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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?

Phantomazi
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    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 '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 '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 '16 at 16:20

3 Answers3

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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.
Community
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mins
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    Any ideas how big the impact is? Ideally speaking of "mainstream" aircraft like the A320, 737 or such. – Phantomazi Mar 16 '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 '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: http://www.b737.org.uk/apu.htm – Paul Smith Mar 16 '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 http://aviation.stackexchange.com/questions/8429/does-the-apu-consume-a-lot-of-fuel-compared-to-a-jet-engine?rq=1 – Agent_L Mar 16 '16 at 16:10
  • @PaulSmith That's pretty amazingly efficient. Modern ground-based powerplants that generate electricity for profit tend not to get that high! – Cort Ammon Mar 17 '16 at 02:49
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    @Agent_L: Your source says 90 kVA, not 90 VA -- a significant difference. (And it's strange to me why it doesn't just say 90 kW). – hmakholm left over Monica Mar 17 '16 at 08:53
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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 '16 at 09:20
  • You don't mention the difference between a fully loaded generator, and the more typical case where all redundant generators are sharing the load (more specifically he asks if you turn something additional on, does it matter, or is the generator using a fixed amount of fuel anyway). Also, as part of your bonus answer, this "wasted" power can be recaptured in some cases if needed (turn the bleeds off for more thrust) and is significant enough to be part of the procedures developed by the manufacturer (takeoff and two of three engines inoperative on the aircraft that I fly). – Lnafziger Mar 17 '16 at 09:25
  • @Lnafziger: That's right, the APU won't probably be used in cruise, and the engines generators will feed different buses. But overall the max power required won't significantly change, the fuel consumption may be a little bit greater (I've updated the answer to clarify, thanks!). The bonus was for fun and food for thought. Still, when the packs are off, no energy is tapped from the engine, heat accumulates in the cabin, but is not reused per se. Or is it in some designs? – mins Mar 17 '16 at 10:15
  • @mins: The reactive load is not something the generator needs to "produce", though. It ought to lead to no average torque on its drive shaft. – hmakholm left over Monica Mar 17 '16 at 10:49
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    @HenningMakholm Yes, the very reason that electricians worry so much about the power factor is that the generator HAS to produce this power. Power that can't be used anywhere but is wasted in the wires of the entire system. – Agent_L Mar 17 '16 at 12:18
  • @Agent_L: Shouldn't power that is wasted in the wires be ohmic load? Otherwise there's no net energy loss over a cycle. – hmakholm left over Monica Mar 17 '16 at 12:20
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    @HenningMakholm "Waste" power (current to be more precise) has to be sent to the receiver, but the receiver doesn't do useful work with it, because voltage isn't there at the moment. But the current flows, so yeah, it is lost as ohmic load in the wires. That's the problem - wires act as the ohmic load instead of the intended receiver. The system as a whole of course can't lose any energy. But it goes into ohmic heating of the wires instead of useful work. Or, to be more precise - into more ohmic heating of the wires than it would happen with optimal power factor of 1. – Agent_L Mar 17 '16 at 12:24
  • @mins Well, if it doesn't tap energy from the engine, then it is available to be used elsewhere (more thrust). That's the whole point of a "bleeds off takeoff". – Lnafziger Mar 17 '16 at 20:59
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    @Phantomazi the first result I found quoted 2500 kg/h flying fuel consumption for a 737-800, so the APU would be using about 1.5% of it max. Or the Amdahl corrolary: no reduction in electrical use can make the plane more than about 1% more efficient. – hobbs Mar 17 '16 at 21:20
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    It's even true in a car. Your car gets less gas milage with the headlights on than off. – user3344003 Mar 18 '16 at 00:14
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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.

jr593
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    This is a fantastic answer, as this explains the reason in such simple terms. +1 – Box Box Box Box Mar 16 '16 at 15:12
  • Welcome to the site, user14035! This is a nice little answer that hits the heart of the matter. If you'd like to stick around, I suggest registering your account so you can pick a more memorable name. I can't remember if you can register an existing account or if you need to create a new one; if you do need to create a new one, contact the site admins to get the two accounts merged. – David Richerby Mar 16 '16 at 16:04
  • And this is exactly why your car has higher fuel consumption when you run the aircon. – David Richerby Mar 16 '16 at 16:04
  • @DavidRicherby, you're not wrong, but I wouldn't say "exactly" because the A/C on a car is a much more of a direct mechanical drain on the engine rather than an electric drain. – JPhi1618 Mar 16 '16 at 18:04
  • @JPhi1618 Wikipedia says that the cooling compressor of car aircon systems can be powered either electrically or by a belt from the engine. I agree that, in the latter case, it's a direct mechanical drain; I'd assumed it was always electrical. – David Richerby Mar 16 '16 at 18:21
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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 '16 at 18:24
  • Is there not excess energy from the engines that could be put to use generating electricity without requiring any more fuel? It doesn't seem like too hard of a problem. – Mohair Mar 16 '16 at 19:34
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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 '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. – user253751 Mar 16 '16 at 21:07
  • @immibis Show me an engine that doesn't produce more energy than it needs. There's always an excess. Put some magnets on the shaft, surround them with coils and you have a generator. The energy drain that would cause is probably irrelevant. – Mohair Mar 16 '16 at 22:58
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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) – user253751 Mar 16 '16 at 23:00
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    @Mohair The "excess" energy released by an engine goes into vibrations and heat loss. If there were a currently-feasible way to capture this (i.e. one that cost less than the fuel savings,) we'd already be using it. The engines on an airplane are not producing significant excess thrust... if they were the plane would climb and/or accelerate. – reirab Mar 17 '16 at 02:12
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    You can never have an excess of energy, all energy produced must be consumed either as useful work (moving the vehicle) or wasted work (vibration and heat loss). If you put more in than you get out the difference will accelerate the vehicle until air resistance etc increases to the point that the system is still in balance. – David Waterworth Mar 17 '16 at 03:55
  • @Mohair: They use pretty much exactly that procedure for eddy current brakes... – DevSolar Mar 17 '16 at 15:04
  • @Mohair, think of this comparison: You can run with a rucksack containing 30 kg of stones. You currently run with 10 kg, you burn a n calories. Your load is increased to 30 kg. Because of this additional load: 1/ you burn more calories and oxygen per minute; 2/ you will be exhausted more quickly; 3/ you won't be able to run on the same distance. While your power is 30 kg, the work you deliver is your actual load multiplied by the distance it is carried on. Energy used is not proportional to the available power, but to the work actually done. – mins Mar 18 '16 at 07:55
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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.

h22
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