Turbine engine ignition systems Turbine engine ignition systems

Turbine engine ignition systems

Turbine engine ignition systems � What do we know about jet fuel compared to avgas?

Turbine engine ignition systems � Jet fuel is less volatile than avgas � Jet fuel has more heat energy per unit of volume � Jet fuel is denser than avgas � With all of these considerations what can be said about the ease at which jet fuel ignites compared to avgas?

Turbine engine ignition systems � Jet fuel requires a hotter spark than found on piston engine ignition applications � To obtain a hotter spark more energy is released over a shorter period of time � This is obtained by the use of a capacitive discharge type system

Turbine engine ignition systems � Turbine engine ignition system can produce potentially lethal amounts of current, always follow manufactures maintenance instructions when working on or around turbine ignition systems

Turbine engine ignition systems �A piston engine ignition system produces a high voltage low amperage spark � Turbine engine produces a high voltage high amperage spark with a high heat intensity. � Jets use “igniters” vice spark plug and spark is generated by “exciter” � Turbine engine ignition system ratings are made in joules as a work unit

Turbine engine ignition systems � 1 joule is a unit of work defined as 1 Watt for 1 second � 1 �A Joule=1 W x 1 Sec typical spark last for a few millionths of a second � Therefore a tremendous amount of power has to be produced to obtain 1 joule � If a spark last. 0001 seconds 10000 watts are required to obtain 1 joule � Some joules � So turbine ignition systems are as high as 20 turbine ignition systems often fire “igniters” continuously until engine combustion is self-sustaining

Turbine engine ignition systems � Low tension ignition system � 28 VDC supplied to exciter circuit � Voltage must be converted to a pulsating DC � Many circuit systems use a vibrator � Voltage must be stepped up through a transformer � This stepped up voltage is used to charge a capacitor

Turbine engine ignition systems � Magnet holds vibrator closed while power is off � Magnetic field created by vibrator coil opens contacts � This collapse the magnetic field inducing current into the secondary coil

Turbine engine ignition systems � High voltage travels through the diode and begins to charge the capacitor � After numerous cycles the capacitor reaches a charge high enough to jump the gap of the discharge tube � The initial arc creates enough heat to ionize the air in the gap allowing current to flow to the igniter

Turbine engine ignition systems � Igniters in a low tension system are know as � � Self Ionizing shunted-gap type � Igniters contain a semi-conductor type material between the center and outer electrodes � Semi-conductor igniter tip allows current to flow � Igniter tip heats up -- resistance increases � Spark gap air ionizes lowering – resistance decreases � When spark gap resistance falls below semi-conductor marterial resistance– spark and massive current dump occurs across spark gap from capacitor

Turbine engine ignition systems � High � tension ignition systems 115 volt 400 Hz AC is applied to transformer exciter unit � Stepped up to approx 2000 volts through transformer Charges the doubler capacitor to 2000 volts � The next alternation will charge the doubler capacitor to 4000 volts � � This travels through a rectifier and charges the storage capacitor

Turbine engine ignition systems Ø Once the storage capacitor is fully charged the air in the gas discharge tube is ionized allowing current to flow to the trigger capacitor. Ø Current through primary coil charges trigger capacitor Ø Current stops and transformer action produces even higher voltage/current that’s able to bridge gap of igniter

Turbine engine ignition systems � Igniters � Differ from spark plugs � Much larger air gap � Designed � Longer � Not � Less to with stand much greater spark intensity service life required to operate continuously susceptible to fouling � Because high energy spark burns off deposits

Turbine engine ignition systems

Turbine engine ignition systems � Most igniters extent into the combustion chamber � Aprox. 0. 1 inch � Constrained gap igniters do not extend into combustion chamber � Center electrode is recessed into plug � This causes spark to jump out in order to jump gap � Constrained gap plugs operate cooler than other types

Turbine engine ignition systems � Glow plugs � Small turbines (apu, turboprop) � Use a resistive coil to generate heat � Fuel flow is ignited by the resistive heater � Once engine is running glow plug is no longer heated

Turbine engine ignition systems � Due to the high intensity of spark generation, turbine engines are often limited to a duty cycle � This allows the igniter plug to cool and avoids damage to the plug

Turbine engine ignition systems � Turbine Engine auto Ignition � Once engine is up and running, ignition is shut off or put into an “Auto” position � Ignition is hooked to a device that senses engine pressure ratio(EPR), torque, ect. . � If a drop in pressure is sensed, the ignition will automatically be turned on to relight the engine.

Inspection and maintenance � Safety…be sure ignition system is discharged before performing maintenance � To remove an igniter � Turn ignition system off � Wait for prescribed for capacitor to discharge � Disconnect igniter lead and ground center connector to chassis � Protects you from lethal discharge of igniter system � Follow manufacture’s instructions on cleaning, dimensional checks and special handling considerations � Semi-conductor material can be delicate

� End of AVM 163….