keep the transfer tubes in proper alignment, secured by two bolts to a mounting boss on the
circumference of the gas generator case. Individual transfer tubes or fuel nozzles can be removed and
replaced without necessarily disconnecting the remainder of the set. The fuel manifold assembly is
illustrated in figure 9.17.
9.19.
IGNITION SYSTEM
The glow-plug type ignition system is used on the T74 engine for quick light-offs (starts),
even at extremely low ambient temperatures. The basic system consists of a current regulator and two
sets of tubes, two shielded plug leads, and two glow plugs. The following subparagraphs discuss the
components in the ignition system.
a.
The current-regulator unit is normally secured to the accessory gearbox housing but can
be remotely mounted if required. The regulator contains four electron tubes, shown in figure 9.18. Each
tube has a pure iron filament surrounded by helium and hydrogen gas and enclosed in a glass envelope
sealed to an octal base. The iron filament, having a negative co-efficient of resistance (resistance
decreases with temperature increase caused by current flow), stabilizes the current flow across the tubes
to a nearly constant value over a wide range of voltages. Each glow plug is wired in series with two
parallel connected ballast tubes. Either glow plug may be selected for starting the engine. The tubes
provide an initial current surge when switched on, which stabilizes to a constant value in approximately
30 seconds. The system heats the glow plugs for fast light-offs.
b.
The glow plug consists of a heating element fitted into a short conventional type plug
body. A cross-section illustration of a glow plug is shown in figure 9.19.
The plugs are secured to the gas generator case in threaded bosses. The heating
element consists of a helically wound coil which lies slightly below the end of the plug body. During
starting procedures, the fuel sprayed by the fuel nozzles runs down along the lower wall of the
combustion chamber liner and into the helical coil in the glow plug body. The fuel is vaporized and
ignited by the hot coil element which heats up to approximately 2,400 F. Three air holes in the plug
body allow compressor discharge air from the gas generator case into the plug body, then past the hot
coil in the combustion chamber liner to produce a hot streak or torching effect which ignites the
remainder of the fuel. The air also serves to cool the coil elements when the engine is running with the
glow plugs switched off.
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