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Propulsion systems
Outcomes
his unit of work addresses aspects of the following syllabus outcomes:
A student:
H1.2 Differentiates between properties of materials and justifies the selection of materials, components and processes in engineering.
H4.1 Investigates the extent of technological change in engineering.
Source: Board of Studies (1999) Stage 6 engineering studies, preliminary and HSC courses. Sydney, Board of Studies
Introduction
What is propulsion? The word comes from the Latin words "pro" meaning before or forwards and "pellere" meaning to drive. There are four principal propulsion systems in aircraft:
- propeller
- jet or turbine
- ram jet
- rocket.
This unit of work focuses only on the jet, or turbine engine.
Turbines have existed historically for many years. People in ancient times used water driven turbines as a primitive power source to grind grain and to power simple machines. Water turbines are used in modern hydro-electricity plants to generate electricity. Other types of electricity plants use coal, oil or a nuclear reactor to produce steam which is then used to drive turbines. The turbines drive generators which produce electricity. Another type of turbine, known as a windmill, uses the power of the wind to generate electricity.
Modern aeroplanes use gas turbines which create a stream or “jet” of hot air and gases to provide thrust for the aircraft. It is this “jet” of hot gas from which these engines get their common name.
Turbines are another form of internal combustion engine. They operate on a four stroke “cycle”: induction, compression, ignition, exhaust, but unlike piston engines they have no reciprocating parts. Thrust may come from the jet stream of the exhaust or from a propeller driven by the turbine mainshaft.
For more information visit:
- NASA
and - How
Stuff Works
Reciprocating engines, as used in the automobile, are limited by their maximum power and speed (limited to below 20 000 rpm). Consequently a vehicle’s performance is also restricted. Also, their power to weight ratio is poor compared to the turbine.
The turbine engine has many advantages over a reciprocating combustion engine. The foremost of these is its ability to reach higher speeds and to work efficiently at greater altitudes. Other advantages of the turbine include reduced vibration stresses due to a lack of reciprocating masses, less complex cooling systems and increased simplicity of controls.
Disadvantages of the turbine engine include cost, high fuel consumption and poor performance at low altitude and slow speeds.
Visit the web site: http://www.howstuffworks.com/turbine2.htm
- Turbojet
A typical turbojet engine has three stages: the compressor stage, the combustion stage and the turbine stage.
A series of fan-like compressor blades draw air into the engine. In this initial compressor section, the air moves through a series of rotor and stator blades. As the air continues to travel through this stage, it moves from low to high pressure.
The air is next propelled into the combustion chamber, in which fuel is mixed and then burnt. This burning has the effect of increasing the temperature and speeding up the gases.
From the combustion stage the high speed gases pass into the turbine stage. Here the gases first hit and turn turbine blades which power the compressor fans. The hot gases are then straightened and streamlined into a concentrated path before being exhausted to the atmosphere creating thrust.
A diagram of the layout of a turbojet engine can be found at the Rolls-Royce web site.
Turbojets are mostly used in air force fighter aircraft, or as the basis for turboprop engines.
- Turbofan
A significant problem with the turbojet engine is the noise created by the high velocity, high temperature exhaust gases. This led to the development of the turbofan engine.
Of the air that enters the turbofan engine, a small proportion passes into the turbine. The turbine is used to drive the engine compressor blades as well as a series of large fan blades. These fan blades accelerate the remaining air over the engine. The total thrust is made up of thrust from the fan, and thrust from the jet exhaust. The fan thrust accounts for up to 80% of the total thrust.
This second type of gas turbine engine has the advantage that it is much quieter than the turbojet engine.
After many years of development turbofans have largely replaced turbojets in commercial passenger and freight aircraft.
Today, turbofans are used in most commercial passenger aircraft and they have been manufactured to provide up to 40 000 kg of thrust, capable of pulling the world’s biggest aircraft, such as the massive 500 seater Airbus A380 (yet to produced).
The general arrangement for a turbofan engine can be found at the following web site:
http://www.howstuffworks.com/turbine4.htm
Source: Rolls Royce Education and Training.
- Turboprop
The third type of turbine engine combines the best features of the turbojet with propeller driven aircraft. The turboprop has improved abilities in its capacity to carry greater loads at slower speeds and lower altitudes.
The turboprop uses a second turbine in the turbine stage to turn a propeller. In doing so, it uses almost all available energy to turn both its compressor and the propeller.
The notable difference between the other turbines and the turboprop is that the aircraft depends on the propeller for thrust, rather than the high velocity gases being exhausted. The speed of the turbine is reduced by a gearbox to drive the propeller.
http://www.nw-ar.com/drakefield/jet_engines.html
Conclusion
In the foreseeable future, the turbofan and turboprop engines will meet the needs of a civilisation that is constantly demanding shorter travelling times over greater distances. Once these engines reach their design limit, it will open the way for "rocket" or even "ramjet" travel.
The ramjet is the simplest of all jet engines as it has no moving parts, however it is still in development. The major flaw in its design is its inability to function at low speeds. It needs to be travelling very fast through the air before being started but it does show a lot of promise for the future.
By what two factors is the efficiency of a gas turbine engine governed?
Activity 2
What are the differences between how an internal combustion piston engine works compared to a turboprop engine?
Activity 3
What are the main advantages of jet engines compared to the reciprocating engine propeller driven aircraft?
For more pictures of jet engines go to:
Additional resources
http://www.rolls-royce.com/education/schools/how_things_work/journey02/flash.html
http://www.videosift.com/video/Rolls-Royce-Blade-Failure-Test
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