Author Topic: Another version of how not to build a model engine  (Read 90327 times)

Offline NickG

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Re: Another version of how not to build a model engine
« Reply #175 on: January 15, 2010, 11:23:48 AM »
I think this is the same sort of cam as you find in a car distributor for opening / closing the points - been so long since I've seen one though i'm not 100% sure!

Nick
Location: County Durham (North East England)

Offline Stilldrillin

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Re: Another version of how not to build a model engine
« Reply #176 on: January 15, 2010, 12:09:18 PM »
A car points cam has a peak (or 4) not a flat (or 4)......

The points should be together for the maximum time..... Not held open!

I think!  :scratch:

David D
David.

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Still modifying bits of metal... Occasionally, making an improvement!

Offline Bluechip

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Re: Another version of how not to build a model engine
« Reply #177 on: January 15, 2010, 02:10:11 PM »
Dave SD

Quite right. I doubt if there is sufficient 'dwell angle' to allow the coil core to reach a high enough magnetic field to give any sort of reasonable spark when the points open, and the field collapses to give said spark. You may need about 120 deg., depends on the coil and the internal resistance of the source of DC. Has yours got a currrent limiter? The instantaneous current may be causing that to kick in, and that may drop your apparent DC input voltage. I had a cheapo Chinese PSU, limited at 5A, if I took pulses out of it at near 5A, the output voltage bounced all over the place. CARP !!
Briefly demoted to battery charger, on it's way to the Wheelie Bin.

Don't know whether you have one, but you should have a capacitor across the points too. About 47nF or 100nF usually work. 100VDC rating will usually survive long enough.

Dave BC

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Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #178 on: January 15, 2010, 03:47:00 PM »
Thanks for the replies and suggestions  :wave:.

Ignition circuit, that's in use, consist:

12V/2,5A PSU from an ancient (DEC/Digital)computer, that has a shortcircuit protection. That bugger seems to last, no matter how hard it is abused used.

standard 12V automotive coil

Velleman's transistor ignition kit K2543

Besides all of those bells and whistles, reducing the ignition cam's flat part/ignition points close/open sequence, as mentioned earlier, has given positive results.

Spark plug stays a lot cleaner than before. Meaning, that I don't have to make a new ptfe insulator every other day anymore :thumbup:.

The vapor carb is still under testing, engine runs with it, but especially the idle adjustment is bit tricky.

Anyway, I'm working on it :dremel:.

Offline Bluechip

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Re: Another version of how not to build a model engine
« Reply #179 on: January 15, 2010, 05:04:50 PM »
Sorveltaja

Not sure what the kit is, but if it uses a power transistor to switch the coil primary, then NO capacitor is needed.
In fact, it usually stops it working if present.

Dave BC
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Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #180 on: January 16, 2010, 06:24:38 AM »
Bluechip, there is no additional capacitor, as the kit's instruction manual emphasizes not to use any. Also the resistor, that was originally attached to the coil, was removed.

Here is the current fuel tank, that is now modified to vapor carb:


To simplify the flowback valve, I removed the previous one, that was outside of fuel tank, and made it to the lower end of the inlet tube:


That is a 4mm(3mm id) brass tube, and it has 3mm steel ball, that acts as valve. Bit of tube shrinking was done, to form the actual closing surface for that ball.

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #181 on: January 21, 2010, 03:27:42 PM »
So far, I haven't been able to get the vapor carb to work good enough. Engine runs with it yes, but only at high rpm.

Numerous setups and combinations were tested. None of the different size fuel hoses makes it any better :scratch:. Be it inlet or outlet hose/tube.

I have seen the vapor carb used successfully on bigger engines. Who knows, maybe it just doesn't work on engines this small.

Also the double sparking -problem still exists, even though I made proper ignition cam with lobe.

If this project had a name, it would be an ML. Murphy's Law.
« Last Edit: January 22, 2010, 04:42:36 AM by sorveltaja »

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #182 on: January 22, 2010, 07:44:47 AM »
Addition to that vapor carb thing, I suspect that the engine ran too lean mixture with it, no matter what :scratch:.

In the meantime, I've changed back to ordinary needle carb, that gives more richer mixture, that the engine seems to prefer.

Offline madjackghengis

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Re: Another version of how not to build a model engine
« Reply #183 on: January 22, 2010, 09:38:41 AM »
Sorveltaja
   A couple of things regarding your fuel mixtures:  I suspect Jan stopped the fuel inlet just before going beneath the surface because of the substantial increase in vacuum necessary to get air flow through the liquid, when there is a layer of fuel vapor hovering above the fuel.  Secondly, there is a need for space between the check valve and the intake, as the intake charge is of low mass and rather low velocity, and it is its inertia which opens the check valve, and it needs room to build up velocity, given the low vacuum presented by the intake stroke.  Check valves on torches are right at the valves because a "backfire" provides plenty of power to the flame that is trying to climb backwards up your hoses and blow you up, by the energy of the ignition and its "pop".  In low velocity systems, inertia is the single most important energy factor, which is why long intake manifolds work well with small engines.  As size goes down by the square, mass and volume go down by the cube, and it is mass which is the smaller of the factors in inertia, and velocity which is the major factor in the energy of the intake charge.  For what its worth, that's my input.  I hope you get it to a happy state soon.
jack

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #184 on: January 22, 2010, 03:04:21 PM »
Jack, thanks for the reply :wave:.

I tested the vapor carb also with vertically adjustable inlet tube, so it was possible to take its lower end as close to fuel surface, as possible. Still no low rpm running though.


As size goes down by the square, mass and volume go down by the cube.

Hmm.. that's interesting. I'm thinking of some simple formulas to calculate needed values, but there seems to be so many factors (and variables?) on the engine and fuel tank, and between them, that I can't figure out how to interpret all that mathematically :smart:.

Offline madjackghengis

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Re: Another version of how not to build a model engine
« Reply #185 on: January 23, 2010, 09:40:56 AM »
Bluechip,
   Just for background information, dwell angles of about thirty degrees are standard on points operated engines, so the flat on the round cam should be sufficient, dwell angle being adjustable by point gap with a degree wheel, a light bulb of the right voltage, and a pointer to set when the bulb goes out, and to show the degrees when it comes on with the points open.  Typical primary voltage in a twelve volt system is two amps for a car engine, anything close to an amp is sufficient for small engines.  Thirty degrees will allow full coil "saturation" at eight grand on a racing engine, thus a full "fat" spark at the plug.  Plenty of dwell for an old Harley.
mad jack

Offline madjackghengis

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Re: Another version of how not to build a model engine
« Reply #186 on: January 23, 2010, 09:53:12 AM »
By the way Sorveltaja, with the vellman transistor kit, you should be able to use a micro switch to trigger the ignition without any problems of burning it up, as there is very little current in the trigger circuit.  I've used one of those conversion kits in a truck to good results.  It should give you a fixed dwell angle established by the circuitry.  Are there any magneto experts out there who have managed a miniature magneto?  I'm looking at the possibility of one for my radial, never made one before so any insight would be appreciated.
   On the fuel tube, you might try putting it right below the surface, with a tiny hole right above surface level, that's what some regular carburetors appear to use to control low speed mixture.  A slash cut end, with just a bare bit of opening showing above the fuel level might work as an alternate.  Quite a lot of dedication, getting this engine running up to snuff, looking good :bang:

Offline NickG

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Re: Another version of how not to build a model engine
« Reply #187 on: January 25, 2010, 08:08:32 AM »
Madjack, I have also wondered about miniature magnetos. I have a book which must have belonged to my grandad, model petrol engines by edgar t westbury. It has a section in there about them. I will see if there's anything useful in there and let you know.

Nick
Location: County Durham (North East England)

Offline madjackghengis

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Re: Another version of how not to build a model engine
« Reply #188 on: January 25, 2010, 09:49:59 AM »
Madjack, I have also wondered about miniature magnetos. I have a book which must have belonged to my grandad, model petrol engines by edgar t westbury. It has a section in there about them. I will see if there's anything useful in there and let you know.

Nick
Hi Nick, I've got an old book of ETW's on miniature ignitions and magnetos I'm studying, it has a lot of information and gives dimensions and numbers for winding miniature coils and the like, as well as some ideas on miniature magnetos.  It was written in the thirties of forties, with the new "rare earth magnets" available, I think I can get a working magneto in scale size certainly for a single cylinder hit or miss, and with one of the chapters on the Scintilla magneto, and how it works, maybe one that would run nine cylinders in at least close to scale size.  ETW was the mother lode for model engines for forty or fifty years, and designed some stirling engine generator/radio sets which were dropped behind occupied lines in WWII and used by the French resistance, to comm with the Brit forces.
It's getting hard to find any of the books he's written, at least here in the states.
mad jack

Offline Bluechip

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Re: Another version of how not to build a model engine
« Reply #189 on: January 25, 2010, 11:11:42 AM »
Folks

Got this myself,   absolutely excellent.

Was sold by Camden in UK, no longer stocked by the looks of it tho.

scroll down the site a bit ..

http://www.bobshores.com/

Dave BC
I have a few modest talents. Knowing what I'm doing isn't one of them.

Offline NickG

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Re: Another version of how not to build a model engine
« Reply #190 on: January 25, 2010, 03:36:33 PM »
Mad Jack,

Wow, that's impressive, didn't know he was that well known!

The bob shores one sounds interesting too BC.

Nick
Location: County Durham (North East England)

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #191 on: January 28, 2010, 08:01:03 AM »
That magneto thing is interesting... how small can be made?

I have ETW's 'Atomag Minor' plans for miniature magneto and the coil. Drawing has the magnets diameter ~30mm(~1.18"), and its material is alnico.

With modern neodymium magnets, maybe even smaller size (and more efficient?)would be possible?

Offline Bluechip

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Re: Another version of how not to build a model engine
« Reply #192 on: January 28, 2010, 10:59:52 AM »
Folks

The principal problem with mags. miniature or not, is the HV coil.

This consists of some 15,000 turns of very fine magnet wire. about 44 AWG, or it's metric/SWG equivalent.

The breaking strain on this stuff is near zero. If it lets go, you start again. I have wound coils on a commercial winder with wire a good bit larger. I broke that! You won't do it on a lathe unless you are very lucky indeed.

(BTW, those coils were not for ignition stuff, just rewinding solenoids).

Assuming you have succeeded in making the coil, you then have to vacuum impregnate it. This needs to be done to give a path through the coil to get rid of the heat. Otherwise, you get a hot spot, and the coil will fail.

Then, there is the problem of sourcing the armature stampings ...

There was. at one time, a UK maker of Mini-Mags .. no longer in business I think. (Jim Shelley).

Some reference here.. by some one we once knew, I think.

http://www.floridaame.org/discus/messages/14/321.html?1182878736

There is a wealth of practical information in Bob Shores book. I strongly suggest anyone even contemplating home building coils/ magnetos gets a copy and reads it before committing any significant time or money.

Dave BC
I have a few modest talents. Knowing what I'm doing isn't one of them.

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #193 on: January 28, 2010, 11:37:42 AM »
Dave, it sure is tricky to wind hair-thin wire. I've been trying that, and there is way too big chance to snap it.

Unless you have a delicate winding setup.

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #194 on: January 28, 2010, 01:40:46 PM »
In the meantime, I've done some more testings with vapor carb.

Earlier I just couldn't get it to work with ethanol. But today I almost accidentally found out, that it's all about temperature.

With gasoline it hasn't been a problem, since it has lower flash point, than ethanol(just a guess).

I remember reading somewhere, that it could be tricky to start the engine, that uses ethanol with vapor carb on the cold circumstances.

I was in doubt, when I tested following:
 

But it seems, that even few degrees increase of temperature makes ethanol to vaporize :ddb:. Once the hand is removed, lukewarm is gone, and soon the engine stops.

Offline Bluechip

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Re: Another version of how not to build a model engine
« Reply #195 on: January 28, 2010, 01:45:49 PM »
Dave, it sure is tricky to wind hair-thin wire. I've been trying that, and there is way too big chance to snap it.

Unless you have a delicate winding setup.

Bob Shore mentions 16,000 turns of  44 AWG  ( 0.05mm  or 0.002")   I can't see it, let alone wind it.   :scratch:

This might be handy ...

http://wires.co.uk/acatalog/conversion.html

Not for me ...

Dave BC
I have a few modest talents. Knowing what I'm doing isn't one of them.

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #196 on: January 28, 2010, 02:25:41 PM »
Once I was interested in making my own pickups for the electric guitar. Wire is somewhat similar size, that is used on ignition coils.

There was the site, where the winding how-to tricks were shown. I think, that same techniques applies to both pickup and the ignition coil winding.

Only the latter being easier, since it's cylindrical..

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #197 on: January 30, 2010, 04:32:10 PM »
I've been testing vapor carb with gasoline and ethanol. Neither provides low rpm, no matter how many setups are tried.

So I moved back to ordinary carb. With it, engine runs at very low speed, but only randomly. Otherwise it likes to run horribly fast.

After all, I have a strong feeling, that it should indeed be possible to get even that small engine to run nice and slow (huff'n'puff'n).

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #198 on: January 31, 2010, 03:57:06 PM »
Last, but hopefully not least, I'm going to make a more standard needle and barrel throttle carb.

I can make the engine run already, and that should be enough, but maybe just one more test, before this project is finalized.

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #199 on: February 01, 2010, 02:21:44 PM »
New carb so far:


On the left side is needle, next is part, that needs an inlet for the fuel to be made. It has 1mm(0.039") hole going through, and piece of smaller tube (od 1mm, id 0.5mm), that goes to carb's throat.

Then is carb body, that has different taper on inlet, and outlet. It has 7mm(0.275") hole for barrel. There is also pair of d-bit reamers, that were made of otherwise scrappy drill bits. Fastening part of the cheap drill bits seems to be softer, allowing them to be filed, if desired. No hardening was done, and still they cut the brass better than I expected :thumbup:.

At the right side is the barrel, that has 2mm(0.078") throat.