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

WWW.SURVIVALEBOOKS.COMFM 1-506level (1) or shortening the duration (t) of the spark.Increasing the energy level results in a heavier, bulkierignition unit. Energy delivered to the sparkplug is about30 to 40 percent of total energy stored in the capacitor.Higher erosion rates on the igniter-plug electrodes occurbecause of heavy current flowing for a comparatively longtime. Much of the spark is wasted since ignition takesplace in a matter of microseconds (microsec). On theother hand, since heat is lost to the igniter-plug electrodesand the fuel-air mixture is never completely gaseous, thespark duration cannot be too short.The relationship between watts and time is shownin the following table. The example is for a 4-jouleignition unit (4 joules appearing at the plug).In an actual capacitor-discharge ignition system,most of the total energy available to the igniter plug isHigh-voltage capacitor ignition system with DCor AC input.Low-voltage capacitor ignition system with DCor AC input.High-Voltage Capacitor System - DC input(more than 5000 VDC to the plug)This system is a typical turbojet engine system in usetoday. This system can include: two exciter units, twotransformers, two intermediate ignition leads, and twohigh-tension leads. Depending on engine configurationa dual ignition is provided on the engine by two separatelymounted exciters or by twin circuits throughout the exciter.As operation begins, the power source delivers28VDC (maximum) input to the system. Each triggeringdissipated in 10 to 100 microsec (0.000010 to 0.000100seconds). The system above would actually deliver80,000 watts if the spark duration was 50 microsec.1w= t4=0.000050= 80,000To review, the spark temperature (a function of thewatts value) is the most important characteristic of anyignition system. All three factors – watts, energy, andtime – must be considered before any ignition systemeffectiveness can be determinedHIGH-ENERGY TYPEIgnition systems for jet engines are divided intoinduction and capacitor discharge types. The capacitordischarge type can be further divided into two basiccategories:circuit is connected to a spark igniter. The operationdescribed here takes place in each individual circuit.Except for the mechanical features of the armature, theoperation is essentially the same in both Units.As a safety factor, the ignition system is actually adual system designed to fire two igniter plugs.Before the electrical energy reaches the exciter unit,it passes through a falter. This filter prevents noise voltagefrom being induced into the aircraft electrical system.The low-voltage input power operates a DC motor, whichdrives one multilobe cam and one single-lobe cam. Atthe same time, input power is supplied to a set of breakerpoints that are actuated by the multilobe cam.From the breaker points, a rapidly interrupted currentis delivered to an automatic transformer. When thebreaker closes, the flow of current through the primarywinding of the transformer establishes a magnetic fieldWhen the breaker opens, the flow of current stops. Thecollapse of the field induces a voltage in the secondarywinding of the transformer. This voltage causes a puke ofcurrent to flow into the storage capacitor. The voltage6-2