17 ATPL Training Gas Turbine Engines #17 Ignition Systems

Summary

This video delves into the intricate workings of high-energy igniter units (HEIUs) used in gas turbine engines, highlighting their dual ignition systems and their critical role in engine starting and preventing flameouts. It covers the mechanics of both high-energy and low-energy ignition systems, the function of igniter plugs, and the safety features built into HEIUs, such as discharge resistors to prevent dangerous capacitor buildup. Additionally, the video discusses the differences between older and modern igniter plug technologies, offering insights into the operational details and safety precautions of ignition systems in aircraft engines.

Takeaways

• 😀 High Energy Igniter Units (HEIUs) are crucial for starting gas turbine engines and are used to initiate combustion.
• 😀 All gas turbine engines feature a dual ignition system, with two high energy igniter units feeding separate igniter plugs.
• 😀 High energy ignition systems typically produce an output of approximately 12 joules, which can be harmful to igniter plugs if used excessively.
• 😀 Continuous ignition systems, which use low energy (3-6 joules), help reduce igniter plug erosion during critical flight situations like takeoff on contaminated runways or flight through heavy precipitation.
• 😀 Some aircraft are equipped with combination ignition systems that offer both high and low energy modes to minimize plug erosion and extend the lifespan of the igniter plugs.
• 😀 The ignition system is activated during engine start and deactivated automatically once the engine reaches self-sustaining speed, often triggered by a speed switch.
• 😀 In the event of an aircraft stall, an automatic ignition system will engage continuous ignition to prevent flameout.
• 😀 The high energy ignition unit operates by charging a large capacitor and discharging it across an igniter plug, creating a spark in the combustion chamber.
• 😀 A capacitor in the ignition system can store dangerous levels of energy, requiring safety measures like discharge resistors to prevent potential harm if the unit is removed or malfunctioning.
• 😀 High energy ignition systems can produce up to 100 sparks per minute, but this occurs in a random, unsynchronized pattern, which can be heard at the jet pipe during engine startup.
• 😀 Modern igniter plugs, like surface discharge igniters, use semiconductor materials for more efficient energy discharge, requiring much lower voltages (around 2,000 volts) compared to older designs (25,000 volts).

Q & A

What is the primary function of high energy igniter units (HEIUs) in gas turbine engines?

The primary function of high energy igniter units (HEIUs) in gas turbine engines is to provide the ignition necessary for engine start-up, ensuring the engine fires properly by creating a spark to ignite the fuel-air mixture in the combustion chamber.

Why do gas turbine engines use a dual ignition system?

Gas turbine engines use a dual ignition system to increase reliability. Each high energy igniter unit feeds a separate igniter plug, providing redundancy in case one unit fails, ensuring the engine starts under a variety of conditions.

What is the typical energy output of high energy ignition systems?

High energy ignition systems typically have an output of approximately 12 joules, which is sufficient to create a spark strong enough to ignite the fuel in the engine's combustion chamber.

In which situations might an aircraft's high energy ignition system be used outside of engine starting?

An aircraft's high energy ignition system may be used during takeoff from contaminated runways or when flying through heavy precipitation. These conditions can cause engine flameout, and the ignition system helps to prevent this by maintaining a steady spark.

What are the consequences of using the high energy ignition system during extended periods?

Using the high energy ignition system for extended periods, such as during takeoff from contaminated runways or flight through heavy precipitation, can cause rapid erosion of the igniter plugs, shortening their working life significantly.

How does a combination ignition system help protect the igniter plugs?

A combination ignition system includes both high energy (12 joules) and low energy (3-6 joules) modes. By switching to the low energy mode for continuous ignition, the system helps reduce the wear on the igniter plugs, preventing excessive erosion.

What safety mechanisms are built into high energy ignition units to prevent injury during servicing?

High energy ignition units have safety features such as discharge resistors that allow any remaining energy in the capacitor to leak safely to earth once the power supply has been disconnected, preventing the risk of a lethal shock during servicing.

What is the role of the capacitor in the high energy ignition system?

The capacitor in the high energy ignition system stores electrical energy until it reaches a sufficient charge to create a spark across the igniter plug. This stored energy is discharged rapidly, producing a high-intensity spark to ignite the fuel mixture.

How does the spark duration in a high energy ignition unit get extended?

The spark duration is extended by a choke, which acts as an inductance to slow down the rate of current flow. This results in a longer, more sustained spark, improving the ignition process in challenging conditions.

What is the difference between the older and newer types of igniter plugs used in aircraft engines?

Older igniter plugs function similarly to piston engine spark plugs but require a much higher voltage (about 25,000 volts) to jump the spark gap. Newer surface discharge igniter plugs, on the other hand, use semiconductor materials that allow electrical leakage and provide a lower resistance path for the spark, requiring only around 2,000 volts.