TheBigOne
Topic Author
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What Is The Difference Between?

Fri Oct 29, 2004 12:53 am

I hope this topic has not been posted before, because I have not been able to find it through the search.

What is the difference between a
Turbojet
Turbofan
Turbo prop
etc?

Sorry if this sounds really stupid, but I'm dying to know.
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kaddyuk
Posts: 3697
Joined: Thu Nov 08, 2001 1:04 am

RE: What Is The Difference Between?

Fri Oct 29, 2004 1:13 am

Okay, you know how an engine works...

A Jet engine, sucks air (alot of it) some of it, (about 20%) is sent through a compresser to increase the pressure and then is burnt, increasing the pressure further, then sent through a turbine to keep it spinning. Then it is exhausted out the back. about 80% of the air is not sent through this combustion process and bypasses around the outside of the core.

This is called a High Bypass Ratio Turbofan engine this type of engine is reasonably quiet compared to a turbojet it is also VERY fuel efficient.

If you send most of the air sucked in through the combustion process then it is called a Turbojet. These engines are very loud and not very fuel efficient.

A Turbo Prop is a prop that uses the same combustion process of a Turbofan to keep the prop spinning (compressor, combustion, turbine) instead of an ordinary prop which uses pistons.

Its not a very good description but should help you out till someone else gets here Big grin
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FredT
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RE: What Is The Difference Between?

Fri Oct 29, 2004 1:25 am

A turbojet engine uses a compressor to compress the air. The air is then heated in the combustion chamber by burning fuel which adds energy. Part of that energy is then extracted in the turbine and used to turn the compressor. The rest is used to propel air out the exhaust, providing thrust (greatly simplified).

Having all this air exit at high speed works, but is inefficient. It is a lot more efficient to accelerate a lot of air a little than to accelerate a little air a lot. To create the former situation, we bolt a huge fan enclosed in a cowling to the front of the compressor of a turbojet engine. This creates a turbofan engine. Most of the thrust comes from the fan air, which is also most of the air (by mass). The basic jet engine is in the turbofan referred to as the gas generator.

A turboshaft engine is what you get if you instead of attaching a fan to the turbojet engine attach a driveshaft. You can then use this driveshaft to drive whatever you wish. These are found in power applications, helicopters etc. Most of the time you have a gearbox on the shaft to convert the RPM to torque.

If you attach a propeller on the other side of that gearbox, you have a turboprop engine.
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Santhosh
Posts: 543
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RE: What Is The Difference Between?

Fri Oct 29, 2004 2:20 am

In a Turbo jet engine. First you have a and inlet diffuser which increases the inlet pressure of the air and then this is compressed by a compressor and this high pressure air then reaches the combustion chamber where its mixed with fuel and then ignited. The ignition causes a very high pressure inside the combustion chamber. So that air rushes out thought a set of turbine making them turn faster and then into the exhaust nozzle. Thus producing thrust

In a Turbofan engine first you have a fan which sucks in air and direct the air into two, one part of the air is bypassed over the casing and the other part is fed into the compressor which compress the air and the high pressure air is mixed with fuel and ignited in the combustion chamber. This combustion product with very high velocity and pressure passes via the Low pressure and High-pressure turbine blades and then passes via the Exhaust Nozzle. The trick here is that Low Pressure turbine is in turn connected to the engine fan thought a drive shaft and thus the Low-pressure turbine is actually supplying power to the fan to rotate. The bypassed air flowing over the casing flows over the exhaust nozzle. The ratio of the cold bypassed air to the hot gases coming from the exhaust nozzle is called by pass ratio.

In Turbo prop engine First you have the propeller behind it you have an inlet diffuser which increases the pressure of the inlet air and then its fed into the compressor where its compressed and then mixed with fuel and ignited thus producing high pressure in the combustion chamber. This high-pressure air passes via the turbine and the exhaust nozzle. Here also the turbine is connected via a shaft and a reduction gear to the propeller. So the turbine is what drives the propeller blades. Here 10 % to 20% thrust is produced by the turbine while 80& to 90% of power is produced by the Propeller.

There are also some other types of engine like Ram Jet and Pulse Jet but there working is quite different and is used for military purposes. Hope this helps to clear you doubt.

George
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Starlionblue
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RE: What Is The Difference Between?

Fri Oct 29, 2004 2:28 am

And then there is the turboramjet, like on the SR-71. This is a turbofan with reheat (an afterburner). At very high speeds, the afterburner handles most of the thrust, effectively acting like a ramjet.
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SlamClick
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RE: What Is The Difference Between?

Fri Oct 29, 2004 2:57 am

TheBigOne there is a lot of info here, and some of it is not exactly right, some of it is pretty confusing. I'd suggest that you need to see pictures. The differences are really pretty easy to understand. I could show you in thirty seconds on a board, but the words just don't convey it.

There are a couple of "how things work" websites. I'd suggest you check them out, or a basic sort of book. Then at some future time come back to this forum for any additional questions.

As it is, I don't think anyone can unscramble the right and wrong above for you.
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Staffan
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RE: What Is The Difference Between?

Fri Oct 29, 2004 3:24 am

This page pretty much explains it with the help of some pictures.

http://www.aerospaceweb.org/question/propulsion/q0033.shtml

Staffan
 
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Starlionblue
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RE: What Is The Difference Between?

Fri Oct 29, 2004 7:35 am

"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
TheBigOne
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RE: What Is The Difference Between?

Fri Oct 29, 2004 11:14 am

Wow....what can I say but thanks!! I guess I'm half way to my Phd in propulsion technology  Wink/being sarcastic

Thanks all!
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