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

WWW.SURVIVALEBOOKS.COMFM 1-506and the start bleed valve for efficient engine operationthroughout the engine speed (Ng ) range. A servo pistonwithin the HMU provides an external output (force andposition) to set the engine VG as a function of Ng and T2This schedule is contoured on the Ng speed servo 3Dcam. Motion of the schedule-follower lever moves a linkpivoted on the actuator piston. This action strokes adouble-acting, spring-loaded, rotating pilot valve. Thevalve then provides high- and low-fuel pressure signals toopposite sides for the VG actuator piston. Motion of theactuator returns the pilot valve to a null position, whichcm-responds to the input position being scheduled. Theactuator provides minimum force output of 280 poundsand is positioned as a function of corrected speed.Engine StartWith the engine shut down, the pressure in theoverspeed and drain valve is equal to ambient pressure,and the inlet and pressurizing valve is spring-loadedclosed. When the engine is cranked over and the PAS isadvanced to the stopcock in the HMU, the HMU delivers,metered flow to the overspeed drain valve inlet. Thismetered flow passes through awash-flow filter, and linepressure builds up until the inlet and pressurizing valveopens. Fuel boost pressure is directed to the top of aselector valve. Higher-pressure W fpasses through theopen selector valve and flows to the 12 fuel injectors forengine lightoff and normal operation.Overspeed and Drain Valve. The T7-GE-701 valvecontrols the sequencing of fuel between the mainmanifold and the primer manifold (Figure 4-15). Thevalve also provides shutoff, drain, primer manifold purgefunctions, and Np overspeed protection. The assemblyconsists of the following componentsShutoff and drain valve.Pressurizing valve.Wash flow filter.Bypass valve (N poverspeed).Fuel servo solenoid valve (N poverspend).Engine ShutdownWhen the PAS is moved to the OFF position, theHMU is stopcocked, fuel flow to the overspeed and drainvalve is stopped, and the inlet and pressurization valve isforced down by spring action. This opens the main fuelline in the overspeed and drain valve to overboard drainpressure. The residual P 3pressure in the combusterforces the fuel injectors and main manifold back throughthe overspeed and drain valve and out the overboarddrain line.4-14