Speaking of model engine ignition systems...

[DICKEYBIRD]

I have this fuzzy daydream that won't go away and am hoping someone here might have the knowledge to help me remove the fuzzy edges.

I've drooled over the Maier butane powered desktop model engines http://www.ministeam.com/acatalog/Maier ... gines.html and want to do something similar but on a MUCH smaller budget. I want to convert one of my old O.S. FS .40 4-stroke model aircraft engines to run on butane or (preferably) propane with a battery powered spark igntion.

This engine will not be used on an airplane...just a desk toy driving a pencil sharpener and a small pm motor (generator) to light an l.e.d. I'm planning to cobble up an electric starter so the engine could be spun up and the butane/propane valve slowly opened until it's running on it's own.

I plan to install a large flywheel to allow a comfortably slow running speed and if necessary adjust the cam timing to get a steady run. My ignition question is about the coil needed with Jerry Howell's really neat electronic ignition kit. He is adamant that the coil must have a primary resistance of 1 ohm. His coil is (to me) too expensive but I have access to piles of fairly small modern distributorless automotive coils which have a higher primary winding resistance. Seems to me that a circuit could be designed or an existing one modified to take advantage of this cheap coil source, using 9 volts or so. Does anyone out there have any ideas or input? The other factor is that I'm no electronics whiz!:(

If this daydream comes to fruition, I would then hope to build an engine from scratch and the fuel & ignition issues would already be worked out. Yeah, that's the ticket! I can dream can't I?:D

[tornitore45]

Jerry web site says the coils are desighed to work between 3.6V and 6V and must have 1 ohm. That means that when the engine is running at low speed the current during the dwell time is most likely limited by the resistance since the primary inductance has had all the time to charge.

3.6/1=3.6Amps and 6/1=6Amps

Since 3.6 amps is sufficient as a minimum value you can use a coil with up to 3 ohms and run it from 12Vdc

The points or the transistor breaking the circuit should be able to stand a minimum of 200V and when it comes to transistor I would not skimp and go for a 400-600V device. The primary voltage being 3.6 or 12 is then not significant in the face of the transistor breakdown rating.

In a nutshell as long as the final current in the coils is the same, it does not matter what the coil resistance.

An if you find a coil with less than 1 ohm simply add an external resistor to bring it up.

[DICKEYBIRD]

Thanks Mauro.

When I read your response late last night, the content went zing, right over my head. With this morning's coffee clearing my head, it makes perfect sense and fits with what what I was thinking.

I'm going to set aside a few coils in the next week or so and see how they measure up. They have rubber coated posts that snap down over the plug but I'll bet the post could be removed and a stranded plug wire grafted on for my use.

[accroul]

I know that this post was put up a long time ago... But i was thinking of looking at a coil and plug out of my paslode nail gun.
It runs on a butane and has a 7.2 volt battery.
I have not tried it, its just an idea.

[Lew Hartswick]

Preface this with I'm not into these models but have had a lot of
electronic experience (50 yrs as an EE ) .
How about a capacitance discharge ignition system for engines like this?
It would eliminate all the concerns about current during point dwell etc.
...lew...

[jones]

With the propane, if you make a demand valve then you don't have to slowly turn it up and down to get more and less fuel for different running speeds, it will run just like on petrol. I forget where but designs for such a valve can be found for free on the internet.

[sorveltaja]

Some of those (if not all)Maier's IC-engines use piezo ignition, without any coils or batteries. Can't get much simpler than that.

Jan Ridders http://heetgasmodelbouw.ridders.nu/index.htm has also used that method, and it seems to work at least on lower-rpm engines.

Since he uses ordinary gas cooker lighters piezo igniter, it definitely can't get cheaper than that either, unless you want to make a diesel or glow engine.

[RAN]

Issue #24 of Model Engine Builder has drawings for a demand type propane carburetor which should be just right.

RAN

[BClem]

Since the coil for a miniature running display engine is usually 'hidden' in a mount/battery box, a magneto coil (pre-electronic) from an old outboard or older version B&S or similar engine will work. I have cut the core pieces that are shaped to pick up the inductance from the flywheel magnets - the coil will work without the core but it is much more efficient with the stacked iron core through its center. Those coils are 'step up' transformers to step the small inductance voltage up to the spark voltage so a low voltage on the primary is interrupted with points or electronically - same as with those expensive little model coils. I used an old Johnson 6HP OB motor magneto coil with a 3V battery and with miniature spark plugs, the spark is quite healthy. The problem with even small automotive coils is that once you establish enough battery power to achieve a spark, the spark is much too intense and will 'jump' to places that it shouldn't.

I have tried winding my own ignition coils with #40 copper wire.....I would not recommend that at all unless a dedicated coil winder is available. I had one but she ran off with the dish washer. (That's an old one!)

Bill C.