Gas Turbines - Fuel Systems — Page 376, Lesson 473

We're starting a new topic now: the fuel system of a gas turbine engine. And I want to begin with the heart of it, the component that decides how much fuel actually reaches the burners. That component has two names, and you'll see both in the manuals: the Fuel Control Unit, or FCU, and the Fuel Flow Regulator, or FFR. They're the same thing. Its job is to control the fuel flow for a given thrust setting. So when you set the throttle, the FCU is what meters the fuel to match that demand. But it's not a simple tap. Inside the FCU there are various devices, and each one adjusts the fuel flow to handle a different condition. The book lists four of them: variations in air intake pressure, engine acceleration control, exhaust gas temperature, and compressor delivery pressure. Let me take each one in turn, because each is a separate little system inside the unit. First, altitude control. Here's the problem: as you climb, the air pressure at the intake changes. The book calls that intake pressure P1. If the fuel flow stayed the same while P1 dropped, the engine wouldn't hold its rpm. So the FCU has to change the fuel flow to the burners so that a fixed rpm is maintained for a selected throttle position at all altitudes and airspeeds. How does it sense altitude? Through a capsule that expands or contracts as P1 changes. That expansion or contraction modifies the fuel flow accordingly. This capsule has a proper name: the barometric pressure capsule, abbreviated BPC. And it lives inside something called the altitude sensing unit, which is built into the FCU. Now the second device: acceleration control. Think about what happens when you push the throttle forward. Adding fuel is what causes the engine to accelerate. But here's the danger — if you add fuel too rapidly, that's the usual cause of compressor stall and surge. So the FCU contains an acceleration control unit. It receives information about two pressures: engine intake pressure, P1 again, and compressor delivery pressure, which for a two-spool engine is called P3. Using those two pressures, it adjusts a fuel metering plunger. That plunger effectively acts as a second throttle valve in series with the main throttle. In series, meaning the fuel has to pass through both. And it regulates the fuel flow to give the maximum engine acceleration without causing stall or surge. So it's a limiter — it lets the engine accelerate as fast as it safely can, but no faster. Third, exhaust gas temperature limiting. And I want you to note what the book says here: exhaust gas temperature is probably the most important parameter in a gas turbine engine. The reason is efficiency. To get maximum efficiency, the engine must be run at the highest possible temperature in the turbine — but without melting the materials the turbine is made from. So there's a constant battle: run hot for efficiency, but not so hot that the metal fails. The FCU has to manage that limit. And the fourth item in that opening list is compressor delivery pressure — that's the P3 I already mentioned, which feeds into the acceleration control unit. So to pull it together: the FCU or FFR is the brain of the fuel system. It takes the thrust setting you select, and then it continuously adjusts the fuel flow using these sensing devices — the barometric pressure capsule for altitude, the acceleration control unit with its fuel metering plunger for surge protection, and the temperature limiting for turbine protection. That figure shows the hydro-mechanical fuel control and how these elements are arranged. Now, one thing I should flag — the excerpt cuts off mid-sentence right at the temperature limiting part. So we've covered what's here: the FCU's role, the altitude sensing with the BPC, the acceleration control with P1 and P3 and the metering plunger, and the principle of EGT limiting. When we continue, we'll pick up the rest of that temperature limiting story.