electronic advance

[Clare Snyder]

> From: "Daniel Yap" <flor1 at mailcity.com>
> Subject: Variable advance for ignition
>
> hi, anyone got any success on increasing the advance for ignition
electronically? Theres no vacuum or static advance on the engine, everything
is triggered electronically by a variable reluctance sensor. I know that one
way is to mount the sensor on a slider and shift it but the engine belongs
to the school and they discourage hacking away at the engine. I looked
around and think that a all pass opamp or phase shift opamp might solve the
problem. The diy_efi archives did mention something, Tommy Palm did do
something like this but his details were a bit sketchy.
> I was thinking of delaying the signal 350 degrees which would make it 10
degrees advanced. The main problem is that the phase shift opamp would delay
from 0 to 180 over a frequency range. I tot of having two together but it
would give a variation of delay as the engine goes from 500rpm to 5000rpm.
Any way of having a static advance introduced electronically?
> Thanks and Merry Xmas
> Daniel Yap
>
The Briggs and Stratton Magnetron has an interesting setup. From the way it
was explained to me by a B&S tech the "trigger" is designed so that the
voltage rises faster when the engine turns fater, and the ignition is fired
when the trigger voltage reaches a given point. At low RPM it takes longer
for the trigger voltage setpoint to be reached, retarding the timing. At
High speeds it takes a shorter "time" (number of degrees of crank rotation)
to reach the setpoint, advancing the timing.

If I was trying to design an electronic timing controller I think I would
set the maximum advance, and have the system retard the timing inversely to
speed using a variable delay. A simple circuit that would do this - not
likely well enough for engine use - would be a 555 timer triggered by the
pickup and firing the coil X ms later. An integrated inverted pickup signal
would decrease in voltage with speed increase - connected to the bypass
terminal of the 555 it would reduce the delay as speed increased. OP Amps
may be required for scaling and conditioning, but the principal is what I,m
trying to illustrate.