Gas Turbines - Auxiliary Power Units and Engine Starting — Page 358, Lesson 448

Let’s pick up right where the starting systems get practical. We’ve covered the electric starter motor and the starter/generator. Now I want to walk you through the normal start cycle for a two-spool turbofan engine, because this is where all the components we’ve discussed come together in a real sequence. First, the key hardware: each engine has an air turbine starter motor. That’s the device that actually rotates the engine’s compressor to get it spinning. It’s supplied with low pressure, high volume air. Notice the contrast with what you might expect—this is not high pressure air. It’s low pressure but a large volume of it, because the starter needs flow, not pressure, to do its work. Where does that air come from? Three possible sources: the APU, a Ground Cart, or the other engine. So on the ground, you might use the APU or an external ground cart. In flight, if you need a restart, you can cross-bleed from the other running engine. Now, the path that air takes. The system uses the normal bleed air ducting—the same ducting that feeds the pneumatic systems. But here’s the clever part: the flow of air is reversed to the starter motor. So instead of bleed air flowing out to the systems, it’s routed backward into the starter. Now, a critical safety question: if you reverse the flow into the starter, why doesn’t that air also flow backward into the engine compressor and cause problems? The answer is two non-return valves. There’s a non-return valve at the LP outlet—that’s the low-pressure compressor outlet. And there’s a non-return valve facility in the HP Shut Off Valve—that’s the high-pressure shut-off valve. These two valves ensure the air supply will not reverse into the engine compressor. So the air goes only to the starter, not back into the engine core. Let me also tie this back to the earlier part of the chapter, because the starting sequence builds on it. The electric starter motor was the original way to start a gas turbine, and it’s still used in smaller executive jets and helicopters. But it’s fallen out of favour in modern larger engines because of its weight. Rapidly becoming more popular on smaller engines is the starter/generator combination, which has a greater usefulness-to-weight ratio because it serves two purposes—starting and generating. In most starting systems, the starter motor is attached to the engine accessory gearbox and drives the compressor when it rotates. Most electric starter motors incorporate an automatic release clutch device to disengage the starter drive from the engine drive. That clutch is a pawl and ratchet type mechanism, very similar to the one in the air starter motor. It performs three functions: first, it prevents excessive starting torque being applied to the engine; second, it acts as an overrunning clutch when the engine accelerates up to idle speed; and third, it disengages the starter from the engine. There’s a known problem with this sprag clutch ratchet called ‘crash re-engagement.’ That occurs when the starter motor is re-energized before the driven spool has slowed sufficiently for the clutch mechanism to engage itself. So if you try to restart too quickly, the clutch can slam back into engagement—that’s the crash re-engagement. Now, the starter/generator connection to the accessory gearbox is different from the straightforward starter motor. It must remain permanently engaged to the gearbox if it’s to perform its function as a generator, and its control circuitry is much more complicated. So to summarise the normal start cycle: air from the APU, ground cart, or other engine flows through the normal bleed ducting, reversed to the air turbine starter, and is prevented from entering the compressor by the non-return valve at the LP outlet and the non-return valve facility in the HP shut-off valve. That’s the sequence that gets the spool rotating toward idle.