CD Ignition

[John Dammeyer]

At 10:48 AM 17/08/1996 -0700, you wrote:
>The CD ignitions might start better under low voltage circumstances, but,
>the short spark length is not as good as could be expected. I have seen a
>couple of articles that say since the duration of time of the CDI's spark is
>so short, it is VERY prone to preignition due to the mixture not being
>totally ignited, and other sorts of problems.

I Suspect most of that would be due to the match between the coil and the
capacitor used etc.  Aren't we really discussing the amount of energy stored
in the coil and how quickly that energy is released when the current is
stopped by the opening of the points.

As I recall the original CD Ignition had several things going for it:
Higher energy spark and minimal breaker point damage from excess arcing.
Once I had the CD ignition I never replaced the points again in 60K plus
miles in addition to good cold weather starting.

Recall too, that some coils require a ballast resistor to limit total
current.  This is similar to the LR method of Stepper Motor control verses
Chopped.  Simple electrical theory states that once a voltage is placed
accros a coil it takes a certain time until the current reaches maximum V/R
where R is the DC resistance of the coil.  

Stepper Motors have this same problem.  By the time the current is up to the
point where maximum torque is reached the motor is already stpping to the
next position so higher stepping speed means less torque.  One solution is
to increase the voltage so the 'pressure' to push those pesky electrons
through the windings is higher and thus maximum current is reached more
quickly.  To limit the current a DC resistance is placed in series with the
coil so that under steady state conditions the coil doesn't burn out (LR).
The other technique is to use a very high voltage and electronically limit
the current (Chopped).

Automotive coils aren't really a lot different other than the fact that they
are a transformer.  The ballast resistor prevents coil burnout as now the
winding isn't a twelve volt winding anymore but perhaps a 3 volt coil.
Current builds up more quickly to maximum and when the points open, zap, (EE
term coined by students in EE labs throwing charged caps at each other).
Perhaps some experiments with coils and CDI used coils with the ballast
resistor still installed.

So,  as far as spark duration goes, if the spark is constructed by the
primary field collapsing, inducing a voltage in the secondary high enough to
break over the spark plug gap, that spark will stay there until the voltage
is too low to sustain the spark.  I suspect that the CD ignition still
produces the spark as the points open or else timing would be all out of
wack but it's unlikely that at that point there is current flowing through
the coil so the spark is created by the inrush from the higher voltage
capacitor; an opposite polarity spark?  Don't know.  Haven't a schematic
handy ergo the original reason for my post.  

>Now MSD's don't have that
>problem, as they will fire several times, up to a certin RPM. 
My ramblings

John.
Pioneers are the ones, face down in the mud,
with arrows in their backs.
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