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I was wondering if I reduce the power consumed by the aircraft, would the generator produce less power and if that would save some fuel.

quiet flyer
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Inah Guimarães
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    Since all the energy available comes from the fuel, reducing the electrical load will save fuel. However, since you can't touch the aircraft systems, that leaves the passenger cabin. Forcing passengers to sit in the dark, eating cold food, with no in-flight entertainment, USB chargers or WiFi will likely cost more in lost revenue than would be saved in fuel. – CatchAsCatchCan Jun 03 '20 at 01:10
  • What type of aircraft? – Dean F. Jun 03 '20 at 02:42

1 Answers1

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Yes, in theory, but you would have to do a total energy analysis to decide if it's worth it. On the surface of it, generators consume fuel not being made into thrust. Watts being made to power things is watts not going out the tailpipe.

A good example of the potential benefit is the switch to LED cabin lighting. A 40w incandescent light is more or less a 35w heater that makes a bit of light. LED lights drop the power consumption by 80-90%, although if you need heat in the same space, you have to replace the heat being made by the lights, so if you did an overall energy balance analysis in that case, you might find the actual saving is negligible for the period when you need to heat the cabin in cruise at altitude (because the reduction in heat from the lights has to be replaced by more bleed air demand).

If you need cooling, it's the opposite because the heat from incandescent lights is working against you, and it's a huge benefit to get rid of the heat produced by the lights, which reduces cooling demand.

So overall, there is probably some fuel burn improvement over time switching to LEDs, very small, but something (say, 200 30w lights consuming 6kW, replaced by 200 5w LEDs consuming 1kW, a 6hp saving, maybe 4lbs/hr - not nothing, but, but as I said you'd have to take more bleed when heating, offsetting a lot of that improvement, and you'd probably have to spread it out over quite a while to get serious money out of it).

Most airlines that switch to LEDs do it mostly for reliability reasons, where labor hours savings and parts costs make a stronger financial business case than just a few pounds of fuel spread out over time, although the fuel-saving does add to the total.

Another area would be advances in avionics that reduce power consumption (you can just imagine the drop in energy demands when avionics went from vacuum tubes to solid-state in the 50s). Even there though, it's the same issue with heat that is not really wasted when the cabin requires more heating than cooling, that steals some of the benefits you thought you were getting.

Reducing electrical demand with efficiency improvements also helps with weight savings (smaller, lighter generators).

On the other hand.... the 787 blows all that out of the water by going to the other extreme, making all sorts of services electrical, requiring really massive generators. I'm not sure that there is a net energy benefit to using electricity to replace bleed for anti-icing for example, and I think the main benefit is reliability (bleed air systems are a massive maintenance and reliability hassle).

Mohsen Alyafei
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John K
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  • Note that the air often does require cooling even in cruise. Compressing the air to maintain cabin pressure is roughly adiabatic, so it also heats the air, and because the atmospheric lapse rate also corresponds to adiabatic expansion, you'll end up with approximately the same temperature as at 8000 ft. If it's hot on the ground, it may easily be more than you want in the cabin. – Jan Hudec Jun 03 '20 at 17:33
  • … but then, note that while heating the air does use some extra bleed air, and therefore energy from fuel, cooling it does not, because that is done by simply running it through heat exchanger with the ambient air (well, may be a little; I am not sure what effect it has on the pressures in the air cycle machine). – Jan Hudec Jun 03 '20 at 17:35
  • Well I think when you take in the interior volume that has to be heated and consider the high velocity -50C flow outside, with maybe 1/4" of insulation thickness, you have a net heating requirement and there is very little flow through the air cycle machines, only what is required to bring the raw bleed temp down to some value that is higher than ambient, and the heat exchangers will do most of that. I've flown in airplanes at altitude with no interiors OR insulation above the cabin floor (corporate prior to completion center) and the cabin temperature at FL340 or so was about -20C. – John K Jun 03 '20 at 18:02
  • On the other hand, the temp of the bleed is the temp of the bleed, and it is quite hot, and much of the excess heat is being dumped overboard in any case, with the flow being only what is needed to keep the pressure hull pumped up, so perhaps the difference is negligible. – John K Jun 03 '20 at 18:05
  • The bleed air is very hot, but also higher pressure than the cabin, and when it expands to the cabin pressure, it cools again. So I don't think that much heat actually gets dumped. – Jan Hudec Jun 03 '20 at 18:43
  • When the air goes through the ACM it goes through several heat exchangers and/or precoolers on most systems. They are bootstrap systems that remove some of the heat while the pressure is high with a precooler, boost the pressure and temp some more (ACM compressor) pass through more heat exchangers, and you end up with high pressure air that has had most of its heat removed. When it finally goes through the ACM turbine, that's when the expansion happens and you get the final temp drop. If you put all the air through an Air Cycle Machine, it's capable of dropping the output to -30 to -40C. – John K Jun 03 '20 at 21:03
  • But some of the air bypasses the ACM and stays hot, and is blended with the cooled output to get the final temp going to the cabin. The controller juggles this balance of direct hot air and cooled air to get the cabin duct temps it wants. – John K Jun 03 '20 at 21:05
  • I am not sure how much it manages to expand the bleed freely rather than adiabatically to keep it hot, but I suppose it can do it enough to be useful, in which case there is probably indeed little difference. – Jan Hudec Jun 04 '20 at 05:26
  • Thank you all for the comments. When I asked this question, I was thinking about the retrofit of cabin ilumination indeed. I'm very interested in how you estimated the 4lbs/hr. Could you give me an exemple in how did you get to that number, utilizing an common aircraft like 737 or A320. Thanks in advance. – Inah Guimarães Jun 04 '20 at 22:01
  • I just converted wattage to hp (746 w per hp), and used a number typical for the fuel burn of turboprops, which is in the region of .6 lbs/hp/hr. That would be a number you might get w a Q400 Dash 8 if it had that many lights. For a turbo fan you'd have to do a conversion using thrust to watts. You'd have to factor in the losses from the power conversion and transmission losses, maybe 5 or 10% or something (I have no idea exactly what), which might reduce the benefit a little. The CRJ "Next Gen"s went to LED lighting and the power drop was significant, but the main attraction was reliability. – John K Jun 05 '20 at 00:16