Ignition is the process of supplying sufficient energy to the reactants to cause them to react and develop into a self-sustaining reaction zone in which exothermic chemical reactions occur. These must provide at least as much heat as that which is lost to the surroundings by heat transfer. In diesel engines, auto-ignition occurs due to the high temperature and pressure caused by compression. In spark ignition and gas turbine engines, an electric ignition system provides, at the spark gap, high temperature plasma which, in turn, provides the energy and active species required to initiate flame propagation. In gas turbines, a fuel spray often is used and, therefore, the ignition system must provide sufficient energy to evaporate the fuel droplets prior to ignition. Gas turbines may require starting at a wide range of operating conditions which range from normal start to altitude relight at which the pressure and temperature are very low. Conversely, the conditions at which reciprocating spark ignition engine combustion is initiated are more favourable, but many ignitions per minute are required and these must be repeatable if cycle to cycle variations in the combustion event are to be minimised.
Shown in Fig. 1 is a schematic of a representative ignition system. An inductor and/or capacitor supply a high voltage. This is delivered through a high voltage cable, having known impedance and containinga radio interference damping resistor, to a spark plug which has its own inductance, capacitance and gap resistance.
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