How does a turbojet engine's performance change as the load on it varies? [closed]

Question

What happens when the load increases on the turbojet engine? What senses the speed change in jet engines and how speed correction takes place?

Answer 1

Taking a very common turbofan as an example, the CFM56-5B used on Airbus A320 family.

---

Indeed the pilot doesn't adjust the throttle when the power demand changes, e.g. when the anti-ice system is turned on, or when the angle air enters the engine varies with aircraft attitude, or when a larger electric power is needed for the galley, or when rain starts falling. All that is regulated.

The regulation is done by a computer in the engine itself, the FADEC (full authority digital engine controller). The crew or the auto-throttle sets a thrust demand, that's the parameter which is important for the flight, and the FADEC manages the engine in order to provide the power to deliver this thrust and the power for secondary uses like electricity.

The first parameter controlled by the FADEC is the fuel flow, the main way to determine how much energy is converted into heat and ultimately into air flow acceleration and propulsive force. But that's not the only parameter under control.

From the engine documentation:

The FADEC provides automatic engine thrust control and thrust parameter limit computation. The FADEC manages power according to two thrust modes:

• manual mode depending on Throttle Lever Angle (TLA),
• autothrust mode depending on autothrust function generated by the Auto Flight System (AFS).

This regulation is done using several means: fuel flow, variable vanes angle of attack in compressor, discharge valves, and turbine blades clearance regulation when temperature changes.

^{Image from CFM56-5B Technical Training Manual by Airbus}

The FADEC manages secondary power demand variation like for anti-icing, cabin pressurization and air conditioning.

The FADEC also provides two idle mode selections: minimum idle and approach idle, obtained when the slats are extended. The idle can also be modulated up to approach idle depending on: air conditioning demand, wing anti-ice demand, engine anti-ice demand and oil temperature (for Integrated Drive Generator (IDG) cooling).

Thrust cannot be measured in flight, it must be evaluated from other parameters. Two usual parameters are:

• N1, the rotation speed of the low pressure spool,

• The engine pressure ratio (EPR), that is the pressure ratio between engine exit and inlet nozzle.

These two parameters, associated with other data, allow the FADEC to adjust the fuel flow and to monitor the engine efficiency. How they are exactly used depends on the engine design.

The FADEC is the blue box in the image below, receiving many inputs (top right) as temperatures and pressures and controlling regulation elements shown on the engine drawing.

^{Image from CFM56-5B Technical Training Manual by Airbus}

A FADEC or an equivalent controller is required on turbine engines due to the complexity and the extreme temperature of the combustion, pushing the combustion chamber and the turbine materials to their limit, so fuel must be regulated very precisely.

A regulation is also needed to obtain the best engine efficiency, and to not exceed engine limitations of many sorts. Before FADEC use, monitoring and controlling the engines was the task a flight engineer, the third crew member:

^{Flight engineer panel on a Boeing 747F, source}

FADEC, source Safran:

Other engines may not have a full authority controller, that is a computer which cannot be overridden by the crew, but a similar controller exists, generally named an EEC (electronic engine controller).

---

Additional information

You can see one aspect of this regulation in this video. Valves open and close in order to prevent the compressor to stall when thrust demand is changed.

Also see these questions:

• Why does the CF6/CFM56 Turbofan Fan Frame contain 12 circular holes between the primary and secondary flow? Here.

• How does the FADEC prevent compressor stall during rapidly increasing thrust demand? Here.

• What is the difference between variable and transient bleed valves? Here.

Answer 2

Because turbines feed air to themselves with a compressor, they generally operate with variable rpm to control power output.

Adding more fuel without changing rpm (airflow) risks overpressuring or overheating the combustion chamber. Opening up the jet exhaust nozzle more helps compensate for the delta Volume caused by delta Temperature. Variable stator vanes help fine tune engine output on the inlet side.

Jets operate using way more air than they need for stoichimetric combustion, so there is some room there for adding more fuel at the same rpm. Turbine helicopter rotor systems are of interest.