Another Atkinson Differential build

[Dave Perreault]

Hi guys.... This is a segment of an email conversation with Ken about getting the engine to run. Thought it might be of interest to someone.

From Dave Perreault

Getting the Mini A engine to run.

Fuel - In the plans I have provided some guidelines as to how to get the engine to run. You must provide oil in the fuel mixture as this engine does not have any other means of lubrication. The fuel mixture that I recommend is what I have found to be the best for this engine.

Electric motor assistance when first starting the engine.
- the Atkinson Differential engine is like no other engine I have built. It has so many variables to get it to run on its own that the use of an electric motor assistance on start-up is a big plus.
- mechanical timing
- valve timing
- fuel mixing valve setting
- ignition timing
- ring seating
- etc.

Having the engine ready to run, ignition ready and lubricant fuel in the tank.
Turn the electric motor on and you can now concentrate on setting things while it is being rotated by the electric motor.
Keep adjusting things until you hear simple ignition of the engine. It will definitely be random but this is telling you that you are close. Make small adjustments to all the above settings to see if it starts to get more regular.
At some point you will notice that it is firing at 90- 95% of the time. This is the point that you can now try to run the motor on its own.
If you can not get the engine to get to this point within a period of about 3 hours of adjustment than you must stop and evaluate everything to see if you have missed something.
YOU SHOULD SEE A STEADY POSITIVE PROGRESS IN GETTING THE ENGINE TO RUN DURING THIS TIME....

Note: No progress = something is not right. This engine will run if everything is right.

There is a reason why so many people can't get the Atkinson Differential engine to run.... It is not your typical internal combustion engine and it does require a different approach.

I have made several model 4 cycle gas engines.
I have designed, made and have 6 Atkinson engines that run well.

Atkinson engines are a special kind!

I hope this helps and if I can be of further assistance just let me know.

Regards
Thanks
Dave

 

[dsage]

(turning it with an electric motor)

>>> "If you can not get the engine to get to this point within a period of about 3 hours of adjustment then you must stop and evaluate everything to see if you have missed something"

Well.
There you have it.

 

[Dave Perreault]

Steamchick:
All interesting Ponderings.
But were you researching the Atkinson "Cycle" engine or the one we are discussing here which is the Atkinson "DIFFERENTIAL" engine? Two very different beasts.
The differential engine we are talking about has two pistons in the same cylinder which for the most part work against each other.
I built the Atkinson cycle engine and it works fine. Not exactly efficient IMHO but it at least runs.
As for some Engineer that has studied it. I would assume Atkinson did. Unless it has been a scam since the beginning. If it weren't for the fact that there are a couple I've seen running I wouldn't believe it could run based on the workings I've observed in the one I'm building.
BTW the version Ken is building is a bit of a cheat from the original design. It has some extra valve gear that may make it better.

Hi, the slide valve design on the Mini A is my approach to simplify the build as making an intake and exhaust valve for this scale would be very hard for the average home builders.
This is a half scale model of my 10" flywheel casting kit that I have available. The valves are .312 diameter so at half scale the valve diameter would be .156 diameter.
Hence the design of the slide valve on the Mini A.
James Atkinson Differential engine.
Didn't use a spark plug used a hot wire.
Had a governor on it.
So yes the Mini A is different!
Cheating?

 

[Ken Brunskill]

Just to add another one of my wild theories which I came up with when I was working on the engine a year or so ago. Since ring sealing is dependent on gases getting behind the ring to force it against the cylinder wall the rings are not being forced against the cylinder wall when the LH piston is moving in the compression stroke. This theory is probably as valid as my theory about phase of the moon.

Gordon

Good point! Believe your analysis point's out and emphasizes the need for both the rings and pistons to have finely machined mating surfaces (fine a finish as possible), with dead square ring grooves with '0' ZERO radius at their bottom. Likely more critical in the Atkinson Differential than most engines. One of our (BAEM Club) very prolific builders has emphasized this very issue many times, I'll bet from experience! This make me think that in addition, some experimentation with Ring pressure against the cylinder wall may prove useful.

Ken B

 

[Dave Perreault]

Good point! Believe your analysis point's out and emphasizes the need for both the rings and pistons to have finely machined mating surfaces (fine a finish as possible), with dead square ring grooves with '0' ZERO radius at their bottom. Likely more critical in the Atkinson Differential than most engines. One of our (BAEM Club) very prolific builders has emphasized this very issue many times, I'll bet from experience! This make me think that in addition, some experimentation with Ring pressure against the cylinder wall may prove useful.

Ken B

Hi Ken, ring pressure on cylinder wall is very critical.... It wants to be a free ring gap on the Mini A should be about .060 ( that is the gap when the ring is out in free air)
And a ring gap of .001 when in the cylinder.
Too much ring pressure will increase the friction of the pistons traveling in the cylinder.

 

[Steamchick]

Hi Gordon. From my experience of ring sealing - and maths - the radial force of ring on cylinder wall is not zero at zero pressure behind the ring, but is the natural radial force of the ring against the cylinder wall from the spring press re of the metal. Always assuming that the rings are made correctly so need some compression to be installed into the bore.
However, you are perfectly correct in that there are moments when the ring is not held dynamically against either top or bottom surface of the ring groove, so there is a tiny leakage there. But as the pressure is quite low when that part of the cycle occurs, and it is for a relatively short time, the leakage is less significant. In production engines, ring thickness to ring groove width is a sliding fit I.E. microns of clearance, not thousandths of an inch. But I recognise that models (parts made on less precise machines without micron measurement!) may have a "thou or 2" of clearance. But I think leakage will be relatively small. (I haven't done calculations on this).
As there are videos of running engines, there are some OK conditions that work. But please strive to make the ring to groove clearance a minimum, within your capabilities.
I remember (40 or 50 years ago) cleaning pistons and rings (carefully) so the rings easily slid into the grooves all the way around, after I had cleaned off the hard carbon on the piston surface. The ring had to be a "neat" fit, just not tight enough to hold it in the groove when turned so gravity could make it fall out. E.g. When rings are on a piston, held with the axis horizontal, rotate the piston around its axis and the rings should naturally fall all the way 'in' to the upper part of the groove all the way around, without binding. But any more (excess) clearance should be avoided. Any tightness will cause the ring to "nip" when warming-up due to frictional heating of rings before the heat can flow into the piston, and the bore expand as it warms. This nip will cause wear, scuffing and possible early seizure. So rings must be free to move in the grooves. But as you indicate, any gap is leakage, however small.
Incidentally, as the gas in front of a ring has to turn 90 degrees 4 times to get to the inside and back out again, this also acts like a labyrinth seal, where there is a pressure drop of something like 20% at each 90 degree transition, acting as a resistance to the leakage gas.
Additionally, any oil on the bore that is swept-up by the ring also helps to block the leading edge (motion direction) of the ring. Some oil (a molecule thickness plus surface inclusions) stays on the bore and ring outer surface, and some is carried across the flat face to get behind the ring. Actually this also will reduce leakage at the lower pressures.
But a good theory! And I like your sketches. Keep up the good work - all ideas accepted.
K

 

[Richard Hed]

Just to add another one of my wild theories which I came up with when I was working on the engine a year or so ago. Since ring sealing is dependent on gases getting behind the ring to force it against the cylinder wall the rings are not being forced against the cylinder wall when the LH piston is moving in the compression stroke. This theory is probably as valid as my theory about phase of the moon.

Gordon

Well what is your theory of the mooN?

 

[Gordon]

When I was working on my engine, which was quite some time ago, I tried using O rings. As I remember I got the engine to fire pretty consistently while it was being turned by the electric motor but the O rings had too much friction to make the engine run on its own. The O rings did not last very long before they no longer worked. I now believe that was due to to the nipping of the ring as it passed the intake and exhaust holes in the cylinder. It is still my belief that adequate compression is the answer.

With all of this conversation I am coming close to taking the engine back off the shelf and trying again. I keep hoping that someone comes up with some answers. Note: My engine is close to the original design and uses atmospheric valves instead of mechanically actuated valves. It is also larger with 1 1/8" bore as opposed to this one with 3/4" bore.

Gordon

 

[Dave Perreault]

Hi guys, a few comments before someone quoted me as saying about getting the Mini A to run for the first time. "After turning it over with the electric motor for 3 hours if it doesn't run on it's own you need to stop and check everything as you missed something"
I want to make sure I get this across right.
By no means am I saying just to attach it to an electric motor and let it run by itself for 3 hours and expect the Mini A to run on its own.
This is what I expect the person who is trying to get the Mini A to run on its own to do during the 3 hour period.
Make sure the following has been set up as best as possible before attaching to the electric motor.
Mini A
- With the spark plug out the motor should rotate free. No additional friction.
- Slide valve adjustment made so it opens and closes the ports at proper timing
- Carb adjustment made to what you think is correct to start.
- Fuel mixture added to fuel tank
- Spark timing is set
- A good working spark plug is installed

Once this is checked
- Connect the engine by belt to the electric motor
- Start it up and it will most likely not be firing at all
- During the entire 3 hour period that I am suggesting to get the Mini A to run you are doing the following to get it to come alive.
- Adust fuel mix air/fuel
- Adjust spark timing
- Adjust slide valve timing
- Make sure the plug is not fouled ... As this can happen often when the engine does not have the right conditions.
- Stop every now and then and think!!!!

The electric motor assistance is not magic.... It is not going to make an out of tune motor run by no means.
The use of the electric motor is one for purpose only.... To rotate the Mini A so you can THINK, ADJUST AND GET IT RUNNING.

My hat is off to anyone that can make an Atkinson Differential engine, fuel it up, give it a few good spins and it runs like a top!

I hope this clears up any confusion I may have caused on the use of the electric assist motor.

Regards Dave

 

[Gordon]

Hi guys, a few comments before someone quoted me as saying about getting the Mini A to run for the first time. "After turning it over with the electric motor for 3 hours if it doesn't run on it's own you need to stop and check everything as you missed something"
I want to make sure I get this across right.
By no means am I saying just to attach it to an electric motor and let it run by itself for 3 hours and expect the Mini A to run on its own.
This is what I expect the person who is trying to get the Mini A to run on its own to do during the 3 hour period.
Make sure the following has been set up as best as possible before attaching to the electric motor.
Mini A
- With the spark plug out the motor should rotate free. No additional friction.
- Slide valve adjustment made so it opens and closes the ports at proper timing
- Carb adjustment made to what you think is correct to start.
- Fuel mixture added to fuel tank
- Spark timing is set
- A good working spark plug is installed

Once this is checked
- Connect the engine by belt to the electric motor
- Start it up and it will most likely not be firing at all
- During the entire 3 hour period that I am suggesting to get the Mini A to run you are doing the following to get it to come alive.
- Adust fuel mix air/fuel
- Adjust spark timing
- Adjust slide valve timing
- Make sure the plug is not fouled ... As this can happen often when the engine does not have the right conditions.
- Stop every now and then and think!!!!

The electric motor assistance is not magic.... It is not going to make an out of tune motor run by no means.
The use of the electric motor is one for purpose only.... To rotate the Mini A so you can THINK, ADJUST AND GET IT RUNNING.

My hat is off to anyone that can make an Atkinson Differential engine, fuel it up, give it a few good spins and it runs like a top!

I hope this clears up any confusion I may have caused on the use of the electric assist motor.

Regards Dave

That is pretty close to what Gingery says in his book. He says between 30 and 40 hours run time with the electric motor before it will start and run on it's own. I suspect that most builders give up before they reach 30-40 hours of assisted run time.

 

[Dave Perreault]

That is pretty close to what Gingery says in his book. He says between 30 and 40 hours run time with the electric motor before it will start and run on it's own. I suspect that most builders give up before they reach 30-40 hours of assisted run time.

Hi Gordon, in my experience 3 hours is a good amount of time and if you are not getting anywhere in the positive direction something is off and you need to scratch your head a little. IMHO

 

[Dave Perreault]

I thought I would share this email that I sent to Ken. It is in reference to the Mini A and the piston locations at TDC and the distance between them.

Good morning Ken!

After thinking about this I believe this is the best way to give you this information.

To start with.....
I want to make sure that everyone understands this.
- The Atkinson Differential engine has like 25 plus dimensions that make the mechanical part of this engine to work right! With that being said...... Every time a part is made and it is off just the smallest amount it effects the stroke of the engine. So these machining errors add and subtract to the total stroke.
Note: If any builder of this Mini A engine substituted bronze bushings for the bearings I specified that is a REAL PROBLEM. Here is why... If you rotate the engine you will notice that the right piston during the compression stroke is retreating to the right.... There are 6 bearing sets that that are in play during this right piston movement. With the ball bearings I have ZERO backlash. If you use bronze bushings Lord only know how much it would have and this backlash will subtract from the compression ratio in a bad way.

Ok, back to the piston to piston distance at top dead center.

On my running engine....
I took the plug out.. by the way make sure you are using a long reach plug.
- When the right piston is to far right the right piston top is aligned with the right side of the spark plug hole.
- When the left piston is to it's far right I have .055 on an inch between them.
See picture.

This is what I would suggest to do if you find that your Mini A piston to piston is different.

- I strongly suggest that once you put the pistons and rings in the engine and started to break them in that you DO NOT REMOVE THEM. I have found this to be a step backwards in ring seating.

- I would make new upper links and change the length of them accordingly to get the proper piston positions at TDC

You don't need to remove any other atms to do this with the exception of the slide valve linkage.




Side note:
A word about calculating displacement

Unlike our standard combustion engine displacement calculation on the Atkinson Differential engine has some added things to consider.
On the Mini A
- Chamfer on the piston
- Distance piston to piston TDC .055
- Spark plug hole
- Inside the spark plug
- When are ALL ports sealed of by the rings...i.e. when does compression official start

Please feel free to share all this information with anyone you like. I don't care if you post it on the forum.


So Ken, I hope this helps!
And as always, feel free to ask more questions as needed.

Regards Dave

 

[Ken Brunskill]

Wow folks, what a great conversation , this has been my 1st foray into the website world and I must say thanks to Dsage for urging me to begin this thread!
I have found it rewarding as well as informative. If you too are enjoying this thread - Thank Dsage.

Now for some technical information, Dave Perreault and I have been trading emails which caused me to figure out how to get the volume of the chamfered area of his POWER piston. Hope this helps anyone interested.

 

[Ken Brunskill]

I thought I would share this email that I sent to Ken. It is in reference to the Mini A and the piston locations at TDC and the distance between them.

Good morning Ken!

After thinking about this I believe this is the best way to give you this information.

To start with.....
I want to make sure that everyone understands this.
- The Atkinson Differential engine has like 25 plus dimensions that make the mechanical part of this engine to work right! With that being said...... Every time a part is made and it is off just the smallest amount it effects the stroke of the engine. So these machining errors add and subtract to the total stroke.
Note: If any builder of this Mini A engine substituted bronze bushings for the bearings I specified that is a REAL PROBLEM. Here is why... If you rotate the engine you will notice that the right piston during the compression stroke is retreating to the right.... There are 6 bearing sets that that are in play during this right piston movement. With the ball bearings I have ZERO backlash. If you use bronze bushings Lord only know how much it would have and this backlash will subtract from the compression ratio in a bad way.

Ok, back to the piston to piston distance at top dead center.

On my running engine....
I took the plug out.. by the way make sure you are using a long reach plug.
- When the right piston is to far right the right piston top is aligned with the right side of the spark plug hole.
- When the left piston is to it's far right I have .055 on an inch between them.
See picture.

This is what I would suggest to do if you find that your Mini A piston to piston is different.

- I strongly suggest that once you put the pistons and rings in the engine and started to break them in that you DO NOT REMOVE THEM. I have found this to be a step backwards in ring seating.

- I would make new upper links and change the length of them accordingly to get the proper piston positions at TDC

You don't need to remove any other atms to do this with the exception of the slide valve linkage.




Side note:
A word about calculating displacement

Unlike our standard combustion engine displacement calculation on the Atkinson Differential engine has some added things to consider.
On the Mini A
- Chamfer on the piston
- Distance piston to piston TDC .055
- Spark plug hole
- Inside the spark plug
- When are ALL ports sealed of by the rings...i.e. when does compression official start

Please feel free to share all this information with anyone you like. I don't care if you post it on the forum.


So Ken, I hope this helps!
And as always, feel free to ask more questions as needed.

Regards Dave

Thanks for this post Dave!

 

[Steamchick]

All good stuff. Just what I like about this thread - the people willing to share lots of good technical stuff.
Thankyou all!
K

 

[Steamchick]

From my musings I think the engine could be improved by reducing the volume of the chamfers.... If these were simply 2 half-drilling in line with the port location - made by pre-drilling the piston before turning to size - then the gases could traverse the piston readily and the compressor would be higher. Would it help Dave P..?
I'll do a sketch if interested?
K

 

[Dave Perreault]

From my musings I think the engine could be improved by reducing the volume of the chamfers.... If these were simply 2 half-drilling in line with the port location - made by pre-drilling the piston before turning to size - then the gases could traverse the piston readily and the compressor would be higher. Would it help Dave P..?
I'll do a sketch if interested?
K

While that will work ....
If you want more compression just move the the left piston further to the by .010

I would proceed with caution.... You may be giving yourself more work. My engine design does work with the chamfers as specified and the pistons at .055 apart from each other TDC.

If you have read my previous post....
You will notice that Colman Camp fuel is only like 50-55 octane and if the compression is too high you will get preignition! I usually try for 5:1 theoretical and end up with like 4:1 because of losses.

IMHO
Dave

 

[Steamchick]

Thanks for that. I was looking at it from the perspective of higher efficiency (thermally) which should help it run for engines less perfect than yours. I have made a few flea-powered engines that I could not get to run, simply because my handiwork is not so perfect. And as an engineer I always want to improve things.... (most people translate "improve" to "interfere"!).
Thanks for your advice,
K

 

[Gordon]

Did some drawing or diagram on this engine disappear on this discussion? I was sure that I remembered a drawing where it showed the pistons at TDC with .050 between them. I was a little bit confused, a common occurrence, and too busy right then to actually analyze it. Later I could not find it. Am I dreaming?

Gordon

 

[Steamchick]

Just so anyone interested can see where I am at with my latest "picture" of the Atkinson's Differential chamber pressure versus volume.
This is a PROGRESS REPORT, not the final thing, so it will change as data is refined - and when I get to do a thorough check that all the correct factors of 10 have been applied in this peculiar Metric system that is reputed to be so easy... (Much easier to be out by tens of thousands than anything, as the numbers are either huge or incredibly tiny...).
I now have a numerical model, that I think gives a useful picture. Though the values shown are purely imaginary (from my head) as an illustration. - This avoids any complications of anyone using data and getting something drastically wrong.
Simply:

1. at the x-axis = 50, there is a vertical line representing ignition.
2. This expands (the pressure drop) to around x = 270-ish, when the exhaust valve opens and the gases exhaust, just before max volume.
3. The graph shows the bottom "loop" as the exhaust (to min volume) to intake phase (back to larger volume), followed by:
4. Compression to the ignition point (min volume).

The pressure is up and down the graph, whereas volume is smallest on the left and largest on the right. The area of the TOP loop expresses the power of the engine, and the area of the bottom loop is the power consumed by pumping gases in and out.... I think? - Any better explanation please?
I hope that makes sense?
The experts will appreciate the differences between this cycle and the Otto and Carnot cycles, so there may be some further "Thread" discussion of that - I hope? - as I want to learn a bit.
I'll be doing a lot of work to make a small presentation around this picture, so can accept useful suggestions if you wish to post anything.
Enjoy!
K