Aircraft Gas Turbine Engine Ignition System Design and Operation

Gas Turbine Engine Ignition System

Turbine ignition systems are typically less problematic than the standard reciprocating engine igniting system since they are only used for a brief period of time during the engine-starting cycle. There is no requirement that the ignition system of a turbine engine ignite at a precise moment in its operational cycle. The fuel in the combustor is ignited using it, and it is then turned off. For some flying conditions, continuous ignition with a lower voltage and energy level is used as a mode of turbine ignition system operation.

In the event that the engine should burn out, continuous ignition would be used. The fuel could be relit by this ignition, preventing the engine from shutting off.

The majority of gas turbine engines are air cooled by fan airflow and have a high-energy, capacitor-type igniting mechanism. Fan air is ducted to the exciter box, where it surrounds and flows around the igniter lead before returning to the nacelle region. When continuous ignition is employed for an extended period of time, cooling is crucial. An alternative to the more straightforward capacitor-type ignition system is the electronic-type ignition system, which is available for gas turbine engines.

The two main types of jet engine igniting systems are the capacitor type, which uses high-energy, extremely high-temperature sparks generated by a condenser discharge, and the induction type (now obsolete), which produces high-tension sparks using standard induction coils. Glow plugs are a third type of ignition system that is not commonly used but is included on some Pratt & Whitney Aircraft PT6A models. The glow plug ignition system has the benefit of not producing the same kind of electromagnetic radiation as the capacitor ignition system, which eliminates the need for a filter to prevent interference with the aircraft's electronic components.

Design Consideration

Even the most knowledgeable observer has problems distinguishing between the many types of gas turbine ignition systems, despite the fact that all of them have the same fundamental components. Most turbine-powered aircraft ignition systems have varying levels of actual performance, design options, and outward manifestations depending on the application. There are numerous models of turbine engine ignition systems in use today for a variety of reasons, including engine operational requirements, combustor designs and performance parameters, operating environments, mounting considerations, FAA requirements, and various design philosophies associated with providing reliable ignition.

Electronic Ignition Systems

For the reasons listed in the article addressing ignition-system requirements, modern engines require not only a high voltage to jump a wide-gap igniter plug, but also a spark of high heat intensity. For gas turbine engines, the high-energy, capacitor-type ignition system has been widely adopted because it offers both a high voltage and an unusually hot spark that spans a sizable area.

Turbine engine igniting is provided by this capacity-type system. It is only necessary to ignite the engine, like other turbine ignition systems; once combustion has started, the flame is constant.

Two storage capacitors are incorporated into each discharge circuit; both are housed in the exciter unit. The voltage across these capacitors is stepped up by transformer units installed in exciter unit. When the igniter plug fires, the gap's resistance is reduced just enough to allow the larger capacitor to discharge across the gap. The second capacitor's discharge has a low voltage but a significant energy content. The end result is a spark with high heat intensity that can burn away any foreign deposits on the plug electrodes in addition to igniting unusual fuel combinations.

At comparatively high altitudes, excellent chances of igniting the fuel-air mixture are guaranteed. The capacitor kind of ignition system is referred to as high energy throughout this section. The intense spark is produced by using very little electric energy for very brief periods of time. Energy is the capacity for doing work. It can be expressed as the product of time and watts of electrical power. Systems for starting gas turbines are measured in joules.

The joule, another way to express energy, is the quantity of energy used up in a second by an electric current of one ampere flowing through a resistance of one ohm. The relationship among these terms can be expressed by the formula:

W =J/t

where W = watts (power)

J = joules

t = time, s

The power level attained determines the temperature of the spark, all other things being equal. The formula shows that a high-temperature spark can be produced by raising the energy level J or by reducing the spark's idle period t.

A heavier, bulkier igniting unit will result from raising the energy level since the energy

delivered to the spark plug is only about 30 to 40 percent if the total energy stored in the 'Capacitor. Higher erosion rates on the igniter plug electrodes would also occur because of the heavy current flowing for such a comparatively long time. Furthermore, much of the spark would be wasted, since ignition takes place in a matter of microseconds. On the other hand, the length of the spark cannot be too short because heat is lost to the igniter plug electrodes and because the fuel-air mixture is never totally gaseous.

Ignition System Components

An ignition system for a turbine engine consists of three parts:

Exciter
Ignition lead
Igniter

Working Principle

The exciter uses the electrical system of the aircraft as its source of input current, boosts the voltage, and sends a high voltage output signal to the igniter through the ignition connection. Once the gap of the igniter is ionised and the field between the centre electrode and igniter shell has disintegrated, the igniter will spark. The ignition systems are optimized to reduce to total cost of ownership for the end user. Many of ignition systems incorporate solid state switching technology for maximized performance, reliability, life and environmental considerations (non-radiation bearing exciters).

Exciter

Most engine exciters can provide a solution that is lower in weight, more powerful, easier repairable, and more dependable than that provided by competitors' exciters by using two fully redundant ignition channels.

Exciter Characteristics:

Higher spark rate for reliable starts under all conditions ( 1 to 3-spark per second solid-state ignition system.
0.7 to 1.2 joules of energy per second to light the engine.
Energy is provided using a non-radiation carrying exciter and solid state components (no spark gap)
High-temperature paint was used to prevent corrosion.
Un-potted design for improved reparability

In the high voltage-switching component of some manufacturers' ignition exciters, a tiny amount of radioactive Tritium gas is present. Spark gaps are straightforward and dependable high voltage switches that have been utilised in aircraft turbine engine ignition systems since the 1940s. They do need a tiny amount of radioactive gas to build them. The core problem is that for the course of the unit's operating life, a radioactive isotope must be present in order to maintain a constant spark gap ionisation voltage. Without the trace amount of tritium gas in the spark gap, the spark gap's ionisation voltage may change significantly over time, potentially lowering the exciter's delivered energy.

Igniters

Optimized use of precious metals, super alloys, coatings, air-cooled tip designs, fuel drain slots (which prevent the igniter from quenching), and high temperature sealing features to withstand today's extreme combustor conditions are just a few manufacturing innovations that have extended the life of the turbine igniter design. In order to respond more consistently to challenging beginning conditions, they are also employing engobe semi-conductive/solid body semiconductor materials.

Modern Igniter Features

Silver-plated shell for mechanical contact protection against corrosion and seizing.

Modern mechanical and glass sealing measures are built outside the combustor casing to prevent gas leakage in high-temperature environments.

Internal configuration designs that protect delicate ignition lead components by maximising heat flow away from terminations.

An inside coating that protects the terminations to stop lead components from seizing.

Firing end design choices that, in some applications, can provide an igniter life of up to 5,000 flight cycles.

Three alternative semiconductor bodies are available to meet the needs of low voltage applications.

External wear and thermal barrier coatings are used to shield the life-limiting features from wear and thermal distress.

Ignition Lead

The engine exciter and igniter are connected via the ignition leads, which are built to provide the best possible conduit. With reduced flashover, better shielding, and more airflow in air-cooled leads, they supply more energy than similarly built rival brands. Modern air-cooled ignition leads have patented features that ensure optimal cooling airflow for the duration of the lead's service life and the longest possible on-wing life when used with the newest igniters.

Features

Inflammer-to-lead and exciter-to-lead interfaces in contact assemblies are made from robust materials to prevent contact arcing.
Choosing materials for conduit designs that best address EMI shielding and durability issues; The air-cooled conduit design minimises innerbraid collapse, a known competitive design flaw that reduces cooling airflow and ultimately results in thermally damaged components.
High voltage designs use conductor wire that complies with MIL-DTL-3702 standards to withstand service temperatures of up to 450°F (232°C); Use of low-voltage designs
The is built in a way that it can withstand service temperatures of up to 572°F (300°C).
Current lead lengths supported range from less than one foot to forty feet.