Low-pressure turbines: The path to world-class manu­facturer

What was MTU’s first step on the path to be­com­ing a lead­ing man­u­fac­tur­er of low-pres­sure tur­bines? Sur­pris­ing­ly, it was the de­vel­op­ment of an ex­haust cas­ing. Ex­act­ly how this hap­pened is some­thing Lud­wig Schweikl knows first­hand, be­cause he and his col­leagues were the ones who built it. “At that time, the ear­ly 1970s, cor­po­rate man­age­ment made a strate­gic de­ci­sion to get in­to the com­mer­cial en­gine busi­ness,” says Schweikl, who had a long tenure as head of de­sign at MTU. The ex­perts al­ready had ex­pe­ri­ence with mil­i­tary en­gines and had de­vel­oped and built the in­ter­me­di­ate-pres­sure tur­bine for the RB199, the en­gine for the Tor­na­do. Over the course of this pro­duc­tion process, MTU came in­to con­tact with Amer­i­can en­gine man­u­fac­tur­er Pratt & Whit­ney. “Some­how this led to an in­vi­ta­tion for the MTU de­vel­op­ment de­part­ment to vis­it the Pratt head­quar­ters in East Hart­ford,” the en­gi­neer re­calls. It would be a great way to get to know one an­oth­er, said the Amer­i­cans. Sounds good, re­spond­ed the Ger­mans. So, Schweikl soon found him­self on a plane mak­ing its way to the US and was thus the first MTU de­vel­op­er to meet his coun­ter­parts at Pratt & Whit­ney.

“They were very skep­ti­cal as to whether or not we had any­thing to of­fer. At that time, Pratt & Whit­ney was the world’s lead­ing en­gine man­u­fac­tur­er and MTU was just a small com­pa­ny,” the 82-year-old says. But he made a good im­pres­sion, and MTU was asked to try its hand at the ex­haust cas­ing for a new ver­sion of what was then the clas­sic medi­um-haul JT8-D. The col­lab­o­ra­tion worked smooth­ly, far ex­ceed­ing Pratt & Whit­ney’s ex­pec­ta­tions. MTU’s skilled en­gi­neers im­pressed the Amer­i­can en­gine man­u­fac­tur­er—which then asked them to de­vel­op the low-pres­sure tur­bine for the JT10-D: a larg­er, more pow­er­ful fol­low-up to the JT8-D. It was an hon­or and an or­der that Schweikl and his de­vel­op­er team grate­ful­ly re­ceived. “This marked the be­gin­ning of the low-pres­sure tur­bine busi­ness for MTU,” Schweikl says. Thus be­gan a true de­vel­op­ment race, with one in­no­va­tion top­ping the oth­er—a race that con­tin­ues to this day.

Ex­perts share the word about MTU’s com­pe­tence

The first low-pres­sure tur­bine for the JT10-D, soon re­named the PW2000, made a shin­ing im­pres­sion thanks to a spe­cial tech­ni­cal re­fine­ment. As the amount of thrust changes, ro­tors and en­gine cas­ings ex­pand and con­tract at dif­fer­ent speeds. This can lead to greater clear­ances and in turn to per­for­mance loss­es. To com­bat this prob­lem, the en­gine in­cor­po­rat­ed the first so­lu­tion for cool­ing the cas­ing, called the ac­tive clear­ance con­trol (ACC) sys­tem. This off­set the dif­fer­ence in the ther­mal ex­pan­sion of the com­po­nents, thus re­duc­ing clear­ance and con­sid­er­ably im­prov­ing the ef­fi­cien­cy of the en­tire as­sem­bly. Pratt & Whit­ney con­tributed the idea and the patent, while MTU made it a re­al­i­ty. “To­day it is stan­dard in every en­gine,” says Schweikl.

Word about MTU’s ex­per­tise in low-pres­sure tur­bines spread through­out avi­a­tion cir­cles. As a re­sult, a short time lat­er Schweikl and his team al­so took on the de­vel­op­ment of the low-pres­sure tur­bines for the V2500 pro­gram. A low-pres­sure tur­bine has a ma­jor in­flu­ence on an en­gine’s over­all per­for­mance. With the V2500, which pow­ers the A320 fam­i­ly and has be­come one of the most im­por­tant pro­grams in MTU’s com­mer­cial port­fo­lio, the en­gi­neers sys­tem­at­i­cal­ly ap­plied what they learned with the PW2000. This meant they could sig­nif­i­cant­ly im­prove the ef­fi­cien­cy of the low-pres­sure tur­bine for the V2500.

Just three stages in the high-speed low-pres­sure tur­bine

Many more MTU low-pres­sure tur­bines fol­lowed: they are found in busi­ness jet en­gines, in the pow­er tur­bines of tur­boshaft en­gines for heavy-lift he­li­copters, in tur­bo­fan en­gines for medi­um- and long-haul air­lin­ers, and even in the GP7000 for the Air­bus A380 mega­lin­er. MTU has num­bered among the glob­al elite in low-pres­sure tur­bine tech­nol­o­gy for a long time now. The Mu­nich com­pa­ny’s cur­rent mas­ter­piece is the high-speed low-pres­sure tur­bine for the Geared Tur­bo­fan™ (GTF): op­ti­miz­ing the aero­dy­nam­ics down to the small­est de­tail has achieved a high­er de­gree of ef­fi­cien­cy. At the same time, the high ro­ta­tion­al speeds that re­sult from the GTF’s re­duc­tion gear­box mean the stages work hard­er. That’s why the pres­sure tur­bine mod­ule in the A320neo en­gine needs just three stages. As a re­sult, space, weight, main­te­nance costs, not to men­tion fu­el con­sump­tion and there­fore CO₂ emis­sions all de­crease.

And as if this weren’t im­pres­sive enough, the new tur­bine is al­so much qui­eter than con­ven­tion­al mod­els. Its noise emis­sions are at high­er fre­quen­cies that are bet­ter ab­sorbed by the at­mos­phere, to the point where the hu­man ear can bare­ly de­tect them. With this world-class low-pres­sure tur­bine, a key com­po­nent with­out which the GTF would not ex­ist, MTU to­day has moved in­to a league of its own. But even this ex­cep­tion­al tur­bine has room for im­prove­ment, through new ma­te­ri­als, pow­er­ful com­put­er sim­u­la­tions, and new­ly op­ti­mized aero­dy­nam­ics. MTU en­gi­neers are cur­rent­ly work­ing at it—and in do­ing so, they will write the next chap­ter in the ex­tra­or­di­nary sto­ry of the low-pres­sure tur­bine’s de­vel­op­ment.