Single Cylinder Opposed 2 Piston Engine

[Roger B]

This discussion isn’t really relevant to Ray’s engine as it is a four stroke but is interesting never the less. The conventional port controlled two stroke is a very simple concept but the inlet (transfer) port and the exhaust port have to be adjacent and open at the same time. It also tended to leave a volume of hot, burnt gas in the cylinder. Various port designs were developed to direct the inlet gas up into the cylinder so it purged the burnt gas and did not escape directly via the exhaust ports, which is wasteful, especially if fuel is lost this way.

To overcome some of these problems two main approaches have been used, firstly induction via ports that are open around BDC and exhaust via valves in the cylinder head. This ensures a good purging of the cylinder and allows separate control of the exhaust valve timing. Lloyd’s engine is based on this concept:

https://www.homemodelenginemachinis...diesel-56cc-2-stroke-will-it-ever-work.31110/

The other option is an opposed piston two stroke where one piston opens the inlet ports and the other opens the exhaust ports. This has the initial benefit that the inlet and exhaust are at the opposite ends of the (rather long) cylinder. It is then possible by playing with the design to open and close the ports at different times. If the engine has two crankshafts like the Fairbanks Morse that you showed or the Junkers Jumo the angle of the exhaust crankshaft can be advanced to open and close the exhaust earlier. This short article refers:

https://achatespower.com/crank-phasing-and-the-impact-on-engine-efficiency/

If the engine has a single crankshaft and side pull rods, the same mechanical design as Ray’s engine, a similar timing can be arranged by having a longer stroke for the exhaust piston than for the inlet piston. Maybe someone good at CAD animation could show this, but very basically the exhaust piston moves further than the inlet piston for the same rotation angle of the crankshaft. The set up has the added advantage that the heavier top piston assembly moves less than the lighter bottom piston and conrod and helps the balancing. This is the basis of the Junkers CLM I am building. The 65mm bore version has an inlet piston stroke of 90mm and an exhaust piston stroke of 120mm. The 85mm bore engine has strokes of 96 and 144mm.

 

[fencer]

Ray I am building the 4 stroke 2 piston and 3 con rod engine by Wolfgang Tepper from images in the Model Engineer magazine.I have finished the cylinder head but for the valve levers which is my next job.This engine has a water jacket with connecting hole through the cylinder head.Like your engine the cylinder is 4" long and after boreing I trued it up with a D bit made from silver steel/drill rod and lapped it .I will look at your construction of the crankshaft and see if it will fit ,as mine has a made up crankshaft to allow bearings to be fitted between the cranks.I will keep watching

 

[Rdean33422]

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.

I have been watching your build but didn't realize the crank had two different strokes. I haven't thought much about 2 cycle engine building yet so that is probably why I didn't catch the significance of the two different strokes.

.

Perhaps Ray can explain his engine when he returns from his other duties in a few days?

The throws on my crank are all the same length with the outside two being 180 degrees offset from the middle throw.

Ray

 

[Rdean33422]

This discussion isn’t really relevant to Ray’s engine as it is a four stroke but is interesting never the less

Yes it really is I am learning some new 2 cycle theory.

Ray I am building the 4 stroke 2 piston and 3 con rod engine by Wolfgang Tepper

I am not familiar with that engine and it sounds interesting. As you can see my crank has 4 dual main ball bearings and is an assembly. Glad to have you along.

Maybe I will have time tomorrow to get back in the shop for some more fun.

Ray

 

[Steamchick]

Thanks guys. You explanations say it all! - Very helpful!
looking forward to watching more of your progress - and finished models running!
Thanks again
K2

 

[Rdean33422]

These are the parts I made for the intake valve.


Parts installed with a brass thumb screw for the idle adjustment and a brass weight/handle to hold the throttle in the closed position.


Thanks for looking
Ray

 

[Steamchick]

Ray, when you say parts for the inlet valve, it looks to me like a carburettor, not an inlet valve? I was assuming for a 4-stroke engine this would have a poppet valve, gear/cam timed, sort of thing? Is it a rotary valve driven from that lever? - It looks to me like the lever is the throttle lever, not a valve operating lever.
Or is it an automatic inlet valve? - the long-stemmed part being the poppet valve?
Can you advise to clarify my confusion?
Thanks, K2

 

[Rdean33422]

The intake valve is atmospheric. The brass knurled knob is the idle stop screw. The brass handle is attached to the throttle arm. Raise the handle and the engine goes fast and let go of the handle and the engine returns to idle by gravity.


Made up the parts for the exhaust valve. They are almost identical to the parts for the intake valve except the carburetor is replaced with an exhaust pipe. the spring on the exhaust valve is much stiffer also.


From the beginning I was planning on using timing gears for the cam shaft well that isn't going to happen. The distance from the crankshaft center to the cam center would require at least three gears be mounted in a row and they would be of considerable size. It would deter from the looks and be very noisy. I have decided to use a timing belt setup so I went looking for parts in my unused horde. I found a 15 tooth and a 30 tooth pulley and the shortest belt I had at 55 teeth.


The pulley bores were the wrong size so I enlarged the holes and pressed in brass bushings with set screws.

I made a crude drawing which showed where I would like the camshaft to be in relation to the crankshaft and measured the center to center distance. I plugged this information into a belt length calculator and it came up with a belt of 56 teeth.
I should buy a lottery ticket today.
I'm sure I can make my 55 tooth belt work just fine.

Thanks for looking
Ray

 

[Steamchick]

Excellent! I am beginning to see how this will work now.
I bought some toothed belt pulleys and belt in miniature sizes - suited the model - to take a drive from a steam engine in a boat to the gear box and then to the prop. shaft. Was the right size according to the design book and worked well.
K2

 

[Rdean33422]

I cut out a couple of camshaft supports and pressed in the ball bearings.


Made up an idler pulley with ball bearings and got everything mounted.




Seems like a lot of work and material for just one cam lobe.

Thanks for looking
Ray

 

[Steamchick]

Ray, You can buy much smaller belt drives (used in photocopiers, and all sorts of printers, scanners, etc!). All you need is a catalogue of "miniature" belt drives. - or the RS website, or something, and a few dollars more...
You may be happier with the appearance of the model afterwards.
K2

 

[Rdean33422]

Yes K2 a smaller version may have looked better on this engine but I already had these parts.

Had to mow the lawn this morning and it got hot in the shop this afternoon so not much done today but some CNC work.

Cutting out the saddle for the exhaust valve rocker arm.


I also made a cam but no picture of the process.


Thanks for looking
Ray

 

[Rdean33422]

Another short day but I did get the cam and the saddle installed.
I also made up the rocker arm and the support.
The exhaust system is now complete.



What do you think?

Thanks for looking
Ray

 

[Steamchick]

Another Wow!

 

[Nerd1000]

Another short day but I did get the cam and the saddle installed.
I also made up the rocker arm and the support.
The exhaust system is now complete.
View attachment 159322

View attachment 159323
What do you think?

Thanks for looking
Ray

I was wondering how you'd do this, it looks good!

 

[Rdean33422]

Thanks K2 and Nerd1000

Just one picture today of the base plate for the engine. I am at the point that I want the engine to be more stable so it is easier to work on.
Nice piece of poplar with a clear finish.


Thanks for looking
Ray

 

[Rdean33422]

I have been concerned about the unbalanced upper piston assembly. I weighs 9 oz including the outer connecting rods. I am going to try to counter balance that load by adding weights to each of the flywheels. I have no way to calculate the weight of a piece of brass 0.312 thick and of this shape. So I just made them to see what they did weight.


This is what they look like installed on the flywheel.

It turns out that each weight is 2.4 ozs or a total of 4.8 oz.

It does help with the vibration but I don't think they are heavy enough being mounted at this location.
I am going to leave them like this for now as it would be possible to remake them and mount them further out on the flywheel later if needed.

Thanks for looking
Ray

 

[Rdean33422]

I put 4 rubber feet on the base and mounted the engine.


I remember now why I have been staining the engine base boards a dark color unlike this natural finish. This will really show any marks or stains and I will be kept busy wiping it down.

I made and installed a magnet wheel and the hall sensor.


Thanks for looking
Ray

 

[Steamchick]

Hi Ray, just an "impression" I get from looking at the photo of the engine. The side con-rods for the top piston look to be "huge" in cross- section. A regular con-rod, I.E. the bottom rod, will take all the compression then firing loads in compression, so needs to be a relatively substantial section to resist buckling when the firing generates the high pressure shock on top of the piston. But the pair of rods for the top piston have very little compressive load by comparison, and I think are almost always in tension, except maybe on the inlet stroke when they must accelerate the piston upwards.
Without doing any calculations, and recognising they are ravel long rods, I would have expected them to be turned-down substantially from your square section, but larger at the mid-point than at the ends.
Otherwise you will have a large mass to counterbalance. Have a look at pictures of the real engines for a clue? I am on holiday, so "too busy" with family to do much research, so may be completely wrong, but I suggest you check it out.
Mass accelerating means stress everywhere, and also mass a plane must carry. So may be worth reducing to a safe minimum?
K2

 

[Rdean33422]

Well as you know this entire engine is an experiment on my part with very little information to go on. I usually overbuild my models to compensate for lack of the structural integrity of the components but I agree the less weight there is to manage the better. Thanks

The original gas tank that I made for my six straight engine was too small so I put it on the shelf and made a larger one for that engine.
It should be about the right size for this engine so I made a 4" tall stand and mounted it on the base.



I set the ignition timing, the valve timing, and now that the fuel system is complete the only thing left to keep it from running is the rest of the ignition system.

Thanks for looking
Ray