Engine Systems Introduction Flashcards
What creates thrust for the aircraft?
Propulsion system
What are the two main ways propulsion systems operate?
- Thrust developed by a gas turbine engine
2. Indirectly by a propeller (rotor) driven by piston or gas turbine engine.
What is the combustion process ?
Air and liquid hydrocarbons are compressed and then ignited.
What cycle do aeronautical piston engines use? What fuel do they use?
- Four stroke petrol cycle.
2. Utilize AVGAS
What is the piston attached to which is then attached to something else?
Piston –> Connecting rod –> Crankshaft
What are the four cycles in a piston engine?
- Intake
- Compression
- Power
- Exhaust
What occurs during the intake cycle of piston engine?
- Intake valve opens and fuel/air mixture is drawn into the cylinder.
- Cylinder pressure reduces as cylinder travels downwards.
What occurs during the compression cycle of piston engine?
- Both valves close
2. Cylinder pressure increases as piston travels upwards again.
What occurs during the power cycle or piston engine?
- Both valves are closed.
- Cylinder reaches TDC and spark plug fires, igniting the compressed fuel/air mixture.
- Resulting explosion forces the piston back down the cylinder.
What occurs during the exhaust cycle of piston engine?
- When piston reaches BDC, exhaust valve opens
- Piston comes back up and forces exhaust gases out exhaust valve
- Exhaust valve closes at TDC and process starts all over again.
What are two methods used to artificially increase the pressure available to the intake manifold by using a compressor on the engine intake air?
- Supercharger
2. Turbochager
What is a supercharger?
Centrifugal compressor directly driven by the engine through a system of gears that rotates the impeller at approximately nine times the engine RPM.
What system can be used to reduce the risk of over-boosting?
Manifold Air Pressure (MAP) Control
What determines the operating speed of a turbocharger?
Exhaust energy and the load on the compressor which is related to MAP.
Which has greater Friction Horsepower (FHP) losses, supercharger or turbocharger?
Supercharger.
What are some advantages of a piston engine?
- Low cost
- Low fuel consumption
- Suitable for smaller engines
- Simple engine and prop controls
What are some disadvantages of a piston engine?
- Higher weight and size for a given thrust output
- Relatively low power output < 300 BHP
- Less reliable and higher maintenance requirements.
What are some gas turbine characteristics?
- Allows higher cruising levels with higher transit speeds for greater economy.
- Utilizes cheaper fuel with the enhanced safety characteristics provided by its higher flash point
- Has fewer moving parts and a higher power-to-weight ratio.
What are the primary categories of gas turbine engines?
- Turbojet
- Turbofan
- Turboprop
- Turboshaft
How does the gas turbine engine move?
Spins smoothly and continuously.
What occurs during the start of a gas turbine engine?
External source is used to spin the engine until there is sufficient airflow for combustion, and then sufficient exhaust flow for the engine to achieved self-sustaining RPM.
What are the 5 stages of a gas turbine engine?
- Air Inlet
- Compression
- Combustion Chamber
- Turbine
- Exhaust
What occurs during the inlet stage?
Air is directed by the air inlet into the compressor.
What occurs during the compression stage?
Air is compressed, volume decreases, increasing temperature and delivered to the combustion chamber.
What occurs in the combustion chamber during engine start/after engine start??
- Fuel is injected and mixed with compressed air in the combustion chamber.
- During engine start, fuel is ignited by igniter plugs
- After engine start, fuel/air mixture is burned continuously with high temperatures.
When does combustion cease?
When the engine is shut down by turning off the fuel supply?
What occurs in the turbine phase?
Power is extracted from the gas flow to drive the compressor.
What is the difference in exhaust gases in a turboprop/turbojet engine?
- Exhaust gases are used to provide thrust in a turbojet
- In a turboprop engine, the exhaust gases are directed through an additional turbine to drive a prop through reduction gearing.
What can occur if there is an overtemperature or overspeed condition that lasts for more than a few seconds?
Internal engine components can be destroyed.
When do the hottest temperatures in the engine cycle occur?
During the starting cycle
What must occur before fuel is introduced into the combustion chamber?
Engine must reach a minimum rotational speed.
What is bypass air in a turbofan?
Air ducted around the hot core of the engine through an inlet fan that is driven by the turbines of the gas generator section.
What is the bypass ratio (BPR)?
Ratio of mass flow of air through the bypass duct to mass flow of air through the primary gas path.
What does a high bypass ratio indicate?
That most of the airflow is directed around the gas generator.
What BPRs do high bypass turbofan engines have?
> 3 : 1
What are there often two stages of compression/power extraction?
To optimize and maximize engine compression.
What BPRs do a low bypass turbofan use?
Less than 1:1
What advantages do low bypass turbofans provide?
- Lower frontal area which reduces drag and enables installation in fuselage.
- Greater efficiency at high speeds
- Reduced rotational inertia enabling more rapid engine power changes
- Increased thrust for a given engine weight.