Principle of a Gas Turbine Engine
Gas Turbine, Subsystems I
In WW-II and shortly thereafter, piston powered aircraft peaked in power, performance and complexity wise. Power went up to over 4000 bhp for large multi-row radial engines. Only to be defeated by the jet, which was developed by (among others) Germany's Dr. Hans von Ohain and separately in the UK by Sir Frank Whittle. Its principles are based on the "Aeolipile" of the ancient Greek scientist Hero and other great thinkers like Leonardo da Vinci and the laws of Isaac Newton.
Compared to a piston, the gas turbine has less parts and the moving parts rotate in only one direction without stopping and accelerating as the pistons normally do in a engine. Thus, a running gas turbine is basically free of the vibrations normally found in piston models, which translates in much longer service life (TBO) and higher reliability.
There are a number of subsystems which assist in keeping the engine running. Start systems, ignition and the fuel system. Without going into great depth we will uncover how they work.
The GTE needs a number of subsystems to start it, supply it with fuel, lubricating oil and monitor its performance to make sure that we are not creating a hot start and destroy the internals of the engine.
Starting a GTE
To start a turbine we need to get the air flow going and at some point we must add fuel and a spark to ignite the mixture. Easier said than done. Usually the starter (also the generator) drives the HP turbine/compressor assembly (N2) to a sufficiently high RPM and airflow so that when fuel is added and ignited it will lite up and accelerate to idle RPM (around 55% N2) and is then self sustaining.
During the start sequence it is important that the pilot monitors the EGT and its rate closely. If an abnormal high rate is observed the fuel flow should be closed immediately! So keep your hand on the fuel control!
Todays GTEs have fully automatic start systems and these monitor, among other things, ITTs and EGTs to make sure the engine starts safely. Without any chance on hot starts, these can be the result of fuel left in the combustion chamber. A ventilation cycle should then be performed.
In flight restarts are performed by bringing the aircraft in the correct speed relight envelope. As the turbine is already windmilling there is no need to use the starter motor, just add fuel and ignition and the engine will start.
Like piston engines GTEs are also fitted with dual ignition systems. These are rated in Joules (watt per second or w/s). The life of the igniters is extended by limiting the amount of energy required.
So if continuous use is necessary, for example in heavy rain, snow and or turbulence, power output is reduced.
Most systems have low and high output igniters and both are used during start where the low output igniter is used for continuous use. Modern electronic igniter systems can be regulated in energy output (J) as required.
A low pressure high volume booster pump delivers fuel from the tanks to the engine driven high pressure pump at a rate high enough to sustain operation under all flight conditions. Vapor lock and cavitation should be prevented by the system. Some installations have a fuel cooled oil cooler, which heats the fuel and any water in the fuel is then dissolved before it could freeze and block the system or filters.
The engine driven high pressure pump raises pressure to several hundred pounds per square inch (psi) for fuel operated servos and for delivery to the combustion cans. Most pumps are the gear type and bypass action (to regulate pressure) is through a spill valve. Other pump models include the plunger type where delivery depends on the speed of rotation and stroke of the plungers (small pistons), pressure can reach 2000 psi and flow rates from 100 to 2000 USG/h are quite common.
Fuel Control Unit
Engine thrust is regulated through a fuel control unit (FCU). This unit is commanded by the throttle and controls fuel flow and takes air density into account. Air density changes are the result of changes in speed (ram air), air temperature, humidity and altitude and fuel flow needs to be compensated to maintain the correct fuel / air ratio.