Since a new thread was started, I'll let that one go back to the original topic, PIA Claims B777-200LR Not Upto Specifications... (by Blrsea May 22 2006 in Civil Aviation)#259 (old)
IF you want to still talk PIA 777's: PIA Claims B777-200LR Not Upto Specifications V2 (by LTU932 May 28 2006 in Civil Aviation)
This one is on engines.
From the last posts: (my text):
I'll do a quick summary.
A geared turbo fan has a fixed gearbox between the turbine that powers the fan and the fan? Now, that same turbine powers the first half of the compressor (called the low compressor). All airfoils operate at their peak efficiency at the same mach number. Rather than explain the speed of sound, think of it as RPM times diameter with a temperature correction. ok?
Now the fan is a really big diameter, so it needs to spin at a low RPM. Overspeeding the fan quickly has huge noise and fuel efficiency penalties. But... fans are usually run 7% to 10% overspeed! About 1400 RPM when you might really want 1300 RPM for the fan.
The fan must be powered by the relatively small diameter turbine (low turbine). That turbine is powered by hot gases coming out the high turbine.
So hot gases mean the turbine needs a higher RPM to be efficient. The smaller diameter and hot gasses mean that the optimum RPM for the turbine is about 4,500 RPM.
The low compressor is most efficient in the 3,500 to 4,000 RPM range.
Ok, so you have one part that is most efficient at 4,500 RPM powering a part that would do best at 1,300 RPM. Simple solution, put in a gear box! Now, since the low compressor is happiest at 4,000 RPM (or less), that's the speed you run the low turbine. Because the fan is slowed down, you gain about 3% improvement in fuel efficiency. Because the turbine is sped up, you gain about 5% in fuel efficiency. We engineers say TSFC rather than fuel efficiency as its a number we can put into our equations. spin
Now TSFC is "Thrust Specific Fuel Consumption."
It the amount of fuel burned per hour (in lbm) divided by the engines thrust (at that setting). Values are dropping (better fuel efficiency) toward a TSFC of 0.510 for cruise. The harder you run a jet engine, the more thrust you get for each pound of fuel (about 7.8 lbm per gallon) so engine makers prefer to quote takeoff TSFC, but for long haul planes cruise is what we discuss. Climb is important too... but one actually looks at dozens of design points... so on a.net we usually just talk cruise to simplify things. spin
A link on TSFC:
Now, if you haven't realized, there is concern over the gearbox durability and reliability. Thus, Rolls Royce has chose another alternative. Their fan is powered by its own turbine, but the fan still runs at the inefficient 1,300 ish RPM.
But, their fix is instead of running the low compressor and its turbine at a very inefficient 1,300 RPM, they put those two on their own shaft and bearings at about 4,000 RPM. This improves fuel efficiency by about 2% (or lowers TSFC by 2%, a good thing).
What it also does is reduce engine weight! By about a ton per engine! Why? Engines are made from lots of nickel and titanium (some other stuff too). Nickel is rather heavy. Well, a more efficient compressor means that less compressor is needed as well as less turbine. This makes the engine shorter. That shorter engine, called a "triple spool" has quite a bit fewer parts in it. Really, the only downside is the extra set of bearings added which can be a bear to service. We call this new shaft the "intermediate spool" even though it has the low compressor on it. RR calls the "low compressor" the "intermediate compressor" so have fun calling it what you want. Smile
Why aren't triple spools the only engines offered? There very heavy for *small* engines. But at 60,000 lbf and above ... they're very light. Thus why the only narrow body with a triple spool is the 757. All the other triple spool engines go on widebodies.
Also, GE and RR have been able to make their double spools as efficient or even more efficient than RR's triple spools by spending money on optimizing the other parts of an engine. On a Boeing 777, if you're flying > 4,000nm get the GE-90. For flights less than 4,000nm get the Trent triple spool. I'm ex-Pratt, but since Pratt blew the Pw4098's fuel efficiency... they're out of the game effectively. cry And the pw4168 is the most efficient engine on the Airbus A330... but one needs 72k of thrust for the latest A330's and its a bit of a story why the Pratt's are stuck at 68k thrust... cry (68k is 68,000 lbf of thrust)
Since engines have to be started, takeoff, climb, cruise, land, be restarted in flight (rare, but important), produce minimum emissions, burn minimum fuel, weigh as little as possible, survive for thousands of takeoffs and hours of operation, not spit out fireballs (awwww...) etc. they are very complicated. Some added cooling to increase cruise fuel efficiency must be turned down or off for large changes in engine thrust. I alluded to that earlier. Some engines are more sensitive to wear and tear, etc.
Please take all RPM's as approximate. I was too lazy to look up exact numbers.
My background: I designed gas turbine engines for almost 5 years at Pratt. I also have some experience at Solar gas turbines. My current job is propulsion related, but my employer strickly forbids me from discussing my job in online forums. What I can say is I'm much more involved on the fuel aircraft system side than before, so I'm getting a different perspective. spin
This thread has been discussing a situation where the better climb durability and increased climb thrust of a triple spool compared to a double spool has been important. Also, all airplanes must fly safely such that if they suddenly lose an engine's thrust everyone is ok. Smile That means you do not fly a 777 or any other two engine plane directly over K2 or everest (unless they're "light") but you're allowed to fly a four engine plane. It happens that the A340 does very well in an engine out situation. Your seeing a bit of argument over how much of a diversion is required by the 777 due to those tall mountains... And some argument on the weights PIA is flying their 777's at and if they're light enough or not to safely fly near K2/Everest. But I know the kind of work Oldaeroguy and Widebodyphotog do, so I'm going to believe them.
Unfortunately, you will find a lot of A vs. B on these forums. I will vouch that neither OldAeroguy nor Widebodyphotog take sides; they both will present their data and tell you the conclusions they make. Most of the time we're making assumptions off incomplete data... so expect some small errors. But this is discussion. I'm certainly not in the market for a 777 or an A340. duck
I hope this helps,
[quote] Oh, I missed a few definitions:
Oh, if it helps, engines work by "Squeeze-bang-blow" You squeeze the air, "bang" fuel (add heat energy) and then "blow" the air/fuel through the turbine to extract work (more work out due to volumetric expansion)
GTF=geared turbo fan
LPC=Low pressure compressor
LPT=Low pressure turbine, usually drives the LPC and fan
HPC=High pressure compressor
HPT=High pressure turbine (only drives the HPC, on the GeNx also a generator)
IPC=Intermediate Pressure compressor. An RR term for the LPC on its own turbine
IPT=In a Triple spool, the turbine that drives the LPC (aka IPC).
RPM=Revolutions per minute (in my posts, there are other uses for this)
Cassing=the pressure vessal that holds the engine together.
Quoting Lumberton (Reply 242):
But doesn't the gear box itself add weight? Are you saying that the weight savings elsewhere compensate for the gearbox weight?
IIRC you posted elsewhere that many crashes win WWII were from gearbox failures? How will the OEM's sell gearbox reliability?
The gearbox more than compensates. By having a slow turning (or more precisely, low mach #) LPC, extra stages must be added to it and the LPT (since it too is turning too slowly for the optimal blade shapes). This stretches out the engine. The pressure vessal (aka casing) of the engine must be made from Inconel 715 (ok, in low pressure designs Inconel 625 is possible, but 715 casts easier in spin casting). That's a really heavy nickel alloy. The longer the engine, the thicker the nickel must be to keep the engine "stiff" (adding weight) *and* the longer the casing must be (adding weight). The gearbox adds a few hundred pounds yet in total on a 60k engine would weigh in like a triple spool. (or net 2,000 lbm weight savings).
You recall correctly on the WWII engines. About half of the engine failures were due to gearbox failures. Why?
1. No temperature sensors. We now have cheap reliable temperature sensors.
2. No synthetic oil. This makes a huge difference!
3. New gear tooth designs (this really helps)
4. Testing, testing, and more testing.
To my knowlege, only Pratt has taken the time to characterize high horsepower gearboxes into the detail required to predict (via oil pressure and temperature) a gearbox failure 100+ cycles before the failure.
...snip of some comment...
IAE is gearing up to launch a GTF for their next engine. (Finally, the "superfan!"). Due to contracts, that locks both RR and Pratt into IAE for the A320NG and 737RS. Thus, that keeps the triple spool out of the < 35k market for a generation. I do belive Boeing will allow IAE to offer an engine on the 737RS. Why? Its either that or Airbus will be the only one with a GTF and you know GE will get its engine on the Airbus too... Lets put it this way, a GTF offers the same drop in fuel burn as composites and bleedless combined! I expect a solution like the 787 where IAE and CFM (or will it be GE solo sans SNECMA) share a nacelle. With bleedless designs, that's quite possible.
ok, I was asked to say if the 787 truly shares a nacelle or not... I cannot answer that. However, Boeing tried to keep control the nacelle to keep it a common part... does anyone know if they did?
Also, the thread ended on why the triple spool doesn't have better efficiency:
my start at my speculation:
***warning, intering speculation zone***
It could also be the turbine technology. While shrouded turbines have theoretical advantages (blade tip sealing) its a bit wasted with the required added cooling for the added metal. I know one reason GE and Pratt avoid shrouded turbines is that with optical measurements minimizing turbine blade tip clearences via active cooling, the real world benifit of shrouded turbines is believed to be negative by their numbers. But then again, by GE's numbers every Pratt PW4090 would have snapped its low spool shaft by now and every GE-90 by Pratt design methodology would have fan surges. So each engine maker has a few tricks they just don't share with *anyone*. Smile
I also think its because RR just doesn't put on a 2nd row of high turbine blades. This can dramatically help the engine efficiency (if paired with one or two more HPC rows of blades) by increasing the engine pressure ratio (higher pressure=more efficient energy extraction of the fuel's heating value). But ohh... it would add weight big time and cost... a lot of cost. Sad
Also, look at the older RR cross sections. e.g., the Trent 800
I see two issues:
1. A very old design pre-diffuser concept (design of the exit of the HPC). This adds 1/2% to fuel burn. Since RR now owns Allison and they have the sweetest pre-diffuser designs I've ever seen... I bet the Trent 1000 fixes this (I can't help but notice this part of the engine is missing on the RR online pages for the future trents... scratchchin so something is changing that they don't want GE to know about...)
2. That long distance between the high turbine and intermediate turbine. That's a killer for engine efficiency. Unfortunately, this distance is to get around the extra set of bearings and this seems to be still a long stretch in the Trent 1000 (but not as extreme). Well... thats *really* hot gas flowing at an high subsonic mach number. So a bunch of added cooling air must be dumped in and the pressure that should be turning a turbine is thrown away to the considerable losses always present in high mach # flow environments. This could increase fuel burn by 2%!!!
But I think the fact that RR tends to go for lower pressure ratios than GE or Pratt is a real part of what is hurting their TSFC. But adding that 2nd row of high turbine blades will add 1,500+ lbm to each engine... so their goes RR's weight advantage. So my guess is that the system engineering folks just went for a design better optimized for the shot hops and lower manufacturing costs. But note, we're in the speculation zone as to why RR made decisions. I've never worked for RR, so I cannot tell you for certain.
Oh, just in case you weren't familiar, the higher the pressure ratio in an engine, the more efficient it is. But with Pratt and GE, they're getting up to the temperature limits of Inconel 715 at the exit of the HPC. wideeyed As Oldaeroguy noted, its a conservative industry. No one is quite ready to make the main casting out of anything more exotic... yet. But the engine after the GenX will have to be made from something new. Are their materials out there? Sure! But they aren't proven to case as nicely as Inconel 715 in *large* spin casting applications. Sad So its possible a 2nd row in the high turbine of the trents would increase the HPC exit temperature beyound what Inconel 715 can handle. But I haven't done the math... I admit this would be a very interesting design study to look into. spin
***Exiting speculation zone***
Then one final unanswered question by Baroque:
Hmm, that had not occurred to me, although the inverse is clearly going to be true - good TSFC will be rather useful on long range aircraft. Can we soon have a thread on high BPR high thrust engines, or are they getting taken out as collateral damage in other threads?
I was intending to have this more of a give and take thread on engines.
Oh, just to be clear, when talking about the weight added to a triple spool to increase its overall pressure ratio, its not just the added row of turbine blades that adds the weight (or its rotor). Its the added HPC rows, added length and required thickness of 3 rotor shafts (all three must get stiffer if they are longer), and the weight of the engine casing (also, this must get stiffer as it gets longer). That is why I *speculate* RR has only gone for a single row high turbine.
So if there are any more questions, I'll answer.
What I would request is more input on the engine selection's impact from OldAeroguy or Widebodyphotog. Most of my career has been "oh, you need more thrust here... WILCO!. At better TSFC? Yep!" In particular, any information on how the Trent 1000's climb performance versus the GenX is effecting customer selection.