FUEL HEATING

96. The spill spray nozzle system, however, involves a somewhat modified type of fuel supply and control system from that used with the previous types. A means has to be provided for removing the

spill and also for controlling the amount of spill flow at various engine operating conditions. A

disadvatage of this system is that excess heat may be generated when a large volume of fuel is being recirculated to inlet. Such heat may eventually leadto a deterioration of the fuel.

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

A proportional flow control system.

Fig. 10-7  A proportional flow control system.

23. H.P. compressor shaft r.p.m. is governed by a hydro-mechanical governor which uses hydraulic pressure proportional to engine speed as its controlling parameter. A rotating spill valve senses the engine speed and the controlling pressure is used to limit the pump stroke and so prevent over-speeding of the H.P. shaft rotating assembly. The controlling pressure is unaffected by changes in fuelspecific gravity.

24. At low H.P. shaft speeds, the rotating spill valve is held open, but as engine speed increases, centrifugal loading moves the valve towards the closed position against the diaphragm loads. This restricts the bleed of fuel to the L.P. side of the valve until, at governed speed, the governor pressure deflects the servo control diaphragm and opens the servo spill valve to control the fuel flow and thereby the H.P. shaft speed. 

25. If the engine gas temperature attempts to exceed the maximum limitation, the current in the L.P. speed limiter and temperature control solenoid is reduced. This opens the spill valve to reduce the pressure on the pressure drop control diaphragm. The flow control spill valve then opens to reduce the pump servo pressure and fuel pump output.

26. To prevent the L.P. compressor from over-speeding, multi-spool engines usually have an L.P. compressor shaft speed governor. A signal of L.P. shaft speed and intake temperature is fed to an amplifier and solenoid valve, the valve limiting the fuel flow in the same way as the gas temperature control (para. 25).

27. The system described uses main and starting spray nozzles under the control of an H.P. shut-off valve. Two starting nozzles are fitted in the combustion chamber, each being forward of an igniter plug. When the engine has started, the fuel flow to these nozzles is cut off by the H.P. shut-off valve.

28. To ensure that a satisfactory fuel pressure to the spray nozzles is maintained at high altitudes, a back pressure valve, located downstream of the throttle valve, raises the pressure levels sufficiently to ensure satisfactory operation of the fuel 

A low pressure fuel-cooled oil cooler,A magnetic chip detector.

Fig. 8-7  A low pressure fuel-cooled oil cooler.

Fig. 8-8 A magnetic chip detector.

25. The air-cooled oil cooler is similar to the fuel-cooled type in both construction and operation; the
main difference is that air is used as the cooling medium.