Specific fuel consumption is the mass of fuel required to sustain 1 lb of net thrust for 1 hour. The unit used in this case is the lb fuel/hr/lb net thrust; or 1 lb fuel/hr/ eshp.
If the SI system used then the SFC will quote as kg fuel/hr/kN net thrust. But the Imperial system one most commonly used.
The SFC quoted for engines is normally that for maximum takeoff thrust setting unless otherwise stated, for example, cruise SFC.
The SFC of an engine influenced by its thermal and propulsive efficiencies and thus reflects the overall efficiency of the engine. SFC can use to compare the overall efficiencies of different engines.
The SFC of an engine initially decreases or improves as altitude increases. The reason for this is that when the temperature of the inlet air decreases it requires less work to compress the cold air than if it were hot.
This improves overall engine efficiency.
You will get more net thrust from a pound of air at altitude than at sea level.
The reduction in SFC with altitude ceases at the higher altitudes, around 36,000ft and above, and increases again as the engine efficiency reduces.
At these high altitudes, the density of the intake air will have fallen to such a degree that the engine RPM will have to increase to maintain the mass airflow.
Net thrust reduces to around 20% of its seal level ISA value at 36,000ft but the overall fuel consumption will have reduced to only 40% of its sea level value.
This means that the amount of fuel used to sustain a pound of net thrust will now higher than it would be at sea level.
For example, the SFC of a typical high by-pass turbo-fan at sea level maximum continuous would be 0.38 lb/hr/lbt. But would rise to 0.65 lb/hr/lbt at 36,000ft at maximum cruise condition.
The overall aircraft fuel consumption in relation to distance traveled. However, will be much reduced due to the high cruising speed achieved.
The subject of Specific Fuel Consumption at altitude treated differently in just about any textbook you pick up.
One will say it decreases, another that it increases, while yet another will say it does not change. I have even found one that says all three. If you hit with this question, it safer to say that the SFC initially decreases with altitude unless the questioner specifically refers to very high altitudes.
When the aircraft speed increases, the net thrust initially falls because of increasing momentum drag so the SFC will rise.
The intake ram ratio progressively recovers the lost thrust. But fuel flow then increased to match the increasing airflow so the SFC will definitely rise with increasing airspeed.
As the airspeed increases the propulsive efficiency of the engine also increase. So the overall efficiency of the engine will increase with airspeed.
The SFC will decrease or improve with increasing engine RPM because the compressor will be operating closer to its design speed.
The inlet air temperature will directly affect the SFC. If the ambient air temperature increases the SFC will also increase.
This will also occur if the engine anti-icing system used as it raises the inlet air temperature.
Increasing the compressor pressure ratio will also improve or reduce SFC.
Turbo-fan or by-pass engines produce cold stream thrust for less work because they move a large mass of air to a low final velocity.
These engines will have a lower SFC than comparable non-bypass engines