Figure 2-52.  APU fuel system schematic. be discharged from the pump element. From the screen, fuel  branches to the metering valve, differential pressure valve,  and the ultimate relief valve. Also shown at this point is a  pump discharge pressure access plug, another point where a  pressure gauge might be installed. EEC is a computer that controls the operation of the engine.  The EEC housing contains two electronic channels (two  separate computers) that are physically separated internally  and is naturally cooled by convection. The EEC is generally  placed in an area of the engine nacelle that is cool during  engine operation. It attaches to the lower-left fan case with  shock mounts.  [Figure 2-53] The EEC computer uses data it receives from many engine  sensors and airplane systems to control the engine operation.  It receives electronic signals from the flight deck to set engine  power or thrust. The throttle lever angle resolver supplies the  EEC with a signal in proportion to the thrust lever position.  The EEC controls most engine components and receives  feedback from them. Many components supply the EEC with  data for engine operation. The differential pressure valve maintains a constant pressure  drop across the metering valve by bypassing fuel to the  pump inlet so that metered flow is proportional to metering  valve area. The metering valve area is modulated by the  torque motor, which receives variable current from the  ECU. The ultimate relief valve opens to bypass excess fuel  back to the pump inlet whenever system pressure exceeds a  predetermined pressure. This occurs during each shutdown  since all flow is stopped by the shutoff valve and the  differential pressure valve, is unable to bypass full pump  capacity. Fuel flows from the metering valve out of the FCU,  through the solenoid shutoff valve and on to the atomizer.  Initial flow is through the primary nozzle tip only. The flow  divider opens at higher pressure and adds flow through the  secondary path. Power for the EEC comes from the aircraft electrical system  or the permanent magnet alternator (PMA). When the engine  is running, the PMA supplies power to the EEC directly. The  EEC is a two channel computer that controls every aspect  of engine operation. Each channel, which is an independent  computer, can completely control the operation of the engine.  The processor does all of the control calculations and supplies  all the data for the control signals for the torque motors and  solenoids. The cross-talk logic compares data from channels  Many large high-bypass turbofan engines use the FADEC  type of fuel control system. The EEC is the primary  component of the FADEC engine fuel control system. The  2-39 FADEC Fuel Control Propulsion Engine ECU Torque motor Metering valve Pump discharge port Pump inlet pressure Metered outlet port Inlet P Valve Filter bypass valve High pressure pump Metered pressure 70 Micron screen Ultimate relief valve 10 Micron inlet filter Fuel shutoff solenoid Ignitor plug Automizer LEGEND Pump discharge pressure Metered pressure Inlet pressure