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


Paul,  The Briggs and stratton 206 cc  5HP flathead as used on WKA gokarts
has an aluminum flywheel and operates up to the 13,600 RPM limit of the
construction rule book. The solid state control is built into the coil.  Put
two coils per flywheel for two plugs and two stacked flywheels for four
plugs. There is weight which can be turned off the  aluminum flywheel as
well as the cooling fan fins. Mount the two flywheels back to back on a
common hub.  For another choice don't forget the Onan used on motor home
generators and riding lawnmowers.which are 180 degree opposed two cylinder
engines. I have had trouble with the Onan coil wires unscrewing from the
coil post and don't know what RPM it could be safely operated. The parts are
also larger than the gokart magneto and the onan is not solid state (has
points). The Briggs is solid state and reliable. Also each coil/plug is a
complete independent ignition and there are plenty of them for free with
rusty contaminated gas tanks  making the carburetors inoperable.

george grimes