Author Topic: miniature 2 stroke gasoline engine  (Read 5826 times)

Offline Old School

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Re: miniature 2 stroke gasoline engine
« Reply #15 on: October 16, 2019, 05:11:34 PM »
 The 40,000 rpm figure is its racing rpm 165 mph, I have not run it under lighter load to see what the maximum rpm is. I don't want to break the engines. The Stelling 2.5cc engines are hard to find and no longer made but still very competitive. They have a steel front housing a heavy cast crank case compared to RC car engines and either drum or Zimmerman induction with a home made pipe.

I have run crowded needle roller big ends in tethered hydroplanes 10cc class they solve the problem of seized big ends, but so does increasing the clearance in the big end.
I also have a 10cc tether car engine with a crowded roller big end mine has never gone particularly fast, but the designers is the second fastest 10cc tether car in the world at around 214 mph, Msts has never told me what his engines rev to

Offline Roger B

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Re: miniature 2 stroke gasoline engine
« Reply #16 on: October 16, 2019, 06:29:58 PM »
Interesting stuff  :ThumbsUp:  :ThumbsUp: I will be following along  :wine1:
Best regards

Roger

Offline lohring

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Re: miniature 2 stroke gasoline engine
« Reply #17 on: October 17, 2019, 03:36:27 PM »
The steel front ends should really help.  It's easier to get good bearing fits when the coefficients of expansion are the same.  Steel is also a lot stiffer than aluminum.  The really fast tether cars should be at the cutting edge of glow engine development.  I'm very interested in the details.

Lohring Miller

Offline Zephyrin

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Re: miniature 2 stroke gasoline engine
« Reply #18 on: October 19, 2019, 02:39:30 PM »
HI,
I have installed the fixing screws between the two cranckase halves, pretty length job but no problem.
it seems to hold well, but it looks fragile, not much material on these legs for screw !
I hope that the tapping will resist upon several assemblies and disassemblies
Reaming (lapping in fact) of the bearings of the two assembled crankcase before the insertion of the bronze bushings to guarantee the concentricity and the proper alignment of both halves.

I have drilled and reamed the bronze bushings held in a collet in the lathe to the final dia (5) then on the same setting, turned their outside dimension, (exactly the same as the bore, 7.11) followed by 5 seconds with Norton microfinishing paper to remove tools marks and facilitate the sliding of the bushings into the aluminium castings.
I still had to force it a little bit with a bolt to get them in, without damaging my castings.

The crankshaft turns between the two crankase halves bolted together without any hard point, phew, not so easy...
I'm happy to have this part done...

https://photos.app.goo.gl/aPDpVnNWrMZHHvAK6

Offline Zephyrin

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Re: miniature 2 stroke gasoline engine
« Reply #19 on: October 24, 2019, 05:47:45 PM »
Today; boring the crankcase for the cylinder, facing of its seating surface and adjusting its height. These tiny castings are hard to hold under the boring head.


Offline Zephyrin

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Re: miniature 2 stroke gasoline engine
« Reply #20 on: October 30, 2019, 11:19:07 AM »
I turned the cylinder in a chunk of cast iron, which was lying around in my garage, not great as a metal.
No particular problem with the machining, but there is a small sealing profile in V on the top of the cylinder that corresponds with a female v on the cylinder head; I put it on, just for fun, because I think it's an unnecessary complication.
The 12 mm bore is then lapped to remove tool marks, up to a nice bore surface, and I get the small extra 0.01mm on the diameter at the bottom of the cylinder, as expected. 

I got confused in the square base of the cylinder; I have to change the position and number of fixing screws, and to make sure that the screw holes do not open into the crankcase!

But at this point of completion, unfortunately, the cylinder fell to the ground, and a fin is broken, it's very ugly, alas.
On reflection, that lasted maybe longer than doing a new cylinder, I'll continue with this one, I'm anxious to see this engine running and I'll do another cylinder if everything goes well…

Offline Admiral_dk

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Re: miniature 2 stroke gasoline engine
« Reply #21 on: October 30, 2019, 10:04:28 PM »
What a shame  :rant:  after all that work and a nice result .....

I hope that the next is without incidents   :cheers:     :popcorn:

Offline Zephyrin

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Re: miniature 2 stroke gasoline engine
« Reply #22 on: January 02, 2020, 05:01:44 PM »
Well, I didn't stay without doing anything…the engine is finished !
pictures of some more pieces!
The finished connecting rods, I had to re-bore a half-crankcase a little bit, to increase the diameter by 0.5mm, for the passage of the connecting rod !
I also had to reduce the size of the screws head, rubbing on the crankcase !
Everything turns well...phew

Machining the flywheel according to the plan, with the coned shape, not easy to measure its thickness ! I like the design of this flywheel I'll surely use it again one day !

The assembly is done by conical fitting on the crankshaft, tightened by a nut. So I turned the cone and thread between centre on the lathe, on a larger engine I prefer a coned bush.
The cylinder heads is also finished, with the V-groove that fits on the cylinder V, a nice part to turn and adjust. The thread for the plug is standard, 1/4" x 32; a bit big; some modellers make their own mini spark plugs, but I never tried it.

Piston is machined in a small cast iron bar, inside finished first, then the outside with 0.5 mm above the actual dia of the cylinder, then drilled for the wrist pin and milled for the port in the skirt and the deflector on the top. then the cylinder is put on mandrel in the lathe for turning the external face, up to the point where the piston start to go into the bottom of the cylinder; the final adjustment  being made with a piece of Norton Q151 microfinishing film, pinched with the fingers between two steel rules on the piston in the lathe, for a series of about 5 seconds pass between testing the fit in the cylinder up to the “end” : i.e., the piston ( dry) enters fully in the cylinder with “some” resistance…hard to be more precise. At this point, the piston is ran in with oil by hand only in the cylinder for 5 min, then mounted in the engine for the test.


Offline Roger B

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Re: miniature 2 stroke gasoline engine
« Reply #23 on: January 02, 2020, 05:14:20 PM »
Very nice  :praise2:  :praise2: Clearances in the crankcase are difficult to determine without sophisticated CAD. I make an approximation and then grind off the bits that cause problems  ::)
Best regards

Roger

Offline Zephyrin

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Re: miniature 2 stroke gasoline engine
« Reply #24 on: January 02, 2020, 05:18:19 PM »
The cylinder is mounted on the crankcase, no paper gasket, just a thin coat of blue loctite.
The transfer cover is screwed with a dab of blue loctite too, the brazed steel carb base is screwed with a Klingerit gasket, to isolate the mixer from the heat of the cylinder.
The mixer is made from brass rod; a steel sewing needle, soft soldered, plugs a 0.35 mm dia fuel port when tightened!
The contact breaker is made of a piece of steel spiral spring with riveted silver contacts, far from ideal but I didn’t find tungsten ones small enough.
The cam is a steel cylinder held on the crankshaft by a cone and a screw, a flat is milled to give about 90° of dwell, and the corner rounded with a few file strokes.


I did two engines from this plan, with very little differences in the port design, just to see if there are perceptible changes in the running properties…
I had to make a new piston in the first one, owing to a poor compression, but I can’t wait to start the test as the second engine being ready to run.


Offline Zephyrin

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Re: miniature 2 stroke gasoline engine
« Reply #25 on: January 02, 2020, 05:23:50 PM »
Here we are, the first running test !

This model started easily, with plain gasoline with 15% motor oil for the running in, and when hot, the motor start with a flip on the flywheel; I’m pretty happy with this !
I have the second engine to get it running ! not yet ready !

However the rpm are not regular, speed increases and decreases continuously, easily perceptible in the video: periodic faster puffs then slowing down.
Amongst possible causes, I first suspect sparks, which may diminish as rpm increases, or poor carburetion, with unstable richness of the fuel ?
<a href="https://www.youtube.com/watch?v=pSHh5WMR_Gk" target="_blank">http://www.youtube.com/watch?v=pSHh5WMR_Gk</a>

« Last Edit: March 17, 2023, 08:47:36 AM by Zephyrin »

Offline fumopuc

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Re: miniature 2 stroke gasoline engine
« Reply #26 on: January 02, 2020, 07:18:34 PM »
I do love the size and the sound. A very nice runner.
Kind Regards
Achim

Offline Admiral_dk

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Re: miniature 2 stroke gasoline engine
« Reply #27 on: January 02, 2020, 08:23:55 PM »
Congratulations - you have a fine runner  :cheers:

The hunting running and sound is not really different to all the European mopeds of my youth (the Japanese were much better), when they ran at low RPM's (idle or a bit above) without any load on them - they usually all cleared their throat when you opened the throttle and the engine pulled you forward (load).
The Japanese had much better carburetors, ignition (the whole electric system) and reed controlled inlet.

Two strokes can be very funny, when all is not perfect - especially the caburation ....  :old:

Best wishes

Per

Offline Zephyrin

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Re: miniature 2 stroke gasoline engine
« Reply #28 on: January 05, 2020, 08:16:23 AM »
I've made a new test yesterday, to continue running in...
sharper noise and blue smoke, speed is improved, and the engine reaches 5400 rpm  under no load.
As the original plan refers to 3000 rpm, I suppose that this engine is finished at this point, higher idle speed would probably disintegrate some parts...

https://photos.app.goo.gl/Swcm75m6mpRP69sr6

Offline MJM460

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Re: miniature 2 stroke gasoline engine
« Reply #29 on: January 05, 2020, 11:21:19 AM »
Congratulations Zephyrin, a beautiful little motor running well.

Will it require some sort of cooling for a longer run?  What sort of load are you planning to drive with it?

MJM460

The more I learn, the more I find that I still have to learn!

 

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