A new look at Opposed Twin I.C. Engine

[Brian Rupnow]

Will it start or not? I don't really know. The engine is wearing the newest edition of cylinder heads, with the sparkplug situated more or less in the center of the cylinder. This of course means that it has new valves and valve cages, and I'm never sure if the valves are going to seal properly or not until I actually try to run the engine. With brand new ball bearings on both shafts, the engine is "stiff" to turn by hand. Of course, if it does take off and run on it's own, this stiffness goes away after the first fifteen minutes of running. I will probably be finished with my design work by tomorrow afternoon, then I will try the engine and see what happens.

 

[Brian Rupnow]

Yesterday I added 1" extensions to the sparkplugs for this engine to get the plug wires up out of the valley they sat in between the rocker arm towers. I don't know for sure that they were shorting out to the towers, but I think they may have been. I was fortunate enough to buy a couple of extra deep sockets as seen in the foreground that let me install these longer sparkplugs with no difficulty. The socket required a little machine work, but they are perfect for this job. I started a separate thread about the lengthening of the sparkplugs. I set the ignition timing yesterday, and sorted out the wiring required for the snowmobile coil which supplies spark to the plugs. Near as I can tell, the engine is ready to run.---This new camera sees far too much, but this picture shows the extended sparkplugs tightened into place in the cylinder heads.

 

[Brian Rupnow]

I didn't want to start pulling things apart, but I had to check and see if my valves were sealing or not. That required that I make a new "blow yer guts out" tool to screw into my engines sparkplug holes. Of course nothing is easy--The tool I have for this wouldn't fit down between the rocker towers, so I had to make a new longer tool. By watching the rocker arms and turning the engine by hand, you can tell when the cylinder you are testing is up on compression stroke. If both valves are sealing properly, you won't be able to blow at all. If you can blow, then you can determine if the air is coming out the intake or exhaust valves. If you can blow but air isn't coming out the carburetor or the exhaust stack, then your rings are bad. One cylinder is sealed tight.--No air escaping at all. The other cylinder is leaking a very small amount from the intake valve, but not enough to keep the engine from firing. I tried to start the engine this morning, but it wouldn't fire at all. My "stretched" sparkplugs aren't shorting out to the rocker towers, but they are working---I checked this out early this morning with the sparkplugs laying out on top of the cylinders----lot's of spark, where it should be.

 

[Brian Rupnow]

I pulled the one cylinder head off, and relapped the offending valve so it was air tight when closed. I have a clue as to what is happening. There is a transparent fuel line between the carburetor and the gas tank. On an engine which has proper valve timing, if you crank the engine while holding your finger over the carburetor intake, you can see the fuel in the transparent line go rushing up to the carburetor. This is not happening for me. In fact it is chasing the fuel back from the carburetor back into the tank. The only thing that can do that is incorrect valve timing. Time to get out my degree wheel and do some timing magic.

 

[animal12]

you made your own " leak down tester " . A very valuable tool for a mechanic .
animal

 

[Brian Rupnow]

I finally had to resort to using the old cylinder heads with the sparkplugs located right at the very bottom of the cylinders. There was just no way I could get the engine to run with my reworked cylinder heads which moved the sparkplug up to the center of the cylinders. Finally, today, after a lot of fuss and bother, the engine ran for it's first time on it's own. It isn't running good, but it's running. My next labour will be to get it fine tuned and idling slower. There has been more work to this than I thought there would be.---Brian

 

[Brian Rupnow]

The engine is running good and starting very easily. The only issue is that I can't coax it into a slow idle. Research shows that a bit of advance on the valve timing will lead to a steadier, more stable idle, but top end rpm will suffer. I am not that interested in reaching a higher rpm, but I do like the slow idle very much. The cam design I am using asks that the cam be set to just start opening the exhaust valve at 40 degrees before the piston reaches bottom dead center. I set everything up on spec, but needed to verify that my valve timing was correct. 40 degrees before bottom dead center is 140 degrees after top dead center. So---accurate measurement should show the valve beginning to open at 140 degrees after top dead center. However, when set up with a timing disc and the end of the dial indicator resting on the end of the rocker arm to clearly identify when the exhaust valve begins to open, it is reading movement beginning at 125 degrees after top dead center. That is too much advance. It makes for a very interesting set-up, so I thought I would post a picture.---Brian

 

[Steamchick]

Hi Brian, having tinkered with valve timing on Triumph twins, of 1950s vintage, you must be very careful to set the tappet clearance correctly. Basically, a few thou extra - or less - clearance at the tappet can move the contact point for valve lift 10 degrees or so earlier or later. The key is to measure the point of start of lift and also the point of valve closure (start of lift in the opposite direction?) in degrees from your datum = TDC.
To make an engine "more suitable" for slow running, the valve opening timing shall be later (bigger angle after TDC - when everything is defined 0 - 360 degrees) than for high revs. All related to TIME, so the gases move at their optimum TIME through the open valves. So I think you should retard the valve timing 5 degrees or so and see if the idle is better?
I assume you have a cam-shaft with both inlet and exhaust cam? The Triumph engines I played with had separate cams for inlet and exhaust. Modern cars have twin cams and usually have an advance device on the INLET cam for 3500rpm and above (varies with engine design, but that is typical). Basically, the cam is timed for low speed intake, then advanced 5~10 degrees for higher revs. Rarely is the exhaust cam altered... as the pressure differences across the valve port when pumping gases out at the end of stroke are much higher than those on the inlet, so the exhaust is less sensitive to timing.
Thinking of your statements about "advancing timing of the exhaust" for a smoother idle, I don't really understand "how" it does that? Considering at idle you have the least fuel = least power developed, I would have expected to need to maximise the expansion of the gases before opening the exhaust valve, so as to extract maximum energy from the small amount available..? - I.E. a later opening exhaust valve than for higher revs. Anyway, 125 degrees after TDC for the exhaust opening sounds too far advanced - from where I am sitting!
Have fun and let us know the result,
Regards,
K2

 

[Brian Rupnow]

So, this is the final final. I made a nice video, and forgot to mention that I had not only changed the base, but had also put sealed ball bearings on both the crankshaft and camshaft. Strangely enough, I couldn't get it to run the way I wanted with my Traxxas 4033 carb, so at the last moment I used a carburetor that was originally designed by Malcolm Stride, and that seemed to do the trick. A person could tune away forever trying to get a slower idle, and yes, it will idle much slower than seen in the video, but it gets a bit unstable at very low idle and will quit right in the middle of making a video. I'm happy to have it up and running again, and yes, I will sell a complete, up to date set of plans for $25 Canadian funds paid to Paypal to [email protected] This engine uses a single set of Dodge ignition points with a dual output two cylinder snowmobile coil. I can take pictures of the ignition set-up I used but really, if you build this engine chances are you will be using a different coil set-up than I used anyways. Thanks for following the build.---Brian

 

[ShopShoe]

Brian,

I like the way that runs now. I have to say again that I admire your stick-to-it-iveness.

I like this engine from a design standpoint and I think you have done an outstanding job. I also like your choice of nice idle over screaming speed.

Thank You for posting, as always.

--ShopShoe

P.S.: Have you ever considered a design with more cylinders? Just asking. At least one curious mind wants to mull over the possibilities.

--SS

 

[CFLBob]

P.S.: Have you ever considered a design with more cylinders? Just asking. At least one curious mind wants to mull over the possibilities.

I'll second that. My plate's kinda full now, but always thinking about what's next.

 

[Jasonb]

Well this engine does have some of it's parentage back in Malcom Strides' Bobcat and Lynx engines. He had also started to draw up a V6 and straight 4 that were to use the same cylinder & head parts so there is a starting point if you were tempted Brian. I think I'm probably the only one with his initial drawings for these which are little more than a basic block and Schillings type crankshafts..

 

[Brian Rupnow]

I have thought of making multiple cylinder engines, but that's not really my thing. I'm happy with the one or two cylinder engines. All of my machines are manual, no CNC . -----Brian

 

[Nerd1000]

Hi Brian, having tinkered with valve timing on Triumph twins, of 1950s vintage, you must be very careful to set the tappet clearance correctly. Basically, a few thou extra - or less - clearance at the tappet can move the contact point for valve lift 10 degrees or so earlier or later. The key is to measure the point of start of lift and also the point of valve closure (start of lift in the opposite direction?) in degrees from your datum = TDC.
To make an engine "more suitable" for slow running, the valve opening timing shall be later (bigger angle after TDC - when everything is defined 0 - 360 degrees) than for high revs. All related to TIME, so the gases move at their optimum TIME through the open valves. So I think you should retard the valve timing 5 degrees or so and see if the idle is better?
I assume you have a cam-shaft with both inlet and exhaust cam? The Triumph engines I played with had separate cams for inlet and exhaust. Modern cars have twin cams and usually have an advance device on the INLET cam for 3500rpm and above (varies with engine design, but that is typical). Basically, the cam is timed for low speed intake, then advanced 5~10 degrees for higher revs. Rarely is the exhaust cam altered... as the pressure differences across the valve port when pumping gases out at the end of stroke are much higher than those on the inlet, so the exhaust is less sensitive to timing.
Thinking of your statements about "advancing timing of the exhaust" for a smoother idle, I don't really understand "how" it does that? Considering at idle you have the least fuel = least power developed, I would have expected to need to maximise the expansion of the gases before opening the exhaust valve, so as to extract maximum energy from the small amount available..? - I.E. a later opening exhaust valve than for higher revs. Anyway, 125 degrees after TDC for the exhaust opening sounds too far advanced - from where I am sitting!
Have fun and let us know the result,
Regards,
K2

My understanding is that the point where the inlet valve closes (some time after BDC) has the biggest impact on running characteristics, so (on a pushrod or SOHC engine) within reason you can move all timing events forward or back to fine tune it with an existing cam. Advancing? (unsure what the correct way of saying it is) the cam will make the inlet close later in the cycle, favouring higher RPM. Of course this messes with the other timing events at the same time so it's nowhere near as good as having DOHC and adjusting the inlet timing independently.

 

[Brian Rupnow]

Graham Vickery---Email me at [email protected] I've got your drawings but don't know your email to send them to.---Brian

 

[IC-man]

Graham Vickery---Email me at [email protected] I've got your drawings but don't know your email to send them to.---Brian

Hi Brian,
Email address is:-
[email protected]
Thanks Brian.

 

[Brian Rupnow]

The first set of 60 .pdf drawings has been sent out to Graham Vickery. This is a fairly complex engine, and I expect Mr. Vickery to have a few questions which I will attempt to answer for him. This engine is a bit more complex than what I usually send out, because it was first designed 5 or 6 years ago, then there were some design changes made about two years ago when I was trying out different cylinder heads, and then the final design changes this year when I made a much shorter base and added ball bearings to the crankshaft and camshaft. Of course the trick for me is to send out only drawings that refer to the current engine as it sets. Good luck Graham, may the force be with you!!!---Brian

 

[Bill Lawson]

Brian, what is your next project? Have you considered an opposed single cylinder dual piston engine similar to a Fairbanks Morse marine engine?
Thanks Bill.

 

[Brian Rupnow]

Bill---I already have a dual opposed piston engine that I built many years ago. I'll try and find a link for you.---Brian
https://www.homemodelenginemachinist.com/threads/a-different-opposed-piston-engine.22399/

 

[Brian Rupnow]

Today I was getting pretty bored with my self imposed "Do Nothingness", so I snuck down to my machine shop and made an "Air Intake Horn" for the carburetor. This really adds nothing to the performance of the engine, it just looks neat. Many years ago I actually bought one of these for a Sno-Jet snowmobile I owned, and was sucked in by the salesman who told me about the performance enhansing properties of such an item. It didn't do a damn thing for performance, but if you sat to close to the engine it would suck in your snowmobile suit and stall the engine.---But----It did look neat.