Ignition Book

[cburian at uiuc.edu]

Bruce Bowling writes:

> A challenge:
> With all of the bright engineers on this group, can someone
> come up with a circuit which does the multiple spark concept
> maybe better than MSD had implemented?

Are MSD schmatics on line anywhere?  I want to design a multi-spark CD
system.

Some of the things I've considered are:  

Can I make a multispark system by simply letting the L-C circuit (capacitor
and ign. coil) ring, or does it need to be retriggered?  (i.e. can I substitute
a triac for the SCR)  Is retriggering normally done internally by detecting
SCR shutoff and subsequent recharging,or does a timing circuit do the 
retriggering?  It looks like common CS systems recharge at 500Hz.  What are
the disadvantages of raising this frequency so that recharging happens faster
(allowing multisparks at high RPM)?  Larger instantaneous currents in the
primary side (and secondary side)?  Easier question:  what guage wire should
I use in the secondary of my step up transformer?

Re: the discussion of how low a battery voltage a CDI will work at:  in the
several schematics I've gathered of "typical" CD ignitions, the big 
functional difference has been that most units have a push-pull oscillator
connected by three wires to the primary of the transformer, and feedback for
the oscillator comes from the opposing transistor.  But in another design
the FB comes from extra windings in the transformer.  The author pointed
out that this will make the oscillator drive to maximum possible output
no matter what input voltage is.  So, the circuit is overengineered to
work at lower voltages, and applying full 14V doesn't cause excessive current
because of the limiting feedback.

My idea is to boost charge freqency to several kHz, then have the computer
start sparking at the proper crank position and continue to retrigger after
the worst case recharge interval multiple times for n degrees of crank 
rotation.  (How far should I go?  Or, considering worst case, is there a
time when it's better to not fire than fire extremely retarded?  Or just 
keep firing until the exhaust opens?  BTW, this is on a dual coil, 4-cylinder
MC engine, not a car engine with distributor, so that doesn't limit me with
worries about pre-ignition of the next cylinder.)  But a retrigger that is
independent of the initial trigger signal (i.e. the trigger is just a gate
to allow sparking to occur) would be preferable.  Any ideas on how to 
implement self-retriggering?  Comparator checking cap voltage?


Chris