Leeham News and Analysis
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Leeham News and Analysis
- FAA understaffing affects air operations oversight; NTSB falls short, too January 13, 2025
- Bjorn’s Corner: Air Transport’s route to 2050. Part 4. January 10, 2025
- The market is ready for a 787F. But is Boeing? January 9, 2025
- 2025 remains another year for recovery in commercial aerospace January 6, 2025
- Boeing touts safety progress but opposes SPEEA initiative January 3, 2025
Bjorn’s Corner: New engine development. Part 16. Compressor air use.
By Bjorn Fehrm
July 19, 2024, ©. Leeham News: We do an article series about engine development. The aim is to understand why engine development now has longer timelines than airframe development and carries larger risks of product maturity problems.
To understand why engine development has become a challenging task, we need to understand engine fundamentals and the technologies used for these fundamentals.
We have covered the problem areas of a compressor and how these achieve power-to-air-pressure conversion efficiencies of over 90% by using advanced 3D airflow modeling. Now, we look at the users of the air from the engin’s compressor.
Figure 1. The gas turbine cycle and its parts. Source: Rolls-Royce: The Jet Engine.
Compressor air use
The engine compressor’s primary purpose is to deliver highly compressed air (more than 40 times higher than ambient air pressure) to the combustor. However, the compressor is also the source of all compressed air used in the engine for cooling, sealing gaps, making sure gases flow in the correct direction, and serving the aircraft’s air conditioning and de-icing with high-energy air.
The amount of air tapped from compressor stages for cooling and other purposes can exceed 20% of the core flow. Here is a list of all the uses of compressor air, with some of the flow paths shown in Figure 2:
- For the cooling of hot engine parts, static channels, and their vanes, and rotating discs with their blades. We will look closer at how this is done when we analyze the turbine section.
- For functioning as part of a seal, the air enters with overpressure on one side of the seal to ensure there is a positive flow of air to stop, e.g., combustion gases from getting into areas that contain combustible oil or oil residues. When this doesn’t work, we have oil fires in the middle of the engine, which have destroyed several engines.
- For clearance control of rotating parts. This is achieved by channeling hot compressor air blended with cooler air through heating/cooling pipes over the turbine’s housing, Figure 3. By regulating the temperature of the turbine housing, the housing’s circular expansion is controlled, and by it, the stator clearance to the turbine blade’s tips.
- For deicing the engine’s intake casing with its vanes.
- Through the aircraft bleed air system:
- Deicing of the aircraft’s wing and empennage leading edges,
- Functioning as the air source for the aircraft’s air conditioning system
- Starting a second engine via its air starter.
The air is always tapped at the earliest possible place in the compressor, which has the appropriate pressure level. The least amount of energy is then invested in the air, and it has the lowest possible temperature.
Figure 2 describes the tapping of air from the high-pressure compressor of the CFM56-5.
Figure 2. The high-pressure compressor of CFM56-5A with tapped air flows. Source: CFM.
As Figure 2 shows, air is tapped from several places on the compressor. This is easy to achieve because each compressor stage’s seal area can be opened to the outer casing at different stages of the compressor.
Figure 3 shows a compressor with seal holes leading to casing pockets with outlets A and B at the sixth and ninth stages (air is tapped from one or the other depending on compressor RPM and, therefore, the stage pressure level).
Figure 3. The compressor cross-section (lower part) has A and B taps of compressed air. The hollow compressor shaft is at the top. Source: Rolls-Royce “The Jet Engine”.
Bleeding air for different uses lowers the compressor’s performance and, hence, the engine’s performance. When bleed air is tapped for both cabin air conditioning and de-icing, it can increase the engine’s fuel consumption by up to 5%.
Active Clearance Control
The CFM56-5 has up to three active clearance control systems:
- For the high compressor rotor (a system called RACC).
- For the High-Pressure Turbine (HPT)
- For the Low-Pressure Turbine (LPT).
All these mix colder air from, e.g., the low-pressure compressor area with hotter air from different parts of the compressor and route the mix to tubes or an annulus in the casings that shall be temperature-manipulated to control stator casing expansion-contraction and thus the blade tip clearance.
Figure 4 shows the turbine Active Clearance Control (ACC) for the Pratt & Whitney V2500, using tubes wrapped around the turbine housing.
Figure 4. The IAE V2500 engine with its Active Clearance Control of the turbine section. Source: IAE.
Aircraft bleed air system
The bleed air tapped from the compressor is too hot to be routed around the aircraft. Therefore, it’s pre-cooled in a pre-cooler that uses the fan’s airstream for cooling.
Figure 5 shows the bleed air plumbing and pre-cooler for the GEnx-2B for the Boeing 747-8i compared to the same engine (in the GEnx-1B variant) when used in the 787 (which does not use bleed air for aircraft systems).
Figure 5. GE’s GEnx-1B for Boeing’s 787 (left) and GEnx-2B 747-8i (right) with bleed air pre-cooler. Source: Boeing.
Conclusion
Compressor air is used for many purposes inside and outside the engine. It is tapped at the lowest pressure level appropriate for its use. The more air needed for cooling and other purposes, the lower the engine’s performance. Materials and seals that allow the use of less compressor air are, therefore, a very active research area by engine OEMs.
Gosh, ever bit of an engine is fascinating. Would love to see more about the Active Clearance Control, and other technology like this that are innovative from one generation of engines to the next
How much efficiency is gained by using electric systems (787) that still need to be powered by the engine versus using bleed air for those functions as in other commercial aircraft?
Theoretially you gain efficiency as an electric generator can be around 90%-95% efficient and you have a direct mechanical drive from the main engine shaft. For the bleed air path of compressor -> bleed cooler -> air drive of air conditioning you have about an 80% efficiency or even a bit less.
But that does not include the heat problem of the power electronics (inverters, etc.) needed downstream of the generator, where you lose another 5% or more. In the end, the gain for the 787 was not large, and it will take further maturity of the whole electric chain before it becomes a common path in aircraft design.
Yes. I see the advantages in some weight and maintenance reductions from those compressed air ducts from engine casing through strut and wing to fuselage.
It may have also been a feature for the 787 reputed ability to change engines from a different manufacturers- although I understand its never happened in practice, but that might be later in life when more planes change fleets or lessors have mixed fleets
The 787 engines of course do use bleed air for the engines own use , but not piped out of the nacelle
Duke has hit on an area that is not been well publissized but very valid, ducts for bleed air use are a desing loss.
While I did buildings, the priciapl is the same. The ducts go in first as there are size and flow have toos and then you wrap the other systesm around your duct paths.
More or all electric, you run your wires as required, sizes are smaller (electronc easier to move than air) and routing is far more flexible. That is a signfiant plus though it has its own costs but everything does. Something like 1.25 Mega Watts (1 millions 250,000 KW) in generaors on the engines and the APU.
Bleed air has to be cooled for AC function so there is that loss.
The MDs had issues with oil burning and getting into the AC system. I flew on one of those and you could smell it. A case where it was downplayed by all but the flgiht crews. And yes I have flown MDs since that they got it stopped.
The only engine change direction on the 787 would be to GE, might be something NZ and ANA do at some point. Seems unlikely. Engines are a huge investment. Probably selling off the RR powered aircraft and buying new ones makes more sense.
Its been interesting to see what they did with the 787 electrically. I worked with those converters in my world. We went from backup for computer power, aka Uninterruptible Power Supply (AC/DC/AC) to driving fans and pumps and GPU on the ramp. Only difference was if they had a battery string in the DC section to maintain the AC output (UPS)
Someplace I have a figures for what an electronic GPU costs to run (peanuts) compared to an engine driven GPU and the highest cost an aircraft APU.
In theory electronic don’t wear out though most inverters require capacitor changes and ours were no exception other than some lasted 9000 hours and some 4000 hours.
Beyond Boeing PR:
Afaics it is a wash under current tech restraints.
Using bleed for things like anti-ice heating is actually rather efficient.
It is a shortish use case. if you go the electric way you have
to provide the short use electric generating power maximum required.
.. for the full aircraft life. generators are weighty.
787 tech level requires water cooling the power distribution/conversion.
weighty.
….
Its all a balance and trade off.
Nothing is free, so bleed air costs X amount and you use more fuel to make electrons if you replace Bleed Air. No free lunch but there can be LESS costly lunches.
The other offsets I listed in the reply to Dukes comment.
I have not seen someone work up some that Duke and I both see such as ducts, having to0 route same, weight of ducts, heat covering at least until its cooled well enough etc.
You should be aware that the 787 uses Starter units integrated with the Generators. I worked with a number of Gen sets that did the same thing. Usually a separate starter but Onnan specifically used a switched setup in its smaller motor home class generators.
As I understand it at least the Bell 407 did the same thing. Dual use on an aircraft makes huge sense as its lost weight. Otherwise its an APU start to put bleed air into engine spinning.
Our hangar had an air feed from outside so they could put the Air Start cart there, hook it up and feed engines inside if they just wanted to spin the engines. On the ramp of course a direct hookup.
Basically Bleed Air is pneumatics and I started my tech career with buildings that used pneumatics to control fans, pumps, air conditioning (at least to start and sometimes load/unload).
As time went on, they used electronics to control that with transducer4s to convert electrical into air power for valves and actuators.
Then the next generation simply used electronic actuators
You had to run power everywhere anyway (and it was everywhere) and a new building did not need to have an air compressor system just to supply what an electronic actuary could do (which by then were direct signaled out of the Controller on the wall if it was a fan, boiler system etc – power to the actuator usually run in the signal run as they did not take a lot of power)
I like the hybrid system that used Pneumatic for the actuators (power) end and transducers to convert the electronic signal to pneumatic power.
Actuators were simple, easy to install, easy to trouble shoot and one size fit lots of applications.
But that not the way the world went, hard to argue with reality.
I have been amazed at how reliable the 787s systems have been. In my world it was a high level of failures to start and then got better as time went by.
‘Using bleed for things like anti-ice heating is actually rather efficient.”
Not so. As mentioned in the story, its too hot so has to go through a heat exchanger first.
And “When bleed air is tapped for both cabin air conditioning and de-icing, it can increase the engine’s fuel consumption by up to 5%”
The electric mats integrated into the wing composite leading edge are more efficient than miles of hot air ducting and brackets and joints needing maintenance
Domain!!! ( and reading comprehension )
this cited sentence
“As mentioned in the story, its too hot so has to go through a heat exchanger first.”
referenced Airconditioning use!
Electric AC actually makes sense. You need the resource for the full flight duration, you do not want contamination from engine lubricants .,…
The very sentence I referred to says
‘When bleed air is tapped for both cabin air conditioning and de-icing, it can increase the engine’s fuel consumption by up to 5%.”
LOL
The images show for the GEnx which is on both 787 and 747 that the pre cooler is within the nacelle.
So all bleed air that is ducted out of the engine is cooled through one point , not just A/c
As others have said, it’s a mixed story between efficiency, maintenance, design benefits.
However, one aspect of electrical systems is that they do tend to fail with precious little warning, after years of service. The classic is that cooling surfaces get dirty, or fans fail, or dirt accumulates where it shouldn’t, capacitors dry up, etc. and finally, years later, it fails.
At least with mechanical systems that are “tricky” to keep running because of known wear rates, known accumulation of contaminants, etc, you have a routine maintenance program to check and an active supply chain of parts and indefinite service life as a result.
Of course, there’s nothing preventing one having a maintenance program for electronics and electrical systems, regular cleaning, regular replacement of fans (where used). Well designed electronics that was built well and kept clean and has had its heat management system well cared for can last a very long time. But even that is a double-edged sword. If customers do care for their systems and they don’t break, there’s no parts supply market and hence no supply. So when a system does finally expire, your aircraft could be grounded.
Back in the old days of valve electronics, pretty much the first step in any maintenance work was cleaning. Texas Instruments used to hose out oscilloscopes with hot soapy water before they’d do anything else.
Even today, things like self-cleaning switches need maintenance; they need to be moved in order to clean themselves. So if there’s a switch on your aircraft they you never, ever move, it’s worth exercising it a few times now and then just to help it stay clean and functional.
Same for contactors in power management systems, anything like that.
Another use of compressor air is boundary layer control, as used on the Blackburn Buccaneer.
With the blowers on that aircraft lost about 20% thrust, but could fly at a whole lot slower speeds (something like 30knots slower than without). That was a whole lot of compressor air taken out the engines and blown over the wings, horizontal stabilizer,
Admittedly, the Bucc was a bit of a one-off in many regards!
One of the things not widely appreciated about that aircraft was that it had interchangeable wing tips; in RN service at sea, they could fit the “low level” wing tips for missions requiring flying down valleys or at low level runs across the ocean, or the “high level” wing tips for high altitude cruise. I don’t know how many times they did this, but it was part of the system concept. I think in later RAF ownership they stuck with just one sort.