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Why do turboprops and jets burn so much more fuel per mile at low level?

I don’t really get the physics behind this.

Also why some turbine engines are less bad than others… shouldn’t they all be subject to the same physics?

Administrator
Shoreham EGKA, United Kingdom

Lots of factors (compressor and by pass design), but for a simple person like me:

- the Fuel Control Unit is reducing fuel as air density decreases
- much lower ambient temperature at intake improves specific fuel consumption
- higher mach/TAS in lower density air

Enstone (EGTN), Oxford (EGTK)

It would be interesting to see Fuel Flow / IAS, as it would remove the density issue.
I imagine it’s that (if you can fly much higher, for the same Fuel FLow / IAS, you get a much lower Flow / TAW), plus engine design which I imagine has an “optimum density altitude” which is set to what’s more typical of the cruise level of these machines.
@Ibra might have some good info on the subject.

Noe wrote:

It would be interesting to see Fuel Flow / IAS, as it would remove the density issue.

Thinking in term of Mach number removes both density and compressibility effects as well (unlike piston, a turbofan/jet for simplicity will “always fly in high density given by the mach number” irrespective of altitude or outside ambient air density)

RobertL18C wrote:

Lots of factors (compressor and by pass design)

=> Use of Mach numbers instead of IAS remove few of these, yes you still have section design but should not be impacted by altitude…

For jet engines (turbofans or jets), the design is mainly optimized for a fixed mach number and efficiency = airflow acceleration inside is determined by section changes (M>1 it has to diverge, M<1 converges)

So you are left with few major variables that RobertL18C mentioned,
- Bunch of Mach/IAS/TAS conversions that depends on height, density….
- Thermal efficiency: improves with low temperatures but this flatten at -60 at some point

You can add two, they are less relevant tough for CAT or GA,
- Aerodynamic drag: reduces with density, so fly high as possible but not much benefit at some point (unlike Balloon or Rockets you still need to support your weight via some “wing lift”)
- Oxygen proportion: that is not related to density, most jet engines without internal oxygen will shutdown at 40km but this will not have much impact on [0,10km] ranges, tough rockets have it and have divergent nozzles as well

For turboprops, it is very tricky as one has to look at “propeller efficiency” (decays with altitude) even if the “compressor density” in the engine remains the same…

I think for sub-sonic, the sweet spot is a height where T = -60 degrees and Mach close to 0.999 (or 0.93 if you allow for dive/cruise protections), higher/faster than that thing don’t improve that much…

Someone who was captain on a budget airline may be able to confirm?

Last Edited by Ibra at 27 Nov 20:40
ESSEX, United Kingdom

Jets produce less thrust as you go higher as they are normally aspirated. Less air, less fuel, less thrust. But drag declines with altitude as well essentially at a greater rate. Most jet engines have an optimum altitude for economy (lbs/nm) which for airliners seems to be in the mid 300s. Light jets at their operating ceiling.

EGTK Oxford

@Peter, I think you are asking why the fuel required per output horsepower decreases for a turboprop engine, while it is (nearly) constant for a turbocharged piston engine?

As I understand it, a turboprop engine operates with nearly constant pressure at the output stage, and when input pressure decreases, the engine compression ratio increases. Since any engine becomes more efficient when the compression ratio increases (that is why Diesel piston engines are more efficient than gasoline ones), the fuel consumption per unit of power decreases.

This compression ratio is in the work cycle of the engine, while the compression of a turbocharger happens before the work cycle, so at constant MP, the engine efficiency of the piston engine remains roughly constant.

The magnitude of the effect depends on what the compression ratio was in the first place, whether it is a free turbine or fixed turbine, and probably other stuff I have no idea about.

BTW – the steady increase in compression ratio also means that the engine is more efficient at high power than at low power, which is why turboprops are flown at much higher proportion of their rated power than piston engines, ideally at max power (which gives close to most HP per pound of fuel, but of course not best IAS per pound of fuel)

Last Edited by Cobalt at 27 Nov 21:19
Biggin Hill

Jets rely on a constant fuel to air ratio in order to burn most efficently. At high altitude, the amount of air decreases and therefore to maintain that ratio, so does fuel flow. At the same time, drag reduces at high altitude as well, so less thrust is required to achieve the same speed. At the same time, the reduced density results in a higher TAS for the same IAS/Mach number. All these effects together create an environment where the engine uses the least fuel for the maximum true air speed.

With turboprops thrust is produced by the prop and therefore prop efficiency is a huge input, that is why props usually have to fly lower. Some turboprops will achieve high altitudes too however.

Given the limitations of most pressurisation systems, which again take thrust from the engine, a sweet spot needs to be defined which satisfy all the factors in to where you want the airplane to do what. Normally for commercial jets this is in between 30-40k ft. If you want to fly lower as a rule (eg Cirrus Jet or other non RVSM jets) you need to adapt the engine to it or take performance penalties. Most biz jet engines will deliver the best power/use ratio at around 40k ft which is also an altitude managable by the pressurisation system.

Last Edited by Mooney_Driver at 27 Nov 21:54
LSZH, Switzerland

JasonC wrote:

Most jet engines have an optimum altitude for economy (lbs/nm) which for airliners seems to be in the mid 300s. Light jets at their operating ceiling.

What determines light jets operating ceiling, pressurisation? aerodynamics?

The 30-40k ft operating range for commercial jets is also related to many other stuff as well (e.g. time to hypoxia if oxygen fails or ironically 1st available FLs from Everest ATC ?), many commercial can still do well in terms of fuel efficiency flying up to FL500 but CAT cruise is limited to FL450 it seems…

Mooney_Driver wrote:

If you want to fly lower as a rule (eg Cirrus Jet or other non RVSM jets) you need to adapt the engine to it or take performance penalties.

Applies to commercial tubes as well, fly at FL500 get a narrow turbojet, fly a bit lower? get a big turbofan, very low? get a turboprop

ESSEX, United Kingdom

Ibra wrote:

What determines light jets operating ceiling, pressurisation? aerodynamics?

Assuming engine power is sufficient in light jets, delta p (pressurisation) and other environmental issues determine certified ceiling. The CJ3 for example can get to 480 but is certified to 450. But as you say time to FL100 is also an issue.

Last Edited by JasonC at 27 Nov 23:40
EGTK Oxford

It’s all about optimizing for different purposes and different mission profiles IMO. Take a marine turbine living all it’s life at sea level. At cruise condition it’s almost as good as a (poor) diesel, but when going slower it’s insanely poor. A piston can maintain the compression ratio no matter the output (or speed), while a turbine looses compression ratio when the output (speed) decreases.

There is no need for a turbine to have low efficiency at low level, but an airplane is optimized for high alt and flying fast, and so is it’s turbine.

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