Atkinson frustrations

[Peter Twissell]

Rather than manipulating a 3d model, it may be easier to create a spreadsheet to calculate the positions of each link at each degree of rotation of the crank. This is not as hard as it may seem, it's just a series of triangles.
The spreadsheet can then calculate the space between the pistons and the compression or expansion of that space.
Once the spreadsheet is written, it is relatively simple to alter the positions of fulcrums and lengths of links to observe the effect on compression and expansion.

 

[dsage]

Rather than manipulating a 3d model, it may be easier to create a spreadsheet to calculate the positions of each link at each degree of rotation of the crank. This is not as hard as it may seem, it's just a series of triangles.
The spreadsheet can then calculate the space between the pistons and the compression or expansion of that space.
Once the spreadsheet is written, it is relatively simple to alter the positions of fulcrums and lengths of links to observe the effect on compression and expansion.

Easy for you maybe. I have no idea how to attack the problem that way. A spreadsheet I could handle maybe but....past that I'm lost on a methematical approach. You'd still have to make changes to the bits and pieces and re-calculate. I'm not sure what I'd be looking at in a spreadsheet full of numbers. Seems to me watching the parts physically move after a change would be more productive. There's a lot of stuff going on.
I can easily animate the 3D model. You just rotate the flywheel and watch it move. Then make a change in the drawing for a piece or spacing to see the effects of changes and to observe the results. It's easy to stop and measure any aspect of the model at any time. The trick is keeping all the connections in tact. Sometimes the 3D model can fall apart.
I think I mentioned I did a bit of this and found that very minor changes can send the motion works into some very wacky gyrations that are obviously wrong. I wouldn't know what the numbers on a spreadsheet were telling me. I'm more of a "try it" kind of guy anyway. Mathematical analysis is not my wheelhouse.
Feel free to have a go though.

 

[bluejets]

I re-read your comments in #54 I have also drawn the engine as a 3D model. I haven't built it yet because I agree that from what I can see the design is very marginal. But the design is VERY critical as to positioning and dimensions of the components. A very SMALL change can make a HUGE difference in the motion. That's why I'm wondering if those that got it running (as per Youtube) unknowingly made minor changes OR they built it perfectly (i.e. didn't inadvertently make mistakes and throw it off).

I would say the former as George drew it using Prodesktop 3d at the time with dimensions from the drawing.
As you say, time waster if not going to work as is, so any drawing development with improvement would be essential I'd imagine.
In George's instance, the time element was primary reason not to go any further, too many other things to do.

Not that it would make any difference in this instance but, any low compression engine can have performance increase but using methanol as a primary fuel with castor as lube and nitro methane added in varying amounts to the methanol volume. Readily available at model aircraft shops.
The nitro introduces oxygen as it burns and adds to the compression. Reason for aircraft use is it tends to widen the needle setting quite considerably, it lowers the idle speed and it improves the transition from idle to increased revs.
Most instances we would add an extra shim under the head to actually lower the compression a small amount for the nitro use on some 4 strokes.
OS were usually ok as primarily designed to run on 10 to 20%.

Usually amounts in 10% increments are the most noticeable.
Byproduct of the nitro is acid which, if left for long periods can attack internal parts so best to give a good wash out later.

 

[dsage]

I would say the former as George drew it using Prodesktop 3d at the time with dimensions from the drawing.
As you say, time waster if not going to work as is, so any drawing development with improvement would be essential I'd imagine.
In George's instance, the time element was primary reason not to go any further, too many other things to do.

Not that it would make any difference in this instance but, any low compression engine can have performance increase but using methanol as a primary fuel with castor as lube and nitro methane added in varying amounts to the methanol volume. Readily available at model aircraft shops.
The nitro introduces oxygen as it burns and adds to the compression. Reason for aircraft use is it tends to widen the needle setting quite considerably, it lowers the idle speed and it improves the transition from idle to increased revs.
Most instances we would add an extra shim under the head to actually lower the compression a small amount for the nitro use on some 4 strokes.
OS were usually ok as primarily designed to run on 10 to 20%.

Usually amounts in 10% increments are the most noticeable.
Byproduct of the nitro is acid which, if left for long periods can attack internal parts so best to give a good wash out later.

An interesting treatise on Nitro. Thanks. I'll try to keep that in mind.
But if I'm to build it It'll have to run on Naptha like the rest of my engines or maybe propane. Since I'm between projects right now I'll do a bit of work to see what can be done if anything to improve the motion works.
What would be nice would be if someone that has one running could make some measurements of all the important pieces and spacings. Of course there is the possibility that the book dimensions are fine (likely) as stated by the one Youtube builder and it's just an exceedingly finicky engine (very likely).
Thanks
http://davesage.ca/

 

[bluejets]

No..... I just spoke to George on this an he even quoted an original builder who had one running at an engineering show in the UK as saying plans were wrong.

Builder had to re-do the drawings.

 

[dsage]

No..... I just spoke to George on this an he even quoted an original builder who had one running at an engineering show in the UK as saying plans were wrong.

Builder had to re-do the drawings.

Interesting. I talked to someone at Cabin Fever one year and he also recalled something he had to change but he didn't (wouldn't ??) say what. Perhaps there is some conspiracy going on here LOL

 

[bluejets]

Interesting. I talked to someone at Cabin Fever one year and he also recalled something he had to change but he didn't (wouldn't ??) say what. Perhaps there is some conspiracy going on here LOL

Well I doubt the people at the UK engineering competition would be full of crap.
They are highly respected both for their work and their expertise.
They don't get to where they are by telling or making up stories.

 

[dsage]

Well I doubt the people at the UK engineering competition would be full of crap.
They are highly respected both for their work and their expertise.
They don't get to where they are by telling or making up stories.

Agreed.
But as mentioned it would be nice to have some clues as to what was changed. Either way I'll play around and see what I come up with. Unless I stumble on it, it won't be any time soon.

 

[bluejets]

Position of the plug was just one of many.

 

[dsage]

Position of the plug was just one of many.

I'm sure the plug position will fall out of the analysis. The first thing will be to get the compression up by changing the motion works to get the pistons closer together at the left end. Once that's achieved it's easy enough to move the plug over the piston ends.
I looked at the problem almost all day yesterday. Fiddling around with the 3D model and making sure I had drawn everything as per the book.
As I once discovered when reading the book there is a big discrepancy between the form of the oscillating arm casting on page 36 and where you are supposed to machine the holes on page 81. They show the connecting rod joining hole placed on a particular radius from the pivot hole and it seems to me you wouldn't be able to achieve that with the castings produced from page 36.
Anyway I made sure my drawing was as per page 81 (because that's what they said to machine). I originally had it more like the casting shape and dimensions on page 36. Changing it didn't make any major difference.
Analysis showed that there is almost zero compression. I looked simply at the distance between the pistons when the intake air is finally trapped in the cylinder and where the pistons come together at the left compression end. I didn't bother with volumetric calculations.
In fact there is expansion of the distance between the pistons in the travel between the two ends so there is a bit of vacuum going on while it's traveling. The spacings at each end were almost the same - so zero compression.
I then went trough various changes in part positions and link lengths and I could get up to 3:1 ratio but it required MAJOR changes that I doubt were undertaken by any builder purposely or by accident.
One major change included making the intake port in the cylinder a slot to take more advantage of the intake stroke and moving the pivot point of the left oscillating arm. That's where I got the 3:1 compression.
As suspected all along changes of only a fraction of an inch shifted positions at both ends basically improving one end and removing the benefits from the other and and generally destroyed the whole geometry.
I assume something quite simple must be at the root of the fix. There are a LOT of things to change. All of them affect the motion significantly.
I'll work on it some more today (maybe).

 

[Gordon]

I'm sure the plug position will fall out of the analysis. The first thing will be to get the compression up by changing the motion works to get the pistons closer together at the left end. Once that's achieved it's easy enough to move the plug over the piston ends.
I looked at the problem almost all day yesterday. Fiddling around with the 3D model and making sure I had drawn everything as per the book.
As I once discovered when reading the book there is a big discrepancy between the form of the oscillating arm casting on page 36 and where you are supposed to machine the holes on page 81. They show the connecting rod joining hole placed on a particular radius from the pivot hole and it seems to me you wouldn't be able to achieve that with the castings produced from page 36.
Anyway I made sure my drawing was as per page 81 (because that's what they said to machine). I originally had it more like the casting shape and dimensions on page 36. Changing it didn't make any major difference.
Analysis showed that there is almost zero compression. I looked simply at the distance between the pistons when the intake air is finally trapped in the cylinder and where the pistons come together at the left compression end. I didn't bother with volumetric calculations.
In fact there is expansion of the distance between the pistons in the travel between the two ends so there is a bit of vacuum going on while it's traveling. The spacings at each end were almost the same - so zero compression.
I then went trough various changes in part positions and link lengths and I could get up to 3:1 ratio but it required MAJOR changes that I doubt were undertaken by any builder purposely or by accident.
One major change included making the intake port in the cylinder a slot to take more advantage of the intake stroke and moving the pivot point of the left oscillating arm. That's where I got the 3:1 compression.
As suspected all along changes of only a fraction of an inch shifted positions at both ends basically improving one end and removing the benefits from the other and and generally destroyed the whole geometry.
I assume something quite simple must be at the root of the fix. There are a LOT of things to change. All of them affect the motion significantly.
I'll work on it some more today (maybe).

I did a lot of playing around with drawings. I only have 2D CAD so I had to just move parts around. There is a similar design by Brooks Pendergast which gives a slightly better compression. The parts are quite a bit different but it still does not make a dependable engine. As you have discovered the compression is generated by one piston moving slightly faster than the other so there is very little compression. Also when the engine fires the pistons are not in a good position to provide power to the crank. The linkage arms are almost horizontal when it fires so not much power is transmitted to the crank. If you want I can give you my CAD drawings and you can play around with them. If interested send me your email address in a message and I will send them to you.

 

[awake]

Just a thought - would it be helpful to print up some models using a 3d printer? 3d CAD is marvelous, but there are times when I find it easier to look at an actual something rather than the visualized object on the screen, and even more so when moving the parts around.

 

[dsage]

Just a thought - would it be helpful to print up some models using a 3d printer? 3d CAD is marvelous, but there are times when I find it easier to look at an actual something rather than the visualized object on the screen, and even more so when moving the parts around.

Gordon / awake:
I think I'm ok with the 3D model / drawings I have. Thanks
The advantage of the 3D computer model is priceless in the ability to manually rotate the flywheel very slowly and watch the pistons move, stop them in a precise position at both ends of the travel and make measurements and look for interferences. I can get through several changes to see the effects in an hour or so.
3D printing is far too slow to make multiple trial changes but it might be useful to get something "in hand" when I'm satisfied I've gone as far as I can.

As an update I found another error in MY DRAWINGS. I was thrown off again by the discrepancy between page 36 and 81 on the oscillating arms. When you set out to re-draw a part from a book you look for something that gives the most information especially when there are defined curves. Page 36 was great because it laid everything out with measurements radii of curves so that's what I used.
Page 81 threw a lot of that away because the spacing between the holes is defined as to be machined.
So I changed my drawing to be as per page 81 dimensions. That made the shape of the arms slightly different that those defined on page 36 (Because the curves no longer fit with the dimensions, but not a significant difference. Now I'm pretty sure I'm working with "the book" dimensions. (however vague and scattered they may be).
All that done unfortunately it made only a bit of difference in the compression (which was previously zero) I though it would have made more.
Currently I'm playing with making the left conrod a bit longer and making the intake port a slot (instead of a hole).
Of course there are the usual "change this and it changes that" problems but lengthening only the left conrod affects the compression directly. The slotted intake port allows more intake charge while the left piston moves left - a good thing because there is quite a bit of movement in that piston alone before the right piston starts to follow - so more useful intake time.
I have the compression ratio (actually the ratio of the pistons gaps at each end of travel) to about 2.5:1. I have not done the volumetric calculations to get the actual compression. It might be higher than 2.5:1 (I doubt less) but it is certainly more than the zero I was getting. The ratio of 2.5:1 is barely enough but it will probably run if everything else supports it. It won't be winning any tractor pulling competitions for sure.
The changes so far are some that can be made easily to an existing engine and are also minor changes if building a new engine. I hope all the required changes follow that idea.
I'm not done yet. I might come across some more simple changes.

 

[bluejets]

I assume something quite simple must be at the root of the fix.

I would not assume anything with this particular engine.

One thing to remember here is this was originally all done with a slide rule, no computers, no digital calculators, just grey matter, pencil and paper.

There is of course the other Atkinson engine which, although does present some setting up problems, does work pretty much from the drawings.
I have one here as does George.

 

[dsage]

I would not assume anything with this particular engine.

One thing to remember here is this was originally all done with a slide rule, no computers, no digital calculators, just grey matter, pencil and paper.

There is of course the other Atkinson engine which, although does present some setting up problems, does work pretty much from the drawings.
I have one here as does George.

I have one of those engines as well which is somewhat why I want to build this one.
http://davesage.ca/
I'm not assuming anything but I am trying to stick to simple changes so that an existing engine can be modified and a new one won't be much different than the original.

End of the day news is that I have the compression up to 4.3 with sort of minor changes. But I have to check carefully because as I said before sometimes when you make a change in a part it comes disconnected from the rest of the engine and it isn't telling the truth about it's motion. But I'm hopeful.
Possibly more tomorrow.

 

[Gordon]

I can tell you that both Ray and I tried different length connecting rods, different linkage, different pivot points, different crank center and everything else we could think of and never got anything which gave us a good running engine. Never got anything but marginal compression. It will kind of run if the moon and the stars are in correct alignment.

 

[dsage]

I think this is where the 3D modelling is helping. I can try a several different "Pieces" in a couple of hours and don't have to make any chips. Rest assured I've had LOTS of failed attempts because what seems like it may be a good idea turns out to be crap for one reason or another. The motion works are vary finicky - as you know. I'm really glad I have this tool because I can imagine your frustration having tried so many real parts. I'll have more to report tomorrow (I hope).

 

[propclock]

Thanks Dave I find this very interesting. Design it and then make it.
The proof is in the pops. 4-1 sounds like most old reliable engines.
I think 4-1 CR will definitely work. Make chips! oh yea it's cold in
Canada. Well then, think about it,h until it is warm enough.
Later Dave.

 

[dsage]

Thanks Paul:
I think that building the engine (according to what I've found) is what I'm going to have to do. The best compression I'm able to achieve is about 4.3 but it requires a few modifications that although could be made to an existing engine are probably best applied to a new build. I wouldn't want to tell anyone to go ahead and modify an existing engine that might have other parameters already modified. As mentioned everything seems to affect the result (mostly for the worse) and combining these modifications with pre-existing mods might result in trashing the engine.
Like medical insurance, preexisting conditions make your policy null and void
For what it's worth the changes to a perfectly "by the book" engine require making the left conrod longer, moving the cylinder center line up a bit, adding a slight bit to the top of the left piston and because of those changes the intake port needs to be made into a slot instead of a hole starting where it is and extending it toward the water jacket.
If one might be satisfied with only about 3.7:1 compression then simply adding some to the top of the piston and making the intake port a slot would probably do.
The intake port slotting seems to be the most effective change. The left piston moves quickly to the left during intake while the right piston moves slowly following it. This is what gives the intake suction. There is quite a bit of rotation where the pistons movement are creating suction but it appears because of the size of the original intake hole in the cylinder the suction is cut off prematurely when the right piston covers it up. With the port slotted by the correct amount full advantage is taken of the vacuum available. After a certain point the pistons move together to the left with a large gap between them so no more vacuum is created. That distance forms one value in the compression ratio because that much gas is trapped between the pistons.
After that the right piston starts to move faster and creates compression on the left end of the cylinder. The gap between the pistons at the left side forms the other value in the ratio. (only a gap ratio but it's pretty close to the actual compression ratio).
I've measured about 0.500 at intake end and .116 gap under compression (4.3:1).
And yes the spark plug hole needs to be moved to the right a bit. But if you are satisfied with the original designs misalignment of the 10mm sparkplug with the hole then it won't bother you. I decided it would be better to use a 1/4" spark plug and make a new boss for it to screw into. Then there is just enough space up against the water jacket to get the sparkplug electrode right down to the hole.
That's enough (vague) info for now. Vague because I don't want anyone to risk their existing engine.
BUT if any of the info has tweaked you interest I can be more specific.
The first caveat would be that you need an engine built exactly by the book. I hope I have it drawn it that way and that I'm starting from the proper place otherwise all bets are off.
That's why it might be better to start a new build with the dimensions I have for the whole engine. I'm confident it will work. I have found CAD to be dead accurate in the past (assuming it's drawn that way).

 

[Gordon]

Thanks Paul:
I think that building the engine (according to what I've found) is what I'm going to have to do. The best compression I'm able to achieve is about 4.3 but it requires a few modifications that although could be made to an existing engine are probably best applied to a new build. I wouldn't want to tell anyone to go ahead and modify an existing engine that might have other parameters already modified. As mentioned everything seems to affect the result (mostly for the worse) and combining these modifications with pre-existing mods might result in trashing the engine.
Like medical insurance, preexisting conditions make your policy null and void
For what it's worth the changes to a perfectly "by the book" engine require making the left conrod longer, moving the cylinder center line up a bit, adding a slight bit to the top of the left piston and because of those changes the intake port needs to be made into a slot instead of a hole starting where it is and extending it toward the water jacket.
If one might be satisfied with only about 3.7:1 compression then simply adding some to the top of the piston and making the intake port a slot would probably do.
The intake port slotting seems to be the most effective change. The left piston moves quickly to the left during intake while the right piston moves slowly following it. This is what gives the intake suction. There is quite a bit of rotation where the pistons movement are creating suction but it appears because of the size of the original intake hole in the cylinder the suction is cut off prematurely when the right piston covers it up. With the port slotted by the correct amount full advantage is taken of the vacuum available. After a certain point the pistons move together to the left with a large gap between them so no more vacuum is created. That distance forms one value in the compression ratio because that much gas is trapped between the pistons.
After that the right piston starts to move faster and creates compression on the left end of the cylinder. The gap between the pistons at the left side forms the other value in the ratio. (only a gap ratio but it's pretty close to the actual compression ratio).
I've measured about 0.500 at intake end and .116 gap under compression (4.3:1).
And yes the spark plug hole needs to be moved to the right a bit. But if you are satisfied with the original designs misalignment of the 10mm sparkplug with the hole then it won't bother you. I decided it would be better to use a 1/4" spark plug and make a new boss for it to screw into. Then there is just enough space up against the water jacket to get the sparkplug electrode right down to the hole.
That's enough (vague) info for now. Vague because I don't want anyone to risk their existing engine.
BUT if any of the info has tweaked you interest I can be more specific.
The first caveat would be that you need an engine built exactly by the book. I hope I have it drawn it that way and that I'm starting from the proper place otherwise all bets are off.
That's why it might be better to start a new build with the dimensions I have for the whole engine. I'm confident it will work. I have found CAD to be dead accurate in the past (assuming it's drawn that way).

As far as adding to the top of the left piston and changing the connecting rod I think the same thing could be accomplished by just making a longer LH connecting rod. The goal is to move the LH piston to the right and just making the connecting rod longer does that unless that causes some interference some other place. As far as slotting the intake I think that you may have something there. You are starting the compression stroke with a smaller initial volume so that you are able to compress the smaller volume with a shorter stroke. What you are doing is creating a shorter stroke. Getting fuel into the chamber is not the problem. Getting the fuel to a state where it is compressed enough to create a combustible mix is the goal. I have played around a little bit with my drawings but I still have not found the sweet spot. As you have found a relatively small change makes a big difference. I found that just the amount of play in the various connecting points can change things.