Engine Fuel Systems Part 2 - Aircraft Gas Turbine Engines #20

Summary

This script discusses the intricacies of gas turbine engine fuel systems, focusing on maintaining optimal fuel flow for various conditions. It covers the barometric pressure capsule, which adjusts fuel flow based on pressure changes, and the acceleration control unit that prevents compressor stall during engine acceleration. The script also details the importance of monitoring exhaust gas temperature to ensure engine efficiency and safety, the power limiter device to protect the compressor casing, and the methods for controlling compressor rotational speed. Additionally, it touches on the high-pressure fuel shutoff valve and the electronic engine control systems, including FADEC, which enhance fuel economy, reliability, and engine life.

Takeaways

• 🌀 The fuel flow to burners must be adjusted to maintain a fixed RPM at various altitudes and air speeds, which is managed by a barometric pressure capsule within the fuel control unit.
• 🚀 To prevent compressor stall and surge during acceleration, an acceleration control unit is used to regulate fuel flow based on engine intake pressure and compressor delivery pressure.
• 🌡️ Exhaust gas temperature is crucial for engine efficiency, and automatic monitoring combined with a top temperature control unit helps to prevent turbine meltdown.
• 🔧 Thermocouple probes measure the temperature at the rear of the engine, and their output is used to control fuel flow through a temperature control signal amplifier.
• 💥 A power limiter device in the fuel control unit prevents compressor casing failure by maintaining a maximum pressure ratio, using signals from intake and compressor delivery pressures.
• ⚙️ Compressor rotational speed is controlled either by an electrical signal from a tachogenerator or by a hydro mechanical governor in the high-pressure fuel pump.
• 🛑 The high-pressure fuel shutoff valve controls the fuel supply to the engine and can be operated manually or electrically.
• 🔄 A dual in-line fuel spray nozzle system with a pressurizing and dump valve prevents fuel from boiling and clogging lines when the engine is shut down.
• 💻 Modern gas turbine engines use electronic engine control systems, such as Supervisory engine control or Full Authority Digital Engine Control (FADEC), for better fuel economy, reliability, and engine life.
• 🔗 FADEC systems monitor all engine parameters, control the engine through various modes of operation, and are considered as essential as the engine's compressor or turbine.

Q & A

What is the purpose of adjusting the fuel flow to burners in a gas turbine engine?

The purpose of adjusting the fuel flow to burners is to maintain a fixed RPM for a selected throttle position at all altitudes and air speeds, ensuring the engine operates efficiently under varying conditions.

How does the barometric pressure capsule influence fuel flow in a gas turbine engine?

The barometric pressure capsule expands or contracts in response to P1 pressure, which in turn modifies the fuel flow to suit the new conditions, ensuring the engine maintains the desired RPM.

Why is it necessary to regulate the fuel flow during engine acceleration?

Regulating the fuel flow during acceleration prevents compressor stall and surge, which can damage the engine. The acceleration control unit adjusts the fuel metering plunger to achieve the fastest engine acceleration without causing these issues.

What role do thermocouple probes play in monitoring exhaust gas temperature in a gas turbine engine?

Thermocouple probes are positioned in the gas stream to measure the temperature at the rear of the engine. Their output is used to control the fuel flow indirectly through a temperature control signal amplifier, ensuring the engine runs at the highest possible temperature without causing material failure.

How does the top temperature control unit prevent meltdown in the turbine assembly?

The top temperature control unit allows the pilot to select full power without risking a meltdown by automatically monitoring exhaust gas temperature and adjusting fuel flow to maintain safe operating temperatures.

What is the function of the power limiter device in a fuel control unit?

The power limiter device prevents compressor casing failure by controlling the fuel flow based on the ratio of compressor delivery pressure (P3) to intake pressure (P1), ensuring the maximum pressure ratio is not exceeded.

How are compressor rotational speeds limited in a gas turbine engine?

Compressor rotational speeds are limited either by an electrical signal proportional to the speed of the compressor shaft, which adjusts fuel flow to prevent exceeding maximum RPM, or by a hydro mechanical governor that uses hydraulic pressure to control the high-pressure fuel pump output.

What is the purpose of the high-pressure fuel shutoff valve in a gas turbine engine?

The high-pressure fuel shutoff valve controls the flow of fuel between the fuel control units and the fuel spray nozzles in the combustion chambers, used for starting or stopping the fuel supply to the engine.

How do electronic engine control systems improve gas turbine engine performance?

Electronic engine control systems, such as Supervisory engine control systems and Full Authority Digital Engine Control (FADEC) systems, improve fuel economy, reliability, and engine life by providing better control over engine parameters and preventing limitations from being exceeded.

Why is a digital computer considered an essential component of the FADEC system?

A digital computer is considered essential in the FADEC system because a complete failure of the FADEC system would result in a complete loss of engine thrust, making the computer as critical as the compressor or turbine.