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American twins,Brit triple spool engines?

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American twins,Brit triple spool engines?

Old 8th Mar 2011, 12:39
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American twins,Brit triple spool engines?

Hi Guys,

While I was reading up on jet engines used on airliners,I've realised that most of the American built engines like the GE CF6, GE90 and the Pratt and Whitney PW4000 are all twin spool engines whereas those of the Rolls Royce Trent Series are three spool engines.

Thus,is there a reason as to why the Americans tend to favour twin spool engines over triple spool engines?What are the advantages of the twin spool and the triple spool engines?

Would really appreciate if you guys could shed some light on this.Thanks.
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Old 8th Mar 2011, 13:02
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Old 8th Mar 2011, 13:29
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3 spool engine can allow a greater differential between the speed of the high pressure compressor and the fan. This should alllow for higher fuel efficiency.

2 spool engines have fewer parts and are less complicated, so they should have lower maintenance costs.

Currently not all Rolls Royce engines are triple spools, only their biggest ones which are used on the biggest planes for intercontinental flying where they will have a relatively high amount of flight hours and a relatively low number of engine cycles. In other words, where cruise efficiency is relatively more important than engine durability, compared to planes that make several more lower distance flights each day.

Pratt and Whitney is working on their own technological equivalent to the triple spool: the geared fan. Just like the triple spool, it adds complexity in order to allow the speed of the high pressure compressor to be further removed from the speed of the fan.

EDIT: I can color in some more information.

The reason that its advantageous to create a bigger difference between the fan speed and the high pressure compressor speed is because the optimal speed for the fan is tied to the speed that the airplane is flying, while the optimal speed for the high pressure compressor is as high as is possible.

The laws of propulsion are that an airplane will be propelled most efficiently when its exhuast is moving at close to the same speed as the airplane. So if the plane is flying 500 mph, you would want the engines to be pushing a large volume of air at 510mph instead of a smaller volume of air at 1000mph. In the latter scenario, even though your engine may be outputting the same exact amount of energy, a lot of it is lost so it is less efficient. This is the reason why you see those massive super-high-bypass turbofans on large planes, and many of those are the Rolls Royce three spools. They need huge fans in order to push a high enough volume of air. This is also why turbofans have completely replaced turbojet engines, even though the former is theoretically bigger, heavier, and more complex. When you think about it, a turbojet engine is essentially just the core of a turbofan without the big, heavy, drag inducing fan at the front. As far as converting fuel to raw energy output, a turbojet is theoretically more efficient than a turbofan. The problem is that the turbojet thrust might be moving at Mach 2+, so if you're trying to push a subsonic airplane, there's a ton of loss there versus the turbofan pushing a higher volume of air at a slower speed.

Interesting trivia: The Concorde's turbojets had higher propulsive efficiency than any turbojet or fan airliner of its time. This is because the Concorde combined the higher raw efficiency of a turbojet engine with cruise speeds high enough to be relatively close to the exhaust speed. The specific fuel consumption per passenger mile would still be really high though because the Concorde had much higher weight and drag per passenger.

Getting back to 2 spool vs 3 spool... the reason that you want the high pressure compressor to go as fast as feasible is because the faster it spins, the higher compression ratio you'll get, and just like your car, higher compression ratio means better efficiency. Also just like your car, the limiting factors are the technology and materials that you're willing to invest in order to achieve those higher ratios, and the amount of extra maintenance that you're willing to endure. For example, an F1 racing engine has a super high compression ratio, but they more or less have to throw it out after each race.

Last edited by tuna hp; 8th Mar 2011 at 15:31.
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Old 8th Mar 2011, 17:27
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Bear in mind that the reason Engine manufacturers produce engines is to make money.

The development of the original, 3 spool RB211 sent Rolls Royce broke!

After government intervention, the bailed out company had the technology to build upon, and now produces the very efficient (if a little finicky- ala BA 38!!) Trent.

It is interesting that the US manufacturers have never gone down the triple spool route, but are leaping past it to the geared fan solution for optimizing low-to-high pressure gradient.
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Old 8th Mar 2011, 17:41
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American = Keep it simple
British = Do it different


Ouch
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Old 8th Mar 2011, 18:43
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The RB211 on the 752 has been a huge sucess, it has logged very high on wing hours before overhaul. As FedEx has been the largest 727 operator for years it will probably be the largest 757 operator in the near future as they can not cut the doors fast enough for them to replace the 727 fleet.
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Old 8th Mar 2011, 19:53
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KISS

keep it simple stupid
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Old 8th Mar 2011, 20:03
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Originally Posted by sevenstrokeroll
KISS

keep it simple stupid
To an extent. But if fuel is 25% of the total cost of your typical 737 flight or 50% of the total cost of your typical 777 flight, then for those long haul international flights it probably makes sense to invest more in more sophisticated engines that might be more expensive to maintain but deliver better fuel economy.
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Old 8th Mar 2011, 20:07
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to invest more in more sophisticated engines that might be more expensive to maintain but deliver better fuel economy.
What engine was it with that A380 near crash disaster?
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Old 8th Mar 2011, 20:10
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The three-shaft design separates the fan from the LOW pressure compressor, not the HP. The LP is then often known as the IP (Intermediate Pressure).

Similarly, the geared fan arrangement allows a speed ratio between the fan and the LP(IP) compressor.

Last edited by sooty655; 8th Mar 2011 at 20:14. Reason: Clarification
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Old 8th Mar 2011, 20:38
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That's confusing the issue!
The RB211 always has had the LP compressor (Fan. The 011 module) The IP compressor and the HP compressor.
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Old 8th Mar 2011, 21:03
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Tuna

Good stuff sir.

Not quite so with F1 motors these days. They have to last 3 (!) races.

Forced induction (usually turbo charged) petrol engines when on boost run very high effective compression ratio motors but not too high .....

....or they become diesel!

Last edited by chris weston; 8th Mar 2011 at 21:43.
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Old 8th Mar 2011, 22:46
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The three-shaft design separates the fan from the LOW pressure compressor, not the HP. The LP is then often known as the IP (Intermediate Pressure).

Similarly, the geared fan arrangement allows a speed ratio between the fan and the LP(IP) compressor.
Semantics. I was thinking about it in terms of wanting to allow the fan and the HP compressor to both spin at closer to their optimal speeds (which for the fan would be slower, the HP compressor faster). Plus not all twin spool engines have a compressor on the LP spool. Plus on a 3 spool, the slowest spool, the one that the fan is on, still gets called the "LP spool".
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Old 8th Mar 2011, 23:40
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if you design an engine for efficiency in terms of fuel use...you get that kind of engine.

there are plusses and minuses ...better a few more drops of fuel used than an engine that isn't robust and reliable.
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Old 9th Mar 2011, 05:28
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Plenty of Pratt & Whitney 3-spools. They just happen to be Turboprops. The PW100 series is a marvelous design with each turbine stage turning a different shaft, Nh, Nl, Np. It manifests the usual 3-spool advantages, impressive Sfc, nimble power response, and easy starting. By the time ones fingers move from the start switch to the fuel control Nh is high enough to introduce fuel, and everything clicks right along. Puts the PT6 to shame.
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Old 9th Mar 2011, 19:32
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I don't agree that large fan three spool engines are more fuel efficient than two spool designs in the same thrust category. In fact, generally, it is just the opposite. The advantage of a three spool design is a shorter and generally lower weight engine. When coupled with a shorter nacelle, less drag is produced. With these two factors combined (shorter engine/shorter nacelle), parity may be reached with two spool engines. While there may be some advantages of better matching of various compressor section in a three spool design, that is only one part of the total package to gain high efficiency (low fuel burn).

If three spool engines were indeed more fuel efficient, wouldn't we all be flying on three spool engines?
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Old 9th Mar 2011, 20:04
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The TRENT weighs a ton less than a comparable GE. It gains six hundred extra pounds back because it labors behind a squashed Titanium cylinder array instead of a Carbon Fibre Wheel. Heavy Fan, lightweight Core.

GE, PW, Roller. An engine comes off wing out of plan, and it loses its SFC advantage (if there is one) in perpetuity.

Emirates chose EA engines over Rolls for its A380's. They are unwise, and don't know the value of Fuel??

More than any simple answer to engine selection, the complicated route would fill several books.
 
Old 9th Mar 2011, 21:05
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Originally Posted by Turbine D
I don't agree that large fan three spool engines are more fuel efficient than two spool designs in the same thrust category. In fact, generally, it is just the opposite. The advantage of a three spool design is a shorter and generally lower weight engine. When coupled with a shorter nacelle, less drag is produced. With these two factors combined (shorter engine/shorter nacelle), parity may be reached with two spool engines. While there may be some advantages of better matching of various compressor section in a three spool design, that is only one part of the total package to gain high efficiency (low fuel burn).
Its all related. The very reason that they can theoretically be shorter and lighter is because they can remove compressor and turbine stages, and the only reason that they can remove stages while increasing efficiency is because all the spools are spinning at much closer to optimal speeds.

Originally Posted by Turbine D
If three spool engines were indeed more fuel efficient, wouldn't we all be flying on three spool engines?
Well come on... ramjets are indeed faster than turbofans, so why aren't we all flying around at Mach 5 [/sarcasm].

I'm no expert, but I would think it has a lot to do with the fact that the 3-spool is an inherently more complicated setup which can induce higher maintenance costs, and so far 3 spools have only made sense for applications where there are relatively many flight hours per engine cycle. However, I have read that Rolls Royce is readying a lower output/ short haul optimized three spool in time to compete for use on the 737 replacement around 2020.
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Old 9th Mar 2011, 21:36
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The TRENT comes off wing in any combination of six "Modules". 1-5, and eight.

The Modularity was highly touted; wags insisted it was necessary due the need for TRENT's bits to be in the shop. Not too far wrong, v/v the 9.

Modularity saved money, a promise Rolls made to its clients. It saved so much, Rolls took the engines back (on "Total Care"), and advantaged themselves of what they thought would be a profit tool. It was, and it wasn't.

So you are correct in saying the Tri-Spool is more complex, (more parts, more pieces) but it is the separation of twin spool shafts into halves that make it easier to r/r, and more susceptible to service isssues relative to joints, mass, and metallurgy.

imo


(I see you changed Turbine D's qualifier "efficient" to faster. Caught that,)
 
Old 9th Mar 2011, 21:47
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Where's the progress?

Why only 3 shafts? Presumably 6 shafts would allow even more optimal speeds.

When the RB211 face came out a typical automobile automatic gearbox would have three gears. Now they have up to eight.

I wonder why aero engines have not made corresponding progress.
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