Full-Authority Digital Engine Control Systems

Electronic controllers for jet engines were developed in the 1980s. Currently, the jet engine controllers use highly customized computers to synthesize the control laws with a number of sensor inputs and actuator outputs. Such electronic con­trollers result in higher engine operating efficiencies by allowing for precise engine control through the use of multiple control loops and improved control laws to reduce transient overshoot or undershoot. It also allows implementation of control algorithms which would be difficult to implement using traditional wired circuitry. Current engine control systems are generally implemented digitally and cannot be overruled by the pilot and for this reason are referred to as full-authority digital engine controllers (FADEC). The primary objective of a FADEC is to operate the engine while avoiding any instabilities over the entire engine while delivering the commanded performance in terms of the thrust generated. Several modes of instability can occur in a jet engine: surge, which is a longitudinal flow oscillation over the length of the compressor and turbine, and rotating stall, which is the lack of pressure rise between the compressor blades. Often rotating stall occurs at low rotor speeds and surge at high rotor speeds. Both surge and rotating stall generate violent axial oscillations of the internal air column which can cause substantial damage to both the compressor and the engine. Fan stalls can be caused by operation with too small a fan duct nozzle area, booster stalls by a throttle reduction to a lower engine rotational speed, and compressor stalls by a rapid throttle increase. The engine and control system must be designed to avoid surge/ stall with sufficient design margin to offset the effects of altitude, increased tip clearances, component deterioration, and engine/airflow operation at high angles of attack. Too much fuel can result in a blowout, where soaking the flame with fuel displaces the oxygen and lowers the temperature enough to extinguish the flame. Depending on the combustor pressure, the combustor can operate in certain regions where they tend to ‘‘blow out.’’ There are two regions of blowout asso­ciated with most jet engine combustors. The high-temperature blowout region is referred to as ‘‘rich blowout’’ and the low-temperature region is referred to as ‘‘lean blowout.’’ Flame-outs which refer to the extinguishing of the flame, although rare, must also be avoided in the combustor as the consequences of flame – outs coupled with other potential instabilities can be quite disastrous.

While higher turbine inlet temperatures can lead to improved specific thrust, a lighter engine but requires expensive turbine materials and a complex turbine cooling system which reduces cruise performance. A proper balance of tempera­ture must be maintained in the engine, and it will depend on the relative impor­tance of specific thrust which sets engine size and weight and fuel requirements in cruise.

Typically, a FADEC must ensure that the following limits are not exceeded:

1. maximum fan speed and compressor speeds,

2. maximum turbine temperature,

3. avoid fan and compressor stall,

4. meet maximum and minimum compressor discharge pressure requirements,

5. Avoid lean and rich blowouts.

To meet these requirements, a minimum set of measurements must be made and these are as follows:

1. High – and low-pressure spool speeds.

2. Engine pressure ratio.

3. Ratio of the fuel flow rate to compressor exit pressure.

4. Low-pressure turbine inlet temperature and

5. The control inputs which could be the fuel-air ratio, the compressor inlet guide vanes, and/or the variable nozzle throat area.

All the states of the system are generally estimated from the measurements, and for this reason, a much larger set of measurements is generally desirable. Jet engine dynamic models may involve as many as 35-40 state variables and four times as many measurements are made in practice.

Primary control strategies are to control the spool shaft speeds, usually of the low-pressure spool so as to indirectly be able to control the thrust. Direct control of thrust, although rare, is also possible in some jet engines. For a more detailed description of the issues involved in the design of the controller, the reader is referred to Spang and Brown (1999).

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