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Turbo prop, fan and jet

 
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flightgearpilot
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PostPosted: Sun Sep 21, 2008 8:49 pm    Post subject: Turbo prop, fan and jet Reply with quote

A basic question. What is the difference between turboprop, turbofan and turbojet engines? Other than the fact, of course, that turboprop drives a propeller, while the other two are used in jet engines. Is there any difference in the way the engine itself works?

I tried looking at Wikipedia and other sources but couldn't really understand the difference, if there is any.

And is the term "turbo" just used to indicate an afterburner? If so, is it the same meaning when used for car engines? There are some diesel cars with an option of a "turbo" engine. From what I understood based on the salesman's speak is that the turbo engine has some kind of an afterburner near the exhaust that makes better utilization of fuel when the engine rpm is very high. But he also claims that the turbo engine makes the car go faster. Why is that so?

Somewhere else I heard that the term "turbo" is used to indicate turbine driven shafts rather than piston driven ones. But I can't imagine a car engine being turbine driven.

So, I'm really confused. Any comments welcome.
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sammyk
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PostPosted: Sun Sep 21, 2008 9:42 pm    Post subject: Reply with quote

I'm sure someone will give you a more detailed explanation but basically it's this:

A turbojet is a jet engine but has very little bypass, if any. Basically all the thrust goes through the core. An example is very early jets like Comets, 707s, etc.

A turbofan has a fan at the front of the engine which allows air to go around the engine, basically bypassing it, hence the bypass name. If you look on modern engines you can see right through the outer area of the fan.

A turboprop is a turbine powered propeller as opposed to piston driven.

In a car, a turbo (or turbocharger to be precise) is in fact a turbine. It is a small turbine that is powered by exhaust gases that turns a compressor which provides more air to the engine. Similar to a supercharger except a supercharger typically gets its power from the engine itself and not the exhaust. The more air/fuel you add to an engine the more power it has.

Now the salesman talking about it being like an "afterburner" is wrong/misleading. In an aircraft an afterburner injects fuel into the stream in the exhaust section to give it a boost in thrust.
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flightgearpilot
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PostPosted: Sun Sep 21, 2008 10:38 pm    Post subject: Reply with quote

Ah! that was great.. thanks! Very Happy

I can imagine the bypass, but maybe I should look back at the Wikipedia pages once again to understand them fully.

So a turbojet doesn't have an intake fan? Or is it just that the core is not bypassed? What is the advantage with the core being bypassed? Better efficiency and greater lifetime for the core, I presume.
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sammyk
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PostPosted: Sun Sep 21, 2008 11:21 pm    Post subject: Reply with quote

flightgearpilot wrote:
Ah! that was great.. thanks! Very Happy

I can imagine the bypass, but maybe I should look back at the Wikipedia pages once again to understand them fully.

So a turbojet doesn't have an intake fan? Or is it just that the core is not bypassed? What is the advantage with the core being bypassed? Better efficiency and greater lifetime for the core, I presume.


It has a fan but it does not send air around the engine, just through the turbine itself. From what I understand. Bypass allows greater efficiency, lower noise, and more power. Sort of like a giant ducted propeller.

http://en.wikipedia.org/wiki/Bypass_ratio
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HAWK21M
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PostPosted: Mon Sep 22, 2008 5:32 am    Post subject: Re: Turbo prop, fan and jet Reply with quote

flightgearpilot wrote:
A basic question. What is the difference between turboprop, turbofan and turbojet engines? Other than the fact, of course, that turboprop drives a propeller, while the other two are used in jet engines. Is there any difference in the way the engine itself works?


As sameer explained.....Quite accurately....I'll add just this.....Currently the Turbofans have a bypass ratio of 5:1 ie 5 times thrust produced is by the fan bypass compared to 1time core engine thrust eg CFM56.
The earlier Turbofan has a bypass of 1:1...almost...eg JT8D.

The Turbojet had no bypass at all.....eg JT3D.

The bypass helps better performance of the Engine,as the aft turbine is concentrically shafted to the Fan.which is driven by the thrust output of the core.These types are much more silent in operation.

The Turboprop is a type that replaces the Fan by an unducted propeller.

regds
MEL
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con spirito
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PostPosted: Tue Sep 23, 2008 8:56 am    Post subject: Reply with quote

Check this out http://aerospaceweb.org/question/propulsion/q0033.shtml
Very informative site.
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Nimish
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PostPosted: Sun Sep 28, 2008 11:51 am    Post subject: Reply with quote

A colleague had a question for me (preparing for his son's school project I guess):

Quote:
I need some info (if possible to obtain) regarding the fuel efficieny of aircrafts (40 - 70 seaters), both turboprop as well as turbofan. Assume twin engines. If you can even give rough numbers or point me in the right direction, that would help.


Any ideas/help anyone? I'm clueless in this area Sad
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texdravid
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PostPosted: Sun Sep 28, 2008 6:27 pm    Post subject: Reply with quote

Nimish,

Here is a detailed explanation from MIT:

http://web.mit.edu/aeroastro/people/waitz/publications/Babikian.pdf
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Nimish
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PostPosted: Sun Sep 28, 2008 7:34 pm    Post subject: Reply with quote

texdravid wrote:
Here is a detailed explanation from MIT:

http://web.mit.edu/aeroastro/people/waitz/publications/Babikian.pdf


Wow Shocked Shocked ! Spot on article, I got the information I needed. Thanks!
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