Flywheel Foolishness

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Brian Rupnow

Design Engineer
Project of the Month Winner
May 23, 2008
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Barrie, Ontario, Canada
I'm bored today, and that is not a good thing. I haven't built much of anything since Christmas, but I woke up this morning thinking about a flat-head hit and miss engine. I did a web search and found a few flat-head Briggs and Stratton engines that had been modified to be hit and miss engines, but no "real" air cooled hit and miss flathead engines from "back in the day". Jason was kind enough to send me some information on water cooled flat-head engines, but the flat head was hidden by a ball shaped water reservoir. Okay---if I build one, it has to be air cooled, and I don't want to run a separate belt driven fan. This takes me back to my air cooled twin cylinder which was cooled by a flywheel with vanes attached to the outside, running inside a metal shroud so it wouldn't become a meat chopper. This time I want a flywheel with something similar to vanes, but with the outer shroud built into the flywheel. So---If I started with a piece of 6" double strong pipe x 1" long, I could make the outer rim from steel something like this, and it would be 6 5/8" outside diameter.
Then, I could build a center hub out of steel, mainly because I'm not fond of using aluminum for hubs because it gets all chewed up by the keyway and the set screws don't stay tight in aluminum.
The center disc of the flywheel can be aluminum. It doesn't really do anything except hold the outer rim and the hub together. All those small holes are for #10 socket head cap screws. The big holes are 1" diameter.
And when it's all bolted together, it looks like this. The easiest way to attach the 1" diameter fan blades in place is to weld them. The last time I tied to weld aluminum with my tig I just made a horrible mess, but these parts are a lot thinner and don't need a lot of weld to keep them in place. This will move a surprising amount of air, which will be directed over the cylinder, and hopefully keep the engine cool.--particularly if the engine is a hit and miss, because all the time the engine is missing, it is pumping ambient temperature air thru the cylinder and out the exhaust.
There are other, different ways to hold those red "vanes" in place, but they are all rather horrible. Welding is the cheapest and easiest, as long as I don't get too excited with my tig welder and melt everything!!!
I'm a flywheel guy too.
I like your idea!
Making the whole thing out of steel would make the welding easier.
Zeb--Stuff like this that is all turned on a lathe is pretty well balanced when it's assembled. Scott---you know, I never even thought of that. Now that I have the tig welder, I'm always thinking of ways to use it. Aluminum weighs almost exactly 1/3 of steel, but the disc is thin so there wouldn't be much weight penalty if I made it all from steel and used my mig welder.
I was thinking it might get a little wobble after welding the louvers. Nothing a briggs couldn't handle though.
I picked up a steel flywheel last weekend at an antique shop. You can never have too many!
In general, cast flywheels cost a lot if you buy them in Canada. They are marginally cheaper in USA but because of their weight and the difference in our dollar values, they still cost outrageously. I have made flywheels from brass---they look wonderful, but the price of brass here has skyrocketed out of reach for hobby machinists. I have made them from steel, but it's a chore hogging out the recess on each side. Lately I have been making them from aluminum with a steel outer rim, which looks and works fine, but the aluminum bores get chewed up quite badly from keys, and the set screws don't want to stay tightened without Loctite. I don't do any metal casting, I machine everything from bar stock or round stock.
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If you have a rotary table, what about making the center solid and then make your 1" holes. Leave a small tab on the inboard and outboard circumference and then simply bend the halves to form louvers.
Make any sense ?

Can't help thinking you are not going to get a lot of air through those holes and what you do is going to line up with the crankcase rather than the head so even less cooling effect. You have said before you don't like doing shrouds and sheet metal so can't even duct the air to where it would have more effect

You would get more air through it if you milled out pockets to leave "spokes" and welded vanes to them but still only going to blow over the crankcase..


Or go for the Maytag look where the fan blades are right at the outside and the rim is really just to contain the blades. Simple enough with a larger say 4" steel hub, some 8" x 1/8" wall tube and some 1" x 1/8" blades. Not only far more area to move air but having the blades further from the ctr gets the air moving over the cylinder. keep the engine layout compact too with the cylinder as close as possible to the crankshaft so it is within the area covered by the fan

fan 2.JPG

Any wobble due to distortion is easily dealt with by leaving the rim a little oversize and the hub bore undersize, Just stick it in the lathe once fabricated and assembled and bore the hub and skim the rim face & sides
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Lots of ideas, but you need very little air to extract a lot of heat. And the more air you "pump", the more "hitting" and less "missing" will occur. A carefully placed deflector should redirect the air-low, without complicated ducting? The more firing (burning fuel) the more waste heat needs to be pumped away, whereas the less firing (with less drag from the fan) the less heat needs to be pumped away = less fan... So I guess Brian has it about right for a novel solution, as a starter - which he will improve as required - rather than any more complex and over engineered solutions?
Think about the crux of pleasure off an Hit n Miss engine, it's the dwell caused by "no load" on the engine = no firing. An alternative could be a simple venturi from the exhaust dragging the cooling air (after it has passed the cylinder) away. But this would be a ducted forced draught, which is what Brian want to avoid.
Another alternative - but it needs water - would be a "drip" cooler... where drips of water are dropped onto a wick around the outside of the combustion chamber, to "steam-off" and extract heat by evaporation (latent heat of vaporisation). If you created a directed exhaust to cause a syphon to lift water into a spray that blows onto the wick... so water is only supplied when the engine "hits" and heat is developed, then there is No extra drag during "miss" cycles! (when the wet-wick can evaporate the wetness from the spray into steam that wafts away...).
But good to see your brain cells working!
Well Brian's engines tend to hit a lot more than mine so he gets a lot more heat generated than mine that go for much longer between hits and also run a lot slower so that was why I was suggesting he needs the airflow where the hottest part of the engine is located. What sort of speed do your H&M engines tend to run at?

In hit and miss circles your drip coolers are usually called "Screen Cooled" where the water passes over a wire mish screen and then into a collection drum ready to be circulated around again. I've done a few engines with this option but pumped rather than thermosyphon, engines still have a good long miss period. On another forum it seemed Brian did not want a water hopper option so I guess this is out too.

You can also do what the likes of Gade did and have a primary and secondary exhaust one of which lets most of the heat of combustion out at BDC so there is less heat going out through the head and during the miss cycles the piston moves more air through the cylinder which draws away heat.
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I have a different method that will move a lot of air. I had a motor that was fan cooled. The fan was plastic and after years it broke a vane.
I turned an aluminum hub from 1-1/4" stock with a 5/8" bore.
Bored 5/8" hole in a 1/8" x 5-1/2" round aluminum that I trepanned. Bolted the disk to the hub with 8/32 SHCS.
Chucked the hub and took light cuts to make the fan round.
1/2" x 3/4" x 1/16" thick angles were pop riveted. to the disk. I installed 12 angles.
This fan pushes a lot of air, been on the motor for about 10 years .
I realize it isn't a fly wheel but could be adapted to look like one.
That sound more like a centrifugal fan that draws it's air from the middle and throws it out the edge, work fine in the usual fan shroud that you find on the end of an electric motor but won't blow axially like Brian wants and he does not like the sheet metal work that a Briggs type shroud would need
Lately I have been making them from aluminum with a steel outer rim, which looks and works fine, but the aluminum bores get chewed up quite badly from keys, and the set screws don't want to stay tightened without Loctite. I don't do any metal casting, I machine everything from bar stock or round stock.

Brian - Couldn't you turn a hub of steel and insert into the aluminum flywheel? The hub would contain the keyway and the setscrew thread.

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