M14.1 Flashcards
A/C engine supplies:
hydraulic, electric, pneumatic
2 types of engines:
Piston type & Gas turbine
Piston type
Advantages:
-simple in design
-more fuel efficient
Disadvantages:
-performance decrease at higher altitude -can’t be on large A/C (weight to power ratio)
Gas Turbines
Advantages:
-operate at high altitudes & speed
-easily provide thrust, torque, & bleed air -powers either a single fan or a set of a counter-rotating fans via a gearbox
Different types of Gas Turbines
- Turbofan engine
- Turbojet engine
- Turboprop engine
- APU
Turbofan engine
-used on modern aircraft
-high A/C speed is possible w/ good engine efficiency @ high altitude
-developed from turbojet
-usually twin or triple spool engine
-fan is always driven by a turbine via drive shaft
-do not have reduction gear to reduce fan speed
Turbofan engine
-converts large part of gas energy into torque to drive the fan and compressor engine -remaining hot gas from core engine is directly converted into thrust
*Total thrust = core engine + fan
-fan accelerates high airflow, low outlet
velocity
-core engine accelerates small quantity of air, high outlet velocity
-fan can produce 80% of total thrust ;
primarily dependent on bypass ratio
Turbojet engine
-first type of gas turbine used on A/C
-give VERY high A/C speed
-VERY LOUD because of extremely high
exhaust gas velocities
-need too much fuel
-all gas energy is converted into thrust
Turboprop engine
-specially design to produce shaft horsepower only which is to drive a propeller
-installed on small commuter A/C
-achievable A/C speed & fuel efficiency
-produce their thrust by a small acceleration of a large quantity of air with propeller
-driven directly on compressor shaft or by a turbine and a long center drive shaft.
-all gas energy into torque
- always required to reduce
the high gas turbine engine rotation to speeds that can be managed by the propeller
Reduction gear
Auxiliary Power Unit:
-small gas turbine engine
-supply A/C w/ electric & pneumatic power if the engine is not available
-A/C is independent of airport equipment
– propelling force, which is generated in the opposite direction to the flow of mass through the jet nozzle.
Jet Propulsion
- engine which uses jet propulsion.
Reaction Engine
this engine uses Newton’s 3rd law of motion (for every force which acts on a body there is an opposite & equal reaction)
Reaction Engine
F=MxA
To accelerate air, air pressure must increase:
2 ways of doing it:
- mechanically w/ compressor
- thermally by increasing volume of air when a fuel/air mixture is burned/heated
Base for all gas turbine engine:
- Hans Van Ohain, 1937
- Frank Whittle, 1941
centrifugal flow compressor driven by radial turbine
Hans Van Ohain, 1937
centrifugal flow compressor driven by axial turbine; these engines were only possible after the development of materials heat-resistant enough for continuous combustion
Frank Whittle, 1941
– generated by the acceleration of ambient air which is forced through the engine.
Thrust Force
2 parameters of THRUST:
- mass of ambient air which is accelerated
- quantity of acceleration itself
series of changes of state of a working medium which occur periodically
Cyclic Process
– technical processes usually used for converting heat into mechanical work (combustion engines) or for heating & cooling by performing work.
Brayton Cycle
– line of equal pressure
Isobars
– lines of equal entropy
Isentropes
– change of state of gases which there is no change in entropy. Energy content is constant.
Isentropic
– a system is transferred from one state to another w/o exchanging thermal energy w/ its environment.
Adiabatic
quantity of airflow to the engine can be changed by
Control Valve
Control Valve – quantity of airflow to the engine can be changed by:
- Nozzle w/ small diameter high outlet velocity; high thrust
- Nozzle w/ medium diameter medium outlet velocity; medium thrust
- Nozzle w/ large diameter low outlet velocity; low thrust
– thrust of the jet nozzle does not change.
Deflector Plate
- Thrust is generated by acceleration of airflow and not by pushing against an object
Deflector Plate
4 main environmental parameters that cause changes in thrust of a jet engine:
- Ambient air pressure
- Air temperature – changes density
- Operating altitude
- Airspeed- speed increases; thrust
decreases. Net effect on thrust is a combo of thrust decrease from acceleration effect & thrust increase from the ram effect.
*1&2 are most important factors that cause a change of mass airflow because these determine the density of air
*lower density creates lower thrust because the airflow contains less mass than a high density airflow.
*higher pressure, higher thrust
*higher temperature, lower thrust
36k ft up to 65k ft – air temperature is constant @ -57C / -70F
- bypass ratio of air that passes through the fan duct & core engine.
Bypass ratio is 4:1 and 9 :1 Older turbofan engine bypass ratio is 1:1
- open rotor engines. Advantage of using up to 20% less than high bypass engine. Has mounting difficulties, not very common on modern jet A/C. Bypass ratio of 90:1
Propfans
Different Methods of Engine Design:
- Non-modular Engine Design
& - Modular Engine Design
- saved weight & fully functional but design was complicated & parts were difficult to access. Usually found on older type of engines and some APUs.
Non-modular Engine Design
- used on all modern A/C. whole engine is spilt up into a set of separate major modules (preassembled &balanced). Designed to be removed and replaced easily. Very often the replacement of a module is considered a minor repair and not a shop repair.
Modular Engine Design
– must be strong as required for their individual task, light, and cheap as possible
Engine Materials
– mainly used for gearbox housing, fan stator casing, and low loaded parts of the fan module
Aluminum alloy
- used in combustion section and in high pressure turbine. Serves as heat & corrosion protection.
Ceramic material
– usually used in the fan module
Composite material
– very heat resistant, can be found in stator of the first stage of the high-pressure turbine.
Cobalt base alloy
- can withstand high centrifugal loads at high temperatures. Used in the high-pressure compressor, in the combustion section & for the high/low pressure turbines
Nickel base alloy
-used for the N1 drive shaft, engine bearings & main structural frames on some engine
Corrosion resistant steel alloy
- strong as steel but only half as heavy. Can withstand high centrifugal loads but expensive. Mainly used in fan, the low-pressure compressor and the front stages of the high-pressure compressor
Titanium base alloy
– operated above idle in parallel with hydromechanical control. Only values which are necessary for determining fuel flow to maintain the desired thrust
Electronic Control
– A centralized computer with all necessary sensors which controls the engine and all its subsystems
- ECU, Fuel metering valve, electrical sensors on the engine
*Electricity is supplied by a control alternator. Which is powered by N2 rotor via accessory gearbox
FADEC (full authority digital engine control)
– digital computer of FADEC. Located at the fan stator case of the engine. Gets demand signal from thrust lever in the cockpit. Power supply from A/C or a small permanent magnet alternator.
- Performs full power management & optimum thrust
- Permanently monitors the engine operation & important system components & gives fault messages to the CMC
- Almost independent from aircraft
Electric control unit
2 independent computers of ECU:
channel A&B
-made by a dual set of electronic cards, 1 set each per channel.
- Signals from each ECU are either electric/pneumatic signals.
channel A&B
- channels A&B always collect the data from the engine and aircraft and they always communicate with each other
Cross-channel data link
- 1 channel at a time has the authority to control the engine
*change in command is automatic.
*all electrical sensors on the engine have individual pick-ups that are connected to the respective ECU channel.
*there are always 2 individual transducers for each pressure line, each channel has its own pressure transducer.
channel in command
– any one time only one channel works together actively with the engine.
Active channel
– carries out all necessary calculations for engine control but its output signal act on a steady load.
*switch over from one channel to another is usually at the beginning of each engine start sequence.
Standby channel
– calculates the thrust command based on the thrust lever angle, ambient air temp, and pressure such as T12 & Ps12. Also uses mach number for info about aircraft speed.
Power management
– connector from the engine to the ECU used for engine identification. Makes sure that the ECU controls the engine to the correct maximum take-off thrust.
Thrust rating plug
– correction factor for the engine efficiency.
Performance factor
– compares the thrust command signal with the actual N1 signal and controls an error signal to the fuel metering valve if the command signal is different to actual N1 signal.
Governing section
– receives the N2, CDP, CIT to make sure that the engine operational limits are not exceeded.
Limiting section
– represents the compressor inlet temp. used in the limiting section to protect the engine against compressor surge.
T25
– represents the compressor discharge pressure. It is used to protect the engine against over boost.
PS3
– receives a position feedback signal from the fuel metering valve to control the necessary fuel flow.
Governing/limiting
– it only receives orders from the ECU to move the fuel metering valve
Fuel metering unit
Components of fuel metering unit:
- Fuel metering valve – located @ HMU
- Bypass valve
- HP shut off valve
- Servo valve with a torque motor
– reduces the fuel flow through the fuel metering valve if the electrical overspeed protection by the ECU fails.
Overspeed governor
– independent of ECU. a fuel metering with these additional servo valves
- Variable stator vanes
- Variable bleed valves
- High/low pressure turbine clearance
control valves
Hydromechanical unit
– transmit the data between FADEC and A/C. also control power supply from the A/C to ECU. Located @ electronic compartment.
- It limits the number of signal wires between aircraft and engine, because 1 single data bus can transmit all digital data. Saves weight, reduces risk of wiring faults.
- Heart of FADEC
*during engine operations the electrical power supply to the ECU is just a back-up to prevent loss of electrical power, normally, a permanent magnet alternator on the engine accessory gearbox supplies the ECU with electrical power
Engine interface unit
– produces the fuel pressure for operating hydraulically powered actuators of the corresponding engine systems
Electro-hydraulic servo valve
– monitors adjustments of the actuated system components, and reports their actual position to ECU
LVDT/RVDT
– supply of electricity to the ignition system as well as certain processes during transition of the thrust reverser.
*temperature increase across LP compressor is always in a fixed relation to the fan speed
Starter valve
What will happen to Force if the mass increases?
The Force will also increase
What will happen to Force if the Acceleration decreases?
The Force will also decrease
What will happen to Pressure if Velocity increases?
Pressure will decrease
The higher altitude the lower the density and the pressure also decrease, causing the Thrust to decrease. What will compensate the Thrust?
- In high altitude the temperature is colder and that will compensate the thrust
- In high altitude the ram air will compensate the Thrust
It is a pressure-volume diagram of a thermodynamic cycle of a turbojet engine.
Brayton Cycle
Section which the pressure reaches the maximum value and volume flow reaches its minimum value.
Section 2 to 3
This process is called isobaric heat addition, which takes place in the combustion section.
Process 3 to 4
How many percent of thrust does the fan can produce and how many percent does the fuel burn in the core engine?
80% Fan; 20% Core Engine
Semi-Electronic Control Systems
PMC; HMC/MEC
FADEC Systems
ECU; HMU/FMU
Second Main Component of the FADEC System
Fuel Metering Unit
Power Supply of ECU
PMA
Location of the ElU (Engine Interface Unit)
Avionics Compartment
Location of the ECU ( Engine Control Unit)
Fan Case
Why are there two independent computers for ECU?
- For redundancy
- For backup
Pressure signals from the engine are what kind of signals?
Pneumatic Signal
The pressures are converted into digital signals by?
Pressure Transducer
It makes sure that the ECU control the engine to the correct maximum take-off thrust. It also provides the engine type identification.
Thrust Rating Plug
it receives control signals from the electronic control unit to move the fuel metering valve.
Torque Motor
It continuously measures the position of the fuel metering valve and transmits a feedback signal to the ECU.
Position Sensor
It is necessary to keep the pressure difference across the fuel metering valve constant so that the fuel flow is proportional to the opening area of the fuel metering valve.
Bypass Valve
In overspeed condition the ECU first tries to decrease the fuel via the ____. If the ECU fails at a slightly higher overspeed the _____ acts as a backup.
Fuel Metering Valve; Overspeed Governor
limits the number of signal wires between the aircraft and engine, because 1 signal data bus can transmit all digital data. It also saves weight and reduces the risk of wiring faults.
EIU
They are connected directly to ECU
PMA, ECAM/EICAS, TLA
During the operation of the ECU, the 2 channels are operating but only 1 channel at a time has the authority to control the engine. This is called
Channel in command
When does the switch-over from one channel to the other one happens?
At the beginning of each engine start sequence
What is the secondary power source of the ECU in case PMA is not available?
Aircraft Power Supply
