Rotary valve leakage

[terryp]

Just finishing up my second engine build. My first was a small wobbler and I wanted a challenge so dove into a two cylinder. Hope to make it to IC some day.
Problem: The rotary valve is one of the reasons I chose this engine build as I did not feel confident in machining gears or cams. BUT that has become the issue...the valve leaks air so that the cylinders are partially charged during the exhaust stroke. This causes too much resistance to rotation during the exhaust stroke and therefore stops the engine at BDC on each cylinder. There is a brass tube pressed into the aluminum head with ports drilled for intake and exhaust. My "valve" is a stainless rod .3745" in diameter running in a .378 ID tube. Do the math and that leaves .004 for running clearance and LEAKING. Has anyone had experience with this situation? Suggestions appreciated! Terry

 

[Charles Lamont]

The leakage path round the periphery of the valve between ports is very short so the valve needs to be a good fit. The clearance needs to be more like one tenth of what you have at the moment. From the photo of the engine I thought I could understand how the timing works, but I cannot reconcile that with the positions of the ports in the brass tube. Can you explain the arrangement, please, or point us to the drawings?

 

[nx06563]

One of my first design and builds was an opposed piston design with both pistons sharing the same cylinder and creating a single combustion chamber with a single spark plug. 'The original plan was to use cogged timing belts to eliminate gears. I tried to use a rotary intake and exhaust valve at the start.
I was also using a bubble carburator. I couldn't get the rotary valve to seal well enough to keep from blowback thru the bubble carburator. I gave up on the combination valve like you are using and went to an atmospheric valve for the intake. one problem solved.
Next problem was I couldn't get compression because of the exhaust valve leakage. After several tries I chucked my exhaust valve shaft in the drill and made a slightly small housing for it. Then using lapping compound I lapped them to a close tolerance fit.
Ok Now I get the thing to try to run halfway and every time it fires really strong my timing belts slip a few teeth and its done running, hence the plastic timing gears shown in the pictures.
I did finally get it to run-But is was very sensitive to the bubble carb setting and barely had enough power to keep running.
Needless to say I have a whole plastic pan of parts from failed efforts trying to fix issues.
Dont give up just keep your tolerances as tight as possible

 

[terryp]

Charles, I had a hard time understanding the valve as well so the first was made out of a 3/8 wood dowel rod. It actually worked probably due to the reduced clearances. Found this engine design on line about 3 years ago and since learned it came from a 1960 vintage Popular Mechanics mag. Using that plan as a basic model my engine is 1.5X larger and sports several design enhancements. Printed below are the pictorial explanations of the valve operation. As stated above, the problem is leakage into the cylinder during the exhaust stroke. Too short a path and loose fit are definitely the culprit here. I am considering remaking the valve out of Delrin for a closer fit but a good bearing surface without binding. Comments? Thanks, Terry

 

[terryp]

One of my first design and builds was an opposed piston design with both pistons sharing the same cylinder and creating a single combustion chamber with a single spark plug. 'The original plan was to use cogged timing belts to eliminate gears. I tried to use a rotary intake and exhaust valve at the start.
I was also using a bubble carburator. I couldn't get the rotary valve to seal well enough to keep from blowback thru the bubble carburator. I gave up on the combination valve like you are using and went to an atmospheric valve for the intake. one problem solved.
Next problem was I couldn't get compression because of the exhaust valve leakage. After several tries I chucked my exhaust valve shaft in the drill and made a slightly small housing for it. Then using lapping compound I lapped them to a close tolerance fit.
Ok Now I get the thing to try to run halfway and every time it fires really strong my timing belts slip a few teeth and its done running, hence the plastic timing gears shown in the pictures.
I did finally get it to run-But is was very sensitive to the bubble carb setting and barely had enough power to keep running.
Needless to say I have a whole plastic pan of parts from failed efforts trying to fix issues.
Dont give up just keep your tolerances as tight as possible

 

[terryp]

nx06563, Thanks for the reply. My goal is to experience myself into an IC engine and I see you are already there. Congratulations. I also appreciate different and non conventional designs and yours definitely qualifies. Do both pistons meet at TDC?
I suppose the easy way out of my dilema would be to shelf my engine and label it "non running learning experience" but that's not me, especially since I have over one year of my life invested in this project. Most likely will lay awake at night thinking of a solution. I believe the main problem here is the .004" clearance between the valve and head so may make a new valve from Delrin for a tighter fit without adding resistance. It sure looks good, though! Terry

 

[nx06563]

Dont worry about a lost year. The one I described took at least a year to get running since I only work on them in the winter. My latest design and build has been going on for two years but I leave in the winter for AZ. and only work on it when the weather is crappy in Iowa in the spring summer and fall.
The engine I described had a combustion chamber between the two pistons at TDC which gave it a compression ratio of about 8 to 1 if I remember right.
Good luck

 

[terryp]

Ha! At my age I worry about lost minutes! Always feel bad about scrapping a part but it's more fun than Sudoku.

 

[terryp]

Well all progress has stopped as it appears after 35 years in this house we are moving! Lots of work to do in the new place including an awesome shop. More to follow but that has become my new full time job. Whew! Moving the shop and machinery will be a huge challenge.

 

[stanstocker]

A better shop is great, but having just moved I don't think I could handle it again! At least for a few years Best of luck and take care of your back and knees!

 

[Shopgeezer]

I groan when I think about moving again. Done it too many times. The old saying “three moves as good as a fire” is very true. Move all your tools yourself. I discovered that the people who come and box everything for a moving company like to take souvenirs. Like an outboard motor and every tool box that looked interesting.