MarkC
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Why 2 Engines More Efficient Than 3, Or 4?

Thu Nov 16, 2006 11:13 am

Am I just missing something here. I have a more than casual knowledge of jet engines.

Why would, say a plane with 2 X 90K thrust engines have better efficiency than the same plane with 3 X 60k thrust engines? Or 4 X 45K thrust engines? Assuming everything else being equal, or is that it, its not equal?

Is it not just TSFC and the power requirements of the aircraft?

And I don't buy the argument that larger engines have a larger bypass ratio. 60K engines could be built with the same bypass and pressure ratio as the latest huge fan engines.

Why is it? I must be missing something.
 
brownbat
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RE: Why 2 Engines More Efficient Than 3, Or 4?

Thu Nov 16, 2006 11:20 am

Quoting MarkC (Thread starter):
Why is it? I must be missing something.

Less parts, less maintenance, less things to go wrong. With the way technology is going we might be able to see single engine airliners, although it probably wouldn't be practical from a physical point of view. Single engine aircraft are already being used for the next generation fighters. Jet engines have become very reliable such that you will most likely never encounter an engine failure after flying all of your life.
 
liedetectors
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RE: Why 2 Engines More Efficient Than 3, Or 4?

Thu Nov 16, 2006 11:29 am

On a realy basic level, more engines equals more drag.

Smaller engines typically have to run faster and hotter inorder to get a given amount of a thrust. It is possible that 4 engines can consume more fuel then 2 engines all running to the same total thrust.

Quoting BrownBat (Reply 1):
Less parts, less maintenance, less things to go wrong

Correct.

Quoting BrownBat (Reply 1):
we might be able to see single engine airliners,

Not likely. Always good to have another engine in the event of an engine failure.
If it was said by us, then it must be true.
 
MarkC
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RE: Why 2 Engines More Efficient Than 3, Or 4?

Thu Nov 16, 2006 12:02 pm

Quoting BrownBat (Reply 1):
Less parts, less maintenance, less things to go wrong.

I know that 3 X PW4000 60K is about half the maintanance of 2 X PW4000 90K. Why does the airframe make up the difference?
 
SlamClick
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RE: Why 2 Engines More Efficient Than 3, Or 4?

Thu Nov 16, 2006 1:00 pm

Another factor is that our performance, that is maximum allowable takeoff weight on a given runway/temperature etc. is predicated on the failure of ONE engine.

This means that four engine designs have to be able to complete the takeoff and climb after the loss of 25% of their total thrust. Two-engine airplanes have to be able to do it after the loss of 50% of theirs. For this reason, the 2-engine A-330 has more total thrust than the 4-engine A-340.
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cancidas
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RE: Why 2 Engines More Efficient Than 3, Or 4?

Fri Nov 17, 2006 12:51 am

a single engine airliner would be cool to see, but would never happen. i'd love to see a 737NG with a single GE-90-115B burried in the tail. Now that would be a fun airplane!
"...cannot the kingdom of salvation take me home."
 
N231YE
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RE: Why 2 Engines More Efficient Than 3, Or 4?

Fri Nov 17, 2006 1:01 am

While this thread is on the subject,

Could someone explain why XJ uses the Avro RJ85? I understand that the BAe-146/Avro RJ was designed for STOL, yet I never saw a need for it in XJ's system. To me it seems as if it is a small 4-engined gas guzzler that is not being used for what it is designed for.
 
David L
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RE: Why 2 Engines More Efficient Than 3, Or 4?

Fri Nov 17, 2006 2:37 am

Quoting BrownBat (Reply 1):
Single engine aircraft are already being used for the next generation fighters.

Single engined jet fighters have been around almost as long as jet engines have been around.  Smile
 
airfoilsguy
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RE: Why 2 Engines More Efficient Than 3, Or 4?

Fri Nov 17, 2006 3:06 am

Quoting BrownBat (Reply 1):
Single engine aircraft are already being used for the next generation fighters. Jet engines have become very reliable such that you will most likely never encounter an engine failure after flying all of your life

That may be so but there is still the chance you could suck in a bird or someone leaves something loose during an inspection. It is always better to have redundancy, especially when you are 30,000 feet above the ground.
It's not a near miss it's a near hit!!
 
bond007
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RE: Why 2 Engines More Efficient Than 3, Or 4?

Fri Nov 17, 2006 3:32 am

Quoting David L (Reply 7):
Single engined jet fighters have been around almost as long as jet engines have been around.

...and quite a few ejections due to engine failures/fires/explosions  Wink


Jimbo
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SlamClick
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RE: Why 2 Engines More Efficient Than 3, Or 4?

Fri Nov 17, 2006 3:41 am

Quoting Cancidas (Reply 5):
a single engine airliner would be cool to see, but would never happen

FAR § 121.159 Single-engine airplanes prohibited.

No certificate holder may pperate a single-engine airplane under this part.


Truly today's jet engines are vastly more reliable than early ones, and especially early piston engines but the issue is redundancy. I don't think in today's political or liability climate anyone would change this rule if they had the opportunity.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
David L
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RE: Why 2 Engines More Efficient Than 3, Or 4?

Fri Nov 17, 2006 4:26 am

Quoting Bond007 (Reply 9):
...and quite a few ejections due to engine failures/fires/explosions

Oh, there's no denying that engines are becoming increasingly reliable. Nevertheless, we're not "just beginning" to see single engine fighters. It all boils down to whether or not you're carrying anyone who can sue.  Smile
 
dakota123
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RE: Why 2 Engines More Efficient Than 3, Or 4?

Fri Nov 17, 2006 9:38 am

As explained to me by a GE engineer (and in addition to the other advantages mentioned), the propulsive efficiency of a larger fan is higher than that of a smaller. Simply, a larger quantity of slower moving air is more efficient than a smaller quantity of faster moving air, although both streams may be the same mass.

Other advantages I didn't see mentioned is structure. Add another engine or two and structural weight and complexity goes up.

Dakota123
 
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lightsaber
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RE: Why 2 Engines More Efficient Than 3, Or 4?

Fri Nov 17, 2006 9:50 am

Quoting MarkC (Thread starter):

And I don't buy the argument that larger engines have a larger bypass ratio. 60K engines could be built with the same bypass and pressure ratio as the latest huge fan engines.

Well... they do.

First, lets go to the basic reason why larger engines have lower fuel burn.

1. Tip spacing. Every engine maker has a technological limit on how close they can make the blade tips to the casings. The larger the diameter of the engine (more thrust), the porportionately less work lost with flow around these leaks. Leaks go as core diameter. Core power goes as core diameter squared. In other words, Twice the diameter has twice the leaks but 4X the power. (Ok, I simplified...) Or in other words, half the inefficiency of the tip leakage goes away!  spin 
a. compressors. Leaks flow backwards and thus air must be recompressed, adding work and reducing engine thrust.
B. Turbines. Leaks result in hot gasses not producing work.
2. Parts. Basically engine costs go as part count. A high thrust engine pays for added part counts (with their efficiency). For example, most high thrust engines either have two turbine stages powering the high spool. This added overall pressure ratio (OPR) improves the thermodynamic efficiency of the cycle. Or, in the case of RR, allows for a triple spool. Below a certain thrust, the triple spool doesn't save weight or improve efficiency. Obviously, added turbine stages are matched with more compressor stages. One exception is the CFM for the A340. There an extra low turbine stage was added to power more low turbine compressor stages (compared to the other CFM56's optimized for shorter hops).
3. The ratio of surface area to flow area within the engine drops as the engine gets larger. Surface area within an engine is DRAG! In other words, a larger engine will have a higher TSFC than a smaller engine if everything else was equal. But it isn't equal (due to tip losses, added turbine/compressor stages, etc.) The advantages just keep adding up for the large engine.
4. Part cost. It usually pays to go with the latest technology in a large engine. Its often not the case in a small engine. For example, fan technologies usually appear first on large engines.
5. Weight. Larger engines weigh less per pound of thrust than a small engine. Weight is the enemy of aircraft. A three/four engine aircraft needs another set of plumbing for bleed/start air. Shaft and bearing weight does not scale up with thrust but rather goes up slowly. In general, the weight of an engine goes up slightly more than the diameter of the engine (parts get thicker), but thrust goes up as the square of the engine diameter. So Large engines have an inherent weight advantage.
6. Mach number. Blades have an optimum mach number that they want to operate in. Yes, technology shifts/broadens that mach number, but that shift is for all engines. A larger engine has a far greater fraction of every airfoil's area (remember, diameter again) near its optimal mach number. So component efficiencies go up with engine size.
7. Nacelle drag. Nacelle drag is purely a function of engine surface area. 3 engines will have more surface area than 2 engines of the same thrust. Just as 4 engines will have more surface area than 3 of the same thrust. Ok, I assume all engines are of the same generation utilizing the same technology.


Because of these efficiencies, the low turbine can do more work in the larger engine. Thus, for the same technology, a larger engine will always have a slightly higher bypass ratio than a smaller engine. Also, 180k of thrust via two engines will *almost* always be cheaper than 3X60k. The exception? If the 60k engine could be produced in much more significant quantities and thus acheive a better "economy of scale" in engineering and production.

Hope this helps,
Lightsaber
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RE: Why 2 Engines More Efficient Than 3, Or 4?

Fri Nov 17, 2006 10:16 am

Quoting MarkC (Thread starter):
Why would, say a plane with 2 X 90K thrust engines have better efficiency than the same plane with 3 X 60k thrust engines? Or 4 X 45K thrust engines? Assuming everything else being equal, or is that it, its not equal?

As Captain Click points out, your proportions are off. Assuming your aircraft has 2x90k engines, an equivalent quad would have 4x30k engines (not 4x45k). This is because engine our requirements are based on one engine failing. So a twin would need 90k left after an engine failure and so would a quad. The equivalent triplet would have 3x45k.

Thus, engines on twins run at a lower proportion of take-off thrust during cruise compared to engines on quads.

I think we will still see quads for a while, at least in the 400+ pax category. At a certain point making a twin, and thus developing an engine even larger than the GE-90 for a very small market starts becoming economically indefensible. Simpler to just stick two extra engines on there and calling it at day. At least that's the case today. In 10-15 years who knows.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
MarkC
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RE: Why 2 Engines More Efficient Than 3, Or 4?

Fri Nov 17, 2006 11:37 am

Thanks for that post Lightsaber.

I look at #5 as convincing me the most.

I never thought to look at tip losses. I'm going to check that out later on.
 
baroque
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RE: Why 2 Engines More Efficient Than 3, Or 4?

Sun Nov 19, 2006 9:47 am

C'est formidable Lightsaber. I was glancing at this thread hoping you would provide a contribution. Highly definitive when it came. Thanks - again!
 
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lightsaber
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RE: Why 2 Engines More Efficient Than 3, Or 4?

Sun Nov 19, 2006 3:24 pm

Quoting Baroque (Reply 16):
C'est formidable Lightsaber. I was glancing at this thread hoping you would provide a contribution. Highly definitive when it came. Thanks - again!

You are welcome.  Smile Thanks for the ego boost.  Wink

I admit to spending less time in the forums... and I'm spending less time answering questions... that have already been answered. But this honest question deserved an honest answer.

Lightsaber
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