Gas Turbines - Ignition Systems — Page 350, Lesson 432

Let’s pick up right where the ignition story gets interesting — the heart of the whole system, the High Energy Ignition Unit, or HEIU. I want you to think of this unit as a device that stores a huge amount of electrical energy and then releases it all at once, in a single violent spark. The principle is simple: it charges up a very large capacitor, and then discharges that capacitor across the face of an igniter plug. That spark is what lights the fuel in the combustion chamber. Now, here’s the critical safety point. That capacitor is so large that it is potentially lethal. The stored charge can kill you if you touch the wrong part. So the designers had to build several safety features into the HEIU just to make it conform to safety regulations. The circuit we’re looking at shows every component inside a HEIU that is supplied by 28 volts DC. Let’s trace the energy path from start to finish. When the supply is connected, the primary coil and the trembler mechanism are both fed with 28 volts DC. The trembler mechanism works just like an electric bell — it vibrates, opening and closing a contact rapidly. That vibration chops the steady DC into a sawtooth waveform. This is a very crude form of AC. Now, because it’s AC, we can use transformer action. The voltage is passed from the primary coil to the secondary coil, and there it is boosted up to 25,000 volts. So now we have 25,000 volts of AC. But the capacitor needs DC to charge. So the rectifiers change that AC back into DC, and that DC begins charging the reservoir capacitor. As the charge builds up in the capacitor, it eventually reaches a level where a spark can jump the discharge gap. That spark is the release of all that stored energy. But here’s the clever part — the energy doesn’t go straight to the igniter. It has to flow through the choke first. The choke acts as a normal inductance, and its job is to slow down the flow of energy. Why? Because slowing it down makes the duration of the spark longer. A longer spark gives the fuel more time to ignite. Only after passing through the choke does the energy reach the igniter in the combustion chamber. Now, let’s talk about the two safety devices, because they protect people and the unit itself. First, the discharge resistors. These are a safety device for servicing. Imagine you’ve removed the HEIU for maintenance. The charge that may still be sitting in the capacitor could be lethal to anyone touching the casing. So the discharge resistors allow that charge to leak through them, draining the capacitor to zero once the supply has been removed. That way, the unit is safe to handle. Second, the safety resistors. These act as a kind of safety valve for a different failure — if the igniter plug becomes disconnected. Think about what would happen: if the plug is disconnected, the energy has nowhere to go. There would be a continued build-up of energy in the capacitor, and eventually it would explode. To prevent that, the safety resistors allow energy in excess of the normal level to flow through them, balancing the charge on the plates of the capacitor. So they bleed off the excess before it becomes catastrophic. Finally, let’s talk about the rate of sparking. The normal rate for the HEIU is between 60 and 100 sparks per minute. And here’s an important detail — that rate is completely random. It’s not a steady, rhythmic beat. So if you’re standing at the jet pipe before engine start, and relight is selected, you should hear an unsynchronized beat if both units on the engine are working correctly. If both units were sparking in sync, you’d hear a single steady rhythm — but because they’re random, you hear that irregular, unsynchronized beat. That’s your audible confirmation that both ignition units are functioning. So to summarize the whole chain: 28 volts DC in, the trembler chops it into a sawtooth AC, the transformer boosts it to 25,000 volts, the rectifiers convert it back to DC, the capacitor stores it, the discharge gap releases it, the choke stretches the spark duration, and the igniter fires in the combustion chamber — all protected by the discharge resistors for servicing and the safety resistors for a disconnected plug.