Turbofan Engine Efficiency at Altitude

[C150J]

Sorry for all of my questions - just getting more and more curious about how our CF34s operate, and want to organize my thoughts.

What primary factors cause increased fuel efficiency at altitude? Is it more than lower turbine temperatures and leaner fuel/air ratios? Any other penalties aside from a loss in thrust?

The general consensus is that FL270 will yield the highest TAS, and anything above that will yield better fuel savings... does everyone agree?

Thanks!
J.

[Polar742]

Originally Posted by C150J

Sorry for all of my questions - just getting more and more curious about how our CF34s operate, and want to organize my thoughts.

What primary factors cause increased fuel efficiency at altitude? Is it more than lower turbine temperatures and leaner fuel/air ratios? Any other penalties aside from a loss in thrust?

The general consensus is that FL270 will yield the highest TAS, and anything above that will yield better fuel savings... does everyone agree?

Thanks!
J.

A turbofan is a normally aspirated engine, so as the density of the ambient air decreases with altitude, so does the required density of fuel to maintain the proper fuel air mixture. Thus higher altitude, lower fuel burn.

[Minuteman]

Here is fodder for tgrayson to hurl back at me ...

In addition to changing engine efficiency, you also have higher nautical air miles per pound of fuel (NAM/LB) at high altitudes.

The dynamic pressure needed to generate lift remains approximately the same at any altitude (CD and CL will change with different Reynolds numbers and compressibility effects, but usually not wildly). As the density of air decreases with altitude, a higher true airspeed is needed to generate a given dynamic pressure (IAS -> CAS -> EAS -> TAS).

Lift and drag are proportional to the dynamic pressure and are roughly the same at sea level or FL350 (for a given CAS), so you end up with a faster speed for a given amount of drag at high altitudes (or, force (thrust) times rate of displacement (TAS) = power).

Fuel flow is proportional to thrust, which is equal to that drag during steady cruise. So you can also say you're getting more mechanical power for a given amount of energy input (which is independent of being propelled by a jet engine).

[Polar742]

There also is a graph for a turbofan engine.

It depicts the % of thrust generated via fan (N1) vs. the core(N2)

The two graphs cross at about 270, which also happens to coincide with Mach crossover too.

I think that explains part of what Minuteman is saying, but as it was found out in the previous thread, I prolly just have a line operator's knowledge from my college degree in aviation and writing training material at the commuter.

[ProudPilot]

Hey guys,

2 cents, what about in general the increase in inlet ram air and decreasing temperatures? Although ambient pressure is decreasing, higher speeds would allow greater inlet ram air pressure, resulting in a more efficient compression, and colder air doing the same to make the engine run better.

Am I correct, or is that just wrong?

[Polar742]

Originally Posted by ProudPilot

Hey guys,

2 cents, what about in general the increase in inlet ram air and decreasing temperatures? Although ambient pressure is decreasing, higher speeds would allow greater inlet ram air pressure, resulting in a more efficient compression, and colder air doing the same to make the engine run better.

Am I correct, or is that just wrong?

I'm not falling for this one again!!!!

[ProudPilot]

Originally Posted by Polar742

I'm not falling for this one again!!!!

What do you mean falling for this one, it's a bloody question?

[Polar742]

Originally Posted by ProudPilot

What do you mean falling for this one, it's a bloody question?

Because last time I commented on an idea you asked about, inspite of not knowing my background, you pretty much crapped on it.

Just be advised you're not the only one with an aviation degree from a well-known school, nor the only person that has taught school on the subject. Just because someone doesn't throw out an equation doesn't mean they only have a line level of knowledge as you assume.

Hope you're more open minded when you have the opportunity to fly in a crew environment. You'd be surprised who you can learn from. Alienate those and that's one less source of knowledge available to you.