TM    55-2840-241-23 c. The power turbine governor lever schedules the governor requirements. The power turbine governor, in turn, schedules the gas producer speed to a changed power  output  to  maintain  output  shaft  speed. d. Fuel flow for engine control depends on com- pressor discharge pressure (Pc), engine speed (gas producer—N1  and/or  power  turbine—N2),  and  lever angle. Fuel flow is a function of Pc as sensed in the fuel control. Variations of fuel flow schedules are ob- tained by modulating the Pc to Px and Py pressures in the  control  through  the  action  of  a  bleed-down  circuit actuated by the governors. (See figure 1-4.) 1-12.   Gas   Producer   Fuel   Control. a. The  gas  producer  fuel  control  has  a  bypass valve,  metering  valve,  acceleration  bellows,  governing and enrichment bellows, manually operated cutoff valve, maximum pressure relief valve, torque tube seal and lever assembly, and a start derichment valve. The  maximum  pressure  relief  valve  protects  the system from excessive fuel pressure. b. Fuel enters the control from the engine fuel pump and filter assembly and is delivered to the metering valve. The bypass valve maintains a cons- tant pressure differential across the metering valve. Also, excess fuel is bypassed to the fuel pump and filter  assembly  through  an  external  line  connecting the pump bypass inlet to the bypass outlet port of the gas producer fuel control. c. The metering valve is operated by lever action through  movement  of  the  governor  and  acceleration bellows, Metering valve area depends on valve travel, Before light -off and acceleration, the metering valve is set at a pre-determined open position by the accelera- tionbellows  under  the  influence  of  ambient  pressure (Pc at zero rpm). d. The  start  derichment  valve  is  open  during light-off and acceleration to a set Pc. The open derich- ment  valve  vents  Py  pressure  to  atmosphere.  Venting Py allows the governor bellows to move the metering valve  against  the  minimum  flow  stop.  At  minimum flow the metering valve provides the required lean fuel schedule  after  light-off.  As  compressor  rpm  increases, the derichment valve is closed by Pc acting on the derichment bellows. When the derichment valve is closed, control of  the metering valve is returned to the normal  operating  schedule. e. During acceleration, the Px and Py pressures are  equal  to  the  modified  compressor  discharge pressure (Pc) up to the point where the speed enrich- ment  orifice  is  opened  by  flyweight  action.  Opening the speed enrichment orifice bleeds Px pressure whil Py remains at a value equal to Pc. Under the influence of the Py minus Px pressure drop across the governor bellows, the metering valve moves toward the max imum flow stop where it increases fuel flow. f. Gas producer speed is controlled by the gas producer fuel control governor. A set of flyweights operate the governor lever which controls the governor bellows  (Py)  bleed  at  the  governing  orifice.  Flyweight operation  of  the  governor  lever  is  opposed  by  a variable  spring  load.  The  spring  force  is  established by the throttle lever acting on a spring scheduling cam. Opening the governing orifice bleeds Py pressure and  allows  Px  pressure  to  control  the  governor bellows.  The  Px  influence  on  the  bellows  moves  the metering valve toward minimum flow and at a posi- tion  where  metered  flow  is  at  steady  state  re- quirements. g.  The  governor  reset  assembly  in  the  gas producer fuel control limits or governs power turbine speed. Control of the reset assembly is derived from the power turbine governor. The power turbine gover- nor also provides quick responding overspeed protec- t ion by bleeding governor servo (Py) pressure from the gas producer fuel control. 1-13.   Power   Turbine   Governor. a. Power turbine speed is scheduled by the power turbine  governor  lever  and  the  power  turbine  speed scheduling cam. The cam sets a governor spring load which opposes a flyweight output. As the desired speed  is  approached,  the  flyweights  operating  against the governor spring move a link to open the power tur- bine governor orifice. The flyweights also open the overspeed bleed (Py) orifice but at a higher speed than the regular governor (Pg) orifice. b. The  governor  orifice  is  downstream  of  a  bleed supplied by a regulated air pressure (Pr). Opening the orifice results in a reduced pressure downstream of the bleed  (Pg)  as  an  inverse  function  of  increasing  speed. Regulated pressure (Pr) and governing pressure (Pg) are applied to opposite sides of a diaphragm in the governor reset section of the gas producer fuel control, The  force  generated  by  Pr  minus  Pg  across  the diaphragm acts on the gas producer power output link through   the   governor   reset   rod.   This   force supplements the weight force in the gas producer fuel governor to reset (reduce) the gas producer speed. Gas producer  speed  cannot  exceed  the  gas  producer  fuel governor  setting.  The  Pr  minus  Pg  diaphragm  is preloaded for establishing the active Pr minus Pg range. Pr pressure is supplied from engine Pc pressure by an air regulator valve. 1-6