ELECTRONIC ENGINE CONTROL

ELECTRONIC ENGINE CONTROL

77. As stated in para. 8, some engines utilize a system of electronic control to monitor engine performance and make necessary control inputs to maintain certain engine parameters within predeter- mined limits. The main areas of control are engine shaft speeds and exhaust gas temperature (E.G.T.) which are continuously monitored during engine operation. Some types of electronic control function as a limiter only, that is, should engine shaft speed or E.G.T. approach the limits of safe operation, then an  input is made to the fuel flow regulator (F.F.R.) to reduce the fuel flow thus maintaining shaft speed or E.G.T. at a safe level. Supervisory control systems may contain a limiter function but, basically, by using aircraft generated data, the system enables a more appropriate thrust setting to be selected quickly and accurately by the pilot. The control system then  makes small control adjustments to maintain engine thrust consistent with that pre-set by the pilot, regardless of changing atmospheric conditions. Full authority digital engine control (FAD.E.G.) takes over virtually all of the steady state and transient control intelligence and replaces most of the hydromechani-cal and pneumatic elements of the fuel system. The  fuel system is thus reduced to a pump and control valve, an independent shut-off cock and a minimum of additional features necessary to keep the engine safe in the event of extensive electronic failure.

78. Full authority fuel control (F.A.F.C.) provides full electronic control of the engine fuel system in the sameway as F.A.D.E.C., but has none of the transient control intelligence capability used to control the  compressor airflow system as the existing engine control system is used for these


Speed and temperature control amplifiers

79. The speed and temperature control amplifier receives signals from thermocouples measuring E.G.T. and from speed probes sensing L.P. and in some cases, L.P. shaft speeds (N1 and N2). The amplifier basically comprises speed and temperature channels which monitor the signals sensed. If either N1, N2 or E.G.T. exceed pre-set datums, the amplifier output stage is triggered to connect an electrical supply to a solenoid valve (para. 47) or a variable restrictor (para. 73) which override the F.F.R.and cause a reduction in fuel flow. The limiter will only relinquish control back to the F.F.R. if the input conditions are altered (altitude, speed, ambient temperature or throttle lever position). The limiter system is designed to protect against parameters exceeding their design values under normal operation and basic fuel system failures.

 Engine supervisory control 
80. The engine supervisory control (E.S.C.) system performs a supervisory function by trimming the fuel flow scheduled by the fuel flow governor (F.F.G.) to match the actual engine power with a calculated engine power for a given throttle angle. The E.S.C. provides supervisory and limiting functions by means of a single control output signal to a torque motor in the F.F.G. In order to perform its supervisory function the E.S.C. monitors inputs of throttle angle, engine bleed state, engine pressure ratio (E.P.R.) and air data computer information (altitude, Mach number and temperatures). From this data the supervisory channel predicts the value of N1 required to achieve the command E.P.R. calculated for the throttle angle set by the pilot. Simultaneously a comparison is made between the command E.P.R. and the actual E.P.R. and the difference is compared with aprogrammed datum.

81. During acceleration the comparitor connects the predicted value of N1 to the limiter channel until the difference between the command and actual E.P.R.is approximately 0.03 E.P.R. At this point the predicted L.P. shaft speed is disconnected and the E.P.R. difference signal is connected to the limiter channel.

82. The final output from the supervisory channel, in the form of an error signal, is supplied to a 'lowest  wins' circuit along with the error signals from the limiter channel. While the three error signals remain positive (N1 and E.G.T. below datum level and actual E.P.R. below command E.P.R.) no output is signalled to the torque motor. If, however, the output stage of the E.S.C. predicts that E.G.T. will exceed datum or that N1 will either exceed its datum or the predicted level for the command E.P.R., then a signal is passed to the torque motor to trim the fuel flow.