Can someone explain jet engines to me?

[]V[]essenjah · Post by **[]V[]essenjah** » Tue Mar 23, 2004 4:45 pm

Hey everyone,

I was wondering if someone here could explain a little bit about how jet engines work? Explain how the turbines, turbofan all work together in a turbojet and what they are for? I know a turbofan is for cooling and somehow works with the turbine but what is the turbine for? Would the turbine be visible if you looked inside of the exhaust? I just am interested because as everyone knows, I like to build 3D models of various ships/jets. So far, I have come up with some interesting designes but I would like to have a better understanding of how it all works.

De Rigueur · Post by **De Rigueur** » Tue Mar 23, 2004 4:55 pm

A turbofan is a type of jet engine having a fan in the front that is larger than the engine itself -- allowing air to bypass the engine and produce extra thrust. The turbine is a rotor which compresses air before it enters the combustion chamber. The expanding gas is also used to cause the rotor to turn. I don't think you can see much looking in the exhaust nozzle. The exhaust itself sometimes has a repeating diamond pattern <><><><>

Tetrad · Post by **Tetrad** » Tue Mar 23, 2004 4:57 pm

http://travel.howstuffworks.com/turbine.htm

Just to get you started.

Flatlander · Post by **Flatlander** » Tue Mar 23, 2004 4:59 pm

http://travel.howstuffworks.com/turbine.htm
(edit - doh!)

[]V[]essenjah · Post by **[]V[]essenjah** » Tue Mar 23, 2004 5:05 pm

Ugh, so on a Pyro's they would kind of be pointless unless your flying around on Earth since you won't find air on most planets. The fan would be good as an alternative form of cooling though.

AceCombat · Post by **AceCombat** » Tue Mar 23, 2004 5:41 pm

doesnt the Pyro use a Mini-Fusion Engine, doesnt need air to work right?

roid · Post by **roid** » Tue Mar 23, 2004 8:04 pm

mob-messenger wrote:Ugh, so on a Pyro's they would kind of be pointless unless your flying around on Earth since you won't find air on most planets. The fan would be good as an alternative form of cooling though.

i might be reading this wrong, coz you already said there's no air in space. but your last sentance...
the fan wouldn't even do anything for cooling if there's no air to blow, you know this right?

but if you're talking about running the engine in an atmosphere. then yeah you are on the right track.
when designing an engine, you want the exhaust of the jet engine to be as cool as possible. this gives it the greatest possible volume of exhaust air, because the cooler air mixes with the hot exhaust air, (steals some of the heat) and expands. lowering the average heat of the exhaust, and boosting the total mass of the exhaust (hot air expands, but only so much, you can keep adding heat and it won't expand much more, so it's best to give that heat to some other cool air to let it expand). i'm not sure how much extra thrust comes from this principle in a turbofan engine (i know in pulsejet engines it adds a lot, viva augmenter!)

in a turbojet engine, simplified: as the fuel burns and is exhausted out the back, it could just shoot out the back as a really fast and HOT stream of gas, and this would provide the thrust.
but what they do in a turbojet is add a turbine into the exhaust gas flow, and it gets spun by the exhaust gases as they shoot past (yes it slows the exhaust down somewhat, lowering it's thrust value). this turbine in turn is connected to a huge FAN, like a propeller, that sucks air in and shoots it out the back (haha look at me, i'm explaining propellers).

the amount of thrust that this propeller can add to the engine is MUCH more than the amount of thrust lost from the turbine which powers it (it slows down the exhaust gas remember?).

it's best to have some kindof diagram, pictures say 1000words and all that. i'm pretty sure the howstuffworks link has good diagrams (even rotatable 3D models). still, feel free to ask more questions here after you've read it. i'm keen

Dedman · Post by **Dedman** » Wed Mar 24, 2004 7:57 am

What De Rigueur said.

I would just like to add a few things. The term we use is: Suck, Squeeze, Bang, Blow. The air gets sucked into the front of the engine, it gets squeezed by the compressor (usually the first 5-15 stages depending on engine type), then the air is mixed with the fuel and burned in the combustion chamber then it is blown out the back end through the turbines (another 3-5 stages).

The compressor has two main sections the low pressure compressor (LPC) and the high pressure compressor (HPC). Likewise, the turbine has two main sections, the high pressure turbine (HPT) and the low pressure turbine (LPT). The air travels through them in this order LPC, HPC, Combustion chamber, HPT, LPT. ). The LPC, HPC, Combustion chamber, HPT, LPT path is call the hot stream or gas path.

Most Pratt & Whitney commercial engines actually have two shafts. It is a shaft within a shaft. The LPT drives the LPC and the Fan and the HPT drives the HPC. The turbines main purpose is to drive the compressor and fan. Rolls Royce commercial engines actually have 3 shafts; a shaft -within a shaft- within a shaft. That allows them to have an intermediate pressure compressor (IPC) and an intermediate pressure turbine (IPT) this gives greater efficiency with a shorter engine.

The fan is what you see at the front of the engine on a turbofan. The fan isnâ??t used for cooling so much as it is for thrust. It actually provides up to 80% of the thrust of the engine depending on the bypass ratio. Turbo jets on the other hand donâ??t have a front fan. They get their thrust from the core, compressors and turbines.

If you look into the exhaust nozzle of most commercial engines you can see the last turbine stage.

Hope this helps.

Admiral Thrawn · Post by **Admiral Thrawn** » Wed Mar 24, 2004 11:40 am

hehe, man! You guys have an uncanny ability to "nerdatize" anything! hehe

Mobius · Post by **Mobius** » Wed Mar 24, 2004 12:41 pm

The Pyro engine is more of a hybrid RBCC engine. Rocket-Based Combined Cycle engines will operate in multiple modes according to the atmosphere (or lack thereof) in which it is flying.

At low speeds the inlets are open and combine atmospheric oxygen to burn fuel in an almost "normal" jet mode, but without turbofans and with the equivalent of an afterburner (but a more efficient variety of it).

At Mach 1 to Mach 5 type speeds it switches to RamJet mode where the engine inlets adjust shape to increase the air pressure, the "jet" function ceases and rammed air, combinedc with a different fuel exit location combine to provide thrust.

At Mach 5 to Mach 18 the device switches to Scramjet mode and once again alters the shape of the inlets and moves the fuel exit point once again.

Beyond Mach 18 the inlet is closed and onboard oxidizser begins being pumped along with fuel, and this is Rocket Mode.

That's about as close as you're going to get to any approaching reality.

In operation an RBCC engine will not change in appearance to a casual observer, however, the sound the engine produces, the appearance of the exhaust will alter dramatically.

Google for Rocket Based Combined Cycle.

Richard Cranium · Post by **Richard Cranium** » Wed Mar 24, 2004 3:55 pm

http://www.gas-turbines.com/images/squi ... ibs)lr.mpg

Crazy

Richard Cranium · Post by **Richard Cranium** » Wed Mar 24, 2004 3:56 pm

If you want to build one your self... This might give you some ideas.

http://www.asciimation.co.nz/turbine/pa ... l#Contents

AceCombat · Post by **AceCombat** » Wed Mar 24, 2004 4:56 pm

Dedman pretty much summed up what Mob needs to know.

in simple terms there are 5 type of "Jet" engines.

Turbo-Fan = Uses a high bypass cold air duct fans powered by the turbines behind the combustion chamber to push air like a large fan as its primary source of thrust. usualy consits of multiple stages of power turbines and 1 or 2 stages of compression fans/turbines. these are the newest types of engines and have a very high fuel effeciency ratio compared to turbo-jets.they are also much quieter than older turbojets. the primary reason behind this is due to the large fan pushing massive amounts of air through the engine but about 4/5 of that air does not get pushed through the combustion chambers as with a turbo-jet about 4/5 or all of the air goes through a combustion chamber, thus being called a high bypass ratio turbofan engine compared to a low bypass ratio turbojet engine

Turbo-Jet = uses a dual stage shaft to power two sets of turbines, one being the intake/compression stage and the other being the power stage. has little or no bypass ducts and relies on high exhaust output as its primary source of thrust.

Turbo-Prop = a propeller system that uses a single stage turbine as its power source to drive gearbox connected to a propeller of 3 or more blades. the reduction gearbox is neccessary as the RPM's of a turbine are far to fast for a propeller and needs to be reduced to about 1/2-1/3 of the turbine's RPM. gearbox also provides a high torque gearing ratio.

RAMJet = a jet system that usualy has no moving turbines but a system that implements a compression cone at the front of the engine to squeeze air through its intake to a speed capable of producing large amounts of thrust capable of speeds in excess of Mach 2 and upto Mach 8. the cone at the front can be adjusted for various speeds and compression needs. the cone also regulates the fuel-air/compression ratio which must be adjusted at different speeds

SCRAMJet = ( Supersonic Combustion RAMJet ) this engine type is virtually the same as a RAMJet except that the air passing through a SCRAMJet is always supersonic in speed terms, as with a RAMJet, once the air is pushed through the engine and gets into the combustion chambers, its moving at subsonic speeds. this engine can only be operated at speeds in excess of mach 2.5 and upto speeds of mach 14. SCRAMJet also use a much more volatile fuel, and these fuels are very expensive and very dangerous to use unless the aircraft or engine itself is used at extremely high altitudes and at extreme speeds that regular JP-4 or JP-7 cannot achieve combustion rapidly enough to produce thrust at the extreme operating ends of the flight envelopes

Dedman · Post by **Dedman** » Wed Mar 24, 2004 6:54 pm

Richard Cranium wrote:If you want to build one your self... This might give you some ideas.

http://www.asciimation.co.nz/turbine/pa ... l#Contents

Yep. A turbo charged internal combustion engine is nothing more than a jet engine with the internal combustion engine acting as the combustion chamber.

There are several companies that make turbo jet engines for model aircraft. Way cool.

http://www.bairdtech.com/bmt/
http://www.dynopower.freeserve.co.uk/homepages/jet.htm
http://www.gasturbine.pwp.blueyonder.co.uk/page2.htm

Richard Cranium · Post by **Richard Cranium** » Wed Mar 24, 2004 9:12 pm

Those little engines are so cool. Those 300MPH+ model planes are awsome.

roid · Post by **roid** » Wed Mar 24, 2004 11:26 pm

i would also add PulseJet engines and PulseDetonation engines to Ace's list of jet engines.

these are jet engines have no turbines, they use thin bendable reed valves. and they operate in cycles (ie: ignition, combustion, intake, blowback/compression) similar to an internal combustion piston engine. it's quite interesting, an explanation with pics is here.

the Pulse Detonation Engine (PDE) is a similar to a PulseJet, but instead of Exploding the fuel, it Detonates it.
the difference is that a Detonation is so powerful it creates a supersonic shockwave (ie: a sonic boom), where as an Explosion just burns the fuel really fast, and it's shockwave isn't supersonic.
supersonic booms are a very great, very dangerous, and very efficient source of propulsion energy. a PDE would produce many (maybe 50) seperate sonic booms every second.
PDEs are the holy grail that has so far not been successfully achieved. research continues.

oh btw, if anyone's interested. here's the link to the first successful SCRAMJet engine demonstration that was done not long ago, only August 2002. (from my own state QLD, i'm proud as punch

.
actually, i think it's still the ONLY SCRAMJet demonstration

(thumbs nose at nasa).

AceCombat · Post by **AceCombat** » Thu Mar 25, 2004 9:39 am

roid wrote:i would also add PulseJet engines and PulseDetonation engines to Ace's list of jet engines.

maybe i should have added the detailing note: "Working and in use Engines"

Lothar · Post by **Lothar** » Thu Mar 25, 2004 11:00 am

Speaking of scramjets, it looks like one is undergoing testing soon, on an aircraft hoped to break mach 7:

http://www.cnn.com/2004/TECH/space/03/2 ... index.html

AceCombat · Post by **AceCombat** » Thu Mar 25, 2004 3:21 pm

i just posted a thread on that issue.

phpbb/viewtopic.php?t=1245

Lothar · Post by **Lothar** » Thu Mar 25, 2004 7:56 pm

me: Posted Thu Mar 25, 2004 9:00 am
you: Posted Thu Mar 25, 2004 1:19 pm

But yeah, good thread.

AceCombat · Post by **AceCombat** » Thu Mar 25, 2004 9:07 pm

Lothar wrote:me:
you: .!.. ..!.
But yeah, good thread.

roid · Post by **roid** » Sat Mar 27, 2004 4:01 am

AceCombat wrote:
roid wrote:i would also add PulseJet engines and PulseDetonation engines to Ace's list of jet engines.
maybe i should have added the detailing note: "Working and in use Engines"

well then why did you mention the SCRAMJet? that's working, but not in use yet.

the PULSEJet engine has been in use since WW2 ace.
it powered the first cruise missile: the german "flying bomb".

AceCombat · Post by **AceCombat** » Sat Mar 27, 2004 11:44 am

roid wrote: well then why did you mention the SCRAMJet? that's working, but not in use yet.

the PULSEJet engine has been in use since WW2 ace.
it powered the first cruise missile: the german "flying bomb".

i stand corrected