How Does Fuel Usage and Design Affect Jet Engine Efficiency?

[bill nye scienceguy!]

i found this on another messageboard and couldn't find an answer. any help?

Question about jet engines...
From: Bisected8 | Posted: 2/6/2006 9:26:42 AM | Message Detail | #001
Is the fuel used to move the fans by way of an internal combustion engine (thus compressing the air and forcing it out). Is it burnt in the compression chamber to provide extra thrust or does this only occur when an afterburner is in use?
---
"Knowledge speaks, but wisdom listens"- Jimi Hendrix
"I wish men had boobs"- Emma Bunton

 

[Danger]

The question is a wee bit vague, Bill. The compressor stage forces air into the combustion chamber, where fuel is injected and ignited. That expansion by combustion provides the rearward thrust and also drives the turbine at the rear which in turn drives the compressor. The fan, on a bypass engine, forces air through a shroud outside of the engine proper. It's also turned by the turbine.
If you mean the starting sequence, I've never heard of an internal combustion motor being used to initiate rotation. Fred can correct me on this if necessary, but I believe that it's primarily compressed air that gets the thing spinning in the first place. There might be some electric starters, but I'm not aware of them.

 

[bill nye scienceguy!]

i think the original question was about the starting sequence. i'll get back to him about electric starters.

 

[Ayrity]

speaking of turbine engines... is it true that they are much more efficient when used at one set speed, and barely changing their rpm? I assume this is true because of the applications that they are used for ie. planes. I am curious because I have a transmission design for a cvt, but it is much better, in my opinion, than the double pulley system out there that they have in things like snowmobiles. In any case i know an engine that runs at a constant rpm would be best for a cvt so i was thinking of matching it up with a turbine engine. any input on that?

 

[Danger]

I was never turbine-rated, but my understanding is that most jet aircraft fly at over 90% throttle for maximum economy.

 

[Ayrity]

well than that could be pretty perfect, because then i would only need to have the gear ratios change by using a clutch and a spring to "feel" the torque and then have the throttle auto adjust to stay at the same rpm no matter what the load, only in neutral, it would idle slowly, maybe even use a wastegate, and at full speed, have it go to 100% throttle, everywhere else it could use its most efficient speed.

another area i might have to touch on is, what about noise, is it possible to make a turbine engine that doesn't require ear protection, muffled maybe? so its possible to run on something with wheels?

 

[Danger]

There are lots of turbine-powered vehicles. In fact, a turbo-prop (aka prop-jet) aeroplane uses gear reduction from a turbine to drive a propellor. A similar system is used in buses in Europe, and in some race cars. The exhaust can be muffled as long as backpressure isn't excessive. I recon you'd probably still get a good whine from the spinning bits, but apparently it isn't too noisy for urban use. I defer to Fred and Brewnog for details.

 

[brewnog]

Gas turbines are not my field whatsoever, sorry dude.

You could Google a motorbike I saw which was powered by a Rolls Royce Allison helicopter engine through a 2 speed box. That looked like a lot of fun.

Fred's your man for GT applications though!

 

[brewnog]

Got excited and did some searching.

Is it just me, or do these numbers sound a little bit scary for something which weighs 225kg, and is sitting between your legs, inches away from your testicles?

Engine: Rolls Royce Allison 250 series gas turbine
Power: 320 HP @52,000 rpm (286 HP @ rear wheel)
Torque: 425 ft/lbs @ 2,000 rpm
Output RPM: 6,000 rpm
Compressor Speed: 54,000 rpm
Weight: 500 lbs
Fuel: Diesel, kerosene
Transmission: 2 speed automatic




Eek!

 

[Cliff_J]

Might as well include a link to the bike:
http://www.marineturbine.com/motorsports.asp

I thought planes flew in cruise at less than 50% throttle, that the remainder was a safety margin for takeoff or manuvers and otherwise not used.

 

[Cyrus]

Ive seen that bike on tv. It seems like a very stupid idea. (1) you will burn yourself or cars beind you, (2) you are going to get lag time as the turbine has to spin up to speed, (3) you will go through a tank of gas in 5 miles, (4) you will never take that thing to a regular shop for maintainace, (5) if debris gets into it your turbine is trash, (6) it costs $150,000 USD. What a waste. I got an idea, how about a turbine powered parachute!?

 

[FredGarvin]

Question about jet engines...

Is the fuel used to move the fans by way of an internal combustion engine (thus compressing the air and forcing it out). Is it burnt in the compression chamber to provide extra thrust or does this only occur when an afterburner is in use?

Well, if I read your question correctly, the answer is that in operation, the air is compressed in the early stages, i.e. the compressor section during which the flow is accelerated. Prior to entering the combustion chamber (the burner) the flow is diffused to slow it down and further increase it's static pressure. In the burner the air is mixed with fuel and ignited. The ignition process is the process responsible for providing the required energy to turn all of the rotors. Without it, there is no working engine. The fan, the compressor sections and turbine sections all turn due to the ignition. An afterburner is technically referred to as "supplemental" thrust in that it is not the primary source and it would not work without the main engine in operation.

You mentioned that it might be a qustion in regards to the starting sequence. In that case, just like a car engine, a supplemental means of moving the rotating groups is required prior to stable ignition. In most smaller applications, that is done via a starter/generator that does the exact same thing as the starter on your car. The only difference is that when up to speed, the unit translates into a generator for DC power. Another widely used form of start power is compressed air. Many of the big boys use this. If you notice an aircraft on the tarmac that has a ground power unit and a big hose going to the aircraft, that's an air start. The SR-71 had a cool start feature which used 2 V-8 engines that turned a shaft that was directly coupled to the engine shafts on the aircraft. Small ones, like our cruise missile engines use a pyrotechnic cartridge that fires and impinges on a turbine rotor to get the shaft up to speed.

 

[FredGarvin]

Got excited and did some searching.

Is it just me, or do these numbers sound a little bit scary for something which weighs 225kg, and is sitting between your legs, inches away from your testicles?

Engine: Rolls Royce Allison 250 series gas turbine
Power: 320 HP @52,000 rpm (286 HP @ rear wheel)
Torque: 425 ft/lbs @ 2,000 rpm
Output RPM: 6,000 rpm
Compressor Speed: 54,000 rpm
Weight: 500 lbs
Fuel: Diesel, kerosene
Transmission: 2 speed automatic

Eek!

If this is the same one, I saw a show where, of course, Jay Leno, was riding his around L.A. It did indeed suffer from the two main problems, noise and throttle lag. Other than that he said it was the coolest bike he had ever owned.

 

[FredGarvin]

speaking of turbine engines... is it true that they are much more efficient when used at one set speed, and barely changing their rpm? I assume this is true because of the applications that they are used for ie. planes. I am curious because I have a transmission design for a cvt, but it is much better, in my opinion, than the double pulley system out there that they have in things like snowmobiles. In any case i know an engine that runs at a constant rpm would be best for a cvt so i was thinking of matching it up with a turbine engine. any input on that?

If you are talking about a turboshaft application, then you are somewhat correct. They are usually held at constant RPM and vary the torque output. My trusty T53's and T55's from my Chinook days are pretty widely used in hydroplane boat applications now.

 

[Ayrity]

hmm, that's interesting. Overall for the mass/power ratio and and their corrisponding most efficient rpm, i believe a turbine engine is more efficent than a conventional ICE, is this correct? I have done a lot of my own research, and have come across many many sites where people designed and build their own turbines with turbochargers and minimal tools and other parts (oil pump, tubeing etc) is this a reasonable project to pursue, or is it better to just purchase one when I know exactly what I need? Any tips on how to design and build my own? has anyone here done it?

 

[FredGarvin]

hmm, that's interesting. Overall for the mass/power ratio and and their corrisponding most efficient rpm, i believe a turbine engine is more efficent than a conventional ICE, is this correct? I have done a lot of my own research, and have come across many many sites where people designed and build their own turbines with turbochargers and minimal tools and other parts (oil pump, tubeing etc) is this a reasonable project to pursue, or is it better to just purchase one when I know exactly what I need? Any tips on how to design and build my own? has anyone here done it?

Turbines can be much more efficient, especially when you can incorporate a recouperator or exhaust gas heat exchanger.

Turbines from turbos are pretty common. They have external burners so they tend to be somewhat homely looking. They are safer for most people because the turbos are already designed to operate at the very high speeds required. This eliminates one of the biggest safety worries because most people have absolutely no idea just how much energy is contained in rotating hardware. They are also clueless when it comes to the necessities of proper bearing selection/mountings and rotating group balancing.

My senior project in college involved a turbo based turbine with regenerative heat exchanger. It was pretty simple, but we had good facilities and took a lot of precautions.

 

[Ayrity]

how did the heat exchanger work? did you use the heat to preheat the fuel, or to boil water for a separate steam turbine? i would love to get some definite reseach on this. the one main are i am having some trouble with is the flame holder hole pattern. everywhere i go, it seems people just say, trial and error, just drill soem holes, then if it doesn't work, add some, take some away or change the configuration. I am a mechE major, there must be a way to calculate the amount of o2 needed for a certain amount of fuel, and therefor how many holes to use, and i could probably figure that part out, the hard part to me is the configuration of the holes. also, what would be the best way to recapture the energy in a turbine and then to a shaft? i suppose I could just attach another turbo blade somewhere down the tube, or maybe weld a shaft off of the compressor turbo. which would be better? is it better to have the turbine you are using to recoup the power into mechanical energy attached or separate from the compressor turbo? my thoughts on this is that if I were to attach this to a vehical, it would be better to just have a separate turbine free to spin on its own down the exhaust tube, that way at idle speed, there could be a waste gate where the engine could still be running, but not spinning the second turbine, thus a neutral built into the engine. ok, this is a lot to write and read ill leave it at that... lol

 

[FredGarvin]

how did the heat exchanger work? did you use the heat to preheat the fuel, or to boil water for a separate steam turbine?

It was used as a combustion inlet air heater and as a fuel pre-heater.

The one main are i am having some trouble with is the flame holder hole pattern. everywhere i go, it seems people just say, trial and error, just drill soem holes, then if it doesn't work, add some, take some away or change the configuration. I am a mechE major, there must be a way to calculate the amount of o2 needed for a certain amount of fuel, and therefor how many holes to use, and i could probably figure that part out, the hard part to me is the configuration of the holes.

Well, there's the trick. I am by no means a burner expert. I have told people here before that burner design is a bit of black magic in a lot of cases. Yes there are calculations that will tell you how much oxygen to get near stoichiometric burning, but it gets a lot more complicated with that, especially when you change the types of fuels you are dealing with. You start to run into problems when you have to visualize the flow in the burner. Depending on the geometry of the burner, you want your central burning zone to be in a specific location. You have to avoid hot and cold spots, etc...There are some very complicated flow visualization techniques used. Even then, it is theory. We have to test all burners to make sure they perform within the envelope they do and have desirable start characteristics. It doesn't take much to burn a hole inside the burner.

The configuration of the holes is going to your way of controling what the distribution of the combustion is going to look like. There are a couple of other aspects to that configuration though. The most important is cooling. Certain holes will be there simply to help cool the burner cover in spots. You would probably freak out if I were to show you the hole patterns in our burner covers and plates. There are thousands of small holes in some pretty weird patterns. In a nutshell, you really are pretty much left with trying to find out what has worked for others and experimenting on your own.

also, what would be the best way to recapture the energy in a turbine and then to a shaft? i suppose I could just attach another turbo blade somewhere down the tube, or maybe weld a shaft off of the compressor turbo. which would be better? is it better to have the turbine you are using to recoup the power into mechanical energy attached or separate from the compressor turbo?

The turboshaft applications I have had experience with all had a specific turbine stage for the power shaft. For example, the T-55's last two turbine stages are for the shaft. The rest of the turbine stages are the gas producer sections. I can't really say what is "better." I would say that it would depend heavily on your application.

 

[Ayrity]

sounds great, thanks very much for all your involvement! sorry for all the specific questions haha. now i just have to find some examples of flame holders

 

[Ayrity]

actually one more question while I am thinking of it, i see that more recent engines use an annular burn ring and not a can, do you think it would be a good idea to try to make an engine with the ring design? It seems they are more effiecent, and on top of that, any sort of formula on air flow, horsepower and the size (diameter of the whole engine and the can diameter) or the nozzle ratios in proportion to the rest of the engine would be helpful thanks