Types of Ignitions

[bcroe at juno.com]

I'll just say a couple things about inductive and capacitive 
(energy storage) ignitions.  You store up energy in a coil 
by applying voltage long enough to get some current 
flowing.  Exceeding the max time will just generate heat,
so max dwell is a good thing.  Once the ckt for primary 
current is broken, the stored energy must find another
place to go.  The "kickback" voltage starts rising, very
fast, and it keeps rising till it finds a path for the amp-turns
(current) that was built up.  If all goes well, it will rise high
enough to break down the spark plug gap, the distributor
gap, and conduct the amp-turns as before.  Note that the
voltage "adjusts" to whatever level is needed to start
current flowing.  Since the new path is in the secondary 
with 100 times as many turns, a constant amp-turns
will give 1/100 the primary current.

Now that the coil is dumping energy instead of taking
it, there will be an (exponential) decay in the current.

Once the gaps are ionized and the current is flowing, the
required voltage will drop way down.  Once again the coil
will "adjust" to just the voltage required to maintain the
current.  At this much lower voltage, the coil energy will
be dissipated much more slowly, giving a decent length
spark.  

With a capacitive discharge ignition, all the energy is 
stored up as voltage in a capacitor.  At the proper time 
the capacitor is connected to a 100:1 transformer and
the output drives the ignition.  No attempt is made to 
store energy in the transformer (or coil), makes you 
wonder why the same design coil should even be used
in this ap.  300 volts in the cap is converted to 30,000 
volts at the ignition.  It doesn't matter if the breakdown 
potential is 2000 volts, or 20,000 volts, or 50,000 volts
(!!!), you get 30,000 volts.  So that number better be
plenty high.

But as soon as the path is established, the voltage needed
drops way down.  Does the capacitor adjust? NO WAY.  Its
like trying to charge a 6 volt battery by connecting it to a
24 volt battery.  The cap quickly dumps all its energy in a
flash trying to pull up to its level.  Some energy is transferred
across, but the mismatch causes a lot to be just wasted as 
heat in the ignition components.  You get a big current but
a very short one.

My experience with these ignitions decades ago pretty much
agreed with the above theory.  The real world is not quite this
simple, and I'm sure practical cap discharge ignitions have
been built.  Still, it just seems like the wrong way to go about
it, and I'll stick with inductive discharge, thankyou.

Bruce Roe

On Wed, 12 Feb 1997 17:04:02 -0500 "Bruce" <nacelp at bright.net> writes:
> 
> Since the recent post about CDIs, and having just 
> posted this on another list, it seems appropriate to 
> mention some things about the different igniton 
> systems, ie the good and back of the various ones.
> Bruce

----- End of forwarded message from owner-diy_efi at diy-efi.org -----
----------------------------------------------------------------------------
To unsubscribe from diy_efi, send "unsubscribe diy_efi" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org