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From: Gary Casey <glcasey@adelphia.net>
Subject: Re: Where has all the power gone?

Lots of good comments on this issue, but I feel the need to clarify a  
few issues.  First, there seems to be a consensus that EI provides a  
"hotter and longer" spark.  sometimes true, but the most common EI  
product is a capacitive discharge (CD) system.  The inductive  
(Kettering) system stores energy by using the inductance of the  
ignition coil and the spark occurs when the current in the primary is  
interrupted.  These system, which include magnetos, are characterized  
by having a relatively slow voltage rise time, but a fairly long  
spark duration at a low current level.  A CD system stores energy at  
high voltage in a capacitor and the spark occurs when this is  
discharged into the coil.  Since the coil isn't required to store the  
total spark energy it can be smaller and of lower inductance.  The  
result is a very fast rise time, high spark current and a very short  
duration.  A short-duration spark works fine as long as there is a  
combustible mixture in the spark gap at the moment of the spark.  CD  
systems work very well in engines that tend to foul plugs, which is  
why their initial popularity was in 2-stroke engines, but they  
typically don't work very well in engines that operate with "poor"  
mixture preparation, such as automotive engines running a lot of  
EGR.  I see no problem using CD technology in aircraft engines as we  
don't typically operate at very light loads with EGR, except at  
idle.  The CD system is far more electronically complex than an  
inductive system, which only requires an electronic switch to  
activate the coil.  The automotive guys have spent billions  
researching ignition systems with the result that all automotive  
engines worldwide still use the original Kettering inductive system.   
The CD coils weigh less, but the electronics weigh more.  I agree  
with Walter that the higher spark current will result in a faster  
initial burn, creating an effective timing advance compared to a  
conventional system.

Gary Casey