In short, the capacitor in an ignition system
manages the rate of change of the voltage in the low tension ignition circuit
when the contact breaker opens, to prevent the energy that is meant to produce
an HT spark at the plug from being wasted as arcing at the contact breaker.
In more detail, the low-tension side of a magneto can be thought of
as a coil (the low-tension or 'LT' winding) spinning in a magnetic field so that
it acts as an alternator, and a switch (the contact breaker or 'CB') that opens
and closes once or twice per revolution of the LT winding. With the CB closed,
an electrical current builds up in the closed circuit formed by the LT winding
and the closed CB. When the CB opens, a large voltage is induced between the
ends of the LT winding (and also across the CB) because it is the nature of a
coil that it doesn't like the current flowing through it to change, and it will
try as best it can to keep the current flowing. The LT winding in combination
with a second coil (the high-tension or HT winding) then act as a transformer to
multiply up the voltage many times so that it can make a spark at the sparking
If the LT and HT windings, the CB and everything
else were perfect, then the voltage induced across the LT winding would rise to
infinity in an infinitesimally short time, and the voltage across the HT winding
would try to become many times infinity. But things aren't perfect. In particular, even
though the CB opens quickly, it takes a finite time, and even when it is fully
open, the CB contacts are only a very small distance apart. Immediately the CB
begins to open, the large voltage that appears across its contacts can be enough
to strike an arc between them so that an electrical current continues to flow
through the CB and the LT winding. As a result, the voltages across the LT and
HT windings become significantly less than infinite. Indeed, the HT voltage may
well be insufficient to cause a spark at the sparking plug, and much of the
electrical energy that was generated by the LT winding spinning in the magnetic
field gets dissipated at the CB as heat, light and damage to the CB contacts.
To prevent, or at least significantly reduce, the
arcing at the CB, a condenser or capacitor is connected between the contacts of
the CB and therefore between the ends of the LT winding. By contrast to a coil
that tries its best to prevent changes in electrical current through it, a
capacitor tries its best to prevent changes in voltage across it. Put the two
together and they dance or resonate with each other at a particular rate
dictated in part by the capacitance of the capacitor. What is important in a
magneto is the very first step of that dance.
So, just before the CB opens, energy has been
stored up in the LT winding in the form of an electrical current flowing through
it and the closed CB, while the capacitor is shorted out by the CB. Immediately
the CB begins to open, the LT winding no longer sees a short circuit between its
ends, but instead sees an uncharged capacitor. The current continues to flow
around this new circuit of LT winding and capacitor, but decays at a rate
dictated in part by the capacitance. Meanwhile, the voltage across the LT
winding and capacitor (and therefore also across the CB) starts ramping up at a
rate dictated in part by the capacitance. So, we can now exercise some control
over how quickly the voltage changes.
The aim is to choose a capacitor such that, while
the CB is opening and the distance between its contacts is increasing, the
voltage across the capacitor (and therefore across the CB) never exceeds the
breakdown voltage of the air between the CB contacts. If that is done, then an
arc and consequent energy loss between the CB contacts is prevented.
The choice of the capacitance value is important.
If it is too small, then arcing will arise at the CB contacts. If it is too
large, then in addition to slowing down the rate of change of the voltage, it
will also reduce the maximum voltage that can be attained so that it may be
insufficient to cause a spark to be struck at the sparking plug. It should also
be borne in mind that the capacitor needs to function satisfactorily over the
whole speed range of the magneto.