Paul,

Correct me if I am wrong please, but if you only wanted to fire the leading plug or both leading and trailing as a set, then one magnet and two coils place 180 degrees apart would do the job right?


Doug in Japan

Yes.
Well perhaps. It all depends on how much charge you will
get out of one magnet going by one of the coils once
per rev at starting RPM's to get enough juice to fire
both plugs.  You need about one microfarad fully charged
with 300 volts to do the job of firing each plug.

The rate the coil charges the 1 ufd capacitor depends on the number of turns on the charging coil, the resistance of the diode, the strength of the magnetic field and the resistance of the spark coil primary. Usually the primary inductance is rather low.

All this assumes the caps are fully charged before you try
to fire a plug. So the trigger should be independent of the charge coil or fire when the charge capacitor reaches 300 volts IF you are using ONLY one magnet, two charge coils and two capacitors for just the leading plugs on a two rotor. A minimal system.

When the plug fires the capacitor is fully discharged.

More magnets generate more charge.

In Franklin's OMC 4 cyl 2 cycle outboard case there are a lot of magnets used to charge the coils. Plus there are two charge coils. These magnets of course are also used to generate 12 volts for general purpose use on the boat with a whole different raft of lower voltage coils. I think it is 6 amps of current.

The key to understanding this is in effect you have two generators. One 300 volts or more and one 12 volts.
If you just want ignition you can do away with the 12 volt
part.

Paul Lamar


Paul,

I'm thinking of a slightly different track for the DIY magneto system.  In lieu of using a milled down outboard flywheel where we would have to mount the stator in a stationary position within the spinning flywheel, perhaps we can move towards a lawnmower version, where the magnets are positioned on the outer part of the 'flywheel' and the coil pickup is placed on the outside of the rotating hub.

First issue;  Does the typical lawnmower magneto through enough volts (300dc) to fire the plug in the rotary?
Second issue:  Have to look for a two piston lawnmower, so magneto orientation will be correctly set up (i.e. 180 degrees apart).  I will still need your circuit to get it to 22 deg. BTDC.

I like the fact that lawnmower hubs / flywheels are quite small and light. Perhaps we can double up / stack them to build redundancy.

Your thoughts?

Franklin

Yes it should work. A spark plug is a spark plug so it takes x amount
of voltage to fire. Compression ratio has a minor effect.

Paul Lamar