Work in progress: a new engine generation

What engines will power our airliners in 10 to 30 years? In two parts we investigate what engine makers, universities and research agencies have in store. This month is the third anniversary of the entry into service of Pratt & Whitney’s Geared Turbofan on the Airbus A320neo. The GTF is seen by many as the next step in civil aero engine technology. But what’s the next step and by whom? Looking into the next decade there will probably be just a handful of clean-sheet airliners entering service: the Chinese/Russian CR929 and Boeing’s New Middle Market aircraft or 797. So it’s not airframe designs that drive new engine technology. That seems to come more from engine makers themselves, pushed by rule-makers and the political (and public…) debate on climate change, reduction of emissions and noise pollution. So what are Pratt & Whitney, General Electric and Rolls-Royce working on for the next decade? Despite P&W’s much-publicized Geared Turbofan (GTF) quality and production issues, the engine itself has delivered on promises of lower specific fuel consumption (SFC), carbon dioxide (CO2), nitrogen oxide (NOx) and less noise. Ironing out the tech gremlins and production delays have been the top priority at P&W for the last two years, but now the focus is shifting towards the near future. “Over the next 30-35 years we believe the GTF will be THE engine and continuously improved engine”, says spokesperson Jenny Dervin. Pratt wishes to extend the power gearbox concept to more products. “The GTF is scalable, up and down. Some of the questions we have to answer is the ratio of turbine speed and fan speed, which currently is 3:1. We believe we can go up from the current 33.000lbs on the A320neo to 50.000lbs with the same gear ratio”, says Dervin. P&W has offered this higher thrust version to Boeing for the NMA, “but it is for them to decide what kind of thrust size they really want’’. It’s just one of many decisions Boeing has to make on the 797, with a go/no go-decision expected in the coming months. Model of the P&W planetary gearbox on the Geared Turbofan. (Richard Schuurman) Except for a bigger version Pratt is working on how to make the GTF better. “On a longer timeline, we’re looking at introducing a GTF 2.0 by the time the other guys are having their first GTF. So we will always be ten years ahead on efficiency”. P&W targets a more efficient core, higher thrust and another 5-7% SFC improvement over the current GTF, which already is a 16-20% improvement over previous generation engines. Pratt’s German partner MTU Aero Engines and Fraunhofer Institute for Production Technology are already testing prototype blisks for a lighter compressor. Instead of being forged they most likely will be 3D printed, as P&W, like its competitors, is heavily investing in additive manufacturing. LEAP When the massive GE9X enters service in 2020 on the Boeing 777X, General Electric’s turbofan family will be up to date again with the GE9X at the top range, the improved GEnx in the middle and the CFM LEAP (together with Safran) at the lower end of the spectrum. Like P&W CFM has offered Boeing a scaled-up version of the LEAP for the NMA. At 50-52.000lb it will be just outside CFM’s agreed thrust limit, but CEO David Joyce thinks it’s still within the spirit of the agreement GE and Safran/Snecma signed many years ago. “It’s gonna be a next-generation engine with an architecture that’s similar to todays. So a direct drive two spool engine with a bigger fan without power gearbox, a smaller core with a higher pressure ratio in the compressor and a by-pass ratio of 10-12:1”, explains GE’s commercial engines marketing manager Bill Brown. CFM LEAP-1B on the Boeing MAX 9 test aircraft at the 2017 Paris Air Show. (Richard Schuurman) GE and Safran are working on an improved LEAP, which in its current form already has an advanced core with ceramic matrix composites (CMCs) and 3D-printed fuel injectors. With GE responsible for the core, Safran is exploring options for the fan and LPT system. But don’t expect a power gearbox here, says Brown: “We are looking at three things: architecture, aerodynamics, and materials. If you focus all your efforts on the architecture you’ll miss two of the other elements that drive half of the performance. That’s core aerodynamics and materials, which means durability. You are talking about more ceramics in the turbine. These technologies give you much more capabilities than how you drive the fan”. Short term it’s GE and Safran’s priority to tidy up LEAP production after both Airbus and Boeing suffered all 2018 from Q1 production delays. The output should increase to 2000 engines next year. Rolls-Royce Rolls-Royce is in the same boat with quality and production issues severely hurting the engine makers’ reputation – and cash position. Affected are the Trent 1000 (Boeing 787), 900 (Airbus A380) as well as the new Trent 7000 (A330neo), with minor hiccups so far on the XWB’s (A350). It says a lot about Rolls-Royce’s strength that it is able to stay focused on the next generation of engines. This will be based on the UltraFan demonstrator, a so-called 2.5 shaft engine with power gearbox and an advanced core. “Propulsive efficiency takes you to higher bypass ratios, a bigger fan and a smaller core. Thermal efficiency takes you to a hotter, higher pressure core. This pretty much describes UltraFan”, says Andy Geer, Chief Engineer and Head of Program for UltraFan. Late in 2013 Rolls-Royce embarked on a roadmap for a successor to the Trent, identifying fourteen key technologies that needed developing, including core layout, a low emission combustor, materials, additive manufacturing plus more recently digitalization. The UltraFan-concept should grow to a family of engines from 25.000-100.000lbs. (Rolls-Royce) The first step towards UltraFan is the Advance3 demonstrator, which is about a new core with ten HPC stages, a lightweight compressor, adaptive turbine cooling, 2-stage HPT with CMC sealing and 1-stage IPT. It's 60:1 overall pressure ratio equals that of the GE9X, while at 11:1 bypass ratio is slightly higher than the Trent 7000’s 10:1. A modified Trent 1000 ALECSys lean combustor has been tested since last January. Coupled to a composite-titanium XWB-84 fan and Trent 1000 LPT, Advance3 ran for the first time in November 2017 and reached full power (84.000lbs) in July 2018. It has so much new technology that Rolls has gone for a stepped approach before moving to the next experiments, says Geer. UltraFan itself gets an updated Advance3 core with extensive use of CMC’s, titanium/aluminide, and nickel/steel to cope with high temperatures. UltraFan is Rolls’ first turbofan with a power gearbox, which lets the turbine run faster and the fan go slower to optimum speed, accommodating for the increase in bypass ratio to 15:1 and overall pressure to 70:1. A multistage IPT-system drives the gearbox, making the LPT obsolete. Four, and soon a five, gearbox prototypes (developed jointly with Liebherr Aerospace) are being tested on the Dahlewitz power rig since May 2017 and have reached 70.000bhp out of the 100,000bhp capability. An all-composite fan system (blades and fan case) features variable blade pitch instead of conventional thrust reverse. The Advanced Low-Pressure System (ALPS) fan was to start additional testing in Q4, while the UltraFan demonstrator itself should begin ground runs in 2021. Airbus is closely involved in powerplant system design, which includes pylon, inlet, nacelle, thrust reverser, and door system, but UltraFan will make its first flight on Rolls’ 747 testbed and not on an Airbus. In 2018 Rolls offered UltraFan to Boeing for its NMA, but on February 28, 2019 announced it had withdrawn from the program after it became apparent that meeting the 2025 deadline for the new aircraft to enter service would be impossible. Rolls-Royce could guarantee that the new engine would have met its own performance and reliability targets and opted to take a couple of years extra for development. With a power band of 25,000-100,000lbs, UltraFan will grow a family well into the next decades. Outside the Big Three, the focus is on Russia and China. United Engine Corporation (UEC) has just been granted Russian certification of the PD-14 turbofan that powers the Irkut MC-21, but entry into service on this new airliner is still a couple of years away. A bigger version called PD-35 is being developed with Aero Engine Corporation of China (AECC) for the CR929 wide body, with a scale model renamed AEF3500 on show at the recent Zhuhai Air Show. Although modern and efficient, Russia’s and China’s latest engines all seem to lack the innovation that their western competitors are working on. Part 2 will survey engine concepts for 2030 and beyond, most from projects funded by the European Union. @ 2019, Richard Schuurman All rights reserved.

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