Model Forum / Radio Controlled / Air Models / November 2006

While doing my first strimmer/weedwacker conversion I've been pondering
on simple ways to make things lighter and more versatile:

The flywheel on the new, unknown make engine I'm converting seems
rather lumpy, even after skimming on the lathe. This is because it
houses some large (ferrite, alnico?) magnets which drive the coil which
directly fires the plug.

It just so happens that I have some neodymium magnets the ideal size to
drive the coil.
They are much lighter and more powerful (although I could use even
smaller magnets).

It means that, since I have to make a prop adapter anyway, I could also
have a much lighter aluminium disc "flywheel", with re-positionable
neo' magnets. To balance the disc, i can either bolt a steel tab to the
opposite half, or drill holes in the disk on the magnet side.

This led me to think about IGNITION TIMING:
1) Basically, the faster a magnet spins against a coil, the higher the
voltage. The spark, however, will jump the gap at a set voltage (more
or less).
This means that at high revs, the spark should occour a little sooner,
which is good. But is it sooner enough? I gather advance spark timings
of 20-25 degrees are quite common.

Since the relatively modern neodymium magnets are much more powerful
and lighter there is now much more scope to experiment with magnet
position and distance from coil than before to optimise auto-adjusting
spark timing.

2) The same applies to transistor fired spark ignition:
Instead of using a hall effect switch to trigger the power transistor,
a small magnet and coil could be used. The advantage of this would be
that ignition timing would be automatically advanced as in 1) instead
of employing more complicated advancing methods (timing circuits or
processors).
The max voltage from the coil could be limited by a zener so as not to
blow the transistor base.

Incidentally, neodymium magnets have a low operating temperature of
only about 120C, so they must'nt get too hot (electric flyers note).

A question: Why are power mosfets not used in ingnition cicuits?