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I know turbofans are less efficient than propellers due to accelerating a smaller mass of air faster. So my question here is not comparing turbofans vs other engines, but compare how inefficient is a turbofan operating at take off and low speeds compared to that same turbofan in its optimal conditions?

In other words, how much is the efficiency loss? Are we talking about 5% or about 50%?

Also, why exactly that loss of efficiency happen (again, compared to the same engine at optimal cruise conditions)? Is it just the speed differential between the exhaust and aircraft airspeed, or is it something to do with the compressor/turbine efficiency?

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    $\begingroup$ Can you define how you quantify efficiency in this context? Fuel efficiency? Thrust generated per lb of fuel per hour? $\endgroup$
    – Ron Beyer
    Jun 6, 2016 at 18:00
  • $\begingroup$ Yes, thrust per unit weight (or volume) of fuel $\endgroup$
    – Southbob
    Jun 7, 2016 at 12:26

1 Answer 1

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To compare the efficiency we need to use the thrust specific fuel consumption (TSFC). It's the fuel per unit time per unit thrust.

enter image description here
(Georgia Institute of Technology)

The lower the y-axis value (TSFC/TSFC0), the more efficient the engine is. TSFC0 being TSFC at sea-level and standstill. Speed increases TSFC, whereas altitude (more precisely: decreasing ambient temperature) lowers it a bit.

Taking the extremes from the example above:

  • M0.4 at sea-level is a ratio of 1.45
  • M0.8 at 11 km (FL360) is a ratio of 1.675

On ground a turbofan engine is more efficient than in cruise when it comes to fuel consumption per thrust given.

But if you're comparing different airplanes, then the distance covered is also taken into account.

Fuel consumption per [distance] can be a more appropriate comparison for aircraft that travel at very different speeds.

enter image description here
(Source)

For supersonic jet engines it's different as shown above.

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    $\begingroup$ Beware, the graph does not give TSFC values, it gives TSFC/TSFC₀, i.e. multiple of the value at stand-still at sea-level. The actual values in lb/(h·lbf) are around 0.3–0.4 at static sea-level and 0.5–0.6 at cruise. $\endgroup$
    – Jan Hudec
    Jun 22, 2016 at 18:01
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    $\begingroup$ I'm somewhat confused. If I understand correctly, jet engines get less efficient as alt and speed increases in terms of TSFC, but jet planes get more efficient in terms of MPG, simply because the plane is moving faster? $\endgroup$
    – Southbob
    Jul 9, 2016 at 0:13
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    $\begingroup$ TSFC is not the whole story. At zero speed, thrust is doing no work. Propulsive efficiency is proportional to true airspeed * TSFC. $\endgroup$
    – pericynthion
    Sep 26, 2016 at 4:31
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    $\begingroup$ Both altitude and speed increase the TSFC: No, if you look at the same Mach number, a higher altitude shows a lower TSFC. Speed increases TSFC whereas altitude (more precisely: decreasing ambient temperature) lowers it a bit. $\endgroup$
    – Peter Kämpf
    May 29, 2017 at 20:00

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