Another Atkinson Differential build

[Ken Brunskill]

Ken:
I enjoy the general discussion. It's all good. But sometimes I wonder if it's getting off topic so I try to be sure we are all talking about the same thing. I get confused.
I am waiting for what to try on my engine next. You are my guinea pig
The pressure is on you Ken
All in good fun.

In the end it is your thread so I should just step aside and let it take it's course as you see fit.
Thanks all.

Ha Ha, Hopefully we can keep the subject related to the Atkinson Differential.

Yet, to that end I feel the discussion regarding 'RINGS' to be pertinent as the compression (or if you prefer 'expansion ratio'), may be much more critical on this specific design vs. that of even the single piston Atkinson Cycle engine.

Keep the pressure on, I need it!! All too often I get sidetracked.

 

[Ken Brunskill]

Although I worked as an Engineer for 45 years, it wasn't studying combustion cycles. Does anyone on the thread understand the "Atkinson cycle" - as shown in the "Pressure/Stroke" diagram? As I do not have a drawing of valve gear - just Wiki sketches of crank to piston linkages - and I am learning from your best info in this thread. - Dsage: Yes, you are correct - the diagram shows the Atkinson Cycle, but as it shows an engine "cycle" it could be related to any configuration of engine that creates the particular "cycle". In Innocence, I read it that the "Atkinson differential engine" is one configuration (probably an early configuration?), followed by the "Atkinson Cycle engine", and latterly the "Atkinson Utilite engine"...? - All of which emulate the "Atkinson cycle" for combustion.
To be able to determine the thermodynamic cycle for an engine needs some knowledge of the valve and ignition timing, as well as the configuration of piston motion - versus shaft rotation, or other datum. As I wasn't planning to "buy the book", I wonder if any of you kind folk have that information in a simple diagram or table? - My interest is purely intellectual, as I am not planning to make a version of any of these engines. I only have experience of "mid-to-late 20th century conventional" engines, and their components, so am curious about some earlier designs.
Ta,
K

K,
Await word from the originator of the Mini A Atkinson Differential. I am confident he will respond in the affirmative. Everyone should note that, going forward, any information shared in this regard will apply to the Mini A specifically, conceptually it may or may not be applied to other designs of the Atkinson Differential concept.

However the best I can do at the moment is refer you to the video I posted on this thread (Refer to page 2, June 13) it is my amateurish narration of the operation, I think it covers the operation. Bear in mind that the major difference between this engine (Mini A) and the Gingery-Pendergast engine is the valve operation. The Mini A, deviates from the original patent design in that it's designer D. Perreault, brilliantly designed a 'Piston Valve' for the intake and exhaust eliminating the 'atmospheric valves' of the 'Gingery-Pendergast engines'. The timing of this valve is very sensitive to it's operation, I have completed my engine, and have yet to get a 'POP'.

Jquevedo & I are both building a Mini A, he has been far more diligent that I have and reports getting some 'POPS', so he is closer to success. Wish us luck!

Cheers,
Ken

 

[Ken Brunskill]

Luckily I have convinced D Perreault (Originator of the Mini A differential design) to review this thread, he has agreed to share a schematic of the mechanics involved. With this our friend Steamchick might be able to generate a pressure diagram of the 'Mini A' design of the Atkinson Differential, this does not mean by any stretch that it will make it easier to get this engine to operate, as I found out today getting the valve timing is very sensitive.

I have created the schematic and passed it onto DP for review, hopefully together we will be able to share it soon.

 

[Steamchick]

Thanks, I'll try my best, when the data is available. But maybe you simply have a assembly drawing of the major linkages with the relevant overall dimensions? And similar for the valves/timing?
Then I'll have a go at working it out (I use pencil and paper, with a smidgen of brain, not CAD).
K

 

[Ken Brunskill]

Thanks, I'll try my best, when the data is available. But maybe you simply have a assembly drawing of the major linkages with the relevant overall dimensions? And similar for the valves/timing?
Then I'll have a go at working it out (I use pencil and paper, with a smidgen of brain, not CAD).
K

I have communicated with the designer of this engine, unfortunately, we could not come up with a way to share the information with out divulging the crucial construction dimensions. I know this would have been interesting to some, I hope that everyone recognizes & respects the proprietary rights involved here.

While these Atkinson Differential engines are arguably the most interesting to watch run with all the motion, the plans for this engine are very reasonably priced and this is a fun build, one in which if done well, is worthy of a prominent place on your display shelf. I'll post a photo of mine when I have it mounted its base.

 

[Steamchick]

I'm currently using a spreadsheet to develop calculations for the pressure of gas and air in a pipe burner, as the gas flows past the orifi of the burner tube.... "WHY?" - because I don't know how to do it but want to make a better model burner than all the "luck, chance, and trial and tribulation" offerings I have ever seen. It seems like a bit of Engineering that I should be able to do (in retirement, as there are no dead-lines, just "death" at some hopefully distant date). As I have never studied Bernoulli's equations I am getting a bit confusticated... so I need a simply geometric problem to distract me, and clear my brain. So the Atkinson volume/pressure diagram sounds simple - after struggling with Bernoulli!
Am I stupid?
Tell me to "stop" - please, someone?
Enjoy modelling!
Ken

 

[Steamchick]

Hi Ken B and Gordon.
I have just watched Ken B's early video on how the engine works - much as I deduced from Wiki and a bit of scalp scratching, but Ken explains the valve action and timing - which I didn't know. That's really good. Now the video Gordon found has shown a working engine! - Great! - Thanks guys!
With some dimensions, I'll have a go at the pressure diagram.
Cheers.
K

 

[Steamchick]

Ken's Cad creation has all I need to approximate the pressure per rotation diagram.
Now I'll have a go. and let you know after a winter of hibernation! (Deep thought?).
K

 

[Steamchick]

I'm currently using a spreadsheet to develop calculations for the pressure of gas and air in a pipe burner, as the gas flows past the orifi of the burner tube.... "WHY?" - because I don't know how to do it but want to make a better model burner than all the "luck, chance, and trial and tribulation" offerings I have ever seen. It seems like a bit of Engineering that I should be able to do (in retirement, as there are no dead-lines, just "death" at some hopefully distant date). As I have never studied Bernoulli's equations I am getting a bit confusticated... so I need a simply geometric problem to distract me, and clear my brain. So the Atkinson volume/pressure diagram sounds simple - after struggling with Bernoulli!
Am I stupid?
Tell me to "stop" - please, someone?
Enjoy modelling!
Ken

At the suggestion of Both Dave Sage & Dave Perreault I created a utube video: (My 1st - sorry for the amateurish narration)

Filename
4th Shot Atkinson operation.MOV

Ken, this video is very clear. Commentary excellent. Don't hide your light under a bushel...
I just wish I could do the CAD....
K

 

[Steamchick]

No diagram, but I can verbally describe it:

The Atkinson cycle is designed around the notion that what makes an engine efficient is the expansion ratio, not the compression ratio per se. As it was conceived at a time when compression ratios were, necessarily, low to avoid knocking, it solves the problem of low compression ratio & high expansion ratio by having a differential action -- the intake & compression strokes are short, followed by long combustion & exhaust strokes, then repeat.

It's basically the Otto cycle, but with different strokes (for different folks?).

My thermodynamics isn't strong enough, but because the expansion stroke is larger than the intake stroke, it ought to be releasing exhaust that's cooler than could be achieved in a fixed-stroke Otto cycle engine. I suppose that in theory, with a whole lot of expansion, the exhaust valve could open at zero pressure differential, or you could even have the thing pull a vacuum at the end of the expansion stroke -- presumably you could have a vacuum and a cylinder filled with rarified air that's cooler than atmospheric pressure, although then you'd be losing efficiency to pumping losses, not gaining it.

I think, though, that the real reason for the Atkinson Differential Engine was to dodge Otto's patents.

Yes Tim, I think you are right - from what I have read - Atkinson did try and overcome the restrictions of the Otto patent by deriving a "more efficient cycle". Which I think it probably is.... but life isn't always like that and the "more power per bang" usually wins initially, before the accountants decide we need "maximum output per dollar of fuel cost"... Hence a hundred years or more before people (Toyota?) seriously looked at the efficiencies of Atkinson's cycle(s). - Excepting the British Gas Engine versions of course, using his later engine arrangements.
Just an aside: I had a college mate who ended up working on using hydraulic actuated poppet valves on regular car engines... so the intake could be restricted when appropriate, and the exhaust timing radically altered between idle, low throttle and high throttle positions - all to improve the efficiency of the engine (Following the 1970's oil crisis!). - But hydraulic operation like that needed microprocessors/engine management computers of the type that were not invented until a decade or 2 later... Yet now we have the search for even higher efficiencies, even to having a steam generator in the exhaust - recovering heat energy - and powering an alternator by a reciprocating or turbine steam engine... (Just downstream of the catalyst there is a relatively high temperature gas stream suitable for a heat recovery boiler to generate adequate steam pressure, quantity and superheat).
K

 

[Ken Brunskill]

I'm currently using a spreadsheet to develop calculations for the pressure of gas and air in a pipe burner, as the gas flows past the orifi of the burner tube.... "WHY?" - because I don't know how to do it but want to make a better model burner than all the "luck, chance, and trial and tribulation" offerings I have ever seen. It seems like a bit of Engineering that I should be able to do (in retirement, as there are no dead-lines, just "death" at some hopefully distant date). As I have never studied Bernoulli's equations I am getting a bit confusticated... so I need a simply geometric problem to distract me, and clear my brain. So the Atkinson volume/pressure diagram sounds simple - after struggling with Bernoulli!
Am I stupid?
Tell me to "stop" - please, someone?
Enjoy modelling!
Ken

Ken, this video is very clear. Commentary excellent. Don't hide your light under a bushel...
I just wish I could do the CAD....
K

Thank you for the compliment.
Ken B

 

[Ken Brunskill]

Ken's Cad creation has all I need to approximate the pressure per rotation diagram.
Now I'll have a go. and let you know after a winter of hibernation! (Deep thought?).
K

I suspect that the distance ratios, would provide reasonable results. Maybe drawing it up 10X to model it?
Keep up the good thoughts, since your efforts will specifically applicable to the Atkinson Differential, I feel that anything you come up with will be a contribution to the overall body of knowledge.
For us handle crankers that have an engine 'almost operating', I hope it provides some insight, leading to helping ease the frustrations faced with getting one of these model engines to operate.
Ken B

 

[Gordon]

I suspect that the distance ratios, would provide reasonable results. Maybe drawing it up 10X to model it?
Keep up the good thoughts, since your efforts will specifically applicable to the Atkinson Differential, I feel that anything you come up with will be a contribution to the overall body of knowledge.
For us handle crankers that have an engine 'almost operating', I hope it provides some insight, leading to helping ease the frustrations faced with getting one of these model engines to operate.
Ken B

Actually I would like to see an analysis of the larger Gingery differential. The mini uses a mechanically operated valve which makes it a different operation. The Gingery more closely follows the original Atkinson design which has atmospheric valves. I have built the larger engine although I did not strictly follow the Gingery design and I have been able to get it to kind of sort of run for very limited cycles. There are a few videos showing the larger engine running but there are a lot more engines setting on the shelf because the builder could not get a consistent run. I think that the secret is in the motion created by the pivot arm lever ratio and the pivot point.

Actually the Perrault video is the only running mini engine I have seen. There may be others which I have not seen.

Note: I do not want to hijack this thread or turn it into another extended discussion on the Gingery/Atkinson original. There are other threads on that already.

Gordon

 

[dsage]

Gordon et. al.
I posted this video on my thread of the Gingery engine. As Gordon says it is more like the original.
So SteamChick you can ponder this one as well. Not much difference really except for the lack of the sliding valve on Kens and it has check valves in the ports instead.
I also brought to attention that when the engine DOES NOT FIRE you can see that there will be a vacuum created in the cylinder when the right side piston moves back in what should be the power stroke. This vacuum creates a false intake vacuum and IMHO it floods the cylinder. Some of that flooding is evident in the fuel dripping out the exhaust port when the exhaust port is uncovered. I think this gets the whole mixture messed up on the next compression stroke. I believe Gordon has had similar results but apparently Gordons' engine runs to a certain extent. I can't get mine to pop even once.
Just more facts to chew on.

 

[Steamchick]

Hi all,
Before I start to study other variants, I need to commit some hours to the first one - Ken's CAD of a slide-valve arrangement.
My first idle musings on the chamber volumes and timing that I have measured from the video (Not precise, but adequate for now):
The intake is miniscule compared to the size of the engine. Really like trying to run 1/0th power for all the friction involved in the linkages - as well as pumping losses. But some people make flea-powered Stirling engines very successfully. There are 2 concerns with this "lack" of intake volume.
The power is proportional to the size of chemical (fuel and air) charge. As some will leak-away during the pumping action, there is less for "firing". Also less for "pre-heating" the charge to get it to firing conditions by the compression stroke. - This is also speed related and this is intended to be a slow engine.
Charge suction and cut-off need very good sealing. I note the "valve" piston does this after the slide valve has closed, but I suspect the slide-valve may also have a leakage that contributes to the effectiveness of the engine, as if the exhaust side leaks it will suck-back some exhaust to corrupt the mixture instead of drawing-in fresh charge?
The ignition point is not well defined, but I assume at TDC of the power piston? with maybe a little advance? - Actually, I need to study the motion a bit more, as the full compression "TDC" of the combustion chamber may not be precisely at TDC of an individual piston.
- Is there any way of producing a table from the CAD where the piston positions can be plotted per 3 degrees of crank rotation?
In another post I recall that you determined a 3 or 4 bar compression? As I cannot deduce the cylinder volume at MAX COMPRESSION and INTAKE VALVE CLOSURE I can only deduce there is considerable volume of piston machined-away for the valve porting? And FLAT topped pistons would instantly improve the compression rather than domed pistons.
Dsage: Just viewed your video, and maybe you can use the CAD to make a table of piston position ve. crank angle - I think 3 degree steps should be adequate - with crank-angles when the valves open and close? - Much better than my "quick and crude" extracts from measuring screen prints!
More musings later.
Now to the spreadsheet calcs... Catch you later!
K

 

[Steamchick]

A first attempt... just a picture of my representation of Chamber Volume versus Pressure.
The peak pressure is just after ignition.... I have assumed "No losses", but "guestimated" a chamber volume at max compression based on 4 bar attainable pressure, and to simplify things uses a 2cm diameter bore. The peak pressure after firing is just a number to make a picture.
2 things: As Atkinson intended, the Pressure-combustion cycle (Top loop) is very large compared to the intake cycle, (bottom loop). Thus the limited charge will do a lot of work efficiently in expanding to ~25% of it's original "fired" pressure before the exhaust is released. Of course, as Otto and Carnot both knew, the compression ratio is "almost everything" and modern petrol engines running at 10:1 compression ratio are twice as efficient as the numerical model I have used.
I haven't plotted curved lines - just for speed to get something of interest here.

Hope this is interesting to some?
More when I have done some tarting-up.
Ken

 

[dsage]

If Ken desires, he should be able to give you your measurements at 3deg intervals. He has the cad drawings. It would be a lot of work rotating and measuring though.
He should at least give you the piston diameter and also the distance between the pistons at compression and at the point the intake valve closes so you know how much volume of intake is trapped in the cylinder. That will give you the compression ratio (more or less).
None of that is proprietary information.
Since you have chosen to analyze Kens engine I wont confuse the issue with the Gingery (my) numbers. The engines are pretty similar except for the slide valve.
I presume your conclusion will be that either engine is pretty sketchy in terms of it's ability to run. Because that's the result in practice.
Looks good so far.
Soldier on man

 

[Gordon]

I pretty much came to the conclusion that the main problem was that due to the marginal operation the problem was that engine did not fire consistently in the final ignition position. This means that now the raw fuel is still in the chamber when it returns, without power, to the exhaust/intake position. At that point there is not enough pressure to completely exhaust the chamber and then it immediately adds more fuel to the already too rich mixture. After a couple of cycles you have raw fuel dripping out of the exhaust. If you are fortunate enough to fire the engine keeps running until it misses a couple of cycles. Unfortunately due to the very marginal operation this happens on a regular basis. The engine is far too dependent on a perfect set of circumstance like correct carburetor setting, adequate compression, proper timing and apparently the correct phase of the moon and alignment of the planets.

Gordon

 

[Steamchick]

Hi Guys!
I hope my musings - as attached - will give you something to ponder? (Hope you can open the file!).
I may easily be wrong with my observations and ideas, but it may give you some new insight to get your engines running?
I realise that changing valve timing and/or porting is no small task.... but it is just my humble idea.
I'm not offended it it is all trash.
Enjoy?
K

 

[Ken Brunskill]

Gordon et. al.
I posted this video on my thread of the Gingery engine. As Gordon says it is more like the original.
So SteamChick you can ponder this one as well. Not much difference really except for the lack of the sliding valve on Kens and it has check valves in the ports instead.
I also brought to attention that when the engine DOES NOT FIRE you can see that there will be a vacuum created in the cylinder when the right side piston moves back in what should be the power stroke. This vacuum creates a false intake vacuum and IMHO it floods the cylinder. Some of that flooding is evident in the fuel dripping out the exhaust port when the exhaust port is uncovered. I think this gets the whole mixture messed up on the next compression stroke. I believe Gordon has had similar results but apparently Gordons' engine runs to a certain extent. I can't get mine to pop even once.
Just more facts to chew on.

Nicely done video Dave, very good explanation.
Cheers, Ken