FM 1-506 Fundamentals of Aircraft Power Plants ... - Survival Books

WWW.SURVIVALEBOOKS.COMFM 1-506necessary interconnecting cables, leads, control switches,and associated equipment for operation are used withthese components.The dynamotor is used to step up the direct currentof the aircraft battery or the external power supply to theoperating voltage of the exciter unit. This voltage is usedto charge two storage capacitors which store the energyused for ignition.In this system, the energy required to tire the igniterplug in the engine burner is not stored in an inductor coilInstead, the energy is stored in capacitors. Each dischargecircuit incorporates two storage capacitors. Bothare located in the exciter unit. The voltage across thesecapacitors is stepped up by transformer units. At theinstant of igniter plug firing, the resistance of the gap islowered sufficiently to permit the larger capacitor todischarge across the gap. The discharge of the secondcapacitor is of low voltage but very high energy. Theresult is a spark of great heat intensity. It is capablenot only of igniting abnormal fuel mixtures but also ofburning away any foreign deposits on the plug electrodes.The exciter is a dual unit, and it produces sparks ateach of the two igniter plugs. A continuous series ofsparks is produced until the engine starts. The batterycurrent is then cut off. The plugs do not tire while theengine is operating.TURBINE ENGINE IGNITERSTurbine engine igniters come in many sizes andshapes depending on what their function is. Theelectrodes of the plugs used with high-energy ignitionsystems must accommodate a much higher energy currentthan the electrodes of conventional sparkplugs. Thehigh-energy current causes more rapid igniter electrodeerosion than in reciprocating engine sparkplugs. This isnot a problem because of the relatively short time aturbine engine ignition system is in operation. This is oneof the reasons for not operating the gas turbine ignitionsystem any longer than necessary. Igniter plug gaps arelarge in comparison with those of conventional sparkplugs. The gaps are large because the operating pressureat which the plug is fired is much lower than that of areciprocating engine.Most igniter plugs are of the annular-gap type. Constrainedgaps are used in some engines. Normally, toprovide an effective spark the annular-gap plug projectsslightly into the combustion chamber liner. The spark ofthe constrained-gap plug does not closely follow theface of the plug. Instead, it tends to jump an arc whichcarries it beyond the face of the chamber liner. Theconstrained-gap plug need not project into the liner. Theresult is that the electrode operates at a cooler temperaturethan the annular-gap plug.The turbojet ignition system is designed for severealtitude conditions common to military operation. It israrely taxed to its full capability by transport use.Flameout is much less common than it was, and flightrelight is not normally required of the ignition system.Ignition problems in general are minor compared to theconstant 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 ignitionarea are —Use of AC power inputs, which eliminates thevibrator, a major source of trouble.Use of solid-state rectifiers.Use of two discharge tubes, which permits thelevel of stored energy per spark to be moreconsistent throughout the life of the exciter.Sealed units.Longer time between overhauls.The advent of short-range jets, which increasedthe ratio of ignition-on time to engine operationand led to development of dual systems.Two types of ignition systems in Army aircraft todayare General Electric’s T-701 and Lycoming's T-55-L-712T-701 GENERAL ELECTRICThe ignition system is an AC-powered, capacitordischarge,low-voltage system. It includes a dual exciterunit mounted on the right-hand side and two igniter plugs(Figure 6-2). The spark rate of each ignition circuit is twosparks per second minimum; energy at the igniter plugsis at least 0.25 joules per spark. The exciter is poweredby one winding of the engine alternator and is connectedto it by the yellow harness. The ignition system must beturned off after starting by shorting the alternator output.For normal starting the aircraft-ignition circuit is tied inwith the aircraft-starting system to de-energize the ignitionsystem at the starter or dropout speedThe igniter plug is a homogeneous semiconductor,surface-gap type spark plug using air cooling of the firingtip (Figure 6-3). It provides a projected electrical dischargefor lightoff of the combustor. An engine setconsists of igniters located at the 4 o’clock and 8 o’clockpositions. They are mounted in the midframe using a6-5