787 air condition/fuel burn

[ElNull]

I'm aware that the 787 doesn't use engine bleed air for air conditioning, instead, they use electrical CACs (Cabin Air Compressors) to supply air to the packs. My question however is, does the CACs/packs have any effect on the fuel consumption?

[tdracer]

Short answer is yes. Driving the gearbox loads - be it hydraulics, big electrical generators, or something else - take energy from the core of the engine and hence increase fuel burn.
Part of the thinking that went into the no-bleed architecture of the 787 was that - by not having to account for high pressure bleed air from the core - the core could be better optimized and hence for fuel efficient. But it didn't really pan out that way - in fact the GEnx-2B engine on the 747-8 uses the same high pressure core as the GEnx-1B engine on the 787 - even though the -2B has conventional engine bleed to run the packs.

[Check Airman]

Is it fair to assume the performance hit is less than the engine bleed design though?

[tdracer]

When you add in all the extra weight of those big electrical generators, compressors, etc., it's pretty much a push. As I noted, the promised optimization of the engine cycle simply didn't happen - IIRC GE ended up even using the same HP compressor casing on the GEnx-1B engine as on the -2B, just with the bleed ports blanked off - and that's where the Boeing R&D guys thought they'd get a big benefit. Worse, those big electrical generators are a nightmare for the accessory gearbox designers - in fact several of the early in-flight shutdowns on the GEnx-1B were due to gearbox failures forcing a major design change.
There is a good reason why they didn't use bleedless architecture on the 777X - it simply isn't worth the trouble.

[oceancrosser]

As always tdracers explanations are excellent. Would it be fair to say that the SFC of the -1B and 2B ended up being not too different, but with the added headache of the generators and heavy electrical stuff on the 787? I have always found it very strange how heavy the 787 ended up.

[tdracer]

The performance types tend to be pretty tight lipped about the various engine TSFC numbers (among other things, it's considered to be highly proprietary). But best I was able to gather, the TSFC numbers between the -1B and the -2B are pretty close with a small advantage going to the -1B, primarily due to it's larger fan (i.e. higher bypass ratio). Both the -1B and -2B were a bit short on fuel consumption numbers at Entry Into Service. But GE quickly put together a "PIP" (Performance Improvement Package) which improved the fuel burn by ~2% which got them to the original target fuel burns.

[Check Airman]

Thanks tdracer. Always appreciate your insight.

[ElNull]

Thanks. I thought it has to do with the engine's variable frequency generators, i.e. CAC's would demand a higher frequency (which will lead to higher engine RPM?) i.e. more fuel burn.

[tdracer]

The new generation of turbofans have big, relatively slow turning fans and small, fast spinning cores. There are a performance and efficiency advantages of spinning a turbine engine faster (hence the carrot of the geared turbofan - spinning the turbine fast while the big fan turns slowly where it works best). So spinning the core fast enough to properly drive the generators is generally not much of an issue.

BTW, when measuring TSFC, normal procedure is to take as much of the accessory loads of the engine as practical - i.e. zero bleed, minimal electrical power off take, etc. The idea is to measure the pure TSFC and aircraft drag. Of course, measuring in-flight thrust is something of a black art (one of my first jobs as a fresh faced engineer right out of college was determining the coefficients for calculating in-flight test for the 767 flight test program). Arguments between the engine company and the airframer regarding thrust and drag are pretty common - i.e. is it the engine's fault or the aircraft's fault that it's not making the desired fuel burn numbers...
NAMS (essentially the aircraft level miles per gallon) is different - with the engines carrying real world accessory loads - to measure representative aircraft fuel mileage numbers.

[Checkboard]

Separate air compressors also avoids some of the air quality perception problems that plagued the 757.

[Fursty Ferret]

Frequency-wild AC is rectified and then inverted by the CSMCs for the CACs, so is independent of engine speed.

[ElNull]

That make sense, but wouldn't that contradicts what has been said in this thread regarding that the CACs uses energy from the engines and increase fuel burn?

[EEngr]

Indirectly, yes. The electrical power needed to drive compressors and fans does indeed come from the engine and impact fuel burn. But the conversion of the engine gearbox energy through the variable frequency generators and then through variable speed motor controllers in the CACs can be done very efficiently (high 90%). And the ability to run fans and compressors at variable speeds can improve the air conditioning cycle efficiency itself.

The same technology is used in home and commercial air conditioning units. Usually referred to as "inverter technology", the compressor is run at the optimal speed for the cooling demand.

[jetpig32]

The 787 has a feature called "Smarter ECS Mode". It will lower the speed and output of the Cabin Air Compressors based upon the number of cabin occupants entered on the cabin control panel. So, in theory, it can reduce electrical load to provide the correct amount of airflow needed. There is reference to this system in a Japan investigation report. We had issues early on with low surge margins in the cacs with low occupants.
I am not allowed to hyperlink due to being new, but you can search "japan AI2020-7" and find the pdf.

In summary, “Smarter ECS Mode” is an operation mode to ensure that airflow across the CAC can be reduced by lowering the CAC rpm so that the PCU can generate minimal Pack Flow required for cabin air according to the number of persons on board input in the Cabin Attendant Panel (CAP).

[TURIN]

ElNull

A generator becomes very hard to turn under high load. So much so that GE (and I assume RR too) had to ensure the engine would not under rotate... below idle speed... when there is a high demand from the electrics. EG engine start with supply from the other running engine. Essentially, I was told, the FCU commands more fuel to maintain idle when under load.

[tdracer]

The external gearbox loads on the HP rotor are quite large. IIRC, we used 100 hp extraction from the HP rotor to simulate the load from the IDG for the 767 and 747-400 engines - and that was only 90 KVA max. The 787 can be as high as 350 KVA/engine, so just that much higher. Of course it's going to use more fuel to drive that sort of external load.
The GEnx FADEC uses a number of idle control schedules - N2, minimum burner pressure (PS3), min fuel flow, and most vary with altitude, airspeed, and temp. The FADEC selects the idle schedule that results in the highest idle, and uses that to schedule fuel flow, so varying gearbox and electrical loads are accounted for automatically. N2 rotor speed is typical on the ground (but not always - min PS3 sometimes comes into play) but up and away it varies considerably with flight conditions. This is a pretty typical design for all FADECs - so I assume Rolls would be similar (obviously N3 instead of N2) but I don't have first hand knowledge.