Ignition Systems - Aircraft Gas Turbine Engines #17

Summary

This script delves into the workings of gas turbine engines' high-energy ignition systems, crucial for starting and preventing flameout in challenging conditions. It explains the dual ignition setup, with two igniter units delivering around 12 joules each. The script also touches on the continuous ignition system for extended use and the automatic ignition system that activates during stalls. The high-energy unit's operation involves charging a large capacitor to 25,000 volts, creating a spark in an evacuated tube for consistent performance. Safety features like discharge resistors are highlighted to prevent potential hazards. The script concludes by contrasting two types of igniter plugs and mentions transistorized ignition devices for AC systems.

Takeaways

• 🔧 All gas turbine engines utilize dual ignition systems with two high-energy igniter units, each providing approximately 12 joules of energy.
• ✈️ The high-energy ignition system is not only used for engine starting but also to prevent flameout during critical operations like takeoff on contaminated runways or in heavy rain.
• 🚫 Prolonged use of the high-energy ignition system can lead to rapid erosion of the igniter plugs, significantly reducing their lifespan.
• 🔌 Some aircraft are equipped with a combination ignition system featuring both low-energy (3-6 joules) and high-energy (6-12 joules) igniters to minimize plug erosion.
• 🔑 Continuous ignition, which energizes the low-energy mode, is typically activated by a switch on the engine start panel, ensuring a constant spark regardless of environmental conditions.
• 🛠️ The starting ignition system is activated by the engine start sequence, either automatically or manually, and is deactivated at self-sustaining speed by a speed switch.
• 🔋 The high-energy ignition unit operates by charging a large capacitor and discharging it across an igniter plug, working with an evacuated tube for consistent spark power.
• ⚠️ Safety features like discharge resistors are integrated into the high-energy ignition unit to prevent dangerous capacitor discharges and potential engine damage.
• 🔌 The unit is supplied by 28 volts DC, and the primary coil's output is transformed into a high voltage AC, which is then rectified back to DC to charge the capacitor.
• 🔌 There are two types of igniter plugs: the traditional type with a large spark gap requiring high voltage, and the modern surface discharge type that operates at lower voltages due to its semiconductor material.

Q & A

What is the purpose of high-energy igniter units in gas turbine engines?

High-energy igniter units are used for engine starting and to prevent engine flameout during critical operations such as takeoff from contaminated runways or flight through heavy precipitation.

How many high-energy igniter units do gas turbine engines typically have?

Gas turbine engines have a dual ignition system fitted, which means they have two high-energy igniter units.

What is the approximate output of each high-energy igniter unit?

Each high-energy igniter unit has an output of approximately 12 joules.

Why might the use of the high-energy ignition system shorten the working life of the igniter plug?

Using the high-energy ignition system during operations other than engine starting can cause the igniter plug to erode quickly, which dramatically shortens its working life.

What is a combination ignition system and why is it used?

A combination ignition system includes both a low-energy continuous selection and a high-energy starting selection. It is used to minimize erosion of the ignitor plugs by providing a lower energy option for continuous ignition.

How is the starting ignition system activated in an aircraft engine?

The starting ignition system is activated when the engine start sequence is initiated, either automatically or by the operation of the high-pressure fuel start lever or fuel and ignition switch.

How does the high-energy ignition unit work?

The high-energy ignition unit works by charging a large capacitor and then discharging it across the face of an igniter plug.

What is the purpose of the discharge resistors in the high-energy ignition unit?

The discharge resistors act as a safety device, allowing energy trapped in the capacitor to leak away to earth once the supply has been removed, preventing potential explosions.

What is the normal spark output rate of the high-energy ignition unit?

The normal spark output rate of the high-energy ignition unit is between 60 to 100 sparks per minute.

What are the two types of igniter plugs mentioned in the script and how do they differ?

The two types of igniter plugs are the older type, which works similarly to a piston engine spark plug but with a much bigger spark gap, and the more modern surface discharge igniter plug, which uses a semiconductor material at the end of the insulator for a lower resistance path and requires approximately 2,000 volts for discharge.