It would. Thus why I think a lower thrust engine will get the GTF
first. Imagine the 737 with an engine that gets 15% lower TSFC... quite a different market, eh?
8% lower TSFC from the Geared Turbo Fan.
3% lower from contra rotation
2% to 3% lower by adding a 2nd High turbine stage (worth it at $70/bbl, not at $30 bbl)
2% due a bunch of incrimental improvements.
|Quoting PVD757 (Reply 237):|
Count me in to this category. My sorry little business degree is being overpowered right now in regards to this engineering discussion, but I'm fascinated nonetheless. I just don't know some of the acronyms that everyone is spitting out. I get the winds and temps and how they affect performance. I even have a basic concept of the engine and the components and how they interact, I just can't get the rest of the GTF, TSFC, and the difference between the double and the triple spool other than I look at it as some sort of overdrive in the gearbox that allows more efficiency at higher rates?
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.
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.
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.
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.
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.
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...
(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.
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.
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.
I hope this helps,