Single Cylinder Opposed 2 Piston Engine

[Lloyd-ss]

Ray, I was wondering about the balancing, too. Some large holes machined in the flywheels diametrically opposite to the weights would improve the balance while also reducing the rotating mass. It could be an option if a reasonable balance is difficult to achieve.
Lloyd
P.S. BTW, excellent work on this intriguing project.

 

[Rdean33422]

Lloyd that was my original thought as well but the center is made of aluminum and only 1/2" thick. I would have to remove much of the center to amount to any appreciable weight and then the problem of the center maybe coming loose from the rim as it is just pressed in the outer rim.

Thank you for the kind words.
Ray

 

[BWMSBLDR1]

Look up the Napier Deltic engine. Intended for the Bristol Brabazon airliner it ended up driving railcars! Bill in Boulder

 

[Rdean33422]

I was going to make a nice box for the ignition module and the batterys but since the engine itself is well exposed I decided to leave them out in the open also.


Yes it does run.
It wasn't the easiest engine to get running but not so bad either.

No video yet even with the brass weights it bucks like a bull rider.
The counter weight calculator I was using told me that I needed 6.6 oz and I had 4.8 oz on the flywheels now so make some bigger weights.
This shows the old and the new counter weights.


The new weights are 3.3 oz and are mounted 1/2" further out on the flywheel and according to the calculator that is the perfect location and size.
I didn't get a picture of them installed but just went right for a trial run. Even thought I had the base held down with a screw clamp it was immediately evident it was a lot worse.
I am thinking the counter balance for a rotating object is different that just being in static balance so in frustration I removed all the flywheel weights and gave it a run.



It still has some vibration as seen in the fuel in the gas tank jumping around but not as bad as I had expected.
It is a success in that it does run but I am not happy with it as is.

This is not over and I will find a way to minimize the bucking and tame the beast.

Thanks for looking
Ray

 

[Roger B]

It looks and sounds great to me for a first start. Don't forget that in an opposed piston engine the other piston supplies most of the balance.

 

[Steamchick]

Hi Ray, Re-reading the post on balance weights: you have a"top" system at 9 oz, but no mention of the lower piston and con-rod, crank-pin, bolts, or whatever. The balance weight on the flywheel should compensate for the difference of top and bottom weights, as they naturally counterbalance each other.

What is the weight of the lower piston pin, rod, etc?
K2

 

[Steamchick]

Ray, if you have a top set of parts at 9oz, and bottom set at 7oz, then it is only a 2 oz difference you need as a flywheel mass, at the same radius as the crank radius. If the radius for crank for top piston is different to the crank radius for the bottom piston then you have to compensate for that as well.
When you have correctly calculated the balance weight needed, if it is not symmetrical about the middle of the cylinder bore, then there will also be a couple set-up where one end of the crank is being pushed up while the other is pushed down then vice-versa. I think all your balance weight is hung on one end of the crank.
I can do drawings and an analysis in a few days if you PM details, weights, etc. But not while I am away on holiday. Meanwhile, have fun and don't break anything.
K2

 

[Rdean33422]

I was completely wrong in my thinking that I could balance this engine by adding weight to the flywheel.

The problem is that the upper piston assembly is heavier than the lower piston assembly so as it moves it creates an up and down unbalance movement. The engine is trying to jump up and down.
To correct this the counter weight must also be of the same the up and down movement.
When a weight is placed on the flywheel it causes an unbalance through out it's rotation not just the up and down directions. It does overcome the weight difference between the upper and lower pistons but only for a short instant when the engine is at bottom dead center or top dead center while causing a great unbalance the rest of the time.
I was in effect building a paint mixer.

I will be looking into adding some weight to the lower piston and installing larger flywheels.

Thanks for looking
Ray

 

[Steamchick]

Hi Ray,
NOT "completely wrong", just "back to front". We have all been there when we put on a tee-shirt back-to-front, or whatever, so don't get in a fettle over it. - The difference in TOP and BOTTOM weights must simply be set 180 degrees from how you set it.
There are 3 MODES of vibration/balancing for crankshafts and single cylinder arrangements.

1. In-line with the "unbalanced Piston/con-rod/crank" masses - We call this Primary balance.
2. The Side-to-side oscillation of the crank-pin/con-rod and counterbalance weights. We call this Secondary balance.
3. Then it gets a bit more complicated due to geometry - we call this Tertiary balance, - BUT DON'T think about this yet.

SO you have learned about that and explained it.
The thing is, Most models are so small and light and relatively slow running that Primary balancing is all you need to do.
There are various standard "balance regimes" I have heard about in the racing motorcycle world. E.g. as Primary balance of 100% of the piston + rod + Crank=pin, etc. is excessive for secondary balance, they use 85% or 75% or 70 % or 60 %, but usually find that around 70~85% is the best range for primary balance vibration versus secondary balance vibration. - Ignoring Tertiary balance.
In your case. Primary IM-balance causes the engine to jump "up-and-down" on the table, Secondary IMbalance causes the engine to rock from side-to-side and walk around the table. So the simple crank counterbalance you are engineering will need a bit of reduction after you have tried it for 100% Primary balancing.
Sorry if this sounds complicated, (but it is).
So reset your crankshaft 180degrees around from where you set it, with the balance weights to compensate for the DIFFERENCE between the "bottom piston+ rod+crank-pin" and the "TOP piston+ rods+crank-pins".
Also don't forget that crank webs make a difference, if the webs for top and bottom are different masses. And there may be nuts and bolts, piston-rings, piston-pins cross-heads, etc. but I have not listed all those as they would make my text too long.
But I am sure you'll work out exactly what bits make the IMbalance between top and bottom arrangements.
Without ALL the details of your machine I cannot do the sums for you, but am willing to, if you wish.
But doing it yourself, and the learning involved, is a good thing too.
Cheers!
K2

 

[Steamchick]

Hi Bill-in-Boulder.
I think you mean this one?
https://en.wikipedia.org/wiki/Napier_Deltic
One of those clever British things that worked fine? Was it also used in the USA? Wiki doesn't mention the Bristol Brabazon ever using a Deltic diesel engine. It was powered by 8 × Bristol Centaurus 18-cylinder radial sleeve-valve piston engines...
K2

 

[Rdean33422]

The last few days have been busy with other breakdowns and problems but I have been able to get some things done on the engine.

This is the old and new lower pistons and connecting rods.
Old is on the left.


Old piston and rod assembly = 1.7oz
New piston and rod assembly= 5.8oz
The new piston is made of cast iron and is 0.250 longer than the original.
The connecting rod is made of brass and is 1/4" wider at the base with a 1/2" long tail and is 5/16" thick instead of 1/4".

This change along with the removal of the flywheel weights has made a tremendous difference but wait I'm not done yet.
I think I can make it even better so stay tuned.

Thanks for looking
Ray

 

[Steamchick]

Glad it works for you.
An unconventional solution to the problem, as I should have followed "Standard practice" and reduced mass for the rods to the top piston (they look huge to my eye) then balanced the difference on the flywheel.
But what works, works.
It is only a model, not subject to commercial constraints.
To get such a model running means you have made some good accurate parts, which some of us find difficult. (E.g. me!).
Well done,
K2

 

[Steamchick]

OOPS! = Message on wrong thread! - Just ignore me for a moment...
K2

 

[Rdean33422]

As you can see I have raised the engine up 1" to allow for these new 6" flywheels. The gas tank was moved up also and the ignition system was rearranged with an on/off switch.




It seems that every time I make a crankshaft I try to make it longer than I need so there won't have any problems later but I still run out of room on one end or the other. Same problem here on the cam drive side but I was able to move the cam and the drive over to make room.

The new flywheels have made an even bigger difference than the piston and rod did. The combined changes to the engine are getting closer to the way I want the engine to run.

There is one more thing I want to do before I make another video of the engine so stay tuned.

Thanks for looking
Ray

 

[Steamchick]

Hi Ray, Just an observation (NOT criticism as you are doing a very good job of developing your knowledge of engine balancing here! - As well as making an interesting engine work well.):
You have made the lower piston moving mass equal to the upper piston moving mass - so have balanced the Primary Balance.
The Secondary balance is the "side-to-side" motion of the masses attached to the cranks. I.E. the rods, crank-shaft big-ends, and bearings, and bolts... - But not the cross-head stuff and top piston.
Now the "catch" is that the centre of gravity of these parts moves "side-to-side" with an amplitude related to only a part of the crank-throw, depending on the position of this centre of gravity along the con-rod length.
I reckon - just from looking at your lower con-rod with large mass below the big-end bearing - that the C-of-G of the lower secondary mass will be relatively closer to the big-end than the C-of-G of the upper set of rods.... so the amplitude of these different masses will not be the same. I say "different masses", because on the primary balance you have made a more massive lower piston than the upper piston, and the upper piston has the straight-running con-rod and cross-head mass to compliment it. BUT I don't know all the weights of these bits, so assume there is a difference in the "linear" moving masses that have no lateral movement.
I'm finding this a bit hard to explain, but you may be lucky, and have balanced the lateral motions adequately, but I really have no idea. It is a bit complex!
Can you detect any lateral vibration? (rocking motion?).
K2

 

[Rdean33422]

I am very happy with the way this engine turned out so without further ado here it is finished.



If you look at the first video I made you can see that I had the engine held down to the table with an aluminum strap and a bolt.
The closeup of the fuel tank now shows almost no vibration and the engine shows no sign of movement and is not held in any way.
The engine is running at 900rpms in the video. It will run slower but then it doesn't fire consistently so if it likes 900 for idle then that's fine with me.

Changes
I had to move the fuel tank over 1/2" because it was in the way for hand starting as I kept catching my hand on the tank.
The weight on the carburetor throttle arm was too light and the engine would speed up by itself so I made a heavier one.
I lowered the compression ratio to just over 5 to 1. That didn't seem to make any difference as to the running of the engine but it is much easier to hand crank now.
You can see that there is a cover on top of the cylinder. I made that to help soften the clapping sound that the upper piston parts make while running. It does help to soften the noise but I am not happy just leaving it unfinished. I need some ideas for a finish for the plastic part. The only thing I have come up with is some color of paint probably black.

Thanks for looking
Ray

 

[Sparky_SC]

That is really sweet ! I have followed a few of your engines now and your work is very impressive.

 

[Rdean33422]

Thank you Doug I am following your build too.

Ray

 

[minh-thanh]

Hi Ray !
Personally, I like something different, strange, unique, your project completely conquered me
Great project, a beautiful and well running engine
Congratulations and thanks for sharing !

 

[Rdean33422]

Thank you Minh your comment is much appreciated.

Ray