First, the speed of the shaft — Page 376, Lesson 476

Let’s pick up right where the fuel system leaves off — we’re now inside the heart of the engine’s speed control. I want to walk you through the two ways the engine keeps its shaft speed in check, and then the valves that sit between the fuel control unit and the burners. First, the speed of the shaft. There are two methods used to control it. The first is an electronic one. A tacho-generator — that’s a small device that produces a voltage proportional to the speed of the shaft it’s driven by — or an electronic speed sensor, driven by the appropriate shaft, sends signals to an amplifier. Now, here’s a nice detail: normally this amplifier is the same one that powers the temperature limiting circuits. So the same electronics that watch over temperature also watch over speed. If the output of the tacho-generator approaches a predetermined level, the fuel flow is adjusted to prevent the maximum rpm from being exceeded. In other words, the system senses the shaft getting too fast and trims the fuel to hold it back. Now, the second method is the one normally used to control the speed of rotation of the HP compressor shaft — that’s the high-pressure compressor shaft. This shaft drives the external gearbox, which is responsible for powering the HP fuel pumps, among other things. Fitted within the HP fuel pump is a hydro-mechanical governor. That’s a device that uses hydraulic pressure proportional to engine speed as its controlling parameter. So the faster the engine spins, the higher that hydraulic pressure climbs. If engine speed exceeds a predetermined maximum pressure, a diaphragm opens valves to bleed off some of the servo piston pressure and limit fuel flow to the burners. So instead of electronics, this one uses pure hydraulic force to cap the speed. Now let’s move to the valves that sit downstream of the fuel control unit. First, the HP fuel cock, also called the HP fuel shut off valve. Its job is to shut off the fuel between the FCU — that’s the fuel control unit — and the fuel spray nozzles, which are the burners. This is the normal control for starting or stopping the engine. It can be mechanically controlled by a lever on the flight deck, or electrically controlled by an actuator, which is itself controlled by a switch on the flight deck. So you have two ways to command it: a physical lever or an electrical switch. Finally, the pressurizing and dump valve. This one is used with a duplex fuel nozzle — that’s a nozzle with two separate fuel passages. At a preset pressure, the pressurizing valve opens and allows fuel to flow into the main manifold as well as the pilot or primary manifold. So at low pressure, only the pilot flow is active; once pressure builds to that preset level, the main manifold joins in. The dump valve does the opposite job on shutdown. When the engine is shut down, it allows the manifold fuel to be dumped into the drains tank. Why? To prevent fuel boiling in the manifold due to residual engine heat. After shutdown, the engine is still hot, and fuel sitting in the manifold could boil — so the dump valve drains it away. So to tie it together: speed is controlled either electronically via the tacho-generator and amplifier, or hydraulically via the hydro-mechanical governor on the HP compressor shaft. Then the HP fuel cock gives you start and stop control, and the pressurizing and dump valve manages the duplex nozzle’s two manifolds — pressurizing for normal running, dumping for safe shutdown.