Engines Flashcards
Bernoulli theorem
Total energy made of 3 forms (potential, pressure/temp and kinetic)
Pressure and kinetic energy proportional
Piston engine process
Induction, compression, combustion (expansion) and exhaust
What is the compression ratio
Ratio to BDC and TDC (total vol / clearance vol)
Different colour smoke meaning
Blue = oil burn in cylinders
Black = carbon granules
White = water content
What does a supercharger do
Delivers more air to the engine cylinder, meaning more fuel, meaning a greater power output
Advantages of propellers
-Extra wing lift due to prop
-slip stream suppresses stall speed (good low speed characteristics)
-slipstream makes fin/rudder more effective
Disadvantages of propeller
Lack of airspeed and range due to limited revolution speed to ensure blade doesn’t become sonic
What decides the prop force
Output of engine and blade angle
3 restrictions on propellers
- Blade length
- Chord size (aspect ratio, centrifugal turning moments)
- blade interference (too many)
Why do blades have washout (twisted)
To maintain blade AoA and prevent it becoming sonic
Three reasons for critical engines
Asymmetric blade affect due to downgoing blade travelling faster
C of G position and blade distance greater on one engine
Slip stream of props
What is specific fuel consumption
Fuel used per hour / thrust of engine
Combustion cycle of jet engine
Induction, compression, combustion, expansion, exhaust
Why were jet engines introduced
-to fly at higher speeds
-more simplistic and reliable
Describe the process of a gas turbine
-air enters and compressed
-ignited in combustion chamber to form more energy
-expansion and passes across turbine
-exhausted through nozzle
Advantages of fuel injection
No vaporisation, uniform delivery, improved fuel/air ratio and instant engine response
Compression ratio for gas turbine
Change in air pressure between inlet and outlet power of a compressor or system
3 benefits of a bypass engine
-greater propulsive efficiency
-greater SFC
-less noise
5 Advantages of a fan engine
-Smaller engine size
-better propulsive efficiency
-better SFC
-reduction in engine noise
-contamination effects reduced
Why flat rate a fan engine
Give it a wide range of operation to keep it within mechanical limits
EPR
Ratio of air pressure measurements
Wet start
Failure to start after fuel delivery due to ignition problems
Hung start
Ignition occurs but fails to reach self sustaining rpm (no starter motor)
Hot start
Ignition and reaches self sustaining rpm but unstable combustion
Surge
Reversal of air through the engine
