Geared Turbo­fan: How the engine of the future was developed

For decades, MTU has been work­ing on ways to make air­craft en­gines con­sume less fu­el, pro­duce low­er emis­sions and be­come qui­eter. The gold­en rule has al­ways been that to ful­fill the am­bi­tious goals of to­mor­row, you must push them through the de­ve­lop­ment process the day be­fore yes­ter­day. This re­quires not on­ly far­sight­ed­ness and con­fi­dence in your own ex­per­tise, but al­so ex­cel­lent part­ners and a good mea­sure of courage. When this bal­ance is right, you can achieve the goals of the fu­ture. A per­fect ex­am­ple of this is the Geared Tur­bo­fan™ (GTF), in­to which vir­tu­al­ly all in­no­va­tions of the past decades have gone.

From ADP to AT­FI to GTF

Back in the 1990s, Amer­i­can man­u­fac­tur­er Pratt & Whit­ney be­gan work­ing with MTU and the for­mer Fi­at Avio on ini­tial pre­lim­i­nary stud­ies for a geared tur­bo­fan en­gine un­der the pro­ject name Ad­vanced Duct­ed Propul­sor (ADP). How­ev­er, the pro­ject was not fur­ther pur­sued for com­mer­cial rea­sons. But over the years that fol­lowed, ris­ing kerosene prices cou­pled with cus­tomer de­mand for qui­eter en­gines that pro­duced low­er emis­sions led to a re-eval­u­a­tion of the geared tur­bo­fan con­cept’s mar­ket po­ten­tial. At the start of the 2000s, the in­ves­ti­ga­tions were re­sumed with the Ad­vanced Tech­nol­o­gy Fan In­te­gra­tor (AT­FI). This demon­stra­tor con­sist­ed of a propul­sion sys­tem with a re­duc­tion gear­box be­tween the fan and the low-pres­sure tur­bine and was test­ed based on a PW6000 core en­gine. In ad­di­tion to the three ADP part­ners, Pratt & Whit­ney Cana­da were now al­so work­ing on the pro­ject. And so en­gine man­u­fac­tur­er Pratt & Whit­ney laid the foun­da­tion for its patent­ed Geared Tur­bo­fan™ pro­gram, which of­fi­cial­ly launched in 2008. At rough­ly the same time as the test phase of the AT­FI, the MTU en­gi­neers de­vel­oped a new high-pres­sure com­pres­sor as part of the En­gine 3E pro­ject. For the lay­out and com­pu­ta­tion of the six-stage HDV12 com­pres­sor, a nu­mer­i­cal 3D Navier-Stokes flow solver was used for the first time. As a re­sult, the com­po­nent achieved a high over­all pres­sure ra­tio of al­most 11.

The HDV12 was to form the ba­sis for the high-pres­sure com­pres­sor of the PW6000 en­gine for the Air­bus A318—and sub­se­quent­ly, like the ADP and AT­FI demon­stra­tors, to pave the way fur­ther for the Geared Tur­bo­fan™. Its core de­vel­op­ment start­ed in 2005, when the part­ners de­cid­ed to de­vel­op and test a demon­stra­tion en­gine. The ini­tial tests of the over­all sys­tem in 2007 im­me­di­ate­ly yield­ed very pos­i­tive re­sults with re­gard to the func­tion­al­i­ty of the crit­i­cal com­po­nents. How­ev­er, it was al­so clear that the new en­gine over­all had one big area of im­prove­ment left: fur­ther de­vel­op­ment would pay off on­ly if it cre­at­ed a whole Geared Tur­bo­fan™ en­gine fam­i­ly. Con­vinced by the con­cept, Pratt & Whit­ney and MTU sys­tem­at­i­cal­ly in­vest­ed fur­ther in the en­tire process chain so as to per­mit the cre­ation of geared tur­bo­fans of dif­fer­ent thrust class­es. Just one year lat­er, the Geared Tur­bo­fan™ took off for the first time on flight tests.

En­gi­neer­ing courage pays off

With the two ma­jor MTU work­shares, the high-speed low-pres­sure tur­bine and the first four stages of the eight-stage high-pres­sure com­pres­sor en­gi­neered in blisk de­sign, the en­gine of the fu­ture al­ready achieves a very high de­gree of ef­fi­cien­cy to­day. The im­pres­sive re­sult: fu­el con­sump­tion and car­bon diox­ide emis­sions are re­duced by 16 per­cent each, and the noise foot­print by 75 per­cent. And as few­er com­pres­sor and tur­bine stages are need­ed, not on­ly are the en­gines lighter, but main­te­nance costs de­cline as well, be­cause few­er com­po­nents are ex­posed to the hot gas. For the key GTF com­po­nent, the high-speed low-pres­sure tur­bine, MTU won two Ger­man in­no­va­tion awards. Ger­many’s lead­ing en­gine man­u­fac­tur­er is the on­ly com­pa­ny in the world to have mas­tered this tech­nol­o­gy. The pa­tience and per­se­ver­ance was worth it: as well as rep­re­sent­ing a tech­no­log­i­cal quan­tum leap, the Geared Tur­bo­fan™ con­cept has al­so been a ma­jor com­mer­cial suc­cess.

To­day, Air­bus of­fers the GTF for the A320neo and the A220 (for­mer­ly Bom­bardier C Se­ries), Mit­subishi has it in their MRJ, and Em­braer has it in the new E-Jets of the E-170 and E-190 fam­i­lies. More­over, Irkut wants the GTF for the MC-21. At present, a to­tal of 80 air­lines world­wide have or­dered more than 8,000 of the GTF en­gines. More new MTU in­no­va­tions have gone in­to the cur­rent ver­sion, such as the first ad­di­tive­ly man­u­fac­tured com­po­nents and brush seals. In ad­di­tion, MTU has looked af­ter a third of the en­tire fi­nal as­sem­bly for the A320neo PW1100G-JM en­gine since the end of 2016. To this end, MTU in­vest­ed some 20 mil­lion eu­ros in a track-guid­ed as­sem­bly line sys­tem, which was de­vel­oped in-house and is unique world­wide. “What we’ve achieved of course makes us proud, but no­body at MTU should rest on these lau­rels,” em­pha­sizes Dr. Jörg-Michael Henne, Se­nior Vice Pres­i­dent En­gi­neer­ing and Tech­nol­o­gy at MTU. Over­all, the GTF has the po­ten­tial to re­duce fu­el con­sump­tion and CO₂ emis­sions by up to 40 per­cent.

Fur­ther de­vel­op­ment al­ready be­ing pre­pared

For in­stance, it would be pos­si­ble to achieve even low­er fan pres­sure ra­tios step by step over the com­ing years, which would fur­ther in­crease the by­pass ra­tio—from the cur­rent 12:1 to as much as 20:1 by 2035. More­over, MTU’s en­gi­neers are work­ing on fur­ther im­prov­ing the core en­gine’s ther­mal ef­fi­cien­cy by in­creas­ing the pres­sure and tem­per­a­ture ra­tios. This will in­volve in­creas­ing the over­all pres­sure ra­tio well be­yond its cur­rent val­ue of about 50:1, while dra­mat­i­cal­ly re­duc­ing the amount of cool­ing air need­ed.

And for 2050 and be­yond, MTU is al­ready de­vis­ing ini­tial stud­ies, con­cepts and ideas in col­lab­o­ra­tion with uni­ver­si­ties and oth­er re­search in­sti­tutes: “We need rev­o­lu­tion­ary ap­proach­es that must go be­yond to­day’s tech­nolo­gies, and above all else, we need new air­craft ar­chi­tec­tures,” says Dr. Ste­fan We­ber, Se­nior Vice Pres­i­dent Tech­nol­o­gy & En­gi­neer­ing Ad­vanced Pro­grams at MTU. Among the op­tions un­der re­view for the en­gine are the use of high­ly ef­fi­cient heat en­gines with ex­treme­ly high pres­sures or the in­te­gra­tion of re­cu­per­a­tive el­e­ments to im­prove the ther­mo­dy­nam­ic cy­cle. Shield­ed pro­pellers or fans dis­trib­uted around the fuse­lage are al­so pos­si­bil­i­ties. In ad­di­tion, there are tech­no­log­i­cal so­lu­tions such as al­ter­na­tive fu­els and steps to­ward tur­bo-elec­tric flight, with­out which fu­ture tar­gets can­not be met.

All im­prove­ments al­ways have the same goal: im­prove ef­fi­cien­cy and there­by min­i­mize fu­el con­sump­tion, emis­sions and noise. The en­gine of the day af­ter to­mor­row has long been in the start­ing blocks—and MTU is al­ready tak­ing re­spon­si­bil­i­ty to­day for mov­ing avi­a­tion to­ward emis­sions-free flight.