Single Cylinder Opposed 2 Piston Engine

[Rdean33422]

Back in the lathe for some cooling fins and a good clean up.


I like shiny things don't you?


Thanks for looking
Ray

 

[Rdean33422]

Back on the engine today after a project for the wife the last couple of days.

I thought it was time the engine stood up by itself so I cut out these feet.


All put together and 10" high but it will need to go higher yet.


Cast Iron liner is next.

Thanks for looking
Ray

 

[Rdean33422]

Turned down a piece of cast iron to fit inside the cylinder and cut to length.


The liner is 5 inches long so started with small drill bits and kept working my way up until I could get a large enough hole for the big boring bar to get through.


This bar has a 3/4" shank and made good time out to the final size.


Now to cleanup the mess.

Thanks for looking
Ray

 

[Roger B]

My cylinder liner is just over 4" long but at 20mm bore I had to cut from both ends so the 'step' was in the combustion chamber.

 

[Rdean33422]

I installed the liner and drilled and taped for the intake valve, the exhaust valve, and the spark plug.
I have given this a lot of thought as to just how I am going to support the upper piston through out it's up and down travel. It also has to be kept from tilting sideways and turning.
This is what I am going to try first and it may not work, or it may wear the sides of the slot or bind up. We will just have to see.

1/2" end mill centered on the cylinder bore to a depth of 1.25"


Well that slot worked out very nicely.
The lower part of the slot extends into the cast iron liner.


So here is the upper piston, connecting rod bolt, cross bar and brass side retaining stops.
I wanted the connecting rod to be adjustable in order to be able to adjust the piston and in turn the compression ratio.



It moves smoothly now but the true test will be when the outer connecting rods and the o-rings are installed.

Thanks for looking
Ray

 

[Rdean33422]

I cut out the two outer connecting rods and installed the bearings in the ends.


Cutting relief slots in the end of the arms for the rod cap bolts.


This is how I drilled the holes for the rod bolts.
It may look a little dodgy but the holes came out straight and true.


Thanks for looking
Ray

 

[Rdean33422]

I sawed through the center of the rod bearings, enlarged the holes in the caps for 4-40 cap screws, threaded the rod ends, reassembled and drilled the rod bearings to size. Then I gave them a little shine.


Here they are installed on the engine. I did have some minor fitting to do on the rods but now they move smoothly. I don't have the o-rings installed yet so things might change but for now all is good.


Thanks for looking
Ray

 

[Rdean33422]

I think two 4" flywheels will be best for this model.
I could make up several reasons for the smaller size being better but really I have the material for the 4" wheels so 4" it will be.


Aluminum inside.


Pressed together, cleaned up, with split tapered retainers.


Next will be to give the flywheels a good coat of paint and the retainers a little shine.

Thanks for looking
Ray

 

[Rdean33422]

The engine is still too low so I made some spacers to allow for the flywheels.


The paint is not completely dry yet on the flywheels. Maybe tomorrow.

Thanks for looking
Ray

 

[Steamchick]

Hi Ray, I'm sure you have a nice precise set-up to make the flywheels and then press in the aluminium centre - finally to set the shaft hole.... - But did it need "trueing-up" after finally making the shaft hole? - Mine always do, so I leave everything oversize and finish the flywheel on a mandrel that is machined in the lathe, then the unfinished flywheel set upon it using the shaft hole as a datum, for final machining.
The other thing I have done, is used castings for flywheels, and where unbalanced, used the natural cast imbalance to compensate for piston, con-rod, etc. balance. A well balanced engine can run much slower in my experience of steam engines. What will you do to balance this engine? I am fascinated by the configuration. - Like a Doxford marine engine?

K2

 

[Rdean33422]

Yes my flywheels usually do need trueing-up to some degree.

One of my main concerns on building this engine was just how to balance the difference in weight of the lower and upper piston assemblies. You now have me thinking of making flywheels that are unbalanced to compensate for the weight difference.
I will probably continue with the flywheels I have now as a test to see just how much offset is needed.

Thank you

Ray

 

[Roger B]

The devil is very much in the details. The main reciprocating forces are fairly well balanced, the pull rods can be quite thin section as they are loaded in tension and don't have significant buckling forces. The Junkers engine I am building has a shorter stroke for the upper (induction) piston which allows the exhaust ports to open earlier for better scavenging and also improves the balance.

 

[Steamchick]

Ray, you can send me a direct message if you want help working out balance weight values. I have a good book on the subject.
K2

 

[Rdean33422]

Roger that looks nice.

K2 thank you and I may do that later.

I installed the flywheels and the o-rings and made several other adjustments today.
Everything is still turning without any binding.


Valves will be next but not for several days as I have other duties.

Thanks for looking
Ray

 

[Steamchick]

Brilliant work!
I'll be interested to know your plans (if any?) for lubrication of bearings and cylinders? Lots of little oil pots?
K2

 

[Rdean33422]

The main bearing are sealed ball bearings. The connecting rod ends are of a special impregnated bronze with 90 weight gear oil.
A drop of oil on each bearing has proven to be satisfactory.
They are sold by McMaster Carr.

Ray

 

[Nerd1000]

The devil is very much in the details. The main reciprocating forces are fairly well balanced, the pull rods can be quite thin section as they are loaded in tension and don't have significant buckling forces. The Junkers engine I am building has a shorter stroke for the upper (induction) piston which allows the exhaust ports to open earlier for better scavenging and also improves the balance.

Wouldn't you offset the exhaust piston crankpins a bit so the exhaust opens first but closes around the same time as the intake?

I suppose that makes the balance issue even stranger.

 

[Steamchick]

Sorry Nerd, I didn't understand that comment. I understand the pair of rods to the top piston to be 180 degrees from the crankpin of the bottom rod n piston. - That's wjat makes this engine work. The valves, timing etc. are separate to Crank pins.
Or have I mixed the Doxford opposed piston engine design with something else?
I understand that modern Ship engines like this are Blown 2-strokes with a separate valve operation to open and blow in the central cylinder port from the supercharger/blower, with valve timed just after the exhaust port (near end of stroke of each piston) has opened, then the inlet valve stays open until after the exhaust port closes. But that is with inlet valve timing from a cam, not one of the cranks. So should not affect the Crank balance...
But the Fairbanks-Morse engine (picture attached) has inlet port at one end of the expanded combustion chamber and Exhaust ports at the other end. So maybe this is what you are referring to? - Except this doesn't need any offset from the 180degree crank arrangement either? - I am confused? - Or wrong?
The Fairbanks-Morse uses a pair of crankshafts, so maybe they are not timed at 180degrees separation? (looking closely at the pictures). - This is all new to me, so I am just a beginner - learning!
So the cylinder ports can be timed by the crank pin timing - a way from 180degrees - as you are suggesting? (IF I understand your ideas?). What did Fairbanks-Morse do?
Perhaps Ray can explain his engine when he returns from his other duties in a few days? Meanwhile we can chat among ourselves - if you wish?
Ken

 

[Roger B]

The Junkers Jumo 204/205aircraft diesel engines were a twin crankshaft design and had the two crankshafts offset by, if I remember correctly, 9°.

 

[Nerd1000]

Sorry Nerd, I didn't understand that comment. I understand the pair of rods to the top piston to be 180 degrees from the crankpin of the bottom rod n piston. - That's wjat makes this engine work. The valves, timing etc. are separate to Crank pins.
Or have I mixed the Doxford opposed piston engine design with something else?
I understand that modern Ship engines like this are Blown 2-strokes with a separate valve operation to open and blow in the central cylinder port from the supercharger/blower, with valve timed just after the exhaust port (near end of stroke of each piston) has opened, then the inlet valve stays open until after the exhaust port closes. But that is with inlet valve timing from a cam, not one of the cranks. So should not affect the Crank balance...
But the Fairbanks-Morse engine (picture attached) has inlet port at one end of the expanded combustion chamber and Exhaust ports at the other end. So maybe this is what you are referring to? - Except this doesn't need any offset from the 180degree crank arrangement either? - I am confused? - Or wrong?
The Fairbanks-Morse uses a pair of crankshafts, so maybe they are not timed at 180degrees separation? (looking closely at the pictures). - This is all new to me, so I am just a beginner - learning!
So the cylinder ports can be timed by the crank pin timing - a way from 180degrees - as you are suggesting? (IF I understand your ideas?). What did Fairbanks-Morse do? View attachment 159200
Perhaps Ray can explain his engine when he returns from his other duties in a few days? Meanwhile we can chat among ourselves - if you wish?
Ken
View attachment 159200

Basically what Roger said. 'Conventional' 2 strokes always have the problem that the exhaust port must be uncovered before the transfer port so as to blow down the exhaust pressure before scavenging begins. If you only have one piston (or two, 180 degrees out of phase) the exhaust must necessarily close after the transfer port. This has the unfortunate effect of allowing some of the charge in the cylinder to be displaced out through the exhaust before the port is covered and compression starts. If you have opposed pistons you can at least in principle arrange to have the piston that covers the exhaust ports run a bit ahead of the piston that covers the transfers, so the exhaust opens before the transfers and closes at the same time as the transfers (or even a bit before).

If you had 2 cranks you would just advance one crank relative to the other. But if you have one crank and side rods you would instead have to adjust the location of the crankpins working the side rods so that the angle between them and the middle crankpin is something other than 180 degrees.