Condensor/Capacitor for Ignition Coil

[Kalle Pihlajasaari]

Hi All,

Long discource on Points capacitors.

Steve Ravet wrote
> "Tony Bryant" <bryantt at psc.fp.co.nz> Wrote:
> | Does the ignition condensor/capactitor serve any function
> | other than to reduce arcing across points? Do I need one
> | if I switch using a transistor? (with 400V zener clamp)
> | 
> | Facts, not theories, please..
> | 
> | ***********************************************************
> | * "Insanity is the only sane response to an insane world" *
> | *                 >> bryantt at fp.co.nz <<                  *
> | ***********************************************************
> 
> Are you talking about electronic ignition or point ignition?  I've never 
> actually worked on a point ignition (not that old), but I'm pretty sure the 
> condensor stores a charge, which is discharged thru the coil to make a 
> spark.  That is CD (capacitive discharge) ignition.  Electronic ignitions 
> usually charge the coil primary during the dwell period, interrupting the 
> charge causes the field to collapse and generate a spark, no capacitor 
> needed.  HEI ignitions (from GM) are electronic, but have a condensor.  The 
> only purpose of the condensor is noise suppression for the radio, however.  
> The car will run fine without it.

There are a few things getting mixed up here.

We have the oldie Ignition coil system that stores the spark energy
in the magnetic field of the SOFT iron in the middle of the coil
the energy is not stored in the windings (primary or secondary) but is 
charged or discharged using these windings.

The Capacitor Discharge Ignition systems store the energy in the 
potential of a capacitor and they are ususally charged to a higher
than battery voltage with some sort of chopper circuit in high energy
devices otherwise you need a large capacitor and a low impedance coil
usually a standard 9 Volt ingnition coil, the high voltage CD 
systems can use smaller high impedance coils with much less Iron in them 
as they do no want to store the energy in the core, they want the spark
when they dump the cap into the coil not when the field collapses.

The electronic points system is a standard Mag storage system with 
just a transistor switch to save on contact errosion on a set of
points.

There is also a high frequency system used in some cars that makes
use of a Ironless low impedance coil and drives it with a high 
frequency to get multiple sparks during the time the distributor
rotor is pointing at the cylinder starting at the firing point
this means that turbulence cannot cause a misfire.


===== NOW =====
That little capacitor in the Mag store systems across the points
OR the transistor switch.  It is required.  Back in the Boys Brigade
{similar to scouts} when I was an ignorant 11, I took an instructor
to task about it being only for suppression but he maintained it was
for storage (we were both wrong).  I was so cock-sure that we ended up 
trooping out to test this on the instructors car and sure enough
it would not run without the cap in place (big blow to my ego). Later
when styding 1st year Circuit theory at university we used the Ignition
coil and Capacitor TANK cicuit as an example for damped occilation
calculations.  The Capacitor and coil form a resonant circuit which
does not allow the magnetic field to collapse too fast.  The two results
of a too fast collapse are firstly that the voltage on the coil primary
may rise to the point that it arcs across the points, this will cause a 
sustained current in the primary and will result in too slow a discharge
of the magnetif field and hence a low voltage long spark.  The other
result you will find in the case of no points (or transistor) arcing
is that the spark will be too high a voltage and will flash over in the
insulation inside or outside the coil and cause the spark to be quenched
before it has a chance to ignite the fuel-air mix.  Plug lead inductance
can cause the voltage to arc first inside the coil before your real fast
spark even gets to the plugs.
===== cut =====

The suppression capacitor is the one you often see attached to the 
B+ terminal on a Alternator and is to get rid of the alternator 
whine from the radio &c.

If you want a faster rise time and higher voltage shorter striking spark
you can try using less capacitance and the rise time will be reduced, you can
try two capacitors in series (buy two at the same time othewise you may
exceed the voltage rating of one) or if you think that a lower strike
voltage is enough you can use two capacitors in parallel.  Remember that
all of this only matters until the arc is formed at the spark plug
at which point you have a constant energy discharge that will dissapate
the stored magnetic field at a much lower sustaining voltage of about
2-5kV whilst the striking voltage is between 8 and 16 kV.

Cheers
-- 
Kalle Pihlajasaari     kalle at data.co.za
Interface Products     Box 15775, Doornfontein, 2028, South Africa
+27 (11) 402-7750      Fax: +27 (11) 402-7751