvounde tghe typingh probledm - my kedboard ijs ghooszeded! - Takes a lot of correction.).
The ring-gap blow-by area is 3thou (ish) by ring radial thickness. The leak area passing the top and back of the ring is maybe 2thou x circumference.... so the gap leakage is small compared to ring and groove clearance. Or so I was taught ... Hence modern engines use simple gaps, not overlap. Yamaha use the simple pin to stop rotation. (They are quite expert at this).
But I am not the expert - I just worked with many...
K

 

[Steamchick]

Try a bit of oil in the fuel? 50:1? That helps the bore to ring seal - when bores are still relatively rough.

 

[Steamchick]

http://modelenginenews.org/techniqu...thod uses a blank,turned and lapped to finish.
All sounds OK for models, although it differs a bit from my "full-sized" industrial experience - except for the Cold Forming bit.
]But at H & GH I witnessed the barrel honing of rings - essentially pre-running-in.
K

 

[dsage]

Sorry I get thickness and width confused as it depends on how your looking at the ring. And I forget the convention.
I consider thickness the vertical height of the ring looking as installed from the side of the piston. Sorry if it's opposite to convention.
With that in mind:
On one hand it would seem a good idea to make the ring thickness and piston groove have almost no side clearance since an aluminum piston is going to "grow" faster than the ring is going to get thicker so the groove will bet wider an the clearance will increase as the engine heats up. (I think).
On the other hand there is a need for some clearance (they say) go gas pressure can get around behind the ring and make it expand and seal against the cylinder bore. Not sure which is correct for a model engine.
I'd be tempted to forget the overlapped ring ends and try almost no side clearance because - as mentioned by others - there is a lot more leakage by area around the ring than there will ever be through a small end gap.
I'd also keep the end gap to an absolute minimum because an aluminum cylinder is going to expand and the ring end gap will increase. Then again the ring might get hotter faster than the cylinder. Again - Not sure.
I'm not sure why this Atkinson engine has so much trouble. I've certainly had no trouble with any of my other engines. Sometimes I think we're overlooking something. Certainly the intake and exhaust ports are problematic and having two pistons to seal at the same time is double trouble.
I've never made a two stroke engine and this engine is more of a two stroke than anything. Maybe a change in thinking is required.
I've stopped work on mine when I discovered the issue with vacuum in the cylinder when it failed to fire. Hence my elaborate solenoid valves (which didn't help).

I've been holding off further work on my engine until I see the results here. I hope you don't mind being the guinea pig Ken

I've been following a guy on Youtube. A channel called "2stroke stuffing". The guy is a genious. He's building his own two stroke 50cc engine 30 horsepower (he figures) from scratch to put in a salt flat racer. The engine starts with conventional reed valves etc. but turns into strictly a tuned resonant system when running at speed. He just got it to run and idle. You have to watch the early videos to watch the progress. Quite motivational.

 

[Steamchick]

dsage: There are many who know a lot more than I do, but my training told me the Axial clearance really needs to be at a minimum. The gas pressure will get behind the ring - so no-one need ever worry about that. The gas pressure forces the ring "down" on the compression stroke, thus giving max area for gas to get past the ring to the inner annular gap. So 0.001" axial clearance is adequate. Assuming the piston crown gets to 400deg.C, (it won't) work out the expansion of the inner diameter of the ring groove and make sure there is a thou or 2 clearance for the ID of a fully compressed ring,. so it doesn't seize. That should be OK for ring gap.
OK?
K

 

[Ken Brunskill]

Dave, Jamie & K; Truly have enjoyed this discourse and the friendly manner of it. I sincerely hope none of us ever feel as if it taking up too much of our time!
Cheers, and stay safe.

 

[Steamchick]

Probably more hours of debate than making bits. But that's why we contribute to the thread. We enjoy the debate and sharing of ideas.
Thanks.
K

 

[jquevedo]

Positive feedback instead of someone trying to make us think certain way goes a long way. Thanks to the inputs provided ideas are formed, some are practical and some are not but all of them deserve consideration..
Debate is needed to challenge old ideas or approaches and to create new ones.

 

[Ken Brunskill]

So far no luck getting a Pop, not discouraged yet, I've just finished assembly, connected the ignition parts, and mixed the fuel. Next step is to build a cranking device so I can continuously crank the engine at a modest RPM while slowly fiddling with Carburetor settings. Of course stopping every few tries to change the spark timing and start the process over again. Any suggestions are appreciated.

After reading and following the other Atkinson Differential threads, another thing has crossed my mind; may be it would be worthwhile to measure the vacuum created at the fuel intake. Any thoughts? Any suggestions of sources for a vacuum gage? Suspect that it will be low vacuum as the compression is in the 10-20 PSI range.

 

[Steamchick]

Just a clue on intake vacuum....
I have a pair of vacuum gauges "for cars" - actually good for anything. They work fine for constant vacuum carburettors, but on my slide carb Moto Guzzi V50 they bounce from 0 ~ 4 or 5 "inches of mercury below atmospheric" and if the throttle slides are lifted they just flutter off the stops. (N.B. Full-ish Vacuum is about 30 inches of mercury below atmospheric, for those unfamiliar with "Grand-dad's measurements").
I guess on a model there will be almost nothing to show on a gauge.
BUT you can try a water column? - Easily made with a clear plastic tube fixed to a plank as a U-shape and HALF filled with water. When vacuum (partial) is applied to one end, with the other open to atmospheric pressure, you can measure "inches water gauge" by the difference of the water levels.
For "hardly any suck" these are great! - see what your lungs can do, and have fun with the kids (OK if they drink the water). The gas man has one for checking the gas pressure supplied to your boiler or gas fire, or maybe the modern ones have digital gauges by now?
K

 

[jquevedo]

Ken:
My experience so far:
Be patient, it will take a while to get the engine to show any signs of life..Low compression, 4 ringsw one cylinder you know the story.
I started with 20 PSI compression and it went down to almost 5-7, then slowly building up again, to almost 25 PSI..
I have been cranking mine for days, short periods, maybe 10-15 minutes at a time.
I have placed the spark setting very close to TDC to avoid kick back in the engine while running in, once the engine showed signs of life, I adjusted spark time to 5 degrees before TDC.
No need to fiddle with gas/air settings for a while..be patient, I used 1/2 to fully open air and 3/4 turn on gas.
Once you get some different noise in the engine and depending on the oil/gas mix you may see some smoke off the exhaust, then it may be time to start trying the air/gas adjustments
Once you start hearing some pops you can also add some oil drops in the carburetor and see if that increase compression and allows you to better tune your engine, one thing you can monitor is cylinder temperature, when the fuel explosion is happening, you will have a steady increase in temperature in the engine..
This is the second cylinder I'm using, first one material was not the best choice, this one is Mehanite and it is taken way longer to get to the point of getting some pops from the engine..
Been two days of short periods of running in to get some pops and finally engine has shown some signs of life..
I have been running the engine at lower rpm's 400 - 600 during the "wear in" period but Dav Perrault thinks it was too slow, 650-750 seems to be a better spot once you are getting some engine pops to do the adjustments.
You may notice some blow by from the rings this will reduce over time and should mostly disappear just before you get engine pops, pump piston is easier to tell when this happens, Power piston is a different story, the intake port gets exposed inside the cylinder, when the piston moves all the way to the right and you get some fuel dripping in the skirt of the piston, which then get flung out the cylinder.. and looking like ring blow by.

Patience is the key, if you get frustrated /impatient, stop go and take a walk and come back to it later, check plug batteries for ignition, fuel etc..
if you prefer give me a ring and we can chat in more detail..

Jaime

 

[dsage]

If you read my thread to the end you will see I was investigating the vacuum issue. I found when the engine does NOT fire there is a vacuum in the cylinder when the pistons move back apart (due to some loss in the original compression). This vacuum is exposed to the carburetor when the piston move farther down the cylinder and sucks fuel in before the actual intake stroke comes and also just before the exhaust valve opens. I found that fuel is sucked in and immediately spat out the exhaust port. I suspect that problem also plays havoc with the real intake stroke that happens later. Gordon also confirmed that his engine seemed to have a lot of fuel coming out the exhaust. The last few posts in my thread show an elaborate valving arrangement with solenoid I tried to relieve that false vacuum from reaching the carb. Of course none it works but it did prove the vacuum in the cylinder issue.
May I suggest putting a bit of fuel right in the cylinder through the spark plug hole. Replace the plug and give it a spin. If things are conducive to starting the engine should at least pop once (or more).

 

[jquevedo]

I will check on the vacuum issue you experienced in your engine design, just need to find how to hook a gas (LPG) electronic device to the intake port or carburetor to see if I have any back blow form the intake.

So far this does not seem to be an issue, engine has a sliding valve which may help to reduce the issue.
My problem with the initial cylinder, intake and exhaust port size was wrong and cast iron material wear inside faster than the rings. My first cylinder got the engine running for short periods I assume the small size in the intake ports was starving the engine, then too much cranking and new set of rings wore the cylinder.

 

[dsage]

I think A couple of people were typing at the same time. So messages might be a bit out of sync.
Following on SteamChicks idea of a vacuum gauge. I did at one point put a column of fuel on the carb fuel line. This is where I notice a suction on the carb before the actual intake stroke as described in detail above. This led to the development of the wacky solenoid valve arrangement to eliminate the issue. Which it did. The only vacuum on the carb was then only due to the intake stroke.

For those following along wondering what's going on - this engine is NOT your typical engine being that it is opposed pistons. So it's probably not exactly accurate to think about it like a typical 4-stroke. Nor is it helpful to compare experiences with a 4-stroke engine break in. Other than in very broad terms. This differential action engine is particularly troublesome.
It might be more useful to think of it like a 2-stroke as far as the intake stroke is concerned. But vacuum in the cylinder due to a misfire is probably not as disruptive on a 2-stroke due to the porting. I'm not a 2-stroke expert.

Jquevedo: Is your engine an Atkinson differential as well. Sorry I haven't been following along to well.

 

[dsage]

Duplicate message.

 

[peterl95124]

Ken, really appreciated seeing your Atkinson Differential at the BAEM meeting, awesome work.

Question, does the intake happen because the the compression piston has created a vacuum in the cylinder so that when the intake port is uncovered the intake is drawn into the cylinder ? if so then then maybe the ring should have more tension against the cylinder wall than in a conventional 4-cycle ? just idle thoughts, ignore if not relevant !

Peter.

 

[Ken Brunskill]

Just a clue on intake vacuum....
I have a pair of vacuum gauges "for cars" - actually good for anything. They work fine for constant vacuum carburettors, but on my slide carb Moto Guzzi V50 they bounce from 0 ~ 4 or 5 "inches of mercury below atmospheric" and if the throttle slides are lifted they just flutter off the stops. (N.B. Full-ish Vacuum is about 30 inches of mercury below atmospheric, for those unfamiliar with "Grand-dad's measurements").
I guess on a model there will be almost nothing to show on a gauge.
BUT you can try a water column? - Easily made with a clear plastic tube fixed to a plank as a U-shape and HALF filled with water. When vacuum (partial) is applied to one end, with the other open to atmospheric pressure, you can measure "inches water gauge" by the difference of the water levels.
For "hardly any suck" these are great! - see what your lungs can do, and have fun with the kids (OK if they drink the water). The gas man has one for checking the gas pressure supplied to your boiler or gas fire, or maybe the modern ones have digital gauges by now?
K

OK this is where I slap myself on the forehead, I certainly know about the U-Tube and water, but did I think about that? NO !

 

[Ken Brunskill]

Ken, really appreciated seeing your Atkinson Differential at the BAEM meeting, awesome work.

Question, does the intake happen because the the compression piston has created a vacuum in the cylinder so that when the intake port is uncovered the intake is drawn into the cylinder ? if so then then maybe the ring should have more tension against the cylinder wall than in a conventional 4-cycle ? just idle thoughts, ignore if not relevant !

Peter.

Peter,
The vacuum occurs while the 'Pump' piston is moving away from the 'Power' piston. They are moving in the same direction, the travel of the (2) pistons vary in speed, see the video I made of the 3D Solidworks Model posted on Page 2 of this thread June 13th. (Please excuse the amateurish narration.)

 

[Ken Brunskill]

Ken, really appreciated seeing your Atkinson Differential at the BAEM meeting, awesome work.

Question, does the intake happen because the the compression piston has created a vacuum in the cylinder so that when the intake port is uncovered the intake is drawn into the cylinder ? if so then then maybe the ring should have more tension against the cylinder wall than in a conventional 4-cycle ? just idle thoughts, ignore if not relevant !

Peter.

Peter, thank you for the complement on the engine, I'd not done that level of machining since the 1960's, it was fun! Sure can see why folks are enamored with the CNC machines. But us old handle crankers are still around, slower though!