Well, I have read many explanations about how the ignition works and why the
two leads at the coil should be interchanged of polarity is changed.
I try to tell you how coil-ignition works:
The contact is closing during the closing angle (I hope this is the right
translation for Schliesswinkel), allowing the current flowing through the
primary windings of the coil. At the time the spark shall occur at the spark
plugs, the contacts open. The purpose is to let the current going to zero as
quickly as possible. The voltage appearing at any of the coil windings is
N.dphi/dt or L.di/dt (induction law). N is the number of turns of the coil,
L is the inductance of the coil, dphi/dt is the rate of magnetical flux
change and di/dt at least is the rate of the current change.
This shows that the voltage is the higher the quicker the current decreases.
Without the capacitor across the contacts the high voltage induced at the
primary windings (calculated by the above given law) will make the gap
between the contacts be crow-bared by a spark, better arc! The energy stored
in the coil will be discharged in this arc. The effect:no spark at the
plugs, contacts burning.
With the capacitor aross the voltage cannot rise quickly across the
contacts. The spark is very short and is not consuming much energy. The
voltage on the secondary winding will rise quickly up to 10 ... 15 kVolts,
the plug sparks, igniton occurs and the magnetic energy makes the arc burn
for 1 ... 2 milliseconds at a voltage of some kVolts until all the magnetic
energy (stored during closing angle) is consumed by the ark. After the arc
has extiguished (and only from this moment on, not earlier) oscillating
occurs with the amplitude of some kVolts.
Now back to the question which stood at the beginning of all the discussions:
"Why not change the direction of the current flow?" Because the voltage at
the plug should be of positive polarity to the ground potential. This has
been found out to be the best ignition polarity. Your ignition will work if
changed, but it is said to be more effective if not.
I hope this helps to clarify the technical situation.
Rainer Wallnig, electrical engineer, TR4 BRG 63 pos.grd. daily
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