Oscillating I.C. Engine

[Brian Rupnow]

Bazmac---Mainly because I don't expect changes at the 5% level. 10%--maybe,20% very probably.

 

[Charles Lamont]

Isn't the most effective way to reduce the flywheel's moment of inertia going to be to reduce the outside diameter?

Have you tried how it responds under load?

 

[Brian Rupnow]

Isn't the most effective way to reduce the flywheel's moment of inertia going to be to reduce the outside diameter?

Have you tried how it responds under load?

Charles--You are correct about the outer rim of the flywheel doing most of the work. that is why I am reducing the weight of the outer rim. I have applied a load to the engine while it runs (by applying thumb pressure against the flywheel)----This makes it "hit" continuously, with no "miss" cycles.

 

[Brian Rupnow]

Positive results--With one side of the outer rim machined as per my previous post, (removing approximately 10.5% of the rims weight), the engine now fires 3 times before it goes into "miss" mode. It was firing 4 times before going into miss mode before I machined one side of the flywheel. The flywheel is now back up on the lathe, removing the same amount of material from the other side.

 

[Brian Rupnow]

With the other side of the flywheel machined away, for a total weight reduction of about 21.5%, I don't at this time see a heck of a lot of difference in the hitting and missing. BOO---HISS!!! This engine has totally different characteristics than my water cooled hit and miss engines. The water cooled engines stay very stable in their set-up demands, requiring that the fuel mixture needs to be slightly leaned out as they warm up, but that is all. This engine gets hot.---Hot enough to burn my fingers with the cylinder. And as the heat gradient changes, so do the set-up parameters. I now have to wait half an hour for it to cool down completely before I mess with it anymore. One thing I know---The little 1/16 cross section x 1/4" i.d. o-ring that I have between the oscillating cylinder and the support bearing is only butyl rubber, and it is probably degrading very rapidly from the exhaust gas passing over it. I may go over today and get a Viton o-ring to replace it. That may help the engine stability.

 

[Brian Rupnow]

I've spent the last few days studying relationships!! No, not that kind of relationships, ya dirty buggers!! The relationship between engine speed, flywheel weight, and governor spring strengths. The heavier a flywheel is, then the greater a "kick" it needs to bring it up to a speed fast enough for the governors to come into play. That is why my engine had to fire five or six times in a row, each time giving another kick to the flywheel until it was spinning fast enough to trip the governors. At this time there were two ways I could have gone--#1-A lighter governor spring, or #2-a lighter flywheel. I chose to go with the lighter flywheel option, and after much dialing and tuning and disassembly/reassembly, I have it tuned to a point where my engine fires only two and sometimes 3 times before the flywheel is going fast enough for the governors to kick in. Also, I note that with the lighter flywheel, the engine runs at a faster speed than previously. I could shave some more weight off the flywheel, but then we get into the opposite side of the equation--The flywheel has to be heavy enough to carry the engine through 5 or 6 "miss" cycles, and still have enough inertia to compress a charge of fuel for firing after the governors disengage. I don't want to take more off my flywheel and have it too light. It is a lot easier to take metal off than it is to put it back on. Another option comes into play here, that I'm thinking about. The faster the flywheel is running, then the closer it is to the governor "trip point", and the less "kick" it needs to speed up to trip the governors.---However, that defeats the "I want my engine to run slowly" side of things. It's easier for me to change the governor spring than it is to remove and machine more off the flywheel, and at least with changing springs it is easier to "undo the change" than it is if too much weight is taken out of the flywheel.

 

[Brian Rupnow]

I am getting very encouraging results by cutting off one coil at a time from the governor spring. The good news is that it is a standard spring from Brafasco, and if I cut it back too far, there are many more of the same where it come from.

 

[gbritnell]

Brian,
As you stated the flywheel mass needs to be great enough to overcome the compression on the compression stroke. There was mention of increasing the compression to give the engine more power but if you were to go that route then you're increasing the problem that you already have. An engine will run probably all the way down to about 4:1 compression. It's not all a matter of ratio but of cylinder/piston sealing. I don't know what your designed compression ratio is but by possibly reducing it you accomplish what I stated in the first sentence. This would just be a matter of making a piston with less material above the wrist pin. It's much easier to make a piston than to take too much off the flywheel and then have to make another.
gbritnell

 

[grapegro]

Hello Brian,
Working from memory, the govenor system you are using is a sliding block or sleeve. When you shorten the goveror spring, could it be that there is a sideways effect that could not allow the sliding movement freedom
Norm

 

[Brian Rupnow]

As designed, this engine has 4.5:1 compression ratio. Norm--the governor spring sets in a cavity between two hinged pieces.

 

[Brian Rupnow]

One final video before I leave this thread behind. I added an anti-backflow valve into the fuel line and shaved away close to 50% of the original flywheel rim thickness to get to the state you see in this final video. ----Brian
[ame]https://www.youtube.com/watch?v=oBSHanHaSRU&feature=youtu.be[/ame]

 

[canadianhorsepower]

Like you say a little bit fast:hDe:
but by the sound of it, it's pretty stable on the number
of hit and the number of miss Thm:Thm:

Cheers

 

[ShopShoe]

That was quite a project to follow.

As always, thanks for posting the design process with explanation, the machine and build process complete with a rethink or two, then the excellent video of another fun-to-watch running engine.

I also enjoyed the detour as you got your new mill set up.

--ShopShoe

 

[Brian Rupnow]

Thought for the day---Imagine a rock, tied to a 10 foot length of string. Hold one end of the string in your hand, and swing the rock around in a horizontal plane. Once you get it going, you can swing that rock pretty darn slowly, and it will keep the sting tight and maintain it's position. Change up to a five foot length of string. You can still keep the rock swinging, but you are going to have to swing it faster to maintain a tight string and keep the string horizontal. Probably not twice as fast, but definitely faster. Now change up to a 1 foot length of string. It becomes impossible to swing the rock fast enough to keep the string tight and horizontal. Whatever forces are at play here, are probably not linear. The same physics apply to model hit and miss engines. My model is a 1" bore with an approximately 5" diameter flywheel. Let's just for the heck of it, translate that to a 3" bore. That would correspond to a 15" diameter flywheel. I don't have a full size 3" bore, single flywheel hit and miss engine here to measure, but I'm pretty darn certain that the flywheel on such a model would probably be at least 24 to 30" diameter. Not everything is scaleable. I did get the engine to slow down some by retarding the ignition a bit, but not a lot slower.---Sure has been fun though!!!

 

[Brian Rupnow]

If anyone decides they would like to have a go at building this engine, then send a request to the email address in my web page (not the forum email) and I will send you a free set of my working drawings which are in Imperial (inch) dimensions.---Brian

 

[jimjam66]

That's a very generous offer, Brian! Thanks again for all you do for us amateur engineers.