Turbine engine ignition systems fall into two general classifications. The induction type produces high-tension sparks by conventional induction coils. The capacitor type causes ignition by high-energy and very high temperature sparks produced by a condenser discharge. A third kind of ignition system not widely adopted uses a glow plug.
Igniter-exciter components are contained in hermetically sealed boxes. With a malfunction, it is necessary to replace the entire exciter. Shielded cable is employed throughout the system to protect against abrasion and ignition system radio interference.
Ignition systems are not only used for engine starting but also for ignition standby protection. Ignition systems would be used to relight the engine if an in flight flameout occurred when operating under potentially unstable flight conditions. Turbine engines sometimes tend to flameout because of an overrich fuel-air ratio caused by a momentary fluctuation of air supply at the compressor inlet. A change in airflow at the compressor inlet or the entrance to the aircraft inlet duct may cause a condition fuel control cannot immediately compensate for. Flameout results. Flying in turbulent air, ingestion of a bird, or ingestion of ice broken loose at the engine inlet may cause such a situation. When one or both of the igniter plugs are operating, the engine will relight automatically after fuel control compensation takes place or the inlet condition corrects itself. The pilot may not be aware that flameout has occurred. When ignition is used as a precaution against flameout, the prescribed ignition use time limitations must be observed. This prevents overheating the ignition system components and enhances the life span of the ignition system.
Early turbine engine ignition systems evolved using the tried principles developed for the reciprocating engine. Some of the early systems employed a vibrator and transformer combination. This was similar to the booster coils used for starting reciprocating engines. Other units substituted a small electric motor driven cam. This provided the necessary pulsating magnetic field to the primary coil of the transformer. Several variations appeared, all using the same basic principle. This principle was high-voltage induction using a transformer to reach the necessary voltage capable of causing an arc across the wide-gap jet igniter plug. An interesting variation of this transformer-type ignition system is the opposite-polarity system. In this circuit two electrodes extend into the combustion chamber. Each electrode alternately becomes highly positively and negatively charged, causing a very high potential difference to exist across the electrodes.
The high-energy, capacitor-type ignition system has been universally accepted for gas turbine engines. It provides both high voltage and exceptionally hot spark which covers a large area. Excellent chances of igniting the fuel-air mixture are assured at reasonably high altitudes.
The term "high energy" is used throughout this chapter to describe the capacitor-type ignition system. Strictly speaking the amount of energy produced is very small. The intense spark is obtained by expending a small amount of electric energy in a very small amount of time.
Energy is the capacity for doing work. It can be expressed as the product of the electrical power (watt) and time. Gas turbine ignition systems are rated in joules. The joule is also an expression of electric energy. It is equal to the amount of energy expended in one second by an electric current of one ampere through a resistance of one ohm. The relationship between these terms can be expressed by the formula--
where w = watts (power)
j = joules
t = time (second)
All other factors being equal, the spark temperature is determined by the power level reached. A high-temperature spark results from increasing the energy level (1) or shortening the duration (t) of the spark. Increasing the energy level results in a heavier, bulkier ignition unit. Energy delivered to the sparkplug is about 30 to 40 percent of total energy stored in the capacitor. Higher erosion rates on the igniter-plug electrodes occur because of heavy current flowing for a comparatively long time. Much of the spark is wasted since ignition takes place in a matter of microseconds (microsec). On the other hand, since heat is lost to the igniter-plug electrodes and the fuel-air mixture is never completely gaseous, the spark duration cannot be too short.
The relationship between watts and time is shown in the following table. The example is for a 4-joule ignition unit (4 joules appearing at the plug).
|TABLE. WATTS AND TIME RELATIONSHIP|
|TIME (SECOND)||POWER (WATTS)|
|0.0001 (ten thousandths)||40,000|
|0.00001 (hundred thousandths)||400,000|
In an actual capacitor-discharge ignition system, most of the total energy available to the igniter plug is dissipated in 10 to 100 microsec (0.000010 to 0.000100 seconds). The system above would actually deliver 80,000 watts if the spark duration was 50 microsec.
To review, the spark temperature (a function of the watts value) is the most important characteristic of any ignition system. All three factors -- watts, energy, and time -- must be considered before any ignition system effectiveness can be determined
Ignition systems for jet engines are divided into induction and capacitor discharge types. The capacitor discharge type can be further divided into two basic categories:
High-Voltage Capacitor System - DC input (more than 5000 VDC to the plug)
This system is a typical turbojet engine system in use today. This system can include: two exciter units, two transformers, two intermediate ignition leads, and two high-tension leads. Depending on engine configuration a dual ignition is provided on the engine by two separately mounted exciters or by twin circuits throughout the exciter.
As operation begins, the power source delivers 28VDC (maximum) input to the system. Each triggering circuit is connected to a spark igniter. The operation described here takes place in each individual circuit. Except for the mechanical features of the armature, the operation is essentially the same in both units.
As a safety factor, the ignition system is actually a dual system designed to fire two igniter plugs.
Before the electrical energy reaches the exciter unit, it passes through a falter. This filter prevents noise voltage from being induced into the aircraft electrical system. The low-voltage input power operates a DC motor, which 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.
From the breaker points, a rapidly interrupted current is delivered to an automatic 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. The collapse of the field induces a voltage in the secondary winding of the transformer. This voltage causes a puke of current to flow into the storage capacitor. The voltage flows 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 4 joules. (One 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 builds up, the contactor is closed by the mechanical action of the single-lobe cam. A portion of the charge flows through the primary of the triggering transformer and the capacitor connected in series 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. This is done together 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, which 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 pukes before discharge.
The employment of the high-frequency triggering transformer, with a low-reactance secondary winding, holds the duration of the discharge to a minimum. This concentration of maximum energy in minimum time achieves an optimum spark for ignition. An optimum spark is 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 and eliminating flashover at altitude due to pressure change. This also ensures shielding against leakage of high-frequency voltage interfering with the radio reception.
Two igniter plugs are mounted in the combustion section outer case. The spark igniters are generally located in two diametrically opposite combustion liners. The igniters receive the electrical output from the ignition exciter unit. The igniters discharge the electrical output from the ignition exciter unit. And they discharge the electric energy during engine starting to ignite the fuel-air mixture in the combustion liners.
Typical specifications for this system are as follows:
|Input voltage:||Normal: 24VDC
Operating limits: 14 to 30 VDC
|Spark rate:||4 to 8 per second at each plug, depending on
|Designed to fire:||2 igniter plugs|
|Accumulated energy:||3 joules|
|Duty cycle:||2 minutes ON, 3 minutes OFF,
2 minutes ON, 23 minutes OFF
High-Voltage Capacitor System - AC Input
Power is supplied to the unit input connector from the 115-volt, 400-cycle source in the aircraft. Power is first led through a filter which blocks conducted noise voltage from feeding back into the airplane electrical system. From the filter, the circuit is completed through the primary of the power transformer to ground.
In the secondary of the power transformer, an alternating voltage is generated at a level approximating 1700 volts. During the first half-cycle this follows a circuit through the doubler capacitor and rectifier A to ground, leaving the capacitor charged. During the second half-cycle when the polarity reverses, this circuit is blocked by rectifier A. The flow of this puke is through ground to the storage capacitor, rectifier B, resistor, doubler capacitor, and back to the power transformer.
With each pulse the storage capacitor assumes a greater charge. By virture of the action of the doubler capacitor, the charge approaches voltage approximately twice that generated in the power transformer. When this voltage reaches the predetermined level calibrated for the spark gap in the discharge tube X (the control gap), the gap breaks down. This allows a portion of the accumulated charge to flow through the primary of the high-tension transformer and the trigger capacitor in series with it. This surge of current induces a very high voltage in the secondary of the high-tension transformer. This surge is enough to ionize the gap in discharge tube Y. The storage capacitor immediately discharges the remainder of its accumulated energy through the spark igniter. This produces a capacitive spark of very high energy.
The bleeder resistors are provided to dissipate the residual charge on the trigger capacitor. This is accomplished between the completion of one discharge at the spark igniter and the succession of the next cycle.
Typical specifications for this system are as follows:
|Input voltage:||Normal: 115-volts, 400-cycle
Operating limits 90 to 1241 volts
|Spark rate:||Normal: 1.50 to 275 per sec
Operating limits: 0.75 to 5.00 per second
|Designed to ignite:||One spark igniter|
|Accumulated energy:||14 to 17 joules|
|Duty cycle:||2 minutes ON, 3 minutes OFF
2 minutes ON, 23 minutes OFF
Low-Voltage Capacitor System-DC Input (less than 1000 volts to the plug)
The basis of operation on which the low-voltage, high-energy ignition system is built is the self-ionizing feature of the igniter plug. In the high-voltage system a double spark is produced. The first part consists of a high-voltage component to ionize (make conductive) the gap between the igniter plug electrodes. The second high-energy, low-voltage portion follows. The low-voltage, high-energy spark is similar except that ionization is effected by the self-ionizing igniter plug.
The main ignition unit changes the amplitude and the frequency characteristics of aircraft power into pulsating DC. To do this, the components in the ignition unit are grouped in stages to filter, amplify, rectify, and store an electric charge.
The spark plugs used in the ignition system are the shunted-gap type, which are self-ionizing and designed for low-tension (relatively low voltage) applications.
Although the spark plug fires at relatively low voltage, a high-temperature spark is obtained from the speed the energy is discharged across the gap. The spark is of short duration (40 microsec), but momentarily expends a great amount of power. Tank capacitor discharge cur-rent from the main ignition unit surges to the spark-plug electrodes. This builds a potential between the center electrode and ground electrode. The semiconducting material shunts the electrodes. When the potential between electrodes reaches approximately 800 volts, it forces enough current through the semiconductor to ionize the air gap between the electrodes. The full-tank capacitor current arcs instantly across the ionized gap, emitting a high-energy spark.
The operation of this system will not be discussed in detail. However, it is mentioned to make you aware of the system.
The ignition system includes one intermittent-duty exciter, one continuous-duty exciter, one intermediate voltage lead, and two high-tension leads. It is designed to tire two spark igniters during ground starts. This is accomplished by the 20-joule intermittent-duty exciter or one spark igniter during flight by the 4-joule continuous duty exciter.
When intermittent operation is employed, DC power is supplied to the input of the intermittent-duty exciter from the 24-volt aircraft electrical system. It is first passed through a radio noise filter to prevent high-frequency feedback. When continuous operation is employed, power is supplied to the input of the continuous-duty exciter from the 115-volt, 400-cycle AC source in the aircraft.
This modified capacity-type system provides ignition for turbojet and turboprop engines. It is required only for starting the engine. Once combustion begins the flame is continuous. Figure 6-1 shows the components of a typical electronic ignition system.
The system consists of a dynamotor/regulator/filter assembly, an exciter unit, two high-tension transformer units, two high-tension leads, and two igniter plugs. The necessary interconnecting cables, leads, control switches, and associated equipment for operation are used with these components.
The dynamotor is used to step up the direct current of the aircraft battery or the external power supply to the operating voltage of the exciter unit. This voltage is used to charge two storage capacitors which store the energy used for ignition.
In this system, the energy required to tire the igniter plug in the engine burner is not stored in an inductor coil Instead, 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, 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 very high energy. The result is a spark of great heat intensity. It is capable not only of igniting abnormal fuel mixtures but also of burning away any foreign deposits on the plug electrodes.
The exciter is a dual unit, and it produces sparks at each of the two igniter plugs. A continuous series of sparks is produced until the engine starts. The battery current is then cut off. The plugs do not tire while the engine is operating.
Turbine engine igniters come in many sizes and shapes depending on what their function is. The electrodes of the plugs used with high-energy ignition systems must accommodate a much higher energy current than the electrodes of conventional sparkplugs. The high-energy current causes more rapid igniter electrode erosion than in reciprocating engine sparkplugs. This is not a problem because of the relatively short time a turbine engine ignition system is in operation. This is one of the reasons for not operating the gas turbine ignition system any longer than necessary. Igniter plug gaps are large in comparison with those of conventional spark plugs. The gaps are large because the operating pressure at which the plug is fired is much lower than that of a reciprocating engine.
Most igniter plugs are of the annular-gap type. Constrained gaps are used in some engines. Normally, to provide an effective spark the annular-gap plug projects slightly into the combustion chamber liner. The spark of the constrained-gap plug does not closely follow the face of the plug. Instead, it tends to jump an arc which carries it beyond the face of the chamber liner. The constrained-gap plug need not project into the liner. The result is that the electrode operates at a cooler temperature than the annular-gap plug.
The turbojet ignition system is designed for severe altitude conditions common to military operation. It is rarely taxed to its full capability by transport use. Flameout is much less common than it was, and flight relight is not normally required of the ignition system. Ignition problems in general are minor compared to the constant attention required by the piston engine system. Airborne ignition analysis equipment is unnecessary. Spark igniter plug replacement is greatly minimized. Only two plugs per engine are used.
The trends taking place in the gas turbine ignition area are--
Two types of ignition systems in Army aircraft today are General Electric's T-701 and Lycoming's T-55-L-712
The ignition system is an AC-powered, capacitor-discharge, low-voltage system. It includes a dual exciter unit mounted on the right-hand side and two igniter plugs (Figure 6-2). The spark rate of each ignition circuit is two sparks per second minimum; energy at the igniter plugs is at least 0.25 joules per spark. The exciter is powered by one winding of the engine alternator and is connected to it by the yellow harness. The ignition system must be turned off after starting by shorting the alternator output. For normal starting the aircraft-ignition circuit is tied in with the aircraft-starting system to de-energize the ignition system at the starter or dropout speed.
The igniter plug is a homogeneous semiconductor, surface-gap type spark plug using air cooling of the firing tip (Figure 6-3). It provides a projected electrical discharge for lightoff of the combustor. An engine set consists of igniters located at the 4 o'clock and 8 o'clock positions. They are mounted in the midframe using a screw-in boss and extend inward through the outer panel of the combustion liner.
The ignition system operates with a maximum output of 7000 volts. To create a spark across the electrode gap with this voltage, the gap surface is a semiconductor material, homogeneous button extending into the tip of the plug. It is, therefore, capable of coping with erosion over a long period of operating time. Consistent with this long life objective, the center electrode is pure tungsten and the outer electrode is tungsten alloy. These electrodes are nickel-plated to prevent oxidation, a problem further minimized by tip cooling. Compressor discharge air provides cooling air which enters through six holes around the tip body and exits through twelve holes at the tip end.
The high potential ignition pulse is developed by the ignition exciter (Figure 6-4). A direct current at 28 volts is applied to the input of the exciter. Current flows through the primary transformer winding the bias coil, and the vibrator points to ground This generates magnetic lines of force which permeate the transformer and bias coil cores, attract the vibrator reed upward, and interrupt the circuit. As current flow ceases, the lines of force collapse, and the reed falls back. This closes the circuit. This cycle repeats at a rate proportional to the input voltage. The resultant current flows in pulses, causing magnetic lines of force to build up and collapse with each pulsation. These lines induce voltage across the secondary. They are transformed to a higher potential by virtue of an increased number of windings comprising the secondary. The diodes rectify the pulsating current back into direct current to charge the capacitors. The charge on the capacitors continues to build up at a rate proportional to input voltage until a potential of 2500 AC volts exists. The calibrated spark gaps ionize at this voltage creating an electrical path for the tiring pulse. The capacitors discharge through this path into the lead coil assembly for distribution to the spark igniters.
Radio frequency energy is generated within the exciter during normal operation. An inductive capacitive filter has been incorporated at the input. This will prevent energy from being fed back onto the 28-volt input line. Radio frequency interference on this line could damage the operation of other electrical accessories. The filter is turned to radio frequencies. It does not offer any appreciable opposition to the flow of 28-volt direct current.
The ignition lead and coil assembly constitutes the high potential ignition wiring. This assembly incorporates two coils fed with high voltage from the two outputs of the ignition exciter. The coil assemblies function as spark splitters distributing high voltage to four igniter plugs. Each coil assembly has one input and two outputs. The coil windings forma transformer having a 1:1 ratio. Any current flowing through either winding will induce a voltage across the other. Even a shortened igniter plug will not short out the high-voltage ignition signal. The entire wiring harness is shielded and grounded at airframe potential to suppress radiation of radio frequency interference.