Bjorn’s Corner: What did we learn in 2015; engines

By Bjorn Fehrm

By Bjorn Fehrm

15 January 2016, ©. Leeham Co: Last week we looked back on what happened in 2015 on the airframe front. We finish the retrospective by looking at what turbofan engine technology came to market in 2015. New engine technology is vital, as it is on the engine side that the quest for higher fuel efficiency has the largest successes.

While advances on the airframe side might bring an additional 5% per generation, the engines typically increase their efficiency per new generation with up to three times that value. Fuel efficiency per delivered thrust unit was improved with a whopping 15% over the engine it replaces for the Pratt & Whitney Geared Turbofan (PW GTF). It was certified for use on the Airbus A320neo in Q4 2015

The competing CFM LEAP-1A shall deliver the same improvement level to the A320neo once it is certified in the summer of this year. This engine has a smaller sister that started ground tests last year, the LEAP-1B, which is developed for the Boeing 737 MAX series.

The engine that is easily forgotten is the Rolls Royce Trent XWB. It entered service on the Airbus A350-900 during the year. It brings an improvement level of around 10% compared to the engines of the aircraft that the A350 replaces (Airbus A340/A330ceo and Boeing’s 777-200 range).

Engine technology of 2015

A 15% improvement in fuel efficiency compared to the engine generation that it replaces is a major feat. It can be instructive to look at how this is achieved and what is the net improvement level from the new engine technology once we combine everything at the aircraft level. The overall story is told in Figure 1.

Engine efficiency gains_

Figure 1. Efficiency gains from modern Turbofans over the engines they replace. Source: Leeham Co.

Engine design

Before we go into the different methods to increase efficiency, we need to understand why I will not give one value for the efficiency gain for each engine technology. Turbofan engines are developed to cover a thrust range. For the engines used on the A320neo, this is 24klbf to 33klbf (lbf for force). This coverage is achieved by producing one engine variant and spinning it to a different maximum RPM to achieve the desired thrust range.

As this is done, the core driving the fan has to work harder. Therefore the By Pass Ratio (BPR) is lower for a high thrust engine variant than an entry level one. Consequently the efficiency gain from increasing the BPR varies between engine variants in a product range.

Improvements in propulsive efficiency

All engines that we look at make the largest efficiency gains by increasing the propulsive efficiency. As I have written several times, this is achieved by accelerating the air less over the engine’s fan. The slower “over-speed” of the air leaving the engine compared to the free stream air gives the engine a higher propulsive efficiency. The lowering of the engine’s air over-speed is called lowering the engine’s specific thrust. It is the engine’s specific thrust that describes an engine’s characteristic and its propulsive efficiency for a certain speed range.

As a lower specific thrust decreases the engine’s thrust for the same air flowing through the engine, the amount of air that is accelerated has to be increased. This is achieved with a larger fan, the engine’s By-Pass Ratio (BPR) is increased to keep the desired thrust level. (Observe that this is the chicken-and-egg order; first one decides on desired specific thrust, then the BPR falls out from the thrust needed).

The different engines use the lower specific thrust technique to varying degree, as it also has drawbacks. The larger fan increases the engine’s diameter and therefore drag and weight. It also requires more shaft horsepower from the low pressure turbine to drive the fan. The fan section and low pressure turbine are the heaviest parts of a turbofan engine, so a high propulsive efficiency means a heavy engine.

There are several ways to combat the weight increase. One is used by the CFM LEAP; it uses an advanced Carbon Fibre Reinforced Plastic (CFRP) fan and fan case. The Pratt & Whitney (PW) GTF uses a fast spinning and therefore smaller low pressure turbine by putting a gear ratio of 3:1 between the turbine and the fan (the fan needs to spin slowly). The Rolls Royce Trent shortens the engine by dividing it into three optimised and therefore short spools, thereby making each spool as light as possible.

The most efficient technique is the geared one. The PW GTF can therefore produce a relatively light engine with a high BPR (14:1) without resorting to CFRP techniques for fan and fan case. The LEAP can stay with a simpler direct drive principle by means of combining an 11:1 BPR (9:1 for the smaller 1B) with a CFRP fan/fan case. The RR Trent XWB uses a hollow Titanium fan to limit the weight increase but will change to CFRP fan/fan case for the next engine generation.

Of the three, the PW GTF uses its geared principle to put the largest efficiency gain in lowered specific thrust, around 8%-9% over previous generation engines. The direct drive LEAP and Trent have gains of around 7%-8%.

Thermal efficiency gains

All engines use advances in overall Pressure Ratio (PR) to increase the combustion efficiency. Cruise pressure ratios grow from the low 30s to low 40s for the single aisle engines and to the mid-40s for the Trent XWB.

Higher PR means higher temperatures for the compressor, combustor and turbines. For the combustor and turbines, compressor air can be tapped for cooling. For the compressor, it can’t. Therefore modern high compression compressors must be made efficient (less heat generated) and use advanced materials. An efficient compressor also uses less horsepower to generate its pressure, therefore requiring a smaller turbine. All manufacturers use advanced 3D aerodynamics to increase the efficiency of their latest compressors.

To keep combustor and turbines within the capabilities of their materials, cooling air is tapped from the engines compressors. Over 20% of the air entering the engines core can be routed away from compressor stages to cool different sections of the engine. The air is always tapped at the lowest pressure possible (it still has to have a higher pressure than the area of its use, otherwise hot gas flows back in the engine). Thereby it has the lowest temperature and not too much work invested in it.

All discussed engines throttle back the amount of cooling air used when the engine is entering a lower power state, like cruise. The techniques vary with the LEAP and Trent throttling both static and rotating parts, whereas the GTF stays with static parts throttling.

The best technique is, of course, if the material used requires no cooling; cooling air zaps the engine’s efficiency. All manufacturers invest in advanced material research. CFM is leading this area, with the LEAP being the first engine to employ Ceramic Matrix Composites (CMC) for the hottest static parts; combustor and first turbine shroud.

A final area of much work is the multitude of leak areas in the engines. At each compressor and turbine stage, there are leak areas where the air is trying to avoid following the path desired by the designer. Air is leaking over the tips and under the feet of blades (the reason for tip clearance control and BLISK=one piece disk+blade stages), into the center of the engine over rotating seals (therefore more advanced rotating seals) and in general everywhere where it’s not wanted.

Especially sensitive areas are where oil is present, like the shaft bearing areas. Special bearing boxes are designed so that high pressure cooling air in the box blocks any hot leaked gases from entering, yet another zap for our precious compressor air.

All engines employ advanced leak preventing technology. Combined with large investments in cooling and material technology, the LEAP and Trent gain around 4% efficiency in this domain; the GTF is content with somewhat less.

Aircraft level losses

As described, the quest for higher propulsive efficiency brings larger and heavier engines (the higher PR also demands a heavier core). This creates losses at the aircraft level. I have written about this before and will not repeat the details here.

Suffice to say that all engines have aircraft level losses due to larger nacelles and higher engine weights. The larger nacelle has a larger wetted area, which causes parasitic drag. The higher engine weight combine with larger heavier nacelles and thrust reversers to increase the aircraft’s empty weight by several tonnes. This increases the aircraft’s drag due to weight, the induced drag.

This reduces an engine efficiency gain of 15% to something like 12%-13% on an aircraft level.

22 Comments on “Bjorn’s Corner: What did we learn in 2015; engines

  1. “Observe that this is the chicken-and-egg order; first one decides on desired specific thrust, then the BPR falls out from the thrust needed”

    I think for many aircraft, the max engines size / ground clearance is a limitation. Meaning maximum BPR is a given and you have to find other ways to get to the required thrust. Without turning the engines into little, red hot, noisy, high OPR, shop queens. Making compromises on fuel burn.

    In my opinion, Pratt has the upper hand at this stage because they can do significant weight saving (carbon fan, fan cowling) and thermal efficiencies enhancements (pressures, ceramic-matrix composites) on a Next Gen GTF series.

    “Hess says P&W has a roadmap to increase the GTF fuel savings to 20-30% from 15% by the middle of the next decade. Some of that will come from bigger fans that increase bypass ratio to 15-18 from around 12 on the PW1000G, but P&W also plans to improve the core.

    This will involve increasing the overall temperature of the engine and require new materials, says Adams. “We will drive thermal efficiency. Core technology will push overall pressure ratio beyond 60, which is the next threshold for this size of engine.”

    “Temperature is a value trade with cash operating cost. We will not arbitrarily drive temperatures too high to gain performance but lose on maintenance cost,” he says. “We are in a good place with the current GTF.”

    I think Safran / GE / RR will have to come with new architecture too.

    • That’s very true regarding GTF vs. LEAP. The LEAP is much more “maxed out” allready. There are discussions about the LEAP – especially the smaller LEAP1-B – not meeting targets [vs. the GTF]:

      However the economical package of the LEAP – including better production processes – counters some of these looses under the current low-oil environment. If oil hits 60-80$ again times for the LEAP (and especially the 737MAX) would get tought.

  2. Bjorn,

    I agree that of all the existing architectures, GTF has the most promise and future potential. CFM and GE are making a big mistake if they keep believing that a CFRP fan module and higher core temperatures/less cooling air resulting from CMCs suffice. The LPT still turns too slowly and has to be massive. Besides, nothing prevents P&W from employing a CFRP fan module and CMCs in its future engines, ALONG with the GTF architecture. That would be an unbeatable engine in terms of overall fuel efficiency. The open rotor concept being researched by Safran and GE (partners in CFM) is likely a non-starter, because of the blade-out and noise issues. NIH (not invented here) syndrome does not serve GE well. Just because its bitter rival P&W did it first does not mean GE cannot graciously embrace “This Changes Everything” revolution in turbofan engine architecture!

    Good article. Enjoyed it. Thanks.

  3. Since GE have CFRP fans and cases and have bought a gearbox company:

    and since RR are introducing CFRP fans and cases on immediate future products and have started up a gearbox company:–and-liebherr-aerospace-announce.aspx

    and since GTF’s have been in airline service since at least 1981:

    and since PW will have to continue to develop their existing GTF for the A320neo and both their competitors will have CFRP fans and cases,

    would anyone like to hazard a guess about the engine configuration that will be offered for the next generation of single aisle aircraft in the 2020’s?

    Perhaps the more interesting question is whether the gearbox is going to be reliable enough. Engines can do 50,000 hours on the wing:
    That’s a tough engineering challenge for a light-weight gearbox shifting maybe 30-40MW of power and without the sort of maintenance time allocated to helicopters and military transports.

    • I think the gearbox issue thats most tricky, isnt the reliability on its own , but at the very high power levels and say 98% efficiency, thats still leaves lost energy as heat that has to go somewhere.

    • You are right w.r.t. the most likely future architecture, a GTF with composite fan module and CMCs in the gas generator, Ti-Al for LPT … …

      If you recall the history of GE/PW rivalry over the decades, ever since P&W pushed GE off the civilian engine market with their very first turbofan, there has been this NIH syndrome. Eventually GE adopted P&W innovations such as directional solidification, snubbers, front fans, twin spools etc. and P&W adopted GE innovations such as variable stators in its compressors, but the bitter rivalry continues, just like A and B. But eventually, no matter what their public posturing is, both get there!

      Open rotors also require gear boxes, and the same reliability and heat dissipation issues turn up there also. Did GE buy the gearbox company so they could secretly build a GTF, while openly pursuing the open rotor? Hard to tell. I wish they would just do what RR did – openly say they will also build a GTF, but a better one with their CFRP fan module and CMCs. But would that be a PR debacle in their mind? Patent issues?

      • Everyone seems to agree that the LEAP iterations of the old B1 bomber engine are the end of the line development-wise, and probably the LEAP engines will become the ultimate refinement of the 2-shaft architecture for single aisle airliners. So whatever GE offers for the new single aisle airliners in the 2020’s, it won’t be another development of that line.

        GE are putting effort into open-rotor, as are RR and presumably PW are as well, although their public face plays down the viability of that. But the fundamental problems of noise and blade-off of open rotor have no current easy solutions and its hard to see either A or B investing in an brand new open rotor airliner in the short or medium term.

        So you can bet your bottom dollar that GE are not going to bet the farm on open rotor to the exclusion of anything else, and you can bet your other bottom dollar that GE are not going to bet the farm on making the B1 bomber engine work a bit better either.

        So GE is developing a GTF and not announcing the fact. It’s just that they will be ten years behind PW and fifty years behind Honeywell when they get there and they will be competing against a clean-sheet design from RR. One hopes it will be worth the wait.

        • GE has bought (the former Fiat) Avio who did the development work for IAE ( who is the name listed on certifying documents, as the builder of the GTF) on the gearbox so you can imagine they have a lot of IP relating to that.

          • One huge impediment is that the ORs have to go on the back of the aircraft. Any gain from efficiency is eliminated by the structural issues (there’s a reason that the rear engine configuration of the DC9 and 727 were not continued (US Centric, Comet as well as far as engine location)

            And a huge factor is the lack of commonality of any proposal. They are all different, any aircraft mfg would be committed to a single engine mfg. No more competition. You better pick right and if you don’t you are in a huge loss position.

            You are stuck (or sunk) with that architecture that can’t be changed even if you could shoe horn someone else’s engine in that was worse than the conceptual engine.

            I do not think the public would like them (they don’t like props of any kind though I am comforted by their twirling) ergo, public views it as second rate, lower tier, not a real aircraft.

            While I thought it was interesting idea when they tried it I have yet to see anything positive about a complete installation. All the efficiency gains are off aircraft and the real world is on aircraft where jet tube win out in all their glory.

    • Assuming a convergence of technologies as Arkay illustrates, would you rather have a lead in a mechanical technology like GTF (of which some technologies have lost patent protection since they’ve been out for a while), or a lead in cutting edge material science like CMCs?

      Some material science will be better understood and integrated in the future and you can let your competitor do the R&D and benefit from the gains by using a more mature version later, but if you can’t use their patented tech when the next generation of engines is introduced, you’re SOL. Integrating a mechanical technology is more of a straightforward R&D exercise and likely has less problems since at least the some of the core technologies has lost patent protection.

      There used to be a little car company called Saab that relied on its lead in mechanical technology like turbocharging, thick pillars and crash safe structures to sell cars at a premium. As soon as the competitors caught up on the tech to meet new crash and fuel efficiency regs, Saab lost its advantages and no longer exists.

  4. Is there any info.on the delay with the pratt engine on the a320.?
    It sounds relatively minor, but is blade case rubbing ever a minor thing?
    You would have thought that problem would have surfaced before service entry.

  5. Very informative as usual, Bjorn. One trivial correction, increasing overall pressure ratio increases thermal efficiency of the cycle, combustion efficiency can be assumed to be 99% or better. While it is correct that using a gearbox allows use of a significantly lighter LP turbine by eliminating 2 or 3 stages , the question becomes is this offset by the weight of the gearbox? It seems to becoming clearer that at very high BPR a gearbox is essential ; large fans may require 50-60 MW at take off, and a gearbox efficiency of 98% would mean heat rejection greater than 1 MW. I have, however, seen claims for gearbox efficiencies above 99%. There have been more than 11,000 TFE731 GTFs in service for forty years so there must be a vast body of knowledge on gearbox reliability. There has also been significant airline use of ALF 502/507 GTF in the BAE 146.

  6. They had the same problem with the F35 engine (rubbing, not when it occurs)

    Yes you would think issue would have popped up though shifting to industrialization vs hand built can have its issues (RR and their engine issue and GE with theirs)

    This will be the third for P&W though and that is not good.

    Like the rest they will solve it but you have to wonder what else lurks under the hood

  7. Just some notes to a good article. A slower turning fan does require less horse power for the same thrust but because of lower rpm and higher torque requires a beefier fan shaft. Power=angular vel* torque.
    I think PW1100G has an Alcoa alu core + carbon fiber fan blades, the low rpm makes the alu possible.
    One important drag contributor can be the engine nacelle fuselage interference drag. Replacing a smaller engine in the same position with a bigger one can cause this drag to increase, hence a new optimal position on the wing exist.

    • I thought I read somewhere that pw 1100g has aluminium fan blades because they don’t need to be as strong owing to their slower speed.Could be wrong.

  8. High fuel prices drove the development of the current generation of engines, which are focused on reducing fuel consumption and extending range. As aviation’s share of global emissions increases (land-based transport and energy have greater substitutability) that pressure will resume. But for the moment, and possibly a number of years, prices are depressed (weakness in China, oversupply).

    So the key factor for engines over the next few years becomes the total cost of ownership. I’m interested to know what the GTF and LEAP mean for maintenance and other costs.

    • Short term no clear answer.

      CFM has done well with their engine as its been extremely reliable, ergo the maint aspect ahs been an owners joy (not sure ontthe VF2500)

      It would seem that the simpler material P&W is more better but maybe not all the savings passed on as money needs to be recovered. At least a flexible edge.

      That assumes the gear system holds up, not new but P&W is having their issues and who knows. Certainly was hard on the A400

  9. What I do not understand and maybe someone can explain is; given what is said in this article about the size of the LP fan. The B373MAX gives away 3cm to the engine on the NEO, NEO’s is larger – yet Boeing claims a better specific fuel consumption.

    How can this be?

    • Boeing can claim all they want but they still have to prove it.

      Re-read the weight part. The 737 LEAP may not be as efficient but it also weights less. Ying and yang. Airbus could go large easy and did, , Boeing could not and has to live with it (and the 737 is a bit lighter than the A320, though you do have to wonder with the fuselage blowouts if they went a bit too far!)

      It occurs to me the $64 question is what happens to the A320 if you put a 737 LEAP on it? Third engine offering anyone?

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