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This page has some very good cut away pictures of a J-33 which show how a gas-turbine engine works, and some pictures
I ran across on the internet. You can enlarge a picture in a new window by clicking on it.
| Turbofan engine animation |
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This is a good animation that shows how a turbofan engine operates. They are used on most of todays commercial aircraft.
The fan below and to the right is off of a JT-8D Low By-pass Turbofan. This engine is used on aircraft like the
Boeing 727 and 737. The fan is attached to the fist stage compressor and acts like a propeller on an aircraft, however it
spins much faster. This provides additonal thrust, and can even be added to an afterurner. The construction of a
fan allows it to spin at super-sonic speeds without loosing much efficiency, where a propeller would become totally
useless. Most engines of this size use what is called a split spool compressor, that means it has two or more spools that
spin at different speeds to allow for maximum efficiency. This is usually achieved by seperate turbines powering the different
compressors. There are several types of fan engines, the low-bypass, high bypass, and the UDF or un-ducted fan. Unducted engines
are not popular in use yet, but with new technology will probably appear in the near future, the problem is that the shock
waves developed by the blades are not channeled out the back of the engine, but thrown into the fuselage of the aircraft,
causing lots of noise and even destruction.
| Cross section of a turbo-jet |
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| JT-8D Low By-Pass Turbofan |
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| J-33 Burner Can |
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The J-33 burner can, this is a cut-away from an engine we have on display at M.T.S.U., please note that the burner can
has expanded from being cut in half and there would normally be much more space for the compressed air to enter the can. You
can see where the circumference of the burner can would be at the fuel nozzle.
| J-33 mid section |
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| Compressor and accessories |
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| Exhaust |
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| Cut away view |
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A J-33 is uses a dual sided centrifugal compressor, if you notice it draws air in from both the left and right sides
and forces it outward, however unlike a turbocharger it uses an axial flow turbine, they do not consume nearly as much energy
from the air, thus allowing it to be used for thrust. On the left side of the engine you can see the accessories, like the
fuel and oil pumps. The J-33 is a very old design, it is heavy, bulky, and very inefficient compared to todays newer turbines.
Basically jet engines work on expansion, and density of gasses. The same amount goes in the front as comes out the back,
you can't make air, you just change it's density by heating it up. People think jet engines are really complicated devices,
but the truth is they are really much simpler than a common recipricating engine. Hopefully after reading the following you
will be able to look at the J-33 pictures above and the cross sections, and be able to figure one out for yourself.
Outside air is pulled in through the chambers in blue on the left, the centrifugal compressor with its rotational force
speeds the air up and changes it to a very high speed air source. The air is then forced outwards into the diffuser, this
converts the high speed air to a slower speed and increases the presure, thanks to Bernuli's principal when you get a pressure
rise the temperature also rises. Some turbojets have a compresser discharge temperature of up to 900 degree's F. The compressed
air is then ducted to the burner cans, the air is pushed in, and around the burner cans to keep them cool so they don't melt.
Inside the burner cans jet fuel (diesel or kerosene) is injected at the far left. An ignitor very similar to a spark plug
ignites the fuel in the early stage of ignition. After the engine comes up to self sustaining speeds, the ignition is usually
switched off, there is no need for it then. The flame takes place in the burner can where the red is, where the warm
air mixture supplied from the compressor is expanded, taking up much more space. Due to this expansion of gasses it increases
the velocity and rushes to the right, through the turbine wheel. The turbine wheel is just like a windmill and spins with
the air rushing across it, this is connected to a shaft that the compressor is also connected to, so the spinning turbine
wheel spins the compressor. The hot gasses are expelled out the back of the engine into the exhaust nozzle which on a thrust
engine is usually of a convergent design, this accelerates the gas even more and produce thrust. An afterburner can be used
to re-heat the air to expand it even more in the exhaust nozzle and create additional thrust.
Turbo-fans, turbo-props and turbo-shafts work by using a very elaborate gear reduction system to take the shaft RPM's
and convert it to shaft horsepower, turbo-fan engines usually have a large fan attached to the the compressor and it
is not geared, however certain turbo-shaft and turbo-props use a "free power turbine" to extract the power. This means another
turbine wheel is placed behind the first and another shaft is run through the center of the main shaft. This allows the second
turbine to spin at slower speeds and not create so much of a strain on the engine. This is usually routed through a gear reduction
box to convert it to a more usefull power. turbo-fans, turbo-shafts, and turbo-prop engines are usually far more efficient
than a Turbo-jet engine, sometimes about twice as efficient. Almost all rotorcraft equiped with jet engines use a free-power
turbine. The PT-6 is the most widely used on bussiness transport aircraft.
| J-33 running |
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This is a J-33 we have at M.T.S.U. that we ran last spring in turbine overhaul class. It is actually running in this
picture but you can't tell. I might post a short video of it running in the future, if anyone shows interest.
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