Martin et al,

I sent the following message to Bob Hoover a couple of days ago and haven't 
heard his opinion yet.  However, your experience has prompted me to go 
public with it (and it's Friday).

After you read it, I hope that you will take the time to open up the gaps 
on you plugs to about .050.  I think that you will find that the 
performance of the CDI will be much better.  At least, if your ignition 
wires hold up.

I welcome anybody else's comments about my 60's research as well.

Thanks,
Mike

Michael A. Radtke  -  Z74               Voice: 602-862-4897
Bull                                      FAX:         4853
13430 N. Black Canyon Hwy.
Phoenix, AZ  85029                      Email: m.radtke@bull.com

----------
>From: 	Michael A. Radtke[SMTP:m.radtke@elm.az05.bull.com]
Sent: 	Tuesday, February 27, 1996 10:29 AM

=========================================================================
Date:         
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         
Subject:      FW: high energy ignition

Bob,

I went off an read your sermons on this.

CREDENTIALS:

I too experimented with CDI in the 60's.  My company (do you remember 
Centralab?) was into ceramic capacitors and since I was into ignitions, we 
looked at building capacitors just for that market.  Thus I got a lot of 
experience.

(The capacitor was a failure.  Our design used a very high dielectric 
strength ceramic that had extensive immunity to wide temperature ranges. 
 Thus we only needed a single plate of about 2x4 cm for about 1 ufd. at 
600v.  The single plate construction was chosen to minimize the temperature 
problems as well.  However, its downfall was the piezo-electric nature of 
the ceramic.  Not only did it sing, but it often fractured itself.  Not a 
good plan.)

POINT:

One thing that I discovered was that all the talk about wider plug gaps was 
not about a benefit, but was about a necessity.  My experience was that for 
narrow gaps, CDI performed worse than Kettering.  An oscilloscope and a few 
experiments soon showed why.

It turns out that time was the overriding issue.  The time constant of a 
inductive discharge system (Kettering) is L/R and a CDI R*C.  Thus, in a 
CDI, a lengthy spark requires a lot of R.  And, since the energy of the 
system (other than the not insignificant radiated impulse) is dissipated in 
R, the R better be in the spark, and not the wiring or other components. 
 The solution is a large gap.

Well, this is easily accomplished, but leads to a second problem.  That is 
very high voltage.  Actually, it's not the high voltage that is the problem 
as much as the spark plug wires not being designed for it.  After my 
<CRL>tm capacitor fiasco, I continued personal involvement with solid state 
ignitions.  I eventually abandoned CDI because my cars had so many problems 
with plug wires in wet weather, which I had plenty of where I was living at 
the time.  I moved to an inductive discharge approach using a power 
transistor biased as a current source to charge the coil and act as the 
switch.  I found this approach every bit as performant as the CDI, and I 
could maintain manufacture's recommended plug gaps and thus keep the plug 
voltages under control.

COMPARISON:

Spark energy:  Inductive Discharge Ignition (IDI) is limited by the  coil 
inductance.  CDI by the discharge capacitor.  Increasing either has the 
side effect of decreasing the maximum spark rate full output can be 
achieved at.

Maximum rate:  IDI is limited by the voltage available, probably 12V and 
the inductance of the coil.  CDI is limited by the high voltage power 
supply current and the size of the discharge capacitor.  Both could be 
limited by point dwell, but both systems electronically ignore this and use 
a fixed spark quench time before beginning the charge cycle.

Spark duration: Discussed above.

My conclusion from this was that electronic IDI was a better system.

TIME FLIES:

I continued to use IDI until my cars came with solid state ignitions.  I 
then jumped to the conclusion that they too were IDI systems and not CDI. 
 However, your notes imply that CDI is the current standard.

THE QUESTION:

Is CDI alive and well in current automotive design?  What was done about 
the high voltage issue?

Thanks,
Mike