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Jan Ridders Pressure-controlled Two-stroke engine

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Jan Ridders02/03/2011 15:27:33
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Hello all,
I just found a video of Mr Stoneman (?) who build my "Pressure Controlled 2-stroke" and with success as you can see on this video.
He made it according to my drawing plan as he stated. I can't remember any mentioned problem or question from him. There must be something we overlook that didn't occur with this engine, accidentally or not!
 
In the meatime I asked my friend Huib Visser by mail about his experiences while building this engine recently and eventually suggestions. As soon as I have his reaction I will post that here.
Friendly greetings from
Jan
 
 
Jim Greethead02/03/2011 18:15:49
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I think I have found the solution.
 
Debra would not stay in the box because I have been convinced all along that the solution is so obvious that I could not see it. So I kept taking it out for one more try.
 
When a friend came over to play with the engine, something was said that convinces me that the problem is:
 
THE FLYWHEEL
 
I have been trying to cure the four cycling and misfiring. But when I looked again at Jan's video, I noticed that his Debbie four cycles and misfires as well. So I have been looking in the wrong place. The difference is that Jan's engine has a larger flywheel to keep it running between the power pulses. A rough estimate is that Jan's flywheel has about 4 times the moment of inertia of the smaller one.
 
I may be wrong again but the diagnosis fits all the known facts. This does not make it right but it is a good start.
 
And now I can't wait to get to the shops to buy a lump of metal to make a larger flywheel.
 
Jim
 
 
 
 
 
Jan Ridders03/03/2011 08:14:12
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Hello Jim
You are talking about the "Simple 2-Stroke Debbie" (horizontal cylinder) and till now all my comments concerned the "Pressure Controlled 2- stroke Sabine" (smaller, with vertical cylinder and two ball valves). The confusion is mine I suppose.
You are right that the fly wheel on my engine is bigger than that on the drawing plan. The reason is that I used my old fly wheel when I rebuild my engine to the new concept according to the present drawing plan. But I increased the width on the drawing plan from 20mm to 25mm to compensated the inertia somewhat. I now calculated the kinetic engery again and the difference is about 2.2 times. So it is well possible that your engine is not running because of this and I sincerely hope this will solve your problem. If so I will change that on the drawing plan with thanks to you.
One observation makes me doubt: Belgian students made this engine with the (smaller) fly wheel as on the drawing plan and that engine runs perfecrly; see from 1.47 minutes on this video:
http://www.youtube.com/watch?v=bnx2RyeSBko&feature=player_embedded
But subtile differences can exist between engines, reason why the one is running with this fly wheel and the other just not.
I wish you a lot of success this time.
Friendly greetings,
Jan
 
Ian S C03/03/2011 11:32:45
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It sound a very likly source of your troubles, have you got enough shaft to put another flywheel , or a larger diameter jacobs chuck, and have a try. I have found with one or two of my hot air engines, and the flame licker that either a larger, or extra flywheel was needed at first, but as they bedded in a lighter flywheel could be used, in fact two of the motors will run without a flywheel.
Jan have you been able to estimate the power out put of "Simple 2- Stroke Debbie", it seems as though it may be quite low powered, so friction would play a major part, similar to the requirements for small hot air motors. Ian S C
John Somers 103/03/2011 12:28:22
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Optimism is high following Jim's eureka moment and fingers are firmly crossed that this at long last may be the answer in overcoming the running problems with both Debra and Debbie.
 
Whilst Jim explores the benefit of a larger flywheel I am planning to make a modified version of Jan's vacuum carb incorporating a variable throttle.Could the misfiring and four stroking characteristics of all these engines be due to fuel starvation ? In an attempt to find out I am planning to construct a revised version of Jan's vacuum carburetor with a throttle device to adjust the volume of vapour fed into the induction tract. I confess that this is a seat of the pants modification based more on gut feel than any technical knowledge.
 
As far as adding an additional flywheel or a jacobs chuck, as Ian has suggested, I am advised by Jim that the problem is that the inertia of the flywheel increases rapidly with diameter but only slowly with width. However as I happen to have a suitable cast iron flywheel 'in stock' there would be no harm in adding this to the crankshaft and observing if there is any improvement.
 
Work continues.
 
John

Jan Ridders03/03/2011 17:41:08
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To Ian:
I did not measure the power of this Debbie 2-stroke engine because I don't have the right equipment for that. But the power is far more greater than that of Stirlings and flame eaters with comparable dimensions. But you are right that friction always plays a role and that friction can decrease somewhat after some hours of running (is that what you mean with "bedded in"?). But there is another counteracting force here in comparison with hot air engines: the much higher compression that also must be overcome by the kinenetic energy of the fly wheel. I don't believe that any IC engine can run without a fly wheel!

To John:
1. The moment of inertia of a fly wheel is k*m*r*r, where m=the mass and r=the radius of the fly wheel (k= a constant depending on the type of fly wheel; bicycle wheel type or massive type). So indeed, the effect of the radius is square and the effect of the mass is only linear.
2. There is a certain effect of starvation of the fuel indeed, but that occurs only after some minutes running and then you can easily compensate that by mixing somwhat less extra air. After that it remains more or less stable for a longer time. This effect is hardly or not present when using Coleman fuel since that contains much less different (volatile) carbon hydrogens.
So this can not be the real reason of the mentioned problems because the engine don't even start with fresh petrol or Coleman fuel as I understand. Furthermore, I (and many others in the meantime) use this carburetor and, apart from the minor mentioned effect of some starvation, I never have any problem with this carburetor; on the contrary.
 
Thanks for your remarks,
Jan
 
 
 
 
Ian S C04/03/2011 10:08:58
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thank you Jan, yes bedding in/ running in, same thing. Yes the diameter is the important point with flywheels, the trick is getting the size right, too big and there is extra load on the motor, and slow running, to small and if the motor goes it runs fast with little power. I agree an IC engine needs a flywheel, well maybe if there are enough cylinders. One of these days I must do some more to my 4 cycle single cylinder open crank engine. It has a 14" fabricated flywheel, the engine is based on one in volume 1 (1898), Model Engineer. Ian S C
Jim Greethead04/03/2011 23:14:54
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Jan
Thank you for your kind words and continuing encouragement as I struggle with your lovely engine. Yours works beautifully, and the Belgian students have made theirs work with the small flywheel so the design is good.
 
Despite the frustrations, I am enjoying building these small engines but I could not do so without you and other people designing the engines and making the plans available. I am grateful to you for all the work you do.
 
Maybe I will get the engine to run, maybe not. Making it and trying to get it to run is the interesting part. Of course, having a finished engine that runs is good too.
Jim
Jim Greethead04/03/2011 23:30:07
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Well, I have made a bigger flywheel (disk 150 dia x 25 width) and put it all together and:
 
IT IS NOT THE FLYWHEEL
 
It still does not run. And of course Ian is right: the flywheel only stores energy and needs only enough inertia to maintain a certain minimum speed between pulses. Any larger (or heavier than needed) just adds to the bearing friction and to the load on the engine.
 
From trying it and thinking about the result and thinking about Ian's comments, I have learned something about flywheels. So I suppose some good has come from it.
 
Jeff - I took your advice about the ignition timing which translates to about 11 degrees BTDC (I have to outsmart the electrickery) and I think it improved the power output. Maybe I need to take the rest of your advice and dive into the spare parts box for an old Bonneville coil and capacitor. We shall see.
 
Now I am waiting to see how John gets on with the Pimento Jar carburettor.
Jim
Jeff Dayman05/03/2011 00:19:44
2189 forum posts
45 photos
Hi Jim,
 
Sorry to hear the flywheel fix wasn't the whole answer. They can have a bad effect on starting if they are too light, for sure. However from John's video I'm still thinking it is a spark timing/strength issue and going back to basics with a simple coil and points will eliminate all doubt. Don't forget a suitable condensor/capacitor across your points - it will give you much longer battery and point life. If you use an ex-car/bike/mower set of points and condensor, costs will be low and parts suited to each other. The plug must be dry when starting - small engines typically can have major problems with wet plugs/ carb flooding too.
 
I just wished I'd saved a few hundred sets of points and condensors when I used to work for my Dad at his garage in the 1960's /1970's - we often did more than 25 tuneups on cars a week in summer, and lots of points not good enough for cars but plenty good enough for models got tossed in the skip. Plenty of small engine ones then too, we fixed everything - it was a small town shop.
 
JD
Jan Ridders14/03/2011 09:27:23
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24 forum posts
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Hi all,
I am not sure that this got enough attention: the air tight fitting of the piston rod in the Teflon bush!
If there is a too great leakage there two things happen:
1. Lower compression and loss of the sucked-in gas mix and, as a result, a poor flushing process above the cylinder.
2. Sucking in some false air below the cylinder togeher with the gas mix from the carburetor; this can disturb the constant ratio of the gas mix.
In both cases this can result in an insufficient fushing process above the cylinder and/or wrong gas mix composition. This can be the reason of irregular ignition (other than circuitry).
Making a Teflon bush (as I suggest on the drawing plan) can be a problem sometimes, because Teflon is rather soft and not very form retaining. To eliminate a possible leak here you can put a small rubber O-ring over the piston rod and press that lightly against the bush. For that make a cavity in the sealing plate for the bush in what the O-ring is fitting well and so that it is pressed for some tenth's of a millimeter.
I now think this is a more reliable solution and I consider to change the drawing plan accordingly. I even think that using a standard bronze bush is more suitable than Teflon.
Please react in case you should have tried this.
Sorry for my bad English,
Jan Ridders
 
 
 
John Somers 114/03/2011 10:14:56
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36 forum posts
Hi Jan
 
This was an area where I also felt an improved seal might be beneficial in minimising the suction of air into the chamber below the piston. As you say this could reduce compression and weaken the mixture.
 
My solution was to split the Teflon bush and insert a seal of graphite string the compression of which can be controlled by the careful tightening of the seal plate. I felt that this would offer less friction than an 'O' ring.
 
This modification seemed to provide an improved seal but sadly little improvement in the ability of 'Debbie' to run unaided.
 
John Somers
 
 
Ian S C14/03/2011 10:15:05
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7468 forum posts
230 photos
This bush is similar to the bush used in the displacer of hot air engines,ie., min imum friction, and minimum leakage, Ihave used Teflon (Carbon impregnated), brass(bronze would be better), and cast iron, mostly running on stainless steel shaft. If an adjustable gland is made, plumbers teflon tape will make a vertually air tight joint, with very little friction.
The air leak problem is a common 2 stroke one. On sump induction motors, the leak occurrs at the point where the crankshaft goes throgh the crankcase, and is cured by fitting new seals, and easily diagnosed by appling grease around the bearing, the forms a tempory seal, and the motor should start and run for a few miniuts until the grease is sucked through the bearings. Learned this years ago when I overhauled a little Villiers. Ian S C
John Somers 114/03/2011 10:19:55
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36 forum posts
I like that idea Ian. I shall try a temporary seal of grease and see what happens. In fact I will go and do it now !
 
John Somers
John Somers 114/03/2011 15:13:20
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Jan/Ian
 
Tried the grease test and there was some evidence of grease being sucked into the cylinder though difficult to be sure due to restricted space. Anyway it prompted me to machine up a phosphor bronze bush, very tight fit which eased after a few turns of the electric starter but no improvement with engine fourstroking as before.
 
Added an inset 'O' ring to the bush which tightened around the piston rod when the seal cover was tightened. Sorry to say still no improvement.
 
Thanks for your continued interest.
 
John Somers
Jeff Dayman14/03/2011 15:27:25
2189 forum posts
45 photos
Hi all,
 
Did anyone do some experiments with back to basics point and coil ignition and timing yet?
 
I still think this may be the root of the problem. If you don't have a healthy spark at exactly the right time any IC engine will run rough.
 
If the timing is right, with a healthy spark, the mixture can be very rich or lean with many engines and it will still run smoothly. The acceptable mixture range on some engines is quite large, for example the humble Briggs and Stratton 3.5 HP mower engines will run from a half turn to 2 turns on the mixture screw. That's almost .060" of needle movement! I'm just saying that adjusting the mixture to an ultra perfect precision is usually not necessary.
 
Good crankcase shaft sealing is important, but with the fixes already suggested re tight running fit, and grease, o-rings etc this will likely be OK.
 
Timing timing and then timing, I think.
 
JD
Richard Parsons14/03/2011 18:14:41
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645 forum posts
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Fuel, spark and compression - a two stroke will run. When they were a bit tricky I would set the points to open at 3/16” Before TDC. I do not really like the trickier electronic ignition. With the old fashioned make and break, a torch bulb and a battery you could set things up well. Leakage in the pre-compression area (crankcase or the Dunnalt under cylinder as used by Mr Ridders) has to be fairly gross before it affects the running. Leaking around the head yes this can cause problems and stop the thing from firing. I had an old Lambretta which ran for years with 2 – 0.020” scores in the cylinder –not much power, but she ran.


J.D. one of the problems is this engine is in the ‘Gnat power’ class. The seals have to be of the small Stirling engine class fits. With PTFE this can be a big problem. On my tide clock which was destroyed by a ‘well oiled’ (as a newt) Hungarian before it was finished. I found it was best to push the PTFE into brass bushes before trying to machine it to size. You needed very sharp tools and even sharper reamers. perhaps two or three narrow flexable seals or the Phillips 'Roll sock' type might do the trick

A little pot of child’s bubble mixture is ideal for testing the gas tightness of the bearings. Drip a little on and see if it blows bubbles.
Ian S C15/03/2011 02:34:53
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7468 forum posts
230 photos
The best way of making Teflon bushes is not by machining them, but by making a little mold , and heat forming them, ' have a go some time and put up some photos. Ian S C
Jim Greethead15/03/2011 09:38:15
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131 forum posts
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At last, IT RUNS!!! And you can see it here
The problem was lack of compression which was diagnosed by my expert friend Win Coupland.
 
The solution was to make a new piston with a flat top and just a small cutaway for the transfer port. I actually designed it to comply with the parameters suggested by Dave Parkes in his articles on two stroke engines in ME. Transfer port opening 100 degrees, exhaust port 120 degrees and compression ratio 8:1.
 
I am not sure how close I got but it certainly has plenty of compression and lots of power. I think I need a throttle control like the one in John Somers' Pimento carburettor.
 
Many thanks to all of you who supported my efforts to get it running and who offered good advice. Your support was really important in keeping me going.
 
And now I have to make it pretty (but not until I have played with it some more).
 
Cheers
Jim
 
John Somers 115/03/2011 09:57:17
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Arise Sir Jim !

Well deserved congratulations ! Your untiring efforts and tenacity have at last paid off. You must be over the moon (or is it under the moon 'down under' ?

You have given renewed hope to all 'Debbie' builders including myself. I can't wait to get into the workshop to make a Jimbo piston.

Best wishes

John Somers

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