Senft "Poppin" engine

[el gringo]

I saw one running (somewhere on this forum) with the two head design and it seemed to do well. I wonder if a thicker valve leaf could be used as well.

On the subject of the Senft cylinder head; I found mine to be leaking.
My 'test' method was to hold a finger on the port as checking the piston fit.
When the piston was pushed toward tdc it slowly crept forward due to losing compression (which it did not do during the same 'test' by covering the end of the cylinder without the head installed) . When tested with the cylinder head under water the leak was obvious.

My fix was Loctite # 518 'gasket eliminator' for aluminum flanges. It is not an adhesive and is easily removable. I have been using it out of the same container for ~ 15 years...some shelf life

Ray M

 

[Brian Rupnow]

I have had a frustrating and dreadful day. My Poppin ran yesterday long enough to get a brief video, and I haven't been able to get it to run again since. I have spent an entire day screwing around with this thing, but no joy. I also found the answer to that age old question "Will a 1/8" wick fit down a 1/8" i.d. tube.--OF COURSE NOT DAMMIT!!! That would have been too easy. I need to get away from this thing for a few days, and I just got a couple of emails about some "real" work, so I'm going to leave this set for a couple of days, before I start banging my head on a rock!!!

 

[el gringo]

Brian, check the cyl head for a leak.... " On the subject of the Senft cylinder head; I found mine to be leaking.
My 'test' method was to hold a finger on the port as checking the piston fit.
When the piston was pushed toward tdc it slowly crept forward due to losing compression (which it did not do during the same 'test' by covering the end of the cylinder without the head installed) . When tested with the cylinder head under water the leak was obvious."...

Also, did you try the previous flame arrangement?

 

[Brian Rupnow]

El Gringo--Last evening after reading your post, I did use a Loctite silicone gasket maker to seal my cylinder head.

 

[Brian Rupnow]

In trying to make an analysis of why I couldn't get the engine to run yesterday, I can think of only a couple of things. I know the timing is correct, because after my first successful run I match marked the flywheel/cam and the crankshaft so I would always be able to find my way back to that position. The only other things that can cause this engine not to run is that either the .002" valve is not sealing the hole in the cylinder head totally, or the machinable graphite piston is worn down to a point where it is not sealing. I don't really know much about machinable graphite but it does seem very soft. I may make a new piston from cast iron. I spun those flywheels at least 100 times yesterday, with no joy. The most unusual thing was that there was no where at all that the engine picked up a little bit or tried to run. That is quite noticeable when you hit the "sweet spot". I have to also rework the new dedicated burner because the 1/8" wick which came yesterday won't fit down the tube I have soldered into the top of the tank.
EDI--EDIT---
No, it isn't the piston worn down. I just rotated the .002 valve out of the way and tried to rotate the engine with my thumb held over the hole in the cylinder head. My thumb over the hole prevents the engine from turning freely. I can feel the suction and compression.

 

[Brian Rupnow]

In case you missed it--I had to change the alcohol burner wick tube. It didn't work as well as I thought it would. I had to open the bore of the wick tube up to about 0.153" so I could persuade the 1/8" wick to go into it, and it didn't seem to work very well with the tube running all the way to the bottom of the tank like I had shown it in a previous post. So--I cut the wick tube off just under the top. Lesson learned--I'll know better next time.

 

[Brian Rupnow]

Today we have one small success to feel good about. I reworked the alcohol burner as per the newest drawing posted, and it appears to work like a charm. I am running 99% medical alcohol in the burner. The flame looks very large and very yellow, but that may just indicate that I need to trim the new 1/8" wick. I see a few videos on YouTube where the flame is mostly blue and has very little yellow corona. I wonder what those guys are burning? I also picked up some 0.003" material to make a new valve.

 

[Brian Rupnow]

Well, I guess I have to acknowledge that my butt has been kicked on this one. I've tried everything I know how to do, and no good results. Timing is good, flywheels are free to spin, and the flame is good but no joy. I've tried pre-heating the cylinder but it doesn't make any difference. This is very unusual for me, but for now, I'm whipped.---Brian

 

[Brian Rupnow]

I have watched about a hundred videos of Poppin engines in the last week. In a number of cases, the spring which holds the roller in contact with the cam is a tension spring connected to the 1/16" diameter arm which supports the valve. It runs vertically from the arm down to a screw tapped into the sub base. This is not what Dr. Senft calls for on his drawings of this engine. I'm curious as to why people do this. Does it put less load on the flywheels? Does it make the engine spin more freely? Is it part of a set of plans similar to Dr. Senft's?--Or is it just that some guys had a tension spring and not a piece of 0.020" music wire to make a spring like Dr. senft calls for?

 

[ninefinger]

Brian,

don't make the spring too strong, one of the unwritten items on the Poppin is that the vacuum holds the valve shut as long as it need to be with a weak spring, too strong and it opens prematurely reducing the effectiveness of the engine and / or making it hard to run. This is why the timing on a Poppin is usually not critical- the exhaust timing is somewhat self regulating and you are only really controlling the intake closure.

Mike

 

[Cogsy]

From what I understand, isopropyl alcohol (likely your 'medical grade') does not burn as hot as denatured alcohol (ethyl alcohol) and your running issues may be as simple as that. I use methylated spirits which is about 96% ethyl alcohol with a dash of methanol to make it undrinkable. Burns nice and blue, with or without a wick.

 

[fcheslop]

Assuming you are using the same fuel when it last ran. I cannot see how it would be a problem
The spring only needs enough tension to keep the follower in light contact with the cam anymore and you are wasting some of the little energy it makes. It wont make any difference to the type
One though comes to mind as youve changed the valve simply make sure in the closed position there is very little overlap as it alters the timing if theres to much. It should only need to cover the port by a 1/64 or less
If the valve is to thick it will prevent adequate exhausting although Iv used 4 thou and prefer to use feeler gauges the metal just seems that bit better
The cylinder head with the groove in for the valve will make it crack very quickly as I found out on my second Poppin
Out of curiosity have you or anyone else come across the water cooled drawings Iv been looking for years to no avail
Well thats my two bobs worth
cheers

 

[Blogwitch]

Frazer,

The only ever water cooled one I have come across is the large Gert Litty # VST 45, which as far as I can remember I sent you the plans for a while back.

Brian,

The fuel that I used for flamelickers was the same as the one Cogsy uses, it burns with a clear light blue flame. I also used industrial methylatd spirit which is basically the same as the purple couloured meths you can buy in the shops over here except it is clear and doesn't have the anti drinking ingredient.
I have also tried using gas to very good effect on my Jan Ridders double one and also this one, a Scott, which is destined to become finished in the near future. The only problem with the Ridders one is that it started to carbon up after about 10 minutes, maybe because I didn't make a clean enough burner, just a hole in tthe end of a pipe, but it did run well on it.

[ame]https://www.youtube.com/watch?v=Mi_T60n52jg[/ame]


John

 

[Brian Rupnow]

The sun is shining---the birdies are singing--All is well at the Rupnow house!!! I finally got my Poppin to run the way I wanted. What did I do to make this happen? First of all I disconnected the valve mechanism completely and removed the con rod and piston. A flip of the flywheels would only make them run for about 15 seconds.---What?--That can't be right. I loosened off the two #2-56 bolts that clamp the straps on top of the bearings and tried again. Flywheels would now spin for over 60 seconds. I remounted the con rod and piston, and then the flywheels would spin for about 12 seconds. I remounted the valve mechanism and a flip of the flywheels would only make it free-wheel for about 3 to 4 seconds. I wasn't happy with the piece of 0.026" spring wire I had mounted as per Dr. Senft's instructions. (actually he called for 0.020" spring wire, but I was only able to get 0.026") I made up a hub for the "off" side of the cross-shaft and hooked up a very light tension spring to replace the .026" spring wire. At first the tension spring was too weak to totally open the valve, but I kept cutting off coils until it was just strong enough. that seems to have done the trick. Engine fired right up as you see in the video. I don't know what the honking and squealing noises are, but I put that down to the 0.002" thick valve. I don't care if it honks a bit.--I've got a runner!!!!!
[ame]https://www.youtube.com/watch?v=UXn6zJBG1h8&feature=youtu.be[/ame]

 

[el gringo]

"If the valve is to thick it will prevent adequate exhausting although Iv used 4 thou and prefer to use feeler gauges the metal just seems that bit better"
Would be interesting to know if anyone out there has tried heat treating shim stock to stiffen it up a tad? When I heat it to a high blue, ~ 600 f, it seems to stiffen but warps because I don't have a controllable heat source.

Ray M

 

[Brian Rupnow]

Ray--If you want stiffer shim stock, cut up a set of feeler gauges. I have had a bit of a chance to calm down since this mornings run, and set the engine up on an old textbook . That cuts down on a lot of the noise generated by the desk it is setting on (Like an echo chamber). The audio pickup on my camera is super sensitive, the engine runs much quieter than it sounds in the video. Most of the honking and squealing sounds have gone away.
[ame]https://www.youtube.com/watch?v=frwkskWA1ZU&feature=youtu.be[/ame]

 

[el gringo]

the engine runs much quieter than it sounds in the video. Most of the honking and squealing sounds have gone away.

The appraisal of the honking and squealing sounds belong in the domain of the eye of the beholder....:hDe:

I expected more of a 'poppin` sound....Looking good ,Brian
Ray M

 

[Brian Rupnow]

So, Ray--are you building one of these, and if so, what stage are you at?---Brian

 

[ShopShoe]

Brian,

As always, I knew you'd get there. That is a pretty and pretty-well-running engine.

Congratulations and thanks for posting the whole build thread.

--ShopShoe

P.S.: If I don't get around to building a whole one of these engines someday, I might copy just your tapered cylinder: I like the look of it.

 

[el gringo]

Yes Brian, I am starting to mess with the valve & timing... any day now I'll put the fire to it.