Electronic APU fuel control System in Aircraft :
Electronic fuel control emulates the mechanical system, however it provides control in a slightly different way. The electronic Control Unit (ECU) monitors the APU speed and EGT continuously and also the low oil pressure switch.
During start the ECU knows the engine speed so will signal the fuel shut off valve to open at 8-10%. At the same time ignition is selected on and the light up will be sensed by the EGT system. The ECU then enters a timed acceleration schedule where EGT and speed are monitored by the ECU. The ECU deselects the starter at 50% and the ignition at 95%. Once up to speed the ECU keeps the engine at 100% and will monitor the EGT and speed to maintain operation throughout the operating envelope of the APU.The Fuel control unit mounted on the APU gearbox is much simpler than the mechanical FCU. It contains a fuel pump, an electronic servo valve and a pressure drop control valve ( [delta] P valve). The electrical shut of valve and the fuel flow divider are retained and work as they did in the mechanical system.
The ECU has Built In Test Equipment (BITE) indicators which will indicate why the APU shut down, however these do not retain the information if power is removed.