Air­lines have their own in­cen­tive to in­crease fuel ef­fi­ciency and re­duce emis­sions, be­cause each tonne of CO2 they avoid adding to the at­mos­phere, rep­re­sents sig­nif­i­cant sav­ing on fuel cost

SP's Airbuz - - Engine Technology - BY JOSEPH NORONHA

THE FLEET DATA­BASE OF CAPA – Cen­tre for Avi­a­tion in­di­cates that there were 31,600 com­mer­cial air­craft in ser­vice world­wide at the end of 2017, an in­crease of 4.1 per cent over 2016. And based on a firm order back­log of over 17,000 planes, the num­ber will con­tinue to rise. The long-term fore­cast is even more bullish. In July this year, Boe­ing es­ti­mated that 42,730 new jets will be needed over the next 20 years to cater for the ris­ing de­mand es­pe­cially from China and In­dia.

While such glow­ing pre­dic­tions bring joy to the air­line in­dus­try, they are un­likely to thrill en­vi­ron­men­tal­ists. Ac­cord­ing to the In­ter­na­tional Air Trans­port As­so­ci­a­tion (IATA), civil avi­a­tion as a whole added around 859 mil­lion tonnes of car­bon diox­ide (CO ) to the at­mos­phere in 2017. This is a small frac­tion (a lit­tle over two per cent) of the 36 bil­lion tonnes of an­nual an­thro­pogenic car­bon emis­sions, but avi­a­tion green­house gases (GHG) have an en­hanced im­pact on the at­mos­phere be­cause they are re­leased at high al­ti­tude. And although the fuel ef­fi­ciency of jets has im­proved sig­nif­i­cantly over the years, emis­sions are steadily ris­ing be­cause of avi­a­tion’s ex­pand­ing foot­print. There­fore, the in­dus­try has set it­self an am­bi­tious tar­get to slash net car­bon emis­sions to half the level in 2005 level by the year 2050. This re­quires huge in­vest­ments and a step change in en­gine tech­nol­ogy. RAISE EF­FI­CIENCY, LOWER EMIS­SIONS. The jet age that be­gan with the de Hav­il­land Comet in 1952, is still go­ing strong. While most sur­face modes of trans­porta­tion are be­com­ing in­creas­ingly de­pen­dent on ve­hi­cles driven by elec­tri­cal power, only very small air­craft are pow­ered by elec­tric­ity. That is be­cause only jet en­gines have enough power to pro­pel large air­lin­ers through the skies at high speeds. But they also spew kerosene fumes into the at­mos­phere.

The avi­a­tion in­dus­try is keen to boost fuel ef­fi­ciency and con­se­quently curb emis­sions. Yet at­tempts to switch to bio­fu­els are well be­hind tar­get and in­stead have in­ten­si­fied en­vi­ron­men­tal prob­lems such as ram­pant de­for­esta­tion and changes in land use. Man­u­fac­tur­ers such as Air­bus and Boe­ing are spend­ing huge sums to re­duce weight through re­design and use of new light­weight ma­te­ri­als in an ef­fort to make air­frames more ef­fi­cient.

But the best medium-term route to cleaner skies is prob­a­bly through ad­vances in en­gine tech­nol­ogy. That is why new fu­el­ef­fi­cient en­gines such as Pratt & Whit­ney’s PurePower PW1000G and CFM In­ter­na­tional’s Lead­ing Edge Avi­a­tion Propul­sion-1 (LEAP-1) tur­bo­fan are sell­ing like hot cake. LEAP­ING AHEAD OF PUREPOWER. Once gas-guz­zling tur­bo­jets evolved into fuel-ef­fi­cient tur­bo­fans in the 1960s, it took only a short time for tur­bo­fans to power prac­ti­cally ev­ery com­mer­cial jet. Mod­ern tur­bo­fans have high by­pass ra­tios of up to 9:1. This is an im­por­tant mea­sure of ef­fi­ciency as it means that nine times as much air flows around the en­gine core as through it. But they con­se­quently re­quire larger in­takes to ac­com­mo­date big­ger and bet­ter fans and the longer fan blades mean their tips are turn­ing at close to the speed of sound where po­ten­tially danger­ous vi­bra­tions could set in.

That is why Pratt & Whit­ney spent about 30 years and over $10 bil­lion to de­velop its PurePower range of high-by­pass geared tur­bo­fan en­gines – the ex­clu­sive en­gine for the Air­bus A220, Mit­subishi Re­gional Jet (MRJ) and Embraer’s E-Jet E2 fam­ily. The com­pany of­fered this en­gine as an op­tion for the Air­bus A320­neo and Irkut MC-21. The PW1100G that pow­ers the A320­neo en­tered com­mer­cial ser­vice with Lufthansa in Jan­uary 2016.

PurePower en­gines use a so­phis­ti­cated gear­box at­tached to the shaft to turn the com­pres­sor fan and low-pres­sure tur­bine each at their most ef­fi­cient speed. This per­mits slower and larger fans (81 inches di­am­e­ter on the A320­neo) and an in­creased by­pass ra­tio of 12:1. As com­pared to the older A320ceo, the A320­neo re­duces fuel burn and CO emis­sions by 16 per cent, gen­er­ates 75 per cent less noise and emits 60 per cent less ni­tro­gen ox­ides (NOx). The down­side in­cludes greater en­gine weight and aero­dy­namic drag, as well as in­creased com­plex­ity of the en­gine which height­ens the risk of fail­ure, as In­diGo’s and GoAir’s trou­bles with their new Air­bus A320­neo air­lin­ers starkly high­light.

So why not sim­ply im­prove the tried and tested tur­bo­fan as much as pos­si­ble? That is ex­actly what CFM In­ter­na­tional has done with its LEAP-1 fam­ily of high-by­pass tur­bo­fan en­gines. LEAP-1 uses ad­vanced de­sign tech­niques, light­weight com­pos­ite ma­te­ri­als, coat­ings, com­bus­tion and cool­ing tech­nol­ogy as also im­proved in­te­gra­tion be­tween the en­gine and air­frame to boost ef­fi­ciency. In the process, it pro­vides 15 per cent lower fuel burn com­pared with its pre­de­ces­sor en­gine, the iconic CFM56-5B. The LEAP-1A en­tered com­mer­cial ser­vice in Au­gust 2016 when Turk­ish bud­get car­rier Pe­ga­sus flew the first Air­bus A320­neo. The LEAP-1B also en­tered rev­enue ser­vice in May 2017 on the Boe­ing 737 MAX plat­form while the Chi­nese Comac C919 pow­ered by the LEAP-1C is in the flight test phase. Th­ese two tech­no­log­i­cal mar­vels, the LEAP-1 and the PurePower en­gines, are set to power sin­gle-aisle air­lin­ers – the vast bulk of the global com­mer­cial fleet – for a cou­ple of decades or more. While the LEAP-1 has at­tracted more than 14,270 or­ders and com­mit­ments, the PurePower en­gine has se­cured or­ders of over 8,000. But although the evo­lu­tion­ary de­sign process has de­liv­ered sat­is­fac­tory re­sults so far, what is needed is a revo­lu­tion­ary in­crease in fuel ef­fi­ciency. REV­O­LU­TION IN THE AIR? The tra­di­tional aero­nau­ti­cal de­sign process has a sig­nif­i­cant pit­fall. Air­frame de­sign­ers build aero­dy­nam­i­cally ef­fi­cient struc­tures and are not un­duly con­cerned about the en­gine. On the other hand, en­gine tech­nol­o­gists in­de­pen­dently build a power plant po­ten­tially com­pat­i­ble with a va­ri­ety of air­frames. Over­all ef­fi­ciency in­evitably suf­fers in the bar­gain. But the tube-and-wing ar­range­ment with twin tur­bo­fans slung un­der-wing, is now near­ing its lim­its of ex­ploita­tion and avi­a­tion tech­nol­o­gists feel com­pelled to switch to in­te­grated en­gine-air­frame de­sign.

In 2008, Aurora Flight Sciences, the Mass­a­chu­setts In­sti­tute of Tech­nol­ogy (MIT) and Pratt & Whit­ney be­gan a NASA-spon­sored ef­fort to rev­o­lu­tionise fu­ture air­craft. The Aurora D8 will be sim­i­lar in size to the Air­bus A-320/Boe­ing 737, but will fea­ture a wide “dou­ble-bub­ble” fuse­lage to gen­er­ate in­creased lift. This means smaller wings, a lighter land­ing gear and a re­duced tail. Con­se­quently, it will need en­gines that are smaller, are of lower weight and and carry less fuel. Un­like the con­ven­tional en­gines that are slung un­der the wing, the D8 will have twin jets in­te­grated with the fuse­lage, mak­ing for a clean high-as­pect-ra­tio wing with low drag. The large fan will have a much higher by­pass ra­tio of 20:1. Fur­ther, since the en­gines will be mounted at the rear, they will reen­er­gise the slow-mov­ing bound­ary layer flow over the fuse­lage and pro­mote Bound­ary Layer Inges­tion (BLI), thus in­creas­ing aero­dy­namic ef­fi­ciency. In­deed, ev­ery part of the air­frame and en­gine will be metic­u­lously con­fig­ured to max­imise ef­fi­ciency and min­imise op­er­at­ing costs. If var­i­ous tech­no­log­i­cal chal­lenges are suc­cess­fully over­come, the D8 may of­fer huge ben­e­fits such as 71 per cent lower fuel burn and emis­sions, Ef­fec­tive Per­ceived Noise level in deci­bels (EPNdB) of 60, and 87 per cent re­duc­tion in land­ing and take­off emis­sions of NOx as com­pared to cur­rent sin­gle-aisle air­craft such as the A-320 or the Boe­ing 737. FOR A CLEAN SKY. Mean­while in Europe, Clean Sky stands out. Clean Sky, a joint un­der­tak­ing of the Eu­ro­pean Com­mis­sion and the Eu­ro­pean aero­nau­tics in­dus­try, aims to get the most out of ev­ery drop of fuel for ev­ery cat­e­gory of air­craft. Ac­cord­ingly, it is study­ing rad­i­cal new ap­proaches, new shapes and new air­craft ge­ome­tries for large pas­sen­ger air­craft, re­gional air­craft, ro­tor­craft and small air trans­port air­craft. Clean Sky 1 ex­ceeded its am­bi­tious tar­gets set in 2007 and Clean Sky 2 is now in full swing.

Safran’s Open Ro­tor demon­stra­tor is a key part of Clean Sky’s plans to de­velop a more fuel-ef­fi­cient propul­sion sys­tem for air­lin­ers by around 2025. It in­volves study­ing novel aero­dy­namic and ma­te­rial tech­nolo­gies to im­prove the in­her­ently noisy prop­fan en­gine. The next gen­er­a­tion prop­fan is ac­tu­ally a gas tur­bine driv­ing two high-speed un­shrouded fans turn­ing in op­po­site di­rec­tions. Ini­tial test­ing prom­ises 15 per cent im­prove­ment in fuel burn and emis­sions over the CFM LEAP-1 en­gine, with com­pa­ra­ble noise lev­els. Rolls-Royce is also de­vel­op­ing prop­fans.

The good news is that apart from green pres­sures, air­lines have their own in­cen­tive to in­crease fuel ef­fi­ciency and re­duce emis­sions, be­cause each tonne of CO2 they avoid adding to the at­mos­phere, cur­rently rep­re­sents a di­rect sav­ing on fuel cost of ap­prox­i­mately $225. In an era of ris­ing oil prices, fuel econ­omy be­comes even more im­por­tant. In In­dia for in­stance, the price of fuel, which ac­counts for 35 to 40 per cent of an air­line’s cost of op­er­a­tions, has surged by nearly 30 per cent over the past year and may rise even fur­ther. Hence, there is good rea­son to make ev­ery drop count us­ing im­proved en­gine tech­nol­ogy.


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