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.

Outlines

00:00

🔥 High-Energy Ignition Systems in Gas Turbine Engines

This paragraph discusses the dual ignition system in gas turbine engines, which includes two high-energy igniter units, each providing about 12 joules of energy to separate igniter plugs. The system is not only used for engine starting but also to prevent flameout during challenging conditions like takeoff from contaminated runways or heavy precipitation. However, continuous use can lead to rapid erosion of ignitor plugs, reducing their lifespan. To address this, some engines feature a combination ignition system with both high and low-energy modes. The high-energy ignition system is activated during the engine start sequence and is automatically deactivated once the engine reaches self-sustaining speed. The igniter units work by charging a large capacitor and discharging it across an igniter plug, and safety features are integrated to prevent potential hazards. The paragraph also describes the components of a high-energy ignition unit, including the trembler mechanism, transformer, rectifiers, and reservoir capacitor, which together generate a consistent spark regardless of environmental conditions.

05:01

🛠️ Safety Features and Types of Igniter Plugs

The second paragraph delves into the safety mechanisms of high-energy ignition systems, particularly the role of safety resistors that act as a safety valve to prevent capacitor explosion in case of an ignitor plug disconnection. It explains how these resistors balance the charge on the capacitor plates to avoid an overload. The normal operation rate of the ignition system is detailed, highlighting the random nature of spark production. The paragraph also contrasts two types of igniter plugs: the older version, which requires high voltage for a large spark gap, and the modern surface discharge igniter plug, which utilizes semiconductor material for a lower-voltage, high-intensity flashover. The summary concludes with a brief mention of transistorized high-energy ignition devices for aircraft with alternating current electrical systems.

Keywords

💡High-Energy Igniter Unit (HEIU)

The High-Energy Igniter Unit (HEIU) is a critical component in gas turbine engines, used for starting the engine and preventing flameout. It operates by charging a large capacitor and then discharging it across an igniter plug. In the video script, the HEIU is described as having an output of approximately 12 joules and is a potentially lethal device due to the high energy it stores, necessitating several safety features to conform to regulations.

💡Dual Ignition System

A Dual Ignition System refers to the presence of two high-energy igniter units in gas turbine engines, each feeding a separate igniter plug. This system provides redundancy and ensures reliable engine ignition. The script mentions that all gas turbine engines have this system, highlighting its importance for engine reliability and safety.

💡Igniter Plug

An Igniter Plug is a device within the engine that ignites the fuel-air mixture. The script explains that the high-energy igniter units feed separate igniter plugs, and the life of these plugs can be dramatically shortened if the ignition system is used in conditions other than engine starting, such as during takeoff from contaminated runways or in heavy precipitation.

💡Continuous Ignition

Continuous Ignition is a feature that provides a lower energy output (3 to 6 joules) for sustained ignition, as opposed to the high-energy ignition used during engine start. The script describes how continuous ignition can be activated by a switch and is designed to minimize erosion of the ignitor plugs, thus extending their life.

💡Flameout

Flameout is a condition where the combustion in a gas turbine engine is extinguished, typically due to insufficient fuel, air, or other operational issues. The script mentions that the ignition system may be selected to prevent flameout during certain flight conditions, such as takeoff from contaminated runways.

💡Reservoir Capacitor

The Reservoir Capacitor is part of the high-energy ignition unit and is responsible for storing electrical energy. The script explains that as the charge in the reservoir capacitor builds up, it eventually reaches a level that causes a spark to jump at the discharge cap, which is essential for igniting the fuel-air mixture in the combustion chamber.

💡Discharge Gap

The Discharge Gap is the space within an evacuated tube where the spark jumps from the discharge cap. The script emphasizes that the tube is evacuated, ensuring that the spark's power remains constant regardless of changes in humidity or altitude, which is crucial for reliable ignition under varying ambient conditions.

💡Choke

A Choke, in the context of the ignition system, acts as an inductance that slows down the current flow, thereby extending the duration of the spark. The script describes how the energy passes through the choke after crossing the discharge gap, which helps in maintaining a sustained spark in the combustion chamber.

💡Safety Resistors

Safety Resistors are components designed to prevent potentially dangerous situations by allowing excess energy to leak away from the capacitor. The script explains their importance in safety, as they act as a safety valve to prevent the capacitor from exploding due to overcharge if the igniter plug becomes disconnected.

💡Surface Discharge Igniter Plug

The Surface Discharge Igniter Plug is a modern type of igniter plug that uses a semiconductor material at the end of the insulator. This allows for a low-resistance path for the energy stored in the capacitor, resulting in a high-intensity flashover that requires less voltage compared to older types of igniter plugs. The script contrasts this with the older type, which operates similarly to a piston engine spark plug but with a much larger spark gap.

Highlights

All gas turbine engines use high-energy igniter units for engine starting.

Dual ignition system with two high-energy igniter units, each feeding a separate igniter plug.

Ignition system output is approximately 12 joules.

Ignition system can be selected to prevent engine flameout during takeoff or heavy precipitation.

Continuous use of high-energy ignition system can dramatically shorten the working life of igniter plugs.

Some aircraft engines have a combination ignition system with low and high energy modes.

Continuous ignition is usually activated by a switch on the engine start panel.

Starting ignition system is activated by the engine start sequence.

Igniters are automatically deactivated after reaching self-sustaining speed.

Automatic ignition system can be triggered by aircraft stall warning system.

High energy ignition unit works by charging a large capacitor and discharging it across an igniter plug.

Safety features are built into the high energy ignition unit to conform to regulations.

The igniter plug operates within an evacuated tube, ensuring constant spark power regardless of ambient conditions.

Discharge resistors act as a safety device to prevent capacitor explosion.

The normal spark output rate is between 60 to 100 sparks per minute.

Transistorized high-energy ignition devices are available for aircraft with alternating current electrical systems.

There are two types of igniter plugs: one with a large spark gap and another with a surface discharge design.

The surface discharge igniter plug uses a semiconductor material for a low-resistance discharge path.

Transcripts

all gas turbine engines use high-energy

igniter units or h e-- I use for engine

starting will investigate the

intricacies of the high energy igniter

unit later

all gas turbine engines have a dual

ignition system fitted which means that

they have two high-energy igniter units

the high-energy igniter units each feed

a separate igniter plug

systems have an output of approximately

12 joules

it may sometimes be necessary to help

prevent engine flameout to have the

ignition system selected in

circumstances other than engine starting

for instance during takeoff from

contaminated runways or flight through

heavy precipitation

the use of the high-energy ignition

system on these occasions will cause the

igniter plug to erode so quickly that it

will shorten its working life

dramatically

to minimize erosion of the ignitor plugs

some aircraft engines are fitted with a

combination ignition system which

includes a low energy three to six

joules continuous selection as well as

the high energy 6 to 12 joules starting

selection

continuous ignition which energizes the

low-energy mode of the igniters is

usually activated by selection of the

appropriate switch on the engine start

panel

the starting ignition system is

activated when the engine starts

sequence is initiated either

automatically or by the operation of the

high-pressure fuel start lever or

fuel and ignition switch

the igniters are automatically

deactivated at some point after

self-sustaining speed usually by the

operation of a speed switch incorporated

in the high-pressure compressor RPM

indicator system

some aircraft have an automatic ignition

system it

all is detected the aircraft stall

warning system will automatically select

the continuous ignition system

the high energy ignition unit works on

the principle of charging up a very

large capacitor and then discharging it

across the face of an igniter plug

the size of the capacitor makes it a

potentially lethal device and several

safety factors have to be built into the

high energy ignition unit so that it

conforms to safety regulations

this diagram shows the high energy

ignition unit mounted on the side of an

engine and also the position of the

igniter within the combustion chamber

the circuit shown here illustrates all

of the components within a high-energy

ignition unit which is supplied by 28

volts direct current

with the supply connected the primary

coil in the trembler mechanism are fed

with 28 volts direct current

the trembler mechanism works on the same

principle as that of an electric Bell by

doing so it causes the primary chord

input to become a sawtooth waveform this

sawtooth waveform is a very crude form

of alternating current

because we've generated alternating

current we can use a transformer to

boost the voltage in the primary coil to

25,000 volts in its secondary coil

the 25,000 volts alternating current is

changed back to direct current in the

rectifiers the direct current commences

charging the reservoir capacitor

as the value of the charge in the

reservoir capacitor builds up it

eventually reaches a level that causes a

spark to jump at the discharge cap

the discharge gap exists within an

evacuated tube the fact that the tube is

evacuated means that changing conditions

of humidity and altitude will have no

effect on the voltage required to jump

the gap within the tube consequently the

power of the spark of the igniter plug

will be constant regardless of ambient

conditions

the electrical energy which has crossed

the discharge gap as then to flow

through the choke the choke acts as an

inductance and slows down the current

flow slowing the rate of current flow

makes the duration of the spark longer

the energy then passes to the igniter in

the combustion chamber

if the unit has to be removed from the

engine for servicing any charge which

may remain in the capacitor even after

the electrical part of the unit has been

disconnected could be lethal to anyone

touching the casing of the high-energy

igniter unit

the discharge resistors act as a safety

device by allowing energy trapped in the

capacitor to leak away to earth once the

supply has been removed

the safety resistors act as a kind of

safety valve if the igniter plug becomes

disconnected

if the ignitor plug did become

disconnected and she would continue to

build up in the capacitor the buildup of

energy in the capacitor would eventually

cause it to explode

to safeguard against a potentially

engine disabling explosion the safety

resistors allow energy in excess of the

normal level to flow through them in an

attempt to balance the charge on the

plates of the capacitor

the normal spark output rate of the

high-energy ignition unit is between 60

to 100 sparks per minute

however the production of Sparks is

completely random and if relight is

selected with the aircraft on the ground

anyone listening at the jet bike before

engine start should hear an

unsynchronized beat if both units on the

engine are working correctly as well as

the direct current high-energy ignition

unit there are transistorized

high-energy ignition devices the

aircraft which have an alternating

current electrical system there are

units which will work on that supply

there are two types of igniter plug

the older of these two types works in a

similar manner to that of the piston

engine sparkplug but it has a much

bigger spark gap

it's approximately 25,000 volts this

extremely high potential requires very

good insulation standards both within

the unit and in the cabling to the

igniter

a more modern version of the igniter

plug is the surface discharge igniter

plug shown here

the end of the insulator of the surface

discharge igniter plug is formed from a

semiconductor material

the semiconductor material allows an

electrical leakage to occur between the

hot electrode to the body of the igniter

the electrical dica CH ionizes the

surface of the semiconductor material

which provides a relatively low

resistance path for the energy stored in

the capacitor

the discharge takes the form of a

high-intensity flashover from the hot

electrode to the body of the igniter

which only requires approximately 2,000

volts this concludes the lesson on

ignition systems

you