Basic Gas Turbine Flashcards
Advantages of a 3 spool
- The engine is physically shorter because it has less compressor and turbine sections.
- Because it is shorter it flexes less, is more rigid, and therefore gives better engine performance over the life of the engine.
- The LP, IP, and HP sections all work closer to their optimum performance levels because they have their own shafts.
- They have better fuel consumption, and lower noise.
- Better thrust to weight ratio.
- Easier to start because only one shaft has to be turned.
- Modular in construction, easier to build and maintain.
- Better propulsive efficiency
- Since most of the fleet is powered by RR, it is cheaper for maintenance.
What things are contained in the accessory gearbox?
- Fuel pump (There may be a number of fuel pumps: low pressure, high pressure and also a speed-sensitive) governor
- Generators, often one for engine systems and one for the aircraft
- Constant Speed Drive to maintain a constant frequency AC generator
- Lubricating oil pumps
- Hydraulic pump
- High-pressure air compressor (undercarriage actuation, etc.)
- Low-pressure air compressor (cabin air conditioning), where this is not provided by tapping engine compressor bleed air.
- Engine starter
- Tachometer sensor drives
How does a jet engine work
The Brayton Cycle
- compression
- Combustion
- Turbine(work)
- Exhaust
How is a jet engine started
- Through the opening of bleed air valves, bleed air is sent to an air turbine starter. These devices typically use the high pressure bleed air to spin and engage a centrifugal clutch connected to the engines accessory drive. This in turn causes the N2 shaft within the engine to spin.
- With the N2 shaft spinning, the N2 compressor and the N2 turbines are spinning. This begins to force air through the engine from front to back.
- With the accessory shaft and N2 shafts spinning, accessories should start working and this can be verified by oil pressure indications on the EICAS.
- With increased N2 rotation, ignition will be turned on. These igniters are located in the hot section of the engine and produce small sparks. There should be an indication on the EICAS that the ignition is active.
- With further increase in N2 rotation, fuel flow will be introduced. This will be verified on the EICAS. Once fuel flow is noted, it is important that the next stop happen fairly soon.
- Light off! The fuel is lit by the ignition and now the fire burning in the hot section supplied by air from the compressor is producing thrust across the N2 and N1 turbines.
- As the engine is producing thrust across on the N1 turbine, the N1 shaft is spinning the N1 fan and the EICAS will note this increase in N1 rotation. N1 and N2 rotation speeds increase.
- Above a N2 threshold, bleed air valves supplying the the air turbine starter will close and the starter disengage. The igniters will turn off at some N2 threshold.
- The engine will settle into a stable idle thrust setting.
N1 vs EPR settings
N1 - Fan speed
Unreliable if fan is damaged, shows fan speed reliable.
EPR - Engine pressure ratio
Measures actual thrust
Unreliable if pitot tubes get blocked
Most EPR engines revert to N1 under failure conditions.
What happens to temperature, pressure and velocity throughout all stages in the turbofan?
Temp and Pressure are increasing through the induction and compression stages. Throughout combustion stage, temp rises rapidly then decreases and pressure starts to decrease slightly. Throughout Turbine and exhaust stages, temperature and pressure is reducing.
Are igniters on all throughout the flight?
Essentially No. On a wet day takeoff or heavy rain, igniters may be selected to continuous operation for an instant relight in the event of flame out. In this case, the energy output of the igniter plugs is lowered to preserve the plugs and the ignition unit.
AUTO IGNITION activates the igniters at high energy for a relight in the case of an inflight flameout.
What is a high bypass ratio?
A high bypass ratio is an engine where majority of the air bypasses the engine core. For example a bypass ratio of 9:1 means that 9kg of air passes around the engine core for every 1kg of air that passes through.
