MSD and Fat Sparks

[Haynes David]

 >   From: Lajber Zoltan  JHT
>   Here, in Europe, several kind of engines and spark plug marks, but, as I 
see:
 >   Each engine prefer that brad of spark plug, which came from same 
counrty.
 >   For example:
 >   Champion - Franche ( Peugeot, Citroen)

I dont know if this idea really has any basis, other than maybe engines 
being designed
around that manufacturers preferred plug maker. On the other hand, my 
Citroen BX
2.0L 16V ran using french plugs (Eyquem FC -3LS) which were triple 
electrode. When
I tried using NGK equivalents  (also triples) I noticed a difference in idle 
quality
when hot, which disappeared on reverting to the old plugs. There was no 
noticeable
difference in performance on the road. The NGK equivalent
part had a longer nose than the Eyquem part (maybe flame shielding ?) and 
thicker
 ground electrodes. The Cit. used to 'eat' spark plugs rapidly,
the original 1.2mm (I think) gap would open to 1.5mm over 12000 miles. The 
centre
electrode of the Eyquem plugs only protuded slightly from the ceramic, such 
that over
12000miles the spark would erode tracks into the ceramic. Since this defines 
the spark
path, it seems that the plug wear would become more rapid when this happens
since the spark connot now ignite across the closest points as would 
normally
happen. (Ignoring variations in the ionisation points of the mixture around 
the plug)
Sadly I returned the NGK plugs for refund (it's handy having friends in 
motor factors!)
so I never found out what the long term might be.

On a different tack :-

When MSD type ignition is used does the spark extinguish between pulses or
is there a continuous ionized path between the electrodes?        IMHO, the
advantage of any MSD arrangement is that the energy per ignition is raised,
without causing breakdown of insulation in the ignition system. In more
conventional systems a given amount of energy is dissipated per spark,
therefore if the plug gap is smaller (less ionization voltage) then the 
spark
duration is longer. However a lower energy spark may not ignited the mixture
so reliably. A larger plug gap will tend to ignite better up to a point, but 
if the
same amount of energy is available, then the spark must collapse more 
quickly
which may again mean that the mixture may not ignite properly.
In an MSD system we can light several 'fat' sparks in rapid succession, thus
the engine will 'see' a high energy spark of long duration, all without
increasing the system voltage, with all the problems that incurs.

If I am correct, then one alternative might be to produce an ac voltage in 
the coil,
and use the 'intelligence' of the electronics to control duration of the 
spark, which
would probably be linked to engine rpm. I dont suggest that this would have 
any
particular advantage over MSD, except that it might be easier to design, and 
it
could be possible to optimise spark duration at different rpm.

I would like to investigate this idea further if/when other commitments 
permit, any
comments will be received with interest.

Dave Haynes

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