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Bleed air is air "stolen" from the compressor of the jet engine and has traditionally been used to pressurize the cabin. The alternative is to use a separate compressor just to provide cabin pressure, which is supposed to be more efficient.

Both systems consume energy to compress air. But bleed air uses a compressor that you already have, while the "bleedless" system requires adding an extra compressor which takes up weight and space. It also requires its own air intake (adding drag) and potentially a more powerful APU to supply power to the compressor. Given all this extra hardware, why is a "bleedless" design more efficient per passenger-km?

TLDR answer:

The fundamental physics is the same: Cabin air doesn't "care" if it is compressed by the main compressor or in an electric one; both of which require engine power.

But the engineering is very different:

  1. Only take the amount you need; don't waste excess bleed air (for some reason they can't slow the "bleeding" with a valve?).
  2. Wires weigh a lot less than pneumatic piping.
  3. Bleed air is provided at too high a pressure/temperature (several hundred C), the dedicated compressor may not need to compress the air this much?
  4. Fume events are eliminated.

More detailed answer accepted below.

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    $\begingroup$ Bleed air is lost air. The last thing you need on an efficient engine is a leaky compressor. $\endgroup$
    – user3528438
    Jul 24 at 6:14
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    $\begingroup$ Aside efficiency considerations, bleed air from the compressor might still contain pollutants from engine that have to be filtered-out and still getting in the cabin in tiny amount. I could clearly smell that in a couple occasions I flew in older jets. $\endgroup$
    – Léa Gris
    Jul 25 at 18:26

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The other answer is on the right track, but doesn't emphasize the real difference (IMO).

Let's say it takes 100 hp (or kW if you prefer) to compress the amount of air you need for a given purpose. The jet engine already has a very efficient compressor, it would be hard to do better with a separate compressor. Let's assume it takes the same power to do the job either way.

In a bleedless system, we have a generator connected to the engine that pulls off 100 hp of shaft work and converts it to 100 hp of electricity. We use that electricity to run a motor to compress the air. These devices each have an efficiency (power loss), but that isn't the important part. Fundamentally, that power comes from the turbine, it was extracted from the engine cycle.

In a bleed system, we pull a certain amount of compressed air out of the compressor ($\dot{m}_\mathrm{\,bleed}$). The compressor runs off of the shaft connected to the turbine, so it still takes 100hp of power from the turbine to compress the air. Roughly speaking, the power extracted from the engine cycle is the same.

However, in a bleed system, you're also stealing air from the cycle. Without bleed, that $\dot{m}_\mathrm{\,bleed}$ would continue to the combustor, would be mixed with fuel, burned, and then to the turbine where energy would be extracted (to help turn the compressor and other accessories) and then expanded out the nozzle producing thrust. I.e. the point of the engine cycle.

The air at the end of the compressor not only took power to reach that state, but it still has an intrinsic value to the engine. The more air flow through the engine, the more ability it has to produce shaft power and thrust. When you steal some of that air flow, you're penalizing the cycle.

In effect, bleed air puts two penalties on the engine (power extracted and mass flow stolen from the cycle). Whereas a bleedless system only puts one penalty on the engine (power extracted).

Most bleed loads do not operate at all times. You need a lot of bleed air during engine startup, but less at cruise. Some is a constant load (the bleed air used for turbine cooling) and some turns on and off -- air conditioning, engine start, etc.

An engine designed for a wide range of bleed across a wide range of operating conditions (very little to a great deal of bleed from flight idle to cruise and takeoff power) will have a lot of accommodations in the engine control -- and will fundamentally be a bigger, heavier, less efficient engine. The bleedless alternative is an engine with a wide range of power takeoff at the wide range of operating conditions (few hp to many hp at all those flight conditions). This is easier, simpler, and more efficient to achieve.

Bleed air for turbine cooling is a relatively constant load (easier to design for) and also keeps the engine a simple, self-contained unit. It is safer to cool an engine with bleed air than to introduce a separate electric compressor to do the job. However, cabin air isn't the same level of criticality and it is easier to add redundancy outside of the engine to make sure everyone has air to breathe.

The 787's 'more electric architecture' tried to move many systems to electric. This was accommodated by a large starter-generator attached to the engine. Instead of using bleed air (first from the APU, but then from the #1 engine) to start the #2 engine, it uses electricity (first from the APU, but then from the #1 engine) to start #2. This allowed the removal of a huge amount of high pressure pneumatic lines from the tail of the aircraft, out the wings, and to each engine -- and from one engine to the other. Those ducts were replaced with copper wires.

Engine start is the largest bleed load -- once that was removed (and replaced with sufficient capacity in a starter/generator (one electric machine (read mass) that does two jobs), then it makes sense to try to do other jobs using electricity instead of bleed.

So, while removing bleed air is a good thing for the engine -- this is also a systems wide tradeoff that can't just be considered in the context of cabin air, but also all the other jobs that compressed air (and electricity) do on the airplane.

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    $\begingroup$ "100 HP of shaft work and converts it to 100 hp of electricity" more like 95 HP of elec and then 90 HP converted back to work by the moter, which is still pretty good. $\endgroup$
    – Kevin Kostlan
    Jul 24 at 0:47
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    $\begingroup$ @KevinKostlan agreed -- there are efficiencies throughout the chain -- and they are pretty good. The main thing I wanted to do was to not focus on an argument of .95 vs. .97 and instead bring to light the changes to the engine cycle when you remove air from it. $\endgroup$
    – Rob McDonald
    Jul 24 at 5:42
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    $\begingroup$ "In effect, bleed air puts two penalties on the engine (power extracted and mass flow stolen from the cycle)." We pay twice either way: in both cases the engine powers the compressor(s) and misses out on the "hotel air" going to the cabin. In both cases each bit of hotel air requires itself and extra engine air to be compressed. The ordinary bleed-air design is "hiding" the secondary compressor as an over-sizing of the engine compressor. But your other arguments, as well as the safety of avoiding fume events, seem like good enough reasons. $\endgroup$
    – Kevin Kostlan
    Jul 25 at 0:03
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Using bleed air isn't free. In order to produce the same amount of power as an engine with no bleed air taken out, the engine needs to compress more air. This takes energy, and means more fuel used for the same amount of power output.

The extra compressor takes up weight and space. But so does all the ducting that moving bleed air around requires, and the intercooler needed to bring it to an appropriate cabin temperature.

Apparently, in at least some airframes, removing the bleed air system is enough of a gain that even after adding a dedicated compressor and air intake there is still an overall gain in efficiency.

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    $\begingroup$ It isn't free either way, but maybe the dedicated compressor is more efficient? $\endgroup$
    – Kevin Kostlan
    Jul 24 at 0:07
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    $\begingroup$ @KevinKostlan My guess is that the bulk of the savings is in removing the extensive ducting required to move the bleed air all through the aircraft. $\endgroup$
    – Chris
    Jul 24 at 0:13
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    $\begingroup$ Another point about removing the ducting: you remove a few severe failure modes. The bleed air typically comes out at around 300~400 deg C, which is hot enough to change the mechanical properties of structural components if a leak happens to hit the structure. Thus, any large aircraft contains several temperature sensors to detect these leaks (and command a bleed shut-off valve) and some diverters, to protect primary structure in case of a leak (these are specially difficult to integrate in the pylons). That adds to the overall weight of the bleed system. $\endgroup$
    – nonDucor
    Jul 24 at 12:27
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Given all this extra hardware why is a "bleedless" design more efficient per passenger-km?

Flexibility and adaptation

But this can only be understood not focusing on the engines only rather by looking at the big picture. Big picture which, in the case for example of a Boeing 787, consists in a shift from pneumatic system to "a greatly expanded electrical system generating twice as much electricity as previous Boeing airplane models"¹.

In a jetliner, the pneumatic system is used for several purposes but, for the sake of simplicity, let's consider only wing's de-icing and air conditioning.

  • In a conventional "bleedful" airplane, the pneumatic power is supplied by the engines; the engine is the boss: if the engine is running at its nominal power then also the pneumatic system is outputting its nominal power; if the engine is running at its maximum power then also the pneumatic system is outputting its maximum power. So what happens now if our airplane is smoothly flying at nominal power, in non-icing conditions and half empty? Well, the penumatic system is still outputting its nominal power but since there's no need to de-ice the wing nor to fully condition the air in the cabin, then not all of the air bleeded from the engines is used and it's simply wasted through modulating valves.

  • In a modern "no-bleed" jetliner, the pneumatic system is supplied by ad-hoc electrical pumps which run according to the pneumatic power needed at that moment. If again our flight takes place in non-icing conditions and with half of the plane empty then these pumps just deliver the requested level of pneumatic power and not the one dictated by the engine: there's no need to heat the wing and the air-conditioning system can be adjusted according to the number of passengers without any waste.

So the shift to a no-bleed system has a positive impact on efficiency due to its flexibility in adjusting its working conditions to the actual needs of the moment and not passively to the needs of the engines.

There's also a weight saving due to the removal of pneumatic ducts, heat exchangers, valves and protection systems. On the B787 for example, the APU doesn't supply compressed air anymore but only electrical energy: the ducts normally connecting the APU with the engines and the air-conditioning system are simply gone. This weight saving is obviously partially offset by the need of bigger and possibly a higher number of electrical generators plus relevant cables and control systems.

¹ source this Boeing pdf.

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  • $\begingroup$ No wasted air is a big one! $\endgroup$
    – Kevin Kostlan
    Aug 9 at 17:54

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