Fuel SG vs jet engine burn efficiency

[lion-g]

Hi guys,

Was just wondering, how does the density of the fuel affect the efficiency of the engine?

Correct me if I am wrong, from a physic view point, the fuel flow into the engine for combustion is just volumetric. How does SG comes into play and affect our fuel burn and fuel in tanks?

From what I have gathered, average jet fuel density is highest in west coast US and lowest in Asia.

Thank you.

Cheers,
Lion-g

[KRviator]

For a higher density fuel, would that not spin the engine at the required RPM for a lower fuel volume? Take two theoretically identical engines and run them side by side. The FADEC will schedule whatever fuel flow it requires to meet the thrust or torque demand, and with a higher density fuel, this flow will be lower due to the higher density/BTU's in that quantity of fuel.

As for how it affects things, the higher the SG, the more weight you have in fuel, but the lower your cruise fuel flow will be, so taking it to the extreme, you could argue you need less fuel for that sector. Is it enough to ever make a difference?

[tdracer]

There is a usually a very close correlation between the density of the fuel and the heating value of the fuel - i.e. higher density fuel has more energy in direct relationship to it's density.

As KR notes, in the FADEC age, the FADEC is going to run closed loop on whatever parameter it's controlling to at the time (e.g. N1, N2/N3, burner pressure, EPR). Even during transients the FADEC will normally control to "N2(or N3) DOT" - that is the rate of change of the higher pressure rotor speed, or sometimes to 'N1 DOT'.
However there are some exceptions. The FADEC will used max and min fuel/air ratios as a limiter (e.g. fuel flow/burner pressure or similar) for protect against surge (accel) or flameout (decel) - that's why an old, tired engine will accel slower even with FADEC - it'll hit the max fuel/air ratio that the FADEC will allow. And that's where fuel density can come into play since the FADEC is using the actual fuel flow (based on the metering value position), not the fuel energy to determine those limits.

Pre-FADEC, everything was based on the fuel metering valve position, so fuel density would directly affect the accel/decal rates of the engine, and possibly even the throttle position needed for a specific EPR/N1.

There are adjustments on the fuel control units to allow adjustment for different density fuels. They are rarely used outside of the shop unless you're switching between significantly different fuels - e.g. going from Jet A to JP4 or visa-versa. Mechanics will sometimes play with the density adjustment to help an old, tired engine meet the required acceleration requirements, but that is not a recommended practice.