Propulsion Flashcards
What is the purpose of the gas generator and which components is it made up of?
The gas generator purpose it to provide high temperature and high pressure gas. Consists of the compressor, the combustor and the turbine.
What is the internal layout of a turbojet?
A turbojet consists of a gas generator with an inlet and nozzle added to either end.
Inlet, Compressor, Combustor, Turbine, Nozzle
What is the internal layout of a turbofan?
A turbofan is similar to a turbojet with a gas generator, inlet and nozzle however it also utilises a fan which is powered by the turbine to accelerate some air flow around the inner gas generator section. Known as a bypass air. More efficient than a pure turbojet.
Inlet, Fan, Compressor, Combustor, Turbine, Nozzle
What is the internal layout of a turboprop/turboshaft?
A turboprop/turboshaft consist of a gas generator driving either the propellor or a shaft respectively
Prop/Shaft, Compressor, Combustor, Turbine
Turboshafts are typically used in vehicles such as helicopters where the engine isn’t mounted in the same plane as propellors.
What is the internal layout of a ramjet?
These engines do not have a gas generator. They have an inlet, a combustion zone and a nozzle. At the start of the combustion zone there is a fuel ring which injects fuel into the inlet air. Mid way through the combustion zone is a flame holder which is used to ignite the fuel.
Piston Engines
What is the compression ratio of the engine? Give the equation as well.
The compression ratio is the ratio between the maximum volume and the minimum volume displaced by the piston.
r = Vmax/Vmin = Vbdc/Vtdc
Piston Engines
What is the mean effective pressure (MEP)?
MEP is a fictitious pressure that, if acted on the piston during the entire piston stroke, would produce the same amount of net work as that which is produced by the actual cycle.
MEP = Wnet/(Vmax - Vmin)
Piston Engines
What are the main layouts of piston engines?
In-line engines: All pistons are in a single row, sharing the same crankcase and crank shaft.
V-class engines: The pistons are placed in a V configuration made up of two in-line cylinder blocks placed at an angle to each other on a common crank case. They share a crankshaft.
Horizontally opposed engines: similar to the V class engines in that there are two in-line cylinder blocks, however they are placed diametrically opposite to each other. Operates using a common crankcase and crankshaft.
Opposed Piston engines: Essentially two engines placed together at the head. Share a common cylinder and combustion space. There are 2 crankcases and crankshafts at each end.
Radial Engines: Several pistons are placed radially around a common crankcase, all acting upon a single crankshaft.
Piston Engines
What are the main two ignition types?
Spark Ignition: A spark plug is used to ignite a fuel-air mixture after it has been compressed
Compression Ignition: The air-fuel mixture is self-ignited by compressing it above it self ignition temperature
Piston Engines
How does a two-stroke engine cycle operate?
A two-stroke engine completes all the events within a single revolution of the crankshaft. During the upstroke, the exhaust port is sealed by the piston and the air fuel mixture is compressed. Whilst this occurs, air/fuel is drawn into the crankcase ready for the next cycle. Just before the piston reaches top dead centre, the fuel is ignited. This causes the downstroke to begin. The downstroke, known as the power stroke, unseals the exhaust port letting the ignited mixture out whilst also sealing the inlet port and compressing the mixture in the crankcase. At the bottom of the stroke, the transfer port becomes uncovered allowing the compressed fuel-air mixture in the crankcase to move to the piston which pushes any leftover exhaust gases out. The upstroke then begins due to the momentum of the crankshaft which begins the cycle again.
Piston Engines
How does a four-stroke engine cycle operate?
A four stoke engine uses mechanically operated valves placed at the top of the cylinder to control the inlet and outlet of gas. The valves are controlled by the camshaft which is driven by belt/chains connected to the crankshaft. A full cycle consists of 2 crankshaft rotations with each stroke stage following the suck, squeeze, bang, blow principle.
Piston Engines
Sketch the P-v diagrams for a two stroke and four stroke engine. Label all the stages.
see OneNote sketches
Piston Engines
What is Hooker’s charge-mass law? Show equations
A method for estimating the charge-mass required by the engine.
Displacement = DelV = StrokeCylinder AreaNumber of cylinders
Engine Volume = Displacement/(1 - 1/r)
Charge-mass flow rate = (nV/(2RTc))(Pc - Pe/r)
check OneNote for clearer demonstration
Piston Engines
What are the main losses of efficiency for piston engines?
- Valves take time to open and close, combustion/exhaust is not instantaneous
- Heat losses to side walls, valves etc
- Finite pumping work
upper limit of internal/mechanical efficiency is approx. 85%
Piston Engines
What is the indicated work and indicated power? Give the equations
Indicated Work is the product of the indicated mean effective pressure and the swept volume (Displacement)
IW = Pei * DelV
Indicated Power is the indicated work multiplied by the number of useful strokes per cycle
IP = (PeiDelVn*k)/N
where k = N/2 for a four stroke and k = N for a 2 stroke
