Gas Turbines - Ignition Systems — Page 350, Lesson 434

Let's talk about igniter plugs, because this is where the ignition energy actually gets delivered into the combustion chamber. There are two types of igniter plug. The older type works in a similar manner to a piston engine spark plug, but with a much bigger spark gap. Because the gap is so large, the potential, or voltage, required to jump that gap is approximately 25,000 volts. That high voltage creates the need for very good insulation within the unit itself and also in the cabling that feeds it. So with this older type, you're dealing with a high-voltage, high-insulation challenge. The more modern version is the surface discharge igniter plug, shown in Figure 23.3. The key difference is that the end of the insulator is formed from a semi-conductor material. Here's how it works: the semi-conductor allows an electrical leakage from the hot electrode to the body of the igniter. That leakage ionizes the surface of the semi-conductor material, which provides a relatively low resistance path for the energy stored in the capacitor. The discharge then takes the form of a high intensity flashover from the hot electrode to the body of the igniter. And critically, this only requires approximately 2,000 volts — a huge reduction compared to the 25,000 volts of the older type. So the contrast is clear: old type, big spark gap, 25,000 volts, heavy insulation demands. New surface discharge type, semi-conductor insulator, ionized low-resistance path, flashover discharge, only about 2,000 volts. Now, where does this all sit physically? Figure 23.1 shows the HEIU — that's the High Energy Ignition Unit — mounted on the side of an engine, and the position of the igniter within the combustion chamber. So the HEIU stores the energy in its capacitor, and the igniter plug delivers that energy as the flashover inside the combustion chamber. One more thing to note: as well as this type of unit, there are transistorized devices, and for aircraft with an AC electrical system, there are units that will work on that type of supply. So the ignition system has to be compatible with the aircraft's electrical architecture — DC or AC.