Turbine Engines--How they work

[hitman]

Since Im on the verge of graduating college and obtaining my A&P license, I know how a turbine functions. Since I need to study for my O&P, I might as well keep my studies here and get some decent info as well.

 

4 types of turbine engines:

Turbojet, Turboshaft, Turbofan, Turbopropeller.

Every one of them perform at high altitudes with exception of turboshaft, which drives a rotor mast.

 

Low bi-pass turbofan engines--found on modern fighters and small airliner aircraft.

 

High Bi-pass turbofans--found on large transport category aircraft

 

Turbojet--found on older aircraft, such as a 727-200 and B-52, not very efficient because its driven by turbine engine only.

 

 

Turbines operate on the Vernoulli principle, also known as venturi effect. The inlet duct is divergent in shape, which increases pressure in front of the front turbine blade. Depending on the type of power produced in the engine, the exit velocity of the gasses will vary accordingly. Turboprops and turboshaft engines are torque producers, therefore dont produce thrust by exiting gases.

 

Subsonic aircraft have convergent exhaust ducts to increase the velocity of exiting gases, which are accellerated in the combustion section. If the aircraft is a supersonic aircraft, the exhaust cone needs to be fully divergent, because at the speed of sound, the exhaust gasses increase in pressure and not velocity at those speeds in a convergent duct. Supersonic aircraft have to have a variable exhaust duct in order to reshape the exhaust gasses to a divergent pattern to increase the exiting speed, and a convergent shape below supersonic speeds.

 

Turbofan engines and turbojet engines can perform better at high altitudes due to the less dense air at altitude, air temperature and moisture content. At altitudes, air is displaced by water vapor , which decreases the weight of the air. However, that water vapor is used to cool the engine, and can increase the pressure inside the compressor section, creating better volumetric efficiency of the air charge in the combustion section. Turbofan engines are far more efficient at high altitudes over turbojets because of an increased air intake, which is driven by the turbine engine exhaust. These engines have between 25%-75% more efficiency over turbojets due to the large fan on the compressor inlet. 25% of the turbofan intake is used in the combustion process, the other 75% is used to directly cool the combustion casing, and helps shape the exhaust gas airflow into a more convergent shape.

 

Turboshaft and turbofan engines have a divergent shape exhaust duct to slow down the exiting gases (you cant get a helicopter to hover with a jet engine that produces thrust). Instead they use an axial or an axial-centrifugal compressor to drive a free floating turbine wheel attached to a rotor mast or a propeller shaft.

 

Afterburning engines (reheaters) consist of a turbine inlet, low pressure and high pressure compressor, which are driven by the high velocity turbine. Aft of the turbine rotors and stators is a variable adjustable exhaust nozzle. This nozzle changes from convergent (low pressure, high velocity subsonic exhaust gases) to divergent (low pressure, high velocity supersonic exhaust gases), and contains an additional combustion section which further heats the exhaust gases. The turbofan first stage compressor supplies the additional oxygen needed for combustion after the first combustion stage. Water may be injected at this point to help cool the fuel/air charge, and burns approximately 1-5 gallons of water per per second (please confirm). The exhaust that is reheated is further expanded, which increases the exhaust velocity above supersonic speeds.

 

<more to come

 

Feel free to correct me with additional info. I have 3 days before graduation, and a few weeks before my O&P.

Edited February 28, 2009 by hitman

[CyBerkut]

Minor correction, most likely a typo, as the letters are adjacent.

 

Vernoulli should be Bernoulli.

 

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

 

Nice, informative post! :thumbup: Good luck on the O&P. :book:

[AlphaOneSix]

Good luck on your oral and practical tests. I am sure you will find them incredibly easy. When I got my A&P, it was based on experience, and I had no real classroom training, so I was very nervous. As it turned out, it was actually quite simple. I'm sure you'll do well, just keep up the studying, and remember, once you've got your A&P, it never expires, so you never have to take any tests again! (Until you go back for your IA, that is.)

[hitman]

The only thing Im stressing right now is my oral. I got a DME that is fairly easy on the questions, but I have to study up on my Gen material and a few powerplant oral questions.

 

Tired of school, I graduate on tuesday :D

[hitman]

Minor correction, most likely a typo, as the letters are adjacent.

 

Vernoulli should be Bernoulli.

 

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

 

Nice, informative post! :thumbup: Good luck on the O&P. :book:

My bad...it is Bernoullis. Ive ben hair pulling and burning FAA books with gas and holy water for the past few days.

[Rhen]

Turbojet engines have a higher range of operating mach numbers than a turbofan. A turbojet may be more inefficient, when it comes to burning JP-8 or Jet-A, but it can operate at higher altitudes and airspeed ranges than a turbofan.

 

Turbofan engines are basically small turbojets that drive a big multibladed prop. The mass of the fan also can act as an airbrake at high altitudes limiting it's speed and altitude. The only thing that separates a turbofan from a turboprop is that the turbofan produces its thrust from the fan + core, while the turboprop only generates thrust from the prop (fan).

 

Just my 0.02$ as a pilot, not a maintainer.

[hitman]

IIRC, the thrust produced in high bi-pass turbofans is greater than in turbojets simply because that big ol' fan produces more thrust than the hot section on a turbine engine. In retrospect, the air force is likely to upgrade the B-52 fleet with high bi-pass turbofan engines. Its in the works, but I havent heard much about it.

 

Turbojets are junk compared to low bi-pass turbofans, and they are only efficient at certain altitudes because of a lack of hydromechanical fuel controllers, EEC, and FADEC. I dont think anyone really uses turbojets anymore...maybe Air Argentina. :P

 

BTW...if I argue, its because Im learning something.

[Rhen]

Again, it depends on the altitude and mach number. The higher the mach number, the better a turbojet might be.

 

Granted, modern jet aircraft almost exclusively use turbofans, it's just that the amount of bypass is the central argument here. The greater the bypass ratio, the less efficient that engine will be at higher mach numbers. Airliners cruise at M0.8-0.9, fighters with lower bypass ratios and an afterburning section operate best at M0.95+. Heck, look at the Concorde. That aircraft had an afterburning turbojet to sustain M2. A turbofan would have increased drag because of it's higher cross section for a given output for that design of aircraft.

[mvsgas]

Again, it depends on the altitude and mach number. The higher the mach number, the better a turbojet might be.

 

Granted, modern jet aircraft almost exclusively use turbofans, it's just that the amount of bypass is the central argument here. The greater the bypass ratio, the less efficient that engine will be at higher mach numbers. Airliners cruise at M0.8-0.9, fighters with lower bypass ratios and an afterburning section operate best at M0.95+. Heck, look at the Concorde. That aircraft had an afterburning turbojet to sustain M2. A turbofan would have increased drag because of it's higher cross section for a given output for that design of aircraft.

Not entirely correct. In my experience, turbofans are more powerful that turbojet because you can have higher temperature in a turbofan and more power (i.e. J57 compered to a F-100-PW-100) Remember not all turbofans are high bypass, low bypass used the extra bypass air to help cool components, increasing pressure and burning temp in the burning chamber.They also produce trust from both the fan and the core (N1 and N2) in low by pass turbo fan (i.e. F-119, F-135,F-100, F-110, etc.). Ever since the F-111 TF30 (which could archive Mach 2 at 500 feet) to the F-22 F-119-PW-100, turbofans engines have rule the ski.

Edited February 28, 2009 by mvsgas

[hitman]

The greater the pressure ratio, the more efficient that engine is. The GE90 has a 40:1 pressure ratio, IIRC, and can (and has) safely made a 747 stable at cruise flight on a single engine.

 

If it were an unducted fan, it would be closer to 85%-90% efficient...buuuut no one likes 110db. People tend to get pissy. :D

[Rhen]

Not entirely correct. In my experience, turbofans are more powerful that turbojet because you can have higher temperature in a turbofan and more power (i.e. J57 compered to a F-100-PW-100) Remember not all turbofans are high bypass, low bypass used the extra bypass air to help cool components, increasing pressure and burning temp in the burning chamber.

 

You're not thinking about the entire range of speeds here, are you? Turbofans are more powerful than turbojets at low speed since they move a large mass of air at low relative speed, versus a turbojet engine, which moves a small mass of air at much higher speeds than the turbofan. At lower mach numbers, the turbofan wins hands down. To get it to produce enough thrust at higher mach numbers, you stick an afterburning can on the back of it to get it to the mach3 range as it's top speed. To go beyond this, you must revert back to the old turbojet.

 

Remember that the area of the inlet also acts as an aerodynamic drag on the aircraft for it's specific fuel consumption at higher mach numbers. There's no one here disputing that a CFM-56 produces a better SFC than a J57! :megalol: The idea being that a jet transport isn't designed for speeds much in excess of mach 0.8. The higher the mach, the lower the bypass ratio to remove parasite drag on the system. You'll not see a big bypass fan on a future fighter anytime soon. :megalol:

 

Yes, we all know that trying to accelerate 200 tons of aircraft with a turbojet will just produce a LOT of noise & heat without moving the aircraft very efficiently down the runway. But, if you want that aircraft to be able to sustain M2 at 40,000 ft, we've got possibilities. The turbojet has a small frontal area compared to a high bypass turbofan. If you need the jet to go barely mach2, then there's a point where a lower bypass turbofan is the appropriate solution - but you'll have to put an afterburner on it, to accelerate something that heavy past mach.

 

 

They also produce trust from both the fan and the core (N1 and N2) in low by pass turbo fan (i.e. F-119, F-135,F-100, F-110, etc.). Ever since the F-111 TF30 (which could archive Mach 2 at 500 feet) to the F-22 F-119-PW-100, turbofans engines have rule the ski.

 

I think I already said that! :smilewink:

 

Pure prop (fan) efficiency is measured as the amount of kinetic energy generated with respect to the surrounding air. The more kinetic energy in it's wake, the less efficient.

 

With regard to overall efficiency we have the following (being VERY general in overall concept):

 

Highest -> Lowest

Prop, turboprop, turbofan, turbojet

 

Fuel consumption per pound of thrust

Prop, turboprop, turbofan, turbojet.

 

Thrust at high mach numbers

Turbojet, turbofan, turboprop, prop

 

Thrust at low mach & altitude

Prop, turboprop, turbofan, turbojet

 

Thrust at high altitude

turbojet, turbofan, turboprop, prop

 

Still, overall efficiency of the turbojet is poor. It's also poor for a ramjet, but it will still produce better thrust at high altitude and mach numbers greater than 3 than a turbofan!

Edited February 28, 2009 by Rhen

[mvsgas]

I may not understand your post, but you keep referring to turbofan on big aircraft and giving examples of High bypass turbofans. The Low bypass turbofans do not increase drag because of the fan. Also, like all modern fighter aircraft jet engines have demonstrated, turbofans are better than turbojets, not only in fuel efficiency but also in power at all speed ranges (AFAIK) and reliability. If turbojet was in any way better than turbofan, how come every mayor engine manufacture stop producing turbojet for the top of the line fighters? What is the last fighter aircraft that carried a turbojet engine? I do not claim to be an expert nor do I know more than anyone. I have been working on turbofan engine for the last 12 years (F-100-PW-220\220E, F-110-GE-100) and after talking to many mechanics, I have form my opinions on this engines. I am not saying you are wrong but the way you present the information seem to me you are only looking or seeing a turbofan engine as a high bypass engine. The F-119-PW-100 push a 30,000lb + aircraft to above mach without afterburner. Name one Turbojet engine that push an aircraft above mach without the use of re-heat or after burner.

Edited February 28, 2009 by mvsgas