Jet engines at High altitudes

[Low-bypass]

Hey Chaps

I'm doing some private studies on Aerodynamics but there's something i'm clearly battling to understand,can anybody explain me why a jet engine is very fuel efficient at High altitudes?

[mattpilot]

http://www.pprune.org/tech-log/10655-why-jet-engines-more-fuel-efficient-high-altitude.html

[Captlg]

At high altitudes,the air is thinner.
1. thermal efficency greater at higher temp differentials, engine is a heat pump the hotter the efflux the more efficient the burn.
2. compressors work better at high rpms, hi altitude flight allows more rpm without exceeding thrust/airframe speed limitations
3. reduced friction/drag at altitudes allowing lower output at altitude, while retaining a set mach/airspeed. better SFC

[WELCO]

In other words, the thrust produced by a jet engine is given as rpm which is a function of air density. Density is dependent on pressure and temperature. With the temperature drop at higher altitudes, the thrust increases and the Specific Fuel Consumption of a jet engine is decreased, the performance is improved and the endurance is increased.


Good luck.

[Lightning Mate]

There is , of course, also the question of propulsive efficiency, as shown by the Froude formula:

TAS x 2 divided by (TAS plus exhaust velocity).

Propulsive efficiency is higher when, for a given exhaust velocity, the true airspeed is higher ie at altitude.