CDI's in Oz

[garfield at pilgrimhouse.com]

On Fri, 22 May 1998 07:30:48 +1200, Mike Morrin <mikem at southern.co.nz>
wrote:

>I must learn not to make throwaway comments...

Not a problem AT ALL. One line zingers like that are fine as long as
you're willing to hang around for the 'encores'. B)  I wasn't really
familiar with CDI's merits/problems but I did understand from the other
guys' previous remarks that there was a potential problem with CDI's
extremely short duration spark. BUT, I didn't understand WHY until your
comment & followup. In my ignorance, I also had skipped over a key
comment in the Saab patents, which I'll mention in a moment!

>Just the reason of CDI sparks being too fast for lean burning motors.
>Typically causing a light throttle misfire. 
>...
>I don't know whether Saab have a way of stretching the spark (the DSE kit
>doesn't), or if they just have an engine design which tolerates short spark
>duration.

I remember reading in one of the Saab patents a comment on the relative
duration of CDI vrs Inductive IGN. Here it is: (from #4,648,367, col. 5,
line 65)
  "At 6000 rpm the CDI spark has a duration equivalent to only 3 to 4
degrees. The spark in the inductive system has a duration equivalent to
about ten times as many degrees at these rpms before it is extinguished.
The measurement window in inductive systems therefore opens much later
than for a capacitive system". They make this observation to say that
CDI gives them a bigger measurement window, a good thang.

BUT, here's a maybe equally important quote, from same patent, which I
would have completely missed were it not for our current "sidebar":
(col. 2, line 5)
  "In a capacitive ignition system, the measuring capacitor in the
ignition circuit causes an EXTENSION [emphasis mine, Gar] of the spark
duration, resulting in a more reliable and smooth combustion in the
engine, particularly before it has attained it's normal working
temperature."

So this may be preventing the "punch through" that Bruce mentions,
without unduly elongating the spark to the length of inductive systems.

Unless I miss my guess, a number of you are saying the same thing I said
when I read that just a few minutes ago, namely, "Well, I'll be
darned!". The "measurement capacitor" they mention is THE capacitor that
gets charged to 400V/80V and keeps the voltage on the plug at this level
once the main spark voltage has subsided. I guess that sucker is doing
double duty! The reason they use a capacitor in the actual circuit,
instead of the "block diagram" depiction of a 400V/80V "voltage source"
(not in the patent, but in the Saab manual), is that they USE the "wilt"
of the capacitor's charge to determine how high and long the ionization
current will be. Helps to get a good measure of the "amount" of residual
ionization right after the spark. Hence the "measurement capacitor".
We'll discuss how the rest of the circuit works later, via a running
commentary on the patent, but I just thot I'd throw that one detail out
so the quotes above would make more sense.

The REALLY neat thing about this "sidebar" of ours on the duration of
CDI sparks, is that once we have ION working on a "lab rat", maybe with
one of these self-built CDI systems, we can actually MEASURE just what
and how much this "spark extension" effect is on a scope, with and
without ION in the circuit! Hey, "research", dudes. That's the adult
word for "going exploring" that used to be okay to say when we were
kids. Now we're so much more "serious". Hee hee.

Gar

P.S. One last thing to mention, for a sanity check amongst ourselves
please, is that if it's indeed true that an inductive spark is "10 times
the duration before being extinguished" compared to CDI, and that means
at say 6000rpm, a spark duration of say 30deg crank (golly that's hard
to believe!), then all kinds of post-ignition bad boy events like say a
25deg late firing could be happening?, and ION would still be "blinded
by the flash" of the inductive spark and unable to detect them.
Whadayall think about THEM apples? I confess I don't know enough to
weigh the relative exposure to "bad events", based on whether they are
pre or post ignition. Anyone offer some insight on this? Obviously this
issue doesn't effect pre-ignition detection at all, but I just don't
have a handle on how bad and "soon" after the ignition pulse the bad
post-ignition events can be.