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How Does A Turbo Jet/prop Work?  
User currently offlineFL370 From United States of America, joined Apr 2006, 252 posts, RR: 0
Posted (7 years 8 months 2 weeks 1 day 5 hours ago) and read 25296 times:

i was reading some papers on turbo jets/props, and my question is...

1) how does the turbo work in the engine?

2) does the turbo kick in after a certain RPM/speed like in a car?

3) and everything in between


fl370

30 replies: All unread, showing first 25:
 
User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 1, posted (7 years 8 months 2 weeks 1 day 5 hours ago) and read 25322 times:

I think you are confusing terms here...

A turboprop is essentially a jet engine with a prop in the front and a gearbox in between the two.

You can't have a turbocharger in a turbofan/jet/prop  


However, in a piston engined GA plane, say, a Columbia 400, C182T, etc., the turbocharger works just like in a car, and usually they have intercoolers as well.

Some are just "turbo-normalized", meaning the engine will produce the same amount of power at sea level as a normally aspirated version of the same engine up to a given altitude. I.E. the engine will run like it is always at sea level. Others are actually boosted, meaning the pressure from the turbocharger is greater than normal atmospheric pressure, giving additional horsepower.

There are also superchargers, but they are rarely seen nowadays, they were really popular for WWII aircraft though.

  

[Edited 2007-01-21 05:41:35]

[Edited 2007-01-21 05:46:12]

User currently offlineZBBYLW From Canada, joined Nov 2006, 1986 posts, RR: 6
Reply 2, posted (7 years 8 months 2 weeks 1 day 4 hours ago) and read 25293 times:

The turbine engine is not like a car what so ever. A car's turbo charger uses exhaust gases in order to spool up a turbine which increases the incoming air to increase the compression inside a regular internal combustion engine.

First I will start off with the turbojet engine. The air is taken from the big opening in the front of the engine, it is then compressed in what is called the compressor. Right after this we get to the combustion chamber where the Jet fuel is now added. This air/fuel now expands as in an internal combustion engine but is redirected backwards and passes through a turbine. This turbine is connected back to the compressor which sends air to the combustion chamber where the fuel is added and ignited, which goes through the turbine spools it back up and have continuous power. If you are interested in military a/c and the afterburner it is basically another combustion chamber which is positioned aft of the turbine but before the engine nozzle (where the exhaust gets directed outwards). Basically just makes another power source that pushes even more air out of the nozzle. Thus making the aircraft have more power.

So basically this is what happens. The turbo jet sucks in air and compresses it. In the combustion chamber the fuel is added and then gets shot through the turbine which makes the compressor spin. Exhaust flows out the back and forward you go.

Ok now for turboprops.

Basically what we have now is like above another jet engine, however it is now attached to a prop. The turbine at the back of the engine is still turned by the hot exhaust which turns a shaft that drives the prop. (Direct drive). There is also another system which is used on other a/c that is similar to a automatic transmission found in a car. Someone else should be-able to know about this though. So again like the turbojet the turboprop has a compressor a combustion chamber and a turbine. The air that runs threw and spools up the turbine also turns the compressor, and thus you have your continuous cycle again. The difference is that the shaft is connected to a transmission which is then connected to a propeller. This propeller is what actually creates most if not all of the trust.

Anyhow thats it for me, I am quite tired so if any of that makes sense great! Cheers Chris



Keep the shinny side up!
User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 3, posted (7 years 8 months 2 weeks 1 day 4 hours ago) and read 25277 times:

Quoting FL370 (Thread starter):
i was reading some papers on turbo jets/props, and my question is...

Wait a sec, didn't you just ask this already a couple of days ago?


User currently offlineAirWillie6475 From United States of America, joined Jan 2005, 2448 posts, RR: 1
Reply 4, posted (7 years 8 months 2 weeks 1 day 3 hours ago) and read 25264 times:

Quoting FL370 (Thread starter):
How Does A Turbo Jet/prop Work?

Suck Squeeze Bang Blow


User currently offlineTheJoe From Australia, joined Oct 2006, 61 posts, RR: 4
Reply 5, posted (7 years 8 months 2 weeks 1 day 2 hours ago) and read 25262 times:

Quoting FL370 (Thread starter):
i was reading some papers on turbo jets/props, and my question is...

1) how does the turbo work in the engine?

2) does the turbo kick in after a certain RPM/speed like in a car?

3) and everything in between

Well, I think you may be a little confused when it comes to the terminology here... A "turbojet" or "turbofan" and all other types of jet engines do not specifically contain a turbo, they essentially are a giant turbo. The name "turbo" on a car comes from "turbo-supercharger" which is just a turbine driven supercharger instead of a direct mechanical drive from the engine. The turbine extracts energy from the exhaust gas of the car which would otherwise just be lost to the atmosphere. This is fundamentally how a "gas turbine" or aeroplane engine works. Well, the names "turbofan", "turboprop" and "turbojet" really just mean a turbine driven fan, a turbine driven propeller and a jet turbine. The reason for the different design is based on the propulsive efficiency of the engine which depends on the speed at which it is designed to operate. "Turboprops" have a higher propulsive efficiency at low speeds. "Turbofans" are better at transonic speeds and "turbojets" are more efficient at supersonic speeds. "Propulsive Efficiency" is just a term which tells us how good an engine is at turning the power is produces in to useful thrust for the aeroplane, normally expressed as a percentage. For example, a "turbofan" may have a "propulsive efficiency" of 80% at a certain forward speed. As this speed gets higher or lower, the efficiency may decrease. Anyway, this all a little off topic. Just a brief insight in to why the engines are named and designed the way they are.

You obviously know how a turbo works in a car. You make this clear in point two. Well, the turbo "kicks in" in a car because it has enough exhaust gas flow for the compressor to start compressing a larger mass of air...

A typical car compressor:
Big version: Width: 782 Height: 599 File size: 59kb
Turbo


If you have a look at this picture of a car turbo, you can see that the blue section is the compressor stage and the red section is the turbine, driven by high velocity exhaust gasses from the engine exhaust.

We'll now compare this to how a "Gas turbine" works. The following diagrams are from a GE J79 "turbojet".

Here is a picture of a typical compressor. It serves the same purpose as the compressor in a car turbo.
Big version: Width: 800 Height: 428 File size: 78kb
Compressor


Note the several stages of compression. One row will be a rotating or "rotor" assembly, followed by a stationary stage or "stator". Each rotor-stator pair, normally referred to as a "stage" compresses the air a little. IIRC, with enough stages, compression ratios of 30-1 are attainable. Simply speaking, for ever pound per square inch (psi) at the intake you will have 30psi at the compressor discharge.

You may ask "why do you want to compress all of that air?". The answer is simple. We burn it! The more air you compress, the more fuel you can burn. The more fuel you burn, the more power you produce. This is where the next section comes in.

The combustor:
Big version: Width: 667 Height: 559 File size: 65kb
Combustor


This is the stage in the engine where the compressed air and fuel is mixed, burnt and propelled out through the turbine. In a car turbo, this stage is achieved by the engine itself and is normally just a waste product. In a gas turbine, the combustor is specifically designed for this task. Note that the combustor does not produce more pressure in the combustion chamber, but rather increases the velocity of the exhaust gasses that are sent through the next stage of the engine. This is where the turbine does it's job.

The turbine:
Big version: Width: 433 Height: 599 File size: 55kb
Turbine


The job of the turbine is to extract as much of the velocity from the air that has passed through the combustion chamber as is required by the engine. Each turbine stage is made up of a nozzle-turbine pair. The turbine spins and extracts the energy from the velocity of the air and the nozzle guide vanes guide the air on to the next stage of turbine. The more energy that is extracted by the turbine stages, the less is left over in the exhaust when it comes out the back of the engine. Now this is where the different types of engines do different things with the exhaust gasses.

A "turbojet" will extract a little bit of energy from the exhaust to drive the compressor. The rest goes out the back as thrust. The "turbofan" extracts more energy to drive the great big fan that sits on the front of the engine. It is this fan that produces most of the thrust of the engine. If we compare two engines, a "turbofan" and a "turbojet" that are producing the same amount of thrust, we will find that the "turbojet" moves a lot less air out the back of the engine at a much higher velocity than the "turbofan". The "turbofan" moves a lot more air but does not accelerate it as quickly. This is where the propulsive efficiency and the operating envelope of an engine determine what type of engine is required.

Finally, the "turboprop" converts as much of the energy of the exhaust gasses as possible into torque for the propeller through some sort of gearbox arrangement. Only a very, very small amount of the thrust produced by the engine is from the exhaust.

All the processes which I have described are happening at the same time inside the engine. The air flows through the engine in the order which I have written in.

Finally, we'll briefly talk about the construction of the engine. Inside the engine have a number of shafts which we call "spools". In modern Aero engines we have up to three spools.

The following is a simplified airflow schematic of a turbofan engine:
Big version: Width: 529 Height: 341 File size: 26kb
Gas Turbine Schematic


A spool is one of the major rotating assemblies of the engine. In the picture provided, the green "spool" is the low pressure spool, containing the low pressure compressor (LPC) and low pressure turbine (LPT). It is driven by the "low pressure turbine". Note this spool also drives the fan. On the 737 engine, which is set up in a similar manner to this one, we call this spool "N1". This is indicated to the pilot in terms of a percentage of "N1", 100% being max power. The purple spool which has the high pressure compressor (HPC) and high pressure turbine (HPT) is the high pressure spool, driven by the high pressure turbine. You guessed it, it's called "N2". This is also indicated to the pilot in terms of percentage of "N2", 100% being max power. These two rotating speeds give the pilot an indication of how much power the engine is producing. Note that a number of turbine stage can drive a number of compressor stages. Three turbine stages may drive seven compressor stages. In the car turbo I have used as an example, one turbine stage drives one compressor stage. In the simplified turbofan diagram above, the last four stages of turbine on the right hand side are the LPT and they drive the fan on the front of the engine (left hand side) and the three stages behind it, which is LPC. The two purple stages on the right hand side of the engine (just before the LPT) is the HPT. This drives the last seven stages of compressor (the HPC). Note that the N1 spool shaft runs down the engine on the inside of the N2 shaft which is also spinning. This is where things can start to get a little complicated!

Anyway, I hope this has given you a little insight in to how a "turbojet/prop/fan" works FL370. If you have any questions, just post them here. I'll keep a look out!

To all those die hard tech/ops people out there, this is just a basic overview of how an engine works. Nothing too in depth. Please feel free to add anything I may have forgotten or correct me if have made any mistakes. Thanks!  Smile


User currently offlineZBBYLW From Canada, joined Nov 2006, 1986 posts, RR: 6
Reply 6, posted (7 years 8 months 2 weeks 16 hours ago) and read 25174 times:

Quoting AirWillie6475 (Reply 4):
Suck Squeeze Bang Blow

Not at all, note a turbojet, turboprop and turbo fan engine are not internal combustion. Suck squeeze bang blow only works with conventional 4 cycle internal combustion engines. Go read thejoes or my response, I think the joes is better though with diagrams and what not.

Cheers Chris



Keep the shinny side up!
User currently offlineHaveBlue From United States of America, joined Jan 2004, 2112 posts, RR: 1
Reply 7, posted (7 years 8 months 2 weeks 16 hours ago) and read 25164 times:

Nicely done TheJoe.  Smile


Here Here for Severe Clear!
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 8, posted (7 years 8 months 2 weeks 14 hours ago) and read 25150 times:

Quoting AirWillie6475 (Reply 4):
Suck Squeeze Bang Blow



Quoting ZBBYLW (Reply 6):
Not at all

Actually, as inexact an explanation as that is, it is equally inadequate for a jet as for a piston engine. Like a recip, a jet engine also has:
intake
compression
(ignition event)
power
exhaust

The least descriptive of these words is "bang" to represent the burning of the fuel. It sounds like an explosion is represented but in a properly running engine the fuel charge does not explode, it burns evenly in a carefully engineered pattern. Of course in the burners of a jet engine it is continuous and in a reciprocating engine it is intermittent, even though it happens neary 200 times per second in any given cylinder in some racing engines. It may be common to refer to these as "strokes" in piston engines but that is somewhat incorrect as intake and exhaust valves do not open and close exactly at the beginning or end of these strokes. It is more proper to refer to them as "cycles" as they are only approximately matched to strokes. Jets have the same cycles.

Quoting ZBBYLW (Reply 6):
turboprop and turbo fan engine are not internal combustion

Well, yes they are. An example of an EXternal combustion engine would be a steam engine where the fuel is burned in the firebox, heating water to make steam, the steam is stored under pressure and used in cylinders at any distance the designers choose, to develop the torque, then exhausted overboard or returned to desuperheaters and condensers. An extreme example would be a "fireless" locomotive such as were used in sugar cane fields. The boiler was back at the motorhouse and the locmotives carried only a steam tank which was serviced on each round trip.

In a jet engine it is all done in the same unit and in the same sequence as in a piston engine.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineZBBYLW From Canada, joined Nov 2006, 1986 posts, RR: 6
Reply 9, posted (7 years 8 months 2 weeks 12 hours ago) and read 25123 times:

Quoting SlamClick (Reply 8):

Right then I stand corrected on all of that...

However just looking for some clarification, while the jet engine is not an external combustion engine, I have always been told that an internal combustion engine is more or less a regular piston engine where everything happens inside these cylinders. Could you go into a bit more depth on where the jet engine lies, weather it meets halfway or is in fact a internal combustion engine.



Keep the shinny side up!
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 10, posted (7 years 8 months 2 weeks 12 hours ago) and read 25121 times:

Quoting ZBBYLW (Reply 9):
Could you go into a bit more depth on where the jet engine lies, weather it meets halfway or is in fact a internal combustion engine.

I think I'd rather leave that to someone with the engineering education who can speak with more actual authority than I can on it. That is what I've been taught in professional level instruction (military and airlines for example) and I've accepted it because it made sense as described. If someone else can add or even dispute what I said, I'd welcome that input.

Another example has occurred to me, and that is the Wankel rotary engine. Again, an internal combustion engine but without pistons. The faces of the trochoid rotors act in a manner similar to piston domes or turbine blade surfaces. The epicycloid chambers (don't you love Mazda brochures?) act in a manner similar to cylinder walls, cylinder heads and the surfaces of the closed valves in piston engines and as the confines of the compressor section and the turbine section of jet engines. All achieve the same intake-compression-ignition-power-exhaust cycles. The rest is just differences in shape.

Some good discussion might be found of this online. Sorry, but I have company coming and cannot look it up myself.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineGoldenshield From United States of America, joined Jan 2001, 6049 posts, RR: 14
Reply 11, posted (7 years 8 months 2 weeks 12 hours ago) and read 25109 times:

Here's an interesting engine being worked on right now.

Quasiturbine

Try debating this one.  Smile



Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.
User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 12, posted (7 years 8 months 2 weeks 9 hours ago) and read 25074 times:

Quoting ZBBYLW (Reply 9):
I have always been told that an internal combustion engine is more or less a regular piston engine where everything happens inside these cylinders.

Well, in jets everything happens inside as well, I personally wouldn't count them as external engines at all, not even with afterburners.

Quoting Goldenshield (Reply 11):
Try debating this one.

That would be internal combustion, like a wankel  

A stirling-cycle engine would qualify as external combustion in my book.

[Edited 2007-01-22 01:34:27]

User currently offlineZBBYLW From Canada, joined Nov 2006, 1986 posts, RR: 6
Reply 13, posted (7 years 8 months 2 weeks 9 hours ago) and read 25069 times:

Quoting FLY2HMO (Reply 12):
Well, in jets everything happens inside as well, I personally wouldn't count them as external engines at all, not even with afterburners.

I can understand how all the steps are the same.... how there is the intake, the compression the power and exhaust, I was always under the impression that it had to be confined in the space. It is hard to wrap your head around a new concept of something you thought you knew, o well. I think I got in now though... That new engine you showed there looks like a glorified Wankel rotary to me.



Keep the shinny side up!
User currently offlineDavid L From United Kingdom, joined May 1999, 9524 posts, RR: 42
Reply 14, posted (7 years 8 months 2 weeks 8 hours ago) and read 25049 times:

Quoting ZBBYLW (Reply 13):
I can understand how all the steps are the same.... how there is the intake, the compression the power and exhaust, I was always under the impression that it had to be confined in the space.

  • In a piston engine, the actions are performed sequentially in time but in the same space. I.e. there's a suck, then a squeeze, then a bang and then a blow, all in the same place.

  • In a turbine, the actions take place sequentially in space but at the same time. I.e. one part of the engine is constantly sucking, one part is constantly squeezing, one part is constantly producing the "bang" and one part is constantly providing the "blow".
There are some excellent details above.

Quoting FLY2HMO (Reply 3):
Wait a sec, didn't you just ask this already a couple of days ago?

Yes, here:
How Does A Turbo Work In A Turbo Jet? (by FL370 Jan 16 2007 in Tech Ops)

... but judging by the thread starter's comments in the first thread, I'm guessing he didn't know it had been moved to Tech/Ops and thought it had been deleted.  

Edit: After all that, I forgot to mention that, as long as all that takes place inside the engine, it's an internal combustion engine.

[Edited 2007-01-22 02:24:09]

User currently offlineJetstar From United States of America, joined May 2003, 1648 posts, RR: 10
Reply 15, posted (7 years 8 months 2 weeks 8 hours ago) and read 25049 times:
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Excellent explanation TheJoe.

I would like to add one thing and that is about the tailpipe and how important it is in a jet engine.

As in your picture of the simplified airflow schematic of a turbofan engine, it shows the tailpipe or nozzle end smaller at the discharge end that the turbine end. Remove the tailpipe and all you have is a gas turbine engine like an APU, the exhaust gas airflow just dissipates without creating much thrust from the exhaust gases. It is the reduced size at the discharge end that increases the velocity of the exhaust gas and creates the thrust that makes the engine into a jet engine.


User currently offlineTheJoe From Australia, joined Oct 2006, 61 posts, RR: 4
Reply 16, posted (7 years 8 months 2 weeks 4 hours ago) and read 25003 times:

Quoting Jetstar (Reply 15):
would like to add one thing and that is about the tailpipe and how important it is in a jet engine.

You raise a very good point Jetstar. A turbojet without a propelling (convergent) nozzle wouldn't be much of a turbojet at all. Thanks for adding that.

Quoting ZBBYLW (Reply 9):
Could you go into a bit more depth on where the jet engine lies, weather it meets halfway or is in fact a internal combustion engine.

Yes, the jet engine is an internal combustion engine, it just follows a different working cycle. Speaking totally from a general point of view, piston engines follow the Otto cycle or "constant volume cycle" while gas turbines follow the Brayton or "constant pressure" cycle. These describe the working cycles of the engine. I don't have enough time to write much about this right now, as an in depth explanation of these cycles is a little complicated and could take a long time! If you're curious, look up "Otto cycle" or "Brayton cycle" on the internet. If anyone else would like to describe the process, feel free!


User currently offlineMrocktor From United States of America, joined Jan 2005, 1668 posts, RR: 50
Reply 17, posted (7 years 8 months 1 week 6 days 16 hours ago) and read 24954 times:

Quoting TheJoe (Reply 16):
Yes, the jet engine is an internal combustion engine, it just follows a different working cycle.

The way I understand it, an internal combustion engine is one where combustion occurs in a chamber not connected to the external environment. This is a case in piston engines (valves are shut when combustion occurs) and false in jet engines (the combustion chamber is open to the environment all the time - in front and in the back).

Incidentally, it also is false in steam engines, where the combustion chamber is also connected to the external environment (by the chimney). The term was created to differentiate between the "new" piston engine type and steam, after all.

[Edited 2007-01-22 18:11:10]

User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 18, posted (7 years 8 months 1 week 6 days 16 hours ago) and read 24947 times:

How about sticking with the terms we learned while studying engineering?

Turbine engines

and

Reciprocating engines

and be done with all the hair splitting?



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 19, posted (7 years 8 months 1 week 6 days 13 hours ago) and read 24906 times:

Quoting Mrocktor (Reply 17):
This is a case in piston engines (valves are shut when combustion occurs)

Um huh, so whats the case when the valves are open?  scratchchin  Big grin


I can't remember any kind of engine that truly is external combustion other than one operating with the stirling cycle, maybe somebody can refresh my memory.


User currently offlineAnalog From United States of America, joined Jul 2006, 1900 posts, RR: 1
Reply 20, posted (7 years 8 months 1 week 6 days 11 hours ago) and read 24880 times:

Quoting FLY2HMO (Reply 19):

I can't remember any kind of engine that truly is external combustion other than one operating with the stirling cycle, maybe somebody can refresh my memory.

Aerospike?

http://en.wikipedia.org/wiki/Aerospike_engine

Maybe also ramjet, as the combustion is in a tube that's open to the outside world, unlike a turbojet, where the combustion is followed by a turbine.


User currently offlineTimz From United States of America, joined Sep 1999, 6839 posts, RR: 7
Reply 21, posted (7 years 8 months 1 week 6 days 11 hours ago) and read 24872 times:

Quoting FLY2HMO (Reply 19):
I can't remember any kind of engine that truly is external combustion other than one operating with the stirling cycle

Steam locomotive?


User currently offlineAnalog From United States of America, joined Jul 2006, 1900 posts, RR: 1
Reply 22, posted (7 years 8 months 1 week 6 days 11 hours ago) and read 24871 times:

Quoting FLY2HMO (Reply 19):

I can't remember any kind of engine that truly is external combustion other than one operating with the stirling cycle, maybe somebody can refresh my memory.

Steam turbines when not nuclear powered (fission is not combustion).


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 23, posted (7 years 8 months 1 week 6 days 6 hours ago) and read 24856 times:

Quasi-Turbines could be quite useful as an automotive engine. It would produce more HP with the same size, and as a result better efficiency

User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 24, posted (7 years 8 months 1 week 6 days 5 hours ago) and read 24848 times:

Quoting TheJoe (Reply 5):

Very nicely put.

Quoting Blackbird (Reply 23):
Quasi-Turbines

Interesting...it looks like a 4-sided rotor derivative of the Wankel engine.

I don't get the efficiency part though. If it is in fact a derivative of the Wankel, then its weight and power are a plus, but its fuel efficiency is a negative.


25 Mrocktor : No combustion is going on...
26 Post contains links TheJoe : According to engineering text, a gas turbine is referred to as an internal combustion engine. http://en.wikipedia.org/wiki/Internal_combustion Read t
27 Post contains images Mrocktor : Interesting. I have not seen that usage and find it conceptually deficient. But I'm done nitpicking
28 Starlionblue : As Timz says, steam locomotives and so forth have external combustion. The fuel is burned, heating water in a boiler. This is external to the engine.
29 Post contains images JetMech : G'day Techies , I have been doing a bit of reading about the internal / external combustion issue, and I think I now understand the fundamental differ
30 Starlionblue : The definition is like this "An external combustion engine is a heat engine which burns fuel to heat a separate fluid (usually water) which then, in
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