Page 122 - The Jet Engine

Fuel system

accurately by the pilot. The control system then E.P.R. and the difference is compared with a
makes small control adjustments to maintain engine programmed datum.
thrust consistent with that pre-set by the pilot,
regardless of changing atmospheric conditions. Full 81. During acceleration the comparitor connects the
authority digital engine control (FAD.E.G.) takes over predicted value of N1 to the limiter channel until the
virtually all of the steady state and transient control difference between the command and actual E.P.R.
intelligence and replaces most of the hydromechani- is approximately 0.03 E.P.R. At this point the
cal and pneumatic elements of the fuel system. The predicted L.P. shaft speed is disconnected and the
fuel system is thus reduced to a pump and control E.P.R. difference signal is connected to the limiter
valve, an independent shut-off cock and a minimum channel.
of additional features necessary to keep the engine
safe in the event of extensive electronic failure. 82. The final output from the supervisory channel,
in the form of an error signal, is supplied to a 'lowest
78. Full authority fuel control (F.A.F.C.) provides full wins' circuit along with the error signals from the
electronic control of the engine fuel system in the limiter channel. While the three error signals remain
same way as F.A.D.E.C., but has none of the positive (N1 and E.G.T. below datum level and actual
transient control intelligence capability used to E.P.R. below command E.P.R.) no output is signalled
control the compressor airflow system as the existing to the torque motor. If, however, the output stage of
engine control system is used for these. the E.S.C. predicts that E.G.T. will exceed datum or
that N1 will either exceed its datum or the predicted
Speed and temperature control amplifiers level for the command E.P.R., then a signal is passed
79. The speed and temperature control amplifier to the torque motor to trim the fuel flow.
receives signals from thermocouples measuring
E.G.T. and from speed probes sensing L.P. and in LOW PRESSURE FUEL SYSTEM
some cases, L.P. shaft speeds (N1 and N2). The
amplifier basically comprises speed and temperature 83. An L.P. system (fig.10-13) must be provided to
channels which monitor the signals sensed. If either supply the fuel to the engine at a suitable pressure,
N1, N2 or E.G.T. exceed pre-set datums, the rate of flow and temperature, to ensure satisfactory
amplifier output stage is triggered to connect an engine operation. This system may include an L.P.
electrical supply to a solenoid valve (para. 47) or a pump to prevent vapour locking and cavitation of the
variable restrictor (para. 73) which override the F.F.R. fuel, and a fuel heater to prevent ice crystals forming.
and cause a reduction in fuel flow. The limiter will A fuel filter is always used in the system and in some
only relinquish control back to the F.F.R. if the input instances the flow passes through an oil cooler (Part
conditions are altered (altitude, speed, ambient 8). Transmitters may also be used to signal fuel
temperature or throttle lever position). The limiter pressure, flow and temperature (Part 12).
system is designed to protect against parameters
exceeding their design values under normal FUEL PUMPS
operation and basic fuel system failures.
84. There are two basic types of fuel pump, the
Engine supervisory control plunger-type pump and the constant-delivery gear-
80. The engine supervisory control (E.S.C.) system type pump; both of these are positive displacement
performs a supervisory function by trimming the fuel pumps. Where low pressures are required at the fuel
flow scheduled by the fuel flow governor (F.F.G.) to spray nozzles, the gear-type pump is preferred
match the actual engine power with a calculated because of its lightness.
engine power for a given throttle angle. The E.S.C.
provides supervisory and limiting functions by means Plunger-type fuel pump
of a single control output signal to a torque motor in 85. The pump shown in fig. 10-14 is of the single-
the F.F.G. In order to perform its supervisory function unit, variable-stroke, plunger-type; similar pumps
the E.S.C. monitors inputs of throttle angle, engine may be used as double units depending upon the
bleed state, engine pressure ratio (E.P.R.) and air engine fuel flow requirements.
data computer information (altitude, Mach number
and temperatures). From this data the supervisory 86. The fuel pump is driven by the engine gear train
channel predicts the value of N1 required to achieve and its output depends upon its rotational speed and
the command E.P.R. calculated for the throttle angle the stroke of the plungers. A single-unit fuel pump
set by the pilot. Simultaneously a comparison is can deliver fuel at the rate of 100 to 2,000 gallons per
made between the command E.P.R. and the actual hour at a maximum pressure of about 2,000 lb. per

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