Why does fuel consumption decrease with increasing aircraft altitude?

Question

I have a chart where the thrust ($F$) and the thrust-specific fuel consumption (TSFC) are plotted against the aircraft flying speed, for several altitudes (i.e. sea level, 3000 meters and 11000 meters). This is for a generic turbojet.

I don't know the source of the image, I apologize for that.

At the left, we see that the thrust decreases with altitude. At the right, we observe that the TSFC actually decreases with altitude as well.

However, I think this is counter-intuitive. I understand that $F$ decreases with altitude, since density (and therefore mass flow rate) decreases when the airplane goes up.

What I don't understand is that the TSFC, defined as: $$\text{TSFC}\equiv\dfrac{\text{fuel mass flow rate}}{F}$$ decreases with altitude. In other words, the higher we fly, the more thermodynamically-efficient the airplane is. How does that happen?

From a pure mathematical point of view, it doesn't make sense that TSFC decreases with altitude since, following the aforementioned formula, $F$ is decreasing, which leads me to think that the fuel mass flow rate diminishes with altitude faster than thrust.

In a nutshell: why does the TSFC decreases with the flying altitude, having the last equation into consideration?

That's a great answer (thanks for linking it), but I was most likely looking a mathematical answer, considering the equation I posted before. I'd like @PeterKampf to chime in and shed some light here! — Jose Lopez Garcia, Jun 01 '16 at 21:03
Hi Jose, this is applied thermodynamics. As usual, Wikipedia has plenty of formulas. I linked to the Carnot cycle; the Brayton cycle is just a special case and its Wikipedia article has fewer formulas. — Peter Kämpf, Feb 08 '17 at 20:15

score 7 · Accepted Answer · answered Jun 02 '16 at 18:05

7

I can't provide the math, but yes, this is correct. Fuel flow decreases as air density decreases. The engine becomes more efficient because of the greater temperature differential between inlet and exhaust gases combined with the lower fuel flow.

answered Jun 02 '16 at 18:05

avianate

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2

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Hey Nate. In order to accept this answer, could you please elaborate more on it at least a bit? You don't need to use maths, a pure "textual" answer will work. Maybe you can provide a chart along with a bit more information? – Jose Lopez Garcia Jun 02 '16 at 18:21
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I'll have to dig through some reference books and see if I can't find something more useful. I'll get back with you on it – avianate Jun 06 '16 at 15:57
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Jose, here are some good references for more information on turbine engine efficiency at altitude:
This video for the maths: https://www.youtube.com/watch?v=E8VfieYhsjg
– avianate Jun 06 '16 at 19:03
This wikipedia article may help you to briefly understand the brayton cycle efficiency of gas turbines: https://en.wikipedia.org/wiki/Thermal_efficiency
And lastly, check out this guys post on the airline pilot's forum about Jet engine efficiency. It's not deep but you may find his notes helpful: http://www.theairlinepilots.com/forum/viewtopic.php?t=476

Hope that helps
– avianate Jun 06 '16 at 19:10
By efficiency are you referring to thermal efficiency? (It's been over 30 years since my Compressible Flow class, vague memories of that needing to be specified ...) – KorvinStarmast Feb 08 '17 at 17:25
To clarify a bit, the engines are designed for cruise altitude and thus must be artificially limited at low altitudes to avoid exceeding temperature and stress limits. At high altitude both the reduced mass flow and reduced inlet temperature allows an increase in inlet compression ratio [with associated relative temperature rise, and a static absolute temperature limit]. – Max Power Feb 07 '22 at 20:32

score 3 · Answer 2 · answered Jan 17 '18 at 19:43

Gas turbine engine works more efficiently at higher altitudes

Altitude increases - Air density reduces - Mass flosw reduces - Maximum thrust reduces.

To maintain thrust as altitude increases - Compressors must rotate faster.

Hight altitude - Less air density - Lesser resistance - Less fuel required to spin the compressor faster.

There is an optimum altitude in reference to speed and thrust which increases as weight reduces.

Altitude increase - Maintaining a constant TAS - Reduction in fuel flow and SFC from sea level up to the optimum altitude. for more information visit http://www.theairlinepilots.com/forum/viewtopic.php?t=476

You've not mentioned that turbine engines are more efficient in lower temperatures. — Max Barraclough, Jan 09 '24 at 20:28

score 2 · Answer 3 · answered Jan 09 '24 at 14:38

I was trying to solve the same question of this topic and I arrived here, I see that this is an old post but but maybe I can contribute a little.

Let's work in the formula quoted by Jose:

Then, according to the formula described in nasa's web page (https://www.grc.nasa.gov/www/k-12/airplane/fuelfl.html) we know that:

If the difference between Tt4 and Tt3 increase, this when the altitude increase, then "f" increase and also TSFC.

I don't have specific values for providing and example, I know that the fuel heating value "Q" is well-known but I don't have the others terms' values.

I hope this could help a little to find the math explanation for the answer.

score 1 · Answer 4 · answered Feb 09 '17 at 12:07

Less oxygen is present at higher altitudes, so not decreasing the fuel flow during climb will eventually throw the air to fuel mixture so far out of balance that the engine will run rough or even shut down or flame out. By definition you will get better fuel economy at higher altitudes but I would not say it is "more efficient" due to the significant reduction in power that can actually be developed due to the lack of oxygen. Any engine will produce more power as a direct result of fuel burn and maximum available oxygen at sea level. However, in jet engines one must take into account EGT temperature differentials and the resulting expansion of exhaust gases that do produce additional thrust dependent on greater temperature differences.

score 1 · Answer 5 · answered Apr 15 '18 at 22:10

Multiple parameters have a role in this, but high altitudes are not the best operating conditions in general for an engine: see hot and high tests, but are the best for an aircraft !

High altitude means - less thrust because of lower air density - less power in the gas generator due to less oxygen density - far less losses from air resistance to the fuselage

Why does fuel consumption decrease with increasing aircraft altitude?

5 Answers5

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