Gte Ignition System-exciter Box

Page-1 of 12 https://www.aircraftsystemstech.com/p/turbineengine-ignition-systems-since.html

Turbine Engine Ignition Systems - Aircraft Engine Ignition and Electrical Systems Since turbine ignition systems are operated mostly for a brief period during the engine-starting cycle, they are, as a rule, more trouble-free than the typical reciprocating engine ignition system. The turbine engine ignition system does not need to be timed to spark during an exact point in the operational cycle. It is used to ignite the fuel in the combustor and then it is switched off. Other modes of turbine ignition system operation, such as continuous ignition that is used at a lower voltage and energy level, are used for certain flight conditions. Continuous ignition is used in case the engine were to flame out. This ignition could relight the fuel and keep the engine from stopping. Examples of critical flight modes that use continuous ignition are takeoff, landing, and some abnormal and emergency situations. Most gas turbine engines are equipped with a high-energy, capacitor-type ignition system and are air cooled by fan airflow. Fan air is ducted to the exciter box, and then flows around the igniter lead and surrounds the igniter before flowing back into the nacelle area. Cooling is important when continuous ignition is used for some extended period of time. Gas turbine engines may be equipped with an electronic-type ignition system, which is a variation of the simpler capacitor-type system. The typical turbine engine is equipped with a capacitor-type, or capacitor discharge, ignition system consisting of two identical independent ignition units operating from a common low-voltage (DC) electrical power source: the aircraft battery, 115 AC, or its permanent magnet generator. The generator is turned directly by the engine through the accessory gear box and produces power any time the engine is turning. The fuel in turbine engines can be ignited readily in ideal atmospheric conditions, but since they often operate in the low temperatures of high altitudes, it is imperative that the system be capable of supplying a high heat intensity spark. Thus, a highvoltage is supplied to arc across a wide igniter spark gap, providing the ignition system with a high degree of reliability under widely varying conditions of altitude, atmospheric pressure, temperature, fuel vaporization, and input voltage. A typical ignition system includes two exciter units, two transformers, two intermediate ignition leads, and two high-tension leads. Thus, as a safety factor, the ignition system is actually a dual system designed to fire two igniter plugs. [Figure 1]

Figure 1. Turbine ignition system components Figure 2 is a functional schematic diagram of a typical older style capacitor-type turbine ignition system. A 24volt DC input voltage is supplied to the input receptacle of the exciter unit. Before the electrical energy reaches

Page-2 of 12 the exciter unit, it passes through a filter that prevents noise voltage from being induced into the aircraft electrical system. The low-voltage input power operates a DC motor that drives one multilobe cam and one single-lobe cam. At the same time, input power is supplied to a set of breaker points that are actuated by the multilobe cam.

Figure 2. Capacitor-type ignition system schematic

From the breaker points, a rapidly interrupted current is delivered to an auto transformer. When the breaker closes, the flow of current through the primary winding of the transformer establishes a magnetic field. When the breaker opens, the flow of current stops, and the collapse of the field induces a voltage in the secondary of the transformer. This voltage causes a pulse of current to flow into the storage capacitor through the rectifier, which limits the flow to a single direction. With repeated pulses, the storage capacitor assumes a charge, up to a maximum of approximately 4 joules. (Note: 1 joule per second equals 1 watt.) The storage capacitor is connected to the spark igniter through the triggering transformer and a contactor, normally open. When the charge on the capacitor has built up, the contactor is closed by the mechanical action of the singlelobe cam. A portion of the charge flows through the primary of the triggering transformer and the capacitor connected with it. This current induces a high-voltage in the secondary, which ionizes the gap at the spark igniter. When the spark igniter is made conductive, the storage capacitor discharges the remainder of its accumulated energy along with the charge from the capacitor in series with the primary of the triggering transformer. The spark rate at the spark igniter varies in proportion to the voltage of the DC power supply that affects the rpm of the motor. However, since both cams are geared to the same shaft, the storage capacitor always accumulates its store of energy from the same number of pulses before discharge. The employment of the high-frequency triggering transformer, with a low-reactance secondary winding, holds the time duration of the discharge to a minimum. This concentration of maximum energy in minimum time achieves an optimum spark for ignition purposes, capable of blasting carbon deposits and vaporizing globules of fuel. All high-voltage in the triggering circuits is completely isolated from the primary circuits. The complete exciter is hermetically sealed, protecting all components from adverse operating conditions, eliminating the possibility of flashover at altitude due to pressure change. This also ensures shielding against leakage of high-frequency voltage interfering with the radio reception of the aircraft.

Capacitor Discharge Exciter Unit This capacity-type system provides ignition for turbine engines. Like other turbine ignition systems, it is required only for starting the engine; once combustion has begun, the flame is continuous. [Figure3] The energy is stored in capacitors. Each discharge circuit incorporates two storage capacitors; both are located in the exciter unit. The voltage across these capacitors is stepped up by transformer units. At the instant of igniter plug firing,

Page-3 of 12 the resistance of the gap is lowered sufficiently to permit the larger capacitor to discharge across the gap. The discharge of the second capacitor is of low-voltage, but of very high energy. The result is a spark of great heat intensity, capable of not only igniting abnormal fuel mixtures but also burning away any foreign deposits on the plug electrodes.

Figure 3. Fan air-cooled exciter

The exciter is a dual unit that produces sparks at each of the two igniter plugs. A continuous series of sparks is produced until the engine starts. The power is then cut off, and the plugs do not fire while the engine is operating other than on continuous ignition for certain flight conditions. This is why the exciters are air cooled to prevent overheating during long use of continuous ignition.

Igniter Plugs The igniter plug of a turbine engine ignition system differs considerably from the spark plug of a reciprocating engine ignition system. [Figure 4] Its electrode must be capable of withstanding a current of much higher energy than the electrode of a conventional spark plug. This high energy current can quickly cause electrode erosion, but the short periods of operation minimize this aspect of igniter maintenance. The electrode gap of the typical igniter plug is designed much larger than that of a spark plug since the operating pressures are much lower and the spark can arc more easily than in a spark plug. Finally, electrode fouling, common to the spark plug, is minimized by the heat of the high-intensity spark.

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Figure 4. Ignitor plugs

Figure 5 is a cutaway illustration of a typical annular-gap igniter plug, sometimes referred to as a long reach igniter because it projects slightly into the combustion chamber liner to produce a more effective spark.

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Figure 5. Typical annular gap igniter plug

Another type of igniter plug, the constrained-gap plug, is used in some types of turbine engines. [Figure 6] It operates at a much cooler temperature because it does not project into the combustion-chamber liner. This is possible because the spark does not remain close to the plug, but arcs beyond the face of the combustion chamber liner.

Figure 6. Constrained gap igniter plug

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Treager GTE Book- Page-362

Trigger transformer: The invention provides an improved turbine engine ignition exciter circuit. Energy stored in an exciter tank capacitor is subsequently switched to the load (igniter plug) through a novel thyristor switching device specifically designed for pulse power applications. The switching device is designed and constructed to include, for example, a highly interdigitated cathode/gate structure. The semiconductor switching device is periodically activated by a trigger circuit which may be comprised of either electromagnetic or optoelectronic triggering circuitry to initiate discharge of energy stored in exciter tank capacitor to mating ignition lead and igniter plug. Likewise, the present invention allows new flexibility in the output PFN (Pulse Forming Network) stage, eliminating need for specialized protective output devices such as saturable output inductors. Due to considerably higher di/dt performance of the device, true high voltage output pulse networks may be utilized without damage to the semiconductor switching device. An exemplary embodiment of invention contains a novel feedback network which causes thyristor timing (trigger) and DC-DC converter circuits to compensate for varying igniter plug wear and dynamic engine combustor conditions, tailoring exciter spark rate, output voltage and energy to account for dynamic load conditions. Turbine engine igniter exciter circuit: An improved turbine engine igniter exciter circuit. An oscillator circuit drives the primary winding of a transformer for charging a capacitor to a predetermined voltage greater than the voltage required to sustain a spark discharge at an igniter but less than the voltage required to establish a spark discharge at the igniter. A timing circuit periodically triggers an SCR to discharge a portion of the energy stored in the capacitor through the primary winding of a step up transformer to create a short duration high voltage ignition pulse for establishing the spark discharge. The remaining lower voltage energy in the capacitor sustains the spark discharge for a predetermined time.

Page-7 of 12 http://aeromodelbasic.blogspot.com/2012/02/starting-and-ignition-ignition.html Starting and ignition – IGNITION: IGNITION 18. High-energy (H.E.) ignition is used for starting all jet engines and a dual system is always fitted. Each system has an ignition unit connected to its own igniter plug, the two plugs being situated in different positions in the combustion system. 19. Each H.E. ignition unit receives a low voltage supply, controlled by the starting system electrical circuit, from the aircraft electrical system. The electrical energy is stored in the unit until, at a pre- determined value, the energy is dissipated as a high voltage, high amperage discharge across the igniter plug. 20. Ignition units are rated in 'joules' (one joule equals one watt per second). They are designed to give outputs which may vary according to requirements. A high value output (e.g. twelve joule) is necessary to ensure that the engine will obtain a satisfactory relight at high altitudes and is sometimes necessary for starting. However, under certain flight conditions, such as icing or take-off in heavy rain or snow, it may be necessary to have the ignition system continuously operating to give an automatic relight should flame extinction occur. For this condition, a low value output (e.g. three to six joule) is preferred because it results in a longer life of the igniter plug and ignition unit. Consequently, to suit all engine operating conditions, a combined system giving a high and low value output is favoured. Such a system would consist of one unit emitting a high output to one igniter plug, and a second unit giving a low output to a second igniter plug. However, some ignition units are capable o! supplying both high and low outputs, the value being pre-selected as required. 21. An ignition unit may be supplied with direct current (D.C.) and operated by a trembler mechanism or a transistor chopper circuit, or supplied with alternating current (A.C.) and operated by a transformer. The operation of each type of unit is described in the subsequent paragraphs.

Fig. 11-10 A D.C. trembler-operated ignition unit. 22. The ignition unit shown in fig. 11-10 is atypical D.C. trembleroperated unit. An induction coil, operated by the trembler mechanism, charges the reservoir capacitor (condenser) through a high voltage rectifier. When the voltage in the capacitor is equal to the breakdown value of a sealed discharge gap, the energy is discharged across the face of the igniter plug. A choke is fitted to extend the duration of the discharge and a discharge resistor is fitted to ensure that any residual stored energy in the capacitor is dissipated within one minute of the system being switched off. A safety resistor is fitted to enable the unit to operate safely, even when the high tension lead is disconnected and isolated.

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23. Operation of the transistorized ignition unit is similar to that of the D.C. trembler-operated unit, except that the trembler-unit is replaced by a transistor chopper circuit. A typical transistorized unit is shown in fig. 11-11; such a unit has many advantages over the trembler-operated unit because it has no moving parts and gives a much longer operating life. The size of the transistorized unit is reduced and its weight is less than that of the trembler-operated unit.

Fig. 11-11 A transistorized ignition unit. and Fig. 11-12 An A.C. ignition unit.

24. The A.C. ignition unit, shown in fig, 11-12, receives an alternating current which is passed through a transformer and rectifier to charge a capacitor. When the voltage in the capacitor is

Page-9 of 12 equal to the breakdown value of a sealed discharge gap, the capacitor discharges the energy across the face of the igniter plug. Safety and discharge resistors are fitted as in the trembler-operated unit.

Fig. 11-13 An igniter plug. 25. There are two basic types of igniter plug; the constricted or constrained air gap type and the shunted surface discharge type. The air gap type is similar in operation to the conventional reciprocating engine spark plug, but has a larger air gap between the electrode and body for the spark to cross. A potential difference of approximately 25,000 volts is required to ionize the gap before a spark will occur. This high voltage requires very good insulation throughout the circuit. The surface discharge igniter plug (fig. 11-13) has the end of the insulator formed by a semiconducting pellet which permits an electrical leakage from the central high tension electrode to the body.

This ionizes the surface of the pellet to provide a low resistance path for the energy stored in the capacitor. The discharge takes the form of a high intensity flashover from the electrode to the body and only requires a potential difference of approximately 2000 volts for operation. 26. The normal spark rate of a typical ignition system is between 60 and 100 sparks per minute. Periodic replacement of the igniter plug is necessary due to the progressive erosion of the igniter electrodes caused by each discharge. 27. The igniter plug tip protrudes approximately 0.1 inch into the flame tube. During operation the spark penetrates a further 0.75 inch. The fuel mixture is ignited in the relatively stable boundary layer which then propagates throughout the combustion system.

Page-10 of 12 FAQ – GTE Ignition System https://quizlet.com/191497187/test-3-turbine-ignition-systems-flash-cards/ When is the ignition system on a turbine needed and why? Turbine engines use continuous combustion and require an electrical spark only during the start sequence to ignite the fuel-air mixture. Once the fire is started, the ignition system can be turned off. Brayton Cycle Inlet -> compressor -> diffuser -> combustion -> exhaust. All steps occur simultaneously EGT exhaust gas temperature. Allows the pilot to know when the fuel has been ignited and the ignition system can be shut down. What are turbine engine ignition systems rated in? Joules (watt-seconds). four-joule systems - some smaller turbine engines 20-joule systems - common on larger turbine engines Why are igniters needed? They need to produce a spark intense enough to vaporize and ignite the fuel (Jet Fuel - kerosine, harder to ignite compared to AVGAS). How much time is needed to ignite the fuel-air mixture? Normally only a few MICROSECONDS (millionths of a second) What are the two types of turbine ignition systems and how are they powered? Two types of high-energy, or capacitor-discharge ignition systems: Low-voltage system: about 1,000 volts at the igniter High-voltage systems: output of more than 5,000 volts at the igniter. Both may be powered with AC or DC What does a typical turbine engine ignition system consist of? Two ignition exciters Two high tension leads Two igniters (the two exciters may be built into a single unit/housing) What is the purpose of EXCITERS? converts 28-volt DC or 115-volt AC into high voltage. Change the input power into pulsating DC (half-wave rectifier changes AC into pulsating DC) that charges a storage capacitor to a specific high voltage. When the voltage is reached, the capacitor discharges across the electrodes of the igniter. What is a duty cycle? Why is it needed in turbine ignition systems? A schedule that allows a device to operate for a given period of time, followed by a cooling down period before the device can be operated again.

Page-11 of 12 The tremendous amount of energy released by turbine ignition systems limits the length of time they can be used without a cooling-down period. Intermittent-duty systems normally used for the main ignition 2 minutes of operation then a cooldown period of 3 minutes after a second 2 minutes of operation, about 20 minutes must be allowed for the system to cool down before a third use. REFER TO THE MANUAL FOR THE PROPER DUTY CYCLE. Continuous duty ignition system has no operating time limit,, normally provide energy to only one igniter and may be kept energized for much of the flight. When are ignition systems normally turned on? For takeoff, landing, or flight into turbulent conditions (may flameout the engine), cross winds, and when the engine anti-ice system is actuated. Pi filter: an electronic filter used to prevent radio frequency energy produced in the ignition exciter from feeding back into the aircraft electrical system. The filter is made of an inductor with a capacitor on its input and output. The name is derived form the resemblance of the three components on a schematic diagram to the greek letter pi. Purpose of a storage capacitor in an exciter? current is stored in the capacitor and trapped by a diode and discharge tube. each pulse of current into the storage capacitor causes the voltage across it to build up until the ionization voltage of the discharge tube is reached. -IT'S LIKE A BUCKET, AND CAN ONLY PASS THE DISCHARGE TUBE ONCE THE BUCKET IS FULL. Purpose of safety and bleed resistors? they bleed off any energy stored in the trigger/storage capacitors when the system is shut down. This prevents voltage buildup to damaging levels, and preventing a technician from severe electrical shock. Can you open an exciter box for servicing? they are sealed units and CANNOT be opened for servicing. DON'T OPEN! only inspect for secure mounting on the engine and all electrical and ground connections should be clean and tight. What is a flameout? a condition of turbine engine operation when the fire unintentionally goes out. Improper fuel-air mixture or interruption of the air flow through the engine can cause a flameout. Auto-ignition systems installed on some turboprop engines to serve as a backup for takeoff and landing and for flight conditions in which the engine could flame out.

Page-12 of 12 What are the basic types of igniters? How do they work? Spark igniters (most popular) - spark, duh Glow plug igniters (used in some smaller engines) - like a car cigarette lighter, coils are heated with current from the ignition exciter until they glow orange-yellow, and fuel from the fuel nozzle spraying on the coil is ignited. especially suited for cold-weather starting. typical types: High-voltage air cooled recessed gap igniter, High-voltage air-cooled surface gap igniter, Low-voltage shunted surface gap igniter, low-voltage glow plug igniter Ignition system servicing before disconnecting the lead form an exciter or igniter, -pull the ignition power circuit breaker -disconnect the power lead to the exciter and observe the time specified in the engine maintenance manual before removing the igniter lead -Normally about 5 minutes, this allows energy stored in the capacitors to bleed safely to ground through the bleeder and safety resistors -when the lead is removed from the igniter, ground the center conductor to the engine to ensure the capacitors are completely discharged.