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I know that while in-flight the electricity is produced by the engines.

What happens if every possible system requiring electricity is in the on state? This should require extra electricity.

Does this usage of electricity increase the fuel consumption by any means?


*Edit to include the additional question from the comments section:

Given that the engines require fuel to produce electricity how big is the impact on fuel consumption, speaking of "mainstream" airliners like the A320?


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2  
If you have ever driven a car in a tropical region during summer, you'd notice that with the air-con turned on, fuel consumption per mile goes WAY up. – kevin Mar 16 at 15:41
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You might get better answers if you specify large transport AC or GA aircraft. I suspect the answer might be very different between a 737 and a C172. – PJNoes Mar 16 at 16:09
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@kevin Its not what you think. The fuel consuption of a car increases while the A/C is on is because the compressor is connected directly to the engine. Info from our sister site. – RogUE Mar 16 at 16:20
up vote 29 down vote accepted

Short answer

In flight, unless you use external energy sources like the Sun, each change in the use of electricity leads to a proportional change in the quantity of fuel burnt in the engines (or in the APU).

On an aircraft, generators only convert fuel potential chemical energy into something else, usually electricity, hydraulic energy or pneumatic energy.

There is no other source of energy:

  • The batteries themselves need to be charged, charging burns fuel.
  • The RAT (ram air turbine) uses energy from the relative wind created by the engines thrust. If the RAT is used because the engines and the APU are down, the relative wind is due to the loss of altitude created by the gravity. This kinetic energy is the restitution of the potential energy accumulated during the climb with the engines, so with fuel.

Details

Maximum power of the electrical equipment

The electrical power needs of the whole aircraft can be completely met by a single engine generator, or by the APU. On the A320 family, the APU is an APS3200 delivering about 90 kVA.

APU fuel consumption

From this PPRuNE discussion, an A320 APU may consume 130 kg per hour on the ground, and 51 kg per hour at FL300. However the APU provides at the same time electricity and bleed air energy.

APU fuel consumption for electrical energy generation

However the fuel quantity used for the sole electricity generation may be approximated:

  • Maximum power required by the aircraft equipment → 90 kVA AC.
  • Assuming a mean cos φ = 0.8 → Apparent power = 110 kW DC.
  • Assuming 25% efficiency for the system (gas turbine + generator) → Fuel power required 440 kW.
  • Fuel energy required → 440 kWh per hour.
  • Converting to joules (1 MJ = 0.28 kWh) → 440 kWh ≈ 1,570 MJ.
  • Kerosene specific energy = 43 MJ/kg → Fuel quantity burnt in an hour = 1,570 / 43 = 36 kg (45 L)

So approximately 36 kg of fuel are required per hour to provide for electric energy on an A320.

Note that in flight all engines generators will be active and will share the total demand (feeding different buses for safety), and the APU will likely be inactive. The fuel consumption should be more or less the same, regardless of the number and nature of the active generators. An inactive or unloaded engine generator can be seen as not impacting the fuel consumption for practical purposes.

Engines burning the maximum rate of fuel

Engines have a maximal power. When the engines are already at full thrust, increasing the electric load will divert an additional amount of energy to the generator. This quantity being removed from the energy usable for aircraft propulsion, the aircraft will slow down.

(Updated, taking into account the multiple helpful comments -- Thanks!)


Bonus

Breathing increases fuel use too:

  • Each time a passenger breathes, they reject heat and CO2 which must be evacuated out of the aircraft.
  • Some quantity of air must be replaced by new air by the conditioning system.
  • This system uses pneumatic or electric energy, and again fuel must be burnt to create this energy.
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3  
Any ideas how big the impact is? Ideally speaking of "mainstream" aircraft like the A320, 737 or such. – Phantomazi Mar 16 at 11:48
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The engine has the limited efficiency. It probably needs to burn at least twice as much fuel to produce that amount of energy. – h22 Mar 16 at 12:28
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The efficiency of a jet turbine is actually about 40-45%. Everything you want to know about APUs is here: b737.org.uk/apu.htm – Paul Smith Mar 16 at 15:00
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737 APU under full load uses 240lb/hr (130l/hr) to generate 90VA. so you're wrong by factor of 13 : D aviation.stackexchange.com/questions/8429/… – Agent_L Mar 16 at 16:10
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@HenningMakholm: VA and W are different units, due to the exisence of a phase difference between current and voltage in a non purely resistive circuit in AC. This involves reactive power and the complex number set. – mins Mar 17 at 9:20

Yes, because (like a car for example), the aircraft is a closed system, and all energy must be provided internally. So, in flight, that energy must come from the engines, and therefore, the engine must either slow down, or use more fuel.

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This is a fantastic answer, as this explains the reason in such simple terms. +1 – Ashish Ahuja ツ Mar 16 at 15:12
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@DavidRicherby, Interesting. I've never seen an electric compressor on a car. Now I really want to know if that exists outside of a electric or hybrid car. – JPhi1618 Mar 16 at 18:24
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@Mohair, no not really. The electrical generator under load acts like a brake on the shaft. If that brake is released, the engine speeds up and produces more thrust. There's no energy "sitting around" waiting for something to use it. – BowlOfRed Mar 16 at 19:45
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@Mohair Suppose the engines were producing more energy than they needed to. Then that would be a waste of fuel, so they'd reduce fuel flow to the engines until the engines weren't producing more energy than they needed to. – immibis Mar 16 at 21:07
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@Mohair Do you understand that the current flowing in the coils produces a magnetic force that slows down the spinning magnets? (If you have nothing connected to the generator, you get no current and no opposing force, but you're not producing electricity either) – immibis Mar 16 at 23:00

It is just not possible otherwise but the effect should be somewhat below 10 % even when maximal amounts of electricity are consumed. Much more power is required and consumed just to fly.

A generator axis must rotate in order to produce electricity, and the resistance to rotate is proportional to the generator load (when the generator is not loaded, only friction remains). That's physics.

An engine that is coupled to the generator will meet more resistance and should consume more fuel when powerful energy consumers are connected to the generator.

However if the A320 generator requires 90 kW (as from another question), it does not make significant part of the engine power. An engine of 747 produces 16275 Kw and even turboprop engines are rated of the order of a few thousand kW (3,362 kW for C-130 here), so the generator is unlikely to have significant impact on the power produced even when running at the full load.

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