Gas Turbine Handbook : Principles and Practices

92 Gas Turbine Handbook: Principles and Practices
Figure 6-2. Typical high energy capacitor-type ignition system.
The low voltage charge in the storage capacitor is not sufficient
to bridge the gap across the spark plug electrodes until the path is
provided by the very high voltage, high tension discharge. Therefore
the high tension capacitor discharges first to bridge the gap across
the electrodes of the spark plug and reduce the resistance to the low
tension discharge. The low tension capacitor then discharges providing
a long, hot spark. One of the drawbacks to this system is that the
leads from the ignition exciter to the spark plugs must be shielded,
heavy duty, leads.
By contrast the low tension ignition system does not have a high
tension transformer, high tension capacitor, or high voltage leads.
However, it does have a semiconductive surface in direct contact
with the center electrode and ground shell of the igniter plug. This
semiconductor is shown schematically as the resistor in Figure 6-3.
The initial flow of current bridges the gap between the center electrode
and the ground shell to reduce resistance to the low tension
discharge. Due to its lower voltages, the low tension system can use
smaller, lighter, ignition cables.
The inductive ignition system (Figure 6-4) utilizes a rapid variation
in magnetic flux in an inductive coil to generate the high energy
required for the spark. While this system produces a high frequency,
high voltage spark, the spark energy is relatively low. This system is
small, light, and less expensive, but it is only suitable for use with
easily ignitable fuels. And, the spark igniter is more prone to fouling
from moisture and carbon build-up.